Case Studies of Photospheric Magnetic Field Properties of Active Regions Associated with X-class Solar Flares

Solar flares are a transient phenomenon occurred in the active region (AR) on the Sun’s surface, producing intense emissions in EUV and soft X-ray that can wreak havoc in the near-Earth space mission and satellite as well as radio-based communication and navigation. The ARs are accompanied with strong magnetic fields and manifested as dark spots on the photosphere. To understand the photospheric magnetic field properties of the ARs that produce intense flares, two ARs associated with X-class flares, namely AR 12192 and AR 12297, occurred respectively on 25 October 2014 and 11 March 2015, are studied in terms of magnetic classification and various physical magnetic parameters. Solar images from the Langkawi National Observatory (LNO) and physical magnetic parameters from the Space-weather HMI Active Region Patches (SHARP) are used in this study. A total of seven SHARP magnetic parameters are examined which are calculated as sums of various magnetic quantities and have been identified as useful predictors for flare forecast. These two ARs are classified as βγδ sunspots whereas their formation and size are quite different from each other. Our results showed that the intensity of a flare has little relationship with the area of an AR and the magnetic free energy; and the temporal variation of individual magnetic parameter has no obvious and consistent pre-flare feature. It is concluded that the temporal variation of individual magnetic parameter may not be useful for predicting the onset time of a flare.

ASYMMETRY IN APPEARANCE OF THE LEADING AND FOLLOWING POLARITIES IN THE PHOTOSPHERIC MAGNETIC FIELD AT THE EARLY STAGE OF ACTIVE REGION FORMATION

We study the evolution of the photospheric magnetic field at the early stage of active region development. We use data on longitudinal component of the magnetic field and line-of-sight velocities from SOHO/MDI and SDO/HMI. The visual inspection of 48 cases of birth of active regions and the detailed analysis of the magnetic flux dynamics in 4 active regions have shown that at the time of emergence of a new magnetic field, the field of the following polarity is the first to be detected in the photosphere. The flux asymmetry of the leading and following polarities persists for several tens of minutes. The observed asymmetry of magnetic fluxes supports the results of the numerical simulation of emergence of the active region magnetic field in the upper layers of the convective zone, which has been carried out by Rempel and Cheung [2014].

ASYMMETRY IN APPEARANCE OF THE LEADING AND FOLLOWING POLARITIES IN THE PHOTOSPHERIC MAGNETIC FIELD AT THE EARLY STAGE OF ACTIVE REGION FORMATION

We study the evolution of the photospheric magnetic field at the early stage of active region development. We use data on longitudinal component of the magnetic field and line-of-sight velocities from SOHO/MDI and SDO/HMI. The visual inspection of 48 cases of birth of active regions and the detailed analysis of the magnetic flux dynamics in 4 active regions have shown that at the time of emergence of a new magnetic field, the field of the following polarity is the first to be detected in the photosphere. The flux asymmetry of the leading and following polarities persists for several tens of minutes. The observed asymmetry of magnetic fluxes supports the results of the numerical simulation of emergence of the active region magnetic field in the upper layers of the convective zone, which has been carried out by Rempel and Cheung [2014].

Non-LTE inversions of a confined X2.2 flare

Context. Obtaining an accurate measurement of magnetic field vector in the solar atmosphere is essential for studying changes in field topology during flares and reliably modelling space weather. Aims. We tackle this problem by applying various inversion methods to a confined X2.2 flare that occurred in NOAA AR 12673 on 6 September 2017 and comparing the photospheric and chromospheric magnetic field vector with the results of two numerical models of this event. Methods. We obtained the photospheric magnetic field from Milne-Eddington and (non-)local thermal equilibrium (non-LTE) inversions of Hinode SOT/SP Fe I 6301.5 Å and 6302.5 Å. The chromospheric field was obtained from a spatially regularised weak-field approximation (WFA) and non-LTE inversions of Ca II 8542 Å observed with CRISP at the Swedish 1 m Solar Telescope. We investigated the field strengths and photosphere-to-chromosphere shear in the field vector. Results. The LTE- and non-LTE-inferred photospheric magnetic field components are strongly correlated across several optical depths in the atmosphere, with a tendency towards a stronger field and higher temperatures in the non-LTE inversions. For the chromospheric field, the non-LTE inversions correlate well with the spatially regularised WFA, especially in terms of the line-of-sight field strength and field vector orientation. The photosphere exhibits coherent strong-field patches of over 4.5 kG, co-located with similar concentrations exceeding 3 kG in the chromosphere. The obtained field strengths are up to two to three times higher than in the numerical models, while the photosphere-to-chromosphere shear close to the polarity inversion line is more concentrated and structured. Conclusions. In the photosphere, the assumption of LTE for Fe I line formation does not yield significantly different magnetic field results in comparison to the non-LTE case, while Milne-Eddington inversions fail to reproduce the magnetic field vector orientation where Fe I is in emission. In the chromosphere, the non-LTE-inferred field is excellently approximated by the spatially regularised WFA. Our inversions confirm the locations of flux rope footpoints that have been predicted by numerical models. However, pre-processing and lower spatial resolution lead to weaker and smoother field in the models than what our data indicate. This highlights the need for higher spatial resolution in the models to better constrain pre-eruptive flux ropes.

Preprocessing of vector magnetograms for magnetohydrostatic extrapolations

Context. Understanding the 3D magnetic field as well as the plasma in the chromosphere and transition region is important. One way is to extrapolate the magnetic field and plasma from the routinely measured vector magnetogram on the photosphere based on the assumption of the magnetohydrostatic (MHS) state. However, photospheric data may be inconsistent with the MHS assumption. Therefore, we must study the restriction on the photospheric magnetic field, which is required by the MHS system. Moreover, the data should be transformed accordingly before MHS extrapolations can be applied. Aims. We aim to obtain a set of surface integrals as criteria for the MHS system and use this set of integrals to preprocess a vector magnetogram. Methods. By applying Gauss’ theorem and assuming an isolated active region on the Sun, we related the magnetic energy and forces in the volume to the surface integral on the photosphere. The same method was applied to obtain restrictions on the photospheric magnetic field as necessary criteria for a MHS system. We used an optimization method to preprocess the data to minimize the deviation from the criteria as well as the measured value. Results. By applying the virial theorem to the active region, we find the boundary integral that is used to compute the energy of a force-free field usually underestimates the magnetic energy of a large active region. We also find that the MHS assumption only requires the x-, y-component of net Lorentz force and the z-component of net torque to be zero. These zero components are part of Aly’s criteria for a force-free field. However, other components of net force and torque can be non-zero values. According to new criteria, we preprocess the magnetogram to make it more consistent with the MHS system and, at the same time close, to the original data.