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Register a new userReconnection fluxes in eruptive and confined flares and implications for superflares on the Sun
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We study the energy release process of a set of 51 flares (32 confined, 19 eruptive) ranging from GOES class B3 to X17. We use H$alpha$ filtergrams from Kanzelhohe Observatory together with SDO HMI and SOHO MDI magnetograms to derive magnetic reconnection fluxes and rates. The flare reconnection flux is strongly correlated with the peak of the GOES 1-8 AA soft X-ray flux (c=0.92, in log-log space), both for confined and eruptive flares. Confined flares of a certain GOES class exhibit smaller ribbon areas but larger magnetic flux densities in the flare ribbons (by a factor of 2). In the largest events, up to $approx$50% of the magnetic flux of the active region (AR) causing the flare is involved in the flare magnetic reconnection. These findings allow us to extrapolate toward the largest solar flares possible. A complex solar AR hosting a magnetic flux of $2cdot 10^{23}, mathrm{Mx}$, which is in line with the largest AR fluxes directly measured, is capable of producing an X80 flare, which corresponds to a bolometric energy of about $7 cdot 10^{32}$ ergs. Using a magnetic flux estimate of $6cdot 10^{23}, mathrm{Mx}$ for the largest solar AR observed, we find that flares of GOES class $approx$X500 could be produced ($E_{rm bol} approx 3 cdot 10^{33}$ ergs). These estimates suggest that the present days Sun is capable of producing flares and related space weather events that may be more than an order of magnitude stronger than have been observed to date.
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We investigate the distinct properties of two types of flares: eruptive flares associated with CMEs and confined flares without CMEs. Our sample of study includes nine M and X-class flares, all from the same active region (AR), six of which are confined and three others are eruptive. The confined flares tend to be more impulsive in the soft X-ray time profiles and show more slender shapes in the EIT 195 A images, while the eruptive ones are of long-duration events and show much more extended brightening regions. The location of the confined flares are closer to the center of the AR, while the eruptive flares are at the outskirts. This difference is quantified by the displacement parameter, the distance between the AR center and the flare location: the average displacement of the six confined flares is 16 Mm, while that of eruptive ones is as large as 39 Mm. Further, through nonlinear force-free field extrapolation, we find that the decay index of the transverse magnetic field in the low corona (~10 Mm) have a larger value for eruptive flares than that for confined one. In addition, the strength of the transverse magnetic field over the eruptive flare sites is weaker than that over the confined ones. These results demonstrate that the strength and the decay index of background magnetic field may determine whether or not a flare be eruptive or confined. The implication of these results on CME models is discussed in the context of torus instability of flux rope.
We investigate the coronal magnetic energy and helicity budgets of ten solar ARs, around the times of large flares. In particular, we are interested in a possible relation of the derived quantities to the particular type of the flares that the AR produces, i.e., whether they are associated with a CME or they are confined. Using an optimization approach, we employ time series of 3D nonlinear force-free magnetic field models of ten ARs, covering a time span of several hours around the time of occurrence of large solar flares (GOES class M1.0 and larger). We subsequently compute the 3D magnetic vector potentials associated to the model 3D coronal magnetic field using a finite-volume method. This allows us to correspondingly compute the coronal magnetic energy and helicity budgets, as well as related (intensive) quantities such as the relative contribution of free magnetic energy, $E_{mathrm{F}}/{E}$ (energy ratio), the fraction of non-potential (current-carrying) helicity, $|H_{mathrm{J}}|/|{H_{V}}|$ (helicity ratio), and the normalized current-carrying helicity, $|H_{mathrm{J}}|/{phi^{prime}}^{2}$. The total energy and helicity budgets of flare-productive ARs (extensive parameters) cover a broad range of magnitudes, with no obvious relation to the eruptive potential of the individual ARs, i.e., whether or not a CME is produced in association with the flare. The intensive eruptivity proxies, $E_{mathrm{F}}/{E}$ and $|H_{mathrm{J}}|/|{H_{V}}|$, and $|H_{mathrm{J}}|/{phi^{prime}}^{2}$, however, seem to be distinctly different for ARs that produced CME-associated large flares compared to those which produced confined flares. For the majority of ARs in our sample, we are able to identify characteristic pre-flare magnitudes of the intensive quantities, clearly associated to subsequent CME-productivity.
Solar flares are sudden and violent releases of magnetic energy in the solar atmosphere that can be divided in eruptive flares, when plasma is ejected from the solar atmosphere, resulting in a coronal mass ejection (CME), and confined flares when no CME is associated with the flare. We present a case-study showing the evolution of key topological structures, such as spines and fans which may determine the eruptive versus non-eruptive behavior of the series of eruptive flares, followed by confined flares, which are all originating from the same site. To study the connectivity of the different flux domains and their evolution, we compute a potential magnetic field model of the active region. Quasi-separatrix layers are retrieved from the magnetic field extrapolation. The change of behavior of the flares from one day to the next -eruptive to confined- can be attributed to the change of orientation of the magnetic field below the fan with respect to the orientation of the overlaying spine, rather than an overall change in the stability of the large scale field. Flares tend to be more-and-more confined when the field that supports the filament and the overlying field gradually become less-and-less anti-parallel, as a direct result of changes in the photospheric flux distribution, being themselves driven by continuous shearing motions of the different magnetic flux concentrations.
The nature of three-dimensional reconnection when a twisted flux tube erupts during an eruptive flare or coronal mass ejection is considered. The reconnection has two phases: first of all, 3D zipper reconnection propagates along the initial coronal arcade, parallel to the polarity inversion line (PIL), then subsequent quasi-2D main phase reconnection in the low corona around a flux rope during its eruption produces coronal loops and chromospheric ribbons that propagate away from the PIL in a direction normal to it. One scenario starts with a sheared arcade: the zipper reconnection creates a twisted flux rope of roughly one turn ($2pi$ radians of twist), and then main phase reconnection builds up the bulk of the erupting flux rope with a relatively uniform twist of a few turns. A second scenario starts with a pre-existing flux rope under the arcade. Here the zipper phase can create a core with many turns that depend on the ratio of the magnetic fluxes in the newly formed flare ribbons and the new flux rope. Main phase reconnection then adds a layer of roughly uniform twist to the twisted central core. Both phases and scenarios are modeled in a simple way that assumes the initial magnetic flux is fragmented along the PIL. The model uses conservation of magnetic helicity and flux, together with equipartition of magnetic helicity, to deduce the twist of the erupting flux rope in terms the geometry of the initial configuration. Interplanetary observations show some flux ropes have a fairly uniform twist, which could be produced when the zipper phase and any pre-existing flux rope possess small or moderate twist (up to one or two turns). Other interplanetary flux ropes have highly twisted cores (up to five turns), which could be produced when there is a pre-existing flux rope and an active zipper phase that creates substantial extra twist.
With the aim of understanding how the magnetic properties of active regions (ARs) control the eruptive character of solar flares, we analyze 719 flares of Geostationary Operational Environmental Satellite (GOES) class $geq$C5.0 during 2010$-$2019. We carry out the first statistical study that investigates the flare-coronal mass ejections (CMEs) association rate as function of the flare intensity and the AR characteristics that produces the flare, in terms of its total unsigned magnetic flux ($Phi$$_{AR}$). Our results show that the slope of the flare-CME association rate with flare intensity reveals a steep monotonic decrease with $Phi$$_{AR}$. This means that flares of the same GOES class but originating from an AR of larger $Phi$$_{AR}$, are much more likely confined. Based on an AR flux as high as 1.0$times$$10^{24}$ Mx for solar-type stars, we estimate that the CME association rate in X100-class ``superflares is no more than 50%. For a sample of 132 flares $geq$M2.0 class, we measure three non-potential parameters including the length of steep gradient polarity inversion line (L$_{SGPIL}$), the total photospheric free magnetic energy (E$_{free}$) and the area with large shear angle (A$_{Psi}$). We find that confined flares tend to have larger values of L$_{SGPIL}$, E$_{free}$ and A$_{Psi}$ compared to eruptive flares. Each non-potential parameter shows a moderate positive correlation with $Phi$$_{AR}$. Our results imply that $Phi$$_{AR}$ is a decisive quantity describing the eruptive character of a flare, as it provides a global parameter relating to the strength of the background field confinement.
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