How can I override apache solr's default search result sorting?
This requires a custom modules where you implement a hook provided by apachesolr. There are several options but hook_apachesolr_modify_query(&amp;$query, &amp;$params, $caller) works as well as any. 

You can modify the $query:

$query-&gt;set_solrsort($field_name, $direction); 


Or you can modify the $params:

$params['sort'] = "$field_name $direction";