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

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 H 2 O + , which is rapidly converted into H 3 O + through proton-transfer reaction with H 2 O in the inner coma. Ion-mass spectrometers on board spacecraft such as Giotto and Rosetta have measured H 2 O + ions in the inner coma, while emission bands of H 2 O + in the optical wavelength region can be observed from ground-based observatories. We present the spatial intensity distribution of the H 2 O + 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 H 2 O + profile scales as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.13</mml:mn> </mml:mrow> </mml:msup> </mml:math> (where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ρ</mml:mi> </mml:math> is the projected distance from the nucleus), in contrast to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>ρ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> profile seen in sunlight reflected by dust grains. We modeled the intensity profile of H 2 O + using a steady-state, radially expanding coma model. Our results indicate that the proton-transfer reaction of H 2 O + with H 2 O dominates the loss process of H 2 O + in the inner coma. Assuming that H 2 O + is produced primarily by photoionization of H 2 O, the modeled intensity profile of H 2 O + 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.