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Register a new userRoles of photospheric motions and flux emergence in the major solar eruption on 2017 September 6
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We study the magnetic field evolution in the active region (AR) 12673 that produced the largest solar flare in the past decade on 2017 September 6. Fast flux emergence is one of the most prominent features of this AR. We calculate the magnetic helicity from photospheric tangential flows that shear and braid field lines (shear-helicity), and from normal flows that advect twisted magnetic flux into the corona (emergence-helicity), respectively. Our results show that the emergence-helicity accumulated in the corona is $-1.6times10^{43}~Mx^2$ before the major eruption, while the shear-helicity accumulated in the corona is $-6times10^{43}~Mx^2$, which contributes about 79% of the total helicity. The shear-helicity flux is dominant throughout the overall investigated emergence phase. Our results imply that the emerged fields initially contain relatively low helicity. Much more helicity is built up by shearing and converging flows acting on preexisted and emerging flux. Shearing motions are getting stronger with the flux emergence, and especially on both sides of the polarity inversion line of the core field region. The evolution of the vertical currents shows that most of the intense currents do not appear initially with the emergence of the flux, which implies that most of the emerging flux is probably not strongly current-carrying. The helical magnetic fields (flux rope) in the core field region are probably formed by long-term photospheric motions. The shearing and converging motions are continuously generated driven by the flux emergence. AR 12673 is a representative as photospheric motions contribute most of the nonpotentiality in the AR with vigorous flux emergence.
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We report on the 2017 September 10 ground level enhancement (GLE) event associated with a coronal mass ejection (CME) whose initial acceleration (~9.1km s^-2) and initial speed (~4300 km/s) were among the highest observed in the SOHO era. The GLE event was of low intensity (~4.4% above background) and softer-than-average fluence spectrum. We suggest that poor connectivity (longitudinal and latitudinal) of the source to Earth compounded by the weaker ambient magnetic field contributed to these GLE properties. Events with similar high initial speed either lacked GLE association or had softer fluence spectra. The shock-formation height inferred from the metric type II burst was ~1.4 Rs, consistent with other GLE events. The shock height at solar particle release (SPR) was ~4.4+/-0.38 Rs, consistent with the parabolic relationship between the shock height at SPR and source longitude. At SPR, the eastern flank of the shock was observed in EUV projected on the disk near the longitudes magnetically connected to Earth: W60 to W45.
We study an evolving bipolar active region that exhibits flux cancellation at the internal polarity inversion line, the formation of a soft X-ray sigmoid along the inversion line and a coronal mass ejection. The evolution of the photospheric magnetic field is described and used to estimate how much flux is reconnected into the flux rope. About one third of the active region flux cancels at the internal polarity inversion line in the 2.5~days leading up to the eruption. In this period, the coronal structure evolves from a weakly to a highly sheared arcade and then to a sigmoid that crosses the inversion line in the inverse direction. These properties suggest that a flux rope has formed prior to the eruption. The amount of cancellation implies that up to 60% of the active region flux could be in the body of the flux rope. We point out that only part of the cancellation contributes to the flux in the rope if the arcade is only weakly sheared, as in the first part of the evolution. This reduces the estimated flux in the rope to $sim!30%$ or less of the active region flux. We suggest that the remaining discrepancy between our estimate and the limiting value of $sim!10%$ of the active region flux, obtained previously by the flux rope insertion method, results from the incomplete coherence of the flux rope, due to nonuniform cancellation along the polarity inversion line. A hot linear feature is observed in the active region which rises as part of the eruption and then likely traces out field lines close to the axis of the flux rope. The flux cancellation and changing magnetic connections at one end of this feature suggest that the flux rope reaches coherence by reconnection shortly before and early in the impulsive phase of the associated flare. The sigmoid is destroyed in the eruption but reforms within a few hours after a moderate amount of further cancellation has occurred.
In this multi-instrument paper, we search for evidence of sustained magnetic reconnection far beyond the impulsive phase of the X8.2-class solar flare on 2017 September 10. Using Hinode/EIS, CoMP, SDO/AIA, K-Cor, Hinode/XRT, RHESSI, and IRIS, we study the late-stage evolution of the flare dynamics and topology, comparing signatures of reconnection with those expected from the standard solar flare model. Examining previously unpublished EIS data, we present the evolution of non-thermal velocity and temperature within the famous plasma sheet structure, for the first four hours of the flares duration. On even longer time scales, we use Differential Emission Measures and polarization data to study the longevity of the flares plasma sheet and cusp structure, discovering that the plasma sheet is still visible in CoMP linear polarization observations on 2017 September 11, long after its last appearance in EUV. We deduce that magnetic reconnection of some form is still ongoing at this time - 27 hours after flare onset.
A recent study carried out on high sensitivity SUNRISE/IMAX data has reported about the existence of areas of limited flux emergence in the quiet Sun. By exploiting an independent and longer (4 hours) data set acquired by HINODE/SOT, we further investigate these regions by analysing their spatial distribution and relation with the supergranular flow. Our findings, while confirming the presence of these calm areas, also show that the rate of emergence of small magnetic elements is largely suppressed at the locations where the divergence of the supergranular plasma flows is positive. This means that the dead calm areas previously reported in literature are not randomly distributed over the solar photosphere but they are linked to the supergranular cells themselves. These results are discussed in the framework of the recent literature.
We studied 101 flux emergence events ranging from small ephemeral regions to large emerging flux regions which were observed with Hinode Solar Optical Telescope filtergram. We investigated how the total magnetic flux of the emergence event controls the nature of emergence. To determine the modes of emergences, horizontal velocity fields of global motion of the magnetic patches in the flux emerging sites were measured by the local correlation tracking. Between two main polarities of the large emerging flux regions with more than around 2 times 10^19 Mx, there were the converging flows of anti-polarity magnetic patches. On the other hand, small ephemeral regions showed no converging flow but simple diverging pattern. When we looked into the detailed features in the emerging sites, irrespective of the total flux and the spatial size, all the emergence events were observed to consist of single or multiple elementary emergence unit(s). The typical size of unitary emergence is 4 Mm and consistent with the simulation results. From the statistical study of the flux emergence events, the maximum spatial distance between two main polarities, the magnetic flux growth rate and the mean separation speed were found to follow the power-law functions of the total magnetic flux with the indices of 0.27, 0.57, and -0.16, respectively. From the discussion on the observed power-law relations, we got a physical view of solar flux emergence that emerging magnetic fields float and evolve balancing to the surrounding turbulent atmosphere. Key words: Sun: magnetic fields - Sun: emerging flux - Sun: photosphere - Sun: chromosphere
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