3-D-geocoronal hydrogen density derived from TWINS Ly-α-data

Abstract

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 R E 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 VidalMadjar (1975).The results for the geocoronal H-densities are compared here both with theoretical calculations based on a MonteCarlo 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 R E ), 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 R E < r < 7 R E 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.

Energetic neutral atoms from the Earth's subsolar magnetopause

“…For example, if the IMF is northward, then ENA fluxes may be higher because the parent proton distribution is anisotropic. However, improvements like these in the model probably do not amount to more than a factor of 2 increase or decrease in exospheric densities derived from the simple model and shown in Table 1. [18] Finally, using Lyman Alpha observations, Østgaard et al [2003] and, more recently, Zoennchen et al [2010] obtained nightside exospheric hydrogen densities that are about a factor of 1.5 -5 times higher than densities in Table 1. However, they indicate evidence of an exospheric tail on the nightside.…”

Exospheric hydrogen density distributions for equinox and summer solstice observed with TWINS1/2 during solar minimum

“…H-density structures of the H-geocorona between June and September 2008 and can be found in Zoennchen et al (2011Zoennchen et al ( , 2010 and Bailey and Gruntman (2011).…”

“…All other corrections (e.g. Earth albedo Lyman-α glow, bright star removements and neutralized subsonic solar wind protons) were done as described in Zoennchen et al (2010). Figure 5 shows the Lyman-α intensities (recalibrated and interplanetary background corrected) against the Earth intersection distances in R E of all TWINS1/2 LAD1/2 lines of sight collected in the datasets of equinox (left) and summer solstice (right).…”

“…A description of the resonant Lyman-α scattering process within the neutral exosphere, the method of enfolding the line of sight integrated TWINS-LAD data into a 3-D neutral H density distribution and details about the TWINS-LAD instrument can be found in Zoennchen (2006), Nass et al (2006), Zoennchen et al (2010), and Bailey and Gruntman (2011).…”

“…The mathematical model used in this work is based on a tomographic enfolding of the line of sight measurements aiming at a data fit by coefficients of a spherical harmonic expansion (Zoennchen, 2006;Zoennchen et al, 2010;Nass et al, 2006;Bailey and Gruntman, 2011) without restricting assumptions for specific (i.e. angle) symmetries.…”