Showing papers on "Heliosphere published in 1997"

The structure and origin of magnetic clouds in the solar wind

Abstract: . Plasma and magnetic field data from the Helios 1/2 spacecraft have been used to investigate the structure of magnetic clouds (MCs) in the inner heliosphere. 46 MCs were identified in the Helios data for the period 1974–1981 between 0.3 and 1 AU. 85% of the MCs were associated with fast-forward interplanetary shock waves, supporting the close association between MCs and SMEs (solar mass ejections). Seven MCs were identified as direct consequences of Helios-directed SMEs, and the passage of MCs agreed with that of interplanetary plasma clouds (IPCs) identified as white-light brightness enhancements in the Helios photometer data. The total (plasma and magnetic field) pressure in MCs was higher and the plasma-β lower than in the surrounding solar wind. Minimum variance analysis (MVA) showed that MCs can best be described as large-scale quasi-cylindrical magnetic flux tubes. The axes of the flux tubes usually had a small inclination to the ecliptic plane, with their azimuthal direction close to the east-west direction. The large-scale flux tube model for MCs was validated by the analysis of multi-spacecraft observations. MCs were observed over a range of up to ~60° in solar longitude in the ecliptic having the same magnetic configuration. The Helios observations further showed that over-expansion is a common feature of MCs. From a combined study of Helios, Voyager and IMP data we found that the radial diameter of MCs increases between 0.3 and 4.2 AU proportional to the distance, R, from the Sun as R0.8 (R in AU). The density decrease inside MCs was found to be proportional to R–2.4, thus being stronger compared to the average solar wind. Four different magnetic configurations, as expected from the flux-tube concept, for MCs have been observed in situ by the Helios probes. MCs with left- and right-handed magnetic helicity occurred with about equal frequencies during 1974–1981, but surprisingly, the majority (74%) of the MCs had a south to north (SN) rotation of the magnetic field vector relative to the ecliptic. In contrast, an investigation of solar wind data obtained near Earth's orbit during 1984–1991 showed a preference for NS-clouds. A direct correlation was found between MCs and large quiescent filament disappearances (disparition brusques, DBs). The magnetic configurations of the filaments, as inferred from the orientation of the prominence axis, the polarity of the overlying field lines and the hemispheric helicity pattern observed for filaments, agreed well with the in situ observed magnetic structure of the associated MCs. The results support the model of MCs as large-scale expanding quasi-cylindrical magnetic flux tubes in the solar wind, most likely caused by SMEs associated with eruptions of large quiescent filaments. We suggest that the hemispheric dependence of the magnetic helicity structure observed for solar filaments can explain the preferred orientation of MCs in interplanetary space as well as their solar cycle behavior. However, the white-light features of SMEs and the measured volumes of their interplanetary counterparts suggest that MCs may not simply be just Hα-prominences, but that SMEs likely convect large-scale coronal loops overlying the prominence axis out of the solar atmosphere.

Cosmic Winds and the Heliosphere

Abstract: Until the advent of space physics, astrophysical plasmas could be studied only using ground-based observations. Although observational methods have advanced over recent decades, the merging of heliospheric physics with astrophysics is far from complete due to the vastly different techniques employed by astronomers and space physicists. That astrophysical plasmas can be studies directly is a major advance in astrophysical research. The solar wind from the Sun is only one of many examples of solar winds, but it provides scientists with a basis for understanding how these formerly disparate disciplines are related. Cosmic Winds and the Heliosphere is a comprehensive sourcebook on conceptually correlated topics in astrophysical winds and heliospheric physics. The contributors review the various kinds of winds, such as solar wind, winds of cataclysmic variables, and winds from pulsating stars. They then examine the physics of wind origin and physical phenomena in winds. including heliospheric shocks, magnetohydrodynamic turbulence, and kinetic phenomena. A final section considers interactions with surrounding media, with contributions ranging from studies of the interstellar cloud surrounding the solar system to considerations of solar wind interaction with comets. Prepared to the scrupulous standards of the University of Arizona Space Science Series, Cosmic Winds and the Heliosphere is an essential volume for astronomers and space physicists.

A hemispherical model of anisotropic interstellar pickup ions

Abstract: We present an analytical model for the distribution function of interstellar pickup ions in a radially directed heliospheric magnetic field which naturally results in anisotropic particle distributions. The model includes the effects of convection and spatial transport in the solar wind, adiabatic deceleration in the radial flow, adiabatic focusing in the radial field, and pitch angle scattering toward isotropy in the frame of the solar wind. The pitch angle scattering is approximated by the hemispherical assumption: we take the scattering to be very efficient within each pitch angle range μ ≷ 0 (where μ is the cosine of the pitch angle) but inhibited between the hemispheres separated by μ = 0. The analytical solution is obtained for the case where the scattering rate across μ = 0 scales as the particle speed divided by the radial position of the fluid parcel. The model distribution functions can be used to interpret recent observations of anisotropic pickup ions.

Anomalously small magnetic field in the local interstellar cloud

Abstract: The solar wind carves out a cavity, known as the heliosphere, in the warm local interstellar cloud, which is itself embedded in a larger hot cloud. It is generally assumed that there is an overall pressure balance between these three regions. Thermal pressure and magnetic field pressure in the local interstellar cloud should therefore balance the inward pressure from the hot cloud1–3, and determine the size of the heliosphere. Here we present direct measurements of the density and speed of interstellar hydrogen and helium ions deep inside the Solar System, from which we derive the thermal pressure in the local interstellar cloud. Combined with the fact that the magnetic field strength in the local cloud is constrained to be less than 4.3 µG (to be compatible with the fact that the Voyager 1 spacecraft has yet to encounter the heliosphere termination shock), the total pressure that we infer is insufficient to balance the inward pressure from the hot cloud. We conclude that either the magnetic field in the local cloud is inhomogeneous, or there is a significant, as yet undetected, non-thermal component to the pressure in the local cloud.

Micron‐sized dust particles detected in the outer solar system by the Voyager 1 and 2 plasma wave instruments

Abstract: During the Voyager 1 and 2 flybys of the outer planets it has been demonstrated that the plasma wave instrument can detect small dust particles striking the spacecraft. In this paper, we examine the Voyager plasma wave data for dust impacts in the interplanetary medium at heliocentric radial distances ranging from 6 to 60 astronomical units (AU). The results show that a small but persistent level of dust impacts exists out to at least 30 to 50 AU. The average number density of these particles is about 2 x 10(exp -8)/cu m, and the average mass of the impacting particles is believed to be a few times 10(exp -11) g, which corresponds to particle diameters in the micron range. Possible sources of these particles are planets, moons, asteroids, comets, and the interstellar medium. Of these, comets appear to be the most likely source. ne number densities are only weakly dependent on ecliptic latitude, which indicates that the particles probably do not originate from planets, moons, or asteroids. Comparisons with interstellar dust fluxes measured in the inner regions of the solar system by the Ulysses spacecraft indicate that the particles are not of interstellar origin.

Solar wind stream interfaces in corotating interaction regions: New SWICS/Ulysses results

Abstract: We have analyzed data from the solar wind ion composition spectrometer (SWICS)/Ulysses instrument taken between August 1996 and May 1997. In this period the Ulysses spacecraft traveled from 28° to 11° N and encountered a highly regular pattern of high-speed streams alternating with slow solar wind. Heliocentric distance varied between 4.3 and 5.1 astronomical units (AU). Using proton and alpha-particle kinetic parameters (density, speed, and kinetic temperatures) as well as charge state and elemental composition data, we identify the stream interfaces in the corotating interaction regions (CIRs) observed in this period. As Wimmer-Schweingruber et al. [1997] previously reported for a similar period in 1992/1993, stream interfaces are the sites of compositional changes between values typical of the slow solar wind and values typical of high-speed streams. During that period, Ulysses traveled from 13° to 34° S and from 4.5 to 5.4 AU. In spite of the similarity of the heliospheric regions probed during 1992/1993 and 1996/1997 the corotating interaction regions observed in 1996/1997 are quite different from those observed in 1992/1993. We observe fewer (11) CIRs in 1996/1997 than in the previous period (15); the CIRs are less evolved, i.e., the kinetic signatures at the stream interface are less clear; and mainly, only 3 out of the 11 CIRs are bounded by forward-reverse shock pairs, whereas in 1992/1993, 13 out of 15 were. This may have important consequences for particle acceleration in CIRs in the inner heliosphere.

Interaction of a nonuniform solar wind with the local interstellar medium: 2. A two‐fluid model

Abstract: Results of a fully three-dimensional model of the interaction of the solar wind with the local interstellar medium are presented. The effects of charge-exchange with interstellar neutral hydrogen are taken into account self-consistently, while the effects of hot solar wind neutral hydrogen, as well as cosmic rays and magnetic fields, are ignored in this study. In accord with solar medium observations by Ulysses, the solar wind is assumed to depend on heliolatitude. Two large, long-lived polar coronal holes, one in the northern hemisphere and the other in the southern hemisphere, are assumed to produce a hot, low-density, high-speed wind which bounds a cooler, higher-density, low-speed ecliptic wind. The solar wind boundary conditions, which allow for a 1.5 increase in solar wind ram pressure over the poles of the Sun compared with the ecliptic plane, are drawn from published Ulysses data [Phillips et al., 1995, 1996]. The results of this simulation are compared with the no-charge-exchange asymmetric solar wind simulation, described by Pauls and Zank [1996]. It is found that the elongation of the heliosphere along the solar poles, resulting from the ram pressure increase with heliolatitude, induces a greater influx of interstellar hydrogen over the poles of the Sun than in the ecliptic plane. This, in turn, reduces the extent of elongation of the heliosphere along the poles of the Sun. The vorticity, found in the no-charge-exchange simulation, is absent in the presence of charge-exchange. Once again, as found by Baranov and Malama [1993] and Pauls et al. [1995], the interaction of the solar wind with the local interstellar medium is influenced strongly by charge-exchange processes.

One- versus Two-Shock Heliosphere: Constraining Models with Goddard High Resolution Spectrograph Lyα Spectra toward α Centauri

Abstract: Redshifted Lyα absorption toward α Cen has been interpreted by Linsky & Wood and Frisch et al. as evidence for decelerated interstellar hydrogen piled up on the upstream side of the heliosphere. We utilize newly developed two-dimensional multifluid models of the solar wind interaction with the interstellar material to corroborate this interpretation by synthesizing the Lyα absorption profile predicted for this "hydrogen wall." Both subsonic and supersonic inflow into the heliosphere are considered, corresponding to one-shock and two-shock global morphologies, respectively. We find that these two extremes give observably different redward absorption characteristics in the Lyα profiles, and our preliminary conclusion is that the Lyα profiles seen toward α Cen favor a barely subsonic model (Mach number 0.9). For such a model to hold, additional interstellar pressure terms, such as cosmic-ray or magnetic pressures, must contribute. To make this conclusion more certain, an extended-model parameter survey is required, coupled with Lyα data along other lines of sight.

MHD simulation of an interaction of a shock wave with a magnetic cloud

Abstract: Interplanetary shock waves, propagating in the heliosphere faster than earlier-emitted coronal ejecta, penetrate them and modify their parameters during this interaction. Using two and one half dimensional MHD simulations, we show how a magnetic cloud (flux rope) propagating with a speed 3 times higher than the ambient solar wind is affected by an even faster traveling shock wave overtaking the cloud. The magnetic field increases inside the cloud during the interaction as it is compressed in the radial direction and becomes very oblate. The cloud is also accelerated and moves faster, as a whole, while both shocks (driven by the cloud and the faster interplanetary shock) merge upstream of the cloud. This interaction may be a rather common phenomenon due to the frequency of coronal mass ejections and occurrence of shock waves during periods of high solar activity.

Radial intensity gradients of galactic cosmic rays (1972–1995) in the heliosphere

Abstract: The radial intensity gradients near the ecliptic plane for galactic cosmic ray He (180-450 MeV/nucleon), H (130-220 MeV), and the integral counting rate of ions >70 MeV are studied using the Pioneer 10/11, Voyager 2, and IMP 6, 7, and 8 observing network over the 1972.25-1996.0 time period at heliocentric distances extending to 64 AU. These gradients display complex temporal and spatial changes that are dominated by the 11-year solar activity cycle but with the 22-year heliomagnetic cycle also having an important role. The data are well ordered out to ∼55 AU using a radial dependence of the intensity gradient g r of the form G 0 r α , where r is the heliocentric distance in astronomical units (AU). With this representation it becomes possible to estimate g r as a function of r at a given time or as a function of time at a given radial distance. Over successive solar minima these values of g r decrease rapidly with increasing heliocentric distance, and for He at 50 AU are 3 times larger in 1987 than in 1977, in general agreement with the expected effects of particle gradient and curvature drifts in the large-scale interplanetary magnetic field. Near solar maximum the gradients increase significantly over their solar minimum values, with a greatly reduced radial dependence for He, while for H, g r increases with increasing r. The radial gradients of the integral rate of ions >70 MeV show a much smaller temporal and spatial variation.

Direct observational evidence for a heliospheric magnetic field with large excursions in latitude

Abstract: Fisk has pointed out that the heliospheric magnetic field in fast solar wind, that is, at the higher heliographic latitudes, may undergo large excursions in heliographic latitudes, and thus that the field will deviate from the expected Archimedes spiral pattern. These excursions result from the interplay between the nonradial expansion of the solar wind in rigidly rotating coronal holes, and the differential rotation of the footpoints of the field lines anchored in the photosphere. In this paper magnetic field data from the Ulysses spacecraft in the southern solar hemisphere are analyzed and show that the field exhibits the systematic variations with latitude and longitude expected from the field configuration predicted by Fisk. We also show that variations about the predicted field should exhibit a unique periodicity of about 20 days at high latitudes and that the predicted periodicity is in fact observed. The observation conformation of the predicted field provides support for several other applications of this theory.

Implications of fluctuations in the distribution functions of interstellar pick-up ions for the scattering of low rigidity particles

Abstract: The distribution functions of interstellar pick-up hydrogen and helium exhibit fluctuations which are correlated An analysis of these fluctuations reveals that they may be the result of variations in, and provide additional evidence for, large mean free paths for these low rigidity particles An analysis of the properties of the distribution function of pick-up hydrogen reveals that these particles do suffer considerable adiabatic cooling, and that the likely cause for the long mean free path is the inability of the particles to scatter through 90° pitch angle

Coronal streamer belt asymmetries and seasonal solar wind variations deduced from Wind and Ulysses data

Abstract: Solar wind measurements from Wind during March 1995 are combined with those from Ulysses' fast latitude scan to construct a map of the streamer belt. On the timescale of coronal change, the map is nearly a snapshot view of solar wind speed contours threaded by the trace of the heliospheric current sheet (HCS) in the ±30° heliolatitude range. The combined set of HCS crossings agrees remarkably well with the neutral line on the corresponding classical source surface map. The neutral line is displaced slightly southward, with latitudinal excursions ranging from -22° to +17°. In contrast, a line running through the locus of minimum speed, although following the general trend of the neutral line, is confined to lower latitudes and displaced slightly northward, ranging from -4° to +13°. The separation between the minimum speed locus and the neutral line marking the HCS was unexpected. Possible interplanetary and solar origins are discussed. The deduced asymmetries as well as coronal change between December and March were responsible for solar wind variations at Earth that mimicked the previously reported seasonal variation in Wind data and would have masked it had the observations been taken during September equinox.

Radial evolution of corotating merged interaction regions and flows between ≃14 AU and ≃43 AU

Abstract: During 1993 and 1994 the solar coronal holes and the heliospheric current sheet were relatively stationary, and recurrent streams and interaction regions with periods of the order of the solar rotation period were present within 5 AU. One expects that during 1994 Voyager 2 (V2) (located at ≈43 AU and at ≈12°S latitude, in the sector zone) would have observed some evolutionary form of corotating streams and interaction regions. The “sector zone” is the latitude band in which a spacecraft observes both positive and negative sectors [Burlaga and Ness, 1996]. We present the observations of the magnetic field strength (B), the speed (V), density (N), and proton temperature (T) made by V2-94, and for reference we also discuss the observations made by V2 at ≈14 AU during 1983 (V2-83) a solar cycle earlier. Correlated, quasiperiodic variations in B and N with a period of ≈26 days (corotating merged interaction regions) were observed at ≈14 AU but not at ≈43 AU. The speed and temperature profiles were irregular at ≈14 AU but quasiperiodic at ≈43 AU. An ƒ−2 spectrum of the magnetic field strength B (indicating the dominance of shocks) was observed at ≈14 AU, but an ƒ−5/3 spectrum (indicating the dominance of Kolmogorov turbulence) was observed at ≈43 AU in the range (2.7 × 10−6 to 2.3 × 10−5)Hz. An ƒ−2.5 spectrum of the speed fluctuations was observed at ≈14 AU, but an ƒ−2 spectrum was observed at ≈43 AU in the range (8.8 × 10−7 to 2.3 × 10−5)Hz. We suggest the hypothesis that the qualitative differences between the observations at ≈14 AU and ≈43 AU represent a change in the state of the solar wind as it moves between these two positions. This change involves a transition from a quasiperiodic (ordered) state in B and N at ≈14 AU to a disordered state at ≈43 AU and from an aperiodic state in V and T at ≈14 AU to a quasiperiodic state at ≈43 AU. The standard MHD models for the radial evolution of corotating streams and interaction regions have not predicted such a transition. Our results suggest that it would be fruitful to develop a new MHD model of such flows, which should include (1) three-dimensional effects, (2) the intermediate-scale fluctuations, and (3) the interstellar pickup ion pressure.

A comparative study of cosmic ray radial and latitudinal gradients in the inner and outer heliosphere

Abstract: The radial and latitudinal intensity gradients of 145–255 MeV/nucleon He, 34–50 MeV/nucleon He and 30–69 MeV H are studied over an extensive range of heliocentric distances and latitudes for the 1993.0–1996.0 time period using data from cosmic ray experiments on the Ulysses, IMP 8, Voyager 1 and 2, and Pioneer 10 spacecraft. The radial gradients are found to decrease rapidly with increasing heliocentric distance and agree with those measured 20 years earlier at a similar phase of the heliomagnetic cycle. The latitudinal gradients measured in the inner and outer heliosphere are in reasonable agreement and positive albeit exceedingly small. In agreement with other Ulysses energetic particle experiments it is found that a shift of heliolatitude by −7° to −10° is necessary to get reasonable symmetry in the measurements at midlatitudes. From the Ulysses data it appears there is a significantly reduced latitudinal variation in the intensity of the three energetic particle components at (magnetic) heliolatitudes above about 50° at this phase of the modulation cycle. Such a reduced entry of cosmic rays over such an extensive area above the solar poles implies a strong modification of the previously assumed cosmic ray transport processes at high latitudes, most probably a considerably increased rate of scattering combined with reduced particle gradient and curvature drifts. A significant higher intensity is observed over the north solar pole than over the south pole for the low-energy components after the corrections have been applied for the temporal changes at the 1-AU baseline.

A self‐consistent determination of the heliospheric termination shock structure in the presence of pickup, anomalous, and galactic cosmic ray protons

Abstract: A self-consistent time-dependent model is used to study the modification of the heliospheric termination shock in the upwind direction under the influence of three suprathermal particle populations, namely pickup, anomalous and galactic cosmic ray protons. By ensuring that the resulting modulated cosmic ray proton spectra are consistent with those observed by the Voyager 2 and Pioneer 10 spacecraft during the solar activity minimum in 1987, two alternative modifications of the shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.

Shock propagation in the outer heliosphere: 1. Pickup ions and gasdynamics

Abstract: Pickup ions are expected to play a fundamental role in determining the global structure of the heliosphere since they dominate the internal energy of the outer solar wind beyond the ionization cavity. The dynamical effect of pickup ions on the propagation characteristics of interplanetary shocks is addressed here. It is found that (1) forward and reverse shocks in a heliosphere mediated by pickup ions have higher speeds than expected of shocks in an adiabatic solar wind without pickup ions; (2) identical initial conditions evolve differently in a pickup ion mediated heliosphere compared to an adiabatic heliosphere, producing more complicated velocity, density and temperature structure, and often with multiple forward-reverse shock pairs; (3) pickup ions act to weaken and dissipate shocks in the outer heliosphere although the velocity jumps can remain large; (4) density fluctuations in the outer pickup ion mediated heliosphere do not correlate with velocity fluctuations as well as they do in the inner heliosphere, and (5) the amplitudes of density fluctuations in a pickup ion mediated heliosphere are considerably smaller than those found in an adiabatic heliosphere without pickup ions.

A Linear Relationship between the Latitude Gradient and 26 Day Recurrent Variation in the Fluxes of Galactic Cosmic Rays and Anomalous Nuclear Components. I. Observations

Abstract: We find that there exists a linear relationship between the magnitude of latitude gradient and the amplitude of 26 day recurrent variations in the fluxes of galactic cosmic rays and anomalous nuclear components. This result is based on energetic charged particle measurements from the University of Chicago High Energy Telescope (HET) in the Cosmic Ray and Solar Particle Investigation (COSPIN) consortium on the Ulysses spacecraft during its mission to the solar poles and on similar measurements from IMP-8 spacecraft orbiting around Earth at 1 AU in the solar ecliptic. The linear relationship holds for recurrent cosmic-ray flux variations observed in the inner heliosphere at all latitudes including the high-latitude regions covered by Ulysses and in the equatorial by IMP-8, and it is roughly independent of particle energy and species. The relationship means that particles with large-latitude flux gradients are also strongly modulated by recurrent solar wind structures such as corotating interaction regions that appear in low- and middle-latitude regions. In the previous solar cycle, when the solar magnetic field had an opposite polarity, cosmic-ray measurements from the Voyager spacecraft and IMP-8 also show a linear relationship between the latitude gradient and the amplitude of recurrent variations even though the latitude gradient had a negative sign. With these observational facts, the three-dimensional flux distributions of cosmic rays in the inner heliosphere can be more easily measured and better understood because the linear relationship implies that there are similarities among different kinds of charged particles in their modulated flux distributions. These observations suggest that there is a common dominant modulation mechanism controlling both the global latitudinal distribution and the short-term temporal variation of cosmic-ray fluxes. A theoretic model to account for these observations and to understand cosmic-ray modulation in the three-dimensional heliosphere will be presented in Paper II.

On the feasibility of imaging coronal mass ejections at radio wavelengths

Abstract: Coronal mass ejections (CMEs) can have a profound impact on the interplanetary medium and the near-Earth environment. We discuss the feasibility of detecting coronal mass ejections at radio wavelengths with a ground-based instrument. In particular, we explore the possibility that a radio telescope employing Fourier synthesis imaging techniques can detect thermal bremsstrahlung emission from CMEs. Using a simulated database from such a telescope, we explore three detection schemes: direct detection, an approximate differential detection scheme, and an “exact” differential detection scheme. We conclude that thermal bremsstrahlung emission from CMEs can be detected by such a telescope provided differential techniques are employed. While the approximate differential detection scheme may be sufficient for CMEs viewed near the solar limb, detection of CMEs against the solar disk may require the more sensitive exact differential scheme. The detection and imaging of nonthermal radio emissions from CMEs is also discussed.

Preacceleration of Anomalous Cosmic Rays in the Inner Heliosphere

Abstract: Because of the apparent difficulty in accelerating previously unaccelerated pickup ions locally at the solar wind termination shock, a model for anomalous cosmic-ray protons is considered in which the initial acceleration of pickup ions to ~10-20 MeV occurs in the inner heliosphere. These accelerated pickup ions are assumed to be accelerated either by propagating interplanetary shocks or by some other, unspecified process. Voyager 2 Low-Energy Charged Particle (LECP) observations at low energies are used to normalize their spectra. The ions are then transported to the termination shock, where they are further accelerated to anomalous cosmic-ray energies. We use a well-established transport model to simulate this process. In this model, the two-dimensional cosmic-ray transport equation is solved using the assumed (and normalized) source of energetic particles (~0.01-20 MeV) located at 10 AU. We show that the computed spectra at higher energies (100 MeV) are consistent with observed anomalous cosmic-ray fluxes and suggest that interplanetary shocks, especially those in the inner heliosphere, may play an important role in the initial acceleration of pickup ions leading to anomalous cosmic rays.

The propagation and survival of interstellar grains

Abstract: In this paper we discuss the propagation of dust through the interstellar medium (ISM), and describe the destructive effects of stellar winds, jets, and supernova shock waves on interstellar dust. We review the probability that grains formed in stellar outflows or supernovae survive processing in and propagation through the ISM, and incorporate themselves relatively unprocessed into meteoritic bodies in the solar system. We show that very large (radii⩾5 μm) and very small grains (radii⩽100 A) with sizes similar to the pre-solar SiC and diamond grains extracted from meteorites, can survive the passage through 100 km s−1 shock waves relatively unscathed. High velocity (⩾250 km s−1) shocks destroy dust efficiently. However, a small (∼10%) fraction of the stardust never encountered such fast shocks before incorporation into the solar system. All grains should therefore retain traces of their passage through interstellar shocks during their propagation through the ISM. The grain surfaces should show evidence o...

Isotopic Abundances of Fe and Ni in Galactic Cosmic-Ray Sources

Abstract: With a new instrument, designed to determine the isotopic composition of Galactic cosmic-ray nuclei, on board the Ulysses spacecraft, we report relative abundance measurements for 54Fe,55Fe,56Fe,57Fe,58Fe and 58Ni,59Ni,60Ni,62Ni in the overall energy range between ~200 and ~420 MeV nucleon-1. These measurements combine excellent mass resolution (σ ~ 0.28 amu at 56Fe) with good statistical significance (more than 6000 Fe events). The high mass resolution is achieved by utilizing solid-state, position-sensing detectors to measure—with less than 1° angular error—the trajectory of each nucleus entering the charged-particle telescope. The cosmic-ray source abundances are derived from the measurements using models for propagation from distributed sources in the Galaxy ("leaky-box" model), taking into account solar modulation during penetration of the heliosphere. Overall, except for 54Fe/56Fe and 57Fe/56Fe, we show that the principal Fe and Ni isotopic source ratios have values close to the solar system ratios derived from meteorites. In particular, we note that 58Fe and 62Ni display no evidence of neutron enrichment. We discuss the role of the predominantly electron-capture nuclide 54Mn, which decays mainly by β- in the cosmic rays, and contributes to the abundance of 54Fe measured in the solar system.

Spectra of energized pick-up ions upstream of the two-dimensional heliospheric termination shock II. Acceleration by Alfvenic and by large-scale solar wind turbulences

Abstract: It is generally envisaged that pick-up ions originat- ingbyionizationprocessesfrominterstellarneutralatomsinthe region of supersonic solar windflow eventually act as seed pop- ulation for anomalous cosmic rays. It is, however, fairly unclear till now where and how the necessary energization from KeV- to 10MeV-energies takes place. Here we consider the continu- ous stochastic acceleration of pick-up ions at their convection to the outer heliospheric regions both by small-scale Alfvenic turbulence and by coherent nonlinear large-scale fluctuations of solar wind velocity and magnetic eld. For these latter fluc- tuations we develope a new turbulence concept by which we describe the average effect of corotating interaction regions in energizing pick-up ions. It is shown that large-scale turbulence isresponsiblefortheaccelerationofpick-upionsfrom10to100 KeV/nucleon while the preceded, primary pick-up ion acceler- ationfrom1to10KeV/nucleonisdonebysmall-scaleAlfvenic turbulence.OurresultsnicelycantULYSSESandVOYAGER data on energetic particles. We also conrm by our theory that a preferable acceleration of helium compared to hydrogen pick- ups occurs to 100KeV/nuc energies. Also the reflection rates at the termination shock are favourable for helium pick-ups, and thus we expect injection rates into the ACR regime favorable for ACR helium. In no case within the frame of our theoretical studies, the typical \anomalous cosmic ray" energies of the or- der of 10 MeV/nucleon will be achieved before arrival of the pick-up ions at the shock. We can show, however, that the per- centage of pick-up ions undergoing reflection at the shock will be much increased by the pre-acceleration operating upstream of the shock. In ongoing reflection processes of rst and higher orders the ions permanently gain in energy till nally they are diffusively decoupled from the shock and can reappear in the inner solar system.

enic and by large-scale solar wind turbulences

Abstract: It is generally envisaged that pick-up ions originat- ingbyionizationprocessesfrominterstellarneutralatomsinthe region of supersonic solar windflow eventually act as seed pop- ulation for anomalous cosmic rays. It is, however, fairly unclear till now where and how the necessary energization from KeV- to 10MeV-energies takes place. Here we consider the continu- ous stochastic acceleration of pick-up ions at their convection to the outer heliospheric regions both by small-scale Alfv´ turbulence and by coherent nonlinear large-scale fluctuations of solar wind velocity and magnetic field. For these latter fluc- tuations we develope a new turbulence concept by which we describe the average effect of corotating interaction regions in energizing pick-up ions. It is shown that large-scale turbulence isresponsiblefortheaccelerationofpick-upionsfrom10to100 KeV/nucleon while the preceded, primary pick-up ion acceler- ationfrom1to10KeV/nucleonisdonebysmall-scaleAlfv´ turbulence.OurresultsnicelycanfitULYSSESandVOYAGER data on energetic particles. We also confirm by our theory that a preferable acceleration of helium compared to hydrogen pick- ups occurs to 100KeV/nuc energies. Also the reflection rates at the termination shock are favourable for helium pick-ups, and thus we expect injection rates into the ACR regime favorable for ACR helium. In no case within the frame of our theoretical studies, the typical "anomalous cosmic ray" energies of the or- der of 10 MeV/nucleon will be achieved before arrival of the pick-up ions at the shock. We can show, however, that the per- centage of pick-up ions undergoing reflection at the shock will be much increased by the pre-acceleration operating upstream of the shock. In ongoing reflection processes of first and higher orders the ions permanently gain in energy till finally they are diffusively decoupled from the shock and can reappear in the inner solar system.

Ulysses observations of the northward extension of the heliospheric current sheet

Abstract: After passing through the northern polar regions of the heliosphere during the summer of 1995, the Ulysses spacecraft has been gradually returning towards lower equatorial latitudes, reaching 30°N in August 1996 at about which time low speed solar wind from the streamer belt was once again observed. This paper reports on the Ulysses magnetic field observations, beginning from this time, concentrating on encounters with the heliospheric current sheet. These encounters restarted at a latitude of 25.5°N, a higher latitude than might be expected at solar activity minimum. We show that this early reappearance was due to a northward deflection of the current sheet caused by a long-lived active region at the corresponding heliographic longitude in the photosphere.

The role of coronal mass ejections and interplanetary shocks in interplanetary magnetic field statistics and solar magnetic flux ejection

Abstract: We examine the role of coronal mass ejections (CMEs) and interplanetary shocks in modifying the large-scale winding of the interplanetary magnetic field (IMF) by extracting CME and shock observations from the ISEE 3 data set and analyzing periods of the disturbed and undisturbed solar wind separately. We use the full ISEE 3 data set representing the entire L1 mission (1978 – 1982). We conclude that CMEs, the shocks upstream of CMEs, and other interplanetary shocks are responsible for the apparent overwinding of the IMF spiral relative to the Parker prediction. The IMF winding angle asymmetry is preserved following the removal of the interplanetary disturbances. We also examine the IMF components, the IMF magnitude and the solar wind speed, and the dependence of those averages and asymmetries on CMEs and shock disturbances. An estimate is obtained for the anomalous azimuthal field contained within CMEs which apparently results from the closed-field topology. We provide new evidence for a nonzero field component crossing the heliospheric current sheet. Last, we examine the role of CMEs and shocks in the measurement of solar magnetic flux ejection. We provide estimates for the average amount of flux transported by CMEs and the error in flux transport analyses that include shock data, and we examine the possible north-south asymmetry of the flux.

New Experimental Data and What It Tells Us About the Sources and Acceleration of Cosmic Rays

Abstract: This paper summarizes new data in several fields of astronomy that relate to the origin and acceleration of cosmic rays in our galaxy and similar nearby galaxies. Data from radio astronomy shows that supernova remnants, both in our galaxy and neighboring galaxies, appear to be the sources of most of the accelerated electrons observed in these galaxies. γ-ray measurements also reveal several strong sources associated with supernova remnants in our galaxy. These sources have γ-ray spectra that are suggestive of the acceleration of cosmic-ray nuclei. Cosmic-ray observations from the Voyager and Ulysses spacecraft suggest a source composition that is very similar to the solar composition but with distinctive differences in the 4He, 12C,14 N and 22Ne abundances that are the imprint of giant W-R star nucleosynthesis. Injection effects which depend on the first ionization potential (FIP) of the elements involved are also observed, in a manner similar to the fractionization observed between the solar photosphere and corona and also analogous to the preferential acceleration observed for high FIP elements at the heliospheric solar wind termination shock. Most of the 59Ni produced in the nucleosynthesis of Fe peak nuclei just prior to a SN explosion appears to have decayed to 59Co before the cosmic rays have been accelerated, suggesting that the59 Ni is accelerated at least 105 yr after it is produced. The decay of certain K capture isotopes produced during cosmic-ray propagation has also been observed for the first time. These measurements suggest that re-acceleration after an initial principal acceleration cannot be large. The high energy spectral indices of cosmic-ray nuclei show a significant charge dependent trend with the index of hydrogen being -2.76 and that of Fe -2.61. The escape length dependence of cosmic rays from our galaxy can now be measured up to ~300 GeV nucl-1 using the Fe sec/Fe ratio. This escape length is ∼P-0.05 above 10 GeV nucl-1 leading to a typical source spectral index of (2.70±0.10) -0.50 = -2.20 for nuclei. This is similar to the source index of -2.3 inferred for electrons within the errors of ±0.1 in the index for both components. Spacecraft measurements in the outer heliosphere suggest that the local cosmic-ray energy density is ~2eV cm-3 – larger than previously assumed. Gamma-ray measurements of electron bremsstrahlung below 50 MeV from the Comptel experiment on CGRO show that fully 20–30% of this energy is in electrons, several times that previously assumed. New estimates of the amount of matter traversed by cosmic rays using measurements of the B/C ratio are also higher than earlier estimates – this value is now ~10 g cm-2 at 1 GeV nucl-1. Thus altogether cosmic rays are energetically a more important component of our galaxy than previously assumed. This has implications both for the types of sources that are capable of accelerating cosmic rays and also for the role that cosmic rays may play in ionizing the diffuse interstellar medium.