Software-Type Wave Particle Interaction Analyzer (S-WPIA)
To understand energy exchange processes between plasma/particles and waves through wave-particle interactions, the newly developed S-WPIA system is installed in the ERG satellite. The time evolution of the kinetic energy of electrons via wave-particle interactions is determined as follows:
where m0 is the electron rest mass, c is the speed of light, γ is the relativistic factor, t is time, q is the charge of electrons, E is the instantaneous electric field vector of plasma waves, ν is the electron velocity, and θ is the relative phase between E and ν. The positive qE(t)⋅ν(t) indicates the acceleration of particles by waves, while the negative qE(t)⋅ν(t) means the growth of waves. The relative phase θ determines the direction of energy flow.
Recently, an innovative method for direct measurement of E⋅ν has been proposed [Fukuhara et al., 2009]. Based on this idea, S-WPIA directly calculates the relative phase θ for each event of particle detection by the onboard particle instruments. S-WPIA has the capability to identify the fraction in the v-θ phase space. The fraction corresponds to the so-called electron hole, which is theoretically expected [Omura et al., 2008]. Thus S-WPIA will quantitatively discriminate which electrons contribute to chorus generation and which electrons are actually accelerated by chorus waves. This will be the first observation to unambiguously identify how energy conversion takes place via wave-particle interactions in space.
Figure 1 schematically shows the data processing of S-WPIA onboard the ERG satellite. All velocity vectors v of particles caught by the particle detector as well as the waveforms E observed by the plasma wave receiver are stored in the onboard memory on the real-time basis. The onboard CPU reads out the stored data from the onboard memory and calculates the phase between a velocity vector and the corresponding instantaneous amplitude. Once the data are sorted according to θ, the distribution of the number of counts can be obtained as shown in Figure 1a. On the other hand, the time variation of ΣE⋅ν provides important information on the energy exchange due to wave-particle interactions. We can compare the energy exchange depending on different pitch angles (Figure 1b).