Ionospheric feedback instability in the coupling of magnetosphere-ionosphere

The ionospheric feedback instability is discussed by using the conductivity argument. We give an exact quantitative description to show that the free energy for this instability comes from the reduction of the Joule dissipation produced by the pre-existing convection electric field through self-consistent changes in ionization and conductivity due to Alfvénic perturbations in the ionosphere. By the over-reflection of Alfvén waves, the energy in the active ionosphere is pumped into the magnetosphere, which is contrary to the usual case whereby energy carried by Alfvén waves is deposited in the ionosphere by Joule dissipation. The ionospheric feedback enhances the electron E × B drift. The electron conductivity is controlled by the ion Perdersen conductivity rather than by the electrons Pedersen conductivity. We also provide a qualitative theoretical explanation to the intense aurora favoured by a lower ambient ionospheric conductivity in the ionospheric feedback instability.