Phase structure of Pc3 waves observed by Cluster and ground stations near the cusp

Ground observations of Pc3 ULF waves at high latitude show enhanced pulsation activity near the cusp. To investigate this, typical Pc3 pulsation events from 0920 to 1010 UT on 1 April 2004 have been selected that are simultaneously observed by the Cluster satellites and Zhongshan, Davis, and Mawson magnetometers, Antarctica. When Cluster was located on closed field lines equatorward of the exterior northern cusp, Zhongshan and Davis were near the ionospheric footprint of the southern cusp. Assuming linear phase variation between the four Cluster spacecraft, the Pc3 waves with a frequency of ∼25 mHz have a wavelength of ∼5200 ± 2400 km and wave vector directed almost perpendicular to the geomagnetic field (94.6° ± 10.4°) pointing toward the Earth. The Pc3 waves are transverse waves over the interval and different from the broadband perturbations and compressional waves generally observed in the cusp and the magnetosheath, respectively. The wave Poynting flux indicates energy is transported predominantly along the magnetic field line toward the ionosphere. These results indicate that Pc3 waves near the cusp are shear Alfvénic waves guided by closed field lines in the low‐latitude boundary of the cusp and reflected from the ionosphere. We suggest that the waves may result from incoming compressional fast waves from the upstream solar wind that couple to shear Alfvén waves guided by the magnetic field at high latitudes and excite field line resonance in various harmonics at middle and low latitudes.