Simultaneous measurements of energetic ion (≥50 keV) and electron (≥220 keV) activity upstream of Earth's bow shock and inside the plasma sheet: Magnetospheric source for the November 3 and December 3, 1977 upstream events

Abstract

Simultaneous observations of energetic ions (≳50 keV) and electrons (≳220 keV) by the IMP 7 and 8 spacecraft, carrying identical instruments and located within the distant (∼37 RE) magnetotail and upstream from the bow shock, have been employed to separate temporal variations from spatial variations during the upstream ion events observed on December 3, 1977 and November 2–3, 1977, in order to determine the source of these particles. The IMP data, when compared with those from ISEE 1 and 2, have also made possible the determination of field‐aligned and flux‐tube intensity gradients in the upstream region, thus enabling the test of specific predictions of the Fermi acceleration model for such events. The analysis of these three‐spacecraft observations and comparison with theory have revealed the following: (1) For each of the observed upstream enhancements, energetic ions and electrons were simultaneously present inside the plasma sheet—successive increases were in excellent time coincidence with substorm injection events seen on ground‐based magnetograms; (2) The low‐energy (≳50 keV) ion intensity profile inside the plasma sheet was relatively flat, while at higher (≳300 keV) energies there was considerable variability, with one case exhibiting an “inverse velocity dispersion” profile; (3) Relativistic electron bursts were seen inside the plasma sheet and also upstream of the shock but at substantially reduced intensities; (4) The ion energy spectrum for the December 3 event, extended to energies ∼2 MeV, was identical in form within the plasma sheet and upstream of the shock and can be described well by dj/dE ∝ E−5.5; (5) Ion anisotropies exhibited typically large dawn‐dusk or dusk‐dawn gradients, depending on spacecraft location, and showed large (up to 20∶1) field‐aligned streaming away from the bow shock; (6) Comparison of IMP and ISEE 2 intensities at ∼60 keV for identical magnetic connections to the bow shock during a period when the interplanetary magnetic field (IMF) was radial showed the IMP 7 intensity to be higher by a factor ∼10, even though IMP 7 was ∼20 RE further from the bow shock, but located toward the dusk side of the earth‐sun line; and (7) Comparison of unidirectional intensities of IMP 8 and ISEE 1 along the same radial IMF line showed the field‐aligned component to be identical at both spacecraft, while the omnidirectional intensities give a gradient of ∼2.5%/RE. The observations are compared with predictions of the Fermi model, and it is found that they are inconsistent with the principal features of the model, including the energy extent, form of the spectrum, anisotropies, scale lengths of acceleration region, IMF connection times, and the presence of electrons.