The role of magnetic field fluctuations in nonadiabatic acceleration of ions during dipolarization

Abstract

[1] We statistically examine changes in the composition of two different ion species, proton and oxygen ions, in the near-Earth plasma sheet (X = À16 R E $ À6 R E ) during substorm-associated dipolarization. We use 10 years of energetic (9-212 keV/e) ion data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particles and ion composition (EPIC) instrument on board the Geotail spacecraft. The results are as follows: (1) Although the percentage increase in the energy density of O + ions before and after a dipolarization exceeds that of H + ions in the low-energy range (9-36 keV/e), this property is not evident in the high-energy range (56-212 keV/e); (2) the energy spectrum of H + and that of O + become harder after dipolarization in almost all events; and (3) in some events the energy spectrum of O + becomes harder than that of H + as reported by previous studies, and, importantly, in other events, the spectrum of H + becomes harder than that of O + . In order to investigate what mechanism causes these observational results, we focus on magnetic field fluctuations during dipolarization. It is found that the increase of the spectrum slope is positively correlated with the power of waves whose frequencies are close to the gyrofrequency of H + or O + , respectively (the correlation coefficient is 0.48 for H + and 0.68 for O + ). In conclusion, ions are nonadiabatically accelerated by the electric field induced by the magnetic field fluctuations whose frequencies are close to their gyrofrequencies.

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…Jones et al 2006). Still, a recent statistical study of Geotail data by Ono et al (2009) showed that at times, the H + spectra can harden more significantly than O + . In this latter case, fluctuations of the dipolarizing field lines on time scales comparable to the H + gyroperiod may be responsible for the high proton energization.…”

Proton and heavy ion acceleration by stochastic fluctuations in the Earth's magnetotail

“…Generally, this is observed during substorms or strong reconnection, as has been reported in the literature (e.g., Nosé et al, 2000a, b;Ono et al, 2009;Keika et al, 2013;Luo et al, 2014). Since the energization process has been observed to happen during substorms and localized dipolarization of the magnetic field, the acceleration process can be considered "local".…”

“…In the presence of substorms and local dipolarization of magnetic field, the most energized species is found to be oxygen, reaching energies larger than 200 keV (Nosé et al, 2000a, b;Ono et al, 2009;Luo et al, 2014). During those periods the magnetic fluctuations are generated by reconnection jets and produce a spatially localized acceleration of ions.…”

“…In our numerical model, instead, only the He ++ ions exceed the 100 keV. This is probably due to the different acceleration mechanism: particles continuously interact with stochastic fluctuations and diffuse in the simulation box, and the perturbed magnetic field is at most 10 nT; conversely, in dipolarization fronts the peak magnetic field δB can be as large as 20-40 nT (Ono et al, 2009;Sergeev at al., 2009), and this is clearly influencing the acceleration rate.…”

“…Grigorenko et al (2009) pointed out that 100 keV ions are accelerated near reconnecting regions, possibly by a process related to time-dependent, patchy reconnection. Spacecraft observations suggest that both electrons and ions are accelerated not only in the vicinity of the reconnection X line but also in a larger area around the reconnection region (e.g., Imada et al, 2007;Grigorenko et al, 2009;Ono et al, 2009). Indeed, during disturbed periods, strong electric (Cattel and Mozer, 1982), velocity and magnetic fluctuations (e.g., Hoshino et al, 1994;Borovsky et al, 1997;Zimbardo et al, 2010), as well as energetic ions, are observed in the magnetotail.…”