Observations of a sky Lyman alpha groove related to enhanced solar wind mass flux in the neutral sheet

We report several observations of the L alpha interplanetary emission recorded by a photometer flown in 1977 on board the soviet spacecraft Prognoz-5. The Prognoz scans sampled emission in a plane perpendicular to the sun-spacecraft line and also traversed the L alpha maximum emission region (MER), a region centered a few AU from the Sun on the upstream interstellar-flow axis. These scans reveal the existence of a 10% decrease in intensity centered near the ecliptic plane and about 30 degrees wide. All the dips form a new feature of the interplanetary emission, a ''groove'' aligned approximately with the ecliptic plane. This groove is present only near the upwind direction, and is interpreted as the result of enhanced ionisation of interstellar H by charge-exchange with the solar wind in a sheet of approximately 30 degrees width in latitude around the average position of the neutral sheet, at this time of solar minimum.