EVIDENCE FOR WIDESPREAD COOLING IN AN ACTIVE REGION OBSERVED WITH THESDOATMOSPHERIC IMAGING ASSEMBLY

Abstract

A well known behavior of EUV light curves of discrete coronal loops is that the peak intensities of cooler channels or spectral lines are reached at progressively later times than hotter channels. This time lag is understood to be the result of hot coronal loop plasma cooling through these lower respective temperatures. However, loops typically comprise only a minority of the total emission in active regions. Is this cooling pattern a common property of active region coronal plasma, or does it only occur in unique circumstances, locations, and times? The new SDO/AIA data provide a wonderful opportunity to answer this question systematically for an entire active region. We measure the time lag between pairs of SDO/AIA EUV channels using 24 hours of images of AR 11082 observed on 19 June 2010. We find that there is a time-lag signal consistent with cooling plasma, just as is usually found for loops, throughout the active region including the diffuse emission between loops for the entire 24 hour duration. The pattern persists consistently for all channel pairs and choice of window length within the 24 hour time period, giving us confidence that the plasma is cooling from temperatures of greater than 3 MK, and sometimes exceeding 7 MK, down to temperatures lower than ~ 0.8 MK. This suggests that the bulk of the emitting coronal plasma in this active region is not steady; rather, it is dynamic and constantly evolving. These measurements provide crucial constraints on any model which seeks to describe coronal heating.

Properties of multistranded, impulsively heated hydrodynamic loop models

“…The loop brightens sequentially from the hotter to the cooler bands as time progresses, as expected for a cooling loop (see, e.g., the light curves of loops 3 and 5 in Winebarger et al 2003). The time delay between the appearance in the channels 195 and 171 of TRACE, or in the corresponding channels 193 and 171 of AIA, is of about 10 min, in agreement with recent SDO/AIA observations of active region loops (e.g., Viall & Klimchuk 2012). The lifetime of the emission in the two 171 filters, assumed to be equal to the FWHM of a Gaussian fitting the light curve (e.g., Winebarger et al 2003) is of about 30 min.…”

Multifractal Solar Euv Intensity Fluctuations and Their Implications for Coronal Heating Models

“…Zero lags are defined at the time-pixel resolution (12 s) and non-zero lags above or below this threshold. As in Viall & Klimchuk (2012) although a particular time signal goes through more than one heating and cooling cycle, the overall tendency is to present a cooling pattern with emission in the hotter waveband leading. For example in the 335-171 pair in the core region we find the following percentages of pixels for the different lags: 53% (positive), 14% (negative), and 30% (zero).…”

“…Alternatively Viall & Klimchuk (2012 have computed the cross-correlations between contemporaneous pairs of coronal Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) extreme-ultraviolet (EUV) light curves in different wavelength bands. These researchers find that in the case of coronal emission, the AR pixels, including those in the loop structures as well as the diffuse emission, the configuration of signal time lags is consistent with the cooling pattern characteristic of an impulsive nanoflare heating process.…”

Euv Flickering of Solar Coronal Loops: A New Diagnostic of Coronal Heating

“…Models of multi-stranded pulse-heated loops have been used to explain the evidence of super-hot plasma and to constrain the heating release (Parker 1988;Cargill 1994Cargill , 2014Klimchuk et al 2008;Viall & Klimchuk 2012;Cargill et al 2012;Reep et al 2013;Antolin et al 2014;Kobelski & McKenzie 2014;Caspi et al 2015;López Fuentes & Klimchuk 2015;Tam et al 2015).…”