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tokens: We would like to thank the referee for constructive comments that helped improved the presentation of this work. We would like to thank A. Casey for discussions that improved this work. K.H. is funded by the British Marshall Scholarship program and the King’s College, Cambridge Studentship. This work was partly supported by the European Union FP7 programme through ERC grant number 320360. U.H. acknowledges support from the Swedish National Space Board (Rymdstyrelsen).