2020 ARTEMIS SCIENCE NUGGETS
Energetic Electron Acceleration by Ion-scale Magnetic Islands in Turbulent Magnetic Reconnection
by San Lu
UCLA EPSS/IGPP
Introduction
How charged particles are accelerated to high energies in astrophysical systems has been a fundamental question for decades, and this question is particularly important for electrons because they can reach extremely high (relativistic) energies. Magnetic reconnection is a process of magnetic energy release and transfer to particles through topological changes in magnetic field lines, and provides a good mechanism for particle acceleration. Reconnection is therefore widely believed to be responsible for energetic particle acceleration in various astrophysical plasma environments. Coherent structures, especially magnetic islands produced by magnetic reconnection, have also been found to play important roles in electron acceleration. Magnetic islands with spatial scale on the order of ion kinetic scales can accelerate electrons via a surfing mechanism. These ion-scale islands are formed by secondary reconnection at the X-lines, therefore, they are referred to as secondary islands. Therefore, electron acceleration in turbulent reconnection, which hosts many such islands, should be more efficient than electron acceleration in laminar reconnection. A systematic examination of this hypothesis using simulations and observations is needed.
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Figure 1. Profiles of Bz (upper panels) and energetic electron energy flux (lower panels) along z=0 for the seven secondary magnetic islands identified at Ωit = 70 . The electron energy flux is defined as  (where ε is the electron kinetic energy), and it is integrated above 10 different energies, εminin ten different high-energy ranges from ε > εmin = 4Te0 to ε > εmin = 40Te0, where Te0 = 0.1miV2A is the initial current sheet temperature for electrons. The electron kinetic energy ε is in units of miV2A, and f(ε) is the electron energy distribution, where ∫f(ε) dε = ne. The electron density ne is in units of n0, so the energy flux is in units of n0miV2A.
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Results
Our particle-in-cell (PIC) simulations show that numerous ion-scale magnetic islands are formed in turbulent magnetic reconnection. In the simulations, for example, at Ωit = 70, there are seven ion-scale magnetic islands, which are characterized by bipolar Bz profiles at z = 0 (Figure 1, upper panels). The spatial scale of these islands is about several ion inertial lengths, di. The lower panels of Figure 1 show spatial profiles of integral high-energy electron energy flux "channels," , integrated above different energies, εmin (different colors represent different channels). The highest channel, ε > εmin = 40Te0, corresponds to a Lorentz factor of γe > 2. The high-energy electron energy flux peaks at or near the center of each secondary island, and the peak value is about one to two orders of magnitude higher than the ambient level. Therefore, such ion-scale secondary magnetic islands in turbulent reconnection can effectively confine and accelerate electrons to high or even relativistic energies.
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| Figure 2. Superposed epoch analysis and event distributions for short electron energy flux peak events. (a), (b) Averaged profiles of normalized electron energy fluxes and magnetic field fluctuations. (c) Distributions of spatial-scales of magnetic field fluctuations. |
We seek confirmation of the above physical process using in-situ observations of magnetic reconnection regions by the two ARTEMIS spacecraft in Earth's magnetotail. These two spacecraft provide an ideal dataset for investigation of reconnection physics in a general current sheet. The ARTEMIS spacecraft detect reconnection regions embedded in both earthward and tailward plasma flows. We focus on the electron energy channel of 30-70 keV and collect a total of 131 peaks of energy flux in this channel. Then we put these 131 peaks into an epoch analysis, as shown in Figure 2. For each interval we determine a typical time-scale T (during which electron energy flux j varies from jmax⁄5 to jmax and then back to jmax⁄5, where jmax is the three-point peak value of electron energy flux) and typical spatial scale L=
. Figures 2a and 2b show the superposed epoch analysis of the normalized electron energy flux and δBz. The 30-70 keV electron energy flux peaks are clearly seen to be statistically associated with bipolar Bz variations, which are indicative of magnetic islands. As evident from Figure 2c, the electron energy flux peaks and the δBz have a spatial scale of L⁄di ~2.5-5 (where the ion inertial length, di, is evaluated using the averaged ion density), confirming that the magnetic islands are indeed ion-scale secondary islands.
Conclusions
We have shown through particle-in-cell simulations that numerous ion-scale secondary magnetic islands formed in the vicinity of a magnetic reconnection region can accelerate energetic electrons effectively. The ARTEMIS spacecraft, in Earth's magnetotail at lunar distance, observed ion-scale peaks of high-energy electron energy fluxes (energies a factor of ~100 higher than the electron temperature) in turbulent reconnection outflows. These electron energy flux peaks have a spatial scale of several ion inertial lengths and correlate with bipolar (+/-) variations of the magnetic field normal to the current sheet, Bz, which is indicative of ion-scale magnetic islands. These observations support our hypothesis from simulations that ion-scale magnetic islands significantly accelerate energetic electrons in turbulent reconnection.
References
Lu, S., Artemyev, A. V., Angelopoulos, V., & Pritchett, P. L. (2020). Energetic electron acceleration by ion-scale magnetic islands in turbulent magnetic reconnection: Particle-in-cell simulations and ARTEMIS observations. The Astrophysical Journal, 896, 105. https://doi.org/10.3847/1538-4357/ab908eBiographical Note
San Lu is an Assistant Researcher in space physics at University of California, Los Angeles. His primary research interests are computer simulations (particle-in-cell and hybrid) of space and laboratory plasma physics.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu