Seasonal changes in day‐to‐day variability of upper air winds near the 100‐km level of the atmosphere

Abstract

The dynamo theory of the magnetic daily variation predicts that winds flowing within the E region at a height of roughly 100 km generate a world‐wide electric current system at this level. The electric currents are generated mainly in middle and high latitudes by the horizontal motion of electrically conducting air across the radial component of the geomagnetic field. Near the equator the geomagnetic field is horizontal so that according to the theory the current above the equator must result from an electrostatic field directed from west to east and overhead near noon, closing the current circuit. The theory also implies that this horizontal electric field must extend to the highly rarefied F region, which will be lifted bodily upwards in response to the crossed electric and magnetic field. Hence day‐to‐day differences in middle latitude winds may be reflected, according to the theory, in day‐to‐day differences, in heights, and hence in ion‐densities of the F region, measured by radio methods.The day‐to‐day differences in ionospheric F region critical frequencies near noon are studied as a measure of corresponding differences in the wind‐generated ionospheric electric field at the magnetic equator. These changes in F region critical frequencies reflect corresponding changes in height of the F region, caused by the electric field of the daily variation. It is shown that the daily variation, though proportional to both the electric field and the electric conductivity, is probably a good indicator of day‐to‐day changes in wind speeds in the E region. The data do not support the view that the ionizing solar radiation fluctuates much from one day to the next. This is in accord also with statistical results obtained by Forbush for solar flares and their effects upon the daily variation. The day‐to‐day changes in wind speed indicated for the E region in middle latitudes are found to be largest in northern winter, and least in northern summer, and are discussed in relation to evidences of variability dependent on season noted from meteorology for tropospheric winds. Changes in wind speed with sunspot cycle are also discussed.The average magnitude of the daily magnetic variation shows a high correlation with sunspots (as high as 0.99 for yearly averages). Thus, having eliminated electric conductivity of the E region as an important cause of the day‐to‐day variability in the magnetic variation, the wind speed in the E region must be able to change by a factor of two or more from one day to the next, as a consequence of solar events.