MULTIPLE FREQUENCY BACKSCATTER OBSERVATIONS OF HEATER-INDUCED FIELD-ALIGNED STRIATIONS IN THE AURORAL E REGION (IONOSPHERE, MODIFICATION)
NOBLE, STEPHEN T.
Doctor of Philosophy
In September 1983 a series of HF ionospheric modification exper- iments were conducted in Scandinavia using the heating facility near Tromso, Norway. The purpose of these experiments was to examine the mechanisms by which high-power HF radio waves produce geo- magnetic field-aligned striations (FAS) in the auroral E region. The vast majority of the backscatter observations were made with radars operating at 47 and 144 MHz (STARE Finland). Additionally, limited observations were conducted at 140 (STARE Norway) and 21 MHz (SAFARI). These radars are sensitive to irregularities having scale lengths between 1 and 7 m across the geomagnetic field lines. During periods of full power O-mode heating, striations having peak cross sections of 40 - 50 dBsm are observed. Striations are not detected during times of X-mode heating. When the heater output is varied a corresponding change in the cross section is measured. The magnitude of the change is most pronounced for heater level changes in the range 12.5 to 50% of full power. These cross sections are significantly larger than those measured at midlatitudes using the Arecibo heater ((TURN)10('1) m('2)). This is consistent with theoretical studies which indicate that it is easier to excite short-scale FAS at places where the geomagnetic dip angle is large. The growth and decay times of the striations are frequency dependent. The growths are variable and can range up to a few seconds if the ionospheric conditions are only marginal for striation development. Under more ideal conditions, growth times are 10('1) - 10('2) ms at VHF frequencies and 10('2) - 10('3) ms at 21 MHz. Decay times follow closely with predictions based upon diffusion across field lines. The times range from 1 s at 21 MHz.