Quiet night Arctic ionospheric D region characteristics

VLF radio propagation recordings are used to determine the characteristics of the nighttime polar lower D region of the ionosphere. Recordings of both VLF phase and amplitude in the Arctic on days within ∼1‐2 weeks of the equinoxes enable their day‐to‐night changes to be determined. These changes are then combined with previously measured daytime polar D region characteristics to find the nighttime characteristics. The previously determined daytime characteristics were measured in the Arctic summer; the NRLMSISE atmosphere model is used to help determine the height change from daytime summer to daytime equinox (∼5 km lower). The principal path used was from the 16.4 kHz Norwegian transmitter JXN (67°N, 14°E) 1334 km northwards across the Arctic Ocean to Ny‐Ålesund (79°N, 12°E), Svalbard. Also used were the 2014‐km path from NRK (37.5 kHz, Grindavik, 64°N, Iceland) to Ny‐Ålesund, the 1655‐km path from JXN to Reykjavik (64°N, Iceland) and the 5302‐km path from JXN across the Arctic Ocean to Fairbanks (65°N) in Alaska. The night values of (the Wait parameters) H' and β were found to average from ∼79 km at equinox down to 77 km near winter solstice (lower than the 85 km at low and mid‐latitudes by ∼7 km) and 0.6 km‐1 respectively. This lower height and its variability are shown to be consistent with the principal source of ionization being energetic electron precipitation.