Influence of the Polar Electrojet on Field‐Aligned Current Estimates From Single Satellite Magnetic Field Measurements

Abstract

There are in general two kinds of intense ionospheric currents at auroral latitudes: one is the field-aligned current (FAC) that flows along the magnetic field lines, and the other is the auroral or polar electrojet (PEJ) that flows horizontally in the ionospheric E-region at about 110 km. As a connection between the magnetosphere and ionosphere, FACs are the dominant transport mechanism for energy and momentum, and they are thus of fundamental importance for our understanding of the solar wind-magnetosphere-ionosphere-thermosphere coupling (e.g., Birkeland, 1908;Iijima & Potemra, 1976b). Differently, the PEJs are part of the horizontally Hall current vortices that close over the polar cap. Along the auroral oval the PEJ currents are concentrated and their intensity is related to the substorm activity (e.g., Akasofu et al., 1965;Kisabeth, 1979).According to Ampere's law, electric currents cause magnetic signatures perpendicular to the current direction. Therefore, measuring the magnetic variations is a possible way to quantify the ionospheric currents. Davis and Sugiura (1966) proposed the auroral electrojet (AE) index to monitor the electrojet activity, which is widely used today as an indicator reflecting the state of polar ionosphere as well as the activity phase of a substorm. Later studies, for example, Kamide et al. (1981) and Friis-Christensen et al. (1985), have also successfully derived the full Hall current vortices from the ground-based magnetic observations. However, one weakness of magnetic observations from ground is that they can only estimate the equivalent currents, representing horizontal currents that close in the ionosphere. Conversely, currents connected with FACs cannot be sensed. An additional limitation is the patchy distribution of ground stations, which restricts the investigation of large-scale current circuits, and they cannot reflect well the global variations. Therefore, magnetic observations taken at satellite altitude have become an important data source for retrieving the ionospheric currents on a global scale, especially for the FAC distribution. Zmuda et al. (1966) first reported the magnetic signatures in the 1963-38C satellite data, shown as persistent perturbations of the east-west magnetic components whenever the satellite crossed the auroral latitudes. These signatures were later identified as the FAC sheet crossings. Potemra (1976a, 1976b) were the

IMF By Effects on the Strength and Latitude of Polar Electrojets: CHAMP and Swarm Joint Observations

“…Polar electrojets (PEJs) play an important role in high‐latitude ionosphere‐magnetosphere coupling processes (Gjerloev & Hoffman, 2014; Wang et al., 2008; Xiong et al., 2023; Zhong et al., 2022). Notably, PEJs flow in the ionospheric E region, transverse to the electric and magnetic fields.…”