Acceleration and transport of energeticparticles at CME-driven shocks

Historically, solar energetic particle (SEP) events are classified in two classes as “impulsive” and “gradual”. Whether there is a clear distinction between the two classes is still a matter of debate, but it is now commonly accepted that in large “gradual” SEP events, Fermi acceleration, also known as diffusive shock acceleration, is the underlying acceleration mechanism. At shock waves driven by coronal mass ejections (CMEs), particles are accelerated diffusively at the shock and often reach > MeV energies (and perhaps up to GeV energies). As a CME-driven shock propagates, expands and weakens, the accelerated particles can escape ahead of the shock into the interplanetary medium. These escaping energized particles then propagate along the interplanetary magnetic field, experiencing only weak scattering from fluctuations in the interplanetary magnetic field (IMF). In this paper, we use a Monte-Carlo approach to study the transport of energetic particles escaping from a CME-driven shock. We present particle spectra observed at 1 AU. We also discuss the particle “crossing number” at 1AU and its implication to particle anisotropy. Based on previous models of particle acceleration at CME-driven shocks, our simulation allows us to investigate various characteristics of energetic particles arriving at various distances from the sun. This provides us an excellent basis for understanding the observations of high-energy particles made at 1 AU by ACE and WIND.     

Optimal FIR filters for time-varying state-space models

An optimal FIR (finite impulse response) filter and smoother is introduced for the time-varying state-space model. The suggested filter has an FIR structure and utilizes finite observation. It is shown that the impulse response of the optimal FIR filter can be obtained by a simple Riccati-type matrix differential equation. Especially for time-invariant systems, this FIR filter reduces to previously known simple forms. For implementation, a recursive form of the optimal FIR filter and smoother is derived by using adjoint variables, and computational algorithms are suggested. It is also shown by sensitivity analysis that the proposed optimal FIR filter alleviates potential divergence characteristics of the standard Kalman filter     

Possible Role of ion Demagnetization in the Plasma Sheet in Auroral arc and Substorm Generation

Ion demagnetization in the plasma sheet causes the formation of field-aligned current that can trigger a magnetosphere-ionosphere coupling feedback instability, which may play an important role in substorm and auroral arc generation. Since field-aligned currents close ionospheric currents, their magnitude is controlled by ionospheric conductivity. The cause of instability is the impact of increasing upward field-aligned currents on ionospheric conductivity, which in turn stimulates an increase in the field-aligned currents. When the magnitude of these currents becomes sufficiently large for the acceleration of precipitating electrons, a feedback mechanism becomes possible. Upward field-aligned currents increase the ionospheric conductivity that stimulates an explosion-like increase in field-aligned currents. It is believed that this instability may be related to substorm generation. Demagnetization of hot ions in the plasma sheet leads to the motion of magnetospheric electrons through a spatial gradient of ion population. Field-aligned currents, because of their effect on particle acceleration and the magnitude of ionospheric conductivity, can also lead to another type of instability associated with the breaking of the earthward convection flow into convection streams. The growth rate of this instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to the formation of auroral arcs with widths of the order of 10 km. This instability is able to explain many features of auroral arcs, including their conjugacy in opposite hemispheres. However, it cannot explain very narrow (less than 1 km) arcs.     

CFAR for homogeneous part of high-resolution imagery

In high-resolution imaging, weak target pixel amplifiers may not be detected in the presence of clutter containing strong nonhomogeneities, when conventional approaches are used. The authors describe a constant false alarm rate (CFAR) approach that avoids the elimination of these significant target returns. The nonhomogeneous clutter as well as the weak target components are detected with this approach. The targets could then be discriminated from the homogeneities by discrimination techniques. It is shown how the lower amplitude components of the background noise and homogeneous clutter (which have Rayleigh statistics) can be detected in the presence of strong homogeneous clutter and targets. The average level of the homogeneous component is then determined using these lower-amplitude components. This CFAR approach avoids having a CFAR on the strong nonhomogeneities as well as the homogeneous component. The avoidance is what yields the ability to detect weak target pixel amplitudes     

A chirp scaling approach for processing squint mode SAR data

Image formation from squint mode synthetic aperture radar (SAR) is limited by image degradations caused by neglecting the range-variant filtering required by secondary range compression (SRC). Introduced here is a nonlinear FM chirp scaling, an extension of the chirp scaling algorithm, as an efficient and accurate approach to range variant SRC. Two methods of implementing the approach are described. The nonlinear FM filtering method is more accurate but adds a filtering step to the chirp scaling algorithm, although the extra computation is less than that of a time domain residual compression filter. The nonlinear FM pulse method consists of changing the phase modulation of the transmitted pulse, thus avoiding an increase in computation. Simulations show both methods significantly improve resolution width and sidelobe level, compared with existing SAR processors for squint angles above 10 deg for L-band and 20 deg for C-band     

Comparative study of spermatogonial survival after x-ray exposure, high LET (HZE) irradiation or spaceflight.

Spermatogonial cell loss has been observed in rats flown on Space Lab 3, Cosmos 1887, Cosmos 2044 and in mice following irradiation with X-ray or with high energy (HZE) particle beams. Spermatogonial loss is determined by cell counting in maturation stage 6 seminiferous [correction of seminferous] tubules. With the exception of Iron, laboratory irradiation experiments (with mice) revealed a similar pattern of spermatogonial loss proportional to the radiation dose at levels less than 0.1 Gy. Helium and Argon irradiation resulted in a 5% loss of spermatogonia after only 0.01 Gy exposure. However, significant spermatogonial loss (45%) occured at this radiation level with Iron particle beams. The loss of spermatogonia during each space flight was less than 10% when compared to control (non-flight) animals. This loss, although small, was significant. Although radiation may be a contributing factor in the loss of spermatogonia during space flight, exposure levels, as determined by dosimetry, were not significant to account for the total cell loss observed.     

U.S. operational space program for climate observations

Satellites provide two important characteristics to earth climate studies not available from other, conventional sources: (1) full global coverage, and (2) consistency within the data set. This latter arises from the fact that the satellite data are usually derived from one instrument (or at least from a small number) whereas other sources involve large numbers of separate instruments and hence exhibit a substantial standard deviation. Satellite data, of course, are more subject to bias and must therefore be carefully validated, usually via ground truth.The ISCCP and ISLSCP are examples of the increasing reliance on satellite data for climate studies. In addition to the multispectral images, quantitative products of importance are: (1) atmospheric temperature structure, (2) snow cover, (3) precipitation, (4) vegetation index, (5) maximum/minimum temperature, (6) insolation, and (7) earth radiation balance. The U.S. civil space program is presently committed to its current geostationary (GOES) and polar (NOAA) programs through this decade and to continue both programs into the next decade with spacecraft carrying improved and augmented instrumentation. GOES VISSR Atmospheric Sounder (VAS) data, presently in research status and available only for special observation periods, will become available operationally in 1987 from the current spacecraft series. GOES-Next will provide additional spectral channels, simultaneous imaging, atmospheric soundings, and possibly increased resolution starting in 1990. The NOAA follow-on spacecraft, in the same time frame, is expected to provide additional spectral channels, improved passive microwave radiometry, and possibly increased spatial resolution. The Landsat program is expected to be continued by a commercial operator following the useful life of Landsat-5. All three follow-on programs are presently at various stages of definition and procurement. Final definition may not be completed until late in 1984. However, their status as of the time of this presentation will be reviewed in detail.     

ISCCP cloud analysis algorithm intercomparison

The International Satellite Cloud Climatology Project (ISCCP) will provide a uniform global climatology of satellite-measured radiances and derive a climatology of cloud radiative properties from these radiances. For this purpose, a pilot study of cloud analysis algorithms was initiated to define a state-of-the-art algorithm for ISCCP. This study compared the results of applying the nine different algorithms to the same satellite radiance data. The comparison allowed for a sharper understanding of the process of detecting clouds and shows that all algorithms can be improved by better information about clear sky radiance values (essentially equivalent to surface property information) and by better understanding of cloud size distribution variations. The dependence of all methods on cloud size distribution led to selection of an advanced bispectral threshold technique for ISCCP because this method is currently better understood and more developed. Further research on cloud algorithms is clearly suggested by these results.     

Aerospace High Voltage Development

High voltage has been used for electrical power system generation, transmission, and distribution for over 75 years and manufacturers have been designing x-rays, radios/television transmitters and receivers for many years with excellent success. High voltage usage in aerospace equipment initiated during World War II with the advent of high power communications and radar for airplanes. About 20 years ago the first high voltage components were built for spacecraft systems. This article is to provide some insight into the status of high voltage for aerospace equipment and the differences between terrestial and aerospace system functions and the attendant problems. What are the basic differences between terrestial/commercial and aerospace equipment? The aerospace environment is defined as that significantly above the Earth's surface: From 5000 feet altitude to deep space. The basic differences are the constraints placed on the user vehicle (airplane, missile, or spacecraft). Constraints include: Atmospheric pressure, temperature, lifting capability, electronic requirements, and volume. Early airplanes needed only radios and mechanical pressurization instruments. Today's sophisticted airplanes require transmitters, receivers, controls, displays, and in the military case, special electronics. The addition of electronic devices has increased the electrical power demand from a few watts (for early aircraft) to well over one megawatt for special applications. There is the need for compact packaging to reduce weight and volume. Spacecraft with booster limitations are ever more restrictive of weight and volume then airplanes while they must maintain complete electrical system integrity for mission durations of several months to years.     

SAMBO-GEOS : Electric field measurements in the disturbed ionosphere and magnetosphere

Measurements of the electric field in the ionosphere and the equatorial plane during the pre-onset and actives phases of a substorm (March 4, 1979) are compared. Correlations and disagrements between the measurements are considered. The preliminary conclusion is reached that the model of electrojet polarisation proposed by CORONITI and KENNEL (1972) could possibly explain part of our observations.