Student: Victoria Chaparro, Undergraduate Student in Physics, Astronomy, and Mathematics, Drake University
Research Mentor: Dr. Charles Nelson
Probing the Lower Solar Chromosphere Via Dynamical Signatures of UV Bursts in Cold Lines
UV Bursts are small scale brightenings identified by dramatic intensification and broadening of emission lines often accompanied by absorption features from cold metallic ions. They can be used as probes of plasma conditions in the lower solar atmosphere since their spectral features suggest they are magnetic reconnection events in the cool lower chromosphere. We present a spatial and temporal analysis of the intensification and broadening of the Si IV 1393.78 Å and Cl I 1351.66 Å lines observed by the Interface Region Imaging Spectrograph (IRIS) to trace magnetic reconnection outflows through the lower chromosphere. We employ a semi-automated UV burst detection algorithm by applying single-Gaussian fits (SGF) to the Si IV 1394 Å line in IRIS sit-and-stare data. We first isolate the likely candidate population in the SGF parameter space, reducing the candidate field from hundreds of thousands to only a few thousand spectra. We then manually search these candidate spectra for Ni II 1393.33 Å absorption. Following detection, we focus on the Cl I 1391.66 Å line to observe the influence of UV bursts over lower chromospheric dynamics. This is done by constructing a series of space-time plots of the peak intensity, Doppler velocity, and line width SGF parameters of Cl I 1391.66 Å to measure time lags between its intensification and broadening, allowing us to estimate downflow velocities in the lower chromosphere under UV burst conditions. This is possible because the peak intensity of Cl I 1391.66 Å is dependent on fluorescence from C II 1335.71 Å, a line strongly emitted by the UV burst source, while its width only broadens when the emitting material is impacted by a material flow. This study establishes the power of UV bursts as probes of dynamical phenomena in the lower chromosphere, a region that is notoriously difficult to observe directly. Furthermore, it lays the groundwork for future exploration of the often overlooked cool emission lines in the IRIS spectral passbands and their potential as physical diagnostics. This work is part of the NSF-REU Solar Physics program at SAO, grant number AGS-1850750.