Geoscience Reference
In-Depth Information
of the waves observed can be drawn. Combined low-frequency electric and
magnetic field measurements on BepiColombo will enable us, for the first
time, to properly diagnose standing ULF waves at Mercury, as well as tell
us more about the conductive properties of the reflective boundary at low
altitude. They will also tell us more about the conductive properties across
the magnetic field close to the planetary surface.
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2.3.
Field-aligned currents
Mariner 10 observed signatures of field-aligned currents.
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Whether these
were persistent or transient is not known, although the generally poorly con-
ducting low-altitude region may make persistent currents unlikely. Study-
ing these with simultaneous measurements of the electric and the magnetic
fields will shed additional light not only on the surface conductivity issue
but also on the more general, and largely unsolved, question of current
closure.
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,
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Depending on the details of the low-altitude closure mechanism
the correlation between the electric and the magnetic “disturbance fields”
will occur for different components, thus enabling diagnostics (Sec. 2.5).
2.4.
Exo-ionospheric conductivity
A conducting layer above the surface of Mercury could, at least in princi-
ple, come about in several ways. If the atmosphere were suciently dense it
would be a plasma created mainly by photo-ionization of the neutral atmo-
sphere. However, another possibility is that of a cloud of electrons, similar
to an electron-rich cathode sheath, made up of electrons photoemitted from
the surface.
2.5.
Current closure
A spacecraft overflying a static structure of field-aligned currents closed by
Pedersen current at low altitude will see a correlation between orthogonal
components of the electric and magnetic vectors transverse to the main
magnetic field (Blomberg
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and references therein). Assuming also homo-
geneous conductivity, orthogonal components of the transverse
E
and
B
are correlated. Such a correlation is often observed when overflying struc-
tures in the terrestrial ionosphere. If photoemission from the surface should
indeed be an important source of charge carriers in the near-Hermean envi-
ronment, there would be a net negative charge present, i.e., there would
be a cathode sheath rather than a plasma. In this case, because of the
