X-ray Diagnostics of Ionizing Plasmas in Collisionless Shocks Cara E. Rakowski Harvard-Smithsonian CfA 60 Garden Street, MS 70 Cambridge, MA 02138 The energy distributions of electrons and ions from thermal to cosmic-ray populations behind a collisionless shock are still under investigation. Anything from pure thermal populations with mass-proportional heating to efficient cosmic-ray acceleration may occur. The primary goal of X-ray observations at the shock front has been to diagnose the post-shock electron temperature, and hope to see signatures of the ion temperatures and any high energy tails. There are many complications to this endeavor: The X-ray emission peaks well behind the shock front, so that the immediate post-shock electron temperature must be inferred from the current temperature. The timescale for this extrapolation must be deduced from how far out of equilibrium the ionization state is. The accuracy of measuring an ionization timescale from the X-ray spectrum depends strongly on our knowledge of the ionization and recomb! ination rates (including DR) as well the emission processes. However, lines that are only strong in a rapidly ionizing plasma could provide a separate proof of non-equilibrium conditions distinct from diagnostics that are indicative of the temperature or ion fractions. Currently, most atomic calculations have focused on either equilibrium or photo-ionizing plasmas. Less attention has been paid to transitions that would only be common in a low density collisionally ionizing plasma, such as radiative decays from high energy states created by inner-shell ionization. In this talk I will present examples of both traditional temperature and ion fraction diagnostics as well as lines that would be undetectable in anything other than a rapidly ionizing plasma.