Complex Active Region Research Articles

The extremely strong non-neutralized electric currents of the unique solar active region NOAA 13664

Context. In May 2024, the extremely complex active region National Oceanic and Atmospheric Administration (NOAA) 13664 produced the strongest geomagnetic storm since 2003. Aims.The aim of this study is to explore the development of the extreme magnetic complexity of NOAA 13664 in terms of its photospheric electric current. Methods. The non-neutralized electric current was derived from photospheric vector magnetograms, provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. The calculation method is based on image processing, thresholding, and error analysis. The spatial and temporal evolution of the non-neutralized electric current of the region as well as its constituent subregions was examined. For context, a comparison with other complex, flare-prolific active regions is provided. Results. Active region NOAA 13664 was formed by the emergence and interaction of three subregions, two of which were of notable individual complexity. It consisted of numerous persistent, current-carrying magnetic partitions that exhibited periods of conspicuous motions and strongly increasing electric current at many locations within the region. These periods were followed by intense and repeated flaring. The total unsigned non-neutralized electric currents and average injection rates reached 5.95 ⋅ 1013 A and 1.5 ⋅ 1013 A/day, and are the strongest observed so far, significantly surpassing other super-active regions of Solar Cycle 24 and 25. Conclusions. Active region NOAA 13664 presents a unique case of complexity. Further scrutiny of the spatial and temporal variation of the net electric currents during the emergence and development of super-active regions is paramount to understand the origin of complex regions and adverse space weather.

The Temporal Evolution of Nonneutralized Electric Currents and the Complexity of Solar Active Regions

We study the evolution of electric currents during the emergence of magnetic flux in the solar photosphere and the differences exhibited between solar active regions of different Hale complexity classes. A sample of 59 active regions was analyzed using a method based on image segmentation and error analysis to determine the total amount of nonneutralized electric current along their magnetic polarity inversion lines. The time series of the total unsigned nonneutralized electric current, I NN,tot, exhibit intricate structure in the form of distinct peaks and valleys. This information is largely missing in the respective time series of the total unsigned vertical electric current I z . Active regions with δ-spots stand out, exhibiting a 1.9 times higher flux emergence rate and 2.6 times higher I NN,tot increase. The median value of their peak I NN,tot is equal to 3.6 × 1012 A, which is more than three times higher than that of the other regions of the sample. An automated detection algorithm was also developed to pinpoint the injection events of nonneutralized electric current. The injection rates and duration of these events were higher with increasing complexity of active regions, with regions containing δ-spots exhibiting the strongest and longest events. These events do not necessarily coincide with increasing magnetic flux, although they exhibit moderate correlation. We conclude that net electric currents are injected during flux emergence but are also shaped drastically by the incurred photospheric evolution as active regions grow and evolve.

Coronagraphic Observations of Si x 1430 nm Acquired by DKIST/Cryo-NIRSP with Methods for Telluric Absorption Correction

We report commissioning observations of the Si x 1430 nm solar coronal line observed coronagraphically with the Cryogenic Near-Infrared Spectropolarimeter at the National Science Foundation’s Daniel K. Inouye Solar Telescope. These are the first known spatially resolved observations of this spectral line, which has strong potential as a coronal magnetic field diagnostic. The observations target a complex active region located on the solar northeast limb on 2022 March 4. We present a first analysis of these data that extracts the spectral line properties through a careful treatment of the variable atmospheric transmission that is known to impact this spectral window. Rastered images are created and compared with extreme-UV observations from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) instrument. A method for estimating the electron density from the Si x observations is then demonstrated that makes use of the forbidden line density-sensitive emissivity and an emission-measure analysis of the SDO/AIA bandpass observations. In addition, we derive an effective temperature and nonthermal line width across the region. This study informs the calibration approaches required for more routine observations of this promising diagnostic line.

The Extreme Space Weather Event of 1872 February: Sunspots, Magnetic Disturbance, and Auroral Displays

We review observations of solar activity, geomagnetic variation, and auroral visibility for the extreme geomagnetic storm on 1872 February 4. The extreme storm (referred to here as the Chapman–Silverman storm) apparently originated from a complex active region of moderate area (≈ 500 μsh) that was favorably situated near disk center (S19° E05°). There is circumstantial evidence for an eruption from this region at 9–10 UT on 1872 February 3, based on the location, complexity, and evolution of the region, and on reports of prominence activations, which yields a plausible transit time of ≈29 hr to Earth. Magnetograms show that the storm began with a sudden commencement at ≈14:27 UT and allow a minimum Dst estimate of ≤ −834 nT. Overhead aurorae were credibly reported at Jacobabad (British India) and Shanghai (China), both at 19.°9 in magnetic latitude (MLAT) and 24.°2 in invariant latitude (ILAT). Auroral visibility was reported from 13 locations with MLAT below ∣20∣° for the 1872 storm (ranging from ∣10.°0∣–∣19.°9∣ MLAT) versus one each for the 1859 storm (∣17.°3∣ MLAT) and the 1921 storm (∣16.°2∣ MLAT). The auroral extension and conservative storm intensity indicate a magnetic storm of comparable strength to the extreme storms of 1859 September (25.°1 ± 0.°5 ILAT and −949 ± 31 nT) and 1921 May (27.°1 ILAT and −907 ± 132 nT), which places the 1872 storm among the three largest magnetic storms yet observed.

High-altitude Spider-type Prominence above the Magnetic Null Point

Rather unique observations of a high-altitude spider-type prominence in 2023 February are presented. The prominence or corresponding filament on the disk was not visible all the time but could appear and disappear in the course of a particular day. However, it persisted during the whole half of a solar rotation, being observable from day to day starting from the east limb of the Sun to the west limb. We show that the prominence was located in sagged coronal field lines just above a coronal magnetic null point. The presence of the null point and magnetic dips above it is confirmed by calculations of the potential magnetic field. The mass of the prominence apparently was appearing due to the condensation of hot coronal plasma after several eruptions that occurred in an active-region complex where the prominence was located. The prominence material flowed down along widely spread large coronal loops as coronal rain and was sometimes swept away by subsequent eruptions.

Slow Solar Wind Connection Science during Solar Orbiter’s First Close Perihelion Passage

The Slow Solar Wind Connection Solar Orbiter Observing Plan (Slow Wind SOOP) was developed to utilize the extensive suite of remote-sensing and in situ instruments on board the ESA/NASA Solar Orbiter mission to answer significant outstanding questions regarding the origin and formation of the slow solar wind. The Slow Wind SOOP was designed to link remote-sensing and in situ measurements of slow wind originating at open–closed magnetic field boundaries. The SOOP ran just prior to Solar Orbiter’s first close perihelion passage during two remote-sensing windows (RSW1 and RSW2) between 2022 March 3–6 and 2022 March 17–22, while Solar Orbiter was at respective heliocentric distances of 0.55–0.51 and 0.38–0.34 au from the Sun. Coordinated observation campaigns were also conducted by Hinode and IRIS. The magnetic connectivity tool was used, along with low-latency in situ data and full-disk remote-sensing observations, to guide the target pointing of Solar Orbiter. Solar Orbiter targeted an active region complex during RSW1, the boundary of a coronal hole, and the periphery of a decayed active region during RSW2. Postobservation analysis using the magnetic connectivity tool, along with in situ measurements from MAG and SWA/PAS, showed that slow solar wind originating from two out of three of the target regions arrived at the spacecraft with velocities between ∼210 and 600 km s−1. The Slow Wind SOOP, despite presenting many challenges, was very successful, providing a blueprint for planning future observation campaigns that rely on the magnetic connectivity of Solar Orbiter.

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.

The connection between starspots and superflares: a case study of two stars

ABSTRACT How do the characteristics of starspots influence the triggering of stellar flares? Here, we investigate the activity of two K-type stars, similar in every way from mass to rotation periods and planetary systems. Both stars exhibit about a hundred spots; however, Kepler-411 produced 65 superflares, while Kepler-210 presented none. The spots of both stars were characterized using the planetary transit mapping technique, which yields the intensity, temperature, and radius of starspots. The average radius was (17 ± 7) × 103 and (58 ± 23) × 103 km, while the intensity ratio with respect to the photosphere was (0.35 ± 0.24) and (0.64 ± 0.15) Ic, and the temperature was (3800 ± 700) and (4180 ± 240) K for spots of Kepler-411 and Kepler-210, respectively. Therefore, spots on the star with no superflares, Kepler-210, are mostly larger, less dark, and warmer than those on the flaring star, Kepler-411. This may be an indication of magnetic fields with smaller magnitude and complexity of the spots on Kepler-210 when compared to those on Kepler-411. Thus, starspot area appears not to be the main culprit of superflares triggering. Perhaps the magnetic complexity of active regions is more important.

Relation between flare activity and magnetic complexity of active regions on the Sun

Relation between flare activity and magnetic complexity of active regions on the Sun

Investigating the associations between solar flares and magnetic complexity of active regions

It is generally believed that solar activities are closely linked with magnetic fields on the solar surface. To learn more about how solar flares are produced in relation to solar magnetic configurations, we examined a total of 37,741 soft-X ray solar flare events observed by a series of GOES satellites. The flare and sunspot data catalogued by NOAA were obtained from 1996 July through 2018 December, covering solar cycles 23 and 24. The investigated flares database consists of 38.26% B-, 55.52% C-, 5.78%M- and 0.44% X-class. In accordance with the Hale magnetic complexity categorisation system, a total of 33,562 sunspot groups comprised of 30.65% α -, 57.7% β-, 8.69% βγ, and 2.96% -βγδ-type active regions were counted based on their daily appearances, from the same source and period. The Multi-Taper and cross-correlations analyses were adopted in carrying out the investigation using their daily numbers. Our findings reveal that B-flares are negatively correlated with all the sunspot groups, in contrast to other flare classes that are positively correlated and synchronous. The β-sunspot and C-flares are found to exhibit the typical 11-year solar cycle, including the double-peaks feature during solar maximum. Correspondingly, the measured daily occurrence rates of β-sunspot and C-flares were the highest during the studied two solar cycles. Meanwhile, a total of 22,121 flare events were observed to be occurring concurrently with sunspot groups, out of which 10,019 flare events were found associated with β-sunspot, from which 5,378 happened to be C-class, the highest number obtained, followed by 4,164 with B-class. The X-flares recorded 2 events with α- sunspot group, the least recorded in the studied period. We also confirm that the βγδ-group is more likely to produce any class of flare. However, constrained by their formation rates, resulting in the β-group producing the most flares by number. Periodicities of 9.9, 10.67, and 9.9 years were obtained for α-, β-, and complex (βγ-and-βγδ) respectively while 4.1-6.5, 10.67, 11.2 and 10.64 years were obtained for B-, C-, M-and-X flares.

Magnetic complexity of active regions during 23–24 solar cycles

Magnetic complexity of active regions during 23–24 solar cycles

Extreme-ultraviolet Late Phase in Homologous Solar Flares from a Complex Active Region

Abstract Recent observations in extreme-ultraviolet (EUV) wavelengths reveal a new late phase in some solar flares, which is seen as a second peak in warm coronal emissions (∼3 MK) several tens of minutes to a few hours after the soft X-ray peak. The origin of the EUV late phase (ELP) is explained by either a long-lasting cooling process in the long ELP loops or a delayed energy ejection into the ELP loops well after the main flare heating. Using the observations with the Solar Dynamics Observatory, we investigate the production of the ELP in six homologous flares (F1–F6) originating from a complex active region (AR), NOAA Active Region 11283, with an emphasis on the emission characteristics of the flares. It is found that the main production mechanism of the ELP changes from additional heating in flare F1 to long-lasting cooling in flares F3–F6, with both mechanisms playing a role in flare F2. The transition is evidenced by an abrupt decrease of the time lag of the ELP peak, and the long-lasting cooling process in the majority of the flares is validated by a positive correlation between the flare ribbon fluence and the ELP peak intensity. We attribute the change in ELP production mechanism to an enhancement of the envelope magnetic field above the AR, which facilitates a more prompt and energetic heating of the ELP loops. In addition, the last and the only confined flare F6 exhibits an extremely large ELP. The different emission pattern revealed in this flare may reflect a different energy partitioning inside the ELP loops, which is due to a different magnetic reconnection process.

Cyclic Variations, Magnetic Morphology, and Complexity of Active Regions in Solar Cycles 23 and 24

Cyclic Variations, Magnetic Morphology, and Complexity of Active Regions in Solar Cycles 23 and 24

Design of resonant cavity thin film structures with complex active layers

A systematic approach to designing resonant cavity distributed Bragg reflector (DBR) structures with complex-index active regions is developed in this paper. The technique is based on the effective reflectance index of multilayer thin films. While the design of DBR structures with real refractive index films is quite straightforward, the use of a complex refractive index in the cavity requires a different approach. In this paper, we show that a complex active region requires an asymmetric reflector geometry along with a separate phase compensator. This technique is illustrated using phase change materials and metals in the resonant cavity.

Can Subphotospheric Magnetic Reconnection Change the Elemental Composition in the Solar Corona?

Abstract 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 effect (I-FIP). Here we observe patches of I-FIP effect solar plasma in AR 12673, a highly complex βγδ active region. We argue that the umbrae of coalescing sunspots, and more specifically strong light bridges within the umbrae, are preferential locations for observing I-FIP effect plasma. Furthermore, the magnetic complexity of the active region and major episodes of fast flux emergence also lead to repetitive and intense flares. The induced evaporation of the chromospheric plasma in flare ribbons crossing umbrae enables the observation of four localized patches of I-FIP effect plasma in the corona of AR 12673. 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 chromosphere. We propose that the waves generating the I-FIP effect plasma in solar active regions are generated by subphotospheric reconnection of coalescing flux systems. Although we only glimpse signatures of I-FIP effect fractionation produced by this interaction in patches on the Sun, on highly active M stars it may be the dominant process.

Temporal and Spatial Evolutions of a Large Sunspot Group and Great Auroral Storms Around the Carrington Event in 1859

AbstractThe Carrington event is considered to be one of the most extreme space weather events in observational history within a series of magnetic storms caused by extreme interplanetary coronal mass ejections from a large and complex active region that emerged on the solar disk. In this article, we study the temporal and spatial evolutions of the source sunspot active region and visual aurorae and compare this storm with other extreme space weather events on the basis of their auroral spatial evolution. Sunspot drawings by Schwabe, Secchi, and Carrington describe the position and morphology of the source active region at that time. Visual auroral reports from the Russian Empire, Iberia, Ireland, Oceania, and Japan fill the spatial gap of auroral visibility and revise the time series of auroral visibility in middle to low magnetic latitudes. The reconstructed time series is compared with magnetic measurements and shows the correspondence between low‐latitude to mid‐latitude aurorae and the phase of magnetic storms. The spatial evolution of the auroral oval is compared with those of other extreme space weather events in 1872, 1909, 1921, and 1989 as well as their storm intensity and contextualizes the Carrington event, as one of the most extreme space weather events, but likely not unique.

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.

Multi-flux-rope system in solar active regions

AbstractMagnetic flux rope (MFR) is closely connected with solar eruptions, such as flares and coronal mass ejections. The classical scenario assumes a single MFR for each eruption, but it is reasonable to expect multiple MFRs in a complex active region (AR). Statistically investigating AR 11897, we verify the existence of multiple MFR proxies during the AR evolution. Recently, AR 12673 in 2017 September produced the two largest flares in Solar Cycle 24. The evolutions of the AR magnetic fields and the two large flares reveal that significant flux emergence and successive interactions between different emerging dipoles resulted in the formations of multiple MFRs and twisted loop bundles, which successively erupted like a chain reaction within several minutes before the peaks of the two flares. We propose that the eruptions of a multi-flux-rope system can rapidly release enormous magnetic energy and result in large flares in solar AR.

Spectral Analysis of the September 2017 Solar Energetic Particle Events.

An interval of exceptional solar activity was registered in early September 2017, late in the decay phase of solar cycle 24, involving the complex Active Region 12673 as it rotated across the western hemisphere with respect to Earth. A large number of eruptions occurred between 4 and 10 September, including four associated with X-class flares. The X9.3 flare on 6 September and the X8.2 flare on 10 September are currently the two largest during cycle 24. Both were accompanied by fast coronal mass ejections and gave rise to solar energetic particle (SEP) events measured by near-Earth spacecraft. In particular, the partially occulted solar event on 10 September triggered a ground-level enhancement (GLE), the second GLE of cycle 24. A further, much less energetic SEP event was recorded on 4 September. In this work we analyze observations by the Advanced Composition Explorer (ACE) and the Geostationary Operational Environmental Satellites (GOES), estimating the SEP event-integrated spectra above 300 keV and carrying out a detailed study of the spectral shape temporal evolution. Derived spectra are characterized by a low-energy break at few/tens of MeV; the 10 September event spectrum, extending up to ~1 GeV, exhibits an additional rollover at several hundred MeV. We discuss the spectral interpretation in the scenario of shock acceleration and in terms of other important external influences related to interplanetary transport and magnetic connectivity, taking advantage of multipoint observations from the Solar Terrestrial Relations Observatory. Spectral results are also compared with those obtained for the 17 May 2012 GLE event.

Testing predictors of eruptivity using parametric flux emergence simulations

Solar flares and coronal mass ejections (CMEs) are among the most energetic events in the solar system, impacting the near-Earth environment. Flare productivity is empirically known to be correlated with the size and complexity of active regions. Several indicators, based on magnetic field data from active regions, have been tested for flare forecasting in recent years. None of these indicators, or combinations thereof, have yet demonstrated an unambiguous eruption or flare criterion. Furthermore, numerical simulations have been only barely used to test the predictability of these parameters. In this context, we used the 3D parametric magnetohydrodynamic (MHD) numerical simulations of the self-consistent formation of the flux emergence of a twisted flux tube, inducing the formation of stable and unstable magnetic flux ropes of Leake et al. (2013, 2014). We use these numerical simulations to investigate the eruptive signatures observable in various magnetic scalar parameters and provide highlights on data analysis processing. Time series of 2D photospheric-like magnetograms are used from parametric simulations of stable and unstable flux emergence, to compute a list of about 100 different indicators. This list includes parameters previously used for operational forecasting, physical parameters used for the first time, as well as new quantities specifically developed for this purpose. Our results indicate that only parameters measuring the total non-potentiality of active regions associated with magnetic inversion line properties, such as the Falconer parametersLss,WLss,Lsg, andWLsg, as well as the new current integralWLscand lengthLscparameters, present a significant ability to distinguish the eruptive cases of the model from the non-eruptive cases, possibly indicating that they are promising flare and eruption predictors. A preliminary study about the effect of noise on the detection of the eruptive signatures is also proposed.