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.

Magnetic records from low and mid latitude stations have been examined to determine their response to solar wind pressure changes. The best correlation between ground level changes and the change in the square root of the solar wind dynamic pressure occurs for stations at latitudes from 15° to 30° such as Tahiti, Honolulu, San Juan and Midway. The horizontal component of the field changes on average 16.5 nT for each change of 1(nPa)½ of the square root of dynamic pressure. This is 50% greater than the vacuum model of Tsyganenko would predict for a non‐conducting Earth and therefore what would be expected for a perfectly conducting interior. Thus, low and mid latitude ground level response to solar wind pressure changes is dominated by the variation of the strength and location of the magnetopause current system and the corresponding induced currents within the Earth rather than ionospheric current sources.

When the interplanetary field is northward and the solar wind dynamic pressure suddenly increases, the increase in the H component magnetic field on the surface of the Earth at low and mid‐latitudes is governed principally by the currents on the magnetopause. Currents in the tail, which act in the opposite sense to the magnetopause currents, are also enhanced. These currents have a smaller effect than the magnetopause currents and have a greater effect at night than in the daytime hours. In this study we examine the response of the H component when the IMF is southward. We find that the dayside response to solar wind pressure increases is over 25% smaller when the IMF is southward than when it is northward, presumably because of the region 1 current system associated with dayside reconnection. At night, the opposite situation ensues. Mid‐latitude bays associated with triggered substorms appear to be responsible for an enhanced nighttime response. Thus the ground level response to sudden pressure changes in the solar wind is very sensitive to the direction of the interplanetary magnetic field.

The response of subauroral H component magnetograms at the time of passage of interplanetary shocks under northward interplanetary magnetic field (IMF) conditions is used to examine the behavior of the magnetosphere when it is suddenly compressed. At subauroral latitudes, near 55 ø or L = 3, the response has some similarities and some important differences from the low-latitude response. The first effect is a preliminary impulse which appears to be due to the Hall current driven in the ionosphere by the arrival of the first Alfven waves along the magnetic field lines from the distant equatorial magnetosphere that has been set in motion by the penetration of the shock wave into the magnetosphere. Next there is a general increase of the magnetic field everywhere as the magnetosphere is compressed to its new size and the information of this compression propagates to the ground. This increase takes about 5 min followed by a 10-min relaxation to the final asymptotic value. This general increase is accompanied by two more localized current systems: one transient and one steady. The transient current appears to be a dual-vortex current system launched from noon toward the nightside. The other steady system is a fixed double-cell convection system. We identify the former with the propagation of the magnetopause indentation associated with the enhanced pressure in the solar wind. We attribute the latter, steady system to high-latitude reconnection with northward IMF.

The observations obtained during the International Magnetospheric Study (IMS) from the magnetometers of the IGS network extending from Cambridge, England, to Tromso, Norway, are used to study the response of subauroral current systems to sudden changes in solar wind dynamic pressure. Observations show that the response is very strong at subauroral latitudes. The preliminary response in the H component is a brief, small increase in the dayside morning sector and a decrease in the afternoon and night sectors. The main response in the horizontal field (the H and D components) is toward the pole except in the dayside morning sector. The inferred ionospheric current is mainly a circulatory system flowing counterclockwise when viewed from the north pole everywhere at subauroral latitudes except the dayside morning sector.