Exploring the Earth's outer atmosphere: an innovative approach to measure in-situ exospheric densities

The Earth's outer atmosphere interacts closely with the magnetosphere - ionosphere - thermosphere system. Solar irradiance and geomagnetic activities heat the ionosphere - thermosphere (IT) and push neutral atoms deep into space. These exospheric neutrals exchange charges with the magnetospheric plasmas and produce energetic neutral atoms (ENAs), which is known as a major loss mechanism for the ring current ions. The exospheric density is a key parameter to understand the IT contribution to the ring current dynamics and to analyze any imaging data based on charge exchange processes, such as the ENA observations from the IMAGE and TWINS missions and the soft X-ray observations from the CuPID, LEXI, and SMILE missions that will be launched in 2020, 2021, and 2023, respectively. Previous studies extracted exospheric densities from the line-of-sight Lyman-alpha observations with an assumption of neutral density distribution along the look directions that may create a large error in their density estimates. We introduce the ISEND (Instrument for studying exospheric neutral density) cubesat mission that carries a neutral mass spectrometer. ISEND will provide unprecedented, in-situ measurements of exospheric neural densities along a highly-elliptical geostationary-transfer orbit with an apogee near geostationary orbit and a perigee less than 500km altitude. ISEND will answer how the Earth's outer atmosphere responds to solar irradiance and geomagnetic activities and help other imaging missions to analyze their data.