S. Perraut

Amplification of ion-cyclotron waves for various typical radial profiles of magnetospheric parameters

Abstract The influence of magnetospheric parameters (such as cold and hot plasma densities, anisotropy and thermal energy of the hot particles) on the growth rate of ion cyclotron waves is investigated in detail. Following the inward penetration of protons having different parallel resonant energies E R , we have computed the integrated growth rate Γ at any radial distance L and plots of iso-Γ contours in the E R - L plane are presented. Conditions for having large amplification inside or outside the plasmasphere are discussed. A comparison between the energy E M of the ‘most interacting’ protons (those which interact with frequencies for which Γ is maximum) and the parallel thermal energy E ∥ of the distribution, has been made in order to determine which part of the distribution function is involved in the interaction. Comparison is also made between E M and the critical energy E c = (B 2 2μ 0 N) A −2 (A + l) −1 and some conclusions regarding the proton precipitation patterns are drawn. The present calculations do not suffer from any approximation and is valid for any cold to hot plasma density ratio.

High-energy electron drift echoes at the geostationary orbit

Abstract Approximately 60 high energy (0.7–2.6 MeV) electron drift echoes, observed with the UCSD omni-directional detectors on board ATS-5, have been analyzed. The period for longitudinal drift around the Earth is a function of energy and verifies the now classical law for relativistic electrons. Empirical relationships have been established, which statistically relate the characteristic period T 0 (the drift period for ~320 keV electrons) with the AE and Kp indices. These results are well interpreted if we adopt a Mead and Williams model (with special coefficients) for the magnetic field, and if we assume that the subsolar distance of the magnetopause R b is mainly a function of Kp , the tail field B T being mainly a function of AE . Evidence is given that these functional dependences agree with other experimental data, such as Coleman and McPherrons (1976) measurements of the midnight magnetic field intensity at the geostationary orbit. It is demonstrated that the origin of drift echoes is less likely to be an injection of high-energy particles, but more likely to be the consequence of a redistribution of particles among different L -shells after a sudden compression or expansion of the magnetosphere, as originally proposed by Brewer et al . (1969).

Amplitude et polarisation des ondes UBF détectées à bord du satellite GEOS-I. Méthodes d’analyse, problèmes rencontrés et solutions pratiques

AnalyseLes auteurs décrivent rapidement une méthode d’analyse de signaux polarisés permettant de déterminer la fréquence, l’amplitude et la polarisation instantanées d’ondesUbfdétectées dans un référentiel de mesure qui tourne à une fréquence proche de celles des ondes étudiées. Cette méthode est appliquée aux données des satellites européensGeos-1et 2, stabilisés par rotation à environ 0,17Hz. Le champ magnétique statique local comporte une forte composante perpendiculaire à l’axe de rotation du satellite, qui induit donc dans les antennes une forte composante à la fréquence de rotation. Une partie de ce signal parasite est éliminée directement à bord par un système despin.Mais la perturbation restante affecte toutes les fréquences du spectre de puissance dans une proportion qui dépend de la valeur relative du temps d’intégration et de la période de rotation. Les auteurs étudient en détail cette modification du spectre et montrent que l’on peut s’en affranchir grâce à une méthode simple de traitement numérique au sol. Les deux méthodes conjointes permettent de produire en routine des sommaires pour expérimentateurs,grâce auxquels on peut suivre, au cours du temps, les principales caractéristiques des ondes détectées.AbstractA method for analysing polarized signals is briefly described. This method is applied to the instantaneous frequency, amplitude and polarization ofUlfwaves detected in a frame of reference rotating at frequency close to the one of the waves being studied. This method is being applied to the data obtained from the European satellitesGeos-1and 2,which are stabilized by rotation at about 0.17Hz. The local static magnetic field has a strong component perpendicular to the satellite rotation axis, therefore inducing in the antennas a strong component at the rotating frequency. Part of this parasitic signal is eliminated directly on board the satellite owing to a despinsystem. But the remaining disturbance affects all the frequencies of the power spectrum with a ratio depending on the relative value of the integration time and the rotation period. This modification of the spectrum is studied in detail and the authors demonstrate that it can be avoided with an easy method of signal processing at the ground. Both methods used jointly allow the production as a routine basis of experimenter summariesenabling to know at each moment the main characteristics of the waves being detected.