A theoretical study of the time-dependent behaviour of O in the mid-latitude plasmasphere

The behaviour of O²⁺ at L = 3 in the plasmasphere is studied. Starting with a low O²⁺ flux-tube content to characterize post-magnetic-storm conditions the time-dependent equations of continuity and momentum for O²⁺ are solved to give densities and fluxes for a period of several days using both sunspotmaximum and sunspot-minimum parameters. Our results show large amounts of O²⁺ near the equator at sunspot maximum but relatively little at sunspot minimum, and emphasize the key role of the collisional process between O²⁺ and O⁺. It is the combined effects of O²⁺---O ⁺ collisions and thermal diffusion that lead to the large O²⁺ densities near the equator at sunspot maximum. Both of these mechanisms have less influence at sunspot minimum. At sunspot maximum the O⁺ layer acts as a collisional barrier below the O²⁺ production region preventing O²⁺ from sinking towards regions of high recombination rate. In this production region the effects of thermal diffusion are small and upward flow of O²⁺ results from the action of the O²⁺ pressure gradient and the polarization electric field. When the upward flowing O²⁺ reaches regions in which thermal diffusion has a strong influence it is accelerated to even higher altitudes. The O ⁺ barrier is so effective that the diurnal variation of the O⁺ layer is reflected in the diurnal variation of O²⁺ near the equator at sunspot maximum. Our sunspot maximum results also indicate that certain types of temperature profiles are more likely to enhance equatorial O²⁺ densities. The existence of large temperature gradients below 1000 km altitude does not help the flow of O²⁺ towards the equator. The associated changes in the O⁺ layer lead to more O²⁺-O ⁺collisions and a smaller O²⁺ thermal-diffusion coefficient, the latter being sensitive to the ratio n(H⁺)/n(O⁺).