Effects of HF multiple scattering in the ionosphere: Experimental observations

N. Zabotin, T. BullettUniversity of Colorado at Boulder

IntroductionThe 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.

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 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 field-aligned irregularities. Calculations have been made at NCAR’s Supercomputing Center.

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.

Boulder VIPIR Radar as a test bed for the theory

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

June 2010 Boulder CO

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.

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).

BD840_2009310012523_Ch3_Fr4

BD840_2009310012019_Ch3_Fr4

0

10

20

30

40

50

60

70

80

90

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

100 km0 km, Boulder

Frequency, kHz

dB

Noise Spectra 13 Oct 2009, Day, Horiz. Only

VIPIR recordings (above) were used to determine temporal trends of the echo amplitude (right).

Frequencies were selected based on the analysis of radio spectrum in the Boulder area.

Routes traveled by mobile setup during measurement sessions

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.

Off-line phase synchronization

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).

GW

1 2 3

Results

Conclusion

-40-30-20-10

0102030405060708090

0 10 20 30 40 50 60 70 80 90 100 110

4171 kHz3388 kHz2727 kHz2050 kHz

Sensor Amplitude(temporal trend removed)

Theory Prediction

Ground Wave

Sensor Amplitude (raw values)

VIPIR Amplitude (time series)

W-E Distance from VIPIR, km

dB

Results of Measurements on 23 October 2009day session

-40

-30

-20

-10

0

10

20

30

40

50

60

-25 -20 -15 -10 -5 0 5 10 15 20 25 30

2570 kHz2440 kHz2250 kHz2050 kHz

Sensor Amplitude(temporal trend removed)

Theory Prediction

Sensor Amplitude (raw values)

VIPIR Amplitude (time series)

Ground Wave

W-E Distance from VIPIR, km

dB

Results of Measurements on 13 November 2009night session

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.

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.