Three‐dimensional magnetosheath plasma ion distributions from 200 eV to 2 MeV

Abstract

We present initial measurements of the three‐dimensional magnetosheath plasma ion flow and spectrum over the energy range of 200 eV to 2 MeV. These observations are from two magnetosheath traversals, one each on the dawn (December 19, 1977) and dusk (July 7, 1978) flanks of the magnetosphere. Data are from three‐dimensional plasma and energetic ion observations from the ISEE 1 satellite. The data show the continuous presence of a high‐energy (≳5 keV) tail in the magnetosheath ion population during these two traversals. Comparisons with plasma sheet data lead us to conclude that for the two cases studied the magnetosheath plasma often has at least two major components, the shocked solar wind seen at energies ≲5 keV and magnetospheric (plasma sheet) particles seen at energies ≳5 keV. At times, however, magnetosheath spectra are observed that indicate the presence of only a shocked solar wind component that includes a high‐energy tail possibly produced either by ambient energetic solar particles or by acceleration processes at the bow shock. The shocked solar wind component at energies ≲5 keV behaves independently of the magnetic field direction, indicating that the frozen‐in field approximation applies. The high‐energy tail at energies ≳5 keV is highly modulated by the magnetic field. In the strongly varying magnetic field case of December 19, 1977, energetic ion transport in the plasma rest frame occurs by diffusive processes which result in an isotropic energetic ion distribution in that frame. The imposition of the plasma bulk velocity on this distribution results in the energetic ion anisotropies observed in the satellite frame. In the much more ordered field of July 7, 1978, energetic ion transport occurs via single‐particle motion along magnetic flux tubes in the plasma rest frame. The transport of flux tubes past the satellite as they change direction and connect/disconnect to remote particle sources causes the variations in energetic ion intensities and anisotropies observed in the satellite frame.

Energy time dispersion of a new class of magnetospheric ion events observed near the Earth's bow shock

“…From these spectra we conclude that a lowenergy ion population was escaping at those times, upstream from the bow shock, under a variety of IMF directions and bow shock structures. Since the E $6 keV ion population was highly anisotropic along the IMF (data not shown in the ®gure) and not dependent on the IMF direction/bow shock structure, we infer that this is more consistent with leakage of magnetosheath superthermal ions from an ion source downstream of the bow shock whose density is independent of the angle h B n (Tanaka et al, 1983;Williams et al, 1988), than with bow shock acceleration (Ipavich et al, 1988). We point out that ®eld aligned beams of higher energy (³50 keV) ions were observed at the onset of all the three November 1, 1984, upstream events of Figs.…”

“…The terrestrial magnetosphere has been known for many years to be an important source of energetic (³30 keV) ions in the magnetosheath (Williams et al, 1988;Kudela et al, 1992;Paschalidis et al, 1994;Karanikola et al, 1999) and the near Earth interplanetary space (Sarris et al, 1978;Anagnostopoulos et al, 1986;. It is also known that dierent estimations for the extent of contribution of the magnetospheric ion population in the magnetosheath and the upstream region have been postulated Anagnostopoulos et al, 1986).…”

IBEX Backgrounds and Signal-to-Noise Ratio

“…For the magnetosheath (the worst case) we used the measured ion fluxes from the CODIF instrument on the Cluster mission for our analysis (shown in Fig. 7), which are similar to magnetosheath measurements reported earlier (Williams et al 1988;Paschalidis et al 1994). Table 2 shows the background fluxes on the conversion surface or conversion foil for the IBEX energy steps under these assumptions and using the cross sections of H + on water (Greenwood et al 2000).…”

Properties and origin of energetic particles at the duskside of the Earth's magnetosheath throughout a great storm

“…In addition, the plasma sheet population of energetic particles alone suces to warrant the lower observable¯uxes in magnetosheath. The density gradient of energetic particles is directed toward the magnetopause (Williams et al, 1988;Traver et al, 1991;Paschalidis et al, 1994;Eastman and Christon, 1995).…”

“…Williams (1979) presented strong evidence for signi®cant but sporadic magnetosheath energy¯ow in the antisolar direction using 50± 220 keV ions. Williams et al (1988) studied two magnetosheath traversals, one on each of the dawn and dusk anks of the magnetosphere. They concluded that the magnetosheath plasma often has at least two major components, the shocked solar wind seen at energies £5 keV (which behaves independently of the magnetic ®eld direction) and the magnetospheric (plasma sheet) particles seen at energies ³5 keV (which are highly modulated by the magnetic ®eld).…”