Individual Wave Propagations in Ionosphere and Troposphere Triggered by the Hunga Tonga-Hunga Ha’apai Underwater Volcano Eruption on 15 January 2022

Abstract

The devastating Hunga Tonga-Hunga Ha’apai underwater volcano erupted at ~04:15 UT on 15 January 2022. We captured the waves that erupted from the volcano propagating in the ionosphere by monitoring total electron content (TEC) perturbations utilizing ground-based global navigation satellite system (GNSS) receivers that receive electromagnetic signals transmitted from the geostationary satellites operated by the BeiDou Navigation Satellite System (BDS). Meanwhile, ground barometers detected unusual enhancements of air pressure traveling in the troposphere. A novel phenomenon shows that the waves can individually propagate with a speed of ~335 m/s in the ionosphere, which is faster than its’ ~305 m/s in the troposphere. We further examined multiple geophysical data at the particular site of the novel instrumental array. Analytical results show that the pressure enhancements traveling in the troposphere not only downward trigger ground vibrations mainly in the horizontal components without obvious time difference, but also upward, leading the secondary TEC perturbations with a ~12-min delay.

1. Introduction

Intense interactions between the Indo-Australian Plate and the Pacific Oceanic Plate cause major earthquakes and volcanoes along the Tonga-Kermadec subduction zone. The devastating Hunga Tonga-Hunga Ha’apai underwater volcano (20.53°S, 175.38°W) erupted at ~04:15 UT on 15 January 2022. The satellite images captured the eruption, which caused a 5 km wide plume of ashes and gases (e.g., himawari8, FY-3E, GOES West, AIRS/Aqua), rising approximately 20 km above the volcano. The eruption excited air blast in the atmosphere. Air pressure increased with ~700 hPa in New Zealand and ~2.5 hPa in Switzerland through long-distance propagation. Meanwhile, the eruption caused intense ground vibrations and tsunami. The intensity of ground vibrations triggered by the eruption is relevant to a Ms 5.8 earthquake. The underwater eruption also generated tsunami that mainly propagated in the Pacific Ocean. The previous studies reported that perturbations in total electron content (TEC) can be triggered by earthquakes, seismic waves, volcano eruptions, and tsunami [1,2,3,4,5]. Heki [6], Dautermann et al. [7], and Shults et al. [8] reported that TEC perturbations with an amplitude of ~0.2 TECU (1 TECU (TEC unit) = 1 × 10¹⁶ el/m²) were triggered by volcano eruptions in Japan, Montserrat, and South America, respectively. The propagation velocity of the TEC perturbations mainly ranged from ~600 to 1200 m/s [6,7,8]. The Hunga Tonga-Hunga Ha’apai volcano under the sea surface differs from the volcanoes on land, as has been reported in previous studies [6,7,8]. The eruption of The Hunga Tonga-Hunga Ha’apai volcano triggered significant blast waves that are quite unique and rare. The severe TEC perturbations could be generated by the eruption of the underwater volcano. In this study, we captured the waves in the ionosphere and troposphere by, respectively, utilizing changes in TEC, and air pressure. We estimated propagation speeds of the waves utilizing the ground-based global navigation satellite system (GNSS) receiver arrays and ground barometers to investigate the potential sources and expose possible interactions between the surface and ionosphere after the eruption.

2. Observation Results from GNSS Arrays

Ground-based GNSS receivers are usually utilized to detect TEC in the ionosphere. An integration of the electron number density between a ground-based receiver and a GNSS satellite is the so-called GNSS sTEC (slant TEC). The GNSS sTEC can be estimated according to the delay and changes in electromagnetic signals transmitted from satellites passing through the ionosphere [9]. The TEC observations from the GNSS, such as the Global Positioning System (GPS), can monitor ionospheric plasma irregularities [10] and traveling ionospheric disturbances [11] happening in the ionosphere frequently. However, most GNSS satellites orbiting the Earth measure TEC depending on elevation cut-off angles as a function of time. The BeiDou Navigation Satellite System (BDS) includes five geostationary (GEO) satellites over the equator in the Asia longitudinal sector [12] (Figure 1). The BDS GEO satellites hang in space at ~36,000 km above the Earth’s surface and continuously provide TEC observations 24 h a day with nearly constant elevation angles. The ionospheric pierce points (IPPs) for the BDS GEO satellites act as space buoys and seismometers to monitor TEC disturbances at particular places 350 km over the Earth’s surface [13]. We retrieved the data recorded at 215 stations (Figure 1) from the Data Sharing Infrastructure of the National Earthquake Data Center in China (http://data.earthquake.cn (accessed on 17 January 2022)) and the Crustal Dynamics Data Information System (CDDIS; https://cddis.nasa.gov/ (accessed on 17 January 2022)). The BDS sTECs in this study were computed by utilizing the pseudoranges and phases of the dual-frequency signals that are transmitted from the BDS GEO satellites (G1, G2, and G3) to the ground-based GNSS receivers [9,14,15,16] and references therein. In other words, this study included 645 IPPs above particular locations in the area of 25°S–55°S, 70°E–160°W to continuously monitor changes in BDS sTECs.

Figure 2a shows the variations in the BDS sTECs at 19 selected IPPs with an equal interval of distances away from the location of the volcano during the period of 04:00 UT–22:00 UT on 15 January 2022. The sTEC exhibits an obvious enhancement of ~10 TECU right after the eruption. The TEC perturbations are weak at distances from ~5000 to ~7500 km due to unknown reasons. The TEC perturbations propagated away from the volcano to the outlying areas at a distance > ~8000 km. The amplitude is ~20 TECU at a distances of ~8000 km and reduces to ~10 TECU at a distance of ~11,000 km. The large amplitude of the sTEC suggests that the perturbations associated with the underwater volcano propagate a long distance > ~11,000 km being significant. To mitigate the influence of TEC diurnal variations and to keep the essentials of the TEC perturbations, simultaneously, we utilized the simple process of subtracting a 1 h running average from the BDS sTECs to illustrate the eruption-triggered perturbations. Figure 2b shows the spatiotemporal distributions of all the TEC utilized in this study (also see the Movie (Video S1: Supp_Mov.avi) in the Supplementary Materials). The front of the perturbations lies on the arrival time with a velocity of ~335 m/s, according to the peaks of the TEC enhancements. The long-lasting disturbance, resonance, or TEC irregularities can be found following the front due to the eruption or tsunami [17,18], which can seriously disturb the electromagnetic wave communication [19] and may cause the disconnection of Tonga after the eruption and hinder rescue operations.

3. Observations of Air Pressure from Barometer Arrays

Air pressure measurements were retrieved from Japan Meteorological Agency (JMA; https://www.jma.go.jp/ (accessed on 17 January 2022)), Crustal Dynamics Data Information System (CDDIS; https://cddis.nasa.gov/ (accessed on 17 January 2022)), Survey and Mapping Office Lands Department, Hong Kong (https://www.geodetic.gov.hk/ (accessed on 17 January 2022)), and the Sichuan meteorological disaster prevention technology center in China. Similarly, we showed the retrieved data at the 26 stations (Figure 1) along with the distance away from the volcano. The sudden pressure enhancements riding on the typical diurnal variations can be found from the data at all the stations with the distance ranging from 0 to ~11,000 km (Figure 2c). The traveling speed of the pressure enhancements is ~305 m/s. In other words, the air pressure enhancements do not contribute to the TEC perturbations with a velocity of ~335 m/s (Figure 2b) due to the TEC perturbations leading the pressure enhancement by ~10% in velocity. To examine the leading phenomenon in detail, the measurements of air pressure and ground vibrations, as well as the BDS sTEC above the particular site of the novel instrumental array, are further compared.

4. Observations in the MVP-LAI System

The novel instrumental array officially started functioning in Leshan (29.6°N, 103.9°E), Sichuan province, China, in September 2021 [20]. The array comprises broadband seismometers, barometers, and ground-based GNSS receivers for Monitoring Vibrations and Perturbations in the Lithosphere, Atmosphere, and Ionosphere (MVP-LAI; please find the detail of the instruments in [20]. Notably, those instruments are installed and operated within a limited area of ~20 by ~20 m. Meanwhile, a ground-based GNSS receiver was established in a sub-location at the Cangxi county (CAXI) (31.8°N, 105.8°E) that is located ~300 km away from the MVP-LAI system. The IPP for the CAXI sTEC retrieved from the BDS G2 satellite is above the MVP-LAI system. Comparisons between the CAXI sTEC from BDS G2, air pressure, and ground vibrations at the MVP-LAI system benefits investigation of vibrations and perturbations in the vertical direction.

Figure 3a shows variations in the CAXI sTEC on 15 January 2022. The sTEC exhibited the typical maxima near the noon-time on 15 January 2022 (Figure 3a). The sTEC decreased after ~07:00 UT and kept at a low level in the nighttime. The CAXI sTEC variations agree with the typical variations in the TEC from the arrays (Figure 2a). Notably, the perturbations of the CAXI sTEC exhibited an obvious enhancement of ~10 TECU at ~12:50 UT, which is ~8 h and 35 min after the volcano eruption. The distance between the MVP-LAI system and the volcano is ~10,275 km. The corresponding traveling speed is ~333 m/s (=10,275,000 m/30,900 s), which agrees with the propagation velocity obtained from the GNSS arrays (Figure 2b). The enhancements are the front of the TEC perturbations propagating in the ionosphere after the eruption.

Figure 3b shows the changes in air pressure at the MVP-LAI system on 15 January 2022. An unusual enhancement with an amplitude of ~2 hPa happened in the air pressure at ~13:30 UT, which is ~40 min later than the front of the TEC perturbations. The delay of ~9 h and 15 min to the volcano eruption suggests that the traveling speed of the air pressure enhancement is ~309 m/s (=10,275,000 m/33,300 s). Figure 3c–e show the ground vibrations recorded by the broadband seismometer at the MVP-LAI system. The ground vibrations occurred in the NS and EW components near noon-time. Notably, the pronounced ground vibrations can be found at the time near the appearance of the air pressure enhancement. The pronounced ground vibrations are not observed in the raw data at the vertical component, but mainly at two horizontal components. Moreover, a secondary peak exists in the CAXI sTEC ~50 min after the front of the TEC perturbations (Figure 3a). The air pressure enhancements lead the secondary peak by ~12 min.

5. Discussion

The TEC perturbations can be attributed to the different sources depending on their propagation speeds. Large earthquakes and tsunami can induce broad-band waves with speeds ranging from a few kilometers per second (e.g., Rayleigh waves) to ~200 m/s (e.g., tsunami waves), and resonance in the ionosphere [16] and persist for several days [17]. Previous studies [6,7,8] have shown that the propagation velocity of the TEC perturbations triggered by the volcanoes ranging from ~600 to 1200 m/s refer to an acoustic wave [5]. In this study, the velocity of ~335 m/s is much slower than that reported in previous studies [6,7,8] and, therefore, acoustic waves are not relevant here. Lin et al. [21] reported that Lamb waves [22] can propagate in the ionosphere with a velocity of ~340 m/s for a long distance, which might be the reason for the significant TEC perturbations observed in this study (Figure 2a). Note that the TEC perturbations, which appeared before the arrivals of the Lamb waves far away from the volcano (beyond ~8000 km), are dominated by the rapid conjugate effect [21]. On the other hand, the pressure perturbations with a propagation velocity of ~310 m/s (Figure 2c) agree with the velocity of the Lamb waves propagating in the troposphere [22,23,24]. This suggests that Lamb waves individually propagate in the troposphere and ionosphere after the eruption. Meanwhile, the pressure enhancements travel in the troposphere with a subsonic speed that is ~10% slower than it in the ionosphere. In other words, the TEC front (Figure 3a) leads the air pressure enhancements in the troposphere.

Propagations of Lamb waves in the troposphere can generate acoustic and gravity waves [23,24]. The secondary TEC peak lags the air pressure enhancement by ~12 min (Figure 3a,b) and is caused by the upward propagation of the acoustic waves, which takes nearly 10 min from the troposphere to the ionosphere [5,25]. We further estimated the arrival time, as a function of the propagation distance, utilizing the speed of 305 m/s by adding the 12 min delay for further comparison with the spatiotemporal distribution of BDS sTEC, as shown in Figure 2b. The comparison shows that the secondary peaks following the front of the TEC perturbations are continuously triggered by the pressure enhancements traveling in the troposphere with a lower speed (i.e., ~305 m/s). The simple, but intense front, benefits us to recognize the TEC perturbations leading the pressure enhancements by ~12 min. These results show that the BDS GEO TECs together with the novel MVP-LAI system is efficient to clarify causal mechanisms of the TEC perturbations. The air pressure underlying IPPs plays an important role in determining the influence from the Lamb waves. The TEC perturbations with a propagation velocity close to a sonic speed (i.e., ~300 to 350 m/s) have generally been referred to as acoustic waves in the past, though this should be reconsidered in the future.

On the other hand, Lamb waves can trigger ground vibrations, simultaneously. Significant ground vibrations in the two horizontal components (Figure 3c,d) could result from Lamb waves that mainly propagate in the horizontal direction. Meanwhile, the air density is much smaller than it of the crust. It is difficult to trigger ground vibrations in the vertical component by Lamb waves mainly propagating in the horizontal direction around the sphere.

6. Conclusions

The global navigation satellite system (GNSS) and air pressure arrays capture the blast waves excited by the Hunga Tonga-Hunga Ha’apai underwater volcano eruption, simultaneously. The waves excited by the volcano individually propagate in the troposphere and ionosphere. The wave speed in the ionosphere is ~335 m/s, which is ~10% faster than its’ ~305 m/s propagating in the troposphere. The multiple data from the MVP-LAI system show that the waves in the troposphere trigger ground vibrations (mainly in the horizontal component) in the lithosphere without an obvious time difference. In contrast, the waves in the troposphere propagate upward and induce perturbations in the total electron content (TEC) in the ionosphere with a delay of ~12 min through the acoustic waves. The geostationary (GEO) satellites operated by the BeiDou Navigation Satellite System (BDS) are space seismometers or buoys to detect the TEC perturbations that can be utilized to support the early warning.