Electrodynamics of a flux transfer event: Experimental test of the Southwood model.

On 12 September 1999, a conjunction between two SuperDARN radars and the Ørsted satellite gave, for the first time, simultaneous access to the ionospheric convection enhancement and the field-aligned currents (FACs) associated with a Flux Transfer Event. The radars observed an azimuthally elongated convection flow burst and the Ørsted satellite observed a series of successive small-scale parallel currents alternating between downward and upward. The most poleward pair of currents, whose directions were in agreement with the Southwood model, was observed when Ørsted crossed the front edge of the flow burst. A quantitative comparison of the current density of each FAC and of the Pedersen current density indicates that the closure current for this FACs pair occurred inside the flow burst, confirming the validity of the Southwood model. The Poynting flux carried by the parallel currents was less than 1% of the power carried by the solar wind plasma

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

Particle and magnetic field observations during a magnetic conjunction Cluster 1-FAST-Søndrestrøm within the field of view of SuperDARN radars on 21 January 2001 allow us to draw a detailed, comprehensive and self-consistent picture at three heights of signatures associated with transient reconnection under a steady south-westerly IMF (clock angle ≈130◦). Cluster 1 was outbound through the high altitude (∼12RE ) exterior northern cusp tailward of the bifurcation line (geomagnetic Bx>0) when a solar wind dynamic pressure release shifted the spacecraft into a boundary layer downstream of the cusp. The centerpiece of the investigation is a series of flow bursts observed there by the spacecraft, which were accompanied by strong field pertur- bations and tailward flow deflections. Analysis shows these to be Alfven waves. We interpret these flow events as being due to a sequence of reconnected flux tubes, with field-aligned currents in the associated Alfven waves carrying stresses to the underlying ionosphere, a view 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 ion steps. The Søndrestrøm radar observed pulsed ionospheric flows (PIFs) just poleward of the convection reversal boundary. As at Cluster, the flow was eastward (tailward), implying a coherent eastward (tailward) motion of the hypothesized open flux tubes. The joint Søndrestrøm and FAST observations indicate that the open/closed field line boundary was equatorward of the convection reversal boundary by ∼2 deg. The unprecedented accuracy of the conjunction argues strongly for the validity of the interpretation of the various signatures as resulting from transient reconnection. In particular, the cusp ion steps arise on this pass from this origin, in consonance with the original pulsating cusp model. The observations point to the need of extending current ideas on the response of the ionosphere to transient reconnection. Specifically, it argues in favor of re-establishing the high-latitude boundary layer downstream of the cusp as an active site of momentum transfer

Amplitude equations for Rayleigh-Benard convective rolls far from threshold

An extension of the amplitude method is proposed. An iterative algorithm is developed to build an amplitude equation model that is shown to provide precise quantitative results even far from the linear instability threshold. The method is applied to the study of stationary Rayleigh-Benard thermoconvective rolls in the nonlinear regime. In particular, the generation of second and third spatial harmonics is analyzed. Comparison with experimental results and direct numerical calculations is also made and a very good agreement is found.Peer reviewe

ESR and EISCAT observations of the response of the cusp and cleft to IMF orientation changes

International audienceWe report observations of the cusp/cleft ionosphere made on December 16th 1998 by the EISCAT (European incoherent scatter) VHF radar at Tromsø and the EISCAT Svalbard radar (ESR). We compare them with observations of the dayside auroral luminosity, as seen by meridian scanning photometers at Ny Ålesund and of HF radar backscatter, as observed by the CUTLASS radar. We study the response to an interval of about one hour when the interplanetary magnetic field (IMF), monitored by the WIND and ACE spacecraft, was southward. The cusp/cleft aurora is shown to correspond to a spatially extended region of elevated electron temperatures in the VHF radar data. Initial conditions were characterised by a northward-directed IMF and cusp/cleft aurora poleward of the ESR. A strong southward turning then occurred, causing an equatorward motion of the cusp/cleft aurora. Within the equatorward expanding, southward-IMF cusp/cleft, the ESR observed structured and elevated plasma densities and ion and electron temperatures. Cleft ion fountain upflows were seen in association with elevated ion temperatures and rapid eastward convection, consistent with the magnetic curvature force on newly opened field lines for the observed negative IMF By. Subsequently, the ESR beam remained immediately poleward of the main cusp/cleft and a sequence of poleward-moving auroral transients passed over it. After the last of these, the ESR was in the polar cap and the radar observations were characterised by extremely low ionospheric densities and downward field-aligned flows. The IMF then turned northward again and the auroral oval contracted such that the ESR moved back into the cusp/cleft region. For the poleward-retreating, northward-IMF cusp/cleft, the convection flows were slower, upflows were weaker and the electron density and temperature enhancements were less structured. Following the northward turning, the bands of high electron temperature and cusp/cleft aurora bifurcated, consistent with both subsolar and lobe reconnection taking place simultaneously. The present paper describes the large-scale behaviour of the ionosphere during this interval, as observed by a powerful combination of instruments. Two companion papers, by Lockwood et al. (2000) and Thorolfsson et al. (2000), both in this issue, describe the detailed behaviour of the poleward-moving transients observed during the interval of southward Bz, and explain their morphology in the context of previous theoretical work

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