A theoretical study of the time-dependent behaviour of O 2+ in the mid-latitude plasmasphere

Abstract

The behaviour of O 2+ at L = 3 in the plasmasphere is studied. Starting with a low O 2+ flux-tube content to characterize post-magnetic-storm conditions the time-dependent equations of continuity and momentum for O 2+ 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 O 2+ near the equator at sunspot maximum but relatively little at sunspot minimum, and emphasize the key role of the collisional process between O 2+ and O +. It is the combined effects of O 2+O + collisions and thermal diffusion that lead to the large O 2+ 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 O 2+ production region preventing O 2+ from sinking towards regions of high recombination rate. In this production region the effects of thermal diffusion are small and upward flow of O 2+ results from the action of the O 2+ pressure gradient and the polarization electric field. When the upward flowing O 2+ 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 O 2+ near the equator at sunspot maximum. Our sunspot maximum results also indicate that certain types of temperature profiles are more likely to enhance equatorial O 2+ densities. The existence of large temperature gradients below 1000 km altitude does not help the flow of O 2+ towards the equator. The associated changes in the O + layer lead to more O 2+-O +collisions and a smaller O 2+ thermal-diffusion coefficient, the latter being sensitive to the ratio n( H +) n( O +) .