Hydrogen atoms in the inner heliosphere: SWAN‐SOHO and MASCS‐MESSENGER observations

Abstract

AbstractWe present here a study made by two instruments, Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) and Solar Wind Anisotropy (SWAN) on SOHO that observed the interplanetary background in 2010. The combination of these two data sets allows us to perform the first study of the distribution of hydrogen atoms inside the Earth's orbit. Triangulation of the position of the maximum emissivity region (MER) was performed for the data of the Ultraviolet and Visible Spectrometer (UVVS) channel of the MASCS‐MESSENGER instrument. We find that the ecliptic longitude of the MER is 253.2°±2.0°. This is the same value that was found from the analysis of the SWAN‐SOHO H cell data obtained in 1996. This strongly suggests that the direction of the interstellar hydrogen wind has not changed between 1996 and 2010. We have also determined the distance of the MER to the Sun. We find that the volume emission rate peaks at 2.37 AU ± 0.2 AU from the Sun. This value is a good test for the solar parameters for total H ionization and radiation pressure used in models. Comparison between the two data sets obtained by the UVVS‐MASCS channel and SWAN on SOHO allow to derive the intensity between the two spacecraft at peak emission. Based on the SWAN‐SOHO calibration, we find an intensity of 80 R ± 36 R. This corresponds to a column density of 1540 m−3 AU × 2.3 ×1014m−2. When divided by the distance between the two spacecraft, we find an average number density of 2300 m−3.