HHT Analysis on ELF-Emission Events Below 10Hz Observed at Lulin ELF Station

Abstract

The ELF magnetic field measurement system in Taiwan was setup by the ISUAL team of National Cheng Kung University at Lulin Observatory managed by National Central University. This system measures magnetic fields at the band of extreme low frequency (ELF). In previous studies, through analyzing the signals using Fast Fourier Transformation (FFT), special emission events with frequency shifts below 10Hz were found to occur sometimes on the frequency-time spectrograms. That is to say, the frequency of emissions with higher intensities can vary from lower-frequency to higher-frequency, or from higher-frequency to lower-frequency.

In this study, other than the method of Fourier analysis, we adopt the Hilbert-Huang Transform (HHT) as the tool to analyze these measured ELF magnetic field signals. The variations of emission frequencies are explored based on data with different time-scales. The characteristics of emission frequencies at frequency-time spectrograms acquired from HHT are also investigated for the time periods defined by the frequency-shift events shown on the time-frequency spectrogram based on FFT.

We analyzed one-year-long data measured from October 2009 to September 2010 and find there do exist two emission bands with enhanced intensities. These emission-bands cover wider ranges in frequencies compared with those derived from FFT. For most frequency-up events, the standard deviations in emission frequencies are found to decrease at the time after their frequencies shift up. Conversely, for most frequency-down events, the standard deviations become larger at the time after their

frequencies shift down. in frequency decreased case. The above features occurred no matter in geomagnetic east-west or north-south directions, but more often in east-west direction. Therefore, from the analysis through HHT, these measured signals are also found to have variations in emission frequencies below 10Hz.

Date of Award	2014
Original language	English
Supervisor	Bing-Chih Chen (Supervisor)