On the morphology of auroral-zone X-ray events—I Dynamics of midnight events

Abstract

Simultaneous multiple balloon measurements have been performed of X-ray bremsstrahlung from electrons ⩾30keV precipitated into the auroral zone during polar magnetic substorms. Observations in the midnight sector have shown that the precipitation region is normally narrow in the north-south direction, but has probably a large extension in the eastwest direction. It has been found that impulsive electron precipitation events frequently occur in the midnight sector near the onset of a negative bay. Indications of rapid poleward motion have been found for such events. During the growing phase of the negative bay, the precipitation region may move equatorward as often as poleward between L ⋍ 5 and L ⋍ 7. Towards the end of the substorm the electron precipitation usually moves poleward to L ⋍ 7 or beyond. When magnetograms indicate the existence of an apparently well-defined electrojet, the precipitation region also seems to be well delimited, and its motions well correlated with simultaneous motions of the auroral electrojet. Following the initial impulsive precipitation, the energy spectrum of the precipitated electrons shows a gradual softening at least throughout the expanding phase of the substorm, irrespective of the direction of motion of the precipitation region. There seems to be a close agreement between the development of an X-ray substorm in the midnight sector and Akasofu's picture of the dynamics of the auroral substorm.