Complex Active Regions Research Articles

Searching for Evidence of Subchromospheric Magnetic Reconnection on the Sun

Within the coronae of stars, abundances of those elements with low first ionization potential (FIP) often differ from their photospheric values. The coronae of the Sun and solar-type stars mostly show enhancements of low-FIP elements (the FIP effect) while more active stars such as M dwarfs have coronae generally characterized by the inverse-FIP (I-FIP) effect. Highly localized regions of I-FIP effect solar plasma have been observed by Hinode's EUV Imaging Spectrometer in a number of highly complex active regions (ARs), usually around strong light bridges of the umbrae of coalescing/merging sunspots. These observations can be interpreted in the context of the ponderomotive force fractionation model, which predicts that plasma with I-FIP effect composition is created by the refraction of waves coming from below the plasma fractionation region in the chromosphere. A plausible source of these waves is thought to be reconnection in the (high-plasma-β) subchromospheric magnetic field. In this study, we use the 3D visualization technique of Chintzoglou & Zhang combined with observations of localized I-FIP effect in the corona of AR 11504 to identify potential sites of such reconnection and its possible consequences in the solar atmosphere. We found subtle signatures of episodic heating and reconnection outflows in the expected places, in between magnetic flux tubes forming a light bridge, within the photosphere of the AR. Furthermore, on either side of the light bridge, we observed small antiparallel horizontal magnetic field components, supporting the possibility of reconnection occurring where we observe I-FIP plasma. When taken together with the I-FIP effect observations, these subtle signatures provide a compelling case for indirect observational evidence of reconnection below the fractionation layer of the chromosphere, however direct evidence remains elusive.

Generation and annihilation of three dimensional magnetic nulls in extrapolated solar coronal magnetic field: data-based Implicit Large Eddy simulation

Three-dimensional (3D) magnetic nulls are the points where magnetic field vanishes and are preferential sites for magnetic reconnection: a fundamental process which converts magnetic energy into kinetic energy, heat, and energy of non-thermal particles along with a rearrangement of magnetic field lines. Reconnection is ubiquitous in nature and plays a major role in various magnetically confined laboratory and space/astrophysical plasmas. In the solar corona, the reconnection manifests as coronal transients including solar flares, coronal mass ejections and coronal jets—often associated with 3D nulls. The nulls are generally found to be collocated with complex active regions on the solar photosphere and merits further attention, particularly in terms of their generation. A recent idealized magnetohydrodynamics simulation initiated with an analytically constructed preexisting proper radial null has identified magnetic reconnection to be responsible for spontaneous generation of these 3D nulls. It is then imperative to further explore the plausibility of spontaneous generation of nulls in naturally occurring plasmas, identify the mechanism and verify the outcome vis-à-vis observations. An apt test bed for such an initiative is the solar atmosphere, as abundant space and ground-based observations are available. In the above backdrop, the paper attempts to investigate 3D null generation by carrying out a data-based simulation of a C6.6 class flare associated with the photospheric active region NOAA 11 977. The simulation confirms spontaneous pairwise generation of 3D nulls with magnetic reconnections as the underlying cause. Importantly, magnetic field lines associated with the spontaneously generated nulls are found to trace observed chromospheric bright points—highlighting their observational relevance. Overall, such spontaneous generation and annihilation of nulls through magnetic reconnections opens up a new avenue for solar coronal and chromospheric heating.

Deep neural networks of solar flare forecasting for complex active regions

Solar flare forecasting is one of major components of operational space weather forecasting. Complex active regions (ARs) are the main source producing major flares, but only a few studies are carried out to establish flare forecasting models for these ARs. In this study, four deep learning models, called Complex Active Region Flare Forecasting Model (CARFFM)-1, −2, −3, and −4, are established. They take AR longitudinal magnetic fields, AR vector magnetic fields, AR longitudinal magnetic fields and the total unsigned magnetic flux in the neutral line region, AR vector magnetic fields and the total unsigned magnetic flux in the neutral region as input, respectively. These four models can predict the production of M-class or above flares in the complex ARs for the next 48 h. Through comparing the performance of the models, CARFFM-4 has the best forecasting ability, which has the most abundant input information. It is suggested that more valuable and rich input can improve the model performance.

Solar active region magnetic parameters and their relationship with the properties of halo coronal mass ejections

The coronal mass ejections (CMEs) from the sun consist of the plasma and strong magnetic fields (∼4–5 G at 1.3 R⊙) and generally, the fast CMEs are often associated with energetic particles of energy few hundreds of MeV. When the fast CMEs are ejected towards the Earth, they can disrupt the flow of solar wind in the heliosphere and impact the Earth's magnetosphere causing known space weather phenomena such as, geomagnetic storms, auroras, communication blackouts, satellite drag and failures. In this work, we have investigated a set of 60 halo coronal mass ejections (CMEs) associated with X-ray flares (≥C1.0) observed in 49 active regions during the period 2010–2016 in solar cycle 24. The X-ray flares and white light CMEs were observed by Geostationary Operational Environmental Satellite (GOES) and the Large Angle Spectrometric Coronagraph (LASCO) coronagraph on the Solar and Heliospheric Observatory (SOHO) satellite, respectively. The active region magnetic properties are obtained from Space - weather HMI Active Region Patch (SHARP) parameters for this set of events. For each active region (AR), the mean value is calculated from a 1-day time series for each of the 16 photospheric magnetic parameters extracted from the vector magnetogram products at a 12-min cadence. The dependency of flare flux and CME kinematics properties on the active region magnetic properties is further confirmed through correlation studies. As a result, we found a moderate to strong correlation value for flare flux and halo CME kinematic properties (like linear speed, space speed and kinetic energy) among the seven (USFLUX, TOTUSJZ, TOTUSJH, ABSNJZH, SAVNCPP, TOTPOT and MEANJZH) SHARP parameters. Most of the events (≥73%) were produced by βγδ, βγ and βδ Hale - type sunspot groups and Ekc/Dkc/Fkc McIntosh sunspot classes. It reveals that the most complex active regions are feasible sources of strong flare-associated halo CME productivity. The front side halo CME kinematic properties depend more on the vertical current helicity and polarity magnetic parameters than the other magnetic parameters. In addition, the flare intensity and speed of CME, calculated using an empirical model with the active region magnetic parameters, are found nearly equal to the observed values. This result will be useful to identify the important magnetic parameters for modelling the occurrence of flare-associated halo CMEs and to understand the physical relationship between them.

A Comprehensive Radiative Magnetohydrodynamics Simulation of Active Region Scale Flux Emergence from the Convection Zone to the Corona

We present a comprehensive radiative magnetohydrodynamic simulation of the quiet Sun and large solar active regions. The 197 Mm wide simulation domain spans from 18(10) Mm beneath the photosphere to 113 Mm in the solar corona. Radiative transfer assuming local thermal equilibrium, optically thin radiative losses, and anisotropic conduction transport provide the necessary realism for synthesizing observables to compare with remote-sensing observations of the photosphere and corona. This model self-consistently reproduces observed features of the quiet Sun, emerging and developed active regions, and solar flares up to M class. Here, we report an overview of the first results. The surface magneto-convection yields an upward Poynting flux that is dissipated in the corona and heats the plasma to over 1 MK. The quiescent corona also presents ubiquitous propagating waves, jets, and bright points with sizes down to 2 Mm. Magnetic flux bundles emerge into the photosphere and give rise to strong and complex active regions with over 1023 Mx magnetic flux. The coronal free magnetic energy, which is approximately 18% of the total magnetic energy, accumulates to approximately 1033 erg. The coronal magnetic field is clearly non-force-free, as the Lorentz force needs to balance the pressure force and viscous stress as well as drive magnetic field evolution. The emission measure from to provides a comprehensive view of the active region corona, such as coronal loops of various lengths and temperatures, mass circulation by evaporation and condensation, and eruptions from jets to large-scale mass ejections.

Evidence of a flare ignited above a low-latitude spotted active region in the ultra fast rotator HK Aqr

Abstract We study the magnetic activity in the ultra fast rotator dMe HK Aqr using tomography techniques with high resolution spectroscopy. We aim to characterise how this magnetic activity appears in a regime of very fast rotation without external forces, given that HK Aqr is, very likely, a single star. We find dark spots located at low latitudes. We also detect prominences below the co-rotation radius and at low latitudes, coinciding with the spot latitudes. This apparent low-latitude activity contrasts with what is typically observed in fast rotators, which tend to form large polar spots. Moreover, we detect a stellar flare which produces an enhancement of the continuum and additional emission in the core of most photospheric and chromospheric lines. We find evidence that the flare is ignited above an active region, as seen in solar flares. This means that, with high probability, the flare is initiated by magnetic reconnection in complex active regions. We also present evidence of bulk red-shifted velocities of about 15 km s−1 during the rise of the flare, and velocities of 5-10 km s−1 during the decay phase. An estimation of the heating during the flare results in about 200 kK close to the peak and in 100 kK at the end of the observations.

The long period of3He-rich solar energetic particles measured by Solar Orbiter 2020 November 17–23

We report observations of a relatively long period of3He-rich solar energetic particles (SEPs) measured by Solar Orbiter. The period consists of several well-resolved ion injections. The high-resolution STEREO-A imaging observations reveal that the injections coincide with extreme ultraviolet jets and brightenings near the east limb, not far from the nominal magnetic connection of Solar Orbiter. The jets originated in two adjacent, large, and complex active regions, as observed by the Solar Dynamics Observatory when the regions rotated into the Earth’s view. It appears that the sustained ion injections were related to the complex configuration of the sunspot group and the long period of3He-rich SEPs to the longitudinal extent covered by the group during the analyzed time period.

Spontaneous Generation of δ-sunspots in Convective Magnetohydrodynamic Simulation of Magnetic Flux Emergence

Abstract Observations reveal that strong solar flares and coronal mass ejections tend to occur in complex active regions characterized by δ-sunspots, spot rotation, sheared polarity inversion lines (PILs), and magnetic flux ropes. Here we report on the first modeling of spontaneous δ-spot generation as a result of flux emergence from the turbulent convection zone. Utilizing state-of-the-art radiative magnetohydrodynamics code R2D2, we simulate the emergence of a force-free flux tube in the convection zone that stretches down to −140 Mm. Elevated by large-scale convective upflows, the tube appears on the photosphere as two emerging bipoles. The opposite polarities collide against each other due to the subsurface connectivity, and they develop into a pair of closely packed δ-spots. The Lorentz force drives the spot rotation and a strong counter-streaming flow of 10 km s−1 at the PIL in δ-spots, which, in tandem with local convection, strengthens the horizontal field to 4 kG and builds up a highly sheared PIL. In the atmosphere above the PIL, a flux rope structure is created. All these processes follow the multi-buoyant segment theory of the δ-spot formation, and they occur as a natural consequence of interaction between magnetic flux and turbulent convection, suggesting that the generation of δ-spots and the resultant flare eruptions may be a stochastically determined process.

Differences in the solar cycle variability of simple and complex active regions during 1996–2018

Aims. Our aim is to examine the solar cycle variability of magnetically simple and complex active region. Methods. We studied simple (α and β) and complex (βγ and βγδ) active regions based on the Mount Wilson magnetic classification by applying our newly developed daily approach. We analyzed the daily number of the simple active regions (SARs) and compared that to the abundance of the complex active regions (CARs) over the entire solar cycle 23 and cycle 24 until December 2018. Results. We show that CARs evolve differently over the solar cycle from SARs. The time evolution of SARs and CARs on different hemispheres also shows differences, even though on average their latitudinal distributions are shown to be similar. The time evolution of SARs closely follows that of the sunspot number, and their maximum abundance was observed to occur during the early maximum phase, while that of the CARs was seen roughly two years later. We furthermore found that the peak of CARs was reached before the latitudinal width of the activity band starts to decease. Conclusion. Our results suggest that the active region formation process is a competition between the large-scale dynamo (LSD) and the small-scale dynamo (SSD) near the surface, the former varying cyclically and the latter being independent of the solar cycle. During solar maximum, LSD is dominant, giving a preference to SARs, while during the declining phase the relative role of SSD increases. Therefore, a preference for CARs is seen due to the influence of the SSD on the emerging flux.

Interactions of Twisted Ω-loops in a Model Solar Convection Zone

Abstract This study aims at investigating the ability of strong interactions between magnetic field concentrations during their rise through the convection zone to produce complex active regions at the solar surface. To do so, we perform numerical simulations of buoyant magnetic structures evolving and interacting in a model solar convection zone. We first produce a 3D model of rotating convection and then introduce idealized magnetic structures close to the bottom of the computational domain. These structures possess a certain degree of field line twist and they are made buoyant on a particular extension in longitude. The resulting twisted Ω-loops will thus evolve inside a spherical convective shell possessing large-scale mean flows. We present results on the interaction between two such loops with various initial parameters (mainly buoyancy and twist) and on the complexity of the emerging magnetic field. In agreement with analytical predictions, we find that if the loops are introduced with opposite handedness and same axial field direction or the same handedness but opposite axial field, they bounce against each other. The emerging region is then constituted of two separated bipolar structures. On the contrary, if the loops are introduced with the same direction of axial and peripheral magnetic fields and are sufficiently close, they merge while rising. This more interesting case produces complex magnetic structures with a high degree of non-neutralized currents, especially when the convective motions act significantly on the magnetic field. This indicates that those interactions could be good candidates to produce eruptive events like flares or CMEs.

The association of aJ-burst with a solar jet

Context. The Sun is an active star that produces large-scale energetic events such as solar flares and coronal mass ejections and numerous smaller-scale events such as solar jets. These events are often associated with accelerated particles that can cause emission at radio wavelengths. The reconfiguration of the solar magnetic field in the corona is believed to be the cause of the majority of solar energetic events and accelerated particles. Aims. Here, we investigate a bright J-burst that was associated with a solar jet and the possible emission mechanism causing these two phenomena. Methods. We used data from the Solar Dynamics Observatory (SDO) to observe a solar jet, and radio data from the Low Frequency Array (LOFAR) and the Nan\c{c}ay Radioheliograph (NRH) to observe a J-burst over a broad frequency range (33-173 MHz) on 9 July 2013 at ~11:06 UT. Results. The J-burst showed fundamental and harmonic components and it was associated with a solar jet observed at extreme ultraviolet wavelengths with SDO. The solar jet occurred at a time and location coincident with the radio burst, in the northern hemisphere, and not inside a group of complex active regions in the southern hemisphere. The jet occurred in the negative polarity region of an area of bipolar plage. Newly emerged positive flux in this region appeared to be the trigger of the jet. Conclusions. Magnetic reconnection between the overlying coronal field lines and the newly emerged positive field lines is most likely the cause of the solar jet. Radio imaging provides a clear association between the jet and the J-burst which shows the path of the accelerated electrons.

Active Longitude and Coronal Mass Ejection Occurrences

The spatial inhomogeneity of the distribution of coronal mass ejection (CME) occurrences in the solar atmosphere could provide a tool to estimate the longitudinal position of the most probable CME-capable active regions in the Sun. The anomaly in the longitudinal distribution of active regions themselves is often referred to as active longitude (AL). In order to reveal the connection between the AL and CME spatial occurrences, here we investigate the morphological properties of active regions. The first morphological property studied is the separateness parameter, which is able to characterize the probability of the occurrence of an energetic event, such as a solar flare or CME. The second morphological property is the sunspot tilt angle. The tilt angle of sunspot groups allows us to estimate the helicity of active regions. The increased helicity leads to a more complex buildup of the magnetic structure and also can cause CME eruption. We found that the most complex active regions appear near the AL and that the AL itself is associated with the most tilted active regions. Therefore, the number of CME occurrences is higher within the AL. The origin of the fast CMEs is also found to be associated with this region. We concluded that the source of the most probably CME-capable active regions is at the AL. By applying this method, we can potentially forecast a flare and/or CME source several Carrington rotations in advance. This finding also provides new information for solar dynamo modeling.

Reconnection brightenings in the quiet solar photosphere

We describe a new quiet-Sun phenomenon which we call "Quiet-Sun Ellerman-like Brightenings" (QSEB). QSEBs are similar to Ellerman bombs (EB) in some respects but differ significantly in others. EBs are transient brightenings of the wings of the Balmer H-alpha line that mark strong-field photospheric reconnection in complex active regions. QSEBs are similar but smaller and less intense Balmer-wing brightenings that occur in quiet areas away from active regions. In the H-alpha wing we measure typical lengths of less than 0.5 arcsec, widths of 0.21 arcsec, and lifetimes of less than a minute. We discovered them using high-quality H-alpha imaging spectrometry from the Swedish 1-m Solar Telescope (SST) and show that in lesser-quality data they cannot be distinguished from more ubiquitous facular brightenings, nor in the ultraviolet diagnostics currently available from space platforms. We add evidence from concurrent SST spectropolarimetry that QSEBs also mark photospheric reconnection events, but in quiet regions on the solar surface.

ON THE ROTATION OF SUNSPOTS AND THEIR MAGNETIC POLARITY

ABSTRACT The rotation of sunspots of 2 yr in two different solar cycles is studied with the data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observataory. We choose the α sunspot groups and the relatively large and stable sunspots of complex active regions in our sample. In the year of 2003, the α sunspot groups and the preceding sunspots tend to rotate counterclockwise and have positive magnetic polarity in the northern hemisphere. In the southern hemisphere, the magnetic polarity and rotational tendency of the α sunspot groups and the preceding sunspots are opposite to the northern hemisphere. The average rotational speed of these sunspots in 2003 is about 0.°65 hr−1. From 2014 January to 2015 February, the α sunspot groups and the preceding sunspots tend to rotate clockwise and have negative magnetic polarity in the northern hemisphere. The patterns of rotation and magnetic polarity of the southern hemisphere are also opposite to those of the northern hemisphere. The average rotational speed of these sunspots in 2014/2015 is about 1.°49 hr−1. The rotation of the relatively large and stable preceding sunspots and that of the α sunspot groups located in the same hemisphere have opposite rotational direction in 2003 and 2014/2015.

ON MAGNETIC ACTIVITY BAND OVERLAP, INTERACTION, AND THE FORMATION OF COMPLEX SOLAR ACTIVE REGIONS

Recent work has revealed an phenomenological picture of the how the $\sim$11-year sunspot cycle of Sun arises. The production and destruction of sunspots is a consequence of the latitudinal-temporal overlap and interaction of the toroidal magnetic flux systems that belong to the 22-year magnetic activity cycle and are rooted deep in the Sun's convective interior. We present a conceptually simple extension of this work, presenting a hypothesis on how complex active regions can form as a direct consequence of the intra- and extra-hemispheric interaction taking place in the solar interior. Furthermore, during specific portions of the sunspot cycle we anticipate that those complex active regions may be particular susceptible to profoundly catastrophic breakdown---producing flares and coronal mass ejections of most severe magnitude.

Complex active regions as the main source of extreme and large solar proton events

A study of solar proton sources indicated that solar flare events responsible for ≥2000 pfu proton fluxes mostly occur in complex active regions (CARs), i.e., in transition structures between active regions and activity complexes. Different classes of similar structures and their relation to solar proton events (SPEs) and evolution, depending on the origination conditions, are considered. Arguments in favor of the fact that sunspot groups with extreme dimensions are CARs are presented. An analysis of the flare activity in a CAR resulted in the detection of “physical” boundaries, which separate magnetic structures of the same polarity and are responsible for the independent development of each structure.

Simulating flaring events in complex active regions driven by observed magnetograms

Context. We interpret solar flares as events originating in active regions that have reached the self organized critical state, by using a refined cellular automaton model with initial conditions derived from observations.

Magnetic and velocity field variations in the active regions NOAA 10486 and NOAA 10488

We study the magnetic and velocity field evolution in the two magnetically complex active regions NOAA 10486 and NOAA 10488 observed during October–November 2003. We have used the available data to examine net flux and Doppler velocity time profiles to identify changes associated with evolutionary and transient phenomena. In particular, we report detection of rapid moving features observed in NOAA 10486 during the maximum phase of the X17.2/4B superflare of October 28, 2003. The velocity of this moving feature is estimated around 40 km/s, i.e., much greater than the usual Hα flare-ribbons’ separation speed of 3–10 km/s, but similar to the velocity of seismic waves, i.e., ∼ 45 km/s reported earlier by Kosovichev & Zharkova (1998).

Two-Dimensional Spectroscopy of Photospheric Shear Flows in a Small δ Spot

In recent high-resolution observations of complex active regions, long-lasting and well-defined regions of strong flows were identified in major flares and associated with bright kernels of visible, near-infrared, and X-ray radiation. These flows, which occurred in the proximity of the magnetic neutral line, significantly contributed to the generation of magnetic shear. Signatures of these shear flows are strongly curved penumbral filaments, which are almost tangential to sunspot umbrae rather than exhibiting the typical radial filamentary structure. Solar active region NOAA 10756 was a moderately complex β δ sunspot group, which provided an opportunity to extend previous studies of such shear flows to quieter settings. We conclude that shear flows are a common phenomenon in complex active regions and δ spots. However, they are not necessarily a prerequisite condition for flaring. Indeed, in the present observations, the photospheric shear flows along the magnetic neutral line are not related to any change of the local magnetic shear. We present high-resolution observations of NOAA 10756 obtained with the 65-cm vacuum reflector at Big Bear Solar Observatory (BBSO). Time series of speckle-reconstructed white-light images and two-dimensional spectroscopic data were combined to study the temporal evolution of the three-dimensional vector flow field in the β δ sunspot group. An hour-long data set of consistent high quality was obtained, which had a cadence of better than 30 seconds and subarcsecond spatial resolution.

Numerical simulations of fast and slow coronal mass ejections

AbstractSolar coronal mass ejections (CMEs) show a large variety in their kinematic properties. CMEs originating in active regions and accompanied by strong flares are usually faster and accelerated more impulsively than CMEs associated with filament eruptions outside active regions and weak flares. It has been proposed more than two decades ago that there are two separate types of CMEs, fast (impulsive) CMEs and slow (gradual) CMEs. However, this concept may not be valid, since the large data sets acquired in recent years do not show two distinct peaks in the CME velocity distribution and reveal that both fast and slow CMEs can be accompanied by both weak and strong flares. We present numerical simulations which confirm our earlier analytical result that a flux‐rope CME model permits describing fast and slow CMEs in a unified manner. We consider a force‐free coronal magnetic flux rope embedded in the potential field of model bipolar and quadrupolar active regions. The eruption is driven by the torus instability which occurs if the field overlying the flux rope decreases sufficiently rapidly with height. The acceleration profile depends on the steepness of this field decrease, corresponding to fast CMEs for rapid decrease, as is typical of active regions, and to slow CMEs for gentle decrease, as is typical of the quiet Sun. Complex (quadrupolar) active regions lead to the fastest CMEs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)