IRIS and SDO observation of recurrent explosive events

Abstract

Observations of recurrent explosive events (EEs) with time scale of 3-5 minutes are reported. These EEs have been observed with the Interface Region Imaging Spectrograph (IRIS) and have a spatial dimension of $\sim1.5"$ along the slit. The spectral line profiles of \ion{C}{2}~$1335/1336$ \AA\ and \ion{Si}{4}~$1394/1403$ \AA\ become highly broadened both in red as well as blue wings. Several absorption lines on top of the broadened profiles were identified. In addition, emission lines corresponding to neutral lines such as \ion{Cl}{1}~1351.66~{\AA}, \ion{C}{1}~1354.29~{\AA}, and \ion{C}{1}~1355.84~{\AA} were identified. The \ion{C}{1}~1354.29~{\AA}, and \ion{C}{1}~1355.84 {\AA} lines were found only during the EEs whereas \ion{Cl}{1}~1351.66~{\AA} broadens during the EEs. The estimated lower limit on electron number density obtained using the line ratios of \ion{Si}{4} and \ion{O}{4} is about $10^{13.5}$ cm$^{-3}$, suggesting that the observed events are most likely occurring at heights corresponding to lower chromosphere. To the best of our knowledge, for the first time we have detected short-period variability (30 s and 60--90 s) within the EE bursts. Observations of photospheric magnetic field underneath EEs indicate that negative polarity field emerges in the neighbourhood of oppositely directed positive fields which undergo repetitive reconnection (magnetic flux cancellation) events. The dynamic changes observed in AIA 1700 \AA, 1600 \AA, \ion{C}{2} 1330 \AA\ and \ion{Si}{4} 1400 \AA\ intensity images corresponded very well with the emergence and cancellation of photospheric magnetic field (negative polarity) on the time scale of 3--5 min. The observations reported here suggests that these EEs are formed due to magnetic reconnection and are occurring in the lower chromosphere.