Whistler‐mode waves inside flux pileup region: Structured or unstructured?

Abstract During reconnection, a flux pileup region (FPR) is formed behind a dipolarization front in an outflow jet. Inside the FPR, the magnetic field magnitude and Bz component increase and the whistler‐mode waves are observed frequently. As the FPR convects toward the Earth during substorms, it is obstructed by the dipolar geomagnetic field to form a near‐Earth FPR. Unlike the structureless emissions inside the tail FPR, we find that the whistler‐mode waves inside the near‐Earth FPR can exhibit a discrete structure similar to chorus. Both upper band and lower band chorus are observed, with the upper band having a larger propagation angle (and smaller wave amplitude) than the lower band. Most chorus elements we observed are “rising‐tone” type, but some are “falling‐tone” type. We notice that the rising‐tone chorus can evolve into falling‐tone chorus within <3 s. One of the factors that may explain why the waves are unstructured inside the tail FPR but become discrete inside the near‐Earth FPR is the spatial inhomogeneity of magnetic field: we find that such inhomogeneity is small inside the near‐Earth FPR but large inside the tail FPR.