Ulysses Mission Research Articles

Modeling accelerated pick-up ion distributions at an interplanetary shock

The acceleration of interstellar pick-up ions as well as solar wind species has been observed at a multitude of interplanetary (IP) shocks by different spacecraft. The efficiency of injection of the pick-up ion component differs from that of the solar wind, and is expected to be strongly enhanced at highly oblique and quasi-perpendicular shock events, in accord with inferences from in situ observations. This paper explores theoretical modeling of the phase space distributions of accelerated ions obtained by the Ulysses mission for the Day 292, 1991 shock associated with a corotating interaction region, encountered before Ulysses’ fly-by of Jupiter. A Monte Carlo simulation is used to model the acceleration process, adapting a technique that has been successfully tested on earlier IP shocks possessing minimal pick-up ion presence. Phase space distributions from the simulation technique for various low mass ions are compared with SWICS and HI-SCALE data to deduce values of a “turbulence parameter” that controls the efficiency of injection, and the degree of cross-field diffusion. Acceptable fits are obtained for the H + and He + populations using standard prescriptions for the pick-up ion distribution; He ++ spectral data was only fit well for scenarios very close to the Bohm diffusion limit. It is also found that the simulation successfully accounts for the observation of energetic protons farther upstream of the forward shock than lower energy pick-up protons, using the same turbulence parameter that is required to achieve reasonable spectral fits.

On the determination of energy spectra of MeV electrons by the Ulysses COSPIN/KET

The Ulysses mission has provided a wealth of data, particularly regarding the transport of low-energy cosmic ray electrons. These data have been used to derive significant constraints for the anisotropic spatial diffusion of these particles. Detailed model simulations allowed, in addition, to determine the relative contributions of galactic and Jovian electrons to the total flux at a given time and position in the heliosphere. Despite these insights, energy spectra have not been reliably determined as yet. This is a consequence of the uncertainty due to a background connected to proton interactions with the spacecraft. Recently, however, it was demonstrated that this uncertainty can, with some difficulty, be reduced, thus opening the opportunity to understand such spectra in the energy range 3–30 MeV, i.e., the part mostly dominated by Jovian electrons. We present results of a corresponding re-analysis of COSPIN/KET data.

The transport of galactic and jovian cosmic ray electrons in the heliosphere

An overview is given on what we know about the cosmic ray diffusion process from the modelling of low-energy (MeV) electron transport in the heliosphere. For energies below ∼300 MeV, these electrons give a direct indication of the average mean free paths because they do not experience large adiabatic energy changes and their modulation is largely unaffected by global gradient and curvature drifts. Apart from galactic cosmic ray electrons, the jovian magnetosphere at ∼5 AU in the ecliptic plane is also a relatively strong source of MeV electrons, with energies up to ∼30 MeV. Therefore, when modelling the transport of these particles in the inner heliosphere, a three-dimensional treatment is essential. By comparing these models to observations from the Ulysses, Pioneer and Voyager missions, important conclusions can be made on e.g., the relative contributions of the galactic and jovian electrons to the total electron intensity, the magnitude of the parallel and perpendicular transport coefficients, and the time dependant treatment thereof.

Solar Orbiter: A unique opportunity for investigating small-scale physical processes at work in the magnetic solar atmosphere

With knowledge gained from the recent ULYSSES, SOHO, TRACE and other missions, a great deal of progress has been made in our understanding of the structure and dynamics of the various regions of the solar atmosphere: e.g., transition region and “quiet” corona, coronal holes and active region loops. However, no clear conclusions can yet be drawn about the fundamental processes that determine the existence and the underlying physics of these regions, especially their observed high temperature and flows. The lack of adequate spatial resolution has limited the study of the continuum between cool/hot, source/sink, and magnetic/non-magnetic regions. We consider here, magnetic reconnection as an example of the basic processes that are most likely involved at many different scales in heating and particle acceleration. Starting with a possible mechanism for reconnection (flux emergence), we consider different remote-sensing signatures of reconnection: flows, field rearrangement, impulsive heating, particle acceleration, wave generation, propagation and dissipation. We discuss their small-scale properties derived from current measurements and ways to improve their observation. We also consider how remote sensing and in situ measurements could be coordinated. To demonstrate how these advances might be achieved, we discuss the potential for discoveries offered by the Solar Orbiter, resulting notably from its relatively high spatial resolution and its capacity to observe for a substantial time in co-rotation with the Sun.

Transition to solar minimum at high solar latitudes: Energetic particles from corotating interaction regions

One of the key questions to be addressed during the solar maximum phase of the Ulysses mission has been the nature of the ubiquitous energetic particle populations observed at all helio‐latitudes from equator to poles. During the current, post‐maximum phase of its mission, Ulysses has encountered the return to more stable solar wind, high speed stream structure, leading to the formation of stream and/or corotating interaction regions, in addition to the coronal mass ejection associated transients. The analysis reported here presents the identification of the first CIR after the recent solar maximum recorded by Ulysses COSPIN/LET at high helio‐latitudes based on energetic particle composition data recorded in the ∼5–30 MeV/n range. The recurrent compression region appears during 3–6 solar rotations in 2002 and is observed from ≈52°N to ≈37°N heliographic latitude at heliocentric distances from ≈3.1 AU to ≈3.8 AU.

Life-cycle emission of oxidic gases from power-generation systems

During the 1980s and 1990s the U.S. sponsored a number of space nuclear power and propulsion programs and studies covering many different mission plans. These space nuclear activities spanned the range from the 300-watt radioisotope thermoelectric generators currently in use on the Galileo mission to Jupiter and the Ulysses mission to explore the polar regions of the Sun to multi-megawatt space nuclear reactor concepts that could be used to provide power for neutral particle beams, free electron lasers, electromagnetic launchers, and orbital transfer vehicles. Studies were undertaken on both nuclear electric propulsion (NEP) and nuclear thermal propulsion (NTP) largely in support the proposed Space Exploration Initiative (SEI). Upgrades to radioisotope power sources involving dynamic (turbine-alternator and linear oscillator) and improved static (thermophotovoltaic and alkali-metal thermal-to-electric conversion) conversion systems were investigated. A major undertaking in the 1980s and early 1990s was the SP-100 space nuclear reactor power system which was designed to span the power range from 10 kWe to 1000 kWe to encompass most high-power missions then being studied. In addition, various efforts to explore the uses of thermionic reactors were undertaken, including an initiative with Russia to test the ENISY (TOPAZ II) thermionic reactor. This paper summarizes these activities and highlights future trends in an era of smaller spacecraft that will be built and launched in shorter periods of time under more tightly constrained budgets.

Alfvén waves, magnetic decreases, and discontinuities in interplanetary space

The Ulysses mission goes to the polar regions of the heliosphere and therefore is an ideal mission to study Alfvén waves and discontinuities. The polar region is dominated by a high speed solar wind with embedded outwardly propagating nonlinear (|Δ B|/ B 0 ∼ 1 to 2) Alfvén waves. Rotational discontinuities are detected at the edges of Alfvén waves and have been shown to be the phase-steepened edges of the waves. Dissipation at these phase-steepened Alfvén waves has been sought in the plasma and magnetic field data, but not found. Another phenomenon detected at high latitudes is magnetic field decreases (MDs) bounded by discontinuities. One-dimensional simulation results suggest that MDs may be a product of the evolution of nonlinear Alfvén waves.

A review of discontinuities and Alfvén waves in interplanetary space: Ulysses results

The Ulysses mission is the first to explore our heliosphere at all latitudes up to ±80° and therefore is an ideal mission to study potential gradients in heliolatitude (and radial distance) of discontinuity occurrence rates and types. Directional discontinuities (DDs) are shown to be dependent on the type of solar wind streams that they are embedded in. The occurrence rate of DDs is 5–10 times higher in high‐speed streams than in slow streams. The explanation is that nonlinear Alfvén waves dominate the high‐speed streams and rotational discontinuities are the phase‐steepened edges of the Alfvén waves. Dissipation at these phase‐steepened Alfvén waves have been sought but not found. An e−(R−1)/5 decrease in discontinuity rate with increasing radial distance (R in units of AU) is partially an artifact of the selection criteria (discontinuity thickening), but dissipation at a relatively slow rate cannot be ruled out at this time. There is no obvious latitudinal gradients in discontinuity types or occurrence rates. Somewhat surprisingly, tangential discontinuities are detected at high latitudes. These have been associated with the edges of local small‐scale magnetic decreases. A pair of slow shocks were detected at 5.3 AU. The speeds are similar to fast mode shock speeds. When Alfvén waves in high‐speed streams impinge upon the Earth's magnetosphere, near‐continuous substorms (called HILDCAAs) occur, leading to the pumping of an extraordinary amount of energy into the nightside ionosphere. Current discontinuity and Alfvén wave research problems are discussed.

Sputtering efficiency of LiF surfaces on impact of low energy neutral helium (20–80 eV): Calibration of the interstellar neutral helium instrument on ULYSSES

For the detection of interstellar neutral helium particles entering the inner solar system, a novel detection technique has been developed using sputtering of secondary ions from lithium-fluoride (LiF) surfaces on the impact of He particles. In order to be able to determine the interstellar He fluxes, the detection efficiency had to be determined in an energy range Ekin≈20 to 200 eV, an energy range for which data were not available in literature. Therefore, a calibration had to be performed in a dedicated accelerator system providing a neutral beam in the required energy range. While precise values could be obtained for energies above about 50 eV, significant uncertainties in the efficiency remained for lower energies, due to the difficulty in determining the absolute flux of the primary beam after acceleration, as elastic scattering of particles out of the beam geometry during the charge exchange process becomes more important. Corrections have been applied using a theoretical simulation of this process. These uncertainties have subsequently been eliminated, using the inflight measurements of neutral He during the ULYSSES mission. Due to the orbital parameters of this spacecraft, the relative energy of the interstellar He particles in the instrument’s reference system varies in the range 20–80 eV. Assuming that the interstellar He flow is stationary, i.e., it shows only negligible time variations, constant count rates would be expected over this energy range. The energy dependence in the actually observed count rates has been used to refine the efficiency calibration at low energies (⩽40 eV).

Solar wind electrons: Parametric constraints

Solar wind electrons are often observed to consist of two distinguishable components, a thermal, more dense core and a suprathermal, less dense halo. In this core/halo model linear Vlasov theory for the whistler heat flux instability predicts dimensionless heat flux thresholds which decrease as the electron core beta, , increases. It has been proposed that this theoretical threshold corresponds to an observable upper bound on the electron heat flux. Linear theory also predicts that there is a critical value of below which the whistler heat flux instability does not have appreciable growth in the solar wind; there is another suggestion that this corresponds to an observable lower bound on . These two proposals are examined by comparison of linear theory and data from the initial in‐ecliptic phase of the Ulysses mission. The instability threshold does provide a statistical constraint on observed solar wind heat fluxes, and the critical of theory is not inconsistent with a statistical lower bound on the observations of that parameter.

The electron heat flux in the polar solar wind: Ulysses observations

Ulysses measurements of the solar wind electron heat flux as a function of heliographic latitude are presented. Contamination from a light leak at the highest latitudes has been removed from the data by interpolating between uncontaminated pixels. After normalization by the radial gradient in the electron heat flux obtained during the inecliptic phase of the Ulysses mission, the electron heat flux is essentially constant from −80° to 80° of heliographic latitude. During brief periods of the fast latitude scan, both uncorrelated and anti‐correlated modulations of the electron heat flux and the solar wind velocity are observed. The implications of the observations with regards to different models of electron heat flux regulation are discussed.

Electron heat flux constraints in the solar wind

Enhanced fluctuations from electromagnetic heat flux instabilities may, through wave-particle scattering, constrain the electron heat flux which flows parallel to the background magnetic field in the solar wind. A corollary of this hypothesis is that instability thresholds should correspond to observable bounds on the heat flux. Here plasma and magnetic field data from February and March 1995 of the Ulysses mission is analyzed in terms of the core/halo electron model to yield scaling relations of dimensionless electron parameters and empirical upper bounds on the dimensionless heat flux as functions of the core β. Use of these scaling relations in linear Vlasov theory for the whistler and Alfvén heat flux instabilities in homogeneous plasmas yields threshold conditions on the dimensionless heat flux which are also functions of the electron core β. The empirical bounds and the theoretical thresholds are similar and are therefore consistent with the hypothesis.

Bulk flows of hot plasma in the Jovian magnetosphere: A model of anisotropic fluxes of energetic ions

Since the Ulysses flyby of Jupiter in February 1992, a consistent picture of the bulk flows of hot plasma has yet to be formed. The present study reports a comprehensive analysis of energetic particle anisotropies from the Ulysses mission. We present an empirical model which separates flow from gradient anisotropies. The model employs a multidimensional minimization routine to fit simultaneously 13 free parameters using 98 measurements. The large number of independent measurements is possible because the Ulysses Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energy (HI‐SCALE) has nearly full angular coverage at high time resolution and moderately high angular resolution. The model uses the full energy range (∼50–3000 keV) of the low‐energy magnetic spectrometers and also the pure proton channels of the composition aperture on the instrument. We present flow results during the inbound pass of Ulysses in which the radial velocity component shows a persistent radial outflow. The azimuthal component is in the sense of planetary rotation but generally below rigid corotation, and the azimuthal velocity is greater near the plasma current sheet than at higher magnetic latitudes. Outbound, the flows in the high‐latitude duskside magnetosphere inferred from the particle anisotropies indicate, surprisingly, sunward flows opposite to planetary rotation. We find evidence of gradient anisotropies especially near the magnetospheric current sheet on the dayside. We present a picture of the global magnetic field configuration and flows of hot plasma, consistent with our model results, dominated by subcorotation at low latitudes but by sunward flow at high latitudes.

A comparison of recurrent energetic ion enhancements observed at Ulysses and at 1 AU by IMP 8 and SAMPEX: Ulysses launch until following the first north polar passage

During the Ulysses mission, the Goddard Space Flight Center instrument on IMP 8 and the University of Maryland low energy ion composition analyzer instrument on SAMPEX observed recurrent MeV/amu ion enhancements in the ecliptic plane at 1 AU. We identify those IMP 8/SAMPEX events related to the recurrent events observed at Ulysses from its launch until mid‐1996, following the first passage over the north solar pole, and compare their properties. Two types of recurrent events were present at Earth. Recurrent enhancements of particles streaming sunward from CIRs in the outer heliosphere were observed most clearly at Earth after the Ulysses Jupiter encounter. Most of the CIRs/particle enhancements at Ulysses, even at high heliolatitudes, can be associated with recurrent events/high‐speed streams at Earth by considering simple corotation delays. This suggests that the events at high latitudes occurred poleward of equatorward extensions of the polar coronal holes which produced corotating high‐speed streams in the ecliptic. The association between events at Ulysses and Earth was less clear in 1996 when there were few major streams from polar coronal hole extensions present near the ecliptic. At the furthest distance of Ulysses from the Sun (5.4 AU), the intensity of ∼1 MeV/amu ions in CIR‐associated events was 1–2 orders of magnitude higher than at Earth. The intensity at Ulysses then declined as the spacecraft moved closer to the Sun and to high latitudes to reach < 0.1 % of that at Earth near polar passage. The observations confirm that the decrease in the intensity of corotating events at high latitudes was predominantly due to the changing spacecraft latitude rather than to a temporal decline in the intensity of corotating events throughout the heliosphere. We suggest that some of the north‐south asymmetries in the particle events observed by Ulysses at high latitudes may be associated with differences in the solar wind structure and energetic particle events at low latitudes during north and south polar passage. In contrast to these events, the prominent recurrent events observed at Ulysses and at Earth during the second half of 1991 were associated with solar activity which, over a period of several solar rotations, was largely confined to a limited range in Carrington longitude.

The acceleration of charged particles in corotating interaction regions (CIR) : A review with particular emphasis on the Ulysses mission

Observations on the acceleration of charged particles to higher energies in corotating interaction regions in interplanetary space are reviewed. Particular emphasis is given to the Ulysses observations which at the moment provide the latest extensive set. By comparison with Pioneer 10/11, Voyager, ISEE 3, and IMP 8, data collected during solar minimum periods, the nature of these phenomena now become much clearer. The introduction describes the formation and peculiarities of corotating interaction regions starting with a brief description of the formation of shocks in the interplanetary medium. Section 2 gives a brief description of the Ulysses mission and its orbit and is followed in Section 3 by the observational material. It is organized such that after earlier observations are introduced upstream effects are described, then we mention the different shapes of energy spectra, and describe what is known on elementary composition. Particular emphasis is given to the observations of the proton to alpha particle ratio, which is related to the important question on the origin of the accelerated particles: Are there energetic seed particles of some 10 keV energy, or do the accelerated particles stem from the solar wind flow reservoir? Are there other sources? After the angular distribution of the accelerated population are described we turn to the question of the efficiency of acceleration by corotating shocks: Is the efficiency a function of certain shock parameters only? Is there a latitudinal dependence? Can the observations be understood by assuming a pure heliodistance dependence? We also discuss the acceleration of electrons in corotating shocks to quite high energies (which is not understood), and mention the effects of corotating structures on extremely high energetic particles like the cosmic rays or the anomalous component before closing this section with a summary of the observational facts. Section 4 briefly summarizes theoretical aspects of the acceleration of particles by CIR-shocks and finally we discuss the material with particular emphasis on three aspects: The latitudinal and/or heliodistance dependence of the phenomena, the elemental composition and the role of pick up ions in the Heliosphere, especially with respect to acceleration in corotating shocks.

Latitude Manifestations of the Solar Wind

AbstractDependencies of solar wind parameters as a function of latitude are presented and discussed, based on measurements of the Ulysses mission during times of declining or minimum solar activity. The heliosphere is found to be structured into a band of slow solar wind at low latitudes and two large high-speed streams from the polar coronal holes. Only small trends of solar wind parameters with heliolatitude are found within these streams. A small but significant temperature difference is seen between the south and the north polar coronal hole.

The Ulysses north polar pass: Latitudinal gradients of anomalous cosmic ray O, N And Ne

A key finding of the Ulysses mission has been the absence of large latitudinal gradients in the Anomalous Cosmic Ray (ACR) component. Studies using data from several satellites have consistently shown a small (∼ 2%/deg.) positive latitudinal gradient for anomalous O, N and Ne in the energy range 4 to 30 MeV/n, and even smaller gradients for ACR He. In this paper we use data from the COSPIN/LET experiment on board Ulysses to investigate (a) the behaviour of the ACR latitudinal gradients during the passage over the northern polar regions, and (b) possible north‐south asymmetries. The descent from the north pole has taken place under solar minimum conditions, providing a unique opportunity to study fully developed latitudinal effects. Our data survey covers three ACR species, O (4 to 20 MeV/n), N (4 to 20 MeV/n) and Ne (4 to 30 MeV/n). ACR gradients have been calculated for three different regions: 0°–20° (∼ 5%/deg.), 20°–60° (∼ 3%/deg.) and 60°–80.2° (∼ 1%/deg.). Comparison of data from the northern and southern hemishere at a given latitude shows an asymmetry with higher fluxes in the north than in the south at a given latitude.

Ulysses observations of whistler waves at interplanetary shocks and in the solar wind

This study of whistler wave emission observed by the Ulysses Unified Radio and Plasma Wave (URAP) experiment between 1 and 5 AU is a complement to previous studies of whistler waves observed by the Helios spacecraft between 0.3 and 1 AU. The Helios spacecraft continuously detected a background of whistlers close to the Sun, and this background was found to decrease in intensity with larger heliocentric distance. Ulysses plasma wave observations confirm this trend. Within a heliocentric distance of approximately 2 AU, whistler waves are routinely observed. Beyond about 3 AU, the waves are usually observed only downstream of interplanetary shocks. Moreover, whistler waves are routinely observed within about 2 AU at all heliographic latitudes of the Ulysses trajectory (−80° to +80°). The combined observations from the Helios and Ulysses spacecraft suggest that whistler emission is always present in the solar wind, although at larger heliocentric distances the wave amplitudes are often below the thresholds of the URAP instrument. Observations throughout the first 5 years of the Ulysses mission show a clear correlation of whistler emission intensity with magnetic field strength, or gyrofrequency, such that increases in wave intensities coincide with increases in gyrofrequency. This correlation is especially evident in observations of interplanetary shocks and high‐speed streams. A possible cause of this correlation is increased whistler wave growth due to enhanced electron temperature anisotropies in regions of compressed magnetic field. A shift of the background whistler spectrum as a function of gyrofrequency could account for the observed decrease in whistler amplitudes with increasing heliocentric distance.

A summary of solar wind observations at high latitudes: Ulysses

The Ulysses mission has provided the first in-situ observations of the solar wind covering all solar latitudes from the equator to the poles in both hemispheres. The measurements from the first polar passes, made at near-minimum solar activity conditions, have confirmed the basic picture established on the basis of remote sensing techniques: the high-latitude wind is fast, and originates in the polar coronal holes. The detailed in-situ observations have, however, revealed a number of features related to the global solar wind structure that were not expected: the transition between slow and fast wind was relatively abrupt, followed by a slight increase in speed toward the poles; the mass flux is almost independent of latitude, with only a modest increase at the equator; the momentum flux is significantly higher over the poles than near the equator, suggesting a non-circular cross-section for the flanks of the heliosphere.

Space nuclear power - An overview

Nuclear power sources have enabled or enhanced some of the most challenging and exciting space missions yet conducted, including missions such as the Pioneer flights to Jupiter and Saturn; the Voyager flights to Jupiter, Saturn, Uranus, and Neptune; the Apollo lunar surface experiments; the Viking Lander studies of Mars; the Ulysses mission to study the polar regions of the sun; and the Galileo mission to orbit Jupiter. This article surveys the early missions and continues to the present Galileo and Ulysses missions and provides a summary of the power technology that was used in each mission. Current activities aimed at improving the specific power and efficiency of space nuclear power sources are summarized. In addition to radioisotope power sources, this article surveys the U.S. space nuclear reactor program focusing on the flight of the SNAP-lOA reactor and the technology developed in the SP-100 space nuclear reactor power system program. The general attributes of space nuclear power sources are described and possible future applications are identified.