Magnetic field measurements made by means of Explorer 10 over geocentric distances of 1.8 to 42.6Reon March 25–27, 1961, are presented. Results from the MIT plasma experiment on the same satellite are referenced in interpretations. The close‐in data are consistent with the existence of a very weak ring current below 3Realong the trajectory, but alternative explanations for the field deviations are possible. Between 8 and 22Rethe existence of a large‐scale field directed away from the sun and earth within the geomagnetic cavity on the evening side of the earth is demonstrated. At distances greater than 22Reand before a storm sudden commencement late in the flight the geomagnetic cavity boundary apparently crossed the satellite trajectory on six principal occasions. Interpretations bearing on the form and boundary conditions of the cavity and solar‐interplanetary fields external to the cavity are treated. Correlations with magnetic variations at the earth's surface are noted under quiet magnetic conditions as well as at the time of the sc. These indicate that changes in the solar wind have a direct influence on surface magnetic activity associated with changes in the structure of the