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 carried out the non-planetary observation at the time when there is no oppotunity for observation of all planets. One of those observations is 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 about 0.5Au of 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. The interplanetary observation is one of powerful tools to recognize the interstellar medium from inside the heliosphere. This study was carried out from the 1970s, but the recent IBEX satellite observation results show that the distribution of interstellar gasses change dinamically. The Hisaki satellite carried out the observaiont of the resonance scattering from inside the helium cone during November and December. In this presentation, the Hisaki's observation results of the helium cone three times in years of 2015 - 2017 are shown, and the change of the wind direction are reported.