A proton of momentum p moving across a magnetic field B follows a circle of radius r = p / (q B) (the gyroradius), with p from the relativistic relation pc = sqrt(T^2 + 2 T m c^2). Crossing a region of width L it is turned by sin(theta) = L / r; if L is greater than r it is reflected. The page integrates the Lorentz force with RK4 and compares the result with that formula.

Solar energetic protons span roughly 10 MeV to several GeV. Earth already screens them with its dipole field: the Stormer vertical cutoff rigidity is approximately Rc = 14.9 GV cos^4(lambda), so only particles with rigidity above Rc reach a given geomagnetic latitude, which is why polar regions receive far more flare protons than the equator. A protective network of magnetic structures is hypothetical; the 3D page is a concept visualisation, and this page shows the physical limit: deflecting a 200 MeV proton needs field-length products of order B L = p / q, i.e. about 2.1 T m for 200 MeV, which is enormous in space.

Simplifications: uniform field with a sharp edge, protons only, no scattering or energy loss, no electric fields, 2D motion perpendicular to B, and the beam is a flat 15-particle fan aimed at an Earth-sized disc. Educational physics of space weather only.