Hard X-ray radiation from several powerful solar flares was detected in October-November, 2003 by the SPR-N hard X-ray polarimeter onboard CORONAS-F satellite. In the October, 29 solar flare (20:40 - 21:00 UT) the X-ray radiation was strongly polarized (more than 70% of photons with energies 40-100 keV). In the flares 28.10.03 and 04.11.03 the emission was not polarized. The upper limits for the part of polarized photons at the level of tens percents were obtained in these flares.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Explosive events and blinkers are two observational classes of transients seen on the quiet Sun and an investigation of the significance of and relationship between such events may be critical for understanding basic processes at work in the solar atmosphere. Our analysis showed that blinkers and explosive events are independent phenomena which have to be explained separately.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We summarize the results of our studies of CME-associated EUV dimmings and coronal waves by ‘derotated’ fixed-difference SOHO/EIT heliograms at 195Å with 12-min intervals and at 171, 195, 284, 304Å with intervals of 6 or 12 hours.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

On the base of the interferometric observations during the total solar eclipse on 11 August 1999 magnetic field strength in a coronal volume at the height of 10$^5$ km above the solar limb has been evaluated using an indirect method. A cold coronal emission (CCE), seen in the lines of $H_{\alpha}$ and K CaII, existed there. The conclusion is made that the magnetic field strength in this bulk, before the investigated CCE-region formed, was about 100-200 G.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We performed the study of a solar flare of GOES class X1.0 which occurred on 2002 August 21, peak time 05:34 UT. A new method was applied for temperature determination of the plasma seen in TRACE images. This method makes use of the detailed knowledge of the TRACE point spread function (PSF) function. Substantial dependence of TRACE PSF on wavelength allows for determination of temperature from a single TRACE image even in these portions of the image which are saturated. We also determined the average temperature of this flare from the soft X-ray spectra measured by RESIK Bragg spectrometer aboard CORONAS-F satellite. The results obtained from TARCE and RESIK data are compared.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The radio and multi-wavelength signature of coronal loop interaction is discussed in the event on November 28, 1998 with X-class flares and CMEs, observed by Nobeyama Radioheliograph (NoRH), YOHKOH/SXT/HXT, SOHO/EIT/LASCO, and Chinese radio spectrometers (2.6-3.8 GHz). The fluctuations at rising phase with bi-directional drift rates may show the signature of magnetic reconnection caused by loop interaction. There are two or three peaks in radio time profiles and spectra, which show that two or three flares are triggered in the process of loop interaction. The 17 GHz sources are composed of three parts with different time profiles and polarization feature. The formation of twisted loops in post-flare phase may indicate the start time of the CMEs in this event.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The comparison of the polarization presence in Ellerman bombs or moustaches and the active region arch structure was carried out. In the most cases of the polarization, interpreted as impact polarization, were detected in that regions where the bright small loops were observed. On the contrary, when a significant polarization wasn't detected the scanning during observation was carried out nearby the large loops.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

RESIK Bragg spectrometer aboard CORONAS-F solar observatory collected spectra of hundreds of solar flares in the unique spectral range between 3.3 Å and 6.1 Å. The instrument's absolute and relative calibrations are now finished which allows for the first time to interpret the observations in terms of the differential emission measure (DEM) distribution. With known DEM shape it is possible to derive and study in detail flaring plasma composition and energy balance. Here we report results of using an improved Maximum Likelihood iterative algorithm (Withbroe-Sylwester) for determination of the DEM distributions for 7 January 2003 flare at 23:30 UT. The study is based on the absolute flux values observed in a number of strong H- and He-like transitions of K xviii, Ar xviii, Ar xvii, S xvi, S xv, Si xiv and Si xiii ions. In this respect, we use line and continuum X-ray emissivities as provided by CHIANTI package from SolarSoft.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Solar flare on 26 June 1999 was accompanied by radio bursts giving an indirect evidence of accelerated superthermal particle beams. It is generally believed that this effect can stimulate an impact linear polarization in Balmer series spectral lines. The flare was observed simultaneously by the Ondřejov Multichannel Flare Spectrograph (MFS) and by the Large Solar Vacuum Telescope (LSVT) using the polarization optics in the H$\alpha$ line. Spectropolarimetric data obtained by the two instruments are analyzed, however no substantial linear polarization was detected. Both spectral and auxiliary data, including radio and X-ray observations are analyzed to discuss possible reasons of the lack of linear polarization.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A preflare situation in a loop structure for stability to the development of plasma waves at the chromosphere part of a current circuit of a loop is investigated. We consider the situation when plasma has the spatial gradients of the density and temperature. In accordance with Heyvarts-Priest-Rust theory such a preflare situation would typically exist when the amplitude of weak electric field in the circuit of an “old” loop in an active region begins to increase as “new” magnetic flux emerges from under the photosphere (Heyvaerts, Priest & Rust (1977)).To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The description of two major flares detected by SONG spectrometer on board the CORONAS-F mission during famous period of solar activity in October- November 2003 and their comparison with soft X-ray measured by GOES-12 is given.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We determine a location of a source of a main acceleration of relativistic particles during the flare on July 14, 2000, adopting that the time of acceleration coincides with a burst of the hard linear and continuum gamma-radiation. The process of this effective acceleration starts when one of flare ribbons reaches the large spot of the group, as it follows from the TRACE data. An estimation shows that the source of acceleration locates on the height of 15–20 thousands of km above the photosphere. Comparison these results with those for other large flares allows us to conclude that during almost 30 powerful events in the last three cycles an additional effective “accelerator” of a moderate scale turns on in diluted regions with large magnetic fields and their gradients.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present the catalog of data for 27 gamma-ray flares detected above 500 keV by the gamma-ray spectrometer SONG (Solar Neutrons and Gamma-Rays) onboard the Russian CORONAS-F satellite. These gamma-ray flares were collected during the period from August 14, 2001 till November 31, 2003 covering the latter half of the 23rd solar sunspot cycle.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The relaxation dynamics of a magnetized plasma system is a subject of fundamental importance in MHD – with applications ranging from laboratory plasma devices like the Toroidal Field Pinch and Spheromaks to astrophysical plasmas, stellar flaring activity and coronal heating. Taylor in 1974 proposed that the magnetic field in a plasma (of small but finite resistivity) relaxes to a minimum energy state, subject to the constraint that its total magnetic helicity is conserved (Woltjer 1958), such that the final magnetic field configuration is a constant $\alpha$ (linear) force-free field – where $\alpha$ is a quantity describing the twist in magnetic field lines. However, a clear signature of this mechanism in astrophysical plasmas remained undetected. Here we report observational detection of a relaxation process, similar to what Taylor (1974, 1986) envisaged, in the magnetic fields of flare-productive solar active regions. The implications of this result for magnetic reconnection and the coronal heating problem are discussed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The CORONAS-F satellite has been launched on 31 July 2001 (Oraevsky et al. 2002). To conduct measurements of the intensity of solar radiation in the EUV spectral region near the hydrogen line $L_{\alpha}(\lambda = 121.6$ nm) the spectrophotometer VUSS was installed onboard CORONAS-F. A vacuum photodiode was used as EUV detector in VUSS with the $CuJ$ cathode and $MgF_2$ entrance window.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Results of modern multi-wavelength observations of large solar non-stationary events are briefly summarized. Such events consist of a puls, an eruption (surge or CME) and a long-living system of giant arches.

Observations demonstrate often that the pulse arises as a result of “avalanche” of interaction of small-scale magnetic elements, while the dynamic system transforms into a state of self-organizing criticality (SOC) on the post-eruptive phase. We developed new code for the avalanche model where it is possible to consider a transition of the active region as a dynamic system in a state of SOC. Derived solutions show, firstly, that the avalanche is naturally developed nearby spots during the pulse. Secondly, during long-duration events, occuring along the neutral line of the longitudinal magnetic field, behaviour of the large-scale coronal magnetic field is characterized by a state of SOC.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The paper is devoted to the coronal bright points. We carried out the analysis of the simultaneous observations in IR He $\lambda$10830 Å and microwaves at 5.2 and 1.76 cm. It was obtained that microwave sources correspond to the dark points in IR He $\lambda$10830 Å with line-of-sight velocities directed from a observer. The possible mechanism of the energy transport within the bright points are suggested.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Coronagraphic spectral observations carried out with UVCS on SoHO above polar coronal holes have shown that the velocity distribution is highly anisotropic. Here we examine the influence of the density stratification on the interpretation of such observations. In particular, we investigate the profiles of O VI spectral lines emitted in polar coronal holes. We find that at distances greater than $1~R_{\odot}$ from the solar surface the widths of the emitted lines are significantly affected by the details of the adopted electron density profiles. In particular, the densities deduced by Doyle et al. (1999) from SoHO data result in O VI profiles whose widths and intensity ratio are relatively close to the values observed by UVCS although only isotropic velocity distributions are employed. Hence we expect that the magnitude of the anisotropy of the velocity distribution deduced from UVCS data depends strongly on the adopted density profile.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We report on a novel investigation of the complex magnetism of the quiet Sun which is based on a realistic three-dimensional modeling of the Hanle effect.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The solar corona is believed to be generally low beta. In a low-beta plasma, energy is mainly stored as magnetic energy or current. To dissipate the stored magnetic energy in the highly conductive corona, anomalous resistivity is needed. Activities of outer layers of X-ray loops in solar flares have been interpreted as the result of reconnection of magnetic field due to anomalous resistivity in a current sheet located above the flaring loop.

However, recent high-cadence EUV observations show many flares without opening up of magnetic field. This means that existence of current sheets above flaring loops is not a necessary element for flares. It is also found for many flares that low temperature prominence plasma are heated in the very early phase of lifting. These observational facts require a new solar flare scenario different from reconnection.

Recent multi-wavelength high-spatial and -temporal resolution observations show that flares originate in the lower atmosphere. We can expect high beta plasma and small curvature magnetic loops in the lower atmosphere. It is also observed high-velocity flows along magnetic loops associated with flares. Bounded motions of charged particles (both thermal motion and flow) along the magnetic lines of force create the centrifugal force. They exceed gravity force under certain conditions (small curvature and high velocity). They can even exceed magnetic tension force depending on beta values of the plasma in the loop.

In the case of the upward acceleration (centrifugal - gravity$>0$), the plasma is balanced by the magnetic tension force if the beta value is not large enough. This is a favorable condition for the interchange instability. Finite beta ($beta \sim 0.1$) plasma in a loop, whose ends are anchored at the photosphere, is unstable against the localized interchange (BALLOONING) mode. It can develop into non-linear phase and explosive phenomena are expected. This is called HIGH-BETA DISRUPTION. The high-beta disruption has many common characteristics to solar flares. Various phenomena related to solar flares such as, high-energy particle acceleration, plasma turbulence, plasma ejections, plasma heating, are interpred as the result of high-beta disruption. High spatial and temporal resolution observations of flares at various wavelengths are used to see the actual instability process. Shibasaki (2001)To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html