Observation of the low-altitude acceleration of auroral protons

Proton data obtained from a rocket flight into the breakup phase of an IBC I aurora are presented. It is shown that over a period of 10 seconds the 140-kev to 250-kev protons were moving predominantly up the magnetic field line away from the atmosphere at an altitude of 320 km; that some of these protons came from apparent mirror points well within the atmosphere; and that the ratio of downward- to upward-moving 250-kev to 450-kev protons outside of the loss cone probably varied during this period. Several natural effects have been investigated and found inadequate for the explanation of these results. In the absence of other possibilities it is concluded that the data offer direct evidence of the acceleration of auroral protons by at least tens of kilovolts at altitudes below about 300 km.