Microjets in the penumbra of a sunspot

Abstract

Context. Penumbral microjets (PMJs) are short-lived jets found in the penumbra of sunspots, first observed in wide-band Ca ii H line observations as localized brightenings, and are thought to be caused by magnetic reconnection. Earlier work on PMJs has focused on smaller samples of by-eye selected events and case studies. Aims. It is our goal to present an automated study of a large sample of PMJs to place the basic statistics of PMJs on a sure footing and to study the PMJ Ca ii 8542 Å spectral profile in detail. Methods. High spatial resolution and spectrally well-sampled observations in the Ca ii 8542 Å line obtained from the Swedish 1-m Solar Telescope (SST) were reduced by a principle component analysis and subsequently used in the automated detection of PMJs using the simple machine learning algorithm k-nearest neighbour. PMJ detections were verified with co-temporal Ca ii H line observations. Results. We find a total of 453 tracked PMJ events, 4253 PMJs detections tallied over all timeframes, and a detection rate of 21 events per timestep. From these, an average length, width and lifetime of 640 km, 210 km and 90 s are obtained. The average PMJ Ca ii 8542 Å line profile is characterized by enhanced inner wings, often in the form of one or two distinct peaks, and a brighter line core as compared to the quiet-Sun average. Average blue and red peak positions are determined at - 10.4 km s-1 and + 10.2 km s-1 offsets from the Ca ii 8542 Å line core. We find several clusters of PMJ hot-spots within the sunspot penumbra, in which PMJ events occur in the same general area repeatedly over time. Conclusions. Our results indicate smaller average PMJs sizes and longer lifetimes compared to previously published values, but with statistics still in the same orders of magnitude. The investigation and analysis of the PMJ line profiles strengthens the proposed heating of PMJs to transition region temperatures. The presented statistics on PMJs form a solid basis for future investigations and numerical modelling of PMJs.