Plasma irregularities in the ionosphere, such as the gradient drift instability dis-cussed in Section 1.4.5, can become intense enough to scatter probing radio signals.
When the wavelength of these irregularities is equal to one-half the radar wave-length, coherent Bragg-like scattering can occur. The wave reflected by each irregu-larity wave-front is in phase with the wave reflected by subsequent wave fronts and a strong signal is received by the radar system. Unlike an incoherent scatter radar, the frequency spectrum recorded by the receiver measures the properties of the wave-like irregularity, not the individual electrons. Therefore, coherent radar systems are not able to directly measure electron density and plasma temperatures, but they can measure the coherency and line-of-sight velocity of the ionospheric irregularities. In the F region, it has been demonstrated that the plasma irregularities drift at the same velocity as the background plasma [Villain et al., 1985] so coherent radars effectively measure the bulk drift of the ionospheric plasma.
One benefit of coherent scatter radar systems over incoherent radars is that,
be-cause of the coherent nature of the returned echo, much less transmitted power is required to provide meaningful backscatter signal. Due to this low power require-ment, coherent radar systems are much less expensive and less complex to build than incoherent radar systems. ISR stations can also detect coherent scatter from iono-spheric irregularities and, due to the much higher initial transmitter power, these result in saturation of the receivers unless the signal is significantly attenuated.
Aside from the lack of information about electron density and temperature, the main drawback to coherent radar use is that measurements can only be made when the radar wave vector is parallel to the plasma irregularity wave vector. Since most irregularities develop parallel to the magnetic field lines of the Earth, this typically requires the radar wave vector to be perpendicular to the magnetic field lines. At VHF and higher radar frequencies, for which there is no refraction of the wave, this aspect sensitivity requirement limits the range of locations at which measurements can be made. At equatorial latitudes, the magnetic field of the Earth is nearly horizontal, so coherent radar measurements of ionospheric irregularities may only be made when the radar beam is roughly vertical. At higher latitudes, VHF radar beams must be nearly horizontal to scatter from irregularities which are aligned along the nearly vertical magnetic field lines. This requirement limits VHF coherent radar observations to the E region at higher latitudes [Fejer and Kelley, 1980; Kelley, 2009].
Coherent scatter radars that use HF (3–30 MHz) instead of VHF or UHF operat-ing frequencies have been developed to overcome the aspect sensitivity limitation for high latitude studies. Since the usual plasma frequency in the ionosphere is between
∼1–10 MHz, radio wave frequencies between ∼10–20 MHz are able to propagate, however, they are significantly refracted by the ionospheric plasma. Waves trans-mitted at frequencies higher than, but comparable to, the plasma frequency in the ionosphere tend to refract downwards resulting in some waves propagating in an es-sentially horizontal direction (over-the-horizon). The refraction to roughly horizontal allows wave propagation that is perpendicular to the magnetic field lines in the F region of the high latitude ionosphere. Using this refractive effect, high latitude HF
radars can obtain coherent scatter, and corresponding ionospheric drift velocities, throughout the auroral and polar ionosphere of the Earth. The magnetoionic and wave propagation theory which describes this refractive effect will be discussed in Section 1.7.
Although the combination of HF radar frequency and the plasma frequency of the ionosphere allow for significant refraction, and therefore an increased field-of-view, Gillies et al. [2009] has shown that this combination can also result in systematic underestimation of the coherent HF radar velocity measurements. All radars deter-mine the velocity of a scattering object vs by measuring the Doppler shift of the returned radar echo and applying the following equation:
vs = ∆fDvp
2f = ∆fD
2f c ns
, (1.10)
where, ∆fD is the Doppler shift, vp is the phase speed of the radar wave at the scattering location, f is the radar frequency, c is the speed of light in a vacuum, and ns is the refractive index at the scattering location. As will be demonstrated in Section 1.7, the refractive index for HF radio waves in a magnetoionic medium, such as the ionosphere, is less than unity. Since there has been no method to measure refractive index in the scattering volume, and it is a highly variable quantity which depends on the electron density, analysis of HF coherent radar data has always been performed by assuming a refractive index of 1.0. In actuality, the refractive index at the location of scatter for a coherent HF radar wave is typically ∼0.7–0.8 [Gillies et al., 2010a]. This indicates that all coherent HF radar measurements that assume the refractive index is 1.0 actually underestimate Doppler velocities by 20–30%.
SuperDARN
The largest array of HF coherent scatter ionospheric radars is the Super Dual Au-roral Radar Network (SuperDARN) [Greenwald et al., 1995; Chisham et al., 2007].
This global array of HF radars has been built over the last two decades to include more than 20 radars spread throughout the northern and southern hemispheres. It is typical to have two SuperDARN radars measure the line-of-sight component of
the plasma drift velocity in the same region of the ionosphere, but from different directions (hence the term ‘Dual’ in SuperDARN). The combination of the two line-of-sight velocity vectors can provide the component of the full velocity vector in the plane perpendicular to the magnetic field of the Earth.
A SuperDARN radar consists of 16 antennas which are phased to transmit radio waves along one of 16 different beam directions. The beams directions are separated by 3.24◦, providing a total azimuthal coverage of 52◦. Each beam typically samples 75 range gates, each with a range resolution of 45 km. In normal operation each beam is sampled for 3 or 7 s, and a full scan of all 16 beams is accomplished in one or two minutes, respectively. The velocity, backscatter power, and spectral width are recorded for each range gate, that receives backscatter, on every radar, every one or two minutes. Along with the main array of 16 antennas, several SuperDARN radars are equipped with an interferometry array of four antennas placed ∼100 m from the main array. The phase difference between the signal received by the interferometry array and the main array is used to provide a measure of the elevation angle at which an echo is received [Andr´e et al., 1998; Milan et al., 1997].
The ∼20 SuperDARN radars spread throughout the northern and southern hemi-spheres operate continuously to provide ionospheric convection velocities in hundreds of range cells every one or two minutes. Ionospheric models are used to fit the line-of-sight velocity data to create large-scale ionospheric convection maps in both hemispheres. As the F-region plasma irregularities follow the E × B drift motion and the magnetic field is relatively constant, these velocity maps can be used to infer ionospheric electric fields [Ruohoniemi and Baker, 1998]. Measurement of iono-spheric electric fields allows creation of high latitude electric potential contour maps which are used to determine the cross polar cap potential (CPCP) (discussed in Section 1.3.3).
A further capability of some of the SuperDARN radars is to operate in a Stereo mode in which two receiving channels can be sampled simultaneously [Lester et al., 2004]. The Co-operative UK Twin Located Auroral Sounding System (CUTLASS) radars are the set of two SuperDARN radars located in Hankasalmi and Pykkvibaer.
The CUTLASS radars were the first to develop the Stereo mode capability and now two southern hemisphere radars also have this capability (Syowa South and TIGER Unwin) [Chisham et al., 2007]. The Stereo mode allows a radar to run two different modes simultaneously. For example, a radar could transmit and receive signal at two different frequencies which can be very useful for providing better coverage and measuring electron density at the scattering location [Gillies et al., 2010a].
Over the years of SuperDARN operation several studies have been performed to confirm that velocities measured by SuperDARN are representative of the actual plasma drift. These studies compared line-of-sight velocities measured by various SuperDARN radars and other ionospheric instruments. As mentioned previously, Villain et al. [1985] confirmed that the irregularities which coherent radars detect do indeed drift at the same velocity as the background electron density. Comparisons between SuperDARN velocities and the EISCAT incoherent radar were performed by Eglitis et al. [1998] and Davies et al. [1999]. These studies found that velocities mea-sured by both SuperDARN and EISCAT were comparable, but velocities meamea-sured by SuperDARN were systematically slower than velocities measured by EISCAT, especially in the Davies et al. [1999] study. Similar results were reported by Xu et al. [2001] and Xu [2003] in studies which compared SuperDARN velocities to veloc-ities measured by the Sondrestrom incoherent scatter radar. Comparisons between line-of-sight velocities measured by various SuperDARN radars and drift velocities measured by DMSP (Defense Meteorological Satellites Program) satellites were per-formed by Drayton et al. [2005] and Drayton [2006]. Like the SuperDARN-ISR comparisons, these studies found that SuperDARN measured similar, but slightly lower, velocities compared to the DMSP satellites. One of the main focuses of this thesis research was to re-examine the comparisons by Davies et al. [1999] and Dray-ton et al. [2005] with the inclusion of an estimate of refractive index.