ESA Science & Technology - Publications Archive

Interstellar dust (ISD) in the solar system was detected in situ for the first time in 1993 by the Ulysses dust detector. The study of ISD is important for understanding its role in star and solar system formation. The goal of this paper is to understand the variability in the ISD observations from the Ulysses mission by using a Monte Carlo simulation of ISD trajectories, with the final aim to constrain the ISD particle properties from simulations and the data. The paper is part of a series of three: Strub et al. describe the variations of the ISD flow from the Ulysses data set, and Krüger et al. focus on its ISD mass distribution. We describe and interpret the simulations of the ISD flow at Ulysses orbit for a wide range of particle properties and discuss four open issues in ISD research: the existence of very big ISD particles, the lack of smaller ISD particles, the shift in dust flow direction in 2005, and particle properties. We conclude that the shift in the dust flow direction in 2005 can best be explained by Lorentz force in the inner heliosphere, but that an extra filtering mechanism is needed to fit the fluxes. A time-dependent filtering in the outer regions of the heliosphere is proposed for this. Also, the high charge-to-mass ratio values found for the heavier particles after 2003 indicate that these particles are lower in density than previously assumed. This method gives new insights into the ISD properties and paves the way toward getting a complete view on the ISD from the local interstellar cloud. We conclude that in combination with the data and simulations, also impact ionization experiments are necessary using low-density dust, in order to constrain the density of the particles.

The Ulysses spacecraft provided the first opportunity to identify and study interstellar dust (ISD) in situ in the solar system between 1992 and 2007. Here we present the first comprehensive analysis of the ISD component in the entire Ulysses dust data set. We analyzed several parameters of the ISD flow in a time-resolved fashion: flux, flow direction, mass index, and flow width. The general picture is in agreement with a time-dependent focusing/defocusing of the charged dust particles due to long-term variations of the solar magnetic field throughout a solar magnetic cycle of 22 years. In addition, we confirm a shift in dust direction of 50° ± 7° in 2005, along with a steep, size-dependent increase in flux by a factor of 4 within 8 months. To date, this is difficult to interpret and has to be examined in more detail by new dynamical simulations. This work is part of a series of three papers. This paper concentrates on the time-dependent flux and direction of the ISD. In a companion paper we analyze the overall mass distribution of the ISD measured by Ulysses, and a third paper discusses the results of modeling the flow of the ISD as seen by Ulysses.

In the early 1990s, contemporary interstellar dust penetrating deep into the heliosphere was identified with the in situ dust detector on board the Ulysses spacecraft. Between 1992 and the end of 2007 Ulysses monitored the interstellar dust stream. The interstellar grains act as tracers of the physical conditions in the local interstellar medium (ISM) surrounding our solar system. Earlier analyses of the Ulysses interstellar dust data measured between 1992 and 1998 implied the existence of a population of "big" interstellar grains (up to 10^-13 kg). The derived gas-to-dust-mass ratio was smaller than the one derived from astronomical observations, implying a concentration of interstellar dust in the very local ISM. In this paper we analyze the entire data set from 16 yr of Ulysses interstellar dust measurements in interplanetary space. This paper concentrates on the overall mass distribution of interstellar dust. An accompanying paper investigates time-variable phenomena in the Ulysses interstellar dust data, and in a third paper we present the results from dynamical modeling of the interstellar dust flow applied to Ulysses. We use the latest values for the interstellar hydrogen and helium densities, the interstellar helium flow speed of ν_ISM∞ = 23.2 km s^-1, and the ratio of radiation pressure to gravity, β, calculated for astronomical silicates. We find a gas-to-dust mass ratio in the local interstellar cloud of R_g/d = 193⁺⁸⁵_-57 and a dust density of (2.1 ± 0.6) × 10^-24 kg m^-3. For a higher inflow speed of 26 km s^-1, the gas-to-dust mass ratio is 20% higher, and, accordingly, the dust density is lower by the same amount. The gas-to-dust mass ratio derived from our new analysis is compatible with the value most recently determined from astronomical observations.
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Observations of solar wind from both large polar coronal holes (PCHs) during Ulysses' third orbit showed that the fast solar wind was slightly slower, significantly less dense, cooler, and had less mass and momentum flux than during the previous solar minimum (first) orbit. In addition, while much more variable, measurements in the slower, in-ecliptic wind match quantitatively with Ulysses and show essentially identical trends. Thus, these combined observations indicate significant, long-term variations in solar wind output from the entire Sun. The significant, long-term trend to lower dynamic pressures means that the heliosphere has been shrinking and the heliopause must be moving inward toward the Voyager spacecraft. In addition, our observations suggest a significant and global reduction in the mass and energy fed in below the sonic point in the corona. The lower supply of mass and energy may result naturally from a reduction of open magnetic flux during this period.

This extract of ESA's Report to the 36th COSPAR Meeting held in Beijing, China in July 2006, covers the Ulysses mission.

Editors: A. Balogh, L.J. Lanzerotti, and S.T. Suess A principal objective of space research has been to increase understanding of the way the Sun changes through its 11-year sunspot cycle, and how these changes affect the space around the Sun - the heliosphere. "The Heliosphere through the Solar Activity Cycle" is the first book to describe and analyze ground and space-based data on the three-dimensional heliosphere taken as the Ulysses spacecraft made orbital passes of the Sun's north and south poles in 1994-1995 and again in 2000-2001. The chapters explain the many different aspects of changes in the heliosphere in response to solar activity, and describe the rise in solar activity from the last minimum in 1998 to its maximum in 2000, and its subsequent decline. "The Heliosphere through the Solar Activity Cycle" was written by leading scientists directly involved with the Ulysses mission and edited by André Balogh, Louis J. Lanzerotti, and Steven T. Suess. It provides the definitive discussion of the subject to date and looks forward to discoveries to be made by Ulysses and potential follow-on missions in the future.

Contents

1.	The heliosphere: Its origin and exploration A. Balogh & L.J. Lanzerotti
2.	Solar Cycle 23 D.H. Hathaway & S.T. Suess
3.	The solar wind throughout the solar cycle R. von Steiger
4.	The global heliospheric magnetic field E.J. Smith
5.	Heliospheric energetic particle variations D. Lario & M. Pick
6.	Galactic and anomalous cosmic rays through the solar cycle: New insight from Ulysses B. Heber & M.S. Potgieter
7.	Overview: The heliosphere then and now S.T. Suess

This paper describes some unanticipated effects of the normal modes of the Sun on engineering and scientific systems. We begin with historical, scientific, and statistical background, then present evidence for the effects of solar modes on various systems. Engineering evidence for these modes was first noticed in an investigation of communications satellite failures and second in a study of excessive dropped calls in cellular phone systems. The paper also includes several sections on multitaper estimates of spectra, canonical coherences, robust, and cyclostationary variants of multitapering, and related statistical techniques used to separate the various components of this complex system. In our attempt to understand this unexpected source of problems, we have found that solar modes are detectable in the interplanetary magnetic fields and energetic particles at the Ulysses spacecraft, five astronomical units from the Earth. These modes couple into the magnetosphere, the ionosphere, the geomagnetic field, and atmospheric pressure. Estimates of the power spectrum of data from solar radio telescopes and induced voltages on ocean cables show what appear to be solar modes at both lower and higher frequencies than the optically measured solar p-modes. Most surprisingly, these modes are easily detected in seismic data, where they literally shake the Earth.

We use the space weather validated 3-D HAFv2 model to help us study the interplanetary propagation of the October/November 2003 solar eruptions from the Sun to >90 AU and over a wide range of heliolongitudes and heliolatitudes. The HAFv2 model predictions at ACE (1 AU), Ulysses (5.23 AU), Cassini (8.67 AU), Voyager 2 (73 AU), and Voyager 1 (93 AU) are compared with available data. These comparisons indicate the importance of asymmetric interplanetary propagation both in heliolongitude and heliolatitude. We recommend that these effects explicitly be taken into account. The HAFv2 results appear to be useful for interpreting the Voyager 2 and Voyager 1 energetic particle data in the outer heliosphere. They are consistent with the effects of the Halloween solar events observed in the energetic particle data at both spacecraft. The HAFv2 results also may be helpful for predicting the plasma wave 2-3 kHz radio emission previously associated with large shocks and their interaction with the heliopause. Our study indicates that the Halloween events may give rise to 2-3 kHz radio emission in early 2005, assuming that the shocks which propagated beyond Voyager 1 will be strong enough.

Eds. Marsden, R., Scherer, K. and Heber, B.

Launched from Cape Canaveral more than 13 years ago, Ulysses is well on its way to completing two full circuits of the Sun in a unique orbit that takes it over the north and south poles of our star. In doing so, the European-built space probe and its payload of scientific instruments have added a fundamentally new perspective to our knowledge of the bubble in space in which the Sun and the Solar System exist, called 'the heliosphere'.

Coordinated data analyses from the initial in-ecliptic phase of the Ulysses mission are presented with special attention given to measurements of particles and fields in interplanetary space. The collection of papers presents the results of observations of large solar flares and effects related to solar wind, and some results are synopsized. The charged-particle instrumentation on the spacecraft is shown to have good dynamic range and energy coverage as demonstrated by a sample proton-energy spectrum.

The Ulysses spacecraft encountered Jupiter in February 1992, passing within 6.31 radii of the planet. For approximately 8 days it was inside the Jovian magnetosphere, and for several days before and after that, Ulysses was in the interaction regions formed by the solar wind (the magnetosheath and boundary layer).

Proceedings of the 28th ESLAB Symposium, held in Friedrichshafen, Germany, April 19-21, 1994. Ed. Marsden, R.G.

Article that appeared in the popular astronomy magazine Sky & Telescope. Unfortunately no preview of the publication exists online. The link from this page, however, will take you to the Sky Publishing website where you purchase a copy of the whole magazine.

For much of the past four decades, the scientific probes sent into space stayed relatively close to the equatorial plane of the Sun, which contains the orbits of Earth and other planets. But a few years ago a single craft, Ulysses, ventured out of that thin zone and into the 'polar regions' of interplanetary space.

Proceedings of the 34th ESLAB Symposium held 3-6 October 2000 at ESTEC, Noordwijk, The Netherlands. Ed. Marsden, R.G.

The Ulysses mission has explored and charted the heliosphere around the minimum in the 11-year cycle of solar activity from a high-latitude perspective. Here, for the first time, is a comprehensive review of the results of that mission. The book furthers our knowledge of the heliosphere near solar minimum and provides the basis for understanding the more complex state of the heliosphere around solar maximum. Amongst other topics, the book covers: the solar wind, the large- and small-scale structure of the heliosphere in 3 dimensions, cosmic rays and energetic particles, interstellar aspects, including cosmic dust. In "The Heliosphere Near Solar Minimum: The Ulysses Perspective", Eds. Balogh, A., Marsden, R.G. & Smith, E.J., Springer-Praxis, Chichester (2001) ISBN 1-85233-204-2, XXV + 411 pp.

Ulysses is now completing its second solar polar orbit, dropping back down in latitude as the Sun passes through its post-maximum phase of the solar cycle. A mid-sized circumpolar coronal hole that formed around solar maximum in the northern hemisphere has persisted and produced a highly inclined CIR, which was observed from ~70°N down to ~30°N. We find that the speed maxima in the high-speed streams follow the same slow drop in speed with decreasing latitude observed in the large polar coronal holes around solar minimum. These results suggest a solar wind acceleration effect that is related to heliolatitude or solar rotation. We also find that the solar wind dynamic pressure is significantly lower in the post-maximum phase of this solar cycle than during the previous one, indicating that while the heliosphere is larger than near solar minimum, it should be smaller than during or after the previous maximum.

Contents:

7.	Ulysses in the Context of the ESA Scientific Programme R.M. Bonnet
10.	Ulysses - A Brief History K.-P. Wenzel & D. Eaton
13.	Ulysses - An ESA/NASA Cooperative Programme W. Meeks & D. Eaton
21.	The Scientific Mission of Ulysses K.-P. Wenzel et al.
29.	The Ulysses Scientific Payload P.J. Caseley & R.G. Marsden
40.	The Ulysses Spacecraft A. Hawkyard & P. Buia
51.	RTGs - The Powering of Ulysses E.F. Mastal & R.W. Campbell
57.	The Ulysses Launch Campaign J.P. Leertouwer & D. Eaton
61.	Post-Launch Operations and Data Production P. Beech & D. Meyer
66.	Orbit Design and Control for Ulysses M. Rosengren
70.	Industrial Cooperation on Ulysses G. Hampel
73.	The Ulysses Storage and Recertification Activities D. Eaton
78.	The International Heliospheric Study D.E. Page