Fast Dust in the Heliosphere

The dynamics of dust particles in the solar system is dominated by solar gravity, by solar radiation pressure, or by electromagnetic interaction of charged dust grains with the interplanetary magnetic field. For micron-sized or bigger dust particles solar gravity leads to speeds of about 30 to 40 km s^–1 at the Earths distance. Smaller particles that are generated close to the Sun and for which radiation pressure is dominant (the ratio of radiation pressure force over gravity F _rad/F _grav is generally termed ) are driven out of the solar system on hyperbolic orbits. Such a flow of -meteoroids has been observed by the Pioneer 8, 9 and Ulysses spaceprobes. Dust particles in interplanetary space are electrically charged to typically +5 V by the photo effect from solar UV radiation. The dust detector on Cassini for the first time measured the dust charge directly. The dynamics of dust particles smaller than about 0.1 m is dominated by the electromagnetic interaction with the ambient magnetic field. Effects of the solar wind magnetic field on interstellar grains passing through the solar system have been observed. Nanometer sized dust stream particles have been found which were accelerated by Jupiters magnetic field to speeds of about 300 km s^–1.     

Measurements of the properties of solar wind plasma relevant to studies of its coronal sources

Interplanetary measurements of the speeds, densities, abundances, and charge states of solar wind ions are diagnostic of conditions in the source region of the solar wind. The absolute values of the mass, momentum, and energy fluxes in the solar wind are not known to an accuracy of 20%. The principal limitations on the absolute accuracies of observations of solar wind protons and alpha particles arise from uncertain instrument calibrations, from the methods used to reduce the data, and from sampling biases. Sampling biases are very important in studies of alpha particles. Instrumental resolution and measurement ambiguities are additional major problems for the observation of ions heavier than helium. Progress in overcoming some of these measurement inadequacies is reviewed.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Acceleration of the solar wind

In this review, we discuss critically recent research on the acceleration of the solar wind, giving emphasis to high-speed solar wind streams emanating from solar coronal holes. We first explain why thermally driven wind models constrained by solar and interplanetary observations encounter substantial difficulties in explaining high speed streams. Then, through a general discussion of energy addition to the solar wind above the coronal base, we indicate a possible resolution of these difficulties. Finally, we consider the question of what role MHD waves might play in transporting energy through the solar atmosphere and depositing it in the solar wind, and we conclude by examining, in a simple way, the specific mechanism of solar wind acceleration by Alfvén waves and the related problem of accelerating massive stellar winds with Alfvén waves.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.On leave from the Auroral Observatory, Institute of Mathematical and Physical Sciences, University of Tromsø, N-9001 Tromsø, Norway.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Exploration of the solar corona by high resolution solar decametric observations

In this review, current state of knowledge of high resolution observations at decameter wavelengths of the quiet Sun, the slowly varying component (SVC), type I to V bursts and noise storms is summarized. These observations have been interpreted to yield important physical parameters of the solar corona and the dynamical processes around 2R from the photosphere where transition from closed to open field lines takes places and the solar wind builds up. The decametric noise bursts have been classified into (i) BF type bursts which show variation of intensity with frequency and time and (ii) decametric type III bursts. The angular sizes of noise storm sources taking into account refraction and scattering effects are discussed. An attempt has been made to give phenomenology of all the known varieties of decametric bursts in this review. Available polarization information of decametric continuum and bursts has been summarized. Recent simultaneous satellite and ground-based observations of decametric solar bursts show that their intensities are deeply modulated by scintillations in the Earth's ionosphere. Salient features of various models and theories of the metric and decametric noise storms proposed so far are examined and a more satisfactory model is suggested which explains the BF type bursts as well as conventional noise storm bursts at decametric wavelengths invoking induced scattering process for 1 t conversion. Some suggestions for further solar decametric studies from the ground-based and satellite-borne experiments have been made.     

A three-dimensional plasma and energetic particle investigation for the wind spacecraft

This instrument is designed to make measurements of the full three-dimensional distribution of suprathermal electrons and ions from solar wind plasma to low energy cosmic rays, with high sensitivity, wide dynamic range, good energy and angular resolution, and high time resolution. The primary scientific goals are to explore the suprathermal particle population between the solar wind and low energy cosmic rays, to study particle accleration and transport and wave-particle interactions, and to monitor particle input to and output from the Earth's magnetosphere.Three arrays, each consisting of a pair of double-ended semi-conductor telescopes each with two or three closely sandwiched passivated ion implanted silicon detectors, measure electrons and ions above 20 keV. One side of each telescope is covered with a thin foil which absorbs ions below 400 keV, while on the other side the incoming <400 keV electrons are swept away by a magnet so electrons and ions are cleanly separated. Higher energy electrons (up to 1 MeV) and ions (up to 11 MeV) are identified by the two double-ended telescopes which have a third detector. The telescopes provide energy resolution of E/E0.3 and angular resolution of 22.5°×36°, and full 4 steradian coverage in one spin (3 s).Top-hat symmetrical spherical section electrostatic analyzers with microchannel plate detectors are used to measure ions and electrons from 3 eV to 30 keV. All these analyzers have either 180° or 360° fields of view in a plane, E/E0.2, and angular resolution varying from 5.6° (near the ecliptic) to 22.5°. Full 4 steradian coverage can be obtained in one-half or one spin. A large and a small geometric factor analyzer measure ions over the wide flux range from quiet-time suprathermal levels to intense solar wind fluxes. Similarly two analyzers are used to cover the wide range of electron fluxes. Moments of the electron and ion distributions are computed on board.In addition, a Fast Particle Correlator combines electron data from the high sensitivity electron analyzer with plasma wave data from the WAVE experiment (Bougeretet al., in this volume) to study wave-particle interactions on fast time scales. The large geometric factor electron analyzer has electrostatic deflectors to steer the field of view and follow the magnetic field to enhance the correlation measurements.     

Interplanetary scintillation

The use of interplanetary scintillations for probing otherwise inaccessible regions of the solar wind is reviewed. A comparison with space-craft observations in the ecliptic is used as a calibration for the scintillation observations. Recent observations at high latitudes and near the Sun are discussed from this viewpoint. A new analysis which uses both scintillation and angular scattering observations to estimate the electron density spectrum is introduced. The spectrum appears to have a high frequency cutoff which varies slowly with solar distances and may also have a relatively flat region just below the cutoff frequency.     

Recent results on the parameters of the interstellar helium from the ULYSSES/GAS experiment

Velocity and direction of the flow of the interstellar helium and its temperature and density have been determined from the measurements of the ULYSSES/GAS experiment for two different epochs: during the in-ecliptic path of ULYSSES, representing solar maximum conditions, and during the south to the north pole transition (11/94-6/95), close to the solar minimum conditions. Within the improved error bars the values are consistent with results published earlier.The determination of the density n of the interstellar helium at the heliospheric boundary from observations in the inner solar system requires knowledge about the loss processes experienced by the particles on their way to the observer. The simultaneous observation of the helium particles arriving on direct and indirect orbits at the observer provides a tool to directly determine the effects of the loss processes assumed to be predominantly photoionization and — for particles travelling close to the Sun — electron impact ionization by high-energy solar wind electrons.Such observations were obtained with the ULYSSES/GAS instrument in February 1995, before the spaceprobe passed its perihelion. From these measurements values for the loss rates and the interstellar density could be derived. Assuming photoionization to be the only loss process reasonable fits to the observations were obtained for an ionization rate = 1.1 · 10^–7 s^–1 and a density n 1.7 · 10^–2 cm^–3. Including, in addition, electron impact ionization, a photoionization = 0.6 · 10^–7 s^–1 was sufficient to fit both observations, resulting in a density n 1.4 · 10^–2 cm^–3.On leave from Space Research Centre, Warsaw, Poland.     

Quiet sun

We underline the diagnostic strength of recent observations of the oscillating quiet Sun. While high quality (k, ) power spectra permit a better knowledge of the convection zone, long and continuous survey of oscillations of the integrated Sun provides an efficient sounding of the inner solar body.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Dynamical theory of the solar wind

This paper is a review of the basic theoretical dynamical properties of an atmosphere with an extended temperature strongly bound by gravity. The review begins with the historical developments leading up to the realization that the only dynamical equilibrium of an atmosphere with extended temperature is supersonic expansion. It is shown that sufficient conditions for supersonic expansion are T(r) declining asymptotically less rapidly than 1/r, or the density at the base of the corona being less than N _b given by (40) if no energy is available except through thermal conductivity, or the temperature falling within the limits given by (18) if T N ^-1 throughout the corona. Less extended temperatures lead to equilibria which are subsonic or static. The hypothetical case of a corona with no energy supply other than thermal conduction from its base is considered at some length because the equations may be solved by analytical methods and illustrate the transition from subsonic to supersonic equilibrium as the temperature becomes more extended. Comparison with the actual corona shows that the solar corona is actively heated for some distance into space by wave dissipation.The dynamical stability of the expanding atmosphere is demonstrated, and in a later section the radial propagation of acoustic and Alfvén waves through the atmosphere and wind is worked out. The calculations show that the magnetometer will probably detect waves more easily than the plasma instrument, but that both are needed to determine the mode and direction of the wave. An observer in the wind at the orbit of Earth can listen to disturbances generated in the corona near the sun and in turbulent regions in interplanetary space.The possibility that the solar corona is composed of small-scale filaments near the sun is considered. It is shown that such filamentary structure would not be seen at the orbit of Earth. It is pointed out that the expansion of a non-filamentary corona seems to lead to too high a calculated wind density at the orbit of Earth to agree with the present observations, unless T(r) is constant or increases with r. A filamentary corona, on the other hand, would give the observed wind density for declining T(r).It is shown that viscosity plays no important role in the expansion of an atmosphere either with or without a weak magnetic field. The termination of the solar wind, presumably between 10–10³ AU, is discussed briefly. The interesting development here is the interplanetary L recently observed, which may come from the interstellar neutral hydrogen drifting into the outer regions of the solar wind.Theory is at the present time concerned with the general dynamical principles which pertain to the expansion equilibrium of an atmosphere. It is to be expected that the rapid progress of direct observations of the corona and wind will soon permit more detailed studies to be carried out. It is important that the distinction between detailed empirical models and models intended to illustrate general principles be kept clearly in mind at all times.This work was supported by the National Aeronautics and Space Administration under Grant NASA-NsG-96-60.     

Study of magnetospheric dynamics using energetic solar particles

Information can be obtained from energetic particle measurements through the chemical composition, energy spectrum, directional anisotropy, temporal and spatial intensity variations. This is equivalent to saying that there is a distribution functionf _k(p,r,t) wherek corresponds to thekth particle species of momentump at positionr and timet.Particle transport is described by the Boltzmann equation, and because the densities are generally low in the case of cosmic rays or energetic solar flare particles, collective transport effects can be neglected. In the absence of magnetospheric motion it is relatively easy to treat the problems of particle transport as simple propagation of charged particles in a stationary magnetic field configuration using, for instance, trajectory calculations in model fields. The method here is to use correlated measurements of the particle distribution at two points along a dynamic trajectory, and in this way to learn something about the geomagnetic field. This approach provides a good basis from which to study magnetospheric dynamics. If the magnetosphere moves, large scale electric fields, turbulent electromagnetic fields and sources and sinks affect the propagation of energetic particles considerably. These effects change the distribution functionf _k(p,r,t) and can thus be detected.In this paper, we shall show the importance of the single particle approximation (trajectories in a reference field) in forming the basis of our understanding of the quiet-time penetration of cosmic rays into the magnetosphere, we shall consider the steady dynamics such as wave-particle inter-action and field line reconnection, which is believed to exist nearly all the time, and finally we shall review the work which has been done in the much more complex and less well-understood field of impulsive dynamics such as geomagnetic storms and substorms. This last topic is only just beginning to be investigated in detail, and it is hoped that the study of impulsive dynamics, using energetic particles, may be as successful as the study of the quiet magnetosphere and the steady dynamics.     

The local interstellar medium

The local interstellar medium can be probed in different ways: by analyzing low energy X-ray data in the range 0.1–0.4 keV, where the radiation is absorbed by the interstellar gas at column densities in excess of about 10²⁰ cm^-2 — and can therefore be regarded as local, by determining the absorption of stellar emission spectra from nearby stars along their lines of sight by intervening gas and by directin situ measurements of those components which penetrate the heliosphere sufficiently far, provided they can be distinguished from interplanetary material. The current status of these different investigations gives the following picture: the solar system is surrounded by a bubble of hot gas (density 0.005cm^-3, temperature 10⁶ K) out to several tens of parsecs. More locally it is embedded in a small warm cloud of density 0.07cm^-3, temperature 7000 K, column density 5 × 10¹⁷ cm^-2 — which gives a mass of about 0.1M . The transition to the heliosphere is governed by solar UV ionization, snowploughing of the interstellar gas by the outwardly expanding solar wind and the bow shock. The heliosphere is the region inside the solar wind terminal shock. Classically it would be regarded as not yet affected by (or aware of) the obstacle ahead. Practically, the existence of the interstellar medium makes itself felt even far inside the heliosphere by the penetration of neutral gas, dust, plasma waves, shock accelerated particles and cosmic rays. These are the local probes of the interstellar medium.     

Alfvenic fluctuations in the solar wind: A case study using Ulysses measurements

We present preliminary results of a study of solar wind Alfvénic fluctuations using magnetometer and plasma measurements from the spacecraft Ulysses. We focus on a particular case study, and use wave language to discuss the fluctuations. The observations are consistent with a mixture of convected structures and Alfvén wave trains. Furthermore, our results tend to suggest that the Alfvénic-fluctuations propagate in the radial direction.     

Co2 and Climate: a Geologist's View

Climate is discussed as an integral part of System Earth, determined by a complex interplay of numerous geological, biological and solar processes. The historical and geological record of changing climate and atmospheric CO₂ pressure does not support the current popular vision that this greenhouse gas is the dominant climate controlling agent. When empirically ante post tested against past global climate changes, the forecasts of the climate models mainly based on forcing by atmospheric CO₂ are not borne out. On the other hand, recent studies show that solar variability rather than changing CO₂ pressure is an important, probably the dominant climate forcing factor.     

Coupling of the coronal he abundance to the solar wind

Models of the transition region — corona — solar wind system are investigated in order to find the coronal helium abundance and to study the role played by coronal helium in controlling the the solar wind proton flux. The thermal force on -particles in the transition region sets the flow of helium into the corona. The frictional coupling between -particles and protons and/or the electric polarization field determines the proton flux in the solar wind as well as the fate of the coronal helium content.     

Energetic particles at high latitudes

We review work on diffusion coefficients of energetic particles with an attempt to extract implications on their behaviour at high latitudes. In the ecliptic plane results from solar energetic particle propagation between the Sun and about 5 AU can be described by an effective radial mean free path _r which is approximately constant as a function of distancer. When particle propagation in three dimensions in the heliosphere is considered it is not sufficient to consider _r only. Jovian electrons can be used as probes to determine the parameters of three-dimensional diffusion. In the polar regions diffusion is dominated by its parallel component. Some predictions how should vary with latitude are discussed. For different choices of this variation we present expectations for intensity-time profiles of solar particle events during the Ulysses polar passages.     

Wolf–Rayet star nucleosynthesis and the isotopic composition of the Galactic Cosmic Rays

There is now strong observational evidence that the composition of the Galactic Cosmic Rays (GCRs) exhibits some significant deviations with respect to the abundances measured in the local (solar neighbourhood) interstellar medium (ISM). Two main scenarios have been proposed in order to account for these differences (`anomalies). The first one, referred to as the `two-component scenario, invokes two distinct components to be accelerated to GCR energies by supernova blast waves. One of these components is just made of ISM material of `normal solar composition, while the other one emerges from the wind of massive mass-losing stars of the Wolf–Rayet (WR) type. The second model, referred to as the `metallicity-gradient scenario, envisions the acceleration of ISM material whose bulk composition is different from the local one as a result of the fact that it originates from inner regions of the Galaxy, where the metallicity has not the local value. In both scenarios, massive stars, particularly of the WR type, play an important role in shaping the GCR composition. After briefly reviewing some basic observations and predictions concerning WR stars (including s-process yields), this paper revisits the two proposed scenarios in the light of recent non-rotating or rotating WR models.     

Electromagnetic ion/ion instabilities and their consequences in space plasmas: A review

This paper reviews recent research on the theory and computer simulations of electromagnetic ion/ion instabilities and their consequences in space plasmas. Ion/ion instabilities are growing modes in a collisionless plasma driven unstable by the relative streaming velocity v ₀of two distinct ion components such that v ₀is parallel or antiparallel to the uniform background magnetic field B ₀0. The space physics regimes which display enhanced fluctuations due to these instabilities and which are reviewed in this paper include the solar wind, the terrestrial foreshock, the plasma sheet boundary layer, and distant cometary environments.     

Magnetic fields,plasmas, and coronal holes: The inner solar system

This paper reviews the recent results concerning streams and magnetic fields in the inner solar system. Specifically, it discusses in situ magnetic field and plasma observations within 1 AU which describe MHD stream flows and Alfvénic fluctuations, and it discusses the latest theories of those phenomena. Observationally, there have been significant advances in our understanding of streams and fluctuations as the result of acquiring nearly complete sets of high resolution plasma and magnetic data simultaneously at two or more points by IMPs 6, 7, and 8, Mariner-Venus-Mercury, HELIOS-1, and HELIOS-2. HELIOS and IMP observations and coronal hole observations demonstrated that streams can have very thin boundaries in latitude and longitude near the Sun. This has necessitated a revision of earlier views of stream dynamics, for it is now clear that magnetic pressure is a major factor in the dynamics of stream in the inner solar system and that nonlinear phenomena are significant much closer to the Sun than previously believed. Simultaneous IMP 6, 7, and 8 observations of Alfvénic fluctuations have shown that they are probably not simply transverse Alfvén waves; they suggest that Alfvénic fluctuations are better described as nonplanar, large-amplitude, general Alfvén waves moving through an inhomogeneous and discontinuous medium, and coupled to a compressive mode.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

ULF waves: Conjugated ground-satellite relationships

We study the simultaneous occurrence of ULF waves observed on board GEOS and at two of its conjugated stations: Husafell (Iceland) and Skibotn (Norway). We try to deduce some properties of the regions in which these waves are generated. The few number of simultaneous observations of pearl events indicates that such structured oscillations can occur only in specific conditions which are not met generally at the geostationary altitude. We introduce a new method for measuring time delays between the satellite and the ground. We show that this time is much higher than it would be expected from a simple extrapolation of measurements done at lower latitudes on structured events.     

Finite Larmor radius effects in the magnetosphere

This article reviews theories and observations related to effects produced by finite (and large) Larmor radii of charged particles in the magnetosphere. The FLR effects depend on =r _H /L, wherer _H is the Larmor radius andL is the spatial scale for field/plasma inhomogeneity. The parameter is a basic expansion parameter for most equations describing plasma dynamics in the magnetosphere. The FLR effects enter naturally the drift approximation for particle motion and represent also non-ideal MHD terms in the fluid formalism. The linear and higher order terms in lead to charge separation, energization of particles, and produce viscosity without collisions. The FLR effects introduce also important corrections to the dispersion relations for MHD waves and drift instabilities. Expansion of plasma into magnetic field leads to filamentation of the plasma boundary and to creation of structures with thickness less than an ion gyroradius. Large Larmor radius effects (1) in curved magnetic field geometry lead to stochastic behaviour of particle trajectories and to deterministic chaos. The tiny scale of the electron and ion gyroradii does not necessarily mean that FLR/LLR phenomena have negligible effect on the macroscopic dynamics and energetics of the whole magnetosphere. On the contrary, the small scale gyro-effects may provide the physical mechanism for gyroviscous coupling between the solar wind and the magnetosphere, the mechanism for triggering disruption of the magnetotail current layer, and the mechanism for parallel electric field that accelerate auroral particles.