Some Like It Hot: Coronal Heating Observations from Hinode X-ray Telescope and RHESSI

Abstract

We have used Hinode X-Ray Telescope observations and RHESSI upper limits together to characterize the differential emission measure (DEM) from a quiescent active region. We find a relatively smooth DEM curve with the expected active region peak at log T = 6.4. We also find a high-temperature component with significant emission measure at log T gsim 7. This curve is consistent with previous observations of quiescent active regions in that it does not produce observable Fe XIX lines. It is different from that generated with X-Ray Telescope (XRT) data alone—RHESSI rules out the possibility of a separate high-temperature component with a peak of approximately log T = 7.4. The strength and position of the high-temperature peak in this XRT-only analysis was, however, poorly determined; adding RHESSI flux upper limits in the 4-13 keV energy range provide a strong high-temperature constraint which greatly improves the multi-thermal findings. The results of the present work as well as those from a growing number of papers on this subject imply that our previous understanding of the temperature distribution in active regions has been limited. Hot plasma (log T ≈ 7) appears to be prevalent, although in relatively small quantities as predicted by nanoflare models. Other models may need to be adjusted or updated to account for these new results.