On the Electrical Coupling Between the Ring Current and the Ionosphere

P.C. Brandt, Y. Zheng, C. Vallat, D.G. Mitchell, A. Ukhorskiy, R. Greenwald, 
and K. Oksavik

The terrestrial ring current is a major player in coupling the inner
magnetosphere with the ionosphere. Its pressure gradients, together with the
magnetic field gradients, are associated with electrical currents (via the force
balance equation) that flow in the equatorial plane as well as through the
ionosphere. This current system is known as the "Region-2" current system and
dominates the inner magnetosphere (sub-auroral ionosphere). Its closure through
the ionosphere is responsible for modifying/creating the quiet-time electric
field of the inner magnetosphere: poleward closure through the ionospheric, 
low-conductance trough region creates Sub-Auroral Polarization Streams  (SAPS);
the Region-2 closure builds up the so-called shielding electric fields that
prevent further spread of the ionospheric electric field to lower latitudes.

We discuss how the temporal and spatial evolution of the ring current, magnetic
field and ionospheric conductance interrelate in the dynamics of the electric
field. How much of the dynamics of the inner magnetospheric electric field is
induced by changes in the magnetic field, and how much is due to the evolution
of the Region-2 current system and ionospheric conductance?

The 3D structure of the Region-2 current system can be derived from global ENA
images of the ring current and compared to in-situ current estimations derived
from magnetic field measurements by the four Cluster spacecraft. The comparison
have revealed an field-aligned current (FAC) component flowing from one
hemisphere to the other.

We also briefly discuss the role of the ring current in the prompt penetration
of equatorial electric fields. Rapid changes in the interplanetary magnetic
field (IMF) have been correlated with dayside ionospheric uplifts, implying an
almost immediate penetration of the electric field induced by the IMF. At the
same time, HENA has observed unusually rapid response to fluctuations in the
solar wind that are about three times faster than ring current models predict.

MI-coupling is put in perspective by reviewing the basic differences between
Saturn and Earth. Earth's magnetosphere is largely a convection driven system,
strongly controlled by external solar wind conditions, where the ring current
pressure plays an important role in the coupling to the ionosphere. Although
Saturn's magnetosphere appears to display storm-substorm like injections, the
transport is dominated by corotation with large abundances of heavy, thermal
ions and neutral gas, which makes interchange between flux tubes with cold dense
plasma and hot tenuous plasma a significant transport mechanism.

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Global Aspects of Magnetosphere-Ionosphere Coupling, 2006 Yosemite Workshop, 
Yosemite National Park, CA, USA, 7-10 February 2006