June 2010 Boulder CO. Introduction
The theory of multiple scattering of MF/HF radio waves by intermediate-scale (0.1-2 km) ionospheric irregularities predicts a very distinctive ground-level spatial distribution of the integral intensity of a signal reflected from the ionosphere with a significant reduction in the vicinity of a ground-based transmitter and an increase at greater distances (see image of the “Cowboy Hat” effect below). Details of the theory can be found in [Zabotin et al., Waves in Random Media, v.8, p.421, 1998]. While there are experimental confirmations of the “anomalous attenuation” effect near the transmitter location, no attempt had been made to track the intensity features at the larger distances. An experiment of this kind, critical for confirmation of the theory, is described here. It has been conducted with Boulder VIPIR installation and a mobile setup of the Radio Vector Field Sensor.
The ionosphere is a multiple-scattering medium for HF radio sounding signals
A qualitative distinction between
Single Scattering and Multiple Scattering
Results by the phase structure function method [Zabotin and Wright, Radio Sci., v.36, p.757-772, 2001]: Typical irregularity amplitudes for the scale length 1 km are 0.3 - 3.0%; typical values of the irregularity power spectrum index are 2.3 – 3.5.
In the case of multiple scattering the spatial redistribution of energy is described by a kind of radiative transfer equation. This treatment is quite different from conventional ray tracing based on geometric optics.
In vertical sounding of the ionosphere, the optical thickness for scattering by intermediate-scale (~100 m – 1 km) irregularities is frequently considerably greater than unity. This implies a multiplicity of scattering that leads to a spatio-angular redistribution of the radio radiation flux.
Properties of ionospheric radio reflection according to the theory, at one of the VIPIR’s frequencies (2.727 MHz)
Angular distribution of the sky radio brightness (ray intensity) for a receiver position shifted (here, Eastward) from the transmitter's magnetic meridian plane, for six shift distances, and for km-scale irregularity amplitude ΔN/N=0.005, at the latitude of Boulder VIPIR Radar. With increasingshift,the nearer-side maximum gradually becomes dominant, but the former central peak continues to play a noticeable role up to some distance. Characteristic three-to-two-maxima structure of the obliquely reflected signal suggests some similarity to the double refraction. This effect is not of magnetoionic nature directly; it is caused by the multiple scattering from field-aligned irregularities.Calculations have been made at NCAR’s Supercomputing Center.
Boulder VIPIR Radar as a test bed for the theory
Off-line phase synchronization
Mobile setup for measuring spatial effects of multiple scattering based on Radio Vector Field Sensor designed and manufactured at the Swedish Institute of Space Physics
Boulder VIPIR radar system allows one to use variable modes of operation, a possibility to work with any desired set of frequencies, a possibility to implement phase synchronization between the radar's signal and the sensor.
A special mode of operation has been implemented: 4.5-minute sessions of continuous pulse sounding (100 pulses per sec) at 4 fixed alternating frequencies (2050, 2250, 2440, 2570 kHz for night; 2050, 2727, 3388, 4171 kHz for day conditions). 8 such sessions per hour, from 17:00 to 22:00 UT in the daytime, from 1:00 to 6:00 UT at night. Four 5-minute windows have been reserved during each hour for sounding sessions of a co-located digisonde which ionograms were used to monitor basic ionospheric structures.
N. Zabotin, T. Bullett
University of Colorado at Boulder
The radar and the sensor did not have means to maintain a phase lock remotely. That is why a phase correction linearly proportional to the time (with adjustable rate) was introduced into the procedure of coherent summation of the pulse signals. A result is illustrated in the left image. The ground wave (GW) and multiple ionospheric reflections are easily determined and can be confirmed by both the phase difference (-90° typical for ordinary echoes) and by ionogram information (right).
Original tripole layout and the battery unit.
Final tripole layout used to distinguish vertical and horizontal components of the radio field and to measure the horizontal components less vulnerable to the broadcast RF noise. See typical response of the sensor’s channels connected to the vertical and horizontal dipoles in Boulder (on the left).
Routes traveled by mobile setup during measurement sessions
Examples of the final results. Raw amplitudes of ionospheric reflections measured by VIPIR and by the sensor are shown. Also, detrended amplitude dependence on the distance is compared with theoretical calculations for various ΔN/N values.
VIPIR recordings (above) were used to determine temporal trends of the echo amplitude (right).
The routes were spanning ~20 km to the West and ~120 km to the East from Boulder. The predicted scale length of the “Cowboy Hat” effect is smaller for the East-West direction (see image in the Introduction). A few powerful broadcast radio stations, representing a saturation threat for the sensor, are located in Boulder and around Brighton. ~75% of the route’s length were radio quiet. The measurements were performed during short-time stops at locations of opportunity along the routes, separated by irregular distances of the order of several kilometers. About 15 long-range rides have been made in Oct-Nov 2009.
Ray tracing in a regular (smooth) ionosphere predicts gradual decrease of the signal amplitude when the distance between the radar and the sensor grows. Our experimental results frequently demonstrate an opposite tendency: the signal amplitude is higher at larger distances within ~100 km range. This fact is in general agreement with the theory describing multiple scattering of HF signals by km-scale irregularities. Our results are consistent with presence of these irregularities at a level of ΔN/N~0.005-0.020 both in day and in night conditions.
Frequencies were selected based on the analysis of radio spectrum in the Boulder area.