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

Area of Science:

• Space Physics
• Plasma Physics
• Astrophysics

Background:

• Magnetic reconnection is a fundamental plasma process driving energy transfer in astrophysical systems.
• Flux ropes are coherent magnetic structures prevalent in space plasmas.
• Understanding these phenomena is key to magnetospheric dynamics.

Purpose of the Study:

• To report the first in situ observation of macroscopic flux rope coalescence.
• To investigate the associated electron diffusion region (EDR) characteristics.
• To explore the role of electron pressure nongyrotropy in supporting electric fields.

Main Methods:

• Utilizing data from the Magnetospheric Multiscale (MMS) mission spacecraft.
• Identifying flux rope coalescence via asymmetric quadrupolar magnetic field signatures.
• Analyzing magnetic field, electric field, and particle distribution data within the current sheet.

Main Results:

• Observed two macroscopic flux ropes (∼1 R_E) coalescing at the subsolar magnetopause.
• Detected an electron diffusion region (EDR) within the merging current sheet.
• Characterized the EDR by intense parallel electric fields, energy dissipation, and suprathermal electrons.

Conclusions:

• The observed EDR, though influenced by a guide field, showed electron pressure nongyrotropy potentially supporting the parallel electric field.
• Suggests the formation of secondary EDRs downstream of the primary reconnection site.
• Provides crucial in situ evidence for flux rope coalescence dynamics in Earth's magnetosphere.