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Using numerical simulations, we study the formation and dynamics of solar flares in a local region of the solar atmosphere. The magnetohydrodynamics (MHD) equations describe the dynamic evolution of flares, including space-dependent and anomalous magnetic resistivity and highly anisotropic thermal conduction on a 2.5 D slice. We adopt an initial solar atmospheric model in magnetohydrostatic equilibrium, with a magnetic configuration consisting of a vertical current sheet, which helps trigger the magnetic reconnection process. Specifically, we study three scenarios, two with only resistivity and the third with resistivity plus thermal conduction. The main results of the numerical simulations show differences in the global morphology of the flares, including the post-flare loops and the current sheet in three cases. In particular, localized resistivity produces more substructure around the post-flare loops that could be related to the Ritchmyer–Meshkov Instability (RMI). Furthermore, in the scenario of anomalous resistivity, we identify the formation of a plasmoid and a jet at coronal heights. On the other hand, in the scenario with resistivity plus thermal conduction, the post-flare loops are smooth, and no apparent substructures develop. Besides, in the z -component of the current density for the Res + TC case, we observe the development of multiple magnetic islands generated due to the Tearing instability in the non-linear regime. [ABSTRACT FROM AUTHOR]

We present a long-term X-ray study of a nearby Active Galactic Nucleus Mrk 6, utilizing observations from XMM–Newton, Suzaku, Swift , and NuSTAR observatories, spanning 22 years from 2001 to 2022. From timing analysis, we estimated variance, normalized variance, and fractional rms amplitude in different energy bands.The temporal study shows fractional rms amplitude (F var) below 10 per cent for the shorter time-scale (∼60 ks) and above 20 per cent for the longer time-scale (∼weeks). A complex correlation is observed between the soft (0.5–3.0 keV) and hard (3.0–10.0 keV) X-ray bands of different epochs of observations. This result prompts a detailed investigation through spectral analysis, employing various phenomenological and physical models on the X-ray spectra. Our analysis reveals a heterogeneous structure of the obscuring material surrounding Mrk 6. A partially ionized absorber exhibits a rapid change in location and extends up to the narrow-line regions or torus. In contrast, another component, located far from the central engine, remained relatively stable. During the observation period, the source luminosity in the 3.0–10.0 keV range varies between (3–15) × 1042 erg s−1. [ABSTRACT FROM AUTHOR]

We consider the M1.1-class solar flare occurred on July 5, 2012, at UT. This event is unique in that a helioseismic perturbation was detected in it despite its low hard X-ray flux in the 25–50 keV energy band and its very soft hard X-ray spectrum. As a rule, most of the known sunquakes have been detected in solar flares with large hard X-ray fluxes at high energies (at least up to 100–300 keV). The event under consideration contradicts the popular hypothesis about the generation of sunquakes by beams of accelerated high-energy electrons. An analysis of the available RHESSI X-ray spectra shows that they can be explained in two ways. The X-ray spectrum in the 25–50 keV energy band is explained by a power-law distribution of accelerated electrons with an power-law index of 7–9 or by the presence of a superhot plasma with a temperature –60 MK. In both cases, we are dealing with electrons of relatively low energies that either were responsible for the sunquake generation or should be considered as a secondary (accompanying) phenomenon with respect to the true cause of the photospheric perturbation. The results of a joint analysis of the X-ray and microwave spectra are presented for the first time for a helioseismically active solar flare. Our analysis shows that the spectra in both ranges can be well explained by the emission of a superhot magnetized plasma and not by accelerated electrons with a soft spectrum. However, the explanation of the spectra when considering partially magnetically trapped accelerated electrons is also possible. We have estimated the parameters of the thermal plasma, accelerated electrons, and energy fluxes of various types. We analyze the dynamics of ultraviolet and X-ray emission sources. We also present an analysis of the magnetic field structure based on vector magnetograms and the nonlinear force-free coronal magnetic field extrapolation. We discuss the mechanisms for the generation of the helioseismic perturbation during the solar flare under consideration. An eruptive process could probably be both primary and secondary causes of the sunquake. The appearance of a superhot plasma in the corona could give rise to propagating thermal fronts into the lower layers of the solar atmosphere, where helioseismic waves are excited. Our analysis does not allow the possibility of the sunquake generation by accelerated electrons with a soft spectrum to be ruled out either. [ABSTRACT FROM AUTHOR]

Image super-resolution is an important subject in image processing and recognition. Here, we present an attention-aided convolutional neural network for solar image super-resolution. Our method, named SolarCNN, aims to enhance the quality of line-of-sight (LOS) magnetograms of solar active regions (ARs) collected by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). The ground-truth labels used for training SolarCNN are the LOS magnetograms collected by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Solar ARs consist of strong magnetic fields in which magnetic energy can suddenly be released to produce extreme space-weather events, such as solar flares, coronal mass ejections, and solar energetic particles. SOHO/MDI covers Solar Cycle 23, which is stronger with more eruptive events than Cycle 24. Enhanced SOHO/MDI magnetograms allow for better understanding and forecasting of violent events of space weather. Experimental results show that SolarCNN improves the quality of SOHO/MDI magnetograms in terms of the structural similarity index measure, Pearson's correlation coefficient, and the peak signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

The solar soft X-ray observations from the GOES satellites now span two full Hale cycles and provide one of the best quantitative records of solar activity, with nearly continuous flare records since 1975. We present a uniform new reduction of the entire time series for 1975 to 2022 at NOAA class C1 level or above, to characterize the occurrence distribution function (ODF) of the flares observed in the 1 – 8 Å spectral band. The analysis includes estimates of the peak fluxes of the 12 flares that saturated in the 1 – 8 Å time series. In contrast to the standard NOAA classifications, these new estimates use the full time resolution of the sampling and have a preflare background level subtracted for all events. Our new estimates include NOAA's latest calibrations for the GOES-1 through GOES-15 data covering 1975 – 2016. For each of the 12 saturated events we have made new estimates of peak fluxes based on fits to the rise and fall of the flare time profile, and have validated our extrapolation schemes by comparing with artificially truncated but unsaturated X10-class events. In this new estimation, SOL2003-11-04 (the most energetic unambiguously observed event) has a peak flux of 4.32 × 10 − 3 W / m 2 . This corresponds to X43 on the new scale, or X30 on the old scale. We provide a list in the Appendix for peak fluxes of all 37 events above 10 − 3 W / m 2 , the GOES X10 level, including the 12 saturated events. The full list now gives us a first complete sample from which we obtain an occurrence distribution function (ODF) for peak energy flux S , often represented as a power-law d F / d E ∝ E − α , for which we find α = 1.973 ± 0.014 in the range M1 to X3. The power-law description fails at the high end, requiring a downward break in the ODF above the X10 level. We give a tapered power-law description of the resulting CCDF (complementary cumulative distribution function) and extrapolate it into the domain of "superflares," i.e., flares with bolometric energies > 10 33 erg . Extrapolation of this fit provides estimates of 100-yr and 1000-yr GOES peak fluxes that agree reasonably well with other such estimates using different data sets and methodology, although there is some tension between our 10,000-yr (the Holocene time-scale) estimate and the GOES class obtained for the out-sized 774 AD solar proton event as inferred from cosmogenic nuclide records. [ABSTRACT FROM AUTHOR]

Statistical relations between the geomagnetic Dst index, cosmic ray variations, and solar wind characteristics are compared for Forbush decreases associated with: (i) coronal mass ejections from active regions (AR-CMEs) accompanied by solar flares, (ii) filament eruptions outside active regions, (iii) corotating interaction regions (CIRs) caused by high-speed streams from coronal holes, (iv) mixed events induced by two or more solar sources. Relationships of geomagnetic indices and parameters of cosmic rays and the solar wind are also compared between sporadic events with or without magnetic clouds (MCs) and between Solar Cycles (SCs) 23 and 24. The results reveal that interplanetary disturbances originated by AR-CMEs associated with an MC are most geoeffective and cause powerful geomagnetic storms, while CIRs create only moderate and weak storms. Sporadic and recurrent events differ in values of the Dst index and southward component of the magnetic field, as well as in the relationship between them. For sporadic events, geomagnetic activity is more affected by the presence or absence of an MC than by the type of solar source. Interplanetary disturbances associated with AR-CMEs are more effective in SC 23 while those associated with other types of solar sources have approximately the same geoeffectiveness in both SCs. [ABSTRACT FROM AUTHOR]

Solar flares are releases of electromagnetic energy generally occurring in active solar regions with magnetic fields, known as sunspots. The burst of radiation released by a solar flare can reach Earth's atmosphere in a few minutes. High-intensity solar flares, M- or X-class flares, can significantly impact some of Earth's activities and technologies, such as satellites, telecommunications, and electrical power systems. Therefore driving efforts in high-intensity solar flare forecasting systems is crucial. A forecasting model that observes the evolution of active regions may analyze a set of attributes that indicate which active regions can be precursors to solar flares. Recent work has focused on deep-learning models that consider the evolution of active regions in the Sun. However, M- and X-class flares are spurious in the solar-cycle period. That situation leads to an imbalanced dataset, increasing the effort to develop machine-learning models for forecasting. Therefore we propose transformers-based models to forecast ≥M-class flares, taking sequences of line-of-sight magnetogram images as input. In addition, we apply data augmentation techniques and other methods to deal with training on imbalanced datasets. Our fine-tuned models outperformed state-of-the-art work using image processing to forecast ≥M-class flares in the next 48 h with an approximate True Skill Statistic (TSS ) of 0.8. Moreover, the data augmentation techniques applied to the training set kept the TSS stable and improved most of the secondary performance metrics analyzed. [ABSTRACT FROM AUTHOR]

We investigated the quasiperiodic pulsations (QPPs) of the X1.2 solar flare (SOL2023-01-06T00:57) based on multi-instrument observations, especially the Hard X-ray Imager (HXI) on board the Advanced Space-based Solar Observatory (ASO-S). The quasiperiod of ≈ 27 s was identified in hard X-rays (HXR) and microwaves using the Markov Chain Monte Carlo method, while no corresponding oscillation was found in soft X-rays. The HXI imaging demonstrates that HXR pulsations arise from the double-footpoint HXR sources. Moreover, the fluctuation of ≈ 27 s period was also present in the nonthermal electron power-law index derived from the X-ray spectra, which is the signature of periodic electron acceleration/precipitation during the impulsive phase. The electron spectral indices not only exhibited the well-known "soft–hard–soft" evolution, but also showed the negative correlation with the HXR pulsations. These results suggest that QPPs directly originate from quasiperiodic injections of accelerated electrons into flare-loop footpoints. We also discuss the possible generation mechanisms for QPPs. [ABSTRACT FROM AUTHOR]

In this study, we performed periodicity analyses of selected daily solar (flare index, coronal index, number of coronal mass ejections), geomagnetic (planetary equivalent range index, disturbance storm time index, interplanetary magnetic field) and cosmic ray indices for the last Solar Cycle 24 (from December 2008 to December 2019). To study the periodic variation of the above-listed datasets, the following analysis methods were applied; multi-taper method, Morlet wavelet, cross-wavelet transform and wavelet coherence analysis. The outcome of our analyses revealed the following. (i) The 25–33 days solar rotation periodicities exist in all datasets without any exception in the MTM power spectra. (ii) Except for the solar rotation periodicity, all periods show data preference, and they appear around the investigated cycle's maximum phase. (iii) When comparing the phase relations between periodicities in the used datasets, they exhibit a gradual transition from small to large periods. For short-term periodicities, there are no phase relations but a mixed phase, whereas for high periodicities, there are complete phase/antiphase transitions. (iv) All identified flare index periodicities are common to all other datasets examined in this investigation. [ABSTRACT FROM AUTHOR]

Here, we performed a statistical analysis of the parameters describing the transverse component of the longitudinal magnetic field gradient (∇ ⊥B z) in the active regions of the AR – the AR-averaged transverse component of the longitudinal magnetic field gradient <∇ ⊥B z >, the average transverse component of the longitudinal magnetic field gradient in the vicinity of a point with its maximum value , the maximum value of the transverse component of the longitudinal magnetic field gradient between pairs of spots in the AR max(∇ ⊥B z) sp. For calculating the denoted parameters, magnetograms of the Bz -components of the magnetic field vector at the level of the Sun's photosphere obtained by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) are used. The statistical sample contains data on 75 ARs. The values of the analyzed parameters obtained over the time of monitoring are compared with the level of flare productivity of the AR (the flare index FI). It is shown that: 1. The ⟨ ∇ ⊥ B z ¯ ⟩ -FI dependence (the overhead line denotes averaging over the time of monitoring the AR) is quadratic with a correlation coefficient k = 0.54 while the spread in values of the quantity ⟨ ∇ ⊥ B z ¯ ⟩ is small (lying within a a range of 0.08-0.12 G km-1 for the overwhelming majority of the studied regions) and differs very little for regions with low and high flare productivity, which may be explained by the dependence of ∇ ⊥B z on the area of the AR. 2. The dependence ⟨ max ∇ ⊥ B z ¯ ⟩ -FI is quasilinear with a correlation coefficient k = 0.61. 3. The dependence max ∇ ⊥ B z ¯ -FI is linear with a correlation coefficient k = 0.63. 4. The threshold values of the analyzed parameters are found: for ⟨ ∇ ⊥ B z ¯ ⟩ ≥ 0.078 G km-1, for ⟨ max ∇ ⊥ B z ¯ ⟩ ≥ 0.983 G km-1, and for the parameter max(∇ ⊥B z) sp ≥ 0.118G km-1. For lower values of the analyzed parameters, no x-ray flares were recorded in any of the ARs of the analyzed sample. [ABSTRACT FROM AUTHOR]

Advancements in space weather forecasting have become crucial for understanding and mitigating the impacts of solar activity on Earth's ionosphere. This research focuses on the prediction of Total Electron Content (TEC) during M-class solar flare events in 2023. TEC is a vital parameter for satellite communications and navigation, making accurate forecasting imperative. Two prediction models, Long Short-Term Memory (LSTM) neural networks and Surrogate Models based on Ordinary Kriging (OKSM), are employed. LSTM, known for capturing temporal dependencies, is contrasted with OKSM, a geostatistical interpolation technique capturing spatial autocorrelation. The study utilizes TEC measurements from the Hyderabad (HYDE) GPS station for model training and evaluation along with solar and geomagnetic parameters. The performance metrics for both models across various solar flare dates are measured using Root Mean Square Error (RMSE), Normalized RMSE, Correlation Coefficient (CC), and Symmetric Mean Absolute Percentage Error(sMAPE). The research interprets the results, highlighting the strengths and limitations of each model. Notable findings include LSTM's proficiency in capturing temporal variations and OKSM's unique spatial perspective. Different solar flare intensities are analyzed separately, demonstrating the model's adaptability to varying space weather conditions. The average performance metrics during M 4.65 SF events for the OKSM model, in terms of Root Mean Square Error is 5.61, Normalized RMSE is 0.14, Correlation Coefficient is 0.9813, and Symmetric Mean Absolute Percentage Error is 14.90. Similarly, for LSTM, the corresponding averages are 10.03, 0.24, 0.9313, and 28.64. The research contributes valuable insights into the predictive capabilities of LSTM and OKSM for TEC during solar flare events. The outcomes aid in understanding the applicability of machine learning and geostatistical techniques in space weather prediction. As society's reliance on technology susceptible to space weather effects grows, this research is pivotal for enhancing space weather forecasts and ensuring the robustness of critical technological infrastructure on Earth. [ABSTRACT FROM AUTHOR]

This study presents the first results on the ionospheric response and occurrence of ionospheric irregularities to the anomalous geomagnetic storm which occurred during period of 27 October 2021–05 November 2021 that was the first strong anomalous storm occurred in the current solar cycle with symmetric horizontal component (SYM-H) peak value of − 118 nT. Multi-site GPS observations from middle latitude of European longitudinal, equatorial region of American and Asian sector were used to analyze variations of total electron content (TEC) and the occurrence of the ionospheric irregularities during the storm period. The storm caused composite consequences on the terrestrial ionosphere. The remarkable positive ionospheric storm with deviation in TEC (ΔTEC) ≈ (120–200) % was occurred at equatorial station over American within longitudinal belt of 65 - 74 ∘ W during the beginning of recovery phase of the storm. In addition, appreciable vertical total electron content enhancement of ΔTEC ≈ (40–60) % was also noticed during recovery phase over European middle latitude with in longitudinal belt of 2 - 20 ∘ E. Regarding, occurrence of ionospheric irregularity the present study concluded that, the generation of ionospheric irregularity was completely suppressed during initial, major and recovery phase of the storm over both equatorial regions of American and Indian longitudinal sector. However, there was a moderate occurrence of ionospheric irregularity over both sectors during 27–31 October 2021, which could be attributed to the effect of 28 October solar flare and less intensity of southward-directed Bz component of the interplanetary magnetic field (IMFBz) during this periods. Moreover, the local time at which the maximum excursion of SYM-H may matter for the suppression of irregularities during the anomalous strong storm of 4 November 2021. In future, it may need further study, using additional observations and numerical modeling which may shed more light on the response of anomalous geomagnetic storm of 4 November 2021. This study is presenting the first result of ionospheric response and the effects of anomalous geomagnetic storm of the current solar cycle on the occurrence of irregularities over low latitude region of South American and Asian sector. [ABSTRACT FROM AUTHOR]

We report an unusual event on absence of high frequency (HF) echoes in ionosonde observations from the ionospheric F2 region during the geomagnetic storm of 23–25 April 2023. This event was observed in both southern and northern hemispheres over two stations, Grahamstown (33.3°S, 26.5°E), South Africa and Pruhonice (50.0°N, 14.6°E), Czech Republic. Significant O/N2 depletion over the stations was observed by TIMED/GUVI, indicating a strong negative ionospheric storm. This is unique since absence of echoes in ionosonde measurements is usually due to strong radio absorption in the ionosphere associated with solar flares. However, there was no flare activity during the periods of "absent" F2 HF echoes. On the other hand, the ionosonde detected echoes from E‐layer. TIEGCM simulation reproduced TIMED/GUVI O/N2 depletion and showed that NmE was larger than NmF2 on dayside over Pruhonice. TIMED/GUVI O/N2 also showed a clear spatial gradient in the O/N2 depleted regions, suggesting F‐region ionosphere was tilted. By estimating the critical frequency of the F2 layer using GNSS observations, we have shown that it wasn't possible for the ionospheric electron density to reach depletion levels prohibiting reflection of HF echoes from ionosondes. We suggest that this phenomena may have been caused by either (a) maximum electron density of E layer exceeding that of F2 layer and/or (b) ionospheric tilting which made the signals to be reflected far away from the ionosonde locations. Plain Language Summary: Solar eruptions such as coronal mass ejections (CMEs) can lead to geomagnetic storms which cause temporal disturbances in the Earth's thermospheric temperature and composition as well as profound changes in ionospheric electron densities. One of the consequences for this phenomena is the negative ionospheric storm where ionospheric electron density is reduced from its "regular" background values. In this study, we report on the temporal absence of high frequency (HF) radio echoes from the ionospheric F2 layer in ionosonde measurements during the geomagnetic storm of 23–25 April 2023. Thermospheric neutral composition observations and simulation suggested that the absence of HF echoes was due to the intense negative ionospheric storm. Key Points: HF signals were not reflected back to the ionosonde location during part of the main phase possibly due to ionospheric tiltingMaximum electron density of the F2 layer derived from GNSS observations correctly identify the negative ionospheric storm effectThe auroral oval moved closer to mid latitudes during the storm main phase [ABSTRACT FROM AUTHOR]

The explosive release of energy in the solar atmosphere is driven magnetically, but the mechanisms that trigger the onset of the eruption remain controversial. In the case of flares and coronal mass ejections (CMEs), ideal or non-ideal instabilities usually occur in the corona, but it is difficult to obtain direct observations and diagnostics there. To overcome this difficulty, we analyze observational signatures in the upper chromosphere or transition region, particularly brightening and dimming at the base of coronal magnetic structures. In this paper, we examine the time evolution of spatially resolved light curves in two eruptive flares and identify a variety of tempo-spatial sequences of brightening and dimming, such as dimming followed by brightening and dimming preceded by brightening. These brightening–dimming sequences are indicative of the configuration of energy release in the form of plasma heating or bulk motion. We demonstrate the potential of using these analyses to diagnose the properties of magnetic reconnection and plasma expansion in the corona during the early stages of the eruption.

We investigated in detail the M5.8 class solar flare that occurred on 2023-03-06. This flare was one of the first strong flares observed by the Siberian Radioheliograph in the microwave range and the Advanced Space-based Solar Observatory in the X-ray range. The flare consisted of two separate flaring events (a “thermal” and a “cooler” ones), and was associated with (and probably triggered by) a filament eruption. During the first part of the flare, the microwave emission was produced in an arcade of relatively short and low flaring loops. During the second part of the flare, the microwave emission was produced by energetic electrons trapped near the top of a large-scale flaring loop; the evolution of the trapped electrons was mostly affected by the Coulomb collisions. Using the available observations and the GX Simulator tool, we created a 3D model of the flare, and estimated the parameters of the energetic electrons in it.

Solar flares are significant occurrences in solar physics, impacting space weather and terrestrial technologies. Accurate classification of solar flares is essential for predicting space weather and minimizing potential disruptions to communication, navigation, and power systems. This study addresses the challenge of selecting the most relevant features from multivariate time-series data, specifically focusing on solar flares. We employ methods such as Mutual Information (MI), Minimum Redundancy Maximum Relevance (mRMR), and Euclidean Distance to identify key features for classification. Recognizing the performance variability of different feature selection techniques, we introduce an ensemble approach to compute feature weights. By combining outputs from multiple methods, our ensemble method provides a more comprehensive understanding of the importance of features. Our results show that the ensemble approach significantly improves classification performance, achieving values 0.15 higher in True Skill Statistic (TSS) values compared to individual feature selection methods. Additionally, our method offers valuable insights into the underlying physical processes of solar flares, leading to more effective space weather forecasting and enhanced mitigation strategies for communication, navigation, and power system disruptions.

The study of stellar flares has increased with new observations from CoRoT, Kepler , and TESS satellites, revealing the broad-band visible emission from these events. Typically, stellar flares have been modelled as 104 K blackbody plasma to obtain estimates of their total energy. In the Sun, white-light flares (WLFs) are much fainter than their stellar counterparts, and normally can only be detected via spatially resolved observations. Identifying the radiation mechanism for the formation of the visible spectrum from solar and stellar flares is crucial to understand the energy transfer processes during these events, but spectral data for WLFs are relatively rare, and insufficient to remove the ambiguity of their origin: photospheric blackbody radiation and/or Paschen continuum from hydrogen recombination in the chromosphere. We employed an analytical solution for the recombination continuum of hydrogen instead of the typically assumed 104 K blackbody spectrum to study the energy of stellar flares and infer their fractional area coverage. We investigated 37 events from Kepler-411 and five events from Kepler-396, using both radiation mechanisms. We find that estimates for the total flare energy from the H recombination spectrum are about an order of magnitude lower than the values obtained from the blackbody radiation. Given the known energy transfer processes in flares, we argue that the former is a physically more plausible model than the latter to explain the origin of the broad-band optical emission from flares. [ABSTRACT FROM AUTHOR]

A significant impediment to solving the coronal heating problem is that we currently only observe active region loops in their cooling phase. Previous studies showed that the evolution of cooling loop densities and apex temperatures is insensitive to the magnitude, duration, and location of energy deposition. Still, potential clues to how energy is released are encoded in the properties of the cooling phase. The appearance of coronal rain, one of the most spectacular phenomena of the cooling phase, occurs when plasma has cooled below 1 MK, which sets constraints on the heating frequency, for example. Most observations of coronal rain have been made by imaging instruments. Here we report rare Hinode/EUV Imaging Spectrometer (EIS) observations of a loop arcade where coronal rain forms following an X2.1 limb flare. A bifurcation in plasma composition measurements between photospheric at 1.5 MK and coronal at 3.5 MK suggests that we are observing postflare-driven coronal rain. Increases in nonthermal velocities and densities with decreasing temperature (2.7–0.6 MK) suggest that we are observing the formation and subsequent evolution of the condensations. Doppler velocity measurements imply that a 10% correction of apparent flows in imaging data is reasonable. Emission measure analysis at 0.7 MK shows narrow temperature distributions, indicating coherent behavior reminiscent of that observed in coronal loops. The limitations on spatio-temporal resolution of EIS suggest that we are observing the largest features or rain showers. These observations provide insights into the heating rate, source, turbulence, and collective behavior of coronal rain from observations of the loop cooling phase. [ABSTRACT FROM AUTHOR]

In the pursuit of enhancing the technological maturity of innovative magnetic sensing techniques, opportunities presented by in-orbit platforms (IOD/IOV experiments) provide a means to evaluate their in-flight capabilities. The Magnetic Experiments for the Laser Interferometer Space Antenna (MELISA) represent a set of in-flight demonstrators designed to characterize the low-frequency noise performance of a magnetic measurement system within a challenging space environment. In Low Earth Orbit (LEO) satellites, electronic circuits are exposed to high levels of radiation coming from energetic particles trapped by the Earth's magnetic field, solar flares, and galactic cosmic rays. A significant effect is the accidental bit-flipping in memory registers. This work presents an analysis of memory data redundancy resources using auxiliary second flash memory and exposes recovery options to retain critical data utilizing a duplicated data structure. A new and lightweight technique, CCM (Cross-Checking and Mirroring), is proposed to verify the proper performance of these techniques. Four alternative algorithms included in the original version of the MELISA software (Version v0.0) are presented. All the versions have been validated and evaluated according to various merit indicators. The evaluations showed similar performances for the proposed techniques, and they are valid for situations in which the flash memory suffers from more than one bit-flip. The overhead due to the introduction of additional instructions to the main code is negligible, even in the target experiment based on an 8-bit microcontroller. [ABSTRACT FROM AUTHOR]

The article investigates the statistical relations between the values of geomagnetic indices and the characteristics of cosmic rays and interplanetary disturbances for Forbush decreases with a sudden and gradual commencement associated with different types of solar sources: (a) coronal mass ejections from active regions accompanied by solar flares; (b) filament eruptions outside active regions; (c) high-speed streams from coronal holes; and (d) multiple sources. Using statistical methods, we also compare the dependence of geomagnetic indices on cosmic ray and solar wind parameters for Forbush decreases in solar cycles 23 and 24. The results show that (a) interplanetary disturbances associated with coronal mass ejections from active regions cause mainly magnetic storms with a sudden commencement, (b) interplanetary disturbances associated with high-speed streams from coronal holes cause mainly storms with a gradual commencement, and (c) interplanetary disturbances associated with filament eruptions outside active regions cause equally likely storms with a sudden and gradual commencement. For sporadic Forbush decreases, the cosmic ray and geomagnetic activity parameters are, on average, larger for sudden commencement events; for recurrent Forbush decreases, the nature of the event commencement does not affect the magnitude of these parameters. For all types of solar sources, the disturbed solar wind parameters are, on average, larger in events with sudden commencement. The geoeffectiveness of interplanetary disturbances is significantly higher in cycle 23 for events associated with ejections from active regions; for other types of disturbances, the difference between cycles is weak. [ABSTRACT FROM AUTHOR]

The correlation between optical and γ-ray flux variations in blazars reveals a complex behaviour. In this study, we present our analysis of the connection between changes in optical and γ-ray emissions in the blazar Ton 599 over a span of approximately 15 yr, from 2008 August to 2023 March. Ton 599 reached its highest flux state across the entire electromagnetic spectrum during the second week of 2023 January. To investigate the connection between changes in optical and γ-ray flux, we have designated five specific time periods, labelled as epochs A, B, C, D, and E. During periods B, C, D, and E, the source exhibited optical flares, while it was in its quiescent state during period A. The γ-ray counterparts to these optical flares are present during periods B, C, and E; however, during period D, the γ-ray counterpart is either weak or absent. We conducted a broad-band spectral energy distribution (SED) fitting by employing a one-zone leptonic emission model for these epochs. The SED analysis unveiled that the optical–ultraviolet emission primarily emanated from the accretion disc in quiescent period A, whereas synchrotron radiation from the jet dominated during periods B, C, D, and E. Diverse correlated patterns in the variations of optical and γ-ray emissions, like correlated optical and γ-ray flares, could be accounted for by changes in factors such as the magnetic field, bulk Lorentz factor, and electron density. On the other hand, an orphan optical flare could result from increased magnetic field and bulk Lorentz factor. [ABSTRACT FROM AUTHOR]

The problem of explaining observed soft X-ray fluxes during solar flares, which invokes acceleration of large fraction of electrons, if the acceleration takes places at the solar coronal loop-top, can potentially be solved by postulating that flare at loop-top creates dispersive Alfven waves (DAWs) which propagate towards the foot-points. As DAWs move in progressively denser parts of the loop (due to gravitational stratification) the large fraction of electrons is no longer needed. Here, we extend our previous results by considering f −1 frequency spectrum of DAWs and add He++ ions using fully kinetic particle-in-cell (PIC) simulations. We consider cases when transverse density gradient is in the range 4–40 c /ωpe and DAW driving frequency is 0.3–0.6ωcp. We find that (i) The frequency spectrum case does not affect electron acceleration fraction in the like-to-like cases, but few times larger percentage of He++ heating is seen due to ion cyclotron resonance; (ii) In cases when counter propagating DAWs collide multiple-times, much larger electron and ion acceleration fractions are found, but the process is intermittent in time. This is because intensive heating (temperature increase) makes the-above-thermal-fraction smaller; Also more isotropic velocity distributions are seen; (iii) Development of kink oscillations occurs when DAWs collide; (iv) Scaling of the magnetic fluctuations power spectrum steepening in the higher-density regions is seen, due to wave refraction. Our PIC runs produce much steeper slopes than the orginal spectrum, indicating that the electron-scale physics has a notable effect on DAW spectrum evolution. [ABSTRACT FROM AUTHOR]

Solar white-light flares (WLFs) are those accompanied by brightenings in the optical continuum or integrated light. The White-light Solar Telescope (WST), as an instrument of the Lyman-alpha Solar Telescope (LST) on the Advanced Space-based Solar Observatory (ASO-S), provides continuous solar full-disk images at 360 nm, which can be used to study WLFs. We analyze 205 major flares above M1.0 from October 2022 to May 2023 and identify 49 WLFs at 360 nm from WST observations, i.e. with an occurrence rate of 23.9%. The percentages of WLFs for M1 – M4 (31 out of 180), M5 – M9 (11 out of 18), and above X1 (7 for all) flares are 17.2%, 61.1%, and 100%, respectively, namely the larger the flares, the more likely they are WLFs at 360 nm. We further analyze 39 WLFs among the identified WLFs and investigate their properties such as white-light enhancement, duration, and brightening area. It is found that the relative enhancement of the white-light emission at 360 nm is mostly (>90%) less than 30% and the mean enhancement is 19.4%. The WLFs' duration at 360 nm is mostly (>80%) less than 20 minutes and its mean is 10.3 minutes. The brightening area at 360 nm is mostly (>75%) less than 500 arcsecond2 and the median value is 225. We find that there exist good correlations between the white-light enhancement/duration/area and the peak soft X-ray (SXR) flux of the flare, with correlation coefficients of 0.68, 0.58, and 0.80, respectively. In addition, the white-light emission in most WLFs peaks around the same time as the temporal derivative of SXR flux as well as the hard X-ray emission at 20 – 50 keV, indicative of the Neupert effect. It is also found that the limb WLFs are more likely to have a greater enhancement, which is consistent with numerical simulations. [ABSTRACT FROM AUTHOR]

Solar Cycle 23 was the most active in ground-level enhancements (GLEs) with 16 events registered by the global neutron monitor network. In this paper, we study a very active period in October–November, 2003, which revealed an intense solar activity burst that led to several eruptive processes and produced a sequence of three GLEs. By applying state-of-the-art modelling to records from the global neutron monitor network as well as space-borne data, we derived the spectral and anisotropy characteristics of accelerated solar protons during the GLE #65 event on 28 October, 2003 and GLE #66 on 29 October, 2003. The spectra and the pitch angle distributions are obtained with a 5-min time resolution, providing their dynamical evolution throughout the event. The spectra are parameterised with a modified power-law rigidity spectrum, whilst the angular distribution with a Gaussian. The constraints and uncertainties of the derived characteristics are evaluated by corresponding modelling. [ABSTRACT FROM AUTHOR]

Since the launch of the Fermi mission in 2008, it has become possible to study high-energy solar γ -rays with an unprecedented imaging capability. In particular, the position of the > 100 MeV γ -ray source can shed light on the origin of high-energy protons that is still controversial. However, the imaging of solar γ -ray sources with the Fermi Large Area Telescope (LAT) is a complex multi-stage process influenced by a number of factors and instrumental effects, which is difficult to fully comprehend a priori. The SOL2014-09-01 behind-the-limb event was significant, for which the γ -ray source position was not firmly established at once. Following the methodology outlined by the Fermi/LAT team, we estimated the proton power-law indices and γ -ray centroid positions at two temporal intervals of this event, separated by one hour. Our estimates for the first interval are comparable to estimates recently updated by the Fermi/LAT team, thereby confirming the consistency of the analysis applied. Although, in the second interval, corresponding to the decay phase of the flare, the proton power-law index clearly hardened, the presumable position of the fading γ -ray source remained unchanged. Its constancy in both temporal intervals and its proximity to the bases of long coronal loops connected to the flare site support the flare origin of high-energy protons injected into these loops along with electrons and trapped there for a long time. Our experience analyzing Fermi/LAT data clarifies their complex handling and will hopefully benefit the solar community in their wider use. [ABSTRACT FROM AUTHOR]

We show using superposed epoch analysis (SEA) that the most energetic protons (> 60 MeV ) in the near-Earth interplanetary magnetic field (IMF) have a peak almost immediately (less than a day) after the peak in solar-flare index (SFI), while protons greater than 10 MeV peak one day after the SFI and protons greater than 1 MeV peak two days after the SFI. The geomagnetic indices AU, -AL, PC, Ap, and -Dst peak after two to three days in SEAs after the peak in SFI. The auroral electrojet indices AU and -AL, however, have only low peaks. In particular, the response of the eastward electrojet, AU, to SFI is negligible compared to other geomagnetic indices. The SEAs of the SFI and cosmic-ray counts (CR) show that the deepest decline in the CR intensity also follows with a 2 – 3-day lag the maximum of the SFI for Solar Cycles 20 – 24. The depths of the declines are related to the SFI strength of each cycle, i.e., the average decline is about 5% for Cycles 21 and 22, but only 3% for Cycle 24. The strongest Cycle 19, however, differs from the other cycles such that it has a double-peaked decline and lasts longer than the decline of the other cycles. The double-superposed epoch analyses show that the response of IMF Bv2, which is about two days, and CR to SFI are quite simultaneous, but sometimes Bv2 may peak somewhat earlier than the decline existing in CR. [ABSTRACT FROM AUTHOR]

Self-organization in continuous systems is associated with dissipative processes. In particular, for magnetized plasmas, it is known as magnetic relaxation, where the magnetic energy is converted into heat and kinetic energy of flow through the process of magnetic reconnection. An example of such a system is the solar corona, where reconnection manifests as solar transients like flares and jets. Consequently, toward investigation of plasma relaxation in solar transients, we utilize a novel approach of data-constrained MHD simulation for an observed solar flare. The selected active region NOAA 12253 hosts a GOES M1.3 class flare. The investigation of extrapolated coronal magnetic field in conjunction with the spatiotemporal evolution of the flare reveals a hyperbolic flux tube (HFT), overlying the observed brightenings. MHD simulation is carried out with the EULAG-MHD numerical model to explore the corresponding reconnection dynamics. The overall simulation shows signatures of relaxation. For a detailed analysis, we consider three distinct subvolumes. We analyze the magnetic field line dynamics along with time evolution of physically relevant quantities like magnetic energy, current density, twist, and gradients in magnetic field. In the terminal state, none of the subvolumes is seen to reach a force-free state, thus remaining in nonequilibrium, suggesting the possibility of further relaxation. We conclude that the extent of relaxation depends on the efficacy and duration of reconnection, and hence on the energetics and time span of the flare. [ABSTRACT FROM AUTHOR]

Self-organized critical avalanche models are a class of cellular automata that, despite their simplicity, can be applied to the modeling of solar (and stellar) flares and generate robust power-law distributions in event size measures. However, bridging the conceptual gap to both magnetohydrodynamics and real flare observations continues to prove challenging. In this paper, we focus on a specific, key aspect of this endeavor: the definition of magnetic energy and its consequences for the model's internal dynamics and energy release statistics. We show that the dual requirement of releasing energy and restoring local stability demands that the instability criterion and boundary conditions be set in a manner internally consistent with a given energy definition; otherwise, unphysical behavior ensues, e.g., negative energy release. Working with three energy definitions previously used in the literature, we construct such internally consistent avalanche models and compare/contrast their energy release statistics. Using the same set of models, we also explore a recent proposal by Farhang et al. (2018, 2019), namely, that avalanches/flares should maximize the amount of energy released by the lattice when instabilities are triggered. This tends to produce avalanches of shorter duration but higher peak energy release, adding to a similar total energy release. For the three energy definitions we tested, these avalanche models exhibit almost identical distributions of event size measures. Our results indicate that the key to reproduce solar-like power-law slopes in these size measures is lattice configurations in which most nodes remain relatively far from the instability threshold. [ABSTRACT FROM AUTHOR]

Charged particles, generated in solar flares, sometimes can attain extremely high energy, above the 500-MeV level, and produce abrupt ground-level enhancements (GLEs) on the ground-based detectors of cosmic rays. The initial flares are strong eruptions and they could originate from active regions (ARs). A list of GLE events and associated flares was initially available, and our aim was to find the hosting AR for each GLE event. Moreover, we aimed to classify the revealed ARs using the magnetomorphological classification (MMC: Abramenko, 2021). We have shown that in 94% of cases such ARs belong to the most complex morphological classes, namely, β γ , β δ , γ δ , β γ δ classes by the Hale classification and B2, B3 classes by the MMC. We also found that the GLE-associated ARs are the ARs with the total unsigned magnetic flux much stronger than the common ARs of the same complexity. The set of GLE-related ARs only partially overlaps with the set of SARs (superactive regions). These ARs seem to be a manifestation of nonlinearities in the regular process of the global mean-field dynamo, the key ingredient to maintain fluctuations and to create critical conditions in different aspects of the solar activity. [ABSTRACT FROM AUTHOR]

Intense solar energetic particle (SEP) events can be observed by neutron monitors (NMs) as so-called ground-level enhancements (GLEs). High-altitude polar NMs have high sensitivity for SEP due to the reduced atmospheric energy cutoff and very low geomagnetic rigidity cutoff compared to other NMs. There is a special class of sub-GLE events, viz. events that are weaker than standard GLEs and can only be observed by high-altitude polar NMs. So far, only one sub-GLE and three candidates are known, all in the period 2012 – 2015. In this work, we inspected the period from March 1964 to December 1969 in the data of the South Pole and Vostok high-altitude polar NMs on the Antarctic Plateau in search of possible sub-GLEs. We found two previously unknown events from 09 June 1968 and 27 February 1969 that formally match the definition of sub-GLE. They were associated with significant enhancements in the integral SEP intensity J (> 60 MeV) measured by space-borne particle detectors, as well as with strong X-class solar flares from the western part of the solar disk. The identified sub-GLEs were analyzed and the corresponding SEP characteristics were estimated. [ABSTRACT FROM AUTHOR]

We present a catalogue of 6266 solar flares detected by the Solar X-ray Monitor onboard the Chandrayaan-2 lunar orbiter between 1.55 and 12.4 keV (1 and 8 Å) from 2019 September 12 to 2022 November 4, including 1469 type A flares. The catalogue represents the first large sample, including both type A, hot thermal flares, and type B, impulsive flares, with a sub-A-class sensitive instrument. We also detect 213 sub-A and 1330 A-class flares. Individual flares are fitted with an exponentially-modified Gaussian function and multi-flare groups are decomposed into individual flares. We validate our findings with flare catalogues made using visual inspection as well as automatic pipelines on Geostationary Operational Environmental Satellite and Solar Dynamics Observatory data. We find a clear bimodality in the ratio of the width to decay time between type A and B flares. We infer a power-law index of α F = 1.92 ± 0.09 for the background-subtracted peak flux distribution of XSM flares, which is consistent with the value ∼ 2 reported in the literature. We also infer α F = 1.90 ± 0.09 for type B, and α F = 1.94 ± 0.08 for type A flares, which has previously not been reported in the literature. These comparable values hint at a similarity in their generative processes. [ABSTRACT FROM AUTHOR]

The transition region is a very thin but most peculiar layer in the solar atmosphere located between the solar chromosphere and the corona. It is a key region for understanding coronal heating, solar eruption triggers, and the origin of solar winds. Here, almost all physical parameters (density, temperature, and magnetic fields) have the maximum gradient. Therefore, this region should be highly dynamic, including fast energy releasing and transporting, plasma heating, and particle accelerating. The physical processes can be categorized into two classes: thermal and non-thermal processes. Thermal processes can be observed at ultraviolet (UV) and extreme ultraviolet (EUV) wavelengths via multi-wavelength images. Non-thermal processes accelerate non-thermal electrons and produce radio emissions via the gyrosynchrotron mechanism resulting from the interaction between the non-thermal electrons and magnetic fields. The frequency range spans from several GHz to beyond 100 GHz, in great number of bursts with narrowband, millisecond lifetime, rapid frequency drifting rates, and being referred to as transition region small-scale microwave bursts (TR-SMBs). This work proposes a new type of Solar Ultra-wide Broadband Millimeter-wave Spectrometer (SUBMS) that can be used to observe TR-SMBs. From SUBMS observations, we can derive rich dynamic information about the transition region, such as information about non-thermal energy release and propagation, the flows of plasma and energetic particles, the magnetic fields and their variations, the generation and transportation of various waves, and the formation and evolution of the source regions of solar eruptions. Such an instrument can actually detect the non-thermal signals in the transition region during no flare as well as the eruptive high-energy processes during solar flares. [ABSTRACT FROM AUTHOR]

Solar flares – the most prominent manifestation of solar activity – typically manifest themselves as a single or a set of luminous arcs (magnetic flux tubes) rooted in regions of opposite polarity in the photosphere. However, a careful analysis of archival data from the Hinode satellite reveals occasional surprising cases of flaring arcs whose footpoints belong to regions of the same polarity or to areas without any appreciable magnetic field. Despite the counterintuitive nature of this phenomenon, it can be reasonably interpreted in the framework of the so-called 'topological model' of magnetic reconnection, whereby a magnetic null point is formed owing to a specific superposition of influences from remote sources rather than by local current systems. As a result, the energy release propagates along the separator of a flipping two-dome structure rather than along a fixed magnetic field line. Therefore, the luminous arc no longer needs to be associated immediately with the magnetic sources. Here, we report observational cases of the above-mentioned type and provide a theoretical model and numerical simulations. [ABSTRACT FROM AUTHOR]

In the article, we have studied the impact linear polarization of the helium D3 line that takes place in the solar chromosphere under the action of protons accelerated in solar flares. The dependence of the energy distribution of protons on the distance traveled inside the chromosphere is calculated. The ratio of the concentrations of nonthermal protons and thermal electrons at different depths is theoretically determined. From the calculation of the degree of linear polarization of the helium line D3 for different layers of the chromosphere, the region of probable formation of the line is determined. [ABSTRACT FROM AUTHOR]

It is known that so-called solar flares systematically occur in the area of sunspots. They are accompanied by radiation in almost all frequency ranges and sometimes by the emission of hot plasma. Observations on the RATAN-600 radio telescope have shown that the radio emission spectrum of plasma clouds heated to values of the order of 106 K erupted from the solar flare region turned out to be adiabatic. The high correlation of the inhomogeneities of the radio emission spectra of active formation over a group of sunspots indicates the stable presence of recombination radio lines in the radiation of active formation. However, the radio emission spectra of hot plasma clouds ejected from the region of solar flares occurring in this group of spots do not show any correlation. [ABSTRACT FROM AUTHOR]

It has been suggested that the ratio of photospheric direct to return current, ∣DC/RC∣, may be a better proxy for assessing the ability of solar active regions to produce a coronal mass ejection (CME) than others such as the amount of shear along the polarity inversion line (PIL). To test this conjecture, we measure both quantities prior to eruptive and confined flares of varying magnitude. We find that eruptive-flare source regions have ∣DC/RC∣ > 1.63 and PIL shear above 45° (average values of 3.°2 and 68°, respectively), tending to be larger for stronger events, while both quantities are on average smaller for confined-flare source regions (2.°2 and 68°, respectively), albeit with substantial overlap. Many source regions, especially those of eruptive X-class flares, exhibit elongated direct currents (EDCs) bracketing the eruptive PIL segment, which typically coincide with areas of continuous PIL shear above 45°. However, a small subset of confined-flare source regions have ∣DC/RC∣ close to unity, very low PIL shear (<38°), and no clear EDC signatures, rendering such regions less likely to produce a CME. A simple quantitative analysis reveals that ∣DC/RC∣ and PIL shear are almost equally good proxies for assessing CME-productivity, comparable to other proxies suggested in the literature. We also show that an inadequate selection of the current-integration area typically yields a substantial underestimation of ∣DC/RC∣, discuss specific cases that require careful consideration for ∣DC/RC∣ calculation and interpretation of the results, and suggest improving photospheric CME-productivity proxies by incorporating coronal measures such as the decay index. [ABSTRACT FROM AUTHOR]

Fast electron scattering on plasma ions due to stimulated Bremsstrahlung is investigated and modeled. Comparison with Coulomb scattering suggests that stimulated Bremsstrahlung scattering can be dominant in low-density, radiation-driven plasmas, provided that the radiation spectrum has a sufficiently high brightness temperature in the neighborhood of the plasma frequency. While stimulated Bremsstrahlung scattering cannot be easily observed in laboratory plasmas due to their small size, it should operate in large-scale astrophysical plasmas, such as those met in the flaring solar corona. The effect of the solar microwave radiation on fast-electron scattering is evaluated through a parameterized flaring corona model. We find that stimulated Bremsstrahlung greatly enhances the fast-electron scattering frequency in the flare magnetic loop, leading the transport of deka-keV electrons to occur in the diffusion regime, characterized by significant precipitation rates. This prediction is consistent with the interpretation of the above-loop-top hard x-ray and microwave emissions from the X3.1 flare of August 24, 2002. Our analysis indicates that stimulated Bremsstrahlung may play an essential role in the dynamics of fast electrons trapped in solar flare loops. [ABSTRACT FROM AUTHOR]

On 3 July 2021, an X1.5 solar flare from the National Oceanic and Atmospheric Administration solar Active Region AR12838 (24°N, 88°W) occurred at 14:18 UT, peaked at 14:29 UT, and decayed at 14:34 UT. The study of this X1.5 solar flare is significant due to its unique geomagnetic crochet feature at high latitudes and its effective signature on Earth. The study examined X‐rays, the extreme ultraviolet spectrum, ionospheric equivalent current (IEC), and geomagnetic field components. The study reveals a sudden increase in IEC during the X1.5 flare episode, forming a zonal current region and producing a geomagnetic crochet signature in geomagnetic field components at high latitudes (50°–80°N) along the 11°–26°E longitude sector during the flare peak time. All three geomagnetic field components show different sensitivity to the solar flare effect (sfe), and the amplitude and phase of the geomagnetic crochet across latitudes (for a given longitude) are consistent with the variations in the IEC. The present study is the first to appraise geomagnetic crochets of low magnitude (8–40 nT) and short duration (10–15 min) at high latitudes, particularly in the polar cusp region, during the X‐class limb flare. Plain Language Summary: The X1.5 solar flare at 14:18 UT on 3 July 2021, occurred in a newly formed solar active region AR12838 (24°N, 88°W) on the sun. The current study analyses the solar flare effect (sfe) signature at high latitude induced by an X1.5 flare, and compares the results from previous paper of Yamauchi et al. (2020, https://doi.org/10.5194/angeo-38-1159-2020). However, while that paper reported geomagnetic crochet of high amplitude (200 nT) and long duration (∼2 hr), the present work reports geomagnetic crochet of small amplitude (40 nT) and short duration (∼15 min). The current study is unique regarding observations of geomagnetic crochet in the polar cusp region. Key Points: An X1.5‐class solar flare was recorded on 3 July 2021, from the newly emerged solar active region NOAA AR12838 over the solar limbThe current study examines the solar flare effect at high latitude magnetometer stations, an aspect that has not been extensively studiedUnique signatures of Cusp geomagnetic crochet of small amplitude and short duration are observed [ABSTRACT FROM AUTHOR]

Deep learning is successful in many fields due to its ability to learn strong feature representations without the need for hand‐crafted features, resulting in models with high representational power. However, many of these models are based on supervised learning and therefore depend on the availability of large annotated data sets. These are often difficult to obtain because they require human input. A common challenge for researchers in space weather is the sparsity of annotations in many of the available data sets, which are either unlabeled or have ambiguous labels. To alleviate the data bottleneck of loosely annotated data sets, unsupervised deep learning has become an important strategy, with anomaly detection being one of the most prominent applications. Unsupervised models have been successfully applied in various domains, such as medical imaging or video surveillance, to distinguish normal from abnormal data. In this work, we investigate how a purely unsupervised approach can be used to detect and extract solar phenomena in extreme ultraviolet images from the NASA SDO spacecraft. We show how a model based on variational autoencoders can be used to detect out‐of‐distribution samples and to localize regions of interest for solar activity. By using an unsupervised approach, we hope to contribute to space weather monitoring tools and further improve the understanding of space weather drivers. Plain Language Summary: Deep learning is a machine learning technique that extracts patterns from data without being explicitly programmed. It has been very successful in many fields because it can learn from large amounts of data and create models that excel at recognizing patterns. However, the standard deep learning method is supervised: It needs a lot of labeled data to work well. The scarcity of labeled data in the space weather domain makes it difficult to apply. To overcome this problem, we present in this article an unsupervised method that does not require labeled data. We use it to detect unusual patterns, or anomalies, in the data. In this research, we develop an unsupervised model that detects and extracts solar features from images taken by the Solar Dynamics Observatory spacecraft. The model is called a context encoding variational autoencoder. It learns to compress and reconstruct solar images. By training this model on a large set of images, we can detect unusual patterns in new images and locate regions of interest for solar activity. With this method, we hope to contribute to space weather monitoring tools and improve our understanding of the causes of space weather effects. Key Points: We introduce a way to extract a low‐dimensional feature representation from SDO images and significantly compress input imagesWe argue that unsupervised learning can support anomaly detection, thus enabling the real‐time flagging of unusual solar activityWe show how to effectively prepare and use the SDO ML version 2 data set to train a deep learning model [ABSTRACT FROM AUTHOR]

We present a photometric characterization of 208 ultra-cool dwarfs (UCDs) with spectral types between M4 and L4, from 20-s and 2-min cadence TESS light curves. We determine rotation periods for 87 objects (⁠|$\sim 42~{{\ \rm per\ cent}}$|⁠) and identify 778 flare events in 103 UCDs (⁠|$\sim 49.5~{{\ \rm per\ cent}}$|⁠). For 777 flaring events (corresponding to 102 objects), we derive bolometric energies between 2.1 × 1030 and |$1.1 \times 10^{34}\ \mathrm{erg}$|⁠ , with 56 superflare events. No transiting planets or eclipsing binaries were identified. We find that the fraction of UCDs with rotation and flaring activity is, at least, 20 per cent higher in M4–M6 spectral types than in later UCDs (M7–L4). For spectral types between M4 and L0, we measure the slope of the flare bolometric energy–duration correlation to be γ = 0.497 ± 0.058, which agrees with that found in previous studies for solar-type and M dwarfs. Moreover, we determine the slope of the flare frequency distribution to be α = −1.75 ± 0.04 for M4–M5 dwarfs, α = −1.69 ± 0.04 and α = −1.72 ± 0.1 for M6–M7 and M8–L0 dwarfs, respectively, which are consistent with previous works that exclusively analysed UCDs. These results support the idea that independently of the physical mechanisms that produce magnetic activity, the characteristics of the rotational modulation and flares are similar for both fully convective UCDs and partially convective solar-type and early-M stars. Based on the measured UCD flare distributions, we find that ultraviolet radiation emitted from flares does not have the potential to start prebiotic chemistry. [ABSTRACT FROM AUTHOR]

It is widely accepted that X-ray emission in luminous active galactic nuclei (AGNs) originates from hot corona. To prevent the corona from overcooling by strong X-ray emission, steady heating to the corona is essential, for which the most promising mechanisms is the magnetic reconnection. Detailed studies of the coupled disc and corona, in the frame of magnetic field transferring accretion-released energy from the disc to the corona, reveal that the thermal electrons can only produce X-ray spectrum with |$\Gamma _{\rm 2-10\, keV}\gt 2.1$|⁠ , which is an inevitable consequence of the radiative coupling of the thermal corona and disc. In this work, we develop the magnetic-reconnection-heated corona model by taking into account the potential non-thermal electrons accelerated in the magnetic reconnection process, in addition to the thermal electrons. We show that the features of the structure and spectrum of the coupled disc and corona can be affected by the fraction of magnetic energy allocated to thermal electrons. Furthermore, we investigate the effects of the power-law index and energy range of non-thermal electrons and the magnetic field on the spectrum. It is found that the X-ray spectrum from the Comptonization of the hybrid electrons can be flatter than that from thermal electrons only, in agreement with observations. By comparing with the observed hard X-ray data, we suggest that a large fraction (⁠|$\gt 40~{{\ \rm per\ cent}}$|⁠) of the magnetic energy be allocated to the non-thermal electrons in the luminous and flat X-ray spectrum AGNs. [ABSTRACT FROM AUTHOR]

Chromospheric oscillations can give us insight into the physical environment in the solar atmosphere, both in quiet Sun and flaring conditions. Many authors have reported increases in the prevalence of 3-minute oscillations which are thought to be excited by events which impact the chromosphere such as flares. In this study, we utilized the Ca  ii 8542 Å line to study the oscillatory behaviour of the chromosphere in an active region which underwent two B-class flares. We analysed oscillations in both intensity and velocity, and found different behaviours in both. Intensity oscillations were most prevalent over the umbrae of sunspots and magnetic pores in the active region, and the extent of the area which contained significant oscillations was found to decrease when comparing times after the flares to before. By measuring the evolution of the magnetic field, we found that this could be because the areas surrounding the umbrae were becoming more 'penumbral' with an increase to the magnetic field inclination. Velocity oscillations were found across the active region both before and after the flares but were observed clearly in areas which were brightened by the second flare. By comparing to EUV imaging, it was seen that strong chromospheric velocity oscillations with 3–4-minute periods occurred at the same time and location as a flare loop cooling 30 min after the second flare peak. This could be evidence of disturbances in the loop exciting a response from the chromosphere at its acoustic cut-off frequency. [ABSTRACT FROM AUTHOR]

Context. Stellar flares cannot be spatially resolved, which complicates ascertaining the physical processes behind particular spectral signatures. Due to their proximity to Earth, solar flares can serve as a stepping stone for understanding their stellar counterparts, especially when using a Sun-as-a-star instrument and in combination with spatially resolved observations. Aims. We aim to understand the disk-integrated spectral behaviors of a confined X2.2 flare and its eruptive X9.3 successor, which had energies of 2.2 × 1031 erg and 9.3 × 1031 erg, respectively, as measured by Sun-as-a-star observations with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N). Methods. The behavior of multiple photospheric (Na D1 & D2, Mg I at 5173 Å, Fe I at 6173 Å, and Mn I at 4031 Å) and chromospheric (Ca II H & K, Hα, Hβ, and He ID3) spectral lines were investigated by means of activity indices and contrast profiles. A number of different photospheric lines were also investigated by means of equivalent widths, and radial velocity measures, which were then related to physical processes directly observed in high-resolution observations made with the Swedish 1-m Solar Telescope (SST) and the Atmospheric Imaging Assembly (AIA) on board of the Solar Dynamics Observatory (SDO). Results. Our findings suggest a relationship between the evolving shapes of contrast profile time and the flare locations, which assists in constraining flare locations in disk-integrated observations. In addition, an upward bias was found in flare statistics based on activity indices derived from the Ca II H & K lines. In this case, much smaller flares cause a similar increase in the activity index as that produced by larger flares. Hα-based activity indices do not show this bias and are therefore less susceptible to activity jitter. Sodium line profiles show a strongly asymmetric response during flare activity, which is best captured with a newly defined asymmetrical sodium activity index. A strong flare response was detected in Mn I line profiles, which is unexpected and calls for further exploration. Intensity increases in Hα, Hβ, and certain spectral windows of AIA before the flare onset suggest their potential use as short-term flare predictors. [ABSTRACT FROM AUTHOR]

Context. The properties of pre-eruptive structures and coronal mass ejections (CMEs) are characterized by those of their footpoints, the latter of which attract a great deal of interest. However, the matter of how to identify the footpoints of pre-eruptive structures and how to do so with the use of ground-based instruments still remains elusive. Aims. In this work, we study an arc-shaped structure intruding in the sunspot umbra. It is located close to the (pre-)eruptive flux rope footpoint and it is expected to help identify the footpoint. Methods. We analyzed this arc-shaped structure, which we call a "sunspot scar", in a CME event on July 12, 2012, and in two CME events from observationally inspired magnetohydrodynamic simulations performed by OHM and MPI-AMRVAC. Results. The sunspot scar displays a more inclined magnetic field with a weaker vertical component and a stronger horizontal component relative to that in the surrounding umbra and is manifested as a light bridge in the white light passband. The hot field lines anchored in the sunspot scar are spatially at the transition between the flux rope and the background coronal loops and temporally in the process of the slipping reconnection which builds up the flux rope. Conclusions. The sunspot scar and its related light bridge mark the edge of the CME flux rope footpoint and particularly indicate the edge of the pre-eruptive flux rope footpoint in the framework of "pre-eruptive structures being flux ropes". Therefore, they provide a new perspective for the identification of pre-eruptive and CME flux rope footpoints, as well as new methods for studying the properties and evolution of pre-eruptive structures and CMEs with photospheric observations only. [ABSTRACT FROM AUTHOR]

Space weather phenomena, including solar flares and coronal mass ejections, have significant influence on Earth. These events can cause satellite orbital decay due to heat-induced atmospheric expansion, disruption of GPS navigation and telecommunications systems, damage to satellites, and widespread power blackouts. The potential of flares and associated events to damage technology and disrupt human activities motivates prediction development. We use Transformer networks to predict whether an active region (AR) will release a flare of a specific class within the next 24 h. Two cases are considered: ≥Cclass and ≥M-class. For each prediction case, separate models are developed. We train the Transformer to use time-series data to classify 24- or 48-h sequences of data. The sequences consist of 18 physical parameters that characterize an AR from the Space-weather HMI Active Region Patches data product. Flare event information is obtained from the Geostationary Operational Environmental Satellite flare catalog. Our model outperforms a prior study that similarly used only 24 h of data for the ≥C-class case and performs slightly worse for the ≥M-class case. When compared to studies that used a larger time window or additional data such as flare history, results are comparable. Using less data is conducive to platforms with limited storage, on which we plan to eventually deploy this algorithm. [ABSTRACT FROM AUTHOR]