The basic principles of incoherent scatter:
- Radar transmits electromagnetic VHF/UHF waves into the ionospheric plasma
- 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 successfully used by several radars, including EISCAT , to study Earth 's ionosphere.
Effects of strong electric fields
Under the influence of a strong electric field (see ionospheric convection) 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).
- 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.