A story of geomagnetic storm and auroral substorm
for the public
J. R. Kan
Geophysical Institute, University of Alaska Fairbanks
Geomagnetic storm is characterized by the magnetic disturbance generated by currents in the inner magnetosphere. This current is called the storm-time ring current. Magnetic fields generated by the ring current measured by ground-based magnetometers at mid-latitudes is the storm Dst index. Storm-time ring current is driven by the global earthward convection in the plasma sheet powered by solar wind-magnetopause (SW-MP) dynamo during prolong southward IMF (interplanetary magnetic field), lasting for ~10 hours or longer. The time scale for the storm main phase is ~5 hours. The recovery phase of storm can last for several days.
On the other hand, auroral substorm is characterized by brightening of onset arc in the ionosphere and dipolarization onset in the near-Earth plasma sheet. The auroral electrojet intensified into Cowling electrojet by the induced southward E-field due to blockage of northward Hall current from closure in the plasma sheet. The Cowling electrojet current for an intense substorm is estimated to be ~ 1 MA (Mega Amp), distributed over ~5˚ latitudinal width between ~60˚ to ~70˚ latitudes in the mid-night sector. The dipolarization onset region is centered between ~9 to 11 RE in the near-Earth plasma sheet. THEMIS observations show that the brightening of the onset arc precedes the dipolarization onset by ~80 sec, which is estimated to be the Alfvén travel time from the ionosphere to the dipolarization onset region.
Both geomagnetic storm and auroral substorm are driven by the global earthward convection of ~20 to ~40 km/s powered by solar wind-magnetopause (SW-MP) dynamo. The dynamo output powers up or down by dayside or tail reconnection. The global earthward convection powered by the SW-MP dynamo during prolonged period of southward IMF leads to the storm-time ring current. At the same time, the global earthward convection intensifies the auroral electrojet to the Cowling electrojet. Divergence and convergence on the dusk and dawn sides of the Cowling electrojet produce upward and downward field-aligned currents. Closure of field-aligned currents in the Alfvén wavefront completes the Cowling electrojet current loop (CECL). Alfvén wavefront, leading the propagation of the CECL, incident on the near-Earth plasma sheet triggers the dipolariza-tion onset to mark the onset of substorm expansion phase. Identifying the SW-MP dynamo as the driver for both storm and substorm, and the tail reconnection as reducing the open field lines and thereby reducing the dynamo power out is the climax of the story.