Theradio window is theregion of theradio spectrum thatpenetrate the Earth's atmosphere. Typically, the lower limit of the radio window's range has a value of about 10MHz (λ ≈ 30 m); the best upper limit achievable from optimal terrestrial observation sites is equal to approximately 1 THz (λ ≈ 0.3 mm).^[1][2]

It plays an important role in astronomy; up until the 1940s, astronomers could only use thevisible andnear infrared spectra for their measurements and observations. With the development ofradio telescopes, the radio window became more and more utilizable, leading to the development ofradio astronomy that providedastrophysicists with valuable observational data.^[3]

The lower and upper limits of the radio window's range of frequencies are not fixed; they depend on a variety of factors.

The upper limit is affected by the vibrational transitions of atmospheric molecules such asoxygen (O₂),carbon dioxide (CO₂), andwater (H₂O), whose energies are comparable to the energies ofmid-infraredphotons: these molecules largely absorb the mid-infrared radiation that heads towards Earth.^[4][5]

The radio window's lower frequency limit is greatly affected by theionospheric refraction of the radio waves whose frequencies are approximately below 30 MHz (λ > 10 m);^[6] radio waves with frequencies below the limit of 10 MHz (λ > 30 m) are reflected back into space by the ionosphere.^[7] The lower limit is proportional to the density of the ionosphere's freeelectrons and coincides with theplasma frequency:$${\displaystyle f_p=9\sqrt{N_e},}$$where${\displaystyle f_p}$ is the plasma frequency in Hz and${\displaystyle N_e}$ the electron density in electrons per cubic meter. Since it is highly dependent on sunlight, the value of${\displaystyle N_e}$ 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 thesolar activity and the geographic position.^[8]

When performing observations, radio astronomers try to extend the upper limit of the radio window towards the 1 THz optimum, since theastronomical objects givespectral lines of greater intensity in the higher frequency range.^[9]Tropospheric water vapour greatly affects the upper limit since itsresonant absorption frequency bands are 22.3 GHz (λ ≈ 1.32 cm), 183.3 GHz (λ ≈ 1.64 mm) and 323.8 GHz (λ ≈ 0.93 mm). The tropospheric oxygen's bands at 60 GHz (λ ≈ 5.00 mm) and 118.74 GHz (λ ≈ 2.52 mm) also affect the upper limit.^[10] To tackle the issue of water vapour, many observatories are built at high altitudes where the climate is more dry.^[11] However, little can be done to avoid the oxygen's interference with radio waves propagation.^[12]

The width of the radio window is also affected byradio frequency interference which hinders the observations at certain wavelength ranges and undermines the quality of the observational data of radio astronomy.^[13]

• Infrared window
• Optical window
• Radio propagation

• Electromagnetic spectrum

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