Editing Radio window (section)

=== Ionosphere ===
The radio window's lower frequency limit is greatly affected by the [[Ionospheric absorption|ionospheric refraction]] of the radio waves whose frequencies are approximately below 30 MHz (λ > 10 m);<ref>{{Cite book| last1=Anderson| first1=John B.| url=http://www.worldcat.org/oclc/56103934|title=Understanding information transmission|last2=Johannesson|first2=Rolf|publisher=[[IEEE Press]], [[Wiley-Interscience]]|year=2005|isbn=978-0-471-67910-3| location=Piscataway, NJ; Hoboken, NJ | pages=110|language=English | oclc=56103934}}</ref> radio waves with frequencies below the limit of 10 MHz (λ > 30 m) are reflected back into space by the ionosphere.<ref>{{Cite book| last1=Torge|first1=Wolfgang|url=http://www.worldcat.org/oclc/987088700|title=Geodesy|last2=Müller|first2=Jürgen|publisher=[[De Gruyter]]|year=2012|isbn=978-3-11-020718-7|location=Berlin| pages=121| language=English| oclc=987088700}}</ref> The lower limit is proportional to the density of the ionosphere's free [[electron]]s and coincides with the [[Plasma oscillation|plasma frequency]]:
<math display="block"> f_p = 9 \sqrt{N_e},</math>
where <math>f_p</math> is the plasma frequency in Hz and <math>N_e</math> the electron density in electrons per cubic meter. Since it is highly dependent on sunlight, the value of <math>N_e</math> changes significantly from daytime to nighttime usually being lower during the day, leading to a decrease of the radio window's lower limit and higher during the night, causing an increase of the radio window's lower frequency end. However, this also depends on the [[Solar phenomena|solar activity]] and the geographic position.<ref>{{Cite book |last1=Warnick |first1=Karl F. |url=http://www.worldcat.org/oclc/1032582026 |title=Phased arrays for radio astronomy, remote sensing and satellite communications |last2=Maaskant |first2=Rob |last3=Ivashina |first3=Marianna V. |author-link3=Marianna Ivashina |publisher=[[Cambridge University Press]] |year=2018 |isbn=978-1-108-42392-2 |pages=5 |language=English |oclc=1032582026}}</ref>