Temporal evolution of chromospheric evaporation: case studies of the M1.1 flare on 2014 September 6 and X1.6 flare on 2014 September 10

TL;DR

This study uses IRIS observations to analyze the evolution of hot chromospheric evaporation flows during two major solar flares, revealing their velocity decay, correlation with energy deposition, and implications for chromospheric condensation speeds.

Contribution

First detailed tracking of the complete evolution of hot evaporation flows in two major flares, linking flow dynamics with energy deposition and chromospheric heating processes.

Findings

Evaporation flows decrease exponentially from ~200 km/s to near stationary.

Flow velocity correlates with energy deposition rate from X-ray observations.

Chromospheric condensation speeds may be higher than previously estimated.

Abstract

With observations from the Interface Region Imaging Spectrograph (IRIS), we track the complete evolution of $\sim$11 MK evaporation flows in an M1.1 flare on 2014 September 6 and an X1.6 flare on 2014 September 10. These hot flows, as indicated by the blueshifted Fe~{\sc{xxi}}~1354.08\AA{}~line, evolve smoothly with a velocity decreasing exponentially from $\sim$200~km~s$^{-1}$ to almost stationary within a few minutes. We find a good correlation between the flow velocity and energy deposition rate as represented by the hard X-Ray flux observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), or time derivative of the soft X-Ray flux observed with the Geostationary Operational Environmental Satellites (GOES) and the HINODE X-ray Telescope (XRT), which is in general agreement with models of nonthermal electron heating. The maximum blue shift of Fe~{\sc{xxi}}~appears approximately at the same time as or slightly after the impulsive enhancement of the ultraviolet continuum and the Mg~{\sc{ii}}~2798.8\AA{}~line emission, demonstrating that the evaporation flow is closely related to heating of the lower chromosphere. Finally, while the hot Fe~{\sc{xxi}}~1354.08\AA{} line is entirely blueshifted with no obvious rest component, cool chromospheric and transition region lines like Si~{\sc{iv}}~1402.77\AA{} are often not entirely redshifted but just reveal an obvious red wing enhancement at the ribbons, suggesting that the speed of chromospheric condensation might be larger than previously thought.