Highlighting the rapid and consistent particle acceleration radiation belts during magnetic storms
An international team, including researchers working at the IRAP 1 and the LPP 2, has just demonstrated the frequently existence of a consistent acceleration, in a few tens of minutes, of the electrons and protons of the radiation belts of the Earth by Ultra Low Frequency Waves 3. These LF waves are associated with the sudden compression of the geomagnetic field during magnetic storms at the Earth orbit. Thanks to the French mission Demeter 4 and to the particle detector onboard, which has an excellent energy resolution, complex energy structures have indeed been for the first time demonstrated and explained. This work is published this month in the Journal of Geophysical Research.
Figure 1: Geographic distribution of an electron flow of 200 keV at an altitude of 650 km (color code), with the magnetic anomaly in the South Atlantic (in red). Here the low geomagnetic field causes the lowering of the mirror point of the particles to the altitude of the Demeter satellite. The (black) regions located north and south of the anomaly present energy bands specific to the resonant acceleration of the particles with the LF waves. It is the asymmetry of the geomagnetic field that allows their detection at low altitude. At higher altitudes, these zones encircle the Earth.
It is the resonance between the drift motion of the particles around the Earth and the electric field of LF waves that causes these structures. These waves could be detected directly on the ground and characterized by comparing the data simultaneously obtained by magnetometers in the United States and Europe, in particular at the magnetic observatory of Chambon-la-Forêt 5. These are quasi-monochromatic pulses of large wavelengths that circulate around the Earth at speeds that depend upon their azimuthal 6 wavelength. Each mode resonates with particles whose drift velocity around the earth is equal to the speed of its phase. It is a consistent phenomenon that carries the particles towards the Earth and accelerates them efficiently. This phenomenon is observed in the inner belt, at distances between 1 to 1.7 earth radii, where the waves have a period of about 20 minutes, and at greater distances (2.5-3.5 earth radii), where the waves in question have a characteristic period of about 1 minute. These accelerations, viewed from Earth in the vicinity of the South Atlantic magnetic anomaly, are common processes that accompany magnetic storms, even of low amplitude.
This resonance had not been demonstrated previously because usual particle detectors were designed to study the variations in the measured flux in large windows or energy bands where the structures were buried. Based on the observations of Demeter (Figure 2), numerical simulations were conducted which couple the movement of particles of high energy with an analytical model of the waves taking into account their main characteristics. These simulations reproduce well the observations (Figure 3).
|Figure 2 : Energy bands detected by the Demeter satellite in the outer part of the inner radiation belt of the Earth during a magnetic storm of low intensity. They reflect a consistent particle acceleration.||Figure 3 : Numerical simulation of the interaction of ULF waves of a period of 67 seconds with the protons of the radiation belts. The ordinate axis shows the energy of the protons. The horizontal axis represents the radial distance in ground radiation, in the magnetic equatorial plane.|
Currently, after the shooting in August 2012 of the two U.S. satellites RBSP 7 designed for the study of the radiation belts of the Earth, the international community has the right tools to determine how the waves are generated, either by direct coupling between the solar wind and the Earth magnetosphere, either by internal instability and how they affect the overall dynamics of the radiation belts of the Earth. Similar processes are expected in the magnetospheres of the giant planets, Jupiter and Saturn.
- Institut de Recherche en Astrophysique et Planétologie (IRAP-OMP/CNRS-Université Paul Sabatier Toulouse III)
- Laboratoire de Physique des Plasmas (LPP-CNRS/École Polytechnique/Université Paris Sud/UPMC)
- Les ondes UBF, pour Ultra Basses Fréquences, sont ici des ondes électromagnétiques avec une période entre 30 et 1200 secondes, une longueur d’onde comprise entre 1000 km et 40000 km.
- Plus d’informations sur le satellite Demeter : http://demeter.cnrs-orleans.fr/dmt/index.html
- Observatoire magnétique national de Chambon-la-Forêt appartenant à l’Institut de Physique du Globe de Paris (IPGP-CNRS/UPMC/Université Paris Diderot): http://www.ipgp.fr/pages/03050204.php
- Les ondes sont quasi-monochromatique (fréquence fixe). Dans le référentiel de chaque onde les particules qui se déplacent autour de la Terre avec la vitesse de l'onde voient un champ électrique constant qui les accélère continûment. Les longueurs d'onde des diverses composantes des ondes sont des sous-multiples de la longueur d'onde maximale et il en est de même pour les vitesses de phase des ondes.
- Plus d’informations sur les satellites Radiation Belt Storm Probes (RSBP) de la NASA : http://vanallenprobes.jhuapl.edu/mission/overview.php
Inner radiation belt particle acceleration and energy structuring by drift resonance with ULF waves during geomagnetic storms, J.-A. Sauvaud, M. Walt, D. Delcourt, C. Benoist, E. Penou, Y. Chen, C. T. Russell, Journal of Geophysical Research, 2013
Jean-André Sauvaud, IRAP-OMP (CNRS/Université Paul Sabatier-Toulouse III)
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