The quiet sunrise E region: Enhancements at high latitudes in winter due to increased nitric oxide

E region electron concentrations derived from European Incoherent Scatter Facility observations at 70°N were found to be much larger than normal for high solar zenith angles during 23 of 32 days in winter. Electron concentrations computed with the Keneshea code are compatible with the observations if nitric oxide concentrations are about 109 cm−3 at 100‐110 km. For these altitudes near ground sunrise, it is shown that numerically, the nitric oxide concentration approximately equals the square of the electron concentration.