Abstract
Auroral substorms are mostly manifestations of dissipative processes of electromagnetic energy. Thus, we consider a sequence of processes consisting of the power supply (dynamo), transmission (currents/circuits) and dissipations (auroral substorms-the end product), namely the electric current line approach. This work confirms quantitatively that after accumulating magnetic energy during the growth phase, the magnetosphere unloads the stored magnetic energy impulsively in order to stabilize itself. This work is based on our result that substorms are caused by two current systems, the directly driven (DD) current system and the unloading system (UL). The most crucial finding in this work is the identification of the UL (unloading) current system which is responsible for the expansion phase. A very tentative sequence of the processes leading to the expansion phase (the generation of the UL current system) is suggested for future discussions. The solar wind-magnetosphere dynamo enhances significantly the plasma sheet current when its power is increased above 10^{18}~mbox{erg}/mbox{s} (10^{11} w).The magnetosphere accumulates magnetic energy during the growth phase, because the ionosphere cannot dissipate the increasing power because of a low conductivity. As a result, the magnetosphere is inflated, accumulating magnetic energy.When the power reaches 3mbox{--}5times 10^{18}~mbox{erg}/mbox{s} (3mbox{--}5times 10^{11} w) for about one hour and the stored magnetic energy reaches 3mbox{--}5times10^{22} ergs (10^{15} J), the magnetosphere begins to develop perturbations caused by current instabilities (the current density {approx}3times 10^{-12}~mbox{A}/mbox{cm}^{2} and the total current {approx}10^{6}~mbox{A} at 6 Re). As a result, the plasma sheet current is reduced.The magnetosphere is thus deflated. The current reduction causes partial B/partial t > 0 in the main body of the magnetosphere, producing an earthward electric field. As it is transmitted to the ionosphere, it becomes equatorward-directed electric field which drives both Pedersen and Hall currents and thus generates the UL current system.A significant part of the magnetic energy is accumulated in the main body of the magnetosphere (the inner plasma sheet) between 4 Re and 10 Re, because the power (Poynting flux [ boldsymbol{E} times boldsymbol{B} ]) is mainly directed toward this region which can hold the substorm energy.The substorm intensity depends on the location of the energy accumulation (between 4 Re and 10 Re), the closer the location to the earth, the more intense substorms becomes, because the capacity of holding the energy is higher at closer distances. The convective flow toward the earth brings both the ring current and the plasma sheet current closer when the dynamo power becomes higher. This proposed sequence is not necessarily new. Individual processes involved have been considered by many, but the electric current approach can bring them together systematically and provide some new quantitative insights.
Highlights
Auroral and Polar Magnetic Substorms1.1 Auroral SubstormsAuroral substorms are a particular global auroral activity, during which auroral displays explosively spread from the midnight sector to the whole dark side of the polar region and beyond (Akasofu 1964); Figs. 1(a) and 1(b)
We have identified the unloading current (UL) current system can explain many aspects of the expansion phase in the earlier sections
Taking the electric current approach, it is found as suggested by Alfven (1967) that this approach can bring a variety of ground-and satellite-based observations in an orderly and systematic way in order to be able to infer the sequence of physical processes leading the expansion phase and can provide a somewhat different view from the magnetic field line approach
Summary
Auroral substorms are a particular global auroral activity, during which auroral displays explosively spread from the midnight sector to the whole dark side of the polar region and beyond (Akasofu 1964); Figs. 1(a) and 1(b). Many arcs or their fragments drift equatorward, and turn eastward, perhaps associated with the large-scale plasma convection Auroral activities spread in the form of westward traveling surges (Fig. 3), while auroral arcs disintegrate and drift toward the morning sky (Fig. 4) and form large-scale folds, called ‘torches’ (Fig. 5). The poleward expansion of auroral arcs during the expansion phase causes large-scale waves which propagate westward (toward the evening sky) along the aurroal oval with a speed of 2 km/s. This phenomenon is called the westward travelling surge; Fig. 3. It is important note that these auroral features during auroral substorms so far have got little theoretical attention (except westward traveling surges; Baumjohann and Opgenoortg 1984) and should be discussed on the basis of what we have learned about magnetospheric processes during substorms
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