Chapter 16 - Formation of Kappa Distributions at Quasiperpendicular Shock Waves

Abstract

Although wave–particle interactions, once formed, may maintain a preexisting kappa distribution throughout the solar wind, an important question is to identify the origin of the kappa distributions. Throughout much of the heliosphere beyond some 2 to 3 AU, PickUp Ions (PUIs) form a suprathermal proton component in the solar wind. The PUI distribution does not equilibrate with the thermal solar wind protons and forms a filled-shell distribution about the colder solar wind protons' Maxwellian core. In this chapter, we show that the suprathermal component is reflected preferentially by the cross-shock electrostatic potential at perpendicular shocks, unlike the colder solar wind proton distribution, which is transmitted through the shock. We show how the velocity distribution function downstream of quasiperpendicular shocks is constructed, and we find that the reflected suprathermal ions form an extended tail to the velocity distribution function. The total proton distribution function closely resembles a kappa distribution. Simulation results are presented that support the general theory for the formation of downstream kappa distributions at quasiperpendicular shocks in the presence of PUIs. Various observational tests of the theory for the formation of kappa distributions by quasiperpendicular shocks are presented. Finally, we discuss the close relationship between the dissipation process that forms kappa distributions at quasiperpendicular shocks and the diffusive shock acceleration of protons at quasiperpendicular shocks, even at 1 AU.