Solved: The Mystery of Earth’s Theta Aurora

The mystery of the northern lights – aurora – spans time beyond history and to cultures of both the southern and northern hemispheres. The mystery involves the lights, fantastic patterns and mystical changes. Ancient men and women stood huddled under them wondering what it meant. Was it messages from the gods, the spirits of loved ones, warnings or messages to comfort their souls?

Aurora reside literally at the edge of space. While we know the basics and even more, we are still learning. A new published work has just added to our understanding by explaining how one type of aurora – the Theta Aurora – is created from the interaction of the charged particles, electric and magnetic fields surrounding the Earth. Their conclusions required the coordination of simultaneous observations of two missions.

The Theta Auroral Oval as observed by the NASA IMAGE FUV camera on September 15, 2005. (Credit: NASA/SWRI)
The Theta Auroral Oval as observed by the NASA IMAGE FUV camera on September 15, 2005 and anlayzed using Cluster data in the paper by Fear et al. (Credit: NASA/SWRI)

We were not aware of Thetas until the advent of the space age and our peering back at Earth. They cannot be recognized from the ground. The auroras that bystanders see from locales such as Norway or New Zealand are just arcs and subsets of the bigger picture which is the auroral ovals atop the polar regions of the Earth. Ground based all-sky cameras and polar orbiting probes had seen what were deemed “polar cap arcs.” However, it was a spacecraft Dynamics Explorer I (DE-1) that was the first to make global images of the auroral ovals and observed the first “transpolar arcs”, that is, the Theta aurora.

They are named Theta after the Greek letter that they resemble. Thetas are uncommon and do not persist long. Early on in the exploration of this phenomenon, researchers have been aware that they occur when the Sun’s magnetic field, called the Interplanetary Magnetic Field (IMF) turns northward. Most of the time the IMF in the vicinity of the Earth points south. It is a critical aspect of the Sun-Earth interaction. The southerly pointing field is able to dovetail readily with the normal direction of the Earth’s magnetic field. The northward IMF interacting with the Earth’s field is similar to two bar magnets turned head to head, repelling each other. When the IMF flips northward locally, a convolution takes place that will, at times, but not always, produce a Theta aurora.

A group of researchers led by Dr. Robert Fear from the Department of Physics & Astronomy, University of Leicester, through analysis of simultaneous spacecraft observations, has identified how the particles and fields interact to produce Theta aurora. Their study, “Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere” in the Journal Science (December 19, 2014, Vol 346) utilized a combination of data from ESA’s Cluster spacecraft mission and the IMAGE spacecraft of NASA. The specific event in the Earth’s magnetosphere on September 15, 2005 was observed simultaneously by the spacecraft of both missions.

Illustrations of the Cluster II spacecraft in orbit and formation around the Earth and the NASA IMAGE spacecraft vehicle design. The two mission's observations were combined to correlate numerous auroral and magnetospheric events. Cluster II remains in operation as of December 2014 (14 yr lifespan). (Credit: ESA, NASA)
Illustrations of the Cluster II spacecraft in orbit and formation around the Earth and the NASA IMAGE spacecraft vehicle design. The two mission’s observations were combined to correlate numerous auroral and magnetospheric events. Cluster II remains in operation as of December 2014 (14 yr lifespan). (Credit: ESA, NASA)

Due to the complexity of the Sun-Earth relationship involving neutral and charged particles and electric and magnetic fields, space scientists have long attempted to make simultaneous measurements with multiple spacecraft. ISEE-1, 2 and 3 were one early attempt. Another was the Dynamics Explorer 1 & 2 spacecraft. DE-2 was in a low orbit while DE-1 was in an elongated orbit taking it deeper into the magnetosphere. At times, the pair would align on the same magnetic field lines. The field lines are like rails that guide the charged particles from far out in the magneto-tail to all the way down to the upper atmosphere – the ionosphere. Placing two or more spacecraft on the same field lines presented the means of making coordinated observations of the same event. Dr. Fear and colleagues analyzed data when ESA’s Cluster resided in the southern lobe of the magnetotail and NASA’s IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) spacecraft resided above the south polar region of the Earth.

Cluster is a set of four spacecraft, still in operation after 14 years. Together with IMAGE, five craft were observing the event. Fear, et al utilized ESA spacecraft Cluster 1 (of four) and NASA’s IMAGE. On that fateful day, the IMF turned north. As described in Dr. Fear’s paper, on that day, the north and south lobes of the magnetosphere were closed. The magnetic field lines of the lobes were separated from the Solar wind and IMF due to what is called magnetic reconnection. The following diagram shows how complex Earth’s magnetosphere is; with regions such as the bow shock, magnetopause, cusps, magnetotail, particle belts and the lobes.

Illustration of the Earth's magnetosphere showing it complexity. The Theta Aurora are now confidently linked to magnetic reconnection events in the lobes of the magnetotail. (Credit: NASA)
Illustration of the Earth’s magnetosphere showing it complexity. The Theta Aurora are now confidently linked to magnetic reconnection events in the lobes of the magnetotail. (Credit: NASA)

The science paper explains that what was previously observed by only lower altitude spacecraft was captured by Cluster within the magnetotail lobes. The southerly lobe’s plasma – ionized particles – was very energetic. The measurements revealed that the southern lobe of the magnetotail was acting as a bottle and the particles were bouncing between two magnetic mirrors, that is, the lobes were close due to reconnection. The particles were highly energetic.

The presence of what is called a double loss cone signature in the electron energy distribution was a clear indicator that the particles were trapped and oscillating between mirror points. The consequences for the Earth’s ionosphere was that highly energetic particles flooded down the field lines from the lobes and impacted the upper atmosphere transferring their energy and causing the magnificent light show that we know as the Northern Lights (or Southern) in the form of a Theta Auroral Oval. This strong evidence supports the theory that Theta aurora are produced by energized particles from within closed field lines and not by energetic particles directly from the Solar Wind that find a path into the magnetosphere and reach the upper atmosphere of the Earth.

A video of an observed major geomagnetic storm (July 15, 2000) taken by the Far Ultraviolet Imaging System (FUV) on IMAGE. IMAGE operated from 2000 to December 2005 when communications were lost. (Credit: NASA/SWRI)  [click to view the animated gif]
A video of an observed major geomagnetic storm (July 15, 2000, southward IMF) taken by the Far Ultraviolet Imaging System (FUV) on the spacecraft IMAGE. IMAGE operated from 2000 until December 2005 when communications were inexplicably lost. (Credit: NASA/SWRI) [click to view the animated gif]
Without the coordination of the observations and the collective analysis, the Theta aurora phenomenon would continue to be debated. The analysis by Dr. Fear, while not definitive, is strong proof that Theta aurora are generated from particles trapped within closed field lines.

The analysis of the Cluster mission data as well as that of many other missions takes years. Years after observations are made researchers can achieve new understanding through study of arduous details or sometimes by a ha-ha moment. Aurora represent the signature of the interaction of two magnetic fields and two populations of particles – the Sun’s field and energetic particles streaming at millions of miles per hour from its surface reaching the Earth’s magnetic field. The Earth’s field is transformed by the interaction and receives energetic particles that it bottles up and energizes further. Ultimately, the Earth’s magnetic field directs some of these particles to the topside of our atmosphere. For thousands and likely tens of thousands of years, humans have questioned what it all means. Now another piece of the puzzle has been laid down with a good degree of certainty; one that explains the Theta aurora.

Reference:

Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere

Transpolar arc evolution and associated potential patterns

Transpolar aurora: time evolution, associated convection patterns, and a possible cause

Related articles at Universe Today:

Guide to Space –

Earth’s Magnetic Field,

Aurora Borealis

Will Aurora Strike Tonight? Here’s What to Expect

(Scroll down for latest update)

Auroras showed up as forecast last night beginning around nightfall and lasting until about 1 a.m. CDT this morning. Then the action stopped. At peak, the Kp index dinged the bell at “5” (minor geogmagnetic storm) for about 6 hours as the incoming shock from the arrival of the solar blast rattled Earth’s magnetosphere. It wasn’t a particularly bright aurora and had to compete with moonlight, so many of you may not have seen it. You needn’t worry. A much stronger G3 geomagnetic storm from the second Earth-directed coronal mass ejection (CME) remains in the forecast for tonight. 

 

Plot showing the Kp index of magnetic activity high in the Earth's magnetic domain called the magnetosphere. The two red bars show the Kp at '5' last night and early this morning (dotted line represents 0 UT or 7 p.m. CDT). Inset is the current detailed forecast in 3-hour increments. Credit: NOAA
Plot showing the Kp index of magnetic activity high in the Earth’s magnetic domain called the magnetosphere. The two red bars show the Kp at ‘5’ last night and early this morning (dotted line represents 0 UT or 7 p.m. CDT). Inset is the current detailed forecast in Universal Time (Greenwich Time) in 3-hour increments. Credit: NOAA

Activity should begin right at nightfall and peak between 10 p.m. and 1 a.m. Central Daylight Time. The best place to observe the show is from a location well away from city lights with a good view of the northern sky. Auroras are notoriously fickle, but if the NOAA space forecasting crew is on the money, flickering lights should be visible as far south as Illinois and Kansas. The storm also has the potential to heat and expand the outer limits of Earth’s atmosphere enough to cause additional drag on low-Earth-orbiting (LEO) satellites. High-frequency radio transmissions like shortwave radio may be reduced to static particularly on paths crossing through the polar regions.

Earth’s magnetic bubble, generated by motions within its iron-nickel core and shaped by the solar wind, is called the magnetosphere. It extends some 40,000 miles forward of the planet and more than 3.9 million miles in the tailward direction. Credit: NASA
Earth’s magnetic bubble, generated by motions within its iron-nickel core and shaped by the solar wind, is called the magnetosphere. It extends some 40,000 miles forward of the planet and more than 3.9 million miles in the tailward direction. Most of the time it sheds particle blasts from the sun called coronal mass ejections, but occasionally one makes it past our defenses and we get an auroral treat. Credit: NASA

If you study the inset box in the illustration above, you can see that from 21-00UT (4 -7 p.m. Central time) the index jumps quickly form “3” to “6” as the blast from that second, stronger X-class flare (September 10) slams into our magnetosphere. Assuming the magnetic field it carries points southward, it should link into our planet’s northward-pointing field and wreak beautiful havoc. A G2 storm continues through 10 p.m. and then elevates to Kp 7 or G3 storm between 10 p.m. and 1 a.m. before subsiding slightly in the wee hours before dawn. The Kp index measures how disturbed Earth’s magnetic field is on a 9-point scale and is compiled every 3 hours by a network of magnetic observatories on the planet.

A lovely rayed arc reflected in Caribou Lake north of Duluth, Minn. on September 11, 2014. Credit: Guy Sander
A lovely rayed arc reflected in Caribou Lake north of Duluth, Minn. on September 11, 2014. Tonight the moon rises around 9:30 p.m. The lower in the sky it is, the brighter the aurora will appear. Hopefully tonight’s lights will outdo what the moon can dish out. Credit: Guy Sander

All the numbers are lined up. Now, will the weather and solar wind cooperate?  Stop back this evening as I’ll be updating with news as the storm happens. For tips on taking pictures of the aurora, please see this related story  “How  to Take Great Pictures of the Northern Lights”.

The auroral oval around 2:30 p.m. CDT this afternoon September 12 shows a southward expansion into the Scandinavian countries and Russia and Iceland. Where the sky is dark, auroras are typically seen anywhere under or along the edge of the oval. Click for current map. Credit: NOAA
The auroral oval at 11:15 p.m. CDT tonight September 12 shows a temporary pullback into northern Canada. Where the sky is dark, auroras are typically seen anywhere under or along the edge of the oval. Click for current map. Credit: NOAA

* UPDATE 8:15 a.m. Saturday September 13: Well, well, well. Yes, the effects of the solar blast did arrive and we did experience a G3 storm, only the best part happened before nightfall had settled over the U.S. and southern Canada. The peak was also fairly brief. All those arriving protons and electrons connected for a time with Earth’s magnetic field but then disconnected, leaving us with a weak storm for much of the rest of the night. More activity is expected tonight, but the forecast calls for a lesser G1 level geomagnetic storm.

* UPDATE 11 p.m. CDT: After a big surge late this afternoon and early evening, activity has temporarily dropped off. The ACE plot has “gone north” (see below). Though we’re in a lull, the latest NOAA forecast still calls for strong storms overnight.

Definite aurora seen through breaks in the clouds low in the northern sky here in Duluth, Minn. After a big surge late this afternoon and during early evening, activity's temporarily dropped off. The ACE plot has "gone north".
Definite aurora seen through breaks in the clouds low in the northern sky here in Duluth, Minn. After a big surge late this afternoon and during early evening, activity’s temporarily dropped off. The ACE plot has “gone north”.

* UPDATE 9 p.m. CDT: Aurora a bright greenish glow low in the northern sky from Duluth, Minn.

* UPDATE 7:45 p.m. CDT September 12: Wow! Kp=7 (G3 storm) at the moment. Auroras should be visible now over the far eastern seaboard of Canada including New Brunswick and the Gaspe Peninsula. I suspect that skywatchers in Maine and upstate New York should be seeing something as well. Still dusk here in the Midwest.

 

Auroras Dance Over Northern U.S. Last Night, May Return Tonight

A burst of energetic particles from the Sun called a coronal mass ejection peppered Earth’s magnetic field yesterday afternoon sparking a modest but beautiful all-night display of the aurora borealis. Another light show may be in the offing tonight for skywatchers living in the northern U.S.,  Canada and northern Europe.

Around 1 a.m. the arc became more active, sending up occasional rays that lasted from about a minute before fading away and being replaced by another. Credit: Bob King
Around 1 a.m. the arc became more active, sending up occasional rays that lasted from about a minute before fading away and being replaced by another. Credit: Bob King

Pale green fingers of light splayed across the northern sky at twilight’s end came as a surprise. NOAA space weather forecasters had predicted little activity. These soon faded but a thick, fuzzy arc persisted throughout the night. It arched from horizon to horizon across the northern sky like a pallid, monochromatic rainbow. Such arcs are common. Often the aurora never gets past this stage and simmers quietly or even fades away during the night.

Not this one. Around local midnight (1 a.m. CDT) here in Duluth, Minn. small bright spots and a series of tall, faint rays punctuated the arc and over the span of a half-hour completely reshaped it into loopy rayed arcs resembling a crown.

I wasn't alone when the northern lights peaked about 1:20-2 a.m. At upper left you'll see the trails of a couple of fireflies. Credit: Bob King
I wasn’t alone when the northern lights peaked about 1:20-2 a.m. At upper left you’ll see the trails of a couple of fireflies. Credit: Bob King

To the eye, the brightest parts of the aurora appeared green, but the taffy-stretched rays were colorless. The camera’s sensitivity coupled with a 30-second time exposure revealed striking pinks and hints of blue. Both pink and green colors are caused by the emission of light from oxygen atoms.

An especially beautiful ray sticks up above the arc. Shorter exposures coupled with shorter shutter speeds are the best way to capture fine details of a northern lights display. Credit: Bob King
An especially beautiful ray sticks up above the arc. Shorter exposures coupled with shorter shutter speeds are the best way to capture fine details of a northern lights display. Credit: Bob King

Bombarded by high-speed solar wind electrons and protons, they get jazzed into higher energy states. When the atoms return to rest, each spits out a photon of green or red light. All those tiny flashes add up. Multiplied by the billions of atoms that exist even in the rarefied air at the aurora’s typical 60-150 mile (100-250 km) altitude and you get heavenly eye candy.

When we see an auroral arc - and associated rays - we really seeing a small section of the much larger, permanent aurora called the auroral oval. The northern oval is centered over the geomagnetic north pole located in northern Canada. Credit: NASA
When we see an auroral arc – and associated rays – we really seeing a small section of the much larger, permanent aurora called the auroral oval. The northern oval is centered over the geomagnetic north pole located in northern Canada. Credit: NASA

I started watching the northern lights at 11 from home then took a drive to darker skies. Even at dawn’s 3 a.m. start, the green arc held its own shot through with rays that occasionally towered halfway up the northern sky. While this display wasn’t a grand spectacle like some auroras, it possessed a certain majesty the same way a long, slow movement concludes a great symphony.

Chances for more of the same continues through tonight and possibly into tomorrow, so keep a watch on the northern sky before you hit the hay tonight. If you see something green and glowing it you might be in for a treat.

In another installment, I’ll share tips on how best to see the northern lights and share several excellent tools you can use for predicting when they might occur.