Kinetic properties of ions at the low-beta, quasi-perpendicular bow shock observed by the Cluster spacecraft

Abstract:

Earth’s bow shock shows different characteristics according to the plasma beta of upstream solar wind. In this study we investigated kinetic properties of ions at a low beta shock using measurements from the Cluster spacecraft. On 27 January 2001 Cluster observed gradual transition of the bow shock from high beta to low beta. For the high-beta shock the temperature of ions increased largely across the shock. However, the velocity space distributions in the downstream region show that the ions consist of two different components. One of them is the solar wind ion beam transmitted into the downstream region across the shock, which is a little bit thermalized from the upstream solar wind. The other is hotter component with bi-Maxwellian distribution. This observation implies that the solar wind ion beam does not instantly thermalize at the shock boundary into the hot ions in the magnetosheath. As the beta of the upstream solar wind becomes lower, the hotter component is substantially reduced and the solar wind ion beam maintains its shape in the velocity space while being slightly thermalized. Moreover, the velocity space distribution of the hotter component becomes more kappa-like, especially along the velocity parallel to the magnetic field. We will compare the level of thermalization of the high-beta and low-beta shocks, which will provide better insight about the thermalization process at collisionless shocks.