Using AIA on SDO to Investigate the Transport of Flare Energy From Its Release Site to the Chromosphere

Coordinated AIA and RHESSI observations of a GOES B4.8 flare on 2010 July 31 show that emission in all seven of AIA's EUV channels brightened nearly six minutes before RHESSI or GOES detected any emission from plasma at temperatures around 10 MK. To help interpret these and AIA flare observations in general, we characterized the expected temporal responses of AIA's 94, 131, 171, 193, 211, and 335 Angstrom channels to solar flare brightenings by combining (1) AIA's nominal temperature response functions available through SSWIDL (Boerner et al. 2011) with (2) existing EUV spectral line flare data obtained on timescales comparable to AIA's image cadence. For the latter we use CDS stare spectra of a flare loop footpoint reported by Brosius and Phillips (2004). These spectra were observed at a cadence of 9.8 s, and their nine emission lines cover a wide range of formation temperature from about 0.05 to 8 MK. The line brightenings that were observed early during the CDS flare occurred at temperatures less than about 0.7 MK, with the largest brightenings around 0.1 MK; this indicates that the flare's energy transport was dominated by nonthermal particle beams. Because all of AIA's EUV channels are sensitive to emission from plasma in this temperature range (0.1 - 0.7 MK), we show that all of AIA's EUV channels are expected to brighten simultaneously during flares like this, in which energy transport is dominated by nonthermal particle beams. Since the 2010 July 31 flare observed by AIA and RHESSI displays this expected behavior, we conclude that such beams began to drive chromospheric evaporation during this event nearly six minutes before flare temperatures around 10 MK were reached. When thermal conduction from a directly-heated, hot (~10 MK) plasma is the dominant energy transport mechanism, the AIA channels that are sensitive to emission from such temperatures (particularly the 94 and 131 channels) will brighten earlier than the channels that are not sensitive to such temperatures (171 and 211). Thus, based on the differences that we expect in AIA's response to flares whose energy transport is dominated by nonthermal particle beams from those whose energy transport is dominated by thermal conduction, AIA may be able to determine the dominant energy transport mechanism for any given event.