FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
Local Sq field behavior around Japan
Masahiro Ichiki1 and Hisashi Utada2
1
2
Research Program for Mantle Core Dynamics, Institute for Frontier Research on Earth Evolution (IFREE)
Earthquake Research Institute (ERI), University of Tokyo, Japan
density of the vertical geomagnetic component at site i, the
transfer function at site i, the power spectrum density of the
vertical geomagnetic component at the reference site and the
period, respectively. Fig. 2 depicts phase responses of the
transfer function at a period of 24 hours in March and July in
1997. No anomalous behavior around KAK is observed in two
seasons. One example of the phase difference between two
sites (KNY-KAK) in each month is shown in Fig. 3. The
phase difference between KAK and KNY is about 20 degrees
in June, and almost vanishes in September.. An obvious seasonal change of the phase difference is observed.
Introduction
Rikitake et al. (1956) analyzed geomagnetic variations in
Japan during a geomagnetically quiet period between 1952
and 1955, and pointed out that there is a phase progress in the
vertical 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 the
phase shift. This was one of the reasons for the interpretation
that this feature is not of external but internal origin. From this
observation, they proposed that the phase shift might be attributed to an anomalous distribution of the mantle electrical conductivity. However, Kuvshinov et al. (1999) presented threedimensional modeling recently, that indicated that most of the
anomalous features in Sq field variations can be explained by
the induction effect of the ocean.
We approached this problem from another point of view.
To quantify the phase shift, analysis in the frequency domain
was carried out. Geomagnetic data observed at more stations
in and around Japan in 1997, when geomagnetic activity was
the most calm in recent years, were analyzed. We will first
show that the relative phase difference around Kanto district at
the period of 24 hours does not show an anomalous tendency,
and that the relative phase difference between two arbitrary
observation sites seems to have a seasonal dependence. These
results indicate that the phase shift is not simply caused by the
conductivity structure, but probably by characteristics of
external variations as well.
In order to examine quantitatively whether the seasonal
dependence is ascribed to the external field, we attempted to
separate the Sq field into external and internal parts. Spherical
cap harmonic analysis (SCH; Haines, 1985) was adopted for
this purpose. We will show both external and internal fields
from SCH, and the seasonal dependence of the phase difference cannot be simply explained by an external origin.
Separation of internal and external Sq field by
SCH
Since the relative phase shift between two observation sites
has seasonal dependence, we examined quantitatively whether
the seasonal dependence can be ascribed to the external field.
If we assumed that the seasonal dependence is attributed to the
external fields, the external source morphology must first be
clarified. We attempted to separate the Sq field into external
and internal parts by using SCH. Our aim was to investigate
regional scale structure rather than global, and SCH for uniformly distributed regional observation sites has an advantage
over an ordinal spherical harmonic analysis for highly nonuniformly distributed world-wide observation sites. Hence, we
adopted SCH for the separation of the internal and external Sq
fields.
In the SCH, we extracted the component of a period of 24
hours in frequency domain, and used the data converted from
this into time domain. The spherical cap radius was 30 degrees,
and the center lay at N30 degrees and E135 degrees. The number of degrees of expansion was 6. We gave the model prior
covariance proportional to the cubic of real degree (Gubbins
and Bloxham, 1985). Fig. 4 shows the results from the data in
March and July, 1997. Unfortunately, the observation data at
most of the sites from September to December in 1997 were
faulty. Fig. 4 indicates that a distinct seasonal change in the
morphology of external field behavior cannot be recognized,
and that the seasonal dependence of the phase difference does
not seem to be explained by an external origin only.
Transfer function
To examine whether the vertical components of the Sq field
are anomalous around Kakioka Magnetic Observatory (KAK
in Fig. 1), the phase shift of Sq field data observed at 14 stations in Japan in 1997 (Fig. 1) were estimated. We chose 5
quiet days in each month by using the Kp index, and calculated a transfer function between a vertical component of Sq
field at each site and that of the reference site. We used OKI
station as a reference site. The transfer function is defined as
follows (e.g., Bendat and Piersol, 1971)
where
Discussion and conclusion
The lack of seasonal change of phase progress of the vertical component of Sq field in the Kanto district provided strong
evidence that the phase shift was attributed to the internal origin of the Sq field. But our transfer function analysis for the
geomagnetic data in 1997 revealed a distinct seasonal change
of the phase progress. This seasonal change may be interpreted as an external field behavior, but SCH analysis indicated
and T denote the power spectrum
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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
that the morphology of the external Sq field does not change
significantly between March and July in 1997. Therefore, it is
obviously doubtful that an external origin only caused the seasonal change of Sq field behavior. The internal Sq field is
quite different between March and July in 1997. An internal
origin may contribute to the seasonal change. Therefore, seasonal changes of the phase shift may provide important information for the internal origin of the Sq field. The problem of
what induces such an internal Sq field – ocean or mantle transition zone – must be investigated in the next step.
We postulated that SCH was superior to the ordinal spherical harmonic analysis, considering the observatory distribution. 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 (Graduate
School of Earth and Planetary Science, University of Tokyo) kindly
made a critical reading of this manuscript. Geographical Survey Institute in Japan and World Data Center provided us their geomagnetic
data.
References
Bendat, J. S., and A. G. Piersol, Random data: analysis and measurement procedures, John Wiley and Sons, Inc., 1971.
Gubbins, D., and J. Bloxham, Geomagnetic field analysis. III. Magnetic 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 induction by Sq and Dst sources in the presence of oceans: bimodal
solutions for non-uniform spherical surface shells above radially
symmetric earth models in comparison to observations, Geophys.
J. Int., 137, 630-650, 1999.
Rikitake, T., I. Yokoyama, and S. Sato, Anomaly of the geomagnetic
Sq variation in Japan and its relation to the subterranean structure,
Bull. Earthquake Res. Inst., 34, 197-235, 1956.
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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1
Figure 3. The difference between KNY’s phase response and KAK’s
one at period of 24 hours in each month in 1997.
Figure 1. Observation site distribution.
Figure 4. Internal and external Sq fields in March and July in 1997
calculated by SCH.
Figure 2. Phase of transfer function at period of 24 hours in the Japanese observation sites. Dotted lines indicate the earth's rotation rate.
Top is the result of March, 1997 and Bottom is that of July, 1997.
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