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

TL;DR

This study combines spectroscopic and X-ray observations to analyze plasma flows during a solar flare, revealing complex, multi-layered evaporation processes that challenge existing models of solar flare dynamics.

Contribution

It provides detailed multiwavelength spectroscopic data showing the complex plasma motions and structures during a solar flare, highlighting limitations of current single-loop models.

Findings

Observation of simultaneous upflows and downflows in different layers.

Detection of high-velocity upflows up to 280 km/s.

Evidence that energy input alone cannot explain all observed plasma motions.

Abstract

We study spectroscopic observations of chromospheric evaporation mass flows in comparison to the energy input by electron beams derived from hard X-ray 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, EUV and X-ray domain during the joint observing campaign JOP171. During the flare peak, we observe downflows in the He\,{\sc i} and O\,{\sc v} lines formed in the chromosphere and transition region, respectively, and simultaneous upflows in the hot coronal Si~{\sc xii} line. The energy deposition rate by electron beams derived from RHESSI hard X-ray 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 CDS slit, the situation is intriguing. The O\,{\sc 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$^{-1}$) 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.