Topic
Very low frequency
About: Very low frequency is a research topic. Over the lifetime, 1540 publications have been published within this topic receiving 24233 citations. The topic is also known as: VLF.
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TL;DR: In this paper, the main mechanisms of excitation of ULF/ELF/VLF waves were reviewed and discussed, such as their electrojet dependency, the location of the source region, continuous and discontinuous waves, the number of HF arrays, and the suitable range of the modulation frequency for different proposed mechanisms.
Abstract: One of the most important effects of ionospheric modification by high power, high frequency (HF) waves is the generation of ultra low frequency/extremely low frequency/very low frequency (ULF/ELF/VLF) waves by modulated heating. This paper reviews the scientific achievements of the past five decades regarding the main mechanisms of excitation of ULF/ELF/VLF waves and discusses their characteristics, such as their electrojet dependency, the location of the source region, continuous and discontinuous waves, the number of HF arrays, and the suitable range of the modulation frequency for different proposed mechanisms. Finally, the outlook for future research in this area is presented.
7 citations
TL;DR: In this article, an approximate wave solution for the VLF modes near cut-off frequencies in the high-latitude Earth-ionosphere waveguide is obtained, where the vertical inhomogeneity of the lower D-region of the ionosphere is included in the form of a simple exponential profile of the parallel conductivity.
7 citations
TL;DR: The vertical electric field component was measured by a group of the Ukrainian Institute of Radio Astronomy on board the Professor Zubov scientific vessel during April 1989 at latitudes from 30°S to 50°N as discussed by the authors.
7 citations
TL;DR: In this article, the phase and amplitude changes of the VLF signals transmitted by the NDK station at 25.2 kHz in North Dakota, USA during the August 21, 2017 solar eclipse were detected.
7 citations
TL;DR: In this paper, the amplitude versus time shape of the pulsations was often triangular or zigzag-like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery.
Abstract: Subionospheric very low frequency and low-frequency (VLF/LF) transmitter signals received at middle-latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag-like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi-static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum.
7 citations