How to get sub category id's under a category in Magento

The following code will provide the necessary steps for fetching the sub-category id's under selected category.

//this will return the category object
$_category = Mage::getModel('catalog/category')->load($categoryId);

$subcategoryIds = array();

//This will return the list of category id's under specified category
$subcategoryIds = split(",", $_category->getChildren());