Abstract
The Hunga Tonga–Hunga Ha’apai (HTHH) undersea volcanic eruption that occurred at 04:15 UT on 15 January 2022 is one of the most explosive events in the modern era, and a vertical plume reached approximately 55 km, corresponding to a height of the lower mesosphere. The intense explosion and subsequent plume generated acoustic and atmospheric gravity waves detected by ground-based instruments worldwide. Because a global-scale atmospheric and ionospheric response to the large volcanic eruption has not yet been observed, it provides a unique opportunity to promote interdisciplinary studies of coupling processes in lithosphere–atmosphere–ionosphere with ground-based and satellite observations and modeling. Further, this event allows us to elucidate the propagation and occurrence features of traveling ionospheric disturbances, the generation of equatorial plasma bubbles, the cause of electron density holes around the volcano, and the magnetic conjugacy of magnetic field perturbations. The most notable point among these studies is that the medium-scale travelling traveling ionospheric disturbances (MSTIDs) have magnetic conjugacy even in the daytime ionosphere and are generated by an external electric field, such as an E-region dynamo field, due to the motions of neutrals in the thermosphere. This advocates a new generation mechanism of MSTIDs other than the neutral oscillation associated with atmospheric gravity waves and electrified MSTIDs, which are frequently observed during daytime and nighttime, respectively. This paper reviews the recent studies of atmospheric and ionospheric disturbances after the HTHH volcanic eruption and summarizes what we know from this extreme event analysis. Further, we analyzed new datasets not shown in previous studies to give some new insights to understanding of some related phenomena. As a result, we also found that 4-min plasma flow oscillations caused by the acoustic resonance appeared with the amplitude of approximately 30 m/s in the northern hemisphere a few hours before the initial arrival of the air pressure waves. The propagation direction was westward, which is the same as that of the daytime MSTIDs with a magnetic conjugate feature. This result suggests that the 4-min oscillations are generated by an external electric field transmitted to the northern hemisphere along magnetic field lines.
Graphical Abstract
Introduction
Natural hazards caused by severe phenomena of geophysical, hydrological, and meteorological origins negatively impact human activity or the environment. Many people all over the world are threatened by natural hazards every year. Although some natural hazards (typhoons, tornadoes, and floods) can be predicted in near real-time, most geophysical natural hazards, such as earthquakes, volcanic eruptions, and tsunamis, are challenging to forecast. Therefore, we need to rapidly and timely detect the signals of natural geophysical hazards to reduce human loss. Among these geophysical hazards, we experienced the Hunga Tonga–Hunga Ha’apai (HTHH) undersea volcano's explosive eruption at 04:15 UT on 15 January 2022. This eruption is one of the most explosive events in the modern era and generated global atmospheric and ionospheric disturbances in a broad atmospheric region from the troposphere to the ionosphere (Wright et al. 2022) in addition to a tsunami in the Pacific Ocean associated with an air pressure wave (Kubota et al. 2022). The HTHH volcanic eruption provides a unique opportunity to promote interdisciplinary studies of coupling processes in lithosphere–atmosphere–ionosphere to reduce disaster risks.
Astafyeva (2019) has reviewed the detection of ionospheric signals associated with earthquakes, tsunamis, and volcanic eruptions and discussed future perspectives for applications of ionospheric observations to detect natural hazards. This paper reviews the studies of atmospheric and ionospheric disturbances observed after the HTHH volcanic eruption, shows some new insights to the understandings of the phenomena by adding new datasets, and summarizes what we know from existing analyses of this severe event. In this section, we describe several basic concepts of the ionosphere and atmospheric waves for understanding atmospheric and ionospheric disturbances caused by the HTHH volcanic eruption.
Basic features of the Earth’s ionosphere
The ionosphere is formed by partial ionization of the upper atmosphere above 60 km altitude due to solar extreme ultraviolet (EUV) radiation and energetic particle precipitation from interplanetary space and the magnetosphere. Percentage of charged particles to neutral atmospheric molecules/atoms increases with altitude. Nevertheless, because the mass of the ionosphere is 1012–1013 times smaller than that of the neutral atmosphere, charged particles in the ionosphere are regarded as a minority component in the upper atmosphere (Astafyeva 2019).
The plasma density distribution in the ionosphere depends strongly on latitude, longitude, altitude, and local time. Based on the height profile of electron density, the ionosphere is classified into several ionospheric regions: D (60–90 km), E (90–150 km), and F (150–1000 km). The height of the maximum electron density is typically located at an altitude of ~ 250–400 km. Such the electron density profile is determined by the ionization of neutral particles, their recombination processes, dynamics, and electrodynamics in the upper atmosphere and ionosphere. Further, the ionosphere highly varies through solar/geomagnetic activities and upward propagating atmospheric waves originating from lower atmospheric disturbances (Hargreaves 1992; Kelley 2009).
Because the ions in the E region frequently collide with the neutral upper atmosphere, this process causes electric current in the ionosphere and a dynamo electric field through momentum transfer by ion-neutral collisions (e.g., Maeda and Kato 1966; Richmond 1979). The dynamo electric field is transmitted along magnetic field lines to the F region of the ionosphere and causes the plasma density perturbations driven by \(\mathbf{E}\times \mathbf{B}\) drift of ionospheric plasma (e.g., Yamazaki and Maute 2017; Shinbori et al. 2022). The spatial inhomogeneity of ionospheric conductivity due to the plasma density perturbations creates a polarization electric field to keep the current continuity. Oscillations of the neutral atmosphere or polarization electric fields can generate traveling ionospheric disturbances (TIDs). Some of the TIDs can subsequently initiate equatorial plasma bubbles (EPBs).
Basic properties of atmospheric acoustic and gravity waves
Transient perturbations near the Earth’s surface associated with earthquakes, Rayleigh waves, volcanic eruption, nuclear explosions, rocket launches, tsunamis, and severe tropospheric events (tropical storms, typhoons, and hurricanes) become a seed of acoustic and gravity waves propagating to the upper atmosphere (e.g., Afraimovich et al. 2001; Afraimovich et al. 2013; Artru et al. 2004; Blanc 1985; Calais et al. 1998; Calais and Minster 1998; Chou et al. 2017; Dautermann et al. 2009a, b; Fritts and Alexander 2003; Heki and ** 2005; Hines 1960; Nishioka et al. 2013; Occhipinti et al. 2013; Pokhotelov et al. 1995; Rolland et al. 2010). Because the atmospheric density exponentially decreases with increasing altitude, the amplitude of both acoustic and gravity waves grows as they propagate upward according to the energy conservation law (Hines 1960). However, a reduction in the wave amplitude also occurs due to the dissipation of the wave energy associated with molecular viscosity and thermal conductivity in the thermosphere (Hines 1960). Therefore, the dam** rate becomes more significant with a decreased atmospheric density. Thus, the amplitude reaches the maximum value at an altitude of ~ 200–300 km for waves with a period of several tens of minutes (Blanc et al. 2010).
The acoustic waves exist above the acoustic cutoff frequency (\({\omega }_{a}\ge\) 3.3 mHz) and lead to pressure change due to compression and rarefaction of the atmosphere (Hargreaves 1992). The wave propagation speed equals the sound speed of ~ 330 m/s on the ground and increases to 800 m/s in the thermosphere at a 250–300 km altitude. Therefore, it takes 8–9 min for the acoustic waves to reach the ionosphere. On the other hand, gravity waves with longer wavelengths displace parcels of atmospheric mass according to the balance between the force of gravity and buoyancy (Fritts and Alexander 2003; Hargreaves 1992). The gravity waves exist below the Brunt-Väisälä frequency (\({\omega }_{ag}\le {\omega }_{b}\cong 2.9\) mHz in the troposphere). They cannot propagate vertically but in the oblique direction with a much slower sound speed. The direction of the group velocity is perpendicular to that of the phase velocity in the vertical direction (Hines 1960; Huang et al. 2019). Because of the slow speed, the travel time of gravity waves with a period of 10–15 min and horizontal phase velocity of 200–310 m/s from the ground to the ionosphere is estimated as 45–60 min (Astafyeva 2019).
When the atmospheric acoustic and gravity waves reach the ionosphere, the ionospheric plasma density fluctuates through dynamical and photochemical processes associated with the neutral atmospheric oscillations of these waves. In the region above 250 km, the atmospheric waves dominantly interact with ionospheric plasmas through momentum transfer by collisions of neutral particles with ions. Because the charged particles can move along the magnetic field line at the same velocity as ions, the velocity \({V}_{i}\) can be written as \({V}_{i}={V}_{n}{\mathrm{cos}}\theta\), where \({V}_{n}\) is the neutral particle velocity and \(\theta\) is the angle between the magnetic field line and direction of the neutral particle oscillation. Figure 1 summarizes schematically the characteristics of upward propagating atmospheric acoustic and gravity waves associated with a volcanic eruption.
Schematic view of generation of atmospheric and ionospheric disturbances associated with a volcanic eruption. After the onset of a volcanic eruption, acoustic and gravity waves are generated by local pressure and density variations. The amplitude grows with increasing altitude. The acoustic waves propagating at the sound velocity (Va) arrive in the ionosphere within ~ 8–9 min after the onset of a volcanic eruption. Because the propagation velocity of the gravity waves is much slower than that of the acoustic waves, the gravity waves reach the ionosphere within ~ 45–60 min after the eruption
An overview of the HTHH volcanic eruption and related phenomena
The HTHH volcano is located at 20.536° S, 175.382° W in the southwest Pacific Ocean and is one of the undersea volcanos rising from the seafloor to the sea surface. Remnant of the northern and western outer edges of the volcanic caldera form two islands each with a length of 2 km. The northern and western islands are named Hunga Tonga and Hunga Ha'apai. The HTHH volcano has been active since an underwater explosion south of Hunga Tonga occurred in 1912. All recorded eruptions were along the rim of the underwater caldera (Fig. 2). During an explosive eruption from December 2014 to January 2015, a tuff cone appeared over the sea level, connecting the Hunga Ha’apai and Hunga Tonga islands (Garvin et al. 2018). The new island existed over the sea surface until a large eruption destroyed it, leaving only the two small islands. On 15 January 2022, the HTHH volcano erupted with two main explosions that released an enormous amount of energy into the atmosphere. Different types of atmospheric and ionospheric disturbances after the eruption were globally observed with other techniques. Due to the extensive feature, many researchers in various fields have studied this event.
Two-dimensional map of the summit platform of the submerged HTHH volcanic edifice. The color bar of the upper left side shows the depth of the sea surface in a unit of meters. The dashed black line indicates a previously undocumented caldera existing 150 to 180 m below the sea surface. The close-up view of the bottom left side represents the detailed locations of the 1988 eruptions (Cronin et al. 2017)
The unique and explosive eruption of the HTHH volcano eruption on 15 January 2022 provided us with a few important aspects: the exact onset time of the eruption, the energy release into the lithosphere and atmosphere, the time evolution of the ash plume, global atmospheric oscillations, and the tsunami caused by this eruption. According to the United States Geological Survey (USGS) earthquake catalog, this event has been recorded as an earthquake with magnitude scale 5.8 that occurred at 04:14:45 UT. The equivalent energy released in the lithosphere was inferred between 4 and 18 Megaton of trinitrotoluene (TNT) (Garvin et al. 2022), while that in the atmosphere and ionosphere was estimated as 50 Megaton (Campbell-Brown et al. 2022), more than 58 Megaton (Kulichkov et al. 2022) and 9–37 Megaton (Astafyeva et al. 2022). The volcanic explosivity index (VEI) value is estimated at 5 (Zhao et al. 2022). The ash plume reached ~ 55 km in the mesosphere and 600 km in diameter, captured by satellite images (National Aeronautics and Space Administration (NASA) 2022). This eruption generated atmospheric waves (e.g., Lamb waves, acoustic waves, and gravity waves), ionospheric disturbances, geomagnetic field variations, tsunamis, and seismic waves observed all over the world (e.g., Aa et al. 2022a; Aryal et al. 2023; Astafyeva et al. 2022; Boyde et al. 2022; Carr et al. 2022; Carter et al. 2023; Chen et al. 2022, 2022) interpreted that the TIDs are driven by electromagnetic coupling through the magnetic field lines in both hemispheres. This finding implies that the atmospheric disturbances triggered by the HTHH volcanic eruption could generate the TIDs with magnetic conjugacy even in the daytime, which had not been previously reported in the literature. To investigate the magnetic conjugate feature of the TIDs, Shinbori et al. (2022) compared the global GNSS-TEC data with the ionospheric plasma flow data obtained from the super dual auroral radar network (SuperDARN) Hokkaido pair of radars (Nishitani et al. 2019). As a result, Shinbori et al. (2022) found the plasma flow perturbations in the F-region corresponding to the TEC perturbations with a magnetic conjugate feature. However, they did not show whether or not an analogous magnetic conjugate feature can be seen in the plasma flow and detrended TEC data when the air pressure wave passed through the field-of-view (FOV) of the SuperDARN radar. For this reason, we investigate the relationship between the northward plasma flow in the northern hemisphere and detrended TEC in the southern hemisphere in addition to the results reported by Shinbori et al. (2022). Figure 6 shows a range-time plots of (a) northward plasma flow, (b) 15-min high-pass filtered northward plasma flow, (c) and (e) temperature deviation in the northern and southern hemispheres, respectively, and (d) detrended TEC in the southern hemisphere. In Fig. 6, the two distinct plasma flow perturbations with an amplitude of ~ 100 m/s were observed around 08:00 and 09:00 UT in the northern hemisphere, simultaneously with the onset of the TEC perturbations after the arrival of air pressure waves in the southern hemisphere (Shinbori et al. 2022). A similar feature of the plasma flow variations in the ionosphere was reported by Zhang et al. (2022a, 2022)
Range-time plots of a northward plasma flow, b 15-min high-pass filtered northward plasma flow, c, e temperature deviation, and d detrended TEC. The first three panels show the data along the beam-8 direction of the SuperDARN radar in the northern hemisphere, indicated by the red line in panels f, g, while the rest panels show the data along the magnetically conjugate beam-8 direction in the southern hemisphere. Panels f, g show the two-dimensional map of temperature deviation at 08:00 and 12:30 UT on 15 January 2022. These times are represented by the vertical dashed lines in the left panel. The blue and red curves indicate the sunset terminator at an altitude of 105 and 300 km, respectively
Rapid changes of the E-region dynamo and thermospheric wind
As described in “Global propagation features of traveling ionospheric disturbances” section, the HTHH volcanic eruption generated global atmospheric and ionospheric disturbances observed with various kinds of ground-based and satellite instruments. This volcanic eruption also caused a rapid change in the E-region dynamo and thermospheric wind pattern. Using ion density and drift velocity data obtained from the ICON satellite, Gasque et al. (2022) found extreme zonal and vertical electric field ion drifts ~ 4000 km away from the HTHH volcano within 1 h of the eruption before the initial arrival of atmospheric disturbances. The period and amplitude of the vertical ion drift were ~ 6 min and ~ 100 m/s, respectively. The observation point was almost consistent with the magnetically conjugate E-region just 400 km from the HTHH volcano. This result suggests that the electric field generated by the E-region dynamo around the HTHH volcano is transmitted along the magnetic field lines. Gasque et al. (2022) also estimated the amplitude of thermospheric wind perturbations as more than 200 m/s based on the observed ion drift velocity and a simple theoretical model.
After the air pressure wave passed through the dayside, the Swarm A and Ionospheric Connection Explorer (ICON) satellites observed the direction change of the equatorial electrojet (EEJ) and neutral wind (90–300 km) (Fig. 7). In Fig. 7g, h, the height profile of the zonal wind shows an enhancement of eastward and westward winds in the Hall (90–110 km) and Pedersen (120–150 km) regions, respectively, when the EEJ is directed westward (Fig. 7f). This relationship between the EEJ and thermospheric neutral wind is almost consistent with that reported by Yamazaki et al. (2014) and Yamazaki et al. (2021). The physical meaning of the anti-correlation between the EEJ and the thermospheric neutral wind is the local generation of the electric field. In the Hall region, an eastward wind generates an upward dynamo electric field which drives the eastward current due to the westward drift of the electrons. Finally, the westward electric field is generated in the Hall region. In the Pedersen region, an eastward wind generates an upward current due to the upward dynamo electric field. This current produces a downward electric field. The electric field is mapped onto the Hall region and drives the westward current off the equatorial region in both hemispheres. Finally, this current generates an eastward electric field. As a result, the westward EEJ is observed near the equatorial region. Le et al. (2022) also showed that strong eastward turning of zonal wind in the E-region leads to the direction change of the EEJ using space- and ground-based observation data. The effect of neutral wind on the EEJ had been predicted by Yamazaki et al. (2014), and a few years later Harding et al. (2022) demonstrated through observations that the neutral wind in the thermosphere plays an important role in the EEJ perturbations based on integrated data analysis of upper atmospheric disturbances after the large HTHH volcanic eruption.
a, b Trajectories of the ICON (red) and Swarm (green) satellites, ground magnetic observatories (yellow), and a wavefront of the air pressure wave from the HTHH volcano with the propagation speed of 318 m/s (black). Dots indicate each satellite location at the given time. c, d Height profiles of zonal wind at the exact locations above (e, f) EEJ observations on each orbit of the Swarm A satellite. g, h Height profiles of zonal wind on each orbit of the ICON satellite. The statistical value of the background wind (gray shaded areas and solid black lines) corresponds to percentiles of the entire data set. The background wind (black dotted line) is calculated with the four previous days (Harding et al. 2022)
Using the data of the X- and Y-components of the geomagnetic field obtained at 13 geomagnetic observatories from mid-latitudes to the equator, Sun et al. (2022a, b, c) showed that the HTHH volcanic eruption caused perturbations of the E-region current density by 22–55 mA/m within a radius of 8000 km from the HTHH volcano. Corresponding to the appearance of the ionospheric currents, the TEC data showed the start of ionospheric electron density variation in the F-region at an altitude of ~ 350 km. From the onset of the magnetic field perturbations, Sun et al. (2022a, b, c) estimated the propagation speed of the leading front as ~ 740 m/s, which corresponds to the acoustic velocity in the ionosphere (thermosphere) at an altitude of 200–300 km (Shinbori et al. 2023). The magnetic field disturbances from several minutes to hours persisted for more than 10 h after the HTHH eruption.
It is well-known that geomagnetic field variation associated with acoustic waves is frequently observed with a period of several mHz range after earthquakes and volcanic eruptions (e.g., Iyemori et al. 2005; Hasbi et al. 2009; Aoyama et al. 2016; Zettergren and Snively 2019). Also, in the case of the HTHH volcanic eruption, the geomagnetic field oscillations were observed in the pulsations continuous (Pc) 5 range (150–600 s or 1.7–6.7 mHz) (Fig. 8). The dominant frequency of the geomagnetic field oscillations was 3.8 mHz, which is almost equal to the acoustic resonance frequency (e.g., Inchin et al. 2020; Kanamori et al. 1994; Lognonné et al. 1998; Matsumura et al. 2012, 2011; Shinagawa et al. 2007; Tahira 1995). This frequency corresponds to the period required for the acoustic waves to go back and forth between the ground and the ionosphere. Yamazaki et al. (2022) described that the amplitude of the geomagnetic field oscillations reached ~ 3 nT at Apia, located 835 km north-northeast of the HTHH volcano, which is much larger than those during other events previously reported. Further, Yamazaki et al. (2022) found that the waveform of the geomagnetic field oscillations at Apia resembles the waveform observed at Honolulu, located near the magnetic conjugate point of the HTHH volcano, as shown in Fig. 8. The amplitude of the geomagnetic field oscillation at Honolulu was much smaller than at Apia. The amplitude difference could be due to the local time dependence on ionospheric conductivities (Iyemori et al. 2022). Iyemori et al. (2022) estimated the Pedersen and Hall conductivities to be 8.3 S and 12.6 S at Apia and 1.9 S and 3.2 S at Honolulu, respectively. However, because the observed amplitude difference cannot be explained only by the ionospheric conductivity difference, Iyemori et al. (2022) suggested that the geomagnetic longitudinal difference may also be one of the causes of the observed amplitude difference. The phase of the geomagnetic field oscillation at Honolulu is opposite to that at Apia. A possible mechanism of the anti-phase oscillation is shown in Fig. 9 (Iyemori et al. (2022)). In this figure, after the HTHH volcanic eruption, an eastward electric field is generated in the dynamo layer at an altitude of the E-region, and the field-aligned currents flow between the northern and southern hemispheres. Then, a polarization electric field accompanying the field-aligned currents is transmitted along the magnetic field lines to another hemisphere. The dynamo and polarization electric fields drive the Hall currents in both hemispheres, shown in Fig. 9. Because the Hall current direction is anti-parallel to each other on the left and right sides of the footprint of the field-aligned current, the phase of the observed geomagnetic field variations shows an anti-correlation, depending on the location of the geomagnetic observatories. Iyemori et al. (2022) and Yamazaki et al. (2022) interpreted that the geomagnetic field oscillations are caused by the effect of the ionospheric dynamo in the sunlit region. Thus, simultaneous detection of the geomagnetic field oscillations at magnetic conjugate points has not been reported before. Yamazaki et al. (2022) pointed out that modeling studies are essential for understanding the three-dimensional distribution of the ionospheric current caused by the HTHH volcanic eruption.
High-pass filtered Y component of the geomagnetic field at Apia, Honolulu, and Pamatai with a cut-off frequency of ~ 0.8 mHz. The green boxes indicate the time interval where the waveform of ΔY at Honolulu is similar to that at Apia (Yamazaki et al. 2022)
A schematic view of the relationship among the HTHH volcanic eruption, geomagnetically conjugate point, inter-hemispheric field-aligned currents, electric field transmission, ionospheric Hall current, and magnetic effect on the ground. In this figure, the symbol ‘V’ represents the neutral wind velocity in the ionospheric dynamo layer (mainly E-layer) caused by the HTHH eruption. Ep and V × B indicate the polarization and Lorentz electric fields (Iyemori et al. 2022)
To give further confirmation and insight of the magnetic conjugate feature of 4-min oscillations reported by Iyemori et al. (2022) and Yamazaki et al. (2022), we analyzed the northward plasma flow observed by the SuperDARN Hokkaido east radar. The result is shown in Fig. 10. In this figure, two negative excursions of the northward plasma flows appeared with their amplitudes of approximately 100 m/s around 08:10 UT and 08:50 UT. These phenomena have already been reported by Shinbori et al. (2022). After the large plasma flow perturbations, the short-period oscillations were observed for approximately 20 min indicated by the horizontal red arrows. The average amplitude and period are 30 m/s and 4 min, respectively. The period of the northward plasma flow variations is almost consistent with that derived from different parameters (e.g., Iyemori et al. 2022). Therefore, it can be thought that the plasma flow oscillations are generated by the acoustic resonance triggered by the air pressure waves. In the range-time plots in Figs. 10a, c, e, the 4-min plasma flow oscillations tend to move in the direction of the decreasing range gate with time. This result indicates that the plasma flow oscillations propagate westward when we consider that the beams-6, 7, and 8 of the SuperDARN Hokkaido east radar are directed in the north-east direction. The propagation direction is almost the same as that of the two main plasma flow variations reported by Shinbori et al. (2022). Considering the propagation feature and occurrence of the 4-min oscillations before the initial arrival of the air pressure wave to the FOV of the SuperDARN radar, the observed 4-min oscillations can be generated by a transmission of an electric field along the magnetic field line from the sunlit southern hemisphere as shown in a model of Shinbori et al. (2022). Because the magnetic field intensity and inclination at an altitude of the ionosphere (300 km) are calculated as 52,084.8 nT and 62.83 degrees with the IGRF-13 model, we can further estimate the eastward electric field of the 4-min oscillations as approximately 1.39 mV/m. This value is smaller than that of the two main plasma flow variations shown by Shinbori et al. (2022).
Range-time plots of the northward plasma flow in the beam-6, 7, and 8 directions observed by the SuperDARN Hokkaido east radar (a, c, e). The color bar of each panel indicates the northward component of the line-of-site velocity in a range from − 150 to 150 m/s. Panels b, d, f show the time-series plots of the northward plasma flow at the range gate number of 17 in each beam. The horizontal red arrows in the panels b, d, f indicate the observed periods of 4-min oscillations. The phase variation of the 4-min oscillations for the range gate number is shown in the dashed blue arrows
Appearance of an ionospheric hole over the HTHH volcano
Electron density depletions (“hole”) in the ionosphere had been observed after large earthquakes such as the 2011 Tohoku-Oki earthquake (e.g., Astafyeva et al. 2013a, b; Kakinami et al. 2012). Astafyeva et al. (2013b) reported that the magnitude and duration of the ionospheric hole depend on the earthquake's magnitude and interpreted that the hole is generated by the rarefaction of the shock-acoustic waves triggered by earthquakes and tsunamis. For the 2011 Tohoku-Oki earthquake, the ionospheric hole was formed after 30–50 min, and the TEC value decreased by − 5 to − 6 TECU from the level before the earthquake (Astafyeva et al. 2013a, b). Such an ionospheric hole appeared with an amplitude of − 18 to − 13 TECU over the HTHH volcano after the large eruption. This phenomenon persisted for at least 1.5–2 h (Astafyeva et al. 2022). The huge ionospheric hole with a magnitude of more than 10 TECU extended around 2000 km away from the epicenter of the HTHH volcanic eruption (Aa et al. 2022a). Astafyeva et al. (2022) proposed that the HTHH volcanic eruption generated stronger shock waves with large amplitude and prolonged the rarefaction phase compared to those from other earthquakes. Aa et al. (2022a) indicated that the ionospheric hole consists of cascading TEC decreases associated with different acoustic waves.
Because the TEC value is an integration of electron density in the ionosphere along the path between the satellite and receiver on the ground, we cannot obtain information on the vertical profile of electron density and its variation from the GNSS-TEC observation. Further, we cannot investigate the temporal and spatial evolution of the TEC variation in most of the ocean areas due to the lack of ground-based receivers. To solve the above problems, Sun et al. (2022a) examined the electron density variations in the vertical direction associated with the HTHH volcanic eruption using the electron density profile obtained from the RO technique onboard the satellites of the FORMOSAT-7/COSMIC2 mission. Figure 11 shows the vertical profiles of the average electron density in the area near the epicenter of the HTHH volcanic eruption. In Fig. 10a, the peak value of the electron density is ~\(10\times {10}^{5}\) el/m3, and the height of the peak value is ~ 330 km during 00:00–04:15 UT on 13–15 January. These electron density profiles are almost consistent with those under normal conditions before the HTHH volcanic eruption at 04:15 UT on 15 January. During 04:15–10:00 UT after the eruption, the electron density profile on 15 January shows a significant decrease above 250 km, compared with that on 13 and 15 (Fig. 11b). The F-region of the ionosphere almost disappeared and did not satisfy the usual Chapman layer (Davies 1990) anymore in the nighttime during 10:00–18:00 UT on 15 January (Fig. 11c). In Fig. 10d, the electron density profile on 15 January shows the recovery to reach the same level as that on other days after sunrise. Sun et al. (2022a) also showed that the disappearance of the F-region of the ionosphere with a horizontal scale of ~ 4000 km persisted for ~ 8 h over the HTHH volcano due to the eruption. However, it has been believed that the ionospheric hole is generated by the vertical motion of the neutral atmosphere due to earthquakes and tsunamis (Kakinami et al. 2012; Kakinami et al. 2012). The disappearance of the Chapman-layer structure was not identified in the case of the 2011 Tohoku earthquake (Liu et al. 2019). Therefore, we need to consider another process that may cause the disruption of the Chapman-layer structure associated with an explosive eruption. Sun et al. (2022a) proposed that the explosive eruption released the molecular particles up to the upper atmosphere, which reduced the ratio of the atomic and molecular densities (e.g., [O]/[N2]) and increased the recombination process of ionospheric plasmas. This process seems to maintain the ionospheric hall or the disruption of the Chapman-layer structure for several hours after the HTHH volcanic eruption. Further, Sun et al. (2022a) concluded that the disappearance of the major ionospheric layers leads to the electron density depression due to the explosive eruption of the HTHH volcano, which is much more effective than the 2011 Tohoku earthquake and tsunami for the upper atmosphere and ionosphere. Accelerometer data from the Gravity Recovery And Climate Experiment—Follow On (GRACE-FO) and Swarm-C observations showed that a large-scale neutral density depletion was formed within a radius of approximately 10,000 km around the epicenter and this structure persisted for a long time (Li et al. 2023a, b). The location of the neutral density depletion was almost consistent with that of the TEC depletion in the ionosphere (Aa et al. 2022a; Astafyeva et al. 2022). On the other hand, He et al. (2023) discussed the relative contributions of the HTHH eruption and the geomagnetic storm that occurred on 14 January 2022 before the HTHH eruption, based on an integrated analysis of GNSS-derived vertical TEC, O/N2 ratio by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED)/Global Ultraviolet Imager (GUVI), ion density and temperature by ICON/ion velocity meter (IVM), and—thermosphere ionosphere electrodynamics general circulation model (TIEGCM). Their results showed that the TEC near the HTHH volcano decreased by ∼80–95% below the quiet-time values. Approximately 20 percent of the TEC depletion was caused by the thermospheric composition changes due to a moderate geomagnetic storm. In the future, the detailed physical mechanism of the formation of ionospheric holes should be examined with the atmosphere–ionosphere coupling model.
Comparison of the average electron density profiles (a) before and b–d after the large HTHH volcanic eruption on 15 January and those on the two days before (13 and 14 January) and the one day after (16 January). The error bar indicates the standard deviation of the electron density at each altitude. The F2 layer vanished between 10:00 and 18:00 UT (Sun et al. 2022a, b, c)
Generation of plasma bubbles in the equatorial and low-latitude ionosphere
Equatorial plasma bubble (EPB) is one of the ionospheric irregularities with a sharp plasma density depletion frequently observed in the nighttime equatorial and low-latitude ionosphere. The EPB is generated by the Rayleigh–Taylor (R–T) instability at the bottom side of the F-region. Atmospheric waves may seed the R–T instability. The growth rate of the R–T instability is enhanced by the prereversal enhancement (PRE) of the eastward electric field around the duskside terminator of the ionosphere (e.g., Kelley 2009; Abdu 2019). As the EPBs are developed, the region of plasma density depletion extends to higher altitudes through the polarization electric field inside the EPBs. The EPB structure is formed along the magnetic field lines and mapped to the low-latitude ionosphere away from the equator (Otsuka et al. 2002; Keskinen et al. 2003; Sori et al. 2022). The plasma density irregularity associated with the EPB causes signal fading, scintillation, and loss-of-lock in satellite-based communication and navigation systems in a wide region (e.g., Seo et al. 2009; Alfonsi et al. 2013). Therefore, it is essential to clarify the occurrence features of EPB to reduce the possible risks of the influence of radio wave propagation. Although the day-to-day variability of the EPB occurrence has not yet been fully understood due to the atmospheric and ionospheric conditions, including seeding sources and driving forces (e.g., Li et al. 2021), the explosive HTHH volcanic eruption provides us with an excellent opportunity to demonstrate that the EPB can be induced by lower atmospheric disturbances originating from the eruption.
For the first time, Aa et al. (2022a) found equatorial plasma troughs and EPBs with small-scale plasma density irregularities over the Asia-Oceania area after the HTHH volcanic eruption from ground-based GNSS-TEC and Swarm/ICON satellite observations. The onset time of an enhancement of the rate of the TEC index (ROTI) enhancement (Pi et al. 1997) that indicated the EPB occurrence was almost coincident with the arrival time of a Lamb wave at a speed of ~ 315 m/s. The ICON-IVM observation data showed upward plasma drifts of 60–120 m/s in the dusk sector, whose ionospheric motion creates one of the favorable conditions of the EPB generation (Aa et al. 2022a). Sun et al. (2022a, b, c) also reported the EPB occurrence at midlatitudes (up to ~ 35° in geographic latitude) over China after the passage of the sunset terminator at an E-region altitude (100 km). Further, Sun et al. (2022a, b, c) pointed out that the EPB occurrence associated with the HTHH volcanic eruption had three interesting aspects compared with that in a usual case. (1) Over the Asian sector, EPB is frequently observed in the equinox of solar maximum (e.g., Shi et al. 2011; Buhari et al. 2017). In the HTHH case, EPB was activated in the northern hemisphere’s winter under the solar minimum condition, and the EPB occurrence was not recognized except for the HTHH eruption day (Shinbori et al. 2023) (2) The EPB usually tends to move eastward (e.g., Fejer et al. 2005), but the EPB associated with the HTHH volcanic eruption moved westward. (3) EPB is mainly observed at low latitudes within \(\pm {20}^{\circ}\) under the solar minimum condition (Li et al. 2021). However, in the HTHH case, the upper limit of the observed latitude of EPB extended to midlatitudes more than \({30}^{\circ}\) N that can be categorized as super plasma bubbles (e.g., Aa et al. 2018; Cherniak and Zakharenkova 2016; Ma and Maruyama 2006; Rajesh et al. 2022; Shinbori et al. 2023). Shinbori et al. (2023) reported that the EPB observed over 100–150° E reached ~ 30° N in geomagnetic latitude after the initial arrival of the air pressure waves propagating in the troposphere (Fig. 12a). The apex altitude is estimated as ~ 3000 km. To confirm the EPB occurrence in the ionosphere, Shinbori et al. (2023) analyzed the electron density data derived from in-situ plasma waves (upper limit frequency of upper hybrid resonance (UHR) waves) observed by the plasma wave experiment (PWE)-high-frequency analyzer (HFA) instrument (Kumamoto et al. 2018) onboard the Arase satellite (Miyoshi et al. 2018a). For the derivation of the electron density data, Shinbori et al. (2023) used magnetic field data obtained from the Arase magnetic field (MGF) instrument (Matsuoka et al. 2018a). The electron density variation shows several depletions of one or two orders of magnitude after the rapid increase associated with the initial arrival of the air pressure waves (Fig. 12b). The occurrence region of electron density depletions is almost consistent with the enhanced ROTI region (Fig. 12c). In this case, the Arase satellite traveled in the evening sector after the sunset terminator in the ionosphere, and the EPB was detected from 400 km (ionosphere) to at least 2000 km (low-altitude plasmasphere). Such super plasma bubbles could be caused by the combination of volcano-induced atmospheric perturbations (Shinbori et al. 2023) and strong PRE in the evening ionosphere (e.g., Rajesh et al. 2022).
Observation of the EPB and air pressure wave between 11:20 to 12:00 UT on 15 January 2022. a Two-dimensional map of ROTI and the Himawari-8 temperature deviation (d3) at 11:40 UT, indicated by the color and gray scales, respectively. The yellow and red lines represent the 105 and 300 km sunset terminators. The horizontal dashed curves show the geomagnetic latitude every 10°. b Electron density observed by the Arase satellite and geographic latitude-time plot of Himawari-8 d3 data along the geographic longitude of the Arase satellite location. c Two-dimensional map of ROTI with the electron density along the Arase satellite path in the same period as in Fig. 11b. The smoothed gray curve is the Arase satellite orbit, while the thick orange line around the smoothed curve is the electron density variation (Shinbori et al. 2023)
To influence the propagation of electromagnetic waves in the ionosphere associated with the EPB, we investigated plasma wave dynamic spectra of the electric field component obtained from the PWE-HFA instrument onboard the Arase satellite. Figure 13 shows the plasma wave dynamic spectra in a frequency range from 3 kHz to 6 MHz when the Arase satellite encountered an air pressure wave propagating in the troposphere shown in Fig. 12b. In Fig. 13, a lot of narrow-banded line spectra appeared with a constant frequency for time above the UHR frequency indicated by the pink line after the encounter of the air pressure wave. These phenomena have been called the hectometric line spectra (HLS) (Hashimoto et al. 2018). The HLS in a frequency range from 1.0 to 1.5 MHz were trapped inside the electron density depletions corresponding to EPBs. Hashimoto et al. (2018) identified the origin of the HLS of the frequency component from 525 kHz to 1.7 MHz as amplitude modulation (AM) broadcasting waves propagating from the ground through the electron density depletions. Further, Hashimoto et al. (2021) confirmed that the HLS were frequently observed in the occurrence region of EPBs or low electron density region in the post-midnight sector using GNSS-TEC and Arase plasma wave observations. Therefore, appearance of such HLS in space found in the present study indicates that broadcasting waves in a wide frequency range escaped to space through the low electron density region produced by the super plasma bubbles triggered by the HTHH volcanic eruption. Therefore, it can be considered that such super EPBs severely impacted the propagation of the HF broadcasting waves between the ground and ionosphere (Fig. 13).
Dynamic spectra of plasma waves in a frequency range from 3 to 6000 kHz obtained from the PWE-HFA instrument onboard the Arase satellite. The pink and red curves indicate the upper limit frequency of the UHR waves and the electron cyclotron frequency, respectively. The vertical dashed line is the time when the Arase satellite encountered an air pressure wave propagating in the troposphere. The right color bar shows the power spectrum of the electric field in a unite of mV2/m2/Hz
Huba et al. (2023) investigated the impact of the HTHH volcanic eruption on the ionosphere and plasmasphere using the coupled SAMI3 model (Sami3 is also a model of the ionosphere/plasmasphere) and the high altitude mechanistic general circulation model (HIAMCM) whole atmosphere model in addition to primary gravity wave effects from the model for gravity wave sources, ray tracing and reconstruction (MESORAC) model. Their simulation results showed that the HTHH volcanic eruption generated a super EPB extending to ∼30° in longitude and up to 500 km in altitude with an electron density depletion of 3 orders of magnitude. Further, Huba et al. (2023) found that the EPB reached high altitudes of more than 4000 km, which is much higher than that shown in previous simulation studies (e.g., Huba and Liu 2020). This result is consistent with the Arase satellite observation in space reported by Shinbori et al. (2023).
Carter et al. (2023) demonstrated the impacts of the HTHH volcanic eruption on the ionosphere and what subsequent impacts they had on precise point positioning (PPP) with a network of ionosondes located throughout the Australian region together with GNSS receivers. As a result, PPP accuracy was not significantly affected by the TIDs and Spread-F under the condition that PPP convergence had already been completed. However, when the PPP procedure was started either shortly before or after the TID arrivals, it took much more time (~ 5 h) to achieve the PPP convergence. Carter et al. (2023) also found that the convergence times were affected by a super EPB which is the largest phenomenon observed over Australia to date. It showed a depression of ∼42 TECU and propagated eastwards at 30 m/s. Based on these results, Carter et al. (2023) concluded that the HTHH volcanic eruption was a good example of how ionospheric disturbances could adversely influence satellite-based precise positioning.
