Hot Plasma in Nonflaring Active Regions Observed by the Extreme-Ultraviolet Imaging Spectrometer on Hinode
The Extreme-Ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft obtains high-resolution spectra of the solar atmosphere in two wavelength ranges: 170-210 and 250-290 Å. These wavelength regions contain a wealth of emission lines covering temperature regions from the chromosphere/transition region (e.g., He II, Si VII) up to flare temperatures (Fe XXIII, Fe XXIV). Of particular interest for understanding coronal heating is a line of Ca XVII at 192.858 Å, formed near a temperature of 6 × 106 K. However, this line is blended with two Fe XI and six O V lines. In this paper we discuss a specific procedure to extract the Ca XVII line from the blend. We have performed this procedure on the raster data of five active regions (ARs) and a limb flare, and demonstrated that the Ca XVII line can be satisfactorily extracted from the blend if the Ca XVII flux contributes to at least ~10% of the blend. We show examples of the high-temperature corona depicted by the Ca XVII emission and find that the Ca XVII emission has three morphological features in these ARs: (1) "fat" medium-sized loops confined in a smaller space than the 1 million degree corona, (2) weaker, diffuse emission surrounding these loops that spread over the core of the AR, and (3) the locations of the strong Ca XVII loops are often weak in line emission formed from the 1 million degree plasma. We find that the emission measure ratio of the 6 million degree plasma relative to the cooler 1 million degree plasma in the core of the ARs, using the Ca XVII to Fe XI line intensity ratio as a proxy, can be as high as 10. Outside of the AR core where the 1 million degree loops are abundant, the ratio has an upper limit of about 0.5.