Coherent Scatter Radars

High Frequency (HF) Coherent scatter radars exploit the spatial coherence of small-scale electron density irregularities in the ionosphere that are sensitive to Bragg scattering. Before the use of HF coherent scatter radars there were geometrical constraints on the ability to sense certain ionospheric regions. Historically, coherent scatter radar measure- ments were usually made with VHF and UHF frequencies. At high latitudes when the magnetic field is almost vertical it is impossible for coherent VHF and UHF to probe above E region altitudes.

Figure 3.1 shows a simple schematic highlighting the principle of Bragg scattering with the three horizontal lines representing three structured planes within a given material. The incident wave front highlights two possible ray paths, each reflecting from a different plane. The ray reflecting at point B travels an additional distance (AB - BC) and thus

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for the ray to interfere constructively with the rays travelling on other ray paths this distance must equate to the incident wavelengthλt(this will give maximum constructive

interference, at other fractions ofλtthe intensity of the reflected wavefront will reduce).

The following formula describes the relationship between the plane separation (d), the incident wavelengthλt and the incident angle (θ).

d= λt

2sin(θ) (3.1)

The necessary conditions for Bragg scattering are:

• The angle of incidence must equal the angle of reflection

• Reflections from several planes must add constructively.

In the Ionosphere, F region field aligned electron density irregularities are produced by a number of processes - some of which are mentioned in the review by Fejer and Kelley (1980). For the high latitude F region the most common plasma instability is the gradient drift instability (Tsunoda, 1988). The gradient drift instability occurs in the presence of electron density gradients and can be formed in two ways. 1) In the auroral zone via structured soft precipitation. 2) Structured convection in the presence of solar EUV ionization gradients. The irregularities last on the order of a few hours (Kelley et al., 1982) and drift at theE~×B~ velocity following the typical ionospheric convection pattern.

F region field aligned irregularities have a wave-vector number that is close to orthogonal to the magnetic field. When the wave-vectorK satisfies the condition K = +/−2Kr

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whereKr is the radar radio wave vector, the wavelength of the ionospheric irregularity

Kis half the transmitted radio waveKr such that the reflected waves will constructively

interfere and produce a signal that is strong enough to be detected. As has been discussed there are two conditions required for Bragg scattering, the first being that the reflected waves from several planes add constructively (mentioned above) and that the angle of incidence is equal to the angle of reflection. In order to achieve a return signal detected by the radar the incident radar wave must reach the ionospheric irregularity at 90 degrees. The HF radar frequency range means that it is able to refract in the ionosphere in such a way to achieve the orthogonality condition.

The variation in electron density at ionospheric altitudes influences radio wave propa- gation and causes the radio waves to refract. In a simple approximation the ionosphere can be thought of as a stack of thin slabs with refractive indices,n1, n2,n3 etc. Snells

law describes the relationship between the angles of incidence and refraction as the elec- tromagnetic wave encounters a boundary between two different mediums (i.e. the thin ionospheric slabs) and is stated as:

sinθ2 sinθ1 =

n2

n1

(3.2)

where sinθ1 is the angle of incidence,n1 is the refractive index of the incident medium,

sinθ2 and n2 refer to the refracted angle and refractive index of the second medium.

The refractive index decreases when moving from a medium of lower to higher electron density. As the plasma frequency increases with height (up to F-region altitudes) as

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Figure 3.2: Ray tracing results to illustrate the HF radio wave propagation into the mid-latitude ionosphere for varying elevation angles (Figure 3 ofde Larquier et al.

(2013)). The electron density is colour-coded from blue to red, pink lines represent the background geomagnetic field lines, rays are shown as gray and the black segments

mark the regions where the orthogonality condition can be met.

Figure 3.3: A schematic taken from Figure 1 ofMilan et al.(1997) to show possible

propagation modes and regions in which backscatter can occur. Three possible ray paths are shown, A, B and C. Ray A a possible E region mode with the possibility of multiple hops. Ray B a higher elevation angles showing a possible F-region mode that produces both near and far backscatter. Ray C shows a ray that penetrates the

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seen in Figure 1.10 the refractive index decreases and causes the ray to gradually bend towards the horizontal.

Figure 3.2 illustrates the effect of the angle of incidence (elevation angle) of a radio wave propagating into the mid-latitude ionosphere. The electron density is shown colour-coded from blue (low) to red (high) with a peak at approximately 250km, the background geomagnetic field is shown by the pink lines, each ray is plotted in gray and black segments mark the regions where the orthogonality condition is met. As the elevation angle increases, the path of the radio wave reaches higher altitudes and experiences less refraction until it is able to penetrate through the ionosphere. At lower elevation angles they are refracted to such an extent that they turn back towards the earth’s surface and hit the ground. It is also possible for the signal to hit the ground and ‘hop’ forward to produce multi-hop waves.

Figure 3.3 shows schematically some possible propagation modes of the transmitted radar wave (taken fromMilan et al. (1997)). The figure highlights three possible radio wave vectors (Kr) emanating from the radar. The magnetic field direction is shown as

the straight arrowed lines directed into the ground. Ray A is transmitted at the lowest elevation angle and is refracted and reflected in the ionosphere at E region altitudes. Ray B has the next highest elevation angle and shows an F region mode. This ray path shows multiple hops and returned backscattered signals from a near and far range. Ray C has the highest elevation angle and is not refracted sufficiently to meet the orthogonality condition and the signal is lost as it penetrates the ionosphere. Signals refracting from E or F region altitudes and returning to ground will produce backscattered signal (ground- scatter). In Figure 3.3 this can be seen at the 1E, 1F and 2F locations, i.e. integer hops.

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Signals returned from the receiver from ground typically have different characteristics than those returned from the ionosphere, namely much smaller velocities and spectral widths, meaning they can be easily identified.