Voyager energetic particle observations at interplanetary shocks and upstream of planetary bow shocks: 1977–1990

The Voyager 1 and 2 spacecraft include instrumentation that makes comprehensive ion (E 28 keV) and electron (E 22 keV) measurements in several energy channels with good temporal, energy, and compositional resolution. Data collected over the past decade (1977–1988), including observations upstream and downstream of four planetary bow shocks (Earth, Jupiter, Saturn, Uranus) and numerous interplanetary shocks to 30 AU, are reviewed and analyzed in the context of the Fermi and shock drift acceleration (SDA) models. Principal findings upstream of planetary bow shocks include the simultaneous presence of ions and electrons, detection of tracer ions characteristic of the parent magnetosphere (O, S, O⁺), power-law energy spectra extending to 5 MeV, and large (up to 100:1) anisotropies. Results from interplanetary shocks include observation of acceleration to the highest energies ever seen in a shock ( 22 MeV for protons, 220 MeV for oxygen), the saturation in energy gain to 300 keV at quasi-parallel shocks, the observation of shock-accelerated relativistic electrons, and separation of high-energy (upstream) from low-energy (downstream) populations to within 1 particle gyroradius in a near-perpendicular shock. The overall results suggest that ions and electrons observed upstream of planetary bow shocks have their source inside the parent magnetosphere, with first order Fermi acceleration playing a secondary role at best. Further, that quasi-perpendicular interplanetary shocks accelerate ions and electrons most efficiently to high energies through the shock-drift process. These findings suggest that great care must be exercised in the application of concepts developed for heliosphere shocks to cosmic ray acceleration through shocks at supernova remnants.     

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.     

Anomalous Cosmic Rays and Solar Modulation

We review aspects of anomalous cosmic rays (ACRs) that bear on the solar modulation of energetic particles in the heliosphere. We show that the latitudinal and radial gradients of these particles exhibit a 22-year periodicity in concert with the reversal of the Sun's magnetic field. The power-law index of the low energy portion of the energy spectrum of ACRs at the shock in 1996 appears to be -1.3, suggesting that the strength of the solar wind termination shock at the helioequatorial plane is relatively weak, with s 2.8. The rigidity dependence of the perpendicular interplanetary mean free path in the outer heliosphere for particles with rigidities between 0.2 and 0.7 GV varies approximately as R2, where R is particle rigidity. There is evidence that ACR oxygen is primarily multiply charged above 20 MeV/nuc and primarily singly-charged below 16 MeV/nuc. The location of the termination shock was at 65 AU in 1987 and 85 AU in 1994.     

Non-BBN Constraints on the Key Cosmological Parameters

Since the baryon-to-photon ratio 10 is in some doubt at present, we ignore the constraints on 10 from big bang nucleosynthesis (BBN) and fit the three key cosmological parameters (h, M, 10) to four other observational constraints: Hubble parameter (ho), age of the universe (to), cluster gas (baryon) fraction (fo fGh3/2), and effective shape parameter (o). We consider open and flat CDM models and flat CDM models, testing goodness of fit and drawing confidence regions by the 2 method. CDM models with M = 1 (SCDM models) are accepted only because we allow a large error on ho, permitting h < 0.5. Open CDM models are accepted only for _M 0.4. CDM models give similar results. In all of these models, large 10 ( 6) is favored strongly over small 10 ( 2), supporting reports of low deuterium abundances on some QSO lines of sight, and suggesting that observational determinations of primordial 4He may be contaminated by systematic errors. Only if we drop the crucial o constraint are much lower values of M and 10 permitted.     

Fourier parameters of heliospheric current sheet and their significance

If the path of the neutral line on the coronal source surface is expressible as a singlevalued function (colatitude vs longitude ), then Fourier analysis of ctn with respect to leads to a simple algorithm for realistically mapping the neutral line outward to model the heliospheric current sheet (HCS) at distancesr1 AU. To be compatible with MHD, the source surface used for this mapping should be prolate (aligned with dipole axis) rather than spherical. Orientation of the Sun's magnetic-dipole moment is indicated by them=1 Fourier amplitude (a ₁ sin +b ₁ cos ) of ctn on the source surface. Physical features (including the neutral line) on a prolate source surface intrinsically map to lower dipole latitudes atr1 AU in the heliosphere, and Ulysses observations of a unipolar field at latitudes beyond 30°S (when the neutral line on the source surface still reached 39°S) confirm the expected geometry.     

Shock acceleration of energetic particles in the heliosphere

The theory of shock acceleration of energetic particles is briefly discussed and reviewed with an emphasis on clarifying the apparent distinction between the V × B and Fermi mechanisms. Attention is restricted to those situations in which the energetic particles do not themselves influence the given shock structure. In particular, application of the theory to the acceleration of energetic particles in corotating interaction regions (CIR) in the solar wind is presented. Here particles are accelerated at the forward and reverse shocks which bound the CIR by being compressed between the shock fronts and magnetic irregularities upstream from the shocks, or by being compressed between upstream irregularities and those downstream from the shocks. Particles also suffer adiabatic deceleration in the expanding solar wind, an effect not included in previous shock models for acceleration in CIRs. The model is able to account for the observed exponential spectra at Earth, the observed behavior of the spectra with radial distance, the observed radial gradients in the intensity, and the observed differences in the intensity and spectra at the forward and reverse shocks.Calculations and resulting energy spectra are also presented for shock acceleration of energetic particles in large solar flare events. Based on the simplifying assumption that the shock evolves as a spherically symmetric Sedov blast wave, the calculation yields the time-integrated spectrum of particles initially injected at the shock which eventually escape ahead of the shock into interplanetary space. The spectra are similar to those observed at Earth. Finally further applications are suggested.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Transient Effects and Disturbed Conditions

In the present phase of the solar cycle no big transients leading to strong modulation had been observed after 1991. Apart from a few minor disturbances cosmic rays were still recovering to a new intensity maximum. It was suggested, therefore, that existing literature from previous cycles should be critically reviewed. The scene was set by the introductory papers on— phenomenology of cosmic ray modulation in successive solar cycles throughout the heliosphere— the present state of models for long term modulation and their shortcomings— the relation between cosmic ray variations and the magnitude of the interplanetary magnetic field (the CR-B-relation)— charge dependent effects.In the discussions, the study of propagating diffusive disturbances and the CR-B-relation played a central role. The difference was stressed between isolated transient disturbances in the inner solar system (Forbush decreases), and the long lasting, step-like decreases caused by merged interaction regions in the outer heliosphere. The recovery rates following the step-like decreases vary with the phase in the 22-year solar cycle. In some cases this requires a modification of existing drift models. In the outer heliosphere, the CR-B-relation leads to the result 1/ between the diffusion coefficient and the field magnitude . This simple result is a challenge for theoreticians to derive the perpendicular diffusion coefficient fromfirst principles. The three articles in this report essentially follow the list of open points and arguments just presented.The article "Observations and Simple Models" is organised around the model of a propagating diffusive barrier, its application to Forbush effects in the inner heliosphere and to decreases caused by merged interaction regions in the outer heliosphere. Acomparison of observed Forbush decreases with model predictions requires a careful separation of the two steps related to the turbulent region behind the shock front and the closed magnetic field regions of the ejecta (the interplanetary counterparts of coronal mass ejections). It is shown that models for propagating disturbances can be used to derive values of the diffusion coefficients phenomenologically, not only during the disturbance, but also in the ambient medium.The "Modeling of Merged Interaction Regions" summarizes the dynamic and time-dependent process of cosmic ray modulation in the heliosphere. Numerical models with only a time-dependent neutral sheet prove to be successful when moderate to low solar activity occurs but fail to describe large and discrete steps in modulated cosmic rays when solar activity is high. To explain this feature of heliospheric modulation, the concept of global merged interaction regions is required. The com-bination of gradient, curvature and neutral sheet drifts with these global merged interaction regions has so far been the most successful approach in explaining the 11-year and 22-year cycles in the long-term modulation of cosmic rays.The "Remarks on the Diffusion Tensor in the Heliosphere" describe available theories of perpen-dicular diffusion and drift, and discuss their relevance to cosmic rays in the heliosphere. In addition, the information about diffusion coefficients and spatial gradients obtained from the analysis of steady state anisotropies at neutron monitor energies is summarized. These topics are intimately related to the other two articles. They are also part of the general discussion about the "Diffusion Tensor throughout the Heliosphere" which played an important role in all working groups.     

The importance of anisotropic interstellar turbulence and molecular-cloud magnetic mirrors for galactic cosmic-ray propagation

Recent studies suggest that when magnetohydrodynamic (MHD) turbulence is excited by stirring a plasma at large scales, the cascade of energy from large to small scales is anisotropic, in the sense that small-scale fluctuations satisfy the inequality k k , where k and k are, respectively, the components of a fluctuations wave vector and to the background magnetic field. Such anisotropic fluctuations are very inefficient at scattering cosmic rays. Results based on the quasilinear approximation for scattering of cosmic rays by anisotropic MHD turbulence are presented and explained. The important role played by molecular-cloud magnetic mirrors in confining and isotropizing cosmic rays when scattering is weak is also discussed.     

Primordial Helium and ΔY/ΔZ from H II Regions and from Fine Structure in the Main Sequence Based on Hipparcos Parallaxes

The primordial helium abundance YP is important for cosmology and the ratio Y/Z of the changes relative to primordial abundances constrains models of stellar evolution. While the most accurate estimates of YP come from emission lines in extragalactic H II regions, they involve an extrapolation to zero metallicity which itself is closely tied up with the slope Y/Z. Recently certain systematic effects have come to light in this exercise which make it useful to have an independent estimate of Y/Z from fine structure in the main sequence of nearby stars. We derive such an estimate from Hipparcos data for stars with Z Z and find values between 2 and 3, which are consistent with stellar models, but still have a large uncertainty.     

Probing the solar wind acceleration region using spectroscopic techniques

Measurements of the intensities and profiles of UV and EUV spectral lines can provide a powerful tool for probing the physical conditions in the solar corona out to 8 R and beyond. We discuss here how measurements of spectral line radiation in conjunction with measurements of the white light K-corona can provide information on electron, proton and ion temperatures and velocity distribution functions; densities; chemical abundances and mass flow velocities. Because of the fundamental importance of such information, we provide a comprehensive review of the formation of coronal resonance line radiation, with particular emphasis on the H i L line, and discuss observational considerations such as requirements for rejection of stray light and effects of emission from the geocorona and interplanetary dust. Finally, we summarize some results of coronal H i L and white light observations acquired on sounding rocket flights.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Interplanetary type III radio bursts observed simultaneously by ICE

We analyze two solar type III radio bursts that were observed simultaneously by the ICE and Ulysses spacecraft. Both bursts originated behind the solar limb as viewed from either spacecraft. At the time of these events, ICE was in the ecliptic plane at 1 AU and Ulysses was 35° south of the ecliptic plane at 4 AU. For one event on 931117, the ratios of the peak flux densities measured at each spacecraft, at each observing frequency, were consistent with the most probable source locations relative to ICE and Ulysses. The second event on 931004 was a complex burst consisting of two distinct components at high frequencies. At low frequencies, the intensity of the first component decreased rapidly at each spacecraft. The second component, however, dominated the low frequency emission observed at Ulysses but not at ICE. These differences in the observed radiation must be related to the different viewing geometries of the two spacecraft. The measured onset times as a function of observing frequency were consistent with a constant exciter speed through the interplanetary medium and suggest that there are significant propagation delays, especially for the radiation propagating within the ecliptic plane.     

Tearing instability of reconnecting current sheets in space plasmas

Magnetic reconnection provides an efficient conversion of the so-called free magnetic energy to kinetic and thermal energies of cosmic plasmas, hard electromagnetic radiation, and accelerated particles. This phenomenon was found in laboratory and space, but it is especially well studied in the solar atmosphere where it manifests itself as flares and flare-like events. We review the works devoted to the tearing instability — the inalienable part of the reconnection process — in current sheets which have, inside of them, a transverse (perpendicular to the sheet plain) component of the magnetic field and a longitudinal (parallel to the electric current) component of the field. Such non-neutral current sheets are well known as the energy sources for flare-like processes in the solar corona. In particular, quasi-steady high-temperature turbulent current sheets are the energy sources during the main or hot phase of solar flares. These sheets are stabilized with respect to the collisionless tearing instability by a small transverse component of magnetic fiel, normally existing in the reconnecting and reconnected magnetic fluxes. The collision tearing mode plays, however, an important and perhaps dominant role for non-neutral current sheets in solar flares. In the MHD approximation, the theory shows that the tearing instability can be completely stabilized by the transverse fieldB _n if its value satisfies the conditionB _n /BS ^–3/4 B is the reconnecting component of the magnetic field just near the current sheet,S is the magnetic Reynolds number for the sheet. In this case, stable current sheets become sources of temporal spatial oscillations and usual MHD waves. The application of the theory to the solar atmosphere shows that the effect of the transverse field explains high stability of high-temperature turbulent current sheets in the solar corona. The stable current sheets can be sources of radiation in the radio band. If the sheet is destabilized (atB _n /BS ^–3/4) the compressibility of plasma leads to the arizing of the tearing instability in a long wave region, in which for an incompressible plasma the instability is absent. When a longitudinal magnetic field exists in the current sheet, the compressibility-induces instability can be dumped by the longitudinal field. These effects are significant in destabilization of reconnecting current sheets in solar flares: in particular, the instability with respect to disturbances comparable with the width of the sheet is determined by the effect of compressibility.     

Latitudinal and radial variation of shock associated ≥30 keV ion spectra and anisotropies at Voyagers 1 and 2

The spectra and anisotropies of ions 30 keV have been measured by the Low Energy Charged Particle experiment on Voyagers 1 and 2 in the vicinity of interplanetary shocks between radial distances of 1–55 AU and heliographic latitudes 11° S-32° N. The spectra and anisotropies associated with a recent corotating (CIR) event at low latitude observed at Voyager 2 (36.6 AU, –9°) are similar to those of another event at high latitude observed at Voyager 1 (49.8 AU, 33.5°). An earlier CIR event observed at Voyager 2 (14 AU) associated with the previous solar cycle produced spectra and anisotropies remarkably similar to the more recent events. The anisotropies are used to calculate the solar wind velocity downstream of shocks where possible using the Compton-Getting effect, allowing the determination of previously unknown velocities at the locations of Voyager 1. For the large shock event observed at Voyagers 1 (38 AU, 30°) and 2 (29 AU, 3°) in mid-1989, the postshock spectra and anisotropies are well described by convected power law distributions. The Voyager 1 and 2 postshock spectra 4 days after the shock passage are nearly identical. The preshock anisotropies at low energy are similar, despite differences in the magnetic field orientation and the low energy spectrum. We find that the 30 keV ion anisotropies are generally well described by convective distributions downstream but not in the upstream region for shocks and many other shock events at Voyagers 1 and 2.     

Intergalactic magnetic fields, and some connections with cosmic rays

An overview is presented of the methods of probing for the geometry, and strength of intergalactic magnetic fields. Recent results are briefly surveyed for galaxy halos, galaxy clusters, and the intergalactic medium on various scales, and some rele vant physical processes and radiation processes are mentioned, as well as the coupling between intergalactic magnetic fields and cosmic rays.The general trend of recent results indicates that, wherever we detect intergalactic hot gas and galaxies, we also find magnetic fields at levels of 10^–7 G, or higher. The hitherto undetected, weaker fields in the ratified i.g.m. and in large intergalactic voids could be probed by both Faraday rotation, and possibly using very energetic CR nuclei (> 10²⁰eV), and/or transient extragalactic ray bursts.     

Electron coronal density irregularity\overline {n^2 } /(\bar n)^2 from measurements of K-coronal and lyman lines brightnesses

We propose a technique to derive the coronal density irregularity factor , wheren is the electron density. The absolute photometric comparison between the intensity of UV lines and the white-light K-coronal polarized brightness (pB) provides an unique constraint on the inhomogeneity of the corona. The ratio of the measured H I Lyman (Ly-) line intensity to the resonant-scattering dominated H I Lyman (Ly-) intensity can be used to extract the collisonal component of the Ly-. This component yields an estimate of . The quantity is then obtained from white-light K-coronal measurements. The use of lines of the same atomic species minimizes the effects due to outflow velocities (i.e., Doppler dimming), and reduces the errors introduced by the uncertainties in the ionization balance, the atomic parameters, and the solar abundances. The UVCS/SOHO unique capability of performing cotemporal and cospatial measurements of the Ly- and Ly- lines, and ofpB makes this instrument ideal for implementing this technique.     

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.     

The Hanle effect applied to magnetic field diagnostics

The Hanle effect is the modification by the local magnetic field of the linear polarization parameters of a spectral line. A brief summary of the theory is presented and it is shown how the complete magnetic vector field can be recovered from the interpretation of the Hanle effect in spectral lines polarized by resonance scattering.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Effects due to an artificial earth satellite in rapid motion through the ionosphere or the interplanetary medium

Conclusion In this paper we have been concerned with the results of theoretical calculations of the interaction between a fast moving body and a tenuous plasma. Particular attention was paid to the case where the velocity of the body is much smaller than the velocity of neutral particles and ions, while the dimensions of the body are sufficiently large in comparison with the Debye radius. Such conditions are realized during the motion of artificial satellites or space rockets through the ionosphere, or through the interplanetary medium in the immediate neighborhood of the earth. Although this case has, on the whole, been investigated quite extensively, there are still a number of problems which require further analysis. Firstly, it is necessary to take into account the effect of the electric field on the motion of ions in the near zone at the rear of the body. Another important problem is that of magnetic disturbances. In the case of scattering of radio waves by the trail of the body, it would be interesting indeed to know the increase in the effective cross-section in the resonance region in which 0. A number of other research problems which arise in the analysis of phenomena in the neighborhood of a moving body have been noted in the Introduction.In the lower layers of the ionosphere it is important to allow for the fact that the dimensions of the body are comparable with the mean free path. Under these conditions there is the further interesting problem of the heating and additional ionization of the plasma, the disintegration of the surface, and the emission of waves. At large distances from the surface of the earth, the dimensions of the body may become comparable with the Debye radius, and the velocity of the body in the given region may become smaller than the thermal velocity of the particles. The character of the various disturbances introduced by the body under these conditions will also require a special investigation.Thus, the interaction of a moving body with plasma leads to special and exceptionally varied effects. Disturbances due to the body are very considerable, so that the physical state of the region surrounding the body is very different from the state of the undisturbed medium.The above results indicate that the phenomena in the neighborhood of satellites and space rockets in the ionosphere, or the interplanetary medium, must be taken into account, in the processing of experimental data when it is required to deduce information about the state of the undisturbed medium. This is particularly important in the analysis of the results of measurements obtained with various types of probes. Considerable errors may be introduced if these effects are not allowed for.Further extensive experimental and theoretical studies of the structure of the disturbed region in the neighborhood of moving bodies in plasma are clearly necessary. Such investigations will, in particular, lead to the development of the most effective methods of studying the properties of the media through which satellites and space rockets travel.Translated from the Russian: Ob effektah vyzyvaemyh iskusstvennym sputnikom bystro dviimsja v ionosfere ili meplanetnoj srede (Uspehi fizieskih nauk 79, 23–80) by Express Translation Service.     

Dual spacecraft observations of energetic particles in the vicinity of the magnetopause,bow shock,and the interplanetary medium

This article presents some of the new and important particle features that have been detected in the energy range 1 keV to 290 keV by the ISEE-1 and -2 spacecraft near the magnetopause, bow shock, and the interplanetary space. Only examples of data from the first few orbits, when the spacecraft were on the front side, are shown.Paper presented at 13th ESLAB Symposium, Innsbruck, Austria (June 5, 1978).     

Characteristics and interpretation of the photometric variability of eta carinae and its nebula

A short review is given on the history of the peculiar variable object Car and on a number of relevant references describing and discussing its physical characteristics and behaviour, based on different types of observational techniques. The star is known to be variable since the 17th century. The excessive mass loss to which it was subject during the 19th century is now visible as an ellipsoidal reflection nebula of 15 diameter: the so-called homunculus. The remainder of the paper is spent on different kinds of problems partly based on the results of a decade of photometric monitoring in the VBLUW photometric system of Walraven. Foreground reddening and reddening by dust in the homunculus are determined and amount to E(B - V) _J = 0 50 and < 6, respectively. Scanning of the homunculus revealed an estimate for the photometric characteristics of the central object, which presumably consists of a massive hot star surrounded by a cooler gas envelope. The total luminosity is derived using fluxes of various sources in the wavelength region 0.15 < < 175 n resulting in M _bol = - 12 3 ± 0 2. The total observed flux corrected for foreground extinction corresponds to a star with R 96 R if T _eff 30 000 K. The mass may be near 150 M . The excess luminosity in 1843, when the star was presumably bolometrically at least 2 5 brighter than at present, may have been caused by envelope-energized pulsations when the star's luminosity was close to its Eddington limit. The temperature should then have been 50 000 K. The mass loss rate, during the excess luminosity phase lasting 30 yr, is estimated to amount to M 4 × 10^-3 M yr^-1. At present the mass loss may be M 10^-4M yr^-1. Since the homunculus is mainly built up from material expelled in the 30 yr interval (from 1830 to 1860), its total mass amounts to M _hom 0.15 M . The historical observations of the colours of Car and a comparison with the characteristics of S Dor type stars, suggest that Car was subject to a number of S Dor type phases similar to those of P Cyg (in the 17th century), S Dor and others. A satisfactory explanation is found for the complete historical light curve. The decrease in light after the 1843 maximum by 9 ^m , presumably consists of a fading of the luminosity excess and the S Dor effect by 2 5 and 3^m, respectively, and a 3 5 extinction by circumstellar dust. The small amplitude light variations which Car showed during the last decade, were studied with the aid of the variations of the Balmer jump. They are presumably caused by temperature variations of the central star.Percy and Welch (1983) (Publ. Astron. Soc. Pacific 95, 491) have observed P Cyg on a number of nights in 1982 and found for the photometric variations a time scale of 30 to 50 days and an amplitude of 0 _. ^m 15.Based partly on observations collected at the ESO, La Silla, Chile.