Thermodynamic and magnetic evolution of an eruptive C-class solar flare observed with SST/TRIPPEL-SP

TL;DR

This study investigates the thermodynamic and magnetic evolution of a C5.1 solar flare and associated filament eruption in AR 12561 on 2016-07-07 by combining high-resolution SST/TRIPPEL-SP spectropolarimetry of Ca II 8542 Å with NLTE inversions (STiC) and non-force-free field extrapolations using SDO/HMI SHARP data. It identifies a localized, deep-heating precursor near a bald-patch region prior to the flare, links pre-flare magnetic free energy buildup to the eruption, and documents strong chromospheric heating and downflows in the flare ribbons during the main phase. The study reveals a coherent magnetic-topology evolution: stored energy in a highly sheared configuration above a pore, selective low-altitude reconnection at bald patches, and a post-flare arcade that signals substantial energy release. These results underscore the importance of photosphere-to-chromosphere magnetic coupling and provide a quantitative energy budget consistent with the observed C-class flare, highlighting the potential of integrated chromospheric diagnostics for understanding and predicting eruption potential.

Abstract

Solar flares are complex phenomena driven by the release of magnetic energy, but a large energy reservoir is not sufficient to determine their eruptive potential; the magnetic topology and plasma dynamics play a key role. We investigate the thermodynamic and magnetic properties of the solar atmosphere during the rise, peak, and decay phases of a C5.1-class flare and filament eruption in active region NOAA 12561 on 2016 July 7, to understand the origin and atmospheric response of this event. High spatial and spectral resolution spectropolarimetric observations of the chromospheric Ca II 8542A line and nearby photospheric lines were obtained with the TRIPPEL-SP spectropolarimeter at the Swedish 1-m Solar Telescope. Using non-local thermodynamic equilibrium (NLTE) inversions and non-force-free field (NFFF) magnetic extrapolations, we followed the event's evolution from its precursor to its decay. Before the flare, our analysis reveals a complex, sheared magnetic topology with a high free energy content ($\sim2\times10^{30}$ erg). In this precursor phase, we detected persistent, localized heating (temperature increase of $\sim$2000 K) with strong downflows ($\sim$10-20 km/s) deep in the atmosphere. This heating was co-spatial with a bald-patch region, suggesting that low-altitude magnetic reconnection could destabilize the filament of the region. The flare's rise phase was marked by the filament's eruption, with a total speed larger than $\sim$70 km/s, when combining inversions and plane-of-sky motions. Following the eruption, the free energy decreased by $\sim$30$\%$ as post-flare loops formed, connecting the flare ribbons and channeling the released energy into the lower atmosphere. The flare ribbons exhibited significant heating to $\sim$8500 K and downflows up to $\sim$10 km/s, consistent with energy deposition along reconnected loops.

Figures (13)

• Figure 1: Time evolution of GOES X-ray flux in the two channels (1$-$8Å and 0.5$-$4Å) on 2016 July 7. Vertical dashed lines mark the approximate peak times of flares originating from AR 12561 (three B-class and one C-class). The gray shaded areas indicate the temporal coverage of the five observed scans.
• Figure 2:
• Figure 3: High-resolution view of the target region from the first SST/TRIPPEL-SP scan (07:37-07:54 UT), taken just before the flare starts. From left to right: Intensity in the far wing of Caii 8542 Å (photosphere); outer wing (upper photosphere); inner wing (lower chromosphere); and at the line core (mid-chromosphere). The last two panels show Stokes $V$ maps in the photospheric Fei 8526.67 Å line and the chromospheric Caii 8542 Å line, respectively. The black arrows indicate the solar North and disk center. The horizontal axis corresponds to the scan direction (left to right). All panels are in units of the average quiet Sun continuum intensity.
• Figure 4: Evolution of the active region as seen in the Caii 8542 Å line core intensity, from the five SST/TRIPPEL-SP scans. The time interval covered by each scan is indicated in the top left corner. The GOES 1-8 Å X-ray flux is plotted below each panel marking the mid-time of each scan. The location of the erupting filament's spine (inferred from multiple wavelengths) is indicated by the white dashed line and the flare ribbons (R1, R2) as dotted lines. Crosses indicate the location of the profiles shown in Fig. \ref{['fig:profiles']}.
• Figure 5: Selection of profiles and their fits from different solar features. Their location is indicated in Fig. \ref{['fig:evolution']}. It shows the Stokes profiles for the Fei 8526.67 Å line (left) and the Caii 8542 Å line (right). Dots represent the observed profiles, while solid lines show the best-fit synthetic profiles from the NLTE inversions.