TY - JOUR
T1 - Plasmoid Instability in Evolving Current Sheets and Onset of Fast Reconnection
AU - Huang, Yi Min
AU - Comisso, Luca
AU - Bhattacharjee, A.
N1 - Funding Information:
Beneficial discussion with Dr.Lijia Guo on IRIS observation is gratefully acknowledged. We thank the anonymous referee for many constructive suggestions. We also thank Dr.Nick Murphy for useful conversations when the paper was under revision. This work is supported by the National Science Foundation, Grant Nos.AGS-1338944 and AGS-1460169, and the Department of Energy, Grant No.DE-SC0016470. Simulations were performed with supercomputers at the Oak Ridge Leadership Computing Facility and the National Energy Research Scientific Computing Center.
Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.
PY - 2017/11/10
Y1 - 2017/11/10
N2 - The scaling of the plasmoid instability maximum linear growth rate with respect to the Lundquist number S in a Sweet-Parker current sheet, γmax ∼ S1/4, indicates that at high S, the current sheet will break apart before it approaches the Sweet-Parker width. Therefore, a proper description for the onset of the plasmoid instability must incorporate the evolving process of the current sheet. We carry out a series of two-dimensional simulations and develop diagnostics to separate fluctuations from an evolving background. It is found that the fluctuation amplitude starts to grow only when the linear growth rate is sufficiently high (γmax τA > O(1)) to overcome advection loss and the stretching effect due to the outflow. The linear growth rate continues to rise until the sizes of plasmoids become comparable to the inner layer width of the tearing mode. At this point, the current sheet is disrupted and the instability enters the early nonlinear regime. The growth rate suddenly decreases, but the reconnection rate starts to rise rapidly, indicating that current sheet disruption triggers the onset of fast reconnection. We identify important timescales of the instability development, as well as scalings for the linear growth rate, current sheet width, and dominant wavenumber at disruption. These scalings depend not only on the Lundquist number, but also on the noise amplitude. A phenomenological model that reproduces scalings from simulation results is proposed. The model incorporates the effect of reconnection outflow, which is crucial for yielding a critical Lundquist number Sc below which disruption does not occur. The critical Lundquist number Sc is not a constant value, but has a weak dependence on the noise amplitude.
AB - The scaling of the plasmoid instability maximum linear growth rate with respect to the Lundquist number S in a Sweet-Parker current sheet, γmax ∼ S1/4, indicates that at high S, the current sheet will break apart before it approaches the Sweet-Parker width. Therefore, a proper description for the onset of the plasmoid instability must incorporate the evolving process of the current sheet. We carry out a series of two-dimensional simulations and develop diagnostics to separate fluctuations from an evolving background. It is found that the fluctuation amplitude starts to grow only when the linear growth rate is sufficiently high (γmax τA > O(1)) to overcome advection loss and the stretching effect due to the outflow. The linear growth rate continues to rise until the sizes of plasmoids become comparable to the inner layer width of the tearing mode. At this point, the current sheet is disrupted and the instability enters the early nonlinear regime. The growth rate suddenly decreases, but the reconnection rate starts to rise rapidly, indicating that current sheet disruption triggers the onset of fast reconnection. We identify important timescales of the instability development, as well as scalings for the linear growth rate, current sheet width, and dominant wavenumber at disruption. These scalings depend not only on the Lundquist number, but also on the noise amplitude. A phenomenological model that reproduces scalings from simulation results is proposed. The model incorporates the effect of reconnection outflow, which is crucial for yielding a critical Lundquist number Sc below which disruption does not occur. The critical Lundquist number Sc is not a constant value, but has a weak dependence on the noise amplitude.
KW - Sun: coronal mass ejections (CMEs)
KW - Sun: magnetic fields
KW - Sun: transition region
KW - magnetic reconnection
KW - magnetohydrodynamics (MHD)
KW - plasmas
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U2 - 10.3847/1538-4357/aa906d
DO - 10.3847/1538-4357/aa906d
M3 - Article
AN - SCOPUS:85034442048
SN - 0004-637X
VL - 849
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2
M1 - 75
ER -