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

More information: Pekka Janhunen (First.Last@fmi.fi), tel. 358 9 1929 4635