Correlation between solar flare productivity and photospheric vector magnetic fields

Studying the statistical correlation between the solar flare productivity and photospheric magnetic fields is very important and necessary. It is helpful to set up a practical flare forecast model based on magnetic properties and improve the physical understanding of solar flare eruptions. In the previous study ([Cui, Y.M., Li, R., Zhang, L.Y., He, Y.L., Wang, H.N. Correlation between solar flare productivity and photospheric magnetic field properties 1. Maximum horizontal gradient, length of neutral line, number of singular points. Sol. Phys. 237, 45–59, 2006]; from now on we refer to this paper as ‘Paper I’), three measures of the maximum horizontal gradient, the length of the neutral line, and the number of singular points are computed from 23990 SOHO/MDI longitudinal magnetograms. The statistical relationship between the solar flare productivity and these three measures is well fitted with sigmoid functions. In the current work, the three measures of the length of strong-shear neutral line, total unsigned current, and total unsigned current helicity are computed from 1353 vector magnetograms observed at Huairou Solar Observing Station. The relationship between the solar flare productivity and the current three measures can also be well fitted with sigmoid functions. These results are expected to be beneficial to future operational flare forecasting models.     

Relative importance of solar X-rays below 10 Å and pulsed HF radiowave absorption at 2.132 MHz frequency over Delhi under solar flare conditions

The computed photoionization rates in the discrete wavelength bands in the XUV spectra under quiet sun, moderate flare, strong flare and outstanding flare conditions have been presented graphically as a function of altitude in the height range 50 – 90 km to show the relative importance of solar X-rays below 10 Å. Ionospheric absorption data, measured by A₁ technique at the Ionosphere Research Centre at the University of Delhi, Delhi (28.6°N) at a single radio wave frequency of 2.132 MHz have been analyzed under solar flare conditions, with a special emphasis on a solar flare event on 3 February, 1983 which started at 1115 IST, reached its peak at 1140 IST and persisted till 1400 IST, during which time the radio wave absorption exceeded 60 or 65 dB for nearly three and half hours. During solar flare, a direct correlation between large enhancements of solar X-ray flux and ionospheric absorption is observed.     

Application of support vector machine combined with K-nearest neighbors in solar flare and solar proton events forecasting

The support vector machine (SVM) combined with K-nearest neighbors (KNN), called the SVM-KNN method, is new classing algorithm that take the advantages of the SVM and KNN. This method is applied to the forecasting models for solar flares and proton events. For the solar flare forecasting model, the sunspot area, the sunspot magnetic class, and the McIntosh class of sunspot group and 10 cm solar radio flux are chosen as inputs; for the solar proton event forecasting model, the inputs include the longitude of active regions, the flux of soft X-ray, and those for the solar flare forecasting model. Detailed tests are implemented for both of the proposed forecasting models, in which the SVM-KNN and the SVM methods are compared. The testing results demonstrate that the SVM-KNN method provide a higher forecasting accuracy in contrast to the SVM. It also gives an increased rate of ‘Low’ prediction at the same time. The ‘Low’ prediction means occurrence of solar flares or proton events with predictions of non-occurrence. This method show promise for forecasting models of solar flare and proton events.     

Evidence of the radio-quiet hard X-ray precursor of the 13 December 2006 solar flare

We report multi-wavelength investigation of the pre-impulsive phase of the 13 December 2006 X-class solar flare. We use hard X-ray data from the anticoincidence system of spectrometer onboard INTEGRAL (ACS) jointly with soft X-ray data from the GOES-12 and Hinode satellites. Radio data are from Nobeyama and Learmonth solar observatories and from the Culgoora Solar Radio Spectrograph. The main finding of our analysis is a spiky increase of the ACS count rate accompanied by surprisingly gradual and weak growth of microwave emission and without detectable radio emission at meter and decimeter wavelengths about 10 min prior to the impulsive phase of the solar flare. At the time of this pre-flare hard X-ray burst the onset of the GOES soft X-ray event has been reported, positive derivative of the GOES soft X-ray flux started to rise and a bright spot has appeared in the images of the Hinode X-ray telescope (XRT) between the flare ribbons near the magnetic inversion line close to the sources of thermal and non-thermal hard X-ray emission observed by Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) during the flare. These facts we consider as evidences of solar origin of the increased pre-flare ACS count rate. We briefly discuss a possible cause of the pre-flare emission peculiarities.     

Microwave imaging observation of an electron stream in a solar flare

We report a Nobeyama Radioheliograph (NoRH) microwave observation of a propagating feature of non thermal emission in a solar flare. The flare had a very extended source well resolved by NoRH. In the rising phase of the microwave burst, a non-thermal gyrosynchrotron source was observed by the high-rate (10 images per second) observations to propagate from one end of the loop to the other with a speed of 9 × 10⁴ km s⁻¹. We interpret this non-thermal propagating source is emitted from streaming electrons.     

The GPS measured SITEC caused by the very intense solar flare on July 14, 2000

This work studies the sudden increases in total electron content of the ionosphere caused by the very intense solar flare on July 14, 2000. Total electron content (TEC) data observed from a Global Positioning System (GPS) network are used to calculate the flare-induced TEC increment, δTEC_f, and variation rate, dTEC_f/dt. It is found that both dTEC_f/dt and δTEC_f are closely related with the solar zenith angles. To explain the observation results, we derived a simple relationship between the partial derivative of the flare-induced TEC, ∂TEC_f/∂t, which is a good approximation for dTEC_f/dt, and the solar zenith angle χ, as well as the effective flare radiation flux I_f, according to the well-known Chapman theory of ionization. The derived formula predicted that ∂TEC_f/∂t is proportional to I_f and inverse proportional to Chapman function ch(χ). This theoretical prediction not only explains the correlation of dTEC_f/dt and δTEC_f with χ as shown in our TEC observation, but also gives a way to deduce I_f from TEC observation of GPS network. Thus, the present work shows that GPS observation is a powerful tool in the observation and investigation of solar flare effects on the ionosphere, i.e., the sudden ionospheric disturbances, which is a significant phenomenon of space weather.     

Reorganization of the solar corona following a C4.7 flare

Yohkoh X-ray images, multifrequency two-dimentional observations of the Nancay Radioheliograph, Kitt Peak and Mees magnetograms provide a unique set of data with which to study a C4.7 long-duration flare that was observed close to the equator (S07, W11) on 25 Oct. 1994 at 09:49 UT. Linear force-free field extrapolations indicate a very high degree of non-potentiality in the active region. The X-ray flare started with the expansion of spectacular twisted loops. Fifteen minutes after the flare onset sporadic radio (type III) bursts were observed spreading over an area of almost 1/3 of the solar disc and two remote X-ray brightenings appeared over quiet regions of opposite magnetic polarity located in on opposite hemispheres of the Sun. In the close vicinity of these remote brightenings two coronal holes formed. The timing and location of these events combined with the overall magnetic configuration provide evidence for a large-scale magnetic reconnection occurring between the expanding twisted loops and the overlying huge loops which inter-connect quiet solar regions.     

Gamma-ray measurements from the space shuttle during a solar flare.

An X2/2B level solar flare occurred on 12 August, 1989, during the last day of the flight of the Space Shuttle Columbia (STS-28). Detectors on the GOES 7 satellite observed increased X-ray fluxes at approximately 1400 GMT and a solar particle event (SPE) at approximately 1600 GMT. Measurements with the bismuth germanate (BGO) detector of the Shuttle Activation Monitor (SAM) experiment on STS-28 showed factors of two to three increases in count rates at high latitudes comparable to those seen during South Atlantic Anomaly (SAA) passages beginning at about 1100 GMT. That increased activity was observed at both north and south high latitudes in the 57 degrees, 300 kilometer orbit and continued until the detector was turned off at 1800 GMT. Measurements made earlier in the flight over the same geographic coordinates did not produce the same levels of activity. This increase in activity may not be entirely accounted for by observed geomagnetic phenomena which were not related to the solar flare.     

Relationship of solar flare accelerated particles to solar energetic particles (SEPs) observed in the interplanetary medium

Observations of hard X-ray (HXR)/γ-ray continuum and γ-ray lines produced by energetic electrons and ions, respectively, colliding with the solar atmosphere, have shown that large solar flares can accelerate ions up to many GeV and electrons up to hundreds of MeV. Solar energetic particles (SEPs) are observed by spacecraft near 1 AU and by ground-based instrumentation to extend up to similar energies as in large SEP events, but it appears that a different acceleration process, one associated with fast coronal mass ejections is responsible. Much weaker SEP events are observed that are generally rich in electrons, ³He, and heavy elements. The energetic particles in these events appear to be similar to those accelerated in flares. The Ramaty high energy solar spectroscopic imager (RHESSI) mission provides high-resolution spectroscopy and imaging of flare HXRs and γ-rays. Such observations can provide information on the location, energy spectra, and composition of the flare accelerated energetic particles at the Sun. Here, preliminary comparisons of the RHESSI observations with observations of both energetic electron and ion near 1 AU are reviewed, and the implications for the particle acceleration and escape processes are discussed.     

New flare model using recent measurements of the solar ultraviolet irradiance

The solar photon output from the Sun, which was once thought to be constant, varies considerably over time scales from seconds during solar flares to years due to the solar cycle. This is especially true in the wavelengths shorter than 190 nm. These variations cause significant deviations in the Earth and space environment on similar time scales, which then affects many things including satellite drag, radio communications, atmospheric densities and composition of particular atoms, molecules, and ions of Earth and other planets, as well as the accuracy in the Global Positioning System (GPS). The Flare Irradiance Spectral Model (FISM) is an empirical model that estimates the solar irradiance at wavelengths from 0.1 to 190 nm at 1 nm resolution with a time cadence of 60 s. This is a high enough temporal resolution to model variations due to solar flares, for which few accurate measurements at these wavelengths exist. This model also captures variations on the longer time scales of solar rotation (days) and solar cycle (years). Daily average proxies used are the 0–4 nm irradiance, the Mg II c/w, F10.7, as well as the 1 nm bins centered at 30.5 nm, 121.5 (Lyman Alpha), and 36.5 nm. The GOES 0.1–0.8 nm irradiance is used as the flare proxy. The FISM algorithms are given, and results and comparisons are shown that demonstrate the FISM estimations agree within the stated uncertainties to the various measurements of the solar Vacuum Ultraviolet (VUV) irradiance.     

Spectroscopic analysis of the solar flare event on 2002 August 3 with the use of RHESSI and RESIK data

We use simultaneous observations from RESIK and RHESSI instruments to compare plasma properties of a major solar flare in its rise and gradual phase. This event occurred on 2002 August 3 (peak time at 19:06 UT). The flare had a very good coverage with RESIK data and well-resolved soft and hard X-ray sources were seen in RHESSI images. Spectra of X-ray radiation from RHESSI images are studied and compared with RESIK measurements in different flare phases. Result shows large differences in flare morphology and spectra between flare rise and gradual phase.     

The morphology and velocity field of the large flare on the solar disc on July 14, 1980

The active region morphology and the features of solar radio bursts and sight-line velocity distribution of a flare of Importance 3B on the solar disc (AR 2562) on 1980 July 14 are introduced in this article.     

Modeling the time-intensity profile of solar flare generated particle fluxes in the inner heliosphere.

It is possible to model the time-intensity profile of solar particles expected in space after the occurrence of a significant solar flare on the sun. After the particles are accelerated in the flare process, if conditions are favorable, they may be released into the solar corona and then into space. The heliolongitudinal gradients observed in the inner heliosphere are extremely variable, reflecting the major magnetic structures in the solar corona which extend into space. These magnetic structures control the particle gradients in the inner heliosphere. The most extensive solar particle measurements are those observed by earth-orbiting spacecraft, and forecast and prediction procedures are best for the position of the earth. There is no consensus of how to extend the earth-based models to other locations in space. Local interplanetary conditions and structures exert considerable influence on the time-intensity profiles observed. The interplanetary shock may either reduce or enhance the particle intensity observed at a specific point in space and the observed effects are very dependent on energy.     

Multi-wavelength study of a strong impulsive solar limb flare on 2002 August 3

We made a detailed study of the impulsive solar flare of GOES class X1.0 which occurred near the west limb on 2002 August 3, peak time 19:07 UT. There is particularly good data coverage of this event, with simultaneous observations in EUV, soft and hard X-rays available. We used TRACE 171 Å images to study the morphology and evolution of this event. Soft X-ray spectra in the wavelength range 3.34–6.05 Å measured by the RESIK Bragg crystal spectrometer on CORONAS-F were used for determination of the evolution of the flare plasma temperature. Data from the RHESSI instrument were used to investigate properties of the higher-temperature plasma during the flare.     

X-ray and gamma-ray observations of a white-light flare

HEAO-1 observed hard radiations (X- and gamma-rays) from a major solar flare on 11 July 1978. The observations showed gamma-ray line and continuum emission extending to the highest energy observed. The lines are identified with the 2.2 MeV line of deuterium formation and the 4.4 MeV line of inelastic scattering on ¹²C, both previously observed in the flares of August 1972 [1]. The 11 July flare was identified as a white-light flare by observations at Debrecen [2]. It thus provides the first opportunity for a detailed examination of white-light flare theories that depend upon proton heating of the photosphere. The line strength over a four-minute integration at 2.2 MeV was 1.00 ± 0.29 ph(cm² sec)⁻¹, and the gamma-ray emission (excluding the 2.2 MeV line which was appreciably delayed) lagged by less than 20 sec approximately after the hard X-ray and microwave fluxes. We conclude that the “second-stage” acceleration of high-energy solar particles must commence promptly after the impulsive phase.     

A statistical study of CME-associated flare during the solar cycle 24

We investigate on the relationship between flares and coronal mass ejections (CMEs) in which a flare started before and after the CME events which differ in their physical properties, indicating potentially different initiation mechanisms. The physical properties of two types flare-correlated CME remain an interesting and important question in space weather. We study the relationship between flares and CMEs using a different approach requiring both temporal and spatial constraints during the period from December 1, 2008 to April 30, 2017 in which the CMEs data were acquired by SOHO/LASCO (Solar and Heliospheric Observatory/Large Angle Spectrometric Coronagraph) over the solar cycle 24. The soft X-ray flare flux data, such as flare class, location, onset time and integrated flux, are collected from Geostationary Environmental satellite (GOES) and XRT Flare catalogs. We selected 307 CMEs-flares pairs applying simultaneously temporal and spatial constraints in all events for the distinguish between two associated CME-flare types. We study the correlated properties of coincident flares and CMEs during this period, specifically separating the sample into two types: flares that precede a CME and flares that follow a CME. We found an opposite correlation relationship between the acceleration and velocity of CMEs in the After- and Before-CMEs events. We found a log-log relation between the width and mass of CMEs in the two associated types. The CMEs and flares properties show that there were significant differences in all physical parameters such as (mass, angular width, kinetic energy, speed and acceleration) between two flare-associated CME types.     

Intermittence of the short-term periodicities of the flare index

Intermittence of the short-term periodicities (25–35 days) of the flare index are investigated using the wavelet transform method for the full-disc and for the northern and the southern hemispheres of the Sun separately over the epoch since 1966 until 2002. The wavelet transform results show that occurence of periodicities of flare index power is highly intermittent in time. The period-averaged wavelet power of the flare index presents this fact very clearly displaying independence of flaring activity on the solar hemispheres in several time intervals over almost four solar cycles under study. Moreover correlations of the period-averaged wavelet power of the flare index for the separate hemispheres and for the full-disc reveal significantly stronger relation between the full-disc and the northern hemisphere than between the full-disc and the southern hemisphere while no significant correlations was found between the hemispheres one another.     

On the motions of RHESSI flare footpoints

The footpoint motions of flare hard X-ray (HXR) sources are directly related to the reconnection scenario of a solar flare. In this work, we tried to extract the information of footpoint motions for a number of flares observed with RHESSI. We found that the RHESSI flare results of the footpoint motions strongly support the classification proposed from the observations of YOHKOH/HXT. Furthermore, it is found that a flare can consist of two types of footpoint motions. We discussed the connections of the footpoint motions with the two-dimensional reconnection models.     

Sensing the Earth’s low ionosphere during solar flares using VLF signals and goes solar X-ray data

An analysis of D-region electron density height profile variations, induced by four isolated solar X-ray flares during period from September 2005 to December 2006, based on the amplitude and the phase delay perturbation of 22.1 kHz signal trace from Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E), coded GQD, was carried out. Solar flare data were taken from NOAA GOES12 satellite one-minute listings. For VLF data acquisition and recordings at the Institute of Physics, Belgrade, Serbia, the AbsPAL system was used. Starting from LWPCv21 code (Ferguson, 1998), the variations of the Earth-ionosphere waveguide characteristic parameters, sharpness and reflection height, were estimated during the flare conditions. It was found that solar flare events affected the VLF wave propagation in the Earth-ionosphere waveguide by changing the lower ionosphere electron density height profile, in a different way, for different solar flare events.