This paper analyses magnetosphere-ionosphere (MI) coupling from a perspective that is independent of inertial reference frame, explicitly acknowledging the role of the principle of relativity in MI coupling. For the first time in the context of MI coupling, we discuss the literature on the low-velocity limit of the theory of special relativity applied to electrodynamics. In many MI coupling theories, a particular low-velocity limit applies, known as the “magnetic limit”. Two important consequences of the magnetic limit are: 1) Maxwell’s equations cannot contain a displacement current and be consistent with the magnetic limit and 2) the magnetic field is not modified by currents created by charge densities in motion, thus charge density is approximately zero. We show how reference frame-independent descriptions of MI coupling require that ion-neutral relative velocities and ion-neutral collisions are key drivers of the physics. Electric fields, on the other hand, depend on reference frame, and can be zero in an appropriate frame. Currents are independent of reference frame and will flow when the electric field is close to zero. Starting with the same momentum equations that are typically used to derive Ohm’s law, we derive an equation that relates the perpendicular current to collisions between ions and neutrals, and electrons and neutrals, without reference to electric fields. Ignoring the relative motion between ions and neutrals will result 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.

An objective of the solar and space physics communities has been to predict the behavior of the interconnected physical systems that bring space weather to Earth. One approach is to use first-principles models that may predict behavior of the various space plasma regimes from the magnetized solar corona to Earth’s upper atmosphere. We focus on space weather forecasts in the thermosphere-ionosphere (T-I), with lead time based on the period following a solar eruption. There are generally 1-4 days lead time before the interplanetary coronal mass ejection (ICME) reaches the Earth’s magnetopause. Forecasting the behavior of the T-I with such multi-day lead times requires new ways of using and assessing first principles models, which are capable of predicting many details of the T-I response, including the time history of the global electron density distribution, neutral densities and neutral winds. All facets of the complex T-I system response must be predicted based on input solar and interplanetary parameters. Another influence on the forecast is the condition of the T-I at the time a forecast is produced (e.g. shortly after the CME eruption epoch). However, the role of such pre-conditioning is not well understood for lead times of a few days. To improve our understanding of these forecasts, we have submitted more than 120 multi-day simulation periods to NASA’s Community Coordinated Modeling Center, spanning three coupled T-I models. Approximately 40 T-I storms have been simulated, driven by solar wind and EUV parameters alone. We will present an analysis that characterizes how T-I models respond to the information content of the solar wind, mediated through climatological models of high latitude forcing, and the possible influence of pre-existing conditions. Smoothing across mesoscale variability is inevitable in this scenario. Analyzing the response across events and across models reveals critical information about the predictability of the T-I system as an ICME approaches.