ALBERT

Description: High-resolution magnetic field and charged-particle data acquired on March 25, 1986 by the Viking and DMSP-F7 satellites, as they traversed the dawn sector auroral zone on nearly antiparallel trajectories within 40 min of each oher, are analyzed. Magnetic field measurements by Viking at 0850 MLT and by DMSP at 0630 MLT indicate the presence of a large-scale earthward-directed region 1 Birkeland current and an upward-flowing region 2 current. Both satellites also observed a third Birkeland current adjacent to and poleward of the region 1 system with opposite flow. This poleward system is about 0.5 deg invariant latitude wide and has a current density comparable to the region 1 and 2 systems. The highest-latitude current is identified as region 0. Its charged-particle signatures were used to infer field line mapping to the equatorial plane.

Description: Results of the computer simulation of the behavior of the inner magnetosphere during the substorm-type event of September 19, 1976, are discussed. The computed electric fields are found to compare satisfactorily with electric fields measured from S3-2, although there are detailed differences. The three general features on which the model and observations are in good agreement are (1) the magnitude and direction of the high-latitude electric field, (2) the degree to which the low-latitude ionosphere is shielded from the high-latitude convection electric field, and (3) the fact that the poleward electric field on the duskside is significantly larger, on the average, than the equatorward electric field on the dawnside. Simple formulas are presented that give rough estimates of global Joule heating rates from observable parameters.

Description: Results of a comprehensive computer simulation of the behavior of the earth's inner magnetosphere during a substorm-type event are reported. It is pointed out that the computer model self-consistently computes electric fields, currents, and plasma distributions and velocities in the inner-magnetosphere/ionosphere system; parallel electric fields and ionospheric neutral winds, however, are not included. The basic equations of the model are derived, and the inputs are described. An overview of the results is also given. The first appendix contains derivations of general, useful laws of bounce-averaged gradient, curvature, and E x B drifts in a plasma with isotropic pitch angle distribution. The second appendix gives a description of the numerical method used in the simulation.

Description: A geomagnetic storm beginning with an sc occurred on Apr. 9, 1971. During the storm the charged particle lunar environment experiment at the Apollo 14 site, the solar wind spectrometer experiment at the Apollo 12 site, and the Ames magnetometers on Explorer 35 took data in the magnetosheath, at the magnetopause, in the plasma sheet, and in the high-latitude geomagnetic tail. The MIT Faraday cup and Ames magnetometers on board Explorer 33 monitored the solar wind. The data show that the storm was caused by a corotating tangential discontinuity in the solar wind, the magnetopause position is strongly dependent on the attack angle of the solar wind, and the tail field strength was indirectly measured to increase from 10 to 14 gamma after the sc. During the main phase the field strength in the tail was observed to increase to between 28 and 34 gamma. This increase is consistent with a thermal and magnetic compression of the tail radius from about 26 to about 16 earth radii.

Description: Data from the DMSP F2 and F4 satellites for the period December 5-10, 1979, have been used to study the diurnal dependence of the high-latitude ion density at 800-km altitude. A 24-hour periodicity in the minimum orbital density (MOD) during a crossing of the high-latitude region is observed in both the winter and summer hemispheres. The phase of the variation in MOD is such that it has a minimum during the 24-hour period between 0700 and 0900 UT. Both the long-term variation of the high-latitude ion density on a time scale of days, and the orbit-by-orbit variations at the same geomagnetic location in the northern (winter) hemisphere for the magnetically quiet time period chosen, show good qualitative agreement with the diurnal dependence predicted by a theoretical model of the ionospheric density at high latitudes under conditions of low convection speeds (Sojka et al., 1981).

Description: The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large-scale high-latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27 to 29, 1992. When the interplanetary magnetic field (IMF) B(sub z) component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross-polar cap potential drops in the two hemispheres are similar. When B(sub z) is positive and absolute value of B(sub y) greater than B(sub z), the convection configurations are mainly determined by B(sub y) and they may appear as normal 'two-cell' patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross-polar-cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of absolute value of B(sub y)/B(sub z) decreases (less than one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the ' merging cell' potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The 'lobe cell' provides a potential between 8.5 and 26 kV and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if absolute value of B(sub y) greater than the absolute value of B(sub z). To estimate the potential drop of the 'viscous cells,' we assume that the low-latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state.

Description: Radar and optical measurements from Sondrestrom are combined with satellite and Goose Bay data in a study of the poleward edge of the nightside auroral oval during a quiet period. The B(sub y) and B(sub z) components of the interplanetary magnetic field were close to zero, and the B(sub x) component was approximately 8 nT for more than 24 hours. On a large scale, the convection and precipitation patterns remained almost constant during this period; on a small scale, however, the conditions were quite dynamic. At 10- to 20-min intervals the arc that marked the poleward auroral boundary intensified, and a new arc appeared poleward of it. About once per hour, stronger intensifications were observed. One such event is examined in detail. The auroral arcs first appeared to dim, and then they brightened, with a factor of 10 increase in E region electron density. At the time of the brightening a new arc formed poleward of all the arcs. The arcs then drifted southward at velocities of approximately 270 m/s. A plasma drift disturbance, characterized by a doubling of the southward velocity and a reversal in the east-west component, propagated westward at 900 m/s through the fields of view of the Sondrestrom and Goose Bay radars. A simultaneous satellite overpass close to the radars revealed the presence of an energetic ion event similar to the 'velocity dispersed ion structures' observed on the Aureol satellite and presumed to be the signature of fast ion beams within the plasma sheet boundary layer. The stronger arc intensification events observed by the Sondrestrom radar are associated with an increase in plasma flow across the boundary between open and closed magnetic field lines. We interpret this increased flow as the ionospheric signature of abrupt, localized increases in the reconnection rate in the midnight sector.

Description: The Rice Convection Model (RCM) is extended and merged with empirical models so as to cover the entire high-latitude ionosphere with the aim of providing precipitation and electric field inputs for ionosphere and thermosphere modelers and producing a model in which the boundaries of the precipitation and electric field patterns maintain physically consistent relationships to each other. The computed auroral electron energy flux, plotted as a function of latitude, exhibited an exaggerated two-peak structure. When no floor was placed under the precipitation rate, the minimum between the two peaks was much too deep to be consistent with typical observations. The regions of excessively weak precipitation map to equatorial distances of 15-35 RE and thus to the regions of the plasma sheet that were not included in previous self-consistent convection calculations.

Description: The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high-latitude ionospheric convection and field-aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the 'snapshot' distributions derived by AMIE allow us to examine in more detail the distinctions between field-aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field-aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field-aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field-aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When B(sub y) is negative, in the northern hemisphere the downward field-aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi-steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.

Description: The relationship of the large-scale dayside Birkeland currents to large-scale particle precipitation patterns, currents, and convection is examined using DMSP and Sondrestrom radar observations. It is found that the local time of the mantle currents is not limited to the longitude of the cusp proper, but covers a larger local time extent. The mantle currents flow entirely on open field lines. About half of region 1 currents flow on open field lines, consistent with the assumption that the region 1 currents are generated by the solar wind dynamo and flow within the surface that separates open and closed field lines. More than 80 percent of the Birkeland current boundaries do not correspond to particle precipitation boundaries. Region 2 currents extend beyond the plasma sheet poleward boundary; region 1 currents flow in part on open field lines; mantle currents and mantle particles are not coincident. On most passes when a triple current sheet is observed, the convection reversal is located on closed field lines.