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.