  We analyse a period spanning 35 years of activity in the Sao Paulo Stock
Exchange Index (IBOVESPA) and show that the Heston model with stochastic
volatility is capable of explaining price fluctuations for time scales ranging
from 5 minutes to 100 days with a single set of parameters. We also show that
the Heston model is inconsistent with the observed behavior of the volatility
autocorrelation function. We deal with the latter inconsistency by introducing
a slow time scale to the model. The fact that the price dynamics in a period of
35 years of macroeconomical unrest may be modeled by the same stochastic
process is evidence for a general underlying microscopic market dynamics.
