Ulysses Space Probe Research Articles

Electromagnetic bounded States and challenges of plasma spectroscopy

We review the foundations and observational support for a new plasma wave mode that has been proposed to explain emissions stimulated by satellite-born high-power radio-frequency sounders. These sounders, which are designed to derive remote electron density profiles from electromagnetic-wave echoes, stimulate plasma resonances in the local plasma. In addition to stimulating resonances at the electron plasma frequency f/sub pe/, harmonics of electron gyrofrequency f/sub ce/, and at the upper hybrid frequency f/sub uh/, resonances are also stimulated at frequencies below f/sub pe/ between the gyroharmonics. The first such resonance, with frequency between f/sub ce/ and 2f/sub ce/, was discovered by Nelms and Lockwood in 1967 using the topside sounder on Alouette 2. From its appearance, it was called a diffuse resonance (D/sub 1/ in modern notation) and was originally thought to have a frequency of 3/2f/sub ce/. Further observations, and theoretical insights, indicated that f/sub D1/ is dependent on f/sub pe/ and f/sub ce/, namely, f/sub D1/=(3//spl pi/)(f/sub pe/f/sub ce/)/sup 1/2/. The theoretical concepts used to obtain this expression indicated that the D/sub n/ resonances (n=1,2,3,...) are electron eigenmodes (electromagnetic bounded states) and led to equations describing the frequencies of the entire D/sub n/ sequence and their subsidiary resonances D/sub n//sup +/ and D/sub n//sup -/. Such formulas, based on a combined theoretical and empirical approach, describe a /spl radic/n spectrum with the frequency of each mode split by the magnetic field analogous to the Zeeman splitting in quantum mechanics. The /spl radic/n spectrum was first predicted for force-free electromagnetic cylindrical plasma oscillations by Osherovich in 1986. The main theoretical challenge remains to derive the frequency f/sub D1/ from first principles. Although the D/sub n/ resonances have been stimulated in the terrestrial ionosphere for decades by topside sounder satellites such as Alouette 2 and by electron guns on several rocket experiments, and in Jupiter's Io plasma torus by the sounder on the Ulysses space probe, and are currently being stimulated in the terrestrial magnetosphere by the sounder on the IMAGE spacecraft, they have yet to be observed in laboratory experiments.

In Situ Measurements of Interstellar Dust

AbstractWe present the mass distribution of interstellar grains measured in situ by the Galileo and Ulysses space probes as cumulative flux. The derived in situ mass distribution per logarithmic size interval is compared to the distribution determined by fitting extinction measurements. Large grains measured in situ contribute significantly to the overall mass of dust in the local interstellar cloud. The problem of a dust-to-gas mass ratio that contradicts cosmic abundances is discussed.

Solar modulation of galactic cosmic rays: The 3D heliosphere

The cosmic ray flux observed with the Kiel Electron Telescope onboard the Ulysses space probe varies with solar activity as well as with heliospheric position. The Ulysses' fast latitude scan performed in less than one year between September 1994 to August 1995 allowed us to obtain unique data about the latitudinal dependence of cosmic ray fluxes close to solar minimum conditions. In this paper we discuss the flux variation of protons, α-particles and electrons in the few GV rigidity range, and compare our measurements with predictions from 2-dimensional drift dominated modulation models. We focus our interest on the energy dependence of the latitudinal gradient of the nuclei component, the North-South distribution of >106 MeV protons and charge dependence of 2.5 GV particles during the fast latitude scan. Whereas the latitudinal gradient for α-particles monotonically decreases with increasing energy, protons show a maximum latitudinal gradient at several hundred MeV. This maximum was not expected to exist, but can be explained in modified modulation models by increasing the diffusion perpendicular to the magnetic field in polar direction. The counting rate of >106 MeV protons in the northern and southern hemisphere is not symmetric with respect to the heliographic equator but with respect to “magnetic” latitude. The e/pratio of 2.5 GV particles shows that electrons and protons have similar temporal modulation and that the latitudinal dependence of this ratio is dominated by the latitudinal proton variation. The modified model can qualitatively reproduce the observed behaviour during the fast latitude scan.

Solar wind corotating interaction regions: The third dimension

A new dimension has recently been added to our understanding of solar wind structure ‐ the dimension out of the solar equatorial plane. Measurements made by the Ulysses space probe at high heliographic latitudes and powerful new 3‐dimensional numerical simulations have revealed that solar wind corotating interaction regions (CIRs) tend to be tilted systematically relative to the solar equatorial plane, at least on the declining phase of the ∼11‐year solar activity cycle. One consequence of this tilt is that with increasing heliocentric distance the forward waves associated with the CIRs tend to propagate toward and across the equator while the reverse waves tend to propagate to higher heliographic latitudes. It has been shown that tilted interaction regions are a natural consequence of the tilt of the solar magnetic dipole relative to the solar rotation axis. This new understanding of the 3‐dimensional nature of CIRs is essential to the interpretation of energetic particle measurements at high solar latitudes, and is useful for explaining certain puzzling aspects of Voyager observations in the very distant heliosphere at lower latitudes. Closer to home, there is an awakening perception of the important role played by CIRs in stimulating recurrent geomagnetic activity. The present paper provides a limited review of recent U.S. work on these topics and on other aspects of corotating interaction regions in the solar wind.

Interplanetary shock waves: Ulysses observations in and out of the ecliptic plane

Between its launch in October 1990 and the end of 1993, approximately 160 fast collisionless shock waves were observed in the solar wind by the Ulysses space probe. During the in-ecliptic part of the mission, to February 1992, the observed shock waves were first caused mainly by solar transient events following the solar maximum and the reorganisation of the large scale coronal fields. With the decay in solar activity, relatively stable Corotating Interaction Regions (CIRs) were observed betwen 3 and 5.4 AU, each associated with at least one forward-reverse shock pair. During the out-of-ecliptic phase of the orbit, from February 1992 onwards, CIRs and shock pairs associated with them continued to dominate the observations. From July 1992, Ulysses encountered the fast solar wind flow from the newly developed southern polar coronal hole, and from May 1993 remained in the unipolar magnetic region associated with this coronal hole. At latitudes beyond 30°, CIRs were associated almost exclusively with reverse shocks only. A comprehensive list of shock waves identified in the magnetic field and solar wind plasma data from Ulysses is given in Table 1. The principal characteristics were determined mainly from the magnetic field data. General considerations concerning the determination of shock characteristics are outlined in the Introduction.

The ulysses mission — a voyage to the poles of the sun

The Ulysses space probe is the first heliospheric mission to carry out in situ observations over all solar latitudes. It is providing a unique set of fields and particles data which will allow a quantitative modelling of the threedimensional heliosphere. This paper reviews highlights of the observations made on board the Ulysses spacecraft over a range of heliolatitudes from near the ecliptic plane to the southern polar regions of the Sun.

Modulation of galactic cosmic ray particles observed on board the ULYSSES spacecraft

The cosmic ray flux observed with the Kiel Electron Telescope on board the ULYSSES spaceprobe varies with solar activity as well as with heliospheric position. Determination of spatial gradients requires a careful analysis of the influences of the current sheet tilt angle, the number of major solar flares, interplanetary shocks and interaction regions evolving in the expanding solar wind. In this paper we concentrate on nuclei with rigidity above 1 GV. We discuss the effects of the variable solar activity in the declining phase of the present solar cycle and the variation with radial distance during the in ecliptic mission period as a basis for separating latitudinal effects. We show that during this phase of the solar cycle modulation of GV nuclei is ordered by temporal evolution, radial distance and small latitudinal effects up to latitudes of 50° South. We discuss the influence on GV nuclei of a disturbance in early 1994 when ULYSSES was beyond 50° South.

Observations of the solar corona with ulysses: Expectations above the polar regions

During its prime mission phase, the Ulysses space probe will fly above both solar poles, providing an unprecedented opportunity to study the high latitude regions of the solar corona. The timing of the polar passes (mid-1994 and mid-1995) coincides with solar minimum, corresponding to a coronal configuration consisting of well-developed polar coronal holes and a near-equatorial heliospheric current sheet. The nine sets of instruments that make up the Ulysses scientific payload are expected to return unique data concerning the high latitude solar wind and interplanetary magnetic field, as well as the solar energetic particles that populate these regions. We present a review of some of these expectations, illustrated by results obtained in 1991 during the in-ecliptic phase of the Ulysses mission.

Dust Measurements in the Outer Solar System

In-situ measurements of micrometeoroids provide information on the spatial distribution of interplanetary dust and its dynamical properties. Pioneers 10 and 11, Galileo and Ulysses spaceprobes took measurements of interplanetary dust from 0.7 to 18 AU distance from the sun. Distinctly different populations of dust particles exist in the inner and outer solar system. In the inner solar system, out to about 3 AU, zodiacal dust particles are recognized by their scattered light, their thermal emission and by in-situ detection from spaceprobes. These particles orbit the sun on low inclination (i ≤ 30°) and moderate eccentricity (e ≤ 0.6) orbits. Their spatial density falls off with approximately the inverse of the solar distance. Dust particles on high inclination or even retrograde trajectories dominate the dust population outside about 3 AU. The dust detector on board the Ulysses spaceprobe identified interstellar dust sweeping through the outer solar system on hyperbolic trajectories. Within about 2 AU from Jupiter Ulysses discovered periodic streams of dust particles originating from within the jovian system.

The intrepid heliospheric explorer

Like its mythological namesake, the Ulysses space probe is breaking new ground across the Solar System. Ulysses is the first satellite to explore the region of space above and below the poles of the Sun – the previously uncharted third dimension of the solar environment, the heliosphere. Explorations like Ulysses are crucial to our understanding of the Sun and its surroundings, but such a mission has not been possible until recently.

Galileo and Ulysses dust measurements: Fz Venus to Jupiter

The Galileo and Ulysses spaceprobes carry two similar dust detectors through interplanetary space from Venus to Jupiter. We report here on impacts which correspond to dust particles above a mass threshold of about 10−13g for which we have complete records. Between December 1989 and January 1992 Galileo repeatedly traversed interplanetary space between 0.7 and 2.26 AU and recorded 374 impacts. The observed impact rates ranged from 0.1 to about 3 impacts per day strongly dependent on whether the spacecraft moved towards or away from the Sun. From October 1990 to January 1992 the Ulysses spacecraft had reached a distance of 5.17 AU from the Sun and had recorded 72 impacts at rates between 0.1 and 0.5 per day. Inside about 2 AU the observed fluxes are compatible with a population of interplanetary dust particles moving on low to moderately eccentric (e = 0.1 to 0.5) and low inclination (i = 0 deg. to 30 deg.) orbits. Outside this distance a dust particle population on different orbits is required in order to explain the Ulysses data.