Deceleration of Interstellar Hydrogen at the Heliospheric Interface

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Abstract
High-resolution spectra of nearby stars show absorption lines due to material in the local interstellar cloud. This cloud is deduced to be moving at 26 kilometers per second with respect to the sun, and in the same direction as the "interstellar wind" flowing through the solar system. Measurements by the Ulysses spacecraft show that neutral helium is drifting through the solar system at the same velocity, but neutral hydrogen appears to be moving at only 20 kilometers per second, a result confirmed by new measurements of the hydrogen emission line taken by the High-Resolution Spectrograph on the Hubble Space Telescope. These results indicate that neutral hydrogen atoms from the local interstellar cloud are preferentially decelerated at the heliospheric interface, most likely by charge-exchange with interstellar protons, while neutral helium is unaffected by the plasma. The magnitude of the observed deceleration implies an interstellar plasma density of 0.06 to 0.10 per cubic centimeter, which in turn implies that the heliospheric shock should be less than 100 astronomical units from the sun.