The response of exospheric neutral hydrogen to weak geomagnetic disturbances between June 12 and 29, 2008

Jochen Hard Zoennchen^1*Gonzalo Cucho-Padin^2,3

• ¹Argelander Institute for Astronomy, Faculty of Mathematics and Natural Sciences, University of Bonn, Bonn, Germany
• ²Space Weather Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA, Greenbelt, Maryland, United States
• ³The Catholic University of America, Washington, D.C., District of Columbia, United States

Stereoscopic remote sensing observations of terrestrial far ultraviolet (Lyman-α at 121.6 nm) emission at solar minimum have been used to retrieve the time dependent response of the exospheric 3-D neutral hydrogen (H) density at radial distances 3-6 Earth radii (Re) to weak geomagnetic disturbances. This study includes continuous observations from the Lyman-α detectors (LADs) onboard NASA's TWINS 1 and 2 satellites between June 12 and 29, 2008, which covers two minor geomagnetic storms (June 15 and 25). For both storms, we derived the 3-D H-density distributions from prior and during the storm involving data from 12 h time intervals. The inversion based on our new H-density model "EXPGRID" and incorporates the effects of Lyman-α absorption within the exosphere and Lyman-α re-emission from Earth's albedo. We found, that the H-density distributions at 3-6 Re are highly variable. They are correlated and varying in phase with exobase temperatures during the geomagnetic events. Furthermore, time dependency and amplitude of the H-density enhancement and the kp index were found to be similar. Since the kp index is assumed to be correlated with the varying size of the plasmasphere, this finding supports theories of a physical interaction between the neutral exosphere and the plasmasphere. The disturbances with a significant effect to the neutral H atoms are initiated by a prior increase of the solar wind flow pressure and exobase temperatures (in particular over the poles). Our time dependent 3-D results indicate that the H atom enhancement is not uniformly distributed over the shells. Instead, we found asymmetries (i.e. dawn/dusk near ecliptic) and temporal evolving zones of regionally strong enhanced H-densities. Among the first affected regions after onset are the vicinities of the geotail (at lower distances) and the north pole (at upper distances). A ~40% exobase temperature increase (NRLMSIS) at the south pole on June 15 correlates with a strong H atom enhancement at the southern hemisphere later on this day. Finally, both storms show, at the upper distance, a remarkably delayed enhancement (peak values as late as ~2 days after onset) of the H atoms density near the sub solar point (dayside "nose" region).