Publication archive

Publication archive

Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH4 and N2). Up to now, it has been assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH4 and N2) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.
Published: 12 May 2007
We examine Cluster observations of a reconnection event at xGSM=-15.7 RE in the magnetotail on 11 October 2001, when Cluster recorded the current sheet for an extended period including the entire duration of the reconnection event. The onset of reconnection is associated with a sudden orientation change of the ambient magnetic field, which is also observed simultaneously by Goes-8 at geostationary orbit. Current sheet oscillations are observed both before reconnection and during it. The speed of the flapping motions is found to increase when the current sheet undergoes the transition from quiet to active state, as suggested by an earlier statistical result and now confirmed within one single event. Within the diffusion region both the tailward and earthward parts of the quadrupolar magnetic Hall structure are recorded as an x-line passes Cluster. We report the first observations of the Hall structure conforming to the kinks in the current sheet. This results in relatively strong fluctuations in Bz, which are shown to be the Hall signature tilted in the yz plane with the current sheet.
Published: 09 May 2007
Oscillating magnetic field lines are frequently observed by spacecraft in the terrestrial and other planetary magnetospheres. The CLUSTER mission is a very suitable tool to further study these Alfvén waves as the four CLUSTER spacecraft provide for an opportunity to separate spatial and temporal structures in the terrestrial magnetosphere. Using a large scaled configuration formed by the four spacecraft we are able to detect a poloidal Ultra-Low-Frequency (ULF) pulsation of the magnetic and electric field in order to analyze its temporal and spatial structures. For this purpose the measurements are transformed into a specific field line related coordinate system to investigate their specific amplitude pattern depending on the path of the CLUSTER spacecraft across oscillating field lines. These measurements are then compared with modeled spacecraft observations across a localized poloidal wave resonator in the dayside plasmasphere. A detailed investigation of theoretically expected poloidal eigenfrequencies allows us to specify the observed 16 mHz pulsation as a third harmonic oscillation. Based on this we perform a case study providing a clear identification of wave properties such as an spatial scale structure of about 0.67 RE, the azimuthal wave number m~30, temporal evolution, and energy transport in the detected ULF pulsations.
Published: 09 May 2007
A close conjunction of several satellites (LANL, GOES, Polar, Geotail, and Cluster) distributed from the geostationary altitude to about 16 RE downstream in the tail occurred during substorm activity as indicated by global auroral imaging and ground-based magnetometer data. This constellation of satellites resembles what is planned for the THEMIS (Time History of Events and Macroscopic Interactions during Substorms) mission to resolve the substorm controversy on the location of the substorm expansion onset region. In this article, we show in detail the dipolarization and dynamic changes seen by these satellites associated with two onsets of substorm intensification activity. In particular, we find that dipolarization at ~16 RE downstream in the tail can occur with dawnward electric field and without plasma flow, just like some near-Earth dipolarization events reported previously. The spreading of substorm disturbances in the tail coupled with complementary ground observations indicates that the observed time sequence on the onsets of substorm disturbances favors initiation in the near-Earth region for this THEMIS-like conjunction.
Published: 09 May 2007
Above the polar cap, at about 5-9 Earth radii (RE) altitude, the PEACE experiment onboard CLUSTER detected, for the first time, electron beams outflowing from the ionosphere with large and variable energy fluxes, well collimated along the magnetic field lines. All these events occurred during periods of northward or weak interplanetary magnetic field (IMF).
These outflowing beams were generally detected below 100 eV and typically between 40 and 70 eV, just above the photoelectron level. Their energy gain can be explained by the presence of a field-aligned potential drop below the spacecraft, as in the auroral zone. The careful analysis of the beams distribution function indicates that they were not only accelerated but also heated. The parallel heating is estimated to about 2 to 20 eV and it globally tends to increase with the acceleration energy. Moreover, WHISPER observed broadband electrostatic emissions around a few kHz correlated with the outflowing electron beams, which suggests beam-plasma interactions capable of triggering plasma instabilities.
In presence of simultaneous very weak ion fluxes, the outflowing electron beams are the main carriers of downward field-aligned currents estimated to about 10 nA/m². These electron beams are actually not isolated but surrounded by wider structures of ion outflows. All along its polar cap crossings, Cluster observed successive electron and ion outflows. This implies that the polar ionosphere represents a significant source of cold plasma for the magnetosphere during northward or weak IMF conditions. The successive ion and electron outflows finally result in a filamented current system of opposite polarities which connects the polar ionosphere to distant regions of the magnetosphere.
Published: 09 May 2007
This paper describes a general-purpose algorithm for computing the gradients in space and time of a scalar field, a vector field, or a divergence-free vector field, from in situ measurements by one or more spacecraft. The algorithm provides total error estimates on the computed gradient, including the effects of measurement errors, the errors due to a lack of spatio-temporal homogeneity, and errors due to small-scale fluctuations. It also has the ability to diagnose the conditioning of the problem. Optimal use is made of the data, in terms of exploiting the maximum amount of information relative to the uncertainty on the data, by solving the problem in a weighted least-squares sense. The method is illustrated using Cluster magnetic field and electron density data to compute various gradients during a traversal of the inner magnetosphere. In particular, Cluster is shown to cross azimuthal density structure, and the existence of field-aligned currents in the plasmasphere is demonstrated.
Published: 09 May 2007
Most regions of the Jovian magnetosphere covered by the Galileo spacecraft measurements undergo quasi-periodic modulations with a time period of several Earth days. These modulations appear in various field and particle properties. Most prominent are periodically recurring ion flow bursts associated with disturbances in the meridional component of the magnetic field in the Jovian magnetotail or variations of the energy spectral shape of the particle distribution associated with the stretching and dipolarization of the magnetic field. Each individual cycle of these modulations is believed to represent a global reconfiguration of the Jovian magnetosphere. We present a simple conceptual model for these periodic processes assuming (1) ion mass loading from internal plasma sources and (2) fast planetary rotation causing magnetotail field line stretching due to centrifugal forces. This leads to a magnetotail configuration favoring magnetic reconnection. Magnetic reconnection causes plasmoid formation and release as well as dipolarization of field lines connected to the planet. Continued mass loading leads again to a stretching of the tail field lines. Our model shows that the suggested intrinsic mechanism can explain the observed periodicities of several days in Jovian substorm-like processes.
Published: 06 May 2007
The solar gravity modes have not been conclusively detected in the Sun as yet due to their small surface amplitudes. They have been actively searched for because they directly probe the solar burning core (below 0.2 solar radius). Using data from the Global Oscillation at Low Frequency instrument we detect a periodic structure, which is in agreement with the period separation predicted by the theory for gravity dipole modes. The detailed study of this structure, compared to simulations including the best physics of the Sun determined through the acoustic modes, would favor a faster core rotation rate than in the rest of the radiative zone.
Published: 04 May 2007
In this paper we report on the third soft gamma-ray source catalog obtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRAL satellite. The scientific data set is based on more than 40 Ms of high-quality observations performed during the first 3.5 yr of Core Program and public IBIS/ISGRI observations. Compared to previous IBIS/ISGRI surveys, this catalog includes a substantially increased coverage of extragalactic fields, and comprises more than 400 high-energy sources detected in the energy range 17-100 keV, including both transients and faint persistent objects that can only be revealed with longer exposure times.
Published: 03 May 2007
Gamma-ray line emission from radioactive decay of 60Fe provides constraints on nucleosynthesis in massive stars and supernovae. The spectrometer SPI on board INTEGRAL has accumulated nearly three years of data on gamma-ray emission from the Galactic plane. We have analyzed these data with suitable instrumental-background models and sky distributions to produce high-resolution spectra of Galactic emission. We detect the gamma-ray lines from 60Fe decay at 1173 and 1333 keV, obtaining an improvement over our earlier measurement of both lines with now 4.9 sigma significance for the combination of the two lines. The average flux per line is (4.4 +- 0.9) x 10-5 ph cm-2 s-1 rad-1 for the inner Galaxy region. Deriving the Galactic 26Al gamma-ray line flux with using the same set of observations and analysis method, we determine the flux ratio of 60Fe/26Al gamma-rays as 0.148 +- 0.06. The current theoretical predictions are still consistent with our result.
Published: 01 May 2007
Multispacecraft measurements in the solar wind are used to determine the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence. The ratio of the parallel to perpendicular correlation lengths is measured by using time-lagged two-point correlations to construct a spatial autocorrelation function. The mean ratio obtained, 1.79 +- 0.36, is significantly greater than unity and therefore consistent with solar wind fluctuations being anisotropic with energy predominantly in wavevectors perpendicular to the large-scale mean magnetic field. In analyzing eight 40-60 minute intervals of multipoint magnetic field data from the four Cluster spacecraft, the degree of variation in the ratio of the parallel to perpendicular correlation lengths about the mean was larger than expected. This variation does not appear to be correlated with the solar wind velocity or the plasma beta. The ratio of parallel to perpendicular correlation lengths was also uncorrelated between different field components.
Published: 01 May 2007
We investigate in detail the breakdown of the frozen-in condition detected by Cluster at the downstream distance of ~19 RE in the midnight sector of the magnetotail during a substorm expansion on 22 August 2001. It is found that the breakdown occurred (1) in a low-density environment with moderate to large proton plasma flow and significant fluctuations in electric and magnetic fields, (2) in regions with predominantly dissipation but occasionally dynamo effect, and (3) at times simultaneously at two Cluster satellites separated by more than 1000 km in both X- and Z-directions. Evaluation of the terms in the generalized Ohm's law indicates that the anomalous resistivity contribution arising from field fluctuations during this event is the most significant, followed by the Hall, electron viscosity, and inertial contributions in descending order of importance. This result demonstrates for the first time from observations that anomalous resistivity from field fluctuations (implying kinetic instabilities) can play a substantial role in the breakdown of the frozen-in condition in the magnetotail during substorm expansions. Consideration of several observed features in the breakdown regions indicates that the breakdown occurs in a turbulent site resembling observed features found in current disruption and dipolarization sites.
Published: 29 April 2007
Double cusps have been observed on a few occasions by polar orbiting spacecraft and ground-based observatories. The four Cluster spacecraft observed two distinct regions, showing characteristics of a double cusp, during a mid-altitude cusp pass on 7 August 2004. The Wind spacecraft detected a southward turning of the Interplanetary Magnetic Field (IMF) at the beginning of the cusp crossings and IMF-Bz stayed negative throughout. Cluster 4 observed a high energy step in the ion precipitation around 1 keV on the equatorward side of the cusp and a dense ion population in the cusp centre. Cluster 1, entering the cusp around 1 min later, observed only a partial ion dispersion with a low energy cutoff reaching 100 eV, together with the dense ion population in the cusp centre. About 9 min later, Cluster 3 entered the cusp and observed full ion dispersion from a few keV down to around 50 eV, together with the dense ion population in the centre of the cusp. The ion flow was directed poleward and eastward in the step/dispersion, whereas in the centre of the cusp the flow was directed poleward and westward. In addition the altitude of the source region of ion injection in the step/dispersion was found 50% larger than in the cusp centre. This event could be explained by the onset of dayside reconnection when the IMF turned southward. The step would be the first signature of component reconnection near the subsolar point, and the injection in the centre of the cusp a result of anti-parallel reconnection in the northern dusk side of the cusp. A three-dimensional magnetohydrodynamic (MHD) simulation is used to display the topology of the magnetic field and locate the sources of the ions during the event.
Published: 20 April 2007
Low power deep space communication technology is an enabling technology for the Interstellar Heliopause Probe Technology Reference Study. Radio wave communication capable of performing this task exists today, but they are heavy and require significant electrical power. Optical communication technology on the other hand is still immature. This short document summarizes the results of a communication subsystem trade performed by Kayser-Threde, as part of the Interstellar Heliopause Probe system design study [Leipold05, Leipold06]. The objective was to identify and investigate optical and radio wave deep space communication systems capable of delivering the required performance of the Interstellar Heliopause Probe TRS.
Published: 18 April 2007
This document provides an overview of the Interstellar Heliopause Probe system design study. The Interstellar Heliopause Probe (IHP) is one of the Technology Reference Studies (TRS) introduced by the Science Payload & Advanced Concepts Office (SCI-A) at ESA. The overall purpose of the TRSs is to focus the development of strategically important technologies that are of likely relevance to potential future science missions. This is accomplished through the study of technologically demanding and scientifically interesting missions, which are currently not part of the ESA science programme. The TRSs subsequently act as a reference for possible future technology development activities. This study overview summarizes the IHP system design study and payload assessment study performed by, respectively, Kayser-Threde [Leipold05], and Cosine Research [Kraft05]. The results of this study have also been presented at a number of conferences and published in several journals (see section 8 for an overview).
Published: 18 April 2007
The electric-field-induced acceleration of ions in current sheets in a collisionless plasma is investigated. The analysis of nonadiabatic ion dynamics provides a universal property of the ion acceleration mechanism, which is independent of the magnetic-field model and the initial particle distribution function. The width of the resonance region is estimated. The theoretical results are compared with the experimental and numerical simulation data.
Published: 16 April 2007
In the April issue of Physics World, Katariina Nykyri reveals how the European Space Agency's Cluster mission, which consists of four identical satellites flying in an elliptical polar orbit around the Earth, has been providing physicists with a 3D map of the structure and dynamics of the plasma surrounding the Earth. Such data could improve our understanding of geomagnetic storms and could even make fusion energy a reality.
Published: 03 April 2007

This document has been prepared to give a concise overview of the studies that have been performed in the framework of the Jovian related Technology Reference Studies. The goal of these studies is the identification of technologies that are required to enable possible low resource missions to the Jovian System. These activities are subdivided in three main topics:

  • The Jovian Minisat Explorer (JME): The exploration of Europa and the Jovian System
  • The Jupiter Entry Probe (JEP): In situ exploration of the Jovian atmosphere up to 100 bar
  • The Jovian System Explorer (JSE): Study of the Jovian magnetosphere and the Jovian System
Published: 31 March 2007
Unusual wave activity in the Pc 1-2 frequency band (0.1-5 Hz) was observed by the Cluster spacecraft in association with the two large geomagnetic storms of late 2003. During the onset of the Halloween storm on October 29, 2003, intense broadband activity between ~0.1 and 0.6 Hz appeared at all 4 spacecraft on both sides of the magnetic equator at perigee (near 1400 UT and 08:45 MLT). Power was especially strong and more structured in frequency in the compressional component: a minimum in wave power was observed at 0.38 Hz, corresponding to the oxygen ion cyclotron frequency. Poynting vector calculations indicated that wave power was primarily directed radially inward rather than along the magnetic field. Narrowband purely compressional waves near 0.15 Hz appeared at higher dayside latitudes in the southern hemisphere. CIS ion spectrometer data during this pass revealed that O+ was the dominant energetic ion. During the recovery phase of the November storm, on November 22, 2003, predominantly transverse 1.8 Hz waves with peak-to-peak amplitude of 10 nT were observed by all four spacecraft near perigee at L=4.4. During this more typical Pc 1 event, wave power was directed along B, toward the northern ionosphere. An unusually polarized 2.3 Hz emission (with power in the radial and compressional, but not azimuthal directions) was observed at L=5.4-5.9, 10-15° south of the magnetic equator. - Remainder of abstract truncated -
Published: 02 March 2007
The article appears translated to English in the Journal of Experimental and Theoretical Physics (JETP) Letters.
Published: 02 March 2007
5-Dec-2020 00:22 UT

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