Marcia,
It is the aluminum which would corrode, not the copper. Technically, they should not be touching, otherwise in a very close way connectible via water. Yea, it is best to have the aluminum above (uphill) from the copper. Here is a helpful link I happened to find today as I researched repairing my coolant system in my subaru and substituting metal hose:
http://fourseasonsroofingandsiding.com/mixmetals.htm
Before you visit that though, you might want to understand that this chimney used in this example is, I'll say for certain, is venting an oil fired furnace, which emits acidic vapors which will end up to condense at times and run down the chimney, and rot flashings. I have 20 years experience in construction, especially the roofing trade, and have had chimney professionals tell me aluminum chimney caps rot in ten years with oil fired furnaces. My chimney cap installations are always stainless. Now under a normal scenario---not directly downhill from a chimney---this being taken from my own professional experience---I presume it would take 100 years or maybe more for this extent of damage to occur, instead of maybe 5-10. I have seen copper and aluminum successfully touch--copper nails into aluminum ridgecaps---now the nails are bright green in patina which takes well over 20 years to form---with nil ill effect thus far, yet no electrolyte falling there aside from the rain, being atop a roof and fully exposed to the stars and New England weather.
Though a major industrial city might give you enough acid in the rainfall, it does not sound like a major worry from here. For right building practice, lead flashing is a common go-to middle-ground metal which may be used--and is---as a separator rather than having direct contact aluminum to copper.
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