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

Abstract

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.

Modeling the electromagnetic ion cyclotron wave‐induced formation of detached subauroral proton arcs

“…Spectra from the Magnetospheric Plasma Analyzer (MPA) [McComas et al, 1993] on these satellites, which measures the distributions of ions and electrons in the energy range $1 eV to 40 keV, are shown in Figure 3 (left) for 23 January and in Figure 3 (right) for 18 June. The nightside geosynchronous observations provide information on the ring current source populations; the hot ion fluxes (energy >1 keV) measured by the MPA on satellite 1994-084 (third panel from top) have larger values between 1800 and 0600 MLT on 18 June (Figure 3, right) reflecting the more elevated magnetic activity than on 23 January (Figure 3, left).…”

Thermal plasmaspheric morphology: Effect of geomagnetic and solar activity

“…Helium ions will be used as a surrogate for total plasma density; it is therefore important to develop a physics-based model that will predict the heliumto-hydrogen ion density ratio. Previously, observations from both geosynchronous satellites [McComas et al, 1993] and polar satellites [Horwitz et al, 1986] have shown a complicated dynamic behavior [e.g., Moldwin et al, 1995], and the role that geomagnetic activity plays in the formation and maintenence of the plasmapause continues to be a subject of considerable interest [Carpenter et 0148-0227/99/1999JA900050509.00 able to detect the plasmapause but will also be able to infer the density variation of the plasma within the plasmasphere, such as radial profiles and asymmetries in local time. The interpretation and analysis of such images require a reliable model for the thermal plasma density morphology.…”

A comprehensive survey of plasmasphere refilling at geosynchronous orbit

“…Los Alamos magnetospheric plasma analyzer (MPA) instruments [Bame et al, 1993;McComas et al, 1993] on board several geosynchronous satellites have now been taking data for a complete solar cycle (i.e., roughly 11 years), which provides an opportunity for a comprehensive statistical survey of plasmasphere refilling at geosynchronous distances. MPAs typically float at a potential that is slightly negative with respect to the ambient plasma, so the low-energy plasmaspheric ions are not repelled by the spacecraft sheath.…”