Geomagnetic oscillations at middle latitudes: Part I. The observational data

Oscillations are defined as comprising only the more regular of the short-period geomagnetic fluctuations (periods from 1 to 200 seconds) and may be recognized as representing electromagnetic energy in narrow frequency bandwidths and identified by objective criteria. The characteristics of these oscillations, as indicated by the available observational data, are examined with a view toward the eventual identification of their resonant sources. Three distinct frequency ‘bands’ are observed. Two of them, a 20-sec band (that is, oscillations having similar characteristics and having periods centered at about 20 sec) and a 70-sec band, are predominantly daytime phenomena, whereas the third, an 8-sec band, occurs at night. The 8-sec oscillations show a strong positive correlation, both in amplitude and duration, with the K index; this correlation is less pronounced for the 20-sec band and is absent (or perhaps negative) for the 70-sec band. An additional nighttime band containing periods longer than about 95 sec is not yet definitely established.