Browsing by Author "Nakanishi, Kunihito"

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    Barometric and magnetic observations of vertical acoustic resonance and resultant generation of field-aligned current associated with earthquakes
    (SpringerOpen, 2013-08) Iyemori, Toshihiko; Tanaka, Yoshikazu; Odagi, Yoko; Sano, Yasuharu; Takeda, Masahiko; Nose, Masahito; Utsugi, Mitsuru; Rosales Corilloclla, Domingo; Choque, Edwin; Ishitsuka, José K.; Yamanaka, Sadato; Nakanishi, Kunihito; Matsumura, Mitsuru; Shinagawa, Hiroyuki
    Three rare occasions are introduced, where the excitation of vertical acoustic resonance between the ground and the ionosphere, and the resultant generation of a field-aligned current, just after earthquakes are observationally confirmed. In the case of two inland earthquakes, barometric observations very close to the epicenters (i.e., only 30 km apart) were available, and they showed a sharp spectral peak which appeared within one hour after the origin time and lasted a few hours. The observed periods of the spectral peaks around 260 seconds are close to the period of the theoretically-expected fundamental mode of the resonance. On the other hand, magnetic observations on the ground showed a dominant period at 220–230 seconds which corresponds to the first overtone among theoretically-expected major resonance peaks. In the third case, i.e., during the 2010 Chile earthquake, a long-period magnetic oscillation in the east-west direction, which has two major resonance periods at 265 and 190–195 seconds, was observed on the night-side magnetic dip equator in Peru, where the distance is more than 2600 km from the epicenter, under a very quiet geomagnetic condition. The oscillation was interpreted as the effect of field-aligned current generated through a dynamo process in the ionosphere over the epicenter caused by the resonance.
    Palabras clave:Acoustic waveField-aligned currentEarthquake
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    Localized field-aligned currents and 4-min TEC and ground magnetic oscillations during the 2015 eruption of Chile’s Calbuco volcano
    (SpringerOpen, 2016-08-31) Aoyama, Tadashi; Iyemori, Toshihiko; Nakanishi, Kunihito; Nishioka, Michi; Rosales Corilloclla, Domingo; Veliz, Oscar; Vidal Safor, Erick
    The Calbuco volcano in southern Chile erupted on April 22, 2015. About 2 h after the first eruption, a Swarm satellite passed above the volcano and observed enhancement of small-amplitude (~0.5 nT) magnetic fluctuations with wave-packet structure which extends 15° in latitude. Similar wave packet is seen at the geomagnetic conjugate point of the volcano. Just after the eruption, geomagnetic fluctuations with the spectral peaks around the vertical acoustic resonance periods, 215 and 260 s, were also observed at Huancayo Geomagnetic Observatory located on the magnetic equator. Besides these observations, around 4-min, i.e., 175, 205 and 260 s, oscillations of total electron content (TEC) were observed at global positioning system stations near the volcano. The horizontal propagation velocity and the spatial scale of the TEC oscillation are estimated to be 720 m/s and 1600 km, respectively. These observations strongly suggest that the atmospheric waves induced by explosive volcanic eruption generate TEC variation and electric currents. The Swarm observation may be explained as a manifestation of their magnetic effects observed in the topside ionosphere.
    Palabras clave:Magnetic fieldMagnetic fluctuationField-aligned current