MODELING THE ARECIBO NIGHTTIME F2 LAYER .2. IONOSPHERIC GRADIENTS

The servo model is extended and used to fit horizontal gradients in th e F-2 layer height and density and to estimate the zonal Pedersen curr ent and its zonal and meridional gradient. Horizontal gradients were m easured from the Arecibo Observatory during the following five nights: August 16-17 and 17-18, 1982; and October 4-5, 5-6, and 9-10, 1983. T he model gradients are driven by nonzero current gradients, which are applied as needed to fit the measured gradients in the F-2 peak. The g radient in the peak height is accurately reproduced; the peak density gradient is calculated self-consistently in the model. The divergence of the Pedersen current can be deduced when the current flows zonally and is found to differ from zero. This is a consequence of zonal diver gence of the model zonal current. Expressions are derived for the dive rgence of the Hall current and for the curl of the current in the pres ence of ionospheric gradients. The vertical vorticity of the F region current is determined from the radar and optical measurements and the mass spectrometer/incoherent scatter (MSIS) neutral densities. Both ne utral and plasma motions generate current vorticity equally as expecte d from the F region dynamo. The measured velocity gradients produce mo re current gradients and vorticity than the measured conductance gradi ents. The measured height gradient in the perpendicular-north plasma d rift (partial derivative(z) nu perpendicular to N) is the dominant ter m in the vorticity and drives the two current shears that cause vortic ity. Geometrical factors increase (partial derivative(z) nu perpendicu lar to N) by about 13% in the zonal gradient of the meridional current over the meridional gradient of the zonal current. This causes antico rrelation between the former current shear and the vorticity. The two measured shears generally follow each other, have opposing vorticities and large uncertainties. The nighttime current may be irrotational or have constant vorticity. Large current gradients occur in conjunction with observed descents of the F-2 peak height. The gradients are inte rpreted as due to the midnight pressure bulge at low latitudes. Short- period gravity waves of meteorological origin are ruled out as they we re not observed and are Limited in their ability to reach ionospheric heights. The harmonic analysis used to obtain horizontal wind gradient s is largely unaffected by spatially uniform wind accelerations. There fore the deduced spatial variations in the measured winds are unlikely to be due to temporal variations.