Suprathermal proton and (alpha)-particle bursts (E/q = 6.5-225 keV/e) observed by the WIND-, ACE- and IMP8-S/C during depressions of the interplanetary magnetic field

The present study deals with suprathermal proton ( E/q=6.5-225 keV/e) and (alpha)-particle bursts measured by the WINDSMS experiment in the interplanetary space. They reach up to ~5-20 times the solar wind speed and last from a few minutes up to ~30 min. Measurements obtained simultaneously by the Solar Wind Ion Composition Sensor SWICS ( E/q=0.5-31.5 keV/e) were also available for this study, as well as magnetic field and particle data recorded by ACE near the Libration point L1 and the IMP8-S/C near the Earth. In order to exclude particles escaping from the magnetosphere or accelerated by the Earthʹs bow shock, interplanetary shocks, coronal mass ejections and corotating interaction regions, we selected ion bursts which were associated with a distinct decrease in the interplanetary magnetic field magnitude and with changes in the azimuthal and tangential field direction. Such changes have been known for a long time as magnetic holes or field depressions. We interpret these signatures as a manifestation of a reconnection process in the interplanetary space near the heliospheric current sheet at about 1 AU distance from the Sun and show for the first time that thermal particles can be accelerated up to ~100 keV/e. The suprathermal particles are most likely accelerated in the electric field of the X-line. Inductive electric fields caused by changes in the field magnitude could also be responsible for the particle acceleration.