Energetic ion composition during reconfiguration events in the Jovian magnetotail

[1] On the basis of the first 15 orbits of the Galileo spacecraft the composition of the energetic ion population of the Jovian magnetosphere is studied on a global scale. Analyzing data from the energetic particles detector onboard Galileo, we study the relative ion abundance ratios of S/O, S/He, O/He and p/He at various energy/ nucleon values. Prominent enhancements of S/O, S/He, and O/He abundance ratios are observed in the predawn sector and are associated with substorm-like events in the Jovian magnetotail. During these reconfiguration events, frequent small-scale variations of the south-north component of the magnetic field are present. Acceleration by such magnetic field variations is examined as a possible mechanism for particle energization in that region. When the timescale of the magnetic field variation is comparable to the particle gyro period, the particle is accelerated by the induced electric field. It is shown that, during the Jovian substorm-like events, the heavier ions are more efficiently energized than the lighter leading to the observed ion abundances. Ion composition measurements in other parts of the magnetosphere have shown quasiperiodic 3-day modulations suggesting that the internally driven processes affect the ion composition in the whole magnetosphere.