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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 & 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.
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| Figure 1. THEMIS-E observations of a reconnection event at the dayside magnetopause on August 6, 2008, with vectors given in the LMN coordinates. The time resolutions for the magnetic field, the electric field, and the plasma data are 0.25 sec, 0.125 sec, and 3 sec, respectively. The electric field data are evaluated in the X-line frame. In the panel (e), the magenta line is the plasma beta (multiplied by 20) and its y-axis is to the right. The green dashed line denotes the location where the BL field component reverses sign. The grey dashed lines mark the start and end of the accelerated flow event, which is identified as the reconnection exhaust region. In the second panel, the horizontal blue dashed line denotes the estimated reconnection guide field of 1.6 nT, and the magenta bar indicates the duration of the Hall magnetic field, where Δt1= 21.0 sec and Δt2= 18.8 sec. |
| Click here to enlarge the image. |
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) x 200 km (length).
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| Figure 2. Two-dimensional cross section of the magnetic island with the axial field BZ in color, using the magneto-hydrostatic GS reconstruction for the force-free conditions. The measured transverse magnetic fields are shown by the red arrows at y= 0. The positive y direction is toward the Earth. |
| Click here to enlarge the image. |
Source
Biographical Note
Wai-Leong Teh is a research associate at the Space Research Insitute in Graz, Austria. Stefan Eriksson and Robert Ergun are LASP researchers. Bengt Sonnerup is an emeritus professor at the Thayer School of Engineering, Dartmouth College, New Hampshire.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu