[P-EM11] Heliosphere and Interplanetary Space

[PEM11-P07] 5-season's optical observations of neutral helium distribution in interplanetary space by Hisaki

*Atsushi Yamazaki¹, Go Murakami¹, Kazuo Yoshioka², Tomoki Kimura³, Fuminori Tsuchiya⁴, Masato Kagitani⁴, Takeshi Sakanoi⁴, Naoki Terada⁵, Yasumasa Kasaba⁶, Ichiro Yoshikawa⁷ (1.Institute of Space and Astronautical Science / Japan Aerospace Exploration Agency, 2.Graduate School of frontier Science, The University of Tokyo, 3.Tohoku University, 4.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 5.Graduate School of Science, Tohoku University, 6.Planetary Plasma and Atmospheric Research Center, Tohoku University, 7.The University of Tokyo)

Keywords:EUV spectral observation, Interstellar wind and gas, Interplanetary neutral helium, HISAKI stellite

The Hisaki (SPRINT-A) satellite has main scientific objectives of the planetary magnetospheric physics and atmospheric evolution by long-term observations, but also carried out an observation of helium atom resonance scattering in interplanetary space. A material in the interstellar medium (ISM) travels into the heliosphere over the heliopause due to the relative velocity between the heliosphere and interstellar gases. The helium atoms move into the neighboring from the sun without ionizing because of its high ionization energy. The helium atoms are bent by sun gravity along the Keplerian orbit and forms a high density region on the down wind side, which is called helium cone. The distribution of helium atoms in the helium cone can estimate the speed and direction of the interstellar wind, and the density and the temperature of the helium atom in interstellar gases. This study has been carried out from the 1970s, but the interplanetary helium atoms observation is one of powerful tools to recognize the interstellar medium from inside the heliosphere. The Hisaki satellite carried out the observation of the resonance scattering in the helium cone during November and December from 2015 to 2019 five times. The interstellar wind direction derived from the comparison between the observation and the model simulation coincide the results of the other satellite observation. It means that the interstellar wind is stable in these years.