Pervasive Faint Fe XIX Emission from a Solar Active Region Observed with EUNIS-13: Evidence For Nanoflare Heating

We present spatially resolved EUV spectroscopic measurements of pervasive, faint Fe XIX 592.2 angstrom line emission in an active region observed during the 2013 April 23 flight of the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS-13) sounding rocket instrument. With cooled detectors, high sensitivity, and high spectral resolution, EUNIS-13 resolves the lines of Fe XIX at 592.2 angstrom(formed at temperature T approximate to 8.9MK) and Fe XII at 592.6 angstrom (T approximate to 1.6MK). The Fe XIX line emission, observed over an area in excess of 4920 arcsec(2) (2.58 x 10(9) km(2), more than 60% of the active region), provides strong evidence for the nanoflare heating model of the solar corona. No GOES events occurred in the region less than 2 hr before the rocket flight, but a microflare was observed north and east of the region with RHESSI and EUNIS during the flight. The absence of significant upward velocities anywhere in the region, particularly the microflare, indicates that the pervasive Fe XIX emission is not propelled outward from the microflare site, but is most likely attributed to localized heating (not necessarily due to reconnection) consistent with the nanoflare heating model of the solar corona. Assuming ionization equilibrium we estimate Fe XIX/Fe XII emission measure ratios of similar to 0.076 just outside the AR core and similar to 0.59 in the core.