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Previous work has shown that solar wind suprathermalelectrons can display a number of features in terms of their anisotropy. Ofimportance is the occurrence of counter-streaming electron patterns, i.e.,with "beams" both parallel and anti-parallel to the local magnetic field,which is believed to shed light on the heliospheric magnetic field topology.In the present study, we use STEREO data to obtain the statisticalproperties of counter-streaming suprathermal electrons (CSEs) in thevicinity of corotating interaction regions (CIRs) during the period March–December 2007.Because this period corresponds to a minimum of solaractivity, the results are unrelated to the sampling of large-scale coronalmass ejections, which can lead to CSE owing to their closed magnetic fieldtopology. The present study statistically confirms that CSEs are primarilythe result of suprathermal electron leakage from the compressed CIR into theupstream regions with the combined occurrence of halo depletion at 90°pitch angle. The occurrence rate of CSE is found to be about 15–20% onaverage during the period analyzed (depending on the criteria used), butsuperposed epoch analysis demonstrates that CSEs are preferentially observedboth before and after the passage of the stream interface (with peakoccurrence rate >35% in the trailing high speed stream), as well asboth inside and outside CIRs. The results quantitatively show that CSEs arecommon in the solar wind during solar minimum, but yet they suggest thatsuch distributions would be much more common if pitch angle scattering wereabsent. We further argue that (1) the formation of shocks contributes to theoccurrence of enhanced counter-streaming sunward-directed fluxes, but doesnot appear to be a necessary condition, and (2) that the presence ofsmall-scale transients with closed-field topologies likely also contributesto the occurrence of counter-streaming patterns, but only in the slow solarwind prior to CIRs

Statistics of counter-streaming solar wind suprathermal electrons at solar minimum: STEREO observations