You shouldn't access Alfresco's Solr directly, why?: - Alfresco uses some specific query parameters (fq's) to respect permissions - Alfresco has his one Query Syntax & Query Handler to handle their dynamic data model
So the way to go is to use the existing REST or CMIS APIs or to write your own Alfresco WebScript to execute the search & just consume this REST API in your search app.
more on Alfresco's search syntax: