Very-Low-Frequency and Particle Rocket Experiment at Kerguelen Islands: 1. Very-low-frequency measurements

Abstract

Three rockets were launched along the magnetic field lines, up to an altitude of ∼400 km, to study the wave-particle interactions that take place during natural VLF emissions. The group delay time was measured for the structured elements of dawn chorus, using a correlation technique. This time is shown to be in good agreement in the ascending part of the flight with the theoretical one deduced from the Appleton-Hartree formula with the use of simultaneous ionospheric soundings giving the electron density above the station. Changes at the end of the flight show discontinuities in the electron density profiles and set an upper limit to the area on which VLF signals detected on the ground are coherent. From the intensity measurements, comparison is made with the theory of attenuation and reflection of VLF waves (f<2 kHz) when passing through the ionosphere. A general agreement was found between the theoretical and experimental results, as far as the shape of the amplitude variation as a function of height is considered. Large discrepancies in the absolute values are observed which are attributed to oblique incidence of the waves. Polarization measurements permit us to ascribe a circular polarization to these waves even at VLF, except in the lower ionosphere (h<120 km) where it shows a tendency to become a elliptical.