Physical Characterization of Low-Frequency Electromagnetic Fields Measured on Board Spacecraft

Measurements of electromagnetic waves on board spacecraft are commonly characterized in terms of energy density and wave polarization. However, when both electric and magnetic field vectors are measured simultaneously—as on board ESA Solar Orbiter, NASA  MMS, and ESA JUICE —it becomes possible to investigate a much larger parameter space by exploiting the 36 auto- and cross-correlations of the complex three-dimensional Cartesian components of the electromagnetic field. These correlations are commonly represented in 6×6 correlation matrices. A limitation of this formalism is that the correlations themselves are not physical observables: they are neither expressed as manifestly covariant space-time tensors nor associated with conserved quantities. By introducing a physically motivated and systematic representation of the 36 degrees of freedom of partially coherent electromagnetic fields, in terms of manifestly covariant space-time tensors, we demonstrate several applications to the analysis of low-frequency electric and magnetic field measurements. The approach increases the amount of physical information that can be extracted from a given data set.