Electron Acceleration and Plasma Heating in an Impulsive Confined C-class Solar Flare

Technical Summary: Electron Acceleration and Plasma Heating in an Impulsive Confined C-Class Solar Flare

Problem Statement
C-class solar flares, despite their lower energy output compared to M- and X-class events, occur frequently and offer a unique opportunity to investigate the fine details of magnetic reconnection, particle acceleration, and plasma heating without the complications of large-scale eruptive dynamics (e.g., Coronal Mass Ejections). However, detailed studies of C-class flares exhibiting Quasi-Periodic Pulsations (QPPs), particularly those combining spectropolarimetric microwave (MW) observations with spatially resolved X-ray imaging, remain scarce. The specific challenge addressed in this work is to characterize the non-stationary energy release, the coupling between thermal plasma and non-thermal electron populations, and the physical origin of QPPs in a confined, impulsive C2.8 flare (SOL2023-03-19T02:12) occurring in a morphologically simple Active Region (AR) NOAA 13256.

Methodology
The study employs a comprehensive multi-wavelength analysis of the flare SOL2023-03-19T02:12, utilizing data from a unique combination of instruments:

• Microwave (MW): Spectral observations (2.8–12 GHz) from the new Solar Radio Spectropolarimeter (SOLARSPEL) and imaging observations from the Siberian Radioheliograph (SRH).
• X-ray: Hard X-ray (HXR) imaging and spectroscopy from Solar Orbiter/STIX and ASO-S/HXI, alongside Fermi/GBM for high-cadence spectral analysis.
• EUV/UV/Magnetograms: Imaging from SDO/AIA and vector magnetograms from SDO/HMI.
• Magnetic Modeling: Nonlinear Force-Free Field (NLFFF) extrapolations were performed using the GX Simulator to reconstruct the 3D magnetic topology of the active region.

The analysis involved:

1. Spectral Fitting: X-ray spectra (6–120 keV) were fitted with a three-component model (thermal bremsstrahlung, thermal line emission, and non-thermal thick-target bremsstrahlung) to derive plasma temperature ($T$), emission measure ($EM$), and accelerated electron parameters (spectral index $\delta$, low-energy cutoff $E_{low}$, and flux $F$).
2. MW Modeling: Gyrosynchrotron modeling was applied to the MW spectrum to constrain electron density and source geometry, cross-validated with HXR data.
3. Temporal Analysis: Fourier and wavelet analyses were conducted on light curves from multiple instruments to identify QPPs.
4. Spatial Analysis: Centroid tracking of MW sources and co-alignment with X-ray and UV sources to map the evolution of energy release sites.

Key Results

• Morphology and Magnetic Topology: The flare occurred in a compact system of low-lying sheared magnetic loops ($\lesssim 10$ Mm) near the polarity inversion line (PIL), where the horizontal gradient of the radial magnetic field reached $\approx 1$ kG Mm$^{-1}$. The event was confined, with no associated CME or large-scale arcade formation. The magnetic configuration suggests reconnection occurred within current sheets possessing a substantial guide-field component.
• QPP Characteristics: The impulsive phase exhibited non-stationary QPPs in MW, HXR, and the time derivative of the Soft X-ray (SXR) flux. The pulsations consisted of 4–5 peaks with a mean period of $11.5 \pm 3.0$ s. Notably, the period decreased from $\approx 15$ s to $\approx 9$ s during the event. The MW source centroid exhibited systematic displacements parallel to the flare ribbons with apparent velocities of 200–800 km s$^{-1}$.
• Thermal-Non-Thermal Coupling: During the early impulsive phase (before hydrodynamic processes fully developed), strong empirical correlations were found:
  • The low-energy cutoff of the accelerated electron spectrum ($E_{low}$) scaled linearly with plasma temperature ($T$): $E_{low} - 10 \text{ keV} \sim 10 \times T \text{ (MK)}$.
  • The total flux of accelerated electrons ($F$) was proportional to the emission measure ($EM$):$F \sim 10^{-12} \text{ cm}^3 \text{ s}^{-1} \cdot EM$.
  • The cumulative energy of accelerated electrons was comparable to the thermal energy of the plasma ($E_{nonth} \approx U_{th}$).
• Acceleration Efficiency: The derived relationship suggests an acceleration probability of roughly 25% for thermal electrons during the initial phase. The authors propose that this efficiency may be modulated by the inflow of chromospheric evaporation plasma, which increases the target density for acceleration.
• Polarization and Narrowband Bursts: A reversal in the sign of circular polarization (Stokes $V$) was observed near 6.7 GHz. Additionally, a short-duration, narrowband coherent burst (4.0–4.6 GHz) was detected 4 seconds after the main peak, likely originating from a denser, lower-altitude source region.

Significance and Claims
The paper claims to provide a rare, detailed view of the "pristine" acceleration process in a solar flare before it is obscured by complex hydrodynamic feedback (chromospheric evaporation). By establishing empirical relationships between thermal and non-thermal parameters in the early impulsive phase, the study offers constraints on the efficiency of electron acceleration in collisionless, guide-field-dominated current sheets.

Regarding the QPPs, the authors conclude that while a definitive mechanism cannot be identified due to resolution limits, the evidence points toward a sequence of episodic magnetic reconnection events. They suggest that oscillatory magnetic reconnection or periodic modulation by slow magnetoacoustic waves reflected from the chromosphere are the most plausible triggers, rather than simple loop oscillations or coalescence instabilities, given the observed period evolution and source displacements.

The study also highlights the capabilities of the new SOLARSPEL instrument and the SRH in resolving fine temporal and spatial structures in confined flares, while acknowledging that future progress requires higher temporal/spatial resolution and advanced data-driven modeling to fully resolve the trigger mechanisms and energy partitioning.