Extended Data Fig. 5: The lateral emission region size as it depends on the Galactic screen distance, d_⊕s1, through the relationship shown on Fig. 3 and the two-screen constraint in equation (9).

The green shaded region shows the allowable lateral emission region sizes and Galactic screen distance combinations for our measured scintillation parameters at 600 MHz: Δν_s2 = 128 kHz and m_s2 = 0.78. The black vertical line indicates the NE2001 prediction²³: d_⊕s1 = 0.64 kpc. The orange shaded region shows the emission region sizes estimated for non-magnetospheric models^5,6,7,8. The grey hatched region shows the parameter space we ruled out based on the apparent diameter of the host galaxy (see Fig. 3).