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| Magnetic field and electric currents in the vicinity of polar cusps as inferred from Polar and Cluster data |
| A detailed statistical study of the magnetic structure of the dayside polar cusps is presented, based on multi-year sets of magnetometer data of Polar and Cluster spacecraft, taken in 1996-2006 and 2001-2007, respectively. Thanks to the dense data coverage in both Northern and Southern Hemispheres, the analysis spanned nearly the entire length of the cusps, from low altitudes to the cusp "throat" and the magnetosheath. Subsets of data falling inside the polar cusp "funnels" were selected with the help of TS05 and IGRF magnetic field models, taking into account the dipole tilt and the solar wind/IMF conditions. The selection funnels were shifted within ±10° of SM latitude around the model cusp location, and linear regression parameters were calculated for each sliding subset, further divided into 10 bins of distance in the range 2<=R<=12 RE, with the following results. (1) Diamagnetic depression, caused by the penetrated magnetosheath plasma, becomes first visible at R~4-5 RE, rapidly deepens with growing R, peaks at R~6-9 RE, and then partially subsides and widens in latitude at the cusp's outer end. (2) The depression peak is systematically shifted poleward (by ~2° of the footpoint latitude) with respect to the model cusp field line, passing through the min{|B|} point at the magnetopause. (3) At all radial distances, clear and distinct peaks of the correlation between the local By and By(IMF) and of the corresponding proportionality coefficient are observed. A remarkably regular variation of that coefficient with R quantitatively confirms the field-aligned geometry of the cusp currents associated with the IMF By, found in earlier observations. |
| Publication date: 02 Apr 2009 |
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| Dual source populations of substorm-associated ring current ions |
| Sources of low-energy ring current ions in the early morning sector (eastward drifting energy domain of about <5 keV) are examined using both statistical analyses and numerical tracing methods (phase-space mapping and simulation). In about 90% of Cluster perigee traversals at 02~07 local time, these low-energy ring current ions have dual ion populations: one is wedge-like energy-dispersed ions, and the other is a band-like ions over different latitudes in a narrow energy range at the upper energy threshold of the wedge-like energy-dispersed ions. Both components are most likely created during past substorm activities. Numerical tracing results strongly suggest that these two components have different sources with different temperatures and elapsed times. The band-like part most likely comes from ions with plasma sheet temperature (~1 keV), and the energy-dispersed part most likely comes from cold ions (temperature <0.1 keV). The source density of the cold component (0.2~0.5x106m-3) is slightly less than that of the hot component (0.5x106m-3), while Cluster observation shows slightly higher density for the wedge-like part than the low-energy band-like part. The hot source component also explains the observed high-energy (>10 keV) ions drifting westward after adiabatic energization in the nightside under time-varying electric field. The wedge-like part has much shorter elapsed time, i.e., less charge-exchange loss, than the band-like part. |
| Publication date: 01 Apr 2009 |
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| Introduction to LISA Pathfinder (LISA-LPF-RP-0002) |
| An introduction to the LISA Pathfinder mission written by Giuseppe Racca (LPF Project Manager) and Paul McNamara (LPF Project Scientist).
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| Publication date: 30 Mar 2009 |
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| Multipoint observations of ions in the 30-160 keV energy range upstream of the Earth's bow shock |
| 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. |
| Publication date: 24 Mar 2009 |
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| Magnetosheath Plasma Turbulence and Its Spatiotemporal Evolution as Observed by the Cluster Spacecraft |
| 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. |
| Publication date: 22 Mar 2009 |
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| Venus Express Mission |
| 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. |
| Publication date: 19 Mar 2009 |
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| Venus cloud top winds from tracking UV features in Venus Monitoring Camera images |
| 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. |
| Publication date: 17 Mar 2009 |
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| The 2008 Annual Report of the ISECG |
This second Annual Report of the International Space Exploration Coordination Group (ISECG) and its sub-working groups provides highlights of their activities during the past twelve-months including the progress of its Workplan, work ahead, the major space exploration accomplishments of its members including future opportunities, and progress in implementing the Themes described in "The Global Exploration Strategy: The Framework for Coordination".
In addition, this ISECG Annual Report, as with the 2007 edition, provides an opportunity for agencies to update the international community on their individual space exploration plans - this information will be found in the Annex.
The Annual Report is intended to keep all exploration stakeholders, including other exploration related coordination groups, better informed of the ISECG's work and progress implementing the Global Exploration Strategy Framework document.
Efficient, beneficial and public supported Space Exploration can only be accomplished as an international endeavour involving a diverse stakeholder community comprising; space agencies and their policy/funding governments, industry, scientific institutions, academia, and non-profit groups. The ISECG is facilitating this dialogue and understanding. |
| Publication date: 15 Mar 2009 |
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| MHD model of the flapping motions in the magnetotail current sheet |
| A new kind of magnetohydrodynamic waves is analyzed for a current sheet in the presence of a small normal magnetic field component (Bz) 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 Bz component increases toward Earth. The current sheet becomes unstable in some regions, where the Bz 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. |
| Publication date: 12 Mar 2009 |
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| Shape, size, velocity and field-aligned currents of dayside plasma injections: a multi-altitude study |
| 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 RE 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 -- |
| Publication date: 12 Mar 2009 |
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| Remote sensing of local structure of the quasi-perpendicular Earth's bow shock by using field-aligned beams |
| 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. |
| Publication date: 02 Mar 2009 |
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| Vortex circulation on Venus: Dynamical similarities with terrestrial hurricanes |
| 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. |
| Publication date: 25 Feb 2009 |
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| Reconnection at High Latitudes: Antiparallel Merging |
| 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. |
| Publication date: 20 Feb 2009 |
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| Strong Release of Methane on Mars in Northern Summer 2003 |
| 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. |
| Publication date: 20 Feb 2009 |
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| Cluster and Double Star multipoint observations of a plasma bubble |
| 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 BY-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 RE of the Earth, consistent with MHD simulations (Birn et al., 2004) are presented. The implications of the presence of a strong BY 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 BY can rotate the field aligned current systems at the edges of the plasma bubble away from its duskward and dawnward flanks. |
| Publication date: 16 Feb 2009 |
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| Magnetosphere response to the 2005 and 2006 extreme solar events as observed by the Cluster and Double Star spacecraft |
| 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 RE). 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. |
| Publication date: 16 Feb 2009 |
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| Galaxies Across Cosmic Time with JWST |
| The James Webb Space Telescope (JWST; Gardner et al. 2006) will be a large, cold, infrared-
optimized space telescope designed to enable fundamental breakthroughs in our understanding of
the formation and evolution of galaxies, stars, and planetary systems (see Astro 2010 white papers
by Gardner et al., Stiavelli et al., Meixner et al., G. Rieke et al., & Sonneborn et al.). In the current
white paper, we describe the great potential of JWST in the theme of Galaxy Assembly. |
| Publication date: 13 Feb 2009 |
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| Direct evidence of solar wind deceleration in the foreshock of the Earth |
| 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. |
| Publication date: 12 Feb 2009 |
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| ESA contribution to the Titan Saturn System Mission |
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
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to prepare for the future competitive industrial assessment studies by identifying critical issues that need addressing at higher priority
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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. |
| Publication date: 12 Feb 2009 |
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| Jupiter Ganymede Orbiter: ESA contribution to the Europe Jupiter System Mission |
| 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.
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| Publication date: 12 Feb 2009 |
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