  We investigate the magicity of the isotopes $^{52}$Ca and $^{54}$Ca, that was
recently confirmed by two experimental measurements, and relate it to
like--particle and neutron--proton tensor effects within a mean--field
description. By analyzing Ca isotopes, we show that the like--particle tensor
contribution induces shell effects that render these nuclei more magic than
they would be predicted by neglecting it. In particular, such induced shell
effects are stronger in the nucleus $^{52}$Ca and the single--particle gaps are
increased in both isotopes due to the tensor force. By studying $N=32$ and
$N=34$ isotones, neutron--proton tensor effects may be isolated and their role
analyzed. It is shown that neutron--proton tensor effects lead to increasing
$N=32$ and $N=34$ gaps, when going along isotonic chains, from $^{58}$Fe to
$^{52}$Ca, and from $^{60}$Fe to $^{54}$Ca, respectively.
  The mean--field calculations are perfomed by employing one Skyrme parameter
set, that was introduced in a previous work by fitting the tensor parameters
together with the spin--orbit strength. The signs and the values of the tensor
strengths are thus checked within this specific application. The obtained
results indicate that the employed parameter set, even if generated with a
partial adjustment of the parameters of the force, leads to the correct shell
behavior and provides, in particular, a description of the magicity of
$^{52}$Ca and $^{54}$Ca within a pure mean--field picture with the effective
two--body Skyrme interaction.
