Hilbert-Huang transform and S-transform of geomagnetic pulsations at auroral expansion onset

The waveform of geomagnetic pulsations at auroral expansion onset looks irregular and is hardly resolved by Fourier transform. Here we perform a novel analysis of the Hilbert-Huang Transform (HHT) to address this problem, focusing on the event investigated in detail by Morioka et al. [2008], in which the AKR (auroral kilometric radiation) breakup was clearly identified. From the HHT analysis of high-latitude search-coil ground magnetometer data, Pi1, Pc3, and Pi2 pulsations are extracted as the first, second, and third intrinsic mode functions, respectively. Amplification of the Pi1 and Pc3 pulsations is first detected as a clear precursor to the AKR breakup. The Pi1 and Pc3 pulsations show sudden enhancement at the AKR breakup. We suggest that the HHT is capable of automatically extracting the Pi1, Pi2, and Pc3 from the irregular high-latitude geomagnetic pulsations, providing a new type of diagnostic tools for understanding the onset mechanism of auroral substorms. A comprehensive time-frequency spectral view is obtained from the instantaneous frequency, especially when complemented with the S-transform, and the instantaneous frequency provides a new objective criterion to identify the type of geomagnetic pulsations such as Pi and Pc. It would be useful to apply the HHT to all the available both ground- and space-based magnetic datasets across all local times and latitudes for diagnosing the wave activities and underlying physics associated with the substorm onset.