get_author_posts_url, how to edit the title attribute?

I want to edit the title attribute generated by get_author_posts_url.

I’ve got as far as this :
<?php $auth = esc_attr( get_the_author() ); ?>
" title="yourtitlehere"><?php the_author(); ?>

… which generates the correct structure, but for the final bit of the URL, which is output as FirstName%20LastName, whereas I need firstname-lastname (author slug must use display name, not user name, and be lower case).

Any ideas gratefully received.