Propagation Velocities and Growth Rates of Short Large‐Amplitude Magnetic Structures (SLAMS) in the Foreshock of Earth

Abstract Short Large‐Amplitude Magnetic Structures (SLAMS) are believed to play important roles in the physics governing the formation of quasi‐parallel shocks. These isolated non‐linear structures, frequently observed in the foreshock, have been suggested to be building blocks for the shock and have been shown to drive shock reformation. In this work, we make a statistical analysis of the propagation velocity of 258 SLAMS in the foreshock of Earth using data from Cluster obtained between the years 2002 and 2012. The results show propagation velocities above both the Alfvén and magnetosonic speeds in the plasma frame, with a propagation direction more strongly dependent on the shock normal than the background magnetic field. We observe a few SLAMS with velocities down to close to 0 km/s in the spacecraft frame, indicating that some SLAMS can merge with the bow shock. The velocity is positively correlated with the amplitude of the structure, and negatively correlated with several upstream parameters. We also determine the growth rate of SLAMS, although the spacecraft configuration limits the analysis to just a few cases.