What Drove the GICs >10 A During the 17 March 2013 Event at Mäntsälä? A Novel Framework for Distinguishing the Magnetospheric Sources.

We combine wavelet analysis and data fusion to investigate geomagnetically induced currents (GICs) on the Mäntsälä pipeline and the associated horizontal geomagnetic field, BH, variations during the late main phase of the 17 March 2013 geomagnetic storm. The wavelet analysis decomposes the GIC and BH signals at increasing "scales" to show distinct multi‐minute spectral features around the GIC spikes. Four GIC spikes >10 A occurred while the pipeline was in the dusk sector—the first sine‐wave‐like spike at ∼16 UT was "compound." It was followed by three "self‐similar" spikes 2 hr later. The contemporaneous multi‐resolution observations from ground‐(magnetometer, SuperMAG, SuperDARN), and space‐based (AMPERE, Two Wide‐Angle Imaging Neutral‐atom Spectrometers) platforms capture multi‐scale activity to reveal two magnetospheric modes causing the spikes. The GIC at ∼16 UT occurred in two parts with the negative spike associated with a transient sub‐auroral eastward electrojet that closed a developing partial ring current loop, whereas the positive spike developed with the arrival of the associated mesoscale flow‐channel in the auroral zone. The three spikes between 18 and 19 UT were due to bursty bulk flows (BBFs). We attribute all spikes to flow‐channel injections (substorms) of varying scales. We use previously published MHD simulations of the event to substantiate our conclusions, given the dearth of timely in‐situ satellite observations. Our results show that multi‐scale magnetosphere‐ionosphere activity that drives GICs can be understood using multi‐resolution analysis. This new framework of combining wavelet analysis with multi‐platform observations opens a research avenue for GIC investigations and other space weather impacts. Plain Language Summary: Geomagnetically Induced Currents (GIC) are produced by complex interaction between the Earth's magnetic field and geological composition during intense geomagnetic storms. These two parameters are often related in frequency domain. In this paper, we analyze the GIC signal from the Finnish natural gas pipeline recorded at Mäntsälä during the 17 March 2013 geomagnetic storm. Four spikes >10 A were recorded between 4:30 and 9:00 p.m. local time. We use wavelet analysis to learn about the frequencies of GIC spikes and then systematically investigate the observations from ground to space (ground‐up approach) to learn what links activity in space to the GICs. Wavelet analysis highlights areas ranging from <1 min to >30 min, which indicates that higher frequency fluctuations are accompanied with longer duration disturbance. Multi‐platform observations help us interpret the physical meaning of the multi‐minute (or multi‐scale) area in the wavelet plot. We find that multi‐scale activity in the magnetosphere and ionosphere, created by fast earthward‐flowing particles (magnetotail mesoscale plasma flows), ultimately drove the significant GIC spikes. This new perspective enabled us to link the magnetospheric activity to GICs through observations and previously published simulations and pave a path for future research. Key Points: Wavelet analysis of GICs at Mäntsälä on 17 March 2013 reveals two features—Pi1/Pi2 pulsations superposed on longer duration disturbancesWavelet decomposition of the GIC and BH signals is consistent with multi‐scale magnetosphere‐ionosphere activity around GIC spikesPi2 pulsations and data fusion suggest mesoscale flow channels (substorm injections) were the underlying cause of four GICs >10 A [ABSTRACT FROM AUTHOR]