Which waves are radiated at an angle toward the ionosphere?

Sky waves are the correct choice because they are a type of radio wave that is transmitted from the Earth's surface and is reflected back to the ground by the ionosphere. The ionosphere, which is a region of the upper atmosphere, contains a high concentration of ions and free electrons, allowing it to reflect certain frequencies of radio waves back toward Earth. This property is particularly useful for long-distance communication, as it enables signals to travel beyond the horizon, effectively bouncing off the ionosphere at an angle.

Sky waves are characterized by their ability to be radiated at angles that facilitate this reflection, allowing for extended communication range compared to ground waves, which tend to follow the curvature of the Earth and are limited to line-of-sight distances. This communication phenomenon is especially prominent at high frequencies, typically in the HF (high frequency) range.

Ground waves, on the other hand, follow the surface of the Earth and diminish in strength over longer distances, while surface waves refer to a different type of wave or signal that travels along the surface of a medium. Radio waves is a more general term that encompasses various types of electromagnetic waves used for communication but does not specifically denote the characteristic of being reflected by the ionosphere as sky waves do.