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  • Title Significance of Wave-Particle Interaction Analyzer for directmeasurements of nonlinear wave-particle interactions

    Author(s)Katoh, Y.; Kitahara, M.; Kojima, H.; Omura, Y.; Kasahara, S.;Hirahara, M.; Miyoshi, Y.; Seki, K.; Asamura, K.; Takashima,T.; Ono, T.

    Citation Annales Geophysicae (2013), 31(3): 503-512

    Issue Date 2013-03-19

    URL http://hdl.handle.net/2433/193729

    Right Author(s) 2013. This work is distributed under the CreativeCommons Attribution 3.0 License.

    Type Journal Article

    Textversion publisher

    Kyoto University

  • Ann. Geophys., 31, 503512, 2013www.ann-geophys.net/31/503/2013/doi:10.5194/angeo-31-503-2013 Author(s) 2013. CC Attribution 3.0 License.

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    DiscussionsSignificance of Wave-Particle Interaction Analyzer for directmeasurements of nonlinear wave-particle interactions

    Y. Katoh1, M. Kitahara1, H. Kojima2, Y. Omura2, S. Kasahara3, M. Hirahara4, Y. Miyoshi4, K. Seki4, K. Asamura3,T. Takashima3, and T. Ono1

    1Department of Geophysics, Graduate School of Science, Tohoku University, Miyagi 980-8578, Japan2Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto 611-0011, Japan3ISAS, JAXA, Kanagawa 229-8510, Japan4Solar-Terrestrial Environment Laboratory, Nagoya University, Aichi 464-8601, Japan

    Correspondence to: Y. Katoh (yuto@stpp.gp.tohoku.ac.jp)

    Received: 26 October 2012 Revised: 24 January 2013 Accepted: 18 February 2013 Published: 19 March 2013

    Abstract. In the upcoming JAXA/ERG satellite mission,Wave Particle Interaction Analyzer (WPIA) will be installedas an onboard software function. We study the statistical sig-nificance of the WPIA for measurement of the energy trans-fer process between energetic electrons and whistler-modechorus emissions in the Earths inner magnetosphere. TheWPIA measures a relative phase angle between the wavevector E and velocity vector v of each electron and com-putes their inner product W , where W is the time variationof the kinetic energy of energetic electrons interacting withplasma waves. We evaluate the feasibility by applying theWPIA analysis to the simulation results of whistler-modechorus generation. We compute W using both a wave elec-tric field vector observed at a fixed point in the simulationsystem and a velocity vector of each energetic electron pass-ing through this point. By summing up Wi of an individualparticle i to give Wint, we obtain significant values of Wintas expected from the evolution of chorus emissions in thesimulation result. We can discuss the efficiency of the energyexchange through wave-particle interactions by selecting therange of the kinetic energy and pitch angle of the electronsused in the computation of Wint. The statistical significanceof the obtained Wint is evaluated by calculating the standarddeviation W of Wint. In the results of the analysis, positiveor negative Wint is obtained at the different regions of ve-locity phase space, while at the specific regions the obtainedWint values are significantly greater than W , indicating ef-ficient wave-particle interactions. The present study demon-strates the feasibility of using the WPIA, which will be on

    board the upcoming ERG satellite, for direct measurementof wave-particle interactions.

    Keywords. Magnetospheric physics (Plasma waves and in-stabilities) Space plasma physics (Wave-particle interac-tions; Instruments and techniques)

    1 Introduction

    Wave-particle interactions in space plasmas occur withintime scales characteristic of plasma particles, such as the gy-roperiod and plasma oscillation period. These time scales are,especially for electrons in the magnetosphere, too short todetect wave-particle interactions in situ using conventionalplasma instruments. As an example, we focus on cyclotronresonant interactions of energetic electrons with a coherentwhistler-mode wave in the magnetosphere. The time scaleassociated with the motion of resonant electrons nonlinearlytrapped by a coherent whistler-mode wave is characterisedby the oscillation period of electrons within the trapping re-gion in velocity phase space (e.g., Matsumoto, 1985), andis one or two orders of magnitude longer than the gyrope-riod of electrons. The trapping oscillations of resonant elec-trons occur within a limited region of velocity phase spacedetermined by the cyclotron resonance condition and withina certain relative phase range with respect to the wave mag-netic field vector. Oscillations of resonant electrons result ina relaxation of the velocity distribution within the trappingregion and saturation of the interactions within time peri-ods of hundreds or thousands of electron gyroperiods (e.g.,

    Published by Copernicus Publications on behalf of the European Geosciences Union.

  • 504 Y. Katoh et al.: Statistical significance of WPIA

    Katoh and Omura, 2004) corresponding to tens of millisec-onds. Using plasma instruments on board spacecraft, whichmostly have time resolutions of a few tens of millisecondsor less, it is impossible to directly measure relaxation of theelectron velocity distribution or energy exchange processesduring wave-electron interactions.

    One observation method that enables wave-particle inter-actions to be investigated is a comparison of the observedwave phase and particles. There have been many attemptsto measure interactions between electron beams and electro-static waves, while previous instruments counted the num-ber of particles by referring the observed wave phase andmeasured the electron distribution in the wave phase space(Ergun et al., 1991, 1998; Gough et al., 1995; Buckley etal., 2000). Recently Fukuhara et al. (2009) proposed a newtype of instrument for direct and quantitative measurementsof wave-particle interactions, which is referred to as Wave-Particle Interaction Analyzer (WPIA). The WPIA uses all ofthe three-dimensional properties of the observed waveformsand particles and quantifies the kinetic energy flow by mea-suring the inner product of the observed instantaneous waveand the particle velocity vectors. To apply the WPIA to spaceplasma measurement, it is necessary to obtain both the wavefield vector and the velocity of each incoming particle on atime scale sufficient to resolve variations in the wave phaseand the gyro-motion of particles. Since the sampling clockin wave and plasma instruments can be adjusted to the orderof microseconds, measurement of the relative phase can becarried out using state-of-the-art instruments. It is not neces-sary for all of waveform and particle data to be transferredto the ground in order to apply the WPIA. Accumulationof the inner product can be conducted onboard, so that thetelemetry data size can be optimised to a realistic size bysending only the results of the accumulation. In the upcom-ing JAXA satellite mission: Exploration of energization andRadiation in Geospace (ERG) project (Miyoshi et al., 2012),WPIA measurements will be carried out using Software-typeWPIA (S-WPIA). One of the primary targets of this missionis to clarify the interaction of relativistic electrons with cho-rus emissions. During the mission, the onboard S-WPIA willbe used to investigate whistler-mode wave-particle interac-tions for the first time.

    Recently, the gyroresonant interaction of relativistic elec-trons with whistler-mode chorus emissions has been investi-gated as a possible generation mechanism for MeV electronsin the outer radiation belt (e.g., Shprits et al., 2008; Thorne,2010; Ebihara and Miyoshi, 2011, for review). Observationshave revealed the possible roles of chorus in controlling theflux of radiation belt electrons (e.g., Miyoshi et al., 2003,2007; Horne et al., 2003; Kasahara et al., 2009). As theo-retical and simulation studies have clarified, nonlinear wave-particle interactions are essential when considering the gen-eration process of coherent chorus elements with rising tones(Nunn, 1974; Nunn et al., 1997; Trakhtengerts, 1995, 1999;Katoh and Omura, 2007a, 2011; Omura et al., 2008, 2009;

    Hikishima et al., 2009; Omura and Nunn, 2011) and withfalling tones

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