A critique of methods of determining the position of the focus of the Sq current system

[1] We examine three methods of determining the latitude of the focus of the Sq current system using data from an array of more than 50 magnetometers operating on the Australian mainland from November 1989 to July 1990. The magnetometer array enables the location of the Sq focus with more certainty than usual. Chains of stations within the array are then used to test the various methods of determining the focus latitude. The most accurate method was that which first determined the time when the magnetic eastward component ΔY passes through zero and then used northward magnetic components ΔX at that time, in a least squares linear fit, to find that latitude at which ΔX went to zero. This was found preferable to an alternative method which uses maximum and minimum values of ΔX. The reliability of these methods was examined on days with some magnetic disturbance present and on days when the focus latitude lay outside the chain of stations being used. The third method uses a principal component analysis to determine eigenvector elements for the daily variations of each station in the chain and fits the associated coefficients of the eigenvector elements to a straight line. This method had several shortcomings, especially if the procedure for determining the focus position were to be automated. For all methods the value of the correlation coefficient associated with the linear fit gave a good indication of the reliability of the estimation.