[S-GC35] Volatiles in the Earth - from Surface to Deep Mantle

[SGC35-P05] Solar flare activity as a new, unknown source of ³He in the atmosphere

*Yuji Sano¹, Teresa M Escobar², Naoto Takahata², Maoliang Zhang³, Daisuke Goto⁴, Daniele L Pinti⁵, Bernard Marty⁶ (1.Center for Advanced Marine Core Research, Kochi University , 2.Atmosphere and Ocean Research Institute, University of Tokyo, 3.Tianjin University, 4.National Institute of Polar Research, 5.University of Quebeck, Montreal, 6.University of Lorraine)

Keywords:Helium Isotopes, Atmosphere, Solar flare

Measurements of helium in the atmosphere suggest a supply of crustal ⁴He from fossil fuel emissions since the onset of industrial activity [1,2]. However, corresponding ³He/⁴He ratio of air appears near-constant [3,4], which calls for an unknown source of ³He to compensate contribution of anthropogenic ⁴He. Here we report ³He/⁴He variations measured in atmospheric air during a solar flare event. Samples were collected in Ny-Ålesund, Svalbard islands near the North Pole. On September 6^th, 2017, a massive solar explosion, or solar flare, occurred. The solar wind was expected to reach the Earth on September 8^th [5]. Therefore, the dates of air sample collection were set for eight days about one week apart, from August 30th to October 18^th, 2017. The ³He/⁴He ratios were measured in the samples and calibrated against the standard atmosphere collected in Kashiwa City, Chiba Prefecture, Japan [6]. The ³He/⁴He ratios varied from -6.1‰ to +7.9‰ compared the air standard. The ³He/⁴He ratios of 12 samples agreed with the atmospheric values of Kashiwa within uncertainties. A total of five samples showed an excess of ³He compared the atmospheric ratio. These samples were collected on September 13^th, 20th, and 27^th, immediately after the solar flare event of September 6^th. If the solar wind, enhanced by solar flares, was fed into the atmosphere by the auroral precipitation mechanism [7], it would increase the polar atmospheric helium isotope ratio. The helium would then be diluted by diffusion and the excess ³He would rapidly disappear. We therefore propose that the unknown source of ³He capable to compensate for anthropogenic addition of ⁴He is the injection of solar wind-derived ³He into the atmosphere.

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