Terrestrial exospheric dayside H-density profile at 3–15 RE from UVIS/HDAC and TWINS Lyman-α data combined

Abstract

Terrestrial ecliptic dayside observations of the exospheric Lyman-α column intensity between 3–15 Earth radii (R_E) by UVIS/HDAC (UVIS – ultraviolet imaging spectrograph; HDAC – hydrogen-deuterium absorption cell) Lyman-α photometer at CASSINI have been analyzed to derive the neutral exospheric H-density profile at the Earth's ecliptic dayside in this radial range. The data were measured during CASSINI's swing-by maneuver at the Earth on 18 August 1999 and are published by Werner et al. (2004). In this study the dayside HDAC Lyman-α observations published by Werner et al. (2004) are compared to calculated Lyman-α intensities based on the 3D H-density model derived from TWINS (Two Wide-angle Imaging Neutral-atom Spectrometers) Lyman-α observations between 2008–2010 (Zoennchen et al., 2015). It was found that both Lyman-α profiles show a very similar radial dependence in particular between 3–8 R_E. Between 3.0–5.5 R_E impact distance Lyman-α observations of both TWINS and UVIS/HDAC exist at the ecliptic dayside. In this overlapping region the cross-calibration of the HDAC profile against the calculated TWINS profile was done, assuming that the exosphere there was similar for both due to comparable space weather conditions. As a result of the cross-calibration the conversion factor between counts per second and rayleigh, f_c=3.285 counts s⁻¹ R⁻¹, is determined for these HDAC observations. Using this factor the radial H-density profile for the Earth's ecliptic dayside was derived from the UVIS/HDAC observations, which constrained the neutral H density there at 10 R_E to a value of 35 cm⁻³. Furthermore, a faster radial H-density decrease was found at distances above 8 R_E ($\approx r^{-\mathrm{3}}$ ) compared to the lower distances of 3–7 R_E ($\approx r^{-\mathrm{2.37}}$ ). This increased loss of neutral H above 8 R_E might indicate a higher rate of H ionization in the vicinity of the magnetopause at 9–11 R_E (near subsolar point) and beyond, because of increasing charge exchange interactions of exospheric H atoms with solar wind ions outside the magnetosphere.