STARE RTI plots
The quicklook RTI (Range-Time-Intensity) plot contains the echo
intensity for beam number 5 (this might change, the number is
mentioned in the upper right corner) in dB units relative to
background noise level as a function of range and time. The range is
the distance of each radar, and is divided into 50 range gates. Range
1 corresponds to the closest point to the radar (the southernmost) and
range 50 the farthest point (the northernmost). The distances are 495 and 1245 km, respectively.
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
- 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 13 dB less than the main lobe in
STARE. If there is a strong (>15 dB, say) echo region at any beam,
there will be some sidelobe contamination at the same range on other
Some references [only some, there are many more]:
- Kustov et al., Electric field and electron density thresholds for
coherent auroral echo onset, J. Geophys. Res., 98, 7729-7736, 1993.
- Janhunen, Implications
of flow angle stabilization on coherent E-region spectra,
J. Geophys. Res., 99, 13203-13208, 1994.
- Uspensky et al., The amplitude of auroral backscatter-III. Effect
of tilted ionospheric layer, J.Atmos.Terr.Phys., 55, 1383-1392, 1993.
- 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.
Back to STARE home page
See also: Interpreting the Doppler velocity
More information: Pekka Janhunen (First.Last@fmi.fi), tel. 358 9 1929 4635