\begin{table}%t1
%\centering
\par
\caption{\label{tabla1}Magnetic field and maximum energies calculated 
for the three cases of~$a$ (see text) for primary electrons and
maximum energy for protons.}
\begin{tabular}{cccc}
\hline \hline \noalign{\smallskip} 
$a$  & $B$ & $E_{\rm max}^{e1}$ & $E_{\rm max}^{p}$ \\  
   & [G] & [eV] & [eV]\\ 
\hline \noalign{\smallskip} 
$0$& $4.0\times10^{-5}$  & $8\times10^{12}$ & -- \\  
$1$& $5.0\times10^{-5}$  & $9\times10^{12}$ &  $9\times10^{12}$ \\  
$100$&$1.4\times10^{-4}$ & $2\times10^{13}$  & $3\times10^{13}$ \\
\hline
\end{tabular}
\end{table}