Incoherent scatter

spaceweb@oulu.fi - last update: 18 May 1999, 1400 UT

Introduction

The basic principles of incoherent scatter:

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.

The received signal is rich of physical content. From the power and the shape of the spectrum one can determine:

electron density
electron temperature
ion temperature
ion drift speed
collision frequencies between ions and molecules

Several quantities can be calculated, e.g.:

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 frictional heating. 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).

References

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, 423-434, 1986.
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.