The Unstable Chromosphere: Effects of the Thermal Farley-Buneman Instability Across a Broad Range of Solar Chromospheric Conditions

TL;DR

This study simulates the Thermal Farley-Buneman Instability (TFBI) in the solar chromosphere, revealing its role in generating turbulence, heating, and current rotation across various conditions, potentially explaining observed discrepancies.

Contribution

First comprehensive simulation of TFBI effects across broad solar chromospheric conditions, linking electric fields to turbulence, heating, and current dynamics.

Findings

TFBI-driven temperature increases are proportional to electric field strength.

Turbulence correlates with the magnitude of the driving electric field.

TFBI causes significant reorientation of current density components.

Abstract

In the coldest regions of the solar atmosphere, lingering discrepancies between models and observations may be caused by the Thermal Farley-Buneman Instability (TFBI). This meter-scale, electrostatic, collisional, multifluid plasma instability converts energy from neutral flows into turbulent motions and heating. In the neutral frame of reference, these neutral flows manifest as an electric field which can drive the TFBI. In this work, we simulate the TFBI across a broad range of solar chromospheric conditions. We find clear proportionality between between TFBI-driven relative temperature increases of charged species ($\Delta T_{s}^{\text{(turb)}} / T_{s}^{(0)}$) and driving electric field strength relative to the theoretical threshold field required for TFBI growth. We also discover a correlation between relative driving field strength and turbulent motions. Additionally, the TFBI consistently causes average current density to rotate towards the driving field, with Pederson/ambipolar component ($\vec{J} \cdot \hat{E}^{(0)}$) increasing by up to roughly 60% while the Hall component ($\vec{J} \cdot \hat{E}^{(0)} \times \hat{B}$) decreases in magnitude by up to roughly 80%. Meanwhile, we find TFBI-driven turbulence increases neutral heating rates by $(43\pm7)\%$ on average, with 85% (28/33) of simulations having mean increase more than 29%, and a flat line of best fit suggesting zero correlation with driving field.