The Origin of Non-Maxwellian Solar Wind Electron Velocity Distribution Function: Connection to Nanoflares in the Solar Corona

TL;DR

This paper proposes that nanoflare-accelerated electron beams trigger instabilities that produce the core-halo electron velocity distribution in the solar wind, explaining observed features and their insensitivity to initial conditions.

Contribution

It introduces a model linking nanoflares to the formation of the core-halo distribution via two-stream instability and kinetic turbulence, supported by theoretical equations and observational consistency.

Findings

Core-halo temperature ratio is insensitive to initial conditions.

The core-halo relative drift is a relic of saturated two-stream instability.

The temperature ratio is nearly independent of heliospheric distance.

Abstract

The formation of the observed core-halo feature in the solar wind electron velocity distribution function is a long-time puzzle. In this letter based on the current knowledge of nanoflares we show that the nanoflare-accelerated electron beams are likely to trigger a strong electron two-stream instability that generates kinetic Alfv\'en wave and whistler wave turbulence, as we demonstrated in a previous paper. We further show that the core-halo feature produced during the origin of kinetic turbulence is likely to originate in the inner corona and can be preserved as the solar wind escapes to space along open field lines. We formulate a set of equations to describe the heating processes observed in the simulation and show that the core-halo temperature ratio of the solar wind is insensitive to the initial conditions in the corona and is related to the core-halo density ratio of the solar wind and to the quasi-saturation property of the two-stream instability at the time when the exponential decay ends. This relation can be extended to the more general core-halo-strahl feature in the solar wind. The temperature ratio between the core and hot components is nearly independent of the heliospheric distance to the sun. We show that the core-halo relative drift previously reported is a relic of the fully saturated two stream instability. Our theoretical results are consistent with the observations while new tests for this model are provided.