Determining riometer quiet day curves - 2. Derivation of filter cutoff frequencies

[1] A method for the determination of riometer quiet day curves which is based on the filtering of a discrete two-dimensional function or matrix has been proposed. The two dimensions of the matrix are siderial time and day number, with each row of the matrix representing one siderial day. The filtering is done in the Fourier domain using a low-pass filter. With a low-pass filter the low frequency Fourier coefficients are retained, while the high frequency Fourier coefficients are discarded. The highest Fourier coefficient that is retained is the cutoff frequency of the filter. In this paper the filter cutoff frequencies are derived directly from the antenna reception pattern as well as from the data itself. A number of methods can be used to infer a cutoff frequency from the data; however, only a number of entropy based methods are investigated in this paper.