Characterization of ULF Pulsations by THEMIS

This paper reports the first use of THEMIS to determine the polarization properties of ULF waves in a non- dipolar magnetic topology: The instrumentation and the alignment at close distances (less than 1 RE) among some of the THEMIS probes (particularly in the first period of its mission) provides unique opportunities to characterize ULF pulsations in the magnetosphere and enables us to validate and parameterize existing models. In the case study presented, electric and magnetic field fluctuations are identified as field line resonances and their modes of oscillation are compared to model estimates. Out of the various modes predicted by the model, the second fundamental mode, or toroidal mode, appears to have most similarities to the measured polarizations. Furthermore, phase-difference calculations using the probes' small separations allow estimates of mode number and propagation characteristics. It is shown that these observations give an excellent source for the verification of model estimates of frequency and polarization of the various modes of field line resonances in the magnetosphere.