Formation of small-scale irregularities in the auroral E region
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In this thesis, knowledge on the production mechanisms of small scale (meter and decameter) irregularities in the auroral E region is advanced, both theoretically and experimentally. In the theoretical part of the thesis, the linear fluid theory of the Farley-Buneman (F-B) and gradient drift (G-D) plasma instabilities is considered. A general 2-D dispersion equation is derived and analyzed. Then, a review of existing nonlinear theories is given. The thrust is on the theory predictions with respect to the phase velocity of plasma waves. As an expansion of the theory, one new effect in the F-B instability evolution is considered, a secondary instability of a turbulent background of the primary F-B waves. It is shown that in a system of F-B modes the energy can flow from the short-wavelength (primary) to long-wavelength (secondary) structures (inverse cascade), contrary to the currently dominating idea that the energy of unstable waves is transferred to the smaller-scale structures. The phase velocity of the secondary waves propagating along the electron flow was found to be close to the electron streaming velocity and not saturated at the ion acoustic speed of the plasma. A possibility of decameter wave generation through this mechanism is envisioned. Experimentally, data of 2 separate experiments carried out in the auroral E region to study phase velocity of meter and decameter irregularities are considered. First, nearly simultaneous measurements of two Super Dual Auroral Radar Network (SuperDARN) HF radars (12 MHz, scatter from decameter waves, λ = 12 m) and one VHF radar (50 MHz, scatter from meter waves, λ = 3 m) at the Antarctic Syowa station are compared. It is demonstrated that HF echoes exhibit quite different characteristics as compared to VHF echoes so that HF echoes with low (< 350 m s-1) and high (> 350 m s-1) Doppler velocities are proposed to consider separately. Observations indicate that the high-velocity HF echoes exhibit properties similar to VHF echoes while the low-velocity HF echoes do not have a VHF counterpart. Other echo characteristics are also studied. For example, the preferential direction for the HF echo occurrence was found to be shifted by 45° from the direction of the electron flow and this shift was related to the existence of the low-velocity echoes in the data statistics. In the second experiment, the characteristics of decameter irregularities at 5 very close scales (between 10 and 16 m) are investigated by considering SuperDARN observations at Prince George, British Columbia, Canada. It is shown that the measured Doppler velocity (and hence the phase velocity of plasma waves) depends on the irregularity scale but only for irregularities propagating within the F-B instability cone. For these directions, the phase velocity was also found to decrease with the aspect angle and the rate of the decrease was found to be scale sensitive. Very little velocity variation with the aspect angle was discovered for observations outside of the F-B instability cone. Several factors potentially contributing to the observed HF and VHF echo characteristics in both experiments are identified and discussed. These are the geophysical conditions during measurements (details of large-scale convection patterns, a possibility of scatter from meteor trails), propagation effects (focusing of the HF radio waves onto various parts of the electrojet layer) and the plasma physics of irregularity formation (effects of strong gradients of the background electron density, ion and neutral particle motions, mode coupling, anomalous collisions). It is argued that the high-velocity HF echoes and VHF echoes are generated through traditional F-B and G-D instabilities, while the low-velocity HF echoes can have additional sources. Finally, several suggestions for further research are presented.