ESA Science & Technology - Publication Archive

We surveyed fast current sheet crossings (flapping motions) over the distance range 10-30 R_E in the magnetotail covered by the Geotail spacecraft. Since the local tilts of these dynamic sheets are large and variable in these events, we compare three different methods of evaluating current sheet normals using 4-s/c Cluster data and define the success criteria for the single-spacecraft-based method (MVA) to obtain the reliable results. Then, after identifying more than ~1100 fast CS crossings over a 3-year period of Geotail observations in 1997-1999, we address their parameters, spatial distribution and activity dependence. We confirm that over the entire distance covered and LT bins, fast crossings have considerable tilts in the YZ plane (from estimated MVA normals) which show a preferential appearance of one (YZ kink-like) mode that is responsible for these severe current sheet perturbations. Their occurrence is highly inhomogeneous; it sharply increases with radial distance and has a peak in the tail center (with some duskward shift), resembling the occurrence of the BBFs, although there is no one-to-one local correspondence between these two phenomena. The crossing durations typically spread around 1 min and decrease significantly where the high-speed flows are registered. Based on an AE index superposed epoch study, the flapping motions prefer to appear during the substorm expansion phase, although a considerable number of events without any electrojet and auroral activity were also observed. We also present statistical distributions of other parameters and briefly discuss what could be possible mechanisms to generate the flapping motions.

In this paper we study flux transfer events (FTE) observed at the post-noon edge of the exterior cusp region by Cluster satellites. During the outbound dayside orbit on 2 February 2003, intense bursts of energetic particles were observed in close conjuction with magnetic field FTE signatures by the RAPID instrument onboard the Cluster 4. The pitch-angle distribution of the particles showed that the enhancements consist of particles flowing antiparallel to the magnetosheath field lines away from the expected reconnection site to the exterior cusp. At the same time Cluster 3 observed enhancements of energetic particles deeper in the exterior cusp with a delay of about 40 s to the Cluster 4 enhancements. The estimated maximum energy gain per particle by reconnection remains below 1 keV, thus clearly below the tens to hundreds of keV energy range observed by the RAPID instrument. These observations support the earlier statistical result of the magnetospheric origin of energetic particles in the exterior cusp. Reconnection near the exterior cusp partly releases the particles in the closed field lines of the adjacent HLPS region into the exterior cusp.

We investigate in detail a reversal of plasma flow from tailward to earthward detected by Cluster at the downstream distance of ~19 R_E in the midnight sector of the magnetotail on 22 August 2001. This flow reversal was accompanied by a sign reversal of the B_z component and occurred during the late substorm expansion phase as revealed by simultaneous global view of auroral activity from IMAGE. We examine the associated Hall current system signature, current density, electric field, Lorentz force, and current dissipation/dynamo term, the last two parameters being new features that have not been studied previously for plasma flow reversals. It is found that (1) there was no clear quadrupole Hall current system signature organized by the flow reversal time, (2) the x-component of the Lorentz force did not change sign while the other two did, (3) the timing sequence of flow reversal from the Cluster configuration did not match tailward motion of a single plasma flow source, (4) the electric field was occasionally dawnward, producing a dynamo effect, and (5) the electric field was occasionally larger at the high-latitude plasma sheet than near the neutral sheet. These observations are consistent with the current disruption model for substorms in which these disturbances are due to shifting dominance of multiple current disruption sites and turbulence at the observing location.

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan's atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titan's atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

Measurements of stratospheric zonal winds on Titan were made in preparation for and during the time of the descent of the Huygens Probe into Titan's atmosphere on 14 January 2005. Fully resolved emission lines from ethane near 11.7 micron were measured on the east, center, and west positions on Titan using the NASA/GSFC Heterodyne Instrument for Planetary Wind And Composition, HIPWAC, mounted on the National Astronomical Observatory of Japan 8.2 m Subaru Telescope on Mauna Kea, Hawaii. Analysis of the Doppler shifts of the emission line shapes yielded mean prograde gas velocity ~60 ± 65 m/s at altitudes below ~120 km (~5 mbar). This result is consistent with retrievals from the Huygens Doppler Wind Experiment and from other observations near this altitude range. Current spectral line shapes, however, differed significantly from those obtained in similar measurements on Subaru in 2004 and on the NASA IRTF in 1993-1996, which retrieved prograde zonal winds 190 ± 90 m/s at 230 km (~0.4 mbar). The cores of the emission lines, which probe the high-altitude region, could not be fitted as before to retrieve wind directly using the accepted atmospheric model for Titan. They imply an approximately tenfold increase in ethane mole fraction (1.2 × 10^-4) with strong wind shear above the stratopause, providing a potential probe of the lower mesosphere and possible evidence of temporal and spatial variability. Results contribute to coordinated measurements of winds by various techniques providing information on the altitude distribution of wind velocity in Titan's atmosphere from near the surface to the lower mesosphere.

Saturn's moon Titan shows landscapes with fluvial features suggestive of hydrology based on liquid methane. Recent efforts in understanding Titan's methane hydrological cycle have focused on occasional cloud outbursts near the south pole or cloud streaks at southern mid-latitudes and the mechanisms of their formation. It is not known, however, if the clouds produce rain or if there are also non-convective clouds, as predicted by several models. Here we show that the in situ data on the methane concentration and temperature profile in Titan's troposphere point to the presence of layered optically thin stratiform clouds. The data indicate an upper methane ice cloud and a lower, barely visible, liquid methane-nitrogen cloud, with a gap in between. The lower, liquid, cloud produces drizzle that reaches the surface. These non-convective methane clouds are quasi-permanent features supported by the global atmospheric circulation, indicating that methane precipitation occurs wherever there is slow upward motion. This drizzle is a persistent component of Titan's methane hydrological cycle and, by wetting the surface on a global scale, plays an active role in the surface geology of Titan.

The presence of dry fluvial river channels and the intense cloud activity in the south pole of Titan over the past few years suggest the presence of methane rain. The nitrogen atmosphere of Titan therefore appears to support a methane meteorological cycle that sculptures the surface and controls its properties. Titan and Earth are the only worlds in the Solar System where rain reaches the surface, although the atmospheric cycles of water and methane are expected to be very different. Here we report three-dimensional dynamical calculations showing that severe methane convective storms accompanied by intense precipitation may occur in Titan under the right environmental conditions. The strongest storms grow when the methane relative humidity in the middle troposphere is above 80 per cent, producing updrafts with maximum velocities of 20 m s^-1, able to reach altitudes of 30 km before dissipating in 5-8 h. Raindrops of 1-5 mm in radius produce precipitation rainfalls on the surface as high as 110 kg m^-2 and are comparable to flash flood events on Earth.

Titan is viewed as a sibling of Earth, as both bodies have rainy weather systems and landscapes formed by rivers. But as we study these similarities, Titan emerges as an intriguingly foreign world.

Huygens provided an unanticipated bistatic radio scattering experiment from Titan's surface. After a successful entry and descent on Titan, on 14 January 2005, the probe remarkably survived the landing and continued radioing from the surface to the overflying Cassini, until the orbiter set below Titan's local horizon. Here we report high-quality measurements of the 2098 MHz (14.3 cm) postlanding radio signal, focusing on the striking variations observed in signal strength. The mechanism that creates this fading pattern is physically interpreted as multipath interference between the direct signal and the signal reflected on Titan's surface. The geophysical aspects that could bear on the signal analysis are described.

Large radio telescopes on Earth tracked the radio signal transmitted by the Huygens probe during its mission at Titan. Frequency measurements were conducted as a part of the Huygens Doppler Wind Experiment (DWE) in order to derive the velocity of the probe in the direction to Earth. The DWE instrumentation on board Huygens consisted of an ultrastable oscillator which maintained the high Doppler stability required for a determination of probe horizontal motion during the atmospheric descent. A vertical profile of the zonal wind velocity in Titan's atmosphere was constructed from the Doppler data under the plausible assumption of generally small meridional wind, as validated by tracked images from the Huygens Descent Imager/Spectral Radiometer (DISR). We report here on improved results based on data with higher temporal resolution than that presented in the preliminary analysis by Bird et al. (2005), corroborating the first in situ measurement of Titan's atmospheric superrotation and a region of strong vertical shear reversal within the lower stratosphere. We also present the first high-resolution display and interpretation of the winds near the surface and planetary boundary layer.

Contents: Hubble News Update; Staff Changes; The AstroAsciiData Python Module; Creating a Legacy Archive for Hubble; The New ST-ECF Web; Scisoft VI

An electronic collisional-radiative model is proposed to predict the nonequilibrium populations and the radiation of the excited electronic states CN(A, B) and N₂ (A, B, C) during the entry of the Huygens probe into the atmosphere of Titan. The model is loosely coupled with flow solvers using a Lagrangian method. First, the model was tested against measurements obtained with the shock-tube of NASA Ames Research Center. Then, the model was applied to the simulation of Huygen's entry. Our simulations predict that the population of the CN(B) state is lower than the Boltzmann population by a factor 40 at trajectory time t = 165 s and by a factor 2 at t = 187 s and that the population of the CN(A) state remains close to the Boltzmann population for both trajectory points. The radiative heat fluxes, driven by the CN(A, B) states, are lower than predictions based on the Boltzmann populations by a factor 15 at t = 165 s and a factor 2 at t = 187 s.

In the proceedings of the 6th International Conference on Space Optics

This paper summarizes the results of an ESA feasibility study of a Wide-Field Optical Infrared Imager (WFI) that would search for Type Ia supernovae at low redshift with the aim to measure the changing rate of expansion of the universe. WFI multi-spectral images of the deep universe could also benefit to many other research area in astrophysics. The WFI payload includes a 2 m class telescope, a 1 square degree field of view imaging camera and a low-resolution integral field spectrometer. A mission concept was identified that consists of a 2000 kg spacecraft launched by a Soyuz-Fregat into a L2 halo orbit. The WFI mission could benefit from the technology developed for the ESA Herschel and Gaia missions and for the NIRSpec ESA instrument. A fully European WFI mission would require improvement of existing European detector and on-board processor technology as well as some effort to support the utilization of the 26 GHz Ka band.

In the context of the Cassini/Huygens mission, we performed supporting ground-based observations to complement the results from the NASA/ESA/ASI space mission to the Saturnian system with particular focus on Titan. On the nights of 18 and 19 December 2004, we conducted adaptive optics observations with VLT/NACO to search for and map the distribution of CO₂ ice deposits on the spatially resolved surface of Titan (65 mas resolution). This experiment became possible because (1) solid CO₂ possesses two narrow and strong absorption lines at 2012 nm and 2070 nm that fall into the 2.05 micron window of Titan's atmosphere and (2) the width of these bands matches the band pass of the Fabry-Perot instrument installed in NACO. We do not detect this chemical compound, but we can derive firm limits on the abundance of CO₂ ice on the surface of Titan at sub-Earth longitudes 284° W and 307° W. With a spatial sampling of 65 mas, we conclude that a partial surface coverage of segregated CO₂ ice does not exceed 7% or 14% for bright or dark surface regions, respectively.

The profile of intense high-altitude electric fields on auroral field lines has been studied using Cluster data. A total of 41 events with mapped electric field magnitudes in the range between 0.5-1 V/m were examined, 27 of which were co-located with a plasma boundary, defined by gradients in particle flux, plasma density and plasma temperature. Monopolar electric field profiles were observed in 11 and bipolar electric field profiles in 16 of these boundary-associated electric field events. Of the monopolar fields, all but one occurred at the polar cap boundary in the late evening and midnight sectors, and the electric fields were typically directed equatorward, whereas the bipolar fields all occurred at plasma boundaries clearly within the plasma sheet. These results support the prediction by Marklund et al. (2004), that the electric field profile depends on whether plasma populations, able to support intense field-aligned currents and closure by Pedersen currents, exist on both sides, or one side only, of the boundary.

Plasmaspheric plumes have been routinely observed by CLUSTER and IMAGE. The CLUSTER mission provides high time resolution four-point measurements of the plasmasphere near perigee. Total electron density profiles have been derived from the electron plasma frequency identified by the WHISPER sounder supplemented, in-between soundings, by relative variations of the spacecraft potential measured by the electric field instrument EFW; ion velocity is also measured onboard these satellites. The EUV imager onboard the IMAGE spacecraft provides global images of the plasmasphere with a spatial resolution of 0.1 R_E every 10 min; such images acquired near apogee from high above the pole show the geometry of plasmaspheric plumes, their evolution and motion. We present coordinated observations of three plume events and compare CLUSTER in-situ data with global images of the plasmasphere obtained by IMAGE. In particular, we study the geometry and the orientation of plasmaspheric plumes by using four-point analysis methods. We compare several aspects of plume motion as determined by different methods: (i) inner and outer plume boundary velocity calculated from time delays of this boundary as observed by the wave experiment WHISPER on the four spacecraft, (ii) drift velocity measured by the electron drift instrument EDI onboard CLUSTER and (iii) global velocity determined from successive EUV images. These different techniques consistently indicate that plasmaspheric plumes rotate around the Earth, with their foot fully co-rotating, but with their tip rotating slower and moving farther out.

Another approach (Multiple Triangulation Analysis, MTA) is presented to determine the orientation of magnetic flux rope, based on 4-point measurements. A 2-D flux rope model is used to examine the accuracy of the MTA technique in a theoretical way. It is found that the precision of the estimated orientation is dependent on both the spacecraft separation and the constellation path relative to the flux rope structure. However, the MTA error range can be shown to be smaller than that of the traditional MVA technique. As an application to real Cluster data, several flux rope events on 26 January 2001 are analyzed using MTA, to obtain their orientations. The results are compared with the ones obtained by several other methods which also yield flux rope orientation. The estimated axis orientations are shown to be fairly close, suggesting the reliability of the MTA method.

BepiColombo is the name of an ESA cornerstone mission to Mercury with a launch scheduled for 2013. After a journey of 6 years, two probes, the Magnetospheric Orbiter (JAXA) and the Planetary Orbiter (ESA), will reach their target orbits. The interplanetary trajectory includes multiple planetary flybys as well as several low-thrust arcs provided by the solar electric propulsion module. The navigation analysis of such a trajectory requires the implementation of complex guidance laws for the low thrust arcs, and the inclusion of trim manoeuvres near each planet encounter. Further challenges are imposed by the occurrence of solar conjunctions, noise introduced by the thrust, limited range and Doppler availability, stringent solar aspect angle constraints, and the definition of recovery solutions in case of thrust outages. A baseline trajectory taking into account all these operational constraints is here presented. The software tools LOTNAV and INTNAV were used to perfom a navigation analysis for the interplanetary trajectory.

BepiColombo is an interdisciplinary mission to the planet Mercury that has been selected as the 5th cornerstone in the Cosmic Vision programme of the European Space Agency (ESA). Owing to the high scientific potential related to the planet and its environment the mission will open a new frontier in the study of our solar system. Planned for launch in 2013, BepiColombo is a collaboration between ESA and the Japan Aerospace Exploration Agency (JAXA). It consists of two scientific orbiters, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO), which will study the origin and evolution of the planet, Mercury's interior dynamics and the origin of the magnetic field. The interplanetary trajectory includes flybys at the Moon, Earth, Venus (twice) and Mercury (twice), as well as several long thrust arcs provided by solar electric propulsion. The Mercury orbit capture and lowering to the operational orbits will be done with chemical propulsion. The launch configuration consists of a stack of the two spacecraft and the chemical and electrical propulsion modules.

A conventional Mercury sample return mission requires significant outbound and return trips, and the large mass of a planetary lander and ascent vehicle. In this paper, it is shown that solar sailing can be used to reduce lander mass allocation by delivering the lander to a low, thermally safe orbit close to the planetary terminator. In addition, the ascending node of the solar sail parking orbit plane can be artificially forced to avoid out-of-plane manoeuvres during ascent from the planetary surface. Propellant mass is not an issue for solar sails, so a sample can be returned relatively easily, without resorting to lengthy, multiple gravity assists. A 275 m square solar sail with a sail assembly loading of 5.9 g m-2 is used to deliver a lander, cruise stage and science payload to a forced Sun-synchronous orbit at Mercury in 2.85 years. The lander acquires samples, and conducts limited surface exploration. An ascent vehicle delivers a small cold gas rendezvous vehicle containing the samples for transfer to the solar sail. The solar sail then spirals back to Earth in one year. The total mission launch mass is 2353 kg, on an H2A202-4S class launch vehicle (C3=0). Extensive launch date scans have revealed an optimal launch date in April 2014 with sample return to Earth 4.4 years later. Solar sailing reduces launch mass by 60% and trip time by 40%, relative to conventional mission concepts. In comparison, mission analysis has demonstrated that solar sail Mars and Venus sample return appears to have only modest benefit in terms of reduced launch mass, at the expense of longer mission durations than conventional propulsion systems.