At AM radio frequencies, most of the radio waves are reflected back to earth or absorbed in the ionosphere. Same concept as how amateur radio HF/MF propagation

At AM radio frequencies, most of the radio waves are reflected back to earth or absorbed in the ionosphere. Same concept as how amateur radio HF/MF propagation allows individuals to communicate with people around the world, past the horizon. (The 160m ham band nearly backs right up to the top end of the AM radio band.) You can even sometimes hear your own echo, apparently, as the signals travel the entire way around the earth, although I've yet to experience this. (Recent Extra class licensed ham radio operator here. 73.) Higher frequencies, on the other hand, are not reflected back by the ionosphere and go straight out into space. That's why you typically use VHF frequencies (30-300 MHz) for things like EME "earth-moon-earth" or "moonbounce" communication, or communication via satellite. So if your concern is about radio waves making our planet a beacon for aliens, you should be more concerned about FM radio towers than AM ones.