Impacts of Variable Solar Activity on the Earth’s Neutral Exospheric Hydrogen Density at Timescales from Half-Hours to Solar Cycle

• Applicant: Dr. Jochen Zönnchen
• Subject Area: Atmospheric Science
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 469043535

Based on 10 years of global exospheric Lyman-α observations by TWINS we propose to investigate the 3D variation of the neutral exosphere of the Earth caused by different solar activity variability (solely solar wind or UV and both combined) on timescales from years (solar cycle) over days (27 day solar rotation period) to hours (geomagnetic storms).The exosphere is the outermost region of the atmosphere and mainly consists of neutral hydrogen (H). As transition zone to the interplanetary space it is of critical importance for the entire historic evolution of the Earth’s atmosphere from the ancient past to the future, for example due to the loss of H from surface water into space. Since directly exposed to radiation and solar activity different space weather events like geomagnetic storms can cause significant impacts on the neutral exosphere. Their quantitative influences and the relevant physical processes are poorly known so far.Exospheric H-atoms resonantly backscatter solar Lyman-α radiation. The scattered intensity is proportional to the local H-density in the optically thin regime above 3 Re (Earth radii). The TWINS satellites collected the unique database of continuous 3D exospheric Lyman-α observation over 10 years, which is just partly analyzed so far.With the usage of tomographic and kinetic modeling techniques we propose to quantify the dynamics of the 3D H-density variation between 3-8 Re as response to varying space weather on different timescales.First, the evolution of the 3D H-density distribution over the solar cycle 2008-2018 will be studied to characterize, how the total number H-density, radial profiles and regional asymmetries around the Earth (polar/equatorial, day/night, dusk/dawn) are coupled with solar cycle.Second, the highly dynamic impacts of geomagnetic storms will be analyzed for the first time in 3D with time resolution of hours to ~30 min using the uniquely large storm sample of TWINS. Monte Carlo simulations will be used to characterize the contributing physical mechanisms responsible for the dynamic H-density response to storms.Third, the influence of solely solar UV variations under the absence of significant solar wind changes will be investigated with two types of variation: the 27 day solar UV-variation (solar rotation cycle) and eruptive UV-flashes at the Sun. We propose to study their impacts on the neutral exosphere, in particular on orbiting H-atoms at larger distances, caused by periodically or rapidly varying photo-ionization rate and radiation pressure.The availability of an H-density model considering the dynamic variations caused by varying space weather would be a significant progress in our understanding of the neutral exosphere. It would also greatly benefit upcoming missions for magnetospheric imaging (like SMILE, LEXI or STORM), since they all need the exospheric H-density at a given point and time for the correct inversion of their ENA- or Soft X-ray measurements.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partners: Professorin Dr. Hyunju Kim Connor; Professor Dr. David Gary Sibeck