Abstract. In this paper we investigate the nonlinear development of the electron acoustic instability that can lead to the transfer of wave energy to frequencies just above the electron plasma frequency (0)pe) and to waves with approximately twice the electron plasma frequency (20)pe). Using plasma conditions in the upstream electron foreshock region based on data from the AMPTE-UKS spacecraft, an electron beam is considered in plasma containing a background of hot and cold electrons. This leads to the linear excitation of large-amplitude electron acoustic waves at frequencies between about 0.8 and 1.00)pe. A modified decay instability then excites waves in the spectrum just above 0)pe. This is followed by a second nonlinear coalescence process that causes the excitation of waves at frequencies just below 2C0pe. The linear and nonlinear properties of the electron acoustic instability are examined for observed conditions using analytical theory, particle-in-cell simulations, and Vlasov simulations. These results have application to observations made inside the electron foreshock region, as well as the polar cap and auroral zone, where plasma oscillations and waves at 2COpe are observed.
[1] The observations in the OEDIPUS-C experiment of bistatic propagation along the lower oblique resonance cone between a separated transmitter and receiver in the ionosphere are explained through detailed calculations of the radiated field of the Vdipole antenna used in the experiment and by a novel theory of the receiving antenna under resonance conditions. Unexpectedly high values of 25 kHz signal observed at the resonance and its structure agree well with calculations of the transmission between the exciter and receiver, when antenna layout and dispersive properties of the plasma at resonance are taken into account.
Symmetric sidebands are observed in the ionosphere by the AUREOL 3 satellite when it passes at a height of 1200 km above the VLF transmitter at the Komsomolsk‐on‐Amur Alpha station (50°5 N, 135° E, frequency 11.90 and 12.65 kHz). The sidebands are about 500 Hz off the carrier frequency of Alpha pulses. They are approximately 20 dB lower than the transmitter signal, and they appear only when ELF natural emission above the local proton gyrofrequency is observed. The data are presented and analyzed. The nonlinear coupling of the VLF transmitter signal to natural ELF emission is invoked to explain the symmetric sidebands. It is shown that the nonlinear current excited by the beats of VLF and ELF waves is strong enough to explain the sideband amplitude.
We show theoretically that the electron density and temperature of a plasma could be deduced from the measurements of the transfer impedance between two small dipole antennae, each much shorter than a Debye length, separated by a distance of ten or more Debye lengths. In contrast to the quadripole probe, this ‘double-dipole probe’ relies on not producing perturbations in the plasma, rather than on minimizing their effects. The plasma is assumed to be warm and isotropic, and the motion of the ions is neglected. First, it is shown that, in a Maxwellian plasma, the frequency response of a double-dipole probe is easier to interpret than that of a quadripole probe with the customary square layout. Then, in a second step, the transfer impedance of the former probe is calculated in a Cauchy plasma, and the results are compared with those previously obtained in a Maxwellian plasma. By so doing, we show that, for large distances between the dipoles, the real part of the transfer impedance is sensitive to the form of the tail of the distribution function.
We study an antenna system consisting of a current loop exciting resonance cones and of an electric dipole generating ion sound waves. The transformation of lower oblique resonance waves on quasineutral density perturbations in the near field of an antenna gives rise to whistler waves on combination frequencies. Their amplitude may exceed several times the amplitude of linear whistler waves excited by the loop itself. The system may be regarded as a parametric antenna for enhanced excitation of whistlers in the ionosphere.
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