The theory that most, if not all, interplanetary shocks are caused by coronal mass ejections (CMEs) faces serious problems in accounting for the strongest shocks. The difficulties include (i) a remarkable absence of very strong shocks during solar maximum 1980 when CMEs were prolific, (ii) unrealistic initial speeds near the Sun for impulsive models, (iii) the absence of rarefaction zones behind the shocks and (iv) sustained high speed flows following shocks which are not easily explained as consequences of CME eruptions. Observations of the proton temperature near 1 AU indicate that strong shock drivers have properties similar to high speed streams emitted by coronal holes. Eruptions of fast solar wind from coronal holes influenced by solar activity can explain the occurrence of the strongest interplanetary shocks.

We present a summary of several studies of transient coronal phenomena based upon high spatial resolution radio imaging data along with Yohkoh SXT and HXT observations. In addition to normal flares the studies also involve such exotic events as active region transient brightenings (ARTB) and coronal jets and bright points. We provide evidence of nonthermal processes in flaring X-ray bright points from spatially resolved meter-wave data, existence and propagation of type III burst emitting electrons in coronal jets, radio signatures of ARTB’s, and beaming of electrons producing microwave and hard X-rays. The implications of these observations are discussed.

Effect of turbulence on interplanetary shock waves propagation is considered. It is shown that background turbulence results in the additional shock wave deceleration which may be comparable with the deceleration due to plasma sweeping. The turbulent deceleration is connected with the energy losses due to the strong turbulence amplification behind the moving shock front.

We discuss the winding of a force-free axisymmetric magnetic field rooted on a heavy conductor on z = 0. In quadrupolar symmetry the field expands in the half-space z > 0 and the toroidal flux concentrates on a conical surface. After a mean twist of 208°, the conical layer hosts large toroidal current loops with reversal of the magnetic flux on either side. The evolution of the field structure is described by scale-free static solutions B ~ r−(p+2), with p taking values between 0 and 2. The large expansion factor of the field structure is suggestive of flaring originating on the solar photosphere.

Large amplitude waves as well as turbulence has been observed in the interplanetary medium. This turbulence is not understood to the extent that one would like to. By means of techniques of nonlinear dynamical systems, attempts are being made to properly understand the turbulence in the solar wind, which is essentially a nonuniform streaming plasma consisting of hydrogen and a fraction of helium. We demonstrate that the observed large amplitude waves can generate solitary waves, which in turn, because of some propagating solar disturbance, can produce chaos in the medium. The chaotic fields thus generated can lead to anomalously large plasma heating and acceleration.

Unlike the solitary waves in uniform plasmas, in nonuniform plasmas we get accelerated solitary waves, which lead to electromagnetic as well as electrostatic (e.g. ion acoustic) radiations. The latter can be a very efficient source of plasma heating.

The mechanisms that could possibly heat the corona are briefly reviewed with emphasis on their observability. Observing enhanced wave flux at footpoints of active regions would confirm wave heating. Observation of nonthermal electrons in tiny coronal events (nanoflares) would confirm dissipation of current sheets. Presence of large scale flows in coronal arcades would underline the importance of turbulent resistivity for coronal heating. A comparison of Hel absorption in quiet and active regions demonstrates the difficulty of interpreting data that connect chromospheric dynamics with coronal heating. Finally, the implications of the search for observations of coronal heating processes are mentioned.

The temperature in the acceleration region of the solar wind remains one of the most elusive parameters to measure. Knowledge of the temperature as well as its gradient in the inner corona is fundamental for placing constraints on physical mechanisms thought to be responsible for the coronal heating process, as well as for understanding the flow properties of the solar wind. Estimates of the helium abundance is essential for understanding the puzzling behavior of heavier ions in the solar wind. As an illustration of the difficulties and uncertainties involved in the inferences of plasma parameters in the solar wind acceleration region, the inference of electron temperature and helium abundance will be described. Prospects for future observations will be briefly discussed.

An improved knowledge of the physical conditions in the low corona and solar wind can only be obtained through careful comparisons between theoretical descriptions of the solar wind expansion and plasma parameters derived from observations. In this review we will present a summary of recent approaches in solar wind modeling. The plasma parameters characterizing the solar wind models will be compared to constraints inferred from in-situ and remote observations. We will then discuss the implications of the results obtained from this study for future model studies and observations. Emphasis will be placed on high-speed solar wind streams originating from large coronal holes.

I discuss the use of Very Long Baseline Interferometer (VLBI) phase scintillations to probe the conditions of plasma turbulence in the solar wind. Specific results from 5.0 and 8.4 GHz observations with the Very Long Baseline Array (VLBA) are shown. There are several advantages of phase scintillation measurements. They are sensitive to fluctuations on scales of hundreds to thousands of kilometers, much larger than those probed by IPS intensity scintillations. In addition, with the frequency versatility of the VLBA one can measure turbulence from the outer corona ~ 5 –10 R⊙ to well past the perihelion approach of the Helios spacecraft. This permits tests of the consistency of radio propagation and direct in-situ measurements of turbulence. Such a comparison is made in the present paper. Special attention is dedicated to measuring the dependence of the normalization coefficient of the density power spectrum, on distance from the sun. Our results are consistent with the contention published several years ago by Aaron Roberts, that there is insufficient turbulence close to the sun to account for the heating and acceleration of the solar wind. In addition, an accurate determination of the relationship could aid the detection of transients in the solar wind.

We present Very Large Array observations at wavelengths of 2, 3.5, 6, and 20 cm, of angular broadening of radio sources due to the solar wind in the region 2-16 solar radii. Angular broadening is anisotropic with axial ratios in the range 2-16. Larger axial ratios are observed preferentially at smaller solar distances. Assuming that anisotropy is due to scattering blobs elongated along magnetic field lines, the distribution of position angles of the elliptically broadened images indicates that the field lines are non-radial even at the largest heliocentric distances observed here. At 5R⊙, the major axis scattering angle is ~ 0.7" at λ = 6 cm and it varies with heliocentric distance as R−1.6. The level of turbulence, characterized by the wave structure function at a scale of 10 km along the major axis, normalized to λ = 20 cm, has a value 20±7 at 5R⊙ and varies with heliocentric distance as R−3. Comprison with earlier results suggest that the level of turbulence is higher during solar maximum. Assuming a power-law spectrum of electron density fluctuations, the fitted spectral exponents have values in the range 2.8–3.4 for scales sizes between 2–35 km. The data suggests temporal fluctuations (of up to 10%) in the spectral exponent on a time scale of a few tens of minutes. The observed structure functions at different solar distances do not show any evidence for an inner scale; the upper limits are 1 km at 2R⊙ and 4 km at 13R⊙. These upper limits are in conflict with earlier determinations and may suggest a reduced inner scale during solar maximum.

The monthly probability of occurrence of southward component of IMF estimated independent of the sector polarity observed near earth is found to change with the magnitude of solar wind velocity. The above analysis is done for each month during two years around sunspot minima and maxima in cycle 21. The results will be interpreted in terms of association of southward Bz events with solar wind flows of distinct solar origin such as low and high speed solar wind.

Until the ULYSSES spacecraft reached high latitude, the only means for measuring the solar wind velocity in the polar regions was from radio scattering observations (IPS), and these remain the only way to measure the velocity near the sun. However, IPS, like many remote sensing observations, is a “line-of-sight” integrated measurement. This integration is particularly troublesome when the line-of-sight passes through a fast stream but that stream does not occupy the entire scattering region. Observations from the HELIOS spacecraft have shown that the solar wind has a bimodal character which becomes more pronounced near the sun. Recent observations from ULYSSES have confirmed that this structure is clear at high latitudes even at relatively large solar distances. We have developed a method of separating the fast and slow contributions to an IPS observation which takes advantage of this bimodal structure. In this paper I will describe the technique and its application to IPS observations made using the receiving antennas of the EISCAT incoherent backscatter radar observatory in northern Scandinavia.

Since the 1950s, a wide variety of radio observations based on scattering by electron density fluctuations in the solar wind has provided much of our information on density fluctuations and solar wind speed near the source region of the solar wind. This paper reviews recent progress in the understanding of the nature of these density fluctuations and their relationship to features on the Sun. The results include the first measurements of fine-scale structure within coronal streamers and evidence for structure in solar wind speed in the inner corona.

In this paper we present observations of two types of solar mass ejections, which seem to be associated with the location of coronal holes. In the first type, a filament eruption was observed near a coronal hole, which gave rise to a strong interplanetary scintillations, as detected by IPS observations. In the second type, several large scale soft X-ray ‘blow-outs’ were observed in the YOHKOH SXT X-ray movies, in all the cases they erupted from or near the boundary of coronal holes and over the magnetic neutral line. It is proposed that the open magnetic field configuration of the coronal hole provides, the necessary field structure for reconnection to take place, which in turn is responsible for filament eruption, from relatively lower heights. While, in the case of X-ray ‘blow-outs’, the reconnection takes place at a greater height, resulting in high temperature soft X-ray emission visible as X-ray ‘blow-outs’.

Coronal flux tubes have in general very large divergence, and the properties of their flow vary greatly inside the flux tube. We present here a general description of the evolution and the characteristics of coronal holes and discuss how these characteristics may lead to erroneous associations between interplanetary and solar structures.

A scenario is presented whereby CMEs and interplanetary shocks are consequences of a large scale rearrangement of the coronal magnetic field induced by the disconnection of field lines from the solar surface due to the emergence of flux with opposite polarity. In this scenario the CME is the mass released from the previously closed structure and the interplanetary shock is formed by the injection of faster solar wind from an extended or newly created coronal hole which results from the opening of the field lines. Here CMEs and interplanetary shocks are associated events, but not cause-effect related. Observational and computational evidence supporting this view is provided.

We review the existing literature on the coronal disconnection events (CDEs) and discuss the importance of these events in understanding coronal structures. We discuss the possible radio signatures of the CDEs and how they may be observed by radio instruments.

Workers in the field of magnetohydrodynamics (MHD) have been interested in the hypothesis that observed solar activities can be utilized in a deterministic way to predict the bulk flow consequences of these activities in the three-dimensional heliosphere. Exploration of this hypothesis, using the conventional/classic initial boundary value approach, will be reviewed against the background of basic, ideal (except for shocks) one-fluid approximations. This work has been divided into two parts: near-Sun simulations in two dimensions of coronal mass ejections (CMEs) as well as interplanetary simulations in 2D and 3D of propagating shocks. In the latter case, the flows behind the shocks should be thought of as interplanetary “ICMEs”, i.e., the interplanetary, evolutionary consequences of the near-Sun simulations.

Initialization of these simulations has been based on observations (optical, soft X-ray, radio) from both ground- and space-based instruments. Simulation outputs have been compared with in situ plasma and field observations and interplanetary scintillations (IPS). Improvements in the initialization procedures - spatial/temporal variations of solar plasma and field parameters at the coronal base - are expected from YOHKOH, SOHO, CORONAS-I, and TRACE experiments. “Ground truth” observations from WIND, SOHO, ACE, and INTERBALL experiments should then be compared with three-dimensional MHD outputs in tests of the fluid hypothesis noted above.