Spectroscopy of Very Hot Plasma in Non-flaring Parts of a Solar Limb Active Region: Spatial and Temporal Properties

In this work we investigate the thermal structure of an off-limb active region (AR) in various non-flaring areas, as it provides key information on the way these structures are heated. In particular, we concentrate on the very hot component (>3 MK) as it is a crucial element to distinguish between different heating mechanisms. We present an analysis using Fe and Ca emission lines from both the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) on board the Solar and Heliospheric Observatory (SOHO) and the EUV Imaging Spectrometer (EIS) on board Hinode. A data set covering all ionization stages from Fe X to Fe XIX has been used for the thermal analysis (both differential emission measure and emission measure, EM). Ca XIV is used for the SUMER-EIS radiometric cross calibration. We show that the very hot plasma is present and persistent almost everywhere in the core of the limb AR. The off-limb AR is clearly structured in Fe XVIII. Almost everywhere, the EM analysis reveals plasma at 10 MK (visible in Fe XIX emission), which is down to 0.1% of EM of the main 3 MK plasma. We estimate the power-law index of the hot tail of the EM to be between -8.5 and -4.4. However, the question about the possible existence of a small minor peak at around 10 MK remains open. The absence in some part of the AR of the Fe XIX and Fe XXIII lines (which fall into our spectral range) enables us to determine an upper limit on the EM at these temperatures. Our results include a new Ca XIV 943.59 Aatomic model.