Showing papers on "Heliosphere published in 1992"

The Ulysses solar wind plasma experiment

Abstract: The scientific objectives of the Ulysses solar wind plasma experiment, termed the Solar Wind Observations Over the Poles of the Sun (SWOOPS) include measurements of the solar-wind global properties, the nonlinear MHD disturbances in the solar wind, the internal state of the solar wind plasma, and the solar-wind interaction with Jupiter's magnetic field. In this paper, special attention is given to the two instrumental packages of SWOOPS experiment that will simultaneously perform measurements on electrons and ions of solar plasma: the ion analyzer and the electron analyzer. Results obtained in the initial phases of the SWOOPS experiment are presented.

The magnetic field investigation on the Ulysses mission - Instrumentation and preliminary scientific results

Abstract: A fundamental feature of the heliosphere is the three-dimensional structure of the interplanetary magnetic field. The magnetic field investigation on Ulysses, the first space probe to explore the out-of-ecliptic and polar heliosphere, aims at determining the large-scale features and gradients of the field, as well as the heliolatitude dependence of interplanetary phenomena so far only observed near the ecliptic plane. The Ulysses magnetometer uses two sensors, one a Vector Helium Magnetometer, the other a Fluxgate Magnetometer. Onboard data processing yields measurements of the magnetic field vector with a time resolution up to 2 vectors/second and a sensitivity of about 10 pT. Since the switch-on of the instrument in flight on 25 October 1990, a steady stream of observations has been made, indicating that at this phase of the solar cycle the field is generally disturbed: several shock waves and a large number of discontinuities have been observed, as well as several periods with apparently intense wave activity. The paper gives a brief summary of the scientific objectives of the investigation, followed by a detailed description of the instrument and its characteristics. Examples of wave bursts, interplanetary shocks and crossings of the heliospheric current sheet are given to illustrate the observations made with the instrument.

The Ulysses mission

Abstract: The Ulysses mission is unique in the history of the exploration of solar system by spacecraft. The path followed by Ulysses will make it possible, for the first time, to explore the heliosphere within a few astronomical units of the sun over the full range of heliographic latitudes, thereby providing the first characterization of the uncharted third heliospheric dimension. Advanced scientific instrumentation carried on board the spacecraft is designed to measure the properties of the heliospheric magnetic field, the solar wind, the sun/wind interface, solar radio bursts and plasma waves, solar energetic particles and galactic cosmic rays, solar X-rays, and interplanetary/interstellar neutral gas and dust. Ulysses will also be used to detect cosmic gamma-ray bursts and search for gravitational waves. The mission, a collaboration between ESA and NASA, was launched in October 1990 and employs a Jupiter gravity-assist to achieve the trajectory extending to high solar latitudes. The paper describes the characteristics of the Ulysses mission in order to establish a framework within which to better understand the objectives and goals of the scientific investigations.

Heliosphere Instrument for Spectra, Composition and Anisotropy at Low Energies

Abstract: The Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies (HI-SCALE) is designed to make measurements of interplanetary ions and electrons throughout the entire Ulysses mission. The ions (E(i) greater than about 50 keV) and electrons (E(e) greater than about 30 keV) are identified uniquely and detected by five separate solid-state detector telescopes that are oriented to give nearly complete pitch-angle coverage from the spinning spacecraft. Ion elemental abundances are determined by Delta E vs E telescope using a thin (5 microns) front solid state detector element in a three-element telescope. Experimental operation is controlled by a microprocessor-based data system. Inflight calibration is provided by radioactive sources mounted on telescope covers which can be closed for calibration purposes and for radiation protection during the course of the mission. Ion and electron spectral information is determined using both broad-energy-range rate channels and a 32 channel pulse-height analyzer for more detailed spectra. Some initial in-ecliptic measurements are presented which demonstrate the features of the instrument.

Low-energy solar electrons and ions observed at Ulysses February-April, 1991 - The inner heliosphere as a particle reservoir

Abstract: Ulysses observations at 2.5 AU of 38-315 keV electrons and 61-4752 keV ions during February-April 1991 suggest in several ways that, during periods of sustained high solar activity, the inner heliosphere serves as a 'reservoir' for low-energy solar particles. Particle increases were not associated one-to-one with large X-ray flares because of their poor magnetic connection, yet intensities in March-April remained well above their February levels. The rise phase of the particle event associated with the great flare of 2245UT March 22 lasted most of two days, while throughout the one-week decay phase, the lowest-energy ion fluxes were nearly equal at Ulysses and earth (IMP-8).

The cosmic radiation in the heliosphere at successive solar minima

Abstract: Cosmic ray observations at 1 AU are compared for the last three solar minimum periods along with the 1977/1989 and 1987 Pioneer 10 and Voyager 1 and 2 data from the outer heliosphere. There is good agreement between the 1965 and 1987 Galactic cosmic ray H and He spectra at 1 AU. Significant and complex differences are found between the 1977/1978 and 1987 measurements of the Galactic and anomalous cosmic ray components at 1 and 15 AU. In the outer heliosphere there are negative latitudinal gradients that reach their maximum magnitude when the inclination of the outer heliosphere current sheet is at a minimum. The radial gradients decrease with heliocentric distance as about 1/r exp 0.7 and do not differ significantly at the successive solar minima. The measured radial and latitudinal gradients are used to estimate the particle transport parameters in the outer heliosphere. Using the local interstellar He spectrum of Webber et al. (1987), it is estimated that the modulation boundary is of the order of 160 AU.

Northern-hemisphere observations of nearby interstellar gas : possible detection of the local cloud.

Abstract: Interstellar Ca II absorption lines on the path to 6 nearby stars from the antigalactic center hemisphere, have been measured at high resolution (R=110000) with the Aurelie spectrometer at the Observatoire de Haute-Provence 1.52 m telescope (France). The aim of this work is the detection of our ambient interstellar cloud, in which the sun is embedded, through 3D reconstruction of the velocity vector. A cloud moving at an heliocentric velocity of 25.7±0.5 km s −1 , in the same direction as the solar system interstellar flow, is the best solution to the observations, with excellent agreements in all directions

Nonlinear magnetosonic waves and mirror mode structures in the March 1991 Ulysses interplanetary event

Abstract: In examining the March 23-25, 1991 Ulysses (2.2 AU) high speed solar wind events, two distinct plasma wave modes are found: steepened magnetosonic waves with whistler precursors and mirror mode structures. These two modes are locally generated by plasma instabilities, presumably associated with anisotropies existing in the energetic shock particles and solar wind plasma, respectively. The magnetosonic waves are generated by a right-hand resonant instability associated with an about 40 keV ion beam. By an extrapolation of the results presented here, assuming microflares and nanoflares at the sun generate shocks in the lower corona and these shocks accelerate energetic ions, it is suggested that the ions, via the right-hand resonant instability, generate magnetosonic waves which steepened to form 'microshocks'. These shocks could, in turn, accelerate more energetic ions, leading to a shock/energetic ion/magnetosonic wave cascade. These newly formed magnetosonic waves and shocks presumably could propagate in a broad range of directions, leading to energy dissipation over a large region of the outer corona.

Constraints on the Acceleration of Anomalous Cosmic Rays

Abstract: The recent determination of an upper limit of 0.2 pc on the distance to the source of the anomalous cosmic rays, and the implied upper limit on their age imposes a significant observational constraint on possible acceleration mechanisms. Adiabatic cooling in the expanding wind makes the constraints significantly more stringent. In the light of these constraints, the rate of second-order Fermi acceleration is much too low to play a significant role. Diffusive shock acceleration, even at the strong termination shock of the solar wind, is sufficient only if one discards the standard expression for the maximum acceleration rate based on the Bohm limit. Only diffusive acceleration at the quasi-perpendicular solar wind termination shock has a high enough acceleration rate to be the source of the anomalous component.

The simulated features of heliospheric cosmic-ray modulation with a time-dependent drift model. I - General effects of the changing neutral sheet over the period 1985-1990

Abstract: A time-dependent drift model is used to simulate the heliospheric modulation of galactic cosmic rays, with emphasis on the effects of the wavy heliospheric neutral sheet (HNS) as a function of time during the period 1985-1990. The model predicted a clearly defined time difference between the time when minimum modulation occurred at earth and at radial distances farther away from the sun, indicating that the HNS plays an important part in establishing this observed time delay. The movements of the Voyager 1 and 2 and Pioneer 10 spacecraft were simulated in order to calculate instantaneous radial and latitudinal gradients. The time dependencies of these gradients were found to follow the observations well. The model study indicates that the HNS cannot be ignored, especially during periods of low to moderate solar activity, when the modulation of cosmic rays is described.

Solar wind Halo electrons from 1–4 AU

Abstract: Observations from the Ulysses solar wind electron spectrometer are used to make a first examination of the evolution of the solar wind suprathermal or halo electron population as a function of heliocentric distance beyond 1 AU. As the core population cools with increasing heliocentric distance, no gap is formed between the core and halo populations. Rather, the halo electrons extend to increasingly lower energies. As predicted previously on theoretical grounds, the ratio of the core electron temperature to the low energy cutoff of the halo population appears to be roughly constant with a value of about 7.5. The total integrated heat flux drops rapidly with increasing heliocentric distance; a best fit power law of R exp -2.36 is found. In addition, it is found that the ratio of the halo to core densities is roughly constant over heliocentric distance with the halo representing 4 percent of the total electron distribution. These results suggest that the halo population may not consist of truly noninteractive test particles over the heliocentric range of 1-4 AU.

Probing the heliosphere with energetic hydrogen atoms

Abstract: The idea of using energetic neutral atoms (ENAs), produced by charge exchange between energetic ions and ambient neutral atoms, as a diagnostic tool to investigate planetary magnetospheres from a distance has been extended to the investigation of the heliosphere. The paper explores what one can reasonably expect of the heliospheric ENA (HSENA) and what criteria would be imposed on HSENA instruments by concentrating on 10-10 exp 3 keV protons in quiet-time interplanetary space, solar-flare events, corotating interaction regions, and populations have distinctive signatures and that the detection of these particles can reveal energy spatial and propagation of ions in 3D interplanetary space, including the solar-wind termination shock. Such breadth of information could not be gained by in situ means.

Cyclic variation of the global magnetic field indices

Abstract: The energetical aspect of solar phenomena of different spatial and time scales has been studied with special attention to global magnetic fields. Cyclic regularities in the heliosphere are determined by energetics of global magnetic fields. The energy variation of global fields consists of a number of maxima and minima coinciding with reference points of the sunspot cycle. The correlations of a number of well-known indices in the heliosphere with Wolf numbers and with indices of energetics of the global magnetic field have been investigated. The results can be used to identify more exactly the reference points of the cycle.

Electron density fluctuations in the local interstellar bubble

Abstract: The interstellar medium in our vicinity forms an elongated cavity of X-ray-emitting gas, extending up to 200 pc from the Sun. The gas inside this local bubble is very hot (~ 10^6 K) and tenuous (electron density n _e ≈ 0.005 cm^(−3)), and is typical of material in the coronal phase of the interstellar medium, thought to occur when hot stars or supernovae blow out holes in cooler and denser interstellar gas. Little is known about turbulent density fluctuations in the coronal gas, but its physical state is important in understanding cosmic ray confinement and the energy balance of interstellar material, as well as the scattering of radio emission from compact sources. To probe turbulence in the local interstellar medium, we have made scintillation measurements at 50 MHz (6 m wavelength) of emission from the nearby pulsar 0950 + 08, which happens to lie near the edge of the local bubble. From the scintillation bandwidth we deduce the amplitude of the electron-density fluctuation spectrum in the coronal gas alone, and find it to be an order of magnitude lower than for any previously measured interstellar line of sight. We conclude that the interior of the bubble is relatively quiescent and that high levels of plasma turbulence—if they exist—must be localized to the cavity boundary.

A Boltzmann-kinetic approach to describe the entrance of neutral interstellar hydrogen into the heliosphere

Abstract: The neutral component of the local interstellar medium can sweep over the plasma borders of the solar system and can enter deeply into the heliosphere prior to ionization. In contrast to helium, hydrogen, while entering, suffers from intensive charge exchange interactions with protons in the perturbed interface plasma flows ahead of the inner heliosphere. This charge exchange influence leads to an important, fractional depletion of interstellar hydrogen and to a modulation of its velocity distribution function dependent on the position in the heliosphere. Here we shall give a quantitative description of these modulations by means of a Boltzmann-kinetic approach to this problem

Intensity variations in the interplanetary magnetic field measured by Voyager 2 and the 11‐year solar cycle modulation of galactic cosmic rays

Abstract: New evidence is presented to support the hypothesis that the 11-yr solar cycle modulation of galactic cosmic rays is caused by strong diffusion inside long-lived merged interaction regions. To test this hypothesis, the 1D force-field approximation of the cosmic ray modulation equation is solved. It is assumed that a constant solar wind speed convects magnetic field compressions and rarefactions unchanged through a model heliosphere. The result is a reasonable simulation of the integrated high-energy cosmic ray intensity profile from about 1982 to mid-1989. This period encompasses both the full recovery portion of the last profile from about 1982 to mid-1989. This model responds to the Voyager 2 magnetic field data by correctly timing the beginning of the new modulation cycle in late 1987. It is concluded that the present hypothesis is consistent with the results of this simulation.

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

Abstract: 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.

Acceleration of the solar wind

Abstract: Different approaches to understanding the physics of solar wind acceleration are reviewed. Particular attention is given to fundamental reasons for a supersonic wind concept; the concept of thermal conduction as the primary energy transport mechanism in the solar wind; coronal holes as the source of wind and alternative acceleration mechanisms; and the state of closure of theory and observation.

Propagation conditions of relativistic electrons in the inner heliosphere

Abstract: The intensity and anisotropy time profiles of six relatively small relativistic electron events observed within 0.65 AU by one of the Helios spacecraft were fitted with the model of focused transport under the assumption of a particle mean free path λ independent of the radial distance. For ∼0.5 MeV electrons it is found that the amount of interplanetary scattering varies from one solar event to the other, with local mean free paths λ between 0.02 and 0.15 AU. Comparison with previous results obtained close to 1 AU shows that there is no marked variation in the average scattering conditions with radial distance

Ulysses: Interplanetary shocks between 1 and 4 AU

Abstract: The complex solar events of March 1991 are evident as a large increase in the rate of occurrence of interplanetary shocks. Using Ulysses magnetic field and plasma measurements, 32 forward shocks and 7 reverse shocks have been identified in the 280 day interval from October 26, 1990 to August 1, 1991. The March events alone have produced 9 shocks, several in association with coronal mass ejections. The shocks have been identified and analyzed to find theta(BN), the speeds in the upstream solar wind, the Mach number, and the inertial speeds along the radial and magnetic field directions.

The interstellar neutral-gas experiment on ULYSSES

Abstract: The properties (density, bulk velocity relative to the solar system, and temperature) of the local interstellar gas, represented by neutral helium penetrating the heliosphere, will be measured in-situ for the first time by the ULYSSES GAS instrument By employing the solar gravitational field as a natural velocity analyser, the bulk velocity relative to the solar system and the temperature of the gas can be derived from the angular distributions of the particles measured in at least two widely separated points in the heliosphere The gas density can be determined if a composition corresponding to cosmic abundances is assumed

Observation and simulation of the radial evolution and stream structure of solar wind turbulence

Abstract: This paper presents a brief overview of the observed evolution in a variety of quantities describing the turbulent evolution of the interplanetary plasma and describes simulation results consistent with many features of the evolution. The turbulence is manifested through a dissipation at small scales in the inner heliosphere with a corresponding evolution in the breakpoint between a relatively flat and a Kolmogoroff spectrum; an evolution from kinetically to (slightly) magnetically dominated energy of the plasma fluctuations; a general decrease in the cross helicity or 'Alfvenicity'; changes in the anisotropy of the fluctuations; and the increasing predominance of quasi-pressure-balanced structures in the compressive component of the fluctuations. MHD simulations with shear layers either side of a central current sheet show that even in the absence of compressibility the lack of a mean field along the direction of the main flow in the current sheet leads to rapid nonlinear evolution and the observed characteristics of 'Elsasser spectra' of the fields in the inner heliosphere. Adding compressibility to the simulations does not greatly change the 'incompressive' quantities but leads in addition to observed correlations between a measure of compression and other quantities.

Galileo and Ulysses dust measurements: Fz Venus to Jupiter

Abstract: The Galileo and Ulysses spaceprobes carry two similar dust detectors through interplanetary space from Venus to Jupiter. Impacts are reported which correspond to dust particles above a mass threshold of about 10 exp 13 g for which complete records exist. Between December 1989 and January 1992 Galileo repeatedly traversed interplanetary space between 0.7 and 2.26 AU and recorded 374 impacts. The observed impact rates ranged from 0.1 to about 3 impacts per day strongly dependent on whether the spacecraft moved toward or away from the sun. From October 1990 to January 1992 the Ulysses spacecraft had reached a distance of 5.17 AU from the sun and had recorded 72 impacts at rates between 0.1 and 0.5 per day. Inside about 2 AU the observed fluxes are compatible with a population of interplanetary dust particles moving on low to moderately eccentric and low inclination orbits. Outside this distance a dust particle population on different orbits is required in order to explain the Ulysses data.

Outer heliospheric radio emissions: 1. Constraints on emission processes and the source region

Abstract: Observations by Voyager 1 and 2 of LF radio emissions near 2 and 3 kHz during the interval 1983-1987 while at heliocentric distances from 15 to 27 AU and 11 to 20 AU, respectively, are reported. Constraints on the emission processes and source characteristics are discussed. Minimum Langmuir wave electric fields in the source region, based solely on the kinematics of the radiation processes, lie in the range 1-100 micro-V/m for nominal source and electron beam parameters. Path lengths for fundamental and harmonic emission processes are calculated. The observed levels of radiation can be produced in path lengths and source dimensions smaller than 1 AU provided the participating Langmuir waves have effective temperatures of about 10 exp 17 K. Previous source models for the radiation are discussed.

Investigation of macroscopic instabilities at the heliopause boundary surface

Abstract: In many papers of the recent past stationary models for the plasma flow configuration in the interface region between the unperturbed interstellar plasma flow and the solar wind plasma flow have been presented and discussed. As a consequence of an MHD-type interaction of these flows a free pressure equilibrium surface, called the heliopause, is formed with a frontal nose part in the upwind direction and a tail region pointing into the downwind direction. It has, however, been left totally unclear in all of these models whether or not the derived stationary heliopause configurations are stable when subject to magnetohydrodynamical or hydrodynamical perturbations connected with time-dependent or turbulent structures in the counter-flowing plasmas

On the role of interplanetary sources in the evolution of low-frequency Alfvénic turbulence in the solar wind

Abstract: The Alfvenic turbulence observed in the interplanetary space is mainly made up of outward traveling waves of solar origin. Inward waves, although a minor component of the turbulence, play an important role in transferring energy between different scales through nonlinear interactions with outward modes. Inward waves can only have an interplanetary origin. A good understanding of how inward waves are generated in the interplanetary space is crucial in modeling the Alfvenic turbulence evolution. Solar wind regions with high differential kinetic energy, which can be converted into turbulent energy, are the best places to search for local sources of Alfvenic turbulence. We have studied one of these regions at different heliocentric distances inside 1 AU, focusing on large-scale fluctuations (periods approximately between 1 and 12 hours). Such a region was indicated by previous investigations as one of the best candidates for turbulence generation. In fact, fluctuations with an inward sense of correlation are observed, but a detailed analysis leads us to favor the hypothesis that they are mainly due to variations in plasma velocity, density, and magnetic field related to structures convected by the solar wind and not to real inward propagating waves. At least in the inner heliosphere, a view in which local sources of waves play a relevant role in driving the Alfvenic turbulence evolution does not seem realistic. We propose that such evolution, especially at the larger scales, is mainly governed not by interplanetary sources but rather by interactions of the outward waves of solar origin with the wind structure.

Outer heliospheric radio emissions: 2. Foreshock source models

Abstract: Observations of LF radio emissions in the range 2-3 kHz by the Voyager spacecraft during the intervals 1983-1987 and 1989 to the present while at heliocentric distances greater than 11 AU are reported. New analyses of the wave data are presented, and the characteristics of the radiation are reviewed and discussed. Two classes of events are distinguished: transient events with varying starting frequencies that drift upward in frequency and a relatively continuous component that remains near 2 kHz. Evidence for multiple transient sources and for extension of the 2-kHz component above the 2.4-kHz interference signal is presented. The transient emissions are interpreted in terms of radiation generated at multiples of the plasma frequency when solar wind density enhancements enter one or more regions of a foreshock sunward of the inner heliospheric shock. Solar wind density enhancements by factors of 4-10 are observed. Propagation effects, the number of radiation sources, and the time variability, frequency drift, and varying starting frequencies of the transient events are discussed in terms of foreshock sources.