Low solar activity means that there is less EUV (extreme ultraviolet) radiation hitting the ionosphere. This leads to weaker plasma formation, as well as shift

Low solar activity means that there is less EUV (extreme ultraviolet) radiation hitting the ionosphere. This leads to weaker plasma formation, as well as shifting geometries of the plasma layer. In general the lower and weaker plasma tends to restrict the range of HF transmissions attempting long distance communications via refraction through ionospheric plasma.