Manual:$wgSecureLogin

Details
If true, forces users to authenticate using https when they come from http, but only if you use a protocol-relative URL in   (otherwise this setting is ignored).

From 1.22 on, use   to set  false by default if you want HTTP users to stay on HTTP after login by default.

Based on an idea by George Herbert on wikitech-l: wikitech-l/2010-October/050071.html.

You will need to set $wgServer as following.