Midlatitude ionospheric D region: height, sharpness and solar zenith angle

VLF radio amplitude and phase measurements are used to find the height and sharpness of the D region of the ionosphere at a mid- to high geomagnetic dip latitude of ~52.5°. The two paths used are both from the 23.4 kHz transmitter, DHO, in north Germany with the first path being northwards and mainly over the sea along the west coast of Denmark over a range of ~320-425 km, and the second, also mainly all-sea, to a single fixed recording receiver at Eskdalemuir in Scotland (~750 km). From plots of the measured amplitudes and phases versus distance for the first of these paths compared with calculations using the US Navy code, ModeFinder, the Wait height and sharpness parameters of the D region at midday in summer 2015 are found to be H’ = 72.8 ±0.2 km and ß = 0.345 ±0.015 km-1 at a solar zenith angle ~33°. From phase and amplitude measurements at other times of day on the second path, the daytime changes in H’ and ß as functions of solar zenith angle are determined from shortly after dawn to shortly before dusk. Comparisons are also made between the modal ModeFinder calculations and wave hop calculations, with both giving similar results. The parameters found here should be useful in understanding energy inputs to the D region from the radiation belts, solar flares or transient luminous events. The midday values may be sufficiently precise to be useful for monitoring climate change.