Shingo NAGAMACHI, Takeshi TOYA and Yuki YOSHIKAKE
Abstract
The strength of the atmospheric electric field observed at Kakioka Magnetic Observatory hasdecreased since 1990. To investigate whether this decrease is related to the growth of a bamboo grovethat has encroached to the observatory, we cut a 12 m~20 m area of the grove to ground level betweenAugust and October 2009. We numerically simulated the atmospheric electric field and determined coefficients of in situ calibration for three boundary conditions: (1) no bamboo grove, (2) before cutting,and (3) after cutting. Case (1) gave a coefficient of in situ calibration of 1.26, which is comparable to the value of 1.3 that has been used since observations commenced at Kakioka; according to old photos,there was no bamboo grove at that time. For case (2) the coefficient was 2.52 and for case (3) it was 2.17. The observed electric field after cutting the bamboo was about 20% higher than that beforecutting. Furthermore, the observed electric field before cutting the bamboo was about 50% lower than that originally recorded before the bamboo grove had developed. These results and comparison to theatmospheric electric field observed at Memambetsu observatory suggest that the recent decreasing trend of the strength of the observed atmospheric electric field at Kakioka is a response to both growthof the bamboo grove and global atmospheric changes.
Received 11 January 2012; accepted 14 September 2012
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