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Cold plasma of ionospheric origin has recently been found to be a much larger contributor to the magnetosphere of Earth than expected 1-3 . Numerous competing mechanisms have been postulated to drive ion escape to space, including heating and acceleration by wave-particle interactions 4 and a global electrostatic field between the ionosphere and space (called the ambipolar or polarization field) 5,6 . Observations of heated O + ions in the magnetosphere are consistent with resonant wave-particle interactions 7 . By contrast, observations of cold supersonic H + flowing out of the polar ionosphere 8,9 (called the polar wind) suggest the presence of an electrostatic field. Here we report the existence of a +0.55 ± 0.09 V electric potential drop between 250 km and 768 km from a planetary electrostatic field (E ∥ ⊕ = 1.09 ± 0.17 μV m -1 ) generated exclusively by the outward pressure of ionospheric electrons. We experimentally demonstrate that the ambipolar field of Earth controls the structure of the polar ionosphere, boosting the scale height by 271%. We infer that this increases the supply of cold O + ions to the magnetosphere by more than 3,800%, in which other mechanisms such as wave-particle interactions can heat and further accelerate them to escape velocity. The electrostatic field of Earth is strong enough by itself to drive the polar wind 9,10 and is probably the origin of the cold H + ion population 1 that dominates much of the magnetosphere 2,3 ., (© 2024. US Government as represented by the Administrator of the National Aeronautics and Space Administration.)