Ulysses Observations Research Articles

Oxygen flux in the solar wind: Ulysses observations

We use the complete set of Ulysses solar wind data to conduct a comprehensive determination of the oxygen flux in relation to the proton flux in the solar wind during a wide range of solar activity levels. The data cover the heliosphere between 1.3 and 5.4 AU and, due to the unique orbit of Ulysses, all of the heliographic latitudes within ±80°. We find log‐normal distributions for O and H daily flux values, but with significant differences between slow and fast wind. Coronal hole‐associated fast wind has a distribution that is approximately three times narrower than the one of slow wind associated with streamers. Finally, we derive the solar oxygen abundance, finding a value of 8.82 ± 0.08, and discuss this value in comparison with the results of spectroscopic determinations.

Modeling the 2003 Halloween events with EMMREM: Energetic particles, radial gradients, and coupling to MHD

present a study of the October/November 2003 Halloween solar energetic particle events with an energetic particle acceleration and propagation model that is part of EMMREM, highlighting the current ability of the framework to make predictions at various locations of the inner heliosphere. We compare model predictions with Ulysses observations of protons at energies above 10 MeV in order to obtain realistic proton fluxes and calculate radial gradients for peak fluxes, event fluences, and radiation dosimetric quantities. From our study, we find that a power law with an index of −3.55 at energy of 200 MeV describes the time‐integrated energetic proton fluence dependence on radial distances beyond 1 AU for the 2003 Halloween events, and an index of −4.18 is appropriate for peak proton fluxes at that energy. Calculations of radiation doses based on these simulations show average power law indices of −4.32 and −3.64 for peak dose rates and accumulated doses, respectively. In an effort to improve the predictions, we have coupled our kinetic code to results from a 3‐D heliospheric magnetohydrodynamic model, WSA/Enlil. While predictions with the coupled model overall show worse agreement than simulations with steady state solar wind conditions for these large events, the capability to couple energetic particle propagation and numerical models of the solar wind is an important step in the future development of space weather modeling.

Propagation of Jovian electron jets in heliospheric flux tube structures

We discuss the propagation of magnetic flux tubes of solar origin through the heliosphere up to the orbit of Jupiter and the possibility that Jovian electron jets result from the interaction of these flux tubes with the magnetosphere of Jupiter. In order to do so, we analyze Ulysses observations in the interplanetary medium along the spacecraft's highly inclined orbit between the orbits of Earth and Jupiter. We apply a method originally developed for the analysis of ACE measurements at 1 AU, in which it was shown that the distribution of the relative change in plasma quantities can actually be attributed to plasma turbulence on the one hand and the existence of individual flux tubes in the solar wind on the other hand. The application to Ulysses data shows that this typical behavior can be observed even up to the orbit of Jupiter. In a first step, we discuss possible consequences of the existence of these flux tubes at such large distances for the transport of charged particles through the inner heliosphere. In a second step, we analyze observations of so‐called Jovian electron jets with Ulysses and Pioneer 10 data with regard to changes in the magnetic field and interpret these as the result of the magnetic interaction of flux tubes with the Jovian magnetosphere.

Asymmetry of solar polar fields and the southward shift of HCS observed by Ulysses

We study the hemispheric asymmetry of high‐latitude unipolar fields and the latitudinal shift of the heliospheric current sheet (HCS) using Ulysses magnetic field observations during the perihelion passes in 1994–1995, 2001–2002, and 2007. Using the cumulative flux density and the best fit lines to its high‐latitude observations in the two hemispheres, we find that the absolute value of the high‐latitude radial field of the Southern Hemisphere is larger than in the north during both minimum time scans in 1994–1995 and 2007. The hemispheric difference is about 0.2 nT during both scans, suggesting that the northern field area is some 5%–10% (5%–15%) larger than the southern area and that the HCS is shifted southward by about −2° during both scans. The results resolve the discrepancy between earlier results in 1994–1995 and clarify similar observations in 2007 in the ecliptic. They also verify the southward shift of the HCS during the exceptional solar cycle 23. We also study the detailed structure of the equatorial region and find that generally the Ulysses observations compare favorably with simultaneous heliospheric magnetic field predictions given by Wilcox Solar Observatory synoptic maps. Using a simple HCS model, we find that even in case of southward shifted HCS, the highest peak of the cumulative flux density could be located above the equator. Thus, Ulysses observations around the heliographic equator cannot alone give an unambiguous information of the HCS shift, which emphasizes the importance of studying the high‐latitude sections of Ulysses orbit.

DEMONSTRATIONS THAT THE SOLAR WIND IS NOT ACCELERATED BY WAVES OR TURBULENCE

The present work uses observations and theoretical considerations to provide both qualitative and quantitative arguments that hydromagnetic waves, whether turbulent or not, cannot produce the acceleration of the fast solar wind and the related heating of the open solar corona. Waves do exist, and can play a role in the differential heating and acceleration of minor ions, but their amplitudes are not sufficient to power the wind, as demonstrated by extrapolation of magnetic spectra from Helios and Ulysses observations. Dissipation mechanisms invoked to circumvent this conclusion cannot be effective for a variety of reasons. In particular, turbulence does not play a strong role in the corona as shown both by observations of coronal striations and other features, and by theoretical considerations of line tying to a nonturbulent photosphere, nonlocality of interactions, and the nature of the kinetic dissipation. We consider possible ways out of the arguments presented, and suggest that in the absence of wave or turbulent heating and acceleration, the chromosphere and transition region become the natural source, if yet unproven, of open coronal energization through the production of nonthermal particle distributions.

PLASMA NEAR THE HELIOSHEATH: OBSERVATIONS AND INTERPRETATIONS

Voyager 2 (V2) has observed heliosheath plasma since 2007 August. The plasma flux decreases by 25% before the termination shock (TS), then, as V2 moved into the heliosheath, the plasma density, temperature, and flux all decreased by an additional factor of 2. We suggest three effects combine to cause these decreases. (1) V2 moved into the lower-flux transition region between the low- and high-speed solar wind. This hypothesis is consistent with Ulysses observations of the transition location, explains the 25% decrease in solar wind flux observed before the TS crossing, and can reconcile discrepancies between the V2 and Voyager 1 heliosheath speeds and between the V2 speeds and model results. (2) The weaker source at the Sun. (3) The heliosheath plasma turning and flowing toward the heliotail.

Introduction to special section on the Earth‐Moon‐Mars Radiation Environment Module

[1] The United States is preparing to return humans to the Moon and is setting the stage for exploration to Mars and beyond. However, it is unclear if long missions outside of low-Earth orbit (LEO) can be accomplished with acceptable risk. The central objective of the NASA Living With a Star Earth-Moon-Mars Radiation Environment Module (EMMREM) is to develop and validate a numerical module for completely characterizing time-dependent radiation exposure in the Earth-Moon-Mars and interplanetary space environments. EMMREM currently provides the capability to predict radiation exposure in the interplanetary environment outside of Earth’s protective atmosphere and magnetosphere and at various heights of a nominal Mars atmosphere. Ongoing and future efforts will fold in the effects of Earth’s atmosphere and the geomagnetic field so that the radiation environment can be predicted near the Earth’s surface, at LEO, and at various geomagnetic latitudes. EMMREM is being designed for broad use by researchers to predict radiation exposure by integrating over time-evolving incident particle distributions from interplanetary space. The EMMREM represents a growing and developing system of coupled models that describe particle acceleration and transport in interplanetary space and secondary transport through shielding materials, atmospheres, and various parts of the human body to determine doses, dose rates, and linear energy transfer spectra. Thus, EMMREM makes the explicit connection from observations and simulations of solar energetic particles and galactic cosmic rays to characterization of the potential hazards of the radiation environment and to acute radiation risks. The papers in this special section will describe the following. [2] 1. Observations near 1 AU from ACE, GOES, and IMP 5 for the historic August 1972 event to characterize the near-Earth radiation environment are used as timedependent boundary conditions near 1 AU in an energetic particle propagation and acceleration module (the Energetic Particle Radiation Environment Module (EPREM)) to predict the radiation environment throughout the inner heliosphere. Observations at Ulysses (near 5 AU) are then used to validate the EPREM predictions. [3] 2. Event-based simulations in the inner heliosphere predict time-dependent estimates of organ exposures for human crews in deep space. [4] 3. A paper provides a risk assessment approaches for solar particle events. [5] 4. Several papers describe a model for prompt solar energetic particle (SEP) dose rate forecasting for the Earth-Moon system. [6] 5. The transmission of SEPs and galactic cosmic rays (GCRs) through the Mars atmosphere predicts the radiation environment at Mars. [7] 6. The coupling between the Energetic Particle Radiation Environment Module and magnetohydrodynamic models improves the predictions of the radiation environment in the inner heliosphere. [8] 7. Ulysses observations and a model of GCRs are used to improve our understanding of recent GCR fluxes and associated GCR dose rates throughout the inner heliosphere. [9] 8. A paper provides predictions of the linear energy transfer spectrum for the Cosmic Ray Telescope for the Effects of Radiation detector during the Lunar Reconnaissance Orbiter mission. [10] 9. Several papers describe predictions of the space radiation environment during gradual solar energetic particle events using physics-based particle acceleration models. [11] Thus, the papers in this special section of Space Weather introduce the EMMREM project and provide a baseline for current understanding of the space environment beyond Earth’s protective atmosphere and magnetosphere.

OBSERVATION OF UNIVERSALITY IN THE GENERALIZED SIMILARITY OF EVOLVING SOLAR WIND TURBULENCE AS SEEN BY ULYSSES

We perform statistical analysis of the fluctuating magnetic field observed in-situ by the Ulysses spacecraft, from the perspective of quantitative characterization of the evolving magnetohydrodynamic (MHD) turbulence. We focus on two successive polar passes around solar minimum which provide extended intervals of quiet, fast solar wind at a range of radial distances and latitudes: the south polar pass of 1994 and the north polar pass of 1995. Fully developed inertial range turbulence has a characteristic statistical similarity property of quantities that characterize the flow, such as the magnetic field components Bk(t), so that the pth moment of fluctuations has power-law dependence on scale t such that similar to tau(zeta(p)). We instead find a generalized similarity similar to g(tau/tau(0))(zeta(p)) consistent with extended self-similarity; and in particular all of these Ulysses observations, from both polar passes, share the same single function g(tau/tau(0)). If these observations are indeed characteristic of MHD turbulence evolving in-situ, then this quantifies for the first time a key aspect of the universal nature of evolving MHD turbulence in a system of finite size, with implications both for theoretical development, and for our understanding of the evolving solar wind.

Cosmic Ray Observations in the 3-D Heliosphere through the Solar Cycle: New Insights from Ulysses

The Ulysses spacecraft has been the first to orbit the Sun over its poles and to explore the heliosphere at high heliolatitudes. It has completed three fast-latitude scans, the first and the most recent one at solar minimum, the second one at solar maximum. The Ulysses observations over 17 of the 22 years of a solar magnetic cycle have produced unexpected surprises. Contrary to pre-mission expectations, the energy spectrum of cosmic ray nucleons was found to be highly modulated even over the solar poles. At solar minimum, only small positive latitudinal cosmic ray gradients were found in the A positive cycle. No latitudinal gradient was seen in the current A negative cycle, At solar maximum, the cosmic ray fluxes were dominated by temporal fluctuations. The Ulysses observations of cosmic rays, the solar wind and the heliospheric magnetic field have significantly contributed to the theoretical understanding of solar modulation in the inner heliosphere, especially at high heliolatitudes.

Suprathermal electron evolution in a Parker spiral magnetic field

Suprathermal electrons (>70 eV) form a small fraction of the total solar wind electron density but serve as valuable tracers of heliospheric magnetic field topology. Their usefulness as tracers of magnetic loops with both feet rooted on the Sun, however, most likely fades as the loops expand beyond some distance owing to scattering. As a first step toward quantifying that distance, we construct an observationally constrained model for the evolution of the suprathermal electron pitch‐angle distributions on open field lines. We begin with a near‐Sun isotropic distribution moving antisunward along a Parker spiral magnetic field while conserving magnetic moment, resulting in a field‐aligned strahl within a few solar radii. Past this point, the distribution undergoes little evolution with heliocentric distance. We then add constant (with heliocentric distance, energy, and pitch angle) ad‐hoc pitch‐angle scattering. Close to the Sun, pitch‐angle focusing still dominates, again resulting in a narrow strahl. Farther from the Sun, however, pitch‐angle scattering dominates because focusing is effectively weakened by the increasing angle between the magnetic field direction and intensity gradient, a result of the spiral field. We determine the amount of scattering required to match Ulysses observations of strahl width in the fast solar wind, providing an important tool for inferring the large‐scale properties and topologies of field lines in the interplanetary medium. Although the pitch‐angle scattering term is independent of energy, time‐of‐flight effects in the spiral geometry result in an energy dependence of the strahl width that is in the observed sense although weaker in magnitude.

Dependence of the Characteristics of Solar Wind Discontinuities and Shocks with Heliolatitude and Radial Distance

The distribution of the shocks in the heliosphere and their characteristic variations are investigated using Ulysses observations. The jumps in solar wind velocity, IMF magnitude, and proton density across the shocks and discontinuities are evaluated and used to characterize them. The distribution of these discontinuities with respect to heliolatitude ± 80° and with radial distance 1 to 5 AU are analyzed during solar minimum and solar maximum to understand their global behavior. It is noticed that the jumps in solar wind parameters associated with shocks and discontinuities are more prominent during the second orbit of Ulysses, which coincided with the maximum phase of solar activity.

Fluid versus simulation model of solitons in solar wind: Application to Ulysses observations

This paper continues recent efforts based on Hall‐MHD theory (Sauer et al., 2007) to explain a new class of magnetically compressive solitary structures in the interplanetary space, observed by the Ulysses magnetometer (Rees et al., 2006). The theoretical basis is extended by a kinetic approach via one‐dimensional hybrid code simulations which reveal deficiencies of fluid theory in describing slow mode‐type solitons in a collisionless finite β plasma. Kinetic solitary structures, resembling obliquely propagating Alfven wave pulses with quasi‐circular or banana‐type polarization are presented which may reproduce most of the observational features. This suggests that the soliton picture provides an adequate theoretical concept for the observed events.

Evolution of the solar wind proton temperature anisotropy from 0.3 to 2.5 AU

We report an analysis of the proton temperature anisotropy evolution from 0.3 to 2.5 AU based on the Helios and Ulysses observations. With increasing distance the fast wind data show a path in the parameter space (β∥p, T⊥p/T∥p). The first part of the trajectory is well described by an anticorrelation between the temperature anisotropy T⊥p/T∥p and the proton parallel beta, while after 1 AU the evolution with distance in the parameter space changes and the data result in agreement with the constraints derived by a fire hose instability. The slow wind data show a more irregular behavior, and in general it is not possible to recover a single evolution path. However, on small temporal scale we find that different slow streams populate different regions of the parameter space, and this suggests that when considering single streams also the slow wind follows some possible evolution path.

Comparative Study of the Initial Spikes of Soft Gamma-Ray Repeater Giant Flares in 1998 and 2004 Observed withGeotail: Do Magnetospheric Instabilities Trigger Large-Scale Fracturing of a Magnetar's Crust?

We present the unsaturated peak profile of the giant flare from SGR 1900+14 on 1998 August 27. This was obtained by the particle counters of the Low Energy Particles instrument on board the Geotail spacecraft. The observed peak profile revealed four characteristic features: an initial steep rise, an intermediate rise to the peak, an exponential decay, and a small hump in the decay phase. From this light curve, we found that the isotropic peak luminosity was 2.3 × 1046 ergs s-1 and that the total energy was 4.3 × 1044 ergs s-1 (E 50 keV), assuming that the distance to SGR 1900+14 is 15 kpc and that the spectrum is optically thin thermal bremsstrahlung with kT = 240 keV. These values are consistent with the previously reported lower limits derived from Ulysses and Konus-Wind observations. A comparative study of the initial spikes of the SGR 1900+14 giant flare in 1998 and of the SGR 1806-20 giant flare in 2004 is also presented. The timescale of the initial steep rise shows a magnetospheric origin, while the timescale of the intermediate rise to the peak indicates that it originates from crustal fracturing. Finally, we argue that the four features and their corresponding timescales provide us with a clue to identify extragalactic soft gamma-ray repeater giant flares among short gamma-ray bursts.

Constraints on the global structure of magnetic clouds: Transverse size and curvature

We present direct evidence that magnetic clouds (MCs) have highly flattened and curved cross section resulting from their interaction with the ambient solar wind. Lower limits on the transverse size are obtained for three MCs observed by ACE and Ulysses from the latitudinal separation between the two spacecraft, ranging from 40° to 70°. The cross‐section aspect ratio of the MCs is estimated to be no smaller than 6:1. We offer a simple model to extract the radius of curvature of the cross section, based on the elevation angle of the MC normal distributed over latitude. Application of the model to Wind observations from 1995 to 1997 (close to solar minimum) shows that the cross section is bent concavely outward by a structured solar wind with a radius of curvature of ∼0.3 AU. Near solar maximum, MCs tend to be convex outward in the solar wind with a uniform speed; the radius of curvature is proportional to the heliographic distance of MCs, as demonstrated by Ulysses observations between 1999 and 2003. These results improve our knowledge of the global morphology of MCs in the pre‐Stereo era, which is crucial for space weather prediction and heliosphere studies.

Ulysses observations of very different heliospheric structure during the declining phase of solar activity cycle 23

This study examines the most recent observations from the solar wind experiment (SWOOPS) on the Ulysses spacecraft. In contrast to observations over the same locations in the declining phase of the previous solar activity cycle, these data show a more complicated heliospheric structure. Around the start of 2003, a band of slow solar wind resided at ∼20°–25°N latitude at all helio‐longitudes. Subsequently, Ulysses failed to observe a well‐organized corotating stream structure as it headed southward from the ecliptic plane, even though, in the previous solar cycle, Ulysses observed a regularly structured stream pattern. These observations show that the three‐dimensional structure of the heliosphere can differ significantly from one solar cycle to the next. Finally, we show that the solar wind dynamic pressure has been relatively stable throughout 2005, indicating that the termination shock distance will also likely be relatively stable throughout 2006.

The Latitudinal Excursion of Coronal Magnetic Field Lines in Response to Differential Rotation: MHD Simulations

Solar energetic particles, which are believed to originate from corotating interacting regions (CIRS) at low heliographic latitude, were observed by the Ulysses spacecraft even as it passed over the Sun's poles. One interpretation of this result is that high-latitude field lines intercepted by Ulysses connect to low-latitude CIRs at much larger heliocentric distances. The Fisk model explains the latitudinal excursion of magnetic field lines in the solar corona and heliosphere as the inevitable consequence of the interaction of a in a differentially rotating photosphere with rigidly rotating coronal holes. We use a time-dependent three-dimensional magnetohydrodynamic (MHD) algorithm to follow the evolution of a simple model of the solar corona in response to the differential rotation of the photospheric magnetic flux. We examine the changes of the coronal-hole boundaries, the redistribution of the line-of-sight magnetic field, and the precession of field lines in the corona. Our results confirm the basic idea of the Fisk model, that differential rotation leads to changes in the heliographic latitude of magnetic field lines. However, the latitudinal excursion of magnetic field lines in this simple tilted dipole model is too small to explain the Ulysses observations. Although coronal holes in our model rotate more rigidly than do photospheric features (in general agreement with observations), they do not rotate strictly rigidly as assumed by Fisk. This basic difference between our model and Fisk's will be explored in the future by considering more realistic magnetic flux distributions, as observed during Ulysses polar excursions.

Ulysses observations of Jovian relativistic electrons in the interplanetary space near Jupiter: Determination of perpendicular particle transport coefficients and their energy dependence

Intensities of ∼1–10 MeV relativistic electrons in several energy channels of the high-energy telescope (HET) on Ulysses increase dramatically during its flybys of Jupiter in 1992 and 2004. Perpendicular diffusion coefficients of these particles are derived by fitting the spatial profile of Jovian electron intensity to a diffusion-convection model of particle transport. It is found that the latitudinal diffusion coefficient during the 2004 Jupiter flyby has to be enhanced from its value during the 1992 Jupiter flyby and it is also enhanced relative to the radial perpendicular diffusion. Such an enhancement of latitudinal particle transport was implied previously through the observations of Jovian electrons, cosmic rays and solar energetic particles at high heliographic latitudes, and now this requirement extends further to low latitude region of the heliosphere. Energy dependence of the perpendicular diffusion coefficient is obtained quite precisely through the variation in the slope of energy spectrum of Jovian electrons. The perpendicular diffusion coefficient increases with energy, which can put a tight constraint on models of the particle transport coefficient. The newest nonlinear guiding center (NLGC) theory of perpendicular diffusion is consistent with this observation, but only when it is combined with a parallel diffusion coefficient from the quasilinear theory in a slab magnetic turbulence without dynamic damping.

Wave acceleration of the fast solar wind

Observations show that the solar wind exhibits two modes of outflow: the slow (∼400 km s −1), high density, highly variable wind that emerges from equatorial solar regions, and the high latitude, fast (∼800 km s −1), low density, steady solar wind. The bi-modal solar wind outflow is most evident near minima of solar activity. Theoretical studies of the fast solar wind in open magnetic structures motivated by SOHO, Ulysses, and Helios observations indicate that both, high frequency kinetic waves, and low frequency MHD waves play a role in its acceleration and heating. Ion-cyclotron waves have been suggested as the main energy source of the solar wind. However, there are theoretical difficulties with the ion-cyclotron wave heating of the protons, and these waves do not heat electrons. Low frequency MHD waves are still the best candidates to transport momentum and energy far from the Sun, to accelerate the solar wind on large spatial scales. I will present recently developed two-dimensional three-fluid model that includes explicit wave acceleration, and visco-resistive dissipation. The model describes electrons, protons, and minor ions as three coupled fluids that are heated by different heating processes with the parameters constrained by observations. I will present the results of 2.5D three-fluid simulations of the fast solar wind plasma that combine the effects of MHD waves self-consistently, and ion-cyclotron waves parametrically on the acceleration and heating processes. I will present the results of hybrid kinetic models of ion-cyclotron wave heating of the heavy ions in the solar wind plasma.

Modelling cosmic ray intensities along the Ulysses trajectory

Abstract. Time dependent cosmic ray modulation in the inner heliosphere is studied by comparing results from a 2-D, time-dependent cosmic ray transport model with Ulysses observations. A compound approach, which combines the effects of the global changes in the heliospheric magnetic field magnitude with drifts to establish a realistic time-dependence, in the diffusion and drift coefficients, are used. We show that this model results in realistic cosmic ray modulation from the Ulysses launch (1990) until recently (2004) when compared to 2.5-GV electron and proton and 1.2-GV electron and Helium observations from this spacecraft. This approach is also applied to compute radial gradients present in 2.5-GV cosmic ray electron and protons in the inner heliosphere. The observed latitude dependence for both positive and negative charged particles during both the fast latitude scan periods, corresponding to different solar activity conditions, could also be realistically computed. For this an additional reduction in particle drifts (compared to diffusion) toward solar maximum is needed. This results in a realistic charge-sign dependent modulation at solar maximum and the model is also applied to predict charge-sign dependent modulation up to the next expected solar minimum.