Cosmic-ray leptons, magnetic fields and interstellar synchrotron emission

Interstellar synchrotron emission depends on Galactic magnetic fields and on cosmic-ray leptons. Observations of radio emission are an important tool for studying cosmic-ray propagation models and interstellar electron spectrum and distribution in the Galaxy. We present the latest developments in our modeling of Galactic synchrotron emission with the GALPROP code, including polarization, absorption, and free-free emission. Using surveys over a wide range of radio frequencies and polarization measurements, we derive constraints on the low-energy interstellar cosmic-ray electron spectrum, magnetic fields and cosmic-ray propagation models. This work is of interest for studies of interstellar gamma-ray emission with Fermi-LAT, and synchrotron for the Planck mission.