Whistler mode noise in Jupiter's inner magnetosphere

Abstract

The distinctive ‘hat shape’ of equatorial pitch angle distributions constructed from Pioneer 10 and 11 Jupiter observations of energetic electrons Ee > 21 MeV and Ee > 31 MeV at L = 3 is examined from the point of view of pitch angle diffusion by resonant interaction with a band‐limited spectrum of whistler mode noise. In this picture, the pitch angle profiles are consistent with whistler mode noise limited to frequencies below an upper cutoff frequency 2.8 ≲ fc ≲ 4.6 kHz. Equatorial linear growth rates of parallel propagating whistlers are evaluated in a fully relativistic manner for the inner region 3 < L < 10 using previously published models for the spatial distribution of thermal plasma and the energetic electron distributions. The maximum frequency for which wave growth is positive at L = 3 is roughly consistent with that implied by the 21‐ and 31‐MeV equatorial pitch angle profiles. Maximum growth rates computed from the respective energetic electron models span the range 0.1 ≲ γmax ≲ 10 s−1 throughout the inner region. The spectral extent of the whistler mode noise, defined by those frequencies for which γ > 0.5γmax, is approximately 2–10 kHz at L = 3 and falls smoothly to 0.2–2 kHz at L = 10. Raytracing of nonducted whistlers of characteristic frequencies 1–4 kHz generated at the magnetic equator and propagated through a finite temperature (100–400 eV) hydrogen plasma centrifugally confined near the magnetic equatorial plane shows that wave phase speeds decrease to near or below electron thermal speeds before high‐latitude reflection can occur. Wave growth in this case is limited to a disclike region centered about the magnetic equator. The amplitude growth of the most unstable whistlers propagating across the growth region parallel to the magnetic field is ∼exp (3–4). By equating energetic electron radial diffusion injection times to the pitch angle diffusion loss times of electrons in cyclotron resonance with waves of frequencies of maximum unstable growth, the power density of whistler mode noise is estimated to be 10−19±1 G² Hz−1. The balance equation for radial diffusive injection and pitch angle diffusive losses occurring in the presence of whistler mode noise of characteristic growths exp (3–4) yields upper limit estimates for the magnitude of fluctuations about the stably trapped limiting intensities in the region 3 < L < 10. This upper limit is approximately at the 10% level, in reasonable agreement with the smooth radial flux profiles observed during the Pioneer‐Jupiter encounters.