Let L be a number field and
let E∕L be an elliptic curve with potentially supersingular reduction at a prime ideal
℘ of L above a rational prime p. In this article we describe a formula for the slopes
of the Newton polygon associated to the multiplication-by-p map in the
formal group of E, depending only on the congruence class of pmod12,
the ℘-adic valuation of the discriminant of a model for E over L, and the
valuation of the j-invariant of E. The formula is applied to prove a divisibility
formula for the ramification indices in the field of definition of a p-torsion
point.