Locating Hot Plasma in Small Flares using Spectroscopic Overlappogram Data from the Hinode EUV Imaging Spectrometer

One of the key processes associated with the “standard” flare model is chromospheric evaporation, a process during which plasma heated to high temperatures by energy deposition at the flare footpoints is driven upwards into the corona. Despite several decades of study, a number of open questions remain, including the relationship between plasma produced during this process and observations of earlier “superhot” plasma. The Extreme ultraviolet Imaging Spectrometer (EIS) onboard Hinode has a wide slot, which is often used as a flare trigger in the He ii emission-line band. Once the intensity passes a threshold level, the study will switch to one focussed on the flaring region. However, when the intensity is not high enough to reach the flare trigger threshold, these datasets are then available during the entire flare period and provide high-cadence spectroscopic observations over a large field of view. We make use of data from two such studies of a C4.7 flare and a C1.6 flare to probe the relationship between hot Fe xxiv plasma and plasmas observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the X-ray Telescope (XRT) to track where the emission comes from and when it begins. The flare trigger slot data used in our analysis has one-minute cadence. Although the spatial and spectral information are merged in the wide-slot data, it is still possible to extract when the hot plasma appears, through the appearance of the Fe x xiv spectral image. It is also possible to derive spectrally pure Fe xxiv light curves from the EIS data, and compare them with those derived from hard X-rays, enabling a full exploration of the evolution of hot emission. The Fe xxiv emission peaks just after the peak in the hard X-ray lightcurve; consistent with an origin in the evaporation of heated plasma following the transfer of energy to the lower atmosphere. A peak was also found for the C4.7 flare in the RHESSI peak temperature, which occurred before the hard X-rays peaked. This suggests that the first peak in hot-plasma emission is likely to be directly related to the energy-release process.