First Observation of Electron Cyclotron Harmonic Waves Inside Mirror Mode Structures in the Earth's Outer Magnetosphere

Electron cyclotron harmonic (ECH) waves are one of the most common plasma waves in the Earth's magnetosphere, and they are considered to be excited by the electron loss‐cone distributions near the magnetic equator. Using the Magnetospheric Multiscale data, we report an unusual event of ECH waves at L = ∼12.4 and MLT = ∼16.0 hr, where the ECH waves are locally excited inside non‐propagating magnetic oscillations, that is, mirror mode structures. The observation reveals that the mirror mode structures are generated by anisotropic ions (Ti⊥/Ti∥ ${T}_{i\perp }/{T}_{i{\Vert} }$ ∼ 1.3 – 1.7), and electron mirror loss‐cone distributions are formed due to the magnetic configuration of the mirror mode structures. Combing with the linear instability analysis, we confirm that the ECH waves are locally excited by the electron mirror loss‐cone distributions inside the mirror mode structures. Therefore, we propose that the mirror mode structures provide another potential source region for ECH waves in the Earth's magnetosphere. Plain Language Summary Electron cyclotron harmonic (ECH) waves are widely observed in the Earth's magnetosphere, which are important for the diffusion auroral precipitation. Previous studies have shown that the main source region of ECH waves in the magnetosphere is near the magnetic equator, and the electron loss‐cone distribution provides the free energy to excite ECH waves. In this study, we reveal that there also exists another potential source region of ECH waves, that is, mirror mode structures. Using the Magnetospheric Multiscale data, we report an unusual event of ECH waves inside the mirror mode structures in the Earth's outer magnetosphere. By analyzing the plasma conditions, we show that the mirror mode structures are generated by anisotropic ions, and electron mirror loss‐cone distributions are formed due to the magnetic configuration of the mirror mode structures. Based on the linear instability analysis, we demonstrate that the ECH waves are excited by the electron mirror loss‐cone distributions within the mirror mode structures. These results provide new insights into the generation of ECH waves in the Earth's magnetosphere. Key Points Using the Magnetospheric Multiscale data, we report the first observation of electron cyclotron harmonic (ECH) waves within mirror mode structures in the Earth's outer magnetosphere The mirror mode structures observed as quasi‐periodic magnetic oscillations are excited by anisotropic ions The simulta