Défense de thèse

Soutenance de thèse de Jessy Matar


Infos

Dates
13 avril 2021
Lieu
Visioconférence
Horaires
13h00

Le mardi 13 avril 2021, Jessy MATAR présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Sciences spatiales) sous la direction de Benoît HUBERT.

Cette épreuve consistera en la défense publique d’une dissertation intitulée :

« Effect of magnetic reconnection over the space environment of the Earth ».

Abstract

The majestic spectacle of polar aurora has fascinated the humankind since the dawn of time. It was understood, already several centuries ago, that the auroral and concurrent magnetic activities were related to solar activity, which was later understood as the source of the resulting solar wind, which carries a frozen-in magnetic field and interacts with the magnetic field of the Earth. Major progresses were realized towards the understanding of the coupled solar wind - magnetosphere - ionosphere system after the beginning of the space era. Since 1958, around thirty satellites were sent to orbit the Earth and to observe the resulting auroral and geomagnetic disturbances and the different processes governing the solar wind - magnetosphere coupling. Space based observations also led to the discovery of a population of electrically charged particles trapped in the geomagnetic field, forming the plasmasphere. Under suitable conditions, interaction between the solar wind and the Earth’s geomagnetic environment causes a reconfiguration of the magnetic field that connects the interplanetary magnetic field to the geomagnetic field producing so-called open magnetic field lines. The solar wind flow then drags these open field lines, giving the magnetotail its elongated shape. Previously opened field lines eventually reconnect in the central region of the magnetotail, releasing energy and reconfiguring the field back again to a closed configuration. This cycle of magnetic field line opening and closure is now understood to be at the heart of the dynamics of the Earth’s space environment and its auroral and magnetic activity, producing auroral substorms and global geomagnetic storms. We investigate several storm and substorm cases in order to understand how the various regions of the magnetosphere and upper atmosphere interact with each other under different solar wind conditions.

This thesis consists of two distinct studies: the first study examines in situ measurements of magnetic reconnection and their relation with remote sensing auroral observations, whilst the second examines the plasmaspheric and auroral responses during storm time. The aim of the first part of the thesis is to study the coupling between the solar wind and the magnetosphere and identify how its consequences materialize in different regions of the system, from the aurora to the space environment of the Earth, with a particular attention being given to the effects of magnetic reconnection. A combination of data from different origins, including satellites, magnetometers and radars, was used to achieve this aim. We combine the NASA-IMAGE satellite observations of the proton aurora with ground-based measurements of the ionospheric convection from SuperDARN to analyze the cycle of magnetic flux opening and closure in the Earth’s magnetosphere. The ESA-Cluster mission provided in situ measurements of the plasma properties at reconnection sites which were concurrent with auroral observations from IMAGE and SuperDARN, and therefore allowed us to investigate the ionospheric consequences of reconnection occurring in the magnetotail on the nightside and at the magnetopause on the dayside. We demonstrated that the reconnection rate, expressed as an electric voltage, determined from ionospheric observation, reliably reflects the physical process occurring in the distant space both on the dayside and on the nightside, a result of fundamental importance.
The impact of intense solar wind coupling with the magnetosphere makes up the second part of my project, devoted to the contrasted storm time response of the plasmasphere density and boundary on one hand, and the ionospheric auroral dynamics on the other hand. The satellite observations of the aurora from IMAGE-FUV and of the plasmasphere from IMAGE-EUV were used in addition to SuperDARN, OMNI, GOES data, and ground-based magnetometer-derived activity indices. We reach several conclusions highlighting the interplay of the different elements of the system: the plasmasphere responds directly to changes in the solar wind properties, the ionospheric convection boundary HMB is magnetically related to the plasmapause reflecting the topology of the system, the plasmasphere density correlates with the open magnetic flux but does not with the dayside and nightside reconnection rates owing to the fact that reconnection varies over shorter time scales. The analysis showed that some parameters can correlate better during the most active phase of the storm and therefore, better describe the direct response of the magnetosphere than the recovery phase.

 
 

Le Jury sera composé de :

M. D. GRODENT (Président), Mme et MM. B. BONFOND, J. DE KEYSER, J.C. GERARD, B. HUBERT, E. JEHIN, S.E. MILAN, V. PIERRARD, Z. YAO.

iconeInfoConformément aux consignes institutionnelles relatives à l’épidémie du coronavirus, cette défense de thèse se fera en visioconférence via Lifesize. Le lien public pour accéder à la défense est :

Adresse : https://call.lifesizecloud.com/8590678
Access code : 86247#

Photo : Somewhere above a road in Scandinavia, the spectacle of polar aurora unfolds in green and red lights in the sky. The green luminescence is emitted by metastable oxygen atoms, while the red glow can be due to transitions of N2+ ions and oxygen atoms.
Photo by Jaanus Jagomägi on unsplash.com

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