The aurora produces a reflecting sheet (or metric sized columns) which tends to lie in a vertical plane. The result of this vertical ionospheric "curtain" is reflection of signals well into the upper VHF band. The reflection is very aspect sensitive. Since the reflecting sheet lies towards the poles, it follows that reflected signals will arrive from that general direction. An active region or coronal hole may persist for some 27 days resulting in a second aurora when the Sun has rotated. There is a tendency for auroras to occur around the March/April, September/October equinox periods, when the geomagnetic field is at right angle to Sun for efficient charged particle coupling. Signals propagated by aurora have a characteristic hum effect, which makes video and audio reception difficult. Video carriers, as heard on a communications receiver, no longer can be heard as a pure tone.