Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission

Abstract

Abstract. Circumterrestrial Lyman-α column brightness observations above 3 Earth radii (R e ) have been used to derive separate 3-D neutral hydrogen density models of the Earth's exosphere for solar minimum (2008, 2010) and near-solarmaximum (2012) conditions. The data used were measured by Lyman-α detectors (LAD1/2) onboard each of the TWINS satellites from very different orbital positions with respect to the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions above 3 R e along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H-column density. We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions. Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity, significantly lower densities were found. Differences in shape and orientation of the exosphere under different solar conditions exist. Above 3 R e , independent of solar activity, increased H densities appear on the Earth's nightside shifted towards dawn. With increasing distance (as measured at 8 R e ) this feature is shifted westward/duskward by between −4 and −5 • with respect to midnight. Thus, at larger geocentric distance the exosphere seems to be aligned with the aberrated Earth-solar-wind line, defined by the solar wind velocity and the orbital velocity of the Earth. The results presented in this paper are valid for geocentric distances between 3 and 8 R e .

The response of the H geocorona between 3 and 8 <i>R</i><sub>e</sub> to geomagnetic disturbances studied using TWINS stereo Lyman-<i>α</i> data

“…The total uncertainties are expected as the sum of the uncertainty in the background-corrected Lyman-α intensities of about ≈ 10-15 % (as discussed in Zoennchen et al, 2015) and the uncertainty in the Gaussian fit, which was found to be ≈ 2-5 %. Therefore, in total we expect uncertainties of ≈ 12-20 %.…”

“…This has been taken into account in the analysis using the methods of LAD sensitivity calibration based on Lyman-α-bright stars and the undisturbed H geocorona (see details in Zoennchen et al, 2015Zoennchen et al, , 2013 as sources of known Lyman-α flux. Nevertheless, the LADs' sensitivity can be assumed to be constant in short time periods from a few hours up to several days, which are used here to analyze exospheric Lyman-α variations during geomagnetic storms.…”

“…The assumption used and the model itself is described in greater detail in Zoennchen et al (2015). The parameters of this model are adjusted such that the interplanetary Lyman-α intensities are in good agreement with maps pro- vided by SOHO/SWAN (see also Bertaux et al, 1998).…”

“…3 that there are the small variations in the daily total solar Lyman-α flux, in particular ≤ 1 % during the solar minimum. Nevertheless, the TWINS LOS data are corrected for these variations by normalization of all LAD observations to the total solar Lyman-α flux of a fixed reference day (see Zoennchen et al, 2015). Additionally, Fig.…”

“…A possible reason for that phenomenon might be an overall lower H density level in the undisturbed exosphere at solar minimum compared to solar maximum (Zoennchen et al, 2015). Within a thinner exosphere, the same number of H atoms transported from lower to larger distances will create a higher-percentage n H peak value.…”