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Spatial Distribution of H2O+ Ions in the Inner Coma of C/2014 Q2 (Lovejoy)

Year: 2025
Type: article
Abstract: Neutral molecules that sublimate from cometary ices (mainly water at ∼1 au from the Sun) are ionized due to solar extreme-ultraviolet radiation and collisions with highly energetic electrons. The primary ionization product of water is H2O+, which is rapidly converted into H3O+ through proton-transfer reaction with H2O in the inner coma. Ion-mass spectrometers on board spacecraft such as Giotto and Rosetta have measured H2O+ ions in the inner coma, while emission bands of H2O+ in the optical wavelength region can be observed from ground-based observatories. We present the spatial intensity distribution of the H2O+ emission in comet C/2014 Q2 (Lovejoy), derived from high-resolution spectra taken on UT 2015 January 11. The observations covered a projected distance of ∼3000 km across the sky along the slit, with a spatial resolution of ∼200 km resulting from the atmospheric seeing. The observed H2O+ profile scales as ρ − 0.13 (where ρ is the projected distance from the nucleus), in contrast to the ρ − 1 profile seen in sunlight reflected by dust grains. We modeled the intensity profile of H2O+ using a steady-state, radially expanding coma model. Our results indicate that the proton-transfer reaction of H2O+ with H2O dominates the loss process of H2O+ in the inner coma. Assuming that H2O+ is produced primarily by photoionization of H2O, the modeled intensity profile of H2O+ remains constant, which is inconsistent with our observations. However, when an electron-impact ionization frequency inversely proportional to distance from the nucleus is assumed, the model successfully reproduces the observed profile.
DOI: 10.3847/1538-4357/adf853
Authors: Hideyo Kawakita, Yoshiharu Shinnaka, Hitomi Kobayashi
Language: English
Open Access status: gold
Estimated APC paid: $4,499
OpenAlex ID: W7082632002
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Cites: 38
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