Evidence for Steady Heating: Observations of an Active Region Core with Hinode and TRACE

The timescale for energy release is an important parameter for constraining the coronal heating mechanism. Observations of “warm” coronal loops (∼ 1MK) have indicated that the heating is impulsive and that coronal plasma is far from equilibrium. In contrast, observations at higher temperatures (∼ 3MK) have generally been consistent with steady heating models. Previous observations, however, have not been able to exclude the possibility that the high temperature loops are actually composed of many small scale threads that are in various stages of heating and cooling and only appear to be in equilibrium. With new observations from the EUV Imaging Spectrometer (EIS) and X-ray Telescope (XRT) on Hinode we have the ability to investigate the properties of high temperature coronal plasma in extraordinary detail. We examine the emission in the core of an active region and find three independent lines of evidence for steady heating. We find that the emission observed in XRT is generally steady for hours, with a fluctuation level of approximately 15% in an individual pixel. Furthermore, we find no evidence for warm emission that is spatially correlated with the hot emission, as would be expected if the high temperature loops are the result of impulsive heating. Finally, we also find that intensities in the “moss,” the footpoints of high temperature loops, are consistent with steady heating models provided that we account for the local expansion of the loop from the base of transition region to the corona. In combination, these results provide strong evidence that the heating in the core of an active region is effectively steady, that is, the heating occurs on very short timescales relative to the relevant radiative and conductive cooling times. Subject headings: Sun: corona Space Science Division, Naval Research Laboratory, Washington, DC 20375 Department of Physics, Alabama A&M, 4900 Meridian Street, Normal, AL 35762 College of Science, George Mason University, 4400 University Drive, Fairfax, VA 22030