Stable reduction to the pole at the magnetic equator

TLDR: In this paper, the Fourier transform of the RTP field is constructed as a model that is maximally smooth and, at the same time, has a power-spectral decay common to all fields produced by the same source.

Abstract: The solution of reduction to the pole (RTP) of magnetic data in the wavenumber domain faces a long standing difficulty of instability when the observed data are acquired at low magnetic latitudes or at the equator. We develop a solution to this problem that allows stable reconstruction of the RTP field with a high fidelity even at the magnetic equator. The solution is obtained by inverting the Fourier transform of the observed magnetic data in the wavenumber domain with explicit regularization. The degree of regularization is chosen according to the estimated error level in the data. The Fourier transform of the RTP field is constructed as a model that is maximally smooth and, at the same time, has a power-spectral decay common to all fields produced by the same source. The applied regularization alleviates the singularity associated with the wavenumber-domain RTP operator, and the imposed power spectral decay ensures that the constructed RTP field has the correct spectral content. As a result, the algorithm can perform the reduction to the pole stably at any magnetic latitude, and the constructed RTP field yields a good representation of the true field at the pole even when the reduction is carried out at the equator.