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

Abstract

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 interfaceCorrespondence to: B. Lavraud (benoit.lavraud@cesr.fr) (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.

The almost monoenergetic ion event on 19 October 2009: SEPT/STEREO observations

“…1f). The appearence of counter-streaming suprathermal electrons strongly supports the idea of its origin in conjunction with a CIR, as was observed by Lavraud et al (2010). These low-energy electrons may be accelerated or/and reflected at the CIR.…”

In Situ Signatures of Interchange Reconnection between Magnetic Clouds and Open Magnetic Fields: A Mechanism for the Erosion of Polar Coronal Holes?

“…By analyzing MCs we can therefore analyze the properties of magnetic loops which have emerged near active regions. Owing to their large velocities, suprathermal electrons propagating parallel (or anti-parallel) to the magnetic field constitute tracers of the heliospheric magnetic topology (see, e.g., Zwickl et al, 1983;Lavraud et al, 2010) and enable us to study the connectivity of MCs at the Sun. Observation of counter-streaming, bi-directional strahl populations is interpreted as the signature of magnetic field lines with both ends connected to the Sun (see, e.g., Pilipp et al, 1987;Lavraud et al, 2009), and is often thought of as a magnetic cloud identifier (Gosling et al, 1987).…”

Counterstreaming electrons in small interplanetary magnetic flux ropes

“…We analyzed the pitch angle spectrograms in the heat energy range of approximately 80 eV to 300 eV to determine the CSEs within the SIMFRs. The CSE events were confirmed through their pitch angle distributions that show higher phase space densities in the field‐aligned and antifield‐aligned directions as compared with the perpendicular (90° pitch angle) direction [ Lavraud et al , ]. The identification of CSE boundaries is important to calculate the percentages of CSEs coincident with the SIMFRs.…”

“…Many observations revealed that counterstreaming suprathermal electrons (CSEs) were one of the characteristics used to identify MCs, and they can be interpreted as signatures of closed magnetic field lines, with both ends connected to the Sun [ Gosling et al , ; Bothmer et al , ; Larson et al , ; Shodhan et al , ; Crooker et al , , ]. CSEs can also be produced by other mechanisms, e.g., connection to the Earth's bow shock [ Feldman et al , ; Stansberry et al , ] and distribution function depletions near the 90° pitch angle [ Gosling et al , , ; Skoug et al , ], CSEs are also frequently observed inside or outside CIRs [ Steinberg et al , ; Lavraud et al , ]. If SIMFRs are the interplanetary manifestations of CMEs produced in the solar corona, and some SIMFRs, like MCs, may still be attached to the Sun at both ends, then these SIMFRs should contain CSEs.…”