In this paper, we study the temperature and density properties of multiple structural components of coronal mass ejections (CMEs) using differential emission measure (DEM) analysis. The DEM analysis is based on the sixpassband EUV observations of solar corona from the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. The structural components studied include the hot channel in the core region (presumably the magnetic flux rope of the CME), the bright loop-like leading front (LF), and coronal dimming in the wake of the CME. We find that the presumed flux rope has the highest average temperature (>8 MK) and density (∼1.0 × 10 9 cm −3 ), resulting in an enhanced emission measure over a broad temperature range (3 T(MK) 20). On the other hand, the CME LF has a relatively cool temperature (∼2 MK) and a narrow temperature distribution similar to the pre-eruption coronal temperature (1 T(MK) 3). The density in the LF, however, is increased by 2%-32% compared with that of the pre-eruption corona, depending on the event and location. In coronal dimmings, the temperature is more broadly distributed (1 T(MK) 4), but the density decreases by ∼35%-∼40%. These observational results show that: (1) CME core regions are significantly heated, presumably through magnetic reconnection; (2) CME LFs are a consequence of compression of ambient plasma caused by the expansion of the CME core region; and (3) the dimmings are largely caused by the plasma rarefaction associated with the eruption.

Abstract:The physical processes resulting in energy exchange between the Sun's hot corona and its cool lower atmosphere are still poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20-60s) of intensity and velocity on small spatial scales (≲500km) at the footpoints of hot and dynamic coronal loops. Comparison with numerical simulations reveal that the observations are consistent with heating by beams of non-thermal electrons and that these beams are generated even in small impulsive (≲30s) heating events called "coronal nanoflares". The accelerated electrons deposit a significant fraction of their energy (≲10 25 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the non-flaring corona. Main Text:Though it is established that the magnetic field plays a major role in the energetics of the bright corona, determining the details of the physical mechanisms that heat the solar corona remains one of the outstanding open issues in astrophysics. There are several candidate physical processes for heating the corona, including dissipation of magnetic stresses via reconnection, and dissipation of magnetohydrodynamic waves (1,2,3). In many heating models, the energy release is characterized by small spatial and temporal scales. For instance, in the "nanoflare" model, random photospheric motions lead to braiding or shearing of magnetic field lines and to reconnection which yields impulsive heating of the coronal plasma (4,5). Several statistical studies of large numbers of solar flares (6-8) have suggested that the mechanisms producing flares are likely similar within a large range from micro-to X-class flares. If nanoflares behave as a scaled down version of larger flares, particles accelerated in the corona by reconnection processes could play a significant role in the heating of plasma even in absence of large flares. Hard X-ray observations of microflares (E~10 27 erg) in active regions reveal the presence of non-thermal particles (8,9), but nanoflare size events (E~10 24 erg) are not currently accessible to hard X-ray studies due to limited sensitivity. As a result, the properties and generation of non-thermal particles in the solar atmosphere and their role in quiescent coronal heating remain poorly known.The observational tracers of the coronal heating are elusive because the corona is highly conductive, washing out the signatures of heating release. However, the emission of the TR, where the temperature steeply increases to MK values in a narrow layer (~1-3 ×10 8 cm), is instead highly responsive to heating since its density, temperature gradients and spatial dimensions change rapidly during heating events (10)(11)(12). This is the also the case for coronal heating events where ...

The frequency of heating events in the corona is an important constraint on the coronal heating mechanisms. Observations indicate that the intensities and velocities measured in active region cores are effectively steady, suggesting that heating events occur rapidly enough to keep high temperature active region loops close to equilibrium. In this paper, we couple observations of Active Region 10955 made with XRT and EIS on Hinode to test a simple steady heating model. First we calculate the differential emission measure of the apex region of the loops in the active region core. We find the DEM to be broad and peaked around 3 MK. We then determine the densities in the corresponding footpoint regions. Using potential field extrapolations to approximate the loop lengths and the density-sensitive line ratios to infer the magnitude of the heating, we build a steady heating model for the active region core and find that we can match the general properties of the observed DEM for the temperature range of 6.3 < Log T < 6.7. This model, for the first time, accounts for the base pressure, loop length, and distribution of apex temperatures of the core loops. We find that the density-sensitive spectral line intensities and the bulk of the hot emission in the active region core are consistent with steady heating. We also find, however, that the steady heating model cannot address the emission observed at lower temperatures. This emission may be due to foreground or background structures, or may indicate that the heating in the core is more complicated. Different heating scenarios must be tested to determine if they have the same level of agreement.

We explore the dependence of the amplitude of stellar dynamo cycle variability (as seen in the Mount Wilson Ca II HK timeseries data) on other stellar parameters. We find that the fractional cycle amplitude Acyc (i.e. the ratio of the peakto-peak variation to the average) decreases somewhat with mean activity, increases with decreasing effective temperature, but is not correlated with inverse Rossby number Ro −1 . We find that Acyc increases with the ratio of cycle and rotational frequencies ωcyc/Ω along two, nearly parallel branches.

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