Cluster Inner Magnetosphere Campaign: Multispacecraft Observations of Equatorial Magnetosonic Waves

The main target of the Cluster inner magnetosphere campaign (July-October 2013) is to identify linear and nonlinear processes that affect EMIC, equatorial magnetosonic and chorus waves. The present study examines equatorial magnetosonic waves that are regularly observed under the arch of the plasmasphere, due to the formation of a natural waveguide like structure. These waves are usually observed in the frequency range from a few tens to hundreds of Hz. The spectrum exhibits a banded structure whose spacing is of the order of the ion gyrofrequency in the source region. This fine structure results in complications for the identification of the wave dispersion using data from multispacecraft separated by distances of around 10-100 km. It is shown how this complication can be resolved. The resulting dispersion of the observed waves is presented and compared with analytical solutions.