16:15 〜 16:30
[PEM12-19] X-ray study of the upper atmosphere density disturbance caused by the explosive eruption of the 2022 Tonga's volcano
キーワード：The 2022 Hunga Tonga-Hunga Ha'apai volcanic eruption, X-ray astronomy satellite, Occultation, Supersonic shock waves, Gravity waves
We present results from X-ray observations of the upper atmosphere in the altitude 90—200 km above the Hunga Tonga-Hunga Ha’apai (HTHH) volcano. Recently, we have been developing a new method to diagnose the upper atmosphere, by using atmospheric occultations of bright celestial X-ray sources observed with X-ray astronomy satellites. This method provides us with a precious opportunity to investigate the neutral density of the lower thermosphere and mesosphere.
The HTHH undersea volcanic eruption that occurred at 04:15 UT on 15 January 2022
was one of the most explosive events in the modern era. At the time of the eruption, the Chinese X-ray astronomy satellite, Insight-HXMT, was observing the supernova remnant Cassiopeia A, one of the brightest X-ray sources in the entire sky. Fortunately, Insight-HXMT captured atmospheric occultations above the HTHH volcano at 3.5, 5, and 6.5 hr after the volcanic eruption, with horizontal distances from the volcano of about 1000, 2000, and 3000 km, respectively. We analyzed X-ray spectral evolution during occultations, obtaining vertical density profiles of combined O and N densities (both atoms and molecules included) for individual occultations. For all the three occultations, the X-ray spectra from Cassiopeia A show relatively low absorption by the atmosphere compared with usual occultations. If we attribute the low absorption solely to the atmospheric density change, then the atmospheric density in the altitude range of 90—120 km must be reduced by a factor of 2 at the 3.5 hr and by a factor of 1.5 at 5 and 6.5 hr after the explosive eruption, respectively. It seems difficult to reproduce such a large density decrease by a hydrostatic GCM, and a nonhydrostatic model with realistic volcanic inputs is necessary to study the density change. Another interesting feature we realized is that the X-ray occultation light curves show some hints of wavy structures with a typical length scale of 10 km. This may be caused by gravity waves triggered by the volcanic eruption.
The HTHH undersea volcanic eruption that occurred at 04:15 UT on 15 January 2022
was one of the most explosive events in the modern era. At the time of the eruption, the Chinese X-ray astronomy satellite, Insight-HXMT, was observing the supernova remnant Cassiopeia A, one of the brightest X-ray sources in the entire sky. Fortunately, Insight-HXMT captured atmospheric occultations above the HTHH volcano at 3.5, 5, and 6.5 hr after the volcanic eruption, with horizontal distances from the volcano of about 1000, 2000, and 3000 km, respectively. We analyzed X-ray spectral evolution during occultations, obtaining vertical density profiles of combined O and N densities (both atoms and molecules included) for individual occultations. For all the three occultations, the X-ray spectra from Cassiopeia A show relatively low absorption by the atmosphere compared with usual occultations. If we attribute the low absorption solely to the atmospheric density change, then the atmospheric density in the altitude range of 90—120 km must be reduced by a factor of 2 at the 3.5 hr and by a factor of 1.5 at 5 and 6.5 hr after the explosive eruption, respectively. It seems difficult to reproduce such a large density decrease by a hydrostatic GCM, and a nonhydrostatic model with realistic volcanic inputs is necessary to study the density change. Another interesting feature we realized is that the X-ray occultation light curves show some hints of wavy structures with a typical length scale of 10 km. This may be caused by gravity waves triggered by the volcanic eruption.