An explanation of the solar transition region

TL;DR

This paper proposes a new explanation for the structure and energy balance of the solar transition region, involving cross-field diffusion of neutral atoms from cool chromospheric threads into the corona, accounting for observed radiative intensities.

Contribution

It introduces a novel model based on neutral atom diffusion and excitation processes to explain the solar transition region's properties, without relying on ad-hoc parameters.

Findings

Estimated H Lα radiative intensities match observations within an order of magnitude.

The model explains the growth of cool threads like fibrils and spicules.

Processes occur efficiently even with high ion gyro-frequencies.

Abstract

Prompted by high resolution observations, I propose an explanation for the 40+ year old problem of structure and energy balance in the solar transition region. The ingredients are simply cross-field diffusion of neutral atoms from cool threads extending into the corona, and the subsequent excitation, radiation and ionization of these atoms via electron impact. The processes occur whenever chromospheric plasma is adjacent to coronal plasma, and are efficient even when ion gyro-frequencies exceed collision frequencies. Cool threads - fibrils and spicules perhaps - grow slowly in thickness as a neutral, ionizing front expands across the magnetic field into coronal plasma. Radiative intensities estimated for H L$\alpha$ are within an order of magnitude of those observed, with no ad-hoc parameters - only thermal parameters and geometric considerations are needed. I speculate that the subsequent dynamics of the diffused material might also explain observed properties of trace elements.