It is shown, for instance, in Loomis's auroral data, which are based on observations at a variety of European and American stations (Ency.
Sunlight is not the only disturbing cause in estimates of auroral frequency.
At Cape Thorsden diffused auroral light had percentages e.
- The frequency of auroral displays is much greater in some years than others.
The auroral data are from Table E of Tromholt's catalogue (5), with certain modifications.