Trapping and Amplification of Unguided Mode EMIC Waves in the Radiation Belt

Electromagnetic ion cyclotron (EMIC) waves can cause the scattering loss of the relativistic electrons in the radiation belt. They can be classified into the guided mode and the unguided mode, according to wave's propagation behavior. The guided mode waves have been widely investigated in the radiation belt, but the observation of the unguided mode waves have not been expected. Based on the observations of Van Allen Probes, we demonstrate for the first time the existence of the intense unguided L‐mode EMIC waves in the radiation belt according to the polarization characteristics. Growth rate analyses indicate that the hot protons with energies of a few hundred keV may provide the free energy for wave growth. The reflection interface formed by the spatial locations of local helium cutoff frequencies can be nearly parallel to the equatorial plane when the proton abundance ratio decreases sharply with L‐shell. This structure combined with hot protons may lead to the trapping and significant amplification of the unguided mode waves. These results may help to understand the nature of EMIC waves and their dynamics in the radiation belt. Plain Language Summary Electromagnetic ion cyclotron (EMIC) waves play an important role in the radiation belt. They can be classified into the guided mode and the unguided mode, according to wave's propagation behavior. Based on the observations of Van Allen Probes, we demonstrate for the first time the existence of the intense unguided mode EMIC waves in the radiation belt. Their energy comes from the hot protons with energies of a few hundred keV. As the spatial variation of ion abundance ratios could form a trapping region, where the unguided mode waves can be amplified significantly. These results may help to understand the nature of EMIC waves and their dynamics in the radiation belt. Key Points Intense unguided mode electromagnetic ion cyclotron waves have been identified in the radiation belt Protons with a few hundred keV provide the free energy for the growth of unguided L‐mode waves Spatial inhomogeneity of ion abundance ratios controls the potential trapping of unguided waves