Simultaneous observations of small multi-scale structures in an auroral arc

Auroral arcs can develop small-scale distortions known as vortex streets or curls. Other common and somewhat larger spatially periodic distortions are auroral folds. In this event study we present simultaneous wide and narrow field imager observations of a third kind of structuring, on even smaller spatial scales. Boundary undulations, or “ruffs”, have been observed to form on the edge of an auroral arc and they occur superimposed on curls, folds or at times of auroral shear. The undulations typically have wavelengths of less than 3 km and amplitudes of less than 800 m. They are observed to move on the edge of the arc, with velocities of about 11 km/s. These observations, with multi-scale deformations, reveal a much more intricate structuring of auroral arcs than previously found.<br/

Large-scale fields and flows in the magnetosphere-ionosphere system

Advances in our understanding of the large-scale electric and magnetic fields in the coupled magnetosphere-ionosphere system are reviewed. The literature appearing in the period January 1991–June 1993 is sorted into 8 general areas of study. The phenomenon of substorms receives the most attention in this literature, with the location of onset being the single most discussed issue. However, if the magnetic topology in substorm phases was widely debated, less attention was paid to the relationship of convection to the substorm cycle. A significantly new consensus view of substorm expansion and recovery phases emerged, which was termed the ‘Kiruna Conjecture’ after the conference at which it gained widespread acceptance. The second largest area of interest was dayside transient events, both near the magnetopause and the ionosphere. It became apparent that these phenomena include at least two classes of events, probably due to transient reconnection bursts and sudden solar wind dynamic pressure changes. The contribution of both types of event to convection is controversial. The realisation that induction effects decouple electric fields in the magnetosphere and ionosphere, on time scales shorter than several substorm cycles, calls for broadening of the range of measurement techniques in both the ionosphere and at the magnetopause. Several new techniques were introduced including ionospheric observations which yield reconnection rate as a function of time. The magnetospheric and ionospheric behaviour due to various quasi-steady interplanetary conditions was studied using magnetic cloud events. For northward IMF conditions, reverse convection in the polar cap was found to be predominantly a summer hemisphere phenomenon and even for extremely rare prolonged southward IMF conditions, the magnetosphere was observed to oscillate through various substorm cycles rather than forming a steady-state convection bay