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Register a new userMagnetic Properties of Solar Active Regions that Govern Large Solar Flares and Eruptions
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Solar flares and coronal mass ejections (CMEs), especially the larger ones, emanate from active regions (ARs). With the aim to understand the magnetic properties that govern such flares and eruptions, we systematically survey all flare events with GOES levels of >=M5.0 within 45 deg from disk center between May 2010 and April 2016. These criteria lead to a total of 51 flares from 29 ARs, for which we analyze the observational data obtained by the Solar Dynamics Observatory. More than 80% of the 29 ARs are found to exhibit delta-sunspots and at least three ARs violate Hales polarity rule. The flare durations are approximately proportional to the distance between the two flare ribbons, to the total magnetic flux inside the ribbons, and to the ribbon area. From our study, one of the parameters that clearly determine whether a given flare event is CME-eruptive or not is the ribbon area normalized by the sunspot area, which may indicate that the structural relationship between the flaring region and the entire AR controls CME productivity. AR characterization show that even X-class events do not require delta-sunspots or strong-field, high-gradient polarity inversion lines. An investigation of historical observational data suggests the possibility that the largest solar ARs, with magnetic flux of 2x10^23 Mx, might be able to produce superflares with energies of order of 10^34 erg. The proportionality between the flare durations and magnetic energies is consistent with stellar flare observations, suggesting a common physical background for solar and stellar flares.
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We compare the coronal magnetic energy and helicity of two solar active regions (ARs), prolific in major eruptive (AR~11158) and confined (AR~12192) flaring, and analyze the potential of deduced proxies to forecast upcoming flares. Based on nonlinear force-free (NLFF) coronal magnetic field models with a high degree of solenoidality, and applying three different computational methods to investigate the coronal magnetic helicity, we are able to draw conclusions with a high level of confidence. Based on real observations of two solar ARs we checked trends regarding the potential eruptivity of the active-region corona, as suggested earlier in works that were based on numerical simulations, or solar observations. Our results support that the ratio of current-carrying to total helicity, $|H_mathrm{J}|/|H_mathrm{V}|$, shows a strong ability to indicate the eruptive potential of a solar AR. However, $|H_mathrm{J}|/|H_mathrm{V}|$ seems not to be indicative for the magnitude or type of an upcoming flare (confined or eruptive). Interpreted in context with earlier observational studies, our findings furthermore support that the total relative helicity normalized to the magnetic flux at the NLFF models lower boundary, $H_mathrm{V}/phi^2$, represents no indicator for the eruptivity.
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.
Flares and eruptions from solar active regions are associated with atmospheric electrical currents accompanying distortions of the coronal field away from a lowest-energy potential state. In order to better understand the origin of these currents and their role in M- and X-class flares, I review all active-region observations made with SDO/HMI and SDO/AIA from 2010/05 through 2014/10 within approximately 40 degrees from disk center. I select the roughly 4% of all regions that display a distinctly nonpotential coronal configuration in loops with a length comparable to the scale of the active region, and all that emit GOES X-class flares. The data for 41 regions confirm, with a single exception, that strong-field, high-gradient polarity inversion lines (SHILs) created during emergence of magnetic flux into, and related displacement within, pre-existing active regions are associated with X-class flares. Obvious nonpotentiality in the active region-scale loops occurs in 6 of 10 selected regions with X-class flares, all with relatively long SHILs along their primary polarity inversion line, or with a long internal filament there. Nonpotentiality can exist in active regions well past the flux-emergence phase, often with reduced or absent flaring. I conclude that the dynamics of the flux involved in the compact SHILs is of preeminent importance for the large-flare potential of active regions within the next day, but that their associated currents may not reveal themselves in active region-scale nonpotentiality. In contrast, active region-scale nonpotentiality, which can persist for many days, may inform us about the eruption potential other than those from SHILs which is almost never associated with X-class flaring.
Dynamical changes in the solar corona have proven to be very important in inducing seismic waves into the photosphere. Different mechanisms for their generation have been proposed. In this work, we explore the magnetic field forces as plausible mechanisms to generate sunquakes as proposed by Hudson, Fisher and Welsch. We present a spatial and temporal analysis of the line-of-sight magnetic field variations induced by the seismically active 2003 October 29 and 2005 January 15 solar flares and compare these results with other supporting observations.
We demonstrate that the current helicity observed in solar active regions traces the magnetic helicity of the large-scale dynamo generated field. We use an advanced 2D mean-field dynamo model with dynamo saturation based on the evolution of the magnetic helicity and algebraic quenching. For comparison, we also studied a more basic 2D mean-field dynamo model with simple algebraic alpha quenching only. Using these numerical models we obtained butterfly diagrams both for the small-scale current helicity and also for the large-scale magnetic helicity, and compared them with the butterfly diagram for the current helicity in active regions obtained from observations. This comparison shows that the current helicity of active regions, as estimated by $-{bf A cdot B}$ evaluated at the depth from which the active region arises, resembles the observational data much better than the small-scale current helicity calculated directly from the helicity evolution equation. Here ${bf B}$ and ${bf A}$ are respectively the dynamo generated mean magnetic field and its vector potential. A theoretical interpretation of these results is given.
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