Betatron Cooling of Halo and Strahl Electrons in the Solar Wind

It is widely recognized that adiabatic acceleration plays an essential role in the dynamics of the solar wind electron distribution. Nevertheless, the role of electron adiabatic cooling remains poorly understood, at least from an observational standpoint. Even the betatron cooling has never been verified in the solar wind. Here, we present a distinct event of simultaneous betatron cooling of halo and strahl electrons in a small-scale magnetic structure, probably due to a local expansion (weakening of magnetic field strength). The betatron cooling results in the drop of electron differential fluxes, which peaks in the quasi-perpendicular direction and smoothly decreases toward the field-aligned direction. The cooling processes of halo and strahl electrons via the betatron mechanism are well reproduced using an ideal adiabatic acceleration/cooling model.