分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要：MHD discontinuities are ubiquitous in the solar wind and are often found at the origin of turbulence intermittency. They may also play a key role in the turbulence dissipation and heating of the solar wind. The tangential discontinuities (TDs) and rotational discontinuities (RDs) are the two most important types of discontinuities. Recently, the connection between turbulence intermittency and proton thermodynamics has been observationally investigated. Here, we present numerical results from a three-dimensional MHD simulation with pressure anisotropy and we define new methods for identifying and distinguishing TDs and RDs. Three statistical results obtained for the relative occurrence rates and heating effects are highlighted: (1) RDs tend to take up the majority of the discontinuities along with time;(2) the thermal states embedding TDs tend to be associated with extreme plasma parameters or instabilities while RDs do not;(3) TDs have a higher average T as well as perpendicular temperature T.. The simulation shows that TDs and RDs evolve and contribute to solar wind heating differently. These results will improve our understanding of the mechanisms that generate discontinuities and cause plasma heating.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Yang, Liping
Zhang, Lei
He, Jiansen
Tu, Chuanyi
Wang, Linghua
Marsch, Eckart
Wang, Xin
Zhang, Shaohua
Feng, Xueshang
摘要：Measurements of solar wind turbulence reveal the ubiquity of discontinuities. In this study we investigate how the discontinuities, especially rotational discontinuities (RDs), are formed in MHD turbulence. In a simulation of the decaying compressive three-dimensional (3D) MHD turbulence with an imposed uniform background magnetic field, we detect RDs with sharp field rotations and little variations of magnetic field intensity, as well as mass density. At the same time, in the de Hoffman-Teller frame, the plasma velocity is nearly in agreement with the Alfven speed, and is field-aligned on both sides of the discontinuity. We take one of the identified RDs to analyze its 3D structure and temporal evolution in detail. By checking the magnetic field and plasma parameters, we find that the identified RD evolves from the steepening of the Alfven wave with moderate amplitude, and that steepening is caused by the nonuniformity of the Alfven speed in the ambient turbulence.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Yang, Liping
Zhang, Lei
He, Jiansen
Peter, Hardi
Tu, Chuanyi
Wang, Linghua
Zhang, Shaohua
Feng, Xueshang
摘要：The Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory has directly imaged the fast-propagating magnetosonic waves (FMWs) successively propagating outward along coronal magnetic funnels. In this study we perform a numerical investigation of the excitation of FMWs in the interchange reconnection scenario, with footpoint shearing flow being used to energize the system and drive the reconnection. The modeling results show that as a result of magnetic reconnection, the plasma in the current sheet is heated up by Joule dissipation to similar to 10 MK and is ejected rapidly, developing the hot outflows. Meanwhile, the current sheet is torn into plasmoids, which are shot quickly both upward and downward. When the plasmoids reach the outflow regions, they impact and collide with the ambient magnetic field there, which consecutively launches FMWs. The FMWs propagate outward divergently away from the impact regions, with a phase speed of the Alfven speed of similar to 1000 km s(-1). In the k-omega. diagram of the Fourier wave power, the FMWs display a broad frequency distribution with a straight ridge that represents the dispersion relation. With the WKB approximation, at the distance of 15Mm from the wave source region, we estimate the energy flux of FMWs to be E similar to 7.0 x 10(6) erg cm(-2) s(-1), which is similar to 50 times smaller than the energy flux related to the tube-channeled reconnection outflow. These simulation results indicate that energetically and dynamically the outflow is far more important than the waves.
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