Field Aligned Currents Derived from Pressue Profiles Obtained from TWINS ENA Images

Abstract

Field aligned currents (FACs) are an important mechanism coupling the Earth's magnetosphere and ionosphere. The Two Wide-Angle Imaging Neutral-atom Spectrometers (TWINS) mission is the first stereoscopic neutral atom imager. Identical energetic neutral atom (ENA) detectors are placed on widely separated satellites, allowing nearly continuous monitoring of the Earth's inner magnetosphere. TWINS ENA measurements, combined with the Tsyganenko and Sitnov 2005 (TS05) magnetic field model, yield ion pressure profiles. Calculations are performed in the quasistatic approximation, allowing FACs to be determined from the ion pressure profiles.

TWINS ENA measurements allow the pitch angle distribution (PAD) of the ions to be determined. These PADs allow anisotropic pressure distributions to be determined. This work calculates FACs using the measured PAD anisotropy from TWINS and compares these calculations to those done assuming an isotropic ion pressure the corotating interaction region (CIR) storm of 01 June 2013. The results are very similar in most cases, but important differences are observed. These differences indicate that pressure anisotropy is important for fine-scale structures in the FACs during geomagnetic stormtime.

The calculation of FACs in the anisotropic pressure case involves an integral with three terms. In the case of isotropic pressure, two of these terms are eliminated. The current densities due to each term are evaluated and compared for several points on the northern ionosphere. In regions where the two cases differ substantially, it is shown that the term appearing in the isotropic case is exceeded by one of the purely anisotropic terms, leading to differences in magnitude and direction of the current.

The evolution of pressure and current density along magnetic field lines is examined at several locations. These indicate that FACs dominate the current near the ionosphere, while currents perpendicular to the magnetic field are more important near the equatorial plane. Current paths are traced through the inner magnetosphere, beginning at several locations on the equatorial plane, and at the ionosphere. These paths indicate that the simple picture of FACs moving along field lines from the ionosphere to the ring current misses important aspects of the current behavior.