Multi‐spacecraft observation of the pre‐substorm long‐lasting poloidal ULF wave

We report global dayside observation of Pc4–5 wave at the late recovery phase of a magnetic storm from seven satellites. Oscillations lasted for 15 hr and revealed the wave appearance before the substorm onset. The continuous observation showed that substorm injections did not affect the wave behavior, and all the wave parameters change was caused by magnetospheric plasma recovery after a magnetic storm. The wave was identified as a second harmonic poloidal Alfvén wave propagating westward with a high azimuthal wave number (|m| ∼ 210) and coupled with 100 keV protons through the drift-bounce resonance. A large radial gradient of phase space density was an energy source for the wave.