Abstract
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&deg.
Highlights
It has been established by observations that the main contribution to the horizontal magnetic field of electromagnetic emissions as observed on the ground in the upper ULF and lower ELF frequency range, that is from 1 to approximately 50 Hz, stems from Schumann resonances, Pc 1 magnetic pulsations and a special type of electromagnetic emission that at times reveals the existence of a weak harmonic structure below and above, but in the vicinity of the first Schumann resonance
Since it is known that in the analyzed frequency range a considerable contribution to the horizontal magnetic field components observed on the ground comes from electromagnetic emissions which are modulated by ionospheric Alfven resonator (IAR), we investigate below to what extent the existing theory of IAR can help us to sort out our observations
The second group of assumptions concerns the derivation of the reflection coefficient R of Alfven waves (see, Eq (2): 1) the inequality kAlH 1 is needed to obtain the expression from a more general equation; it means that the wavelength of the first few eigenmodes should be of the same order as the entire resonator height, 2) for the wave reflection in the upper ionosphere, a violation of the geometrical optics is needed in form of gradients of the refractive index that are to be steep relative to the wavelength in the medium; it yields the inequality nekAlH 1, 3) in the relevant frequency range the optical thickness of the lower ionosphere has to be small
Summary
It has been established by observations that the main contribution to the horizontal magnetic field of electromagnetic emissions as observed on the ground in the upper ULF and lower ELF frequency range, that is from 1 to approximately 50 Hz, stems from Schumann resonances, Pc 1 magnetic pulsations and a special type of electromagnetic emission that at times reveals the existence of a weak harmonic structure (mostly during nighttime) below and above, but in the vicinity of the first Schumann resonance. In this report we step a little bit backward in view of the latter work by addressing only changes in magnetic background polarization properties around the time of the terminator passage over the observation site, leaving gravity wave aspects and other longterm variations in SRS – well present in our data – out of the scope of the paper. This seems justified, since the primary effect has yet to be explored in more detail before implying larger scenarios. This is accompanied by corresponding modelling of suspected terminator effects on SRS and magnetic background noise using the basic SRS theory of Belyaev et al (1989b)
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