Penumbral microjets at high spatial and temporal resolution

Abstract

Context. Sunspot observations in chromospheric spectral lines have revealed short-lived linear bright transients that are commonly referred to as penumbral microjets (PMJs). Details on the origin and physical nature of PMJs are to a large extend still unknown. Aims. We aim to characterize the dynamical nature of PMJs to provide guidance for future modeling efforts. Methods. We analyzed high spatial (0.̋1) and temporal resolution (1 s) Ca II H filtergram (0.1 nm bandwidth) observations of a sunspot that were obtained on two consecutive days with the Swedish 1 m Solar Telescope. Results. We find that PMJs appear to be the rapid brightening of an already existing (faint) fibril. The rapid brightening is the fast increase (typically less than 10 s) in intensity over significant length (several hundreds of kilometers) of the existing fibril. For most PMJs, no clear root or source from where the brightening appears to originate can be identified. After the fast onset, about half of the PMJs have tops that move with an apparent velocity of between 5 and 14 km s−1, most of them upward. No significant motion of the top is observed in the other PMJs. About one-third of the PMJs split into two parallel and coevolving linear features during the later phases of their lifetimes. Conclusions. We conclude that mass flows can play only a limited role in the onset phase of PMJs. It is more likely that we see the effect of a fast heating front.