Mid-latitude sporadic-E clouds (commonly abbreviated as ‘Es’) are a transient feature consisting of thin layers of dense, but patchy, ionization which occur in the E region of the ionosphere. The process of formation is different from that of the rest of the ionosphere and it can produce much higher electron densities, sometimes permitting oblique reflection of radio waves up to 150 MHz.
The mechanism for the oblique reflection of VHF waves from Es layers has not been well described, with candidates including specular reflection, scattering, and magneto-ionic double refraction. The polarization and fading characteristics of waves reflected from Es layers are proposed as a marker for the presence or absence of magneto-ionic effects.
An experimental system has been developed for rapid and accurate polarization and fading measurements at 50 MHz. The overall sensitivity of the system has been optimized by reducing environmental electromagnetic noise, giving the ability to observe weak, short-lived Es propagation events. The effect of the ground reflection on observed polarization has been analyzed and the induced amplitude and phase biases compensated for.
A measurement campaign in the summer of 2018 gathered a large quantity of data, using amateur 50 MHz beacons, at distances between 1,000 km and 1,650 km, as signal sources. The results provide compelling evidence that Es-layer propagation at 50 MHz exhibits the characteristics of magneto-ionic double refraction, but the thin, intense and variable nature of the reflecting region means that the reflected signals can have quite extreme characteristics. Some of the results are surprising, and are yet to be explained convincingly.
In this presentation, an overview of the experimental technique will be given, and the results described. Some of this information has already been published, but much of it is new.