High-cadence Analysis of a UV Burst with IRIS

Ultraviolet bursts (UVBs) are small-scale transient events indicative of magnetic reconnection in the cool lower solar chromosphere. These phenomena are observed as small (~1") compact brightenings in the NUV and FUV slit-jaw passbands of the Interface Region Imaging Spectrograph (IRIS). However, they are best characterized spectroscopically by the intensification and broadening/splitting of FUV emission lines observed by IRIS as well as the presence of cool metallic absorption features, particularly Ni II 1393.3 Å and Fe II 1392.8 Å nearby the Si IV 1393.8 Å emission line. UVBs are critical to understanding how mass and energy flow through the chromosphere. In particular, measuring the rate at which electron density changes with time can approximate the material outflow from a UVB, detailing the facility with which energized plasma can traverse the lower chromosphere. Since UVBs are transient events with lifetimes on the order of minutes at velocities of 100s of km/s, using high-cadence spectroscopic data can provide a robust measurement of how the electron density changes with time. Using an IRIS sit-and-stare observation of a single UVB on 2016 November 11 at 00:33:21 UTC, we measured electron densities using the O IV 1399.8/1401.2 Å line-ratio diagnostic. To verify the presence of a UVB, we use a semi-automated detection technique that isolates characteristic burst spectra based on parameters of single-Gaussian fits to the Si IV 1393.8 Å line, narrowing down the UVB candidate population significantly for manual detection of the Ni II 1393.3 Å absorption feature. After isolating the UV burst, we use spatial binning to produce a high-cadence time series of moderate signal-to-noise O IV 1399.8/1401.2 Å spectra. We find that the electron density derived via the O IV line ratio as a function of time remains largely constant. This result may be attributable to the alignment of the outflow jets along the line of sight; however, the influence of electronic noise introduced by the simultaneous readout of the slit-jaw and spectrograph cameras cannot be discounted. This study and others like it are critical for constraining fluid models of magnetic reconnection in partially ionized plasma such as those found in the chromosphere. This research is funded through the NSF-REU Solar Physics Program at SAO AGS-1850750.