Abstract
Abstract. Circumterrestrial Lyman-α column brightness observations from 3–8 Earth radii (Re) have been used to study temporal density variations in the exospheric neutral hydrogen as response to geomagnetic disturbances of different strength, i.e., Dst peak values between −26 and −147 nT. The data used were measured by the two Lyman-α detectors (LAD1/2) onboard both TWINS satellites between the solar minimum of 2008 and near the solar maximum of 2013. The solar Lyman-α flux at 121.6 nm is resonantly scattered near line center by exospheric H atoms and measured by the TWINS LADs. Along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H column density, assuming optically thin conditions above 3 Re. In the case of the eight analyzed geomagnetic storms we found a significant increase in the exospheric Lyman-α flux between 9 and 23 % (equal to the same increase in H column density ΔnH) compared to the undisturbed case short before the storm event. Even weak geomagnetic storms (e.g., Dst peak values ≥ −41 nT) under solar minimum conditions show increases up to 23 % of the exospheric H densities. The strong H density increase in the observed outer exosphere is also a sign of an enhanced H escape flux during storms. For the majority of the storms we found an average time shift of about 11 h between the time when the first significant dynamic solar wind pressure peak (pSW) hits the Earth and the time when the exospheric Lyman-α flux variation reaches its maximum. The results show that the (relative) exospheric density reaction of ΔnH have a tendency to decrease with increasing peak values of Dst index or the Kp index daily sum. Nevertheless, a simple linear correlation between ΔnH and these two geomagnetic indices does not seem to exist. In contrast, when recovering from the peak back to the undisturbed case, the Kp index daily sum and the ΔnH essentially show the same temporal recovery.
Highlights
The main component of the terrestrial exosphere are neutral hydrogen atoms, in particular in the analyzed regions with geocentric distances between 3 and 8 Earth radii (Re).Besides the study of corresponding exospheric H density distributions to various solar activity level, the analysis of short time H density variations as a direct exospheric response to geomagnetic storms contains ample information of the exosphere’s nature, e.g., about different transport processes initially triggered by sudden solar wind variations hitting the Earth.Since the geocoronal H atoms produce a Lyman-α glow by resonant scattering of solar Lyman-α radiation at 121.6 nm, the observation of geocoronal Lyman-α column brightness and its conversion into exospheric H density models have challenged researchers for decades
In the case of the eight analyzed geomagnetic storms we found a significant increase in the exospheric Lyman-α flux between 9 and 23 % compared to the undisturbed case short before the storm event
3 Re, variations in the exospheric ILy-α are interpretable as equivalent H column density variations along the line of sight (LOS), since the LOSs are completely situated within the optically thin region
Summary
The main component of the terrestrial exosphere are neutral hydrogen atoms, in particular in the analyzed regions with geocentric distances between 3 and 8 Earth radii (Re). Based on TWINS-LAD data Bailey and Gruntman (2013) reported a H density increase for several geomagnetic storms in 2011 by comparing 3-D density fits of the H geocorona near the storm’s maximum with the undisturbed case From their analysis they derived an linear correlation between the peak Dst index of the storm and the relative variation in the exospheric H density, which predicts nearly no exospheric reaction for weak storms with peak values of Dst index > −50 nT. We found that very weak storms (peak values of Dst index > −41 nT) under solar minimum in 2008 cause significant H column density variations (up to 23 %) These storms are best covered with TWINS LAD in terms of spatial and temporal resolution since simultaneously observed LAD data of both TWINS satellites are available from this period
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