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It is extremely difficult to simulate the details of coronal heating and also make meaningful predictions of the emitted radiation. Thus, testing realistic models with observations is a major challenge. Observational signatures of coronal heating depend crucially on radiation, thermal conduction, and the exchange of mass and energy with the transition region and chromosphere below. Many magnetohydrodynamic simulation studies do not include these effects, opting instead to devote computational resources to the magnetic aspects of the problem. We have developed a simple method of accounting approximately for the missing effects. It is applied to the simulation output ex post facto and therefore may be a valuable tool for many studies. We have used it to predict the emission from a model corona that is driven by vortical boundary motions meant to represent photospheric convection. We find that individual magnetic strands experience short-term brightenings, both scattered throughout the computational volume and in localized clusters. The former may explain the diffuse component of the observed corona, while the latter may explain bright coronal loops. Several observed properties of loops are reproduced reasonably well: width, lifetime, and quasi-circular cross section (aspect ratio not high). Our results lend support to the idea that loops are multistranded structures heated by "storms" of nanoflares. [ABSTRACT FROM AUTHOR]

A challenge in characterizing active region (AR) coronal heating is in separating transient (bursty) loop heating from the diffuse background (steady) heating. We present a method of quantifying coronal heating's bursty and steady components in ARs, applying it to Fe xviii (hot 94) emission of an AR observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The maximum-, minimum-, and average-brightness values for each pixel, over a 24 hr period, yield a maximum-brightness map, a minimum-brightness map, and an average-brightness map of the AR. Running sets of such three maps come from repeating this process for each time step of running windows of 20, 16, 12, 8, 5, 3, 1, and 0.5 hr. From each running window's set of three maps, we obtain the AR's three corresponding luminosity light curves. We find (1) the time-averaged ratio of minimum-brightness-map luminosity to average-brightness-map luminosity increases as the time window decreases, and the time-averaged ratio of maximum-brightness-map luminosity to average-brightness-map luminosity decreases as the window decreases; (2) for the 24 hr window, the minimum-brightness map's luminosity is 5% of the average-brightness map's luminosity, indicating that at most 5% of the AR's hot 94 luminosity is from heating that is steady for 24 hr; (3) this upper limit on the fraction of the hot 94 luminosity from steady heating increases to 33% for the 30 minute running window. This requires that the heating of the 4–8 MK plasma in this AR is mostly in bursts lasting less than 30 minutes: at most a third of the heating is steady for 30 minutes. [ABSTRACT FROM AUTHOR]

The responses of Earth's and Mars' thermospheric densities to quasi‐periodic (∼27‐day) solar rotation variations in flux were measured contemporaneously by the Mars Atmosphere and Volatile EvolutioN, Geostationary Operational Environmental Satellites, and Swarm‐C satellites. While large solar rotation variability is found in both planetary thermospheres, correlation analyses performed on over 6 years of observations reveal that, independently of extreme ultraviolet flux level, Earth's daytime density response is about 10%–50% larger than Mars' at a similar density level. Important altitude dependencies in the density sensitivity to the solar rotation in solar flux are found in the Martian thermosphere, while the terrestrial thermosphere is shown to exhibit only small (±5%) day/night and latitude variations in the response. Detailed analyses focused on correlative periods in 2015–2016 and 2020 indicate important solar cycle effects in the sensitivities of both planetary thermospheres, with increased slopes under low solar flux conditions. These results provide important new insights into processes relevant to the interpretation of the sources of short‐term density variability in Mars' and Earth's thermospheres associated with solar drivers and point to the need for targeted modeling efforts along with dedicated data analyses to help resolve current unknowns in thermal balance processes. Plain Language Summary: The quasi‐periodic change in solar extreme ultraviolet (EUV) radiation from active regions on the Sun rotating with a period of about 27 days is one of the largest sources of short‐term variability in the upper atmospheres of both Mars and Earth. This work investigates the response of total mass densities in the upper atmospheres of Mars (∼160–260 km altitude) and Earth (∼450–480 km altitude) to the solar rotation variation in EUV flux, as a tracer of short‐term solar‐driven impacts on neutral densities. We use simultaneous density and solar flux observations from the Mars Atmosphere and Volatile EvolutioN, Swarm‐C, and Geostationary Operational Environmental Satellites from 2014 through 2021. Earth's daytime densities in the middle thermosphere are found to be about 10%–50% more responsive than Mars' for an equivalent altitude. Important dependencies on height are found at Mars, while small day/night and latitude dependencies are found at Earth. Correlation analyses also suggest strong solar cycle effects on the sensitivities, with significantly higher sensitivities during solar minimum. The results of this study can help to better constrain comparative planetary thermosphere simulations and resolve unknowns in thermal balance processes but also suggest the need for additional data‐ and modeling‐driven studies. Key Points: Solar rotation effects on Earth's and Mars' thermospheric densities are quantified in over 6 years of simultaneous satellite observationsEarth's middle thermospheric daytime density is 30%–50% (10%–30%) more responsive than Mars' during solar maximum (minimum) conditionsDensity sensitivities to solar rotation variability in extreme ultraviolet flux show clear dependencies on altitude and solar cycle at both planets [ABSTRACT FROM AUTHOR]

Solar active regions are the main sources of large solar flares and coronal mass ejections. It is found that the active regions producing large eruptions usually show compact, highly sheared polarity inversion lines. A scenario named “collisional shearing” is proposed to explain the formation of this type of polarity inversion lines and the subsequent eruptions, which stresses the role of collision and shearing induced by relative motions of different bipoles in their emergence. However, in observations, if not considering the evolution stage of the active regions, about one-third of the active regions that produce large solar eruptions govern a spot–spot-type configuration. In this work, we studied the full evolution of an emerging AR, which showed a spot–spot-type configuration when producing a major eruption, to explore the possible evolution gap between the “collisional shearing” process in flux emergence and the formation of the spot–spot-type, eruption-producing AR. We tracked the AR from the very beginning of its emergence until it produced the first major eruption. It was found that the AR was formed through three bipoles emerging sequentially. The bipoles were arranged in parallel on the photosphere, shown as two clusters of sunspots with opposite-sign polarities, so that the AR exhibited an overall large bipole configuration. In the fast emergence phase of the AR, the shearing gradually occurred due to the proper motions of the polarities, but no significant collision occurred due to the parallel arrangement of the bipoles nor did the large eruption occur. After the fast emergence phase, one large positive polarity started to show signs of decay. Its dispersion led to the collision to a negative polarity which belonged to another bipole. A huge hot channel spanning the entire AR was formed through precursor flarings around the collision region. The hot channel erupted later, accompanied by an M7.3-class flare. The results suggest that in the spot–spot-type AR, along with the shearing induced by the proper motions of the polarities, a decay process may lead to the collision of the polarities, driving the subsequent eruptions.

The formation of filaments/prominences is still a debated topic. Many different processes have been proposed: levitation, injection of cool plasma, merging filaments, and cooling plasma in hot loops. We take the opportunity to make a multiwavelength analysis of the formation of an active-region filament, combining several UV and EUV observations including the new Ne vii 465 Å filtergrams provided by the Solar Upper Transition Region Imager on board the Space Advanced Technology satellite. The filament is mainly observed at the limb for 3 hr. It is progressively formed through a series of stages, including emergence and cooling of hot loops, reconnection between small filaments, material transfer in a large filament channel, and reconnection between filaments and emerged hot loops. From the observations at 465 Å, we find that the new-formed filaments show bright structures as in 304 Å, while the long-lived stable filaments display dark morphology as in 211 Å. This suggests that the plasma around 0.5 MK would be an essential component of new-formed filaments and the material temperature in filaments would be variable during their evolution. The filament formed by the recombination of two filaments and an emerged hot loop finally erupts. After reconnection, the final filament shows a highly twisted structure of both bright and dark strands, which is surrounded by several weak and dispersive looplike structures. This eruptive filament has a complex multichannel topology and covers a wide range of temperatures.

The Carrington flare in 1859 September is a benchmark, as the earliest reported solar flare and as an event with one of the greatest terrestrial impacts. To date, no rigorous estimate of the energy of this flare has been made on the basis of the only direct observation available, its white-light emission. Here, we exploit the historical observations to obtain a magnitude estimate and express it in terms of its GOES soft X-ray class. From Carrington’s original drawings, we estimated the area of the white-light flaring region to be 116 ± 25 msh. Carrington’s account allows us to estimate the flare blackbody brightness temperature as ≈8800–10,900 K, given the most plausible interpretation of the reported flare brightness. This leads to an unprecedented class estimate of ≈X80 (X46–X126), on the modern revised GOES scale (a factor 1.43 higher than the traditional one). This substantially exceeds earlier estimates but is based on an explicit interpretation of Carrington’s description. We also describe an alternative but less plausible estimation of the flare brightness, as adopted previously, to obtain a class estimate of ≈X14 (X9–X19). This now-deprecated scenario gives an estimate similar to that of with those of directly observed modern great flares. Approximations with “equivalent area,” based on the Hinode observations, lead to comparable magnitudes and approve our estimates, though with a larger uncertainty range. We note that our preferred estimate is higher than the currently used value of X64.4 ± 7.2 (revised) based on indirect geomagnetic measurements.

Information theory is used to characterize the solar active region periodicities and memories from the Carrington map images 1974–2021. The active regions typically evolve and move from one map to the next. In order to track these active region structures in sequences of images, an innovative method based on information theory is developed. Image entropy provides a measure of the organization of structures in the images. The entropy can also be used as a filter to identify structures and partition the active regions, which are then registered for each image. The partitions are used to compute the mutual information and measure the information flow from the active regions from one image to the next. Finally, conditional mutual information is used to give a measure of the information flow from one image to another given the third image. The results suggest the following: (1) there is a long-term memory of two cycles or more; (2) the coherence time of the active regions is ∼2 yr; and (3) the average active region structure scale size carrying the most information is approximately 118 × 10 ^3 –236 × 10 ^3 Mm ^2 . The study has implications to the short- and long-term predictability of active regions and sunspots as well as the nature of flux transport at the Sun. Finally, our innovative method can be similarly applied to stellar data to determine the dynamics of the active regions of stars.

We develop an adaptive method to automatically identify ARs from radial synoptic maps observed by SOHO/MDI and SDO/HMI, calibrate the detections between HMI and MDI data based on identified ARs flux and area and further derive a homogeneous dataset including ARs' area and flux over the last two solar cycles. The data are compared with sunspot number, USAF/NOAA sunspot area, SMARPs and SHARPs and BARD area and flux, which show reasonable agreement. The identified ARs during the overlap period of MDI and HMI have the same areas as a whole while the AR flux based on MDI maps is about 1.36 times as large as that of HMI maps. Based on our dataset, we find strong ARs (| flux | > 1022Mx) contribute most to the difference between cycles 23 and 24 while other ARs (| flux | < 1022Mx) are similar in the two cycles in both area and flux. [ABSTRACT FROM AUTHOR]

We present a study of the spatial distribution of quasi-periodic oscillations (QPOs) of microwave emission in three solar active regions before flares. The following events are considered: SOL2011-09-06T01:50M5.3, SOL2011-09-25T02:33M4.4, and SOL2012-07-08T04:19C6.6. Daily observations on the Nobeyama Radioheliograph at a frequency of 17 GHz are used. In all three cases, pre-flare trains of the QPOs of microwave emission are detected. The length of the trains is 3–4 pulses. In all cases, the most strongly detected oscillations manifested themselves in a compact zone in an active region coinciding with the region of maximum microwave emission brightness during the flare. [ABSTRACT FROM AUTHOR]

This paper analyzes the observations of an original spectroheliograph telescope installed at the Kislovodsk Mountain Astronomical Station of the GAO RAS. The telescope is intended for continuous observation of the activity of the solar chromosphere. The tool makes it possible to register the full profile of the H-alpha spectral line in each pixel of the solar disk with a frequency of approximately 1 minute. We analyzed observational data from September 2015 to July 2020 and found downdrafts with average velocities Vav is approximately 0.5 km/s. The flows are directed predominantly downwards with a small horizontal component. The properties of the streams do not significantly depend on the phase of the solar cycle, the area, or the relative brightness of active regions. We assume that these streams can influence the speed of the solar wind over active regions. [ABSTRACT FROM AUTHOR]

In this paper, we study the development of Forbush decreases associated with coronal mass ejections from active regions accompanied by solar flares and filament eruptions from non-active regions using the database of Forbush effects and interplanetary disturbances created at IZMIRAN. We compared the development of two types of Forbush decreases during solar cycles 23–24, the maxima of these cycles, and the minimum between them. Using statistical methods, we studied the distributions of time intervals from the beginning of the Forbush decrease to registration: the minimum cosmic ray density, the maximum hourly decrease in density, the maximum cosmic ray anisotropy, the maximum solar wind velocity, the maximum strength of the interplanetary magnetic field, and the minimum of the Dst index. The difference in the development of two types of Forbush decreases was compared when the interplanetary disturbance contains or does not contain a magnetic cloud near the Earth. The results showed that flare-associated events develop faster than filament-associated events, even at close values of the solar wind parameters. The difference in the development of two types of Forbush decreases is more noticeable in the case of the presence of a magnetic cloud near the Earth's orbit. The largest difference between the time parameters in the two types of events is observed for the time of registration of the maximum intensity of the interplanetary magnetic field. The main phase of the two types of Forbush decreases is the same at the solar cycle 23 maximum and longer for filament-associated events at the cycle 24 maximum and 23–24 minimum. Considering all time parameters, the difference in the development of the two types of Forbush decreases is more noticeable at the maximum of cycle 23 and at the minimum of cycle 23–24 than at the maximum of cycle 24. [ABSTRACT FROM AUTHOR]

Context. Activity on the far side of the Sun is routinely studied through the analysis of the seismic oscillations detected on the near side using helioseismic techniques such as phase-shift sensitive holography. Detections made through those methods are limited to strong active regions due to the need for a high signal-to-noise ratio. Recently, the neural network FarNet was developed to improve these detections. This network extracts more information from helioseismic far-side maps, enabling the detection of smaller and weaker active regions. Aims. We aim to create a new machine learning tool, FarNet-II, which further increases FarNet's scope, and to evaluate its performance in comparison to FarNet and the standard helioseismic method for detecting far-side activity. Methods. We developed FarNet-II, a neural network that retains some of the general characteristics of FarNet but improves the detections in general, as well as the temporal coherence among successive predictions. The main novelties of the new neural network are the implementation of attention and convolutional long short-term memory (ConvLSTM) modules. A cross-validation approach, training the network 37 times with a different validation set for each run, was employed to leverage the limited amount of data available. We evaluate the performance of FarNet-II using three years of extreme ultraviolet observations of the far side of the Sun acquired with the Solar Terrestrial Relations Observatory (STEREO) as a proxy of activity. The results from FarNet-II were compared with those obtained from FarNet and the standard helioseismic method using the Dice coefficient as a metric. Given that the application of the ConvLSTM modules can affect the accuracy as a function of the position on the sequence, we take this potential dependency into account in the evaluation. Results. FarNet-II achieves a Dice coefficient that improves that of FarNet by over 0.2 points for every output position on the sequences from the evaluation dates. Its improvement over FarNet is higher than that of FarNet over the standard method. Conclusions. The new network is a very promising tool for improving the detection of activity on the far side of the Sun given by pure helioseismic techniques. Space weather forecasts can potentially benefit from the higher sensitivity provided by this novel method. [ABSTRACT FROM AUTHOR]

Using time-sequence vector magnetic field and coronal observations from Solar Dynamics Observatory , we report the observations of the magnetic field evolution and coronal activity in four emerging active regions (ARs). The ARs emerge with leading polarity being the same as for the majority of ARs in a hemisphere of solar cycle 24. After emergence, the magnetic polarities separate each other without building a sheared polarity inversion line. In all four ARs, the magnetic fields are driven by foot point motions such that the sign of the helicity injection (d H /d t) in the first half of the evolution is changed to the opposite sign in the later part of the observation time. This successive injection of opposite helicity is also consistent with the sign of mean force–free twist parameter (α av ). Further, the EUV light curves off the ARs in 94 Å and GOES X-ray flux reveal flaring activity below C-class magnitude. Importantly, the white-light coronagraph images in conjunction with the AR images in Atmospheric Imaging Assembly (AIA) 94 Å delineate the absence of associated Coronal Mass ejections (CMEs) with the studied ARs. These observations imply that the ARs with successive injection of opposite sign magnetic helicity are not favourable to twisted flux rope formation with excess coronal helicity, and therefore are unable to launch CMEs, according to recent reports. This study provides the characteristics of helicity flux evolution in the ARs referring to the conservative property of magnetic helicity and more such studies would help to quantify the eruptive capability of a given AR. [ABSTRACT FROM AUTHOR]

The aim of this study is to shed light on the effects connected with thermal misbalance due to non-equal cooling and heating rates induced by density and temperature perturbations in solar active regions hosting either propagating torsional or shear Alfvén waves. A description for the non-linear forces connected with Alfvén waves in non-ideal conditions is provided, based on the second-order thin flux tube approximation. This provides insight into the effects of Alfvén-induced motions on the boundary of thin magnetic structures in thermally active plasmas. The equations describing the process of generating longitudinal velocity perturbations, together with density perturbations by non-linear torsional Alfvén waves, are obtained and solved analytically. It is shown that the phase shift (compared with the ideal case) and the amplitude of the induced longitudinal plasma motions against the period of the mother Alfvén wave are greater for shear Alfvén waves compared with torsional Alfvén waves, although following the same pattern. The difference in the influence of thermal misbalance on the induced velocity perturbations is governed by the plasma-β although its effect is stronger for shear waves. It is deduced that for a harmonic Alfvén driver the induced density perturbations are left uninfluenced by the thermal misbalance. [ABSTRACT FROM AUTHOR]

In this study, we assume that the magnetic configuration of active regions (ARs) in quiet periods has certain similarities and can be considered “normal” features. While there are some other magnetic features of active regions that are related to strong flares, they can be considered the precursor of strong flares and “anomaly” features. Our study aims to identify those “anomalies” and apply them in strong-flare forecasting. An unsupervised auto-encoder network has been used to understand and memorize these “normal” features, and then, based on the mean squared errors between the pictures of the ARs and the corresponding reconstructed pictures derived by the network, an anomaly detection algorithm has been adopted to identify the precursor for strong flares and develop a strong-flare classification model. The strong-flare classification model reaches an F1 score of 0.8139, an accuracy of 0.8954, a recall of 0.8785, and a precision of 0.7581. Moreover, for those correctly predicted strong-flare events (94 M-class flares and above), the model reaches an average first warning time of 45.24 h. The results indicate that the anomaly detection algorithm can be used in precursor identification for strong flares and help in both improving strong-flare prediction accuracy and enlarging the time in advance. Also, the obtained average maximum warning period for strong-flare prediction (nearly 2 days) will be useful for future applications for space-weather solar flare prediction.

We report the discovery and characterization of high-speed (>100 km s−1) horizontal flows in solar active regions, making use of the Sun-as-a-star spectroscopy in the range 5–105 nm provided by the EVE (Extreme Ultraviolet Variability Experiment) spectrometers on the Solar Dynamics Observatory. These apparent flows are persistent on time-scales of days, and are well observed in lines of Mg  x , Si  xii , and Fe  xvi for example. They are prograde, as evidenced directly by blueshifts/redshifts peaking at the east/west limb passages of isolated active regions. The high-speed flow behaviour does not depend upon active-region latitude or solar cycle, with similar behaviour in Cycles 24 and 25. [ABSTRACT FROM AUTHOR]

This article presents results that challenge the paradigms that (1) the convection zone is the source of the radial magnetic field in the photosphere and (2) that coronal currents are neutralized from the perspective of the photosphere. We demonstrate, using a new analysis tool applied to simulations and observations, that bare or partially dressed current channels are supported by the solar corona and that fingerprints of these coronal current systems can be detected in the photosphere. These coronal current channels can be a significant source of the radial component of the magnetic field in the photosphere. The roots of these coronal current channels in the photosphere are the source of the magnetic field component parallel to the polarity inversion line in active region NOAA 12673. These analyses and observations transform our theoretical understanding of coronal evolution and argue for a reexamination of the present paradigm in which the convection zone is the sole source of the photospheric magnetic field. [ABSTRACT FROM AUTHOR]

We studied an X1.6 solar flare produced by NOAA Active Region 12602 on 2014 October 22. The entirety of this event was covered by RHESSI, IRIS, and Hinode/EIS, allowing analysis of the chromospheric response to a nonthermal electron driver. We derived the energy contained in nonthermal electrons via RHESSI spectral fitting and linked the time-dependent parameters of this call to the response in Doppler velocity, density, and nonthermal width across a broad temperature range. The total energy injected was 4.8 Ă— 1030 erg and lasted 352 s. This energy drove explosive chromospheric evaporation, with a delineation in both Doppler and nonthermal velocities at the flow reversal temperature, between 1.35 and 1.82 MK. The time of peak electron injection (14:06 UT) corresponded to the time of highest velocities. At this time, we found 200 km sâˆ'1 blueshifts in the core of Fe xxiv, which is typically assumed to be at rest. Shortly before this time, the nonthermal electron population had the shallowest spectral index (≈6), corresponding to the peak nonthermal velocity in Si iv and Fe xxi. Nonthermal velocities in Fe xiv, formed near the flow reversal temperature, were low and not correlated with density or Doppler velocity. Nonthermal velocities in ions with similar temperatures were observed to increase and correlate with Doppler velocities, implying unresolved flows surrounding the flow reversal point. This study provides a comprehensive, time-resolved set of chromospheric diagnostics for a large X-class flare, along with a time-resolved energy injection profile, ideal for further modeling studies. [ABSTRACT FROM AUTHOR]

Parker Solar Probe's (PSP's) unique orbital path allows us to observe the solar wind closer to the Sun than ever before. Essential to advancing our knowledge of solar wind and energetic particle formation is identifying the sources of PSP observations. We report on results for the first two PSP solar encounters derived using the Wangâ€"Sheeleyâ€"Arge (WSA) model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model maps. We derive the coronal magnetic field and the 1 R ⊙ source regions of the PSP-observed solar wind. We validate our results with the solar wind speed and magnetic polarity observed at PSP. When modeling results are very reliable, we derive time series of model-derived spacecraft separation from the heliospheric current sheet, magnetic expansion factor, coronal hole boundary distance, and photospheric field strength along the field lines estimated to be connected to the spacecraft. We present new results for Encounter 1, which show time evolution of the far-side mid-latitude coronal hole that PSP corotates with. We discuss how this evolution coincides with solar wind speed, density, and temperature observed at the spacecraft. During Encounter 2, a new active region emerges on the solar far side, making it difficult to model. We show that ADAPT-WSA output agrees well with PSP observations once this active region rotates onto the near side, allowing us to reliably estimate the solar wind sources retrospectively for most of the encounter. We close with ways in which coronal modeling enables scientific interpretation of these encounters that would otherwise not have been possible. [ABSTRACT FROM AUTHOR]

Although the Sun has been a focus of space-based research for several decades, there are still many open questions. One severe gap in solar physics knowledge is the lack of detailed, long-term observations of the solar polar regions to explore the origins of the Sun's magnetism and other connected phenomena. To fill this gap, this paper presents a feasibility study of a mission concept for a solar polar orbiting imaging mission called the Sun Sailing Polar Orbiting Telescope (SunSPOT). SunSPOT utilizes a doppler magnetograph instrument to observe the polar magnetic structure and a large deployable solar sail to reach high inclination orbit. The mission concept is derived from two science objectives investigating the formation of solar active regions and the solar dynamo. These objectives are aligned with the science goals formulated by the National Research Council (NRC) in the 2013 Decadal Survey in Solar and Space Physics and their science closure is shown in the Science Traceability Matrix. The mission and spacecraft design are explained, including a cost and risk analysis, with a special focus on the requirements and risks of the solar sail propulsion system. While the solar sail presents a significant design challenge and financial risk to this and other proposed solar polar missions, it is a necessity to achieve the orbit required for closure of decadal science goals. Although originally conceived of as a Discovery-class mission fitting within a $600 M budget, the solar sail pushes the cost beyond this threshold. The work presented here is the outcome of the NASA Heliophysics Mission Design School (HMDS) 2020 hosted by the NASA Jet Propulsion Laboratory. [ABSTRACT FROM AUTHOR]

We analyze 152 large confined flares (GOES class ≥ M1.0 and ≤ 45° from disk center) during 2010âˆ'2019, and classify them into two types according to the criterion taken from the work of Li et al. “Type I” flares are characterized by slipping motions of flare loops and ribbons and a stable filament underlying the flare loops. “Type II” flares are associated with the failed eruptions of the filaments, which can be explained by the classical 2D flare model. A total of 59 flares are “Type I” flares (about 40%) and 93 events are “Type II” flares (about 60%). There are significant differences in distributions of the total unsigned magnetic flux (ΦAR) of active regions (ARs) producing the two types of confined flares, with “Type I” confined flares from ARs with a larger ΦAR than “Type II.” We calculate the mean shear angle ΨHFED within the core of an AR prior to the flare onset, and find that it is slightly smaller for “Type I” flares than that for “Type II” events. The relative nonpotentiality parameter ΨHFED/ΦAR has the best performance in distinguishing the two types of flares. About 73% of “Type I” confined flares have ΨHFED/ΦAR<1.0 Ă— 10âˆ'21 degree Mxâˆ'1, and about 66% of “Type II” confined events have ΨHFED/ΦAR ≥ 1.0 Ă— 10âˆ'21 degree Mxâˆ'1. We suggest that “Type I” confined flares cannot be explained by the standard flare model in 2D/3D, and the occurrence of multiple slipping magnetic reconnections within the complex magnetic systems probably leads to the observed flare. [ABSTRACT FROM AUTHOR]

Active regions (ARs) on the Sun are discussed as a possible source of the slow solar wind (SW) whose origin is still a subject of debates. In this paper we present experimental evidence for the possible influence of ARs on the SW speed and temperature near the maximum of solar cycle 23 (2000–2002). We have studied separately the characteristics of the SW formed in the periods when ARs were on the central meridian (CM) of the Sun ( of the entire observing time) and the characteristics of the SW formed in the absence of ARs on the CM ( of the observing time). The SW speed in the former case (in the presence of ARs), on average, have turned out to be lower than the speed of the SW formed in the absence of ARs approximately by 1 (434.06 versus 438.09 km s with a measurement error km s ). For the SW temperature the corresponding difference is about 6 ( K versus K with an error K). This result confirms, on average, a lower speed and temperature of the SW forming in ARs in comparison with the SW component forming in coronal holes. [ABSTRACT FROM AUTHOR]

We study the association of solar Type IV radio bursts with the location of active regions on the Sun during Solar Cycle 24. The active regions associated with moving and stationary Type IV bursts are categorized as close to disk center and far from disk center, based on their location on the solar surface (i.e. ≤ 45 ∘ or ≥ 45 ∘ , respectively). The location of active regions associated with Type IV bursts accompanied with coronal mass ejections (CMEs) are also studied. We found that ≈ 30 – 40% of the active regions are located far from disk center for all the bursts. However, it is found that most of the active regions associated with stationary Type IV bursts are close to disk center (≈ 60 – 70%). The active regions associated with moving Type IV bursts are more evenly distributed across the surface, i.e. ≈ 56 % and ≈ 44 % , close to disk center and far from disk center, respectively. The fact that most of the bursts have active regions close to disk center indicates that these bursts can be used to obtain physical properties such as electron density and magnetic fields of the CMEs responsible for geomagnetic storms. [ABSTRACT FROM AUTHOR]

The solar active regions are the regions where various active phenomena occur in the solar atmosphere. Accurate detection and identification of the solar active regions are of great scientific significance to understand the formation mechanism of the solar magnetic field. In this paper, we propose an automatic detection and recognition technology for solar active regions based on the advantages of Scale Invariant Feature Transform (SIFT) and Clustering by Fast Search and Find of Density Peaks (DPC). Firstly, enhance the contrast of longitudinal magnetic image of Helioseismic and Magnetic Imager (HMI) of Solar Dynamics Observatory (SDO). Then extract the feature points by SIFT. Finally, cluster the feature points by fast search and find of density peaks so as to automatically detect and identify the solar active regions. The results show that the combination of SIFT and DPC can accurately detect the solar active region without human-computer interaction. [ABSTRACT FROM AUTHOR]

Analyzing the evolution of magnetic helicity flux at different atmospheric heights is key for identifying its role in the dynamics of active regions (ARs). The three-dimensional (3D) magnetic field of both flaring and nonflaring ARs is constructed using potential field extrapolations, enabling the derivation of emergence, shearing, and total magnetic helicity components at a range of atmospheric heights. An analysis of temporal oscillations of the derived components shows that the largest significant period of the three helicity fluxes are common (within ±2 hr) from the photosphere up to at least 1 Mm for flaring ARsâ€"being consistent with the presence of a coupled oscillatory behavior that is absent in the nonflaring ARs. We suggest that large, energetic solar eruptions may have been produced in ARs when the vertical and horizontal helicity flux components became a coupled oscillatory system in the low solar atmosphere. [ABSTRACT FROM AUTHOR]

Thomson-scattered photospheric light is the dominant constituent of the lower solar corona’s spectral continuum viewed off-limb at optical wavelengths. Known as the K-corona, it is also linearly polarized. We investigate the possibility of using the a priori polarized characteristics of the K-corona, together with polarized emission lines, to measure and correct instrument-induced polarized crosstalk. First we derive the Stokes parameters of the Thomson scattering of unpolarized light in an irreducible spherical tensor formalism. This allows forward synthesis of the Thomson-scattered signal for the more complex scenario that includes symmetry-breaking features in the incident radiation field, which could limit the accuracy of our proposed technique. For this, we make use of an advanced 3D radiative magnetohydrodynamic coronal model. Together with synthesized polarized signals in the Fe xiii 10746 Ă... emission line, we find that an ad hoc correction of telescope- and instrument-induced polarization crosstalk is possible under the assumption of a nondepolarizing optical system. [ABSTRACT FROM AUTHOR]

The paper discusses the diagnostics of plasma jets in the solar corona with the use of data from modern space- and ground-based telescopes observing the Sun in the extreme ultraviolet (EUV) and micro- wave bands. We examine observational parameters of EUV and radio emission in events associated with plasma jets, depending on the mechanism of formation, initiation conditions, and evolution of the jets. The opportunities provided by the study of plasma jets, which relies on simultaneous observations in different bands, are highlighted. For a number of jets, we have measured their primary parameters; and in this paper we present preliminary results of statistical processing of the data obtained. Microwave observations of several specific events, made by ground-based instruments RATAN-600, SRH, and Nobeyama Radioheliograph, are considered in detail. The diagnostic capabilities of these instruments for studying coronal jets are shown. To analyze the three-dimensional structure of the coronal magnetic field, we have used SDO/HMI data, which allowed for the reconstruction of the field in the lower corona. The information gained is compared with the results of diagnostics of the magnetic field at the base of the corona according to RATAN-600 data. The purpose of the methods developed is to determine the physical mechanisms responsible for the generation, collimation, and dynamics of plasma jets in the solar atmosphere.

Multifaceted observations of the Sun reveal that interactions between magnetic field loops expel slow-moving solar wind. [ABSTRACT FROM AUTHOR]

Hα flare index is one of the solar activity indicators to describe the physical properties of the active regions in the solar atmosphere. Sunspot number is a representative indicator of the magnetic activity in the photosphere, which is associated with the energy supply rate to the corona. Thus, it is significant to investigate the phase relationship between Hα flare activity and sunspot activity. In the present work, monthly Hα flare index (FIm), monthly sunspot activity (sunspot numbers (Rs) and group sunspot numbers (Rg)) compiled from the Purple Mountain Observatory are used to find their phase relationship using the cross-correlation analysis. For solar cycles 20–23, there is a very pronounced time lag of about 9, 4, 4.5 and 10 months between the monthly FIm and Rs, respectively. Furthermore, there is a noticeable time lag of about 7, 8, 4.5 and 9 months between FIm and Rg for the same cycles, respectively. If we take the whole time series together, we notice that FIm has a time lag of about 4 months with regard to both Rs and Rg. The possible physical mechanisms for their observed phase lag are discussed. [ABSTRACT FROM AUTHOR]

The Forbush decreases for the period from 1997 to 2020 were studied based on data from the database on Forbush effects and interplanetary disturbances created and maintained at the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN). Using statistical methods, we compared the Forbush decreases associated with coronal mass ejections from active regions of the Sun accompanied by solar flares; filament ejections outside active regions; high-speed streams from coronal holes; and several sources. The difference between Forbush decreases was related to coronal mass ejections when magnetic clouds in the interplanetary disturbances near the Earth were observed or not observed. It has been shown that the distributions of most of parameters are asymmetric for the sporadic Forbush decreases; for the recurrent Forbush decreases, they are nearly symmetric. The strongest correlations between the parameters of Forbush decreases and interplanetary disturbances are observed in the group of coronal ejections from active regions that are accompanied by solar flares and have a structure of magnetic cloud. [ABSTRACT FROM AUTHOR]

This paper examines the response of the upper atmosphere to increased radiation following exceptional solar activity in September 2017. The active region of the Sun AR2673 has caused intense solar-terrestrial disturbance. This active region has generated several powerful X-class solar flares. The strongest outburst of the 24th solar activity cycle X9.3 occurred on 6 September 2017. The second powerful solar flare X8.2 occurred on 10 September 2017. Both flares caused an increase of the solar irradiance in extreme ultraviolet and soft X-ray spectral regions, and Ly- α line. The empirical FISM2 model was used to obtain data on the radiation fluxes from solar flares. The HAMMONIA chemistry–climate model was used to analyze the effect of radiation on the neutral atmosphere. This work presents the results of the effect of solar flares on the chemical composition and ozone of the upper and middle atmosphere. [ABSTRACT FROM AUTHOR]

Necessary conditions for the appearance of decay instability of low-frequency kinetic Alfven waves in loop structures of an active region on the Sun before a flare are obtained. On the basis of the transcendental dispersion equation obtained using the synchronism conditions and additional conservation laws, an expression is obtained for the nonlinear growth rate of the decay instability of the initial kinetic Alfven wave (KAW) to the secondary KAW and the kinetic ion-acoustic wave (KIAW). Boundary values of the reduced amplitude of the initial KAW in the preflare atmosphere of an active region (AR) are obtained. It was assumed in the calculations that the waves involved in the process in the AR under study appear due to the development of instability caused by the presence of a weak large-scale (sub-Dreicer) electric field and drift motions of the plasma due to spatial inhomogeneities of its temperature and density. It is shown that, for a certain type of semiempirical model of the solar atmosphere, kinetic Alfven and kinetic ion-acoustic waves can be generated during the linear stage of instability development. In this case, the process of wave generation can take place both in plasma with purely Coulomb conductivity and in the presence of small-scale Bernstein turbulence in it. To "trigger" the instability, relatively low values of plasma nonisothermality and a very low threshold value of the reduced amplitude of the sub-Dreicer electric field are required. The possibility of the appearance of undamped kinetic waves of small amplitude in the region under study is proven. [ABSTRACT FROM AUTHOR]

Long observational series for bipolar active regions (ARs) provide significant information about the mutual transformation of the poloidal and toroidal components of the global solar magnetic field. The direction of the toroidal field determines the polarity of leading sunspots in ARs in accordance with the Hale's polarity law. The vast majority of bipolar ARs obey this regularity, whereas a few per cent of ARs have the opposite sense of polarity (anti-Hale ARs). However, the study of these ARs is hampered by their poor statistics. The data for five 11-yr cycles (16–18 and 23, 24) were combined here to compile a synthetic cycle of unique time length and latitudinal width. The synthetic cycle comprises data for 14838 ARs and 367 of them are the anti-Hale ARs. A specific routine to compile the synthetic cycle was demonstrated. We found that, in general, anti-Hale ARs follow the solar cycle and are spread throughout the time-latitude diagram evenly, which implies their fundamental connection with the global dynamo mechanism and the toroidal flux system. The increase in their number and percentage occurs in the second part of the cycle, which is in favour of their contribution to the polar field reversal. The excess in the anti-Hale ARs percentage at the edges of the butterfly diagram and near an oncoming solar minimum (where the toroidal field weakens) might be associated with the strengthening of the influence of turbulent convection and magnetic field fluctuations on the arising flux tubes. The evidence of the misalignment between the magnetic and heliographic equators is also found. [ABSTRACT FROM AUTHOR]

In the present research work, we have theoretically analyzed the photonic band-gap properties of one-dimensional photonic structures composed of polycarbonate and non-glass materials. These photonic structures, PC1, PC2, PC3 and PC4, are composed of alternating layers of polycarbonate/Al2O3, polycarbonate/MgF2, polycarbonate/BaF2 and polycarbonate/TiO2 materials, respectively. The period of each photonic structure is made up of a thin non-glass material layer sandwiched between two identical polycarbonate layers. The transfer matrix method has been used to investigate the transmission properties of PC1 to PC4. The comparison between the transmission spectra of PC1 to PC4 shows that the polycarbonate and TiO2-based photonic structure (PC4) possess three PBGs of zero transmission located at far-ultraviolet, visible and near-infrared regions of the electromagnetic spectrum at normal and oblique incidence (θ0 = 55°), both corresponding to TE wave only. The index of refraction of all five materials used in this study was obtained by applying the Sellmeier-type dispersion relationship to ensure accuracy in the results. The purpose of selecting polycarbonate along with Al2O3, TiO2, MgF2 or BaF2 as constituent materials of these photonic structures is due to the heat resistance properties of polycarbonate and the unique optical properties of oxide and fluoride materials with wide transparency from the ultraviolet to the near-infrared regions of the electromagnetic spectrum. The proposed work can be used to design some influential wavelength-selective reflectors composed of 1D PCs behind the active region of the solar cells for improving the photovoltaic performance of solar panels. This study can further be utilized for the fabrication of advanced solar cell designs consisting of 1D photonic mirror-based luminescence and reflection concentrators. The low temperature problem which arises in satellites may also be overcome with the help of smart windows based on the proposed multilayer structures. [ABSTRACT FROM AUTHOR]

We present joint Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) and Interface Region Imaging Spectrograph (IRIS) observations of Fe xii lines in active regions, both on-disk and off-limb. We use an improved calibration for the EIS data, and find that the 192.4 Ă.../1349 Ă... observed ratio is consistent with the values predicted by CHIANTI and the coronal approximation in quiescent areas, but not in all active-region observations, where the ratio is often lower than expected by up to a factor of about two. We investigate a number of physical mechanisms that could affect this ratio, such as opacity and absorption from cooler material. We find significant opacity in the EIS Fe xii 193 and 195 Ă... lines, but not in the 192.4 Ă... line, in agreement with previous findings. As we cannot rule out possible EUV absorption by H, He, and He ii in the on-disk observations, we focus on an off-limb observation where such absorption is minimal. After considering these, as well as possible nonequilibrium effects, we suggest that the most likely explanation for the observed low Fe xii 192.4 Ă.../1349 Ă... ratio is the presence of non-Maxwellian electron distributions in the active regions. This is in agreement with previous findings based on EIS and IRIS observations independently. [ABSTRACT FROM AUTHOR]

A timeâ€"distance measurement technique is derived to isolate phase travel time anisotropy caused by subsurface horizontal magnetic fields; a method that uses the measured anisotropy to estimate the field’s orientation is also derived. A simulation of acoustic waves propagating in a uniform, inclined magnetic field with solar background structure is used to verify the derived technique. Then the procedure is applied to a numerical simulation of a sunspot for which the subsurface state is known to provide context for the results obtained from the study of several sunspots observed by the Helioseismic and Magnetic Imager. Significant anisotropies are detected, on the order of 1 minute, and the subsurface field’s azimuth is estimated and compared with the azimuth of the surface magnetic field. In all cases, the subsurface azimuth is found to be well aligned with that of the surface, and the results from the numerical simulation are used to interpret features in the detected travel time anisotropy. [ABSTRACT FROM AUTHOR]

Duo to the large magnetic Reynolds number, the magnetic helicity originating from the solar interior can be carried away through the photosphere into the corona. However, the relationship between the accumulated magnetic helicity flux through the photosphere and the magnetic helicity in the corona is still unclear. By selecting 36 newly emerging active regions in the 23rd solar cycle, we apply optical flow methods to derive the accumulated magnetic helicity through the photosphere (H m p ) by using the sequential longitudinal magnetograms, we use nonlinear force-free field extrapolation to obtain the 3D coronal magnetic field, and we adopt finite volume methods to calculate the instantaneous relative magnetic helicity in the corona (H m c ) by using vector magnetograms. It is found that the local correlation tracking (LCT)â€"based H m p is larger than H m c in 1″, and that the Differential Affine Velocity Estimatorâ€"based H m p is more consistent with H m c than the LCT-based H m p . H m p is more consistent with H m c in evaluation from 2″ than from 1″. Moreover, H m c âˆ' H m p systematically shows consistency with the Hemispheric Helicity Rule (over 55%), no matter which resolution and method are used. These estimations suggest that the consistency of H m c and H m p is partly dependent on the resolution of the magnetograms and the calculation methods. [ABSTRACT FROM AUTHOR]

The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. Low frequency radio bursts have recently been brought back to light with the advancement of novel radio interferometers. However, their polarisation properties have not yet been explored in detail, especially with the Low Frequency Array (LOFAR), due to difficulties in calibrating the data and accounting for instrumental leakage. Here, using a unique method to correct the polarisation observations, we explore the circular polarisation of different sub-types of solar type III radio bursts and a type I noise storm observed with LOFAR, which occurred during March–April 2019. We analysed six individual radio bursts from two different dates. We present the first Stokes V low frequency images of the Sun with LOFAR in tied-array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental emission, while this trend is either not clear or absent for harmonic emission. The type III bursts studied, that are part of a long–lasting type III storm, can have different senses of circular polarisation, occur at different locations and have different propagation directions. This indicates that the type III bursts forming a classical type III storm do not necessarily have a common origin, but instead they indicate the existence of multiple, possibly unrelated acceleration processes originating from solar minimum active regions. [ABSTRACT FROM AUTHOR]

We analyze the presence of a microwave neutral-line-associated source (NLS) in a super-active region NOAA 12673, which produced a number of geo-effective events in September 2017. To estimate the NLS position, we use data from the Siberian Radioheliograph in a range 4–8 GHz and from the Nobeyama Radioheliograph at 17 GHz. Calculation of the coronal magnetic field in a non-linear force-free approximation has revealed an extended structure consisting of interconnected magnetic flux ropes, located practically along the entire length of the main polarity separation line of the photospheric magnetic field. NLS is projected into the region of the strongest horizontal magnetic field, where the main energy of this structure is concentrated. During each X-class flare, the active region lost magnetic helicity and became a CME source.

Magnetic helicity is a measure of the entanglement of magnetic field lines used to characterize the complexity of solar active region (AR) magnetic fields. Previous attempts to use helicity-based indicators to predict solar eruptive/flaring events have shown promise but not been universally successful. Here we investigate the use of a quantity associated with the magnetic helicity, the magnetic winding, as a means to predict flaring activity. This quantity represents the fundamental entanglement of magnetic field lines and is independent of the magnetic field strength. We use vector magnetogram data derived from the Helioseismic Magnetic Imager (HMI) to calculate the evolution and distribution of the magnetic winding flux associated with five different ARs, three of them with little flaring activity/nonflaring (AR 11318, AR 12119, AR 12285) and two highly active with X-class flares (AR 11158, AR 12673). We decompose these quantities into “current-carrying” and “potential” parts. It is shown that the ARs that show flaring/eruptive activity have significant contributions to the winding input from the current-carrying part of the field. A significant and rapid input of current-carrying winding is found to be a precursor of flaring/eruptive activity, and, in conjunction with the helicity, sharp inputs of both quantities are found to precede individual flaring events by several hours. This suggests that the emergence/submergence of topologically complex current-carrying field is an important element for the ignition of AR flaring. [ABSTRACT FROM AUTHOR]

Precisely localising solar Active Regions (AR) from multi-spectral images is a challenging but important task in understanding solar activity and its influence on space weather. A main challenge comes from each modality capturing a different location of the 3D objects, as opposed to typical multi-spectral imaging scenarios where all image bands observe the same scene. Thus, we refer to this special multi-spectral scenario as multi-layer. We present a multi-task deep learning framework that exploits the dependencies between image bands to produce 3D AR localisation (segmentation and detection) where different image bands (and physical locations) have their own set of results. Furthermore, to address the difficulty of producing dense AR annotations for training supervised machine learning (ML) algorithms, we adapt a training strategy based on weak labels (i.e. bounding boxes) in a recursive manner. We compare our detection and segmentation stages against baseline approaches for solar image analysis (multi-channel coronal hole detection, SPOCA for ARs) and state-of-the-art deep learning methods (Faster RCNN, U-Net). Additionally, both detection and segmentation stages are quantitatively validated on artificially created data of similar spatial configurations made from annotated multi-modal magnetic resonance images. Our framework achieves an average of 0.72 IoU (segmentation) and 0.90 F1 score (detection) across all modalities, comparing to the best performing baseline methods with scores of 0.53 and 0.58, respectively, on the artificial dataset, and 0.84 F1 score in the AR detection task comparing to baseline of 0.82 F1 score. Our segmentation results are qualitatively validated by an expert on real ARs. [ABSTRACT FROM AUTHOR]

The observed primordial energy release of solar flare in the corona is explained by the mechanism of S. I. Syrovatskii, according to which the flare energy is accumulated in the current sheet. The flare release of the current sheet energy causes the observed manifestations of the flare, which are explained by the electrodynamical model of a solar flare proposed by I. M. Podgorny. According to this model, hard X-ray beamradiation on the solar surface is explained by the acceleration of electrons in field aligned currents caused by the Hall electric field in the current sheet. The study of the flare mechanism is impossible without performingmagnetohydrodynamic (MHD) simulations above a real active region (AR), in which the calculation begins several days before the appearance of flares. When setting the problem, noassumptionsweremade about the flaremechanism. An absolutely implicit finite-difference scheme, conservative with respect to the magnetic flux, has been developed, which is implemented in the PERESVET code. MHD simulation in the real scale of time can only be carried out, thanks to parallel computations using compute unified device architecture (CUDA) technology. Methods have been developed that made it possible to stabilize the numerical instability arising near the boundary of the region. Calculation above AR 10365 for low viscosities (Rm = 109, Re = 107, νArt Phoosphere = = - νMagn Art Phoosphere 10-4) showed the appearance of a singular X-type line, in the vicinity of which a current sheet with accumulatedmagnetic energy for a flare can form. Also, by means of MHD simulation the appearance of singular lines above a real AR is shown, in which the magnetic field is a superposition of an X-type field and a diverging magnetic field. In such a superposition of configurations, even if the diverging field predominates, the formation of a current sheet is possible, which can explain the appearance of a flare of not very high power. The coincidence of the position of the source of the flare thermal X-ray radiation with the places of appearance of the current sheets confirms the mechanism of the solar flare, based on the accumulation of energy in themagnetic field of the current sheet. [ABSTRACT FROM AUTHOR]

Flare productivity varies among solar active regions (ARs). This study analyzed 20 ARs of contrasting sunspot areas and flare productivities to understand the super flare productivity of certain ARs. We used the flare index (FI) as an indicator of flare activity. We examined the pattern of morphological evolution of magnetic features. Further, we derived a set of magnetic feature parameters to quantitatively characterize ARs. Our study found that the correlation coefficient is the highest ( r = 0.78) between FI and the length of the strong gradient polarity inversion line (SgPIL), while the coefficient is the lowest ( r = 0.14) between FI and the total unsigned magnetic flux. For the selected ARs, this study also found that the super flare productive ARs have SgPILs ( R value) longer (greater) than 50 Mm (4.5). These results suggest that flare productivity is mainly controlled by the size of the subregion that comprises close interaction of opposite magnetic polarities and is weakly correlated with the size of the whole ARs. Further, even though magnetic flux emergence is important, this study shows that it alone is insufficient to increase flare productivity. New emergence can drive either the interaction of like or opposite magnetic polarities of nonconjugate pairs (i.e., polarities not from the same bipole). In the former case, the magnetic configuration remains simple, and flare productivity would be low. In the latter case, the convergence of opposite magnetic fluxes of nonconjugate pairs results in a magnetic configuration with long SgPIL and an increase in flare productivity.

The plasma composition of the solar corona is different from that of the solar photosphere. Elements that have a low first ionization potential (FIP) are preferentially transported to the corona and therefore show enhanced abundances in the corona compared to the photosphere. The level of enhancement is measured using the FIP bias parameter. In this work, we use data from the EUV Imaging Spectrometer on Hinode to study the plasma composition in an active region following an episode of significant new flux emergence into the preexisting magnetic environment of the active region. We use two FIP bias diagnostics: Si x 258.375 Å/S x 264.233 Å (temperature of approximately 1.5 MK) and Ca xiv 193.874 Å/Ar xiv 194.396 Å (temperature of approximately 4 MK). We observe slightly higher FIP bias values with the Ca/Ar diagnostic than Si/S in the newly emerging loops, and this pattern is much stronger in the preexisting loops (those that had been formed before the flux emergence). This result can be interpreted in the context of the ponderomotive force model, which proposes that the plasma fractionation is generally driven by Alfvén waves. Model simulations predict this difference between diagnostics using simple assumptions about the wave properties, particularly that the fractionation is driven by resonant/nonresonant waves in the emerging/preexisting loops. We propose that this results in the different fractionation patterns observed in these two sets of loops.

The occurrence of X-class solar flares and their potential impact on space weather often receive greater attention than other flares. But predicting when and where an X-class flare will occur is still a challenge. With the multiwavelength observation from the Solar Dynamics Observatory and FengYun-3E satellite, we investigate the triggering of a GOES X1.0 flare occurring in the NOAA active region (AR) 12887. Our results show that this unique X-class flare is bred in a relatively small but complex quadrupolar AR. Before the X-class flare, two filaments (F1 and F2) exist below a null-point topology of the quadrupolar AR. Magnetic field extrapolation and observation reveal that F1 and F2 correspond to two magnetic flux ropes with the same chirality and that their adjacent feet are respectively rooted at nonconjugated opposite polarities. Interestingly, these two polarities collide rapidly, accompanied by photospheric magnetic flux emergence, cancellation, and shear motion in the AR center. Above this site, F1 and F2 subsequently intersect with each other and merge into a longer filament (F3) via a tether-cutting-like reconnection process. As a result, F3 rises and erupts upward, eventually leading to a coronal mass ejection and the X-class flare. This observation suggests that the rapid collision of nonconjugated opposite polarities provides a key condition for the triggering of this X-class flare and also provides a featured case for a flare trigger mechanism and space weather forecasting.