Editing Space weather (section)

===Long-distance radio signals===
Radio waves in the [[high frequency|HF]] band (3 to 30&nbsp;MHz) (also known as the [[shortwave]] band) are reflected by the ionosphere. Since the ground also reflects HF waves, a signal can be transmitted around the curvature of the Earth beyond the line of sight. During the 20th century, HF communications was the only method for a ship or aircraft far from land or a base station to communicate. The advent of systems such as [[Iridium satellite constellation|Iridium]] brought other methods of communications, but HF remains critical for vessels that do not carry the newer equipment and as a critical backup system for others. Space weather events can create irregularities in the ionosphere that scatter HF signals instead of reflecting them, preventing HF communications. At auroral and polar latitudes, small space weather events that occur frequently disrupt HF communications. At mid-latitudes, HF communications are disrupted by solar radio bursts, by X-rays from solar flares (which enhance and disturb the ionospheric D-layer) and by [[Total Electron Content|TEC]] enhancements and irregularities during major geomagnetic storms.

Trans[[polar route|polar airline route]]s are particularly sensitive to space weather, in part because [[Federal Aviation Regulations]] require reliable communication over the entire flight.<ref>FAA Advisory Circular 120-42B, June 6, 2008, Extended Operations (ETOPS and Polar Operations)</ref> Diverting such a flight is estimated to cost about $100,000.<ref name="nap.edu">{{Cite book|title = Severe Space Weather Events--Understanding Societal and Economic Impacts: A Workshop Report {{!}} The National Academies Press|doi = 10.17226/12507|year = 2008|isbn = 978-0-309-12769-1|last1 = Council|first1 = National Research|last2 = Sciences|first2 = Division on Engineering Physical|last3 = Board|first3 = Space Studies|last4 = Workshop|first4 = Committee on the Societal Economic Impacts of Severe Space Weather Events: A.}}</ref>

[[File:Aviation radiation environment.png|thumbnail|All passengers in commercial aircraft flying above {{convert|26,000|ft}} typically experience some exposure in this aviation radiation environment.]]