Filter by entries starting with in Low Search

Question

I have a Low Search form which I'm using to perform simple text searches with on a collection. However I would like to also include the ability to search for all entries beginning with a specific character (A, B, C etc).

MY existing Low Search results tag is as follows:

{exp:low_search:results
        channel="book_authors"
        default:orderby="title"
        default:sort="asc"
        query="{segment_3}"
        result_page="explore/library/authors"
        limit="20"
        }

How would I now pass to this results page a search string which will return all authors with surnames (field name {a_surname}) beginning with 'A' for example?

Update:

Thanks to the pointers from Low I got this working with the following code:

{exp:low_search:results
        collection="book_authors"
        default:orderby="title"
        default:sort="asc"
        {if get:search:a_surname}search:a_surname="^{get:search:a_surname}"{/if}
        result_page="explore/library/authors"
        limit="20"
        }

Answer 1

Use the Field Search filter, and specifically, the starts_with parameter:

search:a_surname="A" starts_with="search:a_surname"

Which is identical to:

search:a_surname="^A"