The polar cap (PC) index is used to measure the geomagnetic activity over the polar caps. Changes in the solar wind are able to cause disturbances in the magnetospheric-ionospheric currents, which in turn cause variations in the PC index. In order to understand how different processes in the solar wind and magnetosphere influence the PC index, it is necessary to separate the effects of solar wind pressure impulses, interplanetary magnetic field (IMF) reorientations, and magnetospheric substorms. We study the variations of the PC index under different solar wind conditions and during substorms and present case analyses and statistical results. The main conclusions are as follows. A solar wind pressure impulse alone, without IMF southward turning or substorm, is to cause a negative spike of ~1 in the PC index within a time interval of 2--6 min and a subsequent increase to a positive value of ~1 within 30 min. The PC index enhances significantly after the IMF turns southward, and the increase of the PC index is 3--5 in most cases and can be as large as 8--9. When a solar wind pressure impulse occurs with a simultaneous southward turning of the IMF, the large increase of the PC index is mainly a response to the southward IMF but not to the solar wind pressure impulse. Magnetospheric substorms have significant effects on the PC index. The response of the PC index to substorms is an increase of 2--4 in most cases and can reach 6 or larger. The effects of IMF southward turnings and substorms on the PC index are much stronger than that of solar wind pressure impulses. On average, the increases of the PC index caused by solar wind pressure impulses, substorm onsets, and IMF southward turnings are 0.8, 3.2, and 3.6, respectively.