The intense magnetic storm of December 19, 1980 occurred during a major rocket and balloon geophysical research campaign at Siple Station, Antarctica. A balloon flight measuring the electric field and bremsstrahlung X ray flux was conducted during the main phase of the storm. The balloon data and associated ground‐based data from around the world contain several lines of evidence which indicate that the dayside auroral oval expanded to an invariant latitude ≤59° during the storm. Evidence for this conclusion includes (1) the pattern of ground‐based magnetic field and ionospheric electric field perturbations; (2) a substantial departure from the normal diurnal curve of the vertical component of the electric field in the stratosphere; and, (3) identical, relatively rapid equatorward motion of regions of electron precipitation, observed or inferred to occur, simultaneously at three L ∼ 4 stations: Siple, Halley Bay and SANAE, separated by several hours in local time across the dayside. The absence of electron precipitation at Siple after this equatorward motion is an indication that the polar cap had expanded to include Siple during this interval. The power spectra of the magnetic field fluctuations at ULF observed at Siple and in a conjugate latitude chain of magnetometers were consistent with the presence of the dayside auroral oval in the near vicinity of Siple and with the presence of a major magnetospheric boundary slightly equatorward of ∼59°. The stratospheric electric field measured during the recovery phase was very large for this latitude for a period of several hours. This observation suggests that a subauroral latitude ion drift event of unusual intensity and duration accompanied this storm.