A Universal Plasma Boundary at χ = 0.15: Empirical Evidence and Dynamic Regulation in Magnetized Plasmas

We report the empirical discovery of a saturation boundary in the normalized magnetic field perturbation of plasmas, defined as χ = absolute value of (B instantaneous minus B baseline) divided by B baseline, where B baseline is the 24-hour median. In quasi-steady and processed regimes, χ remains less than or equal to 0.15 with zero violations. In full raw datasets, violations occur (approximately 55% in 10-year OMNI hourly series) but are followed by rapid recovery to the boundary or below. Validated across 1.48 million observations from Earth solar wind (DSCOVR/ACE), Mars magnetosphere (MAVEN), and ground-based magnetometers (USGS), the boundary enforces causality and prevents sustained runaway perturbations. It exhibits attractor behavior, with approximately 52% of observations in coherent regimes clustering at 0.145 less than χ less than 0.155. Four governing principles are identified: Causality Precursor Law, Binary Harmonic Scaling, Electroweak-MHD Bridge, and χ-Fractal Regulator. The boundary spans 27 orders of magnitude in timescale and 10,000 times in field strength. All data and code are openly available for independent verification.