The response of the azimuthal component of the ionospheric electric field to auroral arc brightening
We have analyzed the response of azimuthal component of the ionospheric electric field to auroral arc activity. We have chosen for analysis three intervals of coordinated EISCAT and TV observations on 18 February, 1993. These intervals include three kinds of arc activity: the appearance of a new auroral arc, the gradual brightening of the existing arc and variations of the arc luminosity. The arcs were mostly east-west aligned. In all cases, the enhancement of arc luminosity is accompanied by a decrease in the westward component of the ionospheric electric field. In contrast, an increase of that component seems to be connected with arc fading. The observed response is assumed to have the same nature as the “short circuit” of an external electric field by the conductor. The possible consequence of this phenomenon is discussed.
The electric field response to the growth phase and expansion phase onset of a small isolated substorm
We capitalise on the very large field of view of the Halley HF radar to provide a comprehensive description of the electric field response to the substorm growth phase and expansion phase onset of a relatively simple isolated substorm (|AL| < 250 nT) which occurred on 13 June 1988. The substorm phases are identified by their standard ground magnetic and spacecraft energetic particle signatures, which provide a framework for the radar measurements. The substorm is preceded by a prolonged period (> 12 h) of magnetic quiescence, such that prior to the start of the growth phase, the apparent latitudinal motion of the radar backscatter returns is consistent with the variation in latitude of the quiet-time auroral oval with magnetic local time. The growth phase is characterised by an increasing, superimposed equatorward motion of the equatorward edge of the radar backscatter as the auroral oval expands. Within this backscatter region, there is a poleward gradient in the Doppler spectral width, which we believe to correspond to latitudinal structure in auroral emissions and magnetospheric precipitation. During the growth phase the ionospheric convection is dominated by a relatively smooth largescale flow pattern consistent with the expanding DP2 (convection) auroral electrojets. Immediately prior to substorm onset the ionospheric convection observed by the radar in the midnight sector has a predominantly equatorward flow component. At substorm onset a dramatic change occurs and a poleward flow component prevails. The timing and location are quite remarkable. The timing of the flow change is within one minute of the dispersionless injection observed at geostationary orbit and the Pi2 magnetic signature on the ground. The location shows that this sudden change in flow is due to the effect of the upward field aligned current of the substorm current wedge imposed directly within the Halley radar field of view.
Pi2 pulsation polarization patterns on the U.K. sub-auroral magnetometer network (SAMNET)
Abstract An array of seven fluxgate magnetometers has been deployed covering mid-latitudes in western Europe. The array is configured to allow simultaneous observations of mid-latitude pulsation characteristics in both latitude and longitude. Here the two-dimensional variations in the horizontal polarization of Pi2 pulsations are related to the substorm current wedge position, deduced from the midtitude bay behaviour, and an estimated plasmapause position. Features of the polarization pattern which are not in agreement with the substorm current wedge model are found. These include unexpected polarization ellipse azimuths and longitudinal phase propagations, and latitudinal variations in polarization ellipse azimuths not associated with ellipticity reversals. An extension of previous travelling wave models to include low latitude effects is considered, which may explain at least some of these discrepancies.