Magnetic reconnection is the process by which oppositely directed magnetic fields can rearrange themselves into new configurations. Reconnection happens almost everywhere in space, but one near-Earth region where it abounds is the magnetopause. Because we are far from understanding the details of magnetic reconnection, NASA’s Magnetospheric Multiscale (MMS) mission was specifically designed to study reconnection around Earth. To do so, its particle detectors are the fastest ever built—as the spacecraft fly around Earth, they measure electron three-dimensional properties every 30 milliseconds. Here Lavraud et al. use MMS measurements of electron energies and the magnetic field to assess the fine-scale motions of electrons near magnetic reconnection events.
There are two components to the motion of an electron in a magnetic field. The electron orbits magnetic “field lines”—a process known as gyration. The electron can also move in either direction along a magnetic field line, as long as it is always gyrating around the same one.
Electrons need energy to move along the field lines: The stronger the magnetic field is, the more energy the electrons will need. As an electron moves into a region of stronger magnetic field, its motion along the field line will slow down, until eventually it is not moving along the field line anymore, only orbiting it. Once it has stopped making forward progress, the force from the magnetic field pushes it back the way it came. If an electron is trapped between two strong regions of magnetic field, then it will “bounce” back and forth. Near reconnection events, the magnetic field strength becomes very weak, and electrons will bounce back and forth through this region of weak field. This low magnetic field bouncing of electrons is shown to add to other proposed bouncing mechanisms such as parallel electric fields.
The researchers found that during reconnection, the region where the magnetic field is getting weaker also becomes sharply bent. Eventually, the field lines become so curved that the electron’s orbit around the magnetic field encompasses more than one part of the field line—breaking the rules. The motion of the electron is then interrupted, and it is scattered away. The more energy an electron has, the larger its orbit will be, and the less bent the field lines need to be to in order to interrupt and scatter it. As electrons bounce back and forth near reconnection sites, electrons with higher energies will scatter sooner.
For the first time, scientists were able to observe this dependence on energy and watch as more energetic, bouncing electrons were scattered farther away from reconnection sites. Observing these small-scale motions is a huge step toward better understanding magnetic reconnection and the behavior of Earth’s magnetopause. (Geophysical Research Letters, doi:10.1002/2016GL068359, 2016)
—Aleida K. Higginson, Freelance Writer
Citation: Higginson, A. K. (2016), Electrons thrown off course in near-Earth magnetic reconnection, Eos, 97, doi:10.1029/2016EO051175. Published on 26 April 2016.