Abnormal Latitudinal Distributions of the F2 Layer Critical Frequency

Abstract

THIS letter deals with the unusual behaviour of the latitudinal distribution of NmF2 obtained from topside sounder iono-grams recorded at low latitudes. The anomalous decrease in the critical frequency at the dip equator compared with that at middle latitudes is well known as the “geomagnetic anomaly”1,2. The minimum at the equator is accompanied by crests near ±25°−30° magnetic dip. Many observers have noted that this behaviour is a daytime phenomenon for geomagnetically quiet conditions and for all levels of solar activity. King et al. (unpublished) have analysed the Alouette-I data and studied various aspects of the anomaly and Eccles and King3 have summarized the results of analyses of equatorial ionospheric data carried out by various workers. In the investigations reported in this letter, latitudinal variations of JFoF2* using Alouette-I satellite data are studied and some abnormalities contrary to earlier observations are noted. We have found a few cases where the equatorial anomaly is absent near local noon. These occurred during quiet geomagnetic conditions at a period of low solar activity. We shall discuss two such abnormal cases with respect to their morphological peculiarities only; a detailed study of this behaviour will be presented in a later publication.