Day-time F region echoes observed by the São Luís radar

This brief report presents unique day-time F region coherent scatter echoes observed by the 30MHz radar over São Luís in eastern Brazil. The radar observation was part of the July 2011 radar campaign designed to detect echoes from day-time F region irregularities. These rare day-time F region radar echoes, scattered from short wavelength plasma irregularities, have quasi patchy features. They are characterized by large upward Doppler velocities (∼100–200m/s). They are very weak (by more than 10dB) when compared to radar echoes observed over São Luís scattered from the night-time F plasma irregularities known as equatorial spread F. Simultaneous GPS vertical total electron content (TEC), and F2 peak frequency (f○F2) and F2 peak height (hmF2) estimated from ionosonde measurements over São Luís have provided additional evidence that the received day-time F region radar echoes were from genuine ionospheric scatterers. This report suggests physical explanations for the day-time radar echoes in terms of the generalized Rayleigh–Taylor instability mechanism operating on large magnetic declination (∼20° westward) of the geomagnetic field geometry in São Luís, conductivity distribution in the winter southern hemisphere and summer northern hemisphere, and the resulting magnetic flux tube electric field configurations.