  The correlation function of a financial index of the New York stock exchange,
the S&P 500, is analyzed at 1 min intervals over the 13-year period, Jan 84 --
Dec 96. We quantify the correlations of the absolute values of the index
increment. We find that these correlations can be described by two different
power laws with a crossover time t_\times\approx 600 min. Detrended fluctuation
analysis gives exponents $\alpha_1=0.66$ and $\alpha_2=0.93$ for $t<t_\times$
and $t>t_\times$ respectively. Power spectrum analysis gives corresponding
exponents $\beta_1=0.31$ and $\beta_2=0.90$ for $f>f_\times$ and $f< f_\times$
respectively.
