Birch tree...storm damage...large limb ripped off...
Cut off limb...
"open wound" about the size of a NFL football.
Should I just 'let it be' ...or...
Spray or patch it with something to help it heal ???
Current best practice is to not use any patches or coverings/medications of any sort. If the branch has been properly trimmed (see link below) the cut will heal from the branch collar and close on its own. Cement, tar and other fillers or dressings that were used in the past have been found through research to promote rot and hinder healing.
Here is a link that might be useful: how to prune
Very interesting information
Here is a link that might be useful: tree wound care
If there are any ragged edges around the wound then you can tidy that up but definitely don't put any sealant on it. Trees by the way don't heal like animals. They grow new tissue around the wound. The wound will always exist inside the tree. You should see the tree starting to close the wound with new growth by next year.
I hope this wasn't a white birch tree. When they are stressed or wounded they send out chemical signals that attract some unwelcome visitors like the bronze birch borer. That can be death sentence for the tree. If the tree was healthy before this happened then you should be OK though.