ALBERT

Description: The eruption of Mt. Pinatubo in June 1991 produced the largest enhancement of stratospheric aerosol loading ever observed by lidar over Hampton, Virginia. Low altitude layers (less than 20 km) were the first to arrive over Hampton in early August, the result of transport associated with a tropospheric anticyclonic cell over North America. The maximum peak scattering ratio, 34 at 22.4 km, and the maximum stratospheric integrated backscatter of 0.0053 sr(exp -1), both at 694 nm, observed since the eruption were measured on February 20, 1992. After decreasing during the spring and summer of 1992, the aerosol burden increased significantly during the winter of 1992-93, evidence of a poleward winter transport from the equatorial reservoir. Over the period from February 1992 to February 1994, the stratospheric aerosol loading decreased with an average 1/e decay time of 10.1 months. The vertical distribution, intensity, and transport of Pinatubo aerosols over this site are described and compared with similar measurements after El Chichon.

Description: T-matrix computations of light scattering by polydispersions of randomly oriented nonspherical aerosols and Mie computations for equivalent spheres are compared. Findings show that even moderate nonsphericity results in suubstantial errors in the retrieved aerosol optical thickness if satellite reflectance measurements are analyzed using Mie theory. On the other hand, the use of Mie theory for nonspherical aerosols produces negligible errors in the computation of albedo and flux related quantities, provided that the aerosol size distribution and optical thickness are known beforehand. The first result can be explained by large nonspherical-spherical differences in scattering phase function, while the second result follows from small nonspherical-spherical differences in single-scattering albedo and asymmetry parameter. No cancellation of errors occurs if one consistently uses Mie theory in the retrieval algorithm and then in computing the albedo for the retrieved aerosol optical thickness.

Description: Azimuthal asymmetries in the atmospheric refractive index can lead to errors in estimated vertical and horizontal station coordinates. Daily average gradient effects can be as large as 50 mm of delay at a 7 deg elevation. To model gradients, the constrained estimation of gradient paramters was added to the standard VLBI solution procedure. Here the analysis of two sets of data is summarized: the set of all geodetic VLBI experiments from 1990-1993 and a series of 12 state-of-the-art R&D experiments run on consecutive days in January 1994. In both cases, when the gradient parameters are estimated, the overall fit of the geodetic solution is improved at greater than the 99% confidence level. Repeatabilities of baseline lengths ranging up to 11,000 km are improved by 1 to 8 mm in a root-sum-square sense. This varies from about 20% to 40% of the total baseline length scatter without gradient modeling for the 1990-1993 series and 40% to 50% for the January series. Gradients estimated independently for each day as a piecewise linear function are mostly continuous from day to day within their formal uncertainties.

Description: Particles leving the neutral sheet in the distant magnetotail at times display adiabatic trajectory sequences characterized by an inflection toward the equator and subsequent mirroring in its vicinity. We demonstrate that this low-latitude mirroring results primarily from a centrifugal deceleration due to the fast direction-changing E x B drift. This effect which we refer to as 'centrifugal trapping' appears both in guiding centre and full particle treatments. It thus does not directly relate to nonadiabatic motion. However, pitch angle scattering due to nonadiabatic neutral sheet interaction does play a role in reducing the parallel speed of the particles. We show that centrifugal trapping is an important mechanism for the confinement of the slowest (typically below the equatorial E x B drift speed) plasma sheet populations to the midplane vicinity.

Description: Measurements of superthermal electron fluxes in the solar wind indicate that field lines within coronal mass ejections, CMEs, near and beyond 1 AU are normally connected to the Sun at both ends. However, on occasion some field lines embedded deep within CMEs appear to be connected to the Sun at only one end. Here we propose an explanation for how such field lines arise in terms of 3-dimensional reconnection close to the Sun. Such reconnection also provides a natural explanation for the flux rope topology characteristic of many CMEs as well as the coronal loops formed during long-duration, solar X-ray events. Our consideration of the field topologies resulting from 3-dimensional reconnection indicates that field lines within and near CMEs may on occasion be connected to the outer heliosphere at both ends.

Description: We examined 11 cases when the interplanetary magnetic field (IMF) was intensely northward (greater than 10 nT) for long durations of time (greater than 3 hours), to quantitatively determine an uppler limit on the efficiency of solar wind energy injection into the magnetosphere. We have specifically selected these large B(sub N) events to minimize the effects of magnetic reconnection. Many of these cases occurred during intervals of high-speed streams associated with coronal mass ejections when viscous interaction effects might be at a maximum. It is found that the typical efficiency of solar wind energy injection into the magnetosphere is 1.0 x 10(exp -3) to 4.0 x 10(exp -3), 100 to 30 times less efficient than during periods of intense southward IMFs. Other energy sinks not included in these numbers are discussed. Estimates of their magnitudes are provided.

Description: The subject of this paper is a self-consistent, magnetohydrodynamic numerical realization for the Earth's magnetosphere which is in a quasi-steady dynamic equilibrium for a due northward interplanetary magnetic field (IMF). Although a few hours of steady northward IMF are required for this asymptotic state to be set up, it should still be of considerable theoretical interest because it constitutes a 'ground state' for the solar wind-magnetosphere interaction. Moreover, particular features of this ground state magnetosphere should be observable even under less extreme solar wind conditions. Certain characteristics of this magnetosphere, namely, NBZ Birkeland currents, four-cell ionospheric convection, a relatively weak cross-polar potential, and a prominent flow boundary layer, are widely expected. Other characteristics, such as no open tail lobes, no Earth-connected magnetic flux beyond 155 R(sub E) downstream, magnetic merging in a closed topology at the cusps, and a 'tadpole' shaped magnetospheric boundary, might not be expected. In this paper, we will present the evidence for this unusual but interesting magnetospheric equilibrium. We will also discuss our present understanding of this singular state.

Description: The stability of collisionless tearing modes is examined in the presence of curvature drift resonances and the trapped particle effects. A kinetic description for both electrons and ions is employed to investigate the stability of a two-dimensional equilibrium model. The main features of the study are to treat the ion dynamics properly by incorporating effects associated with particle trajectories in the tail fields and to include the linear coupling of trapped particle modes. Generalized dispersion relations are derived in several parameter regimes by considering two important sublayers of the reconnecting region. For a typical choice of parameters appropriate to the current sheet region, we demonstrate that localized tearing modes driven by ion curvature drift resonance effects are excited in the current sheet region with growth time of the order of a few seconds. Also, we examine nonlocal characteristics of tearing modes driven by curvature effects and show that modes growing in a fraction of a second arise when mode widths are larger than the current sheet width. Further, we show that trapped particle effects, in an interesting frequency regime, significantly enhance the growth rate of the tearing mode. The relevance of this theory for substorm onset phase and other features of the substorms is briefly discussed.

Description: The second pulsating aurora rocket (PULSAUR 2) rocket was launched into a pulsating aurora on 9 February 1994 and carried a broad range of instruments in order to perform a study of this type of aurora. The rocket measurements were complemented with a set of ground-based measurements. The particle measurements performed in the rocket are related to the ground-based optical measurements performed along the rocket trajectory. It is found that the high energy electrons are largely in phase with the measured luminosity. The EISCAT measurements carried out during the flight are reviewed. The PULSAUR 2 campaign is described. Results concerning the auroral conditions and the particle measurements are presented.

Description: The high-speed correction factor to the O(+)-O collision frequency, resulting from drift velocities between ions and neutrals, is calculated by solving the integral expression in this factor both numerically and analytically. Although the analytic solution is valid for either small or large drift velocities between ions and neutrals, for temperatures of interest and all drift velocities considered, agreement is found between analytic and detailed numerical integration results within less than 1% error. Let T(sub r) designate the average of the ion and neutral temperatures in K, and u = nu(sub d)/alpha, where nu(sub d) is the relative drift velocity in cm/s, and alpha = 4.56 x 10(exp 3) square root of T(sub r) cm/s is the thermal velocity of the O(+)-O system. Then, as u ranges from 0 to 2, the correction factor multiplying the collision frequency increases monotonically from 1 to about 1.5. An interesting result emerging from this calculation is that the correction factor for temperatures of aeronomical interest is to a good approximation independent of the temperature, depending only on the scaled velocity u.

Description: This paper describes an efficient Monte Carlo algorithm for choosing a new direction of a photon after a scattering interaction. The algorithm chooses a scattering angle by linear interpolation in a table of the inverse cumulative scattering probability. A Legendre expansion of the phase function makes it easy to apply Clenshaw's algorithm to build the interpolation table. The points in the table are close enough together that linear interpolation is accurate. With a table of 100,000 entries, we can keep the absolute and relative errors in matching the probability distribution below 10(exp -5).

Description: The relation between the solar wind input to the magetosphere, VB(sub South), and the auroral geomagnetic index AL is modeled with two linear moving-average filtering methods: linear prediction filters and a driven harmonic oscillator in the form of an electric circuit. Although the response of the three-parameter oscillator is simpler than the filter's, the methods yield similar linear timescales and values of the prediction-observation correlation and the prediction Chi(exp 2). Further the filter responses obtained by the two methods are similar in their long-term features. In these aspects the circuit model is equivalent to linear prediction filtering. This poses the question of uniqueness and proper interpretation of detailed features of the filters such as response peaks. Finally, the variation of timescales and filter responses with the AL activity level is discussed.

Description: Empirical data-based models of the magnetosphereic magnetic field have been widely used during recent years. However, the existing models (Tsyganenko, 1987, 1989a) have three serious deficiencies: (1) an unstable de facto magnetopause, (2) a crude parametrization by the K(sub p) index, and (3) inaccuracies in the equatorial magnetotail B(sub z) values. This paper describes a new approach to the problem; the essential new features are (1) a realistic shape and size of the magnetopause, based on fits to a large number of observed crossing (allowing a parametrization by the solar wind pressure), (2) fully controlled shielding of the magnetic field produced by all magnetospheric current systems, (3) new flexible representations for the tail and ring currents, and (4) a new directional criterion for fitting the model field to spacecraft data, providing improved accuracy for field line mapping. Results are presented from initial efforts to create models assembled from these modules and calibrated against spacecraft data sets.

Description: We have investigated the middle atmospheric response to the 27-day and 11-yr solar UV flux variations at low to middle latitudes using a two-dimensional photochemical model. The model reproduced most features of the observed 27-day sensitivity and phase lag of the profile ozone response in the upper stratosphere and lower mesosphere, with a maximum sensitivity of +0.51% per 1% change in 205 nm flux. The model also reproduced the observed transition to a negative phase lag above 2 mb, reflecting the increasing importance with height of the solar modulated HO(x) chemistry on the ozone response above 45 km. The model revealed the general anti-correlation of ozone and solar UV at 65-75 km, and simulated strong UV responses of water vapor and HO(x) species in the mesosphere. Consistent with previous 1D model studies, the observed upper mesospheric positive ozone response averaged over +/- 40 was simulated only when the model water vapor concentrations above 75 km were significantly reduced relative to current observations. In agreement with observations, the model computed a low to middle latitude total ozone phase lag of +3 days and a sensitivity of +0.077% per 1% change in 205 nm flux for the 27-day solar variation, and a total ozone sensitivity of +0.27% for the 11-yr solar cycle. This factor of 3 sensitivity difference is indicative of the photochemical time constant for ozone in the lower stratosphere which is comparable to the 27-day solar rotation period but is much shorter than the 11-yr solar cycle.

Description: The accuracy of Geosat satellite altimetry over the Greenland ice sheet is evaluated by comparing the measured heights to radar elevations from the airborne Greenland Aerogeophysics Project. At the center of the ice sheet where the ice surface is nearly level, surface comparisons show a fit at the 1 to 3 m level as expected, but even at moderately sloping ice regions (0.3 deg-0.6 deg), satellite altimetry mean errors in the range of 10 to 35 m are observed. These errors are found for slope-corrected and waveform-retracked data, so most previous accuracy estimates of current satellite altimetry ice sheet elevations in regions of slopping or undulating ice appear to be too optimistic.

Description: We report here on a number of examples of anomalous enhancements of eastward electric fields near sunrise in the equatorial ionospheric F-region. These examples were selected from the data base of the equatorial satellite, San Marco D (1988), which measured ionospheric electric fields during a period of solar minimum. The eastward electric fields reported correspond to vertical plasma drifts. The examples studied here are similar in signature and polarity to the pre-reversal electric field enhancements seen near sunset from ground-based radar systems. The morphology of these sunrise events, which are observed on about 14% of the morning-side satellite passes, are studied as a function of local zonal velocity, magnetic activity, geographic longitude and altitude. The nine events studied occur at locations where the zonal plasma flow is generally measured to be eastward, but reducing as a function of local time and at satellite longitudes where the magnetic declination has the opposite polarity as the declination of the sunrise terminator.

Description: A nonlinear filtering method is introduced for the study of the solar wind -- magnetosphere coupling and related to earlier linear techniques. The filters are derived from the magnetospheric state, a representation of the magnetospheric conditions in terms of a few global variables, here the auroral electrojet indices. The filters also couple to the input, a representation of the solar wind variables, here the rectified electric field. Filter-based iterative prediction of the indices has been obtained for up to 20 hours. The prediction is stable with respect to perturbations in the initial magnetospheric state; these decrease exponentially at the rate of 30/min. The performance of the method is examined for a wide range of parameters and is superior to that of other linear and nonlinear techniques. In the magnetospheric state representation the coupling is modeled as a small number of nonlinear equations under a time-dependent input.

Description: Magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field is thought to play a major role in the transfer of solar wind momentum and energy to the magnetosphere. As the angle between the IMF and the geomagnetic field is changed at the bow of the magnetosphere, the topological record of the location of the reconnection region should be recorded in the magnetosheath and on the magnetopause along the flanks of the tail, because the super fast flow freezes strong magnetic gradients formed in the bow reconnection regions into the plasma downstream. In this report, we present results from a three-dimensional, magnetohydrodynamic (MHD), global numerical simulation code for the location of the separatrix between unconnected IMF magnetosheath field lines and reconnected field lines which penetrate the magnetopause and connect to the polar ionosphere. The angle between the IMF direction and the line where the separatrix crosses the magnetopause is shown to be a sensitive function of the IMF clock angle. We also explain how this behavior can be used to derive an approximate relation for the dependence of the cross-polar voltage on the IMF clock angle. We conclude with a note of caution concerning the importance of physical boundary conditions in magnetoplasma simulations.

Description: Many problems in geophysical and astrophysical convection systems are characterized by fast rotation and spherical shell geometry. The combined effects of Coriolis forces and spherical shell geometry produce a unique spatial symmetry for the convection pattern in a rapidly rotating spherical shell. In this paper, we first discuss the general spatial symmetries for rotating spherical shell convection. A special model, a spherical shell heated from below, is then used to illustrate how and when the spatial symmetries are broken. Symmetry breaking occurs via a sequence of spatial transitions from the primary conducting state to the complex multiple-layered columnar structure. It is argued that, because of the dominant effects of rotation, the sequence of spatial transitions identified from this particular model is likely to be generally valid. Applications of the spatial symmetry breaking to planetary convection problems are also discussed.

Description: Initial observations of a newly documented type of optical emission above thunderstorms are reported. 'Blue jets', or narrowly collimated beams of blue light that appear to propagate upwards from the tops of thunderstorms, were recorded on B/W and color video cameras for the first time during the Sprites94 aircraft campaign, June-July, 1994. The jets appear to propagate upward at speeds of about 100 km/s and reach terminal altitudes of 40-50 m. Fifty six examples were recorded during a 22 minute interval during a storm over Arkansas. We examine some possible mechanisms, but have no satisfactory theory of this phenomenon.

Description: It is shown from flux transfer event (FTE) occurrence statistics, observed as a function of MLT by the ISEE satellites, that recent 2-dimensional analytic theories of the effects of pulsed Petschek reconnection predict FTEs to contribute between 50 and 200 kV to the total reconnection voltage when the magnetosheath field points southward. The upper limit (200 kV) allows the possibility that FTEs provide all the antisunward transport of open field lines into the tail lobe. This range is compared with the voltages associated with series of FTEs signatures, as inferred from ground-based observations, which are in the range 10-60 kV. We conclude that the contribution could sometimes be made by a series of single, large events; however, the voltage is often likely to be contributed by several FTEs at different MLT.

Description: New investigations of the core ion motion within high-latitude topside ionosphere and near-Earth magnetosphere, using data from the Dynamics Explorer (DE) retarding ion mass spectrometer (RIMS), reveal the existence of significant regions of downward moving O(+). The occurences of downgoing versus upgoing O(+) are not clearly separable in terms of either polar zenith angle or Kp but are well distinquished by the direction of the z component of the interplanetary magnetic field (IMF). On the average, down flow dominates when IMFB2 less than O, while upflow dominates for IMFB2 greater than 0. Combining cross-field convection velocities derived from an empirical convection electric field model with the observed parallel velocities yields a two dimensional ion velocity field. This velocity field is consistent with a senario which has O(+) of cusp/cleft and auroral zone origin concvecting into the polar cap and, because of the dominance of gravitational energy over the upward kinetic energy, falling back into the inonsphere. This provides additional confirmation of the results of studies of the cleft ion fountain. Estimates of the flux of O(+) in the upflowing and downflowing regions for Lambda greater than or equal to 60 deg give a total upflow of approximately 6 X 10(exp 25) ions/sec for IMFB2 greater than 0 and total upflow and downflow of approximately 4 X 10 (exp 25) ions/sec and 1 x 10(exp 25) ions/sec, respectively, for IMFB less than 0. In all cases the magnitude of the dayside outflow is consistent with previous work on upwelling ions. While the magnitudes vary for high and low Kp the ratios of upward to downward flow are roughly the same at approximately 1.7. The downflowing O(+) shows a correlation with the magnitude of the outflow of light ions in the same region but the cause and effect of this relationship is not distinquishable. hable.

Description: We have developed the numerical algorithm for the computation of transient viscoelastic responses in the time domain for a radially stratified Earth model. Stratifications in both the elastic parameters and the viscosity profile have been considered. The particular viscosity profile employed has a viscosity maximum with a constrast of O(100) in the mid lower mantle. The distribution of relaxation times reveals the presence of a continuous spectrum situated between O(100) and O(exp 4) years. The principal mode is embedded within this continuous spectrum. From this initial-value approach we have found that for the low degree harmonics the non-modal contributions are comparable to the modal contributions. For this viscosity model the differences between the time-domain and normal-mode results are found to decrease strongly with increasing angular order. These calculations also show that a time-dependent effective relaxation time can be defined, which can be bounded by the relaxation times of the principal modes.

Description: The effects on double-probe electric field measurements induced by electron density and temperature gradients are investigated. We show that on some occasions such gradients may lead to marked spurious electric fields if the probes are assumed to lie at the same probe potential with repect to the plasma. The use of a proper bias current will decrease the magnitude of such an error. When the probes are near the plasma potential, the magnitude of these error signals, delta Epsilon, can vary as delta Epsilon approx. T(sub e)(Delta n(sub e)/n(sub e)) + 0.5 Delta T(sub e), where T(sub e) is the electron temperature, Delta n(sub e)/ n(sub e) the relative electron density variation between the two sensors, and Delta T(sub e) the electron temperature difference between the two sensors. This not only implies that the error signals will increase linearly with the density variations but also that such signatures grow with Delta T(sub e) i.e., such effects are 10 times larger in a 10-eV plasma than in a 1-eV plasma. This type of error is independent of the probe separation distance provided the gradient scale length is much larger than the distance. The largest errors occur when the probes are near to the plasma potential. During the crossing of a small structure (e.g, a double layer) the error signal appears as a bipolar signature. Our analysis shows that errors in double-probe measurements caused by plasma gradients are not significant at large scale (much greater than 1 km) plasma boundaries, and may only be important in cases where small-scale (less than 1 km), internal gradient structures exist. Bias currents tailored for each plasma parameter regime (i.e., variable bias current) would improve the double-probe response to gradient effects considerably.

Description: The evolution of the volcanic debris plume originating from the June 1991 eruption of Mt. Pinatubo has been monitored since its genesis using a ground-based backscatter lidar facility sited at the Jet Propulsion Laboratory (JPL). Both absolute and relative pre- and post-Pinatubo backscatter observations are in accord with Mie scattering projections based on measured aerosol particle size distributions reported in the literature. The post-Pinatubo column-integrated backscatter coefficient peaked approximately 400 days after the eruption, and the observed upper boundary of the aerosol column subsided at a rate of approximately 200 m/mon.

Description: Using Global Positioning System (GPS) receivers, we reoccupied several leveling benchmarks on the Kenai Peninsula of Alaska which had been surveyed by conventional leveling immediately following the March 27, 1964, Prince William Sound earthquake (M(sub w) = 9.3). By combining the two sets of measurements with a new, high-resolution model of the geoid in the region, we were able to determine the cumulative 1993-1964 postseismic vertical displacement. We find uplift at all of our benchmarks, relative to Seward, Alaska, a point that is stable according to tide gauge data. The maximum uplift of about 1 m occurs near the middle of the peninsula. The region of maximum uplift appears to be shifted northwest relative to the point of maximum coseismic subsidence. If we use tide gauge data at Nikishka and Seward to constrain the vertical motion, then the observed uplift has a trenchward tilt (down to the southeast) as well as an arching component. To explain the observations, we use creep-at-depth models. Most acceptable models require a fault slip of about 2.75 m, although this result is not unique. If the slip has been continuous since the 1964 earthquake, then the average slip rate is nearly 100 mm/yr, twice the plate convergence rate. Comparing the net uplift achieved in 29 years with that observed over 11 years in an adjacent region southeast of Anchorage, Alaska, we conclude that the rate of uplift is decreasing. A further decrease in the uplift rate is expected as the 29-year averaged displacement rate is about twice the plate convergence rate and therefore cannot be sustained over the entire earthquake cycle.

Description: Narrow enhancements of electron precipitation, with energy and flux well above typical values, have been observed with Dynamics Explorer 2 (DE 2) in the cusp/cleft region. The electron flux in the energy range 0.2-1 keV was 2 orders of magnitude higher in these structures than in the magnetosheath and were seen in approximaetly 80% of DE 2 cusp crossings at ionospheric altitudes. Typically, there was more than one electron structure in each cusp crossing. The position of these structures showed a systematic variation: for poleward ion dispersion (energy decreases with increasing latitude), electron structures were seen more often on the equatorial boundary of the cusp, while for equatorward ion dispersion (energy decreases with decreasing latitude), electron structures were more often seen on the poleward boundary. This suggests that the electron structures are associated with newly reconnected field lines. The electron spectra suggest that field-aligned acceleration processes could produce the electron structures, first near the boundary of the cusp/cleft during the reconnection of field lines and then in the cusp/cleft during the motion of reconnected flux tubes through the polar ionosphere.

Description: A permanent Global Positioning System (GPS) receiver at Casa Diablo Hot Springs, Long Valley Caldera, California was installed in January, 1993, and has operated almost continuously since then. The data have been transmitted daily to the Jet Propulsion Laboratory (JPL) for routine analysis with data from the Fiducial Laboratories for an International Natural sciences Network (FLINN) by the JPL FLINN analysis center. Results from these analyses have been used to interpret the on going deformation at Long Valley, with data excluded from periods when the antenna was covered under 2.5 meters of snow and from some periods when Anti Spoofing was enforced on the GPS signal. The remaining time series suggests that uplift of the resurgent dome of Long Valley Caldera during 1993 has been 2.5 +/- 1.1 cm/yr and horizontal motion has been 3.0 +/- 0.7 cm/yr at S53W in a no-net-rotation global reference frame, or 1.5 +/- 0.7 cm/yr at S14W relative to the Sierra Nevada block. These rates are consistent with uplift predicted from frequent horizontal strain measurements. Spectral analysis of the observations suggests that tidal forcing of the magma chamber is not a source of the variability in the 3 dimensional station location. These results suggest that remotely operated, continuously recording GPS receivers could prove to be a reliable tool for volcanic monitoring throughout the world.

Description: A comparison is made of the vertical distribution of high-level cloud tops derived from the Stratospheric Aerosol and Gas Experiment II (SAGE II) occultation measurements and from the International Satellite Cloud Climatology Project (ISCCP) for all Julys and Januarys in 1985 to 1990. The results suggest that ISCCP overestimates the pressure of high-level clouds by up to 50-150 mbar, particularly at low latitudes. This is caused by the frequent presence of clouds with diffuse tops (greater than 50% time when cloudy events are observed). The averaged vertical extent of the diffuse top is about 1.5 km. At midlatitudes where the SAGE II and ISCCP cloud top pressure agree best, clouds with distinct tops reach a maximum relative proportion of the total level cloud amount (about 30-40%), and diffuse-topped clouds are reduced to their minimum (30-40%). The ISCCP-defined cloud top pressure should be regarded not as the material physical height of the clouds but as the level which emits the same infrared radiance as observed. SAGE II and ISCCP cloud top pressures agree for clouds with distinct tops. There is also an indication that the cloud top pressures of optically thin clouds not overlying thicker clouds are poorly estimated by ISCCP at middle latitudes. The average vertical extent of these thin clouds is about 2.5 km.

Description: Global high-level clouds identified in Stratospheric Aerosol and Gas Experiment II (SAGE II) occultation measurements for January and July in the period 1985 to 1990 are compared with near-nadir-looking observations from the International Satellite Cloud Climatology Project (ISCCP). Global and zonal mean high-level cloud amounts from the two data sets agree very well, if clouds with layer extinction coefficients of less than 0.008/km at 1.02 micrometers wavelength are removed from the SAGE II results and all detected clouds are interpreted to have an average horizontal size of about 75 km along the 200 km transimission path length of the SAGE II observations. The SAGE II results are much more sensitive to variations of assumed cloud size than to variations of detection threshold. The geographical distribution of cloud fractions shows good agreement, but systematic regional differences also indicate that the average cloud size varies somewhat among different climate regimes. The more sensitive SAGE II results show that about one third of all high-level clouds are missed by ISCCP but that these clouds have very low optical thicknesses (less than 0.1 at 0.6 micrometers wavelength). SAGE II sampling error in monthly zonal cloud fraction is shown to produce no bias, to be less than the intraseasonal natural variability, but to be comparable with the natural variability at longer time scales.

Description: Enhancements in the fluxes of relativistic electrons trapped within the Earth's magnetosphere have been measured by the high-energy particle spectrometer, part of the particle environment monitor on the upper atmosphere research satellite (UARS). The largest increase in the electron fluxes with energies greater than 1 MeV observed on UARS from October 1991 through July 1994 was in early May 1992. The fluxes of trapped electrons in the drift loss cone and locally precipitating electrons showed differing buildup and decay rates as a function of invariant latitude. Increases of more than 2 orders of magnitude were observed in drift loss cone fluxes at magnetic latitudes of 40 deg-66 deg and in precipitating fluxes from 48 deg to 66 deg. The energy flux contained in the most intense local precipitation observed was approximately 0.1 erg/sq cm/s, entering the atmosphere and creating up to 1000 ion pairs/cu cm/s at 55-km altitude. The daily averaged energy flux from directly precipitating electrons with energies greater than 1 MeV deposited greater than 10(exp 20) erg/d worldwide into the atmosphere for the period May 12-21, 1992, producing greater than 10(exp 31) odd nitrogen molecules below 60-km altitude.

Description: The proposed general analytical model describes the anisotropic, elastoplastic, path-dependent, stress-strain properties of inviscid saturated clays under undrained conditions. Model parameters are determined by using results from strain-controlled simple shear tests on a saturated clay. The model's accuracy is evaluated by applying it to predict the results of other tests on the same clay, including monotonic and cyclic loading. The model explains the very anisotropic shear strength behavior observed for weak marine clays.

Description: The Apollo Soyuz Test Project Geodynamics Experiment was performed to assess the feasibility of tracking and recovering high frequency components of the earth gravity field by utilizing a synchronous orbiting tracking station such as Applications Technology Satellite 6. Two prime areas of data collection were selected for this experiment. The first area is the center of the African continent, and the second area is the Indian Ocean depression centered at latitude 5 N and longitude 75 E. Preliminary results show that the detectability objective of the experiment has been achieved in both areas as well as in several additional anomalous areas around the globe. Gravity anomalies of the Karakoram and Himalayan mountain ranges of ocean trenches, and of the Diamantina depth are specific examples.

Description: The Doppler tracking experiment was designed to test the feasibility of improved mapping of earth gravity field anomalies by means of the low-low satellite-to-satellite tracking method. All prescribed data have been retrieved and are currently being reduced and analyzed. Baseline data taken while the docking module was still attached to the command and service module indicated that the equipment operated satisfactorily. The efficacy of the two frequency ionospheric correction method has been demonstrated, and preliminary reduction of a data sample has successfully removed extraneous signatures down to the 50-millihertz level, where the rotational motion of the docking module is revealed. Photographs of the docking module, taken shortly after jettison, show that its rotation was stable.

Description: A large set of bow shock crossings (i.e., 1392) observed by 17 spacecraft has been used to explore the three-dimensional shape and location of the Earth's bow shock and its dependence on solar wind and interplanetary magnetic field (IMF) conditions. This study investigates deviations from gas dynamic flow models associated with the magnetic terms in the magnetohydrodynamic (MHD) equations. Empirical models predicting the statistical position and shape of the bow shock for arbitrary values of the solar wind pressure, IMF, and Alfvenic Mach number (M(sub A)) have been derived. The resulting data set has been used to fit three-dimensional bow shock surfaces and to explore the variations in these surfaces with sonic (M(sub S)), Alfvenic (M(sub A)) and magnetosonic (M(sub MS)) Mach numbers. Analysis reveals that among the three Mach numbers, M(sub A) provides the best ordering of the least square bow shock curves. The subsolar shock is observed to move Earthward while the flanks flare outward in response to decreasing M(sub A); the net change represents a 6-10% effect. Variations due to changes in the IMF orientation were investigated by rotating the crossings into geocentric interplanetary medium coordinates. Past studies have suggested that the north-south extent of the bow shock surface exceeds the east-west dimension due to asymmetries in the fast mode Mach cone. This study confirms such a north-south versus east-west asymmetry and quantifies its variation with M(sub S), M(sub A), M(sub MS), and IMF orientation. A 2-7% effect is measured, with the asymmetry being more pronounced at low Mach numbers. Combining the bow shock models with the magnetopause model of Roelof and Sibeck (1993), variations in the magnetosheath thickness at different local times are explored. The ratio of the bow shock size to the magnetopause size at the subpolar point is found to be 1.46; at dawn and dusk, the ratios are found to be 1.89 and 1.93, respectively. The subsolar magnetosheath thickness is used to derive the polytropic index gamma according to the empirical relation of Spreiter et al. (1966). The resulting gamma = 2.3 suggests the empirical formula is inadequate to describe the MHD interaction between the solar wind and the magnetosphere.

Description: Radiation resulting from interaction between the effluent cloud of a space shuttle thruster and the ambient atmosphere was observed with a spectograph aboard the shutttle. The spectral measurements were made between 400 and 800 nm with a resolutoion of 3 nm. The primary emissions are identified as NO2, HNO, O(1)D, and O(1)S. These are the first observations od O(1)S emission in the shuttle plume. These data are compared with the previous measurements, and possible excitation mechanisms are discussed. The results are also compared with a Monte Carlo simulation of thruster plume-atmosphere interaction radiation.

Description: We report multi-instrument observations during an isolated substorm on 17 October 1989. The European Incoherent Scatter (EISCAT) radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71 deg Lambda - 78 deg Lambda. Sub-Auroral Magnetometer Network (SAMNET) and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. Interplanetary Monitoring Platform-8 (IMP-8) magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux in the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71 deg Lambda by the time of the expansion phase onset. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the distant neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase.

Description: The impact of the extraterrestrial object that formed the Chicxulub crater in the northwestern Yucatan peninsula of Mexico is the leading suspect for the extinction of the dinosaurs. This article reports on a Planetary Society expedition to Albion Island in the Rio Hondo region of Belize to investigate evidence supporting the impact theory.

Description: Direct current (DC) electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubbles) are used to estimate the vertical neutral wind speed. The measured zonal electric field in a series of density depletions crossed by the San Marco D satellite at 01.47-01.52 Universal Time (UT) on 25 October 1988, can be explained if a downward neutral wind of 15-30 m/s exists. Simultaneously, the F-region plasma was moving downward at a speed of 30-50 m/s. These events appear in the local time sector of 23.00-23.15 in which strong downward neutral winds may occur. Indeed, airglow measurements suggest that downward neutral velocities of 25-50 m/s are possible at time near midnight in the equatorial F-region.

Description: An overview of the observations of backstreaming electrons in the foreshock and the mechanisms that have been proposed to explain their properties will be presented. A primary characteristic of observed foreshock electrons is that their velocity distributions are spatially structured in a systematic way depending on distance from the magnetic field line which is tangent to the shock. There are two interrelated aspects to explaining the structure of velocity distributions in the foreshock, one involving the acceleration mechanism and the other, propagation from the source to the observing point. First, the source distribution of electrons energized by the shock must be determined along the shock surface. Proposed acceleration mechanisms include magnetic mirroring of incoming solar wind particles and mechanisms involving transmission of particles through the shock. Secondly, the kinematics of observable electrons streaming away from a curved shock with an initial parallel velocity and a downstream perpendicular velocity component due to the motional electric field must be determined. This is the context in which the observations and their explanations will be reviewed.

Description: This paper focuses on the comparison of cloud amounts derived from an atmospheric general circulation model (AGCM), Satellite-observed clouds, and Ground-based cloud observations. Unlike Earth Radiation Budget Experiment (ERBE)-type comparisons it does not mix potential errors in the cloud amount with those in the radiation code embedded in the model. Long term cloud climatologies were used to compare global cloud amounts and regional seasonal cycles. The AGCM successfully reproduced the signatures of the warm pool and North Pacific seasonal cycle cloudiness but failed in the low stratus region off the coast of South America, a known problem for AGCMs. The data sets also reproduced the anomaly signature associated with El Nino in the warm pool region, but the model amounts were lower. Global results had a similar success rate, with the model generally producing lower total cloud compared to the satellite and in situ measurements. To compare cloud vertical distributions the cloud height may need to be validated using the corresponding radiation fields. Unfortunately there were also some large discrepancies between the two observed cloud data sets. While tremendously improved over the last decade the character of the observed cloud data sets, must be substantially enhanced before they will be useful in validating AGCMs by any but the crudest levels of comparison.

Description: Phenomena associated with the bowshock, such as its thickness, velocity, and associated waves are described. There are transverse waves radiating away from the shock and at times stationary waves fixed to it. Reflected protons generate waves far upstream. Electrostatic waves occur in the shock.

Description: Heat input calculations were detached from solar extreme UV data and monatomic oxygen densities were derived from simultaneously measured data sets (ion composition 146-191 km) in a study of the heat budget of ionosphere electrons. Earlier inferences that cooling predominates over heating are supported. A search for additional heat sources or a revision of the cooling rates is recommended, by way of balancing the heat budget. Importance is attached to electron cooling by fine structure excitation of monatomic oxygen.

Description: The mesosphere is an atmospheric region characterized by a negative gradient of solar energy absorption and temperature. Although the distribution of most minor constituents is dominated by photochemistry, vertical transport does have a pronounced effect on many of them. The basic dynamic principles are discussed along with their application to the important mesospheric motions: acoustic-gravity waves, tides, planetary-scale waves, and eddy motions. Oxides of nitrogen and hydrogen are also examined which strongly influence the balance of odd oxygen (O and O3). Brief discussions of the chemistry of carbon compounds and of excited species are also included. The chemistry of ionic species in the mesosphere is very important because it strongly influences the propagation and absorption of radio waves. Because of ion clustering and negative-ion formation, such chemistry is extremely complex. The current state of knowledge is discussed in some detail. The principles involved in constructing models for predicting the distribution of minor constituents, both neutral and ionic, are presented.

Description: Electron-density observations made in the topside ionosphere by the Ariel 4 and Isis 2 satellites are examined in conjunction with results obtained by Explorer 45 when it traversed the near-equatorial plasmapause with one hour (both UT and MLT) of the Ariel and Isis traversals of the same L coordinate. Both dusk and night observations are analyzed, and an attempt is made to show that depressions in ionospheric electron density occur in the vicinity of the plasmapause field line. It is concluded that the electron distributions observed in the electron-density troughs at 550 km near dusk by Ariel and at 1400 km near midnight by Isis do not always parallel variations in the light-ion distribution inferred from the Explorer plasmapause traversals and that there appears to be no specific feature of the main ionospheric trough which can be used to identify the plasmapause field line except in a statistical sense.

Description: The possible inhibitory effect of excited ClO produced during chlorine-sensitized ozone decomposition on the stratospheric Cl-ClO-Cl cycle is investigated. Collisional deactivation of excited ClO is discussed, chemical reactions involving excited ClO are examined, and the possible roles of HO2 and NO2 in weakening the Cl-ClO-Cl cycle are evaluated. It is noted that the formation of HOCl through a reaction between ClO and HO2 implies that the Cl-ClO-Cl cycle has not been completed.

Description: The interaction between electromagnetic radiation and the earth's atmosphere is considered, taking into account windows in the electromagnetic spectrum, solar and terrestrial radiation, physical properties of the atmosphere, and clouds. The characteristics of upwelling radiation in the visible wavelengths are discussed along with infrared radiation and microwaves. Attention is given to radiant emittance, radiance, contrast, effects of atmospheric turbulence, ocean color, and questions concerning the classification of surface features with the aid of computers.

Description: The latitudinal width of the magnetospheric whistler duct has been estimated by the first and final invariant latitudes of whistler echoes and the conservation of the magnetic flux for the centered dipole field, using 105 whistler echoes in ISIS VLF data received at Kashima, Japan for 1972-1973. The latitudinal distribution of whistler duct occurrence shows a maximum at invariant latitudes of 40-45 degrees near the maximum occurrence latitude of ground whistlers. The radial width of magnetospheric whistler duct in the geomagnetically equatorial plane increases with invariant latitude of the geomagnetic flux tube in which whistlers propagate.

Description: Observations of lunar occultations of earth at 250 kHz, obtained with the Radio Astronomy Explorer 2 satellite, have been used to derive two-dimensional maps of the location of the sources of terrestrial kilometric radiation (TKR). By examining the two-dimensional source distributions as a function of the observer's location (lunar orbit) with respect to the magnetosphere, the average three-dimensional location of the emission regions can be estimated. Although TKR events at 250 kHz can often be observed at projected distances corresponding to the 250-kHz electron gyro or plasma level (about 2 earth radii), many events are observed much farther from earth (about 5 to 15 radii). On the dayside, emission was apparently observed in the region of the polar cusp and the magnetosheath at a magnetic latitude of about 70 deg; in the night hemisphere, emission is found to be associated with regions of the magnetotail at latitudes of at least 70 deg. The nightside emission is suggestive of a mechanism involving plasma-sheet electron precipitation in the premidnight sector.

Description: A comparative study of thermospheric storms for equinox and winter conditions is presented based on neutral-composition measurements from the Aeros-A neutral-atmosphere temperature experiment. The main features of the two storms as inferred from changes in N2, Ar, He, and O are described, and their implications for current theories of thermospheric storms are discussed. On the basis of the study of the F-region critical frequency measured from a chain of ground-based ionospheric stations during the two storm periods, the general characteristics of the ionospheric storms and the traveling ionospheric disturbances are described. It is suggested that the positive and negative phases of ionospheric storms are different manifestations of thermospheric storms.

Description: Movement of water as vapor in the atmosphere is a fundamental process in the Earth's hydrological cycle. Investigations of spatial and time scales of water vapor transport in the atmosphere are important areas of research. Water vapor transmits energy as a function of its abundance across the spectrum. This is shown in the 400- to 2500-nm spectral region where the transmission of the terrestrial atmosphere has been modeled using the MODTRAN radiative transfer code for a range of water vapor abundances. Based on these model results, spectra measured by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) have been used to investigate the movement of water vapor at 20-m spatial resolution over an 11-by-30-km area at approximately 15-minute time intervals (1.25 hours total). AVIRIS measures the upwelling spectral radiance from 400 to 2500 nm at 10-nm spectral intervals and collects images of 11-by-up-to-1000 km at 20-m spatial resolution. Data are collected at a rate of 1 km of flight line per 4.5 seconds. A set of five AVIRIS flight lines was acquired in rapid succession over Rogers Dry Lake, CA on May 18, 1993 at 18:59, 19:13, 19:29, 19:47, and 19:59 UTC. Rogers Dry lake is located 2 hours north of Los Angeles, CA at 34.84 degrees north latitude and 117.83 degrees west longitude in the Mojave Desert.

Description: The Triad (at a height of 800 km) and Ogo 5 (in the high altitude magnetosphere) magnetic field observations have shown the existence of a field-aligned current system consisting of currents flowing in the polar cap boundary layer and those flowing in another layer located equatorward of the former. In the polar cap boundary layer (identified as the high-latitude boundary of the plasma sheet in the nightside magnetosphere), the current flows into the ionosphere on the morning side and away from the ionosphere on the afternoon side. In the lower-latitude layer, the current directions are reversed. The current in the polar cap boundary layer is considered as the primary field-aligned current system.

Description: The temporal response of the densities of upper-atmospheric ion and neutral constituents to a particular geomagnetic storm is studied using simultaneous ion and neutral-composition data obtained by the OGO 6 satellite during consecutive orbits at altitudes greater than 400 km. The investigated constituents include H(+), O(+), N2, O, He, and H. Derivation of the H density is reviewed, and the main effects of the storm are discussed, particularly temporal and global variations in the densities. It is found that: (1) the H and He densities began to decrease near the time of sudden commencement, with the decrease amounting to more than 40% of the quiet-time densities during the maximum stage at high latitudes; (2) the O and N2 densities exhibited an overall increase which began later than the change in H and He densities; (3) the H(+) density decreased differently in two distinct regions separated near the low-latitude boundary of the light-ion trough; and (4) the O(+) density showed an increase during earlier stages of the storm and decreased only in the Northern Hemisphere during the recovery phase. Certain physical and chemical processes are suggested which play principal roles in the ionospheric response to the storm

Description: The diffusive isothermal partitioning of solute in a layered two-phase material has been analyzed to help elucidate the phenomenon of solute partitioning in multiphase lunar and terrestrial materials and to estimate the cooling history of samples. After reviewing the physical chemistry of partitioning and the case of an infinite one-dimensional diffusion couple, we solve in analytic form the case of a finite one-dimensional couple. The solution can be used to estimate cooling histories or to interpret laboratory experiments on partitioning. A sample calculation is included.

Description: Among the different proposed mechanisms for generating parallel electric fields, magnetic mirroring of charged particles seems to be the most plausible. In the present paper, it is suggested that magnetic mirroring is the basic cause of parallel electric fields in the magnetosphere and that the magnetic mirroring effect may be able to form the basis of an auroral theory that can remove a major portion of the ambiguity of observations. In the model proposed, the parallel electric field is due to a magnetic confinement of a negatively charged hot collision-free plasma. A transfer of electron gyroenergy into wave energy tends to weaken this confinement; if this energy transfer becomes too strong, the parallel potential gradient will break down. Hence, from this model, in contrast to certain other models of parallel electric fields, only a small fraction of the total auroral particle energy may be expected to be transformed into electromagnetic wave energy during the acceleration process.

Description: Thermospheric energetics is examined from the point of view of atomic and molecular processes which convert solar EUV radiative energy into kinetic energy of the ambient electron, ion, and neutral gases. The energy flow from photon to photoelectron-ion pair through energy loss and ion-molecule transfer to eventual electron-ion recombination is traced in detail. Upper and lower bounds are placed on the efficiency of conversion of radiative to thermal energy. Implications for the question of consistency of measured solar EUV fluxes and ionospheric models are discussed.

Description: The paper discusses some phenomena, mainly observed by satellites, which illustrate the use of helium as a tracer for studying the morphology and history of atmospheric responses to energy inputs of varying amplitudes and durations. The effects observed include (1) the annual north-south excursion of the sub-solar point producing the winter helium bulge, (2) the 24-hour diurnal variation, where the helium density peak is phase-shifted to the morning in the lower thermosphere, (3) high latitude magnetospheric heating of the thermosphere, with helium indicating regions of probable upwelling of the heated gas, and (4) gravity wave formation and propagation, with the attendant implications for transport of energy from one region of the atmosphere to another.

Description: Several algorithms have been developed for computing the potential and attraction of a simple density layer. These are numerical cubature, Taylor series, and a mixed analytic and numerical integration using a singularity-matching technique. A computer program has been written to combine these techniques for computing the disturbing acceleration on an artificial earth satellite. A total of 1640 equal-area, constant surface density blocks on an oblate spheroid are used. The singularity-matching algorithm is used in the subsatellite region, Taylor series in the surrounding zone, and numerical cubature on the rest of the earth.

Description: Characteristics of VLF emissions detected by satellite in association with enhancements of ring-current electrons during magnetic storms and substorms are described along with the associated enhancements in electron intensities and the anisotropies of the ring-current electron distribution. It is shown that the emissions are observed only when the satellite is outside the plasmasphere, that the beginning of the emissions coincides with the satellite's encounter with the large electron fluxes in that region, and that the increase in electron intensities associated with the observed emissions is limited only to low-energy electrons. The frequency distributions of emissions with peak frequencies above and below half the electron gyrofrequency at the equator is analyzed. The bimodal frequency distribution of the equatorial whistler-mode emissions is explained in terms of different production regions for emissions at frequencies above and below half the equatorial electron gyrofrequency.

Description: The detection of a stratospheric dust layer of probable volcanic origin over Hampton, Va., on the evening of Jan. 28, 1976, with a 20-inch lidar system is reported. A digitized cathode-ray-tube trace of a return signal from altitudes of 12 to 24 km on January 28 is provided, and it is shown that 38% of the return was due to stratospheric aerosol. Noting that measurements on January 22 and February 2 showed no traces of this stratospheric dust layer, trajectories of dust-laden air parcels are followed from initiation at 0000 GMT on January 24 at the St. Augustine Volcano on Augustine Island, Alaska (59 deg N, 153 deg W). Analysis of the trajectories indicates that dust injected into the stratosphere at 59 deg N, 153 deg W could have been transported to Virginia in about 4 days. It is concluded that if the St. Augustine Volcano was the source of the dust, at least 23 deg of meridional transport from polar latitudes was observed.

Description: Classical linearized gravity wave theory is employed to derive relationships between amplitude ratios and phase angles of atmospheric constituents undergoing acoustic-gravity wave oscillations. These results are compared with recently reported Atmospheric Explorer-C satellite data. Calculated amplitude and phase characteristics, for a large class of pure internal gravity wave oscillations, are in accord with the AE-C satellite measurements.

Description: Absolute intensity measurements of the (8, 6) OH band obtained during 10 flights of the December 1969 NASA Auroral Airborne Expedition are presented. Nightglow intensities higher by a factor of 2 than the usual values are recorded during flights 8, 14, and 15. The OH variations are compared with the evolution of the green line and O2(1 Delta g) emissions measured by other experimenters on board the aircraft. Before sunrise the twilight variations of OH down to a solar depression angle of 5 deg show a rapid decrease. A theoretical prediction of the OH, O I 5577 A, and O2(1 Delta g) emissions is evaluated by means of an extensive time-dependent oxygen-hydrogen model of the 25- to 150-km region. Twilight decrease of the OH emission is interpreted in terms of mesospheric ozone photodissociation. Nighttime variations of the emissions may be reproduced if modifications of the dynamic regime are introduced into the model.

Description: Photometers on the ISIS-II spacecraft provide a view of the atomic oxygen 5577-A and 6300-A emissions and the N2(+) 3914-A emission detected as dayside aurora in the magnetospheric cleft region. The 6300-A emission forms a continuous and permanent band across the noon sector, at about 78 deg invariant latitude, with a defined region of maximum intensity that is never less than 2 kR (uncorrected for albedo) and is centered near magnetic noon. There are significant differences in the intensity patterns on either side of noon and their responses to geomagnetic activity. Discrete 3914-A auroral forms appear within this region at preferred locations that cannot be precisely specified but which tend to the poleward edge of the 6300-A emission in the evening and the equatorward edge in the morning, where the difference between the two emissions is greatest. It is concluded that the discrete auroras observed by all-sky cameras in the day sector follow the 6300-A emission through the cleft region, though a definite cleft boundary is not defined.

Description: An investigation has been conducted of the probable effects of a nearby supernova event on the ozone layer of the earth. It is found that the ozone depletion, although smaller than that estimated by Ruderman (1974), is still significant, and could, as a result of cosmic rays, extend over periods of time from 1000 to 10,000 years. However, the probability of the occurrence of such an event within the past 100 million years appears to be low. The calculated ozone depletion seems to be the major effect of a supernova on a earth-like planet at a distance in the range from 5 to 10 pc.

Description: Studies were made of the electron density profiles at various stages of atmospheric evolution, with the assumption that the atmospheric composition has changed due to the gradual increase of oxygen while the nitrogen level has remained constant. The result of model calculations indicates that when the oxygen content is less than about 1% of the present earth's atmosphere level, the major ions in the F2 region will change from O(+) to N(+). The maximum number density of N(+) ions reaches approximately 10 million per cu cm because of the absence of a rapid loss mechanism for N(+). The height of the N(+) ion density peak is much lower than the height of the F2 layer peak of the current ionosphere.

Description: We have analyzed daytime composition measurements in the thermosphere from Atmosphere Explorer-C by use of a theory of the ion chemistry. Predicted and measured N2(+) densities are compared to infer global variation of the solar ionizing flux. Results indicate that the solar flux of Hinteregger measured on Atmosphere Explorer-C provides an adequate basis for the calculation of solar ultraviolet photoionization rates. The role of metastable species is examined, and theoretical O+(2D) densities are presented.

Description: Several mathematical methods which are available for the description of magnetic fields in space are reviewed. Examples of the application of such methods are given, with particular emphasis on work related to the geomagnetic field, and their individual properties and associated problems are described. The methods are grouped in five main classes: (1) methods based on the current density, (2) methods using the scalar magnetic potential, (3) toroidal and poloidal components of the field and spherical vector harmonics, (4) Euler potentials, and (5) local expansions of the field near a given reference point. Special attention is devoted to models of the magnetosphere, to the uniqueness of the scalar potential as derived from observed data, and to the L parameter.

Description: Total stratospheric ozone values above high-altitude stations in southern California from 1912 to 1950 and northern Chile from 1918 to 1948 are determined using data obtained by the Smithsonian Astrophysical Observatory, including transmission measurements made in the Chappuis band (0.5 to 0.7 micron). The results show that at both sites, total ozone amounts commonly exhibit variations of as much as 20% to 30% on time scales ranging from months to decades. Consideration of the amount of incident solar energy absorbed by the Chappuis band suggests that ozone acts as a shutter on the incoming solar radiation and provides a trigger mechanism between solar activity and climatic change.

Description: The process of nitric oxide formation during atmospheric entry of meteoroids is analyzed theoretically. An ablating meteoroid is assumed to be a point source in a uniform flow with a continuum regime evolving in its wake. The amount of nitric oxide produced by high-temperature reactions of air in the continuum regime is calculated by numerical integration of chemical-rate equations. The results, when summed over the observed mass, velocity, and entry-angle distributions of meteoroids, provide annual global production rates of nitric oxide as a function of altitude. The peak production of nitric oxide is found to occur at altitudes between 90 and 100 km, the total annual rate being about 40 million kg. The present results suggest that the large concentration of nitric oxide observed below 95 km could be attributed to meteoroids instead of photodissociation of nitrogen.

Description: During 12 electron injection events observed by Explorer 45 during the December, 1971, and February, 1972, in magnetic storms it was found that the lowest observable pitch angle particles (down to 7 degrees in one case) arrived first. Pitch angle dispersion and shell-splitting effects are shown to be inadequate explanations for most of these pitch angle distributions. It is suggested that parallel electric fields may be supplying 1- to 5-keV electrons to the magnetosphere from the ionosphere. Various models of parallel electric fields are discussed.

Description: The paper examines temporal variations in electron energy spectra and pitch-angle distributions during a VLF-emission event observed by Explorer 45 in the main phase of a magnetic storm. It is noted that the observed event occurred outside the plasmasphere on the night side of the magnetosphere and that the dusk-side plasmapause had a double structure during the event. It is found that the VLF emissions consisted of two frequency bands, corresponding to the whistler and electrostatic modes, and that there was a sharp band of 'missing emissions' along frequencies equal to half the equatorial electron gyrofrequency. A peculiar pitch-angle distribution for high-energy electrons (50 to 350 keV) is noted. It is concluded that the VLF-producing particles were enhanced low-energy (about 5 keV) ring-current electrons which penetrated into the night side of the magnetosphere from the magnetotail plasma sheet and which drifted eastward after encountering the steep gradient of the geomagnetic field.

Description: The paper investigates two unusual types of ion cyclotron whistlers that were found in the low-latitude topside ionosphere by analyzing ISIS VLF electric-field data received at Kashima, Japan. One type is characterized by an asymptotic frequency equal to one half the local proton gyrofrequency; the asymptotic frequency of the other type corresponds to the minimum proton gyrofrequency along the geomagnetic field line passing through the satellite. The observations are compared with theoretical spectrograms of the ion cyclotron whistlers computed for appropriate model distributions of electrons and ions in the topside ionosphere. It is found that the whistlers with the asymptotic frequency of one half the local proton gyrofrequency are deuteron whistlers and that the other whistlers are due to the transequatorial propagation of proton or deuteron whistlers originating in the southern hemisphere.

Description: A parameterization for the absorption of solar radiation as a function of the amount of water vapor in the earth's atmosphere is obtained. Absorption computations are based on the Goody band model and the near-infrared absorption band data of Ludwig et al. A two-parameter Curtis-Godson approximation is used to treat the inhomogeneous atmosphere. Heating rates based on a frequently used one-parameter pressure-scaling approximation are also discussed and compared with the present parameterization.

Description: Neutral-composition waves excited by auroral heat sources are investigated. For horizontal wavelengths of the order of 1000 km, it is concluded that diffusion processes (1) play a significant role such that deviations from diffusive equilibrium prevail for He throughout the thermosphere; (2) produce phase differences of about 220 deg (or -140 deg) between He and N2, and (3) account for He amplitudes comparable to those of N2. These results basically explain recent AE-C satellite measurements which have revealed an anticorrelation between the heavier and lighter constituents of the thermosphere. The calculations also indicate that temperature and N2 concentration are generally out of phase by about 100 deg.

Description: Far-ultraviolet imagery of the earth in the wavelength ranges from 1050 to 1600 A and from 1250 to 1600 A was obtained from the lunar surface during the Apollo 16 mission on Apr. 21, 1972. The images have an angular resolution of about 2 arcmin (230-km linear resolution) and have been quantitatively analyzed to obtain absolute intensities and spatial distributions of the polar auroras (both wavelength ranges), the day and night airglow, and tropical airglow belts (1250-A to 1600-A wavelength range). The observations are consistent with previous results obtained from the OGO-4 spacecraft, but they have also provided details on the spatial distributions of the various emissions over an entire hemisphere at a single time. A general night airglow, at least in the Northern Hemisphere, is indicated.

Description: It is shown that density and pressure throughout the thermosphere can be adequately described in a logarithmic expansion that provides a sound basis for the application of perturbation theory. This expansion eliminates most of the important nonlinearities associated with density variations. On the basis of this expansion, the validity of perturbation theory can be extended to cover a large variety of atmospheric conditions in which the relative temperature amplitude is less than 0.5 and wind velocities are significantly less than the speed of sound.

Description: We derive a simple expression for the escape flux which takes into account horizontal winds at the exobase. The escape flux is shown to increase with the wind velocity and becomes much larger than the Jeans escape flux for velocities approaching the thermal velocity. This model is applied to terrestrial He-4, where we determine the conditions required for wind-enhanced escape to contribute to the overall He-4 budget. For completeness, we also derive an expression for the escape flux when only a vertical wind is present at the exobase.

Description: The magnetic field of the magnetosheath is most naturally discussed in terms of its steady state and its fluctuating components. The theory of the steady-state field is quite well developed, and its essential features have been confirmed by observations. The interplanetary field is convected through the bow shock, where its magnitude is increased and its direction changed by the minimal amount necessary to preserve the normal component across the shock. Convection within the magnetosheath usually increases the magnitude still further near the subsolar point and further distorts the direction until the field is aligned approximately tangent to the magnetopause. Fluctuations of the magnetosheath field are very complex, variable, and not well understood. Transverse waves are often dominant at frequencies below 0.002 Hz, and compressional waves are often dominant at somewhat higher frequencies. Perturbation vectors of hydromagnetic waves tend to be aligned with the shock and magnetopause surfaces. Magnetosheath waves may be generated upstream, within the magnetosheath, at the bow shock, or at the magnetopause.

Description: The density profiles of stratospheric minor constituents are compiled from several one-dimensional theoretical models that have appeared in the literature. The models are compared with each other and also with observations if data are available. The similarity and disparity of the models are clarified and discussed. If a large difference exists among the models, attempts are made to interpret it as much as possible in terms of the physical and chemical data employed in each model. The inadequacy of one-dimensional models in calculating the realistic ozone density distributions and the problems related to it are briefly discussed.

Description: Mass-spectrometer measurements of O and N2, obtained with the Atmosphere Explorer-C satellite during December 1974 and January 1975, are used to derive the O2 concentration near 250 km from several ionization reactions. In order to separate temperature effects from diffusion effects, the measured O and inferred O2 concentrations are employed to determine, under the assumption of diffusive equilibrium, the O/N2 and O2/N2 ratios at 120 km. It is found that the latitudinal and diurnal variations in the O concentration at 120 km are consistent with previous results obtained with OGO 6 and that the O2 concentration in the Northern (winter) Hemisphere is about twice as high as that in the Southern (summer) Hemisphere. The possible importance of photodissociation in the diurnal O2 variations is discussed.

Description: OGO 6 observations of troughs in the thermal plasma densities in the topside ionosphere are discussed. Ion mass spectrometer measurements were correlated with energetic electron detector and electric field measurements. It is shown that the variation of ion composition at high latitudes is complex and frequently characterized by mid-latitude and high-latitude density depression. Prominent high-latitude troughs in the atomic ion (H, He, O) distributions were seen to lie frequently near the polar cap boundary. This indicates that these troughs are unrelated to the plasmapause which is found on closed magnetic field lines away from the trapping boundary. The production of the high-latitude troughs is shown to be related to enhancements in the soft electron flux and/or to the convection electric field.

Description: Information on both ion density and temperature is obtained from analysis of Retarding Potential Analyzer data from the OGO-4 and Explorer-31 satellites. Results obtained from data in the altitude range of 700-2000 km during medium solar activity are presented. An attempt is made to describe the major altitude variations of ion densities and temperatures at middle and low latitudes. The transition heights, where the heavier and lighter ions are equal, are found to be about 1600 and 1300 km at middle and low latitudes, respectively, for daytime and 700 km at night for middle latitudes. Based on the observed data and using diffusive equilibrium as a first-order approximation, topside ionospheric composition models are given for medium solar activity.

Description: Shock metamorphosed rocks and shock-produced melt glasses from the Wanapitei Lake impact structure have been examined petrographically and by electron microprobe. Eleven clasts exhibiting varying degrees of shock metamorphism and eight impact-produced glasses have been analyzed for Rb, Sr and Sr isotopic composition. Five clasts and one glass have also been analyzed for large ion lithophile (LIL) trace element abundances including Li, Rb, Sr, and Ba and the REE's. The impact event forming the Wanapitei Lake structure occurred 37 m.y. ago based on K/Ar dating of glass and glassy whole-rock samples. Rb/Sr isotopic dating failed to provide a meaningful whole-rock or internal isochron. The isotopic composition of the glasses can be explained by impact-produced mixing and melting of metasediments.

Description: Absorption cross sections for O2, N2, CO2, CH4, N2O, and CO have been measured at each of the lines of the atomic oxygen triplet at 1302, 1305, and 1306 A. Radiation resonantly scattered from oxygen atoms at a temperature of about 300 K was used for the line source. Absorber temperatures were also near 300 K. Direct application of the Lambert-Beer absorption equation yielded pressure-dependent cross sections for carbon monoxide at each line of the O I triplet. Reasons for this apparent dependence are presented and discussed.

Description: During the Skylab 4 mission, the S-193 radar altimeter was operated nearly continuously for a revolution around the world on Jan. 31, 1974. This direct measurement to the sea surface has provided an independent basis for the evaluation of the precision of global geoids computed from satellite-derived earth gravity models. This paper presents comparisons between the Skylab data and several recent gravity models published by Goddard Space Flight Center, the Smithsonian Astrophysical Observatory, and the National Oceanic and Atmospheric Administration. The differences between the altimeter geoid and the satellite geoids were as large as 20 m, rms values ranging from 8 to 10 m. These differences also indicated a systematic long-wavelength variation (about 100 deg) not related to error in the Skylab orbits. Truncation of the models to degree and order 8 did not eliminate the long-wavelength variation, but in every case the rms agreement between the satellite geoids and the altimeter geoid was slightly improved. Orbits computed with the truncated models were found to be inferior to those computed with the complete models.

Description: We have carried out three (piggyback) radon-related projects aboard the KAO. The first, which was limited to upper tropospheric measurements while in level flight, revealed the systematic occurrence of unexpectedly high radon concentrations in this region of the atmosphere. The second project was an instrument development project, which led to the installation of an automatic radon measurement system aboard the NASA ER-2 High Altitude Research Aircraft. In the third, we installed a new system capable of collecting samples during the normal climb and descent of the KAO. The results obtained in these projects have resulted in significant contributions to our knowledge of atmospheric transport processes, and are currently playing a key role in the validation of global circulation and transport models.

Description: The neutral composition and the temperature data obtained from the AEROS-B Neutral Atmosphere Temperature Experiment (NATE) and the Neutral and Ion Mass Spectrometer (NIMS) are compared, and the general validity of inferring gas temperatures from N2 and Ar density profiles is examined by comparing them with the in situ measured values of the neutral kinetic temperature (NATE). At times serious discrepancies are noted between the inferred and the measured temperature. This is particularly evident during periods of increased magnetic activity when the normally observed latitudinal variations are apparently modulated by waves propagating from the polar region to low latitudes. Under these conditions the N2 and Ar densities and temperature oscillations are usually out of phase, and the temperatures inferred from N2 and Ar at a given point become meaningless.

Description: Clear evidence for the existence of a quasi-biennial geomagnetic variation is shown by an analysis of annual averages of the horizontal (H) component of the geomagnetic field observed at five observatories. The analysis uses a numerical filter, which is equivalent to taking the second order time derivative of the time series. The cause for the variation is external to the earth because its amplitude depends on magnetic activity. The second order time derivative of H is well correlated with the corresponding time derivatives of the relative sunspot number and 10.7 cm solar flux. It is suggested that quasi-biennial oscillations observed in the geomagnetic field, cosmic rays, stratospheric zonal wind and temperature, total ozone, and other meteorological parameters could be produced by a common cause on the sun.

Description: The dependence of the charge exchange lifetimes on the mirror latitude for ions mirroring off the geomagnetic equator has been re-computed using improved hydrogen distribution models. The Chamberlain model, with the input parameters determined by recent satellite observations, has been used to define the spatial distribution of the neutral hydrogen environment through which the ring current ions traverse. The resultant dependence of the charge exchange lifetime, tau, on mirror latitude, lambda-m, is best fit by the approximation tau-m = tau-e cos 3.5 lambda-m, where tau-e is the charge exchange lifetime for the equatorial particles.

Description: Spectral analysis is used as an independent test of the reported association between interplanetary-magnetic-field structure and terrestrial weather. Spectra of the Ap geomagnetic activity index and the vorticity area index for the years from 1964 to 1970 are examined for common features that may be associated with solar-related phenomena, specifically for peaks in the power spectra of both time series with periods near 27.1 days. The spectra are compared in three ways, and the largest peak with the smallest probability estimate is found to occur at a period of 27.49 days. This result is considered to be statistically significant at the 98% level. It is concluded that the period derived from the Ap spectrum is related to solar rotation and that the analysis provides supporting evidence for a connection between the vorticity area index and solar activity.

Description: The present theory of electron temperature in the daytime mid-latitude ionosphere is tested by using Atmosphere Explorer-C measurements. In the region below 300 km, where a balance is expected between electron heating by photoelectron impact and electron cooling to ions and neutrals, we find an imbalance in which the cooling rate is consistently higher than the heating rate. The shapes of the altitude profiles also differ substantially. The cooling rate has a sharp peak at about 220 km, while the heating rate exhibits a broad peak about 30 km lower. Improved agreement is achieved at higher altitudes by using an oxygen fine structure loss rate smaller by a factor of 2, based on more recent collision strength calculations. Although this improves the overall agreement of the heating and cooling rates, the shape discrepancy remains, and the new cooling rate falls consistently below the heating rate below 200 km.

Description: One of the dominant electron cooling processes in the ionosphere is caused by electron impact induced fine structure transitions among the ground state levels of atomic oxygen. This fine structure cooling rate is based on theoretical cross sections. Recent advances in the numerical cross section determinations to include polarization effects and more accurate representations of the atomic target result in new lower values. These cross sections are employed in this paper to derive a new fine structure cooling rate which is between 40% and 60% of the currently used rate. A new generalized formula is presented for the cooling rate (from which the fine structure cooling rate is derived), valid for arbitrary mass and temperature difference of the colliding particles and arbitrary inelastic energy difference.

Description: A mid-latitude northern hemisphere model of the daytime ozone distribution in the troposphere, stratosphere, and lower mesosphere has been constructed. Data from rocket soundings in the latitude range of 45 deg N + or - 15 deg N, results of balloon soundings at latitudes from 41 to 47 deg N, and latitude gradients from satellite ozone observations have been combined to produce estimates of the annual mean ozone concentration and its variability at heights up to 74 km for an effective latitude of 45 deg N. This model is a revision for heights above 26 km of the tentative mid-latitude ozone model, included in the U.S. Standard Atmosphere Supplements, 1966, and has been adopted for use in the U.S. Standard Atmosphere, 1976.

Description: A series of proton and electron injections were observed by Explorer 45 associated with several substorms during the main phase of the Feb. 24, 1972 geomagnetic storm. The 1- to 290-keV protons and 1- to 560-keV electrons were observed in the evening quadrant up to L of about 5.2. A model distorted dipole magnetic field and McIlwain's E3 convection electric field were used to backtrack the energy-dispersed electron and proton fluxes to their source at the time of injection. The source turns out to be a region extending over several earth radii outside an injection boundary. In the night magnetosphere, the inferred injection boundary is displaced inward with each successive substorm. The energy dispersion plot of the particles injected during orbit 314 indicates that as the energy of the observed particles decreases there is a smooth transition to the position of the plasmapause. This suggests that for that substorm the injection boundary and the plasmapause were one and the same. The proton 'noses' reported by Smith and Hoffman (1974) are discussed.

Description: An investigation is conducted concerning the feasibility to extrapolate the data of the Ogo 6 empirical composition model to altitudes which are lower than 450 km. Extrapolated Ogo 6 model densities are, therefore, compared with data obtained in the Neutral Atmospheric Composition Experiment (Nace) carried out during the time from April to November 1971. The results of the investigation support the conclusions of an earlier comparison of Ogo 6 and Nace data conducted by Newton et al. (1973).

Description: The behavior of the sodium and hydroxyl nighttime emissions during a stratospheric warming has been studied principally by use of data from the airglow photometers on the OGO 4 satellite. During the late stages of a major warming, both emissions increased appreciably, with the sodium emission returning to normal values prior to the decrease in the hydroxyl emission. The emission behaviors are attributed to temperature and density variations from 70 to 94 km, and a one-dimensional hydrostatic model for that altitude range is used to calculate the effects on the emissions and on the mesospheric ozone densities. These results support the existence of a warming in the upper part of the mesosphere that is correlated with a major stratospheric warming.