We investigate the 2005 August 22 flare event(00:54 UT) exploiting hard X-ray(HXR) observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI) and microwave(MW) observations from the No...We investigate the 2005 August 22 flare event(00:54 UT) exploiting hard X-ray(HXR) observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI) and microwave(MW) observations from the Nobeyama Solar Radio Observatory. The HXR time profile exposes well-damped quasi-periodic pulsations with four sequential peaks, and the MW time profile follows the corresponding peaks.Based on this feature, we derive the time relationship of HXRs and MWs with multifrequency data from the Nobeyama Radio Polarimeter, and the spatially resolvable data from RHESSI and the Nobeyama Radioheliograph. We find that both frequency dependent delays in MWs and energy dependent delays in HXRs are significant.Furthermore, MW emissions from the south source are delayed with respect to those from the north source at both 17 GHz and 34 GHz, but no significant delays are found in HXR emissions from the different sources at the same energies. To better understand all these long time delays, we derive the electron fluxes of different energies by fitting the observed HXR spectra with a single power-law thick-target model, and speculate that these delays might be related to an extended acceleration process. We further compare the time profile of a MW spectral index derived from 17 and 34 GHz fluxes with the flux densities, and find that the spectral index shows a strong anticorrelation with the HXR fluxes.展开更多
From the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESS1) catalog we select events which have approximately the same GOES class (high C - low M or 500-1200 counts s-1 within the RHESSI 6-12 keV energy ...From the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESS1) catalog we select events which have approximately the same GOES class (high C - low M or 500-1200 counts s-1 within the RHESSI 6-12 keV energy band), but with different maximal energies of detected hard X-rays. The selected events are subdivided into two groups: (1) flares with X-ray emissions observed by RHESSI up to only 50 keV and (2) flares with hard X-ray emission observed also above 50 keV. The main task is to understand observational peculiarities of these two flare groups. We use RHESSIX-ray data to obtain spectral and spa- tial information in order to find differences between selected groups. Spectra and images are analyzed in detail for six events (case study). For a larger number of samples (85 and 28 flares in the low-energy and high-energy groups respectively) we only make some generalizations. In spectral analysis we use the thick- target model for hard X-ray emission and one temperature assumption for thermal soft X-ray emission. RHESSI X-ray images are used for determination of flare region sizes. Although thermal and spatial prop- erties of these two groups of flares are not easily distinguishable, power law indices of hard X-rays show significant differences. Events from the high-energy group generally have a harder spectrum. Therefore, the efficiency of chromospheric evaporation is not sensitive to the hardness of nonthermal electron spectra but rather depends on the total energy flux of nonthermal electrons.展开更多
Among the RHESSI flare samples, we concentrated on a kind of flare that presents two successive peaks (that is, it presents both an impulsive phase and a gradual phase) in 12 - 25 keV light curves. Taking the C1.4 f...Among the RHESSI flare samples, we concentrated on a kind of flare that presents two successive peaks (that is, it presents both an impulsive phase and a gradual phase) in 12 - 25 keV light curves. Taking the C1.4 flare on 2002 August 12 as an example, we studied the light curves, spectra, and images in detail. Making full use of the capabilities of RHESSI, we showed some evidence to support the expected causal relationship between these two peaks; the first peak is mainly nonthermal, while the second peak is mainly thermal; the energy carried by nonthermal electrons during the first peak seems to be comparable to the thermal energy of the second peak. The morphologies of X-ray images and their evolutions provide additional evidence for this causality. We conclude that two such peaks in the 12 - 25 keV light curve are good evidence for the chromospheric evaporation. However, the maximum time of the second peak is later than the end time of the first peak, suggesting that for some events, a modification of the traditional Neupert effect could be necessary by inclusion of a time delay, which might be partly related to the filling of the loop by evaporated material.展开更多
The power-law frequency distributions of the peak flux of solar flare X-ray emission have been studied extensively and attributed to a system having self-organized criticality (SOC). In this paper, we first show tha...The power-law frequency distributions of the peak flux of solar flare X-ray emission have been studied extensively and attributed to a system having self-organized criticality (SOC). In this paper, we first show that, so long as the shape of the normalized light curve is not correlated with the peak flux, the flux histogram of solar flares also follows a power-law distribution with the same spectral index as the power- law frequency distribution of the peak flux, which may partially explain why power-law distributions are ubiquitous in the Universe. We then show that the spectral indexes of the histograms of soft X-ray fluxes observed by GOES satellites in two different energy channels are different: the higher energy channel has a harder distribution than the lower energy channel, which challenges the universal power-law distribution predicted by SOC models and implies a very soft distribution of thermal energy content of plasmas probed by the GOES satellites. The temperature (T) distribution, on the other hand, approaches a power-law dis- tribution with an index of 2 for high values of T. Hence the application of SOC models to the statistical properties of solar flares needs to be revisited.展开更多
The frequency distribution for several characteristics of a solar flare obeys a power law only above a certain threshold, below which there is an apparent loss of small scale events presumably caused by limited instru...The frequency distribution for several characteristics of a solar flare obeys a power law only above a certain threshold, below which there is an apparent loss of small scale events presumably caused by limited instrumental sensitivity and th:e corresponding event selection bias. It is also possible that this deviation in the power law can have a physical origin in the source. We propose two fitting models incorpo- rating a power law distribution with a low count rate cutoff plus a noise component for the frequency distribution of the hard X-ray peak count rate of all solar flare sam- ples obtained with HXRBS/SMM and BATSE/CGRO observations. Our new fitting method produces the same power-law index as previously developed methods, a low cutoff of the power-law function and its corresponding noise level, which is consistent with measurements of the actual noise level of the hard X-ray count rate. We found that the fitted low cutoff appears to be related to the noise level, i.e., flares are only recognized when their peak count rate is 3or greater than noise. Therefore, the fitted low cutoff, which is smaller than the aforementioned threshold, might be attributed to selection bias, and probably not to the actual count rate cutoff in flares at smaller scales. Whether or not the actual low cutoff physically exists needs to be checked by future observations with increased sensitivities.展开更多
Temporal and spectral characteristics of X-ray emission from 60 flares of intensity ≥C class observed by the Solar X-ray Spectrometer (SOXS) during 2003-2011 are presented. We analyze the X-ray emission observed in...Temporal and spectral characteristics of X-ray emission from 60 flares of intensity ≥C class observed by the Solar X-ray Spectrometer (SOXS) during 2003-2011 are presented. We analyze the X-ray emission observed in four and three energy bands by the Si and Cadmium-Zinc-Telluride (CZT) detectors, respectively. The number of peaks in the intensity profile of the flares varies between 1 and 3. We find moderate correlation (R ~=0.2) between the rise time and the peak flux of the first peak of the flare irrespective of energy band, which is indicative of its energy-independent nature. Moreover, the magnetic field complexity of the flaring region is found to be highly anti-correlated (R = 0.61) with the rise time of the flares while positively correlated (R = 0.28) with the peak flux of the flare. The time delay between the peak of the X-ray emission in a given energy band and that in 25-30keV decreases with increasing energy, suggesting conduction cooling is dominant in the lower energies. Analysis of 340 spectra from 14 flares reveals that the peak of differential emission measure (DEM) evolution is delayed by 60-360 s relative to that of the temperature, and this time delay is inversely proportional to the peak flux of the flare. We conclude that temporal and intensity characteristics of flares are dependent on energy as well as the magnetic field configuration of the active region.展开更多
This review summarizes new trends in studies of magnetic reconnection in solar flares. It is shown that plasmoids play a very important role in this primary flare process. Using the results of magnetohydrodynamic and ...This review summarizes new trends in studies of magnetic reconnection in solar flares. It is shown that plasmoids play a very important role in this primary flare process. Using the results of magnetohydrodynamic and particle-in-cell simulations, we describe how the plasmoids are formed, how they move and interact, and how a flare current sheet is fragmented into a cascade of plasmoids. Furthermore, it is shown that during the interactions of these plasmoids electrons are not only very efficiently accelerated and heated, but electromagnetic(radio) emission is also produced.We also describe possible mechanisms for the triggering of magnetic reconnection.The relevant X-ray and radio signatures of these processes(such as radio drifting pulsation structures, narrowband dm-spikes, and the loop-top and above-the-loop-top X-ray sources) are then described. It is shown that plasmoids can also be formed in kinked magnetic ropes. A mapping of X-points of the magnetic reconnection on the chromosphere(as e.g. a splitting of flare ribbons) is mentioned. Supporting EUV and white-light observations of plasmoids are added. The significance of all these processes for the fast magnetic reconnection and electron acceleration is outlined. Their role in fusion experiments is briefly mentioned.展开更多
Solar hard X-rays(HXRs) appear in the form of either footpoint sources or coronal sources. Each individual source provides its own critical information on acceleration of nonthermal electrons and plasma heating. Earli...Solar hard X-rays(HXRs) appear in the form of either footpoint sources or coronal sources. Each individual source provides its own critical information on acceleration of nonthermal electrons and plasma heating. Earlier studies found that the HXR emission in some events manifests a broken-up power-law spectrum, with the break energy around a few hundred keV based on spatially-integrated spectral analysis,and it does not distinguish the contributions from individual sources. In this paper, we report on the brokenup spectra of a coronal source studied using HXR data recorded by Reuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI) during the SOL2017–09–10 T16:06(GOES class X8.2) flare. The flare occurred behind the western limb and its footpoint sources were mostly occulted by the disk. We could clearly identify such broken-up spectra pertaining solely to the coronal source during the flare peak time and after. Since a significant pileup effect on the RHESSI spectra is expected for this intense solar flare, we have selected the pileup correction factor, p = 2. In this case, we found the resulting RHESSI temperature(~30MK) to be similar to the GOES soft X-ray temperature and break energies of 45–60 keV. Above the break energy, the spectrum hardens with time from spectral index of 3.4 to 2.7, and the difference in spectral indices below and above the break energy increases from 1.5 to 5 with time. However, we note that when p = 2 is assumed, a single power-law fitting is also possible with the RHESSI temperature higher than the GOES temperature by ~10MK. Possible scenarios for the broken-up spectra of the loop-top HXR source are briefly discussed.展开更多
For the ASO-S/HXI payload, the accuracy of the flare reconstruction is reliant on important factors such as the alignment of the dual grating and the precise measurement of observation orientation. To guarantee optima...For the ASO-S/HXI payload, the accuracy of the flare reconstruction is reliant on important factors such as the alignment of the dual grating and the precise measurement of observation orientation. To guarantee optimal functionality of the instrument throughout its life cycle, the Solar Aspect System (SAS) is imperative to ensure that measurements are accurate and reliable. This is achieved by capturing the target motion and utilizing a physical model-based inversion algorithm. However, the SAS optical system’s inversion model is a typical ill-posed inverse problem due to its optical parameters, which results in small target sampling errors triggering unacceptable shifts in the solution. To enhance inversion accuracy and make it more robust against observation errors, we suggest dividing the inversion operation into two stages based on the SAS spot motion model. First, the as-rigid-aspossible (ARAP) transformation algorithm calculates the relative rotations and an intermediate variable between the substrates. Second, we solve an inversion linear equation for the relative translation of the substrates, the offset of the optical axes, and the observation orientation. To address the ill-posed challenge, the Tikhonov method grounded on the discrepancy criterion and the maximum a posteriori (MAP) method founded on the Bayesian framework are utilized. The simulation results exhibit that the ARAP method achieves a solution with a rotational error of roughly±3 5 (1/2-quantile);both regularization techniques are successful in enhancing the stability of the solution, the variance of error in the MAP method is even smaller—it achieves a translational error of approximately±18μm (1/2-quantile) in comparison to the Tikhonov method’s error of around±24μm (1/2-quantile). Furthermore, the SAS practical application data indicates the method’s usability in this study. Lastly, this paper discusses the intrinsic interconnections between the regularization methods.展开更多
Using the visual inspection and base difference method and data from the X-ray Telescope (XRT) onboard Hinode and TRACE with improved spatial and temporal resolution, we selected 48 X-ray transient brightenings (X...Using the visual inspection and base difference method and data from the X-ray Telescope (XRT) onboard Hinode and TRACE with improved spatial and temporal resolution, we selected 48 X-ray transient brightenings (XTBs) and 237 EUV transient brightenings (ETBs) to study the connection between these two types of transient brightenings (TBs). These ETBs and XTBs have smaller areas (8.42 Mm^2 and 36.3 Mm^2, respectively, on average) and shorter durations (9.0 min and 6.9 min, respectively, on average) than previous studies. These XTBs show three types of morphological structure: point-like, single-loop and multiple-loop. We find only 20% of the ETBs have corresponding XTBs while the other 80% have no X-ray signatures at all. This is presumably due to the small amount of released energy, which is not enough to heat the plasma to coronal temperatures which produce X-ray emission rather than being due to the limitation of spatial resolution and temperature sensitivity of the X-ray instrument. These small ETBs may significantly contribute to the coronal heating.展开更多
Using data-driven algorithms to accurately forecast solar flares requires reliable data sets.The solar flare dataset is composed of many non-flaring samples with a small percentage of flaring samples.This is called th...Using data-driven algorithms to accurately forecast solar flares requires reliable data sets.The solar flare dataset is composed of many non-flaring samples with a small percentage of flaring samples.This is called the class imbalance problem in data mining tasks.The prediction model is sensitive to most classes of the original data set during training.Therefore,the class imbalance problem for building up the flare prediction model from observational data should be systematically discussed.Aiming at the problem of class imbalance,three strategies are proposed corresponding to the data set,loss function,and training process:TypeⅠresamples the training samples,including oversampling for the minority class,undersampling,or mixed sampling for the majority class.TypeⅡusually changes the decision-making boundary,assigning the majority and minority categories of prediction loss to different weights.TypeⅢassigns different weights to the training samples,the majority categories are assigned smaller weights,and the minority categories are assigned larger weights to improve the training process of the prediction model.The main work of this paper compares these imbalance processing methods when building a flare prediction model and tries to find the optimal strategy.Our results show that among these strategies,the performance of oversampling and sample weighting is better than other strategies in most parameters,and the generality of resampling and changing the decision boundary is better.展开更多
We report multi-wavelength observations of four solar flares on 2014 July 07.We firstly select these flares according to the soft X-ray(SXR)and extreme ultraviolet(EUV)emissions recorded by the Extreme Ultraviolet Var...We report multi-wavelength observations of four solar flares on 2014 July 07.We firstly select these flares according to the soft X-ray(SXR)and extreme ultraviolet(EUV)emissions recorded by the Extreme Ultraviolet Variability Experiment and Geostationary Orbiting Environmental S atellites.Then their locations and geometries are identified from the full-disk images measured by the Atmospheric Imaging Assembly(AIA),and the time delays among the light curves in different channels are identified.The electron number densities are estimated using the differential emission measure method.We find that three of four flares show strong emissions in SXR channels and high temperature(>6 MK)EUV wavelengths during the impulsive phase,i.e.,AIA 131 A and 94 A,and then they emit peak radiation subsequently in the middle temperature(~0.6-3 MK)EUV channels.Moreover,they last for a long time and have smaller electron densities,which are probably driven by the interaction of hot diffuse flare loops.Only one flare emits radiation at almost the same time in all the observed wavelengths,lasts for a relatively short time,and has a larger electron density.It is also accompanied by a typeⅢradio burst.The bright emission at the EUV channel could be corresponding to the associated erupting filament.展开更多
Solar type III radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the ...Solar type III radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the solar system. Consequently, they provide information on electron acceleration and transport, and the conditions of the background ambient plasma they travel through. We review the observational properties of type III bursts with an emphasis on recent results and how each property can help identify attributes of electron beams and the ambient background plasma. We also review some of the theoretical aspects of type III radio bursts and cover a number of numerical efforts that simulate electron beam transport through the solar corona and the heliosphere.展开更多
Comparing the ESP/EVE/SDO flux data of 2011 Feb 6, with the counterparts of XRS/GOES and SEM/SOHO, we find that there is an enhancement that is not apparent in the two latter datasets. The enhancement, possibly regard...Comparing the ESP/EVE/SDO flux data of 2011 Feb 6, with the counterparts of XRS/GOES and SEM/SOHO, we find that there is an enhancement that is not apparent in the two latter datasets. The enhancement, possibly regarded as a flare at first glimpse, nevertheless, does not involve an energy-release from the Sun. Based on the enhancement, we combine data from SXI/GOES 15 into a synthesized analysis, and concluded that it arises from a particle-associated enhancement in the channel that measures XUV radiation. Paradoxically, it seems to be somewhat of a particle-avalanching process. Prior to the event, a moderate geomagnetic storm took place. Subsequently, while the event is proceeding, a geomagnetic substorm is simultaneously observed. Therefore, the particles, though unidentified, are probably energetic electrons induced by substorm injection.展开更多
基金Supported by the National Natural Science Foundation of China
文摘We investigate the 2005 August 22 flare event(00:54 UT) exploiting hard X-ray(HXR) observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI) and microwave(MW) observations from the Nobeyama Solar Radio Observatory. The HXR time profile exposes well-damped quasi-periodic pulsations with four sequential peaks, and the MW time profile follows the corresponding peaks.Based on this feature, we derive the time relationship of HXRs and MWs with multifrequency data from the Nobeyama Radio Polarimeter, and the spatially resolvable data from RHESSI and the Nobeyama Radioheliograph. We find that both frequency dependent delays in MWs and energy dependent delays in HXRs are significant.Furthermore, MW emissions from the south source are delayed with respect to those from the north source at both 17 GHz and 34 GHz, but no significant delays are found in HXR emissions from the different sources at the same energies. To better understand all these long time delays, we derive the electron fluxes of different energies by fitting the observed HXR spectra with a single power-law thick-target model, and speculate that these delays might be related to an extended acceleration process. We further compare the time profile of a MW spectral index derived from 17 and 34 GHz fluxes with the flux densities, and find that the spectral index shows a strong anticorrelation with the HXR fluxes.
基金partly supported by RFBR projects 13-02-91165 and 15-32-21078MOST (973 program,2011CB811402)National Natural Science Foundation of China (11233008 and 11427803)
文摘From the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESS1) catalog we select events which have approximately the same GOES class (high C - low M or 500-1200 counts s-1 within the RHESSI 6-12 keV energy band), but with different maximal energies of detected hard X-rays. The selected events are subdivided into two groups: (1) flares with X-ray emissions observed by RHESSI up to only 50 keV and (2) flares with hard X-ray emission observed also above 50 keV. The main task is to understand observational peculiarities of these two flare groups. We use RHESSIX-ray data to obtain spectral and spa- tial information in order to find differences between selected groups. Spectra and images are analyzed in detail for six events (case study). For a larger number of samples (85 and 28 flares in the low-energy and high-energy groups respectively) we only make some generalizations. In spectral analysis we use the thick- target model for hard X-ray emission and one temperature assumption for thermal soft X-ray emission. RHESSI X-ray images are used for determination of flare region sizes. Although thermal and spatial prop- erties of these two groups of flares are not easily distinguishable, power law indices of hard X-rays show significant differences. Events from the high-energy group generally have a harder spectrum. Therefore, the efficiency of chromospheric evaporation is not sensitive to the hardness of nonthermal electron spectra but rather depends on the total energy flux of nonthermal electrons.
基金supported by the National Natural Science Foundation of China (Grant Nos.10773031 and 10833007)the Ministry of Science and Technology of China(Grant No. 2006CB806302)by the CAS project KJCX2-YW-T04
文摘Among the RHESSI flare samples, we concentrated on a kind of flare that presents two successive peaks (that is, it presents both an impulsive phase and a gradual phase) in 12 - 25 keV light curves. Taking the C1.4 flare on 2002 August 12 as an example, we studied the light curves, spectra, and images in detail. Making full use of the capabilities of RHESSI, we showed some evidence to support the expected causal relationship between these two peaks; the first peak is mainly nonthermal, while the second peak is mainly thermal; the energy carried by nonthermal electrons during the first peak seems to be comparable to the thermal energy of the second peak. The morphologies of X-ray images and their evolutions provide additional evidence for this causality. We conclude that two such peaks in the 12 - 25 keV light curve are good evidence for the chromospheric evaporation. However, the maximum time of the second peak is later than the end time of the first peak, suggesting that for some events, a modification of the traditional Neupert effect could be necessary by inclusion of a time delay, which might be partly related to the filling of the loop by evaporated material.
基金supported partially by the Strategic Priority Research Program,the Emergence of Cosmological Structures,of the Chinese Academy of Sciences(Grant No.XDB09000000)MSTC Program2011 CB811402+3 种基金the National Natural Science Foundation of China(NSFC)(Grant Nos.11173063, 11173064,11233008 and 11427803)supported by the NFSC(Grant No.11473070)the Natural Science Foundation of Jiangsu Province (Grant BK2012889)the Youth Innovation Promotion Association,CAS,for financial support
文摘The power-law frequency distributions of the peak flux of solar flare X-ray emission have been studied extensively and attributed to a system having self-organized criticality (SOC). In this paper, we first show that, so long as the shape of the normalized light curve is not correlated with the peak flux, the flux histogram of solar flares also follows a power-law distribution with the same spectral index as the power- law frequency distribution of the peak flux, which may partially explain why power-law distributions are ubiquitous in the Universe. We then show that the spectral indexes of the histograms of soft X-ray fluxes observed by GOES satellites in two different energy channels are different: the higher energy channel has a harder distribution than the lower energy channel, which challenges the universal power-law distribution predicted by SOC models and implies a very soft distribution of thermal energy content of plasmas probed by the GOES satellites. The temperature (T) distribution, on the other hand, approaches a power-law dis- tribution with an index of 2 for high values of T. Hence the application of SOC models to the statistical properties of solar flares needs to be revisited.
基金Supported by the National Natural Science Foundation of China
文摘The frequency distribution for several characteristics of a solar flare obeys a power law only above a certain threshold, below which there is an apparent loss of small scale events presumably caused by limited instrumental sensitivity and th:e corresponding event selection bias. It is also possible that this deviation in the power law can have a physical origin in the source. We propose two fitting models incorpo- rating a power law distribution with a low count rate cutoff plus a noise component for the frequency distribution of the hard X-ray peak count rate of all solar flare sam- ples obtained with HXRBS/SMM and BATSE/CGRO observations. Our new fitting method produces the same power-law index as previously developed methods, a low cutoff of the power-law function and its corresponding noise level, which is consistent with measurements of the actual noise level of the hard X-ray count rate. We found that the fitted low cutoff appears to be related to the noise level, i.e., flares are only recognized when their peak count rate is 3or greater than noise. Therefore, the fitted low cutoff, which is smaller than the aforementioned threshold, might be attributed to selection bias, and probably not to the actual count rate cutoff in flares at smaller scales. Whether or not the actual low cutoff physically exists needs to be checked by future observations with increased sensitivities.
基金the Gujarat Council on Science and Technology(GUJCOST),Dept.of Science&Technology,Govt.of Gujarat under the minor research project grants schemeGUJCOST for the financial assistance+1 种基金supported by NSFC(Grant Nos.41474151,41774150 and 4171101125)the International Postdoctoral Program of USTC
文摘Temporal and spectral characteristics of X-ray emission from 60 flares of intensity ≥C class observed by the Solar X-ray Spectrometer (SOXS) during 2003-2011 are presented. We analyze the X-ray emission observed in four and three energy bands by the Si and Cadmium-Zinc-Telluride (CZT) detectors, respectively. The number of peaks in the intensity profile of the flares varies between 1 and 3. We find moderate correlation (R ~=0.2) between the rise time and the peak flux of the first peak of the flare irrespective of energy band, which is indicative of its energy-independent nature. Moreover, the magnetic field complexity of the flaring region is found to be highly anti-correlated (R = 0.61) with the rise time of the flares while positively correlated (R = 0.28) with the peak flux of the flare. The time delay between the peak of the X-ray emission in a given energy band and that in 25-30keV decreases with increasing energy, suggesting conduction cooling is dominant in the lower energies. Analysis of 340 spectra from 14 flares reveals that the peak of differential emission measure (DEM) evolution is delayed by 60-360 s relative to that of the temperature, and this time delay is inversely proportional to the peak flux of the flare. We conclude that temporal and intensity characteristics of flares are dependent on energy as well as the magnetic field configuration of the active region.
基金supported by grants P209/12/0103(GA CR)the MarieCurie PIRSES-GA-2011-295272 RadioSun project
文摘This review summarizes new trends in studies of magnetic reconnection in solar flares. It is shown that plasmoids play a very important role in this primary flare process. Using the results of magnetohydrodynamic and particle-in-cell simulations, we describe how the plasmoids are formed, how they move and interact, and how a flare current sheet is fragmented into a cascade of plasmoids. Furthermore, it is shown that during the interactions of these plasmoids electrons are not only very efficiently accelerated and heated, but electromagnetic(radio) emission is also produced.We also describe possible mechanisms for the triggering of magnetic reconnection.The relevant X-ray and radio signatures of these processes(such as radio drifting pulsation structures, narrowband dm-spikes, and the loop-top and above-the-loop-top X-ray sources) are then described. It is shown that plasmoids can also be formed in kinked magnetic ropes. A mapping of X-points of the magnetic reconnection on the chromosphere(as e.g. a splitting of flare ribbons) is mentioned. Supporting EUV and white-light observations of plasmoids are added. The significance of all these processes for the fast magnetic reconnection and electron acceleration is outlined. Their role in fusion experiments is briefly mentioned.
基金supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 11790303 41774180, 11703017 and 11873036)the Major International Joint Research Project (11820101002) of NSFCsupport from the Young Elite Scientists Sponsorship Program by the China Association for Science and Technology+2 种基金the Young Scholars Program of Shandong University, Weihaithe Joint Research Fund in Astronomy (U1631242 and U1731241) under the cooperative agreement between NSFC and CASthe “Thousand Young Talents Plan”
文摘Solar hard X-rays(HXRs) appear in the form of either footpoint sources or coronal sources. Each individual source provides its own critical information on acceleration of nonthermal electrons and plasma heating. Earlier studies found that the HXR emission in some events manifests a broken-up power-law spectrum, with the break energy around a few hundred keV based on spatially-integrated spectral analysis,and it does not distinguish the contributions from individual sources. In this paper, we report on the brokenup spectra of a coronal source studied using HXR data recorded by Reuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI) during the SOL2017–09–10 T16:06(GOES class X8.2) flare. The flare occurred behind the western limb and its footpoint sources were mostly occulted by the disk. We could clearly identify such broken-up spectra pertaining solely to the coronal source during the flare peak time and after. Since a significant pileup effect on the RHESSI spectra is expected for this intense solar flare, we have selected the pileup correction factor, p = 2. In this case, we found the resulting RHESSI temperature(~30MK) to be similar to the GOES soft X-ray temperature and break energies of 45–60 keV. Above the break energy, the spectrum hardens with time from spectral index of 3.4 to 2.7, and the difference in spectral indices below and above the break energy increases from 1.5 to 5 with time. However, we note that when p = 2 is assumed, a single power-law fitting is also possible with the RHESSI temperature higher than the GOES temperature by ~10MK. Possible scenarios for the broken-up spectra of the loop-top HXR source are briefly discussed.
基金the Strategic Priority Research Program on Space Science of the Chinese Academy of Sciences,the grant No.XDA15320104,with additional contributions from the Purple Mountain Observatory(PMO)of the Chinese Academy of Sciences and the National Space Science Center(NSSC).
文摘For the ASO-S/HXI payload, the accuracy of the flare reconstruction is reliant on important factors such as the alignment of the dual grating and the precise measurement of observation orientation. To guarantee optimal functionality of the instrument throughout its life cycle, the Solar Aspect System (SAS) is imperative to ensure that measurements are accurate and reliable. This is achieved by capturing the target motion and utilizing a physical model-based inversion algorithm. However, the SAS optical system’s inversion model is a typical ill-posed inverse problem due to its optical parameters, which results in small target sampling errors triggering unacceptable shifts in the solution. To enhance inversion accuracy and make it more robust against observation errors, we suggest dividing the inversion operation into two stages based on the SAS spot motion model. First, the as-rigid-aspossible (ARAP) transformation algorithm calculates the relative rotations and an intermediate variable between the substrates. Second, we solve an inversion linear equation for the relative translation of the substrates, the offset of the optical axes, and the observation orientation. To address the ill-posed challenge, the Tikhonov method grounded on the discrepancy criterion and the maximum a posteriori (MAP) method founded on the Bayesian framework are utilized. The simulation results exhibit that the ARAP method achieves a solution with a rotational error of roughly±3 5 (1/2-quantile);both regularization techniques are successful in enhancing the stability of the solution, the variance of error in the MAP method is even smaller—it achieves a translational error of approximately±18μm (1/2-quantile) in comparison to the Tikhonov method’s error of around±24μm (1/2-quantile). Furthermore, the SAS practical application data indicates the method’s usability in this study. Lastly, this paper discusses the intrinsic interconnections between the regularization methods.
基金supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 10873038 and 10833007)the National Basic Research Program of China (2006CB806302)CAS Project KJCX2-YW-T04
文摘Using the visual inspection and base difference method and data from the X-ray Telescope (XRT) onboard Hinode and TRACE with improved spatial and temporal resolution, we selected 48 X-ray transient brightenings (XTBs) and 237 EUV transient brightenings (ETBs) to study the connection between these two types of transient brightenings (TBs). These ETBs and XTBs have smaller areas (8.42 Mm^2 and 36.3 Mm^2, respectively, on average) and shorter durations (9.0 min and 6.9 min, respectively, on average) than previous studies. These XTBs show three types of morphological structure: point-like, single-loop and multiple-loop. We find only 20% of the ETBs have corresponding XTBs while the other 80% have no X-ray signatures at all. This is presumably due to the small amount of released energy, which is not enough to heat the plasma to coronal temperatures which produce X-ray emission rather than being due to the limitation of spatial resolution and temperature sensitivity of the X-ray instrument. These small ETBs may significantly contribute to the coronal heating.
文摘Using data-driven algorithms to accurately forecast solar flares requires reliable data sets.The solar flare dataset is composed of many non-flaring samples with a small percentage of flaring samples.This is called the class imbalance problem in data mining tasks.The prediction model is sensitive to most classes of the original data set during training.Therefore,the class imbalance problem for building up the flare prediction model from observational data should be systematically discussed.Aiming at the problem of class imbalance,three strategies are proposed corresponding to the data set,loss function,and training process:TypeⅠresamples the training samples,including oversampling for the minority class,undersampling,or mixed sampling for the majority class.TypeⅡusually changes the decision-making boundary,assigning the majority and minority categories of prediction loss to different weights.TypeⅢassigns different weights to the training samples,the majority categories are assigned smaller weights,and the minority categories are assigned larger weights to improve the training process of the prediction model.The main work of this paper compares these imbalance processing methods when building a flare prediction model and tries to find the optimal strategy.Our results show that among these strategies,the performance of oversampling and sample weighting is better than other strategies in most parameters,and the generality of resampling and changing the decision boundary is better.
基金supported by the National Natural Science Foundation of China(Grant Nos.11973092,11790300,11790302,11729301 and12073081)the Strategic Priority Research Program on Space Science,Chinese Academy of Sciences(Grant Nos.XDA15052200 and XDA15320301)+2 种基金supported by the Specialized Research Fund for State Key Laboratories and CAS Strategic Pioneer Program on Space Science(KLSA202003)The Laboratory No.is2010DP173032.supported by DLR(Grant No.50 QL 1701)。
文摘We report multi-wavelength observations of four solar flares on 2014 July 07.We firstly select these flares according to the soft X-ray(SXR)and extreme ultraviolet(EUV)emissions recorded by the Extreme Ultraviolet Variability Experiment and Geostationary Orbiting Environmental S atellites.Then their locations and geometries are identified from the full-disk images measured by the Atmospheric Imaging Assembly(AIA),and the time delays among the light curves in different channels are identified.The electron number densities are estimated using the differential emission measure method.We find that three of four flares show strong emissions in SXR channels and high temperature(>6 MK)EUV wavelengths during the impulsive phase,i.e.,AIA 131 A and 94 A,and then they emit peak radiation subsequently in the middle temperature(~0.6-3 MK)EUV channels.Moreover,they last for a long time and have smaller electron densities,which are probably driven by the interaction of hot diffuse flare loops.Only one flare emits radiation at almost the same time in all the observed wavelengths,lasts for a relatively short time,and has a larger electron density.It is also accompanied by a typeⅢradio burst.The bright emission at the EUV channel could be corresponding to the associated erupting filament.
基金supported by a SUPA Advanced Fellowship (Hamish Reid)the European Research Council under the SeismoSun Research Project No. 321141 (Heather Ratcliffe)the Marie Curie PIRSESGA- 2011-295272 RadioSun project
文摘Solar type III radio bursts are an important diagnostic tool in the understanding of solar accelerated electron beams. They are a signature of propagating beams of nonthermal electrons in the solar atmosphere and the solar system. Consequently, they provide information on electron acceleration and transport, and the conditions of the background ambient plasma they travel through. We review the observational properties of type III bursts with an emphasis on recent results and how each property can help identify attributes of electron beams and the ambient background plasma. We also review some of the theoretical aspects of type III radio bursts and cover a number of numerical efforts that simulate electron beam transport through the solar corona and the heliosphere.
基金supported by the National Basic Research Program of China (973 Program,Grant 2011CB811406)the National Natural Science Foundation of China (NSFC,Grant Nos.11273031,11473040 and 41231066)
文摘Comparing the ESP/EVE/SDO flux data of 2011 Feb 6, with the counterparts of XRS/GOES and SEM/SOHO, we find that there is an enhancement that is not apparent in the two latter datasets. The enhancement, possibly regarded as a flare at first glimpse, nevertheless, does not involve an energy-release from the Sun. Based on the enhancement, we combine data from SXI/GOES 15 into a synthesized analysis, and concluded that it arises from a particle-associated enhancement in the channel that measures XUV radiation. Paradoxically, it seems to be somewhat of a particle-avalanching process. Prior to the event, a moderate geomagnetic storm took place. Subsequently, while the event is proceeding, a geomagnetic substorm is simultaneously observed. Therefore, the particles, though unidentified, are probably energetic electrons induced by substorm injection.