A comparison of the ionospheric equatorial anomaly in the East Asian and the American regions at Sunspot minimum

Abstract

Using ƒoF2 and M(3000)F2 data, taken at stations spread over narrow longitude ranges and giving a good coverage of dip location near the magnetic equator, a comparison has been undertaken of the diurnal variations of the ionospheric equatorial anomaly at different seasons in East Asia and America. The very significant differences observed have been explained in terms of (1) the relative inferred locations of the thermal pressure bulges with respect to the magnetic equator, (2) magnetic declination effects (only significant at high latitudes in South America) and (3) the greater magnetic field in Asia. For both regions maximum symmetry of the equatorial anomaly crests has been observed in the particular month for which the midday overhead sun occurred near the magnetic equator. The equatorial anomaly was found to persist later at the solstices in America but at equinox it persisted until 0300–0400 LT in both regions. Some re-generation of the anomaly seems to take place after sunset but in addition it appears that poleward winds from the so-called ‘midnight pressure bulge’ converge with the winds from the poles thereby delaying the decay of the anomaly crests.