Probing chromospheric fine structures with a Hα proxy using MURaM-ChE

¹ Max Planck Institute for Solar System Research, Justus von Liebig Weg, 37077 Göttingen, Germany
² Institute for Solar Physics, Dept. of Astronomy, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden

^⋆ Corresponding author: chandra@mps.mpg.de

Received: 23 May 2025
Accepted: 6 August 2025

Abstract

Context. Hα observations of the solar chromosphere reveal dynamic small-scale structures known as spicules at the limb and rapid blueshifted and redshifted excursions (RBEs and RREs) on-disc.

Aims. We want to understand what drives these dynamic features, their magnetohydrodynamic (MHD) properties, and their role in energy and heat transport to the upper solar atmosphere. To do this, we aim to develop a proxy for synthetic Hα observations in radiative-MHD simulations to help identify these features.

Methods. We used the chromospheric extension to the MURaM code (MURaM-ChE) to simulate an enhanced network region. We developed a proxy for Hα based on a photon escape probability. This is a Doppler-shifted proxy that we used to identify fine structures in the line wings. We studied on-disc features in 3D, obtaining their 3D structure from the absorption coefficient.

Results. We validate the Hα proxy by comparing it against features detected in the wings of Hα synthesised using MULTI3D. We detect numerous small-scale structures rooted at the network patches, similar to observations in Hα. The dynamics of an example feature (RBE) at a Doppler shift of 37 km/s show that flux emergence and consequent reconnection drive the formation of this feature. Pressure gradient forces build up to drive a flow along the field line carrying the feature, making it a jet. There is strong viscous and resistive heating on the first appearance of the feature associated with the flux emergence. At the same time and location, a heating front appears and propagates along the field lines at speeds comparable to the Alfvén velocity. The feature shows an oscillatory behaviour as it evolves.

Conclusions. We show that a synthetic observable based on an escape probability is able to reliably identify features observed with the Hα spectral line. We demonstrate its applicability by studying the formation, dynamics and properties of an RBE.