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The basic principles of incoherent scatter:
The received signal is rich of physical content. From the power and the shape of the
spectrum one can determine:
- Ionospheric electrons start to oscillate due to the electric field
of the radar transmitted wave
- Oscillating electrons radiate electromagnetic wave
- The frequency of the radiated wave changes according to the movement of each electron
- Electrons are partly following the motion of the much heavier ions
- Radar observes signal from many electrons simultaneously
- Because of the electron movement, spectrum of the observed signal is broad and it has shape
which depends, e.g., on the temperature.
Several quantities can be calculated, e.g.:
- electron density
- electron temperature
- ion temperature
- ion drift speed
- collision frequencies between ions and molecules
- ionospheric electric field
- ionospheric electric currents
- energy and flux of the precipitating particles
Incoherent scatter is succesfully used by several radars, including EISCAT
, to study Earth 's ionosphere.
Effects of strong electric fields
Under the influence
of a strong electric field the directed component of the ion velocity
may become comparable to the thermal speed yielding anisotropic
and non-Maxwellian velocity distribution. This, in turn, may
affect the analysis of incoherent scatter radar measurements!
A correction to Ti is suggested by St.-Maurice
and Schunk (1979) to allow anisotropic ion temperature during
According to the theory, Ti should be replaced
by (2Ti,perp + Ti,par)/3 (Williams and Jain,1986; Glatthor and
Hernandez, 1990; Winser et al., 1990). Note that observations
at aspect angle of 54.7 are not subject to this error (e.g., Lockwood
and Winser, 1988). In addition, for magnetic field aligned incoherent
scatter radar measurements which measure only Ti,par, the assumption
of Maxwellian distribution is accurate to within 5% for field-perpendicular
ion velocities up to 4 km/s (e.g., McCrae et al., 1991). Another
important experimental effect is the change in ion composition
due to increasing Ti: predominantly O+ plasma may change within
2-3 min to predominantly molecular ion plasma (Winser et al.,
1990). Also this affects the analysis of the radar measurements:
when there is a mixture of O+ and NO+, the standard analysis -
assuming 100% O+ - can underestimate the ion temperature significantly
(e.g., Glatthor and Hernandez, 1990).
- Glatthor, N., and R. Hernandez, Temperature anisotropy of
drifting ions in the auroral F-region, observed by EISCAT, J.
atmos. terr. Phys., 52, 545-560, 1990.
- Lockwood, M., and K. J. Winser, On the determination of ion
temperature in the auroral F-region ionosphere, Planet. Space
Sci., 36, 1295-1304, 1988.
- McCrea, I. W., M. Lester, T. R. Robinson, N. M. Wade, and
T. B. Jones, On the identification and occurence of ion frictional
heating events in the high-latitude ionosphere, J. atmos. terr.
Phys., 53, 587-597, 1991.
- St.-Maurice and Schunk, 1979.
- Williams, P. J. S., and A. R. Jain, Observations of the high
latitude trough using EISCAT, J. atmos. terr. Phys., 48,
- Winser, K. J., M. Lockwood, G. O. L. Jones, H. Rishbeth, and
M. G. Ashford, Measuring ion temperatures and studying the ion
energy balance in the high-latitude ionosphere, J. atmos. terr.
Phys., 52, 501-517, 1990.