Presentation #122.05 in the session Coronal Heating: Present Understanding and Future Progress — Poster Session.
The nanoflare model is among the most actively investigated mechanisms used to address the longstanding problem of solar coronal heating. According to this model, the ~2 MK corona is heated by numerous tiny impulsive events, each of which is too small to be detected individually. A key prediction of the model is the widespread presence of small amounts of plasma at temperatures between 6 MK and 10 MK. Although emission from this plasma is expected to be faint, its detection in quiescent active regions would provide compelling evidence for nanoflares as a source of coronal heating. With this in mind, the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket instrument was designed to observe line emission from Fe XVIII, whose maximum ionization fraction occurs at temperatures around 7.1 MK, and Fe XIX, around 8.9 MK. EUNIS observes spectral line emission at 92-115 A in 3rd order, 277-345 A in 1st order, and 525-639 A in 1st order. These wavebands include lines from Fe XVIII at 93.932 and 103.948 A, and Fe XIX at 108.355 and 592.236 A. Overall, EUNIS observes hundreds of lines formed at temperatures that range from several tens of thousands of degrees to 10 MK. We present preliminary results for quiescent solar active region 12824 observed during the flight of 2021 May 18. EUNIS was supported by NASA Heliophysics Low Cost Access to Space award 13-HTIDS13_2-0074.