New phenomena observed by EISCAT during an RF ionospheric modification experiment


  • B. Isham,

  • W. Kofman,

  • T. Hagfors,

  • J. Nordling,

  • B. Thidé,

  • C. LaHoz,

  • P. Stubbe


An ionospheric HF-modification experiment was carried out near Tromsø, Norway, using the Max-Planck-Institut für Aeronomie (MPI) heating facility and the EISCAT 933-MHz incoherent scatter radar (ISR). The MPI heater was normally operated at 4.04 MHz and modulated 20-s on, 40-s off. The ISR observed waves propagating parallel to B0, and chirped as well as normal plasma line observations were performed. Heater-induced plasma lines were observed only in the first 10-s integration interval, indicating a strong overshoot. These lines are unusual in that multiple simultaneous lines were observed, normally originating within one kilometer of the critical region but sometimes from lower heights, and that the frequency of the most constant line is offset some 250 kHz from the heating frequency, with the other lines occurring at greater frequency offsets. The natural, photoelectron-enhanced plasma line was not observed; however, the background plasma was diagnosed via ion line observations and comparisons to chirped observations performed at EISCAT in May 1986 indicate that increased Landau damping may be responsible for both the strong induced-line overshoot and the lack of a distinct natural line. Finally, ion power profile observations show the existence of a topside enhanced ion line at the critical density corresponding to the heater frequency. We believe this is due to strong O to Z-mode coupling parallel to B0 and a low ƒ0F2.