Solar X-ray Flares Research Articles

Characteristics of solar X-ray flares registered by the CORONAS-PHOTON satellite

We examine the time behavior and energy characteristics of the ultraviolet, soft, and hard X-ray emissions of several C-class flares recorded by a set of instruments on board the CORONAS-PHOTON satellite. The difference between their parameters are considered in the thermal and nonthermal radiation ranges. The upper measured limit of the degree of linear polarization of X-rays is determined for the most intense flare.

The geomagnetic solar flare effect identified by SIIG as an indicator of a solar flare observed by GOES satellites

This paper studies the efficiency of geomagnetic solar flare effects (gsfe) in X solar flare detection; so during the period 1999–2007 a comparison between solar flare (sf) observed by satellites of the Geostationary Operational Environmental Satellite (GOES) programme and gsfe published by the Service International des Indices Geomagnetiques (SIIG) is made. Solar flares (sfs) are one of the most powerful manifestations of the solar activity. Because the far UV and soft and hard X-ray solar flare radiation is absorbed by the ionosphere, the ionized gas density increases (ionospheric solar flare effect, isfe) and a sudden enhancement of geomagnetic field near Earth surface (geomagnetic solar flare effect, gsfe) is produced. The solar X-ray flux is systematically recorded since 1975 by GOES satellites, and gsfe are declared by geomagnetic observatories since 1957. We corroborate that the identification of sf using gsfe is affected by several factors: – the intensity and average growth rate of solar flare radiation when quicker is it more easily the sf is detected as gsfe; – the position of the geomagnetic observatory, we found that observatories placed at summer hemisphere identify more easily the sf, so the uneven geographical distribution of observatories make the sf identification difficult; – the existing geomagnetic perturbation previous to sf, and the likeness between the gsfe and other geomagnetic variations. This work shows that gsfe recorded as associated to sf is a poor detector of sf even if it is intense. Some of these inconveniences can be avoided if the distribution of the observatories is improved and the identification of a sf is made using simultaneously gsfe, solar wind parameters and isfe.

Onboard performance of the RT-2 detectors

The RT-2 Experiment onboard the CORONAS-PHOTON satellite is designed to study the spectral, temporal, and spatial details of solar hard X-ray flares in the 15–150 keV range. Above this energy (and upto 1000 keV), it also acts as an omni-directional gamma-ray detector with a capability to study gamma-ray bursts (GRB), bright solar flares, and X-ray pulsars. With an ensemble of hard X-ray detectors with different fields of view and coding devices, it also has the capability to investigate the spectrum of Cosmic Diffuse X-ray Background. The performance of the detectors from 2009 February to November is described in this paper. Results obtained on a few GRBs and solar flares are also briefly discussed.

Global analysis of active longitudes of sunspots

Context. Active longitudes have been found in various manifestations of solar activity. The longitudinal distribution of, e.g., sunspots and solar X-ray flares shows two persistent preferred longitudes separated by roughly 180 degrees. We previously studied solar X-ray flares using an improved version of a dynamic, differentially rotating coordinate system and found enhanced rotational asymmetry and rotation parameter values that are consistent for the three classes of X-ray flares. Aims. We aim to find the optimal values of rotation parameters of active longitudes of sunspots for several different time intervals and separately for the two solar hemispheres. Methods. We perform a global study of the longitudinal location of sunspots (all sunspots and first appearance sunspots) using a refined version of a dynamic, differentially rotating coordinate system. Results. We find that the rotation parameters for sunspots are in good agreement with those obtained for X-ray flares using the same method. The improved method typically finds somewhat faster equatorial rotation with better accuracy. The improved treatment also leads to a larger non-axisymmetry. Rotation parameters for all sunspots and first appearances closely agree with each other, but nonaxisymmetry is systematically larger for all sunspots than for first appearances, suggesting that strong fields follow more closely the pattern of active longitudes. The refined method emphasizes hemispheric differences in rotation. Over the whole interval, the mean rotation in the southern hemisphere is slower than in the north. We also find significant temporal variability in the two rotation parameters over the 136-year interval. Interestingly, the long-term variations (trends and residual oscillations) in solar rotation are roughly the opposite in the northern and southern hemispheres. Conclusions. Rotation parameters vary differently with time in the northern and southern hemispheres. Both sunspots and flares strongly suggest that the northern hemisphere rotated considerably faster but the southern hemisphere slightly slower than the Carrington rotation rate during the last three solar cycles.

Instruments of RT-2 experiment onboard CORONAS-PHOTON and their test and evaluation II: RT-2/CZT payload

Cadmium Zinc Telluride (CZT) detectors are high sensitivity and high resolution devices for hard X-ray imaging and spectroscopic studies. The new series of CZT detector modules (OMS40G256) manufactured by Orbotech Medical Solutions (OMS), Israel, are used in the RT-2/CZT payload onboard the CORONAS-PHOTON satellite. The CZT detectors, sensitive in the energy range of 20 keV to 150 keV, are used to image solar flares in hard X-rays. Since these modules are essentially manufactured for commercial applications, we have carried out a series of comprehensive tests on these modules so that they can be confidently used in space-borne systems. These tests lead us to select the best three pieces of the 'Gold' modules for the RT-2/CZT payload. This paper presents the characterization of CZT modules and the criteria followed for selecting the ones for the RT-2/CZT payload. The RT-2/CZT payload carries, along with three CZT modules, a high spatial resolution CMOS detector for high resolution imaging of transient X-ray events. Therefore, we discuss the characterization of the CMOS detector as well.

Instruments of RT-2 experiment onboard CORONAS-PHOTON and their test and evaluation I: ground calibration of RT-2/S and RT-2/G

Phoswich detectors (RT-2/S & RT-2/G) are major scientific payloads of the RT-2 Experiment onboard the CORONAS-PHOTON mission, which was launched into a polar Low Earth Orbit of around 550 km on 2009 January 30. These RT-2 instruments are designed and developed to observe solar flares in hard X-rays and to understand the energy transport processes associated with these flares. Apart from this, these instruments are capable of observing Gamma Ray Bursts (GRBs) and Cosmic diffuse X-ray background (CDXRB). Both detectors consist of identical NaI(Tl) and CsI(Na) scintillation crystals in a Phoswich combination, having the same diameter (116 mm) but different thicknesses. The normal working energy range is from 15 keV to 150 keV, but may be extendable up to ~1 MeV. In this paper, we present the RT-2/S and RT-2/G instruments and discuss their testing and calibration results. We used different radio-active sources to calibrate both detectors. The radio-active source 57Co (122 keV) is used for onboard calibration of both instruments. During its lifetime (∼3–5 years), RT-2 is expected to cover the peak of the 24th solar cycle.

Statistical analysis of Soft X-ray solar flares during solar cycles 21, 22 and 23

Abstract This paper presents statistical analysis of Soft X-ray (SXR) flares during the period January 1976 to December 2007 covering solar cycles (SCs) 21, 22 and 23. We have analysed north–south (N–S) and east–west (E–W) asymmetry of SXR flares at total (1–90°), low (1–40°) and high (50–90°) latitudes and center meridian distances (CMDs). We have also presented the N–S and E–W asymmetry of different intensity classes (B, C, M and X) during the period of investigation. A slight southern and eastern excess is found after analysis during SCs 21, 22 and 23. N–S distribution shows that the SXR flare events are most prolific in the 11–20° latitude band in the northern and southern hemispheres whereas E–W distribution does not show any prolific band. We found that the N–S and E–W asymmetry which often peaks near the activity minimum is in agreement with the theoretical dynamo models. Our results show that N–S asymmetry is statistically more significant than E–W asymmetry. It is revealed that the SXR flare activity (M and C class flares) during SC 23 is low as compared to the SC 22, indicating the violation of Gnevyshev–Ohl rule. The B class flare activity is higher for SC 23 whereas C, M and X class activities are higher for SC 21. We have also analysed the flare evolution parameters, i.e. duration, rise time, decay time and event asymmetry for total SXR as well as for different classes during last three SCs. The duration, rise time and decay time increase with increasing intensity class. The increase is more pronounced for the duration and decay time than for the rise time for SCs 21 and 22 while for SC 23 it is the same for duration, rise time and decay time. On analyzing event asymmetry indices, we found more positive values during SC 21 ( ≈ 64.86 % ) and SC 22 ( ≈ 54.31 % ) , but for SC 23 we have more negative values ( ≈ 48.08 % ) . Our study shows that during SC 23 we have more SXR flare events having shorter decay time than the rise time as compared to SCs 21 and 22.

Using a new set of space-borne particle monitors to investigate solar-terrestrial relations

Context. We investigated the solar origin of Solar Particle Events (SPEs) which occurred during four time periods of the 23rd solar cycle with intense solar activity and were detected by the Standard Radiation Environment Monitor (SREM) onboard the INTEGRAL satellite. SREM is a second generation ESA particle radiation monitor, already flying onboard seven spacecraft.Aims. The present study attempts to establish the association of recorded SPEs by INTEGRAL/SREM with their solar sources and hence evaluate the potential of SREM units as an alarm system for hazardous SPEs.Methods. X-ray, optical and radio data of solar flares and coronal mass ejections (CMEs) that were observed by several space-based instruments during the aforementioned solar cycle were reduced and thoroughly analyzed to establish the corresponding solar origin of the selected SPEs.Results. The analysis produced clear associations with X-class solar flares and respective CMEs for the events of the October–November 2003, January 2005 and December 2006 periods, while for the events that occurred during September 2005, associations with X-class flares are possible but not straightforward due to the complexity of the registered solar particle fluxes.Conclusions. SREM proves to be a new valuable asset for a comprehensive study of SPEs and a useful alert instrument for explosive geoeffective solar events.

Relationship between CME dynamics and solar flare plasma

The relationship between the velocity of CMEs and the plasma temperature of the associated X-ray solar flares is investigated. The velocity of CMEs increases with plasma temperature (R = 0.82) and photon index below the break energy (R = 0.60) of X-ray flares. The heating of the coronal plasma appears to be significant with respect to the kinetics of a CME from the reconnection region where the flare also occurs. We propose that the initiation and velocity of CMEs perhaps depend upon the dominant process of conversion of the magnetic field energy of the active region to heating/accelerating the coronal plasma in the reconnected loops. Results show that a flare and the associated CME are two components of one energy release system, perhaps, magnetic field free energy.

Positions and sizes of X-ray solar flare sources

We investigate the positions and source sizes of X-ray solar flare sources taking into account Compton backscattering (albedo). Using a Monte Carlo simulation of X-ray photon transport including photo-electric absorption and Compton scattering, we calculate the apparent source sizes and positions of X-ray sources at the solar disk for various source sizes, spectral indices and directivities of the primary source. We show that the albedo effect will alter the true source positions and substantially increase the measured source sizes. The source positions are shifted up to $\sim 0.5"$ radially towards the disk centre and 5 arcsecond source sizes can be two times larger even for an isotropic source (minimum albedo effect) at 1 Mm above the photosphere. X-ray sources therefore should have minimum observed sizes, thus FWHM source size (2.35 times second-moment) will be as large as $\sim 7"$ in the 20-50 keV range for a disk-centered point source at a height of 1 Mm ($\sim 1.4"$) above the photosphere. The source size and position change is the largest for flatter primary X-ray spectra, stronger downward anisotropy, for sources closer to the solar disk centre, and between the energies of 30 and 50 keV. Albedo should be taken into account when X-ray footpoint positions, footpoint motions or source sizes from e.g. RHESSI or Yohkoh data are interpreted, and suggest that footpoint sources should be larger in X-rays than in optical or EUV ranges.

Statistical analysis of Soft X-ray solar flares during solar cycles 21, 22 and 23

This paper presents statistical analysis of Soft X-ray (SXR) flares during the period January 1976 to December 2007 covering solar cycles (SCs) 21, 22 and 23. We have analysed north–south (N–S) and east–west (E–W) asymmetry of SXR flares at total (1–90°), low (1–40°) and high (50–90°) latitudes and center meridian distances (CMDs). We have also presented the N–S and E–W asymmetry of different intensity classes (B, C, M and X) during the period of investigation. A slight southern and eastern excess is found after analysis during SCs 21, 22 and 23. N–S distribution shows that the SXR flare events are most prolific in the 11–20° latitude band in the northern and southern hemispheres whereas E–W distribution does not show any prolific band. We found that the N–S and E–W asymmetry which often peaks near the activity minimum is in agreement with the theoretical dynamo models. Our results show that N–S asymmetry is statistically more significant than E–W asymmetry. It is revealed that the SXR flare activity (M and C class flares) during SC 23 is low as compared to the SC 22, indicating the violation of Gnevyshev–Ohl rule. The B class flare activity is higher for SC 23 whereas C, M and X class activities are higher for SC 21. We have also analysed the flare evolution parameters, i.e. duration, rise time, decay time and event asymmetry for total SXR as well as for different classes during last three SCs. The duration, rise time and decay time increase with increasing intensity class. The increase is more pronounced for the duration and decay time than for the rise time for SCs 21 and 22 while for SC 23 it is the same for duration, rise time and decay time. On analyzing event asymmetry indices, we found more positive values during SC 21 ( ≈ 64.86 % ) and SC 22 ( ≈ 54.31 % ) , but for SC 23 we have more negative values ( ≈ 48.08 % ) . Our study shows that during SC 23 we have more SXR flare events having shorter decay time than the rise time as compared to SCs 21 and 22.

Investigation of Quasi-periodic Variations in Hard X-rays of Solar Flares

The aim of the present paper is to use quasi-periodic oscillations in hard X-rays (HXRs) of solar flares as a diagnostic tool for investigation of impulsive electron acceleration. We have selected a number of flares which showed quasi-periodic oscillations in hard X-rays and their loop-top sources could be easily recognized in HXR images. We have considered MHD standing waves to explain the observed HXR oscillations. We interpret these HXR oscillations as being due to oscillations of magnetic traps within cusp-like magnetic structures. This is confirmed by a good correlation between periods of the oscillations and the sizes of the loop-top sources. We argue that a model of oscillating magnetic traps is adequate to explain the observations. During the compressions of a trap particles are accelerated, but during its expansions plasma, coming from chromospheric evaporation, fills the trap, which explains the large number of electrons being accelerated during a sequence of strong impulses. The advantage of our model of oscillating magnetic traps is that it can explain both the impulses of electron acceleration and quasi-periodicity of their distribution in time.

Global analysis of active longitudes of solar X-ray flares

There is increasing evidence that various manifestations of solar activity are non-axisymmetric and mainly occur in two preferred longitude ranges, so called active longitudes. We have earlier analyzed the longitudinal occurrence of solar X-ray flares observed by GOES satellites using a specially developed dynamic, differentially rotating coordinate system. In this frame, the longitude distribution shows two persistent preferred longitudes separated by about 180 degrees whose strength alternates in time according to the so called flip-flop phenomenon. Here we make the first global statistical analysis to find the best fitting values for parameters describing the differential rotation of active longitudes of X-ray flares. We find that the new analysis greatly improves the earlier values for the rotation parameters, making them consistent between the three different classes of X-ray flares. The improved parameters also yield a systematically higher level of non-axisymmetry for the longitudinal distribution, thus increasing the statistical significance of the existence of active longitudes. Accordingly, a significant amount of X-ray flares of different classes are produced by the same two active longitudes. We also find a significant difference between the rotation rates in the two solar hemispheres, with active longitudes rotating faster than the Carrington rate in the northern hemisphere and slower than the Carrington rate in the southern hemisphere.

Solar flares in soft X-rays detected in the Coronas-F experiment

The RPS-1 spectrometer on the board of the Coronas-F satellite detecting solar X-rays in the range of 3–31.5 keV using a CdTe detector is described and some results of the observation of weak solar flares are presented.

Time structure and energy spectrum evolution of the solar flare X-rays measured by the IRIS spectrometer onboard the CORONAS-F spacecraft

The time structure and energy spectrum evolution of the X-ray emission of solar flares, observed by the IRIS spectrometer onboard the CORONAS-F spacecraft, are investigated. It has been found out that one or two quasi-periodic components with periods of 1–20 s, which are absent in the background preflare emission, appear in the flare soft X rays. It has been indicated that the variation in the shape of the energetic spectra of the C-class flare hard X rays reflects the evolution of the accelerated electron distribution function.

A TEST OF THICK-TARGET NONUNIFORM IONIZATION AS AN EXPLANATION FOR BREAKS IN SOLAR FLARE HARD X-RAY SPECTRA

Solar nonthermal hard X-ray (HXR) flare spectra often cannot be fitted by a single power law, but rather require a downward break in the photon spectrum. A possible explanation for this spectral break is nonuniform ionization in the emission region. We have developed a computer code to calculate the photon spectrum from electrons with a power-law distribution injected into a thick-target in which the ionization decreases linearly from 100% to zero. We use the bremsstrahlung cross-section from Haug (1997), which closely approximates the full relativistic Bethe-Heitler cross-section, and compare photon spectra computed from this model with those obtained by Kontar, Brown and McArthur (2002), who used a step-function ionization model and the Kramers approximation to the cross-section. We find that for HXR spectra from a target with nonuniform ionization, the difference (Delta-gamma) between the power-law indexes above and below the break has an upper limit between approx.0.2 and 0.7 that depends on the power-law index delta of the injected electron distribution. A broken power-law spectrum with a. higher value of Delta-gamma cannot result from nonuniform ionization alone. The model is applied to spectra obtained around the peak times of 20 flares observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI from 2002 to 2004 to determine whether thick-target nonuniform ionization can explain the measured spectral breaks. A Monte Carlo method is used to determine the uncertainties of the best-fit parameters, especially on Delta-gamma. We find that 15 of the 20 flare spectra require a downward spectral break and that at least 6 of these could not be explained by nonuniform ionization alone because they had values of Delta-gamma with less than a 2.5% probability of being consistent with the computed upper limits from the model. The remaining 9 flare spectra, based on this criterion, are consistent with the nonuniform ionization model.

Correlation between decimetric radio emission and hard X-rays in solar flares

Aims. The emission of decimetric flare radiation, in particular na rrowband spikes and pulsations, is generally considered to originate in accelerated, non-thermal particles. On the other hand, non-thermal hard X-rays are also understood to be products of this acceleration. Do radio emission and hard X-ray signatures originate from the same acceleration process? A strong correlation between the light curves in the radio and HXR ranges may help answer this question. Methods. The delay between the radio and hard X-ray emission was determined by cross-correlation. The time profiles of X-ray and radio emission include a wide range of energies and frequencies. Thus, correlation is not simply a yes/no question, but must be systematically searched for in various ranges. The high spectral resolution of RHESSI ensured that it was possible to carefully choose the energy range, excluding thermal emission. The broad bandwidth of Phoenix-2 allowed the selection of any emission in the full decimetre range. The energy range and duration in hard X-rays, and the frequency range in radio spectrograms were chosen to optimize the correlation. The cross-correlation coeffi cient was then analyzed by a Gaussian fitting method. Results. The measured delays have a distribution of FWHM 4.9 s and 4.7 s for pulsations and spikes, respectively, evaluated from su ch a Gaussian fitting method. The mean delay for pulsations was f ound to be−1.4±0.9 seconds (minus indicates that hard X-ray emission comes first), and for narrowband spikes to −2.5± 2.5 seconds. There are broad wings in the distribution, which we interpret as chance coincidences. The delays do not depend on centre frequency, cross-correlation coeffi cient, duration of the correlating sequence, and position on the disk. However, we find an increase in the delay for the spikes with GOES magnitude (peak soft X-ray emission) of the flare and with peak hard X-ray flux. This was not the case for pul sations. Conclusions. In contrast to previous reports, the average delays for all p ulsations and all spike groups are consistent with zero. Thus, correlated decimetric pulsations and spikes are, on average, concomitant with non-thermal X-rays.

The Ulysses Catalog of Solar Hard X-Ray Flares

Ulysses was launched in October 1990, and its Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) has provided more than 13 years of uninterrupted observations of solar X-ray flare activity. Due to the large variation of the relative solar latitude and longitude of the spacecraft orbit with respect to the Earth, the perspective of the GRB instrument often differed significantly from that of X-ray instruments on Earth-orbiting satellites. During extended periods the GRB experiment made direct observations of flares on the hidden face of the Sun, providing a unique record of events not visible to other instruments. The small detector area of GRB and its optimization for very high counting rates minimized the effects of pulse pile-up. We interpret the spectra, time histories, and occurrence distribution patterns of GRB data in terms of “thermal feed-through”, the confusion of thermal soft X-rays and non-thermal hard X-rays. This effect is a systematic problem for scintillation-counter spectrometers observing the solar hard X-ray spectrum. This paper provides a definitive catalog of the Ulysses X-ray flare observations and discusses various features of this unique database. For the equivalent GOES range X2 – X25, we find a power-law fit for the (differential) occurrence frequency at >25 keV with slope −1.61±0.04, with no evidence for a downturn at the highest event magnitudes (for the relatively small sample of such events available in this study). If the nine most intense events are excluded because of concerns about the effects of pulse pile-up, the slope steepens to −1.75±0.08.

HARD-SOFT-HARD FLARE SPECTRA AND THEIR ENERGY DEPENDENCE IN SPECTRAL EVOLUTION OF A SOLAR HARD X-RAY FLARE

In this Letter, we studied the time evolution of the energy-dependent spectral indices for the 2004 November 3 solar hard X-ray flare observed by RHESSI. The common soft-hard-soft (SHS) pattern spectra were found at the lower energies, while a new feature, hard-soft-hard (HSH), was found at higher energies for each subpeak. As the energy increases, the SHS pattern is gradually converted into the HSH pattern. Some possible explanations for the spectral evolution and its energy dependence are discussed, such as the return current.

FARIMA MODELING OF SOLAR FLARE ACTIVITY FROM EMPIRICAL TIME SERIES OF SOFT X-RAY SOLAR EMISSION

A time series of soft X-ray emission observed on 1974-2007 years (GOES) is analyzed. We show that in the periods of high solar activity 1977-1981, 1988-1992, 1999-2003 the energy statistics of soft X-ray solar flares for class M and C is well described by a FARIMA time series with Pareto innovations. The model is characterized by two effects. One of them is a long-range dependence (long-term memory), and another corresponds to heavy-tailed distributions. Their parameters are statistically stable enough during the periods. However, when the solar activity tends to minimum, they change essentially. We discuss possible causes of this evolution and suggest a statistical model for predicting the flare energy statistics.