1Centre d'Etude Spatiale des Rayonnements, Université de Toulouse (UPS), 9 Avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
2Centre National de la Recherche Scientifique, UMR 5187, Toulouse, France
3Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
4Space Environment Physics Group, School of Physics and Astronomy, University of Southampton, UK
5Department of Physics, University of New Brunswick, Fredericton, Canada
6Space Sciences Laboratory, University of California, Berkeley, USA
7Institute of Geophysics and Planetary Physics, University of California, Los Angeles, USA
8Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick, Coventry, UK
9Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
10University of New Hampshire, Durham, USA
Received: 06 Aug 2009 – Revised: 10 Dec 2009 – Accepted: 18 Jan 2010 – Published: 22 Jan 2010
Abstract. Previous work has shown that solar wind suprathermal electrons can display a number of features in terms of their anisotropy. Of importance 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 statistical properties of counter-streaming suprathermal electrons (CSEs) in the vicinity of corotating interaction regions (CIRs) during the period March–December 2007. Because this period corresponds to a minimum of solar activity, the results are unrelated to the sampling of large-scale coronal mass ejections, which can lead to CSE owing to their closed magnetic field topology. The present study statistically confirms that CSEs are primarily the result of suprathermal electron leakage from the compressed CIR into the upstream regions with the combined occurrence of halo depletion at 90° pitch angle. The occurrence rate of CSE is found to be about 15–20% on average during the period analyzed (depending on the criteria used), but superposed epoch analysis demonstrates that CSEs are preferentially observed both before and after the passage of the stream interface (with peak occurrence rate >35% in the trailing high speed stream), as well as both inside and outside CIRs. The results quantitatively show that CSEs are common in the solar wind during solar minimum, but yet they suggest that such distributions would be much more common if pitch angle scattering were absent. We further argue that (1) the formation of shocks contributes to the occurrence of enhanced counter-streaming sunward-directed fluxes, but does not appear to be a necessary condition, and (2) that the presence of small-scale transients with closed-field topologies likely also contributes to the occurrence of counter-streaming patterns, but only in the slow solar wind prior to CIRs.
Lavraud, B., Opitz, A., Gosling, J. T., Rouillard, A. P., Meziane, K., Sauvaud, J.-A., Fedorov, A., Dandouras, I., Génot, V., Jacquey, C., Louarn, P., Mazelle, C., Penou, E., Larson, D. E., Luhmann, J. G., Schroeder, P., Jian, L., Russell, C. T., Foullon, C., Skoug, R. M., Steinberg, J. T., Simunac, K. D., and Galvin, A. B.: Statistics of counter-streaming solar wind suprathermal electrons at solar minimum: STEREO observations, Ann. Geophys., 28, 233-246, doi:10.5194/angeo-28-233-2010, 2010.