Diversity of the morphology of type II spicules in MURaM-ChE simulations

¹ Max Planck Institute for Solar System Research, Justus von Liebig Weg, 37077, Göttingen, Germany
² School of Space Research, Kyung Hee University, Yongin, Gyeonggi, 17104, Republic of Korea

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 February 2026
Accepted: 25 May 2026

Abstract

Context. Spicules are ubiquitous, small-scale features in the solar atmosphere, exhibiting a jet-like appearance most clearly identified by their apparent motion in off-limb observations. While they are often interpreted as narrow, thread-like structures, their true 3D structure remains unknown.

Aims. We aim to uncover the 3D morphology and dynamics of fast-evolving spicules (type II) using a MURaM-ChE simulation.

Methods. We used a Hα proxy that has been developed using non-equilibrium (NE) hydrogen populations in MURaM-ChE. The proxy, modelled as an escape probability, was synthesised to isolate on-disc as well as off-limb Hα wing features. The 3D structure of these features was investigated using the 3D information on opacity in Hα.

Results. We identify type II spicules with unique 3D morphologies, the dominant ones being thread-like and slab-like. The appearance of spicules as slabs or threads is a function of time and Doppler velocity. The spicules extending above the spicule-forest (2–3 Mm above the surface) tend to be located at quasi-separatrix layers (QSLs). We find that the spatially resolved contributions to the opacity of spicules are often similar for spicules synthesised in the horizontal direction, and their on-disc rapid blueshifted excursion (RBE) synthesised in the vertical direction at the same Doppler velocity of 37 km/s. This confirms that RBEs are indeed the on-disc counterparts of spicules. Furthermore, our analysis indicates that cross-field motions can significantly contribute to spicule dynamics.

Conclusions. Spicules exhibit a range of morphologies, including both slab-like and thread-like structures. Their observed appearance depends strongly on line-of-sight (LoS) projection and Doppler sampling. Spicules are preferentially located at QSLs, highlighting the role of magnetic topology in driving spicular dynamics.