Lower hybrid resonances stimulated by the four CLUSTER relaxation sounders deep inside the plasmasphere: observations and inferred plasma characteristics

International audienceThe 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 plas-masphere 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 plasma-sphere 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 Correspondence to: S. Kougblénou ([email protected]) 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

Observations of the relationship between frequency sweep rates of chorus wave packets and plasma density

International audience[1] Chorus emissions are generated by a nonlinear mechanism involving wave‐particle interactions with energetic electrons. Discrete chorus wave packets are narrowband tones usually rising (sometimes falling) in frequency. We investigate frequency sweep rates of chorus wave packets measured by the Wideband data (WBD) instrument onboard the Cluster spacecraft. In particular, we study the relationship between the sweep rates and the plasma density measured by the WHISPER active sounder. We have observed increasing values of the sweep rate for decreasing plasma densities. We have compared our results with results of simulations of triggered emissions as well as with estimates based on the backward wave oscillator model for chorus emissions. We demonstrate a reasonable agreement of our experimental results with theoretical ones. Citation: Macúšová, E., et al. (2010), Observations of the relationship between frequency sweep rates of chorus wave packets and plasma density

New insights into the Non Thermal Continuum radiations: The Cluster Whisper perspective

International audienceThe Whisper instruments on board each of the four Cluster spacecraft exploring the Earth magnetosphere, routinely observe the Non Thermal Continuum emissions (NTC). The various and combined points of view provided by 4 spacecraft in the 2-80 kHz frequency range allow the detection of the different forms of these emissions, narrow band and broad band, emitting from source regions in the plasmapause boundary layer, close to the magnetic equator but also at mid latitude. This paper presents selected examples of new knowledge gained on the NTC radiations, locations and mechanisms, coming from the 15 years of observations of Cluster-Whisper

Détermination des grandeurs caractéristiques des analyseurs électrostatiques toriques. Application à l'optimisation d'analyseurs utilisés en physique spatiale

On présente une méthode numérique de détermination des caractéristiques d'analyseurs électrostatiques de particules, valable dans le cas général d'électrodes toriques. Le champ électrostatique est exprimé sous forme d'un développement limité en fonction de coordonnées réduites, définissant la position d'un point par rapport à l'équipotentielle moyenne. On évalue les coefficients de ce développement jusqu'à un ordre donné quelconque, et la trajectoire d'une particule est déterminée par intégration numérique des équations du mouvement. On aboutit, par exploration des différents cas d'incidence, aux grandeurs caractéristiques de l'analyseur. Les résultats sont comparés avec ceux obtenus au moyen d'une simulation en cuve rhéographique. On présente deux exemples concrets d'utilisation du programme numérique mis au point, exemples qui répondent à des problèmes de mesure en milieu spatial : analyseur à grands pouvoirs de résolution en énergie et en angle, analyseur à grand domaine d'entrée et large bande passante en énergie. La forme torique permet d'élargir de manière très appréciable le cadre des performances obtenues à l'aide d'analyseurs de formes classiques, cylindriques ou sphériques

Electron dynamics and cross-shock potential at the quasi-perpendicular Earth's bow shock

International audience[1] The evolution of the electron distribution function through quasi-perpendicular collisionless shocks is believed to be dominated by the electron dynamics in the large-scale coherent and quasi-stationary magnetic and electric fields. We investigate the electron distributions measured on board Cluster by the Plasma Electron and Current Experiment (PEACE) instrument during three quasi-perpendicular bow shock crossings. Observed distributions are compared with those predicted by electron dynamics resulting from conservation of the first adiabatic invariant and energy in the de Hoffmann-Teller frame, for all pitch angles and all types of trajectories (passing and, for the first time, reflected or trapped). The predicted downstream velocity distributions are mapped from upstream measurements using an improved Liouville mapping technique taking into account the overshoots. Furthermore, for one of these crossings we could take advantage of the configuration of the Cluster quartet to compare mapped upstream velocity distributions with those simultaneously measured at a relatively well magnetically connected downstream location. Consequences of energy and adiabatic invariant conservation are found to be compatible with the observed electron distributions, confirming the validity of electron ''heating'' theories based on DC fields as zeroth-order approximations, but some systematic deviations are found between the dynamics of low-and high-adiabatic invariant electrons. Our approach also provides a way to estimate the cross-shock electric potential profile making full use of the electron measurements, and the results are compared to other estimates relying on the steady state dissipationless electron fluid equations. At the temporal resolution of the instruments, the scales associated to the change of the potential generally appear to be comparable to those of the magnetic field, but some differences between the methods appear within the shock transition. It is argued that potentials evaluated from Liouville mapping rely on less restrictive hypotheses and are therefore more reliable. Finally, we show how, in contrast to methods using electron velocity moments, the technique can be used to produce high-time-resolution electric potentials and discuss the electric potential profiles through the shock. Citation: Lefebvre, B., S. J. Schwartz, A. F. Fazakerley, and P. Décréau (2007), Electron dynamics and cross-shock potential at the quasi-perpendicular Earth's bow shock

Observations of continuum radiations close to the plasmapause: Evidence for small scale sources.

Object identifier 0xc1aa5576 0x0015cbcaISBN 978-3-7001-3691-0 Print EditionISBN 978-3-7001-3763-4 Online Editiondoi:10.1553/0x001231b9International audienceWe briefly present observations of nonthermal continuum (NTC) radiations obtained close to the sources at the plasmapause by the Whisper instruments on Cluster. The examples considered illustrate the variety of the characteristics of the NTC such as the overall bandwidth of the emissions. The omnipresent multiple narrow bandwidth components are further evidence that the emissions could be generated from multiple, closely spaced, short scale regions (a few 10 km)

Modeling of Cluster's electric antennas in space: Application to plasma diagnostics

International audienceThe main characteristics of the long-boom electric antennas installed on board the Cluster satellites are derived from finite element modeling in a kinetic and isotropic space plasma, in the frequency range of about 1-100 kHz. The model is based on the surface charge distribution method in quasi-static conditions. The impedances of both types of antenna, i.e., the double-wire and the double-probe, are computed versus the frequency normalized with respect to the local plasma frequency and for several different Debye lengths. Most of the code outputs are checked using analytic estimations for better understanding of the involved physical mechanisms. As a by-product, the effective length of the double-probe antenna and the mutual impedance between the two antennas are computed by the code. It is shown that if it had been possible to implement such measurements on board, one would have been able not only to determine accurately the electric characteristics of the antennas but also to estimate the local plasma parameters. Nevertheless, an interesting feature predicted by the model has been checked recently in orbit by running a special mode of operation for testing the mutual impedance measurement. The preliminary results are globally consistent with the predictions, except that they suggest that our Maxwellian model for the electron distribution should be revised in order to explain the unexpected low-frequency response. After analysis of the electron flux measurements obtained simultaneously, it appears that a rough adjustment of the electron distribution with a two-component distribution allows us to account for the observations

Spectral Signatures Recorded By WHISPER Sounder Mode On Board CLUSTER2 Mission Inside Different Magnetosphere Regions: Plasma Frequency Determination

International audienceThe Wave of High frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) performs the measurement of the electron density on the four satellites of the CLUSTER2 mission. The two main purposes of the WHISPER experiment are to record the natural waves and to make a diagnostic of the electron density using the sounding technique, the various working modes and the fourier transforms calculated on board provide a good frequency resolution obtained in the bandwidth 2-80 kHz and a well instrumental adaptability to determine the electron density in various plasma. In this presentation, we focus our attention to the active mode when WHISPER is working as a sounder. Cluster2 orbits cross various plasmas around the earth with a weak or a strong magnetic field (solar wind, magnetosheat, cusp, aurora zones, magnetotail etc). Plasma response is strongly different following region. Spectra exhibit numerous resonances where it is difficult to recognize the plasma frequency .We shall show the characteristics features in each region and we shall describe the methods to identify observed resonances as electron gyro frequency and harmonics, plasma frequency, upper-hybrid frequency and Bernstein modes

Medium-latitude sources of plasmaspheric nonthermal continuum radiations observed close to harmonics of the electron gyrofrequency

International audienceNonthermal continuum (NTC) radiation is, with auroral kilometric radiation (AKR), one of the two electromagnetic emissions generated within the Earth's magnetosphere and radiated into space. It is generally believed that NTC is emitted in the plasmapause density gradient after conversion of intense electrostatic waves, present near the magnetic equator, into electromagnetic waves. In this paper, we present a specific type of NTC event, of infrequent occurrence, displaying a finger-like pattern not yet reported: banded emissions peaking at exact multiples of a common frequency, df, which decrease inbound toward the plasmapause boundary layer (PPBL). Analysis is presented that indicates that the corresponding sources are nearby sites of the PPBL where the local electron gyrofrequency f(ce) equals df. The sources are radiating beams of limited cone angle size. The NTC sources for this event are shown to be located at about 20 degrees magnetic latitude. This illustrates that the PPBL is active in radiating NTC waves not only near the magnetic equator but also up to the medium-latitude range