Publication archive

Publication archive

We examine Cluster observations of a so-called magnetosphere crater FTE, employing data from five instruments (FGM, CIS, EDI, EFW, and WHISPER), some at the highest resolution. The aim of doing this is to deepen our understanding of the reconnection nature of these events by applying recent advances in the theory of collisionless reconnection and in detailed observational work. Our data support the hypothesis of a stratified structure with regions which we show to be spatial structures. We support the bulge-like topology of the core region (R3) made up of plasma jetting transverse to reconnected field lines. We document encounters with a magnetic separatrix as a thin layer embedded in the region (R2) just outside the bulge, where the speed of the protons flowing approximately parallel to the field maximizes: (1) short (fraction of a sec) bursts of enhanced electric field strengths (up to <30 mV/m) and (2) electrons flowing against the field toward the X line at approximately the same time as the bursts of intense electric fields. R2 also contains a density decrease concomitant with an enhanced magnetic field strength. At its interface with the core region, R3, electric field activity ceases abruptly. The accelerated plasma flow profile has a catenary shape consisting of beams parallel to the field in R2 close to the R2/R3 boundary and slower jets moving across the magnetic field within the bulge region. We detail commonalities our observations of crater FTEs have with reconnection structures in other scenarios. We suggest that in view of these properties and their frequency of occurrence, crater FTEs are ideal places to study processes at the separatrices, key regions in magnetic reconnection. This is a good preparation for the MMS mission.
Published: 08 February 2011
Supermassive black holes are now thought to lie at the heart of every giant galaxy with a spheroidal component, including our own Milky Way. The birth and growth of the first 'seed' black holes in the earlier Universe, however, is observationally unconstrained and we are only beginning to piece together a scenario for their subsequent evolution. Here we report that the nearby dwarf starburst galaxy Henize 2-10 contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source. From these observations, we conclude that Henize 2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. This nearby dwarf galaxy, simultaneously hosting a massive black hole and an extreme burst of star formation, is analogous in many ways to galaxies in the infant Universe during the early stages of black-hole growth and galaxy mass assembly. Our results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and that by implication so can their primordial counterparts. Moreover, the lack of a substantial spheroidal component in Henize 2-10 indicates that supermassive black-hole growth may precede the build-up of galaxy spheroids.
Published: 03 February 2011
Reference: ESA/SRE(2011)3

This report, the so-called Yellow Book, contains the results of ESA's assessment study (Phase 0/A) of the candidate L-class Cosmic Vision mission LISA.

Published: 03 February 2011
Reference: ESA/SRE(2011)2

This report, the so-called Yellow Book, contains the results of ESA's assessment study (Phase 0/A) of the candidate L-class Cosmic Vision mission IXO.

Published: 03 February 2011
Reference: ESA/SRE(2011)1

This report, the so-called Yellow Book, contains the results of ESA's assessment study (Phase 0/A) of the candidate L-class Cosmic Vision mission EJSM-Laplace.

Published: 03 February 2011
Aurora, commonly seen in the polar sky, is a ubiquitous phenomenon occurring on Earth and other solar system planets. The colorful emissions are caused by high-energy beams of electrons hitting the upper atmosphere, after being accelerated by quasi-static electric fields at altitudes around one Earth radius, or by wave electric fields. Although the aurora was studied by many past satellite missions, Cluster is first to explore the acceleration region with multi-probes, enabling open issues on its nature to be resolved. Here, Cluster data from the upper and lower parts of this region are used to determine the altitude distribution of the acceleration potential above the Aurora Borealis, and to address its stability in space and time. The derived acceleration potential consists of two broad Ushaped potentials in the upper parts of the acceleration region, and a narrower S-shaped potential structure located below, and is stable on a five minute time scale. The results demonstrate that the spatial scale of the electric field is much smaller than the current width in the lower parts, but almost equal in the upper parts of the acceleration region Revealing of these features was possible only by combining data from the two spacecraft.
Published: 01 February 2011
Magnetic reconnection in magnetized plasmas represents a change in magnetic field topology and is associated with a concomitant energization of charged particles that results from a conversion of magnetic energy into particle energy. In Earths magnetosphere this process is associated with the entry of the solar wind into the magnetosphere and with the initiation of auroral substorms. Using data from the THEMIS mission, together with global and test particle simulations, we demonstrate that electrons are energized in two distinct regions: a low-energy population (less than or equal to a few kiloelectronvolts) that arises in a diffusion region where particles are demagnetized and the magnetic topology changes, and a high-energy component (approaching 100 keV) that results from betatron acceleration within dipolarization fronts that sweep towards the inner magnetosphere far from the diffusion region. Thus, the observed particle energization is associated with both magnetic reconnection and with betatron acceleration associated with macroscopic flows.
Published: 30 January 2011
Reference: SRE-PA/2011.002/

This technical review report for the LISA candidate mission presents the outcome of an ESA internal review of this L-class candidate mission in the Cosmic Vision 2015-2025 plan. The review was concluded at the end of the mission assessment phase and carried out in frame of the down-selection for L-class missions to proceed to the definition phase. The review focused on the technical and programmatic elements of the mission.

Published: 29 January 2011
In this investigation we introduce and discuss quantitative parameters of a thin current sheet embedded in the background plasma sheet. We use Cluster statistics and empirical models, as well as self-consistent simulations, to understand the formation and development of embedded current sheets, in particular in the course of substorms. Data and theory show that the embedded sheet thickness is of the order of a proton larmor radius, a constraint equivalent to magnetic flux conservation. The embedded sheet can be essentially described by two dimensionless parameters B0/Bext and F0/Fext. B0 is the magnetic field at the embedded sheet boundary, Bext is the field at the boundary of the background plasma sheet, and F0 and Fext are magnetic flux values. During the growth phase current density in embedded sheet and B0 increase, while thickness decreases. Sheets with the most intense currents (large B0) are observed after onset. The self-consistent anisotropic sheet model, including both electron and proton currents and combined with the Harris-type background shows that when the proton-scale embedded sheet becomes sufficiently thin, an electron-scale current sheet can appear inside it due to enhanced electron curvature drift.
Published: 28 January 2011
Reference: SRE-PA/2011.003/MNCE

This technical review report for the EJSM-Laplace candidate mission presents the outcome of an ESA internal review of this L-class candidate mission in the Cosmic Vision 2015-2025 plan. The review was concluded at the end of the mission assessment phase and carried out in frame of the down-selection for L-class missions to proceed to the definition phase. The review focused on the technical and programmatic elements of the mission.

Published: 27 January 2011
Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6000 galaxies existing just 900-2000 million years (Myr) after the Big Bang (redshifts 6>z>3). The Hubble Ultra Deep Field (HUDF09) data have yielded the first reliable detections of z~8 galaxies that, together with reports of a gamma-ray burst at z~8.2, constitute the earliest objects reliably reported to date. Observations of z~7-8 galaxies suggest substantial star formation at z>9-10. Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z~10 galaxies in the heart of the reionization epoch, only 500 Myr after the Big Bang. Not only do we find one possible z ~10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (~10%) at this time than it is just ~200 Myr later at z~8. This demonstrates how rapid galaxy build-up was at z~10, as galaxies increased in both luminosity density and volume density from z~10 to z~8. The 100-200 Myr before z~10 is clearly a crucial phase in the assembly of the earliest galaxies.
Published: 26 January 2011
Using three-dimensional MHD simulations of magnetic reconnection in the magnetotail, we investigate the fate of earthward bursty bulk flows (BBFs). The flow bursts are identified as entropy-depleted magnetic flux tubes ("bubbles") generated by the severance of a plasmoid via magnetic reconnection. The onset of fast reconnection coincides closely with a drastic entropy reduction at the onset of lobe reconnection. The fact that, in the simulation, the Alfvén speed does not change significantly at this time suggests that the destabilization of ballooning/interchange modes is important in driving faster reconnection as well as in providing cross-tail structure. In the initial phase, the BBFs are associated with earthward propagating dipolarization fronts. When the flow is stopped nearer to Earth, the region of dipolarization expands both azimuthally and tailward. Tailward flows are found to be associated with a rebound of the earthward flow and with reversed vortices on the outside of the flow. Earthward and tailward flows are also associated with expansion and contraction of the near plasma sheet. All of these features are consistent with recent satellite observations by Cluster and the Time History of Events and their Macroscopic Interactions during Substorms (THEMIS) mission.
Published: 22 January 2011
Using the SPICAV-UV spectrometer aboard Venus Express in nadir mode, we were able to derive spectral radiance factors in the middle atmosphere of Venus in the 170-320 nm range at a spectral resolution of R ~ 200 during 2006 and 2007 in the northern hemisphere. By comparison with a radiative transfer model of the upper atmosphere of Venus, we could derive column abundance above the visible cloud top for SO2 using its spectral absorption bands near 280 and 220 nm. SO2 column densities show large temporal and spatial variations on a horizontal scale of a few hundred kilometers. Typical SO2 column densities at low latitudes (up to 50°N) were found between 5 and 50 micron-atm, whereas in the northern polar region SO2 content was usually below 5 micron-atm. The observed latitudinal variations follow closely the cloud top altitude derived by SPICAV-IR and are thought to be of dynamical origin. Also, a sudden increase of SO2 column density in the whole northern hemisphere has been observed in early 2007, possibly related to a convective episode advecting some deep SO2 into the upper atmosphere.
Published: 15 January 2011
Reference: SRE-PA/2010/104

This technical review report for the IXO candidate mission presents the outcome of an ESA internal review of this L-class candidate mission in the Cosmic Vision 2015-2025 plan. The review was concluded at the end of the mission assessment phase and carried out in frame of the down-selection for L-class missions to proceed to the definition phase. The review focused on the technical and programmatic elements of the mission.

Published: 14 January 2011
The modulations of the outer ring current O+ ion fluxes by ULF Pc5 waves are investigated by multisatellite observations during storm times. The O+ ions have energies up to tens of keV. We concentrate on the process in terms of drift-bounce resonance of O+ ions with ULF standing waves to understand whether the ring current O+ ions could be accelerated/decelerated by ULF waves. Two case studies are performed, in which the Cluster satellites travel the outer ring current region in the morning sector with radial distances of about 5.5 RE. Distinct O+ ion flux oscillations are observed associated with fundamental mode ULF standing waves. On 25 October 2002, both satellites SC1 and SC4 observe strong poloidal and toroidal standing waves at approximately the same region one by one with a time lag of 45 min. The O+ ion flux oscillations at around 20 keV are dominantly coherent with the poloidal standing wave at 3.4 mHz with cross phases of near 90° with respect to the magnetic field waves. The O+ phase space density spectra at 10 to 25 keV, measured by both satellites, deviate significantly from the typical power law distribution. We suggest that the O+ ions at 10 to 25 keV are accelerated due to drift-bounce resonance with the poloidal standing wave. On 4 November 2002, satellite SC1 observes considerable poloidal and toroidal standing waves. The O+ ion flux oscillation at around 7 keV is well correlated with both of the two wave modes at 3.7 mHz with cross phases of about 90° with respect to the magnetic field waves. The O+ spectra at 4 to 8 keV deviates remarkably from the background power law distribution. When satellite SC4 closely encounters the same region 40 min later, the wave activities at 3.7 mHz are found to be rather weak and the O+ spectra is close to the background power law distribution. We suggest that the spectra variation of SC1 results from the deceleration of O+ ion at 4 to 8 keV via drift-bounce resonances during the strong wave activities.
Published: 11 January 2011
We report evidence of a fully established galaxy cluster at z = 2.07, consisting of a ~20 sigma overdensity of red, compact spheroidal galaxies spatially coinciding with extended X-ray emission detected with XMM-Newton. We use VLT VIMOS and FORS2 spectra and deep Subaru, VLT and Spitzer imaging to estimate the redshift of the structure from a prominent z = 2.07 spectroscopic redshift spike of emission-line galaxies, concordant with the accurate 12-band photometric redshifts of the red galaxies. Using NICMOS and Keck AO observations, we find that the red galaxies have elliptical morphologies and compact cores. While they do not form a tight red sequence, their colours are consistent with that of a >1.3 Gyr population observed at z ~ 2.1. From an X-ray luminosity of 7.2×1043 erg/s and the stellar mass content of the red galaxy population, we estimate a halo mass of 5.3-8×1013 solar masses, comparable to the nearby Virgo cluster. These properties imply that this structure could be the most distant, mature cluster known to date and that X-ray luminous, elliptical-dominated clusters are already forming at substantially earlier epochs than previously known.
Published: 11 January 2011
The Cosmic Microwave Background fluctuations provide a powerful probe of the dark ages of the universe through the imprint of the secondary anisotropies associated with the reionization of the universe and the growth of structure. We review the relation between the secondary anisotropies and the primary anisotropies that are directly generated by quantum fluctuations in the very early universe. The physics of secondary fluctuations is described, with emphasis on the ionization history and the evolution of structure. We discuss the different signatures arising from the secondary effects in terms of their induced temperature fluctuations, polarization and statistics. The secondary anisotropies are being actively pursued at present, and we review the future and current observational status.
Published: 02 May 2008
In June 2006 Venus Express crossed several times the outer boundary of Venus induced magnetosphere, its magnetosheath and its bow shock. During the same interval the Cluster spacecraft surveyed the dawn flank of the terrestrial magnetosphere, intersected the Earth's magnetopause and spent long time intervals in the magnetosheath. This configuration offers the opportunity to perform a joint investigation of the interface between Venus and Earth's outer plasma layers and the shocked solar wind. We discuss the kinetic structure of the magnetopause of both planets, its global characteristics and the effects on the interaction between the planetary plasma and the solar wind. A Vlasov equilibrium model is constructed for both planetary magnetopauses and provides good estimates of the magnetic field profile across the interface. The model is also in agreement with plasma data and evidence the role of planetary and solar wind ions on the spatial scale of the equilibrium magnetopause of the two planets. The main characteristics of the two magnetopauses are discussed and compared.
Published: 07 May 2010
The forward cascade of decaying whistler turbulence is studied in low beta plasma to understand essential properties of the energy spectrum at electron scales, by using a two-dimensional electromagnetic particle-in-cell (PIC) simulation. This simulation demonstrates turbulence in which the energy cascade rate is greater than the dissipation rate at the electron inertial length. The PIC simulation shows that the magnetic energy spectrum of forward-cascaded whistler turbulence at electron inertial scales is anisotropic and develops a very steep power-law spectrum which is consistent with recent solar wind observations. A comparison of the simulated spectrum with that predicted by a phenomenological turbulence scaling model suggests that the energy cascade at the electron inertial scale depends on both magnetic fluctuations and electron velocity fluctuations, as well as on the whistler dispersion relation. Thus, not only kinetic Alfvén turbulence but also whistler turbulence may explain recent solar wind observations of very steep magnetic spectra at short scales.
Published: 28 December 2010
We present a three-dimensional reconstruction of an eruption that occurred on 2010 April 3 using observations from SWAP on board PROBA2 and SECCHI on board STEREO. The event unfolded in two parts: an initial flow of cooler material confined to a height low in the corona, followed by a flux rope eruption higher in the corona. We conclude that mass off-loading from the first part triggered a rise and, subsequently, catastrophic loss of equilibrium of the flux rope.
Published: 23 December 2010
31-Oct-2020 08:31 UT

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