2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/79644In this paper we consider the stratification on the moduli space of principally polarized abelian surfaces in characteristic $p>0$ defined by the height of the formal group associated to $H^2(X,O_X)$. We compute the cycle classes of the strata and compare these with those of the $p$-rank stratification.Latex, 27 pages, minor correction in section 9Algebraic Geometrytext