Magnetic cage and rope as the key for solar eruptions

Abstract

Solar flares are spectacular coronal events that release large amounts of energy. They are classified as either eruptive or confined, depending on whether they are associated with a coronal mass ejection. Two types of model have been developed to identify the mechanism that triggers confined flares, although it has hitherto not been possible to decide between them because the magnetic field at the origin of the flares could not be determined with the required accuracy. In the first type of model, the triggering is related to the topological complexity of the flaring structure, which implies the presence of magnetically singular surfaces. This picture is observationally supported by the fact that radiative emission occurs near these features in many flaring regions. The second type of model attributes a key role to the formation of a twisted flux rope, which becomes unstable. Its plausibility is supported by simulations, by interpretations of some observations and by laboratory experiments. Here we report modelling of a confined event that uses the measured photospheric magnetic field as input. We first use a static model to compute the slowly evolving magnetic state of the corona before the eruption, and then use a dynamical model to determine the evolution during the eruption itself. We find that a magnetic flux rope must be present throughout the entire event to match the field measurements. This rope evolves slowly before saturating and suddenly erupting. Its energy is insufficient to break through the overlying field, whose lines form a confining cage, but its twist is large enough to trigger a kink instability, leading to the confined flare, as previously suggested. Topology is not the main cause of the flare, but it traces out the locations of the X-ray emission. We show that a weaker magnetic cage would have produced a more energetic eruption with a coronal mass ejection, associated with a predicted energy upper bound for a given region.

Low Geo‐Effectiveness of Fast Halo CMEs Related to the 12 X‐Class Flares in 2002

“…Wang & Zhang, 2007;Falconer et al, 2011;Zuccarello et al, 2017;Kontogiannis et al, 2019). It is crucial to determine if the flare will be eruptive and if particles can escape as demonstrated by Amari, Canou, Aly, Delyon, and Alauzet (2018). Large sunspot groups with a complex magnetic field such as in 2002 are not sufficient to predict geo-effective events (see Figure 2).…”

Flare Energy Release at the Magnetic Field Polarity Inversion Line during the M1.2 Solar Flare of 2015 March 15. II. Investigation of Photospheric Electric Current and Magnetic Field Variations Using HMI 135 s Vector Magnetograms

“…In the event when a fluxrope eruption cannot fully develop, i.g., due to a strongly suppressing overlying magnetic field, the second phase of flare ribbon expansion may be mild or absent, and one observes a confined flare (e.g. Thalmann et al 2015;Amari et al 2018). It should be noted here that the observed speeds of movement of the flare footpoint sources are used to estimate the reconnection electric field (E) and dimensionless reconnection rate in the coronal energy release sources, i.e.…”

Observations and 3D Magnetohydrodynamic Modeling of a Confined Helical Jet Launched by a Filament Eruption

“…A pre-existing flux rope in a bipolar active region becomes ideally unstable (usually to the kink instability), but the overlying field is too strong or has a low decay index and halts the eruption (e.g. Török & Kliem 2005b;Hassanin & Kliem 2016;Amari et al 2018;Liu et al 2018). In some cases the decay index may be high enough, but the development of the instability destroys the coherence of the flux rope before it can erupt (Zhou et al 2019).…”