Plasma Heating During Coronal Mass Ejections Observed in X-Rays

In this work, we study where heating takes place during coronal mass ejections (CMEs). For this purpose, we have used the data of the Mg xii spectroheliograph on board the Complex Orbital Observations Near-Earth of Activity on the Sun (CORONAS)- F satellite. This instrument obtained images of the solar corona in the Mg xii 8.42 Å line, which emits only at temperatures higher than 4 MK. After analyzing the Mg xii data archive from 2001 to 2003, we found ten high-temperature eruptive events. Each of them was associated with a CME and nine were associated with a flare. The eruptive structures had temperatures higher than 4 MK and a characteristic size of 100 – 200 Mm. The events were observed by the Mg xii spectroheliograph for 10 min to 3 h. In the Mg xii images, the peak intensity of the eruptive structures was 0.2 – 14.4% of the peak intensity of the flaring active regions below them. Based on the shape of the events, we divided them into three groups: loop-like, sheet-like, and cloud-like. We interpreted loop-like events as hot flux ropes and sheet-like ones as hot plasma surrounding current sheets. Based on the available data, we cannot determine the nature of the cloud-like events. Their appearance could be caused by projection effects, a postflare reconnection, a shock wave, or a small-scale reconnection in the CME volume. Our estimates suggest that, in solar maxima, plasma should be heated above 4 MK during approximately one out of six CMEs.