What Dominates the Coronal Emission Spectrum During the Cycle of Impulsive Heating and Cooling?

Abstract

The "smoking gun" of small-scale, impulsive events heating the solar corona is expected to be the presence of a hot ( > 5 MK) plasma component. Evidence for this has been scarce, but has gradually begun to accumulate due to recent studies designed to constrain the high temperature part of the emission measure distribution. However, the detected hot component is often weaker than models predict and this is due in part to the common modeling assumption that the ionization balance remains in equilibrium.The launch of the latest generation of space-based observing instrumentation aboard Hinode and the Solar Dynamics Observatory (SDO) has brought the matter of the ionization state of the plasma firmly to the forefront. It is timely to consider exactly what emission current instruments would detect when observing a corona heated impulsively on small-scales by nanoflares. Only after we understand the full e ffects of nonequilibrium ionization can we draw meaningful conclusions about the plasma that is (or is not) present.We have therefore performed a series of hydrodynamic simulations for a variety of different nanoflare properties and initial conditions. Our study has led to several key conclusions. 1. Deviations from equilibrium are greatest for short-duration nanoflares at low initial coronal densities. 2. Hot emission lines are the most affected and are suppressed sometimes to the point of being invisible. 3. The emission detected in all of the SDO-AIA channels is generally dominated by warm, over-dense, cooling plasma.4. It is difficult not to create coronal loops that emit strongly at 1.5 MK and in the range 2 to 5 MK, which are the most commonly observed kind, for a broad range of nanoflare scenarios. 5. The Fe XV (284.16 Å) emission in most of our models is about 10 times brighter than the Ca XVII (192.82 Å) emission, consistent with observations. Our overarching conclusion is that small-scale, impulsive heating inducing a nonequilibrium ionization state leads to predictions for observable quantities that are entirely -3consistent with what is actually observed.

PATTERNS OF NANOFLARE STORM HEATING EXHIBITED BY AN ACTIVE REGION OBSERVED WITHSOLAR DYNAMICS OBSERVATORY/ATMOSPHERIC IMAGING ASSEMBLY

“…This will not be the case for the extremely hot and tenuous plasma that is present shortly after a nanoflare occurs (Bradshaw & Cargill 2006;Reale & Orlando 2008). Since the emission from highly ionized species is diminished in this situation, the high-temperature peaks of the response functions of the 131 channel and, to a lesser extent, the 94 channel predict stronger emission than can actually be expected (Bradshaw & Klimchuk 2011). …”

“…It is important to know whether the count rates observed in the AR core are also consistent with nanoflare storm heating. Bradshaw & Klimchuk (2011) predict count rates in several AIA channels for the loop apex of 14 different nanoflare models computed with the 1D HYDRAD code (Bradshaw & Cargill 2010, and references therein), taking full account of nonequilibrium ionization. The 195/335 ratio ranges between 13 and 446 and has a median value of 46.…”

“…In comparison, the average 195 and 335 count rates in a 30 × 20 pixel area in the core of this observed AR have a ratio of 11. Bradshaw & Klimchuk (2011) did not determine count rates for the 171 channel. The overall magnitudes of the intensities observed in this AR core are consistent with the models.…”

Forward Modelling of a Brightening Observed by AIA

“…We used the same response functions as Bradshaw and Klimchuk (2011); these functions are a product of the plate scale, effective area, and gain of the AIA instrument.…”

Doppler shift of hot coronal lines in a moss area of an active region

“…However, in the case of hot coronal loops, there is observational support for both steady heating (Antiochos et al 2003;Warren et al 2008Warren et al , 2010Winebarger et al 2008Winebarger et al , 2011) and impulsive heating (Tripathi et al 2010b(Tripathi et al , 2011Bradshaw & Klimchuk 2011;Viall et al 2012). Because of the unresolved nature of hot coronal loops (Tripathi et al 2009) it is not possible to isolate a single loop and study its characteristics.…”