Charge Abundances of the Solar Wind Ions Inferred from Cometary X-Ray Spectra

An emission spectrum of X-rays from comet McNaught-Hartley (C/1999 T1) detected with the Chandra X-ray telescope is analyzed on the assumption that it results from electron captures by heavy ions of the solar wind colliding with the atoms and molecules of the cometary atmosphere. An accurate value of the relative abundance of O+8 and O+7 ions is obtained by comparing the spectrum derived from the measurement data with the predicted spectrum. A close match is found for an abundance ratio of O+8 to O+7 of 0.34 ± 0.07, the uncertainty reflecting the measurement statistics. A similar analysis of the contribution from captures by Ne+9 ions leads to an abundance ratio of Ne+9 to O+7 of 0.02 with an uncertainty of 40%. The spectrum consists of many unresolved lines. The brightest feature is composed mostly of the 2 3S1 - 1 1S0 spin-forbidden line of O+6 at 561 eV with 74% of the intensity and the 2 3P1 - 1 1S0 intercombination line at 568 eV with 12%. The strongest of the O+7 lines is the Ly(alpha) transition at 654 eV with an intensity of 29%, relative to the O+6 lines. The sensitivity of cometary Xray spectra to variations in the abundances of highly charged oxygen ions in the solar wind is investigated.