Authors: S. Dorfman (Space Science Institute), K. Zhang (University of California, Los Angeles), L. Turc (University of Helsinki), U. Ganse (University of Helsinki), M. Palmroth (University of Helsinki)

Our understanding of the large-scale structure of waves in the heliosphere is often limited by the small number of available spacecraft. Common single-spacecraft wave vector analysis techniques built around these limitations are widely used, even in regions where the wave amplitude profile may have a strong spatial dependence. We show that in these gradient regions, the divergence-free condition of the magnetic field requires a local modification to the plane of polarization that is incorrectly interpreted as a change in the physical wave vector direction by single-spacecraft techniques. This result is explored in the context of the Earth’s ion foreshock where Ultra Low Frequency (ULF) waves are generated by wave-particle interactions between the background solar wind population and ion beams reflected from the bow shock. Our results form the basis of a new technique for determining the edge of a ULF wave region in which the general wave geometry is well known. More broadly, this technique may aid attempts to infer the global magnetic field topology in other Heliophysics contexts where wave phenomena are present. [Dorfman et. al. JGR 2023].

Supported by NASA Grant 80NSSC20K0801. Vlasiator is developed by the European Research Council Starting grant 200141-QuESpace, and Consolidator grant GA682068-PRESTISSIMO received by the Vlasiator PI. Vlasiator has also received funding from the Academy of Finland. See www.helsinki.fi/vlasiator