Interplanetary Magnetic Field Connection to the L1 Lagrangian Orbit During Upstream Energetic Ion Events

D. K. Haggerty, E. C. Roelof, C. W. Smith, N. F. Ness, R. L. Tokar and R. M. Skoug

Journal of Geophysical Research, 105, 25,123--25,131 (2000).

Abstract:

Measurements from spacecraft orbiting the sunward lagrangian point (L1) show that energetic ions ( >15 keV) moving upstream from the Earth's bow shock frequently appear at L1, approximately 1.5x10^6 km upstream from Earth. The most common orientation of the interplanetary magnetic field (IMF) at L1 during these upstream events is near-radial from the sun, (the nominal direction for connection to the magnetosphere). However, strong unidirectional beams of ions streaming away from Earth, consistent with good magnetic connection to the bow shock, are observed even when the IMF is transverse to the radial direction. Included in this report are observations from: (1) ACE/EPAM, ACE/MAG, and ACE/SWEPAM; (2) WIND/3DP, WIND/MFI; (3) IMP8/EPE, IMP8/MAG; (4) GEOTAIL/EPIC, and GEOTAIL/MAG. The ACE/SWEPAM instrument identifies IMF connection between the L1 point and the Earth's bow shock by the presence of low-energy (272-372 eV) bidirectional electron flux. Instruments on the IMP8 and GEOTAIL spacecraft measure the energetic ion intensity and IMF configuration close to the Earth's bow shock during times of particle enhancements observed at L1. ACE/EPAM observations from launch through December 1998 show that ~30% of upstream events are observed during times non-radial (>38° from the GSE X axis) IMF geometry. Analysis of ACE, WIND, IMP8, and GEOTAIL observations from February through November 1998 confirm this result.