Anomalous ƒoF2variations in the Antarctic

Abstract

The daytime minimum in the F2-layer critical frequency, which is often observed in the Antarctic, is explained on the basis of a vertical shear in the horizontal wind (neutral air) and the resulting movement of charged particles in the presence of the earth's magnetic field. The diurnal variation of the F2-layer critical frequency, as expressed by the noon minus the midnight value, D, is normally positive, but in the case of the daytime minimum is negative. For a given month, the value of D was found to depend upon both the geographic latitude and geomagnetic dip. The D values from the northern hemisphere were compared with the D values of the southern hemisphere for a time separation of 6 months. The comparison gave good agreement in the region of the graphs where there were stations for both hemispheres. (Owing to the asymmetry of the geomagnetic field, several stations in the southern hemisphere are found to have no corresponding stations in the northern hemisphere on the basis of geographic latitude and geomagnetic dip.) It was also noted that the negative D values occurred almost entirely in the southern hemisphere and in that region where there are no corresponding northern-hemisphere stations. A three-level model of the F region was developed for the purpose of demonstrating the effect of a vertical wind shear on the electron concentration. The results of electron-density computations for several cases showed that the characteristic F2-layer critical-frequency variations can be explained on the basis of a wind shear of the order of 25 to 50 m/s per 100 km. The wind shear can account for most of the ‘anomalous’ behavior in the Antarctic.