Cusp Turbulence and Transport and their Impact on Ionospheric Coupling

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The shocked solar wind plasma, or magnetosheath, provides the basic input and outer boundary condition for the Earth's magnetosphere. The most direct access of this magnetosheath plasma into the magnetosphere and ionosphere occurs in the region of the outer polar cusp. Thus, an enhanced understanding of plasma processes at the magnetosheath/cusp interface is essential to the advancing magnetopause and boundary layer physics. Because of tailward bending of the magnetic field near the outer magnetospheric boundary, the entire magnetospheric boundary layer is mapped through this cusp region into the frontside, high- latitude ionosphere near local noon. Although the interaction of plasmas and fields in the outer cusp play a key role in linking the magnetosheath and boundary layer to the ionosphere, very few satellite measurements are available in this region. This project's objective is to advance the knowledge of plasma, field and plasma wave interactions in the outer and mid- altitude polar cusp with a focus on plasma and momentum transport, field fluctuations, and the resulting cusp current system. The investigators will identify and test those plasma and field interactions most relevant to enhancing unique observations available from satellite and used as a baseline sample of particle and field characteristics in the outer cusp and mid-altitude cusp regions. Although very limited, available observations in the outer cusp provide enough information on overall geometry and plasma conditions to apply state-of-the-art 2-D MHD and hybrid simulations of the magnetosheath-cusp interaction region including coupling to the ionosphere. Such simulations will provide information on the local time-dependent flows and magnetic fields in the outer cusp as well as the more nonlocal cusp/ionosphere current system. Understanding anomalous cross-field momentum transport is crucial for developing quantitative models of the cusp and dayside current systems. The PIs will therefore focus on shear-flow driven instabilities and associated turbulence and transport. Realistic 2-D and 3-D hybrid simulations of shear-flow across the magnetosheath/outer cusp and mid-altitude cusp wall boundaries will be completed. Theoretical predictions of electric and magnetic fluctuation spectra will be compared, especially with observed ULF-ELF wave spectra. In turn, cross-field transport will be calculated based on the observed fluctuation spectra to assess the importance of anomalous diffusion processes in the cusp region.