Abstract
Abstract. We show two examples from the CANOPUS array of the optical signatures of auroral arcs produced by field line resonances on the night of 31 January 1997. The first example occurs during local evening at about 18:00 MLT (Magnetic Local Time), where CANOPUS meridian scanning photometer data show all the classic features of field line resonances. There are two, near-monochromatic resonances (at approximately 2.0 and 2.5 mHz) and both show latitudinal peaks in amplitude with an approximately 180 degree latitudinal phase shift across the maximum. The second field line resonance event occurs closer to local midnight, between approximately 22:00 and 22:40 MLT. Magnetometer and optical data show that the field line resonance has a very low frequency, near 1.3 mHz. All-sky imager data from CANOPUS show that in this event the field line resonances produce auroral arcs with westward propagation, with arc widths of about 10 km. Electron energies are on the order of 1 keV. This event was also seen in data from the FAST satellite (Lotko et al., 1998), and we compare our observations with those of Lotko et al. (1998). A remarkable feature of this field line resonance is that the latitudinal phase shift was substantially greater than 180 degrees. In our discussion, we present a model of field line resonances which accounts for the dominant physical effects and which is in good agreement with the observations. We emphasize three points. First, the low frequency of the field line resonance in the second event is likely due to the stretched topology of the magnetotail field lines, with the field line resonance on field lines threading the earthward edge of the plasma sheet. Second, the latitudinal phase structure may indicate dispersive effects due to electron trapping or finite ion gyroradius. Third, we show that a nonlocal conductivity model can easily explain the parallel electric fields and the precipitating electron energies seen in the field line resonance.Key words. Magnetospheric physics (electric fields; energetic particles precipitating; current systems)
Highlights
Numerous ground-based observations have shown that ultralow frequency field line resonances (FLRs) in the 1–4 mHz band influence the formation of some auroral arcs
The comparison between CANOPUS – FAST observations and the theoretical model discussed above demonstrates that FLRs can produce auroral arcs
The CANOPUS data show that the associated FAST event (Lotko et al, 1998) was an observation of an electron accelerator region coincident with a 1.3 mHz FLR
Summary
Numerous ground-based observations have shown that ultralow frequency field line resonances (FLRs) in the 1–4 mHz band influence the formation of some auroral arcs. Lotko et al (1998) attributed the formation of parallel electric fields in a 1.3 mHz FLR to dispersive effects in the Alfvenic structures and regions of anomalous resistivity in the large field-aligned currents associated with the FLR. We shall discuss this schematic in more detail when looking at the experimental data and simulations, but a few comments are in order. A smaller, subkilometer scale structure in the arc could be associated with a filamentation instability in the FAC This process, as well as two other nonlinear mechanisms in the FLR which could be responsible for small-scale structuring, namely tearing in the large FAC above the auroral ionosphere, and shear flow instabilities in the equatorial plane of the FLR, will not be considered here. We consider mechanisms for the formation of parallel electric fields in the FLR, and conclude that kinetic effects, including trapped and precipitating electrons, give the most likely mechanism to produce the observed electron energies and parallel electric fields
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