GALEX FUV Observations of Comet C/2004 Q2 (Machholz): The Ionization Lifetime of Carbon

TL;DR

This study measures the atomic carbon lifetime in interplanetary space using GALEX FUV images of comet Machholz, finding results consistent with solar ionization processes and suggesting shorter parent molecule lifetimes.

Contribution

First direct measurement of atomic carbon lifetime in interplanetary space using UV imaging of a comet, comparing it with physical process models.

Findings

Carbon lifetime is approximately 7.1 to 9.6 x 10^5 seconds at 1 AU.

Results agree with models involving solar photoionization and solar wind interactions.

Indicates possible shorter-lived parent molecules like CH4 in the comet's coma.

Abstract

We present a measurement of the lifetime of ground state atomic carbon, C(^3P), against ionization processes in interplanetary space and compare it to the lifetime expected from the dominant physical processes likely to occur in this medium. Our measurement is based on analysis of a far ultraviolet (FUV) image of comet C/2004 Q2 (Machholz) recorded by the Galaxy Evolution Explorer (GALEX) on 2005 March 1. The bright CI 1561 A and 1657 A multiplets dominate the GALEX FUV band. We used the image to create high S/N radial profiles that extended beyond one million km from the comet nucleus. Our measurements yielded a total carbon lifetime of 7.1 -- 9.6 x 10^5 s (scaled to 1 AU). Which compares favorably to calculations assuming solar photoionization, solar wind proton change exchange and solar wind electron impact ionization are the dominant processes occurring in this medium and that comet Machholz was embedded in the slow solar wind. The shape of the CI profiles inside 3x10^5 km suggests that either the CO lifetime is shorter than previously thought and/or a shorter-lived carbon-bearing parent molecule, such as CH_4 is providing the majority of the carbon in this region of the coma of comet Machholz.