Observations of Small-scale IRIS Bombs (Reconnection Events) in an Evolving Active Region

Abstract:

We present the first Interface Region Imaging Spectrograph (IRIS) observations of small-scale bombs evolving with their host active region. Bombs appear most clearly in the IRIS 1330 Å and 1400 Å slit-jaw images as small (~1 arcsec), compact, intense brightenings at transition region temperatures. Their NUV/FUV emission spectra exhibit dramatic line splitting and strong absorption features indicative of bidirectional flows from magnetic reconnection embedded deep within the cool lower solar atmosphere. The bombs may contribute significantly to the heating of the solar atmosphere in active regions; however, it’s unclear how prevalent the bombs are throughout the lifetime of an active region. Using a semi-automated detection method, we locate bombs within AR 11850 over the course of four observations from 06:00 UT on September 25, 2013 until 11:30 UT the next day. The active region is first observed in an emerging phase and rapidly grows into a mature active region with well-developed sunspots. The bomb occurrence rate drops dramatically as the active region fully emerges. We also find that the bombs fall into two distinct populations: one appears largely during active region emergence and contains a majority of the bombs, while the other population is present regardless of active region age. The first population of bombs is typically found embedded in the low-lying loops prominent in the young active region. Furthermore, we use Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI) line-of-sight magnetograms to show that the bombs associated with the first population occur at the boundaries between the upward and downward flux of small, isolated bipolar regions. These regions dissipate as the active region emerges and reconfigures its magnetic field into two large network patches of upward and downward flux with a clear inversion line. The second, smaller population of bombs usually occurs far from the active region loop structures in the plage and sunspot penumbrae. They are associated with the boundaries between strong upward or downward flux and regions where the magnetic field is perpendicular to the line of sight.