Editing Very low frequency (section)

==Propagation characteristics==
Because of their long wavelengths, VLF radio waves can [[Diffraction|diffract]] around large obstacles and so are not blocked by mountain ranges, and they can propagate as [[ground wave]]s following the curvature of the Earth and so are not limited by the horizon. Ground waves are absorbed by the resistance of the Earth and are less important beyond several hundred to a thousand kilometres/miles, and the main mode of long-distance propagation is an [[Earth–ionosphere waveguide]] mechanism.<ref name="Hunsucker">{{Cite book |last=Hunsucker |first=R.D. |url=https://books.google.com/books?id=IQWHj2bgcxcC&pg=PA419 |title=The high-latitude ionosphere and its effects on radio propagation |last2=Hargreaves |first2=John Keith |publisher=Cambridge University Press |year=2002 |isbn=978-0-521-33083-1 |page=419}}</ref> The Earth is surrounded by a conductive layer of [[electron]]s and [[ion]]s in the upper atmosphere at the bottom of the [[ionosphere]] called the [[D layer]] at {{convert|60|–|90|km|mi|abbr=on}} altitude,<ref name="Ghosh">{{Cite book |last=Ghosh |first=S.N. |url=https://books.google.com/books?id=6Mvf4-gsVycC&pg=PA89 |title=Electromagnetic theory and wave propagation |publisher=CRC Press |year=2002 |isbn=978-0-8493-2430-7 |page=89}}</ref> which reflects VLF radio waves. The conductive ionosphere and the conductive Earth form a horizontal "duct" a few VLF wavelengths high, which acts as a [[waveguide]] confining the waves so they don't escape into space. The waves travel in a zig-zag path around the Earth, reflected alternately by the Earth and the ionosphere, in [[transverse magnetic]] (TM) mode.

VLF waves have very low path attenuation, 2–3&nbsp;dB per 1,000&nbsp;km,<ref name="Hunsucker" /> with little of the "[[fading]]" experienced at higher frequencies.<ref name="Ghosh" />  This is because VLF waves are reflected from the bottom of the ionosphere, while higher frequency shortwave signals are returned to Earth from higher layers in the ionosphere, the [[F1 layer|F1]] and [[F2 layer|F2]] layers, by a refraction process, and spend most of their journey in the ionosphere, so they are much more affected by ionization gradients and turbulence. Therefore, VLF transmissions are very stable and reliable, and are used for long-distance communication. Propagation distances of 5,000–20,000&nbsp;km have been realized.<ref name="Hunsucker" /> However, atmospheric noise ("[[sferics]]") is high in the band,<ref name="Ghosh" /> including such phenomena as "[[whistler (radio)|whistler]]s", caused by [[lightning]].
* VLF waves can penetrate [[seawater]] to a depth of at least {{convert|10|–|40|m|ft|abbr=off|sp=us}}, depending on the frequency employed and the salinity of the water, so they are used to communicate with submarines.
* VLF waves at certain frequencies have been found to cause [[electron precipitation]].
* VLF waves used to communicate with submarines have created an artificial bubble around the Earth that can protect it from [[solar flare]]s and [[coronal mass ejection]]s; this occurred through interaction with high-energy radiation particles.<ref>{{Cite web |last=Susannah Darling |date=17 May 2017 |title=NASA’s Van Allen Probes Spot Man-Made Barrier Shrouding Earth |url=https://www.nasa.gov/missions/van-allen-probes/nasas-van-allen-probes-spot-man-made-barrier-shrouding-earth/ |access-date=11 June 2024 |publisher=NASA}}</ref>
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