\begin{table}
%\centering
\par
\caption{Summary of ionization bulge parameters from observations.}
\label{tab02}
\begin{tabular}{llllll}
\hline\hline
\noalign{\smallskip}
date    & range N & range S & minimum & height N& height S\\%[0.5ex]
\noalign{\smallskip}
\hline
\noalign{\smallskip}
1996.43 & $23.9$  & $-26.7$ & $-1.4$  &  $1.7$  &  $1.5$ \\
1996.93 & $27.6$  & $-33.3$ & $-2.9$  &  $1.6$  &  $1.6$ \\
1997.43 & $27.8$  & $-29.5$ & $-0.9$  &  $1.8$  &  $1.6$ \\
1997.93 & $28.3$  & $-34.2$ & $-2.9$  &  $1.8$  &  $1.6$ \\
1998.43 & $39.5$  & $-32.5$ & $ 3.5$  &  $1.6$  &  $1.5$ \\
1998.93 & $40.6$  & $-42.3$ & $-0.9$  &  $1.9$  &  $1.6$ \\
1999.43 & $43.5$  & $-59.4$ & $-7.9$  &         &        \\
1999.93 & $33.9$  & $-58.1$ & $-12.1$ &         &        \\
\hline
\end{tabular}
\end{table}