The p-rank stratification on the Siegel moduli space with Iwahori level   structure
  Our concern in this paper is to describe the p-rank statification on the Siegel moduli space with Iwahori level structure over fields of positive characteristic. We calculate the dimension of the strata and describe the closure of a given stratum in terms of p-rank strata. We also examine the relationship between the p-rank stratification and the Kottwitz-Rapoport stratification. 
Paul Hamacher
math.AG