Geomagnetic daily variations are produced at a height of about 100 to 130 km due to dynamo action of thermospheric winds in combination with the ionospheric conductivity. They are influxed by longterm variability of thermospheric dynamics and are used as tool for investigating a QBO in thermospheric winds.

Power spectrum analysis has been applied to monthly means of the daily range of the geomagnetic field. In addition to an annual modulation (and subharmonics) there is clear evidence for a sharp peak at about 27 months in the H-component at low-latitude observatories (ΔH≈0.5 nT). At single observatories, statistical significance is at the 99% confidence level, squared coherency between distant observatories is highly significant, too. However, there is no global modulation of the Sq current system because (1) the D-component contain no 27-month periodicity, and (2) the oscillation is not present at mid-latitude observatories.

In order to explain the observations, a 27-month oscillation in the low-latitude prevailing zonal winds with an amplitude of few m/s at a height of about 100--130 km is postulated. Squared Coherency between geomagnetic variations and prevailing winds in the equatorial stratosphere is significant at the 95%-level. ¿ American Geophysical Union 1994