Ulysses Spacecraft Research Articles

Nuclear Power Sources: A Key Enabling Technology for Planetary Exploration

Nuclear power sources for space (NPS) are, according to current physics knowledge, the only power source option for some classes of space missions. Europe has successfully used NPS exploration missions (e.g. Huygens lander on Titan and Ulysses spacecraft). Although some small-scale study and development efforts have been undertaken at the national level during the past 40 years, these did not go beyond study and early prototyping level. The present paper introduces the European situation with respect to the development and use of NPS.

Study of the solar Slow Sonic, Alfvén and Fast Magnetosonic transition surfaces

In this paper we study the shape, extend and time variations of the solar wind transition surfaces using the Lima and Priest (1993) hydrodynamic model adequately adapted for the case of the solar wind flow. The transition surfaces, namely the Slow (Sonic), the Alfvén, and the Fast Magnetosonic surface, are important boundaries around the Sun and play a crucial role in the development of the solar wind and the structure of the inner heliosphere. We determine the shape and dimension of these surfaces as a function of heliographic latitude using measurements from Ulysses spacecraft, and we also study their temporal variation using data from spacecrafts at 1 AU (OMNI database). Furthermore, we establish their dependence with the solar activity, demonstrating their shape and location for the last two solar cycles. From this we noticed that the temporal variation of all transition surfaces follows the 11-year solar cycle. Finally, from the OMNI database, we have studied the temporal variation over the past 40 years of the plasma β parameter, the kinetic to magnetic and the kinetic to thermal energy ratios, at a distance of 1 AU from the Sun.

Scaling law of the reduced magnetic helicity in fast streams

The reduced magnetic helicity is a quantity related to the handedness of the magnetic field fluctuations. In the present paper we study the scaling law of the reduced magnetic helicity in fast streams in the solar wind by using high-resolution magnetic field data by the Ulysses spacecraft. We show that at high frequencies both the left-hand and the right-hand helicity survives, implying that there is no predominance of a single sign. In addition, the scaling law of the magnetic helicity exhibits a strong dependence on the data set analyzed and we do not observe any universal behavior.

OBSERVATIONS OF ANISOTROPIC SCALING OF SOLAR WIND TURBULENCE

Using high-speed solar wind data recorded by the Ulysses spacecraft, we investigate and estimate the anisotropic inertial range scaling of the interplanetary magnetic field. We apply the method of the magnetic structure function (MSF), S-n(tau) = proportional to tau(zeta(n)), to analyze the scaling of solar wind turbulence over the range from 1 s to 10(4) s. By sorting the fluctuations according to the direction of the local mean magnetic field, we obtain a second-order structure function in (r, Theta) coordinates that reveals the scale-dependent anisotropy of the power spectrum. The scale-dependent anisotropy of the MSF indicates that the fluctuation energy tends to cascade toward the direction perpendicular to the local field. The dependence of the MSF scaling index zeta on the direction of the local field is found to be similar to that reported in Horbury et al. and Podesta, with zeta(perpendicular to) = 0.53 +/- 0.18 and zeta(parallel to) = 1.00 +/- 0.14. Furthermore, we estimate and find the scaling law between the perpendicular and parallel scales r(parallel to) proportional to r(perpendicular to)(0.614), which implies the elongation along the parallel direction as the turbulence eddy evolves toward the small lengthscales. These results are in agreement with the predictions of magnetohydrodynamic turbulence theory.

Galileo dust data from the jovian system: 2000 to 2003

The Galileo spacecraft was the first man-made satellite of Jupiter, orbiting the planet between December 1995 and September 2003. The spacecraft was equipped with a highly sensitive dust detector that monitored the jovian dust environment between approximately 2 and 370 R J (jovian radius R J=71 492 km). The Galileo dust detector was a twin of the one flying on board the Ulysses spacecraft. This is the tenth in a series of papers dedicated to presenting Galileo and Ulysses dust data. Here we present data from the Galileo dust instrument for the period January 2000 to September 2003 until Galileo was destroyed in a planned impact with Jupiter. The previous Galileo dust data set contains data of 2883 particles detected during Galileo's interplanetary cruise and 12 978 particles detected in the jovian system between 1996 and 1999. In this paper we report on the data of additional 5389 particles measured between 2000 and the end of the mission in 2003. The majority of the 21 250 particles for which the full set of measured impact parameters (impact time, impact direction, charge rise times, charge amplitudes, etc.) was transmitted to Earth were tiny grains (about 10 nm in radius), most of them originating from Jupiter's innermost Galilean moon Io. They were detected throughout the jovian system and the impact rates frequently exceeded 10 min −1. Surprisingly large impact rates up to 100 min −1 occurred in August/September 2000 when Galileo was far away ( ≈ 280 R J ) from Jupiter, implying dust ejection rates in excess of 100 kg s −1. This peak in dust emission appears to coincide with strong changes in the release of neutral gas from the Io torus. Strong variability in the Io dust flux was measured on timescales of days to weeks, indicating large variations in the dust release from Io or the Io torus or both on such short timescales. Galileo has detected a large number of bigger micron-sized particles mostly in the region between the Galilean moons. A surprisingly large number of such bigger grains was measured in March 2003 within a four-day interval when Galileo was outside Jupiter's magnetosphere at approximately 350 R J jovicentric distance. Two passages of Jupiter's gossamer rings in 2002 and 2003 provided the first actual comparison of in-situ dust data from a planetary ring with the results inferred from inverting optical images. Strong electronics degradation of the dust instrument due to the harsh radiation environment of Jupiter led to increased calibration uncertainties of the dust data.

Measurements of stray antenna capacitance in the STEREO/WAVES instrument: Comparison of the measured voltage spectrum with an antenna electron shot noise model

[1] One of the most accurate techniques for in situ measuring the electron density and temperature in space plasmas is the quasi-thermal noise spectroscopy, which uses the voltage fluctuation spectrum on an electric antenna. This technique has been used successfully on the WIND and ULYSSES spacecraft; however, on STEREO this technique may only work in high-density filamentary structures, where the Debye length is small, because the STEREO/WAVES antennas have a large surface area, so that the resulting shot noise spectrum in the solar wind dominates the power at lower frequencies. In the unperturbed solar wind, we can use instead the electron shot noise to infer the plasma density. For doing so, we use well calibrated WIND particle data to deduce the stray capacitance of the STEREO/WAVES antenna system in a special configuration when the STEREO-B spacecraft was just downstream of WIND. This stray capacitance is also compared to ground experiments done on the flight spare equipment and independent calibrations performed using the galactic radio background.

CHARACTERIZATION OF TRANSITIONS IN THE SOLAR WIND PARAMETERS

The distinction between fast and slow solar wind streams and the dynamically evolved interaction regions is reflected in the characteristic fluctuations of both the solar wind and the embedded magnetic field. High-resolution magnetic field data from the Ulysses spacecraft have been analyzed. The observations show rapid variations in the magnetic field components and in the magnetic field strength, suggesting a structured nature of the solar wind at small scales. The typical sizes of fluctuations cover a broad range. If translated to the solar surface, the scales span from the size of granules (~103 km) and supergranules (~104 km) on the Sun down to ~102 km and less. The properties of the short time structures change in the different types of solar wind. While fluctuations in fast streams are more homogeneous, slow streams present a bursty behavior in the magnetic field variances, and the regions of transition are characterized by high levels of power in narrow structures around the transitions. The probability density functions of the magnetic field increments at several scales reveal a higher level of intermittency in the mixed streams, which is related to the presence of well localized features. It is concluded that, apart from the differences in the nature of fluctuations in flows of different coronal origin, there is a small-scale structuring that depends on the origin of streams themselves but it is also related to a bursty generation of the fluctuations.

Generalized Similarity in Finite Range Solar Wind Magnetohydrodynamic Turbulence

Extended or generalized similarity is a ubiquitous but not well understood feature of turbulence that is realized over a finite range of scales. The ULYSSES spacecraft solar polar passes at solar minimum provide in situ observations of evolving anisotropic magnetohydrodynamic turbulence in the solar wind under ideal conditions of fast quiet flow. We find a single generalized scaling function characterizes this finite range turbulence and is insensitive to plasma conditions. The recent unusually inactive solar minimum--with turbulent fluctuations down by a factor of approximately 2 in power--provides a test of this invariance.

Three years of Ulysses dust data: 2005 to 2007

The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse ( i = 79 ∘ , perihelion distance 1.3 AU, aphelion distance 5.4 AU) since it encountered Jupiter in February 1992. Since then it has made almost three revolutions about the Sun. Here we report on the final three years of data taken by the on-board dust detector. During this time, the dust detector recorded 609 dust impacts of particles with masses 10 − 16 g ⩽ m ⩽ 10 − 7 g , bringing the mission total to 6719 dust data sets. The impact rate varied from a low value of 0.3 per day at high ecliptic latitudes to 1.5 per day in the inner solar system. The impact direction of the majority of impacts between 2005 and 2007 is compatible with particles of interstellar origin; the rest are most likely interplanetary particles. We compare the interstellar dust measurements from 2005/2006 with the data obtained during earlier periods (1993/1994) and (1999/2000) when Ulysses was traversing the same spatial region at southern ecliptic latitudes but the solar cycle was at a different phase. During these three intervals the impact rate of interstellar grains varied by more than a factor of two. Furthermore, in the two earlier periods the grain impact direction was in agreement with the flow direction of the interstellar helium while in 2005/2006 we observed a shift in the approach direction of the grains by approximately 30 ∘ away from the ecliptic plane. The reason for this shift remains unclear but may be connected with the configuration of the interplanetary magnetic field during solar maximum. We also find that the dust measurements are in agreement with the interplanetary flux model of Staubach et al. (1997) which was developed to fit a 5-year span of Ulysses data.

Excess open solar magnetic flux from satellite data: 1. Analysis of the third perihelion Ulysses pass

We use the third perihelion pass by the Ulysses spacecraft to illustrate and investigate the “flux excess” effect, whereby open solar flux estimates from spacecraft increase with increasing heliocentric distance. We analyze the potential effects of small‐scale structure in the heliospheric field (giving fluctuations in the radial component on timescales smaller than 1 h) and kinematic time‐of‐flight effects of longitudinal structure in the solar wind flow. We show that the flux excess is explained by neither very small‐scale structure (timescales < 1 h) nor by the kinematic “bunching effect” on spacecraft sampling. The observed flux excesses is, however, well explained by the kinematic effect of larger‐scale (>1 day) solar wind speed variations on the frozen‐in heliospheric field. We show that averaging over an interval T (that is long enough to eliminate structure originating in the heliosphere yet small enough to avoid cancelling opposite polarity radial field that originates from genuine sector structure in the coronal source field) is only an approximately valid way of allowing for these effects and does not adequately explain or account for differences between the streamer belt and the polar coronal holes.

Scaling Laws of Turbulence and Heating of Fast Solar Wind: The Role of Density Fluctuations

Incompressible and isotropic magnetohydrodynamic turbulence in plasmas can be described by an exact relation for the energy flux through the scales. This Yaglom-like scaling law has been recently observed in the solar wind above the solar poles observed by the Ulysses spacecraft, where the turbulence is in an Alfvénic state. An analogous phenomenological scaling law, suitably modified to take into account compressible fluctuations, is observed more frequently in the same data set. Large-scale density fluctuations, despite their low amplitude, thus play a crucial role in the basic scaling properties of turbulence. The turbulent cascade rate in the compressive case can, moreover, supply the energy dissipation needed to account for the local heating of the nonadiabatic solar wind.

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.

THE FLUX OF OPEN AND TOROIDAL INTERPLANETARY MAGNETIC FIELD AS A FUNCTION OF HELIOLATITUDE AND SOLAR CYCLE

We examine 45 years of near-Earth interplanetary observations as recorded in the National Space Science Data Center OMNI data set and 18 years of observations by the Ulysses spacecraft. We find that recent claims for a 50+ year low in interplanetary magnetic field intensity are substantiated, that the low flux levels can be attributed to open magnetic field lines, and that similar conditions existed in 1963-1964 during the start of the space age. Although the current solar minimum seems prolonged by recent standards and remarkably devoid of transient eruptions, we find no evidence that the present low in magnetic field intensity can be traced to the low in transient ejection rates as measured by toroidal magnetic field levels. We also find that the flux of toroidal magnetic field is approximately zero at high latitudes as measured by the Ulysses spacecraft at all times including both solar minimum and solar maximum conditions. We likely attribute this to the subphotospheric azimuthal magnetic fields that arise due to differential rotation. Our analysis of toroidal flux rates compares favorably with computed estimates of axial flux ejection associated with coronal mass ejections and allows us to place an upper limit on the average latitudinal separation of the footpoints.

Latitudinal dependence of the solar wind density derived from remote sensing measurements using interplanetary Lyman α emission from 1999 to 2002

Abstract The interplanetary Lyman α backscattered emission is an effective tool for remote sensing of the global structure of the solar wind proton flux. This paper reports an attempt to derive the latitudinal dependence of the solar wind density by combining the interplanetary Lyman α measurements of the Nozomi spacecraft for the period 1999–2002 with the solar wind speed data derived from interplanetary scintillation measurements. This approach successfully revealed the slow and dense solar wind over the poles during the period of the solar maximum. Data on the polar solar wind density indicate a significant growth from the middle of 2000, and the polar values of solar wind density are close to those of the equatorial values as a result of the disappearance of the coronal hole. A marked density depletion occurred in the middle of 2001, which can be ascribed to the development of fast winds from the polar coronal hole. To evaluate the remote sensing method, we considered solar wind density data from in situ measurements obtained by the Ulysses spacecraft. We conclude that our method basically agrees with in situ measurements, although we found a significant (a factor of 2) difference between these in the middle of 2001.

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.

The Energy Cascade in Solar Wind MHD Turbulence

Direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), is observed in the solar wind by the Ulysses spacecraft. A linear relation is indeed observed for the scaling of mixed third order structure functions involving Elsasser variables. This experimental result, confirming the prescription stemming from a theorem for MHD turbulence, firmly establishes the turbulent character of low-frequency velocity and magnetic field fluctuations in the solar wind plasma.

Heliospheric sector boundaries: Single or multiple?

Solar wind data were examined for a period when the Ulysses spacecraft moved roughly parallel to the boundary between inward and outward interplanetary fields at a solar distance of 1.4 AU. Multiple crossings of the magnetic sector boundary were observed. Three tests were used to evaluate whether or not the crossings were due to multiple crossings of a single surface or single crossings of separate surfaces: (1) a correlation between the latitude and longitude directions of the normals to the surface, (2) the radius of curvature of the surface calculated from consecutive crossings, and (3) the variation of plasma parameters across the surfaces. Both multiple crossings of a single rippled surface and single crossings of multiple boundary surfaces were observed.

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.

Jovian periodicities (∼10 h, ∼40 min) on Ulysses’ Distant Jupiter Encounter observations around the Halloween CIR events

We analyzed data from four different instruments (HI-SCALE, URAP, SWOOPS, VHM/FGM) onboard Ulysses spacecraft (s/c) and we searched for possible evidence of Jovian emissions when the s/c approached Jupiter during the times of Halloween events (closest time approach/position to Jupiter: February 5, 2004/ R = 1683 R J, θ = ∼49°). In particular, we analyzed extensively the low energy ion measurements obtained by the HI-SCALE experiment in order to examine whether low energy ion/electron emissions show a symmetry, and whether they are observed at north high latitudes upstream from the jovian bow shock, as is known to occur in the region upstream from the south bow shock as well ( Marhavilas et al., 2001). We studied the period from October 2003 to March 2004, as Ulysses moved at distances 0.8–1.2 AU from the planet at north Jovicentric latitudes <75°, and we present here an example of characteristic Jovian periodicities in the measurements around a CIR observed by Ulysses on days ∼348–349/2003 ( R = 1894 R J, θ = 72°). We show that Ulysses observed low energy ion (∼0.055–4.7 MeV) and electron (>∼40 keV) flux and/or spectral modulation with the Jupiter rotation period (∼10 h) as well as variations with the same period in solar wind parameters, radio and magnetic field directional data. In addition, characteristic strong ∼40 min periodic variations were found superimposed on the ∼10 h ion spectral modulation. Both the ∼10 h and ∼40 min ion periodicities in HI-SCALE measurements were present in several cases during the whole period examined (October 2003 to March 2004) and were found to be more evident during some special conditions, for instance during enhanced fluxes around the start (forward shock) and the end (reverse shock) of CIRs. We infer that the Jovian magnetosphere was triggered by the impact of the CIRs, after the Halloween events, and it was (a) a principal source of forward and reverse shock-associated ion flux structures and (b) the cause of generation of ∼10 h quasi-periodic magnetic field and plasma modulation observed by Ulysses at those times.

Observations of recurrent cosmic ray decreases during solar cycles 22 and 23

Abstract. During solar cycle 22, the modulation of several hundred MeV galactic cosmic rays (GCRs) by recurrent and transient cosmic ray decreases was observed by the Ulysses spacecraft on its descent towards the solar south pole. In solar cycle 23, Ulysses repeated this trajectory segment during a similar phase of the solar cycle, but with opposite heliospheric magnetic field polarity. Since cosmic ray propagation in the heliosphere should depend on drift effects, we determine in this study the latitudinal distribution of the amplitude of recurrent cosmic ray decreases in solar cycles 22 and 23. As long as we measure the recurrent plasma structures in situ, we find that these decreases behave nearly the same in both cycles. Measurements in the fast solar wind, however, show differences: in cycle 22 (A&gt;0) the recurrent cosmic ray decreases show a clear maximum near 25° and are still present beyond 40°, whereas we see in cycle 23 (A&lt;0) neither such a pronounced maximum nor significant decreases above 40°. In other words: the periodicity in the cosmic ray intensity, which can be clearly seen in the slow solar wind, appears to vanish there. Theoretical models for drift effects, however, predict quite the opposite behaviour for the two solar cycles. To closer investigate this apparent contradiction, we first put the visual inspection of the data onto a more solid basis by performing a detailed Lomb (spectral) analysis. The next step consists of an analysis of the resulting periodicities at 1 AU in order to distinguish between spatial and temporal variations, so that we can obtain statements about the question in how far there is a correlation between the in-situ data at 1 AU and those measured by Ulysses at larger latitudes. We find a good correlation being present during cycle 22, but not for cycle 23. As one potential explanation for this behaviour, we suggest the difference in the coronal hole structures between the cycles 22 and 23 due to a large, stable coronal hole structure, which is present during cycle 22, but not in cycle 23. We support this possibility by comparing Yohkoh SXT and SOHO EIT maps.