Wavelet Analysis for Automatic Detection of Pi-2 Pulsations during Substorm Onset Along the 210$^{\circ}$ Magnetic Meridian

Abstract

Ground Pi-2 pulsations comprise superpositions of various modal components of shear and fast Alfven waves, field line resonance, and plasmaspheric resonances. These complex waveforms, hard to resolve with Fourier transforms are successfully characterized by wavelet techniques. Wavelet detection employs decomposition and reconstruction modes to characterize time-frequency components. Hence, suitable for the examination of the locality and complexity of natural signal patterns. The current study presents the automatic detection of Pi-2 pulsations using Daubechies and Morlet wavelet transforms. In the study, distinct Pi-2 events from CPMN stations along 210${^\circ}$ magnetic meridian were detected. Global Pi-2 pulsations with harmonious H oscillations and discrete D bays in the sub-aurora zone suggest a common source with diverse tunneling paths. Scalograms of Pi-2 undulations of the frequency band of 6.7-22 mHz were observed despite different kinds of Pi-2s. Auroral Pi-2s were highly localized in local time with clear H and D bays, implying magnetospheric-ionospheric current couplings. Latitudinal and longitudinal Pi-2 propagations are exemplified by 180${^\circ}$ phase-shift (polarization) in EWA and group delay in the mid-latitudes of the northern hemisphere. Overall, Pi-2 wave power from high to low latitudes declined with peak amplitudes of 15 nT to less than 1 nT, respectively. Finally, external influences from sea currents causing signal attenuation due to the station's proximity to the sea were also identified. To conclude, the accuracy and efficiency of wavelet analysis with no computation hassle render it a valuable tool for the study of space events in the magnetospheric community.