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Heliophysics Technology and Instrument Development for Science

The relation of laboratory-generated whistler-mode waves and whistler-mode chorus waves in space

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The relation of laboratory-generated whistler-mode waves and whistler-mode chorus waves in space

1. Introduction Chorus waves are naturally-occurring whistler-mode waves found in the near-Earth space environment, that are typically observed as discrete, coherent emissions in the frequency band between ~0.1 fce and ~0.8 fce, where fce is the equatorial electron gyrofrequency. Recent observations made by NASA’s Van Allen Probes and THEMIS missions have demonstrated that chorus waves play a critical role in creating the relativistic and ultra-relativistic radiation belts the encompass the Earth, as well as important phenomena the impact ionospheric conductivity such as the diffuse and discrete auroral emissions. Despite its recognized importance, the mechanisms that control the basic characteristics of the chorus waves are still poorly understood, or not understood at all. These parameters include the lower and upper frequency limits of the chorus wave, the chirp rate (df/dt), the saturation amplitude, and the wave normal angle of the wave, as a function of frequency. There is not first-principles understanding available of what controls these parameters, yet modelers must put some of these values into their codes using heavy assumptions, in order to calculate diffusion coefficients that control radiation belt dynamics. A recent set of experiments performed by our group at the Large Plasma Device (LAPD) at UCLA has demonstrated that it is possible to produce whistler-mode “chorus” waves in a laboratory plasma, having all the characteristics of naturally occurring chorus waves in space. This is a breakthrough experiment and gives us the ability to carefully measure and understand the parameters that control the chorus wave characteristics. 2. Science goals and objectives The goal of our proposed project is to perform comprehensive measurements and parameter studies at the LAPD at UCLA to understand the basic parameters that control the various chorus wave characteristics, compare our experimental results with linear theory and PIC code simulations, and ultimately test our newfound understand against in-situ observations in space. 3. Brief description of the methodology We excite chorus wave in the LAPD using a LaB6 electron source the creates a fast-electron beam with 3-4 keV energies, propagating through a cool (~1 eV) background plasma. This beam is tilted at an angle to the background B-field, and both the beam and resulting waves are measured several meters further down in the plasma column. We have comprehensive linear and Particle in Cell codes that support the analysis of the measured wave, and the full set of measurements from the (currently operating) THEMIS and Van Allen Probes missions, to compare our lab observations to in-situ observations made in space. Our group has extensive experience in all aspects of this project, including the laboratory experiments, the linear/PIC theory, and observations in space. 4. Relevance to NASA H-TIDes program Consistent with the LNAPP program element, we believe that our proposed project is high relevant and [to quote directly from page B.3-4,5] “The LNAPP program supports studies that … provide benchmarks for integrating theory and modeling with observation in solar and space physics. Laboratory experiments allow the use of a controlled environment to perform reproducible measurements that shed light on key processes with the Heliophysics environment. These experiments are directed toward understanding basic processes.”

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