What causes deviations between Sun and blackbody?

[Sybren]

Hello all,

Does anybody have an idea why the solar spectrum, as measured above the Earth's atmosphere, has such bad agreement with the blackbody curve?

I would think that any deviations such as sunspots would be averaged out because of the distance, and that in general (if any deviation) there should be an overprediction (due to absorption along the way perhaps), but certainly not an underprediction as indicated in the visible range.

(Picture from Wikipedia)

 

[DrBloke]

Ozone is located in the lower stratosphere. It absorbs the harmful high frequency UV light. This why the intensity reaching Earth at the low wavelength region is less than expected, although it means the phrase "at top of the atmosphere" is misleading. When ozone absorbs UV it splits into oxygen atoms and molecules in excited electronic states. Some of these may relax into ground states with an accompanying emission of visible light. This could explain the excess of visible light. That more blue light is present than red suggests that Rayleigh scattering, the cause of the blueness of the sky may be of significance.

 

[Drakkith]

The sun isn't a perfect black body. Not only does it have cool gasses in its outer atmosphere, there is significant temperature difference in its layers. There are a multitude of factors that could cause a deviation in the spectrum emitted by the sun. In addition to the above, inside the core the nuclear fusion releases high energy photons that get degraded as they propegate to the outer layers of the sun and eventually into space. I doubt these photons will exactly match a black body.

 

[triclon]

The light coming from the photosphere (the "surface" of the sun) is approximately a black body but the atoms/ions in the thinner outer layers of the sun (ie. the chromosphere) absorb and re-emit certain wavelengths of light. That's what gives us the sun's absorption lines in its spectrum and causes it to not be a perfect black body.

 

[Bill_K]

there is significant temperature difference in its layers.

Yes, this is apparently the main reason. The light we see comes from the photosphere, which is several hundred km thick and semitransparent. Temperature at the bottom is 6000 °F compared to temperature at the top 4500 °F, so what we wind up seeing is a blend of these.

 

[DrBloke]

Yes, this is apparently the main reason. The light we see comes from the photosphere, which is several hundred km thick and semitransparent. Temperature at the bottom is 6000 °F compared to temperature at the top 4500 °F, so what we wind up seeing is a blend of these.

I see that two black bodies acting together would behave like a black body with a temperature between the two, but shouldn't it still behave like a perfect black body emitter at the intermediate temperature? Shouldn't the emission spectrum be the weighted sum of the two individual spectrum? Why then the excess of emission in the visible wavelengths and a deficit in the ultraviolet wavelengths? This is the main deviation from black body behaviour evident in the graph and this is due to absorption effects of the Earth's atmosphere.

EDIT: forgive me. I'm talking nonsense. I should read the labels more clearly on the graph. the yellow bit is before the light has gone through the atmosphere.

 

[Vanadium 50]

I see that two black bodies acting together would behave like a black body with a temperature between the two, but shouldn't it still behave like a perfect black body emitter at the intermediate temperature?

No. Think about an extreme case where you have very different temperatures.

 

[Drakkith]

EDIT: forgive me. I'm talking nonsense. I should read the labels more clearly on the graph. the yellow bit is before the light has gone through the atmosphere.

Hehe, yes that does help. Nevertheless, you were still correct, as the sun is NOT a perfect black body emitter for various reasons.