Correlated Spatiotemporal Evolution of Extreme-Ultraviolet Ribbons and Hard X-rays in a Solar Flare

Abstract

We analyze the structure and evolution of ribbons from the M7.3 SOL2014-04-18T13 flare using ultraviolet (UV) images from IRIS and SDO/AIA, magnetic data from SDO/HMI, hard X-ray (HXR) images from RHESSI, and light curves from Fermi /GBM, in order to infer properties of coronal magnetic reconnection. As the event progresses, two flare ribbons spread away from the magnetic polarity inversion line. The width of the newly brightened front along the extension of the ribbon is highly intermittent in both space and time, presumably reflecting non-uniformities in the structure and/or dynamics of the flare current sheet. Furthermore, the ribbon width grows most rapidly in regions exhibiting concentrated non-thermal HXR emission, with sharp increases slightly preceding the HXR bursts. The light curve of the ultraviolet emission matches the HXR light curve at photon energies above 25 keV. In other regions the ribbon-width evolution and light curves do not temporally correlate with the HXR emission. This indicates that the production of non-thermal electrons is highly nonuniform within the flare current sheet. Our results suggest a strong connection between the production of non-thermal electrons and the locally enhanced perpendicular extent of flare ribbon fronts, which in turn reflects inhomogeneous structure and/or reconnection dynamics of the current sheet. Despite this variability, the ribbon fronts remain nearly continuous, quasi-one-dimensional features. Thus, although the reconnecting coronal current sheets are highly structured, they remain quasi-two-dimensional and the magnetic energy release occurs systematically, rather than stochastically, through the volume of reconnecting magnetic flux.

Properties and Energetics of Magnetic Reconnection: I. Evolution of Flare Ribbons

“…Particularly in the eruptive flares in this study, ψ has several spikes before F hxr , suggesting episodes of energy release that do not produce significant non-thermal emission. This has been observed in a number of other flares, such as those reported by Warren and Warshall (2001) and more recently by Naus et al (2021) and Qiu (2021). Second, all these flares, eruptive or confined, exhibit the strong-to-weak shear evolution when we consider the connectivity between the major pair of conjugate ribbons.…”

“…Qiu et al (2017) have measured shear evolution as well as the mean parallel and perpendicular motion of flare ribbons; it is suggestive from the properties listed in Table 1 of that study, that flares or episodes of flares with significant HXR emissions tend to exhibit large perpendicular motion v ⊥ in strong magnetic fields B l ≥ 500 G. These observations indicate that the reconnection rate in terms of ψ⊥ or the reconnection electric field E ≈ v ⊥ B l is important, though not always sufficient, for the production of non-thermal emissions (e.g. Naus et al, 2021).…”

“…The study has also revealed enhanced ribbon front width at other locations a few minutes before the onset of the HXR emission. The ribbon front width may be translated into the local reconnection rate, and the findings by Naus et al (2021) therefore suggest that the production of non-thermal electrons is strongly connected with, yet not solely dependent on, the locally enhanced reconnection rate.…”

“…see the discussion in Fletcher and Hudson, 2002;Miklenic et al, 2007;Inglis and Gilbert, 2013;Qiu, 2021). In particular, a recent case study by Naus et al (2021), using high-resolution observations of flare ribbons by the Interface Region Imaging Spectrograph (IRIS: De Pontieu et al, 2014), has characterized in great detail the bursty nature of flare reconnection.…”