AM broadcasting stations have different issues, due to using the mediumwave band. Broadcast range in these stations is determined by ground conductivity, and the proper use and maintenance of grounding radials which act as a ground plane for the mast radiators used. Skywaves reflect off the ionosphere at a much greater distance above Earth's surface at night. This in turn causes mediumwave, most shortwave, and even longwave stations to travel much further at night, which is the side of the Earth where the solar wind pulls the ionosphere (and magnetosphere) away from the planet, instead of pushing toward it as on the day side. Because of this, many AM stations must cut power or go off-air at night, except for the very earliest stations still grandfathered on clear channels. Border blaster stations in northern Mexico also used this effect, along with very high-power transmitters, to extend their nighttime broadcast ranges well over the US/Mexico border and across most of the United States.