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Darrouzet F.,Belgian Institute for Space Aeronomy | Pierrard V.,Belgian Institute for Space Aeronomy | Pierrard V.,Catholic University of Louvain | Benck S.,Catholic University of Louvain | And 6 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

In the present work, we study the relations between the position of the plasmapause and the position of the radiation belt boundaries. The Cluster mission offers the exceptional opportunity to analyze those different regions of the inner magnetosphere with identical sensors on multiple spacecraft. We compare the positions of the radiation belt edges deduced from CIS (Cluster Ion Spectrometry) observations (electrons with energy >2 MeV) with the positions of the plasmapause derived from WHISPER (Waves of HIgh frequency and Sounder for Probing of the Electron density by Relaxation) data (electron plasma frequency). In addition, we compare those results with the edges positions determined from RAPID (Research with Adaptive Particle Imaging Detectors) observations (electrons with energy between 244.1 and 406.5keV). The period of 1 April 2007 to 31 March 2009 has been chosen for the analysis because at that time Cluster's perigee was located at lower radial distances than during the earlier part of the mission. The perigee was then as close as 2 RE, deep inside the plasmasphere and the radiation belts. This time period corresponds to a long solar activity minimum. Differences are observed between the radiation belt boundary positions obtained from the two different instruments: The radiation belt positions are related to the energy bands. The results show that the plasmapause position is more variable than the radiation belt boundary positions, especially during small geomagnetic activity enhancements. A correspondence is observed between the plasmapause position determined by WHISPER and the outer edge of the outer radiation belt of energetic electrons (>2 MeV) observed by CIS. This result is unexpected since previous studies based on other spacecraft observations indicated a correlation between the inner edge of the outer belt and the plasmapause. However, during higher geomagnetic activity time periods, the plasmapause is located closer to the inner boundary of the outer radiation belt. Also, the thickness of the slot region is found to follow the global evolution of the geomagnetic activity. © 2013. American Geophysical Union. All Rights Reserved. Source


Decreau P.M.E.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Aoutou S.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Denazelle A.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Galkina I.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | And 4 more authors.
Annales Geophysicae | Year: 2015

The Cluster multi-point mission offers a unique collection of non-thermal continuum (NTC) radio waves observed in the 2-80 kHz frequency range over almost 15 years, from various view points over the radiating plasmasphere. Here we present rather infrequent case events, such as when primary electrostatic sources of such waves are embedded within the plasmapause boundary far from the magnetic equatorial plane. The spectral signature of the emitted electromagnetic waves is structured as a series of wide harmonic bands within the range covered by the step in plasma frequency encountered at the boundary. Developing the concept that the frequency distance df between harmonic bands measures the magnetic field magnitude B at the source (df = Fce, electron gyrofrequency), we analyse three selected events. The first one (studied in Grimald et al., 2008) presents electric field signatures observed by a Cluster constellation of small size (∼ 200 to 1000 km spacecraft separation) placed in the vicinity of sources. The electric field frequency spectra display frequency peaks placed at frequencies fs = n df (n being an integer), with df of the order of Fce values encountered at the plasmapause by the spacecraft. The second event, taken from the Cluster tilt campaign, leads to a 3-D view of NTC waves ray path orientations and to a localization of a global source region at several Earth radii (RE) from Cluster (Décréau et al., 2013). The measured spectra present successive peaks placed at fs ∼ (n+ 1/2) df. Next, considering if both situations might be two facets of the same phenomenon, we analyze a third event. The Cluster fleet, configured into a constellation of large size (∼ 8000 to 25 000 km spacecraft separation), allows us to observe wide-banded NTC waves at different distances from their sources. Two new findings can be derived from our analysis. First, we point out that a large portion of the plasmasphere boundary layer, covering a large range of magnetic latitudes, is radiating radio waves. The radio waves are issued from multiple sources of small size, each related to a given fs series and radiating inside a beam of narrow cone angle, referred to as a beamlet. The beamlets illuminate different satellites simultaneously, at different characteristic fs values, according to the latitude at which the satellite is placed. Second, when an observing satellite moves away from its assumed source region (the plasmapause surface), it is illuminated by several beamlets, issued from nearby sources with characteristic fs values close to each other. The addition of radio waves blurs the spectra of the overall received electric field. It can move the signal peaks such that their position fs satisfiesfs = (n+α) df, with 0 < α < 1. These findings open new perspectives for the interpretation of NTC events displaying harmonic signatures. © Author(s) 2015. Source


Kougblenou S.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Lointier G.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Decrau P.M.E.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Trotignon J.-G.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | And 4 more authors.
Annales Geophysicae | Year: 2011

The frequency range of the WHISPER relaxation sounder instrument on board CLUSTER, 4-80 kHz, has been chosen so as to encompass the electron gyro-frequency, F ce, and the electron plasma frequency, F p, in most regions to be explored. Measurement of those frequencies, which are triggered as resonances by the sounder, provides a direct estimation of in situ fundamental plasma characteristics: electron density and magnetic field intensity. In the late mission phase, CLUSTER penetrated regions deep inside the plasmasphere where F ce and F p are much higher than the upper frequency of the sounder's range. However, they are of the right order of magnitude as to place the lower hybrid frequency, F lh, in the 4-15 kHz band. This characteristic frequency, placed at a resonance of the medium, is triggered by the sounder's transmitter and shows up as an isolated peak in the received spectrum, not present in spectra of naturally occuring VLF waves. This paper illustrates, from analysis of case events, how measured F lh values give access to a plasma diagnostic novel of its kind. CLUSTER, travelling along its orbit, encounters favourable conditions where F ce is increasing and F p decreasing, such that F ce/F p increases from values below unity to values above unity. Measured F lh values thus give access, in turn, to the effective mass, M eff, indicative of plasma ion composition, and to the core plasmasphere electron density value, a parameter difficult to measure. The analysed case events indicate that the estimated quantities (M eff in the 1.0-1.4 range, N e in the 5 × 10 2-10 4 cm -3 range) are varying with external factors (altitude, L value, geomagnetic activity) in a plausible way. Although covering only a restricted region (mid-latitude, low altitude inner plasmasphere), these measurements are available, since late 2009, for all CLUSTER perigee passes not affected by eclipses (on average, roughly a third of a total of ∼200 passes per year) and offer multipoint observations previously unavailable in this region. © 2011 Author(s). Source


Dudok De Wit T.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Coillot C.,Ecole Polytechnique - Palaiseau | Jannet G.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Krasnoselskikh V.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | And 4 more authors.
Planetary and Space Science | Year: 2011

The ACB search-coil magnetometer for Cross-Scale will measure three components of the AC magnetic field up to 4 kHz, and one component up to 100 kHz. Turbulent and coherent magnetic field fluctuations in that frequency range play an important role in the acceleration, scattering, and thermalisation of particles. ACB will, together with the other instruments of the Cross-Scale wave consortium, allow to address the key science objectives associated with plasma waves. Here, we list some of the important issues, based on the experience drawn from Cluster, and describe the instrument. © 2009 Elsevier Ltd. All rights reserved. Source


Decreau P.M.E.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Kougblenou S.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Lointier G.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | Rauch J.-L.,Laboratoire Of Physique Et Chimie Of Lenvironnement Et Of Lespace Lpc2E | And 6 more authors.
Annales Geophysicae | Year: 2013

The Cluster mission operated a "tilt campaign" during the month of May 2008. Two of the four identical Cluster spacecraft were placed at a close distance (~ 50 km) from each other and the spin axis of one of the spacecraft pair was tilted by an angle of ~ 46°. This gave the opportunity, for the first time in space, to measure global characteristics of AC electric field, at the sensitivity available with long boom (88 m) antennas, simultaneously from the specific configuration of the tilted pair of satellites and from the available base of three satellites placed at a large characteristic separation (~ 1RE). This paper describes how global characteristics of radio waves, in this case the configuration of the electric field polarization ellipse in 3-D-space, are identified from in situ measurements of spin modulation features by the tilted pair, validating a novel experimental concept. In the event selected for analysis, non-thermal continuum (NTC) waves in the 15- 25 kHz frequency range are observed from the Cluster constellation placed above the polar cap. The observed intensity variations with spin angle are those of plane waves, with an electric field polarization close to circular, at an ellipticity ratio e = 0.87. We derive the source position in 3-D by two different methods. The first one uses ray path orientation (measured by the tilted pair) combined with spectral signature of magnetic field magnitude at source. The second one is obtained via triangulation from the three spacecraft baseline, using estimation of directivity angles under assumption of circular polarization. The two results are not compatible, placing sources widely apart. We present a general study of the level of systematic errors due to the assumption of circular polarization, linked to the second approach, and show how this approach can lead to poor triangulation and wrong source positioning. The estimation derived from the first method places the NTC source region in the dawn sector, at a large L value (L ~ 10) and a medium geomagnetic latitude (35° S).We discuss these untypical results within the frame of the geophysical conditions prevailing that day, i.e. a particularly quiet long time interval, followed by a short increase of magnetic activity. © Author(s) 2013. CC Attribution 3.0 License. Source

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