Nuove conoscenze sulla riconnessione magnetica dai dati della missione Cluster. M.F. Marcucci

1. Nuove conoscenze sulla riconnessione magnetica dai dati della missione Cluster. M.F. Marcucci

2. Outline • Introduzione • 3 esempi di osservazioni di Cluster durante la riconnessione • Prospettive future

3. La riconnessione magnetica converte rapidamente l’energia magnetica dei plasmi spaziali in energia termica e cinetica. • E’ un processo universale che si verifica in regioni di plasma diversissime per valori di densità e temperatura. • A tutt’oggi, domande fondamentali riguardanti questo processo non hanno risposta.

4. Simulations show that resistive MHD gives elongated (system size) Sweet-Parker layer and very low reconnection rates (Birn et al. 2001)

5. Quando la riconnessione è di tipo Petschek il tasso di riconnessione raggiunge valori confrontabili con quelli osservati E’ fondamentale descrivere la struttura della regione di diffusione e capire se a guidare il processo è la fisica in questa regione, o le condizioni su grande scala.

6. The ESA cornerstone mission CLUSTER Launched 2000 –Extended to 2009 Cluster is the first spacefleet flying in close formation to study fundamental plasma processes in three dimensions. Cluster explores some of the most important near-Earth regions. During 2004-2007 operated together with the ESA-CSNA Double Star mission. IFSI participated in the design and development of the plasma analyzers in the framework of an international collaboration.

7. 1 Kinetic features of plasma near the X-point ~ 3000 km Bavassano Cattaneo et al., 2006

8. 350000 km Kinetic features of plasma near the X-point 350000 km ~ 3000 km Yokoyama, 2001 Bavassano Cattaneo et al., 2006

9. MP Vt Vn E Reconnection is continous while large scale conditions changes C1C3 C4 Retinò et al. 2005, Bavassano Cattaneo et al. 2006

10. Reconnection is continous while large scale conditions changes C1C3 C4 Spacecraft Separation 1000 km Time Interval 4 hours Retinò et al. 2005, Bavassano Cattaneo et al. 2006

11. Physics of the diffusion region Two component plasma full generalized Ohm’s law The Hall term enables fast reconnection also in collisionless plasma (Birn et al. 2001)

12. Hall currents system quadrupolar out-of-plane magnetic field Hall term signatures (Sonnerup 1979): Adapted from Nakamura, 2005

13. 2 Hall magnetic field observations 2D two fluid MHD numerical simulation of the diffusion region and Cluster configuration. Cluster resolve the spatial and temporal ambiguity always present in the single spacecraft measurements and observe unambiguously the Hall term signature. Spacecraft Separation 100 km Time Interval 4 s Vaivads et al. 2004

14. Observation of an Elongated Electron Diffusion Region 3 Recently particle in cell simulations show that the electron diffusion region develops a distinct two-scale structure along the outflow direction: An out-of-plane current layer of length li A super-Alfvenic outflow jet of electrons extending 10s of li Phan et al. 2007, Shay et al. 2007

15. Observation of an Elongated Electron Diffusion Region Time Interval 15 s Phan et al. 2007, Shay et al. 2007

16. Observation of an Elongated Electron Diffusion Region Time Interval 15 s Phan et al. 2007, Shay et al. 2007

17. Unanswered Questions How is reconnection initiated? – What triggers reconnection within a current sheet? • What parameters control the spatial/temporal characteristics of reconnection site? – How is a reconnection neutral line structured ? • What are the consequences of reconnection? – How are ions and electrons energized as a consequence of reconnection ? It is important to note that reconnection is a multi-scale process

18. Simultaneous 3D measurements on all three scales – electron, ion and fluid – are crucial. This is the scope of the Cross-Scale mission Cross-Scale has been selected by ESA's Space Science Advisory Panel to proceed to the Assessment Phase of Cosmic Vision 2015-2025