Explaining NASA’s magnetosphere observations of electron acceleration with fresh physics and simulation

NASA’s Magnetosphere Multiscale Mission (MMS), the fleet of four spacecraft launched in 2015, has provided unprecedented data on the magnetosphere’s magnetic reconnection, where stored magnetic energy is released, and electrons and ions are accelerated and heated. This particular area in geospace plays an important role in the magnetosphere’s safeguarding the Earth from the sun’s solar wind. How reconnection/electron acceleration occurs in this collisionless zone has been a longstanding question, but in a June 2017 Physics of Plasma a team of researchers from NASA, University of Maryland, and the University of Bergen in Norway offer the fundamental physics.

Co-author Naoki Bessho led formulating the new theory, employing Newtonian equation of motion to ascertain an electron’s meandering/bouncing within the reconnection field. Bessho explains that they then looked to elliptic/oscillating function equations for the time of an electron’s bounce, and then estimated the oscillation period, giving the electron’s acceleration time. Multiplying the force of the electric field by the acceleration time during bounces, the researcher calculated the velocity shift, thus elucidating the broadening crescent shape electron distribution that NASA’s MMS measured.

The team confirmed their theoretical work with simulations on computers at NASA and University of New Hampshire, using a particle-in-cell model, which follows millions to billions of particles in motion with Maxwell equations for the electromagnetic field.

Co-author Li-Jen Chen says their study looks at the particle equation of motion in the magnetic and electric field geometry of the reconnection layer, and succeeds in providing the “smoking gun” signature of collisionless reconnection. Bessho adds that their mathematical analysis and simulations should be very useful to explain MMS reconnection findings, which are surprising and complex.

Source: “The effect of reconnection electric field on crescent and U-shaped distribution functions in asymmetric reconnection with no guide field,” by N. Bessho, L.-J. Chen, M. Hesse, and S. Wang, Physics of Plasmas (2017). The article can be accessed at https://doi.org/10.1063/1.4989737 .