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 a harmonic of the ionospheric electron gyro frequency. Exponential power spectra are a characteristic of deterministic chaos. In the present treatment, a two-fluid model is derived for lower-hybrid (LH) oscillations driven by parametric interaction of the electromagnetic pump field, the electron Bernstein mode, and the 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 patterns. 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.