A NEW SOURCE OF LY-ALPHA EMISSION DETECTED BY VOYAGER UVS - HELIOSPHERIC OR GALACTIC ORIGIN

In 1993 and 1994, the Ultraviolet Spectrometers, aboard both Voyager 1 and 2, were used to study the Ly alpha glow pattern scattered by neut ral hydrogen atoms at great distance from the sun, approximate to 54 A U for Voyager 1 and 40 AU for Voyager 2. These data are characterized by a long integration time (approximate to 10(4) s) and were obtained over a rather short period of time (10 days) to minimize effects of so lar flux variations. When compared to a radiative transfer calculation of the Ly alpha glow pattern assuming a homogeneous distribution of h ydrogen at large distance from the Sun, these maps present an excess o f intensity seen in the direction of the incoming interstellar wind. T his corresponds to a relative increase of approximate to 17% over the expected value for Voyager 1 and approximate to 10% for Voyager 2. An absolute estimate of this Lyman alpha intensity yields 10 to 15 Raylei gh for each spacecraft. The possible effects of longitudinal or latitu dinal solar flux variations on these data, as well as the importance o f the phase function when optical thickness at line center between the spacecraft and the Sun is more than 3, have been ruled out as a cause . We discuss here the possibility that this excess could be the result of a filtration of the neutral hydrogen at heliopause crossing caused by charge exchange between neutral hydrogen of interstellar origin an d protons from the solar wind and the interstellar medium. Yet, this L yman alpha intensity excess may also correspond to the first detection of the galactic Lyman alpha emission as hinted from the similarity of the observations for both spacecraft.