PSTEP Science Nuggets No.16 (20180911)

Energy goes into Earth in a spiral form

Yusuke Ebihara (RISH, Kyoto University)

The disturbances of the magnetosphere and the ionosphere are hazardous to the power grid because of the intensification of the geomagnetically induced current (GIC). Usually, the influence of GIC on the power grid is large at the auroral region where auroral electrojets flow during the substorm that lasts for ~1 hour. Magnetic storms, which last for a few days, are known to intensify the GIC at mid- and low-latitudes such as Japan because of the intensification of the current system, such as the ring current and the field-aligned current. The ultimate source of the energy involved by the substorms and magnetic storms is the solar wind. However, the pathway and conversion from the solar wind to the Earth are little known.

We analyzed simulation results of the global magnetohydrodynamics (MHD) simulation developed by Professor Emeritus Tanaka. When the interplanetary magnetic field (IMF) turns southward, the solar wind kinetic energy starts to be converted to the electromagnetic energy in the dynamo located at high-latitude magnetosphere. The importance of this dynamo (mantle dynamo) has already been pointed out by Prof. Tanaka. We tracked the pathway of the electromagnetic energy entering the magnetosphere. The pathway is shown in Figure 1. Interestingly, the electromagnetic energy shows a spiral with its center moving toward the Earth. The spiral motion implies the presence of the large-scale convection, and the earthward motion implies the presence of the large-scale field-aligned current. For a while, a magnetic field line is reconnected in the nightside magnetosphere, resulting in the redistribution of the magnetospheric structure. As a consequence, additional dynamo region appears in the near-Earth magnetosphere (near-Earth dynamo), which converts the internal energy to the electromagnetic energy by way of the kinetic energy. When the auroral breakup starts, the electromagnetic energy originating from the solar wind, the mantle dynamo, the lobe and the near-Earth dynamo goes into the nightside ionosphere at once as guided by the newly developed field-aligned current on the nightside.

Figure 1: (white line) Pathway of the electromagnetic energy, (blue line) magnetic field lines, (yellow line) magnetic field line of which the auroral breakup starts, (blue surface) the region where the electromagnetic energy is generated (dynamo region) at 15 minutes before the onset of the breakup (left) and at the onset (right).

Ebihara, Y., and T. Tanaka, Energy flow exciting field-aligned current at substorm expansion onset, Journal of Geophysical Research: Space Physics, 122, doi:10.1002/2017JA024294, 2017.