Coalescence of Macroscopic Flux Ropes at the Subsolar Magnetopause: Magnetospheric Multiscale Observations

We report unambiguous in situ observation of the coalescence of macroscopic flux ropes by the magnetospheric multiscale (MMS) mission. Two coalescing flux ropes with sizes of $\sim 1R_E$ were identified at the subsolar magnetopause by the occurrence of an asymmetric quadrupolar signature in the normal component of the magnetic field measured by the MMS spacecraft. An electron diffusion region (EDR) with a width of four local electron inertial lengths was embedded within the merging current sheet. The EDR was characterized by an intense parallel electric field, significant energy dissipation, and suprathermal electrons. Although the electrons were organized by a large guide field, the small observed electron pressure nongyrotropy may be sufficient to support a significant fraction of the parallel electric field within the EDR. Since the flux ropes are observed in the exhaust region, we suggest that secondary EDRs are formed further downstream of the primary reconnection line between the magnetosheath and magnetospheric fields.

Figure 1

Overview of MMS2 observations between $02:10$ and $02:20\mathrm{UT}$. From the top to bottom are: (a) magnetic field vectors, (b) magnetic field strength, (c) ion bulk velocity, (d) ion density, (e) ion parallel (red) and perpendicular temperatures (blue), (f) ion and (g) electron differential energy fluxes. All vectors are in GSM coordinates.

Figure 2

(a)–(c) show the magnetic field and ion bulk velocity observed by MMS2 between $02:14$ and $02:18\mathrm{UT}$. (d) is a schematic of MMS orbits relative to the MP and FRs. (e) depicts the variations of $B_x$ recorded by the virtual MMS spacecraft shown in (d) for two different cases: the upper panel is the case without dissipation while the lower panel is the case with dissipation between the two FRs. (f) is a 3D schematic of field lines of two FRs in GSM coordinates, (g) is a zoomed-in 2D view of FR coalescence and MMS configuration in the $L\text{−}N$ plane.

Figure 3

Electron-scale layer embedded within the merging sheet. (a) magnetic field, (b) current density, (c) electron parallel (blue), perpendicular (red) and average (black) temperatures, (d)–(f) comparison of the three components of the measured electric field (black), $-{\mathbf{V}}_i\times \mathbf{B}$ (blue), and $-{\mathbf{V}}_e\times \mathbf{B}$ (red), (g) parallel electric field, magenta shading indicates the errors associated with the measurements, (h) $\mathbf{J}·{\mathbf{E}}^{\prime }=\mathrm{J}$·($\mathbf{E}+{\mathbf{V}}_e\times \mathbf{B}$), and (i) a measure of electron nongyrotropy. Red and black traces indicate the values that were calculated by using the upper and lower limit of the measured pressure, respectively. Orange shading marks the electron-scale layer with significant $E_{||}$ ($|{\mathrm{E}}_{||}|$ is larger than the error bar), (j) electron PAD, and (k) electron velocity distribution in the plane perpendicular to the magnetic field within the electron-scale layer.

Figure 4

Four spacecraft observations around the EDR. Magnetic fields: (a) $B_L$, (b) $B_M$, (c) $B_N$, electron flow: (d) $V_{\text{el}}$, (e) $V_{\text{eM}}$, (f) $V_{\text{eN}}$, (g) J·($\mathrm{E}+{\mathrm{V}}_e\times \mathrm{B}$), and (h) $|\nabla \times (\mathrm{E}+{\mathrm{V}}_e\times \mathrm{B})|$.