Does TEC react to a sudden impulse as a whole? The 2015 Saint Patrick's day storm event

We provide insights about how the Total Electron Content (TEC), derived by a network of Global Navigation Satellite System (GNSS) receivers over the European longitudinal sector, reacts to a Sudden Impulse (SI). In particular, we investigate how the different components of the TEC variability relates with the ionospheric currents, derived by ground magnetometers and the Piersanti and Villante (2016) model, during the 2015 Saint Patrick's Day storm. To identify the different TEC components, we apply the Ensemble Empirical Mode Decomposition (EEMD) analysis to vertical TEC (vTEC) time series to isolate its SI-related variations over co-located GNSS receiver-magnetometer couples. With such an approach, we were able to identify for the first time a peculiar vTEC mode (WHT∗, T ≃ 45 min) switching on during the SI and propagating southward with a velocity of ∼1.5 km/s. We also found a linear relationship between the estimated ionospheric current and the temporal fluctuations of WHT∗ for both the Preliminary Impulse (PI) and the Main Impulse (MI). The slope has been found to depend only on the electron charge, on the magnetic permeability and on the altitude of the ionospheric E-layer, as expected from first principles equations and as discussed in details.