Inverse First Ionization Potential Effects in Giant Solar Flares Found from Earth X-Ray Albedo with Suzaku/XIS
Satoru Katsuda, M. Ohno, Koji Mori, Tatsuhiko Beppu, Yoshiaki Kanemaru, Tashiro, Makoto S., Terada, Yukikatsu, Sato, Kosuke, Morita, Kae, Sagara, Hikari, Ogawa, Futa, Takahashi, Haruya, Murakami, Hiroshi, Nobukawa, Masayoshi, Tsunemi, Hiroshi, Hayashida, Kiyoshi, Matsumoto, Hironori, Noda, Hirofumi, Nakajima, Hiroshi, Ezoe, Yuichiro, Tsuboi, Yohko, Maeda, Yoshitomo, Yokoyama, Takaaki, Narukage, Noriyuki, 勝田, 哲, 大野, 雅功, 森, 浩二, Beppu, Tatsuhiko, Kanemaru, Yoshiaki, 田代, 信, 寺田, 幸功, Sato, Kosuke, Morita, Kae, Sagara, Hikari, Ogawa, Futa, Takahashi, Haruya, 村上, 弘志, Nobukawa, Masayoshi, 常深, 博, 林田, 清, Matsumoto, Hironori, 野田, 博文, Nakajima, Hiroshi, 江副, 祐一郎, 坪井, 陽子, 前田, 良知, Yokoyama, Takaaki, 成影, 典之
Abstract
We report X-ray spectroscopic results for four giant solar flares that occurred on 2005 September 7 (X17.0), 2005 September 8 (X5.4), 2005 September 9 (X6.2), and 2006 December 5 (X9.0), obtained from Earth albedo data with the X-ray Imaging Spectrometer (XIS) on board Suzaku. The good energy resolution of the XIS (FWHM ∼ 100 eV) enables us to separate a number of line-like features and detect the underlying continuum emission. These features include Si Heα, Si Lyα, S Heα, S Lyα, Ar Heα, and Ca Heα originating from solar flares as well as fluorescent Ar Kα and Ar Kβ from the Earth's atmosphere. Absolute elemental abundances (X/H) averaged over the four flares are obtained to be ∼2.0 (Ca), ∼0.7 (Si), ∼0.3 (S), and ∼0.9 (Ar) at around flare peaks. This abundance pattern is similar to those of active stars' coronae showing inverse first ionization potential (i-FIP) effects, i.e., elemental abundances decrease with decreasing FIP with a turnover at the low end of the FIP. The abundances are almost constant during the flares, with the exception of Si which increases by a factor of ∼2 in the decay phase. The evolution of the Si abundance is consistent with the finding that the i-FIP plasma originates from chromospheric evaporation and then mixes with the surrounding low-FIP biased materials. Flare-to-flare abundance varied by a factor of two, agreeing with past observations of solar flares. Finally, we emphasize that Earth albedo data acquired by X-ray astronomy satellites like Suzaku and the X-Ray Imaging Spectroscopy Mission can significantly contribute to studies of solar physics.