Ulysses Mission Research Articles

Effects of an apparent space dust impact on a position sensing solid state detector aboard the Ulysses spacecraft

The High Energy Telescope (HET) is part of the COsmic and Solar Particle INvestigation (COSPIN) aboard the Ulysses spacecraft. The Ulysses mission is to explore the Heliosphere in three dimensions by passing over the polar regions of the sun. Ulysses achieved it present high inclination (>80°) solar orbit via a gravitational sling-shot around Jupiter. During or proximate to Jovian encounter, the HET suffered an apparent dust impact. The dust particle entered through the telescope aperture windows and damaged the top position sensing solid state detector. This paper discusses the evidence of the impact in the data, and the effect of the impact on the detector.

H2+ pickup ions in the solar wind: Outgassing of interplanetary dust

Heliospheric neutral atoms and molecules can be ionized and picked up by the solar wind plasma flow. A new generation of space plasma instruments allows detailed study of the solar wind pickup ions with high sensitivity. Detection of H2+ pickup ions in the solar wind is an exceptionally sensitive tool for the search of molecular hydrogen in the heliosphere. Possible sources of neutral molecular hydrogen include contributions from the local interstellar medium release from cometary bodies, and outgassing of interplanetary dust. The flux of the solar wind H2+ pickup ions due to outgassing of interplanetary dust is theoretically established in this work. Solar wind ions bombard and penetrate dust grains where they are neutralized and subsequently released in the form of neutral atoms and molecules. The processes of molecular hydrogen interaction with the solar radiation and solar wind plasma are considered in detail and the process rates are determined. Solar wind H2+ pickup ion fluxes are obtained for heliocentric distances < 5 AU for different heliocentric latitudes. These heliocentric radial and latitudinal dependences would provide guidance for search of the solar wind H2+ pickup ions by Ulysses, WIND, and SOHO missions.

Status report on the U.S. space nuclear program

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

Properties of magnetohydrodynamic turbulence in the solar wind as observed by Ulysses at high heliographic latitudes

The Ulysses mission provides an opportunity to study the evolution of magnetohydrodynamic (MHD) turbulence in pure high‐speed solar wind streams. The absence at high heliocentric latitudes of the strong shears in solar wind velocity generally present near the heliocentric current sheet allows investigation of how fluctuations in the magnetic field and plasma relax and evolve in the radially expanding solar wind. We report results of an analysis of the radial and latitudinal variation of the turbulence properties of the fluctuations, especially various plasma‐field correlations, in high latitude regions. The results constrain current theories of the evolution of MHD turbulence in the solar wind. Compared to similar observations at 0.3 AU by Helios, we find spectra that are similar in having a large frequency band with an f¹ power spectrum in the outward traveling component of the waves, followed at higher frequencies by a steeper spectrum. Ulysses observations establish that at high latitudes the turbulence is less evolved (i.e., has a smaller inertial range) than it is in the ecliptic at the same heliocentric distance, apparently due to the absence of strong velocity shear. Once Ulysses is in the polar coronal hole, properties of the turbulence appear to be determined by the heliocentric distance of the spacecraft rather than by its helio‐latitude.

Ulysses observations of cosmic gamma-ray bursts

We review the current status of the Ulysses mission and summarize the results to date of the GRB experiment. This instrument detects bursts at the rate of about one every 3.5 days, and the localization data are being disseminated rapidly via the BACO-DINE and NMSU networks. The mission should operate through 2001, and future missions to Mars starting in 1996 will complete the 3rd Interplanetary Network.

Solar wind double ion beams and the heliospheric current sheet

Double ion beams are often observed in the solar wind, but little work has been done in relating these beams to structures within the solar wind. Double ion beams are observed as beams of a given ion species and charge state occurring at two different energies. We use the three‐dimensional ion plasma instrument on board the Ulysses spacecraft to look for evidence of such beams associated with the heliospheric current sheet. In a subset chosen independently of plasma parameters consisting of 8 of over 47 crossings of the current sheet made during the in‐ecliptic phase of the Ulysses mission we find that these double ion beams are always present on either side of the current sheet. The double beams are present in both the proton and helium species. The secondary beam typically has a higher helium abundance, which suggests that these beams are formed in the helium‐rich corona rather than in interplanetary space. The double beams are not present in the interior of the current sheet. Neither collisions nor effects of plasma beta can account for the disappearance of the double beams inside the current sheet in all eight cases. We postulate that these beams are formed by reconnection occurring near the Sun in the boundary region between the open field lines of the coronal holes and the closed field line region of the heliospheric current sheet. Such a scenario would be consistent with previous X ray measurements which suggest that reconnection is occurring in this region.

Cosmic ray modulation in the 3-D heliosphere

The basic physical processes that lead to the long-term modulation of cosmic rays by the solar wind have been known for many years. However our knowledge of the structure of the heliosphere, which determines which processes are most important for the modulation, and of the variation of this structure with time and solar activity level is still incomplete. Study of the modulation provides a tool for probing the scale and structure of the heliosphere. While the Pioneer and Voyager spacecraft are surveying the radial structure and extent of the heliosphere at modest heliographic latitudes, the Ulysses mission is the first to undertake a nearly complete scan of the latitudinal structure of the modulated cosmic ray intensity in the inner heliosphere (R<5.4 AU). Ulysses will reach latitudes of ~80°S in September 1994 and ~80°N in July 1995 during the approach to minimum activity in the 11 year solar cycle. We present a first report of measurements extending to latitudes of ~52°S, which show surprisingly little latitudinal effect in the modulated intensities and suggest that at this time modulation in the inner heliosphere may be much more spherically symmetric than had generally been believed based upon models and previous observations.

The evolution of the anomalous component of the cosmic rays during the Ulysses mission

We have studied the evolution of the anomalous cosmic ray component at ULYSSES using observations of the quiet time helium fluxes obtained with the COSPIN Low Energy Telescope during the years from launch in October 1990 to the latest data in 1994. Shown are preliminary results of measurements in the energy range from 4.0 to 19 MeV/n. Following the drop in solar activity in late 1991, the low energy cosmic ray helium flux started to increase and continue to rise until the end of 1993, signifying the appearance of the anomalous component at ULYSSES.

Solar wind charge states measured by ULYSSES/SWICS in the south polar hole

The Ulysses mission now has an extensive data base covering several passes of the south polar coronal hole as the spacecraft proceeds to higher latitudes. Using composition measurements from the SWICS experiment on the Ulysses spacecraft, we have obtained charge state distributions, and hence inferred coronal ionization temperatures, for several solar wind species. In particular, we present an overview of Oxygen ionization temperature measurements, based on the O7+/O6+ ratio, for the period January 1993 until April 1994 (∼23°S to ∼61°S heliographic latitude), and detailed Oxygen, Silicon and Iron charge state distributions of the south polar hole during a two month period of nearly continuous hole coverage, Dec 1993–Jan 1994 (∼45°S to 52°S heliographic latitude).

Latitudinal variations in the solar wind electron heat flux

Ulysses measurements of the solar wind electron heat flux as a function of heliographic latitude are presented. The latitudinal in the electron heat flux presented have been normalized by the radial gradient in the electron heat flux obtained during the in-ecliptic phase of the Ulysses mission (qe∼ R−3.0). We find no significant variation in electron heat flux with latitude.

Exploring the heliosphere in three dimensions a keynote presentation

The 28th ESLAB Symposium marks the beginning of the sprint of the Ulysses mission to the very high heliographic latitudes and the pole-to-pole passage. The more than twenty-year quest to understand the Sun and the heliosphere in three dimensions is about to be realized. It is perhaps worthwhile, as we are poised to begin this journey, to ask how history is likely to judge this mission, or equivalently: what questions need to be answered so that the judgment will be kind?

Spatial distribution and orbital properties of interplanetary dust at high latitudes

Although the interplanetary dust cloud is assumed to be mainly concentrated in the ecliptic plane, there is a component of dust particles on highly inclined orbits that forms the out-of-ecliptic distribution. The ULYSSES mission for the first time makes this component accessible to in-situ detection. Evidence for this dust component is also provided from the analysis of the Zodiacal light brightness and especially from the spherical shape of the solar F -corona. An obvious source for an out-of-ecliptic dust population is the activity of comets on high eccentric, highly inclined orbits. We discuss the dynamical conditions of particles under the influence of the radiation pressure when released from the comet and discuss their input to the dust cloud based on brightness analysis and in-situ results.

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.

Magnetic fields in the high latitude heliosphere

Observations in the out-of-ecliptic heliosphere had not been available before the Ulysses mission. Understanding the 3-dimensional structures and properties of the medium is important to model successfully those processes, such as the propagation and modulation of cosmic rays, which are expected to depend on heliolatitude. In this paper, magnetic field observations made onboard the Ulysses spacecraft over a range of heliolatitudes from near the ecliptic plane to the southern polar regions of the Sun are presented and discussed.

Eclipse Photo Captures Solar Magic Act

Of all visual solar effects, the sudden—even mystical—appearance and disappearance of the white light corona at eclipse is the most striking. On November 3, 1994, the High Altitude Observatory eclipse expedition to Putre, Chile, obtained this photograph showing the form of the solar corona late in Solar Cycle 22. This image adds to the long series of drawings and photographs made by many past observers that have recorded the form of the corona at eclipse and thus show coronal variability in the 11‐year solar activity cycle.Several features in this recent solar image are important to understanding the structure of the corona and its extension through the heliosphere. The striking appearance of bright “streamers” separating dark “polar holes” is characteristic of the corona late in a solar cycle. The very long streamer on the SW limb is a cross section of a much larger coronal arcade covering 270° in solar longitude. The long tip of this streamer was visible to the naked eye out to 3 solar radii. This long structure is the cross section of a current sheet separating solar wind flows along field lines of opposite polarity. In addition, the polar plumes at the Sun's south pole show a structure that requires the polar fields to be ‘patchy’ and not uniformly distributed in the polar coronal hole. Both the streamers and the plumes clearly show the departure of coronal structures from a radial orientation. These observations have important implications for the Ulysses mission because the spacecraft was 3 AU behind the southern solar pole and in the polar solar wind at the time of the eclipse.

Ulysses solar wind plasma observations during the declining phase of solar cycle 22

Since launch in October 1990, the Ulysses mission has included an in-ecliptic cruise enroute to Jupiter encounter in February 1992 and a post-Jupiter transit through a wide range of southerly latitudes and heliocentric distances. Here we present results from the solar wind plasma experiment through June 14, 1994, at which time Ulysses was at −68.2° heliographic latitude. During the ecliptic phase of the mission, occurring just after solar maximum, the spacecraft encountered an irregular pattern of solar wind speed and sporadic coronal mass ejections, with mass ejections most prevalent during March 1991. Irregular, small-amplitude solar wind streams prevailed until mid-1992, after which Ulysses encountered a recurrent very high-speed stream from an equatorward extension of the South polar coronal hole. Encounters with the high-density, low-speed plasma from the coronal streamer belt ceased as Ulysses moved to increasing southerly latitudes in 1993. Many forward and reverse shocks associated with corotating interaction regions have been encountered; these shocks all had observable electron foreshocks. The shocks became less prevalent with increasing latitude, with the forward shocks disappearing first because of the tilted streamer belt and the resulting meridional shock propagation. After Ulysses passed −35° in July 1993 the spacecraft encountered only high-speed wind, with a speed range of 700–800 km s −1 and a density, scaled to 1 AU, averaging 3 cm −3. Latitudinal gradients in solar wind fluid parameters generally support previous findings, with the gradient in wind speed offset by a gradient in density such that mass and momentum flux vary relatively little.

The whistler heat flux instability: Threshold conditions in the solar wind

Solar wind electrons are observed often to consist of two components: a core and a halo. The anisotropies and relative average speeds of these two components correspond to a heat flux that has the potential to excite several different electromagnetic instabilities; wave‐particle scattering by the resulting enhanced fluctuations can limit this heat flux. This manuscript describes theoretical studies using the linear Vlasov dispersion equation for drifting bi‐Maxwellian component distributions in a homogeneous plasma to examine the threshold of the whistler heat flux instability. Expressions for this threshold are obtained from two different parametric baselines: a local model that yields scalings as functions of local dimensionless plasma parameters, and a global model based on average electron properties observed during the in‐ecliptic phase of the Ulysses mission. The latter model yields an expression for the heat flux at threshold of the whistler instability as a function of heliospheric radius that scales in the same way as the average heat flux observed from Ulysses and that provides an approximate upper bound for that same quantity. This theoretical scaling is combined with the observational results to yield a semi‐empirical closure relation for the average electron heat flux in the solar wind between 1 and 5 AU.

The heliosphere and the Ulysses mission

The interplanetary medium consists primarily of the supersonic solar wind, carrying the frozen-in magnetic field extending from the solar corona. The properties of this medium are controlled by the state of the corona and by dynamic processes occurring in the medium itself. As a result, there are significant variations in those properties as a function of heliolatitude. In situ observations over the past three decades have been largely confined to the neighbourhood of the solar equatorial plane. While many of the important processes have been identified and studied extensively, observations are required as a function of heliolatitude to define large-scale structures and their dependence on processes in the solar corona. The Ulysses mission, launched in October 1990, is the first space probe dedicated to the exploration of the heliosphere out of the ecliptic plane. By January 1994, the spacecraft had reached a heliolatitude of 50° south. The first results of the mission are summarized here, including the evolution and disappearance of the interplanetary magnetic sector structure; the onset of the dominance of the high-speed solar wind stream originating in the expanding southern coronal hole; observations of the signatures of complex coronal mass ejections; the high-latitude structure of the heliospheric magnetic field, and the evolution of corotating interaction regions as a function of heliolatitude. In particular, the abrupt change in the rotation rate of the sector structure in mid-1992, followed by the equatorward extension of the southern polar coronal hole, represent new observations related to the evolution of large-scale coronal structures and solar magnetic fields and to processes controlling the solar activity cycle.

Symposium explores the high‐latitude heliosphere

The October 6, 1990, launch of the joint European Space Agency (ESA)‐NASA Ulysses mission marked the start of a new era in the study of the heliosphere. For the first time since the dawn of the space age, in situ observations of heliospheric fields and particles are being made in the Sun's polar regions and over the full range of heliographic latitudes.Since shortly after launch, Ulysses has returned an unprecendented data set with which to study the properties of the solar wind, the heliospheric magnetic field, locally accelerated and solar energetic particles, and cosmic rays, as well as important interstellar constituents, dust, and neutral helium. The spacecraft also carries instrumentation to detect solar X rays and cosmic gamma rays, and a radio science investigation to probe the solar corona.

‘Fire and Ice’ Mission looks promising

A future joint U.S.‐Russian “Fire and Ice” mission to the Sun and Pluto is looking “very interesting and very feasible,” according to a joint technical review team on space science that met last week at the Jet Propulsion Laboratory in Pasadena, Calif.Although a concrete plan does not exist, the joint team is evaluating the possibility of sending two probes to explore the Sun and another two probes to fly by Pluto. As in the Ulysses mission to the Sun's poles, Jupiter would be used as a gravitational slingshot to position the probes into appropriate orbits. Each country would develop and monitor one craft in each pair of missions, but reigning technologies would be shared. For instance, the probes would be launched at the same time with the Russian proton rocket, according to Miriam Forman, a NASA cosmic and heliospheric scientist.