Suppression of nonlinear frequency sweeping of resonant interchange modes in a magnetic dipole with applied radio frequency fields

Interchange instabilities excited by energetic electrons trapped by a magnetic dipole nonlinearly saturate and exhibit complex, coherent spectral characteristics and frequency sweeping. When monochromatic radio frequency (RF) fields are applied in the range of 100–1000 MHz, the saturation behavior of the interchange instability changes dramatically. For applied fields of sufficient intensity and pulse-length, coherent interchange fluctuations are suppressed and frequency-sweeping is eliminated. When RF fields are switched off, coherent frequency-sweeping reappears. Since low-frequency interchange instabilities preserve the electron's first and second adiabatic invariants, these observations can be interpreted as resulting from nonlinear resonant wave-particle interactions described within a particle phase-space, (ψ, ϕ), comprised of the third adiabatic invariant, ψ, and the azimuthal angle, ϕ. The frequency-sweeping suppression is understood to result from electron-cyclotron resonant diffusion of energetic electrons in μ-direction, as reproduced by the numerical simulation. Self-consistent numerical simulation is used to study (1) the nonlinear development of the instability, (2) the radial mode structure of the interchange instability, and (3) the suppression of frequency-sweeping. When the applied RF heating is modeled as an “RF collisionality,” the simulation reproduces frequency-sweeping suppression and suggests an explanation for the observations that is consistent with the nonlinear theory by H. Berk and co-workers.