What two-step Type I waves reveal about equatorial E region turbulence
R.K. Choudhary1 and J.-P St.-Maurice2
- Space Physics Laboratory, VSSC, Trivandrum, India (email@example.com)
- ISAS, University of Saskatchewan, Saskatoon, Canada (firstname.lastname@example.org)
It is now well understood that large scale (km) horizontally propagating gradient-drift structures are at times strong enough to excite short scale (m-size) Farley-Buneman (FB) waves in or near the vertical direction in the equatorial electrojet. The presence of these ‘two-step’ FB waves facilitates the study of their properties as a function of altitude and driving conditions in the equatorial E region. A study of their Doppler shift using a VHF radar at Pohnpei, during an event when the echo strength went through exceptionally strong power returns for a short time, has revealed that: 1) the Doppler shift of the vertical echoes increases with power; 2) the largest Doppler shift values increase with decreasing altitude; 3) the Doppler shifts as a whole are stronger off the zenith than at the zenith. In addition, during the same event we have observed that the up-down and east-west power asymmetries became less pronounced as the power increased and would actually be reversed at the higher altitudes under these strongly excited conditions. Our interpretation of the observations is that: 1) the aspect angle of the FB structures was so small that non-isothermal processes played an important role in determining the saturation (threshold) speed. Theoretical calculations have shown that non-isothermal effects become more important at small aspect angles at lower altitudes, which can increase the threshold speed by as much as 50%, in agreement with the observations; 2) The fact that the Type-1 speeds are often smaller than the observed non-isothermal threshold indicates that they are either observed somewhat outside the ‘instability cone’ (our preferred explanation) or that they have a larger aspect angle than their faster counterpart; 3) The east-west and vertical power asymmetries are due to an asymmetry in the nonlinear evolution of gradient-drift structures, which favours larger amplitudes for depletions (holes) than for enhancements (blobs). The larger amplitude structures are associated with a larger rotation of the total electric field away from the vertical. However, if the amplitude of the density structures increases too much, the net electric field inside the structures also weakens, which leads to a reversal of the power asymmetries under such circumstances.