ALBERT

Description: Physical mechanisms proposed to explain the absorption of significant p-mode wave power by sunspots are reviewed, and their viability in view of the current knowledge of the scattering process is discussed. It is concluded that there is no satisfactory theoretical model for the absorption of p-modes by sunspots available at present. It is argued that the resonance absorption model is able to obtain the large absorption coefficients observed for nonaxisymmetric perturbations. For axisymmetric perturbations, departures from perfect cylindrical symmetry or the inclusion of a slight twist in the sunspot flux tube may be able to resolve the problem with the absorption of m = 0 wave modes. Other dissipative models, which do not incorporate the background gradient effects inherent in the resonance absorption mechanism, require inconveniently large dissipation coefficients within the sunspot.

Description: A magnetic cloud associated with a 2N flare on January 1, 1978 was observed by IMP-8, Helios A, Helios B, and Voyager 2. The variation of the magnetic field observed at each spacecraft is represented to good approximation by Lundquist's solution for a cylindrically symmetric force-free magnetic field with constant alpha. A least-squares fit of Lundquist's solution to the data from each spacecraft gives the local orientation of the axis of the magnetic cloud. The times of the estimated boundaries of the magnetic cloud at each spacecraft, together with the speeds of the boundaries and the spacecraft position, give the positions of the boundaries at a given time. From these results the magnetic cloud is determined to resemble a flux rope whose minor radius is approximately 0.15 AU at 1 AU, and whose radius of curvature at 1 AU is approximately 1/3 AU.

Description: Explanations for the observed p-mode absorption in sunspots are examined. It is demonstrated that any dissipative process like radiative, viscous, or resistive dissipation leads to the resonant absorption of acoustic waves incident on the sunspot tube, and that the resultant heating rate can be shown to be consistent with the observed absorption of the p-mode power impinging on an isolated inhomogeneously structured sunspot.

Description: Ulysses, a joint ESA/NASA mission launched in October 1990, will be the first to explore the high latitude heliosphere. Launch will be from the Shuttle and a Jupiter gravity assist will be used to send the spacecraft first over the southern solar pole approximately three and one half years after launch and then over the northern solar pole one year later. Instruments will be carried to study the solar wind, the heliospheric magnetic field, energetic solar particles, galactic cosmic rays, solar X-rays, cosmic gamma rays, cosmic dust and interstellar neutral helium. The radio signals used to track and transmit spacecraft data will be used also to sound the corona and to search for gravitational waves.

Description: The Solar and Heliospheric Observatory (SOHO) is described. The two main objectives of SOHO are to improve understanding of solar coronal phenomena and to study solar structure and interior dynamics from its core to the photosphere. The primary goals of the coronal and solar wind studies are to understand the coronal heating mechanism and its expansion into the solar wind. These goals will be achieved both by remote sensing of the solar atmosphere with high resolution spectrometers and telescopes and by in situ measurement of the composition and energy of the resulting solar wind and the energetic particles that propagate through it. The structure and interior dynamics are to be studied by helioseismological methods and the measurement of solar irradiance variations. The SOHO spacecraft will be three-axis stabilized and located in a halo orbit around the L1 Lagrangian point (approximately 1 percent of the distance from the Earth to the Sun). It is currently scheduled for launch in July 1995.

Description: This document presents in viewgraph form techniques for the optimal design of aerospace vehicles which take into account vehicle shape, aerodynamic performance, and weight minimization. Methods include multilevel optimization with linear decomposition, multi-constraint optimization, global sensitivity matrix, and calculation of sensitivity derivatives using both first and second order derivatives. The effects of including flexibility in the optimization is discussed.

Description: Future utilization of space will require large space structures in low-Earth and geostationary orbits. Example missions include: Earth observation systems, personal communication systems, space science missions, space processing facilities, etc., requiring large antennas, platforms, and solar arrays. The dimensions of such structures will range from a few meters to possibly hundreds of meters. For reducing the cost of construction, launching, and operating (e.g., energy required for reboosting and control), it will be necessary to make the structure as light as possible. However, reducing structural mass tends to increase the flexibility which would make it more difficult to control with the specified precision in attitude and shape. Therefore, there is a need to develop a methodology for designing space structures which are optimal with respect to both structural design and control design. In the current spacecraft design practice, it is customary to first perform the structural design and then the controller design. However, the structural design and the control design problems are substantially coupled and must be considered concurrently in order to obtain a truly optimal spacecraft design. For example, let C denote the set of the 'control' design variables (e.g., controller gains), and L the set of the 'structural' design variables (e.g., member sizes). If a structural member thickness is changed, the dynamics would change which would then change the control law and the actuator mass. That would, in turn, change the structural model. Thus, the sets C and L depend on each other. Future space structures can be roughly divided into four mission classes. Class 1 missions include flexible spacecraft with no articulated appendages which require fine attitude pointing and vibration suppression (e.g., large space antennas). Class 2 missions consist of flexible spacecraft with articulated multiple payloads, where the requirement is to fine-point the spacecraft and each individual payload while suppressing the elastic motion. Class 3 missions include rapid slewing of spacecraft without appendages, while Class 4 missions include general nonlinear motion of a flexible spacecraft with articulated appendages and robot arms. Class 1 and 2 missions represent linear mathematical modeling and control system design problems (except for actuator and sensor nonlinearities), while Class 3 and 4 missions represent nonlinear problems. The development of an integrated controls/structures design approach for Class 1 missions is addressed. The performance for these missions is usually specified in terms of (1) root mean square (RMS) pointing errors at different locations on the structure, and (2) the rate of decay of the transient response. Both of these performance measures include the contributions of rigid as well as elastic motion.

Description: A fundamental problem facing controls-structures analysts is a means of determining the trade-offs between structural design parameters and control design parameters in meeting some particular performance criteria. Developing a general optimization-based design methodology integrating the disciplines of structural dynamics and controls is a logical approach. The objective of this study is to develop such a method. Classical design methodology involves three phases. The first is structural optimization, wherein structural member sizes are varied to minimize structural mass, subject to open-loop frequency constraints. The next phase integrates control and structure design with control gains as additional design variables. The final phase is analysis of the 'optimal' integrated design phase considering 'real' actuators and 'standard' member sizes. The control gains could be further optimized for fixed structure, and actuator saturation constraints could be imposed. However, such an approach does not take full advantage of opportunities to tailor the structure and control system design as one system.

Description: The JPL Control/Structure Interaction Program is developing new analytical methods for designing micro-precision spacecraft with controlled structures. One of these, the Conceptual Design Tool, will illustrate innovative new approaches to the integration of multi-disciplinary analysis and design methods. The tool will be used to demonstrate homogeneity of presentation, uniform data representation across analytical methods, and integrated systems modeling. The tool differs from current 'integrated systems' that support design teams most notably in its support for the new CSI multi-disciplinary engineer. The design tool will utilize a three dimensional solid model of the spacecraft under design as the central data organization metaphor. Various analytical methods, such as finite element structural analysis, control system analysis, and mechanical configuration layout, will store and retrieve data from a hierarchical, object oriented data structure that supports assemblies of components with associated data and algorithms. In addition to managing numerical model data, the tool will assist the designer in organizing, stating, and tracking system requirements.

Description: Ever since the United States space program started some forty years ago, there have been many ideas on how the U.S. should proceed to explore space. Throughout the years, many innovative designs have surfaced for transfer vehicles, space stations, and surface bases. Usually the difference in designs are due to differences in mission objectives and requirements. The problem for Mars is how to choose an architecture for human travel to Mars and what kind of base construction to design for Mars that will be reliable and cost effective. Eventually, if the Space Exploration Initiative is to become a reality, NASA will have to select and fund a single mission architecture involving manned and unmanned Mars fly-by precursors, a Mars landing vehicle, and, ultimately, the plan for constructing a Mars base. The decision to commit to a single architecture is a vital one and, therefore, the design issues, the decision making process, and the analysis tools must be available to explore all of the options that are available. A large part of any space mission architecture is the Earth-to-Mars transfer vehicle. The decision on the type of transfer vehicle to design is a crucial one. The many options must take into account the constraints encountered when assembling the vehicle in earth orbit such as effective joining methods, test and evaluation methods, preventative maintenance measures, etc. Therefore, the process of trading off various designs must include every facet of that design. The on-orbit assembly/construction constraints will drive designs and architectures. This viewgraph presentation highlights the above critical issues so that designs may be evaluated from these viewpoints. Evaluating designs from the issues contained in this paper will help decision makers detect inadequate designs. Stressing these issues in the evaluation procedure will have a great impact on the decisions of future space mission transfer vehicles and consequent architectures.

Description: A cooperative United States/Japan study was made for one year from 1987 to 1988 regarding the feasibility of the Tropical Rainfall Measuring Mission (TRMM). As part of this study a phase-A-level design of spacecraft for TRMM was developed by NASA/GSFC, and the result was documented in a feasibility study. The phase-A-level design is developed for the TRMM satellite utilizing a multimission spacecraft.

Description: Gamma-ray observations of the April 27, 1981 flare have been used to derive elemental abundances of both the ambient gas and the accelerated particle which interact with this gas to produce gamma rays. The results show that the abundances of the accelerated particles which produce the gamma rays are much more consistent with first-class particle abundances than with large proton-flare abundances. Observed bremsstrahlung spectral from 16 flares are compared with calculated bremsstrahlung spectra. It is concluded that the electrons observed in interplanetary space and the electrons responsible for gamma-ray production cannot be drawn from the same accelerated particle population without the former being affected by reacceleration or energy-dependent escape.

Description: A spacecraft-shielding technique is investigated in which the geometrical configuration and material used are emphasized. Ultrathin spaced shield elements are employed to repeatedly shock the impacting projectile to a high energy state that causes melting and vaporization. The ratio of the thickness of the elements to projectile diameter corresponds to a relatively small percentage of debris-plume mass that can be withstood by the backsheet. The strength of the backsheet is thereby reduced and employed in a specific configuration that prevents the debris plume from destroying successive sheets before the particulates reach the sheet. The primary benefit is weight reduction of 30 percent when compared to a 'Whipple shield' fabricated with the same material. The concept is shown to be effective against all impact types tested and produces minimal secondary debris.

Description: Averaging methods are applied to analyzing the transient response associated with adaptive control of large space structures. Using a dominant mode approximation to the plant, an analytical bound is found on the envelope of the adaptive response, characterizing many of the features of the response useful for control design (e.g., peak values, quadratic costs, settling times, etc.). An optimal adaptive design problem is formulated based on minimizing the product of the settling time and peak torque requirement. The resulting nonlinear constrained optimization problem is solved in closed form, and several properties of the optimal adaptive design are discussed.

Description: An approach to the integrated design of linear controlled structures, which is being investigated as part of NASA's controlled structures interaction (CSI) methodology development program, is presented. The approach is integrated in the sense that the structure and its controller are simultaneously designed. The design methodology uses constrained nonlinear optimization procedures based on analytically obtained gradients of the structural responses. Design of the controller is based on the so-called Q-parameterization theory, which parameterizes all closed-loop input/output maps achievable with stabiizing linear controllers. Very general objective and constraint functions are possible, and structural shape can be included in the design variables. This method has been partially implemented and demonstrated; early findings are reported.

Description: The problem of controller design for flexible spacecraft is addressed. Model-based compensators, which rely on the knowledge of the system parameters to tune the state estimator, are considered. The instability mechanisms resulting from high sensitivity to parameter uncertainties are investigated. Dissipative controllers, which use collocated actuators and sensors, are also considered, and the robustness properties of constant-gain dissipative controllers in the presence of unmodeled elastic-mode dynamics, sensor/actuator nonlinearities, and actuator dynamics are summarized. In order to improve the performance without sacrificing robustness, a class of dissipative dynamic compensators is proposed and is shown to retain robust stability in the presence of second-order actuator dynamics if acceleration feedback is employed. A class of dissipative dynamic controllers is proposed which consists of a low-authority, constant-gain controller and a high-authority dynamic compensator. A procedure for designing an optimal dissipative dynamic compensator is given which minimizes a quadratic performance criterion. Such compensators offer the promise of better performance while still retaining robust stability.

Description: Detailed information on solar radiation characteristics on Mars are necessary for effective design of future planned solar energy systems operating on the surface of Mars. Presented here is a procedure and solar radiation related data from which the diurnally, hourly and daily variation of the global, direct beam and diffuse insolation on Mars are calculated. The radiation data are based on measured optical depth of the Martian atmosphere derived from images taken of the sun with a special diode on the Viking cameras; and computation based on multiple wavelength and multiple scattering of the solar radiation.

Description: A simple single-axis testbed is described, and initial experimental results are presented to illustrate collocated and noncollocated control for this structure. The testbed is made up of a pair of single-axis flexible beams attached to a DC servo motor. An optical encoder and strain gauges provide hub and beam position information, respectively. The system is driven by an IBM PC system; with a motor controller, a programmable digital filter processes position error information through user-selected gains and pole-zero configurations. A 25-kHz data acquisition system provides the necessary interface between processor and motor. The control approaches currently being investigated include collocated PD control and noncollocated phase compensation.

Description: Experience with the application of positivity designed multivariable controllers for the NASA ACES flexible beam experiment is discussed. Multivariable controllers were designed using sets of rotational sensors/actuators (rate gyros/torque actuators) and linear sensors/actuators (accelerometers/LMEDs). Experience with this set of controllers demonstrated the difficulty of designing controllers with significant modal uncertainty and significant phase uncertainty. With the aid of multivariable scaling techniques these designs were ultimately able to achieve a high level of closed-loop damping.

Description: Control system design and closed-loop test results for the Mini-Mast truss structure located at the NASA Langley Research Center are presented. The simplicity and effectiveness of a classical control approach to the active structural control design are demonstrated by ground experiments. The concepts of robust nonminimum phase compensation and periodic disturbance rejection are also experimentally validated. The practicality of a sensor output decoupling approach is demonstrated for the inherent, multivariable control problem of the Mini-Mast.

Description: A control algorithm for satellite stabilization using a space leech is presented. The space leech is assumed to have n reaction wheels with known moments of inertia about their axis of rotation. All mass properties of the satellite are assumed to be unknown. The algorithm brings the satellite to a specified attitude trajectory. Simulations were performed to demonstrate the controller. The model parameters and specific algorithm used and the results obtained are presented.

Description: Recent interest in short solar periods has been stimulated by the discovery of a peak period of about 150-160 days in very energetic solar flares and hard X-ray flares. Auroral data are used here as a proxy for solar activity to show the presence of the peak in data from 1570 to 1573 and in some other time periods. The data are also used to show the absence or lack of prominence of this peak at other times in the nineteenth and early twentieth centuries.

Description: In an effort to estimate the highest flow velocity that the solar wind has exhibited at earth during the past 50 years, geomagnetic storms that occurred from 1938 to 1989 were surveyed, and the storms that were preceded by a major proton flare were selected. For each identified flare-storm pair, the average speed ('transit speed') of the associated interplanetary shock from the interval between the flare onset and the sudden commencement of the geomagnetic storm was calculated. In each case, the maximum solar wind flow speed was inferred from an empirical relationship (derived for a sample of recent events) between the shock transit speed and the peak flow velocity of the associated transient stream, obtaining a distribution of maximum solar wind speeds, which presumably corresponds to a sample of the most energetic events of this 50-yr period. Results show no evidence for bulk flow velocities greater than the about 2000 km/sec value deduced by Zastenker et al. (1978) and Grunwaldt (1975) for the August 4, 1972 event.

Description: Numerical studies of tearing modes in a nearly singular forced current sheet equilibrium (Liewer and Payne, 1990) show that the modes saturate quasilinearly when the width of the magnetic island formed by the reconnection is on the order of several times the linear mode width which scales as approximately (kS) exp -2/5, where S is the Lundquist number and k is the wavenumber. The modes saturate quasilinearly by flattening the current profile, converting magnetic energy into plasma energy. The longer wavelength modes, which saturate at higher levels, release the most energy. These modes may, nonlinearly, play a role in coronal heating when sharp current sheets form as a result of global magnetic stresses.

Description: Various configurations of forward and reverse shocks that occur in the outer heliosphere can be classified using catastrophe theory. The existence of a forward shock is associated with a local maximum of a polynomial, and the existence of a reverse shock is associated with a local minimum of a polynomial. A configuration with N forward shocks and N reverse shocks corresponds to a polynomial with N maxima and N minima. The formation of forward and reverse shocks corresponds to the creation of maxima and minima of a polynomial, which is described by the separatrices of the catastrophes. The coalescence of two forward (reverse) shocks corresponds to the situation when two maxima (minima) of a polynomial have equal values, and the interaction of a forward shock with a reverse shock corresponds to a polynomial with a local maximum equal to a local minimum; these situations are described by the Maxwell sets of the appropriate catastrophes.

Description: The Ion Composition Instrument (ICI) aboard the ISEE-3/ICE spacecraft was in the solar wind continuously from August 1978 to December 1982. The results made it possible to establish long-term average solar wind abundance values for helium, oxygen, neon, silicon, and iron. The Charge-Energy-Mass instrument aboard the CCE spacecraft of the AMPTE mission has measured the abundance of these elements in the magnetosheath and has also added carbon, nitrogen, magnesium, and sulfur to the list. There is strong evidence that these magnetosheath abundances are representative of the solar wind. Other sources of solar wind abundances are Solar Energetic Particle experiments and Apollo lunar foils. When comparing the abundances from all of these sources with photospheric abundances, it is clear that helium is depleted in the solar wind while silicon and iron are enhanced. Solar wind abundances for carbon, nitrogen, oxygen, and neon correlate well with the photospheric values. The incorporation of minor ions into the solar wind appears to depend upon both the ionization times for the elements and the Coulomb drag exerted by the outflowing proton flux.

Description: Power spectral analyses of the time series of solar proton events during the past three solar cycles reveal a periodicity around 154 days. This feature is prominent in all of the cycles combined, cycles 19 and 21 individually, but is only weak in cycle 20. These results are consistent with the presence of similar periodicities between 152 and 155 days in the occurrence rate of major solar flares, the sunspot blocking function (Ps), the 10.7 cm radio flux (F10.7), and the sunspot number (Rz). This suggests that the circa 154-days periodicity may be a fundamental characteristic of the sun. Periods around 50-52 days are also found in the combined data set and in the three individual cycles, in general agreement with the detection of this periodicity in major flares in cycle 19, and in Ps, F10.7, and Rz in cycle 21. The cause of the 155 day period remains unknown. The spectra contain lines (or show power at frequencies) consistent with a model in which the periodicity is caused by differential rotation of active zones and a model in which it is related to beat frequencies between solar oscillations, as proposed by Wolff (1974, 1983).

Description: Proton spectra, valid from non- to ultra-relativistic energies, resulting from stochastic Fermi acceleration in solar flares are calculated. These spectra were obtained by numerically solving the Fokker-Planck equation, in which the escape of the particles from the acceleration region is characterized by an energy-independent escape time. In addition to equilibrium spectra, time-dependent energy spectra showing the approach to equilibrium are also presented. These numerical equilibrium spectra are compared with previous results which were obtained either by Monte Carlo simulations or approximate analytical treatments. There are no analytic solutions valid in the transrelativistic regime, which is very important for the production of pions and neutrons in solar flares. The acceleration efficiency is related to physical parameters, in particular the energy density in either magnetosonic or Alfven waves, and a lower limit is placed on either of these energy densities from acceleration times implied by gamma-ray observations. Also discussed is the physical interpretation of the escape time.

Description: The three-dimensional dynamical evolution of twisted magnetic flux tubes is studied using a time-dependent magnetohydrodynamic (MHD) model. The flux tubes are intended to model solar coronal loops, and include the stabilizing effect of photospheric line tying. The model permits the complete evolution of flux tubes to be followed self-consistently, including the formation, equilibrium, linear instability, and nonlinear behavior. Starting from an initial uniform background magnetic field, a twisted flux tube is created by the application of slow, localized photospheric vortex flows. The flux tube evolves quasi-statically through sequences of equilibria with increasing twist, until it becomes linearly unstable to an ideal MHD kink mode. In this paper, the equilibrium properties and the linear stability behavior are discussed. The application of the method to the uniform-twist, Gold-Hoyle field confirms the previous stability threshold for kink instability and provides estimates of the resulting growth rate.

Description: Interplanetary magnetic clouds emerge as a feature of the solar wind at 1 AU, exhibiting enhanced field strength and lower plasma temperature and density than the surrounding plasma. A least-squares program has been developed which fits magnetic field data within a cloud, while estimating such cloud properties as its size, maximum field strength, and axis inclination. The results obtained from a study of 12 clouds observed at 1 AU point to a probable cloud axis direction within 15 deg of the ecliptic plane and about 100 deg from the sun's direction, when projected into the ecliptic plane. A wide variety of orientations is observed; some extend to 80 deg from the ecliptic.

Description: A model is presented which depends on the preferential deposition of heating in the legs of a coronal loop and which produces a stable prominence-scale condensation at the loop top. Dynamic stability is attained by the subsequent adjustment of local parallel gravity by a magnetic inversion at the loop (or arcade) apex. A nonlinear numerical simulation of this process, which includes a deep chromosphere, a heating rate with a fixed dissipation length, and full solar gravity is described.

Description: The ideal and resistive magnetohydrodynamic (MHD) model is used to examine the dynamics and structure of the solar corona. When the coronal magnetic field is deformed by photospheric flow it can evolve to states that become unstable to ideal MHD modes. The nonlinear evolution of these instabilities can lead to the generation of current sheets, field line reconnection, and energy release. The disruption of an arcade field and the kinking of coronal loops is described. The braiding of the large-scale coronal field by convective photospheric motions develops fine-scale structure in the magnetic field and leads to the development of intense current filaments. The resistive dissipation of these currents can provide an efficient coronal heating mechanism.

Description: A method has been presented for restricting the random derived flare temperatures and emission measures. The method is based on three tests and is applicable not only to disk-integrated observations, but also for any high-resolution observations that cannot distinguish the emissions from flaring and nonflaring plasma. The first two tests are based on the assumption that both the temperature and the emission measure should increase as the flare flux rises. The third assumes that the preflare temperature of the plasma that flares should exceed the temperature averaged over all other emitting sources. These tests were applied to two solar flares to illustrate the improvements in the derived flare properties. It is shown that flare properties do not always lie between those derived by the use or subtraction of the total preflare flux. The flare properties can, however, be bounded by subtracting and using the long- and short-wavelength preflare fluxes, respectively.

Description: Changes in solar wind speed and magnetic polarity observed at the Pioneer spacecraft are discussed here in terms of the changing magnetic geometry implied by SMM coronagraph observations over the period 1985-1987. The pattern of recurrent solar wind streams, the long-term average speed, and the sector polarity of the interplanetary magnetic field all changed in a manner suggesting both a temporal variation, and a changing dependence on heliographic latitude. Coronal observations during this epoch show a systematic variation in coronal structure and the magnetic structure imposed on the expanding solar wind. These observations suggest interpretation of the solar wind speed variations in terms of the familiar model where the speed increases with distance from a nearly flat interplanetary current sheet, and where this current sheet becomes aligned with the solar equatorial plane as sunspot minimum approaches, but deviates rapidly from that orientation after minimum.

Description: The radio azimuths for many kilometric type III bursts that originate near or behind the limb of the sun are observed to drift far to the east or far to the west of the spacecraft-sun line. It is shown that the behavior of the observed burst parameters for these events corresponds to the response of a spinning dipole antenna to halolike sources of radiation around the sun. These results provide evidence for a previous suggestion that behind-the-limb type III events should appear as halolike sources of radiation to an observer on the opposite side of the sun, due to scattering of the radiation from the primary source back around the sun.

Description: Off-center vector magnetograms which use all three components of the measured field provide the maximum information content from the photospheric field and can provide the most consistent potential field independent of the viewing angle by defining the normal component of the field. The required transformations of the magnetic field vector and the geometric mapping of the observed field in the image plane into the heliographic plane have been described. Here we discuss the total transformation of specific vector magnetograms to detail the problems and procedures that one should be aware of in analyzing observational magnetograms. The effect of the 180-deg ambiguity of the observed transverse field is considered as well as the effect of curvature of the photosphere. Specific results for active regions AR 2684 (September 23, 1980) and AR 4474 (April 26, 1984) from the Marshall Space Flight Center Vector magnetograph are described which point to the need for the heliographic projection in determining the field structure of an active region.

Description: A 'Spoerer butterfly' method is used to examine solar cycle 22. It is shown from the latitude of active regions that the cycle can now be expected to peak near November 1989 + or - 8 months, basically near the latter half of 1989.

Description: A recent model for solar constant secular changes is used to calculate a 'proxy' solar constant for: (1) the past four centuries, based upon the sunspot record, (2) the past nine centuries, based upon C-14 observations and their relation to solar activity, and (3) the next decade, based upon a dynamo theory model for the solar cycle. The proxy solar constant data is tabulated as it may be useful for climate modelers studying global climate changes.

Description: The level of skill in predicting the size of the sunspot cycle is investigated for the two types of precursor techniques, single variate and bivariate fits, both applied to cycle 22. The present level of growth in solar activity is compared to the mean level of growth (cycles 10-21) and to the predictions based on the precursor techniques. It is shown that, for cycle 22, both single variate methods (based on geomagnetic data) and bivariate methods suggest a maximum amplitude smaller than that observed for cycle 19, and possibly for cycle 21. Compared to the mean cycle, cycle 22 is presently behaving as if it were a +2.6 sigma cycle (maximum amplitude of about 225), which means that either it will be the first cycle not to be reliably predicted by the combined precursor techniques or its deviation relative to the mean cycle will substantially decrease over the next 18 months.

Description: The average rate of growth in sunspot number over selected time intervals and the maximum average value as they both relate to the size of the cycle are examined, in order to predict the size of cycle 22. The predictions are compared with those of Wilson (1990) to determine whether a consensus is apparent. The average rate of growth during the ascending portion of the sunspot cycle, defined as the difference in smoothed sunspot number values between elapsed time t and sunspot minimum divided by t, is shown to correlate with the size of the sunspot cycle, especially for t greater or equal to 18 months. The maximum value of the average rate of growth is also shown to highly correlate (r = 0.98) with the size of the cycle. Using 4.5 as the maximum value of the average rate of growth, a lower limit for R(M) is estimated. The results show that the findings are consistent with the previous single variate predictions for R(M) for cycle 22.

Description: A method is presented for calculating the three orthogonal components of the velocity perturbations of satellite fragments, with a view to ascertaining the nature and intensity of the satellite breakup. The method employs three simultaneous equations furnished by changes in fragment specific energy, specific angular momentum, and plane orientation. Velocity perturbations are thereby calculated for fragments from 20 major satellite breakup events; these results, in conjunction with a technique for determining fragment masses, yield a description of the breakup process.

Description: All spacecraft are susceptible to impacts by meteoroids and pieces of orbiting space debris. An effective mechanism is developed to protect external spacecraft subsystems against damage by ricochet particles formed during such impacts. Equations and design procedures for protective shield panels are developed based on observed ricochet phenomena and calculated ricochet particle sizes and speeds. It is found that the diameter of the most damaging ricochet debris particle can be as large as 40 percent of the original project tile diameter, and can travel at speeds between 24 and 36 percent of the original projectile impact velocity. Panel dimensions are shown to be strongly dependent on their inclination to the impact velocity vector and on their distribution around a spacecraft module. It is concluded that obliquity effects of high-speed impacts must be considered in the design of any structure exposed to the meteoroid and space debris environment.

Description: A Pioneer or Voyager spacecraft will soon pass through one of the last major frontiers in the solar system - the heliospheric terminal shock. Some unknown, but perhaps small, distance beyond the terminal shock is the heliopause, marking the final boundary between solar-wind and Galactic plasmas and the final goal of these spacecraft. This occasion offers an opportunity to obtain a wealth of information on the properties of the Galaxy, the interstellar medium, and the large-scale interactions of that medium with stellar winds. Several tentative remote detections of the shock place it just beyond the present spacecraft locations. However, uncertainties in the physical processes and parameters that determine the location of the boundary lead to equal uncertainties in predicting which of the spacecraft will reach it first.

Description: Results from a high-resolution multi-layer-coated X-ray imaging telescope, part of the Normal Incidence X-ray Telescope sounding rocket payload are presented. Images of the peak of a two-ribbon flare showed detailed structure within each ribbon, as well as the expected bright arches of emission connecting the ribbons. The number of X-ray bright points is small, consistent with predictions based on the previous solar cycle. Topology of the magnetic structure is complex and highly tangled, implying that the magnetic complexity of the photosphere is paralleled in the corona.

Description: Linear tearing modes are studied in a nearly singular forced current-sheet equilibrium, such as could result from global magnetic forces in the solar corona. Growth rates for the tearing modes, determined by solving the linearized reduced MHD (Strauss) equations numerically, were found to scale as (gamma)tau(d) = about S exp 4/5 k(y) exp 4/5, where S is the Lundquist number, k(y) is the wavenumber, and tau(d) is the classical resistive diffusion time. This scaling is in agreement with predictions from analytical theory. Because of the faster S scaling of these modes compared to the tearing modes of a diffuse current-sheet equilibrium, the modes have much higher growth rates (by a factor of about 10,000) for coronal values of S (about 10 to the 12th). For coronal parameters, the growth times of these new modes are estimated to be on the order of several hours to days, as compared to growth times of months to years for the tearing modes in a diffuse current-shear equilibrium. The growth times are comparable to reconnection times scales required in models of coronal heating by magnetic field dissipation.

Description: Recent observational studies of interplanetary field fluctuations and MHD simulation results are used to suggest that the turbulent evolution of the fields over the solar poles leads to a state intermediate between the large fields of the static limit and the small fields given by 'saturation' (deltaB/B = about 1). In particular, assuming the strength of the coronal hole sources of fluctuations are similar in all solar regions, the polar fluctuations in the outer heliosphere are likely to be nearly isotropic, with amplitudes somewhat smaller than those predicted by the WKB extrapolation from 1 AU in the ecliptic plane.

Description: Thermal instability driven by optically thin radiation is believed to initiate the formation of plasma filaments in the solar corona. The fact that filaments are observed generally to separate regions of opposite, line-of-sight, magnetic polarity in the underlying photosphere suggests that filament formation requires the presence of a highly sheared, local magnetic field. Two-dimensional, nonlinear, magnetohydrodynamic simulations of the local genesis and growth of solar filaments in a force-free, sheared, magnetic field were performed, and the evolution of generic perturbations possessing broad spatial profiles was traced. It was found that simulations of the evolution of initial random-noise perturbations produce filamentary plasma structures that exhibit densities and temperatures characteristic of observed solar filaments. Furthermore, in each of these simulations, the filament axis lies at a finite angle with respect to the local magnetic field, consistent with solar observations.

Description: This paper presents the hypothesis that the periodic meridional (north-south) flow observed by Voyager 2 in the neighborhood of 20-25 AU were produced by a heliospheric vortex street. The separation of the vortices was approximately 6 AU in the radial direction, and the vortices were carried away from the sun at a speed of approximately 425 km/s. This hypothesis can account for the observed + or - 5 degree deflections in the flow with a period of 25.5 days, and it predicts small amplitude fluctuations in the bulk speed with two maxima per solar rotation, which is consistent with observations in 1986.

Description: Two corotating streams per solar rotation, separated by the heliospheric plasma sheet, were observed at 1 AU during 1974, and the streams recurred four times during the interval from day 145 to day 255. A single compound stream per solar rotation was observed at 5.5 - 6.0 AU during the corresponding interval from day 165 to day 275, indicating that the two recurrent streams observed at 1 AU coalesced between 1 AU and 6 AU. The average maximum speed of one of the recurrent streams was 805 km/s while that of the other recurrent stream was 705 km/s. The compound stream was not formed by the overtaking of the slow stream by the fast stream. Rather, it was probably formed by a process involving both filtering (due to the fact that the slow stream was 50 percent wider than the fast stream) and a geometrical effect.

Description: The importance of the form of the driving mechanism in MHD simulations of coronal mass ejections is investigated. A model simulation problem is devised, and it is found that the use of a simple form for the initial corona, with an upward moving parcel of cold, dense plasma as the driving mechanism, can produce results that are consistent with many of the features observed by coronagraphs. The results imply that the nature of the driving mechanism may play an important role in determining the dynamical evolution of mass ejections.

Description: A study is presented of the heliocentric distance, frequency, and stream structure dependence of the amplitudes of interplanetary fluctuations in the velocity and magnetic field from 0.3 to nearly 20 AU and for spacecraft-frame periods of 10 days to a few hours. Evidence is presented that, at a given heliocentric distance, the amplitude of the magnetic field fluctuations is proportional to the magnitude of the field, nearly independently of the solar wind speed. The radial evolution of magnetic fluctuations is shown to be nearly consistent with WKB expectations except at smaller scales in the inner heliosphere and at the largest scales in the outer heliosphere. While the large-scale velocity fluctuations are kinetic energy-dominated in the inner heliosphere due to the presence of streams, the magnetic fluctuation energy eventually comes to be slightly dominant over the kinetic energy at all scales. The theoretical implications of the results are considered.

Description: The combination of a sun sensor called a sun gate (SG) and a digital programmable signal processor on the Galileo spacecraft attitude and articulation control subsystem (AACS) will orient the rotation axis of the spacecraft toward the sun to satisfy a new requirement imposed by the new spacecraft trajectory. The combination will continuously monitor the pointing direction of the rotation axis, and any off-sun excursions of more than a preset threshold will be detected, triggering appropriate actions by the flight software to prevent off-sun cone angles of more than 14 deg. The design of the SG is described in detail, its principle of operation is given, and the flight software processing of the SG output is discussed.

Description: A time-dependent MHD simulation was used to investigate the magnetic topology of an idealized streamerlike configuration in three dimensions. Starting from an initially current-free multipole field and a transonic flow, a quasi-steady, streamerlike configuration forms. The simulated streamer exhibits a closed magnetic field region bounded by a current layer and surrounded by open fields. This helmet region is surmounted by a current layer, with an enhanced density in the closed field and current sheet regions. The simulated streamer has a finite longitudinal extent, with the closed field region primarily confined to the center of the structure.

Description: This paper contains a review and discussion of the mathematical basis of the extrapolation techniques involved in using photospheric vector magnetograms to obtain the coronal field above the surface. The two basic techniques employing the Cauchy initial value problem and the variational techniques are reviewed in terms of the mathematical and practical applications. A short review is presented of the current research on numerical modeling techniques in the area of extrapolating vector magnetograms; specifically, algorithms to extrapolate nonlinear force-free magnetic fields from the photosphere are considered.

Description: Theoretical models of current-sheet formation and magnetic heating in the solar corona are examined analytically. The role of photospheric connectivity in determining the topology of the coronal magnetic field and its equilibrium properties is explored; nonequilibrium models of current-sheet formation (assuming an initially well connected field) are described; and particular attention is given to models with discontinuous connectivity, where magnetic singularities arise from smooth footpoint motions. It is shown that current sheets arise from connectivities in which the photospheric flux structure is complex, with three or more polarity regions and a magnetic null point within the corona.

Description: Observations of four flaring solar active regions, obtained during 1980-1986 with the NASA Marshall vector magnetograph (Hagyard et al., 1982 and 1985), are presented graphically and characterized in detail, with reference to nearly simultaneous Big Bear Solar Observatory and USAF ASW H-alpha images. It is shown that the flares occurred where local photospheric magnetic fields differed most from the potential field, with initial brightening on either side of a magnetic-neutral line near the point of maximum angular shear (rather than that of maximum magnetic-field strength, typically 1 kG or greater). Particular emphasis is placed on the fact that these significant nonpotential features were detected only by measuring all three components of the vector magnetic field.

Description: Recent observations of the solar magnetic field and its effects on the solar atmosphere are discussed, with an emphasis on large-scale active regions and their implications for the fine-scale magnetic structure and for activity in the so-called quiet regions. Sample magnetograms, sunlight images, H-alpha images, X-ray images, and spectroheliograms are presented and characterized in detail, and the form and action of the magnetic field in flares are considered. It is pointed out that simultaneous observations of all levels (from the photosphere to the corona) at 100-km (about 100-marcsec) resolution are needed to see the extent of fields looping into the corona and understand their structure and activity; large space-based observatories would be required.

Description: Ground-based video photography of firings of Space Shuttle Primary Reaction Control System (PRCS) engines show optical emissions extending nearly 4 km form the vehicle after steady state is reached. The total intensity and spatial distribution of these emissions depend on the angle between the spacecraft velocity vector and the engine exhaust axis. Candidate sources for this radiation are reviewed and the conclusion is reached that it is most likely due to vibrationally excited OH, formed by the reaction of fast ambient O atoms and H2O molecules in the exhaust.

Description: The interaction of solar p-mode oscillations with solar active regions has been studied in the case of the velocity oscillations observed in the vicinity of NOAA region 5359 on March 10, 1989. A maximum of nearly 70 percent of the power of incident high-degree modes is noted to be absorbed by this exceptionally large sunspot group. The occurrence of a major flare within this region during the observing sequence furnished a unique opportunity for testing the hypothesis that flares can excite photospheric acoustic waves. A search for acoustic 'pulses' emanating from the flare leads no detection of pulses, to a level of about 20 percent of the background power.

Description: A self-consistent numeric two-dimensional code using kinetic-wave particle equations has been developed and applied to the maser dynamics of the solar corona. Time histories generated by the code reveal details of the evolution of the linear phase, the saturation and subsequent relaxation of the loss cone. Quantitative values are obtained for the saturation time, the amount of the converted free energy, the critical wave energy level for onset of maser diffusion, the velocity and pitch angle range of the resonant particles, and the relative importance of the different magnetoionic modes and harmonics. A wide range of initial conditions is covered by varying the loss-cone distribution and the ambient cold plasma parameters.

Description: A mission consisting of an array of small satellites has been proposed to investigate radio sources at frequencies from approximately 2 to 20 MHz, a range unobservable from the ground because of ionospheric absorption and aberrations. Such a mission seems feasible and relatively inexpensive, but further investigation is still necessary. The solar wind is a major concern because it contains turbulence-induced random density variations that cause a fluctuating phase shift in any electromagnetic signal passing through, sometimes making radio source imaging by interferometry impossible. A numerical simulation has been developed which attempts to accurately characterize solar wind density fluctuations. Results have allowed more concrete conclusions to be drawn about observing constraints and have shown that interferometric imaging at radio frequencies below 10 MHz with baselines up to 100 km is possible at solar elongation angles of about 90 deg or more.

Description: Measurements of interplanetary H I Lyman alpha over a large portion of the celestial sphere were made at the recent solar minimum by the Pioneer Venus orbiter ultraviolet spectrometer. These measurements were performed during a series of spacecraft maneuvers conducted to observe Halley's comet in early 1986. Analysis of these data using a model of the passage of interstellar wind hydrogen through the solar system shows that the rate of charge exchange with solar wind protons is 30 percent less over the solar poles than in the ecliptic. This result is in agreement with a similar experiment performed with Mariner 10 at the previous solar minimum.

Description: Fast Fourier analysis of the detrended record of solar irradiance obtained by the Nimbus-7 cavity pyrheliometer shows a rich spectrum of significant frequencies between about 30 and 850 nHz (periods between 13 and 400 days). Wolff and Hickey (1987), elaborating on a model developed by Wolff (1974, 1976, 1983, and 1984), suggest that many of these peaks arise due to interference of rigidly rotating global solar oscillations (r- and g-modes). Their model fit is quite good in the region above about 135 nHz, but less satisfactory below this threshold. It is noted that the FFT spectrum of d-sq L/d-sq t, the second derivative of angular momentum of the solar inertial motion, contains peaks matching the large peaks in the irradiance spectrum below 400 nHz with periods near 0.08, 0.24, 0.65, and about 1 yr. The origins of the peaks in the d-sq L/d-sq t spectra are discussed, and some previous studies are reviewed bearing on the question of a possible relationship of solar motion and solar activity.

Description: Magnetic field structures in the solar wind, characterized by a variation of the field vectors within a plane inclined to the ecliptic ('Planar Magnetic Structures', PMSs), were reported recently (Nakagawa et al., 1989). These PMSs have the property that the plane of variation of the field also contains the nominal Parker spiral direction. An observation of a PMS where the direction of the line of intersection of the plane of field variation with the ecliptic plane makes a large (about 80 deg) angle to the Parker spiral direction is presented. Furthermore, the angular variables of the field (1) vary over a restricted range, and (2) are linearly related. The latter property is related to the former. Currently proposed models for the origin of PMS, inasmuch as they require field configurations which retain strict alignment with the Parker spiral direction from formation to observation, are probably incomplete.

Description: The impulsive mode of particle acceleration is studied by searching for 0.2-2 MeV electron increases associated with intense type III/V bursts. It is found that the presence of a type III/V burst in association with a relatively intense flare event indicates the acceleration and escape of greater than 100 KeV electrons. A list of type III/V electron events is compiled, showing that the majority included greater than 10 MeV proton increases, although they were not followed by a type II burst. The results suggest that there are two different modes of proton acceleration with the second mode becoming significant only for larger, gradual flares.

Description: Observations of solar energetic particles from impulsive flares are reviewed. Consideration is given to observations of electron events, He-3 rich events, and heavy-nuclei-rich events. It is found that these observations can be unified into a description of the particles from impulsive flares. The observations are compared with observations of gamma-ray line in impulsive flares and particles in flares and compared with particles in space. A model for accelerating the unique particle abundances of energetic particles is proposed.

Description: To reach even the nearest stars in a human lifetime requires a ship speed that is a substantial fraction of the speed of light. This means an enormous kinetic energy investment in the ship and suggests that maximizing the efficiency may be more important than minimizing the mass ratio. This paper develops the pertinent relativistic rocket equations and finds the conditions for high kinematic efficiency. Using the limiting efficiency of percent, the minimum energy needed for one-way and for round trip voyages, and to explore all 'good' suns out to a given distance, using prefueled rockets, is determined. Savings due to refueling the rocket (and reloading propellant) at the destination and in flight are both somewhat greater than 2:1.

Description: Recent reports of remote detection of the heliospheric terminal shock place it near 50 AU. These conflict with standard models which, when combined with current data on the local interstellar medium, place the shock beyod 100 AU. Resolution of this discrepancy has led to hypotheses that invoke cosmic ray pressure, momentum exchange with interstellar neutrals, and magnetic field effects between the shock and the contact discontinuity dividing the solar wind from interstellar plasma. These hypotheses depend not only on properties of the interstellar medium, but also on the downstream three-dimensional flow between the shock and the contact discontinuity, in the region called the 'heliosheath'. The downstream flow field in the absence of magnetic fields is examined here under the assumptions that the flow everywhere outside the shock can be approximated as irrotational and incompressible. It is found, in particular, that the distance between the terminal shock and the contact discontinuity is less than the heliocentric distance to the terminal shock, effectively eliminating magnetic field effects in the heliosheath as being dynamically important.

Description: The Helios, IMP 8, ISEE 3, ad Voyager 2 spacecraft are used to examine the solar cycle and heliocentric distance dependence of the correlation between density n and magnetic field magnitude B in the solar wind. Previous work had suggested that this correlation becomes progressively more negative with heliocentric distance out to 9.5 AU. Here it is shown that this evolution is not a solar cycle effect, and that the correlations become even more strongly negative at heliocentric distance larger than 9.5 AU. There is considerable variability in the distributions of the correlations at a given heliocentric distance, but this is not simply related to the solar cycle. Examination of the evolution of correlations between density and speed suggest that most of the structures responsible for evolution in the anticorrelation between n and B are not slow-mode waves, but rather pressure balance structures. The latter consist of both coherent structures such as tangential discontinuities and the more generally pervasive 'pseudosound' which may include the coherent structures as a subset.

Description: Results are presented of a study designed to confirm the suspected relation between Alfven solitons (steepened Afven waves) and rotational discontinuities (RDs) in the solar wind. The ISEE 3 data were used to search for the predicted correlations between the beta value of plasma, the sense of polarization of the discontinuity, and changes of the magnetic field strength and plasma density across the discontinuity. No statistically significant evidence was found for the evolution of RDs from Alfven solitons. A possibility is suggested that the observations made could have been far from the regions in which the RDs were formed.

Description: An Ar/Ca abundance ratio of at least 0.56 is derived using the X-ray spectra of solar flares recorded by the SOLFLEX spectrometers on the P78-1 spacecraft. The method involves determining the contribution of the Ar XVII 1s2 1S0-1s4p 1P1 line to a blended feature at 3.2005 A. This contribution is determined by considering the relationship of the other component of the blended feature, namely the Ca XVIII 1s2 2s 2S(1/2)-1s2p2s 2P(3/2) line, to the dielectronic satellite lines of Ca XVII that fall between 3.215 and 3.24 A. It is demonstrated that a broad-wavelength coverage solar X-ray spectrometer experiment is necessary for precise abundance measurements.

Description: The large-scale temporal behavior and spatial gradients in the heliospheric magnetic field were investigated using magnetic field observations by Pioneer 11 and ISEE 3/ICE satellites, including observations out to a radial distance of 24 AU and over a time interval that includes the recent solar minimum. It was found that, to first order, the gradients in the magnetic field magnitude and in the azimutal component agree well with the Parker (1958) model. The analysis of the Pioneer data confirms previous reports of a magnetic field flux deficit of about 1 percent/AU, so that at 20 AU the relative deficit is of the order of 20 percent. It is argued that most of the magnetic field deficit is in the radial gradient.

Description: Using time-dependent numerical simulations, the proposed rebound shock mechanism for spicules has been examined. At temperatures above a critical value, T(c), the radiation is characteristic of the conditions in the optically thin corona and near optically thin transition region. When T less than T(c), the atmosphere has a radiative cooling time, tau(rad) characteristic of chromosphere. The spicule is initiated with a quasi-impulsive force in the low chromosphere, which drives a train of upward propagating rebound shocks along the rigid magnetic flux tube. These shocks then move the transition region upward. The material below the displaced transition region has temperatures and densities similar to those of spicules when T(c) = 20,000 K or more and tau(rad) = 500 s or more, but not when T(c) = 10,000 K, and probably not when tau(rad) = 100 s. For all the cases where the cross sectional area diverges rapidly with height, the upward velocity of the transition region is less than that of spicules. Moreover, the maximum height is less than that of average spicules. Taller, higher velocity spicules result when the magnetic field cross sectional area is constant. In all cases, the rebound shock mechanism produces substantial motions and temperature and density variations in chromospheric and transition region material. It is suggested that this may be a partial explanation for the continuous dynamic state of the lower solar atmosphere.

Description: Sturrock et al. (1990) have proposed that a distribution of microflares produces the solar coronal and transition-region emission. This paper presents computed EUV and X-ray emission spectra from that distribution, including the effects of time-dependent ionization. The overall agreement of the predicted spectrum with observations is fairly good, although the predicted emission from the upper transition region is high by a factor of 2. Time-dependent ionization greatly enhances the He II 304-A line, improving the agreement with observation compared with ionization equilibrium models. The predicted effects on temperature-sensitive line ratios of other ions cannot yet be tested against observations due to uncertainties in instrumental calibration and atomic rates.

Description: The extremely Zeeman-sensitive 12.32 micron Mg I solar emission line was observed during a 3B/X5.7 solar flare on October 24, 1989. When compared to postflare values, Mg I emission-line intensity in the penumbral flare ribbon was 20 percent greater at the peak of the flare in soft X-rays, and the 12 micron continuum intensity was 7 percent greater. The flare also excited the emission line in the umbra where it is normally absent. The umbral flare emission exhibits a Zeeman splitting 200 G less than the adjacent penumbra, suggesting that it is excited at higher altitude. The absolute penumbral magnetic field strength did not change by more than 100 G between the flare peak and postflare period. However, a change in the inclination of the field lines, probably related to the formation and development of the flare loop system, was seen.

Description: This paper presents the design constraints, flight environments, conceptual design iterations and analytical methods used in the design and flight certification of the Reinforced Carbon-Carbon (RCC) chin panel for the Space Shuttle Orbiter. A discussion of the thermal/structural response of the chin panel to the imposed environments is presented. As with all critical airframe components, it was desirable to conduct a structural test to verify design adequacy. However, with the chin panel, a meaningful test to simulate the critical thermal stresses was not considered feasible.

Description: The nature of the energy release process that produces impulsive bursts of hard X-rays and microwaves during solar flares is discussed, based on new evidence obtained using the method of Crannell et al. (1978). It is shown that the hard X-ray spectral index gamma is negatively correlated with the microwave peak frequency, suggesting a common source for the microwaves and X-rays. The thermal and nonthermal models are compared. It is found that the most straightforward explanations for burst time behavior are shock-wave particle acceleration in the nonthermal model and thermal conduction fronts in the thermal model.

Description: Reexamination of the published sunspot rotation rates from Mount Wilson for the period from 1921 to 1982 suggests that the sun rotates more rapidly when there are fewer sunspots. This behavior is seen over the course of each cycle with the most rapid rotation usually observed at sunspot minimum. It is also seen in hemispheric differences with the southern hemisphere, having fewer spots, rotating more rapidly than the northern hemisphere. Furthermore, the rotation rate averaged over each cycle also shows that the sun rotates more rapidly during cycles with fewer sunspots and less sunspots area. This inverse correlation between sunspot area and rotation rate suggests that during the Maunder minimum the sun may have rotated slightly faster than is observed today.

Description: The results of a comprehensive search of the ISEE 3 energetic particle data for solar electron events with associated increases in elements with atomic number Z = 6 or greater are reported. A sample of 90 such events was obtained. The events support earlier evidence of a bimodal distribution in Fe/O or, more clearly, in Fe/C. Most of the electron events belong to the group that is Fe-rich in comparison with the coronal abundance. The Fe-rich events are frequently also He-3-rich and are associated with type III and type V radio bursts and impulsive solar flares. Fe-poor events are associated with type IV bursts and with interplanetary shocks. With some exceptions, event-to-event enhancements in the heavier elements vary smoothly with Z and with Fe/C. In fact, these variations extend across the full range of events despite inferred differences in acceleration mechanism. The origin of source material in all events appears to be coronal and not photospheric.

Description: The relation between the degree of nonpotentiality of photospheric magnetic fields and the occurrence of gama-ray flares is examined to determine whether there are special signatures of the stressed fields for this type of flare. Observations of the flares in the active region of April 1984 (AR 4474) are analyzed, showing that the big flare initiated at the location on the magnetic neutral line where the field deviated the most from a potential field. The nonpotential signatures of AR 4474 are compared with those of four other regions. The results suggest that gamma-ray flares are associated with strongly nonpotential fields that extend over relatively larger lengths of the magnetic neutral line that the fields associated with flares that do not produce gamma-ray events.

Description: Papers on impulsive solar flares are presented, covering topics such as the optical properties of impulsive flares, soft X-ray spectroscopy of solar flares, the energy release process in impulsive bursts, high-velocity evaporation and a high-speed shock wave during the impulsive phase of the April 24, 1984 flare, nonpotential magnetic fields at sites of gamma-ray flares, and meter-decimeter and microwave radio observations of solar flares. Other topics include rise times in solar radio bursts, removal of the gradual component in analyses of solar impulsive bursts, ion and relativistic electron transport in solar flares, neutrons and gamma-ray emission on June 3, 1982, emission characteristics of three intense solar flares in cycle 21, and solar flare gamma-ray observations with the Hinotori satellite. Additional topics include spectra of relativistic solar proton ground-level events recorded in Antarctica, a 153-d periodicity in the occurrence of solar flares producing energetic interplanetary electrons, a search for solar neutron response in neutron monitor data, neutral beams in two-ribbon flares and in the geomagnetic tail, beam heating in solar flares, and solar flare gamma-ray line shapes.

Description: A new approach for the placement of sensors and actuators in the active control of flexible space structures is developed. The approach converts the discrete nature of the sensor and actuator positioning problem to a nonlinear programming optimization through approximation of the control forces and output measurements by spatially continuous functions. The locations of the sensors and actuators are optimized in order to move the transmission zeros of the system farther to the left of the imaginary axis. The criterion for sensor/actuator placement can be quite useful for optimal regulation and tracking problems, as well as for low-authority controller designs. Two performance metrics are considered for the optimization and are applied to the sensor/actuator positioning of a large-order flexible space structure.

Description: Electret measurements obtained of the particulate contamination environment within the Space Shuttle Orbiter's cargo bay are presently compared with ground measurements of the particulates emitted by the Shuttle's SRBs, as well as with the expected natural particulate environment as measured by Skylab. Chemical analysis is shown to reveal the difference between natural and anthropogenic space debris; the most probable primary source of the Space Shuttle's particulate environment is the SRB exhaust.

Description: Viewgraphs on tethered systems control are presented. Topics covered include: rigid body simplifications; feedback control; quadratic optimal nonlinear control; quadratic optimal nonlinear controllers; Lyapunov-based nonlinear controllers; sensor/actuator options; tether control; and fuzzy logic control.

Description: Viewgraphs on rendezvous guidance are presented. Topics covered include: rendezvous guidance; Lambert Targeting; Linear Quadratic Targeting; unmanned vehicles; rendezvous trajectory; and fuzzy logic control.

Description: Viewgraphs on proximity operations in space station environment are presented. Topics covered include: automated flight; remote manual piloting; task requirements and constraints; problem formulation; and flight parameter flow/pilot interaction for proximity operations task.

Description: Viewgraphs on power system technology discipline for Space Station Freedom are presented. Topics covered include: power generation subsystem; energy storage subsystem; and power distribution.

Description: Viewgraphs on an advanced automation of a prototypic thermal control system for space station are presented. The Thermal Expert System (TEXSYS) was initiated in 1986 as a cooperative project between ARC and JCS as a way to leverage on-going work at both centers. JSC contributed Thermal Control System (TCS) hardware and control software, TCS operational expertise, and integration expertise. ARC contributed expert system and display expertise. The first years of the project were dedicated to parallel development of expert system tools, displays, interface software, and TCS technology and procedures by a total of four organizations.

Description: Viewgraphs on Space Station Freedom central thermal control system (CTCS) evolution are presented. Topics covered include: (1) growth requirements and basic features of research and development and transportation nodes; (2) identifying the principal CTCS hooks and scars at assembly complete to accommodate growth; and (3) describing the general provisions for growth and identifying pertinent design issues.

Description: Viewgraphs and discussion on superfluid helium needs and resupply on space station are presented. Topics covered include: uses of superfluid helium in space; space station He 2 technology issues; resupply - fluid management issues; liquid acquisition devices for on orbit transfer; and liquid acquisition devices for SHOOT.

Description: Subcritical cryogenic fluid management has long been recognized as an enabling technology for key propulsion applications, such as space transfer vehicles (STV) and the on-orbit cryogenic fuel depots which will provide STV servicing capability. The LeRC Cryogenic Fluids Technology Office (CFTO), under the sponsorship of OAST, has the responsibility of developing the required technology via a balanced program involving analytical modeling, ground based testing, and in-space experimentation. Topics covered in viewgraph form include: cryogenic management technologies; nitrogen storage and supply; cryogenic nitrogen cooling capability; and LN2 system demonstration technical objectives.

Description: Viewgraphs and discussion on automated assembly of large space structures are presented. The program research objective is to develop technology and demonstrate the potential for automated inspace assembly of large erectable structures. This is accomplished by merging experience in structural assembly and robotics at LaRC into an interdisciplinary program with focused effort on automated assembly of a generic structural configuration with a standard cell and by building into the system the capability to do expanded research with complex configurations.

Description: PROBE (Precision Reflector Orbital Build Experiment) is a Shuttle flight demonstration experiment designed to study extravehicular activity (EVA) assembly of precision segmented reflectors. PROBE will support missions being considered for NASA's Global Change Technology Initiative as well as other missions in astrophysics and spacecraft optical communications requiring large precision reflectors. Such reflectors are envisioned to consist of a low-mass backup truss to which the optical surface is attached. Because of their large size, these reflectors will be constructed on-orbit from smaller pieces which can be packaged in the launch vehicle. The technology to be developed with PROBE also has application for construction of solar dynamic collectors which are planned for the enhanced configuration of Space Station Freedom. Viewgraphs of PROBE are presented.

Description: Viewgraphs on structures for Space Station Freedom are presented. The structures subsystem, mechanical subsystem, and evolution issues are addressed.