Magnetohydrodynamic equilibria for a plasma in a gravitational field are investigated analytically. For equilibria with one ignorable spatial coordinate, the equations reduce to a single nonlinear elliptic partial differential equation for the magnetic potential A, known as the Grad-Shafranov equation. Specifying the arbitrary functions in the latter equation, one gets a nonlinear elliptic partial differential equation (the sinh Poisson equation). Analytical solutions of this equation are obtained for the case of an isothermal atmosphere in a uniform gravitational field. The solutions are obtained by using the tanh method, and are adequate for describing parallel filaments of diffuse, magnetized plasma suspended horizontally in equilibrium in a uniform gravitational field.

We present the results of numerical simulations of 3D magnetic reconnection driven by photospheric footpoint motions. The model consists of two positive and two negative sources, which are placed on opposite boundaries of the cubic domain. Two different types of photospheric motions are then considered, namely rotating and twisting of the sources. These different footpoint motions result in a difference in the evolution of the magnetic skeleton and the location and efficiency of the energy build up. Both the dynamical evolution and the corresponding potential evolution of each system is investigated and a comparison is made between the energy storage and release that occurs at separators and separatrix surfaces.

The problem of isotropization of energetic protons in the $H_\alpha$ emitting region during flare energy release is considered. It has been shown that absence of the linear polarization of $H_\alpha$ emission in some events can be caused by excitation of small scale Alfven waves by anisotropic protons. Consequences of the proposed model are discussed.

The solar magnetic field that extends through the chromosphere is thought to expand through the transition region into the corona. The strong flux concentrations are located within the boundaries of supergranular convection cells. These boundaries form network lanes, observed in emission lines as bright lanes with varying width throughout the solar atmosphere. These network field concentrations are surrounded by mixed-polarity magnetic field with a scale of the granule diameter, as suggested by observations. We use potential magnetic field extrapolations on synthetic magnetograms to study the magnetic network topology and the effects of background magnetic field on the network expansion through the solar atmosphere. We find that the background magnetic field has a considerable effect on the ratio of network area over field of view. Furthermore we find that the expansion of the network boundaries with height deviate significantly from well-assumed funnel model expansion.

We present spectral analysis methods suitable for diagnostics of flaring plasma from RESIK spectra. RESIK is the uncollimated bent crystal spectrometer aboard the Russian CORONAS-F solar mission. It collected many flare and active region spectra in the wavelength range 3.3 Å–6.1 Å, where strong emission lines of Si, S, Ar, and K are present. Based on a careful instrument calibration the absolute fluxes in the individual spectral lines have been obtained. These fluxes have been used for determination of a set of thermodynamic parameters characterizing the emitting plasma and for studies their time behavior during selected flares.

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.

Mottles and spicules are the most prominent, short-lived, dynamic features residing at the quiet Sun chromospheric network and constitute what is known as chromospheric fine structure. We are reporting a comprehensive study of the dynamical characteristics and physical properties of such structures, from multi-wavelength observations, using line inversion techniques and a wavelet spectral analysis. We are furthermore examining their dynamical evolution and their periodic bi-directional velocity behaviour, their interrelationship and their association with the underlying magnetic field which seems to be their forming and driving mechanism. These studies are crucial to understanding the dynamics of the solar chromosphere, as well as the role such structures play in the mass balance and heating of the overlying solar atmosphere.

We investigate the altitude dependence of hard X-ray (HXR) spectra in solar flares, i.e., whether the HXR spectra are related to the altitudes of reconnection sites. We assume that the reconnection altitude can be scaled by the distance between the two conjugate HXR footpoints in the flare. By searching the RHESSI flare list from 2002 to 2004, we find 42 solar flares below X-class that have enhanced 50–100 keV HXR emission and two well-resolved HXR footpoints at the nonthermal peak time. The preliminary results show that there is a weak correlation ($\sim$ –0.31) between the HXR spectral index and the HXR footpoint distance. We further discuss the possible implications.

Recently, high resolution observations by SOHO and TRACE spacecraft have identified oscillating loops and propagating waves in the solar coronal. These new discoveries established a new discipline that is known as coronal seismology. The importance of this lies in the potential for the diagnostics of coronal structures and knowledge of coronal heating. We present a study of the effects of radiative cooling and heating processes on longitudinal waves in coronal loops. We find that radiation and heating results in a clear modification in the evolution of temperature and pressure perturbations but in slight effect on the decay time of the wave.

Ellerman bombs or moustache are known as short-living fine emission structures associated with a local energy release in solar atmosphere. The relation between the moustaches and flare has been discussed since the moment of the moustaches discovery. The resent results of the Flare Genesis Experiment demonstrated that this problem is still actual and attractive. We took into account the possibility of different mechanisms of their origin (thermal and non-thermal) by using spectro polarimetric data obtained in summer 1999 and carried out analysis of Ellerman bomb's role in flare activity of the Sun by the example of the several active regions.

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.

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.

The Statistics analyses of the microwave type III bursts, coronal mass ejections (CMEs), H$\alpha$flares and relevant events observed with 5200–7600 MHz spectrograph at the Chinese National Astronomical Observatory during the 23rd solar cycle are carried out in this article. Some significant results are obtained from the relevant events. The radiation mechanism of that is also discussed from the observation characteristics.

Evidence of coronal oscillations over the interior of supergranular cells was found through SUMER observations. The observations are rasters of quiet Sun regions and the oscillations were detected, in the Ne ${\sc viii}\, 770 \AA$ Doppler maps, as a characteristic pattern. It should be noted that the Ne ${\sc viii}\,$ ion has coronal formation temperature (650 000 K) and that reports of oscillations in the quiet Sun corona are scarce. Magnetic extrapolation from MDI magnetogram showed that at the location where the oscillation was detected, the gas and magnetic pressures get equalized ($\beta$=1) higher in the atmosphere, compared to the surrounding, non oscillating quiet Sun. This could indicate a non-compressible wave propagating inside the gas dominated medium of the cell causing the detected oscillation.

The determination of the physical parameters of coronal loops remains both an observational and a theoretical challenge. A new diagnostic technique for quiescent dynamically heated coronal loops, based on the analysis of the power spectra of Doppler shift time series, is proposed. It is assumed that a given loop is heated randomly both in space and time by small-scale discrete impulsive events of unspecified nature. It is shown here that, depending on the heliographic position of the loop and the orientation of the observing instrument, various harmonics can be identified in the power spectra of line shift time series. The frequency peaks are sensitive to changes in the average loop temperature and are proposed to use as a temperature diagnostic tool. The analysis of the power spectra also allows to distinguish uniformly heated loops from loops heated near their footpoints.

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.

This is a brief review of the quiescent large scale visible corona with an emphasis on the origin, structure and role of streamers in the solar wind. The review is mostly based on results from the last 10 years of the SOHO mission and the goal is to provide a coherent picture of what is known about streamers at the end of the current cycle.

An overview is presented of large-scale coronal structures as observed in soft X-rays (SXR) and extreme ultraviolet (EUV) wavelengths in the context of their magnetic properties. These structures include large-scale interconnecting and trans-equatorial loops, coronal streamers, coronal holes, filaments and filament channels. Since the general appearance of the corona and its structures change with evolving underlying fields, evolutionary trends and solar cycle dependence of these coronal structures are discussed as well.

The solar magnetic cycle affects all levels of the Sun including the convection zone, photosphere, chromosphere and corona. Recent advances in solar space missions (Yohkoh, SOHO and others) and, also, ground-based observations provide us an excellent opportunity to investigate solar magnetic activity in detail, and to draw a new picture of the solar magnetic cycle. Magnetic field appears on the solar surface as a result dynamo processes in the convection zone, and forms bipolar complexes of solar activity. These complexes can be seen in the photosphere as dark sunspots surrounded by the bright plages extended into chromosphere, with arcades of coronal loops best observed in EUV and soft X-rays. The coronal loops reflect the large-scale magnetic structure of complexes of activity. The new data reveal fundamental changes in the magnetic topology during the solar cycle, and details of the polar magnetic field reversals occurring near the sunspot maximum. The solar synoptic maps obtained from the photospheric and coronal data display a close correlation between the erupted magnetic flux and coronal emissions and show large-scale magnetic connectivities. The brief review of solar cycle studies is presented.

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.