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Ann. Geophys., 21, 933-945, 2003
www.ann-geophys.net/21/933/2003/
© European Geosciences Union 2003
Optical signatures of auroral arcs produced by field line resonances: comparison with satellite observations and modeling
J. C. Samson1, R. Rankin2, and V. T. Tikhonchuk1,*
1Physics Department, University of Alberta, Edmonton, Alberta, Canada
2Institute of Fundamental Physics, University of Bordeaux 1, Talence 33405, France
*on leave from P. N. Lebedev Physics Institute, Russian Academy of Science, Moscow, Russia
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)
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