Editing Ionosphere (section)

==Ephemeral ionospheric perturbations==
{{Unreferenced section|date=July 2024}}

===X-rays: sudden ionospheric disturbances (SID)===
{{Main|Sudden ionospheric disturbance}}
When the Sun is active, strong [[solar flare]]s can occur that hit the sunlit side of Earth with hard X-rays. The X-rays penetrate to the D-region, releasing electrons that rapidly increase absorption, causing a high frequency (3–30&nbsp;MHz) radio blackout that can persist for many hours after strong flares. During this time very low frequency (3–30&nbsp;kHz) signals will be reflected by the D layer instead of the E layer, where the increased atmospheric density will usually increase the absorption of the wave and thus dampen it. As soon as the X-rays end, the sudden ionospheric disturbance (SID) or radio black-out steadily declines as the electrons in the D-region recombine rapidly and propagation gradually returns to pre-flare conditions over minutes to hours depending on the solar flare strength and frequency.

===Protons: polar cap absorption (PCA)===
{{Further|Solar particle event#Polar cap absorption events}}
Associated with solar flares is a release of high-energy protons. These particles can hit the Earth within 15 minutes to 2 hours of the solar flare. The protons spiral around and down the magnetic field lines of the Earth and penetrate into the atmosphere near the magnetic poles increasing the ionization of the D and E layers. PCA's typically last anywhere from about an hour to several days, with an average of around 24 to 36 hours. [[Coronal mass ejection]]s can also release energetic protons that enhance D-region absorption in the polar regions.

===Storms===
{{Main|Geomagnetic storm|Ionospheric storm}}
Geomagnetic storms and ionospheric storms are temporary and intense disturbances of the Earth's [[magnetosphere]] and ionosphere.

During a geomagnetic storm the F₂ layer will become unstable, fragment, and may even disappear completely. In the Northern and Southern polar regions of the Earth [[polar aurora|aurora]]e will be observable in the night sky.

===Lightning===
{{Further|Lightning}}
Lightning can cause ionospheric perturbations in the D-region in one of two ways. The first is through VLF (very low frequency) radio waves launched into the [[magnetosphere]]. These so-called "whistler" mode waves can interact with radiation belt particles and cause them to precipitate onto the ionosphere, adding ionization to the D-region. These disturbances are called "lightning-induced [[electron precipitation]]" (LEP) events.

Additional ionization can also occur from direct heating/ionization as a result of huge motions of charge in lightning strikes. These events are called early/fast.

In 1925, C. T. R. Wilson proposed a mechanism by which electrical discharge from lightning storms could propagate upwards from clouds to the ionosphere. Around the same time, Robert Watson-Watt, working at the Radio Research Station in Slough, UK, suggested that the ionospheric sporadic E layer (E<sub>s</sub>) appeared to be enhanced as a result of lightning but that more work was needed. In 2005, C. Davis and C. Johnson, working at the Rutherford Appleton Laboratory in Oxfordshire, UK, demonstrated that the E<sub>s</sub> layer was indeed enhanced as a result of lightning activity. Their subsequent research has focused on the mechanism by which this process can occur.