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Observation of ELF etc in universe?
First some background and then a question or two.
Some years ago I had a debate with Ted Bunn concerning the stated 411 photons/cm^3 in the universe. I maintained that the no. of photons at low frequencies (ELF ULF etc) probably vastly exceeds the CMBR count of 411. Ted was totally dismissive until he saw the graphic at http://ray.tomes.biz/ref-back.gif from "Cosmology" by Michael Rown-Robinson page 100 after which he agreed that it was possible but said that he still thought that I was wrong. The graph shows that below about 10^9 Hz the intensity of the universal background again begins to rise through to 10^6 Hz where the graph finishes. My questions: Has any observation been made below 10^6 Hz of the general background spectrum? [[Mod. note -- I think very little radio astronomy of any kind has been done at such low frequencies. Indeed, I'm not even sure if the Earth's ionosphere is even transparent at 1 MHz frequencies; if not, the observations would have to be done from space. Try searching on the ADS or scholar.google.com for 'low frequency radio astronomy'. -- jt]] I saw a TV interview with an Australian radio astronomer who said that he was studing low frequency and that the sky was as bright as the Sun all over. Unfortunately I didn't get his name and the frequency range wasn't mentioned. What do other people think when they look at this graph? It rises by 16 orders of magnitude as the frequency reduces by 18 orders of magnitude. It looks like a general trend to me with a few bumps on - CMBR, effect of dust etc. - although admittedly the CMBR is a big bump. Do others see the possibility that from 10^6 Hz through to 10^-17 Hz it could rise at the same average rate and reach 10^20 times as many photons as in the CMBR. Of course the much lower energy per photon would mean that the total energy is not increasing by that much, but it could still be a significant contribution to dark matter. -- Ray Tomes http://ray.tomes.biz/ http://www.cyclesresearchinstitute.org/ |
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Ray Tomes wrote:
My questions: Has any observation been made below 10^6 Hz of the general background spectrum? [[Mod. note -- I think very little radio astronomy of any kind has been done at such low frequencies. Indeed, I'm not even sure if the Earth's ionosphere is even transparent at 1 MHz frequencies; if not, the observations would have to be done from space. Try searching on the ADS or scholar.google.com for 'low frequency radio astronomy'. -- jt]] Even more specifically, check on "Radio Astronomy Explorer", also known as Explorer 49, which unfurled two quite long dipoles in lunar orbit (a few thousand km of rock does manage to block all that nasty emission from stuff om the geomagnetic field). If I read an NRL page correctly, it did measurements down to about 250 kHz, while below 30 kHz, the ISM absorbs the radiation. (That might limit how much ambient density you could have at the lowest frequencies, but this is a radiative-transfer regime I'd be bound to screw up on the back of an envelope). Bill Keel |
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Our esteemed moderator writes
Mod. note -- I think very little radio astronomy of any kind has been done at such low frequencies. The amount of artificial radio (communication) noise would be utterly overwhelming... Indeed, I'm not even sure if the Earth's ionosphere is even transparent at 1 MHz frequencies; Medium wave... The ionosphere is reflective which is why medium wave has very long range as it bounces between earth and sky. However I think long wave (200kHz or so) is transparent since the range is about a wavelength or so. Originally this was chosen because it was expected to get some 'diffraction over [under] the horizon'. VLF circa 10-20kHz is used for very long range submarine communication (one way) since it penetrates 'quite deeply' into seawater. I think many repeater stations are required so presumably the ionosphere is transparent here too. I believe a radio telescope for these frequencies has been proposed, on the far side of the moon. -- Oz This post is worth absolutely nothing and is probably fallacious. Use functions]. BTOPENWORLD address has ceased. DEMON address has ceased. |
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In message , Ray Tomes
writes First some background and then a question or two. Has any observation been made below 10^6 Hz of the general background spectrum? [[Mod. note -- I think very little radio astronomy of any kind has been done at such low frequencies. Indeed, I'm not even sure if the Earth's ionosphere is even transparent at 1 MHz frequencies; if not, the observations would have to be done from space. Try searching on the ADS or scholar.google.com for 'low frequency radio astronomy'. -- jt]] The Voyager planetary radio astronomy experiment apparently covered the range 1.2 kHz to 1.228 MHz http://www.igpp.ucla.edu/pds2/VG_1601/DOCUMENT/PRA/PRAINST.HTM. I see Bill Keel has noted the Radio Astronomy Explorers and absorption in the ISM; I'll just comment that the absorption is mentioned in Arthur Clarke's novel Imperial Earth, where this LF astronomy plays a significant role. -- mail to jsilverlight AT merseia DOT fsnet DOT co DOT uk is welcome. |
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"RT" == Ray Tomes writes:
RT Some years ago I had a debate with Ted Bunn concerning the stated RT 411 photons/cm^3 in the universe. I maintained that the no. of RT photons at low frequencies (ELF ULF etc) probably vastly exceeds RT the CMBR count of 411. RT Ted was totally dismissive until he saw the graphic at RT http://ray.tomes.biz/ref-back.gif from "Cosmology" by Michael RT Rown-Robinson page 100 [...] RT The graph shows that below about 10^9 Hz the intensity of the RT universal background again begins to rise through to 10^6 Hz where RT the graph finishes. RT My questions: RT Has any observation been made below 10^6 Hz of the general RT background spectrum? As Bill Keel has pointed out already, there have been space-based observations below 1 MHz. There were actually two Radio Astronomy Explorers. Also, there have been any number of spacecraft observing either the Sun or the planets that have made observations below 1 MHz. [...] RT What do other people think when they look at this graph? It rises RT by 16 orders of magnitude as the frequency reduces by 18 orders of RT magnitude. It looks like a general trend to me with a few bumps on RT - CMBR, effect of dust etc. - although admittedly the CMBR is a RT big bump. RT Do others see the possibility that from 10^6 Hz through to 10^-17 RT Hz it could rise at the same average rate and reach 10^20 times as RT many photons as in the CMBR. No. Below a certain frequency, the waves cannot propagate. The interstellar (and presumably intergalactic) medium has a plasma frequency, given by 9 kHz*\sqrt{n_e} where n_e is the electron density in units of cm^{-3}. In the local interstellar medium, n_e ~ 0.025 cm^{-3}, so waves with frequencies below 1 kHz just don't propagate. -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
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Thanks to everyone for the many useful comments and references.
Joseph Lazio wrote: No. Below a certain frequency, the waves cannot propagate. The interstellar (and presumably intergalactic) medium has a plasma frequency, given by 9 kHz*\sqrt{n_e} where n_e is the electron density in units of cm^{-3}. In the local interstellar medium, n_e ~ 0.025 cm^{-3}, so waves with frequencies below 1 kHz just don't propagate. What happens to an e/m wave below that cuttoff frequency? The energy must go somewhere. [[Mod. note -- If you have some source radiation below the cutoff frequency, the light will be absorbed by the interstellar medium. Think "light bulb outdoors on a foggy day". -- jt]] -- Ray Tomes http://ray.tomes.biz/ http://www.cyclesresearchinstitute.org/ |
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Ray Tomes wrote:
What happens to an e/m wave below that cuttoff frequency? The energy must go somewhere. And the moderator noted: [[Mod. note -- If you have some source radiation below the cutoff frequency, the light will be absorbed by the interstellar medium. Think "light bulb outdoors on a foggy day". -- jt]] Although the fog mainly scatters the light it cannot absorb it without re-radiating at some other frequency. What I would like to know is the equivalent thing for low frequency e/m waves between the galaxies? There must be a resulting distribution of frequencies that the low frequencies get converted to surely? -- Ray Tomes http://ray.tomes.biz/ http://www.cyclesresearchinstitute.org/ |
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Ray Tomes wrote:
Although the fog mainly scatters the light it cannot absorb it without re-radiating at some other frequency. What I would like to know is the equivalent thing for low frequency e/m waves between the galaxies? There must be a resulting distribution of frequencies that the low frequencies get converted to surely? If EM radiation is below the plasma frequency, it doesn't propagate at all, but is reflected. If the transmitter is embedded in a plasma it doesn't radiate, or (if strong enough) blows a bubble in the plasma due to radiation pressure. Some of the energy also gets dissipated as heat due to the plasma's nonzero resistivity. Paul |
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