Abstract
Abstract. The overall focus on the driver of toroidal Pc5 waves has been on processes located at or acting on the dayside magnetopause and dayside flanks of the magnetosphere. These processes can generate waves that propagate tailward in the magnetosphere. However, an increasing number of studies, both theoretical and experimental, have looked at waves propagating sunward and that are caused by processes in the magnetotail. Here we present an ultra low frequency (ULF) wave observed in the post-midnight/morning sector of the magnetosphere at L=16 RE. The wave has a toroidal mode polarization. We estimate the azimuthal wave number to m=3, consistent with a toroidal mode type pulsation. The positive sign indicates that the wave is propagating sunward and this is confirmed by looking at the Poynting flux of the wave. The frequency of the wave is not constant with time but shows a small increase in the beginning of the event up to over 2.0 mHz. Then the frequency decreases to 1.0 mHz. This decrease coincides with a drop in the total magnetic field strength and we speculate if this is related to an observed reversal of the sign of the interplanetary magnetic field (IMF) By-component. This event occurs during relatively quiet magnetospheric conditions with a solar wind speed of approximately 400 km/s. Thus this event is highly likely to be driven by a source in the magnetotail and the change in frequency is an excellent example that the frequency of an ULF wave may be modulated by changes of the plasma parameters on the resonant field line.
Highlights
ultra low frequency (ULF) waves are common phenomena in the magnetospheric dawn and dusk sectors
One of the central concepts in the theory of ULF waves is that of the field line resonance (FLR), which was first discussed by Tamao (1965) but made widely known to the scientific community by the papers of Southwood (1974) and Chen and Hasegawa (1974)
An FLR is the coupling of energy from a fast mode wave to an Alfven wave at a location where the local Alfven eigenfrequency of the field line matches the frequency of the fast wave
Summary
ULF waves are common phenomena in the magnetospheric dawn and dusk sectors. Despite their frequent appearance the source or sources of these waves are still under considerable debate. Mathews et al (2004) presented multi-instrument observations of periodic, multiple east-west aligned auroral arcs in the dusk sector that were propagating sunward and equatorward from the poleward boundary of the auroral oval They showed that the periodicity of the arcs matched the periodicity of fluctuations of the local magnetic field. They suggested that the observed sunward propagation of the arcs can be explained if they are excited by field line resonances that are driven by waveguide mode waves that propagate sunward from the magnetotail.
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