Modeling the Arecibo nighttime F2 layer: 2. Ionospheric gradients

Abstract

The servo model is extended and used to fit horizontal gradients in the F2 layer height and density and to estimate the zonal Pedersen current and its zonal and meridional gradient. Horizontal gradients were measured 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. The model gradients are driven by nonzero current gradients, which are applied as needed to fit the measured gradients in the F2 peak. The gradient 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 divergence of the model zonal current. Expressions are derived for the divergence of the Hall current and for the curl of the current in the presence 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 neutral and plasma motions generate current vorticity equally as expected from the F region dynamo. The measured velocity gradients produce more current gradients and vorticity than the measured conductance gradients. The measured height gradient in the perpendicular‐north plasma drift (∂zυ⊥N) is the dominant term in the vorticity and drives the two current shears that cause vorticity. Geometrical factors increase (∂zυ⊥N) by about 13% in the zonal gradient of the meridional current over the meridional gradient of the zonal current. This causes anticorrelation 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 F2 peak height. The gradients are interpreted as due to the midnight pressure bulge at low latitudes. Short‐period gravity waves of meteorological origin are ruled out as they were not observed and are limited in their ability to reach ionospheric heights. The harmonic analysis used to obtain horizontal wind gradients is largely unaffected by spatially uniform wind accelerations. Therefore the deduced spatial variations in the measured winds are unlikely to be due to temporal variations.