ULF Waves in the Subauroral Geospace.

Ultra-low frequency (ULF) waves play an important role in electromagnetic coupling between the Earth's ionosphere and magnetosphere. They are involved in energy exchange between ionosphere and magnetosphere, energizing the ionosphere at high latitudes, producing ion outflow, causing formation of the plasma irregularities, density cavities, etc. The ULF waves have the largest amplitudes in the power spectral densities of the oscillations observed during the geomagnetic substorms. It is important to understand the mechanisms driving them, what phenomena cause these oscillations and what are their spatiotemporal properties.

The origin of the ULF waves is suggested to be associated with field line resonances, mechanisms in magnetosphere-ionosphere interactions, reconnection in magnetotail, generation of Kelvin-Helmholtz instabilities in magnetopause or flanks, solar winds' dynamic plasma. They also can be generated in the ionosphere by local disturbances of density/conductivity when large electric field exists in the E-region. Once generated, ULF waves can be trapped inside one of two magnetospheric resonators: 1) the global magnetospheric resonator, formed by the entire magnetic flux tube bounded by the ionosphere and 2) the Ionospheric Alfvén Resonator, formed by the conducting bottom of the ionosphere and a strong gradient in the Alfvén speed at ~1.0 R_E altitude.

Our goal is to determine which of these two resonators under what geomagnetic conditions and in what frequency range contains the main part of ULF waves' power at middle and low latitudes. We present results from simulations of the reduced MHD model containing effects of the active ionospheric feedback at L=1.5-2.5. These results are used for the interpretation of ULF fluctuations detected by the ground magnetometers at the Palmer station in Antarctica (L=2.46) and the Alma-Ata station in Kazakhstan (L=1.87). Comparison of the observed and simulated frequencies of the waves provides valuable information about physical mechanisms generating these waves.