Magnetic field variations in Tongatapu caused by Hunga Tonga–Hunga Ha’apai volcanic activity in January 2022

Abstract The massive eruption of the Hunga Tonga–Hunga Ha’apai volcano on January 15, 2022, triggered various phenomena, such as severe lightning from plume activities, atmospheric Lamb waves, conventional tsunamis, tsunamis caused by sea surface displacement due to Lamb waves, and total electron content variations in the ionosphere. We analyzed the observed magnetic field data at ’Atele, Tongatapu (Tonga), approximately 73 km south–southeast of the volcano, revealing two distinct timescales of variation after the eruption. The first variation, with periods of approximately 256 and 278 s, were predominantly observed in the horizontal magnetic components. Previous studies have reported variations during the same period in Apia (Samoa) and Honolulu (USA). Notably, the amplitude and dominant variation direction at ’Atele and Apia differed. The amplitude at ’Atele was approximately ten times larger than that at Apia, and the dominant variation was observed in the north–south direction at ’Atele but east–west at Apia. These variations were attributed to atmospheric acoustic resonance caused by the eruption, i.e., the electric currents induced by motion in the ionosphere and the electric field propagated along the field line of the geomagnetic main field. The second variation, with a timescale of approximately 2 h, primarily manifested in the vertical magnetic component at ’Atele. In addition, the eastward component shows similar variations at ’Atele and Apia. Simple electric current models of the ionosphere could not explain the magnetic field features at ’Atele and Apia if the variations were related to localized electric current induced in the ionosphere by the volcanic eruption. The variations in the vertical component at the two stations highlight the importance of considering electromagnetic induction within the Earth and discussing the electric currents responsible for these magnetic field variations. Graphical Abstract