Secondary Magnetic Island Observed at the Magnetopause Diffusion Region

Secondary Magnetic Island Observed at the Magnetopause Diffusion Region

Secondary Magnetic Island Observed at the Magnetopause Diffusion Region

Wai-Leong Teh, Stefan Eriksson, Bengt Sonnerup, and Robert Ergun


Magnetic reconnection is a crucial mechanism for converting magnetic field energy into plasma kinetic and thermal energy in the space plasma environment. Physical processes in the ion and electron diffusion regions play a key role in allowing reconnection to occur and in controlling its behavior. Understanding the nature of these processes is therefore of primary importance. Recent numerical simulations indicate that the reconnection rate is correlated with the formation of secondary magnetic islands via the tearing mode instability, operating in the highly elongated diffusion regions. In this report, we present THEMIS observations of a secondary magnetic island within the ion diffusion region during anti-parallel reconnection at the dayside magnetopause. We also derive a two-dimensional (2-D) magnetic field map of the island using magneto-hydrostatic Grad-Shafranov (GS) reconstruction for the special case of force-free conditions.

Observations and Results

Figure 1 shows THEMIS-E observations of a reconnection event at the dayside magnetopause on August 6, 2008, with vectors given in the LMN coordinates. The reconnecting magnetic field (the L-component) is symmetric and anti-parallel whereas the plasma density is asymmetric across the magnetopause, with a ratio of ~8. There are two key findings for this event. First, the ion diffusion region was identified by the quadrupolar Hall magnetic fields and the bipolar Hall electric fields, as shown by the guide-field M-component of the magnetic field (black) in Figure 1b and by the normal N-component of the electric field (red) in Figure 1c. Second, a bipolar signature of the N-component of the magnetic field (as shown by the black line in Figure 1c near 21:14:28 UT) was found within the Hall magnetic field on the magnetosheath side.

The signs of the observed Hall fields are consistent with those predicted for the southward-directed reconnection jet at the magnetopause. The observations show that the Hall magnetic and electric fields are asymmetric and stronger on the magnetospheric side of the magnetopause. Figure 2 shows the 2-D magnetic field map of the island (bipolar BN signature) obtained from magneto-hydrostatic GS reconstruction for force-free conditions, i.e., when the gradient of the plasma pressure is neglected. The reconstruction indicates that the size of the island is only about 100 km (width) × 200 km (length). For more details, see


We present the first (THEMIS) observation of a small-scale secondary magnetic island within the ion diffusion region at the dayside magnetopause for anti-parallel field conditions. This finding indicates that magnetic islands may play an important role in the reconnection process of the diffusion regions, as suggested by recent reconnection simulation studies.

Biographical Note

Wai-Leong Teh is a research associate at the Laboratory for Atmospheric and Space Physics (LASP), University of Colorado at Boulder. Stefan Eriksson and Robert Ergun are LASP researchers. Bengt Sonnerup is an emeritus professor at the Thayer School of Engineering, Dartmouth College, New Hampshire.