On the origin of lower‐ and upper‐frequency cutoffs on wedge‐like spectrograms observed by DEMETER in the midlatitude ionosphere

Abstract

We report observations of unusual lower and upper cutoff frequencies on VLF spectrograms recorded by the DEMETER satellite (orbiting at ∼700 km) during thunderstorm activity. The upper cutoff frequencies in the spectrograms under discussion vary rapidly, approximately in proportion to L−3, where L is the McIlwain parameter on the satellite orbit. On the contrary, the lower cutoff frequencies in the spectrograms are almost constant, so that the cutoffs cross at larger L. Between these cutoffs, which thus form a wedge, intense whistlers are observed, whereas only 0+ whistlers, and probably, ducted whistlers are present outside the cutoffs. Using a model of lower hybrid resonance (LHR) frequency and local measurements of plasma density and ion composition in the ionosphere, we show that during the events under consideration, the satellite is located at altitudes where the height‐dependent variation of the LHR frequency presents a trough. We explain the observed spectrograms on the basis of the features of whistler‐mode wave propagation in the inner magnetosphere. Then the upper cutoff frequency is determined by the limiting trajectories that confine the waves of a given frequency propagating obliquely from a source located in the opposite hemisphere. Since the waves close to the limiting ones propagate in the quasiresonance regime, the intensity and time delay at the upper cutoff should increase, which is the case in observations. As for the lower frequency cutoff, it is determined by the LHR maximum, since quasiresonant waves with lower frequencies originating in the opposite hemisphere do not reach the satellite due to the LHR reflection above it.