- FRONTIER LETTER
- Open Access
Energetic ion acceleration during magnetic reconnection in the Earth’s magnetotail
© Imada et al. 2015
Received: 10 August 2015
Accepted: 11 December 2015
Published: 22 December 2015
In this paper, we present a comprehensive study of the energetic ion acceleration during magnetic reconnection in the Earth’s magnetosphere using the Geotail data. A clear example of the energetic ion acceleration up to 1 MeV around an X-type neutral line is shown. We find that the energetic ions are localized at far downstream of reconnection outflow. The time variation of energetic ion and electron is almost the same. We observe ∼100 keV ions over the entire observation period. We study ten events in which the Geotail satellite observed in the vicinity of diffusion region in order to understand the reconnection characteristics that determine the energetic ion acceleration efficiency. We find that the reconnection electric field, total amount of reduced magnetic energy, reconnection rate, satellite location in the Earth’s magnetosphere (both X GSM and Y GSM) show high correlation with energetic ion acceleration efficiency. Also, ion temperature, electron temperature, ion/electron temperature ratio, current sheet thickness, and electric field normal to the neutral sheet show low correlation. We do not find any correlation with absolute value of outflow velocity and current density parallel to magnetic field. The energetic ion acceleration efficiency is well correlated with large-scale parameters (e.g., total amount of reduced magnetic energy and satellite location), whereas the energetic electron acceleration efficiency is correlated with small-scale parameters (e.g., current sheet thickness and electric field normal to the neutral sheet). We conclude that the spatial size of magnetic reconnection is important for energetic ion acceleration in the Earth’s magnetotail.
The origin of energetic particles is a long-standing problem in space physics spanning over several decades. So far, two major acceleration processes have been proposed. One is the diffusive shock acceleration based on Fermi mechanism at the collisionless shock (Blandford and Ostriker 1978). This acceleration mechanism explains the power-law energy distribution function with an index of 2, which is often observed in the high energy range (e.g., cosmic rays). Therefore, the collisionless shock is widely believed to be one of the sources of energetic particles. The other process is magnetic reconnection through which particles are accelerated by the interaction with a strong inductive electric field (Zelenyi et al. 1990). The stored magnetic field energy can be rapidly released to the particles during the magnetic reconnection. Numerical simulations have tested energetic particle acceleration during magnetic reconnection in various plasma environments, such as solar corona, Earth’s magnetosphere, and pulsar magnetosphere (Drake et al. 2006; Hoshino et al. 2001; Oka et al. 2010; Pritchett 2008; Zenitani and Hoshino 2001), and various mechanisms have been proposed for energetic particle acceleration during reconnection.
The Earth’s magnetosphere has been regarded as a space laboratory for particle acceleration during magnetic reconnection, since precise information on plasma and electromagnetic fields is available through in situ spacecraft observations. At the start of the satellite observations, it was reported that the energetic particles with several 100 keV to 1 MeV are often observed in magnetotail (Fan et al. 1975; Hones et al. 1976; Sarris et al. 1976). The relationship between these energetic particles and magnetic reconnection has been examined since the early stage of space research. Terasawa and Nishida (1976) claimed that energetic electron bursts (0.3–1.0 MeV) occurred close to a magnetic reconnection region, because the southward turning of magnetic field was simultaneously observed with the burst. Baker and Stone (1977) also studied an energetic electron burst (>1 MeV) and concluded that the observed energetic electron bursts are usually associated with neutral sheet crossing. Energetic ion bursts in the magnetotail have also been studied by many authors (Fan et al. 1975; Sarris et al. 1976). The energy spectrum of high-energy ions can be described by the power-law distribution (γ∼4−6), and the typical upper ion energy is ∼300 keV during geomagnetic activity (Baker et al. 1979; Fan et al. 1975; Sarris et al. 1976). Moebius et al. (1983) reported energetic protons in the energy range 30 to 500 keV and energetic electrons >75 keV obtained by ISEE-1. They presented combined thermal plasma, magnetic field, and energetic particle observations for energetic particle bursts in the plasma sheet. The localized sources of energetic ions up to 500 keV have been observed during the passage of the neutral line. They conclude that the energetic ion acceleration took place in the vicinity of an X-type neutral line. Many observations suggest that energetic particle bursts might be related to magnetic reconnection.
Modern satellite observations reveal more precise acceleration sites of energetic electrons during magnetic reconnection. Øieroset et al. (2002) showed the significant electron acceleration up to 300 keV at an X-type neutral line based on a Wind satellite observation. On the other hand, some studies show that the energetic electrons are generated not only at the X-type neutral line but also in the wider region surrounding an X-type neutral line (Asano et al. 2008; Imada et al. 2007). A statistical study by Imada et al. (2005) discussed plasma heating and acceleration in and around magnetic reconnection region. They conclude that the electrons are first energized at an X-type neutral line and further accelerated in the magnetic flux pileup region. Another important finding for energetic electron acceleration is the relationship to small magnetic islands. Chen et al. (2008) showed that energetic electron fluxes peak at sites of compressed density within islands. Retino et al. (2008) also found the energetic electron flux enhancement within a small-scale flux rope that may be associated with flux rope coalescence. Recent spacecraft observations claim that the energetic electrons can be further accelerated at the dipolarization front which locates far downstream of reconnection flow (Fu et al. 2011).
There are some detailed studies of energetic ion acceleration in the magnetotail, using the modern satellite observations (Artemyev et al. 2014; Haaland et al. 2010). These studies indicate the relationship between energetic ion bursts and the geomagnetic activity and/or thin current sheet formation (Luo et al. 2014; Sarafopoulos 2008). However, the precise position of energetic ion acceleration during magnetotail reconnection and acceleration mechanism is still not certain. Further, energetic electron and ion acceleration seems to be different in some cases (Moebius et al. 1983; Øieroset et al. 2002). Imada et al. (2011) studied the favorable conditions for energetic electron acceleration during magnetic reconnection in the Earth’s magnetotail by using ten Geotail magnetic reconnection observations. Their finding is that the energetic electrons are efficiently accelerated in a thin current sheet during fast reconnection events. The aim of this paper is to understand the favorable conditions for energetic ion acceleration by using ten magnetic reconnection events discussed in Imada et al. (2011). We will also reveal the difference between energetic ion and electron acceleration.
We have used the data from comprehensive measurements onboard the Geotail satellite, including the low-energy particles (LEP/EAi, EAe) (Mukai et al. 1994), the energetic particles and ion composition instrument (EPIC/ICS) (Williams et al. 1994), and the magnetic fields (MGF) (Kokubun et al. 1994). We calculated the ion and electron temperature, density, and the three components of the velocity from the distribution functions obtained by the LEP instrument. To obtain ion moments, 12-s time-resolution data are used. We averaged the 12-s time-resolution electron moments into 60-s time resolution to reduce the statistical uncertainty. As for the energetic ions, we used energy spectra with angular distributions of energetic protons with the energy range of 58 keV to 3 MeV obtained every 96 s by EPIC. To investigate the energetic electrons acceleration, we used the integrated electron flux of > 38 (keV) measured by the EPIC instrument with 12-s time resolution. Although EPIC electron observation has two energy channels with energies higher than 38 and 110 keV, we only used the lower-energy channel which can result in enough particle counts during magnetic reconnection. The energetic particle data were integrated over pitch angle by assuming an isotropic velocity distribution function.
Observations of energetic ions: case study
Statistical property of energetic ion acceleration
List of ten reconnection events observed by Geotail
Correlation coefficients between energetic particle acceleration efficiency and reconnection characteristics
Δ B 2
Summary and discussion
The energetic ion acceleration during magnetic reconnection in the Earth’s magnetosphere has been studied using Geotail data. We showed a clear example of energetic ion acceleration up to 1 MeV around an X-type neutral line. The energetic ions are localized at far downstream of reconnection outflow. We also found that a time variation of energetic ion (∼1 MeV) enhancement is almost the same as that of energetic electron (>38 keV). We can observe ∼100 keV ions over the entire observation period. The ten events in which the Geotail satellite observed the vicinity of diffusion region have also been studied to infer what reconnection characteristics determine the energetic ion acceleration efficiency. We found that reconnection electric field, total amount of reduced magnetic energy, reconnection rate, satellite location in the Earth’s magnetosphere (both X GSM and Y GSM) can be categorized into good correlation with energetic ion acceleration efficiency. Ion temperature, electron temperature, ion/electron temperature ratio, current sheet thickness, and electric field normal to the neutral sheet are classified in ambiguous correlation. We cannot find any correlation with absolute value of outflow velocity and current density parallel to the magnetic field. We find that the energetic ions seem to be well correlated with the large-scale parameters (e.g., total amount of reduced magnetic energy and satellite location), although the energetic electron acceleration efficiency seems to be correlated with the small-scale parameters (e.g., current sheet thickness and electric field normal to the neutral sheet).
Let us discuss the plausible scenario of the energetic ion acceleration up to 1 MeV is from the result of our observation. Our observations show that the energetic ions (∼1 MeV) and electrons (>38 keV) seem to be accelerated at the same place. The standard energetic electron acceleration mechanism is that the unmagnetized electrons in the vicinity of the X-type diffusion region can be accelerated by strong inductive electric field during the meandering/Speiser motion. The polarization electric field in a thin current sheet can also contribute to electron pre-acceleration. Furthermore, accelerated electrons, which have a large gyroradius, are then transported outward from the diffusion region and are further accelerated around the piled-up magnetic field region because of ∇B drift and/or curvature drift under the nonadiabatic motion (Hoshino 2005). The reconnection electric field is important for energetic ion and electron acceleration, because ions and electrons get energy from it. For energetic electron acceleration, the pre-acceleration/heating is crucial to get large gyroradius for second-step acceleration. The electron pre-acceleration and/or heating are generally defined by the small-scale plasma condition. This is the reason why the energetic electron acceleration efficiency is well correlated with small-scale parameter of magnetic reconnection. On the other hand, for energetic ion acceleration, the gyroradius is large enough for second-step acceleration. Hot ions at the piled-up magnetic field region can be accelerated without any pre-acceleration. However, the spatial scale size of the acceleration region is conclusive for energetic ion acceleration up to 1 MeV. In the case of reconnection electric field of 10 mV m −1, the spatial scale needs to be at least 10 5 km (∼magnetosphere width) to accelerate ions up to 1 MeV. Energetic ion acceleration efficiency clearly depends on the satellite location Y GSM. This result indicates the importance of the spatial size of the magnetic reconnection in Y GSM. Energetic ion acceleration efficiency also depends on the satellite location X GSM. The scale length in Y GSM direction might be different with location of magnetic reconnection in X GSM. Imada et al. (2008) studied the dawn-dusk asymmetry of energetic particles and claimed the importnace of the spatial diffusion of energetic particles to explain the observed asymmetry. The energetic particles in magnetotail can gain energy larger than the available potential energy by spatial diffusion. Therefore, we conclude that the spatial size of magnetic reconnection is important for energetic ion acceleration in the Earth’s magnetosphere. To strengthen our conclusion, we need to examine more reconnection events. It is also important to compare our results with recent magnetic reconnection observations by Time History of Events and Macroscale Interactions during Substorms (THEMIS). Especially, this is useful for understanding the relationship between the energetic ion acceleration and reconnection location in X GSM, because THEMIS has a large coverage in X GSM. The magnetospheric multiscale (MMS) mission reveals more precise plasma conditions during magnetic reconnection. We expect that MMS observations clarify the relationship between energetic particle acceleration and reconnection conditions thoroughly. This is crucial for understanding the energetic particle acceleration mechanisms during magnetic reconnection.
Typical values for energetic particle acceleration in Earth’s magnetotail and solar corona
V A (m s −1)
E R (V/m)
e ϕ (eV)
The authors would like to thank all members of the Geotail project team. We are most grateful to the Geotail/EPIC team (instrument PI, initially D.J. Williams, R.W. McEntire, and A.T.Y. Lui) for providing us with the energetic particle data of EPIC. We are also grateful to S. R. Nylund for processing the Geotail/EPIC data. We would like to thank H. Isobe, K. Watanabe, M. Oka, T. Minoshima, K. Keika, M. Nose, M. Asgari-Targhi, and T. Mukai for fruitful discussions. This work was partially supported by JSPS KAKENHI Grant-in-Aid for Young Scientist B (24740130), by JSPS KAKENHI Grant-in-Aid for Scientific Research B (23340045), by JSPS KAKENHI Grant-in-Aid for Scientific Research B (26287143), by the JSPS Core- to-Core Program (22001), by JSPS Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers under grant number G2602, and by JSPS KAKENHI under grant number 15H05816.
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