Large‐amplitude (10 to 100 μV m−1 Hz−½) natural radio emissions in a wide frequency range (100 kHz up to 2 MHz) are frequently observed on board the AUREOL/ARCAD 3 satellite at high latitude and at altitudes between 400 and 2000 km. The simultaneous measurement of the local cold plasma density allows the identification of cutoff and resonance frequencies. Three different kinds of wave are observed: (1) electrostatic emissions near the local value of the plasma frequency (fp), (2) electromagnetic whistler mode emissions, sometimes associated with type (1) emissions, and (3) electromagnetic Z mode emissions, also associated with type 1 emissions, but occurring more rarely than the whistler mode emissions and then only when fp is greater than the electron cyclotron frequency (fce). These emissions are always associated with high levels of ELF electrostatic turbulence and a high flux of low‐energy precipitating electrons, extending in energy down to the lower limit of the detectors (∼100 eV). The statistical distribution of the emissions in geomagnetic coordinates shows an occurrence greater than 80% in the polar cusp region and between 25% and 60% in the nightside auroral zone. A generation mechanism for such emissions is proposed, based on the calculation of the growth rate of the kinetic Cherenkov instability, associated with a beamlike suprathermal tail in the parallel distribution of the bulk electron population. In particular, suprathermal, downward electron beams of about 100–200 eV energy, with a thermal spread of the same order, are found to be responsible for the generation of whistler mode and Z mode HF emissions in a source region extending down to 1000 km of altitude. It is suggested that such intense radio emissions should be considered as one of the energy dissipation processes resulting from magnetosphere‐ionosphere coupling, through interdependent electrodynamic mechanisms such as current systems, parallel anomalous resistivity, plasma turbulence, energy diffusion, and heating.
Mutual impedance experiments have been developed to constrain the plasma bulk properties, such as density and temperature, of ionospheric and later space plasmas, through the electric coupling between an emitter and a receiver electric antennas. So far, the analytical modeling of such instruments has enabled to treat ionospheric plasmas, where charged particles are usually well characterized by Maxwellian electron distributions. With the growth of planetary exploration, mutual impedance experiments are or will be used to constrain space plasma bulk properties. Space plasmas are usually out of local thermodynamic equilibrium; therefore, new methods to calibrate and analyze mutual impedance experiments are now required in such non‐Maxwellian plasmas. To this purpose, this work aims at modeling the electric potential generated in a two‐electron temperature plasma by a pulsating point charge. A numerical method is developed for the computation of the electrostatic potential in a sum of Maxwellian plasmas. After validating the method, the results are used to build synthetic mutual impedance spectra and quantify the effect of a warm electron population on mutual impedance experiments, in order to illustrate how the method could be applied for recent and future planetary space missions, such as Rosetta, BepiColombo, and JUICE. In particular, we show how it enables to separate the densities and temperatures of two different electron populations using in situ measurements from the RPC‐MIP mutual impedance experiment on board Rosetta.
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