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

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 $\sim 0.1\mathrm{msec}$ 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.

Figure 1

Twenty milliseconds of whistler electric and magnetic field data showing nonlinear spikes in the parallel electric field that produce net parallel potentials.

Figure 2

(a)–(c) Components of $\mathrm{\Delta }B$ in magnetic field aligned coordinates. (d)–(e) Spectra of the total electric and magnetic fields, respectively. (f) The number flux of 2.3 MeV electrons, which shows that the acceleration processes being discussed also produced relativistic electrons.

Figure 3

Phase space densities as a function of time for electrons observed on VAP-B and LANL-04A, where VAP-B was about 0.8 Earth radii earthward of LANL-04A.

Figure 4

(a) The whistler electric field amplitude as a function of time. (b) The electric field power at 300 Hz, which is a proxy for the occurrence frequency of the TDS. (c) The electron number flux ($\text{counts}/{\mathrm{cm}}^2\text{ster}\text{sec}\text{keV}$) measured by the particle experiments on the satellite. Because the spacecraft was at a positive potential of 6–8 V during these measurements, the actual electron energies are 6–8 eV greater than given in the figure.

Figure 5

Pitch angle distributions as a function of time for electrons with energies from 0.1 to 10.4 keV. The dynamic range of the color scale in all plots is a factor of 10.

Figure 6

Test particle calculations showing the range of output energies achieved by incident electrons with fixed energies and random phases with respect to the whistler wave with which they are in resonance.