Magnetic reconnection-driven energization of protons up to 400 keV at the near-Sun heliospheric current sheet

TL;DR

NASA's Parker Solar Probe observed energetic protons up to 400 keV accelerated by magnetic reconnection at the heliospheric current sheet near the Sun, revealing a new source of solar wind energization.

Contribution

This study provides the first direct in-situ evidence of proton acceleration up to 400 keV by magnetic reconnection at the near-Sun heliospheric current sheet, supported by simulations.

Findings

Protons up to ~400 keV are accelerated within reconnection exhausts.

The energy spectrum follows a power-law with index about -5.

Magnetic islands and guide fields facilitate proton trapping and acceleration.

Abstract

We report observations of direct evidence of energetic protons being accelerated above ~400 keV within the reconnection exhaust of a heliospheric current sheet (HCS) crossing by NASA's Parker Solar Probe (PSP) at a distance of ~16.25 solar radii (Rs) from the Sun. Inside the extended exhaust, both the reconnection-generated plasma jets and the accelerated protons propagated toward the Sun, unambiguously establishing their origin from HCS reconnection sites located beyond PSP. Within the core of the exhaust, PSP detected stably trapped energetic protons up to ~400 keV, which is approximately 1000 times greater than the available magnetic energy per particle. The differential energy spectrum of the accelerated protons behaved as a pure power-law with spectral index of about -5. Supporting simulations using the kglobal model suggest that the trapping and acceleration of protons up to ~400 keV in the reconnection exhaust is likely facilitated by merging magnetic islands with a guide field between ~0.2-0.3 of the reconnecting magnetic field, consistent with the observations. These new results, enabled by PSP's proximity to the Sun, demonstrate that magnetic reconnection in the HCS is a significant new source of energetic particles in the near-Sun solar wind. The discovery of in-situ particle acceleration via magnetic reconnection at the HCS provides valuable insights into this fundamental process which frequently converts the large magnetic field energy density in the near-Sun plasma environment and may be responsible for heating the sun's atmosphere, accelerating the solar wind, and energizing charged particles to extremely high energies in solar flares.