<textarea style="display:none;" name="myTextArea" id="myTextArea" cols="100" rows="14"></textarea>
<iframe name="richTextField" id="richTextField"></iframe>
<button id="article_btn" onclick="saveArticle()">Save Article</button>

function saveArticle(){
    var theForm = _("writearticle");
    theForm.elements["myTextArea"].value = window.frames['richTextField'].document.body.innerHTML;
    theForm.submit();
}