Spatially resolved plasma composition evolution in a solar flare – The effect of reconnection outflow

¹ ESTEC, European Space Agency, Keplerlaan 1, PO Box 299 NL-2200 AG Noordwijk, The Netherlands
² University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
³ Department of Physics & Astronomy, George Mason University, 4400 University Drive, Fairfax, VA 22030, USA
⁴ Institute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan
⁵ Department of Earth and Planetary Science, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, Japan
⁶ National Solar Observatory, 3665 Innovation Drive, Boulder, CO 80303, USA
⁷ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁸ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege út 15-17., H-1121 Budapest, Hungary
⁹ Centre for Astrophysics & Relativity, School of Physical Sciences, Dublin City University, Glasnevin, D09 K2WA Dublin, Ireland
¹⁰ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast, BT7 1NN Northern Ireland, UK
¹¹ Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA

^⋆ Corresponding authors; andysh.to@esa.int, imada@eps.s.u-tokyo.ac.jp

Received: 16 January 2024
Accepted: 26 September 2024

Abstract

Context. Solar flares exhibit complex variations in elemental abundances compared to photospheric values. These abundance variations, characterized by the first ionization potential (FIP) bias, remain challenging to interpret.

Aims. We aim to (1) examine the spatial and temporal evolution of coronal abundances in the X8.2 flare on 2017 September 10, and (2) provide a new scenario to interpret the often observed high FIP bias loop top, and provide further insight into differences between spatially resolved and Sun-as-a-star flare composition measurements.

Methods. We analyzed 12 Hinode/Extreme-ultraviolet Imaging Spectrometer (EIS) raster scans spanning 3.5 hours, employing both Ca XIV 193.87 Å/Ar XIV 194.40 Å and Fe XVI 262.98 Å/S XIII 256.69 Å composition diagnostics to derive FIP bias values. We used the Markov Chain Monte Carlo (MCMC) differential emission measure (DEM) method to obtain the distribution of plasma temperatures, which forms the basis for the FIP bias calculations.

Results. Both the Ca/Ar and Fe/S composition diagnostics consistently show that flare loop tops maintain high FIP bias values of > 2–6, with peak phase values exceeding 4, over the extended duration, while footpoints exhibit photospheric FIP bias of ∼1. The consistency between these two diagnostics forms the basis for our interpretation of the abundance variations.

Conclusions. We propose that this variation arises from a combination of two distinct processes: high FIP bias plasma downflows from the plasma sheet confined to loop tops, and chromospheric evaporation filling the loop footpoints with low FIP bias plasma. Mixing between these two sources produces the observed gradient. Our observations show that the localized high FIP bias signature at loop tops is likely diluted by the bright footpoint emission in spatially averaged measurements. The spatially resolved spectroscopic observations enabled by EIS prove critical for revealing this complex abundance variation in loops. Furthermore, our observations show clear evidence that the origin of hot flare plasma in flaring loops consists of a combination of both directly heated plasma in the corona and from ablated chromospheric material; and our results provide valuable insights into the formation and composition of loop top brightenings, also known as EUV knots, which are a common feature at the tops of flare loops.