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Register a new userThe Statistical Relationship between White-light Emission and Photospheric Magnetic Field Changes in Flares
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Added by
J. S. Castellanos Dur\\'an
Publication date
2020
fields
Physics
and research's language is
English
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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.
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Abrupt and permanent changes of photospheric magnetic fields have been observed during solar flares. The changes seem to be linked to the reconfiguration of magnetic fields, but their origin is still unclear. We carried out a statistical analysis of permanent line-of-sight magnetic field ($B_{rm LOS}$) changes during 18 X-, 37 M-, 19 C- and 1 B-class flares using data from Solar Dynamics Observatory/Helioseismic and Magnetic Imager. We investigated the properties of permanent changes, such as frequency, areas, and locations. We detected changes of $B_{rm LOS}$ in 59/75 flares. We find that strong flares are more likely to show changes, with all flares $ge$ M1.6 exhibiting them. For weaker flares, permanent changes are observed in 6/17 C-flares. 34.3% of the permanent changes occurred in the penumbra and 18.9% in the umbra. Parts of the penumbra appeared or disappeared in 23/75 flares. The area where permanent changes occur is larger for stronger flares. Strong flares also show a larger change of flux, but there is no dependence of the magnetic flux change on the heliocentric angle. The mean rate of change of flare-related magnetic field changes is 20.7 Mx cm$^{-2}$ min$^{-1}$. The number of permanent changes decays exponentially with distance from the polarity inversion line. The frequency of the strength of permanent changes decreases exponentially, and permanent changes up to 750 Mx cm$^{-2}$ were observed. We conclude that permanent magnetic field changes are a common phenomenon during flares, and future studies will clarify their relation to accelerated electrons, white light emission, and sunquakes to further investigate their origin.
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.
The observations of magnetic field variations as a signature of flaring activity is one of the main goal in solar physics. Some efforts in the past give apparently no unambiguous observations of changes. We observed that the scaling laws of the current helicity inside a given flaring active region change clearly and abruptly in correspondence with the eruption of big flares at the top of that active region. Comparison with numerical simulations of MHD equations, indicates that the change of scaling behavior in the current helicity, seems to be associated to a topological reorganization of the footpoint of the magnetic field loop, namely to dissipation of small scales structures in turbulence. It is evident that the possibility of forecasting in real time high energy flares, even if partially, has a wide practical interest to prevent the effects of big flares on Earth and its environment.
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.
Using observations by the Solar Dynamics Observatory from June 2010 to December 2017, we have performed the first statistical investigation of circular-ribbon flares (CFs) and examined the white-light emission in these CFs. We find 90 CFs occurring in 36 active regions (ARs), including 8 X-class, 34 M-class, 48 C- and B-class flares. The occurrence rate of white-light flares (WLFs) is 100% (8/8) for X-class CFs, $sim$62% (21/34) for M-class CFs, and $sim$8% (4/48) for C- and B-class CFs. Sometimes we observe several CFs in a single AR, and nearly all of them are WLFs. Compared to normal CFs, CFs with white-light enhancement tend to have a shorter duration, smaller size, stronger electric current and more complicated magnetic field. We find that for X-class WLFs, the white-light enhancement is positively correlated with the flare class, implying that the white-light enhancement is largely determined by the amount of released energy. However, there is no such correlation for M- and C-class WLFs, suggesting that other factors such as the time scale, spatial scale and magnetic field complexity may play important roles in the generation of white-light emission if the released energy is not high enough.
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