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Effects of HF multiple scattering in the ionosphere:
Experimental observations
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 groundbased 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.
Boulder VIPIR Radar as a test bed for the theory
N. Zabotin, T. Bullett
University of Colorado at Boulder
A qualitative distinction between
Single Scattering and Multiple Scattering
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.
The ionosphere is a multiple-scattering medium for HF radio sounding signals
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 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 О”N/N=0.005, at the latitude of Boulder VIPIR
Radar. With increasing shift, 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 fieldaligned irregularities. Calculations have been made at NCAR’s Supercomputing Center.
Mobile setup for measuring spatial effects of multiple
Off-line phase synchronization
scattering based on Radio Vector Field Sensor designed and
manufactured at the Swedish Institute of Space Physics
1
3
GW
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).
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 (-90В° typical
for ordinary echoes) and by ionogram information (right).
Results
Results of Measurements on 23 October 2009
day session
60
80
70
VIPIR Amplitude
(time series)
50
40
VIPIR Amplitude
(time series)
50
Routes traveled by mobile setup during measurement sessions
4171 kHz
3388 kHz
2727 kHz
2050 kHz
Ground Wave
60
Ground Wave
30
40
20
dB
30
20
Sensor Amplitude
(raw values)
2570 kHz
2440 kHz
2250 kHz
2050 kHz
10
Theory Prediction
10
Sensor Amplitude
(raw values)
Theory Prediction
0
BD840_2009310012019_Ch3_Fr4
Results of Measurements on 13 November 2009
night session
90
dB
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 5minute windows have been reserved during each
hour for sounding sessions of a co-located
digisonde which ionograms were used to monitor
basic ionospheric structures.
2
0
-10
-10
-20
-20
Sensor Amplitude
(temporal trend removed)
-30
-40
BD840_2009310012523_Ch3_Fr4
0
10
20
30
40
50
60
70
80
W-E Distance from VIPIR, km
VIPIR recordings
(above) were used to
determine temporal
trends of the echo
amplitude (right).
Noise Spectra 13 Oct 2009, Day, Horiz. Only
90
80
100 km
0 km, Boulder
70
dB
60
50
40
30
20
10
0
0
1000
2000
3000
4000
5000
6000
7000
Frequency, kHz
8000
9000
10000
11000
12000
Frequencies
were selected
based on the
analysis of
radio spectrum
in the Boulder
area.
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.
90 100 110
-30
Sensor Amplitude
(temporal trend removed)
-40
-25 -20 -15 -10
-5
0
5
10
15
20
25
30
W-E Distance from VIPIR, km
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 О”N/N values.
Conclusion
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 О”N/N~0.005-0.020 both in day and in night conditions.
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