Including the Temporal Dimension in the SECS Technique.

The equivalent source method of Spherical Elementary Current Systems (SECS) has contributed valuable results for spatial magnetic interpolation purposes where no observations are available, as well as for modeling equivalent currents both in the ionosphere and in the subsurface, thus providing a separation between external and internal sources. It has been successfully applied to numerous Space Weather (SW) events, whereas some advantages have been reported over other techniques such as Fourier or Spherical (Cap) Harmonic Analysis. Although different modalities of SECS exist (either 1‐D, 2‐D, or 3‐D) depending on the number of space dimensions involved, the method provides a sequence of instantaneous pictures of the source current. We present an extension of SECS consisting in the introduction of a temporal dependence in the formulation based on a cubic B‐splines expansion. The technique thus adds one dimension, becoming 4‐D in general (e.g., 3‐D + t), and its application is envisaged for, though not restricted to, the analysis of past events including heterogeneous geomagnetic data sets, such as those containing gaps, different sampling rates or diverse data sources. A synthetic model based on the SW Modeling Framework is used to show the efficacy of the extended scheme. We apply this method to characterize the current systems of past and significant SW events producing geomagnetically induced currents, which we exemplify with an outstanding geomagnetic sudden commencement occurred on 24 March 1991. Plain Language Summary: Spherical Elementary Current Systems is a mathematical technique that allocates an electrical current system as the source of the geomagnetic perturbations typically recorded by ground magnetometers. Such a current system is assumed to flow on a sheet at a certain height/depth from the surface. Traditionally, Spherical Elementary Current Systems is a purely spatial technique, which is appropriate if we rely on homogeneous time series at each observation site. Otherwise, the information is typically concentrated at certain timestamps and sparse at others, resulting in uneven source currents being modeled. We have extended the traditional technique by including a time expansion, thus allowing spatial information to be shared across time. This produces a smoother, more realistic equivalent source current, as shown by comparing both techniques with the results of a synthetic model. We have also applied the extended method to an outstanding geomagnetic sudden commencement occurred on 24 March 1991. Key Points: The technique of Spherical Elementary Current Systems has been extended by including a temporal dependence based on cubic B‐splinesImprovement is achieved when dealing with heterogeneous data sets consisting of geomagnetic ground data sampled at diverse ratesThe method can be used to characterize the equivalent current systems of past and significant Space Weather events [ABSTRACT FROM AUTHOR]