Abstract
Detailed measurements of the parameters of a shear Alfvén wave in the earth's magnetosphere have allowed a limit of ≲ 2.4 × 10−48 gm to be placed on the rest mass of the photon (Mph). This limit is comparable to that determined recently by Hollweg from statistical properties of Alfvén waves in the solar wind (≲1.3 × 10−48 gm) and, together, the two limits represent the best present upper limits to Mph as derived from measurements of magnetohydrodynamic waves.