Recently, with data from European Space Agency (ESA)’s Cluster spacecraft, SIGMA team under State Key Laboratory of Space Weather, National Space Science Center (NSSC), have made the first direct observations of multiple small-scale plasmoids during a substorm. Using a magnetohydrodynamic model, the team unraveled how the magnetic field properties in the magnetotail affected the formation of the plasmoids.
During a substorm, the production of small-scale plasmoids associated with Hall Effect in the magnetotail is considered as the signature of multiple X-line collisionless reconnection. However, no direct observation of such plasmoids has ever been made so far.
Using ESA’s Cluster spacecraft, for the first time the research team reported three small plasmoids during a single magnetic substorm (Figure 1). Results show that traveling plasmoids with a scale length of ~1 Earth radii are produced by multiple X-line reconnection. And through a series of Hall-MHD simulations, they managed to reproduce the main characteristics of small-scale plasmoids observed by Cluster, i.e., the quadrupole Hall magnetic field near the X-line, the core field in the plasmoids, and the plasma convection generation in plasmoids (Figure 2, 3).
Based on the modeled dynamics and the Cluster observations, the SIGMA team further concluded that the Hall Effect and the occurrence of a cross-tail magnetic field in the magnetotail are playing a crucial role in the formation of different types of plasmoids.
“The study on the small-scale plamoids will achieve major scientific significance for further understanding of the dynamical processes in the magnetotail” said LIU Chaoxu, one of SIGMA team members and lead author of the research article, entitledStudy of small-scale plasmoid structures in the magnetotail using Cluster observations and Hall MHD simulations.
The article was published on Journal of Geophysical Research (JGR) (Volume 118, doi:10.1002/jgra.50248,2013), and was chosen as one of American Geophysical Union (AGU)’s “Research Spotlights” published on AGU's weekly newspaper Eos,Vol. 94, No. 24, 11 June 2013.
Figure 1: Cluster observation of three successive plasmoids during the interval 13:13–13:18 UT on 12 September. Three vertical dashed lines denote plasmoid-1, 2, and 3. A bursty bulk flow after plasmoid-2 has been observed by the Cluster spacecrafts 1 and 3.
Figure 2: (a) The magnetic field line (solid line) and the velocity vector of the plasma flow (arrows), (b) contours of our-of-plane magnetic field By (color plot) for Case 1 with By0 = 0 at t = 10.5𝜏A, and (c) to (f) time variations of the magnetic field and plasma velocity at the given point.
Figure 3: Time evolution of the contour maps of out-of-plane magnetic field By for Case 2 with By0 = 0.5. Time variations of Bx, Bz, By and Vx for Case 2 at given point.