The speed of light, a fundamental physical constant and thought to be independent of frequency, is tested here with naturally occurring radio waves in the atmosphere at extremely low frequencies. It is shown that the speed of light in the frequency range 5–50 Hz is known with an accuracy determined by perturbations of the ionospheric reflection height associated with space weather phenomena, which place an upper limit on the photon rest mass $m_{\gamma }\lesssim 4\times {10}^{-52}\mathrm{k}\mathrm{g}$ to date.

• Received 18 August 2003

DOI:https://doi.org/10.1103/PhysRevLett.93.043901