OBSERVATIONS OF THE ELECTRIC-FIELD FINE-STRUCTURE ASSOCIATED WITH THEWESTWARD TRAVELING SURGE AND LARGE-SCALE AURORAL SPIRALS

The characteristics of the fine scale electric field associated with t he 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 ov al crossings by the Freja satellite. Three of the crossings were assoc iated with signatures of auroral substorms and one crossing went direc tly through the head of a surge close in time and space to substorm on set. Three passes were adjacent to auroral spiral formations, one pole ward of and one equatorward of such forms and one through the multiple are region near the front of an extended region of auroral activity. The ambient electric field was found to intensify in the direction tow ard the spiral head (or the center of the amoral activity region) over a region comparable to the size of the visible auroral forms. These r esults confirm previous findings that the spiral dr surge head is asso ciated with negative space charge and an intense upward field-aligned current. The fourth pass, directly through the surge head reveals a ve ry complicated structure of the surge region. Narrowly structured, int ense (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 mu A/m(2)) are seen nea r 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 f ields and field-aligned currents. According to some existing models of the surge, a pronounced westward electric field component and a south ward polarisation electric field is expected within the entire high-co nductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward sur ge current is closed by distributed downward field-aligned currents fr om the near surroundings. The Freja electric field is typically seen t o intensify at the edges of or in-between bright auroral structures an d to decrease within the arcs similar to what is observed in the ionos phere. The surge electric field is, however, much more intense than pr eviously observed or anticipated at these altitudes with characteristi cs 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.