An Explanation for Arecibo Plasma Line Power Striations

Measurements of plasma lines by the Arecibo incoherent scatter radar are known to have sharp striations in power, varying with the plasma frequency and magnetic aspect angle of the radar beam. We explain these power striations as the manifestation of a suprathermal electron population with peaks in energy at approximately 15, 25, and 45 eV. These energies correspond to sharp features in the photoelectron energy spectra measured by rockets and spacecraft. A new theory is developed to predict the plasma line power for an arbitrary, magnetized suprathermal distribution. The magnetization terms in this theory are shown to contribute substantially to the enhancement of plasma line power through inverse Landau and cyclotron damping of the suprathermal peaks. The theory is applied as a forward model to measurements obtained at Arecibo for different magnetic field aspect angles, showing general agreement with the data. At large magnetic aspect angles the theory reproduces the upper-hybrid instability which can cause 150 km echoes. The developed theory allows for the suprathermal distribution at a given altitude to be probed across a wide range of energies and pitch angles.

Automated summary

Measurements of plasma lines by the Arecibo incoherent scatter radar show sharp power striations that vary with plasma frequency and magnetic aspect angle, which are explained as manifestations of a suprathermal electron population with peaks at specific energy levels. A new theory has been developed to predict plasma line power for arbitrary magnetized suprathermal distributions, showing that magnetization terms significantly enhance the power through inverse Landau and cyclotron damping. This theory has been applied to Arecibo measurements, demonstrating general agreement with the data and allowing for probing of suprathermal distributions across a wide range of energies and angles.