Rocket observations in the equatorial electrojet : current status and critical problems

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The current status of in-situ investigations in the equatorial electrojet is reviewed, including observations of zero-order electrodynamic and plasma parameters as well as measurements of plasma instabilities and ‘turbulent-like’ irregularities. Several key outstanding questions are discussed from the point of view of what future rocket-borne observations might help resolve. These include : (1) the relation of the vertical polarization field to the electrojet current and the electron number density ; (2) investigation of the puzzling square shapes of the large amplitude kilometer-scale horizontal electric field structures and their modulation (limitation?) of the driving polarization electric field ; (3) measurements of the complete (including λ < 1 m) δ E and δn primary two-stream wave spectrum (including phase velocity vs k) as a function of the driving d.c. electric field ; (4) investigation of the intense vertical, meter-scale waves observed on the topside of the electrojet associated with horizontal laminar-like primary two-stream waves ; (5) measurements of upgoing and downgoing secondary two-stream and gradient drift wave packets driven by δ E × B drifts ; (6) investigation of the non-linear meter-scale ‘turbulence’ with small mean phase velocities observed by radars at altitudes outside the regions of high Cowling conductivity (e.g. in the bottomside electrojet) ; and (7) investigation of wave-particle heating by the plasma instabilities. Future in-situ experiments are discussed that include new electric field probe arrangements and geometries designed to address these critical problem areas. Experiments such as these should be conducted in conjunction with ground-based radar backscatter measurements, both to ensure the desired launch conditions and to provide vital observations needed to successfully interpret the in-situ data.