Artifacts in the results of contourf()

Hi all,
I'd like to ask you several questions related to a figure I've generated ; I
apologize in advance for the forthcoming messages, because I've chosen to
start a new thread for each question...

My figure is located at .
Beware that the file is quite large (over 400 kb).

Now, my first question is: notice the artifacts on the contours of Bz and I at
y=0.3 and y=0.55, and on the contours of Q for multiples values of y. These
doesn't come from data, as they disappear when zooming in. Do you know where
do they come from ? And, more importantly (to me), how to get rid of them ?