27

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1

Introduction

Rikitake et al. (1956) analyzed geomagnetic variations inJapan during a geomagnetically quiet period between 1952and 1955, and pointed out that there is a phase progress in thevertical components of the Sq field around the Kanto district(e.g., KAK in Fig.1) relative to those at other observatories.Moreover, they showed there is no seasonal dependence of thephase shift. This was one of the reasons for the interpretationthat this feature is not of external but internal origin. From thisobservation, they proposed that the phase shift might be attrib-uted to an anomalous distribution of the mantle electrical con-ductivity. However, Kuvshinov et al. (1999) presented three-dimensional modeling recently, that indicated that most of theanomalous features in Sq field variations can be explained bythe induction effect of the ocean.

We approached this problem from another point of view.To quantify the phase shift, analysis in the frequency domainwas carried out. Geomagnetic data observed at more stationsin and around Japan in 1997, when geomagnetic activity wasthe most calm in recent years, were analyzed. We will firstshow that the relative phase difference around Kanto district atthe period of 24 hours does not show an anomalous tendency,and that the relative phase difference between two arbitraryobservation sites seems to have a seasonal dependence. Theseresults indicate that the phase shift is not simply caused by theconductivity structure, but probably by characteristics ofexternal variations as well.

In order to examine quantitatively whether the seasonaldependence is ascribed to the external field, we attempted toseparate the Sq field into external and internal parts. Sphericalcap harmonic analysis (SCH; Haines, 1985) was adopted forthis purpose. We will show both external and internal fieldsfrom SCH, and the seasonal dependence of the phase differ-ence cannot be simply explained by an external origin.

Transfer function

To examine whether the vertical components of the Sq fieldare anomalous around Kakioka Magnetic Observatory (KAKin Fig. 1), the phase shift of Sq field data observed at 14 sta-tions in Japan in 1997 (Fig. 1) were estimated. We chose 5quiet days in each month by using the Kp index, and calculat-ed a transfer function between a vertical component of Sqfield at each site and that of the reference site. We used OKIstation as a reference site. The transfer function is defined asfollows (e.g., Bendat and Piersol, 1971)

where and T denote the power spectrum

density of the vertical geomagnetic component at site i, thetransfer function at site i, the power spectrum density of thevertical geomagnetic component at the reference site and theperiod, respectively. Fig. 2 depicts phase responses of thetransfer function at a period of 24 hours in March and July in1997. No anomalous behavior around KAK is observed in twoseasons. One example of the phase difference between twosites (KNY-KAK) in each month is shown in Fig. 3. Thephase difference between KAK and KNY is about 20 degreesin June, and almost vanishes in September.. An obvious sea-sonal change of the phase difference is observed.

Separation of internal and external Sq field bySCH

Since the relative phase shift between two observation siteshas seasonal dependence, we examined quantitatively whetherthe seasonal dependence can be ascribed to the external field.If we assumed that the seasonal dependence is attributed to theexternal fields, the external source morphology must first beclarified. We attempted to separate the Sq field into externaland internal parts by using SCH. Our aim was to investigateregional scale structure rather than global, and SCH for uni-formly distributed regional observation sites has an advantageover an ordinal spherical harmonic analysis for highly non-uniformly distributed world-wide observation sites. Hence, weadopted SCH for the separation of the internal and external Sqfields.

In the SCH, we extracted the component of a period of 24hours in frequency domain, and used the data converted fromthis into time domain. The spherical cap radius was 30 degrees,and the center lay at N30 degrees and E135 degrees. The num-ber of degrees of expansion was 6. We gave the model priorcovariance proportional to the cubic of real degree (Gubbinsand Bloxham, 1985). Fig. 4 shows the results from the data inMarch and July, 1997. Unfortunately, the observation data atmost of the sites from September to December in 1997 werefaulty. Fig. 4 indicates that a distinct seasonal change in themorphology of external field behavior cannot be recognized,and that the seasonal dependence of the phase difference doesnot seem to be explained by an external origin only.

Discussion and conclusion

The lack of seasonal change of phase progress of the verti-cal component of Sq field in the Kanto district provided strongevidence that the phase shift was attributed to the internal ori-gin of the Sq field. But our transfer function analysis for thegeomagnetic data in 1997 revealed a distinct seasonal changeof the phase progress. This seasonal change may be interpret-ed as an external field behavior, but SCH analysis indicated

Local Sq field behavior around Japan

Masahiro Ichiki1 and Hisashi Utada2

1 Research Program for Mantle Core Dynamics, Institute for Frontier Research on Earth Evolution (IFREE)2 Earthquake Research Institute (ERI), University of Tokyo, Japan

28

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1

that the morphology of the external Sq field does not changesignificantly between March and July in 1997. Therefore, it isobviously doubtful that an external origin only caused the sea-sonal change of Sq field behavior. The internal Sq field isquite different between March and July in 1997. An internalorigin may contribute to the seasonal change. Therefore, sea-sonal changes of the phase shift may provide important infor-mation for the internal origin of the Sq field. The problem ofwhat induces such an internal Sq field – ocean or mantle tran-sition zone – must be investigated in the next step.

We postulated that SCH was superior to the ordinal spheri-cal harmonic analysis, considering the observatory distribu-tion. We should confirm this proposition quantitatively soon.

Acknowledgements. We would like to thank Drs. H. Shimizu(ERI) and T. Koyama (Center for Data and Sample Analyses, IFREE)for valuable discussion and comments. Prof. Y. Hamano (GraduateSchool of Earth and Planetary Science, University of Tokyo) kindlymade a critical reading of this manuscript. Geographical Survey Insti-tute in Japan and World Data Center provided us their geomagneticdata.

References

Bendat, J. S., and A. G. Piersol, Random data: analysis and measure-ment procedures, John Wiley and Sons, Inc., 1971.

Gubbins, D., and J. Bloxham, Geomagnetic field analysis. III. Mag-netic fields on the core-mantle boundary, Geophys. J. Roy. astr.Soc., 80, 696-713, 1985.

Haines, G. V., Spherical cap harmonic analysis, J. Geophys. Res., 90,2583-2591, 1985.

Kuvshinov, A. V., D. B. Avdeev, and O. V. Pankratov, Global induc-tion by Sq and Dst sources in the presence of oceans: bimodalsolutions for non-uniform spherical surface shells above radiallysymmetric earth models in comparison to observations, Geophys.J. Int., 137, 630-650, 1999.

Rikitake, T., I. Yokoyama, and S. Sato, Anomaly of the geomagneticSq variation in Japan and its relation to the subterranean structure,Bull. Earthquake Res. Inst., 34, 197-235, 1956.

29

FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1

Figure 1. Observation site distribution.

Figure 2. Phase of transfer function at period of 24 hours in the Japan-ese observation sites. Dotted lines indicate the earth's rotation rate.Top is the result of March, 1997 and Bottom is that of July, 1997.

Figure 3. The difference between KNY’s phase response and KAK’sone at period of 24 hours in each month in 1997.

Figure 4. Internal and external Sq fields in March and July in 1997calculated by SCH.