Modeling studies have predicted that the acoustic resonance of the atmosphere during geophysical events such as earthquakes and volcanos can lead to an oscillation of the geomagnetic field with a frequency of about 4 mHz. However, observational evidence is still limited due to scarcity of suitable events. On January 15, 2022, the submarine volcano Hunga Tonga-Hunga Ha’apai (20.5˚S, 175.4˚W, Tonga) erupted in the Pacific Ocean and caused severe atmospheric disturbance, providing an opportunity to investigate geomagnetic effects associated with acoustic resonance. Following the eruption, geomagnetic oscillation is observed at Apia, approximately 835 km from Hunga Tonga, mainly in the Pc 5 band (150-600 s, or 1.7-6.7 mHz) lasting for about 2 hours. The dominant frequency of the oscillation is 3.8 mHz, which is consistent with the frequency of the atmospheric oscillation due to acoustic resonance. The oscillation is most prominent in the eastward (Y) component, with an amplitude of ~3 nT, which is much larger than those previously reported for other events (<1 nT). Comparably large oscillation is not found at other stations located further away (>2700 km). However, geomagnetic oscillation with a much smaller amplitude (~0.3 nT) is observed at Honolulu, which is located near the magnetic conjugate point of Hunga Tonga, in a similar wave form as at Apia, indicating interhemispheric coupling. This is the first time that geomagnetic oscillations due to the atmospheric acoustic resonance are simultaneously detected at magnetic conjugate points.

The Hunga Tonga-Hunga Ha’apai volcano in the Pacific Ocean erupted on January 15, 2022. The energy released by this submarine eruption caused waves propagating through the lithosphere, ocean and atmosphere. Less than 10 minutes after the eruption, pulsation-like geomagnetic disturbances started at the geomagnetic observatory Apia, approximately 835 km from Hunga Tonga, and lasted for about 2 hours. These disturbances were most prominent in the Y (east) component, with an oscillation amplitude of ~3 nT and dominant periods of 276, 254 and 219 s. Comparable geomagnetic disturbances are absent at neighboring as well as high-latitude geomagnetic observatories, indicating that the disturbances are localized and not related to solar wind energy input. Tide gauge data show that tsunami waves arrived at Apia more than one hour after the eruption. This leaves ionospheric currents as the likely cause of the geomagnetic disturbances.

This paper examines the response of the upper atmosphere to equatorial Kelvin waves with a period of ~3 days, also known as ultra-fast Kelvin waves (UFKWs). The whole atmosphere model GAIA is used to simulate the UFKW events in the late summer of 2010 and 2011 as well as in the boreal winter of 2012/2013. When the lower layers of the model below 30 km altitude are constrained with meteorological data, GAIA is able to reproduce salient features of the UFKW in the mesosphere and lower thermosphere as observed by the Aura Microwave Limb Sounder. The model also reproduces ionospheric response, as validated through comparisons with total electron content data from the GOCE satellite as well as with earlier observations. Model results suggest that the UFKW produces eastward-propagating ~3-day variations with zonal wavenumber 1 in the equatorial zonal electric field and F-region plasma density. Model results also suggest that for a ground observer, identifying ionospheric signatures of the UFKW is a challenge because of ~3-day variations due to other sources. This issue can be overcome by combining ground-based measurements from different longitudes. As a demonstration, we analyze ground-based magnetometer data from equatorial stations during the 2011 event. It is shown that wavelet spectra of the magnetic data at different longitudes are only in partial agreement, with or without a ~3-day peak, but a spectrum analysis based on multipoint observations reveals the presence of the UFKW.