Decomposition of $L^{2}$-vector fields on Lipschitz surfaces: characterization via null-spaces of the scalar potential

For $\partial Ω$ the boundary of a bounded and connected strongly Lipschitz domain in $\mathbb{R}^{d}$ with $d\geq3$, we prove that any field $f\in L^{2} (\partial Ω; \mathbb{R}^{d})$ decomposes, in an unique way, as the sum of three silent vector fields---fields whose magnetic potential vanishes in one or both components of $\mathbb{R}^d\setminus\partial Ω$. Moreover, this decomposition is orthogonal if and only if $\partial Ω$ is a sphere. We also show that any $f$ in $L^{2} (\partial Ω; \mathbb{R}^{d})$ is uniquely the sum of two silent fields and a Hardy function, in which case the sum is orthogonal regardless of $\partial Ω$; we express the corresponding orthogonal projections in terms of layer potentials. When $\partial Ω$ is a sphere, both decompositions coincide and match what has been called the Hardy-Hodge decomposition in the literature.

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