[P-EM18] Heliosphere and Interplanetary Space

[PEM18-P04] 4-season's optical observation 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 Kasaba⁴, Ichiro Yoshikawa⁶, Hisaki (SPRINT-A) Project team (1.Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science , 2.The University of Tokyo, Graduate School of Frontier Sciences, 3.Tohoku University, 4.Tohoku University, Planetary Plasma and Atmospheric Research Center , 5.Tohoku University, Graduate School of Sciense, 6.The University of Tokyo)

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

The Hisaki (SPRINT-A) satellite has a main scientific topic of the planetary magnetspheic physics and atmospheric evoluiton by long-term observations,
but also carried out an observation of helium atom resonance scattering from 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 was 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 observaiont of the resonance scattering from inside the helium cone during November and December.
The interstellar parameters derived from The Hisaki's observation results of the helium cone four times in years of 2015 - 2018 are reported.