Evolution Of Solar Active Regions Research Articles

Solar active region evolution and imminent flaring activity through color-coded visualization of photospheric vector magnetograms

Context. The emergence of magnetic flux, its transition to complex configurations, and the pre-eruptive state of active regions are probed using photospheric magnetograms. Aims. Our aim is to pinpoint different evolutionary stages in emerging active regions, explore their differences, and produce parameters that could advance flare prediction using color-coded maps of the photospheric magnetic field. Methods. The three components of the photospheric magnetic field vector are combined to create color-combined magnetograms (COCOMAGs). From these, the areas occupied by different color hues are extracted, creating appropriate time series (color curves). These COCOMAGs and color curves are used as proxies of the active region evolution and its complexity. Results. The morphology of COCOMAGs showcases typical features of active regions, such as sunspots, plages, and sheared polarity inversion lines. The color curves represent the area occupied by photospheric magnetic field of different orientation and contain information pertaining to the evolutionary stages of active regions. During emergence, most of the region area is dominated by horizontal or highly inclined magnetic field, which is gradually replaced by more vertical magnetic field. In complex regions, large parts are covered by highly inclined magnetic fields, appearing as abrupt color changes in COCOMAGs. The decay of a region is signified by a domination of vertical magnetic field, indicating a gradual relaxation of the magnetic field configuration. The color curves exhibit a varying degree of correlation with active region complexity. Particularly the red and magenta color curves, which represent strong, purely horizontal magnetic field, are good indicators of future flaring activity. Conclusions. Color-combined magnetograms facilitate a comprehensive view of the evolution of active regions and their complexity. They offer a framework for the treatment of complex observations and can be used in pattern recognition, feature extraction, and flare-prediction schemes.

Light Bridges and Solar Active Region Evolution Processes

The formation mechanism of light bridges (LBs) is strongly related to the dynamic evolution of solar active regions (ARs). To study the relationship between LB formation and AR evolution phases, we employ 109 LB samples from 69 ARs in 2014 using observational data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. LBs are well matched with the weak field lanes (WFLs), except those aligned on the polarity inversion line of δ sunspots. For penumbral intrusion (type-A) and umbral-dot emergence (type-C) LBs, the WFLs represent the splitting of magnetic flux systems. The sunspots tend to decay and split into several parts after type-A and type-C LBs are formed. For sunspot/umbra-merging (type-B) LBs, the declining WFLs are caused by collisions of flux systems. The sunspots merged and remained stable after type-B LBs formed. We conclude that type-B LBs are formed by collisions of flux systems, while type-A and type-C LBs are generated by splits. The time differences (δ T) between LBs appearing and ARs peaking have an average value of 1.06, −1.60, and 1.82 days for type-A, B, and C LBs, with the standard deviations of 3.27, 2.17, and 1.89, respectively. A positive value of δ T means that the LB appears after the AR peaks, whereas a negative δ T means it appears before the peak. Type-A LBs tend to form in the decaying phase or around the peak time. Type-B LBs are more likely to be formed in the developing phase. Type-C LBs mostly take shape in the decaying phase of ARs.

MHD Simulation of a Solar Eruption from Active Region 11429 Driven by a Photospheric Velocity Field

Data-driven simulation is becoming an important approach for realistically characterizing the configuration and evolution of solar active regions, revealing the onset mechanism of solar eruption events, and hopefully achieving the goal of accurate space weather forecasting, which is beyond the scope of any existing theoretical modeling. Here we performed a full 3D MHD simulation using the data-driven approach and followed the whole evolution process from the quasi-static phase to eruption successfully for solar active region (AR) NOAA 11429. The MHD system was driven at the bottom boundary by a photospheric velocity field, which is derived by the DAVE4VM method from the observed vector magnetograms. The simulation shows that a magnetic flux rope was generated by a persistent photospheric flow before the flare onset and then triggered to erupt by torus instability. Our simulation demonstrates a high degree of consistency with observations in the preeruption magnetic structure, the timescale of the quasi-static stage, the pattern of flare ribbons, as well as the time evolution of the magnetic energy injection and total unsigned magnetic flux. We further found that an eruption can also be initiated in the simulation driven by only the horizontal components of the photospheric flow, but a comparison of the different simulations indicates that the vertical flow at the bottom boundary is necessary for reproducing more realistically these observed features, emphasizing the importance of flux emergence during the development of this AR.

Kronecker-structured covariance models for multiway data

Many applications produce multiway data of exceedingly high dimension. Modeling such multi-way data is important in multichannel signal and video processing where sensors produce multi-indexed data, e.g. over spatial, frequency, and temporal dimensions. We will address the challenges of covariance representation of multiway data and review some of the progress in statistical modeling of multiway covariance over the past two decades, focusing on tensor-valued covariance models and their inference. We will illustrate through a space weather application: predicting the evolution of solar active regions over time.

Evolution and motions of magnetic fragments during the active region formation and decay: A statistical study

Context. The evolution of solar active regions is still not fully understood. The growth and decay of active regions have mostly been studied in case-by-case studies. Aims. Instead of studying the evolution of active regions case by case, we performed a large-scale statistical study to find indications for the statistically most frequent scenario. Methods. We studied a large sample of active regions recorded by the Helioseismic and Magnetic Imager instrument. The sample was split into two groups: forming (367 members) and decaying (679 members) active regions. We tracked individual dark features (i.e. those that are assumed to be intensity counterparts of magnetised fragments from small objects to proper sunspots) and followed their evolution. We investigated the statistically most often locations of fragment merging and splitting as well as their properties. Results. Our results confirm that statistically, sunspots form by merging events of smaller fragments. The coalescence process is driven by turbulent diffusion in a process similar to random-walk, where supergranular flows seem to play an important role. The number of appearing fragments does not seem to significantly correlate with the number of sunspots formed. The formation seems to be consistent with the magnetic field accumulation. Statistically, the merging occurs most often between a large and a much smaller object. The decay of the active region seems to take place preferably by a process similar to the erosion.

Evolution of Solar Active Regions Before Large Flares: Overview of the Events of 2010–2017

The paper presents a study of the evolution of the photospheric magnetic field of active regions (ARs) of the Sun in which flares (larger than the M9 class in the Geostationary Operational Environmental Satellite (GOES) X-ray classification) occurred in 2010–2017. The purpose of this paper is to detect the precursors of flares. Thirteen ARs at a distance of not more than 45 degrees from the central meridian are selected for analysis out of 31 ARs in which flares larger than M9.0 were detected in the specified period. The magnetographic characteristics of the selected ARs are studied based on the Solar Dynamics Observatory (SDO) data. A flare index is proposed. It is calculated from the data of the Helioseismic and Magnetic Imager of SDO (SDO/HMI) and reflects the distance between regions of opposite magnetic polarity computed between the field boundaries by the threshold value. The analysis showed that a sharp increase in the flare index is detected in all the studied ARs 2–3 days before large flares. Flares larger than the M9.0 class occurred 5–20 h after the global or local maximum of the flare index. It is shown using the example of individual events that regions of the opposite polarity first converged and then relatively quickly separated from each other before large flares. The revealed features of evolution of ARs before large flares can be used to develop methods for predicting them.

Numerical Simulations of the Evolution of Solar Active Regions: the Complex AR12565 and AR12567

We have performed numerical magnetohydrodynamic (MHD) simulations of two closed active regions (AR). The input magnetic field values were the coronal magnetic field computed as extrapolation coronal from observations of the photospheric magnetic field. The studied active regions, NOAA AR12565 and AR12567, were registered as different bipolar region. Our investigation, the 3D coronal extrapolations, as well as the numerical MHD experiments, revealed that actually they evolved together as a quadrupolar active region. The second region emerged later under the loops system of AR12565 and separated from this one. A natural current sheet formed between then and it plays an important role in the explosive events (flares and coronal mass ejections) occurrence.

MAGNETIC FLUX TRANSPORT AND THE LONG-TERM EVOLUTION OF SOLAR ACTIVE REGIONS

With multiple vantage points around the Sun, STEREO and SDO imaging observations provide a unique opportunity to view the solar surface continuously. We use He II 304 A data from these observatories to isolate and track ten active regions and study their long-term evolution. We find that active regions typically follow a standard pattern of emergence over several days followed by a slower decay that is proportional in time to the peak intensity in the region. Since STEREO does not make direct observations of the magnetic field, we employ a flux-luminosity relationship to infer the total unsigned magnetic flux evolution. To investigate this magnetic flux decay over several rotations we use a surface flux transport model, the Advective Flux Transport (AFT) model, that simulates convective flows using a time-varying velocity field and find that the model provides realistic predictions when information about the active region's magnetic field strength and distribution at peak flux is available. Finally, we illustrate how 304 \AA\ images can be used as a proxy for magnetic flux measurements when magnetic field data is not accessible.

Evolution of solar active regions: Detecting the emergence of new magnetic field through multifractal segmentation

New emerging magnetic flux is identifed using a multifractal segmentation method used to investigate regularities in the evolution of active regions and activity complexes. The SOLIS, MDI SOHO, and SOT Hinode magnetograms were used. Observations of the active region NOAA 10488 on October 26–28, 2003 indicate considerable variability of areas of newmagnetic flux on a characteristic time scale of 1–2 h. Broadening of the multifractal spectrum in magnetogram sections corresponding to areas of new flux leads to their detection on segmented multifractal images corresponding to the minimal fractal dimensions. New regularities in the emergence of magnetic flux in the 24th solar cycle are studied using data obtained in 2010–2014. A positive correlation of the time variations of areas of new magnetic flux of N and S polarities is predominant at all stages in the development of the cycle. A gradual decrease in the correlation coefficient during 2010–2014 is associated with the progressive complexity of the relationships among active regions, which leads to their clustering into activity complexes.

Evolution and Flare Activity of δ $\delta$ -Sunspots in Cycle 23

The emergence and magnetic evolution of solar active regions (ARs) of beta-gamma-delta type, which are known to be highly flare-productive, were studied with the SOHO/MDI data in Cycle 23. We selected 31 ARs that can be observed from their birth phase, as unbiased samples for our study. From the analysis of the magnetic topology (twist and writhe), we obtained the following results. i) Emerging beta-gamma-delta ARs can be classified into three topological types as "quasi-beta", "writhed" and "top-to-top". ii) Among them, the "writhed" and "top-to-top" types tend to show high flare activity. iii) As the signs of twist and writhe agree with each other in most cases of the "writhed" type (12 cases out of 13), we propose a magnetic model in which the emerging flux regions in a beta-gamma-delta AR are not separated but united as a single structure below the solar surface. iv) Almost all the "writhed"-type ARs have downward knotted structures in the mid portion of the magnetic flux tube. This, we believe, is the essential property of beta-gamma-delta ARs. v) The flare activity of beta-gamma-delta ARs is highly correlated not only with the sunspot area but also with the magnetic complexity. vi) We suggest that there is a possible scaling-law between the flare index and the maximum umbral area.

A statistical analysis of current helicity and twist in solar active regions over the phases of the solar cycle using the spectro-polarimeter data of Hinode

Abstract Current helicity and twist of solar magnetic fields are important in characterizing the dynamo mechanism working in the convection zone of the Sun. We have carried out a statistical study on the current helicity of solar active regions observed with the spectro-polarimeter (SP) of the Hinode Solar Optical Telescope (SOT). We used SOT-SP data of 558 vector magnetograms of a total of 80 active regions obtained during the period from 2006 to 2012. We have applied spatial smoothing and division of data points into weak and strong field ranges to compare the contributions from different scales and field strengths. We found that the current helicity follows the “hemispheric sign rule” when weak magnetic fields (absolute field strength < 300 gauss) are considered and no smoothing is applied. On the other hand, the pattern of current helicity fluctuates and violates the hemispheric sign rule when stronger magnetic fields are considered and smoothing of 2${^{\prime\prime}_{.}}$0 (modeled on ground-based observations) is applied. Furthermore, we found a tendency that weak and inclined fields conform to the hemispheric sign rule and strong and vertical fields violate it. These different properties of helicity through the strong and weak magnetic field components give important clues in understanding the solar dynamo as well as the mechanism of formation and evolution of solar active regions.

Solar flare forecasting based on sequential sunspot data

It is widely believed that the evolution of solar active regions leads to solar flares. However, information about the evolution of solar active regions is not employed in most existing solar flare forecasting models. In the current work, a short-term solar flare forecasting model is proposed, in which sequential sunspot data, including three days of information about evolution from active regions, are taken as one of the basic predictors. The sunspot area, the McIntosh classification, the magnetic classification and the radio flux are extracted and converted to a numerical format that is suitable for the current forecasting model. Based on these parameters, the sliding-window method is used to form the sequential data by adding three days of information about evolution. Then, multi-layer perceptron and learning vector quantization are employed to predict the flare level within 48h. Experimental results indicate that the performance of the proposed flare forecasting model works better than previous models.

MAGNETIC ENERGY AND HELICITY BUDGETS IN THE ACTIVE-REGION SOLAR CORONA. II. NONLINEAR FORCE-FREE APPROXIMATION

Expanding on an earlier work that relied on linear force-free magnetic fields, we self-consistently derive the instantaneous free magnetic energy and relative magnetic helicity budgets of an unknown three-dimensional nonlinear force-free magnetic structure extending above a single, known lower-boundary magnetic field vector. The proposed method does not rely on the detailed knowledge of the three-dimensional field configuration but is general enough to employ only a magnetic connectivity matrix on the lower boundary. The calculation yields a minimum free magnetic energy and a relative magnetic helicity consistent with this free magnetic energy. The method is directly applicable to photospheric or chromospheric vector magnetograms of solar active regions. Upon validation, it basically reproduces magnetic energies and helicities obtained by well-known, but computationally more intensive and non-unique, methods relying on the extrapolated three-dimensional magnetic field vector. We apply the method to three active regions, calculating the photospheric connectivity matrices by means of simulated annealing, rather than a model-dependent nonlinear force-free extrapolation. For two of these regions we correct for the inherent linear force-free overestimation in free energy and relative helicity that is larger for larger, more eruptive, active regions. In the third studied region, our calculation can lead to a physical interpretation of observed eruptive manifestations. We conclude that the proposed method, including the proposed inference of the magnetic connectivity matrix, is practical enough to contribute to a physical interpretation of the dynamical evolution of solar active regions.

Magnetic Energy and Helicity Budgets in the Active Region Solar Corona. I. Linear Force‐Free Approximation

Expanding on an earlier work that relied on linear force-free magnetic fields, we self-consistently derive the instantaneous free magnetic energy and relative magnetic helicity budgets of an unknown three-dimensional nonlinear force-free magnetic structure extending above a single, known lower-boundary magnetic field vector. The proposed method does not rely on the detailed knowledge of the three-dimensional field configuration but is general enough to employ only a magnetic connectivity matrix on the lower boundary. The calculation yields a minimum free magnetic energy and a relative magnetic helicity consistent with this free magnetic energy. The method is directly applicable to photospheric or chromospheric vector magnetograms of solar active regions. Upon validation, it basically reproduces magnetic energies and helicities obtained by well-known, but computationally more intensive and non-unique, methods relying on the extrapolated three-dimensional magnetic field vector. We apply the method to three active regions, calculating the photospheric connectivity matrices by means of simulated annealing, rather than a model-dependent nonlinear force-free extrapolation. For two of these regions we correct for the inherent linear force-free overestimation in free energy and relative helicity that is larger for larger, more eruptive, active regions. In the third studied region, our calculation can lead to a physical interpretation of observed eruptive manifestations. We conclude that the proposed method, including the proposed inference of the magnetic connectivity matrix, is practical enough to contribute to a physical interpretation of the dynamical evolution of solar active regions.

The birth and evolution of solar active regions

The growth of solar active regions is a well-observed surface phenomenon with its origins concealed in the solar interior. We review the salient facts about the emergence of active regions and the consequences of their growth on the solar atmosphere. The most powerful flares, the ones which display a range of phenomena that still pose serious challenges for high-energy astrophysics, are associated with regions of high magnetic complexity. How does that degree of complexity arise when the vast majority of active regions are simple bipolar entities? In order to gain some insight into that problem, we compare the emergence of magnetic flux in ordinary regions with an instance when magnetic complexity is apparent from the very first appearance of a new region - clearly a subsurface prefabrication of complexity - and with others wherein a new region interacts with a pre-existing one to create the complexity in plain view.

A compact dopplergraph/magnetograph suitable for space-based measurements of solar oscillations and magnetic fields

A compact Dopplergraph/magnetograph placed in a continuous solar-viewing orbit will allow us to make major advancements in our understanding of solar internal structure and dynamics. An international program is currently being conducted at JPL and Mt. Wilson to develop such an instrument. By combining a unique magneto-optical resonance filter with CID and CCD cameras we have been able to obtain full- and partial-disk Dopplergrams and magnetograms. Time series of the velocity images are converted into k-ω power spectra which show clear- the solar nonradial p-mode oscillations. Magnetograms suitable for studying the long-term evolution of solar active regions have also been obtained with this instrument. A flight instrument based on this concept is being studied for possible inclusion in the SOHO mission.