Ionospheric modification from under-dense heating by high-power HF transmitter

Under-dense HF heating experiments were conducted near local solar noon as well as in the nighttime with the HF heater transmitting at 9.1 MHz directed along the geomagnetic zenith and run at 2 min on and 2 min off. The effective isotropic radiated power of the HF transmitter exceeded 3 GW. The Digisonde operated in a fast mode was used to monitor the temporal evolution of the ionospheric electron density distributions in the bottomside of the ionosphere (in the ranges from 90 to 190 km in the noontime and from 230 to 350 km in the nighttime). The electron temperature distributions were then evaluated. The results show that the electron density distributions are modified continuously over the experimental periods. In the noontime, the electron density decreases/increases in time in the region below/above a height at about 140 km, manifesting the change of the balance between the photoionization and the electron-ion recombination and the electron-oxygen dissociative attachment losses by the heating. In the nighttime, the ionosphere was lifted by 30 to 50 km through continuously upward expansion, resulting in the drop of the electron density in the bottomside of the ionosphere in time. A comparison with the ionogram, height, and electron density distribution of unheated ionosphere with similar background conditions further elaborates the observation of thermal expansion.