We study the MHD processes related to a flare/CME event in the lower solar corona using numerical simulations. Our initial state is an isothermal gravitationally stratified corona with an embedded flux rope magnetic field structure. The eruption is driven by applying an artificial force to the flux rope. The results show that as the flux rope rises, a shock structure is formed, reaching from ahead of the flux rope all the way to the solar surface. The speed of the shock quickly exceeds that of the driving flux rope, and the shock escapes from the driver. Thus, the shock exhibits characteristics both of the driven and blast wave type. In addition, the temperature distribution behind the shock is loop-like, implying that erupting loop-like structures observed in soft X-ray images might be shocks. Finally, we note that care must be taken when performing correlation analysis of the speed and location of type II bursts and ejecta.

Within a period of intense activity (20 October to 5 November 2003), the injection and propagation of near relativistic electrons, resulted in hundreds of type III bursts recorded by the ARTEMISIV radio spectrograph (20–650 MHz). For a number of these type III events association with GOES SXR/Hα flare and/or SOHO/LASCO CME was established. We study the variation of characteristic type III parameters and their relationship with features of the associated flares and/or CMEs.

We have performed multiwavelength analysis on an event with a metric type II burst, which appeared first as fragmented emission lanes in the radio dynamic spectrum. The start frequency was unusually high. Since type II bursts are thought to be signatures of propagating shock waves, it is of interest to know how the shocks, and the type II bursts, are formed. This radio event was associated with a flare and a coronal mass ejection (CME), and we investigate their connection. Observations suggested that a propagating shock was formed due to the erupting structures, and the observed radio emission reflects the high densities in active region loops. We then utilised numerical MHD simulations, to study the shock structure induced by an erupting CME, in a model corona including dense loops. Our simulations show that the fragmented part of the type II burst can be formed when a coronal shock driven by a CME passes through a system of dense loops overlying an active region. To produce fragmented emission, the conditions for plasma emission have to be more favourable inside the loop than in the inter-loop area. The obvious hypothesis, consistent with our simulation model, is that the shock strength decreases significantly in the space between the denser loops. Outside the active region, the type II burst dies out when the changing geometry no longer favours the electron shock-acceleration.

A method is developed for estimation of the vertical structure of the magnetic field in active regions using multi-wave spectral-polarization measurements of radio waves which gives not only the dependence of magnetic field strength on height but also determines two-dimensional form of a magnetic flux tube, emitted in the microwave range of wavelengths.

Study of solar oscillations has provided us detailed information about solar structure and dynamics. These in turn provide a test of theories of stellar structure and evolution as well as theories of angular momentum transfer and dynamo. Some of these results about the solar structure and its implication on the recent revision of heavy element abundances are described. Apart from these the solar cycle variations in the rotation rate and its gradients are also discussed.

The variations of the global mass of heliosphere in the 23rd cycle of the solar activity are described. The results are derived from solar corona observations and from ‘in situ’ measurements made by the space probes SOHO, VOYAGER2, ACE, WIND, and ULYSSES. It has been revealed that though the total mass of corona fluctuates during the solar activity cycle approximately in a ratio of 1 : 3, the specific mass flow (q) in the solar wind does not change in the ecliptic plane. In the polar regions the q decreases during the minimum in a third of the original value and the velocity of expansion is roughly double. These findings are valid for the 23rd solar cycle.

Stereographic projection of Hopf field on the 3-sphere into Euclidean 3-space is used as a model of 3D steady flow of ideal compressible fluid in MHD. In such case, flow lines are Villarceau circles lying on tori corresponding to the levels of Bernoulli function. Existence of an optimal torus with minimal relative surface free energy is shown. Beat of oscillations with wave numbers corresponding to structural radii of optimal torus leads to scaling of optimal tori. Spatial intersection of homothetic tori within one torus result in formation of cluster with the size depending on scaling factor. Optimal tori are considered as precursors of planetary orbits.

Magnetic helicity is a quantity that describes the linkage and twistedness/shear in the magnetic field. It has the unique feature that it is probably the only physical quantity which is approximately conserved even in resistive MHD. This makes magnetic helicity an ideal tool for the exploration of the physics of eruptive events. The concept of magnetic helicity can be used to monitor the whole history of a CME event from the emergence of twisted magnetic flux from the convective zone to the eruption and propagation of the CME into interplanetary space. In this article, I discuss the sources of the magnetic helicity injected into active regions and the role of magnetic helicity in the initiation of solar eruptions.

Forbush decrease (or, in a broader sense, Forbush effect) - is a storm in cosmic rays, which is a part of heliospheric storm and very often observed simultaneously with a geomagnetic storm. Disturbances in the solar wind, magnetosphere and cosmic rays are closely interrelated and caused by the same active processes on the Sun. Thus, it is natural and useful to investigate them together. Such an investigation in the present work is based on the characteristics of cosmic rays with rigidity of 10 GV. The results are derived using data from the world wide neutron monitor network and are combined with relevant information into a data base on Forbush effects and large interplanetary disturbances.

The Photometric-Magnetic Dynamical model handles the evolution of an individual sunspot as an autonomous nonlinear, though integrable, dynamical system. The model considers the simultaneous interplay of two different interacted factors: The photometric and magnetic factors, respectively, characterizing the evolution of the sunspot visible area A on the photosphere, and the simultaneous evolution of the sunspot magnetic field strength B. All the possible sunspots are gathered in a specific region of the phase space (A, B). The separatrix of this phase space region determines the upper limit of the values of sunspot area and magnetic strength. Consequently, an upper limit of the magnetic flux in an active region is also determined, found to be ≈7.23 × 1023 Mx. This value is phenomenologically equal to the magnetic flux concentrated in the totality of the granules of the quite Sun. Hence, the magnetic flux concentrated in an active region cannot exceed the one concentrated in the whole photosphere.

In this work we present the first results of study and comparison of the parameters of quasi-periodic long-term oscillations of microwave emission of large (>0.7 arcmin) sunspots as a result of simultaneous observations with two radioheliographs – NoRH (17 GHz) and Siberian Solar Radio Telescope (SSRT) (5.7 GHz) with 1 minute cadence. Radioheliographs have been working with quite large time overlap (about 5 hours) and have the high spatial resolution: 10 arcsec (NoRH) and 20 arcsec (SSRT). We have found that quasi-periodic long-term oscillations are surely observed at both frequencies with the periods in the range of 20–150 min. We detected common periods for common time of observations with two radioheliographs and interpret this as the consequence of the vertical-radial quasi-periodic displacements of sunspot as a whole structure.

The theory of plasma emission and of electron cyclotron maser emission, and their applications to solar radio bursts and to Jupiter's decametric radioation (DAM) and the Earth's auroral kilometric radiation (AKR) are reviewed, emphasizing the early literature and problems that remain unresolved. It is pointed out that there are quantitative measures of coherence in radio astronomy that have yet to be explored either observationally or theoretically.

In this paper we show that different locations of acceleration/injection sites in flaring loops may produce very different types of pitch-angle distributions of accelerated electrons and, as a consequence, different spatial, spectral and polarization properties of the loop microwave emission. It is shown that these properties can be detected using spatially resolved microwave observations of specific flaring loops and be used to choose the most suitable electron acceleration model.

Coronal dimmings often develop in the vicinity of erupting magnetic configurations. It has been suggested that they mark the location of the footpoints of ejected flux ropes and, thus, their magnetic flux can be used as a proxy for the ejected flux. If so, this quantity can be compared to the flux in the associated interplanetary magnetic cloud (MC) to find clues about the origin of the ejected flux rope. In the context of this interpretation, we present several events for which we have done a comparative solar-interplanetary analysis. We combine SOHO/Extreme Ultraviolet Imaging Telescope (EIT) data and Michelson Doppler Imager (MDI) magnetic maps to identify and measure the flux in the dimmed regions. We model the associated MCs and compute their magnetic flux using in situ observations. We find that the magnetic fluxes in the dimmings and MCs are compatible in some events; though this is not the case for large-scale and intense eruptions that occur in regions that are not isolated from others. We conclude that, in these particular cases, a fraction of the dimmed regions can be formed by reconnection between the erupting field and the surrounding magnetic structures, via a stepping process that can also explain other CME associated events.

The numerical method developed by Veselovsky & Ivanov (2006), together with magnetograms of the Sun obtained at the photospheric level were used to calculate the coronal magnetic field with open, closed and intermittent topology during March-December 2007. The results of the modelling are compared with stereoscopic images and movies of the corona observed by EUV telescopes onboard STEREO and SOHO spacecraft. The sources of the permanent and transient high speed solar wind streams as well as the sector structure and the heliospheric plasma sheet observed at the Earth's orbit by the ACE and STEREO spacecraft are discussed.

The gravitational stratification effect on magnetohydrodynamic waves at a single interface in the solar atmosphere has been studied in the penumbral region of the sunspot recently. The existence of slow and fast magneto acoustic gravity waves and their characteristics has been discussed. The effect of flows on magneto acoustic gravity surface waves leads to modes called flow modes or v-modes. The present geometry is that of a plasma slab moving with uniform velocity surrounded by a plasma of different density. As is applicable to the corona, we assume that the plasma β to be small. The dispersion characteristics change significantly with a change in the value of G (gravity) and uniform flow.

Using SOHO/EIT Fe XII λ195 Å observations the new type of oscillations in coronal loops was detected. The oscillation corresponds to wave propagated to outer area of atmosphere of active area. As opposed to most kind of oscillations associated with coronal loops the waves are observed at non-flare stage of active areas evolution. Velocities of the wave propagation were 8-20 km s−1 and had quasi-perpendicular direction with magnetic field. Such waves were detected in active areas located on solar disk and loops structures outside solar limb. Investigation of EIT data shows the waves are not result of changes of topology of a magnetic field and loops configuration. The nature and probable sources of waves are discussed.

We examine the source properties of X-class soft X-ray flares that were not associated with coronal mass ejections (CMEs). All the flares were associated with intense microwave bursts implying the production of high energy electrons. However, most (85%) of the flares were not associated with metric type III bursts, even though open field lines existed in all but two of the active regions. The X-class flares seem to be truly confined because there was no material ejection (thermal or nonthermal) away from the flaring region into space.

A numerical simulation with ENLIL+Cone model was carried out to study the propagation of the shock driven by the 2005 May 13 CME. We then conducted a statistical analysis on a subset of similar events, where a decameter-hectometric (DH) type II radio burst and a counterpart kilometric type II have been observed to be associated with each CME (DHkm CME). The simulation results show that fast CME-driven shocks experienced a rapid deceleration as they propagated through the corona and then kept a nearly constant speed traveling out into the heliosphere. Two improved methods are proposed to predict the fast CME-driven shock arrival time, which give the prediction errors of 3.43 and 6.83 hrs, respectively.

The two Helios probes traveled at variable longitudinal and radial separations through the inner heliosphere. They collected most valuable high resolution plasma and magnetic field data for an entire solar cycle. The mission is still so successful that no other missions will collect the same kind of data in the next 20 years. One of the subjects studied after the success of the Helios mission was the identification of more than 390 shock waves driven by Interplanetary Coronal Mass Ejections (ICMEs). Combining the data from both probes, we make a statistical study for the extension of the shock waves in the interplanetary medium. For longitudinal separations of 90° we found a cutoff value at this angular separation. A shock has 50% of chance to be observed by both probes and the same probability for not being observed by two spacecrafts at the same time, when the angle between them is around 90°. We describe the dependence of the probability for shocks to be observed by both probes with decreasing spacecraft separation. Including plasma data from the ISEE-3 and IMP-8 spacecrafts improves our statistical evaluation substantially.