Modelling the time-dependent ionospheric electric field

Abstract:

It has been well established for over two decades that the high-latitude ionospheric electric field pattern cannot be parameterised by the concurrent interplanetary conditions alone. Despite this, most empirical models of the electric field (or equivalent plasma convection pattern) do just that. In fact, the pattern consists of two time-dependent components, one driven by magnetopause coupling (which may be parameterised by the interplanetary conditions) and one driven by magnetotail dynamics, but the latter has rarely been considered in empirical models. We present a new statistical model of the ionospheric electric field that uses novel parameterisations to capture this time-dependence. Our model is derived from line-of-sight plasma velocity measurements from the Super Dual Auroral Radar Network (SuperDARN) with electric field patterns produced using an established technique that models the electric field as a spherical harmonic expansion of the ionospheric electric potential. Three specific sources of time-dependence are included. The first relates to the time-history of the upstream solar wind conditions, specifically the interplanetary magnetic field or solar wind electric field. The time-dependence of the magnetosphere-ionosphere system means it is not static under continuous driving by the solar wind but evolves with time, even if the solar wind conditions themselves remain steady. We account for this by defining a solar wind steadiness timescale with which we parameterise the electric field patterns. The second source of variability relates to the storage and release of energy in the magnetosphere that is associated with the substorm. The electric field evolves throughout the substorm cycle and its morphology is strongly influenced by the location of substorm onset. We therefore also parameterise the electric field patterns by substorm onset location. Lastly we account for the variability introduced by geomagnetic storms. The ionospheric electric field evolves differently through each phase of a storm and this we account for by parameterising the electric field patterns by storm phase. We discuss the details of the model, and assess its performance by comparison to other models and to observations.