Ultra-low frequency waves, magnetic pulsations, and the ionospheric Alfven resonator.

Abstract

Magnetic pulsations are the signatures of ionospheric currents which are driven by magnetospheric ultra low frequency (ULF) waves. The characteristics of ULF waves with frequencies near 1 Hz is strongly dependent on the structure of the plasmas in the ionosphere and near-Earth magnetosphere, particularly those of a region known as the Ionospheric Alfvén resonator (IAR).

The IAR is an inhomogeneous plasma region bounded below by a conducting ionosphere and above by a sharp increase in the Alfvén speed. The Alfvén speed reaches a minimum near the ionosphere, but the location of the minimum is not strictly coincident with the ionospheric boundary, being located at the F2 density peak some 100's of kilometers above. The particular structure of the IAR allows for the existence of two particular ULF eigenmodes, cavity and waveguide modes. The cavity eigenmode is a localized shear oscillation of a particular magnetic field line, while the waveguide mode is a transversely propagating compressional oscillation near the Alfvén speed minimum. Each IAR eigenmode is characterized by a particular spectrum of resonant frequencies. The cavity and waveguide modes are coupled by the action of ionospheric Hall currents, which also produce detectable signatures below the ionosphere.

Using realistic models of the IAR Alfvén speed and geomagnetic field, we have studied the properties of IAR and its effects on ULF waves both inside and outside the IAR. Our results indicate that models which do not account for the separation of the Alfvén speed minimum from the ionosphere may incorrectly predict the resonance structure of the IAR. In addition to this eigenmode analysis, we present results from a threedimensional finite difference time domain (FDTD) model. This model is unique in its ability to self-consistently calculate electromagnetic fields both above and below the ionosphere, thus providing an accurate representation of electrodynamic processes at the ionospheric boundary and in the IAR. Results from this simulation demonstrate that the structure of the ionospheric boundary is itself an important factor in determining the properties and lifetime of IAR cavity modes and their ionosphere-mediated coupling the the waveguide mode.