On the North‐South Symmetry of the Chorus Source Region: Conjugate Observations by the Cluster Spacecraft

Chorus waves are arguably the most important electromagnetic emissions in the Earth's magnetosphere, being responsible for both energization and loss of radiation belt electrons. Chorus generation, inherently related to nonlinear wave‐particle interactions, as well as the frequency‐time shapes of individual elements forming it, are not yet fully understood. We use multipoint high‐resolution measurements performed by the Cluster spacecraft to address the north‐south symmetry of the source with respect to the magnetic equator. The analyzed event offers a unique situation in which the Cluster spacecraft move along nearly the same field line, with one located north and the other south of the magnetic equator. A detailed wave analysis employing lower‐resolution data is used to demonstrate that the emissions indeed propagate away from the equator, as expected. We then show that the structure of the upper band chorus is generally different at the two spacecraft, while the lower‐band chorus emissions detected by both spacecraft are very similar. This indicates that the lower‐band chorus source radiates nearly symmetrically toward both hemispheres, while the upper‐band chorus does not.