Abstract
The Earth constantly loses matter, mostly in the form of H+ and O+ ions, through various outflow processes from the upper atmosphere and ionosphere. Most of these ions are cold (below 1 eV in thermal energy), but can still escape and travel farther out along the magnetic field lines into the magnetospheric tail lobes1,2. The outflow has previously been measured close to the Earth3,4,5,6,7. To understand what fraction does not return but instead escapes, the measurements should be conducted at larger geocentric distances. However, at high altitudes the cold ions are normally invisible to spacecraft measurements, because the potential of a sunlit spacecraft exceeds the equivalent energy of the ions8,9. Here we show that cold ions dominate in both flux and density in the distant magnetotail lobes, using a new measurement technique on the Cluster spacecraft10,11. The total loss of cold hydrogen ions from the planet is inferred to be of the order of 1026 s−1, which is larger than the previously observed more energetic outflow12,13. Quantification and insight of the loss processes of the Earth’s atmosphere and ionosphere are also important for understanding the evolution of atmospheres on other celestial bodies.
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Acknowledgements
Magnetic field data from the Cluster FGM instrument (principal investigator E. Lucek) has been used in the analysis.
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E.E. analysed the data, contributed to method development and had the main responsibility for writing the paper, A.I.E. developed the method and contributed to analysing the data and writing the paper, C.C. contributed to analysing the data and writing the paper, M.A. had the main responsibility for the EFW instrument and contributed to writing the paper, R.T. had the main responsibility for the EDI instrument and R.T. and H.V. provided EDI data and comments on the EDI instrument operation.
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Engwall, E., Eriksson, A., Cully, C. et al. Earth’s ionospheric outflow dominated by hidden cold plasma. Nature Geosci 2, 24–27 (2009). https://doi.org/10.1038/ngeo387
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DOI: https://doi.org/10.1038/ngeo387
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