To quote the article,
Synesthesia is a neurological phenomenon in which activation of one sensory processing system (e.g., numbers or written language) leads to the automatic engagement of a second, distinct sensory processing system (e.g., color) to create a "crossed" sensory perception. For example, as in my case, numbers appear to have their own colors. Or, in other forms of synesthesia, sensory processing is "crossed" with emotion processing, imbuing letters, words, days of the week or months with their own personalities.
The author further says:
[...] people had colors, too. Not everyone. But many people did, whether I liked them or not, whether I knew them well or not. I didn't have to think about it; it was just so. As apparent to me as their eye color.
For much of my childhood, I enjoyed my synesthesia. [...] I'd turn my eyes to a teacher halfway through class to bask in her lavender glow. [...] I'd heard about auras -- radiant light surrounding a person, somehow revealing or reflecting his or her soul -- and I figured the color I saw from certain people, or numbers, was an aura. Had I somehow gained access to a spiritual dimension?
So the so-called "aura" (or, rather, the ability of some people to see it) may actually be a natural phenomenon. Of course, it is useless for any practical purpose -- you can't extract any real information from it -- because it is purely subjective. Any two synesthetes will see the same person's auras in different colors and attribute different emotional qualities to it.