Collisionless shocks accelerate particles through a messy upstream region called the foreshock, where reflected ions interact with the incoming plasma to create compressive magnetic structures. These structures are well-studied at Earth's bow shock, where spacecraft have decades of observations. Interplanetary shocks — traveling through the solar wind far from any planet — should produce the same physics. They don't produce the same structures.
Raptis et al. (arXiv:2603.17882) compare foreshock observations from two missions: MMS at Earth's bow shock and Solar Orbiter at an interplanetary shock. The initiation physics is identical. Foreshock compressive structures begin forming at the same normalized distance upstream of both shocks, triggered when the suprathermal ion density reaches roughly 1% of the background. The seed is universal.
But the interplanetary shock's structures never fully develop. Earth's bow shock produces Short Large Amplitude Magnetic Structures — nonlinear, fully evolved features that efficiently scatter and accelerate particles. The interplanetary shock produces only the precursors. The growth zone — the spatial region where structures can amplify — extends roughly 135 ion inertial lengths. For the fast-moving interplanetary shock, this corresponds to an observational window of under 10 seconds. There isn't enough time or space for full nonlinear development.
The missing ingredient is geometry. Earth's bow shock is curved, allowing energetic ions from different regions to exchange laterally, feeding the growth of structures across the shock surface. An interplanetary shock lacks this global curvature. Each region evolves in isolation, and the isolation starves the nonlinear growth.
Same physics, different geometry, different outcome. The structures that accelerate cosmic rays depend not on the shock's strength but on its shape.