Deterministic chaos in modulated multi-cell drifts of localized lower hybrid oscillations excited by high frequency waves in the ionosphere

Abstract. The prominent broad upshifted maximum (BUM) feature in electromagnetic emissions stimulated by powerful high frequency radio waves in the ionosphere exhibits an exponential spectrum for pump frequencies near an harmonic of the ionospheric electron gyro frequency. Exponential power spectra is a characteristic of deterministic chaos. In the present treatment, a two-fluid model is derived of lower hybrid (LH) oscillations driven by parametric interaction of the electromagnetic pump field, electron Bernstein mode and upper hybrid mode as previously proposed to interpret the BUM. In two dimensional geometry across the geomagnetic field, LH oscillations localized in cylindrical density depletions are associated with multi-cell plasma drift paterns. The numerical simulations show that topological modulations of the drift can give rise to approximately Lorentzian shaped pulses in the LH time signal. For parameter values typical of the ionospheric experiments, the exponential power spectrum of the Lorentzian pulses has a slope that is consistent with the slope of the BUM spectrum. The BUM spectral structure is therefore attributed to deterministic chaos in LH dynamics.