PROBLEMS OF GEOCOSMOS

Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)
Introduction
FRACTAL CHARACTERISTICS OF ULF EMISSIONS
REGISTERED IN THE HIGH LATITUDE
SEISMIC-QUIET REGION OF SPITSBERGEN ISLAND
N.A. Smirnova, A.A. Isavnin
Institute of Physics, St.Petersburg University, St.Petersburg, 198504, Russia, e-mail:
nsmir@geo.phys.spbu.ru
Abstract. ULF frequency range (f = 0.001-5Hz) is now considered as a promising one for manifestation
of electromagnetic earthquake precursors. To support the registration of ULF emissions in seismicactive
areas, and to exclude magnetospheric effects from the lithospheric ones, the reference stations in
seismic quiet but magnetic active regions are required. Here we are starting to analyze the magnetic
field data obtained with high resolution (10 Hz sampling rate) in the high latitude seismic-quiet region
of Spitsbergen Island (Barentsburg station, Φm=76° N; Λm=115°). Fractal analysis of the ULF data has
been fulfilled using Higuchi method and Burlaga-Klein approach. The first preliminary results of the
fractal analysis are presented. Variations of the fractal dimensions of the ULF emissions time series are
considered along the local time. The possible physical interpretation of the results obtained is suggested.
A possibility of using the Barentsburg observatory as a reference station in seismo-electromagnetic
research is discussed.
One the most important tasks in geophysics is the study of precursors of earthquakes. Nowadays it is known that
earthquakes are related to weak seismogenic ULF emissions (see Hayakawa and Fujinawa (eds), 1994). But the
problem of analyzing these signals is that they are strongly screened by ULF emissions of the magnetospheric
origin (Smirnova, 1999). So the analysis may be strict only during the calm “space weather” time. In this work
we are starting to analyze the magnetic field data in the high latitude seismic-quiet region of Spitsbergen Island.
This region is situated at the cusp latitude – the most sensitive area to magnetic field disturbances. Because of its
extremely high sensitivity to the solar wind conditions it can be used as a reference station for the research of
seismogenic electromagnetic emissions.
There is also an important task to monitor the position and size of the cusp area.
Experimental data
In Fig.1 you can see a principle scheme of the magnetosphere of the Earth. There are two cusp areas in the
magnetosphere corresponding to two poles of the dipole field of the Earth. Each of the cusps is situated between
closed and opened magnetic field lines. It is clear that dependent on the solar wind conditions the geographical
position of the station can fall into the zone of opened or closed magnetic field lines. So it is important to learn
out if the station is situated in the cusp during the
analyzed period of time (the magnitude gap of
cusp area is very narrow). For this aim we will
use the Tsyganenko model of magnetosphere to
project the cusp area on the Earth's surface
according to parameters of the solar wind.
Comparing the magnetic field behavior during the
periods when the station falls into the projection
of the cusp area on the Earth's surface with
parameters of solar wind we will exclude the
fluctuations of the magnetic field caused by
inhomogenities in the solar wind. After such
filtering we will obtain the magnetic field data
with most of fluctuations in ULF frequency range
Fig.1. The scheme of the Earth's magnetosphere. One can
see the polar cusps between opened and closed magnetic
field lines (the so-called points of bifurcation).
caused by lithospheric effects, not magnetospheric. Analyzing such “clean” data and confronting it in time with
registered lithospheric events and finding correlation parameters between them could give us some sort of
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