The Dual Auroral Radar Network (DARN) is a global-scale network of HF and VHF radars capable of sensing backscatter from ionospheric irregularities in the E- and F-regions of the high latitude ionosphere. The current network of radars was given the acronym SuperDARN since it was derived from the DARN concept but is significantly larger in scope. The HF radars are based upon the design of the Goose Bay HF radar system described by Greenwald et al. [GBHH85]. A good general discussion about SuperDARN can be found in the paper by Greenwald et al. [GBD 95].
The radars use ionospheric refraction to achieve orthogonality with the magnetic field since a key factor is magnetic aspect sensitivity. To adjust for the effects on refraction of changes in the electron density, the radar is capable of operating over an extended frequency range from 8 to 20 MHz. The main antenna array consists of 16 log-periodic antennas which are fed by electronically phased signals that allow for beam-forming and beam-steering over a azimuthal range. Only a single beam is produced at any given time. The half-power beamwidth depends on the frequency and ranges from at 20 MHz to at 8 MHz. The range resolution of the SuperDARN measurements is determined by the transmitted pulse length. It is typically (so is normally ). However pulses as short as can be used for a range resolution of . Since the radar beam can be steered, a rapid scanning through a number of beam directions is possible.
The radar uses a multipulse transmission sequence consisting of 5 to 9 pulses. The backscatter signals returned from these pulses are sampled and are processed to generate the complex autocovariance function (ACF) as a function of time delay between pulses for all ranges. The multipulse sequence is repeated about 60 times for a given beam position, so the 60 ACFs can be integrated and averaged to increase the signal to noise ratio. These ACFs can then be used to determine a Doppler power spectrum for each range. The ACFs can also be used to determine the backscattered power, mean Doppler velocity and spectral width. Data from a radar pair with overlapping fields of view can be used to determine convection maps of the F-layer plasma from the individual Doppler velocities. Because of the large field-of-view for each radar pair, SuperDARN data are ideal for analysis in combination with data obtained by other instruments. Figure 3.2 shows a map with the field of view of a SuperDARN radar pair and the locations of other instruments.
Figure 3.2: The field of view of the Saskatoon and Kapuskasing SuperDARN radar pair, Sapphire VHF radars, all-sky imagers and magnetometer sites (EU - Eureka, RB - Resolute Bay, SH - Sachs Harbor, CB - Cambridge Bay, YL - Yellowknife, BL - Baker Lake, RI - Rankin Inlet, LCR - LaCrete, RL - Rabbit Lake, CH - Churchill, GI - Gillam).
The combination of several radar pairs can be used to draw a global picture of the convection. Conjugate studies are possible due to the existence of radars in the northern and southern hemisphere. Table 3.1 lists all SuperDARN radars that are operational (or will be in the near future). The first six radars constitute the three northern hemisphere pairs, the last three will act as a triplet in Antarctica.
Table 3.1: Geographic locations of SuperDARN sites
One of the main data processing steps with SuperDARN data is the merging of data sets from two different radars with an overlapping field of view into a vector velocity map [RGB 89]. This is done on a grid of 16 by 16 cells and will yield actual plasma speeds at these locations [SC94]. From such velocity maps, one also can derive maps of field-aligned currents. One can show that the vorticity (the curl of the plasma convection velocity) is a direct measure of the ``magnetospheric'' component of the field-aligned current:
where is the height integrated Pedersen conductivity and the magnetic field [SGB95]. Figure 3.3 is an example of such an analysis, showing both the 2D convection map and the resulting field aligned current system that has been calculated.
Figure 3.3: SuperDARN curl plot showing the large-scale distribution of the plasma convection velocity and the magnetospheric field aligned current component