Observation of Unusual Chorus Elements by Van Allen Probes

Whistler mode chorus waves play an important role in the radiation belt dynamics, which usually appear as discrete elements with frequency sweeping. Finer structure analysis shows that a chorus element is composed of several frequency‐sweeping subelements, and such two‐level structures can be successfully reproduced by modeling based on nonlinear theories. Previous observations and models suggest that an element and its subelements should have the same frequency‐sweep direction. However, we here present two unexpected chorus rising tone events within which the subelements exhibit clearly reversed, falling frequency‐sweep. Moreover, the subelements consist of several wave packets that also show falling frequency‐sweep features. The three‐level structured chorus elements are distinctly different from all the reported observations and seem to bring challenges to the existing theories. We propose a possible scenario that the falling tone subelements are formed by nonlinear process with much shorter timescale and the starting frequency of each subelement is controlled by fast varying electron distribution. This study may inspire more studies toward a thorough understanding of the chorus generation process. Plain Language Summary Whistler mode chorus is a very important plasma wave in the magnetosphere. The frequency‐time spectrogram of chorus waves shows discrete elements with frequency sweeping. Wave data analysis with higher time resolution shows that the frequency‐sweeping element is composed of several frequency‐sweeping subelements. Such two‐level structures have been successfully reproduced by modeling based on nonlinear models. It is widely accepted so far by the researchers that the subelements should have the same direction of frequency sweep with the whole element that they formed. However, in this study, we report two unexpected chorus rising tone events in which the frequency‐sweep direction of subelements is clearly opposite. Moreover, we find that the subelements consist of even finer structures that have the same frequency‐sweep direction as the subelements. Such a three‐level chorus wave structure is distinctly different from all the previous observations and seems to contradict the existing theories. We propose a possible scenario that the falling tone subelements are formed by nonlinear process with much shorter timescale and the starting frequency of each subelement is controlled by fast varying linear electron distribution. Our finding provi