Someone on Pinterest had been hiking in a mountainous area, and had observed that his shadow, on an opposite hillside, was surrounded by a halo. He had never observed this before, and, in fact, had never heard of it. He wondered what caused it. It's not too difficult to figure out or explain. When the sun is behind you, and you are facing away from it, and you can see where your shadow is (or should be), against any irregular surface, the planes in that surface that are directly facing you, and therefore perpendicular to the light from the sun, are most brightly illuminated when they are closest to you. At the same time, some of the planes not as close to perpendicular are going to be more shaded, and those will be more visible, the farther they are from the centre of the shadow. All together, the area immediately around the shadow is brightest, and the illumination fades the further away (in angular terms) from the centre of the shadow.
This can be observed in many ways. As noted, in mountainous terrain, when you can see your shadow against an opposite slope. When you are flying over some forested area, and can see where the shadow of the plane is (or should be). When you are flying over a cloud bank, and can see where the shadow of the plane is (or would be). You can even use this effect to figure out where the shadow of your plane should be, if you can't make it out. Just look for the center of the brightest area.
You can even observe this on a sidewalk. When you are walking, facing away from the sun, and can see your shadow, note that the area on the sidewalk is brightest immediately around your head, adjacent to your shadow. The sidewalk, particularly if it is old, isn't a regular surface even if the irregularities are quite small. So, the surfaces facing you most directly, are most brightly illuminated by the sun, when the sun's rays are closest to your head.
You're welcome.
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