Interplanetary magnetic field connection to the L1 Lagrangian orbit during upstream energetic ion events

Abstract

Energetic ions (>15 keV) moving upstream from the Earth's bow shock frequently appear at the sunward Lagrangian point (L1) ∼1.5 × 106 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. This study includes for the first time observations of upstream events from as many as four upstream spacecraft. Included in this report are energetic ion and magnetic field measurements from (1) ACE/Electron, Proton, and Alpha Monitor (EPAM) and ACE/Magnetometer Instrument (MAG); (2) Wind/Three‐Dimensional Plasma Analyzer (3DP) and Wind/Magnetic Fields Investigation (MFI); (3) IMP 8/Energetic Particle Experiment (EPE) and IMP 8/MAG; and (4) Geotail/Energetic Particle and Ion Composition (EPIC) and Geotail/MAG. In addition, the ACE/Solar Wind Electron, Proton, and Alpha Monitor (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 IMP 8 and Geotail spacecraft simultaneously measure the energetic ion intensity and IMF configuration close to the Earth's bow shock during times of particle enhancements observed at L1. Analysis of ACE, Wind, IMP 8, and Geotail observations from February through November 1998 shows that >40% of upstream events are observed during times of nonradial (>30° from the GSE X axis) IMF orientation. A second and independent study of ACE/EPAM observations of over 500 upstream energetic ion events from launch through November 1998 confirms this result. We find similar distributions of radial and nonradial IMF orientations both close to the bow shock and at L1. This implies that the IMF configurations during upstream events are large‐scale spatial structures with radii of curvature of the order of the Earth‐L1 distance, i.e., 0.01 AU. These spatial structures are being convected through the L1 point into the Earth's foreshock.