Aurora and the Earth’s Magnetotail Part 3: THEMIS and beyond

Power transformer in New Jersey, US, burnt
out due to induced current caused by a
geomagnetic storm in March 1980.
(courtesey PSE&G)

In March 1998 there was a strong magnetic storm which created currents in the Earth's magnetosphere and induced currents also in the electricity network. Strongly changing magnetic fields will induce electric fields in conductors, and thereby electric currents. If the electrical system is not optimized or switched off (which mostly happens with satellites in space) then during strong events the induced current can become so great that it burns transformers, like the example in New Jersey. In order to avoid this kind of damage one needs to study Space Weather (apart from updating the power grid to modern standards).

THEMIS view of the Magnetotail

The THEMIS spacecraft in the clean room
mounted on their launch platform.

On 17 February 2007 the 5-spacecraft mission THEMIS (Time History of Events and Macroscale Interactions during Substorms) were launched, which had the goal to study the dynamic processes in the Earth's magnetotail at 5 different locations along the tail.They were put into elliptical orbits with apogees (furthest distance from Earth) at 10 (2x), 12, 20 and 30 Earth radii, and with every full orbit of the outermost satellite, the five would line up along the tail.This way, the development of the explosive events leading to aurorae, substorms, can be studied in space and time through simultaneous observations by identical spacecraft.

All the processes in the Earth's magnetotail that lead to the reconfiguration of the magnetic field, through stretching, reconnection, fast flows and generation of the aurora are called a magnetospheric substorm. The word substorm is a bit antiquated, because in the beginning people thought that a geomagnetic storm (another energetic phenomenon of the Earth's magnetosphere, which lasts much longer) was build up from a set of smaller substorms. Although this idea was proven wrong, the name substorm stuck.

The "outside-in" model for substorms

There has been, and still is, a longstanding discussion about the processes during such a substorm, where basically two schools stand face-to-face. One side says: "A substorm starts by reconnection far down the tail and then processes closer to the Earth happen and the aurora is created," which is the so-called "outside-in" group. The other side, unsurprisingly, says: "At a substorm first something happens close to the Earth, which sends a signal out, which sets on reconnection further down the tail and then the aurora is created," which is the "inside-out" group. THEMIS was supposed to solve this problem.

The "inside-out" model for substorms

In 2008 the first and conclusive paper was published in Science by the PI team: Tail Reconnection Triggering Substorm Onset. This should show that the "outside-in" model was the correct interpretation of how substorms develop. However, one can imagine that this was not the end of the story, the other group wanted to have a say too. After many (heated) discussions at scientific conferences a paper was published in the Journal of Geophysical Research in 2011: Revisiting Time History of Events and Macroscale Interactions during Substorms (THEMIS) substorm events implying magnetic reconnection as the substorm trigger, where some critical notes were made on the interpretation of the original paper, showing that the case may not be so clear as originally was thought and that the "inside-out" model could be the correct interpretation. Well, fortunately, one can find still other events that do not seem to care at all about these two schools and do not adhere to either. This will keep space physicists busy for the next coming years, if not decades. And there is a important thing to be learned here, also in space physics things are not black or white, in complicated processes like substorms there are at least 50 shades of grey. Don't get hung up on just one interpretation.

Unfortunately THEMIS is no longer. The spacecraft are still in space and working, but the mission has changed. The two outermost spacecraft have had their orbits majorly changed and were send to the Moon in 2011, where they now operate under the name ARTEMIS. The three innermost spacecraft remained in their near-Earth orbits.

The next big multi-spacecraft mission for the Earth's magnetotail is going to be NASA's Magnetospheric Multi Scale (MMS) mission, which is going to be another 4 spacecraft mission, similar to Cluster in a tetrahedron configuration, but now the spacecraft will be much closer together, 30 - 400 km. MMS will look at the dynamic processes in the tail at the "electron scale." Basically, this mission will "zoom in" on e.g. the reconnection process that was measured by the Cluster mission and get a more detailed view on smaller scales of what is going on. The planned launch is in October 2014.