Synchrotron Depolarization

Synchrotron radiation from relativistic electrons spiraling in the Galactic magnetic field is intrinsically highly linearly polarized, but observations of diffuse synchrotron emission from the Milky Way reveal that the polarization fraction decreases dramatically at low frequencies and along certain lines of sight, a phenomenon called depolarization. Depolarization can arise from beam averaging (integration of regions with different polarization angles within the telescope beam), depth depolarization (integration along the line of sight through regions with varying Faraday rotation and emissivity), or differential Faraday rotation across the bandwidth. The observed spatial distribution of depolarization across the sky shows large-scale patterns that are only partially reproduced by models of the Galactic magnetic field and electron density derived independently from pulsar dispersion measures and rotation measures, suggesting that the magnetic field structure is more complex or more coherently organized than standard random-turbulence models predict.

Successive Collision Theory explains the large-scale coherent pattern of synchrotron depolarization through the angular momentum inheritance of the Galactic magnetic field. In SCT, the Milky Way's magnetic field is not purely the product of a small-scale turbulent dynamo operating on post-collision gas; it traces the organized angular momentum structure of the collision debris field, with large-scale ordered components aligned along the angular momentum strata that set the disk geometry, the warp, and the filamentary arms. This organized large-scale field produces coherent rotation measure gradients over angular scales of tens of degrees — larger than standard random-turbulence magnetic field models predict — creating depth depolarization patterns that extend over the full angular scale of the ordered field coherence. The observed large-scale depolarization structures aligned with the Galactic spiral arm pattern and with the outer disk warp reflect this angular-momentum-organized magnetic topology.

The pre-existing stellar populations contribute to synchrotron depolarization through their compact remnants — pulsars, magnetars, and stellar-mass black holes with associated magnetospheric emission — that were incorporated into the Galactic magnetic field environment from the collision epoch. These objects contribute small-scale but high-amplitude Faraday rotation measure fluctuations localized around their positions, producing depolarization in their vicinities at levels that depend on their inherited spin rates and magnetic field configurations. The spatial distribution of these depolarization enhancements traces the angular momentum-organized debris field structure, being concentrated along the same filamentary and ring-like density enhancements (such as the Monoceros Ring and outer disk warp) that are explained by the same angular momentum inheritance mechanism. This connection between synchrotron depolarization patterns and other large-scale ISM structures provides a multi-wavelength consistency test of the SCT angular momentum field.