Observations of the electric field fine structure associated with the westward traveling surge and large‐scale auroral spirals

Abstract

The characteristics of the fine scale electric field associated with the westward traveling surge and large‐scale auroral spirals and surges are investigated using high‐resolution electric field, magnetic field, particle and UV imager observations from four eveningside auroral oval crossings by the Freja satellite. Three of the crossings were associated with signatures of auroral substorms and one crossing went directly through the head of a surge close in time and space to substorm onset. Three passes were adjacent to auroral spiral formations, one poleward of and one equatorward of such forms and one through the multiple arc region near the front of an extended region of auroral activity. The ambient electric field was found to intensify in the direction toward the spiral head (or the center of the auroral activity region) over a region comparable to the size of the visible auroral forms. These results confirm previous findings that the spiral or surge head is associated with negative space charge and an intense upward field‐aligned current. The fourth pass, directly through the surge head reveals a very complicated structure of the surge region. Narrowly structured, intense (up to 700 mV/m) and mostly converging electric fields associated with intense electron precipitation (of both high and medium energies) and balanced field‐aligned currents (up to 30 μA/m2) are seen near the edge of the surge head and adjacent to auroral structures in the wake. These narrow regions are embedded within more extended regions of intense high‐energy electron precipitation but very weak electric fields and field‐aligned currents. According to some existing models of the surge, a pronounced westward electric field component and a southward polarisation electric field is expected within the entire high‐conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field‐aligned currents from the near surroundings. The Freja electric field is typically seen to intensify at the edges of or in‐between bright auroral structures and to decrease within the arcs similar to what is observed in the ionosphere. The surge electric field is, however, much more intense than previously observed or anticipated at these altitudes with characteristics rather similar to those observed in the auroral acceleration region. Since the particle data indicate that most of the acceleration takes place above Freja altitudes, it seems as if Freja traversed the lower part of the auroral acceleration region associated with the surge.