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Ann. Geophys., 23, 2647-2655, 2005
www.ann-geophys.net/23/2647/2005/
© European Geosciences Union 2005
Possible ionospheric preconditioning by shear flow leading to equatorial spread F
D. L. Hysell1, E. Kudeki2, and J. L. Chau3
1Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA
2Department of Electrical and Computer Engineering, University of Illinois, Urbana, Il 61801, USA
3Radio Observatorio de Jicamarca, Instituto Geofísico del Perú, Lima, Perú
Abstract. Vertical shear in the zonal plasma drift speed is apparent in
incoherent and coherent scatter radar observations of the bottomside
F region ionosphere made at Jicamarca from about 1600–2200
LT. The relative importance of the factors controlling the shear,
which include competition between the E and F region
dynamos as well as vertical currents driven in the E and F
regions at the dip equator, is presently unknown. Bottom-type
scattering layers arise in strata where the neutral and plasma drifts
differ widely, and periodic structuring of irregularities within the
layers is telltale of intermediate-scale waves in the
bottomside. These precursor waves appear to be able to seed
ionospheric interchange instabilities and initiate full-blown
equatorial spread F. The seed or precursor waves may be
generated by a collisional shear instability. However, assessing the
viability of shear instability requires measurements of the same
parameters needed to understand shear flow quantitatively -
thermospheric neutral wind and off-equatorial conductivity
profiles.
Keywords. Ionosphere (Equatorial ionosphere; ionospheric
irregularities) – Space plasma physics (Waves and instabilities)
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