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Ann. Geophys., 22, 1989-2000, 2004
www.ann-geophys.net/22/1989/2004/
© European Geosciences Union 2004
Dispersive changes in magnetic background noise polarization at 0.1 to 6Hz during sunset and sunrise at L=1.3
T. Bösinger1 and S. L. Shalimov2
1Department of Physical Sciences, POB 3000, FIN-90014 University of Oulu, Finland
2Institute of Physics of the Earth, 123810, Moscow, B. Gruzinskaya, 10, Russia
Abstract. Polarization properties of the magnetic background noise (MBN) and
the spectral resonance structure (SRS) of the ionospheric Alfvén
resonator (IAR) below the first Schumann resonance but above 0.1 Hz
are measured by a sensitive pulsation magnetometer at the island
of Crete (L=1.3) and analyzed using the existing SRS theory by
Belyaev et al. (1989b). The focus of the paper is on the
systematic changes in the MBN and SRS properties associated with
the transition from a sunlit to a dark ionosphere (sunset) and
vice versa (sunrise). We are able to pinpoint in observations an
E-region and F-region terminator effect and to simulate it by
means of a simple ionosphere model, implying the formalism given by
Belyaev et al. (1989b). The E-region terminator effect is associated with
an apparent control for the SRS presence or absence with no clear
frequency dispersion in polarization properties, whereas the F-region
terminator effect exhibits strong frequency dispersion,
especially in the low frequency range. This yields a change in the
ellipticity of MBN, starting as early as 2 to 3h ahead of the
"zero-line" of the terminator. In a 24h presentation of the
ellipticity versus frequency and time, the sunrise/sunset effect
produces a sharp, dispersive boundary between night and day (day
and night). Only inside this boundary, during the night hours, is SRS
observed, at times accompanied by a large quasi-periodic long
period modulation in the azimuthal angle of the major axis of the
polarization ellipse. Attention is also paid to peculiarities in
the low frequency range (~0.1Hz), where especially large
changes in the polarization properties occur in association with
the passage of the terminator. The F-region effect is very
distinct and well reproduced by our simple model. Changes in the
azimuth associated with the E-region terminator effect are of
the order of 20°.
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