Three meter striations are produced in response to the heating of the ionosphere by a powerful high frequency (HF) radio wave. The striations are electron density perturbations aligned parallel to the geomagnetic field in the heated region of the ionosphere. They are formed within seconds as a direct consequence of the heating, and disappear rapidly after the heater has been turned off.
Recent experiments were conducted to investigate the evolution times of the striations. The experiments combined the use of the new HF facility at the Arecibo Observatory with a portable 50 MHz radar located on either the island of St. Croix or Guadeloupe. Striations were observed in both the E and the F regions of the ionosphere.
The conclusions of our investigation can be summarized as follows. First, relationships have been determined between the striations' rise and fall times and the electron collision frequency, temperature, and density. Second, the rise times of the striations are found to be dependent on the HF electric field. The nature of this dependency is presented and interpreted. Third, both the E and the F region data verify previous theoretical predictions that the striations' decay times are directly proportional to the electron diffusion across the magnetic field. Finally, the temporal interaction between the striations and the enhanced plasma line, another HF-induced phenomenon, is studied. For the first time, a relationship between the rise times of the striations and the overshoot of the enhanced plasma line has been experimentally determined.
This last finding addresses the primary motivation of plasma heating experiments; that is, an understanding of the mechanism by which energy is transferred to and through the plasma. Our results suggest that energy cascades from the large scales of initial HF excitation to the smaller scales associated with the striations.