Statistical study on propagation characteristics of Omega signals (VLF) in magnetosphere detected by the Akebono satellite

📝 Abstract

This paper shows a statistical analysis of 10.2 kHz Omega broadcasts of an artificial signal broadcast from ground stations, propagated in the plasmasphere, and detected using an automatic detection method we developed. We study the propagation patterns of the Omega signals to understand the propagation characteristics that are strongly affected by plasmaspheric electron density and the ambient magnetic field. We show the unique propagation patterns of the Omega 10.2 kHz signal when it was broadcast from two high-middle-latitude stations. We use about eight years of data captured by the Poynting flux analyzer subsystem on board the Akebono satellite from October 1989 to September 1997. We demonstrate that the signals broadcast from almost the same latitude (in geomagnetic coordinates) propagated differently depending on the geographic latitude. We also study propagation characteristics as a function of local time, season, and solar activity. The Omega signal tended to propagate farther on the nightside than on the dayside and was more widely distributed during winter than during summer. When solar activity was at maximum, the Omega signal propagated at a lower intensity level. In contrast, when solar activity was at minimum, the Omega signal propagated at a higher intensity and farther from the transmitter station.

💡 Analysis

This paper shows a statistical analysis of 10.2 kHz Omega broadcasts of an artificial signal broadcast from ground stations, propagated in the plasmasphere, and detected using an automatic detection method we developed. We study the propagation patterns of the Omega signals to understand the propagation characteristics that are strongly affected by plasmaspheric electron density and the ambient magnetic field. We show the unique propagation patterns of the Omega 10.2 kHz signal when it was broadcast from two high-middle-latitude stations. We use about eight years of data captured by the Poynting flux analyzer subsystem on board the Akebono satellite from October 1989 to September 1997. We demonstrate that the signals broadcast from almost the same latitude (in geomagnetic coordinates) propagated differently depending on the geographic latitude. We also study propagation characteristics as a function of local time, season, and solar activity. The Omega signal tended to propagate farther on the nightside than on the dayside and was more widely distributed during winter than during summer. When solar activity was at maximum, the Omega signal propagated at a lower intensity level. In contrast, when solar activity was at minimum, the Omega signal propagated at a higher intensity and farther from the transmitter station.

📄 Content

Suarjaya et al. Earth, Planets and Space (2017) 69:100 DOI 10.1186/s40623-017-0684-5 FULL PAPER Statistical study on propagation characteristics of Omega signals (VLF) in magnetosphere detected by the Akebono satellite I Made Agus Dwi Suarjaya1,2, Yoshiya Kasahara1* and Yoshitaka Goto1 Abstract  This paper shows a statistical analysis of 10.2 kHz Omega broadcasts of an artificial signal broadcast from ground stations, propagated in the plasmasphere, and detected using an automatic detection method we developed. We study the propagation patterns of the Omega signals to understand the propagation characteristics that are strongly affected by plasmaspheric electron density and the ambient magnetic field. We show the unique propagation pat- terns of the Omega 10.2 kHz signal when it was broadcast from two high–middle-latitude stations. We use about eight years of data captured by the Poynting flux analyzer subsystem on board the Akebono satellite from October 1989 to September 1997. We demonstrate that the signals broadcast from almost the same latitude (in geomagnetic coordinates) propagated differently depending on the geographic latitude. We also study propagation characteristics as a function of local time, season, and solar activity. The Omega signal tended to propagate farther on the nightside than on the dayside and was more widely distributed during winter than during summer. When solar activity was at maximum, the Omega signal propagated at a lower intensity level. In contrast, when solar activity was at minimum, the Omega signal propagated at a higher intensity and farther from the transmitter station. Keywords:  Whistler mode wave, Omega signal, Ionosphere, Plasmasphere, Akebono Satellite, Wave propagation © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/) , which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Introduction The Earth’s plasmasphere is located in the inner part of the magnetosphere and is mainly filled with cold plasma. Very low frequency (VLF) waves such as whistlers (Car- penter 1963) and Omega signals propagate in the magne- tosphere as whistler mode waves and are strongly affected by the electron density profile. The major species of ions in the plasmasphere are protons, helium ions, and oxygen ions. The composition ratio of these ions plays an impor- tant role in the propagation effect of VLF waves such as the subprotonospheric whistler and the magnestospheri- cally reflected whistler (Kimura 1966). Hence, it is very important to clarify the spatial distribution of electron density as well as the ion constituents in the magneto- sphere to understand the propagation characteristics of VLF waves. In other words, the propagation characteris- tics of the whistler mode wave in the VLF range could be an important clue to study the electron density profile in the magnetosphere and the impact of these waves on the wider system. Several studies had previously been conducted to determine the electron density profile around the Earth. Some studies of in  situ satellite observations revealed features of the plasmasphere such as “notches” (Sandel et  al. 2001, 2003). A particularly long-lived (2–3  days) depleted region [magnetic local time (MLT) >1.5–2  h) or “notch” extended out from L ~ 3 in the plasmasphere (Kotova et al. 2004). Remote sensing from ground-based magnetometers enabled the comparison of mass density and electron density between different L-shells using multiple observation stations along the 330° magnetic Open Access *Correspondence: kasahara@is.t.kanazawa‑u.ac.jp 1 Graduate School of Natural Science and Technology, Kanazawa University, Kakuma‑machi, Kanazawa 920‑1192, Japan Full list of author information is available at the end of the article Page 2 of 14 Suarjaya et al. Earth, Planets and Space (2017) 69:100 longitude, spanning the L-shell between 1.5 and 3.4 (Chi et  al. 2013). It was also demonstrated that mass den- sity decreased within a few hours by 50% or more at 0.5 radius of the Earth or further inward of the plasma- pause during moderately disturbed periods on December 10–15, 2003 (Menk et al. 2014). Measurement of electro- magnetic wave data is also important to determine the electron density. For example, dispersion of electromag- netic waves originating from lighting discharge, known as whistlers (Helliwell 1965), is useful for estimating the spatial electron density profile in the plasmasphere, because the propagation velocity of whistler mode waves depends on the electron density profile around the Earth (Crouchley 1964; Singh et al. 2003). Statistical studies of whistlers have been performed using gr

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