By Hans Huybrighs (Dublin Institute for Advanced Studies)
Io is a moon of Jupiter with active volcanoes and an atmosphere of which the structure and variability is poorly understood. A fascinating object on its own, neutral gas from Io also serves as the main source of plasma in Jupiter's magnetosphere. Improving our understanding of Io's atmosphere will allow us to better understand the precise link between Io's neutral environment and the plasma torus surrounding Jupiter. In this work we analyse data from the historic Galileo spacecraft that encountered Io. Specifically, we analyse regions close to Io where losses of energetic protons occur.
Using a comparison between the Galileo data and particle tracing model we find that charge exchange between the protons and Io's atmosphere along with the effect of the electric and magnetic fields can cause the measured decreases. We show that charge exchange with the atmosphere is either a major or the dominant cause of the losses, depending on the flyby altitude. The distributions of the proton loss region are related to the structure of Io’s atmosphere and hint at an atmosphere that is more extended on Io's day side (also the downstream side from the plasma flow's point of view), doesn't collapse fully on the night (upstream) side, and appears to be more extended than we assumed.
See publication for further information:
2024). Energetic proton losses reveal Io's extended and longitudinally asymmetrical atmosphere. Journal of Geophysical Research: Space Physics, 129, e2023JA032371. https://doi.org/10.1029/2023JA032371
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During periods of northward interplanetary magnetic field (IMF), the magnetospheric structure and dynamics are dramatically different and less understood compared to the southward IMF case.
Under northward IMF magnetic reconnection occurs at higher latitudes tailward of the cusp, known as lobe reconnection (Dungey, 1963). Lobe reconnection can occur on the same IMF field line in both hemispheres, known as dual lobe reconnection. It is thought that dual lobe reconnection can result in either the partial or complete closure of the magnetosphere (Milan et al., 2022). The auroral oval is also contracted to higher latitudes reflecting the reduced open flux content of the magnetosphere. The auroral emission is dimmer and distinct auroral features are observed poleward of the auroral oval such as cusp-aligned arcs, horse collar aurora and transpolar arcs.
Using Cluster and ARTEMIS in-situ data, we examined a period of prolonged northward IMF during which multiple instances of cusp-aligned arcs were observed poleward of the auroral oval. The Cluster observations showed trapped plasma on closed flux in the high latitude magnetotail (|ZGSE| ~ 13 RE) in regions which would typically be expected to be open magnetotail lobe void of plasma under southward IMF. Meanwhile, the ARTEMIS spacecraft observed simultaneous high electron and ion fluxes in the distant magnetotail (XGSE ~ - 60 RE). The plasma in both magnetotail regions was observed coincidently with observations of cusp-aligned arcs in the auroral data. We interpret these observations of trapped plasma on closed field lines as providing the source population for the cusp-aligned arc emission in the polar region.
During this interval we suggest that the magnetosphere was almost entirely closed as a result of dual lobe reconnection. The magnetotail is closed or partially closed but extends at least as far as ∼ 60 RE downtail. The occurrence of plasma in the magnetotail and the closure of the magnetosphere resulted in distinct changes to the magnetotail structure including a reduction in the magnetic field strength and pressure as well as a narrowing of the tail by approximately 20 RE.
See the full papers for further details:
Milan, S. E., Mooney, M. K., Bower, G. E., Taylor, M. G. G. T., Paxton, L. J., Dandouras, I., et al. (2023). The association of cusp-aligned arcs with plasma in the magnetotail implies a closed magnetosphere. Journal of Geophysical Research: Space Physics, 128, e2023JA031419. https://doi. org/10.1029/2023JA031419
Mooney, M. K., Milan, S. E., & Bower, G.E. (2024). Plasma observations in the distant magnetotail during intervals of northward IMF. Journal of Geophysical Research: Space Physics, 129,e2023JA031999. https://doi.org/10.1029/2023JA031999