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Thursday 07 May 2020A fresh look at waves in ion-electron plasmas: reformulating the textbook treatments

Prof Rony Keppens - Centre for mathematical Plasma Astrophysics, KU Leuven

Online - contact A. Hillier for webinar link 14:30-15:30

In many -- if not all -- plasma physics textbooks, a two-fluid treatment of linear waves supported by a uniform, cold ion-electron plasma serves as an introduction to motivate more general, fully kinetic treatments. The ideal two-fluid approach lacks important velocity-phase- space effects like Landau damping, and is thus categorized as being physically incomplete. The latter statement is even more pertinent for the one-fluid ideal magnetohydrodynamic (MHD) model, where only Slow, Alfven, and Fast waves remain. In a recent paper [1], we revisited the classical textbook treatments for a cold ion-electron plasma, to make more direct contact between the ideal two-fluid approach and the MHD limit. It became evident that all textbook treatments are unduly complex, since they classify waves at fixed frequency and pay particular attention to exactly parallel (to the magnetic field) or exactly perpendicular propagation. We will contrast the textbook view, which emphasizes the Clemmow-Mullaly-Allis diagram (an intricate manner to show wave diversity), with our new categorization in S(low), A(lfven), F(ast), M(odified) electrostatic mode, and electromagnetic O and X branches [1,2,3]. This six-way categorization directly connects with MHD theory, and elucidates the shortcomings of the textbook approach. It allows to visualize phase and group speed diagrams at all wavelengths, and demonstrates the intricate wave transformations that exist at parallel and perpendicular orientations. Since linear MHD theory uniquely addresses the extension of wave diversity from spatially uniform plasmas, to spatially inhomogeneous configurations, an entirely new research field comes into focus: two-fluid wave spectroscopy.

[1] Keppens & Goedbloed, 2019, Frontiers in Astronomy and Space Science 6, 11(15pp), doi:10.3389/fspas.2019.00011

[2] Keppens & Goedbloed, 2019, Journal Plasma Physics (Letter) 85, 175850101 (11pp), doi:10.1017/S0022377819000102

[3] Keppens, Goedbloed & Durrive, 2019, Journal Plasma Physics 85, 905850408 (28pp), doi:10.1017/S0022377819000552

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