Auroral Substorms: Search for Processes Causing the Expansion Phase in Terms of the Electric Current Approach

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.(1) The solar wind-magnetosphere dynamo enhances significantly the plasma sheet current when its power is increased above 10 18 erg/s (10 11 w). (2) 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. (3) When the power reaches 3-5 × 10 18 erg/s (3-5 × 10 11 w) for about one hour and the stored magnetic energy reaches 3-5 × 10 22 ergs (10 15 J), the magnetosphere begins to develop perturbations caused by current instabilities (the current density ≈3 × 10 −12 A/cm 2 and the total current ≈10 6 A at 6 Re). As a result, the plasma sheet current is reduced. (4) The magnetosphere is thus deflated. The current reduction causes ∂B/∂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. (6) 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.

A Review of the Current Understanding in the Study of Geomagnetic Storms

“…Chapman and Ferraro [1] was the first to consider the impact of solar plasma flows on the earth's dipole field, and their theory became the first theory of the forma- The relationship among the sun (corona), the solar (plasma) wind, the magnetosphere/ionosphere and geomagnetic storms. This chain of processes can be considered as an input-output problem, in which the solar wind energy is converted into electric power by the solar wind-magnetosphere dynamo which generates the three storm current systems [29]. April 17-18, 1965 [39].…”

The electric current approach in the solar–terrestrial relationship

“…(b) The reconstructed of the helical magnetic structure (Saito et al, 2007). (c) Comparison of results between the observation and the reconstructed helical structure (Saito et al, 2007;Akasofu, 2017). clear helical magnetic structure, one of the initial conditions should specifically include the expanding loop current rather than a thermal expansion or magnetic reconnection alone.…”

“…In a sense, DD corresponds to the magnetic arch current and UL to the loop current in solar flares. (Axford and Hines, 1961); (d) the expected convection pattern of ionospheric plasma, which is basically the equipotential pattern (Akasofu, 2017); (e) SuperDARN result (Bristow and Jensen, 2007); (f) the realistic ionospheric DD current (convection) derived from the magnetometer data (Akasofu, 2017). The UL current: (g) the UL component of the ionospheric current (Akasofu, 2017); (h) Bostrom's 3-D current system made up of azimuthal and meridional components (Akasofu, 2017).…”

“…Changes in the earth's magnetic configuration when magnetic energy is accumulated centered around 6 R E (red dot); (a) the earth's field change and (b) the distribution of the magnetic field produced by the model ring current. (c) The distortion (inflation) of the magnetic field lines Akasofu, 2017). ability of energy is not enough.…”

Observations of loading-unloading process at Saturn’s distant magnetotail

“…The unloading process is responsible for the substorm expansion phase, when the magnetospheric currents divert into the ionosphere; current sheet expands in north‐south direction and the lobe magnetic field decreases. An unloading process is usually much more rapid than a loading process (see a recent review paper by Akasofu (2017)).…”