Coronal O VI emission observed with UVCS/SOHO during solar flares: Comparison with soft X-ray observations

Abstract

In this work, we derive the O vi 1032 Å luminosity profiles of 58 flares, during their impulsive phase, based on off-limb measurements by the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the SOlar and Heliospheric Observatory (SOHO). The O vi luminosities from the transition region plasma (here defined as the region with temperatures 5.0 ≤ log T (K) ≤ 6.0) were inferred from the analysis of the resonantly scattered radiation of the O vi coronal ions. The temperature of maximum ionization for O vi is log T max (K) = 5.47.By comparison with simultaneous soft X-ray measurements, we investigate the likely source (chromospheric evaporation, footpoint emission, or heated prominence ejecta) for the transition region emission observed during the impulsive phase. In our study, we find evidence of the main characteristics predicted by the evaporation scenario. Specifically, most O vi flares precede the X-ray peaks typically by several minutes with a mean of 3.2 ± 0.1 min, and clear correlations are found between the soft X-ray and transition region luminosities following power laws with indices ∼0.7 ± 0.3. Overall, the results are consistent with transition region emission originating from chromospheric evaporation; the thermal X-ray emission peaks after the emission from the evaporation flow as the loops fill with hot plasma. Finally, we were able to infer flow speeds in the range ∼20−100 km s −1 for one-third of the events, 14 of which showed speeds between 60 and 80 km s −1 . These values are compatible with those found through direct spectroscopic observations at transition region temperatures by the EUV Imaging Spectrometer (EIS) on board Hinode.

An Observational Test of Solar Plasma Heating by Magnetic Flux Cancellation

“…In panel (c), we find that dW/dt shows similar variations on both shortterm (a few to several tens of minutes) and long-term (a few hours) scales as seen in the EUV intensity profiles, sometimes with a time lag of ∼5 -10 minutes. This time lag between the magnetic energy release and EUV intensity can be explained by the heating scenario of chromospheric evaporation (first proposed by Neupert 1968) in which the rapid downward acceleration of non-thermal particles produced by reconnection, as well as the onset of the consequent evaporation, precedes thermal (soft X-ray and EUV) coronal emissions typically by several minutes (Antonucci et al 1984;Silva et al 1997;Veronig et al 2002;Kamio et al 2005;Mancuso et al 2016). We finally explore how the estimated magnetic energy released as heat by flux cancellation is converted into (or contributes to) radiative energy losses of electrons in the ROI as a function of temperature.…”