Ionospheric effects of the Mt. Kirishima volcanic eruption as seen from subionospheric VLF observations A. Rozhnoi a , M. Hayakawa b,n , M. Solovieva a , Y. Hobara c , V. Fedun d a Institute of Physics of the Earth, Russian Academy of Sciences, B. Gruzinskaya 10, Moscow 123995, Russia b University of Electro-Communications (UEC), Advanced Wireless Communications Research Center, 1-5-1 Chofugaoka, Chofu Tokyo 182-8585, Japan c UEC, Graduate School of Informatics and Communication Engineering, Chofu Tokyo, Japan d Department of Automatic Control and Systems Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK article info Article history: Received 9 July 2013 Received in revised form 24 October 2013 Accepted 24 October 2013 Available online 11 November 2013 Keywords: Subionospheric VLF propagation Ionospheric perturbation Volcano eruption Atmospheric gravity waves abstract Data from the Pacific network of VLF receivers have been used to study the response of the lower ionosphere to the January 2011 Mt. Kirishima (South Japan) volcanic eruption. A major explosive eruption occurred on January 27, which was preceded by several small eruptions. Perturbations of nighttime subionospheric VLF signals have been detected on the day of the first small eruption on January 18 (UT) with the maximum observed about 1.5 h after the eruption. The nighttime signal remained disturbed during the subsequent pre-eruptive and eruptive activity of Mt. Kirishima. The daytime perturbations were not observed. The frequency of the maximum spectral amplitude was found to be in the range of periods of 6–30 min, which corresponds to the periods of internal gravity waves. These results suggest that the observed VLF ionospheric effects can possibly be produced by the penetration of gravity waves caused by the volcanic activity into the ionosphere. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction There has been recently reported a lot of convincing evidence on the presence of ionospheric disturbances as seen by subiono- spheric VLF/LF propagation caused by different kinds of agents including earthquakes, tsunami and volcano eruptions. Waves in the VLF frequency (3–30 kHz) and LF frequency (30–300 kHz) ranges are trapped between the lower ionosphere and the Earth and are reflected from the boundary between the upper atmosphere and lower ionosphere at altitudes of  70 km in the daytime and  90 km at night. When measured by a VLF/LF receiver, such signals inherently contain information on the reflection region of the iono- sphere and its variability. As for the seismo-ionospheric perturbations, there have been established a statistical correlation between the VLF ionospheric perturbations and earthquakes with large magnitude greater than 6 and with shallow depth (e.g., Molchanov and Hayakawa, 2008; Hayakawa et al., 2010; Hayakawa, 2011), which seems to be very promising for the short-term earthquake prediction. Then, the second topic of tsunami-associated ionospheric perturbation, Rozhnoi et al. (2012a) have reported the first report on the tsunami-related VLF perturbation, in which they have detected the VLF ionospheric perturbation in possible association with the tsunami of the 2011 Japan earthquake. The third topic is the VLF/LF ionospheric perturbation in association with a volcanic activity, which is the subject of this paper and to our knowledge there have been no report on VLF/LF study on this topic. Volcanic eruptions are likely to generate acoustic and gravity waves that propagate upward into the ionosphere where they induce electron density fluctuations which can be detected by electromagnetic waves in different frequency range. The ionospheric response of the Mt. Pinatubo volcanic eruption in June 1991 was found from observations of the total electron content (TEC) variations in Taiwan (Cheng and Huang, 1992) and in Japan as well (Igarashi et al., 1994). Cheng and Huang (1992) studied signals from stations located to the north of the volcano. The period of the ionospheric disturbances was determined to be around 16–30 min. Ionospheric fluctuations with periods of about 20 min were found in the records of HF Doppler and TEC data by Igarashi et al. (1994). The surface pressure fluctuations due to the passage of atmospheric and gravity waves were confirmed by the microbarograph chain data in Japan. The similar period was also reported from the Doppler shift observation in the Beijing area (Hao et al., 2006). Heki (2006) used data from the dense array of Global Position- ing System (GPS) in Japan in order to estimate an explosive energy of the Asama Volcano eruption in September 2004. The distur- bance had a period of one and a quarter minutes, which indicates its origin as the acoustic wave generated by the explosion. TEC Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2013.10.014 n Corresponding author at: University of Electro-Communications, Advanced Wireless Communications Research Center, 1–5-1 Chofugaoka, Chofu Tokyo 182– 8585, Japan. E-mail address: hayakawa@hi-seismo-em.jp182-8585 (M. Hayakawa). Journal of Atmospheric and Solar-Terrestrial Physics 107 (2014) 54–59