SUDDEN IMPULSES AT LOW-LATITUDE STATIONS - STEADY-STATE RESPONSE FOR NORTHWARD INTERPLANETARY MAGNETIC-FIELD

An examination of the response of the low-latitude H component of the Earth's magnetic field during the passage of interplanetary shocks whe n the interplanetary magnetic field is northward reveals that this res ponse can be understood quantitatively in terms of the compression of a simple vacuum magnetospheric model. The compression at the surface o f the Earth at 20 degrees latitude at noon in the absence of equatoria l electrojet effects is found to be 18.4 nT/(nPa)(1/2). Stations below 15 degrees latitude and above 40 degrees appear to have additional bu t variable sources of current which magnify this effect. The diurnal v ariation of the compression is larger than expected from the simple va cuum magnetosphere, +/-20% about the mean instead of +/-l0%. We interp ret this difference to indicate that tail currents, not in the vacuum model, are as important as the magnetopause currents in determining th e diurnal variation of the field at the surface of the Earth.