A 27.3‐day period in the aurora borealis

Abstract

In the course of a general study of the frequency of occurrences of aurora in Scotland during the years 1858–1938 a well‐marked period of length about 27.3 days was found to exist. Commander W.R. Priston has found a similar period in other meteorological elements, and it was his work (not yet published) which prompted the present investigation. The period is more than the well known 27‐day recurrence observed in many magnetic and associated phenomena, since that usually continues for only a few solar rotations, whereas this period has persisted for 80 years. The data used in this investigation were simply the dates on which aurora was seen from one or more of the meteorological stations in Scotland, including Orkney and Shetland. From 1858 to 1919 these are available in the Journal of the Scottish Meteorological Society and since 1924 they have been included in the Lerwick section of the Observatories' Year Book. For the intervening years the Reports of the Registrar General for Scotland were consulted, the required information being included in the meteorological notes appended thereto. In a few cases, chiefly in the earlier years, the original climatological returns have been referred to. This was necessary when the published information was indefinite, for example “Aurora was seen on nearly every night in October.” There were 5224 nights on which aurora was seen, an average of 66 nights per year and 5.1 per 27.3‐day period. The number of nights varied from 161 in the year July 1869 to June 1870 to five in 1912–1913.