Effects of notch-filtering on the ELF sferics and the physical parameters

For ELF stations using hardware notch-filters to suppress power-grid emissions, the amplitude and the phase of the recorded sferics will inevitably be distorted. The phase shift causes the event time to move and hinders cross-comparing of events between different platforms. Notch-filtering also create fictitious dips in the current moment spectrum of the ELF sferics that lead to error in deducing the charge moment change (CMC) of the source discharge. To alleviate the effects of notch-filtering, we use an elementary signal processing method to reconstruct the source signals and use lab-generated ELF-like signals to check the limitations of the reconstruction. The results indicate that the delay of signals due to the notch-filtering module can be reliably restored, while the amplitude is less; but the reconstruction does restore the low frequency components of the sferics that are important for the CMC determination. Using the corrected event time, 20 associated sprites recorded by the ISUAL/FORMOSAT-2 experiment during June to September of 2008 were found. Comparing with the notch-filtered sferics, the reconstructed sferics are found to increase the current moment amplitude Iods by 69 ± 55%, reduce the time constant τ by 52 ± 15%, and lowers the CMC by 22 ± 21%, respectively. From the linear-correlation of the sprite brightness and the CMC of the sprite-producing positive lightning, a threshold CMC of ∼900 C-km for sprite initiation is inferred based on the reconstructed sferic data; this threshold value is also 25% lower than that inferred using the notch-filtered sferics.