Dag, Thanks for your tip. I didn't know that I could use a numpy array directly. I tried that out, and it works like a charm. :-) Jonathan, Glad that you mentioned spacepy. I actually tried spacepy before I decided to write my own wrapper. However, on the one hand, I can't install spacepy on my mac (10.8 OS X). On the other hand, spacepy has too much stuff that I don't need, and I don't think I can rip spacepy.pycdf off the whole package. So, I eventually decided to write my own wrapper. However, as a space physics scientist, I'd like to see spacepy become better and more generic. For the moment, it seems to me that spacepy is more tailored to suit LANL's needs, especially about radiation belt research, than being a generic tool for space physics. Cheers, Jianbao