by Vassilis Angelopoulos

The Sun [Movie 1] constantly spews out charged particles moving at a million miles per hour past Earth and distorts Earth's magnetic field region, the magnetosphere, from the familiar bar-magnet shape into a windsock-shaped object. On the Sunward side of that interaction, when the magnetic field of the solar wind points south, that is, opposite to Earth's field, the two fields snap and connect again. The result is that magnetic field lines of the solar atmosphere now connect on one side directly to Earth's upper atmosphere. This way, Earth's magnetic field lines are peeled off like onion layers on the dayside and are transported into Earth's magnetic tail, the magnetotail. The transport stores magnetic energy into the magnetosphere and drives intense electrical currents in space. At times, the input power is equivalent to the total power-generating capacity in the U.S.

Eventually, something "gives", triggering a transformation of the stored magnetic energy into hot jets of charged particles streaming towards Earth. Around the same time the aurora brighten suddenly, advancing towards the poles. This 3-4 hour long cycle of solar wind energy storage and sudden release is called a substorm.

Substorms can release energy equivalent to a magnitude 5-6 Earthquake, and can be part of large magnetic storms, energizing charged particles in the radiation belts, and disrupting communications. Like meteorologists who study tornadoes to understand the most severe thunderstorms, we want to study substorms to gain insight into the most intense space storms and better protect astronauts and space systems.

What is a substorm?
Movie 1. Substorms begin with solar wind energy
bombarding and contorting Earth's magnetosphere.
Click here to play the animation.

The THEMIS question

But what triggers this energy release? This question has puzzled scientists since the beginning of the space age. Two main models exist. The two models involve identical processes but differ only in the sequence these processes happen in.

The first model, [Figure 1] called the "current disruption model," suggests that the intense space currents that had built up during the energy storage phase are disrupted, as if space electricity has blown a fuse. The magnetotail implodes, causing the aurora to brighten in spectacular flashes, a minute or two later. The disruption starts at ~ 1/6 of the way to the moon and releases a pressure wave, which evacuates particles from further down the tail, at 1/3 of the way to the moon, setting off magnetic reconnection there.

The Current Disruption model of a substorm
Figure 1. The current disruption model of the propagation
of substorms.
Click here to enlarge the image.

The other model, the "reconnection model," [Figure 2] suggests that the spontaneous onset of magnetic reconnection happens first, at about 1/3 of the way to the moon, resulting in fast flows in its aftermath, like an explosion. The flows collide with the near-Earth region, resulting in current disruption within 1-2 minutes, and an intense aurora yet 1-2 minutes later.

The reconnection model of a substorm
Figure 2. The reconnection model of the propagation
of substorms.
Click here to enlarge the image.

By timing the sequence of auroral brightening, current disruption and reconnection, we can distinguish between the models. This is precisely what we set out to do with THEMIS. We recently were able to show, in several cases, that magnetic reconnection triggered substorm onset.

Results of our study

At the top of Figure 3 you see images of the aurora from several ground stations. At the bottom you see the positions of the five THEMIS satellites (or probes), which were along the same local time meridian as the aurora, but in space. Two of the satellites, P1 and P2, colored red and green, were on either side of the reconnection site. Two other satellites, P3 and P4, colored ciel and blue, were near the current disruption region.
Ground images of the aurora and THEMIS probe positions
Figure 3. Ground images of aurorae and satellite positions.
Click here to enlarge the image.

The magnetic field and flows from these satellites are shown in Figure 4. Satellites P1 and P2 (in red and green) observed changes in the magnetic field and flow indicated by the vertical bar at TRx. The northward, or Z-component of the magnetic field reduced at P1 and increased at P2, showing that reconnection started to occur between these two spacecraft at the time marked "1st".

About 1.5 minutes later, the aurora brightened. This is shown in the Auroral Intensity panel, the black trace, which is constructed from auroral images like the ones we saw earlier. The auroral intensity increased sharply at time TAI, marked "2nd".

Next, satellite P3 (in ciel) observed classical signatures of current disruption: The northward magnetic field component shown suddenly increased toward a value close to that of Earth's bar-magnet, as expected from the reduction of the space current, at TCD.

This happened 1.5 minutes after auroral intensification and a full 3 minutes after reconnection. So, cause and effect requires an interpretation in which reconnection triggered this substorm and the auroral brightening.

Sequence of events observed by THEMIS probes during a substorm event.
Figure 4. The sequence of events observed by the probes
during a substorm event.
Click here to enlarge the image.
What was very surprising to us was that the aurora brightened almost immediately after reconnection and before current disruption. This defies our old paradigms, as it's not one of the sequences that was shown in Figures 1 and 2 earlier. It suggests that the aurora is linked much more directly to the reconnection process than we previously thought.

Movie 2 shows an artist's rendition of the substorm onset based on these findings. The movie shows how the outer THEMIS probes observed the reconnection process and how the accelerated particles may result in the prompt auroral brightening and poleward motion that we observed in this event. We will be looking at more of these events in the next year to see how general these findings are.

THEMIS findings depicted in animation
Movie 2. An artist's rendition of a substorm based on
THEMIS findings.
Click here to play the animation.
Future work

Where do we go from here? Solving this age-old mystery is expected to galvanize research in modeling the fundamental process of substorms, eventually contributing to better space weather prediction tools. Moreover, now that we see that reconnection starts the substorm process, we want to understand what causes reconnection to begin in the first place, in the Earth's magnetotail. What we learn from Earth's space environment can lead to a better understanding of energy releases in other astrophysical settings where reconnection occurs, such as the solar wind, the Sun, and other planetary and stellar systems.

Biographical note

Vassilis Angelopoulos is an Associate Professor at the department of Earth and Space Sciences at UCLA, and a member of the Institute of Geophysics and Planetary Physics. He is the principal investigator of the THEMIS mission.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu