edit: At the Solar Dynamics Observatory (SDO) website, I just found the image sdo.gsfc.nasa.gov/assets/img/latest/latest_1024_HMIIC.jpg. The color gradient of the limb darkening seems very similar to the Wikimedia image below. I've discovered that it is called a "colorized intensitygram" and the color gradient is purely artificial - the data is single channel intensity. The limb darkening is certainly real (compare to the artificially "flattented" display!)
I would like to try to start to understand what the sun actually "looks like", with human vision, assuming the brightness has been reduced.
Like the gas giant planets, the sun doesn't have an abrupt surface, it just gets denser and hotter and denser and hotter as you go deeper.
One consequence of this is limb-darkening. As the material becomes denser the deeper you go, it becomes more opaque, so - roughly speaking - the light you see, including the color and brightness, is determined by the layers above that point.
If you look at center of the solar disk, you can see deeper and hotter parts. If you look near the edge of the sun, or solar limb, the incidence is oblique and you are not seeing as deep or as hot.
The solar physics and photon transport theory is complicated, but in the visible part of the spectrum it may be OK to think of blue light scattering much more strongly than red light. In the thick, dense solar atmosphere, it scatters enough to attenuate. So in blue light you are seeing even shallower, which is even colder, and therefore dimmer in blue.
Right now I'd just like some good analytical approximation of the wavelength dependent limb darkening of the sun, or some actual linear images (before web-processing) of the sun at various visible wavelengths so I can make my own approximation.
Here is an image from Wikimedia titled 2012_Transit_of_Venus_from_SF which I've separated into RGB channels and plotted brightness across horizontal (solid) and vertical (dashed) diameters, 20 pixels wide. You can see the dramatic difference in behavior of different wavelengths. Since the image is "from the internet" I've no information about the linearity, so it's an illustrative example, but not what you'd think of as data.
import numpy as np import matplotlib.pyplot as plt img = plt.imread("sun limb darkening.png") # FROM: https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/2012_Transit_of_Venus_from_SF.jpg/600px-2012_Transit_of_Venus_from_SF.jpg w, c, hw = 600, 300, 10 # image happens to be 600x600 pixels square! hor = img[ c-hw:c+hw].sum(axis=0) / (2*hw) ver = img[ :, c-hw:c+hw].sum(axis=1) / (2*hw) plt.figure() plt.imshow(img) r, g, b = c - 0.67*c*hor.T plt.plot(r, '-r') plt.plot(g, '-g') plt.plot(b, '-b') r, g, b = c - 0.67*c*ver.T plt.plot(r, '--r') plt.plot(g, '--g') plt.plot(b, '--b') plt.plot([0, w], [c, c], '-k') plt.xlim(0, 599) plt.ylim(599, 0) plt.text(150, 137, 'R', fontsize=18) plt.text(156, 192, 'G', fontsize=18) plt.text(170, 272, 'B', fontsize=18) plt.show()