On the north-south symmetry of the equatorial whistler-mode chorus source region

Chorus waves are among the most important electromagnetic whistler-mode emissions in the Earth’s inner magnetosphere and are responsible for both the energization and loss of energetic electrons in the Van Allen radiation belts. The generation of chorus is inherently related to nonlinear wave-particle interactions around the min-B equator, which result in the formation of chorus fine structure composed of individual elements sweeping in frequency. However, the details of the formation mechanism and the explanation for the spectral gap typically observed at half of the electron cyclotron frequency are still missing. One of the open questions concerns the symmetry of the generated emissions with respect to the min-B equator. We address this issue using multipoint, high-resolution measurements performed by the Cluster spacecraft. These measurements allow us to analyze a unique event in which the Cluster spacecraft move along nearly the same magnetic field line, with one spacecraft located northward and the other southward of the equator. Wave analysis based on available multicomponent measurements reveals that the waves propagate away from the equator, consistent with an equatorial source location. The structure of the upper-band chorus north and south of the equator is found to be rather different. However, the lower-band chorus emissions detected by both spacecraft are very similar, indicating that the source radiates nearly symmetrically towards both the north and south.