Editing Ionosphere (section)

===D layer===
The D layer is the innermost layer, {{convert|48|to|90|km|mi|abbr=on}} above the surface of the Earth. Ionization here is due to [[Lyman series]]-alpha hydrogen radiation at a [[wavelength]] of 121.6 [[nanometre]] (nm) ionizing [[nitric oxide]] (NO). In addition, [[solar flares]] can generate hard X-rays (wavelength {{nowrap|< 1 nm}}) that ionize N{{sub|2}} and O{{sub|2}}. Recombination rates are high in the D layer, so there are many more neutral air molecules than ions.

Medium frequency (MF) and lower high frequency (HF) [[radio wave]]s are significantly attenuated within the D layer, as the passing radio waves cause electrons to move, which then collide with the neutral molecules, giving up their energy. Lower frequencies experience greater absorption because they move the electrons farther, leading to greater chance of collisions. This is the main reason for [[Ionospheric absorption|absorption of HF radio waves]], particularly at 10&nbsp;MHz and below, with progressively less absorption at higher frequencies. This effect peaks around noon and is reduced at night due to a decrease in the D layer's thickness; only a small part remains due to [[cosmic rays]]. A common example of the D layer in action is the disappearance of distant AM [[broadcast band]] stations in the daytime.

During [[solar proton event]]s, ionization can reach unusually high levels in the D-region over high and polar latitudes. Such very rare events are known as Polar Cap Absorption (PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region.<ref name="Rose1962">{{cite journal|last1=Rose|first1=D.C.|last2=Ziauddin|first2=Syed|title=The polar cap absorption effect|journal=Space Science Reviews|date=June 1962|volume=1|issue=1|page=115|doi=10.1007/BF00174638|bibcode=1962SSRv....1..115R|s2cid=122220113}}</ref> In fact, absorption levels can increase by many tens of dB during intense events, which is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24 to 48 hours.