One of the observational facts about sunspots is that magnetic field strength of sunspots varies in narrow interval $\approx$ from 1 to 4–5 kG. Why sunspot equilibrium is stable only under these values? To answer this question, we propose the energetic model of sunspot, which involves the gravitational energy of the system. Formation of a sunspot is followed by redistribution of gas density along the vertical: cooled plasma flows down, inside the “magnetic funnel”, from above the sunspot to the “low boundary of magnetic flux tube”, i.e. to the region where the magnetic field of sunspot decreases abruptly with depth. Theoretical model gives the interval of stable sunspot equilibria exactly between 0.8–1.0 and 4–6 kG, which suites the observations. The “depth of sunspot” turns to be about 2–4 Mm – this fact now is approved by helioseismology. Also, the long-term (T$\,{>}\,$20 min) eigen oscillations of sunspot, as a whole, are studied. These oscillations were measured by different methods, being observed as quasi periodical vertical displacements of the body of sunspot followed by variations of its radius and magnetic field strength.

We analyze time series scans of the solar limb performed by the spectrometer SUMER on-board the SoHO satellite. The analyzed dataset consists of observations obtained in the C III 977 Å line. After applying the standard SUMER data reduction procedure, we perform a one line Gaussian fit to the profiles to obtain different parameters for each pixel within the scans. Studying the variations of the line width from the disk to the limb and off-limb we find that it increases off-limb and shows a peak at $\sim15\prime\prime above the limb. We suggest that this increase is due to the random directions of several spicules found along the line of sight.

A kinetic analysis is performed here considering close relationship between plasma jets in the solar corona and a consequent acceleration of electrons. Solar jets are formed when a plasma stream is shooted out from a reconnection site. In some cases these jets are temporally and spatially associated with type III radio bursts, the latter being the radio signatures of nonthermal electron beam propagation through the corona. This theoretical work on jet associated type III burst argue the possibility of regarding the plasma jet as a trigger mechanism for electron energization (heating or/and acceleration). While the jet propagates in the solar corona, it creates wave-like disturbances in the background plasma, that are in general unstable. Further on we are interested in how a test electron will behave in such (electrostatic) wave field. The settings for the jet and the test electron are done under typical coronal conditions.

The manuscript presents a review of the results obtained by the author from fast sequential H-alpha filtergrams taken for a quiescent filament together with similar results of other authors. The revealed structure clearly shows that the foot elements of the filament are surrounded by a fishbone-like plage area (magnetic region). Some parts of the filament exist at the boundary of large supergranular cells of about 40′′ in diameter. It seems that the filament is formed due to mixing of magnetic elements of different polarities.

The large-scale magnetic field of the Sun and solar-type stars is probably generated near the interface of the radiative core and the convection zone. One of the well-known difficulties of interface-type and other dynamo models is to explain the latitudinal distribution of magnetic fields as observed on the Sun. In this contribution new results of numerical MHD simulations of transport coefficients of magnetic fields in the rapid rotation regime, relevant for the base of the solar convection zone, are presented that may contribute to explaining the latitudinal distribution of magnetic fields observed on the Sun. A brief outlook on further numerical simulations of transport coefficients and their relevance for mean-field dynamo theory is given.

A suggested method is proposed to forecast the general features of the 11- year solar activity principle cycle. It is based upon the count of lengths and durations of spotless events, prevailing in the preceding minimum of the coming new cycle. The method has been successfully applied to predict the strengths and time of rises for the $22^{nd}$ and $23^{rd}$ 11-year cycles. The proposed precursor technique is further developed to make preliminary prediction of the maximum relative sunspot number and the time of rise of the $24^{th}$ 11-year cycle. The predicted values of these parameters are found to be $90.7\pm9.2$ and $4.6 \pm1.2$ year respectively. In addition, neural, Fuzzy neural and genetic algorithms have been also applied for the confirmation of the predicted results. A comparison with the early predictions used by other methods is given.

Major properties of the solar atmosphere and wind are not understood. The energy distribution of the solar atmosphere perturbations, over nearly ten orders of magnitude in energy, is close (within one order of magnitude) to a power-law of index -2, even though there is no agreement on the detailed shape. There is no agreed explanation of the origin of the solar wind, either, nor of the fact that the total wind energy flux is independent of speed, latitude, and phase of solar cycle, with an average base flux of 70 W/m$^2$ – a figure that is similar for a number of other cool stellar winds, and is close to the total observed energy flux of solar atmosphere perturbations. A major theoretical difficulty is that both coronal heating and wind production depend fundamentally on the heat flux, and the solar atmosphere is not collisional enough for classical transport theory to hold. Heat transport thus behaves non classically and the particle velocity distributions may have significant high energy tails, which should dramatically affect coronal heating and solar wind acceleration, in a way that is outside the scope of standard MHD.

Fourier local correlation tracking (FLCT) was applied to time series of the normal magnetic field in vector magnetograms of NOAA AR 8210, from 17:13–21:29 UT on 1998 May 1, to derive photospheric flows. These flows, combined with both the observed horizontal magnetic field and an inferred horizontal potential magnetic field, can be used to derive the flux of free magnetic energy across the photosphere. Here, we present the formalism used, as well as a map of the time-averaged free energy flux. Over the time interval covered by the magnetograms, ${\sim} 10^{31}\,$erg of free magnetic energy flowed upward across the photosphere, a significant fraction of the energy typically released in large flares coronal mass ejections (CMEs). A flare and coronal mass ejection began at 22:30 UT. We acknowledge the kind support of AFOSR's MURI program.

We report on a complex study of a typical large W-limb CME event occurring on December 2, 2003 in the vicinity of AR 0508. It is associated with a prominence eruption which has been observed with the Pic du Midi Ha flux coronagraph as well as in EUV by both the SOHO/EIT and the Coronas/SPIRIT space telescopes. The eruption started with the emergence of a fast expanding loop between 9 and 10 UT, followed by a heating and an acceleration of the erupted material and resulted in three-part CME observed by LASCO after 10:50 UT. A temporal analysis of the prominence motion and the EUV dimming light curve have shown that both the frontal structure and the core of a CME were initiated simultaneously with the peak of the X-ray C7.2 flare. The total mass of the prominence including the H$\alpha$ filament and EUV filament channel is close to the mass of the core but significantly less than the total CME mass.

Here we present an analysis of microwave emission spectra from flare-productive active regions (FPAR). For that we used regular observations in a wide frequency range with high spectral resolution and polarization sensitivity made with the RATAN-600 radio telescope. The multi-frequency observations make possible to detect small changes of magnetic field structures at different heights of the solar atmosphere. Observations with RATAN-600 and other large radio heliographs revealed an existence of narrow-band irregularities in the circular polarization spectra of FPARs radio emission. The new receiver complex, recently installed at RATAN-600, significantly improves its technical capabilities for the FPARs further study. Now it provides 1% (or 100 MHz) resolution in the frequency range from 6 GHz to 18 GHz in simultaneous registration. We present some observational results obtained with this new equipment.

We present studies on a series of ICMEs detected by Ulysses and for which the solar sources on the Sun could be identified. EUV and white light data are used in order to correlate characteristics seen during eruption with those measured in-situ. Particularly, an attempt was made to find solar features that show a relationship with the type of ICME seen later (i.e. cloud or non-cloud ICME). For magnetic clouds (MC) the chirality of the magnetic field was then analyzed. Finally, the charge states of oxygen ions contained in ICMEs were used to obtain freezing-in temperatures and then compare these with the presence of flares occurring close (spatially and temporally) to the CME eruption.

We have found no solar feature that could be used to predict the presence of a MC in interplanetary space, they occur with the same frequency for cloud and non-cloud ICMEs. The chirality of the clouds seems to follow only weakly the hemisphere rule. The presence of solar flares do not seem to be correlated with the oxygen freezing-in temperatures seen in-situ.

We examine the influence of different electron density stratifications on the coronal LOS-integrated profiles of H I Ly-$\alpha$ and O VI lines at 103nm. We find that the widths of the emitted lines are significantly affected by the details of the adopted electron density profiles. Densities deduced 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 kinetic temperatures are employed. Hence we expect the magnitude of the anisotropy to depend strongly on the density stratification adopted when analyzing the data.

Observations of the large scale magnetic field in the photosphere taken at the Wilcox Solar Observatory since 1976 up to 2005 have been analyzed to deduce its latitudinal and longitudinal structures, its differential rotation, and their variability in time. Basic topology of solar magnetic field was reconstructed in coordinate system rotating with different rate.

Employing the Poincaré index of isolated null-point in a vector field, we work out a mathematical method of searching 3D null-points in coronal magnetic fields. After introducing the theory of differential topology, we test the method by using the analytical model of Brown & Priest. The location of null-point identified by our method coincides precisely with the analytical solution. Finally we apply the method to search 3D null-points in coronal magnetic fields reconstructed based on observed magnetograms of two active regions (NOAA AR 10488 and AR 10720). We find that the 3D null-points seem to be a key element to the magnetic topology associated with flare occurrence.

We report the eruption of a small filament on 2001 September 28. Followed by a two-ribbon flare, the eruption was associated with a partial halo coronal mass ejection. During the eruption, coronal bipolar double dimmings were formed on the regions of opposite magnetic polarities. The optical counterparts of the coronal dimmings, i.e., the $H_{\alpha}$ dimmings, were clear and showed similar appearance and consistent development. A remarkable fact is that the two dimmings were preceded by impulsive EUV and $H_{\alpha}$ brightenings at the flare's rise phase, suggesting that the eruption can be explained by the tether-cutting model.

From Yohkoh and SoHO observations, the magnetic reconnection is considered as a main energy release mechanism of the solar active phenomena. In this study, we try to reproduce GOES X-ray light curves (1–8 $\AA$) of solar flares using a model incorporating the radiative and conductive cooling and the magnetic reconnection heating.

The Prototype Brazilian Decimetre Array (PBDA) consists of 5 element alt-azimuth mounted parabolic dishes of 4-m diameter, having baselines up to 216 m in East-West direction. We present initial solar observations carried out with the PBDA during the period 22$^{\hbox{\scriptsize{nd}}}$ November to 11$^{\hbox{\scriptsize{th}}}$ December, 2004. The frequency of observation was 1.6 GHz. The temporal and spatial resolution were 100 ms and 3 arcmin, respectively.

The propagation of surface and body linear MHD modes in a twisted magnetic flux tube embedded in a magnetically twisted plasma environment is considered. The dispersion relation for surface and body modes is derived assuming constant external twisted field. Analytic approximate solutions to the dispersion equation are found for the long and short wave length cases.

It was found, that in case the twisted component of the magnetic field in the environment of the flux tube is constant the index of Bessel functions $\nu$ in the corresponding dispersion relation is not integer. In the particular case of a homogen magnetic twist the total pressure is found to be constant across the boundary of the flux tube.

We use the spectral shapes of the EUV line profiles to study the plasma dynamics, acceleration and heating, in polar plumes (PP). We find that the observed profiles are reproduced fairly well when considering low plume wind speeds and velocity turbulence $(\alpha_S)$ at low altitudes followed by a rapid acceleration and heating of the plasma to reach the properties of inter-plumes (IP) by $\approx3-4~R_\odot$. We also find that plumes very close to the pole give narrow profiles at all heights that are not observed above $\approx2.5~R_\odot$. This suggests a tendency for plume footpoints to lie more than 10° away from the pole. High resolution magnetograms of SOLIS and EUV images support this hypothesis.

Longitudinal structure of the photospheric magnetic field over last tree solar cycles has been studied. The reconstruction of the longitudinal structure in the heliographic system rotating rigidly with Carrington rate was performed and compared with longitudinal distributions of random origin.