12:15 〜 12:30
[PEM36-13] 地磁気日変化に見られる超高層大気の長期変動
キーワード：地磁気日変化, 太陽活動, 長期変動, 地磁気永年変化, 電離圏電気伝導度, 地球温暖化
It has been well-known that the geomagnetic field on the ground shows a regular variation with a fundamental period of 24 hours during a solar quiet day. This daily variation depends on local time, latitude, season and solar cycle and has been called solar quiet (Sq) geomagnetic field daily variation. The Sq variation is mainly produced by magnetic effects due to ionospheric currents flowing in the E region of the ionosphere around 105 km. The global pattern of the Sq variation of the H-component shows positive and negative changes in the equatorial and middle-latitude regions around noon, respectively. The Sq current system expected from the geomagnetic field perturbations consists of two large current vortices: one is an anticlockwise current in the northern hemisphere and the other is a clockwise current in the southern hemisphere. The Sq current is dominant in the daytime ionosphere where ionospheric conductivity is relatively large, and is driven by electric fields originating from the ionospheric dynamo via the interaction between ionized and neutral particles. According to the Ohm's law, the main variables in the Sq amplitude are the ionospheric conductivity, the polarization electric field, the solar diurnal tide, and the intensity of the ambient magnetic field at the E-region height. Then, to investigate the long-term variation in the Sq amplitude is important for understanding the physical mechanism of long-term variation in the upper atmosphere related to solar activity and lower atmospheric change such as global warming. In this study, we investigated long-term variation in the Sq amplitude using 1-hour geomagnetic field data obtained from 184 geomagnetic observation stations within a period of 1947-2012 in order to clarify the physical mechanism of long-term variation in the upper atmosphere. For the analysis of long-term observation data obtained from a lot of geomagnetic stations, we took advantage of the IUGONET data analysis system (metadata database search system and data analysis software). The Sq amplitude is defined as a difference of the H-component of geomagnetic field between the maximum and minimum values each solar quiet day. We identified the solar quiet day as the day when the maximum Kp value is less than 4 for each day. As a result, the Sq amplitude observed at all the geomagnetic stations showed a clear dependence on the 11-year solar activity and it tended to be enhanced significantly during solar maximum. The Sq amplitude became the smallest around the minimum of 23/24 solar cycle in 2008-2009. The relationship between the Sq amplitude and F10.7 solar activity index was not linear but nonlinear. This nonlinearity could be interpreted as the decrease of production rate of electrons and ions in the ionosphere for the strong extreme ultraviolet (EUV) and ultraviolet (UV) fluxes. In order to minimize an effect of solar activity including the long-term variation in the Sq amplitude, we calculated second orders of fitting curve between the F10.7 solar index and Sq amplitude during 1947-2012, and examined the residual Sq amplitude defined as the deviation from the fitting curve. As a result, majority of the residual Sq trends passed through the trend test showed a negative value without dependence on geographical latitude and longitude. The tendency was strong in India, the southern part of Africa, and the northern part of America and Europe. In a region of northern part of America and Europe, the secular variation of magnetic inclination becomes relatively large, compared with other regions. Therefore, the long-term trend in the residual Sq amplitude could be linked to a change in the ionospheric conductivities associated with the secular variation of the ambient magnetic field and the upper atmosphere and electro motive force (UxB) via the interaction between ionized and neutral particles.