Observations of field line resonances, auroral arcs, and auroral vortex structures

Abstract

Ultralow frequency (1–4 mHz) shear Alfvén, field line resonances are often found in the nightside magnetosphere on field lines threading the equatorward region of the auroral zone. The morphologies of the electric fields and field‐aligned currents in these field line resonances is very similar to those in discrete auroral arcs. This leads to the possibility that some types of auroral arcs might be produced by field line resonances. The field line resonances would produce arcs with latitudinal 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 and electric field antinodes in the equatorial magnetosphere, leads to very large radial shears in azimuthal plasma flows in the equatorial plane. These shear flows might be nonlinearly unstable to the Kelvin‐Helmholtz instability leading to vortex structures which map to longitudinal scale sizes of the order of hundreds of kilometers in the auroral ionosphere. In this study we present detailed observations from one night of data from the Canadian CANOPUS array of magnetometers, meridian scanning photometers, and digital all‐sky imager showing strong evidence for auroral arcs and auroral vortex structures formed by field line resonances. The structure of the discrete arcs within the field line resonances is particularly clear in the all‐sky imager data, showing latitudinal scale sizes of the order of 10–15 km. Meridian scans of the all‐sky imager data give clear evidence for a periodic restructuring of the arc system, with a characteristic frequency of about 2.8 mHz. Measurements of 6300/5577‐Å ratios indicate that the arc has an “inverted V” energy structure, with maximum energies ranging from several hundred eV to one or two keV. The all‐sky imager data also show clear evidence of large scale (hundreds of kilometers) vortex formation evolving from the arcs associated with the field line resonances, giving relatively strong evidence for nonlinear effects in the resonances.