Editing Interstellar medium (section)

== Radiowave propagation ==
[[File:Micrwavattrp.png|thumb|400px|Atmospheric attenuation in [[decibel|dB]]/km as a function of frequency over the EHF band. Peaks in absorption at specific frequencies are a problem, due to atmosphere constituents such as water vapor (H<sub>2</sub>O) and carbon dioxide (CO<sub>2</sub>).]]

Radio waves are affected by the plasma properties of the ISM. The lowest frequency radio waves, below ≈ 0.1&nbsp;MHz, cannot propagate through the ISM since they are below its [[plasma frequency]].  At higher frequencies, the plasma has a significant refractive index, decreasing with increasing frequency, and also dependent on the density of free electrons. Random variations in the electron density cause interstellar [[Twinkling|scintillation]], which broadens the apparent size of distant radio sources seen through the ISM, with the broadening decreasing with frequency squared. The variation of refractive index with frequency causes the arrival times of pulses from [[pulsars]] and [[Fast radio bursts]] to be delayed at lower frequencies (dispersion). The amount of delay is proportional to the column density of free electrons (Dispersion measure, DM), which is useful for both mapping the distribution of ionized gas in the Galaxy and estimating distances to pulsars (more distant ones have larger DM).<ref>{{cite web | url=https://www.youtube.com/watch?v=kvKDoQVevAI | archive-url=https://ghostarchive.org/varchive/youtube/20211114/kvKDoQVevAI| archive-date=2021-11-14 | url-status=live| title= Interstellar Medium Interference (video) | author=Samantha Blair | work=SETI Talks| date=15 June 2010}}{{cbignore}}</ref>

A second propagation effect is [[Faraday rotation]], which affects [[Linear polarization|linearly polarized]] radio waves, such as those produced by [[synchrotron radiation]], one of the most common sources of radio emission in astrophysics. Faraday rotation depends on both the electron density and the magnetic field strength, and so is used as a probe of the interstellar magnetic field.

The ISM is generally very transparent to radio waves, allowing unimpeded observations right through the disk of the Galaxy. There are a few exceptions to this rule. The most intense [[spectral lines]] in the radio spectrum can become opaque, so that only the surface of the line-emitting cloud is visible. This mainly affects the carbon monoxide lines at millimetre wavelengths that are used to trace molecular clouds, but the [[21-cm line]] from neutral hydrogen can become opaque in the cold neutral medium. Such absorption only affects photons at the line frequencies: the clouds are otherwise transparent. The other significant absorption process occurs in dense ionized regions. These emit photons, including radio waves, via thermal [[bremsstrahlung]]. At short wavelengths, typically [[microwaves]],  these are quite transparent, but their brightness approaches the [[black body]] limit as <math>\propto \lambda^{2.1}</math>, and at wavelengths long enough that this limit is reached, they become opaque. Thus metre-wavelength observations show H&nbsp;II regions as cool spots blocking the bright background emission from Galactic synchrotron radiation, while at decametres the entire galactic plane is absorbed, and the longest radio waves observed, 1&nbsp;km, can only propagate 10-50 parsecs through the Local Bubble.<ref>{{Cite journal |last1=Novaco |first1=J. C. |last2=Brown |first2=L. W. |date=1978 |title=Nonthermal galactic emission below 10 megahertz |url=http://adsabs.harvard.edu/doi/10.1086/156009 |journal=The Astrophysical Journal |language=en |volume=221 |pages=114 |doi=10.1086/156009 |bibcode=1978ApJ...221..114N |issn=0004-637X}}</ref> The frequency at which a particular nebula becomes optically thick depends on its ''emission measure''
:<math>EM = \int n_e^2\, dl</math>,
the [[column density]] of squared electron number density. Exceptionally dense nebulae can become optically thick at centimetre wavelengths: these are just-formed and so both rare and small ('Ultra-compact H&nbsp;II regions')

The general transparency of the ISM to radio waves, especially microwaves, may seem surprising since radio waves at frequencies > 10&nbsp;GHz are significantly attenuated by Earth's atmosphere (as seen in the figure). But the column density through the atmosphere is vastly larger than the column through the entire Galaxy, due to the extremely low density of the ISM.