EUNIS and IRIS: A Tale of Two UV Spectrographs and an Explosively Heated Solar Corona

Recent results from NASA’s Interface Region Imaging Spectrograph (IRIS) Small Explorer satellite and Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket instrument provide new insight in to the elusive mechanism(s) that heat the solar corona to millions of degrees. The broad spectral coverage and unprecedented dynamic range of the EUNIS 2013 observations includes emission lines of ionization stages from He I to Fe XX, and thus a wide temperature range of 0.03 to 10 MK. Pervasive, faint Fe XIX 592 Å line emission was observed in active regions. Comparison of observed line intensities with calculations demonstrates that the Fe XIX emission, formed at temperatures around 8 MK, is evidence of the faint hot emission predicted by impulsive heating models of the solar corona, that is, ‘nano-flares’, whether they be driven by reconnection events or resonant absorption of magneto-hydrodynamic waves. Extremely high spectral- and spatial-resolution IRIS observations at longer UV wavelengths resolve the chromospheric response to some of these events individually, providing examples of individual small-scale reconnection events, and constraints on the associated electron beam acceleration properties.

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