Kun Zhang

In space physics, wave measurements typically use a small number of datapoints due to limited 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 by single‐spacecraft techniques. We explore the consequences of this result in the Earth's ion foreshock using both Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun spacecraft data and a 2.5‐D hybrid Vlasov simulation conducted using the Vlasiator code. The observed foreshock ultralow frequency waves have a finite extent in the direction perpendicular to the Interplanetary Magnetic Field, and incorrect application of standard minimum variance techniques at the boundary yields a false wave vector orientation that may be used as a novel edge detection method. This method may be particularly useful for analysis of the ion foreshock at other planets where only a single spacecraft is available. More broadly, our results stand as a cautionary tale for wave analysis in other space physics contexts where the wave geometry is less clear.