[1] 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 northsouth 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.

We analyzed ground magnetometer data for the 10 International Quiet Days during 1996–2007. The data were obtained from 19 stations along 210°magnetic meridian (MM) of the Circum‐pan Pacific Magnetometer Network (CPMN) covering both the Northern Hemisphere and Southern Hemisphere. From the daily variations of the geomagnetic field, we deduced the latitude‐local time (LAT‐LT) diagram of the equivalent Sq current system, which can be regarded as the superposition of the following three current systems: Sq0, Sq1, and Sq2. The Sq0, Sq1, and Sq2 current systems are equivalent current systems for the yearly average, annual variation, and semiannual variation of the Sq field, respectively. We have examined temporal and spatial features of these current systems. The principal features are as follows: (1) The total current intensities of the Sq1 and Sq2 current systems are about 35% and 15% of that of the Sq0 current system, respectively. (2) The Sq0 and Sq2 current systems have a dayside vortex in each hemisphere, while the Sq1 current system has a single vortex centered at the equatorial region in the morning sector (∼1000 LT).

[1] We reexamined the daily S q -equatorial electrojet (EEJ) relationship based on these extended magnetometer networks in the east Asian region: (1) the Circum-pan Pacific Magnetometer Network (CPMN), (2) the International Real-time Magnetic Observatory Network (INTERMAGNET), and (3) the World Data Center for Geomagnetism, Kyoto (WDC). Daily variations of the geomagnetic field for geomagnetically quiet days (Kp ≤ 2+) from 1996 to 2005 were analyzed. Noontime eastward S q current intensities were estimated by latitudinally integrating the north-south component of the S q field. The corresponding EEJ intensities were estimated from the daily geomagnetic field variations observed at Davao station (dip latitude of −0.84°deg). We discovered that these intensities of daily S q and EEJ are well correlated on a long-term basis (r = 0.80). The dependences on the solar activity (as indicated by F10.7) and season (the day number) of S q and EEJ variations were examined. It was demonstrated that both daily S q and EEJ intensities are correlated to F10.7 with similar sensitivities. F10.7 is known to show similar variations with solar EUV radiation which causes ionization and heating of the ionosphere. For seasonal dependence, both daily S q and EEJ intensities show predominant semiannual variations with similar spring-fall asymmetry. The effect of seasonal changes of the EUV flux into the low-latitude ionosphere is considered. Our results indicate that the daily values of S q and EEJ react, in the same manner, to temporal changes of solar ionization and heating of the ionosphere.

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[1] The total current intensity (J total ) of the equivalent S q current system along the 210°magnetic meridian shows the following variations: (1) solar activity variations, (2) seasonal variations, and (3) day-to-day variations. These variations arise from different physical mechanisms. The main objective of the present paper is to determine the relative amount contributed by each variation to the J total . First, the empirical S q field model by Yamazaki et al. (2011) is analyzed to examine the impact of solar activity and seasonal variations. The results show that J total changes by ±33% with the change of solar radiation activity in one solar cycle and by ±17% with the change of season. Next, observation data are analyzed to examine day-to-day variations. The daily values of J total from 2000 to 2002 are derived after removal of the solar activity and seasonal contributions. The results show that J total changes by ±14% from day to day. Therefore, we conclude that variations in J total are mainly controlled by solar radiation activity, while the impact of seasonal effects is about half of the solar activity contribution and the impact of day-to-day effects is also about half of the solar activity contribution.Citation: Yamazaki, Y., K. Yumoto, T. Uozumi, and M. G. Cardinal (2011), Intensity variations of the equivalent S q current system along the 210°magnetic meridian,