Multiwavelength imaging and spectroscopy of chromospheric evaporation in an M-class solar flare

Veronig A.M.¹, Rybak J.², Gömöry P.^2,3, Berkebile-Stoiser S.¹, Temmer M.^1,4, Otruba, W.³, Vrsnak B.⁵, Pötzi, W.³, Baumgartner, D.³

¹ Institute of Physics, University of Graz, Universitaetsplatz 5, A-8010 Graz, Austria
² Astronomical Institute of the Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia
³ Institute of Physics/Kanzelhoehe Observatory, University of Graz, A-9521 Treffen, Austria
⁴ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrase 6, A-8042 Graz, Austria
⁵ Hvar Observatory, Faculty of Geodesy, Kaciceva 26, 1000 Zagreb, Croatia

Abstract:
We study spectroscopic observations of chromospheric evaporation mass flows in comparison with the energy input by electron beams derived from hard X-ray (HXR) data for the white-light M2.5 flare of 2006 July 6. The event was captured in high-cadence spectroscopic observing mode by SOHO/CDS combined with high-cadence imaging at various wavelengths in the visible, extreme ultraviolet, and X-ray domain during the joint observing campaign JOP 171. During the flare peak, we observe downflows in the He I and O V lines formed in the chromosphere and transition region, respectively, and simultaneous upflows in the hot coronal Si XII line. The energy deposition rate by electron beams derived from RHESSI HXR observations is suggestive of explosive chromospheric evaporation, consistent with the observed plasma motions. However, for a later distinct X-ray burst, where the site of the strongest energy deposition is exactly located on the Coronal Diagnostics Spectrometer (CDS) slit, the situation is intriguing. The O V transition region line spectra show the evolution of double components, indicative of the superposition of a stationary plasma volume and upflowing plasma elements with high velocities (up to 280 km/s) in single CDS pixels on the flare ribbon. However, the energy input by electrons during this period is too small to drive explosive chromospheric evaporation. These unexpected findings indicate that the flaring transition region is much more dynamic, complex, and fine structured than is captured in single-loop hydrodynamic simulations.