My initial guess would be the immense electric field around the lines, that causes hair to get charged and due to each hair having the same charge they start to repel each other.
So what is exactly happening here?
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I believe the mechanism is somewhat different: the electric field polarizes, rather than charges, hair, and then acts on the resulting electric dipoles, judging by the formulas in: "Proceedings of the 2005 IEEE Engineering in Medicine and Biology 27th Annual Conference, Shanghai, China, September 1-4, 2005", p. 4266. "Analysis of Body Hair Movement in ELF Electric Field Exposure", H. O. Shimizu, K. Shimizu. According to the formulas, it is essential that the electric field is not uniform. The authors claim good agreement with experimental results.
Its simple. The electric line and the hair carry same charges, and since like Charges repel each other,this charges tends to push very far away from each other, and so this causes the hair to rise. The principle is 'the more the quantity of charges the more the hair tends to rise straight'.
Just like in the case of the Van de Graaff generator, when a person touches the charged metal ball (dangerous) the excess stored electron on the ball travel into the person's body, now, the charges can't get to the ground(since the person's standing on a rubber). You are now filled up with electrons. The electrons don't like each other and are trying to get as far away from each other as possibly. Usually this makes your hair stand up because it is filled with electrons that are repelling each other.