Characterization of UV Bursts Using Line-Ratio Density Diagnostics

Abstract:

We compare abundance-sensitive and abundance-insensitive electron density diagnostics for a large sample of UV burst spectra. These bursts are compact (~1 arcsec), short-lived (~minutes to hours) active-region phenomena, forming in the cool lower solar atmosphere and capable of reaching nearly 100,000K. They exhibit significant intensification and broadening/splitting of the Si IV 1393.76/1402.77 Å lines, which we believe are signatures of magnetic reconnection. Bursts also display absorption from cool metallic ions Ni II and Fe II, suggesting that they form deep in the chromosphere. These unusual phenomena are critical to understanding energy and mass transfer within the solar atmosphere, possibly contributing to upper-chromospheric and coronal heating and affecting the energetics and dynamics of the lower atmosphere. We employ a semi-automated detection algorithm to assemble a sample of over 9,800 UV burst spectra for Interface Region Imaging Spectrograph (IRIS) observations of two active regions spanning several days.

We perform line-ratio density diagnostics using O IV and S IV pairs as well as a Si IV / O IV ratio. Since bursts are characterized by strong Si IV 1402.77 Å emission and O IV 1401.16 Å also appears strongly in a majority of burst spectra, the Si IV / O IV diagnostic has the potential to be a robust density indicator. There is a significant uncertainty around the use of different-species diagnostics due to their strong dependence on relative atomic abundances, which are poorly constrained in the chromosphere. We compare the Si IV / O IV diagnostic with the abundance-insensitive ratios of O IV 1399.78 Å / O IV 1401.16 Å, O IV 1401.16 Å / O IV 1404.81 Å, and S IV 1404.85 Å / S IV 1406.06 Å and comment on the viability of the Si IV /O IV method in burst regions. We also find the rates of detection for these lines in our spectra. Our density analysis provides useful constraints on UV burst formation altitudes and aids in the comparison of observations to models.

This work is supported by the NSF-REU Solar Physics program at SAO, grant number AGS-1850750 and NASA H-SR grant number 80NSSC18K1124.