Measurement of Charge Exchange and X-Ray Emission Cross Sections for Solar Wind–Comet Interactions

Abstract

X-ray emission from a comet was observed for the first time in 1996. One of the mechanisms believed to be contributing to this surprisingly strong emission is the interaction of highly charged solar wind ions with cometary gases. Reported herein are total absolute charge-exchange and normalized line-emission (X-ray) cross sections for collisions of high-charge state (+3 to +10) C, N, O, and Ne ions with the cometary species H2O and CO2. It is found that in several cases the double charge-exchange cross sections can be large, and in the case of C3+ they are equal to those for single charge exchange. Present results are compared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission intensities is emphasized.

X-ray emission cross sections following charge exchange by multiply charged ions of astrophysical interest

“…The trends exhibited by the high npö 1s transitions in Figure 1 leave clear trends in R, where now the CTMC results tend to overestimate the data due to the strong 5pö1s emission obtained for H 2 O, CO 2 and CH 4 . At higher energies, where JPL data (H 2 O and CO 2 ) [10] as well as KVI data (H 2 ) [11] are available, we see that the CTMC is in very good agreement.…”

“…Hardness ratio R as a function of collision energy for O 8+ projectiles. The experimental data corresponds to Beiersdorfer [9], Greenwood et al [10] and Suraud et al [11]. …”

“…Our theoretical results are first compared to high-resolution data obtained with the EBIT machine at LLNL at low collision energies (10 eV/amu) [9]. Then, they are compared with data measured for similar reactions but for impact energies which are in accord with the astrophysical observations (~ 1-3 keV/amu) [10,11].…”

Variability of Cometary X-Ray Emission Induced by Solar Wind Ions

“…We employ a more detailed description of the capture and radiative processes and argue that X-ray emissions from comets C/1990 K1 (Levy), P/Encke 1997, and C/1995 O1 (Hale-Bopp) were induced by slow wind ions. We demonstrate that forbidden transitions give rise to intense emission lines, which, because of the long lifetimes of metastable states, are absent from laboratory beam spectra (Suraud et al 1991;Greenwood et al 2000).…”

“…Highly charged ions moving at solar wind velocities capture electrons into a range of excited states with a distribution that depends on the charge and energy of the ion and on the identity of the target species that, for comets, is primarily the water molecule with lesser abundances of CO, CO 2 , OH, O, and H. Multielectron charge transfer can occur at low collision velocities (Posthumus & Morgenstern 1992;Kamber et al 1999;Beiersdorfer et al 2000;Greenwood et al 2000;Moretto-Capelle, Bordenave-Montesquieu, & Bordenave-Montesquieu 2000), but the resulting excited ions decay mostly by emission of energetic Auger electrons (Posthumus & Morgenstern 1992;Kamber et al 1999;Moretto-Capelle et al 2000) and rarely produce Xrays. X-ray emission arises from single-electron capture into specific excited states (Janev & Winter 1985;Suraud et al 1991;Janev 1995;Greenwood et al 2000;Beiersdorfer et al 2000).…”