Etude des processus de transport et conversion d'énergie dans la magnétosphère terrestre à partir des observations THEMIS

Les magnétosphères sont des objets universels qui résultent de l'interaction entre un écoulement plasma et un obstacle magnétisé. Leurs structures reposent sur un système de courant qui assurent la transition entre différents régimes plasma et à travers lesquels l'énergie électromagnétique peut être convertie en énergie cinétique et thermique et inversement. Dans le cas de la Terre, c'est la couche de courant s'écoulant dans le plan médian de la queue magnétosphérique qui est la clef de voûte du système et c'est là que se développe une instabilité majeure autour de laquelle s'organise le cycle énergétique du système. A la suite d'une instabilité globale, l'énergie transmise par le vent solaire et accumulée dans la queue magnétosphérique est dissipée de façon explosive lors de périodes perturbées appelées sous-orages magnétosphériques. Les missions multi-points telles Cluster, Double Star ou THEMIS permettent une analyse multi-échelle de la queue magnétosphérique. Une étude de cas de trois sous-orages successifs montre que, les changements observés dans la queue (dipolarisation se propageant vers la queue et injections de particules vers la Terre) correspondent à une disruption de courant se propageant en direction anti-solaire. La comparaison des données avec des simulations cinétiques (PIC, Particle In Cell) suggère que les signatures observées peuvent être celle du processus de reconnexion magnétique initié dans la queue proche se propageant en direction antisolaire. Les dipolarisations sont des signatures caractéristiques des sous-orages magnétosphériques. L'analyse d'une série de huit dipolarisations montre qu'elles ne sont pas toujours associées aux sous-orages mais à de petites perturbations très localisées en région aurorale. Une analyse statistique de ce type de dipolarisations a permis de caractériser les changements de l'état de la couche de plasma qu'elles provoquent (épaississement, réduction de la densité de courant, augmentation de la densité d'énergie).Magnetospheres are universal objects which result from the interaction between plasma flows and a magnetized obstacle. Their structures are based on a system of currents which insure the transition between various plasma regimes and through which the electromagnetic energy can be converted in kinetic and thermal energy. In the case of the Earth, the current sheet flowing in the median plan of the magnetotail is the keystone of the system and the place where a major instability occur. The energy cycle of the system is organized around it. Following the global instability, the energy transmitted by the solar wind and accumulated in the magnetotail is dissipated in an explosive way during perturbed periods called magnetospheric substorms. Multi-points missions such as Cluster, Double Star or THEMIS, permit a multi-scale analysis of the magnetotail. A case study of three successive substorms show that, the changes observed in the tail (dipolarisations propagating towards the tail and injections of particles towards the Earth) correspond to a current disruption propagating in anti-solar direction. Comparing data with kinetics simulation (PIC, Particle In Cell) suggests that the signatures observed can also be interpreted as magnetic reconnexion initiated in the near tail and propagating in antisolar direction. Dipolarisations are characteristic signatures of substorms. The analysis of eight successive dipolarisations shows that they are not always associated with substorms but with very localized disturbances in the auroral region. A statistical analysis of this type of dipolarisations allows characterizing the changes they make in the plasma sheet (thickening, current density reduction, increase of the energy density)

Modulation of the Substorm Current Wedge by Bursty Bulk Flows: September 8, 2002 - Revisited

The ultimate formation mechanism of the substorm current wedge (SCW) remains to-date unclear. In this study, we investigate its relationship to plasma flows at substorm onset and throughout the following expansion phase. We revisit the case of September 8, 2002, which has been defined as "the best textbook example for a localized substorm onset observation" because of its excellent coverage by both spacecraft in the magnetotail and ground-based observatories is revisited. We found that a dense sequence of arrival of nightside flux transfer events (which can be understood as the lobe magnetic signature due to a bursty bulk flow travelling earthward in the central plasmasheet) in the near-Earth tail leads to a modulation (and further step-like built-up) of the SCW intensity during the substorm expansion phase. In addition, we found that small SCWs are created also during the growth phase of the event in association with another less intense sequence of NFTEs. The differences between the sequence of NFTEs in the growth and expansion phase are discussed. We conclude that the envelope of the magnetic disturbances which we typically refer to as an intense magnetic substorm is the result of a group or sequence of more intense and more frequent NFTEs

Signatures of wedgelets over Fennoscandia during the St Patrick s Day Storm 2015

During the long main phase of the St Patrick's Day storm on March 17, 2015, we found three separate enhancements of the westward electrojet. These enhancements are observed in the ionospheric equivalent currents computed using geomagnetic data over Fennoscandia. Using data from the IMAGE magnetometer network, we identified localised field-aligned current (FAC) systems superimposed on the pre-existing ionospheric current system. We suggest that these localised current systems are wedgelets and that they can potentially contribute to a larger-scale structure of a substorm current wedge (SCW). Each wedgelet is associated with a negative BX spike. Each spike is recorded at a higher latitude than the former one and all three are very localised over Fennoscandia. The first spike occurred at 17:34 UT and was observed at Lycksele, R rvik and Nurmij rvi, the second spike was recorded at 17:41 UT and located at Lycksele and R rvik, whereas the last spike occurred at 17:47 UT and was observed at Kevo and Abisko. Simultaneous optical auroral data and electron injections at the geosynchronous orbit indicate that one or more substorms took place in the polar ionosphere at the time of the wedgelets. This study demonstrates the occurrence of small and short-lived structures such as wedgelets at different locations over a short time scale, 15 min in this case

Signatures of wedgelets over Fennoscandia during the St Patrick’s Day Storm 2015

During the long main phase of the St Patrick’s Day storm on March 17, 2015, we found three separate enhancements of the westward electrojet. These enhancements are observed in the ionospheric equivalent currents computed using geomagnetic data over Fennoscandia. Using data from the IMAGE magnetometer network, we identified localised field-aligned current (FAC) systems superimposed on the pre-existing ionospheric current system. We suggest that these localised current systems are wedgelets and that they can potentially contribute to a larger-scale structure of a substorm current wedge (SCW). Each wedgelet is associated with a negative BX spike. Each spike is recorded at a higher latitude than the former one and all three are very localised over Fennoscandia. The first spike occurred at 17:34 UT and was observed at Lycksele, Rørvik and Nurmijärvi, the second spike was recorded at 17:41 UT and located at Lycksele and Rørvik, whereas the last spike occurred at 17:47 UT and was observed at Kevo and Abisko. Simultaneous optical auroral data and electron injections at the geosynchronous orbit indicate that one or more substorms took place in the polar ionosphere at the time of the wedgelets. This study demonstrates the occurrence of small and short-lived structures such as wedgelets at different locations over a short time scale, 15 min in this case