changing height\depth of sub- and superscripts

I *think* I've seen this discussed in an earlier post, but despite my
better efforts, I can't seem to track it down.

I'm interested in techniques/options for changing the height (for
superscripts) or depth (for subscripts) from the LaTeX defaults - in
other words, change (overrride) the default vertical spacing rules for
sub- and superscripts. I ask because quite often, my students make
mistakes on assignments by confusing what should have been subscripts
as separate variables in an equation.

For example, $\rho\gamma_{PH}$. The PH product should be a subscript
for \gamma, but in \cm font, if you're not paying attention when
reading, it looks a lot like rho * gamma* PH. In other words, the
subscript depth for {PH} isn't 'deep' enough for the reader to
unambiguously see that the {PH} is a subscript. THis would be solved,
I think, if I could make the subscript a bit deeper (same being true
in reverse for superscripts).

Suggestions? Pointers to the obvious?

Thanks in advance...

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