Some of the most mysterious physics in all of space occurs tens of thousands of kilometers above the Earth, where the Sun’s magnetic field merges with that of Earth. This region pulses with currents and fields as the two fields tangle and reconnect, especially during solar storms, when reconnection soars and sends currents surging down into Earth’s magnetic field, causing hazardous geomagnetic storms.
Scientists have labored for decades to understand what happens here, but in October 2015 they got a significant break: For the first time, a fleet of NASA satellites flew directly through a reconnection event. Now a new study by Dai et al. explains these data further and suggests that one electric field important to reconnection is triggered by a certain type of plasma wave. The work advances our understanding of magnetic reconnection, which is critical to forecasting geomagnetic storms.
NASA’s Magnetospheric Multiscale (MMS) mission consists of four spacecraft launched in March 2015. Flying in a tight pyramid-shaped formation, they soar through the fringes of Earth’s magnetic field hunting for reconnection sites, taking high-resolution measurements as they fly in and out of the swirling currents and fields.
One of the most prominent fields that appears during reconnection is the Hall electric field, which points across the boundary of Earth’s magnetic bubble. MMS revealed that this field is caused by the pressure of solar wind ions outside of Earth’s magnetic field pushing against it. The Earth’s magnetic field generates the Hall electric field to balance against the intruding ions.
But what allows these ions to intrude in the first place? And why only positively charged ions and not electrons, too, which would result in no separation of charge and no electric field? As the authors detail, this change in ion pressure is mathematically related to the vibrations in the magnetic field lines themselves. These vibrations, called kinetic Alfvén waves, are akin to those caused by plucking on a string.
According to the equations that govern plasma particles, both electrons and ions gyrate around the field lines. But protons have roughly 2000 times the mass of electrons, with a corresponding amount of additional inertia. So although electrons in the solar wind remain tightly coiled around the Sun’s magnetic field lines (typically within a few kilometers), positively charged ions perform gyrations as wide as hundreds of kilometers. This allows them to penetrate farther into regions than electrons. This effect also shows up in the equations for kinetic Alfvén waves, suggesting that these waves trigger the Hall electric field in reconnection events.
The authors note that kinetic Alfvén wave physics can also explain several other phenomena observed at magnetic reconnection sites, including currents that flow along the magnetic field as well as the formation of additional smaller magnetic fields that run perpendicular to those of the Earth and Sun. In addition, the strength of the Hall electric field relative to these perpendicular magnetic fields happens to be very close to the speed of kinetic Alfvén waves as they propagate along magnetic field lines, strengthening the case that they play an important role in magnetic reconnection. (Geophysical Research Letters, https://doi.org/10.1002/2016GL071044, 2017)
—Mark Zastrow, Freelance Writer