A Study of Pre-Flare Solar Coronal Magnetic Fields: Magnetic Flux Ropes (1908.08643v1)

Abstract: Magnetic flux ropes (MFRs) are thought to be the central structure of solar eruptions, and their ideal MHD instabilities can trigger the eruption. Here we performed a study of all the MFR configurations that lead to major solar flares, either eruptive or confined, from 2011 to 2017 near the solar disk center. The coronal magnetic field is reconstructed from observed magnetograms, and based on magnetic twist distribution, we identified the MFR, which is defined as a coherent group of magnetic field lines winding an axis with more than one turn. It is found that 90% of the events possess pre-flare MFRs, and their three-dimensional structures are much more complex in details than theoretical MFR models. We further constructed a diagram based on two parameters, the magnetic twist number which controls the kink instability (KI), and the decay index which controls the torus instability (TI). It clearly shows lower limits for TI and KI thresholds, which are $n_{\rm crit} = 1.3$ and $|T_w|{\rm crit} = 2$, respectively, as all the events above $n{\rm crit}$ and nearly 90% of the events above $|T_w|_{\rm crit}$ erupted. Furthermore, by such criterion, over 70% of the events can be discriminated between eruptive and confined flares, and KI seems to play a nearly equally important role as TI in discriminating between the two types of flare. There are more than half of events with both parameters below the lower limits, and 29% are eruptive. These events might be triggered by magnetic reconnection rather than MHD instabilities.

• The paper establishes that approximately 90% of solar flare events exhibit complex pre-flare magnetic flux ropes, underlining their role as eruption precursors.
• The study uses a dual-parameter diagram of magnetic twist and decay index to set empirical thresholds (n_crit = 1.3 and |T_w|_crit = 2) that discriminate over 70% of eruptive versus confined flares.
• The paper also reveals that 29% of events below the classical instability thresholds still erupt, suggesting key influences from non-ideal MHD processes such as magnetic reconnection.

The Study of Pre-Flare Solar Coronal Magnetic Fields: Magnetic Flux Ropes

This paper examines the configuration and instabilities of magnetic flux ropes (MFRs) as crucial structures in solar eruptions, focusing on significant solar flares recorded from 2011 to 2017. The paper reconstructs the coronal magnetic field from observed magnetograms to identify MFRs based on their magnetic twist distribution.

Key Findings

1. Prevalence of MFRs: Approximately 90% of analyzed solar flare events contain pre-flare MFRs, denoting their importance as precursors to solar eruptions. The 3D configurations of these MFRs are considerably more intricate than what is suggested by theoretical models.
2. Instability Thresholds: The paper introduces a dual-parameter diagram employing the magnetic twist number, associated with the kink instability (KI), and the decay index, associated with torus instability (TI). Empirical lower limits are established for the TI and KI thresholds: decay index n_crit = 1.3 and twist number |T_w|_crit = 2. Flares exceeding these thresholds more frequently result in eruptions.
3. Discrimination Capability: Over 70% of the analyzed events can be effectively discriminated between eruptive and confined flares using these critical parameters. KI is revealed to have nearly as significant a role as TI, which contrasts with some previous assumptions that differentiated only the role of the decay index.
4. Non-Instability-Triggered Eruptions: A noteworthy fraction, specifically 29% of events that are below both the TI and KI thresholds, still manage to erupt. These occurrences are posited to be driven by non-ideal MHD processes, particularly magnetic reconnection, rather than the classical MHD instabilities.

Implications

The implications of this paper are multifaceted, affecting both theoretical and applied solar physics. The empirical thresholds provided can enhance the predictive accuracy of solar eruption modeling. Additionally, the apparent role of magnetic reconnection in events that do not meet classical instability criteria offers new research directions regarding the non-linear processes active in solar flare genesis.

On a theoretical level, this paper underscores the importance of complex and realistic magnetic configurations over simplified theoretical models. These detailed reconstructions provide deeper insights into the potential feedback mechanisms in solar eruptions, supporting the development of more advanced modeling techniques in solar physics.

Speculation for Future Research

Future research could benefit from integrating more extensive datasets across different solar cycles to refine these instability thresholds further. Additionally, advancements in high-resolution magnetographic data and extrapolation techniques could provide even finer detail of the MFR structures, potentially linking specific topological features to the likelihood of eruption or confinement. Finally, the role of magnetic reconnection presents a compelling avenue for studies focusing on the onset mechanisms for solar flares that challenge traditional MHD instability-driven paradigms.