We present a measurement of the lifetime of ground state atomic carbon , C ( ^ { 3 } P ) , 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 C i 1561 Å and 1657 Å multiplets dominate the GALEX FUV band .
We used the image to create high signal-to-noise ratio radial profiles that extended beyond 1 \times 10 ^ { 6 } km from the comet nucleus .
Our measurements yielded a total carbon lifetime of 7.1 – 9.6 \times 10 ^ { 5 } s ( ionization rate of 1.0 – 1.4 \times 10 ^ { -6 } s ^ { -1 } ) when scaled to 1 AU .
This 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 C i profiles inside 3 \times 10 ^ { 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 .