Abstract
The ultra low frequency (ULF) electromagnetic (EM) wave activity usually recorded on Earth’s ground has been found to depend on various types of space weather. In addition ULF waves observed before an earthquake have been hypothesized to be a result of geotectonic processes. In this study we elaborate for the first time the origin of sub-ULF (<1 msec) magnetic field waves before an earthquake (Chi-Chi/Taiwan, 20.9.1999) by comparing simultaneously obtained measurements in the interplanetary space (ACE satellite) and on the Earth’s ground (Taiwan). The most striking result of our data analysis, during a period of 7 weeks, is that the detection of four groups of sub-ULF waves in Taiwan coincide in time with the quasi-periodic detection of two solar wind streams by the satellite ACE with approximately the solar rotation period (∼28 days). The high speed solar wind streams (HSSs) in the interplanetary space were accompanied by sub-ULF Alfvén wave activity, quasi-periodic southward IMF and solar wind density perturbations, which are known as triggering agents of magnetic storm activity. The four HSSs were followed by long lasting decreases in the magnetic field in Taiwan. The whole data set examined in this study strongly suggest that the subULF magnetic field waves observed in Taiwan before the Chi-Chi 1999 earthquake is a normal consequence of the incident of HSSs to the magnetosphere. We provide some observational evidence that the sub-ULF electromagnetic radiation on the Earth was most probably a partner to (not a result of) geotectonic processes preparing the Taiwan 1999 earthquake.
Highlights
The Origin of Terrestrial Ultra Low Frequency Electromagnetic Waves and the Taiwan Chi-Chi 7.7Mw 1999 EarthquakeUltra low frequency (ULF) waves observed by terrestrial observatories is a matter of scientific research of great interest
It is obvious that the two corotating interaction regions (CIRs), which are associated in time with high solar wind speeds, strong magnetic storm activity and the ultra low frequency (ULF) wave groups II and IV separated by a period of ∼28 days, before the Chi-Chi EQ, strongly suggest that the two ULF magnetic field wave groups II and IV observed in Taiwan were caused by the incidence of a CIR on the Earth’s magnetosphere (Kivelson and Russell, 1995; Fraser, 2009; Richardson, 2018)
Such solar conditions allow high solar wind speed streams to emanate from coronal holes and form structures rotating in the interplanetary space with the ∼27-days period of solar rotation
Summary
Ultra low frequency (ULF) waves observed by terrestrial observatories is a matter of scientific research of great interest. Given that there exist several reports on the existence of ULF EM waves before the Chi-Chi earthquake, in this study we investigate the hypothesis that the ULF geomagnetic fluctuations in certain frequency bands may be originated from solar wind. To this end we reasonably address the question whether sub-ULF geomagnetic anomalies are not the result of the microseismicity observed for ∼7 seven weeks before the Chi-Chi earthquake, but an agent of micro-earthquakes. We compare space and terrestrial observations in order to identify the origin of the sub-ULF (
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