3-D-geocoronal hydrogen density derived from TWINS Ly-α-data
Abstract. Based on Ly-α-line-of-sight measurements taken with two Ly-α detectors onboard of the satellite TWINS1 (Two Wide-angle Imaging Neutral-atom Spectrometers) density profiles of the exospheric, neutral geocoronal hydrogen were derived for the time period between summer solstice and fall equinox 2008. With the help of specifically developed inversion programs from Ly-α line of sight intensities the three-dimensional density structure of the geocoronal hydrogen at geocentric distances r>3 RE could be derived for the period mentioned characterized by very low solar 10.7 cm radiofluxes of ≈65–70 [10−22 W m−2 Hz−1]. The time-variable, solar "line-centered"-Ly-α-flux was extracted on the basis of daily (terrestrial) NGDC 10.7 cm radioflux data using the models from Barth et al. (1990) and Vidal-Madjar (1975).
The results for the geocoronal H-densities are compared here both with theoretical calculations based on a Monte-Carlo model by Hodges (1994) and with density profiles obtained with the Geocoronal Imager (GEO) by Østgaard and Mende (2003). In our results we find a remarkably more pronounced day-/night-side asymmetry which clearly hints to the existence of a hydrogen geotail (i.e. a tail structure with comparatively higher hydrogen densities on the night side of the earth for geocenctric distances >4 RE), and a only weakly pronounced polar depletion. These unexpected features we try to explain by new models in the near future. The derived 3-D-H-density structures are able to explain the line-of-sight (LOS) dependent Ly-α intensity variations for all LOS seen up to now with TWINS-LAD. The presented results are valid for the region with geocentric distances 3 RE<r<7 RE and are based on the reasonable assumption of an optically thin H-exosphere with respect to resonant Ly-α-scattering allowing the use of single scattering calculations.