The Earth’s ionosphere ranges from ~ 60 to > 800 km in altitude and is characterized by large number of free electrons. Ionospheric conditions are controlled by solar radiation and often disturbed by geomagnetic activities. In addition to such disturbances caused by space weather, the ionosphere is disturbed by events below (Blanc 1985), such as earthquakes (Heki 2021), tsunami (Occhipinti et al. 2013), human-induced explosions (Kundu et al. 2021), and volcanic eruptions.
Ionospheric total electron content (TEC) can be easily measured by comparing the phases of two microwave carriers from global navigation satellite system (GNSS) satellites, such as Global Positioning System (GPS) (e.g., Hofmann-Wellenhof et al. 2008). Ground GNSS networks have been deployed to monitor crustal movements, and these networks were found useful to study ionospheric disturbances by volcanic eruptions. There are two types of ionospheric TEC responses to volcanic eruptions.
The first type is the long-lasting harmonic TEC oscillations (Fig. 1). They are atmospheric modes excited by continuous acoustic waves generated typically by Plinian eruptions. The interference of upward and downward acoustic waves between the ground surface and the mesopause causes resonant oscillation of atmosphere. They have prescribed frequencies reflecting the vertical atmospheric structure (Tahira 1995). It was found after the 13 July 2003 eruption of the Soufrière Hills volcano, Montserrat, in the Lesser Antilles (Dautermann et al. 2009a, b).
This type of disturbance also occurred after the February 2014 eruption of the Kelud volcano, eastern Java Island, Indonesia (Nakashima et al. 2016). They reported that harmonic oscillations caused by atmospheric resonance excited by the Plinian eruption of the Kelud volcano lasted for ~ 2.5 h after the eruption started. Shults et al. (2016) found similar TEC oscillations after the 2015 April Plinian eruption of the Calbuco volcano, Chile. Cahyadi et al. (2020) also found such harmonic TEC oscillations lasting ~ 20 min following the 2010 November 5 eruption of the Merapi volcano, central Java Island. Although the onsets of these continuous eruptions are not always clear, the TEC oscillations emerge 20–30 min after the eruptions started. Cahyadi et al. (2020) also suggested that the TEC oscillation amplitudes relative to background TEC represent the mass eruption rate, and the products of such amplitudes and the duration provides a new index for the total amount of the ejecta. Recently, Shestakov et al. (2021) reported the TEC oscillations lasting for an hour during the 2009 eruption of the Sarychev Peak volcano, Kuril Islands, Russia.
The second type of disturbances occur 8–10 min after volcanic explosions by short pulses of acoustic waves propagating upward from the surface to the ionospheric F region (Fig. 1). They make short-term N-shaped impulsive TEC responses as Heki (2006) observed with the GPS-TEC method after the Vulcanian explosive eruption of the Asama volcano, Central Japan, on September 1, 2004. Despite many reports of the first type of disturbances, this second type of disturbances have not been reported since the Asama eruption. In this study, we report four new examples of this type and compare their amplitudes together with the 2004 Asama case.
Intensity of a volcanic explosion has been studied by atmospheric pressure changes associated with the airwave (infrasound) generated by the eruption (e.g., Matoza et al. 2019). However, geometric settings of such sensors relative to volcanoes are diverse, and amplitudes of such airwaves are difficult to serve as a universal index to describe the explosion intensity. Volcanic explosivity index (VEI) is used to describe the intensity of the eruptions (Newhall and Self 1982). However, this index is determined by the amount of ejecta and does not directly indicate the explosion intensities. In this study, we explore the possibility to use the amplitude of ionospheric disturbance that occur ~ 10 min after a large explosion as the new index. For this purpose, we compare ionospheric TEC responses to five recent explosive volcanic eruptions of four volcanoes in Japan 2004–2015 comparing the GNSS-TEC data from GEONET (GNSS Earth Observation Network), a continuous GNSS network in Japan.