Magnetic reconnection at the solar wind current sheets as a possible cause of strahl electrons acceleration and SEP dropouts

Abstract:

According to the shape of the electron velocity distribution function, there are two populations of suprathermal electrons: halo and strahls (beams). The halo electrons are omni-directional, and strahls are magnetic field aligned beams of electrons that predominantly move in the anti-sunward direction. Properties of strahls represent a great interest, because this population is most energetic, but its origination is still unclear. Usually, it is supposed that strahls is a focused part of halo electrons, non-scattered during their propagation from the Sun.

We demonstrate a possibility to better understand nature of strahls if to suggest their acceleration directly in the solar wind due to a magnetic reconnection, occurring at current sheets. We use results of our PIC-simulation of particles behaviour at reconnecting current sheets (Zharkova, Khabarova, ApJ, 2012) in order to explain such effects as:

- mismatches between a position of suprathermal electrons pitch-angle changes and real crossing of the heliospheric current sheet,

- correlation between heat flux/solar energetic particles dropouts and high plasma beta,

- occurrence of counterstreaming electrons at the ICME front and at corotating shocks at r > 2 AU,

- radial evolution of strahls/halo density.

Multi-spacecraft observations (STEREO, ACE, Ulysses) of properties of suprathermal electrons attributed to crossings of the heliospheric current sheet as well as smaller-scale current sheets during SEP events and CME-CIR interactions will be discussed.