Investigation of naturally occurring and radio wave induced ionospheric plasma turbulence

Abstract

We have been conducting experiments at Arecibo, Puerto Rico and Gakona, Alaska to investigate naturally occurring and high power radio wave induced ionospheric plasma turbulence in the past two decades. Ionospheric turbulence can have a broad range of scale length from several hundred kilometers to centimeters. Zero-frequency modes associated with plasma density fluctuations (irregularities) are highly field-aligned. Singled out for discussion in this paper are large-scale (tens of kilometers to hundred kilometers) acoustic gravity waves created by tsunamis [Lee at. al., 2008], natural anomalous heat sources and HF waves [Pradipta et al., 2011], medium-scale (hundred meters to a few kilometers) field-aligned plasma sheets generated by HF heater waves [Lee et al., 1998; Cohen et al., 2010], and short-scale [a few meters to ten meters) lower hybrid waves excited by whistler waves [Lee et al., 1997; Labno et al., 2007]. Lower hybrid waves can effectively accelerate electrons (ions) along (across) the earth's magnetic field in the ionopsheric F region. VLF whistler waves can be launched from a ground-based transmitter or HF amplitude-modulated electrojet currents. Extensive radio and optical diagnostic instruments have been used in these experiments, including incoherent backscatter radars, ionosondes, GPS satellites, all sky imagining system, magnetometers, Fabry-Perot interferometer etc. Because of the highly field-aligned structures of aforementioned large- and medium-scale ionospheric plasma turbulence, they can act as ionospheric ducts to favorably guide whistler waves to propagate from the ionosphere into the radiation belts. These ducted whistler waves can interact with trapped energetic charged particles and change their pitch angles, to cause their precipitation into the lower ionosphere [Pradiptraet al., 2007]. Radars and optical instruments were used to monitor acceleration of charged particles caused by whistler waves and charged particle precipitation from radiation belts. Theories developed to understand these physical processes will be presented.