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Ann. Geophys., 22, 2515-2523, 2004
www.ann-geophys.net/22/2515/2004/
© European Geosciences Union 2004
Isolated electrostatic structures observed throughout the Cluster orbit: relationship to magnetic field strength
J. S. Pickett1, L.-J. Chen1, S. W. Kahler1, O. Santolík1,2, D. A. Gurnett1, B. T. Tsurutani3, and A. Balogh4
1Department of Physics and Astronomy, The University of Iowa, Iowa City, IA, USA
2Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
4The Blackett Laboratory, Imperial College, London, UK
Abstract. Isolated electrostatic structures are observed throughout
much of the 4RE by 19.6RE Cluster orbit. These structures are
observed in the Wideband plasma wave instrument's waveform data as bipolar
pulses (one positive and one negative peak in the electric field amplitude)
and tripolar pulses (two positive and one negative peak, or vice versa).
These structures are observed at all of the boundary layers, in the solar
wind and magnetosheath, and along auroral field lines at
4.5-6.5RE.
Using the Wideband waveform data from the various Cluster spacecraft we have
carried out a survey of the amplitudes and time durations of these structures
and how these quantities vary with the local magnetic field strength. Such a
survey has not been carried out before, and it reveals certain
characteristics of solitary structures in a finite magnetic field, a topic
still inadequately addressed by theories. We find that there is a broad range
of electric field amplitudes at any specific magnetic field strength, and
there is a general trend for the electric field amplitudes to increase as the
strength of the magnetic field increases over a range of 5 to 500nT. We
provide a possible explanation for this trend that relates to the structures
being Bernstein-Greene-Kruskal mode solitary waves. There is no corresponding
dependence of the duration of the structures on the magnetic field strength,
although a plot of these two quantities reveals the unexpected result that
with the exception of the magnetosheath, all of the time durations for all of
the other regions are comparable, whereas the magnetosheath time durations
clearly are in a different category of much smaller time duration. We
speculate that this implies that the structures are much smaller in size. The
distinctly different pulse durations for the magnetosheath pulses indicate
the possibility that the pulses are generated by a mechanism which is
different from the mechanism operating in other regions.
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