The LUF and MUF curves show a normal, daily variation. A SWF can occur over the entire sunlit hemisphere of the Earth facing the sun and it can last anywhere from tens of minutes to an hour or two. This enhanced absorption can become strong enough to close the HF propagation window completely, which is referred to as a Short Wave Blackout. The result is signal fading and data drop-outs on satellite command uplinks, data downlinks, or on communication signals. Ionospheric radiowave scintillation is very similar to the visual twinkling of starlight or heat shimmer over a hot road caused by atmospheric turbulence.
Scintillation tends to be a highly localized effect.
Only if the signal path penetrates an ionospheric region where these small scale electron density irregularities are occurring will an impact be felt. Low latitude, nighttime links with geosynchronous communications satellites are particularly vulnerable to intermittent signal loss due to scintillation. Scintillation tends to be most severe within 20 degrees of the geomagnetic equator. It is also strongest from local sunset until just after midnight, and during periods of high solar activity. In the auroral and polar regions, scintillation is strong, especially at night, and its influence increases with higher levels of geomagnetic activity.