2:15 PM - 2:30 PM
[PCG20-13] Optimization of Absorption Cell Filters for Stable and Precise Isotopic Observations of Planetary Hydrogen Coronae
Keywords:planetary corona, Lyman-alpha
Planetary hydrogen corona represents emissions arising from the resonant scattering of solar radiation by hydrogen atoms in the exospheres of planetary bodies. Its spatial distribution, temperature, and deuterium-to-hydrogen (D/H) ratio are important indicators for understanding atmospheric escape processes and the formation history of these bodies. As optical filters capable of making these isotope measurements, absorption cell filters have been developed.
The absorption cell filters are designed to absorb hydrogen or deuterium Lyman-alpha emissions (H Ly-α 121.567 nm, D Ly-α 121.534 nm) which enter the cells by resonant scattering. They are cylindrical glass cells with MgF2 windows sealing both ends and filled with hydrogen or deuterium gas. Heating tungsten filaments in the cell filters—installed in the optical path of an imager—causes thermal dissociation of hydrogen or deuterium molecules into atoms, which resonantly scatter incoming H or D Ly-α emissions, resulting in selective absorption of these emissions in the line of sight. This allows separate measurement of hydrogen and deuterium densities in the corona.
In the past, there have been several attempts to use these filters for observations of various planetary bodies including the Earth, but only a few have succeeded in deriving the line shape of Ly-α or D/H ratio of the geocorona. The primary factor that makes these observations difficult is the purity of the hydrogen and deuterium gases filled in the cells. Contaminants such as H2O and O2 can oxidize the filaments, causing changes in the filter’s optical depth and also shorten its lifetime. Moreover, hydrogen contamination in the D cell causes unwanted absorption of H Ly-α, leading to an overestimate of the measured D Ly-α brightness. To address these issues, a palladium filter was used for selective gas permeation, along with a cold trap and titanium/zirconium getters for the removal of gases upon filling the cells. With the cells where gases were filled, durability test of the filaments and measurement of Ly-α absorptance were conducted. We present the experimental results, discussing the measurement accuracy of D/H ratio and stability of absorption cell filters for future missions
The absorption cell filters are designed to absorb hydrogen or deuterium Lyman-alpha emissions (H Ly-α 121.567 nm, D Ly-α 121.534 nm) which enter the cells by resonant scattering. They are cylindrical glass cells with MgF2 windows sealing both ends and filled with hydrogen or deuterium gas. Heating tungsten filaments in the cell filters—installed in the optical path of an imager—causes thermal dissociation of hydrogen or deuterium molecules into atoms, which resonantly scatter incoming H or D Ly-α emissions, resulting in selective absorption of these emissions in the line of sight. This allows separate measurement of hydrogen and deuterium densities in the corona.
In the past, there have been several attempts to use these filters for observations of various planetary bodies including the Earth, but only a few have succeeded in deriving the line shape of Ly-α or D/H ratio of the geocorona. The primary factor that makes these observations difficult is the purity of the hydrogen and deuterium gases filled in the cells. Contaminants such as H2O and O2 can oxidize the filaments, causing changes in the filter’s optical depth and also shorten its lifetime. Moreover, hydrogen contamination in the D cell causes unwanted absorption of H Ly-α, leading to an overestimate of the measured D Ly-α brightness. To address these issues, a palladium filter was used for selective gas permeation, along with a cold trap and titanium/zirconium getters for the removal of gases upon filling the cells. With the cells where gases were filled, durability test of the filaments and measurement of Ly-α absorptance were conducted. We present the experimental results, discussing the measurement accuracy of D/H ratio and stability of absorption cell filters for future missions