Nonlinear Waves and Solitary Structures from MMS
Solitary waves are a commonly observed form of electric fields parallel to the background magnetic field in space plasmas. It has been repeatedly suggested that they have a significant influence on the surrounding environment, either individually or as a collective. At a minimum, nonlinear waves appear to be signposts for important plasma processes such as magnetic reconnection and turbulence. Understanding the effects of solitary waves on the macro-scale first requires us to characterize their structure on the micro-scale. We have investigated solitary waves of several flavors using a combination of simulations and high time resolution data from the Magnetospheric Multiscale (MMS) mission. Timing between multiple spacecraft is used to precisely measure the structures' size and speed. In all tested cases, solitary waves defy our expectations based on ideal physical models. In one instance, we identify unusually large-scale, quasi-neutral structures which have a combination of properties expected from both electron solitons and ion phase-space holes. Their parallel velocity and size perpendicular to the background magnetic field are on electron scales despite having nearly equal ion and electron density perturbations. In another case, electron phase-space holes are unambiguously measured in three-dimensions for the first time. We observe a "dimple" structure in their perpendicular electric field which is not predicted by existing 3D models. Finally, mixing between magnetosphere and magnetosheath plasma at reconnection sites is expected to generate strong electrostatic signatures often seen in MMS data. We use 1D Vlasov simulations to show that difficult to observe, low energy, "cold" features in the electron distribution may explain the unusual aspects of these structures.