K. R. Moore

Magnetospheric plasma analyzer: Initial three‐spacecraft observations from geosynchronous orbit

The first three magnetospheric plasma analyzer (MPA) instruments have been returning data from geosynchronous orbit nearly continuously since late 1989, 1990, and 1991. These identical instruments provide for the first time simultaneous plasma observations from three widely spaced geosynchronous locations. The MPA instruments measure the three-dimensional velocity space distributions of both electrons and ions with energies between ∼1 eV/q and ∼40 keV/q. MPA capabilities and observations are summarized in this paper. We use the simultaneous observations from three longitudinally separated spacecraft to synthesize a synoptic view of the morphology of the magnetosphere at geosynchronous orbit over a 6-week interval in early 1992. The MPA observations indicate that the spacecraft encountered seven regions with characteristic plasma populations during this period: (1) the cool, dense plasmasphere (13.1% of the data); (2) a warmer, less dense plasma trough (22.5%); (3) the hot plasma sheet (40.3%); (4) a combination of plasma trough and plasma sheet (18.6%); (5) an empty trough region, devoid of plasma sheet, plasmasphere, or plasma trough populations (4.3%); (6) the magnetosheath and/or low-latitude boundary layer (0.7%); and (7) the lobe (0.3%). The local time distributions of these regions are examined. For example, as suggested by previous authors, we find that at geomagnetically quiet times (Kp < 2) geosynchronous orbit can lie entirely within the plasmasphere while at more active times only the afternoon to evening portions of the orbit are typically within the plasmasphere. We also find that the plasma convection inside the plasmasphere is generally sunward in the corotating (geosynchronous spacecraft) reference frame, independent of activity level, in contrast to previous studies. In addition to such statistical results, the simultaneous data sets at different local times allow us to at least partially separate spatial from temporal variations. In particular, we use these observations to examine the instantaneous shapes of the plasmapause and magnetopause as they pass over geosynchronous orbit. As expected, the plasmapause is found to have a highly variable shape, at various times showing (1) a stable dusk side bulge, (2) a variable bulge which expands, contracts, and moves, (3) an overall expansion and contraction of the plasmasphere, and (4) even more complicated behavior which is best accounted for by large-scale structure of the plasmapause and/or disconnected plasma blobs. During the 6 weeks of data examined, the magnetosheath was encountered on several occasions at synchronous orbit, preferentially on the prenoon side of the magnetosphere. For the first time, simultaneous prenoon and postnoon observations confirm this asymmetry and demonstrate that the magnetopause shape can be highly asymmetric about the Earth-Sun line.

An examination of the structure and dynamics of the outer plasmasphere using multiple geosynchronous satellites

The structure and the dynamics of the plasmaspheric bulge are examined using in situ three-dimensional plasma observations from magnetospheric plasma analyzers onboard multiple geosynchronous satellites. We identify the plasmasphere by the presence of high fluxes of low-energy (≈ few eV) ions (corresponding to densities of ≈10s up to ≈100 cm−3). The results from one year (1991) of nearly continuous plasma measurements from two longitudinally and latitudinally separated spacecraft are presented. This study corroborates many of the features and statistical behavior of the plasmaspheric bulge evidenced in past ground-based and single spacecraft data sets, except we often find a more complex outer plasmasphere than earlier studies have suggested. By using multipoint, simultaneous observations to separate spatial from temporal changes, this study extends previous examinations of the plasmasphere at synchronous orbit. We find that the width and location of the plasmaspheric bulge can differ significantly for the two spacecraft (separated by 6-8 hours in time), particularly during quiet geomagnetic conditions. The very different plasmaspheric morphologies seen by the two spacecraft lead us to conclude that the outer plasmasphere is often highly structured even during steady geomagnetic conditions and that the simple teardrop model of the bulge rarely, if ever, adequately describes the duskside plasmasphere.

Composition from fast neutrons: Application to the Moon

Planetary neutron leakage fluxes provide a measure of surface composition. However to be used in geological studies, a quantitative relationship between measured fluxes and surface composition is needed. The present work shows that neutron production is expected to be a function of the atomic mass, and that the fast leakage flux in the energy range between 0.6 and 8 MeV is linearly related to the average soil atomic mass. This result is consistent with laboratory measurements, and with Lunar Prospector observations of the Moon. When calibrated with returned lunar samples, this relationship is used to construct a map of the average atomic mass of lunar soils.

Circulatory Patterns of Air Pollutants within the Barcelona Air Basin in a Summertime situation: Lidar and Numerical Approaches

This work examines circulatory patterns of airpollutants in the area of Barcelona (Spain), a regionwith strong coastal and orographic influences. Thiswas achieved through exploitation ofelastic-backscatter lidar data and by numericalsimulation of the atmosphere with a meteorologicalmesoscale model (MEMO). Lidar data were acquired inJuly 1992 during a collaborative campaign between LosAlamos National Laboratory (LANL) and the PolytechnicUniversity of Catalonia (UPC). The lidar providedinformation about the distribution of aerosols and theprevailing winds, determined by application of amaximum cross-correlation algorithm toelastic-backscatter lidar data. Lidar winds are usedto evaluate high altitude winds simulated by themodel. This study showed that circulatory patterns inBarcelona are correlated with daytime convectivevertical mixing, sea-breeze circulations, and verticalforcing caused by mountain thermal and mechanicaleffects.

The magnetospheric lobe at geosynchronous orbit

On rare occasions, satellites at geosynchronous altitude enter the magnetospheric lobe, characterized by extremely low ion fluxes between 1 eV and 40 keV and electron fluxes above a few hundred eV. One year of plasma observations from two simultaneously operating spacecraft at synchronous orbit is surveyed for lobe encounters. A total of 34 full encounters and 56 apparent near encounters are identified, corresponding to ∼0.06% of the total observation time. Unlike energetic particle (E>40 keV) dropouts studied earlier, there is a strong tendency for the lobe encounters to occur postmidnight, as late as 07 local time. The two spacecraft encounter the lobe with different rates and in different seasons. These occurrence properties are not simply explicable in terms of the orbital geometry in either the solar magnetic or the geocentric solar magnetospheric coordinate system. A composite coordinate system which previously organized more energetic particle dropouts is somewhat more successful in organizing the lobe encounters, suggesting that solar wind distortion of the magnetic equatorial plane away from the dipole location and toward the antisolar direction may be largely responsible for these dropouts. Our results further suggest that this distortion persists even sunward of the dawn-dusk terminator. However, a simple dawn-dusk symmetric distortion does not fully account for all the seasonal and local time asymmetries in the occurrence of the lobe encounters; thus there is probably an additional dawn-dusk asymmetry in the distorted field. The lobe encounters are strongly associated with magnetospheric activity and tend to occur in association with rare magnetosheath encounters at synchronous orbit. It thus appears that the presence of the lobe at geosynchronous orbit is the result of major, probably asymmetric modifications of the magnetospheric field geometry in times of strong disturbance.

Novel low-energy neutral atom imaging technique

Recently proposed low-energy neutral atom (LENA) imaging techniques rely on collisional processes to convert LENAs into ions to separate the neutrals from the intense UV radiation background. At low energies, these collisional processes have poor conversion efficiencies and limit the angular resolution of these devices. However, if the intense UV light background can be suppressed, direct LENA detection is possible. They present results from a series of experiments designed to develop a novel filtering structure based on free-standing gold transmission gratings. If the grating period is sufficiently small, the gratings can substantially polarize UV light in the wavelength range 300 to 1,500 [angstrom]. If a second grating is placed behind the first grating with its axis of polarization oriented perpendicular to that of the first, considerable attenuation of the UV radiation is achievable. The neutrals pass through the remaining open area of two gratings and are directly detected. They have obtained nominal 2,000-A-period (1,000-[angstrom] bars with 1,000-[angstrom] slits) gratings and measured their UV and atomic transmission characteristics. The geometric factor of a LENA imager based on this technology is comparable to that of other proposed LENA imagers, with a significantly better angular resolution.

Fundamentals of low-energy neutral atom imaging

Imaging of the space plasma environment via low-energy neutral atoms (LENAs) promises to revolutionize the way in which largescale space plasma phenomena are viewed and understood. LENAs are produced by charge exchange between plasma ions (less than tens of kilo-electron-volts) and cold geocoronat neutrals; these LENAs radiate outward in all directions from their points of origin. Previously developed methods for imaging higher energy neutrals are not suitable for observing the majority of the terrestrial magnetosphere, which is comprised primarily of lower energy plasma populations. This paper briefly describes both the direct and indirect techniques that have been suggested for imaging LENAs to date. We then examine in more detail the most advanced of these techniques appropriate for magnetospheric imaging, indirect detection based on ionization of LENAs as they transit ultrathin foils. Such a LENA imager consists of four basic components: (1) a biased collimator to remove the ambient charged particles and set the azimuthal field of view; (2) an ultrathin foil, which ionizes a portion of the incident LENAs; (3) an electrostatic analyzer to reject UV light and set the energy passband; and (4) a coincidence position detector to measure converted LENAs while rejecting noise and penetrating radiation.

Mars odyssey neutron sensing of the south residual polar cap

[1] Measurements from the neutron spectrometer instrument of the Mars Odyssey Gamma-Ray Spectrometer (GRS) package for solar aereocentric longitude between 340° and 360° are used to estimate the CO 2 mass, dust content, and covered area of the south residual polar cap. Two models for the cap are explored; a cap uniformly covered in dust-free CO 2 and a Swiss cheese model with an exposed H 2 O ice substrate and 8m mesas composed of CO 2 and dust. A range of 1.0 to 3.0 x 10 17 g is found for the mass of residual CO 2 on the cap, or about 3 to 9% of the total CO 2 seasonally cycled in and out of the atmosphere. For the Swiss cheese model, if the CO 2 is dust-free then 60% of the cap area must be the exposed H 2 O ice substrate. This area decreases with increasing dust concentration in the CO 2 . For example if 45% of the cap area is exposed water ice then 224 g/cm 2 of dust is in the CO 2 and for a typical dust deposition rate of 0.9 x 10 -3 g/cm 2 /yr, the dust can be deposited in ∼25,000 yrs.

Low-energy neutral atom emission from the Earth's magnetosphere

Imaging of the terrestrial magnetosphere is possible through the detection of low-energy neutral atoms (LENA5) produced by charge exchange between magnetospheric plasma ions and neutral atoms of the Earths geocorona. We present calculations of both hydrogen and oxygen line-of-sight LENA fluxes expected on orbit for various plasma regimes as predicted by the Rice University Magnetospheric Specification Model. To decrease the required computation time, we are in the process of adapting our code for massively parallel computers. The speed gains achieved from parallel algorithms are substantial, and we present results from computational runs on the Connection Machine CM-2 data parallel supercomputer. We also estimate expected image count rates and image quality based on realistic instrument geometric factors, energy passbands, neutral atom scattering in the instrument, and image accumulation intervals. The results indicate that LENA imaging instruments will need a geometric factor ( G ) on the order of 0.1 cm 2 sr eV/eV to be capable of imaging storm time ring currents, and a G of 1.0 cm 2 sr eV/eV in order to image the quiet time ring current fluxes, ion injections from the tail, and subsequent ion drifts toward the dayside magnetopause.

Imagers for the magnetosphere, aurora, and plasmasphere

We present a small Explorer mission, Imagers for the Magnetosphere, Aurora, and Plasmasphere (IMAP), to provide the first global magnetospheric images that will allow a systematic study of major regions of the magnetosphere, their dynamics, and their interactions. The mission objective is to obtain simultaneous images of the inner magnetosphere (ring current and trapped particles), the plasmasphere, the aurora, and auroral upflowing ions. The instruments are (1) a Low Energy Neutral Particle Imager for imaging H and O atoms, separately, in the energy range of ~1 to 30 keV, in several energy passbands; (2) an Energetic Neutral Particle Imager for imaging H atoms in the energy range ~15 to 200 keV and, separately, O atoms in the energy range ~60 to 200 keV, each in several energy passbands; (3) an Extreme-Ultraviolet Imager to obtain images of the plasmasphere (the distribution of cold He + ) by means of He + (30.4 nm) emissions; and (4) a Far-Ultraviolet Imaging Monochromator to provide images of the aurora and the geocorona. All images will be obtained with time and spatial resolutions appropriate to the global and macroscale structures to be observed. IMAP promises new quantitative analyses that will provide great advances in insight and knowledge of global and macroscale magnetospheric parameters. The results expected from IMAP will provide the first large-scale visualization of the ring current, the trapped ion populations, the plasmasphere, and the upflowing auroral ion population. Such images, coupled with simultaneously obtained auroral images, will also provide the initial opportunity to globally interconnect these major magnetospheric regions. The time sequencing of IMAP images will also provide the initial large-scale visualization of magnetospheric dynamics, both in space and time.