When the Cassini spacecraft passed by Earth en route to the outer solar system, the magnetic field magnitude was measured with extremely high accuracy by the onboard helium magnetometer operating in the scalar mode. Magnitude measurements of the near-equatorial field were obtained inbound from 4 to 1.17 RE at local times from postnoon to dusk and outbound to 4 RE between dusk and midnight. The observed field consisted of both the internally generated field of Earth and the field of external magnetospheric currents which were enhanced because of an ongoing magnetic storm. The contribution of the internal field was removed using a current version of the International Geomagnetic Reference Field (IGRF). An excellent characterization of the disturbance field was then obtained as a function of radial distance and local time. The observations were compared with available models of the disturbance field. Reasonable agreement was obtained, but discrepancies were found that were large enough to require changes in some of the model parameters. After removing the disturbance field, accuracy of measurement of high-degree spherical harmonic moments of a planetary field, an important objective at Saturn, was investigated. The contribution of the sum of high-degree moments to the Earth's field magnitude was obtained from the IGRF. The corresponding contribution to the measured field magnitude was derived using a theorem developed for that purpose. A close correspondence between the Cassini measurements and the IGRF was then found. The physics underlying the scalar measurements and the implementation of this mode in the Cassini magnetometer, the first time on a planetary mission, are described. ¿ 2001 American Geophysical Union