Why isn't the atmosphere completely ionized?

We have carried out experiments on electron attachment and positive ion / negative ion neutralization at thermal energies using the VENDAMS method (variable electron and neutral density attachment mass spectrometry, which is an extension of the flowing-afterglow Langmuir-probe technique). The VENDAMS method allows us to determine rate coefficients for ion-ion mutual neutralization along with neutral product branching fractions. The method is limited at present to noble gas positive ions. A major advantage of the method is that electron attachment to labile molecules such as POCl$_{2}$ or C$_{2}$F$_{5}$ may be studied, if these molecules are present in the apparatus as products of the primary neutralization reaction. Measurements have been made on electron attachment to SF$_{2}$, SF$_{3}$, SF$_{4}$, SF$_{5}$, and SF$_{6}$, for example, along with rate coefficients and neutral product distributions for SF$_{5}^{-}$ and SF$_{6}^{-}$ neutralization by Ar$^{+}$[1] and for SF$_{4}^ {-}$, SF$_{5}^{-}$, and SF$_{6}^{-}$ neutralization by Ar$^{+}$ and Kr$^{+}$ at 300 K. These and other electron attachment and ion-ion neutralization results, measured over a temperature range 300-600 K, will be presented. In addition, we have identified a new plasma process in which electrons act as a third body to remove energy from an orbiting ion-ion complex, which enhances the neutralization rate coefficient. Details of this process, electron-catalyzed mutual neutralization, have been recently published,[2] and new results will be presented for various species.\\[4pt] Collaborators in this work: Nicholas S. Shuman, Albert A. Viggiano, Jeffrey F. Friedman, Connor M. Caples, Raymond J. Bemish, and J\"{u}rgen Troe.\\[4pt] [1] N. S. Shuman, T. M. Miller, and A. A. Viggiano, J. Chem. Phys. \textbf{133}, 234304 (2010).\\[0pt] [2] N. S. Shuman, T. M. Miller, R. J. Bemish, and A. A. Viggiano, Phys. Rev. Lett. \textbf{106}, 018302 (2011).