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.     

Interpretation of hard X-ray images during the impulsive phase of a limb flare

We show that the observations of a limb flare, in which a hard X-ray (16–30 keV) source is seen at the boundary between two interacting magnetic structures, indicate the presence of hot (T ? 6 × 10⁷ K) plasma within the region. Non thermal bremsstrahlung processes do not agree with these observations. We discuss the possible causes of the heating.     

Multi-wavelength study of a coronal mass ejection: a flare event from AR#9393

The active region, AR#9393, produced a number of intense flares during March–April 2001. In this paper, we report the analysis of an X1.1 flare event of April 2, 2001 and its associated coronal mass ejection. The timing and location of the H_α eruption, radio burst activities, and the onset of mass ejection suggest an energy release that occurred close to the surface of the sun. At this region, as shown by the magnetogram, X-ray and EUV images, the field configuration was complex and the 3-D extrapolation revealed the presence of a magnetic null point. Results also suggest that the energy release is followed by the magnetic reconnection between the low-lying loops near the separator point and outlying loops. This study provides the support for the magnetic break-out process to trigger the energy release in eruptive flare event.     

Onset study of impulsive solar energetic particleevents

Impulsive solar energetic particle (SEP) events are associated with impulsive X-ray flares, energetic electrons,and enhanced heavy ion abundances. Using instruments on ACE, we have examined the composition and origin of twelve impulsive SEP events from November 1997 to June 2000. All selected impulsive SEP events have enhanced ³He/⁴He ratios compared with the solar wind values. The range of ³He/⁴He ratios varies from 0.01 to 7.8. By assuming scatter-free propagation at zero degree pitch-angle, we fitted the minimum particle path lengths (from 1.2 to 1.4 AU, as expected), and estimated the ion event release time back at the Sun to within better than 30 minutes in most cases. We found only four events in which the release times agree for both 38–50 keV electrons and <1 MeV/nucleon ions. Five of our events have significant differences (>40 minutes) between the electron and ion onset times, all with ions injected later. Three impulsive ion events have no association with any impulsive electron event. Seven events have associated solar electromagnetic signatures (Type III radio bursts and/or X-ray flares).     

An experiment to study solar flare effects on radio-communication signals

The occurrence of radio signal fading events caused by ionospheric absorption plays an important role in the performance of radio-communication systems. It is necessary to know the magnitude and time-scale of such events in order to specify technical parameters of the communication system to be used. Generally, fading events are associated with solar flares, which are characterized by sudden increase in the solar X-ray flux that causes an increase in the ionization in the lower ionosphere. The abrupt increase of ionization causes the absorption of radio waves propagating in the Earth–ionosphere wave-guide and is reported as radio signal fading events. A simple experiment to monitor the behavior of lower ionosphere has been carried out at the Southern Space Observatory-SSO/INPE (29.43°S, 53.8°W), located in southern Brazil. The experiment is basically a computer controlled radio receiver that records the received signal strength of Amplitude Modulated (AM) radio signals in the HF (High Frequencies) range. We analyzed data of the 6 MHz beacon signal that has been transmitted by a broadcasting radio station located about 400 km from the observation site. In this work we present initial results of daily variation of the received signal strength and fading events associated with solar flares observed in the 6 MHz signal monitored by the experiment during 2001. X-ray solar flux data from the GOES-8 satellite were used to identify X-ray solar bursts associated with solar flares. Based on the one-year data collected by the experiment, a statistical summary of fading occurrences and their correlation with solar flares, as well as the distributions of time-scales and magnitudes of such events are presented.     

Hard X-ray and high-frequency decimetric radio observations of the 4 April 2002 solar flare

Hard X-ray and high frequency decimetric type III radio bursts have been observed in association with the soft X-raysolar flare (GOES class M 6.1) on 4 April 2002 (1532 UT). The flare apparently occurred 6 degrees behind the east limb of the Sun in the active region NOAA 9898. Hard X-ray spectra and images were obtained by the X-ray imager on RHESSI during the impulsive phase of the flare. The Brazilian Solar Spectroscope and Ondrejov Radio Telescopes recorded type III bursts in 800–1400 MHz range in association with the flare. The images of the 3–6, 6–12, 12–25, and 25–50 keV X-ray sources, obtained simultaneously by RHESSI during the early impulsive phase of the flare, show that all the four X-ray sources were essentially at the same location well above the limb of the Sun. During the early impulsive phase, the X-ray spectrum over 8–30 keV range was consistent with a power law with a negative exponent of 6. The radio spectra show drifting radio structures with emission in a relatively narrow (Δf ≤ 200 MHz) frequency range indicating injection of energetic electrons into a plasmoid which is slowly drifting upwards in the corona.     

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.     

Hard X-ray and metric/decimetric spatially resolvedobservations of the 10 April 2002 solar flare

The GOES M8.2 flare on 10 April 2002 at 1230 UT was observed at X-ray wavelengths by RHESSI and atmetric/decimetric wavelengths by the Nançay Radioheliograph (NRH). We discuss the temporal evolution of X-ray sources together with the evolution of the radio emission sites observed at different coronal heights by the NRH. While the first strong HXR peak at energies above 50 keV arises from energy release in compact magnetic structures (with spatial scales of a few 10⁴ km) and is not associated with strong radio emission, the second one leads to energy release in magnetic structures with scales larger than 10⁵ km and is associated with intense decimetric/metric and dekametric emissions. We discuss these observations in the context of the acceleration sites of energetic electrons interacting at the Sun and of escaping ones.     

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.     

Analysis of the physical conditions in a strong X-ray flare

The temperature distribution of the hot plasma emission measure in a large but slowly developing flare has been investigated using the following data obtained from the INTERCOSMOS 4 satellite: (1) the X - ray spectra in the range 1.7 – 1.9 Å, (2) the hard X - ray fluxes in the range 10 – 40 keV. It has been found that all the data can be explained by a consistent thermal model of the emitting region.     

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.     

Multi-wavelength observations of the pre-cursor phase of solar flares

Observational studies of the pre-cursor phase of solar flares have shown that there are many and varied signatures that may or may not indicate the probable onset of a flare. Combining data from Yohkoh, SOHO and TRACE and more recent observations from RHESSI, SOHO and TRACE we, investigate the relationships between the different manifestations of pre-flare behaviour in two solar flares with a view to determining how they are related to the subsequent flare energy release. We find that in one case the preflare activity seems strongly related to the subsequent flare and probably represents a build-up of energy in the active region prior to flare onset. The second case we find to be less clear cut suggesting that significant further work remains to be done in order to determine which pre-flare signatures are most useful in indicating the build-up to flare onset.     

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.     

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.     

RHESSI observation of the M4.0 flare on 17 March 2002

The M4.0/SF flare on 17 March 2002 is a good example of the early observations with RHESSI. We presenthard X-ray images, light curves and energy spectra of individual hard X-ray sources, the spatial relationship between the hard X-ray sources and the H emission regions, and comparisons of light curves observed by RHESSI and GOES. We found that the picture exhibited by RHESSI is consistent with the general cartoon of a solar flare. In particular, we showed that the hard X-ray image spectra could be explained by a power-law electron beam with a lower energy cutoff E_c. The derived E_c could be as high as 40 keV, larger than the usually value of 20 keV.     

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.     

Impact linear polarization observed in a UV chromospheric line during a solar flare

Linear polarization was observed in the S I 1437Å line in bright flaring points during the soft X-ray emission. The degree of polarization is about 25% and is detected at a signal to noise ratio of 2.9. The polarized electric vector is directed towards disk center to within 3°.This polarization could be due to collisional excitationm of S I by energetic electrons beamed in the vertical direction. We suggest that the heat flux in the region interconnecting the transition zone to the high chromosphere during the gradual phase of a flare could lead to an anisotropic excitation. Then the observed polarization would be due to vertical motions of the transition zone sweeping the preexisting chromosphere.     

Physics of the impulsive phase of solar flares

The physics of the impulsive phase of solar flares is discussed in relation to high resolution microwave, hard X-ray and ultraviolet observations. High spatial resolution observations of the structure of microwave flaring loops and their interpretation in terms of arcades of loops as the sites of primary energy release are presented. Theoretical interpretation of the confinement of microwave producing energetic electrons in the coronal part of loops is discussed. High temporal and spatial resolution measurements in hard X-rays, as well as observations of the spectral evolution of the hard X-ray emission are presented. Observations of the relative locations of microwave and hard X-ray emitting regions are presented and their significance with respect to the energy release site and electron acceleration is discussed. The relative timing of the peaks of impulsive hard X-ray and microwave burst is discussed. The significance of ultraviolet measurements in obtaining the density of flaring regions is discussed. Possible diagnostics of impulsive phase onsets from cm-λ polarization data are presented, and the role of the emergence of new flux and of the current sheet formed between closed loops in producing impulsive energy release at centimeter wavelengths are analyzed. Decimeter and meter wave manifestations of preflash phase and millisecond pulsations at centimeter and decimeter wavelengths and the relevant physical processes involved are discussed.     

Information on particle acceleration and transport derived from solar flare spectropolarimetry

The hydrogen Hα line has been found to be linearly polarized at some locations and times during a June 15th 2001 flare observed with THEMIS. This flare was accompanied by radio pulses and hard X-ray emission. Linear polarization is below the noise level in the flare kernels. However, it is present at the edges of these kernels, in the line center and near wings where the polarization degree exceeds 4%. The directions of polarization are not random but close within ±15° to the tangential and radial directions. This polarization can be due either to electron beams and their associated return currents or to electron and proton beams.