On the divergence of the auroral electrojets

Abstract

[1] The current configuration in the auroral region is known to consist typically of downward and upward field-aligned current (FAC) sheets, connected in the ionosphere by meridional Pedersen currents, while divergence free electrojets (EJs) flow azimuthally as Hall currents. This configuration of the auroral current circuit was introduced by Boström (1964) and labeled as "Type 2," while he suggested also an alternative "Type 1" configuration, with filamentary FACs connected in the ionosphere by azimuthal Pedersen currents. By using an updated version of the recently developed ALADYN technique, we investigated the divergence of the auroral electrojets for a few FAST crossings over the auroral oval in the 20-22 MLT sector, two of which are presented in detail. Although a precise estimate of the electrojet divergence is difficult, because of several error sources, the results suggest that this divergence can be significant over certain latitude ranges, comparable with the FAC density. Direct FAC-EJ coupling appears to contribute to the ionospheric current closure not only during active times, as already known, but also during rather quiet periods. The quiet time FAC-EJ coupling is likely to be achieved in a mixed "Type 1/Type 2" configuration, with the FAC sheet (Type 1) azimuthally connected to the Pedersen component of the EJ (Type 2). This configuration requires a non-zero tangential component of the electric field, and is therefore more likely realized inside or near the Harang region. At the same time, the divergence of the Hall current is presumably negligible, and likewise the ionospheric polarization, consistent with statistical results published recently. During more active intervals and possible reconfigurations of the auroral current circuit, our results suggest that the FAC-EJ coupling could be also achieved by Hall currents. We conclude by exploring a tentative scenario for the integrated evolution of the ionospheric current closure and Cowling mechanism during the substorm cycle. A systematic examination of more experimental evidence is needed to validate this scenario.

Ionospheric conductances and currents of a morning sector auroral arc from Swarm‐A electric and magnetic field measurements

“…Our example event was a Swarm crossing of a morning sector auroral arc with 1‐D properties (i.e., negligible azimuthal gradients and vanishing eastward electric field) allowing SECS analysis with single satellite electric field data. Analysis of more complicated arc cases, where, e.g., the Hall and Pedersen current systems are coupled with each other [ Marghitu et al , ], is left for future studies with electric field data from two Swarm satellites. In our case the structure of the derived conductances, electric fields, and currents associated with the arc were shown to be in good agreement with those expected for morning sector arcs: upward FAC, enhanced Hall and Pedersen conductances, and relatively weak electric field coincided with the arc.…”

Use of fractal approach to investigate ionospheric conductivity in the auroral zone

“…As it was mentioned above (see section 3), the asymptotically path‐connected behavior is possible for the Hall current in the auroral ionosphere. It is known that Hall current flows in auroral arcs [ Haerendel et al , ; Frey et al , ; Marghitu et al , ]. The multiple arcs system is observed often rather than one auroral arc.…”

Lower-thermosphere–ionosphere (LTI) quantities: current status of measuring techniques and models

“…For so-called high-top models, with upper boundary in the thermosphere, the boundary conditions can be defined by empirical models based on satellite data (e.g. Marsh et al, 2004), which depend on geomagnetic indices, day of the year, and solar flux. However, current models are based on temporally limited data and do not cover full solar cycles and/or differences between solar cycles, and recent studies indicate a need for improvements (Hendrickx et al, 2018;Kiviranta et al, 2018).…”

“…By using the EISCAT ISR measurements, it has been established that auroral arcs are often associated with narrow intense electric fields just outside of the auroral arcs and related increased electron densities due to auroral electron precipitation (Aikio et al, 2002). Cluster satellite measurements showed that those electric fields develop rapidly in a timescale of minutes (Marklund et al, 2001;Aikio et al, 2004). Additionally, intense flow channels of ionospheric plasma have been found on the dayside in the cusp region (Oksavik et al, 2004), in the polar cap (Nishimura et al, 2014), and at high latitudes on the nightside in association with magnetospheric bursty bulk flows (Pitkänen et al, 2013).…”