Saturday, July 25, 2015

Limb darkening: SDO 160 nm

I wrote a lot about limb darkening in the previous posts and tried to carefully explain that the Sun gets darker when you go from the center of the disk to the limb. This effect is stronger for shorter wavelengths (ultraviolet) than for longer wavelengths (infrared). Guess what: Today I show you that this is not true.

Well, to understand that I have to explain that what I wrote about previously is correct for the photosphere of the star. I mentioned this in another blog already. If we talk about the photosphere, everything I wrote and showed before is correct. However, if I show you how the limb darkening of the Sun really looks like in an image, you will probably notice that the solar disk is brighter in the UV than in visual light.

Again I show a picture of the Solar Dynamics Observatory (SDO), however, this time not in the visual but in the ultraviolet (160 nm, see left image). Compared to the optical, the Sun has much more structure in this image.

In the top figure I present the brightness of the Sun in this wavelength from one edge to the other. To create this diagram, I used several dozen SDO images taken over a month, and averaged their brightnesses. Shown is the mean brightness and the error bars indicate how variable this brightness was in the course of one month. If you compare this graph to the one for the visual, you will notice that the limb darkening of the Sun is actually weaker in the UV. So comparing these two images you see the opposite of what I told you in the beginning and all the previous post on limb darkening.

The solution to this problem is: What you seen in this UV image of the Sun is not really the photosphere. You see a different part of the solar atmosphere which is slightly above the layer you see in the visual image of a previous post. You start seeing the chromosphere. And the chromosphere is different. There the Sun is hotter and a lot of light is emitted in shorter wavelengths. Because you see the emitting regions of the chromosphere better at the limb than in the center, you get more light from the limb. And the combination of all the light coming from different layers in the Sun in this UV wavelength interval makes up the figure I show in the top.

In this wavelength the brightness is still not higher in the limb than in the center, but in the next post I will show you a wavelength regime where we do not have limb darkening anymore - but limb brightening. With your naked eyes you will never see that because this is light you cannot receive with the human eye. So for us, the Sun has limb darkening.

I maybe should mention in the end, that the absolute brightness of the light coming from the chromosphere is much less than what you get from the photosphere. I always show normalized brightnesses, so this is something you cannot see in the figures. However, in absolute numbers there is much more light coming from the photosphere - which is basically the reason why we see it with the eye. But the relative contribution in the UV grows, which is the reason why we start detecting limb brightening for smaller wavelengths.