电子重联中电磁场结构的粒子模拟
doi: 10.11728/cjss2024.06.2024-yg31 cstr: 32142.14.cjss.2024-yg31
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胡市航 男, 2001年3月出生于河北省武安市, 现为中国科学技术大学地球和空间科学学院在读硕士研究生. 主要研究方向为磁场重联的实验研究和数值模拟. E-mail: shihang_hu@mail.ustc.edu.cn -
陆全明 男, 1969年生, 现为中国科学技术大学地球和空间科学学院教授, 博士生导师. 长江学者特聘教授, 国家杰出青年科学基金获得者. 主要研究方向为空间等离子体物理和粒子模拟高性能计算. E-mail: qmlu@ustc.edu.cn
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Particle-in-cell Simulation of Electromagnetic Field Structure in the Electron-only Reconnection
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摘要: 采用2.5D全粒子模拟程序研究了强引导场情况下的电子重联, 分析了重联率最大时刻霍尔磁场、霍尔电场及电子流速的空间分布特征. 重联率最大时刻, 观察到分离线两侧的电子入流及出流, 并且有四极霍尔磁场生成. 与早期标准重联的研究结果不同, 霍尔磁场并未在引导场的作用下发生明显扭曲. 同时分离线区域发生电荷分离, 并生成近似对称分布的霍尔电场. 此外, 研究了电子流速空间分布的时间演化. 在重联前期, 电子主要沿磁力线流动, 即 $ \boldsymbol{v}_{{\mathrm{e}}}\approx v_{\mathrm{e}y}\boldsymbol{B}/B_y $; 在重联后期, 电子运动由电场漂移所主导, 即 $ \boldsymbol{v}_{{\mathrm{e}}}\approx\boldsymbol{E}\times\left(B_y\boldsymbol{e}_{{{y}}}\right)/B_y^2 $. 模拟结果表明, 电子重联中可以发生极强的电荷分离, 生成近似对称分布的霍尔电场. 在引导场下, 这种霍尔电场将导致电子的电场漂移, 并主导电子流速的面内空间分布, 进而生成近似对称分布的霍尔电流和四极霍尔磁场.Abstract: Standard collisionless magnetic reconnection couples with both electron and ion dynamics. Recently, a new type of magnetic reconnection, electron-only magnetic reconnection without ion outflow, has been observed. Using $ 2.5 $D particle-in-cell simulation, the electromagnetic field structure in the electron-only reconnection with a strong guide field was studied. At the moment of the maximum reconnection rate, the electron inflow and outflow are observed on either side of the separatrix. The spatial distribution of the electron bulk velocity is approximately symmetric, which generates the nearly symmetrically distributed Hall current, resulting in the quadrupole Hall magnetic field. The Hall magnetic field is not obviously distorted despite the presence of the strong guide field. Meanwhile, the charge separation is caused in the separatrix region, which generates the nearly symmetrically distributed Hall electric field. Besides, the evolution of the spatial distribution of the electron bulk velocity was studied. The equation of motion for the frozen electrons was analytically obtained: $ \boldsymbol{v}_{\rm{{e}}}=v_{\mathrm{e}y}\boldsymbol{B}/B_y+\boldsymbol{E}\times\left(B_y\boldsymbol{e}_{{{y}}}\right)/B_y^2 $. According to the equation of motion for the electrons, we divide the electron-only reconnection with a strong guide field into two stages. In the first stage, the $ \boldsymbol{E}\times \boldsymbol{B} $ drift is negligible because of the weaker Hall electric field, and then the electrons flow mainly along the magnetic field line following the equation $ \boldsymbol{v}_{\rm{{e}}}\approx v_{\mathrm{e}y}\boldsymbol{B}/B_y $. In the second stage, the Hall electric field is so strong that the motion of electrons is dominated by the $ \boldsymbol{E}\times \boldsymbol{B} $ drift following the equation $ \boldsymbol{v}_{\rm{{e}}}\approx\boldsymbol{E}\times\left(B_y\boldsymbol{e}_{{{{y}}}}\right)/B_y^2 $. The simulation shows that extremely strong charge separation can be caused in electron-only reconnection, which generates the nearly symmetrically distributed Hall electric field. With a guide field, this Hall electric field leads to the $ \boldsymbol{E}\times \boldsymbol{B} $ drift, which dominates the spatial distribution of the electron bulk velocity. Therefore nearly symmetrically distributed Hall current is formed, which generates the nearly symmetrically distributed quadrupole Hall magnetic field.
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图 1 重联点磁通量$ \varPsi $和重联率$ \mathrm{d}\varPsi /\mathrm{d}t $随时间的演化
Figure 1. Temporal evolution of the reconnected magnetic flux $ \varPsi $ and the reconnection rate $ \mathrm{d}\varPsi /\mathrm{d}t $ at the X-line
图 2 $ {\varOmega }_{\mathrm{i}}t=0.45 $时刻面外重联电场$ {E}_{y} $, 电子面外电流$ {J}_{\mathrm{e}y} $, 离子面外电流$ {J}_{\mathrm{i}y} $和$ {J}_{\mathrm{e}y}{E}_{y} $面内空间分布 (黑色实线表示磁力线)
Figure 2. Color contours of the reconnection electric field $ {E}_{y}$, the out-of-plane current of electrons $ {J}_{\mathrm{e}y} $, the out-of-plane current of ions $ {J}_{\mathrm{i}y} $, and $ {J}_{\mathrm{e}y}{E}_{y} $ at $ {\varOmega }_{\mathrm{i}}t=0.45 $ (The black lines represent the magnetic field lines)
图 3 $ {\varOmega }_{\mathrm{i}}t=0.45 $时刻$ x $方向离子流速$ {v}_{\mathrm{i}x} $, $ z $方向离子流速$ {v}_{\mathrm{i}z} $, $ x $方向电子流速$ {v}_{\mathrm{e}x} $和$ z $方向电子流速$ {v}_{\mathrm{e}z} $的面内空间分布(黑色实线表示磁力线)
Figure 3. Color contours of the ion bulk velocity in the $ x $ direction $ {v}_{\mathrm{i}x} $, the ion bulk velocity in the $ z $ direction $ {v}_{\mathrm{i}z} $, the electron bulk velocity in the $ x $ direction $ {v}_{\mathrm{e}x} $, and the electron bulk velocity in the $ z $ direction $ {v}_{\mathrm{e}z} $ at $ {\varOmega }_{\mathrm{i}}t=0.45 $ (The black lines represent the magnetic field lines)
图 4 $ {\varOmega }_{\mathrm{i}}t=0.45 $时刻霍尔磁场$ {\Delta }{B}_{y}={B}_{y}-{B}_{\mathrm{g}} $, 电荷密度$ \rho $, 面内电场$ {E}_{x} $和面内电场$ {E}_{z} $的面内空间分布 (黑色实线表示磁力线)
Figure 4. Color contours of the Hall magnetic field $ {\Delta }{B}_{y}={B}_{y}-{B}_{\mathrm{g}} $, the charge density $ \rho $, the electric field $ {E}_{x} $, and the electric field $ {E}_{z} $ at $ {\varOmega }_{\mathrm{i}}t=0.45 $ (The black lines represent the magnetic field lines)
图 5 $ {\varOmega }_{\mathrm{i}}t=0.10 $和$ {\varOmega }_{\mathrm{i}}t=0.45 $时刻$ x $方向电子流速$ {v}_{\mathrm{e}x} $和$ z $方向电子流速$ {v}_{\mathrm{e}z} $的面内空间分布 (黑色实线表示磁力线)
Figure 5. Color contours of the electron bulk velocity in the $ x $ direction $ {v}_{\mathrm{e}x} $ and the electron bulk velocity in the $ z $ direction $ {v}_{\mathrm{e}z} $ at $ {\varOmega }_{\mathrm{i}}t=0.10 $ and $ {\varOmega }_{\mathrm{i}}t=0.45 $ (The black lines represent the magnetic field lines)
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