H. E. Spence1 and J. B. Blake2
1Center for Space Physics and Department of Astronomy,
Boston University, 725 Commonwealth, Avenue, Boston, MA 02215,
USA, E-mail: email@example.com
2Space and Environmental Technology Center, The Aerospace Corporation, P.O. Box 92957, MS M2/259, Los Angeles, CA 90009-1055, USA
We report on the first observations of magnetospheric energetic particle populations by the Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD) experiment on the recently launched NASA POLAR spacecraft. The CEPPAD Imaging Proton Spectrometer (IPS) experiment has been returning excellent measurements of 15 keV ions with high angular, temporal, and spectral resolution since its activation shortly after the POLAR spacecraft launch. The POLAR spacecraft is routinely sampling both previously explored and "new" regions and the high sensitivity CEPPAD instruments observe exciting phenomena in virtually every portion of the POLAR orbit. In this paper, we report on CEPPAD IPS measurements of both familiar magnetospheric regions (cusp, plasma sheet, ring current, radiation belt) as well as the unfamiliar (the appearance of energetic particle near apogee at very high magnetic latitudes). We propose that newly discovered polar energetic particles (PEPs) are the signature of either; (1) accelerated magnetosheath particles on open field lines, or (2) hot particles of a severly distorted plasma sheet, both of which occur during periods of northward interplanetary magnetic field (IMF).
With the launch of POLAR, the Global Geospace Science (GGS) program (Acuña et al., 1995) is now fully operational. The GGS program includes not only POLAR, but also the WIND spacecraft, a comprehensive suite of ground-based instruments, and a supporting theory element. GGS is the United States' contribution to the International Solar Terrestrial Physics (ISTP ) program. The ISTP program now includes an impressive armada of space- and ground-based instruments providing unprecedented global measurements of the near-Earth space environment in a highly coordinated manner. In this paper, we preview several interesting measurements made by the CEPPAD sensors on the POLAR spacecraft that relate to global structure and dynamics of the magnetosphere.
We find that there are fascinating energetic ion (>15 keV) phenomena seen in virtually every portion of the POLAR orbit. In the dayside high-L regions we see velocity-dispersed ions associated with the magnetic cusp and dayside merging. In the nightside high-L regions we observe the high-latitude extension of the plasma sheet and its dynamics. At lower L (< 10) we frequently observe substorm and storm-related particle energization and Alfvén-layer dynamics associated with the inner plasma sheet and ring current populations. However, the most suprising result is a newly discovered population of energetic particles seen at very high-L, away from traditional magnetospheric boundaries, nominally occuring within the tail lobe region where energetic particles are not anticipated. In this short paper, examples of these regions and their signatures are briefly summarized.
THE POLAR MISSION AND CEPPAD EXPERIMENT OVERVIEW
The NASA POLAR spacecraft was launched into a ~2 x ~9 RE near-polar orbit on February 24, 1996 with apogee over the northern hemisphere. Over the first six months of operations, the POLAR orbital plane has moved through the noon-midnight meridian (mid-April) and more recently through the dawn-dusk meridian (mid-July), thereby sampling vast regions of magnetospheric particle populations as it slices across L-shells in its orbit every ~18 hours. POLAR is a cylindrical spacecraft , with a despun platform containing three auroral imagers and a spinning section (~10 rpm) housing the eight in situ particles and fields experiments (Harten and Clark, 1995). The CEPPAD experiment package (Blake et al., 1995) is mounted on the rotating portion of POLAR. CEPPAD is a multiple sensor experiment that provides high-time-resolution measurements of energetic particle pitch angle from moderate (~15 keV) to very (~80 MeV) energetic electrons and ions, comprised by the Imaging Proton Spectrometer (IPS), the Imaging Electron Spectrometer (IES), and the HIgh Sensitivity Telescope (HIST).
The focus of this paper are measurements from the IPS which is described briefly here (please see Blake et al., 1995 for a more thorough description). The IPS uses a monolithic ion-implanted solid-state detector that is discretely segmented into multiple pixels, a customized design by Micron Semiconductor. The detector sits behind a collimation stack at the "focal plane" of a "pin-hole camera", thereby imaging a slice of phase space. Three identical heads, each with three non-overlapping look directions (20 deg x 12 deg) provide collectively an instantaneous snapshot of a 180 degrees x 12 degrees wedge of phase space. As a consequence of spacecraft rotation, the IPS maps out a full 4pi steradian image each spin period (~6 seconds). A low energy threshold of ~15 keV is the result of an extremely thin detector "dead layer" and very low-noise electronics. Sixteen energy bins span the low energy threshold to ~1.5 MeV. The IPS response to different magnetospheric regions follows.
THE MAGNETOSPHERE ACCORDING TO CEPPAD /IPS
The Magnetospheric Cusp and the Dayside Boundary Layer
As POLAR passes through the high-latitude dayside cusp, we observe velocity-dispersed
ion signatures (see Figure 1 at ~6:15UT) during southward IMF characteristic
of dayside merging reported previously using Dynamics Explorer 1 and 2
data (Burch et al.,1982). The POLAR orbit is such that it encounters
the cusp at higher altitudes than did DE 1 and thus transits this plasma
entry site much more slowly. In other cusp passages we see clear evidence
of multiple VDIS that have been interpreted both in terms of magnetosheath
plasma variability as well as pulsed dayside reconnection. During periods
of northward IMF we observe the expected "inverse" velocity dispersion.
These observations are allowing us to probe the efficiency and location
of the acceleration mechanism as well as constraining the temporal and
spatial extent of the reconnection regions. More strikingly, we often find
a second velocity dispersed signature equatorward of the cusp that frequently
starts at energies up to ~1 MeV and often lasts 1-2 hours (see Figure 1
at ~5:15 UT). This feature resides within the magnetospheric cavity and
may be in some way related to the cusp merging line acceleration process.
Fig. 1. Velocity dispersion of cusp at poleward edge of outer dayside plasma sheet (~6 UT). The top three panels show IPS energy (~15 to 1500 keV log spaced)-time spectrograms for three different look directions relative to the spacecraft spin vector. The fourth panel shows the electron background and the bottom plot shows L trajectory.
The Plasma Sheet and Ring Current
As POLAR passes through the high magnetic latitude extension ("horns") of the plasma sheet it routinely monitors boundary location and dynamics. An example of this behavior is shown in Figure 2 which plots both IPS and IES(similar in form and function to the IPS) energy-time spectrograms for a pass through the near-midnight plasma sheet on 14-15 April 1996. (Note that the band at nearly fixed energy in the IES spectrogram is the energy base-line of the sensor.) Dynamics likely reflect both a global mode response of the magnetotail to the solar wind driver as well as to internal reconfigurations (i.e., substorms). This particular pass occurred during a moderate geomagnetic storm which showed dramatic plasma sheet expansions and heatings and concurrent exotic pitch-angle distributions in the inner magnetosphere (see paper by Sheldon and Spence  for a description of this storm). The ability to have excellent sensitivity of energetic particle populations at high time and pitch angle resolution will allow us to use such boundary crossings as a powerful remote sensing tool of distant acceleration processes.
Fig. 2. Illustration of plasma sheet and ring current dynamics on April 14-15, 1996 during a moderate storm.
The Polar Cap
Perhaps the most interesting energetic particle populations seen on POLAR are those occuring in the most unlikely and unexpected region. Near apogee, POLAR often sees energetic (<200 keV) magnetosheath-like ions at very high magnetic latitudes in regions not traditionally associated with energetic particles (see Figure 3). These Polar Energetic Particles (PEPs) may persist in the lobe regions for many hours at a time with complex pitch angle distributions (PADs) and internal structure. We find that the most prominent PEPS occur predominantly during periods of IMF +Bz, especially with large Bx. Early analysis reveals two possible sources of this population: (1) the entry of accelerated magnetosheath plasma onto open lobe field lines via merging well poleward of the cusp and; (2) and a greatly inflated plasmasheet during periods of strongly northward IMF. Both mechanisms are needed to explain the spectral and pitch angle observations: mechanism (1) explains that class of PEPs seen at the highest magnetic latitudes while mechanism (2) explains that class of PEPs seen nearer to the expected location of the plasma sheet.
Fig. 3. Polar Energetic Particles (PEPs) seen in the polar cap during 4/9/96 with energies up to ~100 keV.
The POLAR orbit is probing both old and new regions of geospace with state-of-the-art instrumentation from the CEPPAD IPS and IES sensors. This combination is providing new insights on "old" problems (e.g., ring current processes) as well as new views of how the solar-terrestrial system is coupled (e.g., PEPs). We are currently in the process of relating the energetic particle measurements from POLAR with the complete suite of ISTP resources as we develop a more comprehensive understanding of geospace and its interaction with the Sun.
The authors gratefully acknowledge the countless engineers and scientists listed in the Blake et al. reference who toiled many years to make the CEPPAD experiment a reality. Without their dedicated efforts, CEPPAD science would not be possible. This work was supported by NASA contract NAS5-30368.
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