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A new measurement of the interplanetary hydrogen density with ALAE/ATLAS 1

Abstract : The ALAE Lyman α spectrophotometer has been flown on the ATLAS 1 mission aboard the space shuttle Atlantis from March 24 until April 2, 1992. During this mission, when the cargo bay of the shuttle was pointing toward the zenith, observations of the interplanetary Lyman α glow were made possible with the help of a hydrogen absorption cell that absorbed most of the geocoronal emission. Here, we analyze these zenith data. The calibration factor of the instrument has been determined by studying the full geocoronal emission seen at the zenith. Intercomparison of radiative transfer calculations of both the interplanetary and geocoronal Lyman α emissions allows for a precise determination of the interplanetary hydrogen density. This estimate is independent of the value of the solar Lyman α flux at line center for the time of observation. The hydrogen number density obtained here is 0.15+/-0.05cm^-3^. Using solar wind ion flux measurements available from the NSSDC database to estimate the ionization rate of neutral hydrogen, we find a more accurate value equal to 0.165+/-0.035cm^-3^. Previous measurements obtained from other space experiments are also reviewed. Those obtained independently of solar flux/calibration factor cluster in the range 0.11-0.17cm^-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 works. This relatively high number density is a further indication that the heliospheric shock, compressed by the interstellar flow, might be nearer than previously estimated.
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Eric Quémerais, Jean-Loup Bertaux, B. R. Sandel, Rosine Lallement. A new measurement of the interplanetary hydrogen density with ALAE/ATLAS 1. Astronomy and Astrophysics - A&A, EDP Sciences, 1994, 290, pp.941-955. ⟨insu-01711438⟩

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