A general method for computing thermal magnetic noise arising from thin conducting objects

Thermal motion of charge carriers in a conducting object causes magnetic field noise that may interfere with sensitive measurements near the object. In this paper, we describe a method to compute the spectral properties of the thermal magnetic noise from arbitrarily shaped thin conducting objects. The method is based on modeling divergence-free currents on a conducting surface using a stream function and calculating the magnetically independent noise-current modes. By doing this, we obtain the power spectral density of the thermal magnetic noise as well as its spatial correlations and frequency dependence. We also describe a numerical implementation of the method and verify it against analytic formulas. We provide the implementation as a part of the free and open-source software package bfieldtools.