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Software-type Wave–Particle Interaction Analyzer on board the Arase satellite
- Yuto Katoh1Email authorView ORCID ID profile,
- Hirotsugu Kojima2,
- Mitsuru Hikishima3,
- Takeshi Takashima3,
- Kazushi Asamura3,
- Yoshizumi Miyoshi4,
- Yoshiya Kasahara5,
- Satoshi Kasahara6,
- Takefumi Mitani3,
- Nana Higashio7,
- Ayako Matsuoka3,
- Mitsunori Ozaki5,
- Satoshi Yagitani5,
- Shoichiro Yokota8,
- Shoya Matsuda4,
- Masahiro Kitahara1 and
- Iku Shinohara3
© The Author(s) 2018
- Received: 31 August 2017
- Accepted: 25 December 2017
- Published: 8 January 2018
- Radiation belts
- Whistler-mode chorus
- Wave–particle interactions
The Arase (ERG) satellite was launched from the Uchinoura Space Center on December 20, 2016 to explore the dynamics of the terrestrial radiation belts. One of the prime objectives for this satellite mission is the investigation of the energization process of relativistic electrons by whistler-mode chorus emissions. Whistler-mode chorus emissions are coherent electromagnetic plasma waves observed mainly on the dawn side of the inner magnetosphere (e.g., Summers et al. 1998). Previous studies showed that chorus emissions play crucial roles in the reformation of the outer radiation belt during the recovery phase of geomagnetic storms (e.g., Miyoshi et al. 2003). Recent theoretical and simulation studies have revealed that chorus emissions emerge from a band of whistler-mode waves in regions close to the magnetic equator through nonlinear wave–particle interactions (e.g., Katoh and Omura 2007, 2011, 2013, 2016; Omura et al. 2008, 2009). Chorus emissions propagate away from the equator, and their propagation characteristics vary depending on the plasma environment in the inner magnetosphere (e.g., Katoh 2014). In the generation process of chorus emissions, an electromagnetic electron hole is formed in a specific range of the velocity phase space due to the nonlinear Lorentz force acting on resonant electrons. Simulation studies have revealed that most resonant electrons lose their kinetic energy, contributing to the generation of chorus emissions, and that a fraction of the resonant electrons is trapped inside the hole and is effectively energized through a special form of nonlinear wave trapping called relativistic turning acceleration (Omura et al. 2007) and ultra-relativistic acceleration (Summers and Omura 2007).
Wave–particle interactions in the magnetosphere occur over the timescale of the characteristics of plasma waves and particles. During the interaction between coherent whistler-mode waves and energetic electrons, relaxation of the velocity distribution function of resonant electrons occurs within hundreds or thousands of electron gyro-periods (e.g., Katoh and Omura 2004), corresponding to tens of ms for typical parameters of the Earth’s inner magnetosphere. Since the time resolution of conventional plasma instruments on board a spacecraft is usually a few tens of ms or less, it is difficult to measure the relaxation of the velocity distribution or the energy exchange between wave and particles.
To overcome the difficulty in the direct measurement of wave–particle interactions, previous studies have used the observed wave phase as a reference to count the number of particles in order to obtain the distribution as a function of the relative phase angle between waves and particles (Ergun et al. 1991, 1998; Gough et al. 1995; Buckley et al. 2000). In sounding rocket experiments, their attempts successfully identified wave–particle correlations between Langmuir waves and electrons, with a statistical significance (Kletzing et al. 2017). Fukuhara et al. (2009) proposed a new type of instrument for the direct and quantitative measurement of the energy exchange between waves and particles, which is referred to as the Wave–Particle Interaction Analyzer (WPIA). The WPIA uses the three components of observed waveforms and particle velocity vectors to quantify the energy flow by measuring the inner product of the observed instantaneous wave and velocity vectors, corresponding to Joule heating of particles by plasma waves (Katoh et al. 2013). The feasibility of the WPIA for the Arase satellite has been studied using pseudo-observations based on simulations with self-consistent plasma particle codes, which reproduce the process of chorus generation (Katoh et al. 2013; Hikishima et al. 2014). Kitahara and Katoh (2016) suggested that the WPIA is also capable of measuring the pitch angle scatter of particles by plasma waves directly and quantitatively. Recently, Shoji et al. (2017) showed that the WPIA can directly measure the formation of an ion hole through interactions of electromagnetic ion cyclotron waves and energetic ions in the inner magnetosphere.
In this paper, the implementation of the Software-type Wave–Particle Interaction Analyzer (S-WPIA) on the Arase satellite is described. Since the Arase satellite is the first application of the WPIA in space, we installed the Software-type WPIA because of its flexibility in choosing processing algorithms and optimization. The principles and significance of the WPIA are discussed in “Principles of the WPIA and its significance” section. Details of the S-WPIA implementation in the Arase satellite are described in “S-WPIA implemented on the Arase satellite” section, and a summary is present in “Summary” section.
Specifications of instruments on board the Arase satellite for implementing the S-WPIA
For the WPIA, it is essential to ascertain that the time resolution of t i , indicating the detection time for the i-th particle, is shorter than the timescale for the wave–particle interactions. For the S-WPIA on board the Arase satellite, the requirement of the relative time accuracy for each instrument used in the direct measurement of interactions between the chorus and energetic electrons in the inner magnetosphere is studied. The relative phase angle between the electromagnetic field vector for the wave (Ew and Bw) and the velocity vector v⊥ for the energetic electrons should be resolved in order to identify the sign of W correctly for each detected electron. Here, θ represents the relative phase angle between Ew and v⊥ (Fig. 1a, b), and ζ denotes the angle between Bw and v⊥. In addition, identifying of the presence of an electromagnetic electron hole in the velocity phase space is one of the primary goals of the S-WPIA. While the hole is formed in the specific range of ζ (e.g., Omura et al. 2008; Katoh et al. 2013), which rotates in time with the wave period, the wave phase variation needs to be resolved on a timescale that is sufficiently shorter than the wave period. In the inner magnetosphere, chorus emissions appear in a frequency range lower than the electron cyclotron frequency: typically, from 0.2 to 0.5 Ωe0 for the lower band chorus and from 0.5 to 0.8 Ωe0 for the upper band chorus, where Ωe0 is the electron gyrofrequency at the magnetic equator. Assuming 10 kHz as the highest electron cyclotron frequency along the Arase orbit at the equator, the wave period of the chorus is approximately 100 μs. An accuracy greater than 10 μs resolves the wave phase on the order of a few tens of degrees. The same accuracy should be utilized for the synchronization between wave and particle instruments in order to identify θ and ζ correctly.
The instruments on board the Arase satellite meet the requirements for direct measurements of interactions between chorus and energetic electrons by the S-WPIA. Chorus emissions are often observed on the dawn side of the inner magnetosphere and outside the plasmapause. The typical frequency range of chorus emissions is covered by the waveform capture receiver (WFC) of the plasma wave experiments (PWE) on board the Arase satellite (Kasahara et al. 2018a). Furthermore, since the ratio between the plasma frequency (fp) and the electron cyclotron frequency (fce), fp/fce, is typically less than 10, the minimum resonance energy based on the first-order cyclotron resonance condition is estimated to be in the energy range of hundreds of eV to a few keV for the upper band chorus and from a few keV to tens of keV for the lower band chorus, respectively. The resonance energy changes depending on the pitch angle of the resonant electrons and increases to over MeV for large pitch angle ranges. These estimations show that the kinetic energy range of resonant electrons, particularly for the lower band chorus, is covered by the medium-energy particle experiments (MEP-e) (Kasahara et al. 2018b), the high-energy electron instruments (HEP) (Mitani et al. submitted to Earth, Planets and Space), and the extremely high-energy electron experiment (XEP) (Higashio et al. submitted to Earth, Planets and Space) on board the Arase satellite.
Estimation of the required integration time for the S-WPIA
For the direct measurements of wave–particle interactions by the S-WPIA, a certain number of particles detected in the region of interest need to be collected in order to obtain a statistically significant Wint and/or a non-uniform distribution of particles in the wave phase space caused by the presence of an electromagnetic electron hole. Assuming that the distribution of energetic electrons as a function of ζ is changed by 10% from the average due to the presence of an electromagnetic electron hole and that the statistical fluctuation follows a Poisson distribution for which the standard deviation is expressed as N1/2/N for a particle count N, at least more than 100 particles need to be collected in each bin. If the distribution of particles as a function of relative phase angle ζ is analyzed at every 30°, i.e., if 12 bins are assumed for ζ from 0° to 360°, then the collection of 1200 particles would be required to assess each of the kinetic energy and pitch angle ranges.
The required time interval for the S-WPIA based on the estimation shown in Fig. 2c is evaluated. If the required number of particles is set at 2000 as estimated earlier, the required particle count can be collected by MEP-e within one spin period in the pitch angle range from 60° to 120°. However, additional restrictions and limitations should be taken into account for the S-WPIA. If the number of particles required in order to increase the statistical significance of the obtained results is set at 12,000, the accumulation time should be greater than six spin periods in the pitch angle range from 60° to 120°. In addition to using a large particle count to achieve statistical significance, in order to increase the signal-to-noise ratio for the S-WPIA, the count at the time of the whistler-mode chorus enhancements should be used. We expect that both the net increase of Wint and modulation of the particle distribution as a function of the relative phase angle ζ can only be measured in the presence of chorus emissions. Considering that the statistical fluctuation of the particle count is expressed as N1/2/N, the particle count detected in the absence of chorus emissions only increases the statistical fluctuations without increasing the amount of the modulation due to wave–particle interactions. By selecting the interval of chorus emissions, we expect that the detected count will increase both N1/2/N and the amount of the modulation of the distribution, and therefore, we expect the signal-to-noise ratio to increase. Since chorus elements appear in the spectra intermittently with a timescale of less than 1 s, it can be roughly assumed that one-third of the detected particles are accompanied by chorus elements. Taking these assumptions into account, the required accumulation time for the S-WPIA is estimated to be at least 18 spin periods, corresponding to 144 s. The expected duration of the S-WPIA measurements is more than 3 min in the region of interest, and this expectation is considered in the operation planning for the Arase satellite.
In order to realize the S-WPIA in the Arase satellite, a dedicated mission network system for the synchronization of wave and particle instruments was developed. In this section, the implementation of the S-WPIA on the Arase satellite is described.
Mission network based on the Spacewire
Each scientific instrument writes the observed data into the MDR in the S-WPIA data format through the mission network. The communication among the scientific instruments is conducted by a relay packet. The data packed in the relay packet are transferred by each instrument according to the routing information, which is decided in advance by commands. Through the relay packet, the S-WPIA activates the generation of the data designated to the S-WPIA for each instrument. Each instrument reports its readiness and generation statuses for the S-WPIA data through the relayed packet. The total bandwidth of the mission network is 12 Mbps, and a specific bandwidth is allocated to each instrument based on the bit rate of the data generation by commands.
Accuracy of relative observation time
Operation and output of the S-WPIA
The S-WPIA computes Wint from the observational data of the PWE, XEP, HEP, MEP-e, and MGF stored on the MDR. Because of the vast amounts of observed raw data for electromagnetic waveforms and individual particle counts, the S-WPIA measurement is intermittent and has a short duration for each orbit of the Arase satellite. First, we set the command to activate the generation of the raw data for each instrument only in the region of interest. After the observation, by referring to quick-look plots of the PWE, we determine the time interval subject to the computation of the S-WPIA. Then, we set the command for the computation of the stored data observed in the time interval to obtain Wint, σW, and N as functions of K, α, and ζ. The output of the S-WPIA, Wint(K, α, ζ), σW(K, α, ζ), and N(K, α, ζ), is transferred to the ground, and the raw data used for the S-WPIA output can also be downlinked for verification and investigation of the S-WPIA algorithm. Details of the S-WPIA calculation and the specifications of the S-WPIA software applications are described in Hikishima et al. (submitted to Earth, Planets and Space).
In this report, the principle of the WPIA (Fukuhara et al. 2009) and its significance for the direct measurements of wave–particle interactions in the Arase mission were described. The WPIA computes an inner product W(t i ) = qE (t i )·v i , where t i is the detection time for the i-th particle, E(t i ) is the wave electric field vector at t i , and q and v i represent the charge and the velocity vectors for the i-th particle, respectively. Since W(t i ) denotes the gain or the loss of the kinetic energy for the i-th particle, summing W for detected particles allows the net amount of the energy exchange in the region of interest to be acquired. By referring to the specifications of the MEP-e and by assuming a count rate of 5000 cps for each detector of the MEP-e, we estimated that the number of particles required to obtain statistically significant results by the S-WPIA can be collected during 18 spin periods.
The implementation of the S-WPIA on the Arase satellite is next described. The S-WPIA was installed on the Arase satellite as a software function running on the mission data processor. It uses an electromagnetic field waveform measured by the WFC of the PWE and velocity vectors detected by the MEP-e, HEP, and XEP. The primary goal of the S-WPIA is measuring the energy exchange between the whistler-mode chorus emissions and energetic electrons in the inner magnetosphere. It is essential for the S-WPIA to synchronize instruments with a relative time accuracy that is better than the time period of the plasma wave oscillations. Since the typical frequency of chorus emissions is a few kHz in the inner magnetosphere, a relative time accuracy better than 10 μs should be maintained in order to measure the relative phase angle between wave electromagnetic field and velocity vectors with an accuracy sufficient to correctly detect the sign of W. In the Arase satellite, a dedicated system has been developed in order to obtain the required time resolution for inter-instrument communication. Both the time index distributed to all instruments through the satellite system with a time resolution of 15.6 ms and the S-WPIA clock signal, which is distributed from the PWE every 1.9 μs to particle instruments through a direct line, are used. The S-WPIA has been successfully implemented on the Arase satellite with instrument specifications and mission networks suitable for the direct measurement of interactions between chorus and energetic electrons in the inner magnetosphere. The S-WPIA software on board the Arase satellite is described in detail in an accompanying paper by Hikishima et al. (submitted to Earth, Planets and Space).
YK and MK contributed theoretical consideration and data analysis. HK, MH, TT, and KA contributed discussion of the implementation. YM, YK, SK, TM, NH, AM, MO, AY, SY, SM, and IS contributed discussion of specifications of instruments.
The authors express their sincere gratitude for numerous efforts made by all members of the ERG project. This study is supported by Grants-in-Aid for Scientific Research (23224011, JP15H05747, JP15H05815, JP15H03730, JP16H06286, and 17K18798) of Japan Society for the Promotion of Science. This work was also supported by Toray Science and Technology Grant of Toray Science Foundation. This work was carried out by the joint research program of the Institute for Space-Earth Environmental Research (ISEE), Nagoya University. The authors wish to express their sincere appreciations to Emeritus Professor Takayuki Ono for valuable discussion and continuous encouragement on this study.
The authors declare that they have no competing interests.
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