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 (unpublished) have analysed the Alouette-I data and studied various aspects of the anomaly, and Eccles and King³ have summarized the results of analyses of equatorial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.