Substorms: Ionospheric Manifestation of Magnetospheric Disturbances

Abstract:

In conventional treatments of magnetosphere-ionosphere-thermosphere coupling, it is assumed that the magnetic field of the ionosphere can be treated as static because the perturbed field is much smaller than the background field. This argument holds for evaluating the JxB force in the momentum equations but not for Faraday’s law, in which the time derivative of the perturbed magnetic field is always greater than that of the static field because the latter is zero. When inductive and inertial effects of the ionosphere are considered, ionospheric manifestations of magnetospheric disturbances become substantial, especially if the sources of the disturbances (e.g., magnetopause or nightside reconnection) are not very close to the ionosphere. Since many magnetic storm and substorm signatures are measured on the ground or from the bottom side of the ionosphere, they include the ionospheric manifestations in addition to the magnetospheric disturbances. Disturbances propagating downward from the magnetosphere are reflected and refracted as they encounter the ionosphere, and signals which penetrate into the ionosphere may undergo mode conversion. For example, antisunward Alfvenic motion in the magnetosphere may become a fast mode perturbation in the ionosphere, which can quickly produce a global ionospheric response. Ionospheric manifestations also amplify the amplitude of the magnetospheric disturbances during a transient interval of around 20-30 min, which is of the order of the time a substorm lasts, as a result of reduction of the propagation speed and superposition with the reflection.