What provides signal paths between the transmitter and receiver in sky wave propagation?

In sky wave propagation, the ionosphere plays a crucial role as it reflects and refracts radio waves back toward the Earth, creating signal paths between the transmitter and receiver. The ionosphere is a region of the Earth's upper atmosphere, filled with charged particles, that can vary in its reflective properties depending on factors like time of day, solar activity, and the frequency of the transmitted signal. This characteristic enables long-distance communication by allowing signals to bounce off the ionosphere, reaching receivers that would otherwise be out of range due to the curvature of the Earth or other obstacles.

The other options do not facilitate the same type of signal path for sky wave propagation. Ground waves, for instance, travel along the Earth's surface and are limited to shorter distances. The troposphere, while significant for certain communication methods, primarily affects signals traveling in the line-of-sight range and does not contribute to the bouncing back of radio waves like the ionosphere. Similarly, the stratosphere is not typically involved in radio wave reflection or refraction in a way that supports long-distance communication; instead, it is layered above the troposphere and does not have the same interaction with radio frequencies.