Using the data of the DMSP F6 and F7 satellites, we show that there is a systematic difference between ion precipitation in the auroral oval and in the polar cap. The oval ion precipitation is smooth, and the ratio of the total electron number flux to the total ion flux is less than 20. The polar cap ion precipitation is patchy, and the ratio is more than 20. The boundary between these two types of ion precipitation usually can be detected for both northward and southward interplanetary magnetic field by a sharp fall of the ion total number flux below the level 5¿105--106 ions (cm2 s sr)-1. Since this determination of the poleward boundary of the auroral oval is efficient for any conditions, we propose to define the polar cap as a region bounded by this boundary. The polar cap determined in this way has roughly the shape of an ellipse aligned along the 1100--2300 MLT meridian with daytime and nighttime boundaries at 80¿ and 72¿, respectively. Four patterns of electron precipitation in the polar cap are identified for northward IMF conditions: (1) symmetric precipitation in both polar caps with no distinct regularity. (2) symmetric precipitation with a prevalence of electron fluxes in the morning sector of both polar caps, (3) asymmetric distribution with no correspondence between the patterns in the opposite polar regions, and (4) asymmetric distribution where the increased electron fluxes are observed in opposite sectors (morning or evening) of the opposite polar caps depending on the polarity of the BY IMF component. The energy spectra of electrons even in neighboring spikes in the same polar cap may be greatly different, and therefore discrepancy in electron spectra in opposite polar caps cannot be regarded as unique proof for the open character of the magnetic field lines. ¿ American Geophysical Union 1992