  Charge exchange X-ray emission provides unique insights into the interactions
between cold and hot astrophysical plasmas. Besides its own profound science,
this emission is also technically crucial to all observations in the X-ray
band, since charge exchange with the solar wind often contributes a significant
foreground component that contaminates the signal of interest. By approximating
the cross sections resolved to $n$ and $l$ atomic subshells, and carrying out
complete radiative cascade calculation, we create a new spectral code to
evaluate the charge exchange emission in the X-ray band. Comparing to
collisional thermal emission, charge exchange radiation exhibits enhanced lines
from large-$n$ shells to the ground, as well as large forbidden-to-resonance
ratios of triplet transitions. Our new model successfully reproduces an
observed high-quality spectrum of comet C/2000 WM1 (LINEAR), which emits purely
by charge exchange between solar wind ions and cometary neutrals. It
demonstrates that a proper charge exchange model will allow us to probe
remotely the ion properties, including charge state, dynamics, and composition,
at the interface between the cold and hot plasmas.

High Energy Astrophysical Phenomena;Earth and Planetary Astrophysics;Astrophysics of Galaxies;Instrumentation and Methods for Astrophysics;Atomic Physics