1. Department of Earth and Planetary Sciences, Kyushu University, Hakozaki, Japan
  2. Space Environment Research Center, Kyushu University, Hakozaki, Japan
  3. Centre for Space Physics, University of Newcastle, Callaghan, New South Wales, Australia
  4. Guam Magnetic Observatory, U.S. Geological Survey, Dededo, Guam
  5. Institute of Space Science, National Central University, Chung-Li, Taiwan
  6. Now at Institute of Seismology and Volcanology, Hokkaido University, Sapporo, Japan
  7. Ionospheric Systems Research, Noosaville, Queensland, Australia
  8. Ionospheric Prediction Service Radio and Space Services, Bureau of Meteorology, Sydney, New South Wales, Australia
  9. Manila Observatory, Quezon, Philippines
  10. Applied Electromagnetic Research Center, National Institute of Information and Communications Technology, Tokyo, Japan
  11. V. I. Il'ichev Pacific Oceanological Institute, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok, Russia
  12. Department of Physics, University of San Carlos, Cebu City, Philippines
  13. National Institute of Aeronautics and Space, Bandung, Indonesia
  14. Institute of Cosmophysical Research and Radio Wave Propagation, Far Eastern Branch of the Russian Academy of Sciences, Paratunka, Russia
  15. Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan

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.

Yamazaki, Y., 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, and A. Yoshikawa, An empirical model of the quiet daily geomagnetic field variation, J. Geophys. Res., 116, A10312, doi:10.1029/2011JA016487, 2011. (October 18, 2011)