The Unstable Chromosphere: Predicting Thermal Farley–Buneman Instability Growth Across a Broad Range of Solar Chromospheric Conditions

Abstract In the coldest regions of the solar atmosphere, models struggle to match observations. Recent work suggests the Thermal Farley–Buneman Instability (TFBI) may play an important role. This meter-scale, electrostatic, collisional, multifluid plasma instability converts energy from neutral flows into turbulent motions and heating. In this work, we improve upon previous methods for solving the linear theory across single-fluid MHD simulation outputs, and present the new tfbi_theory code. We predict much less TFBI growth within a 2D Bifrost plage model than previously suggested. To bypass some limitations of single-fluid MHD models, we also solve TFBI theory across a 5D grid of chromospheric parameters. We find that the TFBI can grow across a wide range of chromospheric densities, temperatures, hydrogen ionization fractions, and magnetic field strengths. We also discover that the minimum ∣ E ∣/∣ B ∣ speed in the neutral reference frame required to drive the TFBI in the chromosphere is less than 3.2 km s −1 in some regions, but always more than 1 km s −1 .