An empirical model of the quiet daily geomagnetic field variation

Abstract

[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.

Sq and EEJ—A Review on the Daily Variation of the Geomagnetic Field Caused by Ionospheric Dynamo Currents

“…This is owing to the difference in the lengths of a solar day (= 24.00 h) and a lunar day (= 24.84 h). The phase of the daily variation of L tends to be fixed at a certain lunar time τ , not solar time t (e.g., Rastogi and Trivedi 1970;Yamazaki et al 2011b). It is obvious from Fig.…”

“…35 (from Yamazaki et al 2010). Figure 36a shows month-to-month variations of J total derived from the empirical model of Yamazaki et al (2011b). It can be seen that the semiannual variation in J total with equinoctial maxima is more pronounced under higher solar flux conditions.…”

“…The harmonic coefficients a m and b m vary temporally with season and solar activity, and spatially with latitude and longitude (e.g., Rastogi and Iyer 1976;Campbell 1982;Campbell et al 1989Campbell et al , 1992Yamazaki et al 2011b). It is important to understand these variations because they can reveal the physical processes that govern the ionospheric dynamo.…”

“…According to the satellite measurements reported by , semidiurnal lunar tidal winds in the dynamo region have peak amplitudes of 15-20 m/s, which is remarkably consistent with the earlier estimates by Maeda and Fujiwara (1967) based on the analysis of geomagnetic lunar variations. The amplitude of L is typically one tenth of the amplitude of Sq (e.g., Yamazaki et al 2011b). The smaller amplitude of L is owing to weaker driving winds.…”

Magnetic Signatures of Ionospheric and Magnetospheric Current Systems During Geomagnetic Quiet Conditions—An Overview

“…Pedatella et al (2011); and Chulliat et al (2016) derived climatological models of the mid latitude Sq. However, the Sq currents exhibit significant day-to-day variations (e.g., Yamazaki et al 2011), that clearly need to be considered for single event analyses.…”