As Galileo passed through the wake of Io it encountered a number of magnetic depressions that have been interpreted to be the result of the mirror-mode instability. Herein we examine the magnetic signatures of these structures and simultaneous measurements of the electron density and temperature. These structures have phase fronts that propagate at large angles to the magnetic field and scale sizes of several ion gyroradii. The inferred density enhancements that accompany the magnetic field depressions range up to 200% of the background density. The spread in normal directions suggests that the depressions are cylindrical and not sheet-like. A companion paper [Huddleston et al., this issue> discusses the theoretical aspects of these waves. ¿ 1999 American Geophysical Union