DMSP/F2 electron observations of equatorward auroral boundaries and their relationship to magnetospheric electric fields

Energetic electron measurements from a particle detector on the DMSP/F2 satellite have been used to determine the corrected geomagnetic latitude Λ CGM of the equatorward boundary of the auroral oval. The satellite was launched into a nearly sun‐synchronous, polar orbit centered on the 0700–1900 MLT meridian. Because of the wobble of the dipole and a very slow precessional motion of the orbit more than 6000 boundary crossings could be studied in the 1600–2300 and 0400–1000 MLT sectors. The detectors, which have large geometric factors, look radially away from the earth and detect precipitating electrons with energies between 50 eV and 20 keV. In the evening sector the equatorward boundaries are precisely determined from the rise in the total electron flux. The morningside boundary cannot always be clearly delineated. Data were divided into 1‐hour magnetic local time bins with ∼400 samples per bin. In each of the MLT bins, Λ CGM was found to be linearly correlated to the KP index. Regression values of Λ CGM were projected to the magnetic equatorial plane by using the Mead‐Fairfield magnetic field model. The projected boundaries are not in good agreement with the injection boundaries of Mauk and McIlwain (1974). They are best fit to boundaries derived by using Volland‐Stern type electric fields with the axis of symmetry rotated into the evening sector for KP <2 and into the afternoon sector for KP ≥2.