A study of the equatorial anomaly at American longitudes during sunspot minimum

Abstract

A detailed study of the equatorial anomaly, during low sunspot years (October 1962–December 1964), using ƒoF2 data obtained in the American sector by means of the topsidesounder-satellite Alouette 1 shows the seasonal differences in the form and development of the anomaly. The equatorial anomaly becomes observable at the same early hours (0800–1000 hr local time) in the American sector as it does in the Asian sector. The anomaly disappears earlier during the evening hours in D-months (November, December, January and February) as compared to E-months (March, April, September and October) or J-months (May, June, July and August). Data, regarding the position of anomaly crests in both northern and southern hemispheres at different times of the day, are given for each seasonal group separately. Asymmetries in the form of the anomaly generally occur near solstitial months; the winter crest of the anomaly being larger than the summer one in the daytime. At night the sense of the asymmetry is reversed. The diurnal development of the asymmetrical anomaly is shown to be consistent with the hypothesis of a neutral wind blowing across the magnetic equator from the summer to the winter hemisphere. During geomagnetic disturbances, the equatorial anomaly either gets reduced in magnitude or disappears completely. The weakening of the strength of the equatorial electrojet during disturbed conditions is a possible cause of this effect.