Role of a Senior Editor I have submitted a paper to one of the IEEE journals. I had to choose a Senior Editor.
What is the exact role of the Senior Editor? As far as I know, the Editor-in-Chief assigns an Associate Editor who determines the reviewers but not sure about the role of Senior Editor. [Whom do typically people refer to by just "Editor"?]

Also in my cover letter, by mistake I mentioned the name of my chosen SE instead of the EiC. Should I contact the journal to modify it or just wait for their response?
Different journals are set up differently. Some journals have an editor-in-chief who assigns an associate editor who assigns a member of the editorial board. Others have the journal office assign an editor who assigns the reviewers. Yet others could let the authors choose.

It sounds like you've submitted your paper to a journal using the last model. My guess would be that the chosen editor will be the one handling your paper (unless she declines). So don't worry about it and let the process run its course.

About the roles of all these editors, the word "editor" has many possible meanings and it's not really possible to tell without insider information what everyone does. Editor-in-chief is generally the highest-ranking position, and is responsible for everything published in the journal, but beyond that anything goes. "Managing editor", "Senior editor", "Consulting editor", "Assistant editor", etc - they could all be doing the same thing. It all depends on how the journal is set up.

Finally, about the salutation, I wouldn't worry about it. Chances are the SE will be reading your cover letter. If it's the EiC (or journal office) reading it instead, they're not likely to take offense.