Radar Thomson scatter studies of photoelectrons in the ionosphere and Landau damping

Abstract

The velocity distribution of daytime ionospheric photoelectrons in the energy range 7–20 ev has been investigated at altitudes near 600 km by measuring the intensity of radar Thomson scatter from the steady-state level of plasma waves in the ionospheric plasma. This intensity depends on the velocity distribution through plasma wave-particle interactions such as Landau damping. The measurements were carried out at the Arecibo Ionospheric Observatory (L ≈ 1.5, geographic latitude 18°N). The experimental values for the vertically upward flux of photoelectrons are in the range (3–10) × 108 cm−2 sec−1, and, for the upward energy flux, (3–13) × 109 ev cm−2 sec−1. The experiments show that the daytime downward flux is equal to the upward flux to within experimental error (±50%) at Arecibo. Some December 1966 observations during the presunrise and postsunset periods have detected photoelectrons originating at the magnetic conjugate point. The effect of a weak (ωp > ωc) magnetic field on the Landau damping of plasma waves by a Maxwellian velocity distribution was investigated by changing the angle θ between the radar beam and the magnetic field. The theoretical prediction that the Landau damping is orders of magnitude larger than the field-free case and depends strongly on angle was verified experimentally.