Magnetosphere-Ionosphere Coupling and the Evolution of Field-Aligned Currents

Abstract:

Changes in field-aligned currents are mediated by the propagation of shear Alfvén waves along geomagnetic field lines. The interaction of Alfvén waves with the ionosphere results in the reflection and/or absorption of these waves, depending on the ionospheric conductance. The ionospheric Hall conductivity couples shear Alfvén waves to compressional waves, modifying the reflection coefficient and leading to the ground signatures of these currents. While current continuity implies that only gradients of the Hall conductivity are important, the more complete inductive ionosphere model shows that even a uniform Hall conductivity affects the reflection of Alfvén waves and thus the structure of the field-aligned currents. Multi-satellite observations from missions such as Cluster and Swarm can lead to a better understanding of the fine-scale structure of these field-aligned currents. These interactions will be illustrated by means of a three-dimensional model for the propagation of MHD waves in the dipolar regions of the magnetosphere that includes conductivity differences between day and night sides. In particular, near the terminator under solstice conditions quarter-wave modes can be excited due to the difference in conductivity in the two hemispheres.