ACE News #31
June 18, 1999

Non-radial IMF connection between the Earth's bow shock and L1

Observations at both ACE and WIND show that ions moving upstream from the Earths bow shock frequently appear at the sunward Lagrangian point (L1). To observe these ions at L1 the interplanetary magnetic field (IMF) must somehow connect to the bow shock and the expected way is on field lines that are radial between L1 and the bow shock. The figure shows an example during which upstream ions were observed at ACE, WIND, and GEOTAIL (July 30, 1998 18:07 - 19:04 UT). Shown are the XY components of the IMF during times that these upstream ions were observed. The IMF components have been color coded in time from purple through the spectrum at each spacecraft so that blue vectors at ACE are observed at the same time as blue vectors at WIND and GEOTAIL. The figure shows that the IMF was not radial at L1 nor close to Earths bow shock during this event, yet an upstream ion event was observed. A study of the IMF from February through November 1998 at ACE, WIND, GEOTAIL, and IMP8 shows that 30% of the upstream events occurred when the IMF was > 30 degrees from the radial direction. This study shows that although a radial connection between L1 and the bow shock is preferred, IMF connection and upstream ion events do occur on highly non-radial field lines. The probable explanation for this effect is upstream ion interaction with convecting spatial structures in the solar wind, i.e. magnetic field discontinuities and/or flux ropes, that are convecting past the spacecraft. These energetic particle events will provide a valuable tool to help understand these interplanetary structures and the L1 to bow shock magnetic field connection.

Contributed by:
D. K. Haggerty and E. C. Roelof of The Johns Hopkins University Applied Physics Laboratory,
C. W. Smith of Bartol Research Institute, University of Delaware,
R. L. Tokar and R. M. Skoug of Los Alamos National Laboratory

See the EPAM Home Page at JHU/APL for more information about the EPAM instrument.

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Last modified June 21 1999, Andrew Davis
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