Statistical Properties of Short Large-Amplitude Magnetic Structures (SLAMS) in the Foreshock of Earth From Cluster Measurements

Abstract

Short Large-Amplitude Magnetic Structures (SLAMS) are non-linear isolated magnetic field structures commonly observed in the foreshock region of quasi-parallel collisionless shocks. In this work, we use an automated algorithm to create a database of SLAMS detections made in the foreshock of Earth by the Cluster mission between the years 2002–2012. We define SLAMS to have amplitudes of at least two times the background magnetic field, leading to a detection of 1736 SLAMS during the studied period. Subsequently, the statistical properties of the SLAMS in the database are studied, such as their amplitude and temporal scale size. Correlations with the upstream environment are also studied, together with the conditions required for SLAMS formation and solar cycle dependencies. We find a mean temporal scale size of 3.3 s and an amplitude normalized by the background field, $\Delta B/B_{bg}$, varying between 2 and 9, with a mean value of 2.9. 81% of the SLAMS are right-hand polarized in the spacecraft frame. We find that the magnetosonic and Alfvén Mach numbers are important for SLAMS formation, with an increasing observation rate with increasing Mach numbers. Higher Mach numbers also tend to increase $\Delta B/B_{bg}$ and decrease the temporal scale size of the structures. SLAMS are often associated with peaks in the plasma density, and we find a positive correlation between the amplitude of the magnetic field peaks and the amplitude of the density peaks, confirming the fast magnetosonic nature of SLAMS.