Dynamics of Subarcsecond Bright Dots in the Transition Region above Sunspots and Their Relation to Penumbral Micro-jets

Abstract

Recent high-resolution observations reveal that subarcsecond bright dots (BDs) with sub-minute lifetimes appears ubiquitously in the transition region (TR) above sunspot penumbra. The presence of penumbral microjets (PMJs) in the chromosphere have also been reported earlier. It was proposed that both the PMJs and BDs are formed due to magnetic reconnection process and may play an important role in heating of the penumbra. Using simultaneous observation of the chromosphere from the Solar Optical Telescope (SOT) aboard Hinode and the TR from the Interface Region Imaging Spectrograph (IRIS), we study the dynamics of BDs and their relation with PMJs. We find two types of BDs, one which is related to PMJs and the others which do not show any visible dynamics in the SOT Ca II H images. From a statistical analysis we show that these two types have different properties. The BDs which are related to PMJs always appear at the top of the PMJs, the vast majority of which show inward motion and originate before the generation of the PMJs. These results may indicate that the reconnection occurs at the lower coronal/TR height and initiates PMJs at the chromosphere. This formation mechanism is in contrast with the currently believed formation of PMJs by reconnection in the (upper) photosphere between differently inclined fields.

The Dark Side of Penumbral Microjets: Observations in Hα

“…8 agrees with the results presented by Reardon et al (2013), in particular, with those provided by Samanta et al (2017). The assertion by Samanta et al (2017) that PMJs host a preceding brightening phase seen in the upper chromosphere and transition region is confirmed by our results. However, the conclusion drawn by Samanta et al (2017) that PMJs are triggered by a reconnection event in the transition region is not supported by our results.…”

“…The gradual brightening phase illustrated in Fig. 8 agrees with the results presented by Reardon et al (2013), in particular, with those provided by Samanta et al (2017). The assertion by Samanta et al (2017) that PMJs host a preceding brightening phase seen in the upper chromosphere and transition region is confirmed by our results.…”

Solar Ultraviolet Bursts

“…They have a progressively weaker signal in lower atmosphere layers, with most events having no signature in Hα . Tian et al (2014a) suggested that bright dots have a connection to the Ca II penumbral microjets which was largely confirmed by , but Samanta et al (2017) identified a distinct class of bright dots without Ca II signature. Penumbral bright dots were also found in 193 Å images from the Hi-C rocket flight ), which were not considered to be coronal features but contributed by cooler TR emission lines in the 193 Å passband.…”

The transition region above sunspots

“…Simultaneous observations in the chromosphere and TR (Vissers et al 2015;Samanta et al 2017) show that some of these TR bright dots rest at the tips of chromospheric penumbral micro-jets (Katsukawa et al 2007;Tiwari et al 2016). Vissers et al (2015) suggested that these TR bright dots are the heating signature of the penumbral micro-jets, whereas Samanta et al (2017) showed that the majority of the TR dots originate before the generation of the chromospheric jets. These observations left us with an unclear and confused understanding about their origination process and indicated the further need for high-quality imaging and spectroscopic observations.…”

“…These observations left us with an unclear and confused understanding about their origination process and indicated the further need for high-quality imaging and spectroscopic observations. Interestingly, many bright dots show no detectable signatures in chromospheric images, which might be caused by cooling downflows from the overlying corona (Deng et al 2016, Samanta et al 2017. Another category of small-scale brightenings observed with IRIS is narrow-line-width UV bursts (Hou et al 2016).…”