Evidence for anisotropic non-Maxwellian velocity distributions in the solar transition region

Abstract

Spectroscopic observations of the solar transition region have long shown excess line broadening and non-Gaussian profiles, but their spatial distribution and prevalence are not well established, and their physical origin remains the subject of debate. Here we analyze SiIV line profiles in full-disk mosaics of observations with Interface Region Imaging Spectrograph (IRIS; De Pontieu et al. 2014), and find that non-Gaussian k-like profiles, indicative of suprathermal velocity distributions, are pervasive (comprising ~60% of the observed profiles). In addition, we find that their occurrence depends strongly on the angle between the line of sight and the magnetic field as inferred from NLFFF extrapolations. k-like profiles become increasingly prevalent when the magnetic field is oriented transverse to the observer, whereas Gaussian profiles become more prevalent when the field is aligned with the line of sight. This geometric dependence provides evidence that the observed velocity distributions in the transition region are non-Maxwellian and anisotropic. Our findings provide evidence for the importance, in the solar transition region, of kinetic processes, and the key role the magnetic field plays in the superposition of signals, and/or the generation or relaxation of non-Maxwellian particle distributions, likely caused by magnetic energy release through reconnection. These effects are not captured by the magnetohydrodynamic approaches that are typically invoked to study the solar atmosphere. Our results highlight the need and provide constraints for more advanced multi-fluid and/or kinetic modeling of the solar transition region.