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Accueil > A propos du LPP > Communication > Actualités archivées > 2016 > A special issue of GRL journal for MMS mission first results

A special issue of GRL journal for MMS mission first results

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Figure 1
Vue d’artiste de la mission MMS ©NASA

MMS consists of four identical satellites placed into an Equatorial orbit flying with a tetrahedral configuration through the Earth magnetosphere [1]. As of September 1st, 2015, the mission entered the scientific analysis phase to explore the key regions of the Earth’s magnetosphere. It is divided into two main orbital phases : phase 1 is dedicated to the study of the dayside magnetopause, the flanks and the near-Earth magnetotail (apogee of 12 Earth radii, RE) ; phase 2 focuses on the study of the geomagnetic tail at average distance from Earth (apogee 25 RE).

Its next-generation instruments and inter-satellite distances aim to study these physical processes at the scale of electron dynamics. The average inter-satellite distance of the tetrahedral configuration varies from 160 km to 10 km complementing the scales studied by Cluster (10 000 km to 100 km). In addition, particle measurements are freed from the limitation linked to satellite rotation (4 s on Cluster) by increasing the number of sensors. Electron distribution functions are thus measured with a temporal resolution of 30 ms and of 150 ms for ions. The strategy for managing telemetry data volume is to configure the instrument into the burst mode when the spacecraft crosses the targeted region, to store data in the onboard memory and to select, from the low time resolution data downlinked to the ground, high time resolution periods to transmit first. The level-2 database (physical quantities) has been open to the public since March 1st, 2016, and the data must be continuously produced within one month of receipt.

The French contribution includes the search-coil magnetometer (SCM) – which were designed, built and calibrated by the LPP [2] within the “FIELDS” consortium [3] – and the supply and calibration of micro-channel plates (MCP) by IRAP for the dual ion spectrometer (DIS) of the Fast Plasma Investigation (FPI) [4].

First results gathered during the commissioning phase

In May 2015 while MMS satellites were flying in a “string of pearls” configuration on the dusk side of the Earth’s magnetotail at about 11 RE with only Fields instruments turned on, a dipolarization front (DF) was captured [5]. DF are associated with a sharp increase of the magnetic field in the direction of the Earth’s dipole and with fast plasma jets. They could be formed due to interchange instability of the magnetic flux tubes or produced by magnetic reconnection occurring farther in the magnetotail. Without any particle instruments turned on, the bulk plasma velocity has been estimated from the electric drift (ExB/B2, E and B being the electric and magnetic field respectively). From this estimate, it has been shown that the plasma moved at 100 km/s in the direction normal to the front. Furthermore, intense whistler mode wave emissions were detected behind the front (Figure 2) and propagating from the magnetic equator toward the auroral regions. Thanks to the specific MMS configuration, it has been possible to show the changes of the orientation of the fronts (normal direction) and of the intensity of the whistler wave emissions at the ion time scales during the front propagation. Such a phenomenon had not be shown by Cluster.

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Figure 2
A gauche : composantes verticale et module du champ magnétique. Spectrogramme dynamique des fluctuations magnétiques mesurées par SCM entre 1 et 128 Hz pour chacun des quatre satellites. A droite : rotation de l’orientation du front lors de la propagation vers la Terre.

The magnetopause at ion and electron scales

On October 16th, 2015, at 13:05:40 UT, MMS crossed the magnetopause (Figure 3) while the satellites were within 10 km of each other. A plasma jet heading south along the magnetopause was detected in accordance with the formation of a magnetic reconnection region located north of MMS.
Before it crossed the magnetopause, a nearly stationary whistler mode wave emission propagating oblique to the north was identified thanks to a polarization analysis using SCM measurements [6]. These whistler mode waves have a parallel electric field component capable of accelerating resonant electrons into the ionosphere. This emission is interrupted just before magnetic field lines open and the Hall electric field (jxB/(qe ne)) appears, leading to ion decoupling with magnetic field lines and electrons. The Hall electric field is obtained by calculating the electric current thanks to the four-point-measurements of the magnetic field as on the Cluster mission. The good agreement between this current and that obtained independently from particle measurements rules out artifacts. Open magnetic field lines are deduced from the disappearance of energetic electrons in the direction antiparallel to the magnetic field.
The first results delivered by the MMS mission demonstrate the need for a very high temporal resolution of wave and particle instruments as well as a fine spatial resolution (small inter-satellite distance <100 km) in order to understand the physical processes that occur at the interface between two plasmas such as in the terrestrial magnetopause, or in a turbulent plasma such as in the magnetosheath. The future phases of the mission in the close and distant tail as well as in the magnetopause will provide the international community with a comprehensive set of measurements at the scale of electron dynamics in all key regions of the Earth’s magnetosphere.

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Figure 3
Composantes du champ magnétique dans le repère LMN lié à la magnétopause. L est dirigé vers le Nord, N vers le Soleil et M complète le trièdre. Les fluctuations électriques et magnétiques sont filtrées entre 32 et 4096 Hz. Densités spectrales de puissance des fluctuations électriques et magnétiques. Les courbes blanches indiquent la 0,1fce, 0,5fce et fce fréquence de giration des électrons. Les panneaux suivants présentent l’angle de propagation et l’ellipticité fournis par l’analyse de polarisation. La composante du vecteur de Poynting, les températures parallèle et perpendiculaire des électrons, le paramètre alpha -1 avec alpha = Tperp,e/Tpara,e, le beta parallèle des électrons, les courants parallèle, perpendiculaire et total obtenus à partir des mesures particules, la DAA des électrons pour trois gammes d’énergie.

The full set of first MMS results can be found in the Geophysical Research Letters (GRL) social issue.

References

[1] Burch, J. L., et al. (2015), Magnetospheric Multi-scale Overview and Science Objectives, Space Sci. Rev.
[2] Le Contel, O., et al. (2014), The Search-Coil Magnetometer for MMS, Space Sci. Rev.
[3] Torbert, R. B., et al. (2014), The FIELDS Instrument Suite on MMS : Scientific Objectives, Measurements, and Data Products, Space Sci. Rev.
[4] Pollock, C., et al. (2016), Fast Plasma Investigation for Magnetospheric Multiscale, Space Sci. Rev.
[5] Breuillard, H., et al. (2016), Multispacecraft analysis of dipolarization fronts and associated whistler wave emissions using MMS data, Geophys. Res. Lett.
[6] Le Contel, O., et al. (2016), Whistler mode waves and Hall fields detected by MMS during a dayside magnetopause crossing, Geophys. Res. Lett.

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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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