Role of unstable sporadic‐E layers in the generation of midlatitude spread F

There is growing experimental evidence to suggest that mesoscale spread F is linked to the occurrence of midlatitude coherent backscatter from patchy sporadic‐ E layers, which are unstable to the gradient‐drift and Farley‐Buneman plasma instabilities. To validate this suggestion, we have compared E ‐region backscatter and spread‐ F ionosonde recordings from about 100 days of joint operation during summer and found a one‐to‐one relation in the occurrence of both phenomena. Also, midlatitude backscatter studies over the last few years have shown the existence of enhanced electric fields inside patchy sporadic E . These are believed to be polarization fields set up locally by neutral winds that transport the plasma patches horizontally, and by the relatively large Hall‐to‐Pedersen conductivity ratios at E ‐region altitudes. Moreover, midlatitude echoes were found to be associated with mostly westward drifting sporadic‐ E patches with typical scale lengths from 10 to more than 100 km and perturbed eastward electric fields from a few to maybe more than 10 to 15 mV/m. We propose that the enhanced polarization fields set up inside unstable sporadic‐ E patches can easily map up the magnetic field lines to the F region and thus contribute to the formation of midlatitude spread F . This new mechanism for spread‐ F generation is basically an image process that can account for key observational properties of the phenomenon. These include the rapid plasma upwelling and the abrupt changes in height (uplifts) of the F layer, as well as the scale sizes involved and morphological characteristics.