Geoscience Reference
In-Depth Information
Fig. 1. ULF pulsations recorded at Mercury by the Mariner 10 magnetometer, assumed
to be the fourth harmonic of a standing wave (after Russell
9
).
of the “surface” conductivity may reside in a photoelectron cloud just above
the surface. By comparing observations in different local time sectors the
two components can be separated.
In the terrestrial magnetosphere (damped) standing ULF waves are
quite frequently generated by a Kelvin-Helmholtz instability at the flanks
of the magnetosphere as the solar wind streams past. A similar effect is
conceivable at Mercury, and there have been some indications that it does
indeed occur
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(see Fig. 1). This class of waves is often referred to as field
line resonances, since they involve a large-scale fluctuating motion along
the entire length of a set of magnetic field lines. Depending on the con-
ducting properties of the planet and its immediate environment, the waves
will be reflected either above, at, or below the surface. Also, depending on
whether the conductance at the reflection boundary is greater or less than
the waveguide conductance either the magnetic field or the electric field of
the wave will change phase when it is reflected. Mariner 10 did not mea-
sure the electric field and, thus, no firm conclusions regarding the nature
