The Earth’s exosphere is the outermost layer of our atmosphere that ranges from ~500km altitude (exobase) to the Moon’s orbit. Atomic hydrogen (H) becomes the dominant species above an altitude of ~1500km. The exosphere gains and loses hydrogen atoms as a result of the Sun – solar wind – magnetosphere – upper atmosphere interaction via physical processes like atmospheric upwelling, photoionization, and charge-exchange between neutrals and plasmas. The exosphere links the terrestrial ground atmosphere with the interplanetary space and has important influence on the whole evolutionary history of the Earth’s atmosphere. Study of the hydrogen density distribution and its variation is a key to understanding the past, present, and future of the Earth’s exosphere, its role in Sun – Earth interactions, and the relevance of its physical drivers to other planetary atmospheric escape and evolution.

Due to the lack of in-situ exospheric density measurements, scientists have studied the exosphere using remote sensing observations of solar Lyman-alpha photons resonantly scattered by the Earth’s exospheric hydrogen atoms. Several spacecraft missions provide extensive geocoronal observations from various viewpoints. TIMED covers an optically thick region below 2 Earth Radii (2 RE, ~12,750 km) altitude where a photon experiences multiple scattering due to a dense exosphere. TWINS cover an optically thin region in 2 ~ 7 RE altitude where photon is likely to scatter only once. SOHO and PROCYON cover the global exosphere from their deep space vantage points, but their datasets are limited to just a few images. Recent studies reported that 20 years of soft X-ray observations from the XMM Newton astrophysics missions and 10 years of energetic neutral atom observations from the IBEX heliophysics mission are capable of revealing the exospheric density above 7 RE altitude. Therefore, it is timely to gather scientists with expertise in different parts of the Earth’s exosphere and discuss the exosphere as a complete system.

By having a one-week meeting per year over a period of two years in Bern, Switzerland, our ISSI team proposes to answer the following science questions:

  1. How is the exospheric hydrogen density (NH) distributed around the Earth?

  2. How and why does the density vary during a solar cycle?

  3. How and why does the density vary during a geomagnetic storm?

We will gather up-to-date understanding of the above questions and improve the currently available empirical NH models by considering space weather impact and/or expanding its spatial coverage. Additionally, we will discuss current gaps in our knowledge and suggest future directions, such as modeling and mission planning, to fill the gaps. After the ISSI activity, our team will publish the results of our discussions in a peer-reviewed journal.