Engineering topics

Ahmadi, Narges

Publications and source records attributed to Ahmadi, Narges.

4 records

DOE OSTI2019

Magnetic Reconnection in Three Dimensions: Modeling and Analysis of Electromagnetic Drift Waves in the Adjacent Current Sheet

Magnetic reconnection at the subsolar magnetopause is persistently accompanied by strong fluctuations of the magnetic field (B), plasma density (n), and all components of the electric field (E) and current (J). The strongest fluctuations are at frequencies below the lower hybrid frequency and observed in a thin, intense current sheet adjacent to the electron diffusion region. In this current sheet, the background magnitudes of B and n are changing considerably, creating an inhomogeneous plasma environment. In this paper, we show that the fluctuations in B and n are consistent with an oscillatory displacement of the current sheet in the surface normal direction. The displacement is propagating parallel to the magnetic reconnection X-line. Wavelengths are on the order of or longer than the thickness of the current sheet to which they are confined, so we label these waves electromagnetic drift waves. E and J fluctuations are more complex than a simple displacement. They have significant variations in the component along B, which suggest that the drift waves also may be confined along B. The current sheet is supported by an electron drift driven by normal electric field, which, in turn, is balanced by an ion pressure gradient. We suggest that wave growth comes from an instability related to the drift between the electrons and ions. We discuss the possibility that drift waves can displace or penetrate into the electron diffusion region giving magnetic reconnection three-dimensional structure. Drift waves may corrugate the X-line, possibly breaking the X-line and generating turbulence.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2017

Simulation of magnetic holes formation in the magnetosheath

Magnetic holes have been frequently observed in the Earth's magnetosheath and are believed to be the consequence of the nonlinear evolution of the mirror instability. Mirror mode perturbations mainly form as magnetic holes in regions where the plasma is marginally mirror stable with respect to the linear instability criterion. We present an expanding box particle-in-cell simulation to mimic the changing conditions in the magnetosheath as the plasma is convected through it that produces mirror mode magnetic holes. We show that in the initial nonlinear evolution, where the plasma conditions are mirror unstable, the magnetic peaks are dominant, while later, as the plasma relaxes toward marginal stability, the fluctuations evolve into deep magnetic holes. Finally, while the averaged plasma parameters in the simulation remain close to the mirror instability threshold, the local plasma in the magnetic holes is highly unstable to mirror instability and locally mirror stable in the magnetic peaks.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2016

Reply to comment by Remya et al. on “Effects of electron temperature anisotropy on proton mirror instability evolution”

In the comment to our paper, Remya et al. (2017) state that we conclude that their theory is incorrect; however, no such conclusion is in our paper. In fact, as stated in their paper, we agree with their theory that shows the impact of heavy ions and electron temperature anisotropy on the competition of the ion anisotropy instabilities. While their linear theory is correct, our paper focused on the nonlinear evolution, where one needs to be careful in assuming a given electron anisotropy, because electrons themselves can be unstable to the electron whistler instability, which quickly lowers the anisotropy to levels where, in the absence of heavy ions, it is not sufficient to significantly change the balance between proton cyclotron and mirror mode. We agree that the electron whistler instability will not lead to complete isotropization of the electrons but only lower it to the instability threshold. In the parameter regime addressed, this limited isotropization will still eliminate the dominance of mirror mode and restore the usual dominance of the proton cyclotron mode, so our point still stands. Our simulations showed an isotropization of the electrons beyond the electron whistler threshold. In this paper, we will show that there are two contributing reasons: The nonlinear evolution of the mirror instability affects the electron anisotropy, as does unphysical numerical heating due to the limited resolution of a particle-in-cell simulation. Finally, we further discuss the coexistence of electron whistler instability and mirror instability, and we agree that both instabilities can be present in the magnetosheath.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

The Plasma Simulation Code: A modern particle-in-cell code with patch-based load-balancing

This work describes the Plasma Simulation Code (psc), an explicit, electromagnetic particle-in-cell code with support for different order particle shape functions. We review the basic components of the particle-in-cell method as well as the computational architecture of the psc code that allows support for modular algorithms and data structure in the code. We then describe and analyze in detail a distinguishing feature of psc: patch-based load balancing using space-filling curves which is shown to lead to major efficiency gains over unbalanced methods and a previously used simpler balancing method.

Computer Science↗