Engineering topics

Li, T. C.

Publications and source records attributed to Li, T. C..

5 records

DOE OSTI2019

Three-Dimensional Magnetic Reconnection With a Spatially Confined X-Line Extent: Implications for Dipolarizing Flux Bundles and the Dawn-Dusk Asymmetry

Using 3-D particle-in-cell simulations, we study magnetic reconnection with the X-line being spatially confined in the current direction.We include thick current layers to prevent reconnection at two ends of a thin current sheet that has a thickness on an ion inertial ($d_i$) scale. The reconnection rate and outflow speed drop significantly when the extent of the thin current sheet in the current direction is $≲ O$(10$d_i$). When the thin current sheet extent is long enough, we find that it consists of two distinct regions; a suppressed reconnecting region (on the ion-drifting side) exists adjacent to the active region where reconnection proceeds normally as in a 2-D case with a typical fast rate value ≃ 0.1. The extent of this suppression region is $≃ O$(10$d_i$), and it suppresses reconnection when the thin current sheet extent is comparable or shorter. The time scale of current sheet thinning toward fast reconnection can be translated into the spatial scale of this suppression region, because electron drifts inside the ion diffusion region transport the reconnected magnetic flux, which drives outflows and furthers the current sheet thinning, away from this region. This is a consequence of the Hall effect in 3-D. While the existence of this suppression region may explain the shortest possible azimuthal extent of dipolarizing flux bundles at Earth, it may also explain the dawn-dusk asymmetry observed at the magnetotail of Mercury, which has a global dawn-dusk extent much shorter than that of Earth.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2018

Orientation and Stability of Asymmetric Magnetic Reconnection X Line

Here, the orientation and stability of the reconnection x line in asymmetric geometry is studied using three-dimensional (3-D) particle-in-cell simulations. We initiate reconnection at the center of a large simulation domain to minimize the boundary effect. The resulting x line has sufficient freedom to develop along an optimal orientation, and it remains laminar. Companion 2-D simulations indicate that this x line orientation maximizes the reconnection rate. The divergence of the nongyrotropic pressure tensor breaks the frozen-in condition, consistent with its 2-D counterpart. We then design 3-D simulations with one dimension being short to fix the x line orientation but long enough to allow the growth of the fastest growing oblique tearing modes. This numerical experiment suggests that reconnection tends to radiate secondary oblique tearing modes if it is externally (globally) forced to proceed along an orientation not favored by the local physics. Finally, the development of oblique structure easily leads to turbulence inside small periodic systems.

3-D magnetic reconnection↗

DOE OSTI2014

Dynamics of double layers, ion acceleration, and heat flux suppression during solar flares

Observations of flare-heated electrons in the corona typically suggest confinement of electrons. The confinement mechanism, however, remains unclear. The transport of coronal hot electrons into ambient plasma was recently investigated by particle-in-cell (PIC) simulations. Electron transport was significantly suppressed by the formation of a highly localized, nonlinear electrostatic potential in the form of a double layer (DL). In this work large-scale PIC simulations are performed to explore the dynamics of DLs in larger systems where, instead of a single DL, multiple DLs are generated. The primary DL accelerates return current electrons, resulting in high velocity electron beams that interact with ambient ions. This forms a Buneman unstable system that spawns more DLs. Trapping of heated return current electrons between multiple DLs strongly suppresses electron transport. DLs also accelerate ambient ions and produce strong ion flows over an extended region. This clarifies the mechanism by which hot electrons in the corona couple to and accelerate ions to form the solar wind. These new dynamics in larger systems reveal a more likely picture of DL development and their impact on the ambient plasma in the solar corona. They are applicable to the preparation for in situ coronal space missions like the Solar Probe Plus.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

Coronal electron confinement by double layers

In observations of flare-heated electrons in the solar corona, a longstanding problem is the unexplained prolonged lifetime of the electrons compared to their transit time across the source. This suggests confinement. Recent particle-in-cell (PIC) simulations, which explored the transport of pre-accelerated hot electrons through ambient cold plasma, showed that the formation of a highly localized electrostatic potential drop, in the form of a double layer (DL), significantly inhibited the transport of hot electrons. The effectiveness of confinement by a DL is linked to the strength of the DL as defined by its potential drop. In this work, we investigate the scaling of the DL strength with the hot electron temperature by PIC simulations and find a linear scaling. We demonstrate that the strength is limited by the formation of parallel shocks. Based on this, we analytically determine the maximum DL strength, and also find a linear scaling with the hot electron temperature. The DL strength obtained from the analytic calculation is comparable to that from the simulations. At the maximum strength, the DL is capable of confining a significant fraction of hot electrons in the source.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1965

Investigation to define the propagation characteristics of a finite amplitude acoustic pressure wave, phase 1 final report, 29 jun. 1964 - 29 jul. 1965

The contribution of high entropy production regions to the generation and propagation characteristics of a finite amplitude pressure is considered. Preliminary analysis indicates that, for nozzles where pressure rations are above critical, the predominant contribution may come from the shock layer formation in the exhaust region. Temperature effects indicate high dependence of the forcing function upon the initial temperature of the media.

WAVE PROPAGATION↗