The role of the ionosphere in aurora and space weather

Recent research strongly suggests that the ionosphere plays a crucial role in the dynamics of space weather. Although the ionosphere is by volume only a small fraction of the magnetosphere, it serves as a variably conducting boundary, modulating the global electrodynamic circuit in crucial ways. A s triking example is the behavior of intense aurora, which have recently been discovered to occur only when the background ionospheric conductivity is l ow. It is now clear that auroral acceleration occurs at the interface betwe en the ionosphere and the magnetosphere and is controlled by magnetospheric -ionospheric coupling, with the solar cycle variations arising from a surpr ising source: variations in solar EUV flux. The discovery of diverging elec tric fields with their possibly corresponding black aurora provides a new s ymmetry to magnetosphere-ionosphere coupling processes. The far-reaching sc ope of the ionosphere in space weather problems is illustrated here in seve ral ways. Ionospheric convection is suggested to be a major player in space weather, by creating global coherence in the magnetosphere on timescales n ot otherwise practical. Even a problem seemingly as far removed as possible from the ionosphere, namely, that of charge neutrality in polar rain (supe rthermal solar wind electron) entry into the distant magnetotail, is shown to be coupled to the problem of polar wind outflow from the ionosphere.