Numerical modeling tools can be used for a number of reasons yielding many benefits in their application to planetary upper atmosphere and ionosphere environments. These tools are commonly used to predict upper atmosphere and ionosphere characteristics and to interpret measurements once they are obtained. Additional applications of these tools include conducting diagnostic balance studies, converting raw measurements into useful physical parameters, and comparing features and processes of different planetary atmospheres. This chapter focuses upon various classes of upper atmosphere and ionosphere numerical modeling tools, the equations solved and key assumptions made, specified inputs and tunable parameters, their common applications, and finally their notable strengths and weaknesses. Examples of these model classes and their specific applications to individual planetary environments will be described.

Abstract. Photochemical processes in the upper atmosphere are known to create significant amounts of energetic oxygen atoms or "hot O". In this research we simulate the effects of ionized hot oxygen, hot O + , on the ionosphere. We find that hot O + is not able to maintain a temperature substantially above the ambient ion temperature at most altitudes, the exception being around the F-region ion density peak. However, the thermalization of hot O + , due to Coulomb collisions, represents an important heating process for the ambient ions. A time-dependent, fluid-kinetic model of the ionosphere (TRANSCAR) is used to self-consistently simulate hot O + by considering it to be a separate species from O + . A Maxwellian neutral hot O population having characteristics consistent with current knowledge is added to TRANSCAR. The production of the hot O + is then computed by considering ion charge exchange with the neutral hot O population that we have assumed. Loss of hot O + results from these charge exchange reactions and from reactions with molecular atoms.