5:15 PM - 7:15 PM
[PCG19-P02] Estimating the effect of bounce electron absorption on whistler mode wave growth through the formation of a loss-cone velocity distribution around Europa
Keywords:Jupiter magnetosphere, Europa, Linear growth rate, whistler mode wave, loss-cone distribution
Galileo observations have revealed whistler-mode waves near Europa and Ganymede with wave power up to 106 times greater than those observed in the absence of satellites (Shpritz et al., 2018). The detailed mechanism of the significant difference in the wave amplitude has not been fully explained yet. On the other hand, observations by Cassini suggest that Rhea, a satellite of Saturn, can act as a driver of wave excitation by absorbing high-energy electrons in the Saturnian magnetosphere, leading to temperature anisotropy (Santolík et al., 2011). In this study, based on the hypothesis that Europa absorbs high-energy electrons bouncing along Jovian magnetic field lines, creating temperature anisotropy that drives an instability to generate whistler-mode waves, we investigated the dependence of the wave growth rate on Europa's magnetic latitude and the wave frequency through simulations.
We assume a loss-cone distribution as the anisotropic particle distribution and vary the parameter q, which characterizes the anisotropy, to reproduce various situations about the relative distance between Europa and the magnetic equator. Furthermore, we estimated the energy range of electrons colliding with Europa based on the relationship between the bounce time of the particles and the time Europa spends traversing the magnetic field lines, as well as considerations regarding the particles' equatorial pitch angles (Clark et al., 2022). We compute the linear growth rate according to Xiao et al. (1998). The simulation results show that, at a wave frequency of 0.2 Ωe, where Ωe represents the electron gyrofrequency and is equal to 12000 [/s], the growth rates obtained were ωi = 4.82 [/s] at Europa's magnetic latitude (Mlat) of 10°, ωi = 16.5 [/s] at Mlat = 7°, and ωi = 0.652 [/s] at Mlat = 3°. These results suggest that as Europa's magnetic latitude decreases, two opposing effects are present: (1) the temperature anisotropy of the velocity distribution function increases, leading to an increase in the growth rate, and (2) the number of particles resonating with the waves decreases due to collisions with the satellite, leading to a decrease in the growth rate. Similar trends were observed at frequencies of 0.4 Ωe and 0.6 Ωe, with the growth rate being maximum at Mlat = 7° and minimum at Mlat = 3°. The obtained growth rates are considered sufficient to explain the significant increase in the wave amplitude observed around the Europa orbit reported by Shpritz et al. (2018).
We assume a loss-cone distribution as the anisotropic particle distribution and vary the parameter q, which characterizes the anisotropy, to reproduce various situations about the relative distance between Europa and the magnetic equator. Furthermore, we estimated the energy range of electrons colliding with Europa based on the relationship between the bounce time of the particles and the time Europa spends traversing the magnetic field lines, as well as considerations regarding the particles' equatorial pitch angles (Clark et al., 2022). We compute the linear growth rate according to Xiao et al. (1998). The simulation results show that, at a wave frequency of 0.2 Ωe, where Ωe represents the electron gyrofrequency and is equal to 12000 [/s], the growth rates obtained were ωi = 4.82 [/s] at Europa's magnetic latitude (Mlat) of 10°, ωi = 16.5 [/s] at Mlat = 7°, and ωi = 0.652 [/s] at Mlat = 3°. These results suggest that as Europa's magnetic latitude decreases, two opposing effects are present: (1) the temperature anisotropy of the velocity distribution function increases, leading to an increase in the growth rate, and (2) the number of particles resonating with the waves decreases due to collisions with the satellite, leading to a decrease in the growth rate. Similar trends were observed at frequencies of 0.4 Ωe and 0.6 Ωe, with the growth rate being maximum at Mlat = 7° and minimum at Mlat = 3°. The obtained growth rates are considered sufficient to explain the significant increase in the wave amplitude observed around the Europa orbit reported by Shpritz et al. (2018).