Fully Kinetic Simulations of Ion‐Acoustic Waves in Asymmetric Reconnection With Cold Ions

Abstract In collisionless plasma, ion‐acoustic wave (IAW) is an important candidate for enhancing resistivity and viscosity, yet their presence and role in magnetic reconnection remain poorly understood. Here, we employ 2.5‐D particle‐in‐cell simulations to investigate IAWs in asymmetric magnetopause reconnection involving a cold ion population. Electrostatic waves propagating along the magnetic field are observed on the magnetospheric separatrix, with frequencies below the ion plasma frequency and satisfying the IAW dispersion relation. The free energy driving these waves originates from the electron outflows produced by reconnection. The IAWs mediate energy transfer by accelerating cold ions and heating hot ions, playing a crucial role in particle energization within the separatrix. In contrast, simulation without cold ions exhibits no IAW activity, confirming that their presence is essential for wave excitation. These results clarify the excitation and role of IAWs and indicate that they offer an additional pathway for cross‐species energy coupling in magnetic reconnection.