A NEW MEASUREMENT OF THE INTERPLANETARY HYDROGEN DENSITY WITH ALAE ATLAS-1/

The ALAE Lyman alpha spectrophotometer has been flown on the ATLAS 1 m ission aboard the space shuttle Atlantis from March 24 until April 2, 1992. During this mission, when the cargo bay of the shuttle was point ing toward the zenith, observations of the interplanetary Lyman alpha glow were made possible with the help of a hydrogen absorption cell th at absorbed most of the geocoronal emission. Here, we analyze these ze nith data. The calibration factor geocoronal emission seen at the zeni th. Intercomparison of radiative transfer calculations of both the int erplanetary and geocoronal Lyman alpha emissions allows for a precise determination of the interplanetary hydrogen density. This estimate is independent of the value of the solar Lyman alpha flux at line center for the time of observation. The hydrogen number density obtained her e is 0.15 +/- 0.05 cm(-3). Using solar wind ion flux measurements avai lable from the NSSDC database to estimate the ionization rate of neutr al hydrogen, we find a more accurate value equal to 0.165 +/- 0.035 cm (-3). Previous measurements obtained from other space experiments are also reviewed. Those obtained independently of solar flux/calibration factor cluster in the range 0.11-0.17 cm(-3), which is nearly a factor of two higher than most previous estimates. This is mainly due to the use of an incorrect ionization rate for neutral hydrogen in earlier w orks. This relatively high number density is a further indication that the heliospheric shock, compressed by the interstellar flow, might be nearer than previously estimated.