IFS Seminar

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 depend-

ing 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 Alfv´en 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 for-

mation 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-Alfv´enic 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 mod-

els demonstrate the growth, nonlinear saturation, and relaxation of the

instabilities, resulting in generation of kinetic wave spectra, and even-

tual 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 colli-

sionless wave-particle interactions, that leads to heating and associated

acceleration of the solar wind plasma.