Kinetic Properties of Solar Wind Discontinuities at 1 AU Observed by ARTEMIS

Because solar wind plasma flow transports the entire spectrum of magnetic field fluctuations (from low-frequency inertial range to electron kinetic range), it is a natural laboratory for plasma turbulence investigation. Among the various wave modes and coherent plasma structures that contribute to this spectrum, one of the most important solar wind elements is ion-scale solar wind discontinuities. These structures, which carry very intense current, have been considered as a free energy source for plasma instabilities that contribute to solar wind heating. Investigations of such discontinuities have been mostly focused on their magnetic field signatures; much less is known about plasma kinetics around them. Using statistics of such discontinuities observed around the Earth (similar to 1 AU) by two spacecraft from the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission, we demonstrate that they are accompanied by density and temperature variations with a spatial scale of tens of ion inertial lengths. Inversely correlated density and temperature variations suggest the discontinuity configuration is nearly force free. The discontinuities are typified by two spatial scales, an intense current layer (>1 nA/m(2)), and a much broader current layer structure enveloping them. The magnetic field rotation occurs at the large scale, whereas at the small (inner) scale the plasma pressure gradients provide the pressure balance. We find that the electron kinetic behavior around the discontinuities strongly depends on electron energy: Whereas the pitch-angle distribution of near-thermal electrons (10-30 eV) changes significantly across discontinuities, hot (100-1,000 eV) electrons retain their properties on the two sides, suggesting they are able to cross the discontinuities freely. Exploring these and other kinetic characteristics of discontinuities, we discuss possible mechanisms of formation/evolution of these structures.