Abstract
Magnetic reconnection plays a key role in the conversion of magnetic energy into the thermal and kinetic energy of plasma. On either side of the diffusion region in space plasma, the conditions for the occurrence of reconnections are usually not symmetric. Previous theoretical studies have predicted that reconnections under asymmetric conditions will bear different features compared with those of symmetric reconnections, and numerical simulations have verified these distinct features. However, to date, the features of asymmetric reconnections have not been thoroughly investigated using in situ observations; thus, some results from theoretical studies and simulations have not been tested with observations sufficiently well. Here, spacecraft observations are used in a statistical investigation of asymmetric magnetic reconnection exhaust at the dayside magnetopause. The resulting observational features are consistent with the theoretical predictions. The results presented here advance our understanding of the development of reconnections under asymmetric conditions.
Highlights
Region R1 corresponds to the magnetosheath, which has a southward magnetic field, low magnetic intensity, low temperature and high plasma density; Region R2 corresponds to the magnetosphere, which has a northward magnetic field, high magnetic intensity, high temperature and low plasma density; and Region R3 corresponds to the exhaust region, which presents reconnection jets and has a moderate plasma density and temperature
In our statistical investigation of the separation of the BL rotation point (BRP) and the plasma jump point (PJP) in the exhaust region, we found the BRP is tilted towards the magnetosheath side and the PJP is tilted towards the magnetospheric side, in agreement with the features of the BRP and the PJP in the diffusion region[4,23]
We presented an analysis of the fast flow under asymmetric conditions based on TC-1 satellite observations of the dayside magnetopause (10LT–14LT) for the four-year period from 2004 to 2007
Summary
To evaluate the background conditions of the magnetosphere side and the magnetosheath side, the average of 5 consecutive stable data points (16 sec time intervals without great fluctuations between the data points) adjacent to the exhaust region on either side are considered to be representative of the sheath and magnetosphere plasma. The shear angle (θ°) of the magnetic field lines between the sheath side and the magnetosphere side is 174.8° ± 8.9°, as estimated by θ° = acos (B1 ⋅B2) The reconnection, in this case, developed under highly asymmetric and nearly antiparallel conditions. As shown by the Walen test results in Methods section later, in most cases of this study, the observed reconnection jets can match the local Alfven speed; it is reasonable that we take the observed jets as the proxy of the out-flow speed when compared to the predicted value. In the case of an antiparallel reconnection, the BM can approximately be the proxy of the Hall field (Fig. 1); in this case, the Hall fields are on the sheath side of the exhaust region, but nearly disappear on the magnetosphere side
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