2016 THEMIS SCIENCE NUGGETS
In situ evidence of electron energization in the electron diffusion region of magnetotail reconnection
by Mitsuo Oka
UC Berkeley Space Sciences Laboratory
Introduction
Magnetic reconnection is a fundamental plasma process that converts magnetic energy to particle energies. It plays an important role in explosive energy-release phenomena such as solar flares and terrestrial substorms. During reconnection, magnetic field lines of opposite directions "break" and "reconnect" in the diffusion region (Fig. 1). As a result, bi-directional outflow jets are produced centered around the diffusion region. Thus, this diffusion region is the key to a better understanding of magnetic reconnection and hence the energy-release phenomena.
Because plasma is an ionized gas consisting of ions and electrons, the diffusion region has a two-scale structure. The electron-scale diffusion region (Fig. 1, the dark gray region) is much smaller than the ion-scale region (Fig. 1, the light gray region). Because of its minuscule size, there is probably not much chance for a spacecraft to encounter the electron diffusion region in Earth's magnetotail. Here we report a fortuitous encounter of the electron diffusion region by a THEMIS spacecraft in Earth's magnetotail.
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| Figure 1. (upper panel) A schematic illustration, showing two-scale structure of the magnetic reconnection diffusion region. Earth is to the left and the magnetotail extends to the right in this schematic. (lower panels) 3s sampling electron data from P1, showing an evolution of electron pitch angle distribution on 7 February 2009. The angles 0° and 180° correspond to parallel and anti-parallel to the magnetic field line, respectively. |
Observations
On 7 February 2009, P1 (the outermost probe of the five THEMIS probes) was in the midnight sector of Earth's magnetotail and detected a tailward passage of the magnetic reconnection region. A passage was identified by a reversal of the detected outflow plasma from tailward to earthward, associated with a reversal of magnetic field direction from southward to northward (e.g. relative to the reconnection region, P1 moved from right to left in the schematic of Fig.1).
During the correlated reversals, P1 observed different patterns of electron distribution functions (Fig.1, lower panels). In the immediate upstream of the electron diffusion region (labeled a), P1 observed lower-energy (~1 keV) and bi-directional electrons along the field line focused around 0° and 180° pitch angles. Since charged particles generally spiral along magnetic field lines, "pitch angle" is the angle between the direction of the magnetic field and a particle's spiral trajectory. In the immediate downstream of the electron diffusion region (labeled c), P1 observed higher-energy (~5 keV) and isotropic electrons. These types of distribution have been already reported by previous spacecraft missions, and a detailed analysis has been performed in the framework of magnetic reconnection.
What is new in our study is the detection of the third type of distribution (labeled b). It appears to be a mixture of both lower-energy, bi-directional electrons and higher-energy electrons. The higher-energy electrons appeared only around 45-90° pitch angles. Previously, plasma particle simulations have predicted that, in the electron diffusion region, electrons would be energized centered around the 90° pitch angle. The observed features are consistent with the prediction and can be regarded as evidence of entry into the electron diffusion region. Further analysis suggests that electrons were demagnetized (i.e., electron motion was not ordered by the local magnetic field line) and that a total of more than an order of magnitude energization was achieved across the diffusion region region (from far upstream to immediate downstream).
Conclusion
THEMIS obtained in-situ evidence of the electron diffusion region during magnetotail reconnection. We found significant energization of electrons in the electron diffusion region, consistent with a prediction by simulations. Previously, there have been conflicting theories as to whether the electron diffusion region can be a place of rapid energization of plasmas. Our results demonstrate that, despite its minuscule size, the electron diffusion region does indeed contribute to the overall process of electron energization via magnetic reconnection.
Reference
Oka, M., T.-D. Phan, M. Øieroset, and V. Angelopoulos (2016), In situ evidence of electron energization in the electron diffusion region of magnetotail reconnection, J. Geophys. Res. Space Physics, 121, doi:10.1002/2015JA022040.Biographical Note
Mitsuo Oka is an Assistant Research Physicist at Space Sciences Laboratory, University of California Berkeley. His primary research interests are observations and simulations of electron acceleration in various environment including solar flares and Earth's bow shock.
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Emmanuel Masongsong / emasongsong @ igpp.ucla.edu