You have a document that is composed of (short) title and (long) body fields and want body to be highlighted.
In order to avoid highlighted body field to be empty, you can use hl.alternateField parameter.
Although you want to set f.body.hl.alternateField=body, you may set f.body.hl.alternateField=title,
because response time is awful when the body values are big. But the title field provides users with
information smaller than body field.
In this case, you can use f.body.hl.alternateFieldLen=100 to limit the body length to 100 characters.