We study the electromagnetic coupling of a neutrino that propagates in a two-stream electron background medium. Specifically, we calculate the electromagnetic vertex function for a medium that consists of a normal electron background plus another electron stream background that is moving with a velocity four-vector vμ\documentclass[12pt]{minimal}
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\begin{document}$$v^\mu $$\end{document} relative to the normal background. The results can be used as the basis for studying the neutrino electromagnetic properties and various processes in such a medium. As an application, we calculate the neutrino dispersion relation in the presence of an external magnetic field (B\documentclass[12pt]{minimal}
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\begin{document}$$\mathbf {B}$$\end{document}), focused in the case in which B is inhomogeneous, keeping only the terms of the lowest order in 1/mW2\documentclass[12pt]{minimal}
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\begin{document}$$1/m^2_W$$\end{document} and linear in the B and its gradient. We show that the dispersion relation contains additional anisotropic terms involving the derivatives of B\documentclass[12pt]{minimal}
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\begin{document}$$\mathbf {B}$$\end{document}, such as the gradient of k^·(v×B)\documentclass[12pt]{minimal}
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\begin{document}$${\hat{k}}\cdot (\mathbf {v}\times \mathbf {B})$$\end{document}, which involve the stream background velocity, and a term of the form k^·(∇×B)\documentclass[12pt]{minimal}
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\begin{document}$${\hat{k}}\cdot (\nabla \times \mathbf {B})$$\end{document} that can be present in the absence of the stream background, in addition to a term of the form k^·v\documentclass[12pt]{minimal}
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\begin{document}$${\hat{k}}\cdot \mathbf {v}$$\end{document} and the well known term k^·B\documentclass[12pt]{minimal}
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\begin{document}$${\hat{k}}\cdot \mathbf {B}$$\end{document} that arises in the constant B\documentclass[12pt]{minimal}
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\begin{document}$$\mathbf {B}$$\end{document} case. The derivative-dependent terms are even under a CP transformation. As a result, in contrast to the latter two just mentioned, they depend on the sum of the particle and antiparticle densities and therefore can be non-zero in a CP-symmetric medium in which the particle and antiparticle densities are equal.
