The magenta is a sunset as seen from space.
In situations where the light that comes to your eye is not direct from the source, that light is scattered or bounced off of the medium it is traveling through. The more the light scatters and bounces, the more of it can change path and make it to your eye. Short (blue, indigo, violet) wavelengths scatter and bounce more. That is why the sky is blue - it is lit indirectly and more of the blue bounces back down to your eye. That is why water is blue - of all the light entering the water, the short wavelengths come back out to your eye.
(I just realized this phenomenon explains why a pure note whistled in a stairwell will echo slightly sharper! Woo!)
The converse is true for a sunset or sunrise. The light is coming straight at your eye from the source. When coming through an atmosphere, shorter wavelengths scatter away but the long wavelengths (reds) punch through whatever is scattering the light and keep going. That is why sunsets are red and smoke / dust make redder sunsets.
- The planet - I conclude it is being lit from behind and we are seeing the equivalent of a sunset refracted around through the atmosphere. The refracted light has travelled a straight (although refracted) path and so is redder. Some of the blue is still present scattering back which tints the red towards magenta. You can sometimes see this in the sunset.
At sunrise and sunset, light from the Sun must take a much longer path
through the Earth's atmosphere than it does during the middle part of
the day. This means more of the blue and indigo light of sunlight is
scattered away because these shorter wavelengths of visible light are
more affected by air molecules in the atmosphere. This often allows
more of the red and orange light to reach the Earth's surface. Other
factors -- including dust, pollution, haze, and cloud formations – may
also affect the colors of a sunset, creating a more complicated
palette of light as the Sun dips below the horizon
What a great picture. I think it is unmodified because they are using it as exactly the example I want.
Being pedantic - I do not think we would see the illuminated swath at 10 oclock if the only available light were such as to produce the magenta sunset effect. I conclude that this celestial body must have 2 light sources - one illuminating the top left and the other almost completely behind the body, refracting around the edge as the sunset.