THE INSTITUTION PROPOSES A NEW SOUNDING ROCKET MISSION TO PROVIDE THE FIRST DIRECT MEASUREMENTS OF THE INTRINSIC AMBIPOLAR ELECTRIC POTENTIAL GENERATED BY EARTH'S IONOSPHERE ACROSS THE CRITICALLY IMPORTANT EXOBASE TRANSITION REGION (200KM - 800KM); ENERGY INPUT INTO WHICH IS MORE EFFECTIVE IN GENERATING IONOSPHERIC OUTFLOW THAN IF IT WERE INPUT AT HIGHER ALTITUDES WERE THE PLASMA FLOW IS SUPERSONIC (LEER AND HOLZER 1980). ENDURANCE WILL BE THE FIRST MISSION CAPABLE OF RESOLVING EARTH'S ELECTRIC POTENTIAL DROP DETERMINING ITS VERTICAL STRUCTURE AND INVESTIGATING KEY UNDERLYING PHYSICS.