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After much reading, I think I'm now getting a much better understanding of how all this fits together ….
The Big Bang/Lambda CDM Theory says that following the Reionisation Epoch (near z~6), (by which time, most of the intergalactic medium had been photoionised to a temperature of about 10^4K), the predominant heating mechanism was through thermal shocks that developed as large-scale density waves collapsed in the dark matter. This pushed the temperature up to the 10^5K region in the volume for z<1. Systems and galaxies develop with temperatures of 10^7 to 10^8K, which can be observed by X-Ray telescopes.
So, what was predicted by the Big Bang/Lambda CDM model, was that these 'WHIM' filaments may be detected at X-ray wavelengths at a volume of z<1, and would be produced by thermal bremmstrahlung emission.
So what they've now observed is:
- soft X-Ray emissions in two energy bands: 0.9-1.3 keV and 0.5-2.0 keV. ('Soft' X-Ray emission is from about 0.12 keV to 12 keV).
- the thermal temperature range predicted by theory, translates to 0.0086-0.86 keV (10^5 to 10^7 K), so what they've detected falls (somehwhat) within these bands.
The thermal bremmstrahlung radiation is given off by electrons, as they are scattered by the strong electric field near the other high speed nuclei (ionised) in the filaments. These X-rays have a continuous spectrum. So, this is the source of the X-Ray emissions.
Now, I personally think that the term 'Birkeland currents' is hugely generalised and not particulary well-defined. The spectral emission energies in Earth's Auroras, can range anywhere from 1 to 200 keV, which still overlaps with what Amelia et al have detected. (As avery rough example of a well-known Birkeland Current phenomenon).
As such, it appears that the only reasons I can find, to rule out that what Amelia et al have observed may be a 'Birkeland Current', is a well-defined, empircally plausible, cause mechanism (ie: some evidence of a huge, self-sustaining intergalactic battery at either end of the filaments, to create the initial current flow in the filament). The self-sustaining mechanism which ensues, requires charge separation to be maintained continuously in the filament and it is a complete mystery as to how this can happen, unless energy is continuously added to the filament. If reconnection conditions occur, energy would be lost, and distinctive emissions would be able to be detected. So where could this big-battery possibly come from ? .. all sorts of story-telling then ensues, with little/no specific observational evidence, for the case in hand .. ie: for example, take the filament from Abell 2829 to Abell 0118 .. ~ 20 Mpc in length ? Are Abell 2829 and Abell 0118 somehow able to act together as a giant anode/cathode pair ?
The BB/Lambda CDM model on the other hand, defines a cause mechanism and made the prediction, and Amelia et al found the smokin' gun !
That's my take on it all.
Has anyone ever seen any papers which classify the spectra of Birkeland Currents having typical space-bound plasma environments ? (This information may help to differentiate a BC from an ionic gas plasma filament, at least spectroscopically).
Cheers
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