Abstract
Abstract. On 20 February 2005, Cluster in the outer magnetosphere and Double Star-2 (TC-2) at mid-altitude are situated in the vicinity of the northern cusp/mantle, with Cluster moving sunward and TC-2 anti-sunward. Their magnetic footprints come very close together at about 15:28 UT, over the common field-of-view of SuperDARN radars. Thanks to this conjunction, we determine the velocity, the transverse sizes, perpendicular and parallel to this velocity, and the shape of three magnetic flux tubes of magnetosheath plasma injection. The velocity of the structures determined from the Cluster four-spacecraft timing analysis is almost purely antisunward, in contrast with the antisunward and duskward convection velocity inside the flux tubes. The transverse sizes are defined from the Cluster-TC-2 separation perpendicular to the magnetic field, and from the time spent by a Cluster spacecraft in one structure; they are comprised between 0.6 and 2 RE in agreement with previous studies. Finally, using a comparison between the eigenvectors deduced from a variance analysis of the magnetic perturbation at the four Cluster and at TC-2, we show that the upstream side of the injection flux tubes is magnetically well defined, with even a concave front for the third one giving a bean-like shape, whereas the downstream side is far more turbulent. We also realise the first quantitative comparison between field-aligned currents at Cluster calculated with the curlometer technique and with the single-spacecraft method, assuming infinite parallel current sheets and taking into account the velocity of the injection flux tubes. The results agree nicely, confirming the validity of both methods. Finally, we compare the field-aligned current distribution of the three injection flux tubes at the altitudes of Cluster and TC-2. Both profiles are fairly similar, with mainly a pair of opposite field-aligned currents, upward at low-latitude and downward at high-latitude. In terms of intensity, the field-aligned currents at Cluster are two to three times less intense than at TC-2 for the first two flux tubes, in agreement with magnetic field line convergence. For the third flux tube, the intensity is equal, which is explained by the fact that TC-2 crosses the tube on its edge. Finally, the analysis of the ion and electron moments at Cluster shows that the field-aligned currents result from a small difference between upward ion and electron fluxes.
Highlights
Pulsed magnetic reconnection known as Flux Transfer Event (FTE) at the magnetopause and direct plasma injection in the polar cusp are responsible for a large part of mass and momentum transfer from the solar wind to the magnetosphere
The transverse sizes are defined from the Cluster-the outer magnetosphere and Double Star-2 (TC-2) separation perpendicular to the magnetic field, and from the time spent by a Cluster spacecraft in one structure; they are comprised between 0.6 and 2 RE in agreement with previous studies
The velocity of the flux tubes determined by the Cluster four-spacecraft timing analysis, is almost purely antisunward, in contrast with the antisunward and duskward convection velocity inside the flux tubes
Summary
Pulsed magnetic reconnection known as Flux Transfer Event (FTE) at the magnetopause and direct plasma injection in the polar cusp are responsible for a large part of mass and momentum transfer from the solar wind to the magnetosphere. The capabilities of the four-Cluster spacecraft mission have recently allowed refined studies of FTE size from 0.3 to 2 RE, together with estimations of their velocity varying from 70 to 200 km s−1 at the magnetopause (Owen et al, 2001; Bosqued et al, 2001; Sonnerup et al, 2004; Hasegawa et al, 2006). In the high-altitude cusp, the transverse size of reconnected flux tubes has been estimated with Cluster in the range 700–1500 km (for the longitudinal extent) with velocities of the tubes between 7 and 20 km s−1 during quiet IMF conditions, in agreement with ionospheric conjugate SuperDARN measurements (Vontrat-Reberac et al, 2003; Marchaudon et al, 2004a). In the magnetosphere a purely cylindrical or elliptical transverse shape has almost always been assumed, apart from Sonnerup et al (2004) who used a Grad-Shafranov reconstruction with Cluster data to obtain a map of a FTE cross section at the magnetopause, showing its irregular shape
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
