Abstract
The aurora shows explosive activities a few times in 24 hours on a moderately active day. This specific phenomenon is called the auroral substorm, which consists of the growth, expansion and recovery phases; the explosive activities occur during the expansion phase. As an introduction, the explosive activities of the aurora are morphologically described first on the basis of ground-based all-sky and satellite images. In terms of theoretical understanding, the processes for the explosive activities have been considered almost exclusively in terms of “the magnetic field line approach” in the past, including the process of magnetic reconnection. Instead, in this paper, we consider the processes in terms of “the electric current line approach”. This approach requires that the whole process of auroral substorms should be considered as a chain of processes, which consists of power supply (dynamo), transmission (currents/circuits) and dissipation (auroral substorms). An increased power of the solar wind-magnetosphere dynamo (≈ 5x1018 erg/s or 1011w) intensifies the electric currents mainly in the main body (just outside of the ring current) of the magnetosphere, resulting in accumulating energy in its inductive circuit and inflation of the magnetosphere. When the accumulated energy reaches about 5x1022 ergs (5x1015 J), the magnetosphere tends to become unstable (because of current instabilities). As the current intensity is reduced as a result, the magnetosphere is deflated. It is suggested that it is in this deflation process, during which the accumulated energy is unloaded, and an earthward electric field (5-50 mV/m) is produced on the equatorial plane, establishing the current system (the UL current system), which is responsible for the expansion phase, including the most characteristic features of the expansion phase, such as the poleward advance of the aurora and the development of the auroral electrojet. The electric current approach is rather new and needs much more effort to develop.
Highlights
SOLAR- TERRESTRIAL RELATIONSHIPWhen we consider auroral phenomena and the accompanying geomagnetic storms, it is worthwhile to consider first the whole solar-terrestrial relationship, since these phenomena are the last part of the solar-terrestrial relationship, in which the solar energy is dissipated mainly in the ionosphere as the Joule heating
(12) It is tentatively suggested that the cause of the earthward electric field is caused by the deflation of the magnetosphere after the inflation caused by the accumulated energy; the concept of the frozen-in field breaks down at this crucial time
The electric current approach provides a different view on the processes of auroral substorms from that of the magnetic field line approach
Summary
When we consider auroral phenomena and the accompanying geomagnetic storms, it is worthwhile to consider first the whole solar-terrestrial relationship, since these phenomena are the last part of the solar-terrestrial relationship, in which the solar (wind) energy is dissipated mainly in the ionosphere as the Joule heating. The magnetic field lines are nearly equi-potential, so that the morning half of the auroral oval becomes positive (+), and negative (–) in the evening half, establishing an electric field across the polar cap Another way to describe this situation is that the dawn-todusk electric field across the magnetosphere drives an earthward plasma convection in the equatorial plane (Figure 7A) and induces the convection of the ionospherc plasma; the equipotential contour (along which the convection occurs) produced in the ionosphere is shown in Figure 7B (Sun et al, 2002); the accuracy of this current distribution is confirmed by (Bristow and Jensen, 2007). Since EJ is negative only in the equatorial part of the circuit, the location where E is generated must be mainly on and near the equatorial plane In this consideration, it is crucial to confirm that the UL current system is the current system responsible for the expansion phase, Figure 10A shows time variations of the DD and UL components, together with the power (expressed in terms of ε). It is unlikely that magnetic reconnection in the magnetotail provides a substantial extra energy; this is confirmed by a satellite observation, which shows no sufficient energy flows from the magnetotail, which is needed for a substorm (Miyashita et al, 2012)
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