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Ann. Geophys., 23, 487-498, 2005
www.ann-geophys.net/23/487/2005/
© European Geosciences Union 2005
An investigation of the field-aligned currents associated with a large-scale ULF wave using data from CUTLASS and FAST
H. C. Scoffield1, T. K. Yeoman1, D. M. Wright1, S. E. Milan1, A. N. Wright2, and R. J. Strangeway3
1University of Leicester, Leicester, UK
2University of St. Andrews, St. Andrews, UK
3University of California, Los Angeles, California, USA
Abstract. On 14 December 1999, a large-scale ULF wave event was observed by
the Hankasalmi radar of the SuperDARN chain. Simultaneously, the FAST
satellite passed through the Hankasalmi field-of-view, measuring the
magnetic field oscillations of the wave at around 2000km altitude, along
with the precipitating ion and electron populations associated with these
fields. A simple field line resonance model of the wave has been created and
scaled using the wave's spatial and temporal characteristics inferred from
SuperDARN and IMAGE magnetometer data. Here the model calculated field-aligned
current is compared with field-aligned currents derived from the
FAST energetic particle spectra and magnetic field measurements. This
comparison reveals the small-scale structuring and energies of the current
carriers in a large-scale Alfvén wave, a topic, which at present, is of
considerable theoretical interest. When FAST traverses a region of the wave
involving low upward field-aligned current densities, the current appears to
be carried by unstructured downgoing electrons of energies less than 30eV.
A downward current region appears to be carried partially by upgoing
electrons below the FAST energy detection threshold, but also consists of a
mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric
electrons of energies <30eV, with the hotter upgoing electrons
presumably representing those upgoing electrons which have been accelerated
by the wave field above the low energy detection threshold of FAST. A
stronger interval of upward current shows that small-scale structuring of scale
~50km has been imposed on the current carriers, which are downgoing
magnetospheric electrons of energy 0-500eV.
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