Abstract
Abstract. The Lyman-α Detectors (LAD) on board the two TWINS 1/2-satellites allow for the simultaneous stereo imaging of the resonant emission glow of the H-geocorona from very different orbital positions. Terrestrial exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation. During the past solar minimum, relevant solar parameters that influence these emissions were quite stable. Here, we use simultaneous LAD1/2-observations from TWINS1 and TWINS2 between June 2008 and June 2010 to study seasonal variations in the H-geocorona. Data are combined to produce two datasets containing (summer) solstice and (combined spring and fall) equinox emissions. In the range from 3 to 10 Earth radii (RE), a three-dimensional (3-D) mathematical model is used that allows for density asymmetries in longitude and latitude. At lower geocentric distances (< 3 RE), a best fitting r-dependent (Chamberlain, 1963)-like model is adapted to enable extrapolation of our information to lower heights. We find that dawn and dusk H-geocoronal densities differ by up to a factor of 1.3 with higher densities on the dawn side. Also, noon densities are greater by up to a factor of 2 compared to the dawn and dusk densities. The density profiles are aligned well with the Earth–Sun line and there are clear density depletions over both poles that show additional seasonal effects. These solstice and equinox empirical fits can be used to determine H-geocoronal densities for any day of the year for solar minimum conditions.
Highlights
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A description of the resonant Lyman-α scattering process within the neutral exosphere, the method of enfolding the line of sight integrated TWINS-Lyman-α Detectors (LAD) data into a 3-D neutral H density distribution and details about the TWINSLAD instrument can be found in Zoennchen (2006), Nass et al (2006), Zoennchen et al (2010), and Bailey and Gruntman (2011)
Both of the TWINS1/2 spacecraft are equipped with two LAD-sensors each, which are mounted on an actuator platform
Summary
The measured Lyman-α intensity is the sum of the geocoronal and the interplanetary Lyman-α glow. With respect to the geocentric Earth intersection distances of the LAD-lines of sight, the interplanetary glow usually accounts for 10–50 percent of the measured intensities. The fall- and spring equinox are assumed to lead to similar exospheric hydrogen density distributions under the assumption of nearly identical solar activity conditions, because both equinox seasons have the same solar tilt angle with respect to the Earth Equator of 0◦. 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 TWINSLAD instrument can be found in Zoennchen (2006), Nass et al (2006), Zoennchen et al (2010), and Bailey and Gruntman (2011)
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