Abstract :
[en] 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 analyse 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.
