The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons

TL;DR

This study uses Liouville mapping of electron distributions from Parker Solar Probe data to infer the interplanetary potential, revealing its critical role in solar wind proton acceleration and halo electron evolution.

Contribution

It introduces a novel method to determine the interplanetary potential from electron distributions, linking it to proton acceleration and halo formation.

Findings

The inferred potential accounts for nearly 100% of proton acceleration.

Electron halo evolution is consistent with scattering-free Liouville's theorem.

The same potential explains both proton acceleration and halo electron dynamics.

Abstract

We provide evidence that the sunward-propagating half of the solar wind electron halo distribution evolves without scattering in the inner heliosphere. We assume the particles conserve their total energy and magnetic moment, and perform a "Liouville mapping" on electron pitch angle distributions measured by the Parker Solar Probe SPAN-E instrument. Namely, we show that the distributions are consistent with Liouville's theorem if an appropriate interplanetary potential is chosen. This potential, an outcome of our fitting method, is compared against the radial profiles of proton bulk flow energy. We find that the inferred potential is responsible for nearly 100% of the proton acceleration in the solar wind at heliocentric distances 0.18-0.79 AU. These observations combine to form a coherent physical picture: the same interplanetary potential accounts for the acceleration of the solar wind protons as well as the evolution of the electron halo. In this picture the halo is formed from a sunward-propagating population that originates somewhere in the outer heliosphere by a yet-unknown mechanism.