Geotail observation of upper band and lower band chorus elements in the outer magnetosphere

Using the Geotail observation of upper band and lower band (dual‐band) rising tone chorus emissions in the dayside outer magnetosphere, we evaluate the mechanism of creating their frequency gap bandwidths, on the basis of a nonlinear damping mechanism at half the local gyrofrequency proposed by Omura et al. (2009). For ELF dual‐band chorus elements observed in the dayside of the equatorial outer magnetosphere ( L ∼ 10.5), the lower cutoff frequency of the upper band elements follows half the local gyrofrequency. On the other hand, the upper cutoff of the lower band elements is almost consistent with half the gyrofrequency at a minimum‐B pocket (a possible chorus generation region) along the field lines connecting Geotail according to the Tsyganenko geomagnetic TS05 model. This is consistent with the scenario that the rising tone wave packet initially excited in a wide frequency range suffers from damping at half the local gyrofrequencies during quasi‐parallel propagation. Since the local gyrofrequency gradually increases away from the generation region, the upper cutoff of a lower band element should represent half the gyrofrequency at the generation region, whereas the lower cutoff of an upper band element should follow half the local gyrofrequency. We confirm that the frequency sweep rates and amplitudes of the observed chorus wave packets are consistent with those predicted by the nonlinear growth theory of chorus emissions, except for the frequency gap. This indicates an observational evidence of nonlinear chorus generation in the minimum‐B pocket in the outer magnetosphere and creation of dual‐band elements due to nonlinear damping through quasi‐parallel wave propagation.