SH023-0001
5-Season's Optical Observation of Neutral Helium in Interplanetary Space by Hisaki

Thursday, 10 December 2020
Poster
Atsushi Yamazaki1, Go Murakami1, Kazuo Yoshioka2, Tomoki Kimura3, Fuminori Tsuchiya4, Masaki Kuwabara5, Hajime Kita6, Masato Kagitani4, Takeshi Sakanoi4, Naoki Terada7, Yasumasa Kasaba7 and Ichiro Yoshikawa2, (1)JAXA Japan Aerospace Exploration Agency, Sagamihara, Japan, (2)The University of Tokyo, Tokyo, Japan, (3)Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Japan, (4)PPARC, Tohoku University, Sendai, Japan, (5)ISAS/JAXA, Kanagawa, Japan, (6)ISAS/JAXA, Sagamihara, Japan, (7)Tohoku University, Sendai, Japan
Abstract:
Relative motion existence between the heliosphere and interstellar medium results into interstellar medium flow to the heliosphere as interstellar wind. The major constituents of interstellar medium are hydrogen and helium, and their neutrals can penetrate the heliosphere over the heliopause. Once the neutrals are ionized by the solar ultraviolet light in the heliosphere, they are picked up by the solar wind and return back to the heliopause. Only neutral helium, while remaining neutral due to the high ionization energy, can penetrate deep near the sun.

The motion of neutral particles in the heliosphere is determined by solar gravity and solar radiation pressure. The orbit of helium atoms becomes Keplerian motions because the term of radiation pressure is almost negligible to form dense regions on the downwind side of the interstellar wind, which is called a helium cone. The helium distribution in this region is dependent on the velocity and direction of the interstellar wind and the density and temperature of helium atoms in the interstellar medium. Although optical observation of neutral helium is a traditional study that has been done since the 1970s, it is a valuable method even today because it is possible to estimate the parameters of interstellar medium from the observation in interplanetary space.

The Hisaki (SPRINT-A) satellite was launched by Epsilon rocket vehicle in 2013 to insert low earth orbit with the altitude of about 1000 km. It has an extreme ultraviolet spectrograph which can detect resonance scattered light of helium atoms. The main aim is observational studies the plasma and energy transportation in planetary magnetospheres, ionospheres and atmospheres over a long period. Furthermore, helium atom resonance scattering emission in interplanetary space were observed at the time when Hisaki passed through the helium cone between November and December from 2015 to 2019.

We estimate interstellar wind direction by comparing the observations with the emission distribution calculated from the simple model of helium cone formation. As a result, it is confirmed that the direction is consistent with recent observation by other satellites and temporally stable for these 5 years.

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