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Thomson scatter radars have successfully measured plasma parameters in the ionosphere for over 60 years. Fundamentally, the radars measure increased power returns when the Bragg scattering condition is met by a source of density fluctuations in the plasma. Typically, wave modes of the plasma provide the source of structuring, and the radars measure strong power returns at the ion line which is associated with the ion-acoustic mode, the gyro line which is associated with the electrostatic whistler mode, and the plasma line that comes from the Langmuir mode. However, the existence of an ion-acoustic mode or electrostatic whistler mode is not guaranteed in the ionosphere. In this study, a formalism is developed to explain non-resonant wave modes as features occurring at frequencies where the dielectric function has a local minimum as opposed to a root corresponding to the typical resonant wave mode. With this formalism, the frequency of non-resonant waves is numerically solved as a function of basic plasma parameters. By solving for minima of the dielectric function, the frequency and intensity of gyro lines is determined for a wide range of plasma temperatures and densities. This analysis explains why Arecibo gyro lines are typically weak in intensity and result from non-resonant waves. For VHF systems like EISCAT, gyro lines are shown to be strong spectral peaks corresponding to standard resonant solutions for electrostatic whistler waves.
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Solution of the wave equations for a cylindrical whistler duct
The coupled equations governing whistler waves propagating along a duct with cylindrical cross section are derived and then solved numerically. These equations are expressed in terms of magnetic and current flux functions and show that it is possible to have a solution where the waves are finite in the duct and decay exponentially outside the duct. This solution has the property of having zero radial Poynting flux everywhere, so, as required for whistler waves to bounce back and forth losslessly between magnetically conjugate terrestrial hemispheres, no wave power leaks from the duct. The coupled equations are solved numerically for a tangible realistic situation by dividing the radial domain into an inner and an outer region, where the interface between these regions is at a mode conversion location, where fast and slow modes inside the duct merge and effectively reflect. The result of this effective reflection is that there are fast and slow standing waves in the duct. In the region external to the duct, the wave solutions are also a form of standing waves, but with a strong exponential decay and a radial wavelength that is intermediate between that of the fast and slow waves in the duct. The numerical solution is shown to be in good quantitative agreement with estimates made from analytic models. Detailed examination of the solutions in the vicinity of the mode conversion location shows that the classic plane wave assumption fails to describe the true nature of the modes.
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- Award ID(s):
- 2403814
- PAR ID:
- 10652922
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Physics of Plasmas
- Volume:
- 32
- Issue:
- 12
- ISSN:
- 1070-664X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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