The polarization of Short Large-Amplitude Magnetic Structures (SLAMS) in the foreshock of Earth

Short Large-Amplitude Magnetic Structures (SLAMS) are isolated non-linear magnetic field signatures frequently observed in the foreshock of quasi-parallel shocks. They are believed to form due to a non-linear growth of ultra-low frequency (ULF) waves, but the detailed formation mechanisms are still highly uncertain. SLAMS have been suggested to be important for the formation of the quasi-parallel shock, and understanding the nature of these structures is hence important in order to fully understand the physics of collisionless shocks.

The majority of SLAMS (about 80%) are right-hand polarized in the spacecraft frame, corresponding to a left-hand polarization in the plasma frame. This polarization is opposite from that of the ULF waves from which they are believed to grow. The reason for this polarization change is unknown. Not all SLAMS are however right-hand polarized in the spacecraft frame. About 20% are left-hand polarized, and the reason for these two different groups of SLAMS and their underlying formation mechanisms are also unknown.

In this work, we make a statistical analysis of the polarization properties of SLAMS in the foreshock of Earth using data from the Cluster mission. The aim is to investigate the difference between right-hand and left-hand polarized SLAMS, studying differences in the ion distribution and the general properties of the plasma environment. This can give clues about the underlying formation mechanisms. We also study the evolution of the polarization as the ULF waves grow into SLAMS.