Observations were made of the HF-modified ionosphere over Arecibo during the daytime on March 4, 1987 and April 6--8, 1988, and during the nightime on April 13 and 14 and 14 and 15, 1988, using chirped incoherent scatter radar (ISR). The daytime observations show a difference in the frequencies of the HF-induced plasma line (HFPL) and the photoelectron-enhanced (natural) plasma line (PEPL) at the same height of the order of 100 kHz. In addition, the daytime HFPL originates within about 200 m of the reflection height (that is, the O mode cutoff), often within 20 ms of HF turn-on, and may remain at that location for many seconds. A second HFPL source region was sometimes observed to form between approximately 1 and 4 km below the usual source region after 5 to 10 s of heating. During the nighttime, rapidly changing regions of HF-induced plasma lines spread over more than 3 km in height were observed; the existence of these regions appears to be strongly dependent on HF heating power. At times, frequency shifting and broadening of the PEPL during heating was noted; the shift in frequency is due to increasing electron temperature and the frequency broadening to the appearance of short-scale density striations. These effects continued even during periods when the heater remained on and the HF-induced line was not observed by the radar. ¿ American Geophysical Union 1993