How Alfvén waves set the largescale structure of magnetic reconnection. *

Kinetic Alfvén waves (KAWs) has been postulated as a possible source of energy source for the aurora[1]. The simulation performed in the earlier studies were on small length and time scales, but in large simulation domain we observe that these waves do not propagate all the way into the exhaust.  The simulation domain used in this study is large (200d_i × 30d_i) helping us study the nature of waves which carry the reconnection signature further downstream into the exhaust. We observe that the large-scale structure or the Hall field spreads into the inflow perpendicular to the field lines due to the propagation of waves, generated at the separatrix. Near the X-line these waves have wavelengths significantly smaller than ion inertial lengths d_i, and hence are dispersive in nature. Away from the X-line as the wave propagates in the exhaust the wavenumber k decreases, hence decreasing their propagation velocity. From the extended simulation domain, it is clear that these waves are super-Alfvénic (~2V_a0) and dispersive near the separatrix but they become Alfvénic (1.2V_a0) in the exhaust. Electrons accelerated by these Alfvénic waves precipitate in the ionosphere and cause the aurora.

[1] Shay M.A. et al. PRL 107, 065001 (2011)