KINETIC PROPERTIES OF MAGNETIC DECREASES OBSERVED IN THE SOLAR WIND AT ~1 AU

Abstract

In this study, we investigate the kinetic properties of magnetic decreases observed in the solar wind at~1 AU using the Cluster observations. We study two different magnetic decreases: one with a short observation duration of~2.5 minutes and stable structure and the other with a longer observation duration of~40 minutes and some fluctuations and substructures. Despite the contrast in durations and magnetic structures, the velocity space distributions of ions are similar in both events. The velocity space distribution becomes more anisotropic along the direction parallel to the magnetic field, which differs from observations obtained at high heliographic latitudes. On the other hand, electrons show different features from the ions. The core component of the electrons shows similar anisotropy to the ions, though the anisotropy is much weaker. However, while ions are heated in the magnetic decreases, the core electrons are slightly cooled, especially in the perpendicular direction. The halo component does not change much in the magnetic decreases from the ambient solar wind. The strahl component is observed only in one of the magnetic decreases. The results imply that the ions and electrons in the magnetic decreases can behave differently, which should be considered for the formation mechanism of the magnetic decreases.