We examine the spatial and temporal behaviour of the quiet Sun transition region (TR) using high resolution SUMER observations in O VI 1032 Å. A detailed study of raster images allows us to differentiate the network and internetwork through Doppler shift and line width measurements. The oscillatory nature of the TR is investigated using wavelet analysis over a series of spectra obtained with high temporal cadence.

Using coordinated TRACE and EIT observations, we discussed which pair of coronal abundances and ionization equilibrium assumption will give the least difference of temperature and emission maps. It is found that the combination meyer_coronal.abund and shull.ioneq gives the smallest difference between TRACE and EIT observations among the 42 pairs of combination, while the difference between each pair is not much. The difference between temperature or emission maps from coordinated TRACE and EIT observations may be regarded as an estimation of the accuracy of the temperature or emission maps using this kind of method. Under this assumption, the error for temperature diagnosis is estimated at Δ(log10T) = 0.01 and the error for emission diagnosis is estimated at Δ(log10EM) = 0.08 at largest.

While TRACE data have provided us much information of transition region and coronal structures, many TRACE data users would like to have a knowledge of emission heights of TRACE bands. By analyzing TRACE limb observations, we give an average estimation of emission heights of TRACE 171, 195 and 1216 bands for different features like quiet Sun regions, active regions and coronal holes. Average emission heights over the limb are also discussed. Previous equator-to-pole height variation is further confirmed by TRACE data when averaging on quiet Sun regions. If averaging for all fluxes, a reverse equator-to-pole height variation is shown.

We describe the fine structure of white light polar plumes observed using SOHO/LASCO C2 coronagraph. The evolving helical structures of different scales are clearly seen on processed images (the processing is made to reveal the faint contrast objects). The observed structures trace the magnetic field lines, so the electric currents flow along the axes of plumes. An MHD model of a plume which takes into account field-aligned electric currents is developed. The model permits to understand the existence of high-density plasma inside the plume due to the balance between the magnetic forces and the transverse pressure gradient.

The study of a possible connection between magnetic activity and orbital period variation in close binaries is a very interesting work. Recently, the orbital periods of four chromospherically active binaries, ER Vul, UV Psc, AR Lac and BH Vir, are analyzed. It is discovered that the orbital periods of UV Psc and BH Vir oscillate with periods of 61 and 9.12 years, and the orbital periods of ER Vul and AR Lac show periodic variations with periods of 31 and 47 years respectively while they undergo secular decrease. The mechanisms that could explain the changes in the orbital periods of the four systems have been studied. The period variation of UV Psc may be caused by the cyclical magnetic activity in the primary component, and the magnetic activity in secondary component of AR Lac can explain its periodic component in the orbital period changes. For the other two systems, BH Vir and ER Vul, the cyclical magnetic activity in one or both of the components can explain the cyclical orbital period changes of BH Vir and the periodic component in the changes of the orbital period of ER Vul. These results suggest that the periods of the orbital period oscillations in the four systems may be the magnetic activity cycles.

The active region NOAA 8151 observed between February 11–13, 1998 exhibits a filament eruption linked to the disappearance of a sigmoidal structure. Using vector magnetograms from IVM (Mees Observatory, Hawaii), we perform a non linear force-free reconstruction of the coronal magnetic field above this active region. This reconstruction allows to determine the distribution of electric currents, the magnetic energy and the relative magnetic helicity. The reconstructed magnetic field lines are compared to the soft X-rays (SXT, Yohkoh) observations.

There are two major types of solar wind. The steady fast wind originating on open magnetic field lines in coronal holes, and the unsteady slow wind coming from the bulk or boundary layer of streamers, which are mostly magnetically closed but may open up temporarily. Many observations of the solar wind have in the past been made in situ and remotely. Key empirical constraints, which are imposed on the models by the Helios (near-Sun, in-ecliptic) and Ulysses (high-latitude) interplanetary measurements and by the Solar and Heliospheric Observatory (SOHO) plasma diagnostics, are discussed with respect to the fluid and kinetic properties of the wind. The majority of the models is concerned with the fast wind, essential properties of which can be reproduced by 1-D multi-fluid models involving broad-band waves. Owing to its time-variable nature, no robust understanding of the slow wind exists. Selected results from modelling and observations are discussed.

Energy release in the solar Corona is characterized by a sequence of space and time localized events, whose intensity follows power-law distributions. In quiet Sun regions, small energy events, possibly under the detection threshold, dominate, thus supporting the “nanoflare” scenario of coronal heating. Two complementar models of heating are discussed, in connection with the above observational features. The first model is based on Alfvénic wavepackets dissipation in 3D force-free magnetic fields; the presence of regions of chaoticity of magnetic lines allows for a fast wave dissipation, within a fraction of a solar radius. The second model describes a MHD turbulence in low-β plasma, in which magnetic energy is continuously furnished by slow photospheric motions. Energy release events corresponds dissipation of current sheets, often associated with magnetic reconnection. The resulting distribution of dissipated power follows a power law, similar to observations.

We report on results from the Solar Eclipse Coronal Imaging System (SECIS), an instrument using fast CCD cameras designed to search for short-period modulations in visible-light emission from the corona during an eclipse or with a coronagraph. The instrument was successfully used during the total eclipse of 1999 August 11 from a site in Bulgaria. This paper summarizes both the instrument and preliminary results.

The content of coronal material in the quiet Sun is not constant as soft X-ray and high-temperature EUV line observations have shown. New material, probably heated and evaporated from the chromosphere is occasionally injected even in the faintest parts above the magnetic network cell interiors. Assuming that the smaller events follow the pattern of the well observed larger ones, we estimate the total energy input. Various recent analyses are compared and discussed. The results using similar EUV data from EIT/SOHO and TRACE basically agree on the power-law exponent when the same method is used. The most serious deviations are in the number of nanoflares per energy unit and time unit. It may be explained at least partially by different thresholds for flare detection.

The last two decades have seen the emergence of a new field in stellar astrophysics, the study of stellar coronae mostly in X-ray domain. With the advent of soft X-ray imagery X-ray emission was found from many thousands of solar-like stars. I will summarize the most important findings of X-ray surveys of late type stars and put the results in the context of the solar-stellar connection.

We report the results from a spectroscopic study of the solar corona using both ground-based and space-based data. Some of the current topics in coronal physics, such as the plume-interplume differences in coronal holes and wave propagation in the corona have been examined. The distribution of emission line intensities, Doppler velocities and line widths were obtained from the spectroscopic observations from Norikura Solar Observatory of National Astronomical Observatory of Japan. The coronal images in Fe IX,X 171 Å and Fe XII 195 Å from SOHO EIT were used to get the temperature map of the corona. Combining both results, we have obtained the nonthermal velocities in the coronal hole region, without the usual assumption of a uniform ion temperature. It has been found that the histogram distributions of Doppler velocities and nonthermal velocities for coronal hole and quiet regions are markedly different. The nonthermal velocities have been found to be larger by about 27 %at the interplume regions as compared to plumes which supports the view that the interplume regions are the source regions of the fast solar wind. The analysis of the time sequence spectral data shows signatures of coronal oscillations at localized regions. Periods of the order of a few minutes have been found mainly in Doppler velocities.

We study two nonlinear processes that are efficient in the spectral redistribution of Alfvén wave energy - second harmonic generation, and scalar decay. Both processes transport the energy of AWs towards smaller scales and/or higher frequencies, and can accelerate the energization of space plasmas implied by recent SOHO observations.

We propose a coronal heating theory based on the magnetic twisting, which inevitably produces charge imbalance. The resulting electric field creates supra-thermal electron beams. Beams are then thermalized by classical collisions. The dissipation rate is enough to heat the corona and to accelerate the solar wind.

There is extensive evidence from SoHO and other data that “micro-events” play an important role in sustaining at least some components of the solar corona. These are often termed coronal micro-” heating events” though a major part of their role is feeding coronal loops through chromospheric evaporation. We consider what can be learnt from these data concerning the energy release and transport mechanisms driving the evaporation, including thermal conduction and fast particles. We conclude, from one large event and the statistics of many small ones, that conductive evaporation alone does not fit observations and that fast particles or some other nonthermal driver must be involved.

Since the start of the SoHO mission, EIT -the Extreme ultraviolet Imaging Telescope- offers a global view of the solar corona over the whole rising phase of the current activity cycle. Such a dataset is unprecedented. We give here the current results of an on-going investigation over the entire EIT archive. In the Fe XV images (2 MK), the on-disk and off-disk intensity distributions have been evaluated, and their evolution is described. Additionally, we developed an image processing technique that extracts the smallest detectable features. The cosmic ray hits are statistically disentangled from the solar point-like phenomena, and the trends in both rates are assessed.

We use the Coronal Diagnostic Spectrometer instrument on board the Solar and Heliospheric Observatory to analyse coronal helmet streamer structures observed close to the solar minimum / maximum on the 1996 July 8 / 1999 August 4-5th. The radial variation of peak electron temperature is extracted out to 2 solar radii. These are found to agree well with Yohkoh observations close to the solar maximum, but are found to be reduced by around half a million close to the solar minimum.

Magnetic flux tubes reaching from the solar convection zone into the chromosphere have to pass through the relatively cool, and therefore highly non-ideal (i.e. resistive) photospheric plasma. It is shown that stationary MHD equilibria of magnetic flux tubes which pass through this region require an inflow of plasma into the tube and a deviation from isorotation along the tube axis. Although for characteristic parameters of thick flux tubes the effect is negligible, a scaling law indicates its importance for small-scale structures. The relevance of this inflow for the expansion of flux tubes above the photosphere is discussed.

The nonlinear evolution of parametric instability of a non-monocrhomatic Alfvén wave is studied using numerical simulations. After a linear stage the instability saturates. For β ~ 1 the initial mode remains dominant, except at large scales, where an inverse cascade of backscattered Alfvénic modes is present. A comparison with solar wind data gives a qualitative agreement, indicating that parametric instability could play an important role in the evolution of solar wind turbulence.

We study the evolution of the emissivity correlated with magnetic flux density of an active region from its birth until its decay throughout all atmospheric layers. We analyse multi-wavelength data obtained from SOHO, Yohkoh, GOES, SOLSTICE and 10.7 cm radio data from DRAO, Canada. We utilise our results to understand the scaling laws in different atmospheric layers. We confirm that the relationship between the emitted excess flux (flux - basal flux) and photospheric magnetic flux density ΔF(< f B >) follow power laws, and the powers depend on the formation temperature of the line(s) involved.