Wavelet Transform of GIC and Magnetic Field Signal during March 17, 2013, Event - a Time-Frequency Perspective into the Geomagnetic Storm.

We expand the analysis of ground magnetic data into the spectral domain to study Field Aligned Currents (FACs) associated perturbations at the poleward boundary of the auroral zone during the late main phase of the 17 March 2013 event. Wavelet analysis sheds light on the frequencies or periodicities within the ground-based signal that we assert can be traced to mesoscale disturbances in the magnetosphere. The analysis also provides insights into the nuances of the storm as seen at specific locations in the context of GICs which is the last link of the space weather chain. We perform a wavelet analysis of the Nurmijärvi (NUR) geomagnetic field data and the Geomagnetically Induced Current (GIC) signal at the nearby Mantsala pipeline station to compare their spectra for the event. We discuss the results in the context of what has previously been published and suggest a ground-to-magnetosphere picture of the event. Strong geomagnetic storms tend to have global and local effects. This study looks a local effects that tend to appear in the post-dusk region during strong storms. We use Continuous Wavelet transform (CWT), to identify periodicities associated with the signals and Wavelet cross-spectrum (XWT), and Wavelet coherence (WTC) to detect common time-localized oscillations and the distribution of power of two signals along with relative lag between the two-time series.[Xu et al, 2022; Torrence et al. 1998]

The GIC data is gathered from the Finnish Pipeline and the recommended magnetometer (NUR) data is taken from the IMAGE magnetometer array. IMAGE 2-D equivalent current shows the ionospheric current over NUR and Mantsala location. SuperMAG shows the global nature of the auroral electrojet and DMSP, AMPERE, and SuperDARN provide insights into the input from the magnetosphere. OMNIweb is used to describe the influence of the solar wind. Four GIC spikes > 10 A are recorded at 15:56, 18:04, 18:28, 18:45 UT. During this time interval, Mantsala is in the dusk region (18-20 MLT). A vortex feature can be seen in IMAGE array data [Belkhovsky, 2019], SuperMAG, SuperDARN, and AMPERE plots during these spikes between 1800-1900 UT. Keograms of AMPERE FACs during 15:00-19:00 UT show splitting of Region1 current systems which temporally correlates to the four spikes. Movie maps of SuperMAG show rapidly forming, high-intensity vortices, which can also be seen in SuperDARN convection maps between 1800-1830 UT. We propose that this vortex structure has its roots in the magnetosphere during the formation of a duskside double-auroral oval [Ohtani, 2012]. Although the storm has been extensively studied in the past from different perspectives, in this paper, we found that a line of inquiry generated by frequency analysis led to the discovery of this vortex structure that temporally correlates with the GIC spikes, The ground-up approach, with a novel perspective using the wavelet analysis, not only brings those perspectives together and forms a more complete picture of the storm, but also proves the importance of the wavelet analysis as tool for understanding the nature and drivers of GICs.