# Astronomical-wavelength radio (AWR) transmissions between cosmic plasmas?

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My son asked me if electromagnetic waves longer than radio exist. I told him that even though physics permits such waves, there are no antennas long enough to radiate or detect them.

However, on further thought I realized that is not quite correct.

Supernovas like the Crab Nebula, as well as other astronomical phenomena such as cosmic jets, routinely create continuous plasmas with conductive lengths ranging from planetary distances to light years across. Such plasmas are also often in violent motion. Without (yet) attempting any detailed calculations, I'm pretty confident that violent motion in such giant conductive constructs should cause them to radiate in astronautical wavelength radio bands (AWR, and yes, I just now made that up), possibly powerfully.

But how would one detect such emissions, even if they do exist?

That's where the hair-of-the-dog-that-bit-you principle comes in: A sufficiently powerful AWR broadcast should be received and converted into large-scale plasma currents by any nearby cosmic-scale plasmas of the same general type.

So, my question if anyone who is willing to take a whack at something this speculative:

If AWR emissions exist and are powerful enough to create slowly alternating, long-duration current flows within cosmic-scale plasmas, are there experimentally detectable optical or short-radio phenomena -- e.g. polarization effects, or faster cooling of transmitting plasmas due to unexpected energy losses, or unexpected energy gains in receiving plasmas -- by which the emission or receipt of AWR could be detected?

I realize this is as much an astronomy question as a physics question, but if my physics argument is drastically wrong somewhere, or is correct but does not produce experimentally detectable phenomena, the astronomy part doesn't matter much.

2013-06-08 -- The discussion is interesting, but perhaps drifted a bit towards the question of much shorter (and directly detectable) VLF waves. Eduardo make the excellent point that the interstellar medium is opaque to VLF. By simple generalization of VLF behavior to far lower frequencies, that may also mean that astronomical-scale EM waves are also not possible.

It's worth emphasizing that even if something within the astronomy zoo is able to generate powerful AWR band emissions, and even if such transmissions could make it to earth, I'm pretty sure no antenna on earth would be able to pick up those transmissions.

That in turn emphasizes that the only readily apparent way to detect AWR band energy transfers, if they exist at all, would be by them enabling unexpected modes of transfer of energy between visible astronomical plasmas that would act as both AWR generators and receivers.

Eduardo's excellent point about the opacity of the interstellar plasma may provide the simplest answer: AWR cannot exist because interstellar plasmas will never allow it to.

However, that same VLF opacity issue brings up at least two more questions:

1. Opacity at VLF may not necessarily guarantee interstellar opacity in the AWR band, which after all would be orders of magnitude lower in frequency. Speculating wildly, AWR might for example be so gentle in its ion-level impact that the interstellar plasma ends up being largely transparent to it. And yes, that really is just a wild speculation, nothing more.

2. Even if the interstellar plasma is highly absorbing of AWR, any major generator of AWR band energy would still likely result in some sort of unexpectedly higher or faster outward transfer of energy surrounding the AWR generator. More specifically: if (big if_ AWR exists and can be generated at high power levels by self-excitation of currents in violently moving astronomically sized plasmas (think Crab Nebula), but is also absorbed very quickly by the interstellar medium, it should still be indirection detectable by its enabling of otherwise inexplicable increases in the rate or speed at which energy dissipates outward from the event into the surrounding interstellar plasma. Such an effect would be indirectly detectable by a lack of explanation for it from other known and well-modeled energy transfer modes.

I realize this has likely turned into an unfair question at this point, since the range of issues that would need to be analyzed is pretty high. It seems likely the idea of a AWR band that could (maybe!) exist, and even have indirectly observable effects on visible phenomena such as supernova cloud expansions, has not been explored much, if at all. That's interesting all by itself.

I'll probably either award Eduardo the answer soon for best coverage of the issue so far, or perhaps I'll even try a bonus. I am a bit intrigued by this one.

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Terry Bollinger
related: physics.stackexchange.com/q/65068/4552

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Ben Crowell
If one of these plasmas was going to radiate such a wave, the charges would have to be moving coherently. Also, aren't plasmas opaque to electromagnetic waves? Any such waves might get reabsorbed near the source.

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Ben Crowell
Ben, thanks... good reference, and both good points. For coherency, my suspicion (nothing more) is that the large-scale similarities in the geometry of e.g. an expanding supernova plasma sphere may in turn create similarly large-scale coherent current flows, with of course currents at many other smaller scales also. Very good point about transparency... though hmm, I suspect that the the wavelength itself of an AWR band emission might make the situation not that different from an EM-opaque metallic antenna radiating and receiving. In fact, the plasma will have to be partially opaque to absorb.

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Terry Bollinger
IIRC the ISM becomes opaque for frequencies at and below the kilohertz-ish band (somewhere around there is the typical plasma frequency for the typical free electron density).

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Chris White
Some discussion here cv.nrao.edu/course/astr534/Pulsars.html of the Crab Nebula pulsar. Its dipole moment is misaligned with its rotational axis, so it's an incredibly powerful 30 Hz dipole radiator. According to WP en.wikipedia.org/wiki/Extremely_low_frequency this 30 Hz radiation doesn't reach us because it's below the plasma frequency of the interstellar medium. It's immediately reabsorbed by the nebula, and this is what causes the nebula to glow.

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Ben Crowell
@BenCrowell, the book I quoted (Choudhuri 2010) says "The plasma frequency of the Earth's ionosphere is about 30 MHz. Radio waves from cosmic sources can penetrate through the ionosphere only if the frequency is higher than 30 MHz"

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Eduardo Guerras Valera
@BenCrowell your assertion about what causes the nebula to glow is not correct - that is what WP states is not correct. The emission that we see at lower frequencies in the nebula proper is merely caused by lower energy plasma; plasma which has cooled via synchrotron/inverse Compton emission and the adiabatic expansion of the nebula bubble. The thermal emission way out past the termination shock in the nebula proper has little or nothing to do with the pulsed radiative emission from within the magnetosphere.

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Killercam

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This is not an answer, but merely some information to put you on the right track, too long to be in a comment. I am curious too, to see what other users with more information about the question may answer.

Cold plasma models (where the analysis is not complicated by thermal motions) consist essentially in electrons that are nearly free to move among heavier ions. There is an associated quantity $\omega_{p}$, the plasma frequency:

$$\omega_{p}^2 = \frac{n e^2}{\epsilon_0 m_e}$$

where $n$ is the number density of electrons. For a propagating electromagnetic wave, this relation holds:

$$\omega^2=\omega_p^2 + k^2 c^2$$

as derived in 8.13 in Choudhuri 2010

So that, for an EW with much bigger frequency than $\omega_p$,

$$\omega=k c$$ as with any usual EW, which means that the electrons are too massive to respond to such a high frequency, thus the plasma is transparent for such waves.

But if the frequency is lower than $\omega_p$, there is no real solution for $k$. I guess that means that the incident wave is partially absorbed as an evanescent wave, and the rest is reflected back, but I am no expert on this. The question is that low frequency waves cannot propagate within plasma, because the electrons are light enough to "dance" with the frequency of the incoming wave, thus absorbing (and re-radiating?) the energy.

Another question is the movement of a supernova plasma sphere, as you say in your comment... well, that's a bit like cheating. In the same sense, Voyager II is a metalic object that has been slowly moving along 1 light-day during the last decades. That is a loooong, (incredibly weak) EW what it has created, right? (yeah, -1 for this paragraph!). If I understood your question, you probably are wondering if there is in nature something big enough, electrically non-neutral at that size scale, that moves fast enough to create that giant EW. A binary black hole?

Books have data about $\omega_p$ for ionospheric plasma and things like that. I wonder what is the value of that free electron density of interstellar medium in Chris's comment (again, I am no expert on this). But I remember to have read that there is a delay in pulsar signals for low frequencies, that is attributed to the interaction with free electrons along the line-of-sight. I would be happy to read something more technical from any user.

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Eduardo Guerras Valera
answered Jun 3, 2013 by (435 points)
Eduardo, thanks, many interesting suggestions in that. I think on the scale thing the image in my mind was the earth's self-exciting generation of electrical currents and magnetic fields due to massive movements and circulations of conductive molten metal. @ChrisWhite, that sounds right; see e.g. the last paragraph on ELF blocking around magnetars in this Wikipedia article. So would the same blocking effect definitely extend this far down, or would new transparency windows emerge for some reason? Dunno...

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Terry Bollinger
@TerryBollinger, +1, very interesting link! I had always wondered how signals were sent to submarines...

This post imported from StackExchange Physics at 2014-06-14 12:58 (UCT), posted by SE-user Eduardo Guerras Valera

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