ESA Science & Technology - Publication Archive

An introduction to the LISA Pathfinder mission written by Giuseppe Racca (LPF Project Manager) and Paul McNamara (LPF Project Scientist).

We use multipoint observation data by Cluster during time periods when the interspacecraft separation distance was between 1 and 1.5 Earth radii in order to study the physical processes related to diffuse ions at <200 keV/e. For our analysis we use data from the Research with Adaptive Particle Imaging Detectors (RAPID) experiment onboard Cluster SC1 and SC3. We determine spatial ion density gradients by using proton intensities in the 27.7-159.7 keV energy range and helium intensities in the 137.8-235.1 keV energy range as a function of distance from the bow shock along the magnetic field. Our results show that the diffuse ions are subject to diffusive transport and the ion partial densities decrease exponentially with increasing distance from the bow shock. By complementing RAPID data with Cluster Ion Spectrometry measurements at lower energies (from 10 to 32 keV) from the same upstream ion event we find that the e-folding distance of energetic ion density increases almost linearly with energy. This effect is also seen in the hardening of the particle spectra with increasing distance from the bow shock. We determine the spatial diffusion mean free path and the diffusion coefficient as a function of ion energy by assuming that upstream diffusion is balanced by downstream convection.

We study the plasma turbulence, at scales larger than the ion inertial length scale, downstream of a quasiparallel bow shock using Cluster multispacecraft measurements. We show that turbulence is intermittent and well described by the extended structure function model, which takes into account the spatial inhomogeneity of the cascade rate. For the first time we use multispacecraft observations to characterize the evolution of magnetosheath turbulence, particularly its intermittency, as a function of the distance from the bow shock. The intermittency significantly changes over the distance of the order of 100 ion inertial lengths, being increasingly stronger and anisotropic away from the bow shock.

Venus Express is well and healthy and has now been providing exciting new data from Venus, our nearby twin planet, for over 2 years. Many of the new results are presented and discussed in the subsequent papers in this special section. The overall scientific objective of Venus Express is to carry out a detailed study of the atmosphere of Venus, including the interaction of the upper atmosphere with the solar wind and the interaction of the lowest part of the atmosphere with the surface of the planet. In addition, the plasma environment and magnetic fields as well as some aspects of the surface of the planet are addressed. For the first time, investigations make systematic use of the transparent infrared spectral windows in order to probe the atmosphere in four dimensions: three spatial dimensions plus time. The spacecraft design is taken from Mars Express with some modifications necessary owing to the specific environment around Venus. The payload is composed of three spectrometers, a camera, a magnetometer, an instrument for detecting energetic particles, and a radio science package. The orbit is polar and highly elliptic, with a pericenter altitude of about 200 km over the northern polar region and an apocenter altitude of 66,000 km. Presently, the coverage of the southern hemisphere is very good, but important gaps still do exist. The coverage of the northern hemisphere is much less dense. Venus Express is a part of the European Space Agency's program for the exploration of the inner solar system, which includes missions to study the Sun, Mercury, Venus, the Moon, Mars, and comets and asteroids.

To date dynamical observations of the Venus clouds have delivered mainly either only short-term or long-term averaged results. With the Venus Monitoring Camera (VMC) it finally became possible to investigate the global dynamics with a relatively high resolution in space and time on a long-term basis. Our findings from manual cloud feature wind tracking in VMC UV image sequences so far show that the details of the mesospheric dynamics of Venus appear to be highly variable. Although the general rotation of the atmosphere remained relatively stable since Mariner 10, more than 30 years ago, by now, there are indications of short-term variations in the general circulation pattern of the Venus atmosphere at cloud top level. In some cases, significant variations in the zonal wind properties occur on a timescale of days. In other cases, we see rather stable conditions over one atmospheric revolution, or longer, at cloud top level. It remains an interesting question whether the irregularly observed midlatitude jets are indeed variable or simply become shielded from view by higher H₂SO₄ haze layers for varying time intervals. Winds at latitudes higher than 60°S are still difficult to obtain track because of low contrast and scarcity of features but increasing data is being collected. Over all, it was possible to extend latitudinal coverage of the cloud top winds with VMC observations. Thermal tides seem to be present in the data, but final confirmation still depends on synthesis of Visible and Infrared Thermal Imaging Spectrometer and VMC observations on night and dayside. Although poorly resolved, meridional wind speed measurements agree mainly with previous observations and with the presence of a Hadley cell spanning between equatorial region and about 45°S latitude.

On 20 February 2005, Cluster in the outer magnetosphere and Double Star-2 (TC-2) at mid-altitude are situated in the vicinity of the northern cusp/mantle, with Cluster moving sunward and TC-2 anti-sunward. Their magnetic footprints come very close together at about 15:28 UT, over the common field-of-view of SuperDARN radars. Thanks to this conjunction, we determine the velocity, the transverse sizes, perpendicular and parallel to this velocity, and the shape of three magnetic flux tubes of magnetosheath plasma injection. The velocity of the structures determined from the Cluster four-spacecraft timing analysis is almost purely antisunward, in contrast with the antisunward and duskward convection velocity inside the flux tubes. The transverse sizes are defined from the Cluster-TC-2 separation perpendicular to the magnetic field, and from the time spent by a Cluster spacecraft in one structure; they are comprised between 0.6 and 2 R_E in agreement with previous studies. Finally, using a comparison between the eigenvectors deduced from a variance analysis of the magnetic perturbation at the four Cluster and at TC-2, we show that the upstream side of the injection flux tubes is magnetically well defined, with even a concave front for the third one giving a bean-like shape, whereas the downstream side is far more turbulent. We also realise the first quantitative comparison between field-aligned currents at Cluster calculated with the curlometer technique and with the single-spacecraft method, assuming infinite parallel current sheets and taking into account the velocity of the injection flux tubes. The results agree nicely, confirming the validity of both methods. Finally, we compare the field-aligned current distribution of the three injection flux tubes at the altitudes of Cluster and TC-2. -- Remainder of abstract truncated --

A new kind of magnetohydrodynamic waves is analyzed for a current sheet in the presence of a small normal magnetic field component (B_z) varying along the sheet. For the initial undisturbed state, a simplified model of the current sheet is considered with a Harris-like current density distribution across the sheet. Within the framework of this model, an analytical solution is obtained for the flapping-type wave oscillations and instability, related to the gradient of the normal magnetic field component along the current sheet. The flapping wave frequency is found to be a function of the wave number, which has an asymptotic saturation for large wave numbers. This frequency is pure real in a stable situation for the magnetotail current sheet, when the B_z component increases toward Earth. The current sheet becomes unstable in some regions, where the B_z component decreases locally toward Earth. In the stable region, the "kink"-like wave oscillations are calculated for an initial Gaussian perturbation localized to the center of the current sheet. The flapping wave propagations are analyzed for two cases: (1) the initial perturbation is fixed, and (2) the source is moving toward Earth. In the last case, the Mach cone is obtained for the propagating flapping waves. The source for the flapping waves is associated with the fast plasma flow originated from the reconnection region.

Field-aligned ion beams (FABs) originate at the quasi-perpendicular Earth's bow shock and constitute an important ion population in the foreshock region. The bulk velocity of these FABs depends significantly on the shock normal angle, which is the angle between shock normal and upstream interplanetary magnetic field (IMF). This dependency may therefore be taken as an indicator of the local structure of the shock. Applying the direct reflection model to Cluster measurements, we have developed a method that uses proton FABs in the foreshock region for remote sensing of the local shock structure. The comparison of the model results with the multi-spacecraft observations of FAB events shows very good agreement in terms of wave amplitude and frequency of surface waves at the shock front.

The present issue of Spatium is devoted to meteorites, more specifically however to the good-natured class of small extraterrestrial bodies that continue reaching the Earth without doing major harm. Science has learned to appreciate them as cosmic messengers that faithfully recount the billion year history of our solar system. This role of meteorites was the topic of an exciting lecture by Dr. Beda Hofmann (Natural History Museum of Bern and Institute for Geology, University of Bern) for the Pro ISSI audience on 28 October 2008.

Some dynamical and morphological similarities exist between the vortex organization of the atmosphere in the northern and southern hemispheres of Venus and the tropical cyclones/hurricanes on Earth. An S-shape feature detected in the center of the vortices on Venus from Pioneer Venus Orbiter and Venus Express observations has also been seen in tropical cyclones. This feature can be simulated with an idealized nonlinear and non-divergent barotropic model and, like in the vortices on Venus and in tropical cyclones, it is found to be transient. Given the challenges in measuring the deep, atmospheric circulation of Venus, the morphological similarities provide clues toward understanding the processes involved in the maintenance of Venus' atmospheric super rotation.

New investigations have renewed the debate on the occurrence of magnetic reconnection of Earth's dayside magnetopause. Here, we show for the first time strong evidence for a high-latitude reconnection site, located on initially closed field lines, where the magnetic field orientations inside and outside the magnetopause are close to antiparallel. The evidence centers on repeated sampling of the ion diffusion region and associated null magnetic field by four spacecraft in formation, together with simultaneous monitoring of the local magnetosheath behavior by a fifth spacecraft.

Living systems produce more than 90% of Earth's atmospheric methane; the balance is of geochemical origin. On Mars, methane could be a signature of either origin. Using high-dispersion infrared spectrometers at three ground-based telescopes, we measured methane and water vapor simultaneously on Mars over several longitude intervals in northern early and late summer in 2003 and near the vernal equinox in 2006. When present, methane occurred in extended plumes, and the maxima of latitudinal profiles imply that the methane was released from discrete regions. In northern midsummer, the principal plume contained ~19,000 metric tons of methane, and the estimated source strength (>=0.6 kilogram per second) was comparable to that of the massive hydrocarbon seep at Coal Oil Point in Santa Barbara, California.

Depleted flux tubes, or plasma bubbles, are one possible explanation of bursty bulk flows, which are transient high speed flows thought to be responsible for a large proportion of flux transport in the magnetotail. Here we report observations of one such plasma bubble, made by the four Cluster spacecraft and Double Star TC-2 around 14:00 UT on 21 September 2005, during a period of southward, but B_Y-dominated IMF. In particular the first direct observations of return flows around the edges of a plasma bubble, and the first observations of plasma bubble features within 8 R_E of the Earth, consistent with MHD simulations (Birn et al., 2004) are presented. The implications of the presence of a strong B_Y in the IMF and magnetotail on the propagation of the plasma bubble and development of the associated current systems in the magnetotail and ionosphere are discussed. It is suggested that a strong B_Y can rotate the field aligned current systems at the edges of the plasma bubble away from its duskward and dawnward flanks.

The four identical Cluster spacecraft, launched in 2000, orbit the Earth in a tetrahedral configuration and on a highly eccentric polar orbit (4-19.6 R_E). This allows the crossing of critical layers that develop as a result of the interaction between the solar wind and the Earth's magnetosphere. Since 2004 the Chinese Double Star TC-1 and TC-2 spacecraft, whose payload comprise also backup models of instruments developed by European scientists for Cluster, provided two additional points of measurement, on a larger scale: the Cluster and Double Star orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The Cluster and Double Star observations during the 2005 and 2006 extreme solar events are presented, showing uncommon plasma parameters values in the near-Earth solar wind and in the magnetosheath. These include solar wind velocities up to ~900 km/s during an ICME shock arrival, accompanied by a sudden increase in the density by a factor of ~5 and followed by an enrichment in He⁺⁺ in the secondary front of the ICME. In the magnetosheath ion density values as high as 130 cm^-3 were observed, and the plasma flow velocity there reached values even higher than the typical solar wind velocity. These resulted in unusual dayside magnetosphere compression, detection of penetrating high-energy particles in the magnetotail, and ring current development following several successive injections of energetic particles in the inner magnetosphere, which "washed out" the previously formed nose-like ion structures.

Submitted to 'Astro2010: The Astronomy and Astrophysics Decadal Survey', (http://sites.nationalacademies.org/bpa/BPA_049810), Science White Papers, no. 248

JWST will play a central role, along with other new capabilities such as Herschel, ALMA, and large groundbased telescopes, in advancing our understanding of the four key questions:

• How do interstellar clouds of gas and dust begin their collapse into stars?
• What processes regulate the star formation following this collapse?
• How do planets form in dense disks of gas and dust around young stars?
• What is the subsequent evolution of planetary systems?

Although the solar wind deceleration in the terrestrial foreshock was noticed three decades ago, previous studies show some conflicting results. This paper presents direct evidence of solar wind deceleration in the foreshock of the Earth by using the data of two Cluster satellites. On 2 February 2003, the two satellites (C1 and C3) of Cluster missions are inside and outside the foreshock, respectively, approximately along the solar wind flow line, which can effectively exclude the uncertainty caused by the spatial and temporal changes of solar wind itself. Comparison of the plasma data recorded by two satellites shows that the solar wind velocity decreases in the foreshock and the largest deceleration reaches 22 km/s. The velocity distribution of ions in the phase space indicates that the solar wind ions undergo pitch angle scattering in the foreshock. The solar wind deceleration is associated with diffuse ions and ULF wave activities. The diffuse ion density reached 0.25 cm^-3, about 7% of the solar wind density. The interaction of ULF waves with solar wind also deflects the solar wind away from the bow shock in both the ecliptic and the meridian planes. Meanwhile, the solar wind deceleration is accompanied by thermalization during which the solar wind temperature can reach 240 eV.

This document gives a concise overview of the ESA assessment study based on the "Laplace" proposal for the Cosmic Vision 2015-2025 call. It describes the work performed during the assessment study, which includes the Concurrent Design Facility (CDF) study and subsequent scientific and engineering work. The aim of this study was to quickly assess the technical feasibility, cost and schedule in the framework of a Cosmic Vision 2015-2025 L-class mission and to prepare the follow-on industrial studies. The transition from the submitted "Laplace" proposal to the ESA/NASA/JAXA joint mission called "Europa/Jupiter System Mission" (EJSM) is documented and the baseline of the ESA contribution to this mission, the Jupiter Ganymede Orbiter (JGO), is described in detail.

This report describes the updated baseline of the internal assessment study that was performed with the assistance of the ESTEC Concurrent Design Facility (CDF) in June and July 2008. The main goals of the CDF study were:

• to assess the feasibility of the proposed mission
• to prepare for the future competitive industrial assessment studies by identifying critical issues that need addressing at higher priority
• to provide the building blocks and feed-back for further interaction with the science team for formulation of feasible mission goals.

To this end, the CDF study established bottom-up designs for three elements: a montgolfière, a long-lived lander (powered by an ASRG), and a minimum sized small lander of limited lifetime (powered by battery).

This report describes the design of the chosen baseline after the first technical iteration and incuding an updated mission profile providing enhanced mass capability of the delivery of the in situ elements, after the addition of a solar electric propulsion stage of the NASA orbiter.

This report is the result of an independent ESA internal assessment study.

This document is the XEUS proposal submitted to ESA in response to the Cosmic Vision call for proposals. It includes descriptions of the scientific objectives, mission profile, instrument payload, spacecraft, operations, and data archiving of XEUS.

JEO is the NASA Element of the Europa Jupiter System Mission (EJSM).

The objective of this study, to assess the science merit, technical risk and qualitative assessment of relative cost of alternative architectural implementations as applied to a first dedicated mission to Europa, was accomplished by 1) reviewing results from previous and current studies and 2) examining alternative architectural options relative to the science objectives defined by the 2007 EE SDT. This report summarizes a number of Europa mission and system concepts studied over the last decade as well as the results from assessing alternative architectural options in light of current science requirements.