Showing papers in "Annales Geophysicae in 1990"

Latitudinal variation of perturbation electric fields during magnetically disturbed periods - 1986 Sundial observations and model results

Abstract: F-region incoherent scatter radar drift observations from Millstone Hill and Jicamarca, h-prime F observations from Huancayo, and high latitude ground-magnetometer measurements taken during the Sundial 1986 campaign are used to study the relationship between plasmaspheric electric field perturbations and high latitude currents during disturbed periods. The observations are in good agreement with numerical results from a Rice Covection Model run that involved a sharp increase in the polar cap potential drop followed by a subsequent decrease. The zonal disturbance electric field pattern is latitude independent, and the corresponding amplitudes change approximately as L exp n (where n is about 1.5). The meridional electric field patterns and amplitudes have larger latitudinal variations. The mid-, low, and equatorial electric fields from the Rice Convection Model are in good agreement with previous results from the semianalytic, Senior-Blanc (1987) model. Also discussed are three physical mechanisms (over-shielding, fossil winds, and magnetic reconfiguration) that contribute to the long lasting (1-2 h) equatorial zonal electric field perturbations associated with a sudden northward turning of the IMF. It is predicted that the penetration of high latitude electric fields to low latitudes should, in general, be closely related to the rate of motion of the shielding layer and the equatorward edge of the diffuse aurora.

Spatial structures in high-speed streams as signatures of fine structures in coronal holes.

Abstract: Les donnees du champ magnetique et du plasma obtenues par les 2 sondes Helios sont utilisees pour determiner si les structures fines observees dans les trous coronaux peuvent etre preservees dans l'ecoulement interplanetaire rapide du vent solaire. Un ensemble de 41 courants rapides du vent solaire mesures entre 0.3 et 1 UA pour la periode 1974 a 1977 sont analyses. Une estimation du diametre angulaire moyen de ces structures est egalement calculee a partir d'une analyse statistique realisee sur cet ensemble

Hot hydrogen and oxygen atoms in the upper atmospheres of Venus and Mars

Abstract: Optical observations of hot atoms in the atmospheres of Venus and Mars are briefly reviewed. A summary of hot hydrogen and oxygen production and loss processes is given. Results of some recent model calculations as well as a number of new results of the hot hydrogen and oxygen populations are presented and their implication in terms of solar wind interaction processes is discussed.

World map of ELF/VLF emissions as observed by a low-orbiting satellite

Abstract: The worldwide distribution of ELF and VLF emissions is presented in order to gain an understanding of the relative importance of the VLF ground-based transmitters and thunderstorm activities. Data is obtained from filterbanks with the frequency range of observations extending from tens of Hz to 15 Hz. The geomagnetic as well as geographical plots of the data at different frequency are provided, the influence of VLF ground-based transmitters and thunderstorm activities are discussed, and the results are compared with a theoretical work identifying the spatial distribution of electron precipitation induced by ground-based transmitters. The geomagnetic representations indicate that the data recorded via filterbanks is low-altitude ELF hiss and VLF hiss, while the geographical plots show intense emissions over North America at all frequencies.

Giotto magnetic field observations at the outbound quasi-parallel bow shock of Comet Halley

Abstract: The investigation of the outbound bow shock of Comet Halley using Giotto magnetometer data leads to the following results: the shock is characterized by strong magnetic turbulence associated with an increasing background magnetic field and a change in direction by 60 deg as one goes inward. In HSE-coordinates, the observed normal turned out to be (0.544, - 0.801, 0.249). The thickness of the quasi-parallel shock was 120,000 km. The shock is shown to be a new type of shock transition called a 'draping shock'. In a draping shock with high beta in the transonic transition region, the transonic region is characterized by strong directional variations of the magnetic field. The magnetic turbulence ahead of the shock is characterized by k-vectors parallel or antiparallel to the average field (and, therefore, also to the normal of the quasi-parallel shock) and almost isotropic magnetic turbulence in the shock transition region. A model of the draping shock is proposed which also includes a hypothetical subshock in which the supersonic-subsonic transition is accomplished.

Mesospheric observations with the EISCAT UHF radar during polar cap absorption events. I - Electron densities and negative ions

Abstract: Les observations de la densite des electrons mesospheriques ont ete realisees avec le radar UHF EISCAT durant plusieurs evenements d'absorption par la calotte polaire en 1989 Les observations continues sur plus de 2 jours durant un evenement permettent une etude des variations dans la densite des electrons durant 4 intervalles crepusculaires On montre qu'au lever du soleil, au-dessus de 70 km, les electrons sont liberes par photodetachement UV d'un ion negatif a haute affinite, probablement NO 3 − Les profils nocturnes du rapport de la densite des ions negatifs a la densite des electrons sont etablis, et les relations empiriques de la concentration des electrons entre 60 et 70 km sont determinees comme fonction des flux de protons observes simultanement

Electric potential patterns deduced for the SUNDIAL period of September 23-26, 1986

Abstract: High-latitude electric potential and ionospheric conductance patterns are presented and discussed for the Northern Hemisphere during the SUNDIAL period of September 23-26, 1986 using the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique of Richmond and Kamide (1988). Data sources used for the model are satellite and ground magnetometers, electron precipitation instruments, incoherent scatter radars, and ionospheric coherent radars. The period was characterized by many substorms, and a wide variety of instantaneous patterns of electrodynamic parameters on a hemispheric scale are derived throughout this period, of which only a few examples are displayed. The entire set of electric potentials and conductances are being made available through the NCAR CEDAR Data Base for further analysis and utilization in simulation models.

Giotto IMS measurements of the production rate of hydrogen cyanide in the coma of comet Halley.

Abstract: The ion composition measurements in the ionosphere of Comet Halley by the ion mass spectrometer (IMS) experiment on the Giotto spacecraft are used to estimate the relative abundance of HCN. From a comparison of the normalized number density of ions with mass-to-charge (M/q) ratio of 28 AMU/e with steady-state photochemical models, it can be determined that the production rate of HCN directly from the central nucleus is Q(HCN) is less than about 0.0002 Q(H2O) at the time of Giotto encounter. The related photochemical- model calculations also indicate that Q(NH3)/Q(H2O) at the time of Giotto encounter. The related photo-chemical model calculations also indicate that Q(HN3)/Q(H2O) equals about 0.005, in agreement with recent determination from ground-based observations. The estimated value of Q(HCN) is lower than the relative abundance of Q(HCN)/Q(H2O) of about 0.001, as derived from radio observations of the 88.6 GHz emission of the J = 1 - 0 transition of HCN. The difference may be the result of time variations of the coma composition and dynamics, as well as other model-dependent effects.

Excitation of periodic magnetic field oscillations in the ULF range by amplitude modulated HF waves

Abstract: Les ondes radio HF a haute puissance rayonnees dans l'ionosphere renforcent fortement la temperature electronique. Ce chauffage des electrons peut causer des perturbations dans le tenseur de conductivite via une augmentation de la frequence de collision ou de la densite des electrons, dependant de la frequence de modulation de l'onde HF. Deux experiences avec des frequences de modulation entre 0.7 et 9 Hz, et une avec des frequences de 1425 Hz, 1 Hz, et 8.33 mHz sont etudiees en detail et comparees avec des calculs numeriques

A possible common nature of equatorial half-gyrofrequency VLF emissions and discrete plasmaspheric emissions

Abstract: Une tentative est realisee de connecter les emissions TBF dans une bande de frequence juste au-dessus de la moitie de la gyrofrequence electronique observees dans la region equatoriale magnetospherique au-dela de la plasmapause et les emissions plasmaspheriques discretes avec une frequence de coupure inferieure proche d'un quart de la gyrofrequence electronique equatoriale, observees sur des satellites a basse altitude dans la plasmasphere

Water group plasma in the magnetosphere of Saturn.

Abstract: The paper deals with the question of the composition and spatial distribution of water group plasma in the magnetosphere of Saturn. It is suggested that the problem of the coexistence of neutral atomic hydrogen and such a plasma can be resolved if the dominant ion is taken to be H2O(+) or H3O(+). It is also suggested that this ion may provide a means of radiative cooling of the inner magnetosphere plasma which is possibly easier to realize than transport of nitrogen from Titan. As a source for this plasma in the region inward of the icy satellites, the ring atmosphere is suggested. The depleted plasma density in the domain between the Dione-Tethys torus and the ring plane crossing of Voyager may provide an explanation for the survival of the E-ring. Data are presented which indicate that H2O(+) or H3O(+) is also a favored candidate in the outer magnetosphere and that it can diffuse without loss through the hot electron gas.

Ion dynamics in the plasma mantle

Abstract: Une analyse statistique est realisee sur les donnees du manteau de plasma de l'experience EGD a bord du satellite ISEE 2 et de l'experience ICE a bord de ISEE 1. L'attention est focalisee sur les traversees manteau de plasma-feuillet de plasma afin d'elucider le role du plasma du manteau dans le remplissage du feuillet de plasma

Linear prediction theory in studies of solar wind-magnetosphere coupling

Abstract: La technique de prediction lineaire est appliquee pour modeliser la reponse magnetospherique en utilisant les donnees du vent solaire obtenues par le satellite ISEE 3 et les indices geomagnetiques AL, AU, et Dst. Les fonctions de reponse impulsionnelle calculees possedent les caracteristiques d'un filtre passe-bas avec des retards de temps qui dependent de la force de l'entree d'energie