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Polarization properties of the magnetic background noise (MBN) and\udthe spectral resonance structure (SRS) of the ionospheric Alfvén\udresonator (IAR) below the first Schumann resonance but above 0.1 Hz\udare  measured by a sensitive pulsation magnetometer at the island\udof Crete (<i>L</i>=1.3) and analyzed using the existing SRS theory by\udBelyaev et al. (1989b). The focus of the paper is on the\udsystematic changes in the MBN and SRS properties associated with\udthe transition from a sunlit to a dark ionosphere (sunset) and\udvice versa (sunrise). We are able to pinpoint in observations an\udE-region and F-region terminator effect and to simulate it by\udmeans of a simple ionosphere model, implying the formalism given by\udBelyaev et al. (1989b). The E-region terminator effect is associated with\udan apparent control for the SRS presence or absence with no clear\udfrequency dispersion in polarization properties, whereas the F-region\udterminator effect exhibits strong frequency dispersion,\udespecially in the low frequency range. This yields a change in the\udellipticity of MBN, starting as early as 2 to 3h ahead of the\ud"zero-line" of the terminator. In a 24h presentation of the\udellipticity versus frequency and time, the sunrise/sunset effect\udproduces a sharp, dispersive boundary between night and day (day\udand night). Only inside this boundary, during the night hours, is SRS\udobserved, at times accompanied by a large quasi-periodic long\udperiod modulation in the azimuthal angle of the major axis of the\udpolarization ellipse. Attention is also paid to peculiarities in\udthe low frequency range (~0.1Hz), where especially large\udchanges in the polarization properties occur in association with\udthe passage of the terminator. The F-region effect is very\uddistinct and well reproduced by our simple model. Changes in the\udazimuth associated with the E-region terminator effect are of\udthe order of 20°

Dispersive changes in magnetic background noise polarization at 0.1 to 6Hz during sunset and sunrise at L=1.3