Neutral Hydrogen in the Terrestrial Thermosphere and Exosphere

The exosphere is the interface between the Earth s neutral atmosphere and interplanetary space. Our understanding of this important interface, through observations of its mean state and its response to external forcing, will provide important constraints as we seek to develop a complete picture of our complicated space-atmosphere system. This chapter will highlight the contributions of ground-based geocoronal hydrogen observations to our understanding of this system. Observations are made throughout the night; the base of the Earth s shadow is used as a first-order probe of the exosphere s altitude structure. Major areas of scientific focus include (1) high resolution observations of the geocoronal hydrogen Balmer α line profile and its relation to excitation mechanisms, effective temperature, and exospheric physics; (2) retrieval of geocoronal hydrogen parameters such as the hydrogen column abundance [H], the hydrogen density profile [H]( z ), and the photochemically initiated hydrogen flux ϕ (H); and (3) long-term observations of the geocoronal hydrogen column emission intensity for the investigation of natural variability, such as solar cycle trends, and of potential anthropogenic change due to increases in atmospheric concentrations of greenhouse gases.