HAO 2010 PROFILES IN SCIENCE: Dr. Ingrid Cnossen
Dr. Ingrid Cnossen is a Post Doc I at the High Altitude Observatory at NCAR. She is studying the effects of long-term changes in the Earth's magnetic field on magnetosphere-ionosphere interactions and on the ionosphere itself, using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model.
How is the ionosphere affected by changes in the strength of the Earth's magnetic field?
The Earth's magnetic field strength is slowly decreasing, and its effects can complicate the search for ionospheric changes associated with trends in atmospheric temperature. The changing field strength affects the electrodynamics of the coupled ionosphere-magnetosphere system. Ingrid Cnossen and colleagues performed simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model, first using a dipole moment of 8·1022 Am2, close to the present-day value ("strong dipole"), and then a dipole moment of 6·1022 Am2, which may be its value a few centuries in the future ("weak dipole"). Both simulations were run under the same solar wind conditions, for intermediate solar activity (F10.7 = 150), at equinox. Substantial differences are predicted for the ionosphere. The left column of the figure shows ionospheric quantities for the "strong dipole", averaged over the two hours 13-15 UT: the height of the peak of the F2 layer, hmF2 (top); the vertical component of the neutral wind parallel to the magnetic field, vn,par,v (middle), which forces the plasma up and down field lines; and the vertical component of the ExB drift (bottom). The right column shows the changes in these quantities when the dipole moment is reduced. Changes in vn,par,v, explain the changes in hmF2 over the Pacific, off the south-east coast of Australia, and over east Asia. The changes in the neutral wind themselves are caused partly by an enhancement in ExB ion drift velocities (through ion-neutral collisions) and partly through an increase in the meridional pressure gradient, in response to stronger high-latitude Joule heating. Changes in the vertical component of the ExB drift, which is most effective in changing hmF2 at low latitudes, may be responsible for the decrease in hmF2 over the Indian Ocean. There is also a global mean uplift of the ionosphere, due to an increase in global mean temperature, which causes the thermosphere to expand.
Cnossen, I., A.D. Richmond, M. Wiltberger, W. Wang, and P. Schmitt. 2011: The response of the coupled magnetosphere-ionosphere-thermosphere system to a 25\% reduction in the dipole moment of the Earth's magnetic field. J. Geophys. Res.. Submitted, doi:10.1029/2011JA017063.