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# On the Role of Longitudinal Currents in Radiating Systems of Charges

preprint

posted on 2023-01-12, 14:55 authored by Nikita Nemkov, Vassili FedotovThe time derivative of the charge density is linked to the current density by the continuity equation. However, it features only the longitudinal part of a current density, which is known to produce no radiation. This fact usually remains unnoticed though it poses a seemingly serious paradox, suggesting that the temporal variation of a charge density should be also irrelevant for radiation. We resolve this paradox by showing that the effective longitudinal currents are not spatially confined even when the time-dependent charge density that generates them is. This enforces the co-existence of the complementary (i.e. transverse) part of the current density through the Helmholtz decomposition. We illustrate the mechanics of the underlying non-locality of the Helmholtz decomposition in the case of a dynamic electric dipole, discussing its practical implication for underwater antenna communications. More generally, we explain the role of the Helmholtz decomposition in shaping the structure of the conventional multipole expansion.