  We report the discovery of a geocoronal solar wind charge exchange (SWCX)
event corresponding to the well-known 2006 December 13th coronal mass ejection
(CME) event. Strong evidence for the charge exchange origin of this transient
diffuse emission is provided by prominent non-thermal emission lines at
energies of $\rm O^{7+}$, $\rm Ne^{9+}$, $\rm Mg^{11+}$, $\rm Si^{12+}$, $\rm
Si^{13+}$. Especially, a 0.53 keV emission line that most likely arises from
the $\rm N^{5+}$ $1s^1 5p^1 \to 1s^2$ transition is detected. Previously, the
forecastability of SWCX occurrence with proton flares has been disputed. In
this particular event, we found that the SWCX signal coincided with the arrival
of the magnetic cloud inside CME, triggered with a time delay after the proton
flux fluctuation as the CME shock front passed through the Earth. Moreover, a
spacecraft orbital modulation in SWCX light curve suggests that the emission
arises close to the Earth. The line of sight was found to always pass through
the northern magnetospheric cusp. The SWCX intensity was high when the line of
sight passed the dusk side of the cusp, suggesting an azimuthal anisotropy in
the flow of solar-wind ions inside the cusp. An axisymmetric SWCX emission
model is found to underestimate the observed peak intensity by a factor of
about 50. We suggest this discrepancy is related to the azimuthal anisotropy of
the solar-wind flow in the cusp.
