OBSERVATIONS OF FIELD LINE RESONANCES, AURORAL ARCS, AND AURORAL VORTEX STRUCTURES

Ultralow frequency (1-4 mHz) shear Alfven, field line resonances are o ften found in the nightside magnetosphere on field lines threading the equatorward region of the auroral zone. The morphologies of the elect ric fields and field-aligned currents in these field line resonances i s very similar to those in discrete auroral arcs. This leads to the po ssibility that some types of amoral arcs might be produced by field li ne resonances. The field line resonances would produce arcs with latit udinal scale sizes of the order of 10 km, longitudinal scale sizes of many time zones, and with periodic restructuring on timescales of the order of minutes. The standing wave nature of the field line resonance , with electric field nodes in the northern and southern ionosphere an d electric field antinodes in the equatorial magnetosphere, leads to v ery large radial shears in azimuthal plasma flows in the equatorial pl ane, These shear flows might be nonlinearly unstable to the Kelvin-Hel mholtz instability leading to vortex structures which map to longitudi nal scale sizes of the order of hundreds of kilometers in the auroral ionosphere. In this study we present detailed observations from one ni ght of data from the Canadian CANOPUS array of magnetometers, meridian scanning photometers, and digital all-sky imager showing strong evide nce for auroral arcs and auroral vortex structures formed by field lin e resonances. The structure of the discrete arcs within the field line resonances is particularly clear in the allsky imager data, showing l atitudinal scale sizes of the order of 10-15 km. Meridian scans of the all-sky imager data give cleat evidence for a periodic restructuring of the are system, with a characteristic frequency of about 2.8 mHz. M easurements of 6300/5577-Angstrom ratios indicate that the ate has an ''inverted V'' energy structure, with maximum energies ranging from se veral hundred eV to one or two keV. The all-sky imager data also show clear evidence of large scale (hundreds of kilometers) vortex formatio n evolving from the arcs associated with the field line resonances, gi ving relatively strong evidence for nonlinear effects in the resonance s.