None. An aura is defined as a field of energy or essence surrounding something or someone, and is generally a fictional concept with zero evidence to support it.

If you view a human through an infrared scope you will see a thermal “aura” surrounding the person, or if you photograph them with a Kirlian camera you can see their EM field.

But the eye can only see light in the visible spectrum.

Light reflects off of the outer moisture layer covering the eye, the same way light reflects off of water. The inner cones and rods structures of the eye aren’t numerous enough to reflect light except in extreme light levels, such as in a camera flash which is what gives the “red eye” effect in older film style pictures. Thesse cones and rods are far more numerous in animal eyes, which allows them to see in extremely low light conditions, and is why dog and cat eyes seem to “glow” at night in bright light.

If you are having problems seeing the “sparkle” in someone else’s eyes, it is due to two factors: 1. their eyes aren’t hydrated. Light doesn’t reflect well off of a dry pool.
2: they are emotionally drained. When people are experiencing what is called a “flat affect” emotionally, their eye muscles remain relaxed which causes the moisture in the eye to remain spread out and diffuse. When we are happy our muscles contract around the eye, causing the moisture to “pile up” over the eye and reflecting more light as the front of the eye becomes more hydrated.

Our eyes dry out due to a myriad of factors, and dry eyes are a common and chronic problem for a great number of people. It isn’t due to being demonically possessed.