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STARE RTI plots

The echo intensity depends in a complicated way on several factors and usually it cannot be alone used for any physical studies. But whenever the echo intensity is clearly above the noise level, the radar can measure the Doppler velocity and the power spectrum. Thus, whenever both radars measure a echo intensity which is clearly above the background (more than 5 dB, say), we can have an estimate for the electron flow velocity in the E-region and thus the ionospheric electric field.

At least the following factors affect the echo intensity:

• If the ionospheric electric field is below the Farley-Buneman threshold, we usually have no echo at all. The threshold value depends on the ion temperature and corresponds approximately to the ion-sonic point of the ExB velocity. A typical value for the threshold is 17 mV/m.
• If everything else remains the same, the echo intensity is linearly proportional to the electron density. (But notice that usually everything else does NOT remain the same if the electron density changes. It is more typical that e.g. the electric field and electron density are anticorrelated.)
• Above the Farley-Buneman threshold the echo intensity first raises and then starts slowly to decrease when the field is 2-3 times larger than the threshold.
• The echo intensity depends strongly on the aspect angle (the angle between the radar k vector and the geomagnetic field). Strongest echo is obtained for close to 90 degree aspect angle. If the aspect angle is e.g. 85 deg, the echo intensity is already very much reduced. The geometry of the radar experiment has been designed in such a way that perpendicularity is usually achieved, but during severe geomagnetic disturbances the field line direction may change enough to affect the aspect angle so that the observed echo intensities are changed in a very complicated manner. Also ray propagation effects can occur (refraction or forward scattering), though these can normally be ignored on 150 MHz radars such as STARE.
• The echo intensity depends on the flow angle. Flow angle is the angle between the ExB velocity (the electron flow) and the radar k vector. Generally the largest echo is obtained in the direction of the flow (for zero flow angle). This is why the Norwegian radar usually sees a stronger signal: it is looking more parallel with the electrojet than the Finnish radar, which is looking more or less perpendicularly to the electrojet. However, the flow angle dependence is an unresolved problem, and some simulation studies indicate that it might be an asymmetric function which is caused by nonlinear interactions. Electrojet turbulence and auroral structures cause a further spread in the flow angle dependence.
• The sidelobes are only about 20 dB less than the main lobe in STARE. If there is a strong (>25 dB, say) echo region at any beam, there will be some sidelobe contamination at the same range on other beams also.

1. Kustov et al., Electric field and electron density thresholds for coherent auroral echo onset, J. Geophys. Res., 98, 7729-7736, 1993.
2. Janhunen, Implications of flow angle stabilization on coherent E-region spectra, J. Geophys. Res., 99, 13203-13208, 1994.
3. Uspensky et al., The amplitude of auroral backscatter-III. Effect of tilted ionospheric layer, J.Atmos.Terr.Phys., 55, 1383-1392, 1993.
4. Williams et al., The relationship between E region electron density and the power of auroral coherent echoes at 45 MHz, Radio Sci., 34, 449-457, 1999.