The evolution of the emission measure distribution in the core of an active region

Abstract

We study the spatial distribution and evolution of the slope of the emission measure (EM) between 1 MK and 3 MK in the core of the active region (AR) NOAA 11193, first when it appeared near the central meridian and then again when it reappeared after a solar rotation. We use observations recorded by the Extreme-ultraviolet Imaging Spectrometer (EIS) aboard Hinode, with a new radiometric calibration. We also use observations from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). We present the first spatially resolved maps of the EM slope in the 1-3 MK range within the core of the AR using several methods, either from approximations or from the differential emission measure (DEM). A significant variation of the slope is found at different spatial locations within the active region. We selected two regions that were not greatly affected by lower temperature emission along the line of sight. We found that the EM had a power law of the form EM ∝ T b , with b = 4.4 ± 0.4, and 4.6 ± 0.4, during the first and second appearance of the active region, respectively. During the second rotation, line-of-sight effects become more important, although difficult to estimate. We found that the use of the ground calibration for Hinode/EIS and the approximate method to derive the EM, used in previous publications, produce an underestimation of the slopes. The EM distribution in active region cores is generally found to be consistent with high frequency heating, and does not change much during the evolution of the active region.

Understanding Heating in Active Region Cores through Machine Learning. I. Numerical Modeling and Predicted Observables

“…However, while this quantity retains information about the frequency of energy deposition, drawing conclusions about the heating based solely on the observed emission measure slope, particularly for a small number of pixels may be misleading. As shown here and in Del Zanna et al (2015), the slope may vary significantly across a given active region. Additionally, calculating EM(T ) from observations is non-trivial due to several factors, including the mathematical difficulties of the ill-posed inversion (Craig & Brown 1976;Judge et al 1995Judge et al , 1997, uncertainties in the atomic data (Guennou et al 2013), and insufficient constraints from spectroscopic observations (e.g.…”

Solar UV and X-ray spectral diagnostics

“…The revised Hinode EIS calibration of Del Zanna (2013b) substantially increased the slopes b to values around 5, which increase even further if foreground/background emission is taken into account (Del Zanna 2013a; Del Zanna et al. 2015d). These results suggest that low-frequency nanoflare modeling should be ruled out.…”

Nonequilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind (Invited Review)

“…The DEMs were only shifted towards higher T as a result of the behavior of the ionization equilibrium. This is of importance since in active region cores, the DEM slope at temperatures lower than its peak are thought to provide constraints on the recurrence frequency of the coronal nanoflares (e.g., Viall and Klimchuk, 2011;Bradshaw, Klimchuk, and Reep, 2012;Warren, Winebarger, and Brooks, 2012;Winebarger, 2012;Cargill, 2014;Del Zanna et al, 2015b). Since the low-temperature slope of the DEM κ does not change with κ, the constraints on nanoflare timescales derived from observations do not change with respect to the relative number of accelerated particles (i.e., different κ) present in the active region corona.…”