Despite ~50 years of compact object studies in the Galaxy, the physics of relativistic jet launching remains an active and open field of research. There has been important recent progress in development of state-of-the-art MHD simulations, but how these determine the emission properties is now a major focus. A deeper understanding requires us to probe rapidly changing conditions in the outflowing plasma close to the jet base, for which only piecemeal studies have been possible to-date. A key observational feature, the spectral break above which the jet becomes optically thin, provides a crucial link between light and plasma properties. It is a variable feature that needs to be tracked and linked to other jet properties, and is known to pass through the mid-infrared band.

We propose to detect this jet spectral break, and monitor rapid flux and spectral shape variations in outbursting Galactic black hole binaries using the MIRI LRS mode, to provide estimates of the variable magnetic (B) field strength and physical dimensions of the jet base. There is evidence that these properties are subject to violent variations on timescales <~1 seconds, which can have important consequences for our understanding of the inner jet B field dynamics and particle acceleration mechanisms. We will measure the B field strength and radiative energy of the jet, while any variability in flux and frequency will provide unprecedented constraints on the speed and magnitude of B field changes. The break is predicted to lie in the infrared regime during typical jetted outbursts of transient black hole X-ray binaries, so this science is ideally suited for MIRI LRS observations.