Discontinuities in the solar wind

The nonuniform emission of the solar wind from the sun means that conditions are established which favor the development of discontinuities in the plasma parameters. Since the solar wind is in rapid proper motion with respect to the sun and the earth, examination of these discontinuities requires that the wind velocity be transformed away. Then it is found that they satisfy the conditions of magnetohydrodynamics and can be treated as shock waves and the stationary contact surfaces consisting of either tangential or contact discontinuities. The collision-free structure of the solar wind suggests that the tangential discontinuity is the more likely contact surface as it is more capable of inhibiting diffusion which is required for a lifetime sufficient for the structure to be carried to the neighborhood of the earth.Either the shock wave or the contact surface can create signals that are detectable at the surface of the earth. The simplest surface signal to detect is the sudden impulse (SI) but other signals may be found. The existence of a field of MHD discontinuities in the solar wind should make possible the generation of ensembles of shocks and contact surfaces. Various possibilities are explored and these are discussed from the standpoint of combinations of sudden impulses at the earth's surface which are both positive and negative. Some of these are recurrent with a 27-day period; the interplanetary M region shock ensemble associated with this is discussed and the development of these structures in space is reviewed.Lastly observational evidence for interplanetary shock waves is given together with the analytic technique for establishing their geometry and comparing the derived and measured jump parameters. The applicability of the geometrical construction of the general class of MHD discontinuity to their analysis is indicated and shows the way in which the structural content of the solar wind can be classified by the use of magnetometers and plasma probes. A parametric study of the jump conditions through a shock wave can be used to verify the correctness of field measurements because of the redundancy in measurements. This also allows the details of shock structure to be examined including the intrinsic partitioning of the internal energy of the shocked plasma.     

The interplanetary medium and its interaction with the Earth's magnetosphere

This paper reviews the principal results of direct measurements of the plasma and magnetic field by spacecraft close to the Earth (within the heliocentric distance range 0.7–1.5 AU). The paper gives an interpretation of the results for periods of decrease, minimum and increase of the solar activity. The following problems are discussed: the interplanetary plasma (chemical composition, density, solar wind flow speed, temperature, temporal and spatial variation of these parameters), the interplanetary magnetic field (intensity, direction, fluctuations and its origin), some derived parameters characterizing the physical condition of the interplanetary medium; the quasi-stationary sector structure and its connection with solar and terrestrial phenomena; the magnetohydrodynamic discontinuities in the interplanetary medium (tangential discontinuities and collisionless shock waves); the solar magnetoplasma interaction with the geomagnetic field (the collisionless bow shock wave, the magnetosheath, the magnetopause, the Earth's magnetic tail, the internal magnetosphere characteristics), the connection between the geomagnetic activity and the interplanetary medium and magnetosphere parameters; peculiarities in behaviour of the interplanetary medium and magnetosphere during geomagnetic storms; energetic aspects of the geomagnetic storms.     

Unsolved problems of solar wind expansion: Can we learn anything from SOHO?

Some of the main problems of solar wind expansion are addressed. Emphasis is placed on solar wind acceleration and the mass flux problem. It is demonstrated how these two properties of the flow depend on other plasma parameters such as temperature, density and helium abundance. The importance of placing constraints on a given solar wind flow in the inner corona and at larger distances from the sun simultaneously, is also shown. Whether and how these constraints can be derived from observations carried out by SOHO instruments is then discussed.     

The solar wind: A turbulent magnetohydrodynamic medium

MHD turbulence properties in the solar wind are briefly reviewed. The evolution of fluctuations of alfvénic type in near-ecliptic regions of the inner heliosphere is described. The role of interplanetary sources and the influence of interactions with structures convected by the solar wind are discussed. Turbulence features at high latitudes and in the outermost regions of the heliosphere are finally highlighted.     

UVCS Observations of Temperature and Velocity Profiles in Coronal Holes

The spectroscopic observations of the Ultraviolet Coronagraph Spectrometer (UVCS), on board the SOHO observatory, allow the study and the full characterization of the expansion of the solar atmosphere by means of measurements of the outflow speeds and the physical properties of the wind, directly in the region where the solar plasma is heated and accelerated: the extended corona. During solar minimum, when the magnetic configuration of the corona is rather simple, the open magnetic fields emerging from the wide polar coronal holes channel toward the heliosphere both the fast and the slow wind. The fast wind flows along flux tubes with lower areal divergence than the slow wind which is guided by flux tubes characterized by non-monotonic areal expansion functions. Differences in the physical properties, such as kinetic temperature, electron density, composition and density fluctuations, of the fast and slow wind in the corona are discussed.     

Solar Wind Models from the Sun to 1 AU: Constraints by in Situ and Remote Sensing Measurements

There are three major types of solar wind: The steady fast wind originating on open magnetic field lines in coronal holes, the unsteady slow wind coming probably from the temporarily open streamer belt and the transient wind in the form of large coronal mass ejections. The majority of the models is concerned with the fast wind, which is, at least during solar minimum, the normal mode of the wind and most easily modeled by multi-fluid equations involving waves. The in-situ constraints imposed on the models, mainly by the Helios (in ecliptic) and Ulysses (high-latitude) interplanetary measurements, are extensively discussed with respect to fluid and kinetic properties of the wind. The recent SOHO observations have brought a wealth of new information about the boundary conditions for the wind in the inner solar corona and about the plasma conditions prevailing in the transition region and chromospheric sources of the wind plasma. These results are presented, and then some key questions and scientific issues are identified. This revised version was published online in June 2006 with corrections to the Cover Date.     

Decay of Magnetic Field Irregularities Observed by Ulysses

We study the solar cycle, radial, and latitudinal dependence of the characteristics of magnetic field irregularities in the Heliosphere. The frequency of magnetic field discontinuities is determined, using high time resolution magnetic field observations by Ulysses, covering the time interval from 1992 to 2000. The quasi-linear scattering mean free path of particles is also calculated. These investigations aim at understanding/exploring transport properties of energetic charged particles in the Heliosphere. We find that the travel time of solar wind plasma from the Sun to the observer is the key parameter of the process, by controling the decay of the irregularities. This revised version was published online in August 2006 with corrections to the Cover Date.     

Plasma kinetics in the solar wind

An account is given of the observations and theoretical ideas concerning the role of kinetic processes in the solar wind. This includes, first of all, the measurements on distribution functions of plasma electrons and protons, the relation of the observed non-thermal electron features with the concept of an exospheric expansion of the solar corona, and the connection of non-thermal proton distributions with bulk flow inhomogeneities of the wind. A discussion is given of the present understanding of the connection between observed features of the particle distributions and anomalous values of some plasma transport coefficients, which in turn determine the actual values of macroscopic plasma parameters.A further topic of the review is that of possible kinetic processes occurring within small scale structures in the solar wind, like collisionless shocks, various types of discontinuities and D-sheets.     

The multifuid solar wind termination shock and its influence on the threedimensional plasma structure upstream and downstream

The solar wind termination shock is described as a multi-fluid phenomenon taking into account the magnetohydrodynamic self-interaction of a multispecies plasma consisting of solar wind ions, pick-up ions and shock-generated anomalous cosmic ray particles. The spatial diffusion of these high energy particles relative to the resulting, pressure-modified solar wind flow structure is described by a coupled system of differential equations describing mass-, momentum-, and energy-flow continuities for all plasma components. The energy loss due to escape of energetic particles (MeV) from the precursor into the inner heliosphere is taken into account. We determine the integrated properties of the anomalous cosmic ray gas and the low-energy solar wind. Also the variation of the compression ratio of the shock structure is quantitatively determined and is related to the pick-up ion energization efficiency and to the mean energy of the downstream anomalous cosmic ray particles. The variation of the resulting shock structure and of the solar wind sheath plasma extent beyond the shock is discussed with respect to its consequences for the LISM neutral gas filtration and the threedimensional shape of the heliosphere.     

Direct observations of solar-wind particles

Direct observations of solar-wind particles are discussed in detail. A well-defined quiet state of the solar wind is indicated by observations made from 1962 to 1967. The plasma properties in this quiet state are compared with those predicted by hydrodynamic models of the coronal expansion. While the basic flow parameters are predicted with reasonable accuracy by these models, the thermal properties of the solar-wind particles remain largely unexplained. As the interplanetary plasma is not in thermodynamic equilibrium, the thermal properties are determined by the specific energy-transfer mechanisms operative in the plasma. The observed magnitude of the magnetic-field-aligned anisotropy of proton random motions is interpreted as evidence for the presence of instability-generated waves; the effect of these waves on the thermal properties is examined. The observed chemical compositon of the solar wind is discussed and related to the solar origin of the inter-planetary material. Finally, the spatial and temporal structure of the medium is investigated through consideration of patterns of variation in the plasma properties.     

Self-Consistent Models of the Solar Wind

The origins of the hot solar corona and the supersonically expanding solar wind are still the subject of much debate. This paper summarizes some of the essential ingredients of realistic and self-consistent models of solar wind acceleration. It also outlines the major issues in the recent debate over what physical processes dominate the mass, momentum, and energy balance in the accelerating wind. A key obstacle in the way of producing realistic simulations of the Sun-heliosphere system is the lack of a physically motivated way of specifying the coronal heating rate. Recent models that assume the energy comes from Alfvén waves that are partially reflected, and then dissipated by magnetohydrodynamic turbulence, have been found to reproduce many of the observed features of the solar wind. This paper discusses results from these models, including detailed comparisons with measured plasma properties as a function of solar wind speed. Some suggestions are also given for future work that could answer the many remaining questions about coronal heating and solar wind acceleration.     

Unusually Distant Bow Shock Encounters at Mars: Analysis of March 24, 1989 Event

Detailed analysis of disturbances observed on 24 March, 1989 far upstream of the usual Martian bow shock position was completed with the use of the planetary obstacle and bow shock models relevant for the period of Phobos 2 observations and for low Mach numbers, respectively. It is proven that the system of discontinuities observed in the solar wind between 18:42 and 19:36 UT was the consequence of unusually distant planetary bow shock excursions. The cause was unusually small ρV ² and M _a values in the solar wind flow.     

THE INFLUENCE OF THE LOCAL INTERSTELLAR MEDIUM ON THE SOLAR WIND DYNAMICS IN THE INNER HELIOSPHERE

The consequences of the interaction between the solar wind and the local interstellar medium for the wind region enclosed by the heliospheric shock are reviewed. After identifying the principal mechanisms to influence the dynamics of the solar wind, an approach allowing the simultaneous incorporation of neutral atoms, pick-up ions, cosmic rays and energetic electrons into a multifluid model of the expanding wind plasma is outlined. The effects of these particle species are discussed in detail, with special emphasis on the electron component which behaves more like a quasi-static hot gas rather than an expanding fluid. This electron gas is effectively trapped within a three-dimensional trough of a circumsolar electric potential whose outer fringes are possibly determined by the density distribution of anomalous cosmic rays. The electrons are proven to be a globally structered component of great importance for the solar wind momentum flow contributing to a triggering of the solar wind dynamics by asymmetric interstellar boundary conditions. Finally, the consequences for the relative motion of the Sun and the local interstellar medium as well as for the solar system as a whole are described.     

Vlasov theory of the equilibrium structure of tangential discontinuities in space plasmas

Extensive theoretical work has been performed on the equilibrium structure of tangential discontinuities (TDs) in collisionless plasmas. This paper reviews kinetic models based on steady-state solutions of the Vlasov equation. It is shown that most of the existing models are special cases of a generalized multi-species model. In this generalized model all particle populations -from both outer regions and from inside the layer — are described using a unique formalism for the velocity distribution functions. Because of their historical importance, the Harris and Sestero models are reviewed and deduced from the generalized model. The Lee and Kan model is also a special case of the generalized model. The generalized model, however, is also able to describe TDs with velocity shear and large angles of magnetic field rotation. Such a multi-species model with a large number of free parameters and different gradient scales illustrates many observable features of TDs, including their multiscale fine structure. Particular attention is paid to the magnetopause. Observed magnetopause crossings are simulated. The effects of the relative flow velocity and asymmetrical magnetic field profiles on the structure of the magnetopause and on its stability with respect to tearing perturbations are discussed. We also present calculations that demonstrate the potential of the generalized model in explaining the origin of discrete auroral arcs. Numerical simulations of solar wind TDs with heavy ions and a large spectrum of thicknesses are also feasible. This indicates that such a model is of fundamental importance for understanding the detailed structure of solar wind TDs, like those observed by the interplanetary spacecraft ULYSSES. The problems associated with the one-dimensional, time-independent Vlasov approach are discussed and a variational principle is suggested to reduce the arbitrariness resulting from the large number of free parameters.     

The Solar Wind and the Sun in the Past

Exposure to the solar wind can have significant long term consequences for planetary atmospheres, especially for planets such as Mars that are not protected by global magnetospheres. Estimating the effects of solar wind exposure requires knowledge of the history of the solar wind. Much of what we know about the Sun’s past behavior is based on inferences from observations of young solar-like stars. Stellar analogs of the weak solar wind cannot be detected directly, but the interaction regions between these winds and the interstellar medium have been detected and used to estimate wind properties. I here review these observations, with emphasis on what they suggest about the history of the solar wind.     

Physical reasons and consequences of a three-dimensionally structured heliosphere

In this article we have discussed reasons both of solar and of interstellar origin giving rise to a pronounced three-dimensional structure of the expanding solar wind and thus of the global configuration of the heliosphere. Our present observational knowledge on these structurings is reviewed, and all attempts to theoretically model these solar wind structures are critically analysed with respect to their virtues and flaws. It is especially studied here by what mechanisms interstellar imprints on the actual type of solar wind expansion can be envisaged. With concern to this aspect it hereby appears to be of eminent importance that the solar system maintains a relative motion with a submagnetosonic velocity of about 23km/sec with respect to the ambient magnetized interstellar medium corresponding to a magnetosonic Mach number of about 0.5.A heliopause closing the distant heliospheric cavity within a solar distance of about 100AU on the upwind side and opening it into an largely extended tail on the downwind side results as a first consequence from this relative motion. As a second consequence an asymmetric heliospheric shockfront with upwind distances smaller than downwind distances by ratios between 1/3 and 2/3 is most likely provoked which gives rise to at least two important upwind-downwind asymmetric processes influencing the supersonic solar wind expansion downstream from the shock: the anomalous cosmic ray diffusion into the solar wind, and high energetic jet electrons originating at the shock and moving inwards up to an inner critical point at around 20AU. As we shall demonstrate both processes are influencing the solar wind expansion beyond 20AU, however, more efficiently in the upwind hemisphere as compared to the downwind hemisphere. In the region inside 20AU other mechanisms are operating to propagate the interstellar imprint on the solar wind expansion further downstream into the inner heliosphere because here even the original solar wind electrons, in view of the solar wind bulk velocities, behave as a subsonic plasma constituent which can modify the solar wind solutions by means of an appropriate detuning of the circumsolar electric polarisation field. We give quantitative estimates for these effects.What concerns the theory of a solar wind expansion into a counterflowing ambient interstellar medium, some flaws of the present theoretical attempts are identified impeding that the interstellar influence on the actual solar wind solutions can become visible. We thus conclude that there is a clear need for three-dimensional and time-dependent solar wind models with a free outflow geometry taking into account the multisonicity of the solar wind plasma with different eigenmodes for a perturbation propagation.     

The solar wind in the outer solar system

The properties of the solar wind including magnetic fields, plasma, and plasma waves are briefly reviewed with emphasis on conditions near and beyond the orbit of Jupiter. An extrapolation of the steady-state wind to large distances, evolution of disturbances and structure, modulation of cosmic rays, interactions with planetary bodies (bow shocks and magnetosheaths), and interactions with interstellar neutral helium and hydrogen are briefly discussed. Some comments on instrumentation requirements to observationally define the above phenomena are also included.This is one of the publications by the Science Advisory Group.     

Helios: Evolution of Distribution Functions 0.3–1 AU

The radial evolution of the velocity distribution functions of the protons, electrons and ions, as they were measured during the Helios mission in the solar wind between 0.3 and 1.0 AU, is discussed and analysed. Emphasis is placed on the detailed plasma measurements, and on the non-thermal features of the particles and the kinetic processes they undergo in the expanding solar wind. As the plasma is multi-component and nonuniform, complexity prevails and the observed distributions exhibit, owing to their low number densities, significant deviations from local thermal equilibrium, and reveal such suprathermal particles as the strahl electrons, as well as ion beams and temperature anisotropies. The distribution functions still carry imprints of their solar boundaries that are reflected locally, but also have ample free energy driving in situ plasma instabilities which are triggered and modulated by wave-particle interactions. The ion temperatures and their anisotropies and the non-adiabatic radial evolution of the solar wind internal energy are discussed in detail.