Auroral Substorm: THEMIS Pinpoints Location of Magnetic Reconnection in Magnetotail

The THEMIS satellites lined up inside the Earth's magnetotail. A reconnection event occurred between the 4th and 5th satellites (NASA)
The THEMIS satellites lined up inside the Earth's magnetotail. A reconnection event occurred between the 4th and 5th satellites (NASA)

Although magnetic reconnection is one of the bedrock theories within the field of space plasma physics, it has been very difficult to observe. We know that magnetic instabilities and electric currents operate within the plasma environment, but the triggering mechanism is difficult to understand. Reconnection occurs near the surface of the Sun and it occurs where the solar wind interacts with the sunward geomagnetic field. It also happens in the magnetotail (i.e. in the shadow of the Earth, where the magnetosphere is swept behind our planet by the pressure of the solar wind) and is most commonly linked with a terrestrial phenomenon: the aurora. Now, for the first time, scientists have located the point at which the magnetic field lines snap, blasting a huge amount of energy right at the Earth…

Auroral substorms occur when energy is released within the terrestrial magnetic field after being “charged up” by a solar storm (most active during solar maximum). A threshold is reached where the magnetic field lines in the magnetotail are compressed and forced together. The resulting reconnection snaps the field lines together, liberating huge amounts of energy, injecting trapped solar plasma into the upper polar atmosphere (generating aurorae). Until now it has been unclear where in the magnetotail the reconnection event occurs, but now, with the help of a chain of satellites embedded inside the magnetosphere called Time History of Events and Macroscale Interactions during Substorms (or “THEMIS”), a reconnection event has been observed, confirming theory and helping scientists understand the nature of auroral substorms.

The THEMIS mission is composed of five satellites, all orbiting within the magnetotail of the Earth (see the graphic above). Each satellite has electric, magnetic, ion and electron detectors on board, ensuring the characteristics of the magnetotail environment are accurately measured. THEMIS collaborates with 20 ground-based observatories in the northern US and Canada which constantly observe the polar sky for auroral activity and magnetic disturbances. The THEMIS satellites are situated in very precise orbits so that every four days, the satellites line up. So one satellite is a third of the way to the Moon, another is a fourth of the distance and the rest are about a sixth of the distance. This way, the outermost satellite will orbit every four days and align with the other four, creating a line of in-situ observatories.

Observations of aurorae above North America by a series of all-sky cameras (NASA)
Observations of aurorae above North America by a series of all-sky cameras (NASA)

It was during one of these satellite alignments that a reconnection event occurred. On February 26th, 2008, the fourth and fifth satellites registered a huge increase in ion flux with the electric and magnetic characteristics of a reconnection event. At a distance of about a third of that from the Earth to the Moon, the magnetic flux had been forced together, generating a reconnection event which liberated 1015 Joules of energy in an instant. The point at which reconnection occurred is known as the “X” point. Two huge bubbles of highly energetic ions (NASA’s news release calls these “plasma bullets” – I’m not convinced this is a particularly accurate description) travelled from the X point, along the magnetic field lines, toward and away from the Earth. The outermost THEMIS satellite registered a “hit” and the Earthward satellite registered a “hit”. The Earth-bound plasma bubble passed through the other three THEMIS locations and eventually entered the Earth’s North Pole auroral oval (as observed by the ground based observatories. View the movie of the auroral eruption through the all-sky camera locations in the US and Canada).

THEMIS scientists are obviously thrilled that they have connected the phenomenon of an auroral substorm event with reconnection in the magnetotail of the Earth:

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.” – Vassilis Angelopoulos, THEMIS principal investigator.

For more information on this stunning discovery, check out the excellent UCLA press release (at Physorg.com), or the not-so-detailed NASA press release

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