Auroral and sub-auroral phenomena: an electrostatic picture

Abstract

Abstract. Many auroral and sub-auroral phenomena are manifestations of an underlying magnetosphere-ionosphere coupling. In the electrostatic perspective the associated auroral current circuit describes how the generator (often in the magnetosphere) is connected to the load (often in the ionosphere) through field-aligned currents. The present paper examines the generic properties of the current continuity equation that characterizes the auroral circuit. The physical role of the various elements of the current circuit is illustrated by considering a number of magnetospheric configurations, various auroral current-voltage relations, and different types of behaviour of the ionospheric conductivity. Based on realistic assumptions concerning the current-voltage relation and the ionospheric conductivity, a comprehensive picture of auroral and sub-auroral phenomena is presented, including diffuse aurora, discrete auroral arcs, black aurora, and subauroral ion drift. The electrostatic picture of field-aligned potential differences, field-aligned currents, ionospheric electric fields and plasma drift, and spatial scales for all these phenomena is in qualitative agreement with observations.

Monopolar and bipolar auroral electric fields and their effects

“…At the outset, it is assumed that the magnetosphere harbours sufficiently long-lived electric potential structures that drive auroral current systems, i.e., the quasi-electrostatic picture is adopted here (as done by Lyons, 1980Lyons, , 1981Roth et al, 1996;De Keyser, 1999;Echim et al, 2008;De Keyser and Echim, 2010). Such magnetospheric potential structures can act as generators and are connected via field-aligned currents to the ionosphere, which behaves as a load.…”

“…The current-voltage relation indicates how such currents relate to the field-aligned electric potential difference φ = φ −φ between the ionospheric (φ) and magnetospheric (φ) electric potentials. The present study employs a phenomenological approach by considering (piecewise) linear current-voltage relations (as in De Keyser and Echim, 2010). The fundamental principle is that the field-aligned current grows with φ for particles of one sign, while particles of the opposite sign experience an electric potential barrier.…”

“…The individual currents are not independent from one another as charge neutrality must be maintained, and that would translate in a certain relationship between the different Knight constants, as well as a possible deviation from linearity. Nevertheless, the linear approximation can already capture a number of interesting properties of the actual physical system (e.g., De Keyser and Echim, 2010).…”

“…The observed electric field profiles can have various forms. One way of classifying these structures is to distinguish "convergent" and "divergent" electric fields, which correspond to V-shaped or nested V-shaped magnetospheric potential profiles (e.g., Lyons, 1980Lyons, , 1981De Keyser and Echim, 2010). Another classification considers three Correspondence to: J.…”

“…The paper also addresses the general properties of the auroral current circuits associated with such monopolar and bipolar electric fields. These can be understood in terms of the structures identified for convergent/divergent electric fields (De Keyser and Echim, 2010), since monopolar and bipolar electric field profiles can be regarded as a succession of convergent and/or divergent electric field configurations. Because of their frequent occurrence in the auroral regions, however, monopolar and bipolar fields merit a more detailed study.…”