Solar wind electron halo depletions at 90° pitch angle

Strong depletions in solar wind electron halo distribution functions, centered on and roughly symmetric about 90 degrees pitch angle, are present in at least 10 % of data obtained by the Los Alamos SWEPAM experiment on the ACE spacecraft. Sunward-directed conics are also present in the electron data, occur in conjunction with the 90 degrees depletions, but are less common than depletions. We suggest that on open field lines both depictions and conics arise from focusing and mirroring effects associated with field line connection to magnetic field enhancements further out in the heliosphere. The focusing and mirroring produces field-aligned, counterstreaming fluxes of suprathermal electrons somewhat similar to that observed on closed field lines. Our observations provide strong evidence that the antisunward-directed portion of the electron halo on open field lines at times results primarily from mirroring of back scattered strahl and/or shock-heated electrons from far out in the heilosphere.