An examination of the response of the low-latitude H component of the Earth's magnetic field during the passage of interplanetary shocks when the interplanetary magnetic field is northward reveals that this response can be understood quantitatively in terms of the compression of a simple vacuum magnetospheric model. The compression at the surface of the Earth at 20¿ latitude at noon in the absence of equatorial electrojet effects is found to be 18.4 nT(nPa)1/2. Stations below 15¿ latitude and above 40¿ appear to have additional but variable sources of current which magnify this effect. The diurnal variation of the compression is larger than expected from the simple vacuum magnetosphere, ¿20% about the mean instead of ¿10%. We interpret this difference to indicate that tail currents, not in the vacuum model, are as important as the magnetopause currents in determining the diurnal variation of the field at the surface of the Earth. ¿ American Geophysical Union 1994