Onset Mechanism of M6.5 Solar Flare Observed in Active Region 12371

Abstract: We studied a flare onset process in terms of stability of a three-dimensional (3D) magnetic field in active region 12371 producing an eruptive M6.5 flare in 2015 June 22. In order to reveal the 3D magnetic structure, we first extrapolated the 3D coronal magnetic fields based on time series of the photospheric vector magnetic fields under a nonlinear force-free field (NLFFF) approximation. The NLFFFs nicely reproduced the observed sigmoidal structure which is widely considered to be preeruptive magnetic configuration. In particular, we found that the sigmoid is composed of two branches of sheared arcade loops. On the basis of the NLFFFs, we investigated the sheared arcade loops to explore the onset process of the eruptive flare using three representative magnetohydrodynamic instabilities: the kink, torus, and double arc instabilities (DAI). The DAI, recently proposed by Ishiguro & Kusano, is a double arc loop that can be more easily destabilized than a torus loop. Consequently, the NLFFFs are found to be quite stable against the kink and torus instabilities. However, the sheared arcade loops formed prior to the flare possibly become unstable against the DAI. As a possible scenario for the onset process of the M6.5 flare, we suggest a three-step process: (1) double arc loops are formed by the sheared arcade loops through the tether-cutting reconnection during an early phase of the flare, (2) the DAI contributes to the expansion of destabilized double arc loops, and (3) finally, the torus instability makes the full eruption.

• The paper demonstrates that double arc instability (DAI) triggers the M6.5 flare when traditional instabilities remain stable.
• It employs NLFFF extrapolation of time-series photospheric data to reconstruct the sigmoidal magnetic configuration preceding the flare.
• The authors propose a three-step eruption process, offering insights that improve predictive models for solar flare onset.

Onset Mechanism of M6.5 Solar Flare in Active Region 12371

The paper "Onset Mechanism of M6.5 Solar Flare Observed in Active Region 12371" conducted a comprehensive study on the mechanisms triggering solar flares through the analysis of the three-dimensional magnetic structure in solar active region (AR) 12371. This region produced a significant eruptive solar flare, classified as M6.5, on June 22, 2015. This research offers valuable insights into the nature of magnetic instability that leads to such solar phenomena.

Methodology

The investigation employed Nonlinear Force-Free Field (NLFFF) extrapolation to model the three-dimensional magnetic fields based on time-series data of photospheric vector magnetic fields. Through NLFFFs, the observed sigmoidal structure associated with pre-eruptive magnetic configurations was successfully reproduced. The paper focused on diagnosing the stability of this structure against three well-known magnetohydrodynamic (MHD) instabilities: kink instability (KI), torus instability (TI), and the recently proposed double arc instability (DAI).

Key Findings

1. Sigmoidal Structure and Sheared Arcade Loops: The study identified that the sigmoidal structure preceding the flare was composed of two branches of sheared arcade loops. This configuration provides crucial insights into the structure's stability before a flare onset.
2. Instability Analysis:
   • The magnetic field was found to be stable against the KI and TI instabilities. The magnetic twist values did not surpass critical limits, nor did the decay index reach thresholds associated with TI.
   • A potential instability was identified through the DAI model. The sheared arcade loops were assessed to become unstable when analyzed under the DAI criteria. Therefore, DAI offers an explanation for the eruption mechanism when traditional instabilities fail to provide satisfactory insights.
3. Eruption Scenario: The authors proposed a three-step flare onset process:
   • Initially, tether-cutting reconnection forms double arc loops from the sheared arcade loops.
   • The DAI subsequently contributes to the expansion of these loops.
   • Finally, when the loops reach a region where the TI criteria are satisfied, a full eruption occurs.

Implications

The paper's analysis on the applicability of DAI in explaining the solar flare's onset presents a pivotal theoretical advancement. This clarified a scenario in which eruptions can transpire even when standard instability conditions are unmet. Practically, understanding the complex mechanisms of flare initiation can enhance space weather forecasting accuracy, often critical for numerous Earth-based and space-dependent technologies.

Future Directions

Further research endeavors could refine the critical thresholds involved in DAI to delineate more precise eruption predictive models. Additionally, future studies can aim for observational validations using higher spatial resolution data to track the magnetic topology changes preceding flare occurrences in greater detail. Improved computation methods that enable real-time monitoring and prediction could also be developed based on these findings.

Through this detailed examination of MHD instabilities in solar active regions, the paper underscores the potential roles differing instabilities might play in complex magnetic interactions leading to significant solar events. As such, it represents a significant contribution to our understanding of solar physics and flare dynamics.