In 2015, NASA launched a fleet of four spacecraft that seek out the strange physics that happen at the fringes of Earth’s magnetic field. The Magnetospheric Multiscale Mission (MMS) spacecraft fly in tight formation just kilometers apart, hunting for sites of magnetic reconnection, where Earth’s magnetic field lines link up with the Sun’s field, causing them to explosively snap and realign. This process is still poorly understood, but it is what creates powerful geomagnetic storms that can threaten power grids and astronauts alike.
When MMS flew right through the heart of one such event later that year, one of the signatures that scientists used to recognize the storm was the specific pattern of the scattershot of electrons flying away from the site, propelled along the snapping field lines. Over the past few years, simulations had predicted that most electrons would slowly orbit the Earth, but some would “meander” in orbits that tilt north or south away from the reconnection site. When this velocity distribution is mapped, it produces a “heat map” in a characteristic crescent shape, which was replicated in the MMS data. These simulations also led scientists to think that this behavior is produced by complex disturbances in the electric fields at the reconnection site.
But now a fresh analysis of the MMS data from Lapenta et al. has revised this picture: Although magnetic reconnection is responsible, it appears no electric field is required. This new explanation is simpler and means that this behavior might be more widespread than previously thought.
Originally, scientists had ascribed the electrons’ slow orbit around the globe to a well-known phenomenon for charged particles. In a magnetic field, they spiral along the field lines in a perfect helix, but when they also encounter an electric field, it pushes them off course, sending the electrons and ions drifting in the same direction, in slow but steady orbits around the globe.
But when the authors reexamined the MMS data, they noticed something was off: The electrons and ions were drifting in the opposite direction, a possible incompatibility with previous models. And upon closer inspection, they realized that the speed at which the particles were drifting wasn’t the predicted speed either.
They followed up this observation with calculations and high-resolution computer simulations. The results replicated their analysis of the MMS data and showed that electric fields aren’t as important as previously thought. Instead, the magnetic field reversing itself symmetrically at the X point is enough to send the electrons meandering. The presence of an electric field changes the extent of the crescent, but it’s not required to create it.
If they’re right, then crescent-shaped electron velocity distributions should exist in more places, in particular, high above the nightside of Earth where the solar wind blows the planet’s magnetic field out into a tail. At the tip of the tail, Earth’s field lines reconnect symmetrically and should send electrons meandering. MMS should return data from the magnetotail soon, the authors write, which will put this theory to the test. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2016JA023290, 2017)
—Mark Zastrow, Freelance Writer