Neutral Hydrogen in the Terrestrial Thermosphere and Exosphere: Ground-Based Observations

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 talk will highlight the application of ground-based, high-throughput interference spectroscopy to the study of this important interface. 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) observations of the geocoronal hydrogen column emission intensity for the investigation of natural variability. Recent work from two unique spectrometers located in Wisconsin and Chile will be reported, with results highlighting each of these three areas of focus. Special emphasis will be placed on high spectral resolution line profile observations of the Balmer α emission line and the forward-model analysis of these data using the lyao_rt radiative transport code of Bishop [1999]. For example, an observed decrease in effective temperature with increasing shadow altitude is found to be a persistent feature for nights in which a wide range of shadow altitudes are sampled. This result will be interpreted in the context of the lyao_rt code. This work is supported by National Science Foundation award AGS-2050077.