The homologous flare sites and the general solar activity

The homologous flares observed in the same region of a spotgroup testify the existence and the duration of a permanent instability. However, they also attest that the general magnetic configuration is not destroyed by these flares and that it changes $\text{slowly}$ up to the death of the site.The study of every flaring sites where more than ten flares occur has been performed in Meudon for the 1974–1980 period.One hundred and sixty-six sites have been analysed from the rotation where the A.R. is observed up to five rotations ahead. The basis of the study are the “Synoptic Maps”. A relation is found between the presence of crossing of “filament-phantom” corridors and the location of the homologous flare sites.¹     

A homologous microwave flare on May 29, 1980

Two homologous solar bursts were recorded on May 29, 1980 at 1028 UT and at 1147 UT from the Hale region 16864. The measurements were done at 8 mm wavelength at the Metsähovi Radio Research Station using a 14 meter radio telescope. The time series of the bursts were similar even in the small details. The rise time of both bursts was about 10 seconds and the peak flux density was 3.3 sfu at 1028 UT and 1.2 sfu at 1147 UT. Both bursts were composed of several elementary spikes which were typically 3 seconds apart from each other. The maximum of the gyro-synchrotron type spectrum was close to 15 GHz. The time profile of the bursts, elementary spikes and the frequency spectrum indicated that the origin of these homologous microwave bursts was in a magnetic structure with several loops and that the same complex loop structure was producing energy during both bursts.     

Large-scale reconnection in a large flare

Some specific features of the large-scale magnetic reconnection in large solar flares are briefly reviewed. In particular, the large-scale structure and dynamics of the 3B/X5.7 flare on 14 July 2000 are interpreted in terms of the topological model. The role of the betatron effect in collapsing magnetic traps, that are created by reconnection in the solar corona, is considered. We discuss some possibilities to observe the collapsing trap acceleration in solar flares.     

Role of newly emerging flux in the flare process

New flux emerging from below the photosphere is believed to give rise to small flares and also to be capable of triggering large events when extra energy is stored in the overlying field. A summary is given of the observations of emerging flux, together with the current theoretical ideas on its behaviour.     

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.     

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.     

Numerical study of solar flare particle propagation in the heliosphere

Numerical solutions are presented for the propagation of solar cosmic rays interplanetary space, including the effects of pitch-angle scattering and adiabatic focusing. The intensity-time profiles can be well fitted by a simple radial spatial diffusion equation with scattering mean-free path λ_fit. For low-rigidity particles the radial mean-free path so obtained is significantly larger than the mean-free path calculated from the scattering coefficient due to the inapplicability of the diffusive approximation early in the event. The well-known discrepency between λ_fit and the theoretical predictions may be resolved by these calculations.     

Particle charge interchange during acceleration in flare regions

We have examined the conditions for the establishment of charge equilibrium of solar particles during their acceleration. We derive criteria for charge interchange with the atomic and ionized hydrogen at the particles'sources, for two different acceleration mechanisms. It is found that charge interchange is established whenever a particle event is produced. The implications related to mass and charge spectra of particles are discussed. The measured charge state of solar particles cannot in general be directly used for diagnosis of the source temperature, so we suggest another alternative based on the emitted radiation from electron capture.     

Proton flare and magnetic storm effect in the vicinity of the Earth.

We have developed a model and associated computational procedure for estimating energetic proton exposures during a major solar proton event that occur in combination with a large magnetic storm. Transmission functions for solar protons are computed using geomagnetic vertical cutoff data for quiescent and disturbed conditions. Predicted exposures in low altitude polar orbit are found to be orders of magnitude greater for severe magnetic storm conditions than are corresponding exposures in the absence of major disturbances. We examine the response scenario for the events of November 1960 as an example.     

Solar EUV flux variation and flare brightenings

On 2010 February 8, the Extreme ultraviolet (EUV) flux variation in 195 Å and flare brightening has been examined in different sizes of active regions by using SOHO/EIT, MDI and Hα$\alpha$ observational data. These three active regions represent a large active region with a sunspot group, a moderate active region without a sunspot and a small region with weak plage in Hα$\alpha$ band respectively. Our study shows that the main full disk EUV flux comes from active regions, especially from large active regions. The sudden increases of EUV flux are corresponding to the EUV flare brightenings. For the large active region, the local EUV 195 Å flux peaks are well correlated to that of the GOES X-ray flux. The EUV 195 Å flux peaking time of M-class flares delay GOES X-ray flux a few minutes. For the moderate active region, the local EUV 195 Å flux is not well correlated to GOES X-ray flux. The EUV 195 Å flare brightenings in the moderate active region appeared in the duration of sudden increase of its own local EUV flux. For the small active region, the local EUV 195  Å flux varied almost independently of the GOES X-ray flux. Our study suggests that for an active region its local EUV 195 Å flux is more closely correlated to the EUV flare brightening than the full disk GOES X-ray flux.     

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.     

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.     

On soft electron beams in solar active and flare region

On the basis of the experimental data obtained from the high resolution X-ray spectra for solar flares and active regions the Suprathermal electron model (SEM) was proposed. This model suggests the existance of the multitemperature structure of the solar plasma emitting Fe and Ca X-rays and the presence of additional electrons with low energies E ? 10 keV and small densities ～ 1–5% relative to the thermal component.     

Direct evidence for chromospheric evaporation in a well-observed compact flare

We have observed the flare of 1980 May 7 1456 UT with several Solar Maximum Mission instruments, in coordination with the Sacramento Peak Observatory Vacuum Tower Telescope. From the X-ray data we determine the total amount of plasma at T > 2 × 10⁶ K, commonly attributed to chromospheric evaporation. From Hα we have determined the amount of plasma that has been evaporated from the chromosphere. We find that enough material has been evaporated from the chromosphere to account for the X-ray plasma. Taken together, the Hα, soft and hard X-ray images suggest that chromospheric evaporation is driven both by flare accelerated electrons, during the impulsive phase, and conduction, during the thermal phase.     

The thermal X-ray spectrum of the 2003 April 26 solar flare

Observations and their analysis of the thermal X-ray spectrum of the M2 flare on 2003 April 26 are described. The spectrum observed by the RHESSI mission cover the energy range from ∼5 to ∼50 keV. With its ∼1-keV spectral resolution, intensities and equivalent widths of two line complexes, the Fe line group at 6.7 keV (mostly due to Fe xxv lines and Fe xxiv satellites) and the Fe/Ni line group at 8 keV (mostly due to higher-excitation Fe xxv lines and Ni xxvii lines) were obtained as a function of time through a number of flares. The abundance of Fe can also be determined from RHESSI spectra; it appears to be consistent with a coronal value for at least some times during the flare. Comparisons of RHESSI spectra with those from the RESIK crystal spectrometer on CORONAS-F show very satisfactory agreement, giving much confidence in the intensity calibration of both instruments.     

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.     

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.     

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.     

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.     

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.