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[1] The spatial distribution of electric fields, conductances, and currents of steadily drifting medium-scale (15–50 km) arcs in the evening sector (20–23 magnetic local time (MLT)) is obtained from European Incoherent Scatter Radar (EISCAT) and optical ground-based measurements. The current systems of stable arcs residing in the northward convection electric field region show a consistent pattern: currents flow downward on the equatorward side of the arcs, then poleward, and upward from the arcs. In one event where the arcs are located in a region of convection reversals, the current pattern is more complicated. Most of the arcs are associated with an enhanced northward-directed electric field region on the equatorward side of the arc, colocated with downward field-aligned currents (FACs) and suppressed E and F region electron densities. The width of the region of the enhanced electric field is one to four times the width of the arc. In some cases, the electron density reduction is so pronounced that the region can be described as an auroral ionospheric density cavity. The electrostatic magnetosphere–ionosphere coupling model of arcs predicts that the width L of an arc is related to the ionospheric Pedersen conductance ΣP and the “field-aligned conductance” K by . This study shows that stable medium-scale arcs in the evening sector obey this equation. A value of K = 2 × 10−8 S m−2 is obtained for 15–35 km wide arcs. It is argued that the large value of the field-aligned conductance cannot be interpreted in terms of the adiabatic theory. Possibly the high value of K results from nonadiabatic processes acting on the current-carrying electrons.

Received 14 January 2002; revised 22 July 2002; accepted 9 August 2002; published 11 December 2002. [1] Super Dual Auroral Radar Network (SuperDARN) observations, ultraviolet imaging from the Polar satellite (ultraviolet imager [UVI]), and particle precipitation data from Defense Meteorological Satellite Program (DMSP) satellites have been used to investigate the electrodynamics of the postnoon auroral oval in the Northern hemisphere. We show that: (1) For negative interplanetary magnetic field (IMF) By, the convection reversal (CR) was colocated with the maximum of auroral luminosity, but during positive IMF By, the convection reversal was poleward of the auroral oval up to several degrees in latitude. (2) Postnoon auroral oval was associated with a large-scale upward field-aligned current (FAC) of the order of 6 � 10 � 7 Am � 2 in magnitude (the FAC was inferred from the SuperDARN and UVI data). For negative IMF By, the maximum of the auroral intensity coincides in latitude with the maximum of the upward FAC. However, for positive IMF By, the maximum of the upward FAC was shifted to the poleward edge of the auroral oval. (3) In response to the IMF By turning from positive to negative, the maximum of the auroral luminosity did not change its position noticeably, but the position of the CR changed considerably from 80� –81� to about 76� magnetic latitude (MLAT), and the maximum of FAC moved from 77� –78� to about 76� MLAT. Thus, after the IMF By turns negative, both the FAC maximum and CR tend to coincide with the auroral maximum. (4) The IMF Bz positive deflection was followed by a decrease in both FAC intensity and auroral luminosity. However, the decrease in the auroral luminosity lags behind the FAC decrease by about 12 min. Firstly, these observations allow us to suggest that the IMF By-related electric field can penetrate into the closed magnetosphere and produce convection and FAC changes in the region of the postnoon auroral oval. Secondly, we suggest that the interchange instability is a promising mechanism for the postnoon auroras. INDEX TERMS: 2463 Ionosphere: Plasma convection; 2407 Ionosphere: Auroral ionosphere (2704); 2708 Magnetospheric Physics: Current systems (2409); 2784 Magnetospheric Physics: Solar wind/ magnetosphere interactions; KEYWORDS: postnoon aurora, ionospheric convection, field-aligned currents, IMF changes, boundary plasma sheet

[1] We present observations of the near-noon high-latitude poleward moving auroral arcs, which show essential differences from well-known poleward moving auroral forms associated with reconnection and flux transfer events. We suggest that the observed arcs are a specific class of dayside auroras referred to in the paper as poleward moving auroral arcs. The arcs were investigated together with the ionospheric plasma flows derived from the European Incoherent Scatter VHF measurements over Svalbard. The auroral arc motion was monitored at 1000–1300 magnetic local time by the all-sky camera on 9 December 1998. It has been found that the noon auroral arcs move poleward at the velocity of the order of 150–350 m s−1, and this velocity does not show any dependence on the velocity of the ionospheric plasma convection along the same direction. The arcs appear at 10–20 min after strong changes (up to 3 km s−1) in the east-west plasma flow, which resulted from variations in the interplanetary magnetic field. Ionospheric plasma drift and ground magnetometer data show the location of the auroras equatorward of the convection reversal. Spectrums of the northward electric field variations measured by the radar demonstrate clear peaks corresponding to the magnetic field line eigenfrequency oscillations with amplitude on the order of 10 mV m−1. The period of the oscillations increases from 7 min at 74.3° magnetic latitude (MLAT) to 14 min at 76.4° MLAT, and corresponding spectral characteristics were observed in the magnetic field measured on the ground. The observed features allow us to suggest that the noon auroral arcs arise on closed magnetic field lines as a result of interference between Alfven field line eigenmode toroidal oscillations on different L shells. The field line resonance oscillations at the near-cusp L shells are excited by the Alfven impulse associated with the convection disturbance following after variations in interplanetary parameters.

A mechanism for the transformation of a magnetoacoustic wave into an Alfvén wave is proposed. During the compression of the magnetosphere by the solar wind the inner edge of the plasma sheet and the contours of B=const move in different ways. In the case of asymmetrical compression, the contours of B=const will cross the inner edge of the plasma sheet. To close the drift currents - that flow in the plasma sheet along the contours of B=const - the appearance of the field-aligned currents is necessary. This appearance corresponds to the generation of the Alfvén wave.

This thesis is based on five publications studying the terrestrial and Martian ionospheres by making use of versatile instruments and of modelling techniques. The terrestrial ionosphere is a complex system strongly coupled to the magnetosphere and hence very sensitive to solar wind driving. Various kinds of instruments may be used to study the ionosphere, from ground-based instruments to satellite-borne systems. Two papers study the response of the auroral and subauroral ionosphere to solar wind high-speed streams, which originate from coronal holes at the surface of the Sun. These two studies make use of the superposed epoch analysis method, which enables to derive the statistical behaviour of the studied parameters. For the first study, which focuses on the F-region peak electron density measured by the Sodankylä ionosonde (at L = 5.2), the superposed epoch method has been modified so that a study of the effects of high-speed streams in the F region in different magnetic local time sectors becomes possible. The modified method is called phase-locked superposed epoch analysis. The second paper focuses on energetic (>30 keV) electron precipitation during high-speed streams by making use of cosmic noise absorption measurements from a chain of riometers located between L = 3.8 and L = 5.7. A third study reveals for the first time pulsation signatures in cosmic noise absorption data during a pulsating aurora event. This indicates that the electron precipitation flux is modulated simultaneously over a broad range of energies (from a few keV to several tens of keV) in relation to pulsating aurora. The fourth and fifth articles study the Martian ionosphere. They present a novel analysis method for Mars Express radio-occultation data. Contrary to the classical inversion approach, this new method is based on a direct simulation of the radio wave propagation between the ground-based station at Earth and the Mars Express spacecraft, in a modelled Martian environment. The parameters determining the properties of the neutral atmosphere and the ionosphere of Mars are adjusted in order for the simulated radio-occultation data to fit the measured data. The optimal set of parameters provides the retrieved neutral temperature and density profiles in the atmosphere, and the ion and electron density profiles in the ionosphere near the occultation point.This doctoral training is co-supervised by the University of Oulu (Finland) and the Université Toulouse III Paul Sabatier (France).; Cette thèse est basée sur cinq publications étudiant les ionosphères terrestre et martienne en s'appuyant sur la combinaison d'observations provenant d'instruments variés ainsi que sur des techniques de modélisation. L’ionosphère terrestre est un système complexe fortement couplé à la magnétosphère et est par conséquent affectée par les perturbations provenant du vent solaire. De nombreux types d’instruments peuvent être utilisés pour étudier la variabilité de l’ionosphère, qu'il s'agisse de systèmes d'observation au sol ou d'instruments à bord de satellites. Deux des articles se focalisent sur les réponses de l'ionosphère terrestre aurorale et subaurorale aux courants de vent solaire rapide émanant des trous coronaux à la surface du soleil. Ces deux études sont basées sur la méthode des époques superposées, qui permet d'obtenir un comportement statistique des paramètres considérés. Pour la première étude, qui s'intéresse à la concentration électronique du pic de la région F de l'ionosphère à l'aide de l'ionosonde de Sodankylä (Finlande, L = 5.2), la méthode des époques superposées a été modifiée en ajoutant un verrouillage de phase permettant de distinguer les réponses de la région F dans différents secteurs de temps magnétique local. La deuxième étude s'intéresse aux précipitations d'électrons énergétiques (>30 keV) durant les courants de vent solaire rapide, en s'appuyant sur des mesures d'absorption du bruit cosmique par des riomètres situés entre L = 3.8 et L = 5.7. Une troisième étude met en évidence pour la première fois des signatures de pulsations dans les données riométriques durant une aurore pulsante. Cela révèle que le flux de précipitation d'électrons est modulé simultanément sur une grande plage d'énergies – de quelques kiloélectronvolts à plusieurs dizaines de kiloélectronvolts – durant une aurore pulsante. Les quatrième et cinquième articles traitent de l'ionosphère martienne. Ils présentent une nouvelle méthode d'analyse des données d'occultation radio fournies par la sonde Mars Express, qui s'appuie non pas sur une inversion des mesures tel qu'effectué classiquement, mais sur une modélisation directe de l'environnement martien – atmosphère neutre et ionosphère – et de la propagation des ondes radio entre la station sol sur Terre et la sonde Mars Express. L'ajustement des paramètres dont dépendent l'atmosphère et l'ionosphère martiennes permet d'obtenir des données d'occultation radio simulées s'approchant le plus possible des données mesurées. L'ajustement optimal donne alors les profils de température et de concentration des neutres ainsi que les profils de concentrations ioniques et électronique dans l'ionosphère martienne au voisinage du point d'occultation. Cette thèse est préparée en cotutelle entre l'Université d'Oulu (Finlande) et l'Université Toulouse III Paul Sabatier (France).