Leon Ofman (Catholic University of America & NASA-Goddard Space Flight Center)

"Observations and modeling of ion kinetic instabilities in the solar"

Abstract: The solar wind is a stream of hot magnetized plasma from the Sun that forms the background state of the heliosphere. The main constituents are electron, protons, and ⇠ 4% of fully ionized helium (i.e., alpha particles) that reach velocities of 400-800 km/s depending on coronal sources. The solar wind is variable, and the physical mechanisms that accelerate and heat the solar wind are still debated. Past studies and models of the solar wind formation relied primarily on fluid and MHD description, large scale MHD or Alfvén waves and incompressible turbulence models, motivated by observations at 0.3-1AU. The Parker Solar Probe (PSP) mission launched into the inner heliosphere provides observations close to the Sun (¡0.1AU) in the formation region of the solar wind and discovered abundance of ion-scale kinetic wave activity, proton and alpha particle beams, temperature anisotropies and unstable ion velocity distributions (VDFs) deduced from the Solar Probe ANalyzer for Ions SPAN-I instrument. Recently, PSP provided data at perihelia from the sub-Alfvénic solar wind out-flow regions finding non-Maxwellian proton and alpha particle VDFs, and high temperature anisotropies. Motivated by these observations we employ 2.5D and 3D hybrid-PIC models of proton-alpha solar wind plasma and investigated the onset and nonlinear evolution of the ion kinetic instabilities that arise from the unstable ion VDFs. The models demonstrate the growth, nonlinear saturation, and relaxation of the instabilities, resulting in generation of kinetic wave spectra, and eventual anisotropic heating of the solar wind plasma. The results of the models are used to produce the kinetic waves dispersion in the solar wind frame where the various resonant and non-resonant branches are evident. The study demonstrates the role of ion-kinetic instabilities in the transfer of energy from fluid to dissipation scales through collisionless wave-particle interactions, that leads to heating and associated acceleration of the solar wind plasma.