AAVSO: American Association of Variable Star Observers

Measuring Decay Timescales of Downflows in Solar Flare Footpoints: Testing the One-minute Theory (Abstract)

Volume 48 number 1 (2020)

Alexander K. Beltzer-Sweeney
NSO/San Diego Mesa College, 5191 68th Street, San Diego, CA 92115; a.beltzer.sweeney@gmail.com
Elizabeth Butler
address correspondence to Alexander K. Beltzer-Sweeney, 5191 68th Street, San Diego, CA 92115; a.beltzer.sweeney@gmail.com
Adam Kowalski
address correspondence to Alexander K. Beltzer-Sweeney, 5191 68th Street, San Diego, CA 92115; a.beltzer.sweeney@gmail.com
Gianna Cauzzi
address correspondence to Alexander K. Beltzer-Sweeney, 5191 68th Street, San Diego, CA 92115; a.beltzer.sweeney@gmail.com

Abstract

(Abstract only) In 1989 George Fisher found analytically that chromospheric downflows in flare footprints should slow down to background detection levels within ~ 1 minute regardless of the initial energy injected. We set to test this theory by measuring downflows in flare kernels that were observed by the IRIS satellite between 2014 and 2017. The GOES classification system was used as a proxy for the energy of the nonthermal electron beam that is thought to heat the flare footprint. The redshift evolution of a Mg II triplet line was measured in twenty-six C, M, and X class flares to determine the timescale of deceleration of the chromospheric plasma in response to explosive flare heating. Two different methods for measuring the decay of the redshift as a function of time, bisector and gaussian, were used to test the robustness of the inferred downflow gas velocities across the wide variety of flares. Results of the analysis show that downflow velocities reached 30 ~ 50 km/s, which is consistent with previous results with a derived Mach number of 4 ~ 5. The times of half-maximum velocity were found to be between 15 ~ 30 seconds, indicating a rapid slowing. At later times, the Mg II line profiles exhibit prolonged redshifts with inferred speeds of 5 to 7 km/s.