Abstract
<p>High Frequency radio waves reflected by the ionosphere can provide a relevant amount of information within the composite received signal. The ionosphere is indeed a frequency dispersive, bi-refractive, absorbing medium, in which multipath propagation occurs due to disturbance on a varied time-spatial scale. On the time-spatial level of Small Scale Disturbances (SSD) the ionosphere dynamics, detectable by Vertical Ionospheric Sounding (VIS), is mainly dependent on wrinkled layers acting as multi-reflectors. The present paper discusses different aspects of the effects of multipath fading suffered by the wave along the propagation path and potentially associated with SSD. To achieve these objectives, a VIS campaign at a fixed frequency of 3.0 MHz was conducted at the ionospheric observatory in Rome (Latitude 41.8 N; Longitude 12.5 E), by collecting a series of measurements of the power variations in received echo signals recorded between two consecutive ionograms whose sounding repetition rate was set to 15 min. The obtained results show that: 1) the fading suffered by the wave follows either a Rayleigh trend or a Nakagami-Rice trend, or a mix of them, the mixed case being the most frequent (about 65 % of the analysed cases); 2) the predominant periodicities characterizing the power variation are less than 25 s; such values are compatible with the small scale ionospheric disturbances; 3) for all the 24 hours of the day the ionospheric reflector is pretty stable and for time intervals of 10-30 s the periods of stability occur with a percentage of occurrence ranging between 55% and 95 %; for time intervals of 190- 210 s the periods of stability occur instead with a percentage of occurrence ranging between 5% and 54 %.</p>
Highlights
Vertical Ionospheric Sounding (VIS) is a consolidated technique that was first applied nearly a century ago to unveil the existence of the ionosphere, i.e. the density and altitude of the ionospheric layers
The VIS technique uses radio waves propagated in the High Frequency (HF) range and reflected by the ionospheric layers acting like mirrors, with the corresponding echo delay related to the reflection virtual height of the reflecting layers
This paper addresses the ones related to Small Scale Disturbances (SSD), which are characterized by random processes, by exploiting PVH measurements
Summary
Vertical Ionospheric Sounding (VIS) is a consolidated technique that was first applied nearly a century ago to unveil the existence of the ionosphere, i.e. the density and altitude of the ionospheric layers. The AIS-INGV (Advanced Ionospheric Sounder - Istituto Nazionale di Geofisica e Vulcanologia) ionosonde, with minor modifications and calibrations, was adapted to measure only Power in dBm and Virtual Height in km (PVH), at a fixed frequency, assuming that the critical frequency and reflection altitude of the ionospheric layers to be investigated were known from routine VIS measurements [Bianchi et al 2013a]. When radio waves are reflected from a concave iso-density surface, the waves will undergo a focusing gain; when the surface is convex there is a loss due to the defocusing effect The latter phenomenon, together with multipath fading, defines the behavior of the ionospheric irregularities considered in this paper, where a series of measurements of radio echo power were performed to study and monitor some dynamic characteristics of the ionospheric propagation path and related reflecting layers potentially associated with SSD. Applying a gating of 10 km, if the height is 100 km, the area of the ionospheric layer investigated is some hundreds of square km
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