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Emma J Bunce
for the following first-authored paper published in
Planetary & Space Science, Bunce, E.J., and S.W.H. Cowley,
Divergence of the equatorial current in the dawn sector of Jupiter's
magnetosphere: analysis of Pioneer and Voyager magnetic field data,
Planet. Space Sci., 49, 1089-1113, 2001. |
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Emma Bunce's work is concerned with the large-scale
structure of Jupiter's magnetosphere, and in particular with the
current system which couples angular momentum between the magnetosphere
and ionosphere. Her work has for the first time combined the
magnetic data from all five pre-Galileo spacecraft fly-bys, from the
Pioneer, Voyager, and Ulysses spacecraft, to quantify these
large-scale currents. Her first study (also published in P&SS in
2001) examined the azimuthal current in the equatorial current
sheet, showing for the first time that significant asymmetries exist, the
currents being stronger on the nightside than on the dayside, an effect
ascribed to the day-night asymmetry imposed by the solar wind. An
empirical model was derived from the data, which serves to quantify the
azimuthal current and its divergence. The above paper then went on to
study the radial dependence of the radial current along the fly-by
trajectories, thus establishing the second component of the equatorial
current divergence. Combining the results of both studies in the above
paper thus allowed a determination of the overall divergence of the
equatorial current, and hence a first estimate to be made of the
field-aligned current density flowing between the equatorial plasma and
the ionosphere. Mapped to the ionosphere, these currents were found to be
~0.1-1 mA m-2, an appreciably large current, directed principally out of
the ionosphere into the current sheet.
Emma then showed that in the tenuous jovian
environment such currents require large field-aligned accelerating
voltages in order to make them flow, of order ~100 kV, such that the
precipitating electron energy flux will also be very large, ~100 mW m-2,
sufficient to produce an intense ~MR aurora. She thus proposed that this
precipitation produces the bright 'main oval' auroras at Jupiter, which
have received extensive recent study using the HST. Previous proposed
mechanisms fall short of the required energy flux by several orders of
magnitude. Emma's results published in the above paper have therefore not
only quantified Jupiter's magnetic field and currents, but also provided a
basis for understanding the main jovian auroral (and radio) emissions,
which are now under intensive subsequent theoretical follow-up.
Emma is an excellent young scientist who has what
it takes to make a most successful career. |