Quiet time localization of geomagnetic data for Japanese regions

Geomagnetic data are continuously recorded at observatories during 24 hours a day with particular sampling frequency (1-2 Hz). Despite quiet places are mandatory for international standard observatories, noise may not be avoided due to natural conditions, such as HF signals arising from Sun's activities and ULF ones arising from Earth's activities, as well as anthropogenic sources from human activities during day times. On the other hand, geomagnetic signal processing, particularly for determinations of earthquake precursors needs uncontaminated data which are often regarded as night time values. While many papers using geomagnetic data have selected their own time span, actually no obvious limits have been commonly determined for the quietest time so far. Hence each region are necessary have their unique time span, since differences of socio-cultural and human behaviors over the world. This paper applies wavelet transformation to determine the quietest time range of geomagnetic signal processing for Japanese regions by de-noising of declination (the most sensitive geomagnetic values) of three INTERMAGNET observatories (i.e. Kakioka (KAK), Kanoya (KNY), and Memambetsu (MMB)). Comprehensive evaluations which are taken over selected days during ten years (2001-2010) resulted in the quietest time span of 21:48:00-00:53:00 at the Local Time. These results are in line with the time used by Hayakawa et al. (1996), e.g. 22:00:00- 02:00:00 LT, which are mostly referred by many researchers.