<blockquote>
  <p>Do some ELF (Extremely low frequency) radio waves pass the atmosphere reach Earth surface and then are reflected passing the atmosphere again to reach outer space?</p>
</blockquote>

<p>In space <a href="https://en.wikipedia.org/wiki/Plasma_(physics)" rel="nofollow">plasmas</a>, <a href="https://en.wikipedia.org/wiki/Extremely_low_frequency" rel="nofollow">ELF</a> is used to describe waves in the frequency band ~3 Hz to ~3 kHz.  <a href="https://en.wikipedia.org/wiki/Very_low_frequency" rel="nofollow">Very low frequency (VLF)</a> is used to describe waves in the frequency band ~3 kHz to ~30 kHz.</p>

<p>Generally the <a href="https://en.wikipedia.org/wiki/Cutoff_frequency" rel="nofollow">cutoff frequency</a> of the Earth's <a href="https://en.wikipedia.org/wiki/Ionosphere" rel="nofollow">ionosphere</a> prevents waves at such low frequencies from freely passing through it.  In plasmas, this tends to be any wave at or below the local electron <a href="https://en.wikipedia.org/wiki/Plasma_oscillation" rel="nofollow">plasma frequency</a>, which can be approximated as $f_{pe} \approx 8.9787 \ \sqrt{n_{e} \left[ cm^{-3} \right]}$ kHz (where the $\left[ cm^{-3} \right]$ implies that the electron number density, $n_{e}$, should be in units of $cm^{-3}$).  The densities in the ionosphere are ~$10^{3} - 10^{6} \ cm^{-3}$, which corresponds to $f_{pe}$ ~ 280 kHz to ~9 MHz.</p>

<p>Early <a href="http://adsabs.harvard.edu/abs/1953RSPTA.246..113S" rel="nofollow">studies</a> found a relationship between <a href="https://en.wikipedia.org/wiki/Lightning" rel="nofollow">lightning</a> and a VLF emissions called <a href="https://en.wikipedia.org/wiki/Whistler_(radio)" rel="nofollow">whistlers</a>.  It is now known that high energy electron beams, associated with the lightning, escape into the <a href="https://en.wikipedia.org/wiki/Magnetosphere" rel="nofollow">magnetosphere</a>.  These beams radiate the whistlers through a plasma <a href="https://en.wikipedia.org/wiki/Instability" rel="nofollow">instability</a> (i.e., free energy due to a fast beam streaming through an otherwise homogeneous plasma).  So the free energy sources can escape the lower atmosphere, but the electromagnetic emissions at such low frequencies cannot.</p>

<blockquote>
  <p>And once in outer space can they travel indefinetely in it or the interplabetary plasma would stop them?</p>
</blockquote>

<p>Generally, waves with $f &lt; f_{pe}$ experience <a href="https://en.wikipedia.org/wiki/Damping" rel="nofollow">damping</a> as they propagate through a plasma.  This is because the wave fields oscillate slow enough to interact with the particles, thus adding an effective inertia term to the system.  ELF and VLF waves generally damp through <a href="https://en.wikipedia.org/wiki/Electron_cyclotron_resonance" rel="nofollow">cyclotron</a> and Landau resonances.  Note, however, that both of these resonances are also the processes responsible for wave growth in the presence of sufficient free energy (e.g., free energy from beams is often related to a Landau-like or <a href="https://en.wikipedia.org/wiki/Cherenkov_radiation" rel="nofollow">Cerenkov</a>-like emission).</p>

<blockquote>
  <p>This occurs only when when the plasma is excited?</p>
</blockquote>

<p>Not necessarily.  We can artificially induce VLF emissions using <a href="https://en.wikipedia.org/wiki/Lualualei_VLF_transmitter" rel="nofollow">ground-based transmitters</a>.  But otherwise yes, there needs to be free energy for the radiation of electromagnetic waves.</p>

<blockquote>
  <p>When it is not, these radio waves would be able to make it through?</p>
</blockquote>

<p>See my comments above about the cutoff frequency.</p>

CC BY-SA 3.0
2016-08-10T13:33:02.577Z
2016-08-10T13:33:02.577Z
honeste_vivere
<p>...reach Earth surface and then are reflected passing the atmosphere again to reach outer space? And once in outer space can they travel indefinetely in it or the interplabetary plasma would stop them? This occurs only when when the plasma is excited? When it is not, these radio waves would be able to make it through?</p>

<universe><radio-astronomy><plasma-physics>
Do some ELF (Extremely low frequency) radio waves pass the atmosphere
noduagg
# Do some ELF (Extremely low frequency) radio waves pass the atmosphere

(Asked by: username_0 on 2016-08-09)

...reach Earth surface and then are reflected passing the atmosphere again to reach outer space? And once in outer space can they travel indefinetely in it or the interplabetary plasma would stop them? This occurs only when when the plasma is excited? When it is not, these radio waves would be able to make it through?

**Tags:** universe, radio-astronomy, plasma-physics


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## 1 answers

### Answer by username_1 on 2016-08-10. Score: 2

> Do some ELF (Extremely low frequency) radio waves pass the atmosphere reach Earth surface and then are reflected passing the atmosphere again to reach outer space?

In space [plasmas](https://en.wikipedia.org/wiki/Plasma_(physics)), [ELF](https://en.wikipedia.org/wiki/Extremely_low_frequency) is used to describe waves in the frequency band ~3 Hz to ~3 kHz. [Very low frequency (VLF)](https://en.wikipedia.org/wiki/Very_low_frequency) is used to describe waves in the frequency band ~3 kHz to ~30 kHz.

Generally the [cutoff frequency](https://en.wikipedia.org/wiki/Cutoff_frequency) of the Earth's [ionosphere](https://en.wikipedia.org/wiki/Ionosphere) prevents waves at such low frequencies from freely passing through it. In plasmas, this tends to be any wave at or below the local electron [plasma frequency](https://en.wikipedia.org/wiki/Plasma_oscillation), which can be approximated as $f\_{pe} \approx 8.9787 \ \sqrt{n\_{e} \left[ cm^{-3} \right]}$ kHz (where the $\left[ cm^{-3} \right]$ implies that the electron number density, $n\_{e}$, should be in units of $cm^{-3}$). The densities in the ionosphere are ~$10^{3} - 10^{6} \ cm^{-3}$, which corresponds to $f\_{pe}$ ~ 280 kHz to ~9 MHz.

Early [studies](http://adsabs.harvard.edu/abs/1953RSPTA.246..113S) found a relationship between [lightning](https://en.wikipedia.org/wiki/Lightning) and a VLF emissions called [whistlers](https://en.wikipedia.org/wiki/Whistler_(radio)). It is now known that high energy electron beams, associated with the lightning, escape into the [magnetosphere](https://en.wikipedia.org/wiki/Magnetosphere). These beams radiate the whistlers through a plasma [instability](https://en.wikipedia.org/wiki/Instability) (i.e., free energy due to a fast beam streaming through an otherwise homogeneous plasma). So the free energy sources can escape the lower atmosphere, but the electromagnetic emissions at such low frequencies cannot.

> And once in outer space can they travel indefinetely in it or the interplabetary plasma would stop them?

Generally, waves with $f < f\_{pe}$ experience [damping](https://en.wikipedia.org/wiki/Damping) as they propagate through a plasma. This is because the wave fields oscillate slow enough to interact with the particles, thus adding an effective inertia term to the system. ELF and VLF waves generally damp through [cyclotron](https://en.wikipedia.org/wiki/Electron_cyclotron_resonance) and Landau resonances. Note, however, that both of these resonances are also the processes responsible for wave growth in the presence of sufficient free energy (e.g., free energy from beams is often related to a Landau-like or [Cerenkov](https://en.wikipedia.org/wiki/Cherenkov_radiation)-like emission).

> This occurs only when when the plasma is excited?

Not necessarily. We can artificially induce VLF emissions using [ground-based transmitters](https://en.wikipedia.org/wiki/Lualualei_VLF_transmitter). But otherwise yes, there needs to be free energy for the radiation of electromagnetic waves.

> When it is not, these radio waves would be able to make it through?

See my comments above about the cutoff frequency.


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