NDLI: Kelvin–Helmholtz instability in an active region jet observed with Hinode

Content Provider	Springer Nature Link
Author	Zhelyazkov, I. Chandra, R. Srivastava, A. K.
Copyright Year	2016
Abstract	Over past ten years a variety of jet-like phenomena were detected in the solar atmosphere, including plasma ejections over a range of coronal temperatures being observed as extreme ultraviolet (EUV) and X-ray jets. We study the possibility for the development of Kelvin–Helmholtz (KH) instability of transverse magnetohydrodynamic (MHD) waves traveling along an EUV jet situated on the west side of NOAA AR 10938 and observed by three instruments on board Hinode on 2007 January 15/16 (Chifor et al. in Astron. Astrophys. 481:L57, 2008b). The jet was observed around $\operatorname{Log} T_{\mathrm{e}} = 6.2$ with up-flow velocities exceeded $150~\mbox{km}\,\mbox{s}^{-1}$ . Using Fe xii $\lambda186$ and $\lambda195$ line ratios, the measured densities were found to be above $\operatorname{Log} N_{\mathrm{e}} = 11$ . We have modeled that EUV jet as a vertically moving magnetic flux tube (untwisted and weakly twisted) and have studied the propagation characteristics of the kink ( $m = 1$ ) mode and the higher $m$ modes with azimuthal mode numbers $m = 2, 3, 4$ . It turns out that all these MHD waves can become unstable at flow velocities in the range of $112\mbox{--}114.8~\mbox{km}\,\mbox{s}^{-1}$ . The lowest critical jet velocity of $112~\mbox{km}\,\mbox{s}^{-1}$ is obtained when modeling the jet as compressible plasma contained in an untwisted magnetic flux tube. When the jet and its environments are treated as incompressible media, the critical jet velocity becomes higher, namely $114.8~\mbox{km}\,\mbox{s}^{-1}$ . A weak twist of the equilibrium magnetic field in the same approximation of incompressible plasmas slightly decreases the threshold Alfvén Mach number, $M_{\mathrm{A}}^{\mathrm{cr}}$ , and consequently the corresponding critical velocities, notably to $114.4~\mbox{km}\,\mbox{s}^{-1}$ for the kink mode and to $112.4~\mbox{km}\,\mbox{s}^{-1}$ for the higher $m$ modes. We have also compared two analytically found criteria for predicting the threshold Alfvén Mach number for the onset of KH instability and have concluded that one of them yields reliable values for $M_{\mathrm{A}}^{\mathrm{cr}}$ . Our study of the nature of stable and unstable MHD modes propagating on the jet shows that in a stable regime all the modes are pure surface waves, while the unstable kink ( $m = 1$ ) mode in untwisted compressible plasma flux tube becomes a leaky wave. In the limit of incompressible media (for the jet and its environment) all unstable modes are non-leaky surface waves.
Starting Page	1
Ending Page	14
Page Count	14
File Format	PDF
ISSN	0004640X
Journal	Astrophysics and Space Science
Volume Number	361
Issue Number	2
e-ISSN	1572946X
Language	English
Publisher	Springer Netherlands
Publisher Date	2016-01-07
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Sun: EUV jets MHD waves: dispersion relation Kelvin–Helmholtz instability Astrophysics and Astroparticles Astronomy, Observations and Techniques Cosmology Extraterrestrial Physics, Space Sciences Astrobiology
Content Type	Text
Resource Type	Article
Subject	Astronomy and Astrophysics Space and Planetary Science

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