Statistical Study of the Propagation Velocity of Short Large-Amplitude Magnetic Structures (SLAMS) in the Foreshock of Earth

The bow shock at Earth is created when the super-Alfvénic solar wind interacts with Earth’s magnetosphere and is slowed to sub-Alfvénic velocities. Depending on the angle θ_Bn between the interplanetary magnetic field and the bow shock normal, the shock is defined to be either quasi-perpendicular (θ_Bn > 45°) or quasi-parallel (θ_Bn < 45°). In the quasi-parallel regime, the upstream region magnetically connected to the shock, called the foreshock, is highly dynamic and characterized by various plasma instabilities and wave activity. Short Large-Amplitude Magnetic Structures (SLAMS) are non-linear isolated magnetic field signatures commonly observed in this region. They are believed to grow from ultra-low frequency (ULF) waves which are common in the foreshock.

SLAMS are suggested to be important for the formation of the quasi-parallel shock. They are propagating upstream towards the sun, but generally with a propagation velocity smaller than the solar wind velocity. Consequently, they are convected downstream towards the shock. If they obtain high propagation velocities they should, however, be able to become stationary in the bow shock frame of reference and studies have suggested that the shock itself is composed of a patchwork of SLAMS.

In this work, we use multipoint measurements made by the Cluster mission to make a statistical analysis of the propagation velocity of SLAMS in the foreshock of Earth. We study the dependence on other properties of the SLAMS, such as their amplitude, and parameters related to the upstream environment.