My MIL has it. You're so very lucky that after 10 years, your mom is still able to go out and do things with you--even if she doesn't quite know who you are, where she is, or what's going on . I'd ask this, though--are you absolutely certain it's Alzheimers? Had a friend who's mother was diagnosed with it, but it wasn't progressing the way the disease usually does (by 10 years, people with it are often getting to the later stages where they are beginning to lose physical abilities). Turns out, friend's mother was misdiagnosed. She had something else entirely, and spent years getting treated for the wrong thing, because some of these conditions are so very hard to pin down.
MIL (diagnosed about 9 years ago) has been pretty much a vegetable for the past 3-4 years. Unfortunately, you also need to be aware of, and protect against the fact that someone with that disease is very likely to be victimized by someone unscrupulous. Hope that doesn't happen to your mom. It did in our case, and we've been in a very heart-breaking fight to protect her interests.
I'm so pleased for you, though, that you were able to take her out and enjoy--as much as possible--the day. Hang in there.