Excitation of Low‐ and High‐Frequency Magnetosonic Whistler Waves Associated With SLAMS in the Terrestrial Foreshock.

Based on observations from the Magnetospheric Multiscale mission, this study presents an analysis of a short large‐amplitude magnetic structures (SLAMS) event with simultaneous occurrence of low‐ and high‐frequency magnetosonic whistler waves. It was found that low‐frequency magnetosonic whistler waves around the lower‐hybrid frequency emerge in the presence of solar wind ions and local low‐energy ions in the trailing region of SLAMS. Additionally, counter‐propagating whistler waves (the high‐frequency branch of the magnetosonic whistler wave) are observed within SLAMS, coinciding with a perpendicular temperature anisotropy in the electron population. Instability analyses demonstrate that these low‐frequency waves are induced by the two‐stream instability associated with the cross‐field relative velocity between low‐energy ions and electrons, while whistler waves are locally generated by the whistler anisotropy instability. Our results shed light on the impact of SLAMS on particle and wave dynamics in the terrestrial foreshock. Plain Language Summary: Short large‐amplitude magnetic structures (SLAMS) are frequently observed within the terrestrial foreshock. They play a significant role in particle dynamics, often leading to the formation of unstable ion and electron velocity distributions. Consequently, plasma waves are excited across both ion and electron scales. Despite their importance, direct observational evidence linking SLAMS to the local excitation of plasma waves has been lacking. In this study, we utilized observations from the Magnetospheric Multiscale mission to investigate the local excitation of magnetosonic whistler waves associated with SLAMS. We revealed the presence of counter‐propagating whistler waves within SLAMS. We attributed the generation of these waves to the local whistler anisotropy instability. Furthermore, we found the occurrence of low‐frequency magnetosonic whistler waves in the trailing region of SLAMS. We identified their excitation mechanism as the two‐stream instability relating to local low‐energy ions. These results offer valuable insights into the intricate interplay between SLAMS and particle‐wave dynamics within the terrestrial foreshock. Key Points: MMS observes both low‐ and high‐frequency magnetosonic whistler waves associated with SLAMS in Earth's foreshockCounter‐propagating high‐frequency magnetosonic whistler waves are proposed to be excited by the whistler anisotropy instability in SLAMSLow‐frequency magnetosonic whistler waves are excited by the cross‐field relative velocity between low‐energy reflected ions and electrons [ABSTRACT FROM AUTHOR]