Dependence of solar-wind/magnetosphere/ionosphere coupling on the geomagnetic dipole strength

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Friday, November 2, 2012

The possible effects of geomagnetic-field changes over periods of millennia on solar wind/magnetosphere/ionosphere/thermosphere coupling was examined, by altering the strength of the geomagnetic dipole in the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. These changes affect ionospheric conductivity and magnetosphere-ionosphere coupling.

Sample template image
The stand-off distance Rs (top) and the ratio between the stand-off to the flank and nose of the magnetosphere (bottom) as a function of dipole moment for solar minimum (blue), medium (green) and maximum (red) conditions. The black line indicates the theoretical relationship Rs  M1/3, where M is the geomagnetic dipole moment.

The strength of the Earth's magnetic field changes over time. We use simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere model to investigate how the magnetosphere, upper atmosphere, and solar quiet (Sq) geomagnetic variation respond as the geomagnetic dipole moment M varies between 2⋅10²² and 1⋅10²² Am². We find that the magnetopause stand-off distance and the cross-polar cap potential increase, while the polar cap size decreases, with increasing M. Their dependence on M is stronger than predicted by previous studies. We also show for the first time that the shape of the magnetosphere starts to change for M ≤ 4⋅10²² Am². This may be due to enhanced magnetopause erosion and/or to strong changes in the ionospheric conductance, which affect the field-aligned currents and the magnetic fields they create in the magnetosphere, thus modifying the magnetic pressure inside the magnetosphere. E × B drift velocities, Joule heating power, the global mean thermospheric temperature and the global mean height of the peak of the ionospheric F₂ layer, hmF₂, all increase with increasing M for low dipole moments, but all decrease with increasing M for larger dipole moments. The peak electron density of the F₂ layer, NmF₂, shows the opposite behavior. The Sq amplitude decreases with increasing M and this dependence can be roughly described by a power law scaling. Most scaling relations show a weak dependence on the solar activity level, which is likely associated with a change in the relative contributions to the Pedersen conductance from the upper and lower ionosphere, which scale differently with M.

Reference: Cnossen, I., A. D. Richmond, and M. Wiltberger (2012a), The dependence of the coupled magnetosphere-ionosphere-thermosphere system on the Earth's magnetic dipole moment, J. Geophys. Res., 117, A05302, doi:10.1029/2012JA017555.