EGS Young Scientists' Publication Awardee - 2002
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.

. 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.