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Particle and magnetic field observations during amagnetic conjunction Cluster 1-FAST-S\uf8ndrestr\uf8m within the field of viewof SuperDARN radars on 21 January 2001allow us to draw a detailed, comprehensive and self-consistent picture atthree heights of signaturesassociated with transient reconnection under a steady south-westerlyIMF (clock angle ≈130°).Cluster 1 was outbound through the high altitude (~12RE) exterior northern cusptailward of the bifurcation line (geomagnetic Bx>0)when a solar wind dynamic pressure releaseshifted the spacecraft into a boundary layerdownstream of the cusp.The centerpiece of the investigation is a series of flow bursts observed there by the spacecraft, which were accompanied by strongfield perturbations and tailward flow deflections.Analysis shows these to be Alfv\ue9n waves. We interpret these flow events as being due to a sequence of reconnected flux tubes,with field-aligned currentsin the associated Alfv\ue9n waves carrying stressesto the underlying ionosphere, aview strengthened by the other observations.At the magnetic footprint of the region of Cluster flow bursts, FAST observed an ion energy-latitude disperison of the stepped cusp type, with individual cusp ion steps corresponding to individual flow bursts. Simultaneously, the SuperDARN Stokkseyri radar observed very strong poleward-moving radar auroral forms(PMRAFs)which were conjugate to the flow bursts at Cluster.FAST was traversing these PMRAFs when it observed the cusp ionsteps.The S\uf8ndrestr\uf8m radar observed pulsed ionospheric flows(PIFs)just poleward of the convection reversal boundary. As at Cluster, the flow waseastward (tailward), implying a coherenteastward (tailward) motion of the hypothesized open flux tubes.The joint S\uf8ndrestr\uf8m and FAST observations indicate that theopen/closed field line boundary was equatorward of theconvection reversal boundary by ~2°. The unprecedented accuracy of the conjunction argues strongly forthe validity of the interpretation of the varioussignatures as resulting from transient reconnection. In particular, the cusp ion steps ariseon thispass from this origin, in consonance with the original pulsating cuspmodel. The observations point to the need of extending currentideas on the response of the ionosphere to transient reconnection. Specifically,it argues in favor of re-establishing the high-latitude boundary layerdownstream of the cusp as an active siteof momentum transfer

annales geophysicae

Pulsed flows at the high-altitude cusp poleward boundary, and associated ionospheric convection and particle signatures, during a Cluster - FAST - SuperDARN- Søndrestrøm conjunction under a southwest IMF