Electric fields and electron precipitation during a sharp negative H perturbation in the early evening sector

The relationship between electric fields and energetic electron precipitation for an intense substorm during the very early evening (1700–1800 MLT) was examined using balloon‐ and ground‐based data at Syowa Station, Antarctica. The electron precipitation (>25 KeV) was implied by the Bremsstrahlung X rays measured at the balloon altitudes. A large and prompt increase in X rays occurred simultaneously with the onset of a sharp H perturbation of −1000 nT, when the azimuthal direction of the electric field changed from southwest (poleward) to northwest (equatorward). This intense precipitation occurred during the developing stage of the negative H perturbation. Comparative studies of a time sequence of the electric field, electron precipitation, and auroral electrojet currents indicate that the precipitation region of intense energetic electrons (>25 KeV) in the very early evening sector was confined latitudinally to the vicinity of the region, where the enhanced westward electrojet pushes away the eastward electrojet.