Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland 20723, USA
Robert B. Decker,
Stamatios M. Krimigis,
Edmond C. Roelof &
Matthew E. Hill
Academy of Athens, Athens 11527, Greece
Stamatios M. Krimigis
R.B.D. performed the data analysis and contributed to the text; S.M.K. contributed to the text; E.C.R. contributed to the text and provided theory interpretation; and M.E.H. analysed elemental composition.
Competing financial interests
The authors declare no competing financial interests.