Observation and Simulation of Solar Wind Proton Energization by Magnetic Switchback Pulse

Abstract

<p>Magnetic switchbacks are believed to play an important role in the heating of solar corona and solar wind as well as the acceleration of solar wind in the inner heliosphere. To address this issue, we conduct a comprehensive study involving observational analysis and numerical simulation.</p> <p>In the first part, we analyze magnetic field data and plasma data measured by PSP during its encounters, and select 71 switchback events with reversals of the radial component of the magnetic field and unchanged electron-strahl pitch angles. We investigate the anisotropic thermalization of switchbacks in a statistical study of the measured proton temperatures in the parallel and perpendicular directions as well as proton density and specific proton fluid entropy. We apply the ``Genetic Algorithm" (GA) method to directly fit the measured velocity distribution function (VDF) in field-aligned coordinates using a two-component bi-Maxwellian distribution function. We find that the protons in most switchback events are hotter than outside the switchbacks, with characteristics of parallel and perpendicular heating. Specifically, significant parallel and perpendicular temperature increases are seen for 45 and 62 of the 71 events, respectively. We find that the density of most switchback events decreases rather than increases, which indicates that proton heating inside the switchbacks is not caused by adiabatic compression, but is probably generated by non-adiabatic heating caused by field-particle interaction. Accordingly, the proton fluid entropy inside the switchbacks is higher than that in the ambient solar wind. In PSP observation, a significant portion of the proton VDFs is obscured by the spacecraft’s heat shield and therefore lies outside SPAN’s field of view, so simulation is an extra crucial method to study the heating effect of switchbacks.</p> <p>In the second part, we perform a test-particle simulation with a background magnetic field, and introduce test-particles to explore how magnetic switchbacks affect their energization. We introduce square waves to imitate recurrent switchback intervals and their ambient quiet intervals between which fluctuating magnetic field have different angles. Besides, Boris algorithm is employed to advance the motion of test-particles in the time varying electromagentic fields. We report the following three findings after analyzing the simulation results. (1) When the amplitude of simulated switchback increases, the heating effect of test-particles is more prominent. (2) The velocity distribution of test-particles is temporarly modulated by the encouter of switchback interval. (3) Preferential perpendicular heating of test-particles can be observed.</p>