What now for NASA's magnetic field probes?


Only a few months into their mission, four spacecraft accidentally but fortuitously wandered into a magnetic field reconnection in Earth orbit. With a little more than a year to go, what else can we expect from them? Alan Duffy explains.


An artist's rendition of the probes travelling around Earth, sampling magnetic field interactions.
NASA
NASA's Magnetosphere Multiscale (MMS) mission, currently zooming around Earth, directly measured magnetic field lines as they snapped apart and reconnected, unleashing blasts of energy and particles.

The announcement, published in Science last week, is the first step in NASA’s quest to learn more about how magnetic fields act in space. So how does the mission work – and what else can it tell us?

The technology behind the mission is, on the face of it, fairly straightforward.

Launched in March 2015, the four MMS probes fly in a pyramid formation as close as 10 kilometres and as far as 160 kilometres apart.

Onboard instruments measure two components to magnetic reconnection: plasma and magnetic fields.

The first component – plasma – is unfamiliar to us on Earth, but it comprises as much as 99% of the normal matter in the Universe. Somewhere between liquid and gas, plasma forms when hot atoms between stars lose or gain electrons (or become “ionised”).

Their motion, then, is guided by magnetic fields – charged ions run along a magnetic field line like water running through a hose. And they can, in turn, generate their own magnetic fields.

An artist's rendition of the MMS probes measuring how the magnetic fields between Earth and the Sun interact.
NASA

The MMS can take “snapshots” of electron flow, a bit like a high-speed camera, every 30 milliseconds (and heavier, positively charged ions at 150 milliseconds).

By sampling 100 times faster than before, the MMS can turn what would previously have been a blurred “image” into a clear time lapse.

The speedy sampling rate is critical, too, as the spacecraft have less than a second to take measurements as they fly through reconnection points.

The second component, magnetic and electric fields, are measured through eight deployable boom antennae. They hang as far as 60 metres out from each craft and measure voltage – the electric field – in the plasma. Magnetic field sensors are held 15 metres away to prevent interference from the spacecraft itself.

This means the four spacecraft can create 3-D maps of fields and plasma wherever they fly.

The experiment is designed to run for two-and-a-half years. But only seven months after launch, the mission found itself in a sweet spot of sorts: the centre of a magnetic field reconnection event.

Usually plasma stays within its magnetic field “hose” but sometimes, those magnetic fields violently smash together, snap apart and reconnect.

What does this look like to the MMS?

First, magnetic field strength drops as the field lines connect. Then the ions accelerate in the opposite direction.

Finally, magnetic field energy is converted into heat and kinetic energy (movement), accelerating electrons to hundreds of kilometres a second.

This point is known as the electron dissipation region. We see it as the Earth’s and Sun’s magnetic fields connect. Plasma from the solar wind – charged particles streaming from the Sun – and the edge of Earth’s magnetic field, called the magnetosphere, mix.

These interactions cause beautiful northern and southern lights. But if they’re strong enough, they can damage power grids.

While the MMS was lucky enough to fly through a magnetic field reconnection in Earth’s magnetosphere at just the right time, the next phase of the mission will see it head away from the Sun, as far as 25 times Earth’s radius, to explore our stretched, comet-like geomagnetic tail.

There, physicists hope to see a perhaps even more energetic reconnection event as the solar wind and Sun’s magnetic field snaps and rejoins Earth’s.

Why the extra energy? Earth’s night-side magnetic field stores more energy because it’s stretched – like an elastic band stretched to its limit.

And the more reconnections the MMS can measure, the better we can harness the information on Earth.

One big problem with nuclear fusion power is the fluctuations in the reactor as magnetic fields snap apart and reconnect.

So while the mission is critical to astrophysics, there are plenty of “down to Earth” reasons to be interested in magnetic reconnection events.

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Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. Twitter | @astroduff
  1. http://science.sciencemag.org/content/early/2016/05/10/science.aaf2939
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