E3D-BRITE: EISCAT_3D-based reconstruction of ionosphere-thermosphere electrodynamics

In many experimental studies of ionosphere-thermosphere (IT) coupling and ionospheric electrodynamics, the limitations of existing observational data sets require one to represent the three-dimensional IT system as an infinitely thin, two-dimensional sheet. This 2D representation of the coupled IT system cannot however represent some of its most basic properties, such as the existence of different ionospheric layers. On the other hand, observing systems that are capable of probing the altitudinal structure of relevant quantities, such as plasma density and drift, often can only provide information within a very narrow overhead volume. This limitation typically requires one to assume that these quantities have no horizontal gradients. Measurements from the upcoming EISCAT_3D incoherent scatter radar therefore present an unprecedented opportunity to probe the 3D IT system in three dimensions and on relatively short time scales (of order minutes). Here we present a data assimilation technique, EISCAT_3D-based reconstruction of ionosphere-thermosphere electrodynamics (E3D-BRITE), for routine estimation of all three components of the ionospheric current density and their uncertainties. We illustrate the technique using synthetic EISCAT_3D measurements of the plasma density and ion drift. We describe how the E3D-BRITE technique can also be used to simultaneously estimate the neutral wind and the perpendicular electric field. The technique relies on a 3D generalization of curl-free and divergence-free Cartesian elementary current systems. We also discuss the limitations imposed on this technique by the geometry of the three EISCAT_3D sites in Skibotn, Karesuvanto, and Kaiseniemi.