Direct observations of pass-through and coalescence in collisions between electron phase-space holes.

Electron holes are Debye-scale kinetic structures that mediate particle trapping, field-aligned electric fields, and energy conversion in collisionless plasmas. How such coherent structures interact with one another, however, has remained difficult to resolve in situ. Using four-spacecraft Magnetospheric Multiscale electric-field measurements, we identify two head-on encounters between positive-potential electron holes in the terrestrial magnetotail. In both events, the holes accelerate toward one another during approach, a behavior consistent with the effective negative-mass dynamics of phase-space holes rather than with the traditional repulsion expected for isolated like-signed positive-potential structures. The subsequent outcomes differ: one fast-slow pair passes through, whereas one slow-slow pair coalesces into a single evolving structure. Targeted one-dimensional Vlasov-Poisson simulations reproduce both outcomes and show that attraction alone does not determine the collision result. Instead, the outcome is controlled by a dimensionless phase-space overlap parameter that compares the trapped-electron velocity width with the relative drift speed. An energetic diagnostic based on trapped-electron kinetic and potential energies supports this boundlike versus unboundlike interpretation. These observations and simulations demonstrate that electron holes can behave as attractive kinetic quasiparticles whose pairwise interactions follow a simple phase-space merger rule.