Solar X-ray Flares Research Articles

Energy Spectra and Time Delays of Hard X-Rays of Solar Flares in Konus-Wind and RHESSI Experiments

This paper analyzes hard X-ray radiation of solar flares detected by Konus-Wind and RHESSI spacecrafts. X-ray quanta are measured using high energy, time (Konus-Wind), and spatial (RHESSI) resolution. Spectra of accelerated electrons are calculated from the spectra of hard X-rays both via the forward-fitting method and the Tikhonov reconstruction method. The X-ray data for the solar flare of July 8, 2013, 0120:24 UT are analyzed. In Konus-Wind data the irregularities and inversions are detected in the energy spectrum. According to RHESSI data, the hard X-ray (HXR) energy spectrum is smoother, although it contains breaks. The X-ray energy spectrum determined by the forward-fitting method is hard at the peak of the flux, and the spectrum index is γ ≈ 3.5. Over time, the HXR spectrum is significantly softened, and the spectrum index increases to γ ≈ 7.5. X-ray time delays are determined by a correlation analysis of the time series of different energies. The time delay spectrum is U-shaped with a 47-keV break in the energy for the Konus-Wind data and 57 keV for the RHESSI data.

Electron Acceleration and Subsecond Time Delays of Hard X-Rays of Solar Flares According to Lomonosov Satellite Data

On the basis of satellite observations obtained with the help of X-ray and gamma-ray detector units (BDRG/Lomonosov in Russian), using digital filtering methods, the analysis of time delays between the second pulsations of time profiles of hard X-ray radiation of different frequency bands from the solar flare that occurred on July 21, 2016 has been made. It is shown that the count rate in energy channels of 10–20, 20–35, and 35–60 keV detected by BDRG/Lomonosov are correlated with an accuracy up to 0.1 s. The result agrees well with the observations of the Gamma-ray Burst Monitor on board the Fermi satellite and supposes an effective acceleration of charged particles over the entire length of the flare magnetic arc.

Statistical study of GNSS L-band solar radio bursts

Intense L-band solar radio bursts (SRBs) can affect acquisition and tracking of signals from the Global Navigation Satellite Systems (GNSS) by receivers located in the sunlit hemisphere of the earth and have been regarded as a potential threat to GNSS stability and performance. To obtain the occurrence characteristics and evaluate the probability of GNSS L-band SRB events, we take a survey using the latest 20-year event set (1997–2016). The bursts data were archived and provided by the Space Weather Prediction Center (SWPC) of National Oceanic and Atmospheric Administration (NOAA). The results indicate that, during this period, a total of 2384 L-band radio bursts were observed, including 1384 minor events, 859 moderate events, 120 strong events, 17 severe events, and 4 extreme events. As expected, these bursts are generally closely related to solar activity and solar cycle. But, for the severe and extreme events, bursts occurrence is sporadic and random, which not only occurs at solar maximum but also at other intervals. Compared to X-ray solar flares, the L-band SRB events do not occur frequently, and the ratio of annual SRB events with peak flux exceeding 100 SFU to X-ray flares is approximately 0.12. Statistically, the occurrence frequency of strong, severe and extreme SRB events is conservatively about 6.00, 0.85, and 0.20 bursts per year, respectively. The data fitting shows that L-band SRB events yield a power law distribution with an exponent of about – 1.791.

Attenuation of decameter wavelength sky noise during x-ray solar flares in 2013–2017 based on the observations of midlatitude HF radars

Based on a joint analysis of data from 10 midlatitude decameter wavelength radars effects are investigated during 80 x-ray flares that occurred in the period 2013–2017. For the investigation nine mid-latitude SuperDARN radars of the northern hemisphere (Adak Island West and East radars, Blackstone radar, Christmas Valley East and West radars, Fort Hays East and West radars, Hokkaido East radar and Wallops radar) and Ekaterinburg coherent decameter radar of ISTP SB RAS are used. All the radars work in the same 8–20 MHz frequency band and have similar hardware and software. During the analysis the temporal dynamics of noise from each of the radar direction and for each flare is investigated separately. As a result, on the basis of about 13000 daily measurements we found a strong anticorrelation between noise power and x-ray flare intensity, indicating that short-wave sky noise can be used to diagnose the ionospheric effects of x-ray solar flares. It is shown that in 88.3% of cases an attenuation of daytime decameter radio noise is observed during solar flares, and the attenuation correlates with the temporal dynamics of the solar flare. The intensity of decameter noise anticorrelates well (the Pearson correlation coefficient better than −0.5) with the shape of the X-ray flare in the daytime (for solar elevation angle >0) in 33% of cases, the average Pearson correlation over the daytime is about −0.34. Median regression coefficient between GOES 0.1–0.8 nm x-ray intensity and daytime sky-noise attenuation is about −4.4⋅104[dB⋅m2/Wt]. Thus, it is shown that measurements of the sky noise level at midlatitude decameter radars can be used to study the ionospheric absorption of high-frequency waves in the lower ionosphere during x-ray solar flares. This can be explained by the assumption that the larger part of the decameter sky noise detected by the radars is produced by ground sources at distances of the first propagation hop (∼3000 km).

Analysis of the Relationship Between the Solar X-Ray Radiation Intensity and the D-Region Electron Density Using Satellite and Ground-Based Radio Data

Increases in the X-ray radiation that is emitted during a solar X-ray flare induce significant changes in the ionospheric D region. Because of the numerous complex processes in the ionosphere and the characteristics of the radiation and plasma, the causal-consequential relationship between the X-ray radiation and ionospheric parameters is not easily determined. In addition, modeling the ionospheric D-region plasma parameters is very difficult because of the lack of data for numerous time- and space-dependent physical quantities. In this article we first give a qualitative analysis of the relationship between the electron density and the recorded solar X-ray intensity. After this, we analyze the differences in the relationships between the D-region response and various X-ray radiation properties. The quantitative study is performed for data observed on 5 May 2010 in the time period between 11:40 UT – 12:40 UT when the GOES 14 satellite detected a considerable X-ray intensity increase. Modeling the electron density is based on characteristics of the 23.4 kHz signal emitted in Germany and recorded by the receiver in Serbia.

Deep Learning Based Solar Flare Forecasting Model. I. Results for Line-of-sight Magnetograms

Abstract Solar flares originate from the release of the energy stored in the magnetic field of solar active regions, the triggering mechanism for these flares, however, remains unknown. For this reason, the conventional solar flare forecast is essentially based on the statistic relationship between solar flares and measures extracted from observational data. In the current work, the deep learning method is applied to set up the solar flare forecasting model, in which forecasting patterns can be learned from line-of-sight magnetograms of solar active regions. In order to obtain a large amount of observational data to train the forecasting model and test its performance, a data set is created from line-of-sight magnetogarms of active regions observed by SOHO/MDI and SDO/HMI from 1996 April to 2015 October and corresponding soft X-ray solar flares observed by GOES. The testing results of the forecasting model indicate that (1) the forecasting patterns can be automatically reached with the MDI data and they can also be applied to the HMI data; furthermore, these forecasting patterns are robust to the noise in the observational data; (2) the performance of the deep learning forecasting model is not sensitive to the given forecasting periods (6, 12, 24, or 48 hr); (3) the performance of the proposed forecasting model is comparable to that of the state-of-the-art flare forecasting models, even if the duration of the total magnetograms continuously spans 19.5 years. Case analyses demonstrate that the deep learning based solar flare forecasting model pays attention to areas with the magnetic polarity-inversion line or the strong magnetic field in magnetograms of active regions.

Frequencies of Flare Occurrence: Interaction between Convection and Coronal Loops

Abstract Observations of solar and stellar flares have revealed the presence of power-law dependences between the flare energy and the time interval between flares. Various models have been proposed to explain these dependences and the numerical value of the power-law indices. Here, we propose a model in which convective flows in granules force the footpoints of coronal magnetic loops, which are frozen-in to photospheric gas, to undergo a random walk. In certain conditions, this can lead to a twist in the loop, which drives the loop unstable if the twist exceeds a critical value. The possibility that a solar flare is caused by such a twist-induced instability in a loop has been in the literature for decades. Here, we quantify the process in an approximate way with a view to replicating the power-law index. We find that, for relatively small flares, the random walk twisting model leads to a rather steep power-law slope that agrees very well with the index derived from a sample of 56,000+ solar X-ray flares reported by the GOES satellites. For relatively large flares, we find that the slope of the power law is shallower. The empirical power-law slopes reported for flare stars also have a range that overlaps with the slopes obtained here. We suggest that in the coolest stars, a significant change in slope should occur when the frozen-flux assumption breaks down due to low electrical conductivity.

The Instruments and Capabilities of the Miniature X-Ray Solar Spectrometer (MinXSS) CubeSats

The Miniature X-ray Solar Spectrometer (MinXSS) CubeSat is the first solar science oriented CubeSat mission flown for the NASA Science Mission Directorate, with the main objective of measuring the solar soft X-ray (SXR) flux and a science goal of determining its influence on Earth’s ionosphere and thermosphere. These observations can also be used to investigate solar quiescent, active region, and flare properties. The MinXSS X-ray instruments consist of a spectrometer, called X123, with a nominal 0.15 keV full-width at half-maximum (FWHM) resolution at 5.9 keV and a broadband X-ray photometer, called XP. Both instruments are designed to obtain measurements from 0.5 – 30 keV at a nominal time cadence of 10 s. A description of the MinXSS instruments, performance capabilities, and relation to the Geostationary Operational Environmental Satellite (GOES) 0.1 – 0.8 nm flux is given in this article. Early MinXSS results demonstrate the capability of measuring variations of the solar spectral soft X-ray (SXR) flux between 0.8 – 12 keV from at least GOES A5–M5 (5 times 10^{-8},mbox{--},5 times10^{-5}~mbox{W},mbox{m}^{-2}) levels and of inferring physical properties (temperature and emission measure) from the MinXSS data alone. Moreover, coronal elemental abundances can be inferred, specifically for Fe, Ca, Si, Mg, S, Ar, and Ni, when the count rate is sufficiently high at each elemental spectral feature. Additionally, temperature response curves and emission measure loci demonstrate the MinXSS sensitivity to plasma emission at different temperatures. MinXSS observations coupled with those from other solar observatories can help address some of the most compelling questions in solar coronal physics. Finally, simultaneous observations by MinXSS and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) can provide the most spectrally complete soft X-ray solar flare photon flux measurements to date.

Solar are and geomagnetic activity relations

Two x-ray solar flare parameters (number of flares and peak flux) are compared to geomagnetic and space weather activity indices. The first solar flare parameter, X-ray solar flare number (SFN), is the total number of solar flares observed in a given month. The second one is the monthly maximum of X-ray solar flare peak fluxes (SFP). In order to compare these two parameters, both of them are cross-correlated with some other indices of geomagnetic activity (aa, Dst, and AE indices) and space weather (maximum coronal mass ejection speed index and solar wind speed). Maximum correlation was found ( ∼ role=presentation> ∼ ∼ \sim 0.63) between both flare time series (SFN and SFP) and maximum coronal mass ejection speed index with no lag. Our results show that X-ray solar flare peak flux better describes those geomagnetic and space weather activity indices than X-ray solar flare numbers. This is mostly because of high time delays between flare numbers and studied indices.

MODIFICATIONS OF MESOSPHERIC CONDUCTIVITY IN SUDDEN IONOSPHERE DISTURBANCES AND CHANGES OF THE SCHUMANN RESONANCE FREQUENCIES

We present a technique for detecting modifications of the peak frequencies of Schumann resonance during the sudden ionospheric disturbances (SID) caused by the solar X-ray flares. A SID is associated with a sharp lowering of ionosphere at the sunlit hemisphere during the solar flares, which decreases the average ionosphere height increases Schumann resonance frequencies. The resonance frequencies and the characteristic heights of ionosphere are computed using the full wave solution for SID of different intensity. Power spectra of EZ field are computed using the 2D telegraph equations for the uniform distribution of thunderstorms in the cavity for several observer positions relative to the solar terminator. The weighted average frequency was found and linear dependence was found of this frequency on the SID intensity and/or on alteration of magnetic characteristic height. Results might be used in interpretations of Schumann resonance observations and estimation of ionosphere modification.

Forecasting Solar Energetic Particle (SEP) events with Flare X-ray peak ratios

Solar flare X-ray peak fluxes and fluences in the 0.1–0.8 nm band are often used in models to forecast solar energetic particle (SEP) events. Garcia (2004) [Forecasting methods for occurrence and magnitude of proton storms with solar soft X rays, Space Weather, 2, S02002, 2004] used ratios of the 0.05–0.4 and 0.1–0.8 nm bands of the X-ray instrument on the GOES spacecraft to plot inferred peak flare temperatures versus peak 0.1–0.8 nm fluxes for flares from 1988 to 2002. Flares associated with E > 10 MeV SEP events of >10 proton flux units (pfu) had statistically lower peak temperatures than those without SEP events and therefore offered a possible empirical forecasting tool for SEP events. We review the soft and hard X-ray flare spectral variations as SEP event forecast tools and repeat Garcia’s work for the period 1998–2016, comparing both the peak ratios and the ratios of the preceding 0.05–0.4 nm peak fluxes to the later 0.1–0.8 nm peak fluxes of flares >M3 to the occurrence of associated SEP events. We divide the events into eastern and western hemisphere sources and compare both small (1.2–10 pfu) and large (≥300 pfu) SEP events with those of >10 pfu. In the western hemisphere X-ray peak ratios are statistically lower for >10 pfu SEP events than for non-SEP events and are even lower for the large (>300 pfu) events. The small SEP events, however, are not distinguished from the non-SEP events. We discuss the possible connections between the flare X-ray peak ratios and associated coronal mass ejections that are presumed to be the sources of the SEPs.

Global response of Magnetic field and Ionosonde observations to intense solar flares on 6 and 10 September 2017

Intense X-ray fluxes during solar flares are known to cause enhanced ionization in the Earth’s ionospheric D, E and F region. This sudden change of ionospheric electron density profile is serious problem to radio wave communication and navigation system. The ground magnetograms often record the sudden change in the sunlit hemisphere during the enhanced X-ray flux, due to the sudden increase in the global ionospheric current system caused by the flare-induced enhanced ionospheric conductivity. These geomagnetic field disturbances are known as ‘‘solar flare effects’’ (SFEs) or geomagnetic crochets [Campbell, 2003]. The typical SFE is increase variation on the equatorial magnetic data. On Ionosonde observation during solar flare event, the High-Frequency (HF) radio wave blackout is often detected in ionogram due to the sudden disturbance in ionosphere. Two intense X-class solar flares occurred on 6 and 10 September 2017. We investigated the magnetic field and Ionosonde responses to the intense solar flare events. Dayside magnetic field variations sudden increased due to the ionospheric disturbance resulting from solar flare. There is no response in night side magnetometer data. The magnitude of SFE (magnetic field) is independent of solar flare x-ray magnitude. We found HF radio wave blackout in ionogram at dayside Ionosonde stations. The duration of blackout is dependent of latitude and local time of Ionosonde stations. There is the different feature of ionogram at night side.

A Compressed Sensing-based Image Reconstruction Algorithm for Solar Flare X-Ray Observations

Abstract One way of imaging X-ray emission from solar flares is to measure Fourier components of the spatial X-ray source distribution. We present a new compressed sensing-based algorithm named VIS_CS, which reconstructs the spatial distribution from such Fourier components. We demonstrate the application of the algorithm on synthetic and observed solar flare X-ray data from the Reuven Ramaty High Energy Solar Spectroscopic Imager satellite and compare its performance with existing algorithms. VIS_CS produces competitive results with accurate photometry and morphology, without requiring any algorithm- and X-ray-source-specific parameter tuning. Its robustness and performance make this algorithm ideally suited for the generation of quicklook images or large image cubes without user intervention, such as for imaging spectroscopy analysis.

Catalog of hard X-ray solar flares detected with Mars Odyssey/HEND from the Mars orbit in 2001–2016

The study of nonstationary processes in the Sun is of great interest, and lately, multiwavelength observations and registration of magnetic fields are carried out by means of both ground-based telescopes and several specialized spacecraft (SC) on near-Earth orbits. However the acquisition of the new reliable information on their hard X-ray radiation remains demanded, in particular if the corresponding SC provide additional information, e.g. in regard to the flare observations from the directions other than the Sun-Earth direction. In this article we present a catalog of powerful solar flares registered by the High Energy Neutron Detector (HEND) device designed in the Space Research Institute (IKI) of Russian Academy of Sciences. HEND is mounted onboard the 2001 Mars Odyssey spacecraft. It worked successfully during the flight to Mars and currently operates in the near-Mars orbit. Besides neutrons, the HEND instrument is sensitive to the hard X-ray and gamma radiation. This radiation is registered by two scintillators: the outer one is sensitive to the photons above 40 keV and the inner one to the photons above 200 keV. The catalog was created with the new procedure of the data calibration. For most powerful 60 solar flares on the visible and on the far sides of the Sun (in respect to a terrestrial observer), we provide time profiles of flare radiation, summed over all the channels of X-ray and in some cases of gamma-ray bands as well as the spectra and characteristics of their power law approximation. We briefly discuss the results of the previous articles on the study of the Sun with HEND instrument and the potential of the further use of these data.

Physics of erupting solar flux ropes: Coronal mass ejections (CMEs)—Recent advances in theory and observation

Solar eruptions, observed as flares and coronal mass ejections (CMEs), are the most energetic visible plasma phenomena in the solar system. CMEs are the central component of solar eruptions and are detected as coherent magnetized plasma structures expanding in the solar wind (SW). If they reach the Earth, their magnetic fields can drive strong disturbances in the ionosphere, causing deleterious effects on terrestrial technological systems. The scientific and practical importance of CMEs has led to numerous satellite missions observing the Sun and SW. This has culminated in the ability to continuously observe CMEs expanding from the Sun to 1 AU, where the magnetic fields and plasma parameters of the evolved structures (“ejecta”) can be measured in situ. Until recently, the physical mechanisms responsible for eruptions were major unanswered questions in solar and by extension stellar physics. New observations of CME dynamics and associated eruptive phenomena are now providing more stringent constraints on models, and quantitative theory-data comparisons are helping to establish the correct mechanism of solar eruptions, particularly the driving force of CMEs and the evolution of their magnetic fields in three dimensions. Recent work has demonstrated that theoretical results can simultaneously replicate the observed CME position-time data, temporal profiles of associated solar flare soft X-ray emissions, and the magnetic field and plasma parameters of CME ejecta measured at 1 AU. Thus, a new theoretical framework with testable predictions is emerging to model eruptions and the coupling of CME ejecta to geomagnetic disturbances. The key physics in CME dynamics is the Lorentz hoop force acting on toroidal “flux ropes,” scalable from tokamaks and similar laboratory plasma structures. The present paper reviews the latest advances in observational and theoretical understanding of CMEs with the emphasis on quantitative comparisons of theory and observation.

Effect of Solar Flares on the Schumann-Resonance Frequences

We present the results of analyzing the response of the Schumann-resonance frequencies to the Xray solar flares. A special technique, by which a sharp variation that is present in all the resonant modes, but is hidden by regular variations combined with stochastic fluctuations is singled out using the weighted average frequency, has been developed. On the basis of comparison of the response shape and the flare intensity, the parameters of the statistical relationship between the frequency variations and the X-ray radiation intensity were analyzed and determined for the nonlinear regression model. It has been shown that an increase in the resonant frequency is proportional on the average to the logarithm of the X-ray radiation intensity to the power of 1.7. The magnitude of changes in the cavity frequency is related with the corresponding changes in the characteristic “magnetic” height of the conductivity profile of the lower ionosphere and compared with the results of the independent analysis performed by measuring VLF radio signals.

Diagnostics of plasma in the ionospheric D-region: detection and study of different ionospheric disturbance types

Here we discuss our recent investigations of the ionospheric plasma by using very low and low frequency (VLF/LF) radio waves. We give a review of how to detect different low ionospheric reactions (sudden ionospheric disturbances) to various terrestrial and extra-terrestrial events, show their classification according to intensity and time duration, and present some methods for their detections in time and frequency domains. Investigations of detection in time domain are carried out for intensive long-lasting perturbations induced by solar X-ray flares and for short-lasting perturbations caused by gamma ray bursts. We also analyze time variations of signals used in the low ionospheric monitoring after earthquake events. In addition, we describe a procedure for the detection of acoustic and gravity waves from the VLF/LF signal analysis in frequency domain. The research of the low ionospheric plasma is based on data collected by the VLF/LF receivers located in Belgrade, Serbia.

Monitor and Prediction of Near-Earth Radiation Environment in the Frame of Space Monitoring Data Center at Moscow State University

AbstractRadiation environment of near-Earth space is one of the most important factors of space weather. Space Monitoring Data Center of Moscow State University provides operational monitor and forecast of radiation conditions both at Geostationary Orbits (GEO) and at Low Earths Orbits (LEO) of the near-Earth space using data of recent space missions (Vernov, CORONAS series) and current (Lomonosov, Meteor-M, Electro-L) ones. Internet portal of Space Monitoring Data Center of Skobeltsyn Institute of Nuclear Physics of Lomonosov Moscow State University (SINP MSU - [swx.sinp.msu.ru]) provides possibilities to monitor and analyze the space radiation conditions in the real time mode together with the geomagnetic and solar activity including hard X-ray and gamma-emission of solar flares.

On the classifications of the solar active regions (ARs); Case study for solar cycles 23 and 24

The declining phase of solar cycle 23 is an important case of study. Many high energetic solar flares occurred in 2003. The occurrence of high energetic solar X-ray flares (X-type) is related to solar cycle’s phases and the state of the active regions producing them. In some cases, the declining phases may be more active than the maximum phase. A statistical study is performed for solar flares (X-class) occurrence at declining and maximum phases of solar activity. The relationships between the X-class flares, SSN and active regions (ARs) will be investigated. The active region productivity of X-flares is in coherence with the number of days of (β – γ – δ) magnetic field. The active regions energies are consumed in the acceleration of protons and production of X-flares among other things (e.g. CME lifting up). The higher the energy of X-flares the lower the proton flux (pfu @ >10MeV) and vice versa.

The effects of solar activity: Electrons in the terrestrial lower ionosphere

Solar flare X-ray energy can cause strong enhancements of the electron density in the Earth’s atmosphere. This intense solar radiation and activity can cause sudden ionospheric disturbances (SIDs) and further create ground telecommunication interferences, blackouts as well as some natural disasters and caused considerable material damage. The focus of this contribution is on the study of these changes induced by solar X-ray flares using narrowband Very Low Frequency (VLF, 3–30 kHz) and Low Frequency (LF, 30–300 kHz) radio signal analysis. The model computation and simulation were applied to acquire the electron density enhancement induced by intense solar radiation. The obtained results confirmed the successful use of applied technique for detecting space weather phenomena such as solar explosive events as well for describing and modeling the ionospheric electron density which are important as the part of electric terrestrial-conductor environment through which external-solar wind (SW) electrons can pass and cause natural disasters on the ground like fires.