Plasma sounding at the upper hybrid frequency

A sounder measures the density of plasmas in various parts of the solar sys tem. The sounder emits wave pulses into the ambient plasma and listens to t he response. Intensity peaks in the wave response are typically related to two mechanisms. One is provided by waves that are reflected off plasma inho mogeneities and propagate back to the emitting antenna, where they are then detected. The second is provided by waves propagating with the same group velocity as that of the receiving antenna. In the second case the waves sta y close to the antenna and thus yield a long-lasting response. Response pea ks to sounding at the upper hybrid (UH) frequency have, in most cases, been related to reflected waves. In this work we examine if accompanying waves can give rise to the UH response peak. We examine quantitatively how the pl asma response to sounding at the UH frequency depends on the plasma density , on the electron temperature, and on the emission amplitude. For the first two parameters this is done by solving the linear dispersion relation. The well-known property of the UH waves to change from having a zero group vel ocity to propagating waves, depending on how the electron density compares to the electron cyclotron frequency, is applied to Alouette sounder data. I t is discussed how the change in the group velocity may affect the spectral profile of the UH resonance. We present results from numerical particle in cell (PIC) simulations which show that in the case of nonpropagating UH wa ves, energy can be coupled into the plasma even though the vanishing group velocity of the UH waves should not allow this. The PIC simulations and sou nder data from the Alouette mission show that in the case of propagating UH waves the response duration to sounding may be used to determine the elect ron temperature. Emission amplitudes that are typical for plasma sounders a re also shown to suppress the generation of certain electron cyclotron harm onic waves.