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I've read some papers using the term "Rayleigh-Jeans tail" but cannot find a general definition. I would infer from context that it refers to the blackbody emission spectrum in the range of wavelengths that are long enough that the emission can be approximated by the Rayleigh-Jeans law. Is this correct?

Example references: "We show that, despite stringent constraints on the shape of the main part of the cosmic microwave background (CMB) spectrum, there is considerable room for its modification within its Rayleigh-Jeans (RJ) end, ω ≪ TCMB.". PHYSICAL REVIEW LETTERS 121, 031103 (2018).

"However, in the FUV band, the Rayleigh-Jeans (RJ) tail of the ∼ 10e5 K surface emission may be dominant and detectable by the HST." https://arxiv.org/abs/1901.07998#:~:text=Assuming%20a%20blackbody%20spectrum%2C%20we,models%20of%20old%20neutron%20stars.

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As far as I know, the two ends of the black-body radiation curve is historically described using the Rayleigh-Jeans law and the Wien law, as seen on Wikipedia for example

Picture from https://en.wikipedia.org/wiki/Rayleigh%E2%80%93Jeans_law

The Rayleigh-Jeans law is the low frequency limit of the full curve, where the spectral radiance is inversely proportional to the wavelength to the fourth power.

I am not entirely sure that this is the "tail" that you are referencing to, but I've personally encountered the term Rayleigh-Jeans tail in the context of the low frequency end of the radiation curve (in general, not only for black-body radiation).

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The Rayleigh-Jeans tail (of a blackbody distribution) is simply where you can assume $h\nu \ll k_BT$, where $\nu$ is the frequency.

In this regime, you can simplify the Planck function, by allowing $\exp(h\nu/k_BT) \approx 1 + h\nu/k_BT$, to show that the specified intensity is proportional to $\nu^2$.

It is effectively the "classical regime", where the energy quantum of light is much smaller than the equipartition energy.

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