The velocities of HF-induced short scale striations
Coster, A. J. (Anthea J.)
Gordon, William E.
Master of Science
Short scale striations are among the phenomena that occur when the ionosphere is heated by high frequency (HF) radio waves with ordinary mode polarization. These striations are electron density irregularities aligned along the magnetic field lines in and near the heated region of the ionosphere. They are a direct consequence of the heating and disappear within seconds after the heater has been turned off. The striations are detected to be moving in the heated region. Experiments were performed in June 1977 to investigate short scale striations. The experiments combined the use of a portable 5 MHz radar located on the island of Guadeloupe with heating facilities at the Arecibo Observatory. The 5 MHz radar had a line of sight that was orthogonal to the magnetic field lines in the F region above Arecibo and was used to detect the striations and their velocities. The 43 MHz radar at Arecibo was used for additional diagnostics. Varying the power of the HF radio wave did not appear to affect the striation velocities. Instead, their velocities appeared to be well correlated with general F-region ionization drifts. On two evenings, shortly after ionospheric sunset, dramatic changes in the striation velocities were observed. These F-region velocity disturbances occur in conjunction with the intensification of existing sporadic-E regions. Only during these disturbed time periods were reflections from sporadic E detected at greater than 5 MHz. A mechanism whereby the sporadic-E regions cause the alternate coupling and decoupling of the E and F regions in the early evening hours is presented. This mechanism can explain changes in local F-region electric fields which would cause the velocity disturbances.