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Register a new userWhat is the relationship between photospheric flow fields and solar flares?
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We estimated photospheric velocities by separately applying the Fourier Local Correlation Tracking (FLCT) and Differential Affine Velocity Estimator (DAVE) methods to 2708 co-registered pairs of SOHO/MDI magnetograms, with nominal 96-minute cadence and ~2 pixels, from 46 active regions (ARs) from 1996-1998 over the time interval t45 when each AR was within 45^o of disk center. For each magnetogram pair, we computed the average estimated radial magnetic field, B; and each tracking method produced an independently estimated flow field, u. We then quantitatively characterized these magnetic and flow fields by computing several extensive and intensive properties of each; extensive properties scale with AR size, while intensive properties do not depend directly on AR size. Intensive flow properties included moments of speeds, horizontal divergences, and radial curls; extensive flow properties included sums of these properties over each AR, and a crude proxy for the ideal Poynting flux, the total |u| B^2. Several magnetic quantities were also computed, including: total unsigned flux; a measure of the amount of unsigned flux near strong-field polarity inversion lines, R; and the total B^2. Next, using correlation and discriminant analysis, we investigated the associations between these properties and flares from the GOES flare catalog, when averaged over both t45 and shorter time windows, of 6 and 24 hours. We found R and total |u| B^2 to be most strongly associated with flares; no intensive flow properties were strongly associated with flares.
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Continuum emission, also called white-light emission (WLE), and permanent changes of the magnetic field ($Delta{B}_{{rm{LOS}}}$) are often observed during solar flares. But their relation and their precise mechanisms are still unknown. We study statistically the relationship between $Delta{B}_{{rm{LOS}}}$ and WLE during 75 solar flares of different strengths and locations on the solar disk. We analyze SDO/HMI data and determine for each pixel in each flare if it exhibited WLE and/or $Delta{B}_{{rm{LOS}}}$. We then investigate the occurrence, strength, and spatial size of the WLE, its dependence on flare energy, and its correlation to the occurrence of $Delta{B}_{{rm{LOS}}}$. We detected WLE in 44/75 flares and $Delta{B}_{{rm{LOS}}}$ in 59/75 flares. We find that WLE and $Delta{B}_{{rm{LOS}}}$ are related, and their locations often overlap between 0-60%. Not all locations coincide, thus potentially indicating differences in their origin. We find that the WL area is related to the flare class by a power law and extend the findings of previous studies, that the WLE is related to the flare class by a power law, to also be valid for C-class flares. To compare unresolved (Sun-as-a-star) WL measurements to our data, we derive a method to calculate temperatures and areas of such data under the black-body assumption. The calculated unresolved WLE areas improve, but still differ to the resolved flaring area by about a factor of 5-10 (previously 10-20), which could be explained by various physical or instrumental causes. This method could also be applied to stellar flares to determine their temperatures and areas independently.
We have analyzed multi-wavelength observations and magnetic-field data for the solar flare of May 10, 2012 (04:18 UT) and have detected a sign inversion of the signal in the line-of-sight magnetic measurements in the umbra of a small spot. This effect is associated, at least partly, with the emergence of a new magnetic field. Almost at the same time, a burst of hard X-rays was recorded, and a wave in the vacuum ultraviolet (EUV) range (a sunquake) was generated due to the impact of the disturbance in the energy release range on the photosphere. At the beginning of the event, a sigmoid flare was recorded, but it did not spread, as it usually does, along the polarity inversion (neutral) line. SDO/HMI full-vector measurements were used to extrapolate the AR 11476 magnetic field to the corona, and the distribution of vertical currents $j_z$ in the photosphere was obtained. The distribution of currents in the active region shows that the relationship between them and the occurrence of flares is very intricate. We have corroborated that the expected ideal behavior of the current system before and after the flare (e.g., see (Sharykin and Kosovichev, 2015)) is observed only in the sigmoid region. The results obtained were compared with the observations of two other flares recorded in this AR on the same day, one of which was similar to the flare under discussion and the other was of different type. Our results confirm that the formation and eruption of large-scale magnetic flux ropes in sigmoid flares are associated with the shear motions in the photosphere and the emergence of twisted magnetic tubes, as well as with the subsequent development of the torus instability.
Many previous studies have shown that magnetic fields as well as sunspot structures present rapid and irreversible changes associated with solar flares. In this paper we first use five X-class flares observed by SDO/HMI to show that not only the magnetic fields and sunspot structures do show rapid, irreversible changes but also these changes are closely related, both spatially and temporally. The magnitudes of the correlation coefficients between the temporal variations of horizontal magnetic field and sunspot intensity are all larger than 0.90, with a maximum value of 0.99 and an average value of 0.96. Then using four active regions in quiescent times, three observed and one simulated, we show that in sunspot penumbra regions there also exists a close correlation between sunspot intensity and horizontal magnetic field strength, in addition to the well-known one between sunspot intensity and normal magnetic field strength. Connecting these two observational phenomena, we show that the sunspot structure change and the magnetic field change are the two facets of the same phenomena of solar flares, one change might be induced by the change of the other due to a linear correlation between sunspot intensity and magnetic field strength out of a local force balance.
We report on the spatial relationship between solar flares and coronal mass ejections (CMEs) observed during 1996-2005 inclusive. We identified 496 flare-CME pairs considering limb flares (distance from central meridian > 45 deg) with soft X-ray flare size > C3 level. The CMEs were detected by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). We investigated the flare positions with respect to the CME span for the events with X-class, M-class, and C-class flares separately. It is found that the most frequent flare site is at the center of the CME span for all the three classes, but that frequency is different for the different classes. Many X-class flares often lie at the center of the associated CME, while C-class flares widely spread to the outside of the CME span. The former is different from previous studies, which concluded that no preferred flare site exists. We compared our result with the previous studies and conclude that the long-term LASCO observation enabled us to obtain the detailed spatial relation between flares and CMEs. Our finding calls for a closer flare-CME relationship and supports eruption models typified by the CSHKP magnetic reconnection model.
There are still debates whether particle acceleration in solar flares may occur due to interruption of electric currents flowing along magnetic loops. To contribute to this problem, we performed the first statistical study of relationships between flare hard X-ray (HXR; $50-100$ keV) sources observed by the textit{Ramaty High-Energy Solar Spectroscopic Imager} (RHESSI) and photospheric vertical electric currents (PVECs, $j_{r}$) calculated using vector magnetograms obtained with the Helioseismic and Magnetic Imager (HMI) on-board the textit{Solar Dynamics Observatory} (SDO). A sample of 48 flares, from C3.0 to X3.1 class, observed in central part of the solar disk by both instruments in 2010--2015 was analyzed. We found that $approx 70$% of all HXR sources overlapped with islands or ribbons of enhanced ($left| j_{r} right| gtrsim 10^{4}$ statampere~cm$^{-2}$) PVECs. However, less than $approx 40$% of the HXR sources overlapped with PVEC maxima, with an accuracy of $pm 3^{primeprime}$. More than in half of the flares there were HXR sources outside regions of enhanced PVECs. We found no correlation between intensity of the HXR sources and PVEC density or total PVEC under them. No systematic dissipation of PVECs under the HXR sources was found during the flares. Collectively, the results do not support the current-interruption flare models. However, the results indicate the importance of the presence of longitudinal currents in flare regions. Understanding of their specific role in the processes of energy release, plasma heating, and acceleration of particles requires further investigation.
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