Possibility of VLF focusing during a solar eclipse

During a solar eclipse the ionosphere approaches a nighttime state and the effective height of the waveguide channel increases. This increase of effective height leads to a decrease in the phase velocity of VLF waves. The phase front deforms in such a way as to make possible the focusing of VLF waveguide modes at a definite distance from the lunar shadow spot. In the Kirchhoff approximation and taking into account the earth's sphericity, the focal distance and the gain coefficient of the 'lens' created during the solar eclipse are calculated. The VLF focusing effect is verified using a hydroacoustic model.