Local proton heating at magnetic discontinuities in Alfvenic and non-Alfvenic solar wind

TL;DR

This study explores how magnetic discontinuities in solar wind influence local proton heating and kinetic features, revealing differences between Alfvénic and non-Alfvénic streams through observations and hybrid simulations.

Contribution

It provides new insights into proton energization mechanisms at small-scale structures in different solar wind regimes, supported by combined observational and simulation data.

Findings

Hottest protons are localized around magnetic structures in both wind types.

Alfvénic wind shows parallel temperature enhancements at smaller scales.

Non-Alfvénic wind exhibits perpendicular temperature enhancements and hot proton beams.

Abstract

We investigate the local proton energization at magnetic discontinuities/intermittent structures and the corresponding kinetic signatures in velocity phase space in Alfv\'enic and non-Alfv\'enic wind streams observed by Parker Solar Probe. By means of the Partial Variance of Increments method, we find that the hottest proton populations are localized around compressible, kinetic-scale magnetic structures in both types of wind. Furthermore, the Alfv\'enic wind shows preferential enhancements of $T_\parallel$ as smaller scale structures are considered, whereas the non-Alfvenic wind shows preferential $T_\bot$ enhancements. Although proton beams are present in both types of wind, the proton velocity distribution function displays distinct features. Hot beams, i.e., beams with beam-to-core perpendicular temperature up to three times larger than the total distribution anisotropy, are found in the non-Alfv\'enic wind, whereas colder beams in the Alfv\'enic wind. Our data analysis is complemented by 2.5D hybrid simulations in different geometrical setups, which support the idea that proton beams in Alfv\'enic and non-Alfv\'enic wind have different kinetic properties and origins. The development of a perpendicular nonlinear cascade, favored in balanced turbulence, allows a preferential relative enhancement of the perpendicular plasma temperature and the formation of hot beams. Cold field-aligned beams are instead favored by Alfv\'en wave steepening. Non-Maxwellian distribution functions are found near discontinuities and intermittent structures, pointing to the fact that the nonlinear formation of small-scale structures is intrinsically related to the development of highly non-thermal features in collisionless plasmas.