Directional Changes in the Irregularity Drift during Artificial Generation of Striations

Resonance instabilities generated by high power HF radio waves have been observed to produce 1 meter field-aligned electron density depletions in the high latitude E-region. The striations have in a smaller portion of the experiments given a clockwise turning in the artificial irregularity drift velocity with respect to the natural drift of irregularity waves in the electrojet.

Two sets of conditions in the experiments offer explanations of the clockwise turning. One theory takes into account the steady state conductivity changes by assuming that the striations can be presented as homogeneous plasma columns elongated in the magnetic field direction. The electrostatic polarization electric field due to changes in temperature and density modifies the artificial irregularity wave direction.

The idealized model of the striations explains the observations for parameters expected by the theory. The simple model gives maximum changes in the altitude of 95 km for |ΔNe/Ne0| > 10-2. The driving term in the production of the electric polarization field is ΔNe/Ne0, while enhanced temperatures reduce the effect of ΔNe/Ne0.

Another consideration takes into account the possibility that the formation of striations takes place in a different altitude than the electrojet height. This condition gives a twisting in the drift direction in comparison to the phase velocity direction of natural two-stream and gradient drift irregularities when the striations follow the maximum growth direction of natural irregularities.

Height differences between striations and electrojet show the largest effect for altitudes above 110 km. Calculations show that this effect is minor with respect to the above proposed for altitudes less than 108 km.