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Abstract Beat wave (BW) high frequency (HF) ionospheric heating experiments were conducted to generate very low frequency (VLF) waves. The VLF waves were registered with a VLF receiver located ∼15 km east of the European Incoherent Scatter (EISCAT) heating facility in Tromsø, Norway. A fluxgate magnetometer was used to monitor auroral electrojet current, and ionospheric conditions were measured using a Dynasonde. Correlation coefficients between VLF amplitudes and the deviation of geomagnetic north–south components were calculated. Experimental results show that strong and positive correlation exists the majority of the time, but sometimes no correlation or even a negative correlation occurred. This is consistent with similar past experiments that took place with exclusively AM generation. These results therefore support the conclusion that BW generation of VLF waves is no different than with AM, likely occurring in the D or lower E ionospheric region. [ABSTRACT FROM AUTHOR]

This paper presents the experimental results of modulation and heating of the ionosphere using the European Incoherent Scatter facility in Troms ϕ , Norway, heating facility. The frequency characteristics of the excited extremely low frequency (ELF)/very low frequency (VLF) radiation sources in the amplitude modulated (AM) and dual-beam beat-wave (BW) modes are compared, and the polarization of the horizontal ELF/VLF magnetic field received at 15 km from the heating facility is studied. The results show that in the AM mode, the amplitude of the ELF/VLF signal decreases as the radiation frequency increases when the amplitude of the ELF/VLF signal reaches the maximum at 2017 Hz. In the BW mode, the maximum appears at 2017 Hz, but the overall amplitude of the ELF/VLF signal increases as the radiation frequency increases. The AM and BW modes have different frequency characteristics on ELF/VLF radiation sources formed by modulation and heating of the ionosphere. When the frequency is low, the amplitude of the ELF/VLF signal excited by the AM mode is greater than that excited by the BW mode, but as the frequency increases, the BW mode performs better than the AM mode. Under the same background conditions, the polarization characteristics of the ELF/VLF radiation signals excited by the AM and BW modes are different. The former tends to produce circularly polarized waves, while the latter produces linearly polarized waves. [ABSTRACT FROM AUTHOR]

Modulated high frequency heating of the polar ionosphere in the presence of a polar electrojet current (PEJ) can generate extremely/very low frequency (ELF/VLF, 3 Hz–30 kHz). In order to overcome the dependence of the radiation intensity on the variable PEJ, researchers have focused on electrojet-independent approaches. One potential method is so-called "beat-wave" (BW) ELF/VLF generation invoking nonlinearity of the F-region ionospheric plasma, but this F-region source mechanism remains controversial. We perform polarization analysis of BW generated ELF/VLF waves using EISCAT heating facility near Tromsø. The results show that ELF/VLF waves can be decomposed into right- (R) and -left (L) handed circularly polarized waves with comparable strengths, and the polarization ellipse tends to be parallel to the well-known dominant background electric field, which is fully consistent with the recognized features of ELF/VLF waves generated by PEJ modulation. Hence, the results demonstrate that the BW generated current is located in the D/E region, and not the F region. • The polarization analysis of BW generated ELF/VLF waves is performed to examine its generation mechanism. • The features of R and L modes and polarization ellipse are consistent with AM generated ELF/VLF waves. • The polarization characteristic favors the PEJ modulation mechanism, not the F-region source mechanism. [ABSTRACT FROM AUTHOR]