Non-diffusive resonant acceleration of electrons in the radiation belts
Résumé
We describe a mechanism of resonant electron acceleration by oblique high-amplitude whistler
waves under conditions typical for the Earth radiation belts. We use statistics of spacecraft
observations of whistlers in the Earth radiation belts to obtain the dependence of the angle h
between the wave-normal and the background magnetic field on magnetic latitude k. According to
this statistics, the angle h already approaches the resonance cone at k 15 and remains close to it
up to k 30–40 on the dayside. The parallel component of the electrostatic field of whistler
waves often increases around k 15 up to one hundred of mV/m. We show that due to this
increase of the electric field, the whistler waves can trap electrons into the potential well via wave
particle resonant interaction corresponding to Landau resonance. Trapped electrons then move with
the wave to higher latitudes where they escape from the resonance. Strong acceleration is favored
by adiabatic invariance along the increasing magnetic field, which continuously transfers the
parallel energy gained to perpendicular energy, allowing resonance to be reached and maintained.
The concomitant increase of the wave phase velocity allows for even stronger relative acceleration
at low energy <50 keV. Each trapping-escape event of electrons of 10 keV to 100 keV results in
an energy gain of up to 100 keV in the inhomogeneous magnetic field of the Earth dipole. For
electrons with initial energy below 100 keV, such rapid acceleration should hasten their drop into
the loss-cone and their precipitation into the atmosphere. We discuss the role of the considered
mechanism in the eventual formation of a trapped distribution of relativistic electrons for initial
energies larger than 100 keV and in microbursts precipitations of lower energy particles.
Domaines
Planète et Univers [physics]Origine | Fichiers éditeurs autorisés sur une archive ouverte |
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