Effects of bursty bulk flows on global-scale current systems

Bursty bulk flows (BBFs) are often considered as the products of the near-Earth reconnection which ejects opposite plasma flows at high speed, playing effective and crucial roles in transporting the mass, momentum, and energy in the tail magnetosphere. Following the reconnection, current system often experiences large perturbations and the global magnetic field is reconfigured. Although the MHD model employed in many studies can capture BBFs and associated phenomena, a global view of the 3-D current system commonly follows schematic illustrations inferred from the simulations. Furthermore, due to the lack of kinetic physics in the MHD models, few simulation studies have unveiled the impact of BBFs on the ring current system in the inner magnetosphere.

Recently, Professor Yiqun Yu and her collaborators from the School of Space and Environment at Beihang University utilized a coupled modeling framework, in which the kinetic physics of the inner magnetosphere represented in a ring current model was coupled to the MHD model, to investigate the BBFs caused by the tail reconnection and their subsequent influences on the global-scale current system, particularly on the ring current. The powerful global model enables a full 3-D view of various current systems and their intermediate relationships.

It is found that the penetrated flows from the tail generate multiple vortices in the inner magnetosphere after they brake, significantly modulating the westward-flowing ring current. The vortices cause radially inward intrusion of the ring current and disturbances on the pressure distribution, which gives rise to field-aligned component of the current system. When the current is largely disturbed towards the high latitude, it is diverted into the ionosphere as a form of field-aligned current (FACs), leading to current eddies there.

Such a diversion of FACs from the inner magnetosphere ring current system, rather than from the tail current widely known as substorm current wedge, is discovered with the aid of a combination of a global MHD model and a kinetic ring current model.  The simulation reveals important mesoscale structures in the inner magnetosphere associated with the tail substorm injections, which is very difficult to reconstruct even with multi-spacecraft observations.




Fig. Three-dimensional view of the current systems (colored streamlines) following fast flows from the tail brake around X=-10 Re. Two types of vortices (contours in the equatorial plane) are formed, one at the edge of the braking region, and the other in the inner magnetosphere. The westward ring current is disturbed to arch towards high latitudes, giving rise to field-aligned currents (FACs) to be connected to the ionosphere. This connection between the westward ring current and ionosphere current via FACs is new in addition to the substorm current wedge that connects the tail currents with the ionosphere via FACs.



Yiqun Yu, professor, school of space and environment, Beihang University, E-mail:



Yu, Y., J. Cao, H. Fu, H. Lu, and Z. Yao (2017), The effects of bursty bulk flows on global-scale current systems, Journal of Geophysical Research-Space physics, 122 6139-6149 doi:10.1002/2017JA024168.