Expected EMIC Wave Generation and Unexpected MS Wave Disruption in a Magnetic Dip.

Magnetic dip plays a potentially important role in the ring‐current‐radiation‐belt‐coupling. Here, we report a proton‐driven magnetic dip event accompanied by the expected generation of electromagnetic ion cyclotron (EMIC) wave and the unexpected disruption of fast magnetosonic (MS) wave observed by the Van Allen Probe B on 03 November 2015. The effect of magnetic dip on the EMIC wave generation and MS wave propagation is analyzed. The analysis of EMIC wave instability demonstrates that EMIC waves are more easily excited in the magnetic dip. Moreover, the cold plasma density shows a highly positive correlation with the magnetic field fluctuation, and they are closely related to the disappearance of MS wave during the magnetic dip structure, which is confirmed by theoretical estimation. Our results indicate that magnetic dip may play different roles in the activities of EMIC and MS waves, which has rarely been discussed before. Plain Language Summary: Magnetic dip, a localized minimum of magnetic field intensity, is a frequently observed magnetic field structure in the terrestrial magnetosphere. The formation of magnetic dips is due to the increasement of ∼10 keV proton and electron fluxes in terms of pressure balance. In turn, the magnetic dips can change the distributions of protons and electrons and further the generation and propagation of electromagnetic waves. Previous studies have found that magnetic dips favor the generation of electromagnetic ion cyclotron waves and magnetosonic waves. However, in this study, we find magnetic dips can modulate the cold electron density and indirectly inhibit the propagation of MS waves, which is not reported yet. These findings will advance our understanding of the role of magnetic dip in the inner magnetosphere dynamics. Key Points: The expected generation of electromagnetic ion cyclotron (EMIC) waves and the unexpected disruption of magnetosonic (MS) waves are simultaneously observed in a magnetic dipThe magnetic dip plays different roles in the activities of EMIC and MS wavesThe cold plasma density correlated with the magnetic field fluctuation may account for the disruption of MS waves [ABSTRACT FROM AUTHOR]