\begin{table}%t2
\caption{Description of data sets.}\label{tab data sets}
\small%\centerline
 {
\begin{tabular}{c c c c}
\hline
\hline
&&& \\[-8pt]
      Data set$^{{a}}$ &  Date   &  Position$^{{b}}$  &  $R_\odot${} (arcsec)$^{{c}}$ \\
\hline
1         &   30 May 2002   &  $X=0$, $Y=1150$  &  956.5   \\      
2         &   28 May 2002   &  $X=0$, $Y=-1150$ &  956.5   \\  
3         &   19 Nov. 2003   & $X=0$, $Y=1145$   &  980.0   \\  
4         &   12 June 2004  & $X=0$, $Y=-1140$  &  960.0   \\   
\hline  
\end{tabular}}
\medskip
$^{{a}}$ Observations are all made with the $1\arcsec$~$\times$ $300\arcsec$ slit, and detector~A. In data sets~2$-$4, only the \ion{Mg}{x}~625~\AA\ line was analysed in the present paper, for comparison purpose. \\ 
$^{{b}}$ Position of the centre of the slit, in arcseconds from disc centre. \\
$^{{c}}$ Solar radii as seen from SOHO. 
\end{table}