Electron Distribution Functions Near Interplanetary Shocks

This paper examines the signatures of electron velocity distribution functions near several interplanetary shocks as observed by the Vector Electron Spectrometer on the WIND spacecraft. These interplanetary shocks are moderate in strength and have values of thetabn between 60 and 90 degrees. We concentrate on a quasi-perpendicular shock observed by WIND on August 26, 1998 which was associated with an interplanetary type II radio burst. WIND observations of this shock have previously been studied by Bale et al. [GRL, June 1, 1999] who found the first evidence for electron beams in the source region of a type II burst in the upstream region of this shock. Our focus will be on comparing the electron distribution functions before and after the WIND encounter with the shock, showing the evidence for electron acceleration and heating from the shapes of the distributions. In the upstream, loss cone and bump on tail distributions are observed that are characteristic of magnetic mirroring in the rising magnetic field of the shock layer. The dispersion in arrival times of the upstream electrons has been used to measure the distance along the magnetic field lines connecting the spacecraft and the shock. On traversing the shock layer from the upstream to the downstream, the distributions broaden, first in the direction perpendicular to the magnetic field, followed by broadening of the parallel distribution and the formation of parallel beams until the distribution is nearly isotropic. The downstream distributions of this shock, as well as several of the other stronger interplanetary shocks in this study, have the typical flat top signature of electron heating and the electron beams found immediately downstream of the Earth's bow shock. Observations by the WAVES experiment on WIND also show typical broadband, impulsive signatures in the shock. Further downstream the distributions again become slightly anisotropic with the perpendicular temperature exceeding the parallel. We compare the observed distributions with results of theoretical works, particularly the recent work of Hull et al. [JGR, August 1, 2001].