The rapidly changing spectacle of an auroral storm dazzles the eye with intense explosions of colors and forms, but their origins are still not fully understood.
Figure 1: Picture of an auroral storm over Canada taken from the DMSP satellite at night. (Courtesy - DMSP).
Sensitive magnetic instruments, and even compass needles, occasionally register a sudden increase and decrease in Earth's magnetic field strength. These changes often occur during the most violent solar storms and can lead to spectacular Northern and Southern Lights. What scientists still don't understand is the cause of the Aurora as part of the exact sequence of events that take place during a magnetic storm. Exactly how does a magnetic storm cause particles to rush into the Polar Regions and create an Auroral storm?
The Northern Lights have been observed for thousands of years, but for nearly all of that time, observers have merely watched and wondered what could possibly be causing them. Folk lore and supernatural explanations abounded, but precious few scraps of data to help guide the speculation. By the 1700's, disturbances in Earth's magnetic field became familiar to the fledgling scientific community of those times, and later christened by Alexander von Humbolt (1769-1859) as 'Magnetic Storms'. Observatories were set up around the world to monitor Earth's strange magnetic gyrations, which eventually led to the discovery that the strongest disturbances coincided with aurora borealis sightings.
At first, aurora were blamed on atmospheric discharges of static electricity. Scientists soon proposed that an electrical current flowed between the North and South Poles through the atmosphere, and when this current was disturbed, electrical discharges on a planetary scale would soon follow, producing aurora. During the latter part of the 1800's, the observed connection between aurora and magnetic storms grew into a firm correlation, and was soon joined by the observation that aurora sightings followed the sunspot cycle and the appearance of large sunspots. This led to the idea that aurora and magnetic storms, were by some means triggered by beams of particles emitted by sunspots, impinging on Earth's magnetic field.
Figure 2: An aurora from the ground at Yellowknife, NT in December 2004. Without magnetic storms, aurora would be featureless glows in the sky. Magnetic fields provide the pathways for currents to flow into the atmosphere in particular patterns controlled by the magnetic forces and their changes through time. (Courtesy: Andrew Eaton)
During the first half of the 20th Century, the solar origins for aurora, and a refined understanding of solar activity and the dynamics of Earth's magnetic field, led to the first precise mathematical models for magnetic storms. Although aurora were not part of this modeling, the models did show a terrestrial magnetic field severely distorted by the solar wind, which consisted of charged particles (plasma) and pieces of the sun's own magnetic field. As this medium flowed past Earth, it distorted earth's field into a comet-like shape with a long magnetic tail extending millions of miles behind Earth. This tail was capable of storing up enormous amounts of energy, but there was still no sign of a process for creating aurora.
By the 1950's, models describing Earth's magnetic field began to include additional flows of charged particles (called currents) trapped within the magnetic field, but changing in strength as Earth's field became more distorted. One of these is called the Ring Current: an invisible river of particles flowing above Earths' equator during the most severe magnetic storms...and then it vanishes when the storm is over. Between 1965 and 1975, satellite observations confirmed observations by ground-based observers that aurora and magnetic storms evolve by passing through specific kinds of stages.
Another important idea that made its appearance was Magnetic Reconnection through the theoretical work on solar flares by James Dungey in 1961, and later by Eugene Parker in 1963 and Harry Petschek (1930-2005) in 1964.
This is the idea that a complicated magnetic field in which a lot of magnetic energy is stored can suddenly change its shape into a more simpler one with less energy by altering how its magnetic lines of force are connected. In some ways, this is like taffy or a rubber band stretched to its limit, which suddenly snaps and rearranges itself. This process was later confirmed in discoveries made by spacecraft such as Geotail (1997) and Cluster (2006).
Figure 3: A sequence of pictures taken by the IMAGE satellite of an aurora borealis sequence. From space, the aurora appear as a halo of light centered on the magnetic north and south poles. (Courtesy: IMAGE-FUV)
How does magnetic reconnection help us to understand how aurora, and auroral storms, are created? Because this rearrangement provides a way of releasing magnetic energy and changing the way that charged particles are moving over times as short as a few minutes. The problem is that, almost immediately, two very different ideas arose from this magnetic reconnection proposal, and suggested different sequences of events. The sequence of events occur so quickly, literally within a few minutes, and span such a huge extent of space…nearly 100,000 kilometers, that earlier satellite studies were overwhelmed by the scale of the process and could only observe small parts of it at a time.
To understand how they work, it is convenient to use a unit called the Re (pronounced by just saying the letters 'R' and 'E'). One Re is a distance equal to the radius of the Earth or 6,378 kilometers. 10 Re is a distance of 63,780 kilometers.
Scenario I - When Earth's field becomes distorted, a magnetic reconnection event begins in the outer portion of the magnetic tail, perhaps 20 - 30 Re from Earth. This event causes intense magnetic storms at the latitudes of Earth where the lines of magnetic force pass through the ground. The reconnection event causes an intense current to flow directly towards Earth along the line connecting the magnetic tail to Earth's center. This current, traveling at thousands of kilometers a second, gets to within 6 Re of Earth before it is disrupted by collisions with particles already present in this region of space. Space Scientists call this the Current Disruption Region, and it is the disruption of that inflowing current that causes the spectacular auroral intensifications we see from the ground.
At this point, and only 90 seconds after the reconnection event occurred, the diverted current is now forced to flow along lines of magnetic force that connect with the North and South Polar regions. The currents flow along these lines of magnetism as though they were invisible magnetic pipes, and eventually collide with the upper atmosphere. At this point they cause the aurora about 120 seconds after the reconnection event. This model was proposed in 1978-1979 based on data from multiple satellites, but the events were too fast to really confirm the theory.
Scenario II - Instead of triggering a reconnection event, an electrical current may be spontaneously created in the Current Disruption Region. This creates the conditions for an auroral storm about 30 seconds later, and the particles then trigger the magnetic reconnection event about 60 seconds later. The point is that the particle flows from the reconnection event arrive at the Current Disruption Region AFTER auroral storm has already begun. This idea was proposed in 1985.
Neither hypothesis could be tested before, because there have never been satellites that can observe both the flows and the currents at the same time. The THEMIS satellites are designed to be exactly where they need to be to monitor the regions of space where these events take place. By seeing how the particles and magnetic fields change from second to second, THEMIS is expected to help scientists decide which of the two scenarios is more correct. Of course, they may also discover that both scenarios at different times, or that neither scenario is correct! Unexpected discoveries are what make science so exciting!!!
Figure 4: Five THEMIS satellites will observe the two major regions that could trigger an auroral storm to see which sequence of events accounts for the process: Current Disruption then Aurora then Reconnection or Reconnection then Current Disruption then Aurora. (Courtesy: THEMIS)
THEMIS tests two theories - http://ds9.ssl.berkeley.edu/themis/mission_models.html
What is a sub-storm? http://ds9.ssl.berkeley.edu/themis/mission_substorm.html
What is an Aurora? http://image.gsfc.nasa.gov/poetry/educator/Aurora79.html