Multi-point observations of whistler mode waves in the outer radiation belt region

Abstract : Natural emissions of whistler-mode electromagnetic waves, especially chorus and hiss, can influence the dynamics of the Van Allen radiation belts via quasi-linear or nonlinear wave particle interactions. They can play a role in complex processes of the energy transfer between different electron populations. To separate temporal and spatial variations of their characteristics, measurements at different points in space are necessary. We show initial results of measurements of the Cluster space fleet during the period of close separations between Cluster 3 and Cluster 4 from September to December 2018. As the separations of these spacecraft decrease down to 3 km in the equatorial source region of chorus we can perform systematic phase measurements of nonlinear wave packets of chorus. The embedded fine structure of sub-packets is particularly interesting in this context because their spatio-temporal scales have been shown to be much shorter compared to scales of the chorus wave packets. We use detailed waveform measurements of the WBD instruments onboard Cluster 3 and Cluster 4 close to the equator, with Cluster 1 and/or Cluster 2 located at much larger separations at higher latitudes. These detailed measurements are put in the context of overall wave propagation and polarization patterns by multi-component data of the STAFF-SA instruments onboard all the 4 Cluster spacecraft, with the FGM instruments used to define the magnetic field-aligned coordinate system and to normalize the measured frequencies by the electron cyclotron frequency.