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Ionospheric signature of a magnetic flux rope in the magnetotail

A flux rope was detected in the near‐Earth magnetotail at X ≈ −20 RE by Geotail with simultaneous viewing of the northern polar region by Polar on January 11, 1997. This offers the opportunity to ascertain the optical signature in the ionosphere of a flux rope for the first time. The magnetotail flux rope was associated with tailward streaming of energetic particles and had an estimated core radius of ∼ 1.5 RE. The composition of energetic ions within the flux rope showed a temporal increase in the abundance of ionospheric N+ and O+ ions. Simultaneous with the flux rope encounter were auroral activations at ∼23 MLT (substorm onset) and ∼ 18 MLT. An extensive consideration of magnetic field projection of the Geotail location to the ionosphere and the associated auroral and particle signatures of this event lead to the inference that the optical signature of the flux rope was an auroral arc system at ∼ 18 MLT, which had an ∼2 h extent in MLT and was moving progressively poleward and westward.