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Direct measurements of a pronounced latitudinal variation in the exospheric ion composition have been obtained from the radio frequency ion spectrometer experiment on the Polar Orbiting Geophysical Observatory (OGO 2). Measurements of thermal positive ions obtained in a nearly polar dawn‐dusk orbit during mid‐October 1965 show that in the altitude range of 415–1525 kilometers the major ion are O+ and H+, and the minor constituents are N+ and He+. Consistent with this period of low solar activity, He+ is at all altitudes a minor ion, relative to H+. Evidence of pronounced solar and geomagnetic control of the ion distributions is further examined by translating the data along magnetic field lines to both (1) a constant 1000‐kilometer reference level and (2) the dipole equator, applying chemical and diffusive equilibrium theory. At 100 kilometers O+ dominates in both the northern and southern polar ionospheres, yielding at lower latitudes where H+ dominates. The resultant mean ion mass distribution, about 14–16 amu at the poles, and about 4 amu at the equator, is generally consistent with theory and other measurements. The latitudinal asymmetry in the distributions of O+ and N+ reflects the control of seasonal temperature differences, while the greater symmetry in the distributions of H+ and He+ reveals the strong influence of solar‐geomagnetic control of the light ions. The high‐latitude ionosphere is marked by two dominant features: (1) a persistent, major trough in n(H+) and n(He+) where n(H+) drops to about 10² ions/cm³ near 60° dipole and (2) a variable poleward peak in which the total ion concentration Ni approaches 104 ions/cm³ a near 80° dipole. The pronounced light ion trough, which correlates well with the whistler cutoff, is believed to mark the high‐latitude boundary of the thermal plasma that diffuses upward along closed field lines to populate the plasmasphere. Poleward of the trough, the strong fluctuations in the composition and the variability of the amplitude and position of the ionization peak suggest that the polar exosphere is decoupled from the plasmasphere and is probably linked with the solar wind‐magnetospheric tail system. Comparison of the extrapolated equatorial Ni profile with thermal ion distributions measured directly in the magnetosphere further supports this interpretation.

We present wavepacket dynamics simulations of above threshold dissociation (ATD) in Na2 + and Li2 +. We consider resonant two-photon processes in which both the intermediate and the final state belong to the dissociative continuum. We show that in both molecular ions ATD experiments can be planned with moderately high laser intensities. If a vibrational initial state other than v = 0 is selected and/or a two-color excitation is envisaged, a wider range of processes can be observed, such as interference phenomena and Rabi oscillations between continuum states.