An Analysis of Magnetosphere-Ionosphere Coupling That Is Independent of Inertial Reference Frame

This paper analyses magnetosphere-ionosphere (M-I) coupling from a perspective that is independent of inertial reference frame, and delineates how physical theories of M-I coupling are affected by the principle of relativity. For the first time in the context of M-I coupling, we discuss the literature from the 1970s on how the low-velocity limit of the theory of special relativity is applied to electrodynamics. In most M-I coupling theories, a particular low-velocity limit applies, known as the "magnetic limit." Two important consequences of this literature are: (a) significant displacement currents in Maxwell's equations break the Galilean invariance of the equations and (b) magnetic fields are not generated by currents created by a net charge density in motion. We show how reference frame-independent descriptions of M-I coupling require that ion-neutral relative velocities and ion-neutral collisions are key drivers of the physics. Currents are independent of reference frame whereas electric fields depend on reference frame. Starting with the same momentum equations that are typically used to derive Ohm's law, it is possible to express the perpendicular ionospheric current as depending on collisions between ions and neutrals, and electrons and neutrals, without reference to electric fields. Ignoring the relative motion between ions and neutrals results in errors exceeding 100% for estimates of high latitude Joule heating during significant geomagnetic storms, when ion-neutral velocity differences are largest near the initiation of large-scale ion convection.