Auroral substorms – a space weather mystery is solved

Artist's impression of an auroral substorm (image: NASA)

Around the world there are space scientists who have built and sustained their careers on the back of what seemed an endless controversy over the origin of the majestic light shows known as auroral substorms. But the mystery has finally been solved, thanks to a multi-spacecraft mission called THEMIS.

Auroral substorms are short periods of intense auroral activity that result from localised disturbances of the Earth’s magnetic field at high latitudes. Observers on the ground will typically see bands of green-white light stretching from east to west across the sky. These arcs of auroral light brighten as they move southwards, following which they break up into smaller, multi-coloured features that dance around as they reverse direction and move north.

An auroral substorm is an amazing sight to behold. For a decade or so I made a living studying the phenomenon in northern Scandinavia, and mathematically modelling it in digital space.

Typical auroral display (image: NASA/Jan Curtis)

The motivation for studying auroral substorms is both aesthetic – the northern and southern lights are among the world’s greatest natural wonders – and practical. Huge amounts of charged-particle radiation are dumped on the Earth as a result of storms in space. Such space weather events can disrupt terrestrial communications, trip power transmission lines, trigger military early warning radars and destroy satellite subsystems.

We have long known that events on the surface of the Sun propagate outwards in the supersonic stream of protons and electrons known as the solar wind and disturb the Earth’s magnetic field, giving rise to the aurora. But what hasn’t been fully understood until now is the location of the substorm trigger within our planet’s magnetic envelope.

Some scientists held that the origin of substorms was to be found relatively close to the Earth in the so-called magnetotail, which is a comet-like tail on the nightside of the Earth, formed by the interaction of the solar wind with the magnetosphere. At around one-sixth of the distance from the Earth to the moon, large electric currents across the magnetotail can become disrupted, and the energy in these currents released in an explosive plasma instability.

Typical auroral display (image: NASA/Jan Curtis)

An alternative hypothesis is that the trigger is located further out at around one-third of the distance to the moon, where magnetic field lines swept behind the Earth by the solar wind merge in the magnetotail. These magnetic field lines reconnect, releasing magnetic energy in the form of accelerated charged particles which move around and along the field lines toward the Earth, where they finally impact with oxygen and nitrogen in the upper atmosphere. The atmospheric atoms and molecules in turn release light of various colours as they relax back to their normal energy state.

Data from the five THEMIS spacecraft have shown that magnetic reconnection is the substorm trigger. UCLA space physicist and THEMIS Principal Investigator Vassilis Angelopoulos explains:

“Our data show clearly and for the first time that magnetic reconnection is the trigger. Reconnection results in a slingshot acceleration of waves and plasma along magnetic field lines, lighting up the aurora underneath even before the near-Earth space has had a chance to respond. We are providing the evidence that this is happening.”

The Earth’s magnetosphere is a vast region, and being in the right place at the right time to study an auroral substorm is a considerable challenge. You can record substorms from the ground with magnetometers, cameras and radar systems, and you can fly single spacecraft through the magnetosphere. But to build up an accurate, three-dimensional picture requires a fleet of satellites flying in tightly-controlled formation, and some very sophisticated data analysis. It is otherwise impossible to separate structures with spatial extent from variations in time as the spacecraft move through an active region.

But spacecraft alone, even with the most advanced magnetometers and particle detectors available, are not enough. In the case of THEMIS, ground-based instruments are a key part of the mission. My old friend Trond Trondsen and his colleagues at the University of Calgary in Canada built the THEMIS all-sky cameras installed in an array of 20 ground-based observatories sited from the east coast of Canada to the west coast of Alaska.

THEMIS was launched in February 2007, at a total cost of around $200m. The mission is expected to observe around 30 substorms during its nominal lifetime.

Images: NASA

See the THEMIS website for more images, and animations that illustrate concepts such as magnetic reconnection in ways that words alone cannot match.