A nonlinear mechanism for the damping of fast magnetoacoustic wave in the solar corona is studied. It is shown that the nonlinear coupling of finite-amplitude fast waves to small-scale Alfven waves can be much faster than damping mechanisms involving classic transport coefficients.

We discuss recent collaborative observations of coronal mass ejections and related activity using the Very Large Array (VLA) and the Solar and Heliospheric Observatory (SOHO). VLA observations show the onset of intense 400 cm burst emission during an X1 GOES soft X-ray burst observed by the Yohkoh spacecraft and prior to a CME observed by the Large Angle Spectroscometric C2 Coronagraph (LASCO) on board SOHO. VLA snapshot maps show that the 400 cm burst site varied discontinuously throughout the course of the flare, possibly reflecting abrupt changes in the structure of the coronal magnetic fields along which the energetic particles traveled.

Launched in October 1990, the ESA-NASA Ulysses mission has conducted the very first survey of the heliosphere within 5 AU of the Sun over the full range of heliolatitudes. The first polar passes took place in 1994 and 1995, enabling Ulysses to characterise the global structure of the heliosphere at solar minimum, when the corona adopts its simplest configuration. The most important findings to date include a confirmation of the uniform nature of the high-speed (~ 750 km s−1) solar wind flow from the polar coronal holes, filling two-thirds of the volume of the inner heliosphere; the sharp boundary, existing from the chromosphere through the corona, between fast and slow solar wind streams; the latitude independence of the radial component of the heliospheric magnetic field; the lower-than-expected latitude gradient of galactic and anomalous cosmic rays; the continued existence of recurrent increases in the flux of low-energy ions and electrons up to the highest latitudes.

Recent in-situ measurements of the composition of the solar wind and heliospheric particles from SOHO, ACE, Wind and Ulysses are providing valuable and often unique new information on origins and acceleration processes in the Sun and heliosphere. This paper reviews some of the more interesting recent findings and their implications.

The magnetic cloud of November 17-18, 1975 is analyzed and it is shown that measurements of magnetic field and solar wind plasma are consistent with the interpretation that this magnetic cloud is a dual-polarity flux rope which consists of a core and an annular region. The core has a strong magnetic field; the annular region has a higher plasma density and opposite magnetic polarity.

Suprathermal electron populations are generated efficiently in uniform space plasmas by resonant Landau interaction with broadband Alfvén wave spectra. Based on a simulation of the particle acceleration in response to the wave energy input within the Fokker-Planck formalism, details of the time evolution of the velocity space distribution towards commonly observed solar wind power law spectra are discussed. Furthermore, it is shown that the diffusion properties and consequently the particle energization change significantly in astrophysical environments with magnetic field and density gradients, since synergetic effects become dominant. Electrons are accelerated resonantly out of the bulk of the distribution such that they interact again with a wave packet of higher phase velocity, leading to a multi-stage energization. Hence, a unique acceleration mechanism can be achieved without postulating pre-acceleration by ad hoc mechanisms. Depending on the density and magnetic field profile, significant enhancement of energetic particles as well as stagnation in a saturated stage of suprathermal, non-Maxwellian velocity space distributions is possible. The importance of synergetic acceleration in complex solar flare structures and for solar wind heating mechanisms is discussed in relation to spacecraft observations.

Examples of the acceleration of energetic ions and electrons are given for the Sun, the middle heliosphere, and the outer heliosphere. The characteristics of the acceleration are revealed only after the effects of energetic particle propagation are understood.

We investigate the acceleration of charged particles in the framework of collisionless reconnection. A steady reconnection scenario is considered, with a two dimensional X-point magnetic field geometry having also a uniform field component transverse to the plane of the X-point field, and an inductive electric field generating an inflow of particles. Test particle trajectories are studied, and the energy spectra of the accelerated particles are determined.

On the basis of the Yohkoh data on the site and mechanism of magnetic energy transformation into thermal and kinetic energies of superhot plasmas and accelerated particles, a model is developed that explains observed properties of collisionless 3D reconnection in active regions. The model makes intelligible the observed relation between the S-like morphology and eruptive activity.

Data from the Mass and Charge Time-of-Flight spectrometers MTOF and CTOF on board SOHO have been accumulated for time periods in which coronal hole and non-coronal hole type plasma has been detected by using different methods in order to determine the abundance ratios of magnesium isotopes in these two different source regions of the solar wind. As indicators for the two different solar wind type plasma we have used several plasma parameters such as, the freeze-in temperatures, the Fe/O and the He/H ratio, which are known as reasonable indicators. For this study we have used the refractory element Mg which is an authentic whitness for matter in the solar nebula because temperatures of the Sun never have been high enough to change their isotopic composition by nuclear burning. In addition Mg has three isotopes with abundance ratios greater than about 10% and therefore they are easy to observe.

Solar Orbiter will provide, at very high spatial (35 km pixel size) and temporal resolution, novel observations of the solar atmosphere and unexplored inner heliosphere, which will be made in the heliosynchronous segments of the orbits at heliocentric distances near 45 solar radii and out of the ecliptic plane at high heliographic latitudes up to 38°. The Solar Orbiter will achieve its wide-ranging scientific aims with a suite of sophisticated instruments through an innovative orbit design.

It is shown that high energy electrons propagate in solar plasma as beam-plasma structure, a new nonlinear object.

Direct solar flare neutrons are a valuable diagnostic of high-energy ion acceleration in these events, and COMPTEL improves over all previous cosmic neutron detectors in its capacity for neutron energy measurement. Previous studies of COMPTEL neutron data have worked with an incomplete model of the instrumental response, applying energy-by-energy detection efficiencies. Here we employ statistical regularisation techniques with the full (Monte Carlo simulation derived) response matrix to produce improved estimates of neutron numbers and energy distribution. These techniques are applied to data from the well-observed 15 June 1991 flare. Our improved treatment of the instrumental response results in a reduction of 73% in total neutron numbers, compared with previously deduced values. Implications for the picture of primary ion acceleration in this flare are briefly discussed.

Spacecraft measurements of energetic electrons in the vicinity of the Earth's bow shock and interplanetary shocks are analyzed and compared with theoretical calculations. It is concluded that shock drift acceleration of electrons is very modified by an additional process, probably by strong pitch angle scattering. Calculations including this effect are presented.

IPS observations have recently begun at Miyun Station, Beijing Astronomical Observatory. This paper briefly describes the radio telescope at Miyun Station, discusses the observation and the data reduction procedures and presents the preliminary results of observations on IPS source 3C48.

The heliospheric magnetic field configuration is largely determined in the solar atmosphere. The interplanetary magnetic field is therefore intimately linked with the coronal structure and evolution during the solar cycle. We summarize recent experimental results from active satellite experiments on Ulysses and the Advanced Composition Explorer (ACE). These results provide constraints on the sources of the solar wind and also the magnetic structure of the heliosphere and the corona. These results suggest the relevance of reconnection processes and differential rotation effects close to the Sun. This leads to large perturbations from a standard Archimedean spiral configuration which cannot be successfully modeled using coronal models which assume a potential magnetic field.

HST, EUVE, and Ca II spectra are providing critical velocity and column density data needed to identify individual stuctures (clouds) of warm gas in the local ISM. The Sun is located very close to the edge of and will soon leave the Local Interstellar Cloud (LIC). We will summarize the properties of the LIC and other nearby warm clouds.

We use HST observations of the nearest star system (the α Cen system) to search for Lyα absorption from both heliospheric material around the Sun and analogous astrospheric material around the stars which has been heated by the collision between the solar/stellar winds and the interstellar medium. For α Cen A and B, both heliospheric and astrospheric absorption are detected, whereas for α Cen's distant companion star Proxima Cen only heliospheric absorption is detected, suggesting that Proxima Cen's astrosphere must be smaller and its wind therefore weaker than that of α Cen. Based on observations of these stars taken many years apart, we find that neither the heliospheric absorption nor the astrospheric absorption detected toward α Cen AB varies significantly.

We analyse the new ISCCP (International Satellite Cloud Climatology Project) D2 cloud data to ascertain whether or not a connection between cosmic ray flux and cloud cover exists. Our results indicate that only the low-level cloud follows solar activity over the full period, 1983-1994. Using several proxies for solar activity and the radiative forcing for the ISCCP cloud types, we estimate the possible impact that such a solar-terrestrial connection may have on climate. We conclude that, possibly excluding the most recent decades, much of the warming of the past century can be quantitatively accounted for by the direct and indirect effects of solar activity.