COSTEP/SOHO OBSERVATIONS OF ENERGETIC ELECTRONS FAR UPSTREAM OF THE EARTH’S BOW–SHOCK

摘要

We have analyzed 124 electron bursts at energies above 0.25 MeV observed with the COSTEP/EPHIN instrument onboard the SOHO spacecraft far upstream of the Earth’s bow–shock. Some of these upstream bursts were accompanied by energetic protons (< 1 MeV). Most of the bursts were observed during low solar activity (in 1996–1997 and in 2005) and were not associated with solar particle events. The event occurrence number shows a distinct seasonal variation with maxima around equinoxes and minima near solstices. This together with a close correspondence between the event occurrence number and enhancements in both solar wind speed (Vsw) and geomagnetic activity index (Ap) indicates that the observed events can be explained in terms of leakage of magnetospheric particles during enhanced geoactivity rather than as acceleration at the Earth’s bow–shock. We compare these data with measurements of particle bursts as well as magnetic field and solar wind parameters obtained with theWind spacecraft.