Electrodynamics of midlatitude spread F 1. Observations of unstable, gravity wave-induced ionospheric electric fields at tropical latitudes

In part 1 of our series exploring the role of electrical forces in midlatitude spread F, we present observations of an electrodynamically driven traveling ionospheric disturbance which passed over Arecibo Observatory between 22 and 24 AST on January 26, 1993. The total electric potential differences driving the wave were of the order of 1 kV. Our analysis indicates that this disturbance is the result of a midlatitude F region plasma instability seeded by a thermospheric gravity wave. Two novel measurements, in addition to typical incoherent scatter observations, were crucial to this determination: the use of 6300 Å airglow images from the coupling, energetics, and dynamics of atmospheric regions (CEDAR) all-sky imager to track the two-dimensional, mesoscale dynamics of the disturbance and the use of a portable ionosonde to simultaneously measure the fieldline integrated ionospheric conductivity in the conjugate hemisphere. We have also determined that this disturbance, like several previously observed midlatitude disturbances, is consistent with our theoretical knowledge of the basic instability of the midlatitude ionosphere described originally by Perkins [1973].