Ulysses Measurements Research Articles

Ion Charge States in a Time-Dependent Wave-Turbulence-Driven Model of the Solar Wind

Ion fractional charge states, measured in situ in the heliosphere, depend on the properties of the plasma in the inner corona. As the ions travel outward in the solar wind and the electron density drops, the charge states remain essentially unaltered or "frozen in". Thus they can provide a powerful constraint on heating models of the corona and acceleration of the solar wind. We have implemented non-equilibrium ionization calculations into a 1D wave-turbulence-driven (WTD) hydrodynamic solar wind model and compared modeled charge states with the Ulysses 1994-5 in situ measurements. We have found that modeled charge state ratios of $C^{6+}/C^{5+}$ and $O^{7+}/O^{6+}$, among others, were too low compared with Ulysses measurements. However, a heuristic reduction of the plasma flow speed has been able to bring the modeled results in line with observations, though other ideas have been proposed to address this discrepancy. We discuss implications of our results and the prospect of including ion charge state calculations into our 3D MHD model of the inner heliosphere.

Ulysses and IBEX Constraints on the Interstellar Neutral Helium Distribution

We relax the usual assumption of Maxwellian velocity distributions in the interstellar medium (ISM) in the analysis of neutral He particle data from Ulysses and the Interstellar Boundary Explorer (IBEX). For Ulysses, the possibility that a narrow component from heavy neutrals is contaminating the He signal is considered, which could potentially explain the lower ISM temperature measured by Ulysses compared to IBEX. The expected heavy element contribution is about an order of magnitude too small to resolve that discrepancy. For IBEX, we find that modest asymmetries in the ISM velocity distribution can potentially improve the quality of fit to the first two years of data, and perhaps improve agreement with the Ulysses measurements.

Stardust Interstellar Preliminary Examination X: Impact speeds and directions of interstellar grains on the Stardust dust collector

AbstractOn the basis of an interstellar dust model compatible with Ulysses and Galileo observations, we calculate and predict the trajectories of interstellar dust (ISD) in the solar system and the distribution of the impact speeds, directions, and flux of ISD particles on the Stardust Interstellar Dust Collector during the two collection periods of the mission. We find that the expected impact velocities are generally low (<10 km s−1) for particles with the ratio of the solar radiation pressure force to the solar gravitational force β > 1, and that some of the particles will impact on the cometary side of the collector. If we assume astronomical silicates for particle material and a density of 2 g cm−3, and use the Ulysses measurements and the ISD trajectory simulations, we conclude that the total number of (detectable) captured ISD particles may be on the order of 50. In companion papers in this volume, we report the discovery of three interstellar dust candidates in the Stardust aerogel tiles. The impact directions and speeds of these candidates are consistent with those calculated from our ISD propagation model, within the uncertainties of the model and of the observations.

OBSERVATIONS OF THE INTERPLANETARY HYDROGEN DURING SOLAR CYCLES 23 AND 24. WHAT CAN WE DEDUCE ABOUT THE LOCAL INTERSTELLAR MEDIUM?

Observations of interstellar helium atoms by the Interstellar Boundary Explorer (IBEX) spacecraft in 2009 reported a local interstellar medium (LISM) velocity vector different from the results of the Ulysses spacecraft between 1991 and 2002. The interplanetary hydrogen (IPH), a population of neutrals that fills the space between planets inside the heliosphere, carries the signatures of the LISM and its interaction with the solar wind. More than 40 yr of space-based studies of the backscattered solar Lyα emission from the IPH provided limited access to the velocity distribution, with the first temporal evolution map of the IPH line-shift during solar cycle 23. This work presents the results of the latest IPH observations made by the Hubble Space Telescope's Space Telescope Imaging Spectrograph during solar cycle 24. These results have been compiled with previous measurements, including data from the Solar Wind Anisotropies instrument on the Solar and Heliospheric Observatory. The whole set has been compared to physically realistic models to test both sets of LISM physical parameters as measured by Ulysses and IBEX, respectively. This comparison shows that the LISM velocity vector has not changed significantly since Ulysses measurements.

Interpretation of the coronal magnetic field configuration of the Sun

The origin of the heliospheric magnetic flux on the Sun, and hence the origin of the solar wind, is a topic of hot debate. While the prevailing view is that the solar wind originates from outside the coronal streamer helmets, there also exists the suggestion that the open magnetic field spans a far wider region. Without the definitive measurement of the coronal magnetic field, it is difficult to unambiguously resolve the conflict between the two scenarios. We present two 2-dimensional, Alfvénic-turbulence-based models of the solar corona and solar wind, one with and the other without a closed magnetic field region in the inner corona. The purpose of the latter model is to test whether it is possible to realize a picture suggested by polarimetric measurements of the corona using the Fe XIII 10747 Å line, where open magnetic field lines seem to penetrate the streamer base. The boundary conditions at the coronal base are able to account for important observational constraints, especially those on the magnetic flux distribution. Interestingly, the two models provide similar polarized brightness (pB) distributions in the field of view (FOV) of SOHO/LASCO C2 and C3 coronagraphs. In particular, a dome-shaped feature is present in the C2 FOV even for the model without a closed magnetic field. Moreover, both models fit the Ulysses data scaled to 1 AU equally well. We suggest that: 1) The pB observations cannot be safely taken as a proxy for the magnetic field topology, as is often implicitly assumed. 2) The Ulysses measurements, especially the one showing a nearly uniform distribution with heliocentric latitude of the radial magnetic field, do not rule out the ubiquity of open magnetic fields on the Sun.

Model of the Jovian magnetic field topology constrained by the Io auroral emissions

[1] The determination of the internal magnetic field of Jupiter has been the object of many studies and publications. These models have been computed from the Pioneer, Voyager, and Ulysses measurements. Some models also use the position of the Io footprints as a constraint: the magnetic field lines mapping to the footprints must have their origins along Io's orbit. The use of this latter constraint to determine the internal magnetic field models greatly improved the modeling of the auroral emissions, in particular the radio ones, which strongly depends on the magnetic field geometry. This constraint is, however, not sufficient for allowing a completely accurate modeling. The fact that the footprint field line should map to a longitude close to Io's was not used, so that the azimuthal component of the magnetic field could not be precisely constrained. Moreover, a recent study showed the presence of a magnetic anomaly in the northern hemisphere, which has never been included in any spherical harmonic decomposition of the internal magnetic field. We compute a decomposition of the Jovian internal magnetic field into spherical harmonics, which allows for a more accurate mapping of the magnetic field lines crossing Io, Europa, and Ganymede orbits to the satellite footprints observed in UV. This model, named VIPAL, is mostly constrained by the Io footprint positions, including the longitudinal constraint, and normalized by the Voyager and Pioneer magnetic field measurements. We show that the surface magnetic fields predicted by our model are more consistent with the observed frequencies of the Jovian radio emissions than those predicted by previous models.

Solar winds along curved magnetic field lines

Both remote-sensing measurements using the interplanetary scintillation (IPS) technique and in situ measurements by the Ulysses spacecraft show a bimodal structure for the solar wind at solar minimum conditions. At present what makes the fast wind fast and the slow wind slow still remains to be answered. While a robust empirical correlation exists between the coronal expansion rate $f_c$ of the flow tubes and the speeds $v$ measured in situ, further data analysis suggests that $v$ depends on more than just $f_c$. We examine whether the non-radial shape of field lines, which naturally accompanies any non-radial expansion, could be an additional geometrical factor. We solved the transport equations incorporating the heating due to turbulent Alfv\'en waves for an electron-proton solar wind along curved field lines given by an analytical magnetic field model, representative of a solar minimum corona. The field line shape is found to influence substantially the solar wind parameters, reducing the asymptotic speed by up to $\sim 130$ km s$^{-1}$, or by $\sim 28%$ in relative terms, compared with the case neglecting the field line curvature. This effect was interpreted in the general framework of energy addition in the solar wind: Relative to the straight case, the field line curvature enhances the effective energy deposition to the subsonic flow, resulting in a higher proton flux and a lower terminal proton speed. Our computations suggest that the field line curvature could be a geometrical factor which, in addition to the tube expansion, substantially influences the solar wind speed. Furthermore, at solar minima although the field line curvature unlikely affects the polar fast solar wind, it does help make the wind at low latitudes slow, thereby helping better reproduce the Ulysses measurements.

Study of high-beta supra-thermal plasma events in the vicinity of magneto-hydrodynamic shock-waves

The ratio value – known as plasma parameter “beta” (β) - of the interplanetary-plasma pressure (thermal pressure) to the magnetic field’s (magnetic pressure) or of their energy densities, is critical for the space plasmas and has important consequences to their properties. Although in the scientific literature the contribution of the supra-thermal particles to the plasma pressure is generally assumed negligible, we deduced, by analyzing energetic particles and magnetic field measurements recorded by the Ulysses spacecraft, that in a series of events, the energy density contained in the supra-thermal tail of the particle distribution is comparable to or even higher than the energy density of the magnetic field, creating conditions of high-beta plasma. In particular, in this paper we analyze Ulysses measurements of the energy density ratio (parameter βep) of the supra-thermal (20 keV to ~5 MeV) ions’ (by the HI-SCALE instrument) to the magnetic field’s (by the VHM/FGM instrument) in order to find occurrences of high-beta (β ep >1) supra-thermal plasma conditions in the vicinity of interplanetary shock-waves. These high-beta ion events are associated with energetic particle intensity enhancements which have been produced by reacceleration at the shock fronts.

On the relationship between coronal heating, magnetic flux, and the density of the solar wind

The stark differences between the current solar minimum and the previous one offer a unique opportunity to develop new constraints on mechanisms for heating and acceleration of the solar wind. We have used a combination of numerical simulations and analysis of remote solar and in situ observations to infer that the coronal heating rate, H, scales with the average magnetic field strength within a coronal hole, Bch. This was accomplished in three steps. First, we analyzed Ulysses measurements made during its first and third orbit southern and northern polar passes (i.e., during near‐solar minimum conditions) to deduce a linear relationship between proton number density (np) and radial magnetic field strength (Br) in the high‐speed quiescent solar wind, consistent with the results of McComas et al. (2008) and Ebert et al. (2009). Second, we used Wilcox Solar Observatory measurements of the photospheric magnetic field to show that the magnetic field strength within coronal holes (Bch) is approximately correlated with the strength of the interplanetary field at the location of Ulysses. Third, we used hydrodynamic simulations to show that np in the solar wind scales linearly with H. Taken together, these results imply the chain: H ∝ np ∝ Br ∝ Bch. We also explored ideas that the correlation between np and Br could have resulted from interplanetary processes, or from the superradial expansion of the coronal magnetic field close to the Sun, but find that neither possibility can produce the observed relationship. The derived heating relationship is consistent with (1) empirical heating laws derived for closed‐field line regions and (2) theoretical models aimed at understanding both the heating and acceleration of the solar wind.

MODULATION OF GALACTIC COSMIC RAY PROTONS AND ELECTRONS DURING AN UNUSUAL SOLAR MINIMUM

During the latestUlyssesout-of-ecliptic orbit the solar wind density, pressure, and magneticfield strength have been the lowest ever observed in the history of space exploration. Since cosmic-ray particles respond to the heliospheric magnetic field in the expanding solar wind and its turbulence, the weak heliospheric magnetic field as well as the low plasma density and pressure are expected to cause the smallest modulation since the 1970s. In contrast to this expectation, the galactic cosmic-ray (GCR) proton flux at 2.5 GV measured by Ulysses in 2008 does not exceed the one observed in the 1990s significantly, while the 2.5 GV GCR electron intensity exceeds the one measured during the 1990s by 30%‐40%. At true solar minimum conditions, however, the intensities of both electrons and protons are expected to be the same. In contrast to the 1987 solar minimum, the tilt angle of the solar magnetic field has remained at about 30 in 2008. In order to compare the Ulysses measurements during the 2000 solar magnetic epoch with those obtained 20 years ago, the former have been corrected for the spacecraft trajectory using latitudinal gradients of 0.25% deg # 1 and 0.19% deg # 1 for protons and electrons, respectively, and a radial gradient of 3% AU # 1 . In 2008 and 1987, solar activity, as indicated by the sunspot number, was low. Thus, our observations confirm the prediction of modulation models that current sheet and gradient drifts prevent the GCR flux to rise to typical solar minimum values. In addition, measurements of electrons and protons allow us to predict that the 2.5 GV GCR proton intensity will increase by a factor of 1.3 if the tilt angle reaches values below 10 .

Decrease in heliospheric magnetic flux in this solar minimum: Recent Ulysses magnetic field observations

The Ulysses spacecraft has traveled from the solar equator at 1.3 and 5.3 AU to above the polar caps at 2.2 AU three times during the last 17 years and has provided measurements of the solar–heliospheric magnetic field. The open magnetic flux, i.e., the radial component, BR, multiplied by the square of the radial distance, r, is independent of latitude at both solar minimum and maximum. Measurements of r2 BR contain information about the average polar cap field strength when allowance is made for the non‐radial expansion of the magnetic field and solar wind near the Sun that eliminates the latitude gradient in magnetic pressure. Recent Earth‐based magnetograph observations indicate that the Sun's polar cap field strength, BP, has decreased by a factor of about two between the previous and present latitude scans. Ulysses measurements show that the average value of r2 BR has decreased from 3.6 nT (AU)2 in 1993.5 to 1995.0 to 2.3 nT (AU)2 in 2006.1 to 2007.4, a decrease by 0.64. The two Ulysses scans are not precisely at solar minimum. However, in‐ecliptic BR is highly correlated with the Ulysses measurements at all latitudes and can be used to determine the open flux at the two solar minima. Averages of BR at the two solar minima are 2.82 and 2.45 nT. This decrease is contrary to the suggestion based on previous solar cycles that BR returns to the same value of ≈ 3 nT at solar minimum. The ratio of BP to the expansion factor, fE, is proportional to the measured open flux and observed and assumed values of BP are used to determine the corresponding values of fE. Another property of the fast solar wind is that it is highly turbulent compared to lower latitudes. To determine if the decrease in r2 BR and BP has affected the intensity of the magnetic fluctuations, the total variances in the magnetic field fluctuations are derived and found to decrease by a factor of 0.75.

Statistical study of low‐energy heliosphere particle fluxes from 1.4 to 5 AU over a solar cycle

Throughout the entire Ulysses mission, the Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies (HI‐SCALE) has collected measurements of low‐energy interplanetary ions and electrons. Time series of electron, proton, and ion fluxes have been obtained since 1990. We present statistical studies of high‐resolution ion and electron energy spectra (∼50 keV to ∼5 MeV) as measured by the HI‐SCALE instrument on the Ulysses spacecraft over a time interval longer than a solar cycle (1990 to 2004). Ulysses is the only spacecraft that continually measured the inner (∼1.4 to ∼5 AU) heliosphere particle population during these years. The data thus provide measures of the lower‐energy population of particles that a spacecraft traveling outward from Earth would have encountered and that also could have impacted the atmosphere and surface of Mars and of its satellites during this interval. Comparisons of Ulysses particle fluxes with those from the Electron, Proton, and Alpha Monitor (EPAM) instrument on the Advanced Composition Explorer (ACE) spacecraft (the HI‐SCALE backup instrument) have shown that azimuthal and heliolatitude dependencies of particle fluxes in the inner heliosphere following solar events are not as extreme as might be expected. Thus the Ulysses measurements, while taken over a range of heliolatitudes, can provide important statistical information that can be used to estimate the low‐energy radiation dosages and potential sputtering fluxes to planetary surfaces and to heliosphere spacecraft surfaces and solar arrays over a solar cycle.

Solar magnetic field reversal as seen at Ulysses

The rapid motion of the Ulysses spacecraft from high southern to high northern latitudes in 2000–2001 provided an excellent opportunity to make inferences regarding the solar magnetic dipole's behaviour around solar maximum. A simple dipole model is fitted to Ulysses measurements of the polarity of the heliospheric magnetic field mapped back to the solar wind source surface. Although higher order components of the field are ignored, the gradual reversal in orientation of the dipole field component can be followed during solar maximum, with the dipole axis crossing the solar equator during early 2000–early 2001. The dipole appears to exhibit a rotation at a slower rate than the Carrington frame of reference, similar to previous measurements made around solar maximum in the solar equatorial regions.

Cassini between Venus and Earth: Detection of interstellar dust

We report the successful in situ measurement of interstellar dust particles inside the orbit of the Earth with the Cosmic Dust Analyzer (CDA) on the Cassini spacecraft. The impact ionization subsystem of the CDA is similar to the instruments on Ulysses and Galileo. As the heliocentric velocity and the direction of the interstellar dust flux are well known from Ulysses measurements, a combined analysis of the impact charge signals together with geometric and kinematic spacecraft data allowed us to separate interplanetary impacts from interstellar ones. The mean interstellar flux between 0.7 and 1.2 AU derived from our analysis is 2.5 ± 0.5 · 10−5 m−2s−1, in a mass range of 5 · 10−17 kg to 10−15 kg which is in good agreement with the interstellar dust flux measured by Ulysses at 3 AU during the same time period [Landgraf et al., 2003]. The simultaneous detection of interstellar grains by Ulysses at 3 AU and approximately 1 AU by Cassini proves that big interstellar grains (radius greater than 0.4 μm), can penetrate deeply into the inner solar system.

On the heliolatitudinal variation of the galactic cosmic-ray intensity. Comparison with Ulysses measurements

Abstract. We study the dependence of cosmic rays with heliolatitude using a simple method and compare the results with the actual data from Ulysses and IMP spacecraft. We reproduce the galactic cosmic-ray heliographic latitudinal intensity variations, applying a semi-empirical, 2-D diffusion-convection model for the cosmic-ray transport in the interplanetary space. This model is a modification of our previous 1-D model (Exarhos and Moussas, 2001) and includes not only the radial diffusion of the cosmic-ray particles but also the latitudinal diffusion. Dividing the interplanetary region into "spherical magnetic sectors" (a small heliolatitudinal extension of a spherical magnetized solar wind plasma shell) that travel into the interplanetary space at the solar wind velocity, we calculate the cosmic-ray intensity for different heliographic latitudes as a series of successive intensity drops that all these "spherical magnetic sectors" between the Sun and the heliospheric termination shock cause the unmodulated galactic cosmic-ray intensity. Our results are compared with the Ulysses cosmic-ray measurements obtained during the first pole-to-pole passage from mid-1994 to mid-1995.Key words. Interplanetary physics (cosmic rays; interplanetray magnetic fields; solar wind plasma)

Evolution of magnetic field fluctuations in high‐speed solar wind streams: Ulysses and Helios observations

Ulysses magnetic field measurements in high‐speed streams emanating from the Sun's polar coronal holes reveal a complex combination of fluctuations on different scales. Measurements of the wavenumber dependence of radial and latitudinal power scaling, spectral index, and anisotropy of undisturbed polar magnetic field fluctuations between 1.4 and 4.1 astronomical units (AU) are presented. These results confirm earlier work and reveal the presence of three populations: structures at large scales, Alfvén waves at intermediate scales, and turbulent fluctuations at the smallest scales. A significant decrease in power with latitude, as a result of overexpansion of polar coronal holes, has consequences for energetic particle diffusion coefficients. Other results, in particular those related to field magnitude variations, are not well understood at this time. A similar analysis of magnetic field fluctuations in high‐speed streams at low heliolatitudes between 0.3 and 1 AU, measured by the Helios spacecraft, shows consistency with Ulysses measurements, suggesting a common origin for fluctuations in all high‐speed streams.

Connecting the Sun and the Solar Wind: Comparison of the Latitudinal Profiles of Coronal and [ITAL]Ulysses[/ITAL] Measurements of the Fast Wind

A quantitative comparison of the latitudinal profile of polarized brightness (pB) measurements nearest the Sun at 1.15 R☉ by the Mauna Loa Solar Observatory K-Coronameter and Ulysses interplanetary measurements of the fast solar wind during its first south polar pass, at the declining phase of the solar cycle, is made for the first time to identify the sources of the fast solar wind in the context of coronal density structure. Both profiles are found to have the same shape. At the Sun, the minimum coincides with the radial extension of the coronal hole boundaries. The slight rise and plateau following this minimum toward lower latitudes are identified with the coronal extension of the quiet Sun. The corresponding profile of the in situ measured velocity has a maximum within the angular extent of the polar coronal hole and decreases gradually beyond its boundaries. The latitudinal profile of the proton flux mimics the density profile, implying that the mass-loss rate is lowest within the angular extent of the polar coronal hole. The association of the fast wind with a density profile that reflects the polar coronal hole and the surrounding quiet Sun suggests that the fast wind observed by Ulysses originates from both regions. That these conclusions differ from earlier published analyses of the same Ulysses measurements is a consequence of the quantitative and systematic comparison made between Ulysses and coronal measurements at 1.15 R☉.

An Estimation of the Shape and Location of the Heliospheric Termination Shock

We calculate semianalytically the distance to the heliospheric termination shock, and we use Ulysses measurements as data input for a realistic application. Therefore, we can find directly the consequences of the solar wind latitudinal variation on the shape of the termination shock. The model that we use is an approximate gasdynamic model. Our purpose is to develop a semianalytical model that can use any spacecraft measurements as data input to give the distance of the termination shock from the Sun, avoiding the complicated but more accurate numerical simulations.

Ulysses measurements of variations in the solar wind‐interstellar hydrogen charge exchange rate

Charge exchange with solar wind protons is the primary ionization process for interstellar hydrogen traveling through the heliosphere. In this study we use Ulysses solar wind observations to examine and quantify variations in charge exchange. We find that these variations are largest within stream interaction regions where the charge exchange rate is tens of times larger in compressional, corotating interaction regions (CIRs) than in rarefactions. Similarly, we find enhanced charge exchange in compression regions ahead of fast CMEs. As a consequence, much of the interstellar hydrogen at low latitudes is picked up in pulses as CIRs and fast CMEs sweep past. Observations over Ulysses' first full polar orbit indicate that the charge exchange rate is higher at low than at high latitudes, and that this rate drops off more slowly than the inverse square of heliocentric distance. We believe that this is due to the growth of CIRs as they propagate out through the inner heliosphere. The variability of hydrogen charge exchange has implications for the analysis of scattered Lyman‐α observations and produces variations in the pickup rates of newly ionized interstellar hydrogen, which is a seed population for anomalous cosmic rays.

Relationship between Ulysses plasma observations and solar observations during the Whole Sun Month campaign

In this report, we summarize measurements made by the plasma experiment on the Ulysses spacecraft during the period designated as “Whole Sun Month” (WSM, August 10 to September 8, 1996). This interval coincided with the return of solar wind variability at Ulysses. Ulysses was located at ∼ 28°N heliographic latitude, at a heliocentric distance of 4.25 AU, and on the opposite side of the Sun from Earth. In particular, we explore the evolution of the solar wind between the Sun and Ulysses for several rotations surrounding WSM. Specifically, we map Ulysses measurements back toward the Sun by applying a two‐dimensional inverse MHD algorithm. This approach is compared with the commonly used constant speed (or ballistic) approximation. We find that the MHD mapping technique produces substantially better results when compared with solar observations. Both the Ulysses MHD‐mapped results and the solar observations are consistent with a picture of a modestly tilted streamer belt (< 10°) that was deformed northward by an active region at 240° –; 270° longitude.