Investigation of group velocities of vertically propagated high frequency (HF) waves in the ionosphere

The group velocity, propagation velocity of the modulated wave in a medium, varies depending on refractive index. Group velocity is important for the investigation of the propagation of HF radio waves in the ionosphere. The vertically propagated HF radio waves in the ionosphere are weakened due to collision of the electrons with other neutral and charged particles. The refractive index of HF waves affected by collisions is greater than the refractive index at which collisions are neglected. As a result of this, the group velocities and reflection heights of the waves in different frequencies and modes vary in the ionosphere. In this study, it is assumed that the vertically propagated HF waves enter the ionosphere at an altitude of 80 km with speed of light and group velocities (Vg) were calculated for 3, 4, 5, 6 and 7 MHz frequencies and ordinary, extraordinary, right-hand, and left-hand waves. The variations are calculated based on the solstice and equinox times where the collisions are neglected or not. According to the results, it was found that group velocities increase depending on the frequency and high frequency waves reach higher heights than low frequency waves. In addition, the refractive index of the medium changing by the collisions also affect the group velocities. As a result of this situation, it was calculated that the group velocities without collision were greater than collisional medium. The group velocities of the waves vary according to their frequency and the conditions of the medium. The reasons for the variations are time, the conditions of the medium and the effects of the collisions.