Influence of environmental change near the atmospheric electric field observation hut at Kakioka - numerical calculations to estimate the effect of a nearby bamboo grove-
Nagamachi, S., Toya, T. & Yoshitake, Y.
The strength of the atmospheric electric field observed at Kakioka Magnetic Observatory has decreased since 1990. To investigate whether this decrease is related to the growth of a bamboo grove that has encroached to the observatory, we cut a 12 m×20 m area of the grove to ground level between August 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 before cutting. 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 the atmospheric 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 growth of the bamboo grove and global atmospheric changes.