An investigation of the field-aligned current associated with a large-scale ULF wave in the morning sector,, Planet. Space Sci., 55, 770, 2007.

Previous work by Scoffield, H.C., Yeoman, T.K., Wright, D.M., Milan, S.E., Wright, A.N., Strangeway, R.J. [2005. An investigation of the field aligned currents associated with a large scale ULF wave using data from CUTLASS and FAST. Ann. Geophys. 23, 487-498) investigated a large-scale ULF wave, occurring in the dusk sector (-1900MLT). The wave had a period of similar to 800s (corresponding to 1.2mHz frequency), an azimuthal wave number of similar to 7 and a full-width at half-maximum (FWHM) across the resonance of 350km. IMAGE ground magnetometer and SuperDARN radar observations of the wave's spatial and temporal characteristics were used to parameterise a simple, two-dimensional field line resonance (FLR) model. The model-calculated field-aligned current (FAC) was compared with FACs derived from the FAST energetic particle spectra and magnetic field measurement. Here the authors use the same method to investigate the FAC structure of a second large-scale ULF wave, with a period of similar to 450 s, occurring the dawn sector (similar to 0500 MLT) with an opposite sense background region 1-region 2 current system. This wave has a much larger longitudinal scale (m similar to 4.5) and a smaller latitude scale (FWHM = 150 km). Unlike the dusk sector wave, which was dominated by upward FAC, FAST observations of the dawn sector wave show an interval of large-scale downward FAC of similar to 1.5 mu A m(-2). Downgoing magnetospheric electrons with energies of a few kcV were observed, which are associated with upward FACs of similar to 1 mu A m(-2). For both wave studies, downward currents appear to be carried partially by upgoing electrons below the FAST energy detection threshold (5eV), but also consist of a mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric electrons of energies 30 eV-1 keV. Strong intervals of upward current show that small-scale structuring of scale similar to 50 km has been imposed on the current carriers. In general, this study confirms the findings of Scoffield, H.C., Yeoman, T.K., Wright, D.M., Milan, S.E., Wright, A.N., Strangeway, R.J. [2005. An investigation of the FACs associated with a large-scale ULF wave using data from CUTLASS and FAST. Ann. Geophys. 23, 487-498). (c) 2006 Elsevier Ltd. All rights reserved.