By Julia E. Stawarz, Department of Physics, Imperial College London, UK.
In Stawarz et al. , we examine large- and small-scale properties of three ion-scale flux ropes in Earth’s magnetotail. Evidence of variability in the flux rope orientations is found and an electron-scale vortex is discovered inside one of the flux ropes.
Magnetic reconnection, which releases stored magnetic energy and converts it into particle motion, is a key driver of dynamics in Earth’s magnetosphere. However, it is still not fully understood how particles are accelerated and energy is partitioned both within the reconnection diffusion region, where particles decouple from the magnetic field, and within reconnection outflows. Helical magnetic fields known as flux ropes are one type of structure generated by reconnection and often observed within reconnection outflows [Borg et al., 2012; Eastwood & Kiehas, 2015; Sharma et al., 2008], which are both theoretically [Drake et al., 2006; Dahlin et al., 2017] and observationally [Chen et al., 2008] linked with particle energization. Previous observations have shown flux ropes can have substructure and intense electric fields [e.g., Eastwood et al., 2007], but the nature of these electric fields have not been previously determined. Recent high-time-resolution, mutispacecraft measurements with electron-scale separations from NASA’s Magnetospheric Multiscale (MMS) mission finally allow us to examine the detailed substructure of flux ropes.
The three closely spaced flux ropes examined in Stawarz et al.  are observed near a reconnection diffusion region and have different orientations, indicating significant spatiotemporal variability and highlighting the three-dimensional nature of the overall reconnection event. One of the most intense electric fields in the event is found within one of the flux ropes and is linked with an electron vortex (Fig. 1). The intense electric field is perpendicular to the magnetic field and the vortex consists of electrons that are frozen-in and ions that are decoupled from the fields. The resulting difference in motion between the ions and electrons drifting in the electromagnetic fields drives a current perpendicular to the magnetic field that produces a small-scale magnetic enhancement. The presence of such vortices may contribute to accelerating particles, either through inferred parallel electric fields at the ends of the structure or the excitation of waves, and points to the necessity of better understanding the substructure of flux ropes in order to characterize particle energization in magnetic reconnection.
For more information, see our paper below:
Stawarz, J. E., J. P. Eastwood, K. J. Genestreti, R. Nakamura, R. E. Ergun, D. Burgess, J. L. Burch, S. A. Fuselier, D. J. Gershman, B. L. Giles, O. Le Contel, P.-A. Lindqvist, C. T. Russell, & R. B. Torbert (2018), Intense electric fields and electron-scale substructure within magnetotail flux ropes as revealed by the Magnetospheric Multiscale mission, Geophys. Res. Lett., 45. https://doi.org/10.1029/2018GL079095
Figure 1: Overview of the electron vortex. (a) Electron-scale perturbation to the magnetic field with a 1s running average removed as observed by the four MMS spacecraft. (b,c) Components of the electric field perpendicular to the magnetic field as observed by the four MMS spacecraft. (d,e) Components of the current perpendicular to the magnetic field based on the curl of the magnetic field (black), moments of the ion and electron distribution functions (blue), and assuming the current is driven by electrons drifting in the electric and magnetic fields (red). (f) Diagram of the electron vortex encountered inside of one of the flux ropes. The observed profiles of the electric field and current are consistent with the indicated trajectories through the structure.