On the Nature of Candle-Flame-Shaped Solar Flares and Sub-Alfv\'enic Supra-Arcade Plasma Downflows

TL;DR

This study uses a 3D MHD model to analyze candle-flame-shaped solar flares, revealing that observed plasma downflow speeds are significantly underestimated and that magnetic topology changes do not always align with the apparent cusp tip.

Contribution

The paper provides new insights into the magnetic topology and plasma flow speeds in candle-flame-shaped solar flares using advanced 3D MHD simulations and synthetic EUV imaging.

Findings

Y-points do not necessarily match the apparent cusp tip.

Observed downflow speeds underestimate true Alfvén speeds by at least a factor of two.

Projection effects influence measurements of plasma downflow speeds.

Abstract

Certain solar flares exhibit a distinctive candle-flame or cusp-shaped feature above the bright flare arcade visible in extreme ultraviolet (EUV) and X-ray channels sensitive to high-temperature plasma. The presence of a cusp-like structure is generally regarded as a key piece of morphological evidence for magnetic reconnection to power explosive energy release in solar flares. In addition, downward-propagating plasma flows above the flare arcade have often been interpreted as outflows driven by magnetic reconnection. However, the relationship between the observed candle-flame-shaped morphology and the underlying magnetic field geometry for reconnection remains unclear. Likewise, the observed speed of the plasma downflows has been found to be too low compared to the upstream Alfv\'en speed predicted by reconnection theories. With the help of a recently developed three-dimensional magnetohydrodynamics (MHD) model, we examine the locations where magnetic topology changes from antiparallel to closed (Y-points) in a candle-flame-shaped flare, compare the observational emission features with synthetic EUV images generated from the model, and analyze their time evolutions. We also investigate the role of projection effects and line-of-sight integration in the measurements of plasma downflow speeds. Our analysis reveals that the Y-points do not necessarily coincide with the apparent cusp tip. Also, the apparent speeds of the supra-arcade downflows, as derived from tracks in the time-distance plots, underestimate the true Alfv\'en speeds in the reconnection inflow region by at least a factor of two up to an order of magnitude.