The adjutant general may assign the number of assistant adjutants general that are authorized by National Guard Bureau rules and regulations from the ground forces and the air forces of this state. The assistant adjutants general shall, if they qualify therefor^*, hold military rank as may be authorized and approved for the positions by the National Guard Bureau of the United States. The assistant adjutants general, at the time of their appointment, shall have not less than ten years of military service as a commissioned officer with the military forces of this state, another state or territory, the District of Columbia or the United States, or in any or all such services combined, five years of the service being in field grade. The assistant adjutants general shall serve at the pleasure of the adjutant general and perform such duties as are assigned by the adjutant general. During any period when the adjutant general is unable to perform such duties, the senior assistant adjutant general may, under the direction of the governor, perform the duties of the adjutant general.