Direct Observation of Radiation-Belt Electron Acceleration from Electron-Volt Energies to Megavolts by Nonlinear Whistlers

Abstract : The mechanisms for accelerating electrons from thermal to relativistic energies in the terrestrial magnetosphere, on the sun, and in many astrophysical environments have never been verified. We present the first direct observation of two processes that, in a chain, cause this acceleration in Earth's outer radiation belt. The two processes are parallel acceleration from electron-volt to kilovolt energies by parallel electric fields in time-domain structures (TDS), after which the parallel electron velocity becomes sufficiently large for Doppler-shifted upper band whistler frequencies to be in resonance with the electron gyration frequency, even though the electron energies are kilovolts and not hundreds of kilovolts. The electrons are then accelerated by the whistler perpendicular electric field to relativistic energies in several resonant interactions. TDS are packets of electric field spikes, each spike having duration of a few hundred microseconds and containing a local parallel electric field. The TDS of interest resulted from nonlinearity of the parallel electric field component in oblique whistlers and consisted of ∼0.1 msec pulses superposed on the whistler waveform with each such spike containing a net parallel potential the order of 50 V. Local magnetic field compression from remote activity provided the free energy to drive the two processes. The expected temporal correlations between the compressed magnetic field, the nonlinear whistlers with their parallel electric field spikes, the electron flux and the electron pitch angle distributions were all observed. Rapid acceleration of electrons up to relativistic energies occurs in different plasma configuration on all scales from the laboratory to astrophysics. The Van Allen radiation belts around Earth contain such relativistic electrons that are trapped in Earth's magnetic field. Because of intrinsic interest in the acceleration mechanism, because these electrons may be prototypical of relativistic electron acceleration in other environments, and because they present a danger to space travelers and spacecraft, it is important to understand their origin and acceleration. Two possible sources of these electrons that have been discussed are injections into the local environment of electrons that were energized by moving earthward from the tail into a stronger magnetic field while conserving their first two adiabatic invariants [1], and local acceleration in the region of the satellite measurements. While both mechanisms occur, the local acceleration mechanism has been shown to be more important, at least for major, rapid, relativistic flux increases [2–4]. Simulations of rela-tivistic electron acceleration via the whistler mode resonance have produced relativistic electrons from seed populations of hundreds of keV electrons [5,6]. This work has left open the question of the source of such seed populations. Meanwhile, observations have been made in Earth's radiation belts of parallel (to the local magnetic field) electric fields in the form of packets of spikes, each spike having a duration the order of 100 msec, and each packet containing hundreds of such spikes [7]. These spikes, dubbed time-domain structures, have at least five different forms that satisfy the above description and they have been suggested as the mechanism for producing the ∼100 keV electrons that are the seed population for whistler wave acceleration to highly relativistic energies [7]. This suggestion has not been verified by detailed comparison of particles and fields before the studies described in this Letter that show, for the first time, both that low energy electrons can be accelerated up to keV energies by the parallel electric fields in time-domain structures and that such keV electrons can be further accelerated to relativistic energies via the whistler mode resonance even though their initial energies are significantly less than ∼100 keV. The data in this Letter were collected on May 2, 2013 on Van Allen probe B (VAP-B) by the electric field experiment [8], the magnetic field experiment [9], and the Energetic particle, Composition, and Thermal plasma (RBSP-ECT) Suite experiments [10–12]. The spacecraft was at a magnetic latitude of 2°, a magnetic local time of midnight, and a geocentric radial distance of 5.8 Earth radii during these measurements. Figure 1 illustrates three components of the electric and magnetic fields in the background-magnetic-field-aligned coordinate system during a 20 msec interval in which two packets of nonlinear whistlers were observed. Panels 1(a) and 1(b) give the two perpendicular (to the background magnetic
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F.S. Mozer, O Agapitov, V Krasnoselskikh, S Lejosne, G.D. Reeves, et al.. Direct Observation of Radiation-Belt Electron Acceleration from Electron-Volt Energies to Megavolts by Nonlinear Whistlers. Physical Review Letters, American Physical Society, 2014, 113 (3), pp.035001. ⟨10.1103/PhysRevLett.113.035001⟩. ⟨insu-01256166⟩



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