Keywords

The Solar Polar-orbit Observatory (SPO), proposed by Chinese scientists, is designed to observe the solar polar regions in an unprecedented way with a spacecraft traveling in a large solar inclination angle and a small ellipticity. However, one of the most significant challenges lies in ultra-long-distance data transmission, particularly for the Magnetic and Helioseismic Imager (MHI), which is the most important payload and generates the largest volume of data in SPO. In this paper, we propose a tailored lossless data compression method based on the measurement mode and characteristics of MHI data. The background out of the solar disk is removed to decrease the pixel number of an image under compression. Multiple predictive coding methods are combined to eliminate the redundancy utilizing the correlation (space, spectrum, and polarization) in data set, improving the compression ratio. Experimental results demonstrate that our method achieves an average compression ratio of 3.67. The compression time is also less than the general observation period. The method exhibits strong feasibility and can be easily adapted to MHI.

Using the data on magnetic field maps and continuum intensity for Solar Cycles 23 and 24, we explored 100 active regions (ARs) that produced M5.0 or stronger flares. We focus on the presence/absence of the emergence of magnetic flux in these ARs 2–3 days before the strong flare onset. We found that 29 ARs in the sample emerged monotonically amidst quiet-Sun. A major emergence of a new magnetic flux within a pre-existing AR yielding the formation of a complex flare-productive configuration was observed in another 24 cases. For 30 ARs, an insignificant (in terms of the total magnetic flux of pre-existing AR) emergence of a new magnetic flux within the pre-existing magnetic configuration was observed; for some of them the emergence resulted in a formation of a configuration with a small δ-sunspot; 11 out of 100 ARs exhibited no signatures of magnetic flux emergence during the entire interval of observation. In six cases the emergence was in progress when the AR appeared on the Eastern limb, so that the classification and timing of emergence were not possible. We conclude that the recent flux emergence is not a necessary and/or sufficient condition for strong flaring of an AR. The flux emergence rate of flare-productive ARs analyzed here was compared with that of flare-quiet ARs analyzed in our previous studies. We revealed that the flare-productive ARs tend to display faster emergence than the flare-quiet ones do.

The solar flare is one of the most violent explosions, and can disturb the near-Earth space weather. Except for commonly single-peaked solar flares in soft X-ray, some special flares show intriguing a two-peak feature that is deserved much more attentions. Here, we reported a confined two-peaked solar flare and analyzed the associated eruptions using high-quality observations from Educational Adaptive-optics Solar Telescope and Solar Dynamics Observatory. Before the flare, a magnetic flux rope (MFR) formed through partially tether-cutting reconnection between two sheared arches. The flare occurred after the MFR eruption that was confined by the overlying strong field. Interestingly, a small underlying filament immediately erupted, which was possibly destabilized by the flare ribbon. The successive eruptions were confirmed by the analysis of the emission measure and the reconnection fluxes. Therefore, we suggest that the two peaks of the confined solar flare are corresponding to two episodes of magnetic reconnection during the successive eruptions of the MFR and the underlying filament.

Coronal magnetic fields evolve quasi-statically over long timescales and dynamically over short timescales. As of now there exist no regular measurements of coronal magnetic fields, and therefore generating the coronal magnetic field evolution using observations of the magnetic field at the photosphere is a fundamental requirement to understanding the origin of transient phenomena from solar active regions (ARs). Using the magneto-friction (MF) approach, we aim to simulate the coronal field evolution in the solar AR 11429. The MF method is implemented in the open source Pencil Code along with a driver module to drive the initial field with different boundary conditions prescribed from observed vector magnetic fields at the photosphere. In order to work with vector potential and the observations, we prescribe three types of bottom boundary drivers with varying free-magnetic energy. The MF simulation reproduces the magnetic structure, which better matches the sigmoidal morphology exhibited by Atmospheric Imaging Assembly (AIA) images at the pre-eruptive time. We found that the already sheared field further driven by the sheared magnetic field will maintain and further build the highly sheared coronal magnetic configuration, as seen in AR 11429. Data-driven MF simulation is a viable tool to generate the coronal magnetic field evolution, capturing the formation of the twisted flux rope and its eruption.

In this present study, we have analyzed different types of X-ray solar flares (C, M, and X classes) coming out from different classes of sunspot groups (SSGs). The data which we have taken under this study cover the duration of 24 yr from 1996 to 2019. During this, we observed a total of 15015 flares (8417 in SC-23 and 6598 in SC-24) emitted from a total of 33780 active regions (21746 in SC-23 and 12034 in SC-24) with sunspot only. We defined the flaring potential or flare-production potential as the ratio of the total number of flares produced from a particular type of SSG to the total number of the same-class SSGs observed on the solar surface. Here we studied yearly changes in the flaring potential of different McIntosh class groups of sunspots in different phases of SC-23 and 24. In addition, we investigated yearly variations in the potential of producing flares by different SSGs (A, B, C, D, E, F, and H) during different phases (ascending, maximum, descending, and minimum) of SC-23 and 24. These are our findings: (1) D, E, and F SSGs have the potential of producing flares ≥8 times greater than A, B, C and H SSGs; (2) The larger and more complex D, E, and F SSGs produced nearly 80% of flares in SC-23 and 24; (3) The A, B, C and H SSGs, which are smaller and simpler, produced only 20% of flares in SC-23 and 24; (4) The biggest and most complex SSGs of F-class have flaring potential 1.996 and 3.443 per SSG in SC-23 and 24, respectively. (5) The potential for producing flares in each SSG is higher in SC-24 than in SC-23, although SC-24 is a weaker cycle than SC-23. (6) The alterations in the number of flares (C+M+X) show different time profiles than the alterations in sunspot numbers during SC-23 and 24, with several peaks. (7) The SSGs of C, D, E, and H-class have the highest flaring potential in the descending phase of both SC-23 and 24. (8) F-class SSGs have the highest flaring potential in the descending phase of SC-23 but also in the maximum phase of SC-24.

Accurate measurement of magnetic fields is very important for understanding the formation and evolution of solar magnetic fields. Currently, there are two types of solar magnetic field measurement instruments: filter-based magnetographs and Stokes polarimeters. The former gives high temporal resolution magnetograms and the latter provides more accurate measurements of magnetic fields. Calibrating the magnetograms obtained by filter-based magnetographs with those obtained by Stokes polarimeters is a good way to combine the advantages of the two types. Our previous studies have shown that, compared to the magnetograms obtained by the Spectro-Polarimeter (SP) on board Hinode, those magnetograms obtained by both the filter-based Solar Magnetic Field Telescope (SMFT) of the Huairou Solar Observing Station and by the filter-based Michelson Doppler Imager (MDI) aboard SOHO have underestimated the flux densities in their magnetograms and systematic center-to-limb variations present in the magnetograms of both instruments. Here, using a sample of 75 vector magnetograms of stable alpha sunspots, we compare the vector magnetograms obtained by the Helioseismic and Magnetic Imager (HMI) aboard Solar Dynamics Observatory (SDO) with co-temporal vector magnetograms acquired by SP/Hinode. Our analysis shows that both the longitudinal and transverse flux densities in the HMI/SDO magnetograms are very close to those in the SP/Hinode magnetograms and the systematic center-to-limb variations in the HMI/SDO magnetograms are very minor. Our study suggests that using a filter-based magnetograph to construct a low spectral resolution Stokes profile, as done by HMI/SDO, can largely remove the disadvantages of the filter-type measurements and yet still possess the advantage of high temporal resolution.

Magnetic helicity is an important concept in solar physics, with a number of theoretical statements pointing out the important role of magnetic helicity in solar flares and coronal mass ejections (CMEs). Here we construct a sample of 47 solar flares, which contains 18 no-CME-associated confined flares and 29 CME-associated eruptive flares. We calculate the change ratios of magnetic helicity and magnetic free energy before and after these 47 flares. Our calculations show that the change ratios of magnetic helicity and magnetic free energy show distinct different distributions in confined flares and eruptive flares. The median value of the change ratios of magnetic helicity in confined flares is −0.8%, while this number is −14.5% for eruptive flares. For the magnetic free energy, the median value of the change ratios is −4.3% for confined flares, whereas this number is −14.6% for eruptive flares. This statistical result, using observational data, is well consistent with the theoretical understandings that magnetic helicity is approximately conserved in the magnetic reconnection, as shown by confined flares, and the CMEs take away magnetic helicity from the corona, as shown by eruptive flares.

Solar flares and coronal mass ejections (CMEs) are thought to be the most powerful events on the Sun. They can release energy as high as ∼10³² erg in tens of minutes, and also can release solar energetic particles (SEPs) into interplanetary space. We explore global energy budgets of solar major eruptions that occurred on 2017 September 6, including the energy partition of a powerful solar flare, and the energy budget of the accompanying CME and SEPs. In the wavelength range shortward of ∼222 nm, a major contribution of the flare radiated energy is in the soft X-ray (SXR) 0.1–7 nm domain. The flare energy radiated at wavelengths of Lyα and mid-ultraviolet is larger than that radiated in the extreme ultraviolet wavelengths, but it is much less than that radiated in the SXR waveband. The total flare radiated energy could be comparable to the thermal and nonthermal energies. The energies carried by the major flare and its accompanying CME are roughly equal, and they are both powered by the magnetic free energy in the NOAA AR 12673. Moreover, the CME is efficient in accelerating SEPs, and the prompt component (whether it comes from the solar flare or the CME) contributes only a negligible fraction.

Following our previous work, we studied the partial eruption of a large-scale horse-shoe-like filament that had been observed in a decaying active region on the solar disk for more than 4.5 days. The filament became active after it was broken into two pieces, P1 and P2 seen in Hα, by magnetic reconnection between the magnetic field around it and that of a newly emerging active region nearby. P1 eventually erupted 13 hr after the breaking and escaped from the Sun, developing to a fast coronal mass ejection, and P2 stayed. But the mass in P1 falling down to P2 in the eruption suggests that the global magnetic fields over P1 and P2 were still connected to each other prior to the eruption. The reconnection process breaking the filament occurred outside the filament, and P1 and P2 were located almost at the same altitude, so the fashion of the filament partial eruption studied here differs from that of the "double-decker model" and that of reconnection inside the filament. Analyzing the decay indices of the background fields above P1 and P2, n₁ and n₂, showed that the altitude where n₁ exceeds the critical value of n_c = 1.5 for the loss of equilibrium or the torus instability is lower than that where n₂ > n_c, and that n₁ > n₂ always holds at all altitudes. Combining this fact with that the eruption occurred 13 hr after filament was broken by reconnection, we conclude that the eruption of P1 was triggered by the loss of equilibrium or the torus instability in the configuration, and magnetic reconnection breaking the filament helped weaken the confinement of the background field on P1, allowing P1 to erupt. Detailed features of the eruption and the corresponding physical scenario were also discussed.

Eruption of solar flares is a complex nonlinear process, and the rays and high-energy particles generated by such an eruption are detrimental to the reliability of space-based or ground-based systems. So far, there are not reliable physical models to accurately account for the flare outburst mechanism, but a lot of data-driven models have been built to study a solar flare and forecast it. In the paper, the status of solar-flare forecasting is reviewed, with emphasis on the machine learning methods and data-processing techniques used in the models. At first, the essential forecast factors strongly relevant to solar flare outbursts, such as classification information of the sunspots and evolution pattern of the magnetic field, are reviewed and analyzed. Subsequently, methods of resampling for data preprocessing are introduced to solve the problems of class imbalance in the solar flare samples. Afterwards, typical model structures adopted for flare forecasting are reviewed from the aspects of the single and fusion models, and the forecast performances of the different models are analyzed. Finally, we herein summarize the current research on solar flare forecasting and outline its development trends.