Resonant excitation of whistler waves by a helical electron beam

Geophysical Research Letters RESEARCH LETTER 10.1002/2015GL067126 Key Points: • Chorus-like whistler mode waves are excited in a laboratory plasma to study excitation process • The resonance structure of whistler wave is experimentally resolved for the first time • Linear theory shows consistent behavior in both intensity and wave normal angle Supporting Information: • Supporting Information S1 • Movie S1 • Movie S2 • Movie S3 • Movie S4 Correspondence to: X. An, xinan@atmos.ucla.edu Citation: An, X., B. Van Compernolle, J. Bortnik, R. M. Thorne, L. Chen, and W. Li (2016), Resonant excitation of whistler waves by a helical electron beam, Geophys. Res. Lett., 43, 2413–2421, doi:10.1002/2015GL067126. Received 21 NOV 2015 Accepted 21 JAN 2016 Accepted article online 25 JAN 2016 Published online 29 MAR 2016 ©2016. American Geophysical Union. All Rights Reserved. AN ET AL. Resonant excitation of whistler waves by a helical electron beam X. An 1 , B. Van Compernolle 2 , J. Bortnik 1 , R. M. Thorne 1 , L. Chen 3 , and W. Li 1 1 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA, 2 Department of Physics, University of California, Los Angeles, California, USA, 3 Physics Department, W. B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, Texas, USA Abstract Chorus-like whistler mode waves that are known to play a fundamental role in driving radiation belt dynamics are excited on the Large Plasma Device by the injection of a helical electron beam into a cold plasma. The mode structure of the excited whistler wave is identified using a phase correlation technique showing that the waves are excited through a combination of Landau resonance, cyclotron resonance, and anomalous cyclotron resonance. The dominant wave mode excited through cyclotron resonance is quasi-parallel propagating, whereas wave modes excited through Landau resonance and anomalous cyclotron resonance propagate at oblique angles that are close to the resonance cone. An analysis of the linear wave growth rates captures the major observations in the experiment. The results have important implications for the generation process of whistler waves in the Earth’s inner magnetosphere. 1. Introduction Naturally generated whistler mode chorus waves in the Earth’s near-space environment are known to play a key role in the acceleration and loss of the relativistic electrons that comprise the radiation belts [e.g., Thorne, 2010; Thorne et al., 2013]. They appear in various forms, such as discrete rising or falling tones, and broad- band hiss emissions [Pope, 1963; Burtis and Helliwell, 1969; Cornilleau-Wehrlin et al., 1978; Koons, 1981; Santolik et al., 2009; Li et al., 2012]. These chorus waves are commonly separated into two distinct bands with a gap at 0.5Ω e ( Ω e is the electron gyrofrequency) [Tsurutani and Smith, 1974; Hayakawa et al., 1984; Li et al., 2013]. The lower band ( 𝜔∕Ω e 0.5) has a broad range of wave normal angles between 0 ∘ and 60 ∘ [Li et al., 2013]. Several mechanisms have been proposed to account for the banded structure of chorus [Omura et al., 2009; Liu et al., 2011; Fu et al., 2014; Mourenas et al., 2015; Fu et al., 2015]. Among the proposed mechanisms, Mourenas et al. [2015] suggested that the less frequently occurring but statistically significant very oblique lower band chorus waves can be gener- ated through a combination of cyclotron resonance and Landau resonance with low-energy electron beams having an energy of a few keV. But these theoretical ideas remain to be tested by satellite observations and laboratory experiments. In order to study the excitation of chorus-like whistler waves in a laboratory plasma, an electron beam is used as the free energy source, injected into a cold background plasma. In a beam-plasma system, vari- ous instability processes can excite a variety of waves including whistler mode waves [Bell and Buneman, 1964], Bernstein-mode waves [Kusse and Bers, 1970; Mizuno et al., 1971], and Langmuir waves [O’Neil et al., 1971; Gentle and Lohr, 1973]. Whistler mode emissions by beam-plasma interaction have been studied exten- sively in the past, such as in the generation of auroral hiss [Maggs, 1976; Gurnett et al., 1983; Sazhin et al., 1993], in active experiments in the space environment [Lavergnat and Pellat, 1979; Tokar et al., 1984; Gurnett et al., 1986; Farrell et al., 1988; Neubert and Banks, 1992], and in controlled laboratory settings [Stenzel, 1977; Kraﬀt et al., 1994; Starodubtsev and Kraﬀt, 1999; Starodubtsev et al., 1999; Van Compernolle et al., 2015]. During active experiments of the Spacelab 2 mission, for instance, beam-generated whistler mode emissions were observed to propagate near the resonance cone and were attributed to Landau resonance [Gurnett et al., 1986; Farrell et al., 1988]. In controlled laboratory experiments, whistler mode emissions were also gener- ated through both Landau resonance [Kraﬀt et al., 1994] and cyclotron resonance [Starodubtsev and Kraﬀt, 1999] using a density-modulated electron beam. However, if the electron beam is not modulated, WHISTLER WAVE EXCITATION