The formation and heating of chromospheric fibrils in a radiation-MHD simulation

Abstract

Aims. We examine the movements of mass elements within dense fibrils using passive tracer particles (corks) in order to understand fibril creation and destruction processes. Methods. Simulated fibrils were selected at times when they were visible in an Hα image proxy. The corks were selected within fibril Hα formation regions. From this set, a cork was selected, and the field line passing through it was constructed. Other fibrilar corks close to this fieldline were also selected. Pathlines were constructed, revealing the locations of the mass elements forward and backward in time. The forces acting on these mass elements were analysed. Results. The main process of fibrilar loading in the simulation is different to the mass loading scenario in which waves steepen into shocks and push material upwards along the fieldlines from near their footpoints. Twisted low lying fieldlines were destabilised and then they untwisted, lifting the material trapped above their apexes via the Lorentz force. Subsequently, the majority of the mass drained down the fieldlines towards one or both footpoints under gravity. Material with large horizontal velocities could also elevated in rising fieldlines, creating somewhat parabolic motions, but material was not generally moving upward along a stationary magnetic fieldline during loading. Conclusions. The processes observed in the simulation are plausible additional scenarios. Criteria for observing such events are described. It is desirable that our simulations can also form more densely-packed fibrils from material fed from the base of field footpoints. Experimental parameters required to achieve this are discussed.

Physical properties of a fan-shaped jet backlit by an X9.3 flare

“…Kayshap et al (2013) reported a much lower value of 4.1 × 10 9 particles cm −3 , but this is for a much hotter surge of 2 MK. We can also compare our densities to similar chromospheric structures; for example, a Bifrost fibril has a typical density of around 10 −9 to 10 −10 kg m −3 (Druett et al 2021b), and spicule densities of around 2.2 × 10 10 particles cm −3 have been reported (Sterling 2000;Shimojo et al 2020).…”