Geoscience Reference
In-Depth Information
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16
Incoherent Scatter Radar — Spectral Signal
Model and Ionospheric Applications
Erhan Kudeki
1
and Marco Milla
2
1
University of Illinois at Urbana-Champaign
2
Jicamarca Radio Observatory, Lima
Peru
1. Introduction
Doppler radars find a widespread use in the estimation of the velocity of discrete
hard-targets
as described elsewhere in this volume. In case of
soft-targets
— collections of vast numbers of
weakly scattering elements filling the radar beam — the emphasis typically shifts to collecting
the
statistics
of random motions of the scattering elements — i.e., Doppler spectral estimation
— from which thermal or turbulent state of the target can be inferred, as appropriate.
For instance, in case of a plasma in thermal equilibrium, e.g., the quiescent
ionosphere
,a
Doppler radar of sufficient power-aperture-product can detect, in addition to the plasma
drift velocities, the densities, temperatures, and even current densities of charged particle
populations of the probed plasma — such Doppler radars used in ionospheric research are
known as
incoherent scatter radars
(ISR). In this chapter we will provide a simplified description
of ISR spectral theories (e.g., Kudeki & Milla, 2011) and also discuss magnetoionic propagation
effects pertinent to ionospheric applications of ISR's at low latitudes. A second chapter in this
volume focusing on in-beam imaging of soft-targets by Hysell & Chau (2012) is pertinent to
non-equilibrium plasmas and complements the topics covered in this article.
The chapter is organized as follows: The working principles of ISR's and the general theory
of incoherent scatter spectrum are described in Sections 2 and 3. ISR spectral features
in unmagnetized and magnetized plasmas are examined in Sections 4 and 5, respectively.
Coulomb collision process operating in magnetized ionosphere is described in Section 6.
Effects of Coulomb collisions on particle trajectories and ISR spectra are discussed in Sections
7 and 8. Finally, Section 9 discusses the magnetoionic propagation effects on incoherent
scattered radar signals. The chapter ends with a brief summary in Section 10.
2. Working principles of ISR's
The basic physical mechanism underlying the operation of ISR's is
Thomson scattering
of
elecromagnetic waves by ionospheric free electrons. Thomson scattering refers to the fact that
free electrons brought into oscillatory motions by incident radar pulses will re-radiate like
Hertzian dipoles at the frequency of the incident field. The total power of scattered fields in
an ISR experiment is a resultant of interference effects between re-radiated field components
arriving from free electrons occupying the radar field of view. Furthermore the frequency
spectrum of incoherent scatter signal is shaped by the same interference effects in addition
