Intensity map of the interplanetary hydrogen Lyman alpha The Imaging Ultraviolet Spectrometer onboard the Japanese spacecraft, NOZOMI, is observing the interplanetary Lyman alpha intensities during her cruising phase to Mars. Lyman alpha emissions emitted from the Sun are scattered by interplanetary hydrogen atoms and brighten the interplanetary space like the blue sky on the Earth. The interplanetary hydrogen originates from an interstellar wind or a flow of galactic medium. The brightest area on the map indicates an upwind direction of the interstellar wind. As approaching to the Sun the interstellar hydrogen atoms are gradually ionized by the solar ultraviolet radiation and become protons, which no longer scatter the Lyman alpha. Therefore, the interplanetary Lyman alpha intensity has a minimum in the downwind direction. The observation by NOZOMI has also shown correlation between the solar activity and the interplanetary Lyman alpha intensity.

The Heliosphere The solar system is located at a distance of 30,000 light years from the galactic center, orbiting around it together with the other stars of which the Galaxy consists. The solar system moves with a relative speed of 20km/s against the local interstellar medium. This relative motion of the local medium is observed as the interstellar wind. The solar magnetic field is brought by the solar wind to compose the solar magnetosphere. The magnetic field lines connect to the galactic field lines. Solar ultraviolet radiation ionizes interstellar hydrogen and helium atoms, which are removed from the solar magnetosphere by the solar wind. The region where the solar influence is significant (about 100AU) is called the heliosphere. The far side of the heliosphere has not been explored and, therefore, we have little knowledge about it.

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