Observations and Modeling of Unstable Proton and α Particle Velocity Distributions in Sub-Alfvénic Solar Wind at Parker Solar Probe Perihelia

Abstract

Past observations show that solar wind (SW) acceleration occurs inside the sub-Alfvénic region, reaching the local Alfvén speed at typical distances ∼10–20 solar radii (R s ). Recently, Parker Solar Probe (PSP) traversed regions of sub-Alfvénic SW near perihelia in encounters E8–E12 for the first time, providing data in these regions. It became evident that the properties of the magnetically dominated SW are considerably different from the super-Alfvénic wind. For example, there are changes in the relative abundances and drift of α particles with respect to protons, as well as in the magnitude of magnetic fluctuations. We use data of the magnetic field from the FIELDS instrument, and construct ion velocity distribution functions (VDFs) from the sub-Alfvénic regions using Solar Probe ANalyzer for Ions data, and run 2.5D and 3D hybrid models of proton-α sub-Alfvénic SW plasma. We investigate the nonlinear evolution of the ion kinetic instabilities in several case studies, and quantify the transfer of energy between the protons, α particles, and the kinetic waves. The models provide the 3D ion VDFs at the various stages of the instability evolution in the SW frame. By combining observational analysis with the modeling results, we gain insights on the evolution of the ion instabilities, the heating and the acceleration processes of the sub-Alfvénic SW plasma, and quantify the exchange of energy between the magnetic and kinetic components. The modeling results suggest that the ion kinetic instabilities are produced locally in the SW, resulting in anisotropic heating of the ions, as observed by PSP.

The Alpha-proton Differential Flow in the Alfvénic Young Solar Wind: from Sub-Alfvénic to Super-Alfvénic

“…However, sub-Alfvénic solar wind measurements are not incorporated into their study, and the impacts of Alfvénic fluctuations on the differential flow are not discussed. Ofman et al (2023) studied the nonlinear evolution of ion kinetic instabilities and the transfer of energy between ions and waves in the sub-Alfvénic regime, which suggests that the locally produced ion instabilities contribute to the anisotropic heating of alphas. One of the striking observations of PSP is the common presence of Alfvénic switchbacks in the young solar wind, which are characterized by a deflection in the magnetic field direction and an increase in the radial velocity (Bale et al 2019; examined the 3D nature of the velocity fluctuations of alphas and protons in switchbacks using PSP observations from encounters 3 and 4.…”

Electrostatic Bursts Generated by the Ion–Ion Acoustic Instability with Solar Wind Plasma Parameters

“…The presence of multiple proton populations can result in a number of instabilities. Sources of instability-driving free energy can in fact include both ion temperature anisotropy and the presence of drifting ion populations (Gary 1993;Matteini et al 2012;Ďurovcová et al 2019;Klein et al 2019Klein et al , 2021Martinović et al 2021;Mostafavi et al 2022;Martinović & Klein 2023;Ofman et al 2023). Examples of instabilities driven by multiple particle populations include the currentdriven instability, caused by the relative drift between electrons and ions; the ion-acoustic heat flux instability, caused by the relative drift between strahl and core electrons; and the ion-ion acoustic instability, caused by the relative drift between two ion populations (Gary 1993).…”