High-power electromagnetic waves can excite a variety of plasma instabilities in Earth's ionosphere. These lead to the growth of plasma waves and plasma density irregularities within the heated volume, including patches of small-scale field-aligned electron density irregularities. This paper reports a statistical study of intensity distributions in patches of these irregularities excited by the European Incoherent Scatter (EISCAT) heater during beam-sweeping experiments. The irregularities were detected by the Co-operative UK Twin Located Auroral Sounding System (CUTLASS) coherent scatter radar located in Finland. During these experiments the heater beam direction is steadily changed from northward to southward pointing. Comparisons are made between statistical parameters of CUTLASS backscatter power distributions and modeled heater beam power distributions provided by the EZNEC version 4 software. In general, good agreement between the statistical parameters and the modeled beam is observed, clearly indicating the direct causal connection between the heater beam and the irregularities, despite the sometimes seemingly unpredictable nature of unaveraged results. The results also give compelling evidence in support of the upper hybrid theory of irregularity excitation.