Geoscience Reference
In-Depth Information
2. Scientific Topics
We here briefly review a number of scientific topics related to the low-
frequency electric field in the plasma environment of planet Mercury. For a
more thorough overview of the Mercury environment the reader is referred
to, e.g., Cumnock and Blomberg. 3
2.1. Plasma convection and cross-polar potential
Plasma convection in a magnetosphere is controlled by the DC electric
and magnetic fields. Thus, to determine plasma circulation and plasma
transport across boundaries the static component of the electric field needs
to be measured. The electric field and plasma flow inside the magnetosphere
also give important information on the interaction of the solar wind with
the magnetosphere.
In Earth's magnetosphere there is a saturation effect so that the poten-
tial drop inside the magnetopause does not increase linearly with an increas-
ing potential drop across a similar distance in the solar wind. 4 - 7 Hill's
explanation of the saturation effect is based on the idea that field-aligned
currents are set up that create a magnetic field opposing the field at the
magnetopause and thus weakening reconnection, eventually limiting it at
some fixed plateau value. The eciency of this feedback process depends
on the ionospheric conductivity. However, at Mercury it is di cult to con-
ceive of a similar process given the low conductivity near the planet and
the typically corresponding low ability of its magnetosphere to sustain any
appreciable field-aligned currents. Thus, for strong solar wind electric fields,
a significantly higher fraction may penetrate into the Hermean magneto-
sphere than would penetrate into the Terrestrial one, as further discussed
elsewhere. 8 In situ measurements are needed to clarify the saturation mech-
anism at Mercury.
2.2. ULF pulsations
There are indications from Mariner 10 magnetometer data of the existence
of field-line resonances at Mercury. 9 A field-line resonance is a fundamental
response of the planetomagnetic field to the solar wind's interaction with the
magnetopause. At Mercury they are interesting also because they depend
on the (electromagnetic) reflective properties of the surface and thus may
be used to estimate the surface conductivity. This requires simultaneous
measurements of the electric and the magnetic fields at low frequency. Some
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