STARE Doppler velocities
Are STARE Doppler velocities correct?
The Doppler velocity is the main scientific product of STARE, and is
currently computed using the so-called cosine law, which is the
simplest way to compute the electron flow from two radars viewing the
same volume from different angles.
At least the following error sources exist:
-
The plasma waves move somewhat slower than the electron flow. How much
slower, depends on the collision frequencies, aspect angle, and
possibly the strength of the electric field and amount of electrojet
turbulence.
-
The radar gather signal from many different altitudes. Some of these
altitude layers have higher collision frequencies and less ideal
aspect angles than the "main" scattering layer. The net result is a
reduction of the mean measured Doppler speed.
-
Often one can see short-lived peaks of very high Doppler velocity in
STARE data. The origin of these false signals is unknown.
Very roughly, STARE usually underestimates the electric field if the
electric field is clearly above the threshold (a usually quoted value
for the threshold is 17 mV/m, the threshold depends on the ion and
electron temperature). The amount of underestimation is likely to
increase if the scattering volume becomes more turbulent (here
turbulence refers to any electron density/electric field variations
whose scale size is more than 1 m but less than the size of the
scattering volume, about 15 km). The underestimation factor can be
something like 1.2-1.4, but could be more than 2 in some cases. In
other cases, STARE can also overestimate the electric field,
especially if the field is close to the threshold.
Thus, one can say that STARE prefers to display Doppler velocities
that are close to the Farley-Buneman threshold, regardless of the
value of the electric field. Heuristically, the reason is that various
nonlinear processes occur the waves to have about zero growth rate in
an average sense. According to linear theory, zero growth rate is
achieved when the phase velocity is close to the sound speed, i.e. the
FB-threshold. This means that most wave power propagates at one of the
edges of the linearly unstable cone, and the radar picks these signals
up most easily. If the scattering volume becomes more turbulent and/or
if altitude mixing occurs due to ray propagation or magnetic field
deformation effects, signals close to acoustic speed dominate the
spectrum more and more.
But even if this explanation gives the correct picture, we cannot use
it to "invert" the relationship, because the amount of scattering
volume turbulence and other factors remain essentially unknown.
See also: Factors affecting the backscattered
signal strength.
Some references [only some, there are many more]:
- Haldoupis and Schlegel, Direct comparison of 1-m irregularity
phase velocities and ion acoustic speeds in the auroral E region
ionosphere, J. Geophys. Res., 95, 18989-19000, 1990.
- Kustov and Haldoupis, Irregularity drift velocity estimates in
radar auroral backscatter, J. Atmos. Terr. Phys., 54, 415-423, 1992.
- Janhunen, Implications
of flow angle stabilization on coherent E-region spectra,
J. Geophys. Res., 99, 13203-13208, 1994.
Back to STARE home page
Pekka Janhunen (First.Last@fmi.fi)
Updated 17 December, 1999.