Polar format algorithm for bistatic SAR

Matched filtering (MF) of phase history data is a mathematically ideal but computationally expensive approach to bistatic synthetic aperture radar (SAR) image formation. Fast backprojection algorithms (BPAs) for image formation have recently been shown to give improved O(N/sup 2/ log/sub 2/N) performance. An O(N/sup 2/ log/sub 2/N) bistatic polar format algorithm (PFA) based on a bistatic far-field assumption is derived. This algorithm is a generalization of the popular PFA for monostatic SAR image formation and is highly amenable to implementation with existing monostatic image formation processors. Limits on the size of an imaged scene, analogous to those in monostatic systems, are derived for the bistatic PFA.     

UVCS/SOHO Observations of Spectral Line Profiles in Polar Coronal Holes

Ultraviolet emission line profiles have been measured on 15-29 September 1997 for H I 1216 Å, O VI 1032, 1037 Å and Mg X 625 Å in a polar coronal hole, at heliographic heights ? (in solar radii) between 1.34 and 2.0. Observations of H I 1216 Å and the O VI doublet from January 1997 for ? = 1.5 to 3.0 are provided for comparison. Mg X 625 Å is observed to have a narrow component at ? = 1.34 which accounts for only a small fraction of the observed spectral radiance, and a broad component that exists at all observed heights. The widths of O VI broad components are only slightly larger than those for H I at ? = 1.34, but are significantly larger at ? = 1.5 and much larger for ? > 1.75. In contrast, the Mg X values are less than those of H I up to 1.75 and then increase rapidly up to at least ? = 2.0, but never reach the values of O VI.     

Streamer Evaporation

Streamer evaporation is the consequence of heating in ideal MHD models because plasma is weakly contained by the magnetic field. Heating causes inflation, opening of field lines, and release of solar wind. It was discovered in simulations and, due to the absence of loss mechanisms, the ultimate end point is the complete evaporation of the streamer. Of course streamers do not behave in this way because of losses by thermal conduction and radiation. Heating is also expected to depend on ambient conditions. We use a global MHD model with thermal conduction to examine the effect of changing the heating scale height. We also extend an analytic model of streamers developed by Pneuman (1968) to show that steady streamers are unable to contain plasma for temperatures near the cusp greater than ∼ 2 × 10⁶ K. This revised version was published online in June 2006 with corrections to the Cover Date.     

2001 Mars Odyssey Mission Summary

The 2001 Mars Odyssey spacecraft, now in orbit at Mars, will observe the Martian surface at infrared and visible wavelengths to determine surface mineralogy and morphology, acquire global gamma ray and neutron observations for a full Martian year, and study the Mars radiation environment from orbit. The science objectives of this mission are to: (1) globally map the elemental composition of the surface, (2) determine the abundance of hydrogen in the shallow subsurface, (3) acquire high spatial and spectral resolution images of the surface mineralogy, (4) provide information on the morphology of the surface, and (5) characterize the Martian near-space radiation environment as related to radiation-induced risk to human explorers. To accomplish these objectives, the 2001 Mars Odyssey science payload includes a Gamma Ray Spectrometer (GRS), a multi-spectral Thermal Emission Imaging System (THEMIS), and a radiation detector, the Martian Radiation Environment Experiment (MARIE). THEMIS and MARIE are mounted on the spacecraft with THEMIS pointed at nadir. GRS is a suite of three instruments: a Gamma Subsystem (GSS), a Neutron Spectrometer (NS) and a High-Energy Neutron Detector (HEND). The HEND and NS instruments are mounted on the spacecraft body while the GSS is on a 6-m boom. Some science data were collected during the cruise and aerobraking phases of the mission before the prime mission started. THEMIS acquired infrared and visible images of the Earth-Moon system and of the southern hemisphere of Mars. MARIE monitored the radiation environment during cruise. The GRS collected calibration data during cruise and aerobraking. Early GRS observations in Mars orbit indicated a hydrogen-rich layer in the upper meter of the subsurface in the Southern Hemisphere. Also, atmospheric densities, scale heights, temperatures, and pressures were observed by spacecraft accelerometers during aerobraking as the spacecraft skimmed the upper portions of the Martian atmosphere. This provided the first in-situ evidence of winter polar warming in the Mars upper atmosphere. The prime mission for 2001 Mars Odyssey began in February 2002 and will continue until August 2004. During this prime mission, the 2001 Mars Odyssey spacecraft will also provide radio relays for the National Aeronautics and Space Administration (NASA) and European landers in early 2004. Science data from 2001 Mars Odyssey instruments will be provided to the science community via NASA’s Planetary Data System (PDS). The first PDS release of Odyssey data was in October 2002; subsequent releases occur every 3 months.     

Mars Global Surveyor Observations of Solar Wind Magnetic Field Draping Around Mars

We examine the magnetic field in the martian magnetosheath due to solar wind draping. Mars Global Surveyor provided 3-D vector magnetic field measurements at a large range of altitudes, local times, and solar zenith angles as the spacecraft orbit evolved. We choose orbits with very clean signatures of draping to establish the nominal morphology of the magnetic field lines at local times of near-subsolar and near-terminator. Next, using a compilation of data from Mars Global Surveyor, we determine the average magnetic field morphology in the martian magnetosheath due to the solar wind interaction. The topology of the field is as expected from previous observations and predictions. The magnetic field magnitude peaks at low altitude and noon magnetic local time and decreases away from that point. The magnetic field has an inclination from the local horizontal of 5.6° on average in the dayside magnetosheath and 12.5° on the nightside. The inclination angle is closest to zero at noon magnetic local time and low altitude. It increases both upward and to later local times. The magnetic field in the induced magnetotail flares out from the Mars—Sun direction by 21°. Finally, we compare the observations to gasdynamic model predictions and find that the shocked solar wind flow in the martian magnetosheath can be treated as a gasdynamic flow with the magnetic pileup boundary as the inner boundary to the flow.     

透博梅卡公司探索新的涡轴发动机

虽然透博梅卡公司现在每年销售发动机超过650台,但它们却认为已经到了研制替换轴功率为450～670千瓦级发动机的时候了.     

Ionization processes in the heliosphere — Rates and methods of their determination

The rates of the most important ionization processes acting in interplanetary space on interstellar H, He, C, O, Ne and Ar atoms are critically reviewed in the paper. Their long-term modulations in the period 1974 – 1994 are reexamined using updated information on relevant cross-sections as well as direct or indirect data on variations of the solar wind/solar EUV fluxes based on IMP 8 measurements and monitoring of the solar 10.7 cm radio emission. It is shown that solar cycle related variations are pronounced (factor of 3 between maximum and minimum) especially for species such as He, Ne, C for which photoionization is the dominant loss process. Species sensitive primarily to the charge-exchange (as H) show only moderate fluctuations 20% around average. It is also demonstrated that new techniques that make use of simultaneous observations of neutral He atoms on direct and indirect orbits, or simultaneous measurements of He⁺ and He⁺⁺ pickup ions and solar wind particles can be useful tools for narrowing the uncertainties of the He photoionization rate caused by insufficient knowledge of the solar EUV flux and its variations.     

THE WIDE-BAND PLASMA WAVE INVESTIGATION

As part of the Cluster Wave Experiment Consortium (WEC), the Wide-Band (WBD) Plasma Wave investigation is designed to provide high-resolution measurements of both electric and magnetic fields in selected frequency bands from 25 Hz to 577 kHz. Continuous waveforms are digitised and transmitted in either a 220 kbit s^-1 real-time mode or a 73 kbit s^-1 recorded mode. The real-time data are received directly by a NASA Deep-Space Network (DSN) receiving station, and the recorded data are stored in the spacecraft solid-state recorder for later playback. In both cases the waveforms are Fourier transformed on the ground to provide high-resolution frequency-time spectrograms. The WBD measurements complement those of the other WEC instruments and also provide a unique new capability for performing very-long-baseline interferometry (VLBI) measurements.     

Near Infrared Spectrometer for the Near Earth Asteroid Rendezvous Mission

The Near-Infrared Spectrometer (NIS) instrument on the Near-Earth Asteroid Rendezvous (NEAR) spacecraft is designed to map spectral properties of the mission target, the S-type asteroid 433 Eros, at near-infrared wavelengths diagnostic of the composition of minerals forming S asteroids. NIS is a grating spectrometer, in which light is directed by a dichroic beam-splitter onto a 32-element Ge detector (center wavelengths, 816–1486 nm) and a 32-element InGaAs detector (center wavelengths, 1371–2708 nm). Each detector reports a 32-channel spectrum at 12-bit quantization. The field-of-view is selectable using slits with dimensions calibrated at 0.37° × 0.76° (narrow slit) and 0.74° × 0.76° (wide slit). A shutter can be closed for dark current measurements. For the Ge detector, there is an option to command a 10x boost in gain. A scan mirror rotates the field-of-view over a 140° range, and a diffuse gold radiance calibration target is viewable at the sunward edge of the field of regard. Spectra are measured once per second, and up to 16 can be summed onboard. Hyperspectral image cubes are built up by a combination of down-track spacecraft motion and cross-track scanning of the mirror. Instrument software allows execution of data acquisition macros, which include selection of the slit width, number of spectra to sum, gain, mirror scanning, and an option to interleave dark spectra with the shutter closed among asteroid observations. The instrument was extensively characterized by on-ground calibration, and a comprehensive program of in-flight calibration was begun shortly after launch. NIS observations of Eros will largely be coordinated with multicolor imaging from the Multispectral Imager (MSI). NIS will begin observing Eros during approach to the asteroid, and the instrument will map Eros at successively higher spatial resolutions as NEAR's orbit around Eros is lowered incrementally to 25 km altitude. Ultimate products of the investigation will include composition maps of the entire illuminated surface of Eros at spatial resolutions as high as 300 m.