Magnetospheric and Auroral Science

While much of Stan Cowley’s early scientific career concerned theoretical and data analysis studies of the Earth’s outer plasma environment, involvement in work on the gas giant planets began with his participation in the analysis of Ulysses energetic particle and magnetic field data obtained during the 1992 Jupiter flyby. Subsequently this research strand extended into more general studies of the structure of Jupiter’s magnetosphere using Pioneer, Voyager, Ulysses, and Galileo magnetic field data, undertaken in collaboration with his then PhD student Emma Bunce, now herself Professor of Planetary Plasma Physics in the Department of Physics and Astronomy at Leicester.

In turn, this led to considerations of the coupling between the magnetosphere and the planet’s ionosphere and the origins of Jupiter’s bright auroras. Starting from earlier ideas on the transfer of spin momentum from the planet to the surrounding plasma medium proposed by Tom Hill (Rice University, Texas) and Vytenis Vasyliunas (Max Planck Institute, Lindau), Stan Cowley and Emma Bunce constructed a mathematical model which was the first to show how Jupiter’s ‘main auroral oval’ could be generated.

A key feature of the theory was the inclusion of a realistic model of Jupiter’s extended equatorial magnetic field, which resulted in a tenfold increase in the density of the currents connecting the ionosphere to the magnetosphere than in previous models, sufficient, as they showed, to require acceleration of magnetospheric electrons along the magnetic field lines into the ionosphere to energies of ~50-100 keV, thus producing the bright auroras observed. This model, together with its subsequent refinements, is at present the commonly accepted, but so far experimentally unproven, theory of the origin of Jupiter’s ‘main oval’ auroras, which led to Professor Cowley’s invitation to become a Co-Investigator on the Juno mission in 2003.

Subsequently in 2003/4, Emma Bunce, Stan Cowley, and Tim Yeoman also developed a theoretical model of Jupiter’s bright, highly variable and highly structured dayside polar auroras, based on the occurrence of dynamical plasma processes at the outer boundary of Jupiter’s magnetosphere where it meets the solar wind outflow from the Sun.

Further related work at Leicester has included further development of the Jupiter magnetosphere-ionosphere coupling model followed by application of the model to predictions of magnetic field and auroral particle observations by Juno using the initial planning orbits that were available in 2007. In 2012, systematic predict plots were made and provided to the international Juno team for each of the ~30 main mission spacecraft orbits, which are currently in process of being updated at Leicester by Research Fellow Dr Gabrielle Provan with the latest orbit predict information.

Figure 1 shows the trajectory of the spacecraft through Jupiter’s magnetic field (black lines) and field-aligned current layers (green lines) on one Juno orbit, blue dots being shown at intervals of one (Earth) day, while Figure 2 shows the predicted components of the magnetic field over an interval of 5 (Earth) days either side of closest approach to the planet, indicated by the black dot near the planet (the red sphere) in Figure 1.

The spacecraft itself does not, of course, oscillate up and down in its orbit around the planet as appears to be suggested in Figure 1, the figure actually shows the motion of the spacecraft relative to Jupiter’s magnetic field, and the field wobbles up and down across the spacecraft over the course of the Jovian day, due to a ~10 degree offset between the spin and magnetic poles of the planet similar to that present at Earth.

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