Following particle precipitation at high latitudes that occurs as a result of geomagnetic disturbances, the electron density of the E region of the ionosphere undergoes substantial variation. These variations modify the E layer propagation characteristics which adversely affect HF propagation paths both unexpectedly and in many cases for long periods of time. This paper presents two case studies of how the enhancement of E region electron density immediately following storm sudden commencement affects high-latitude HF communications links in and through the auroral zone. Using data gathered from ionosondes, incoherent scatter radar and an oblique sounder, the Advanced Composition Explorer Satellite, and riometers and magnetometers, the physical effects of particle precipitation on the ionospheric E layer and the resulting effect on HF links is revealed. Both case studies show that besides the expected occurrence of severe short-term absorption and the reduction in the critical frequency of the F layer caused by a redistribution of the electron density in the upper layers of the ionosphere, there is the likelihood a sporadic E layer will form and/or be enhanced because of significant and sustained particle precipitation. When the critical frequency of the sporadic E layer increases above the norm and in cases exceeds the critical frequency of the F layer, there are severe implications for many HF radio communication systems used at high latitudes unless corrective action is taken.