Researchers have demonstrated the potential use of a new way to measure properties of Earth's magnetosphere, the magnetic bubble that surrounds the planet.Zhai et al. used a property known as Faraday rotation for radio tomographic imaging of the magnetosphere. Faraday rotation occurs when a linearly polarized light wave travels through a magnetized medium such as the magnetosphere. The magnetic field causes the plane of polarization to rotate, and the amount of rotation is directly proportional to the electron density in the medium and to the magnetic field. Because Earth's magnetic field is known, researchers therefore can use measurements of Faraday rotation to reconstruct electron density in the magnetosphere.