Y. Yamazaki, K. Yumoto, M.G. Cardinal, B.J. Fraser, P. Hattori, Y. Kakinami, J.Y. Liu, K.J.W. Lynn, R. Marshall, D. McNamara, T. Nagatsuma, V.M. Nikiforov, R.E. Otadoy, M. Ruhimat, B.M. Shevtsov, K. Shiokawa, S. Abe, T. Uozumi, A. Yoshikawa
An empirical model of the quiet daily geomagnetic field variation has been constructed based on geomagnetic data obtained from 21 stations along the 210 Magnetic Meridian of the Circum-pan Pacific Magnetometer Network (CPMN) from 1996 to 2007. Using the least squares fitting method for geomagnetically quiet days (Kp ≤ 2+), the quiet daily geomagnetic field variation at each station was described as a function of solar activity SA, day of year DOY, lunar age LA, and local time LT. After interpolation in latitude, the model can describe solar-activity dependence and seasonal dependence of solar quiet daily variations (S) and lunar quiet daily variations (L). We performed a spherical harmonic analysis (SHA) on these S and L variations to examine average characteristics of the equivalent external current systems. We found three particularly noteworthy results. First, the total current intensity of the S current system is largely controlled by solar activity while its focus position is not significantly affected by solar activity. Second, we found that seasonal variations of the S current intensity exhibit north-south asymmetry; the current intensity of the northern vortex shows a prominent annual variation while the southern vortex shows a clear semi-annual variation as well as annual variation. Thirdly, we found that the total intensity of the L current system changes depending on solar activity and season; seasonal variations of the L current intensity show an enhancement during the December solstice, independent of the level of solar activity. Copyright 2011 by the American Geophysical Union.
Department of Earth and Planetary Sciences, Kyushu University, Hakozaki 812-8581, Japan; Space Environment Research Center, Kyushu University, Hakozaki 812-8581, Japan; Centre for Space Physics, University of Newcastle, Callaghan, NSW 2308, Australia; Guam Magnetic Observatory, U.S. Geological Survey, Dededo 96912, Guam; Institute of Space Science, National Central University, Chung-Li 32001, Taiwan; Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan; Ionospheric Systems Research, Noosaville, QLD 4566, 16 Heritage Dr., Australia; V.i.il'Ichev Pacific Oceanological Institute, Far Eastern Branch, Russian Academy of Sciences, Vladivostok 690041, Russian Federation; Department of Physics, University of San Carlos, Cebu City 6000, Philippines; National Institute of Aeronautics and Space, Bandung 40173, Indonesia; Institute of Cosmophysical Research and RadioWave Propagation, Far Eastern Branch, Russian Academy of Sciences, Paratunka 684034, Russian Federation; Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan; Ionospheric Prediction Service Radio and Space Services, Bureau of Meteorology, Sydney, NSW 2000, 477 Pitt St., Australia; Manila Observatory, Quezon 1101, Philippines; Applied Electromagnetic Research Center, National Institute of Information and Communications Technology, Tokyo 184-8795, Japan
