'tis the Season for Auroras
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Spring and Fall are good times to spot Northern Lights, and scientists
would like to know why.
October 26, 2001: Last Sunday, Oct. 21st, a cloud of magnetized gas from the Sun (a "coronal mass ejection") swept past Earth and rocked our planet's magnetic field. Northern sky watchers were delighted as red and green lights rippled across the sky. It was the aurora borealis -- breaking out for the third time this month.
"The auroras were probably the most spectacular I have ever witnessed," says Ryan Kramer, an observer in North Dakota. "It was like being under a giant canopy. Northern Lights filled the sky -- including directly above and even within 30 degrees of the southern horizon."
Above: Sky watcher Duane Clausen photographed these colorful Northern Lights over Menominee, Michigan (USA), on Oct. 22, 2001. [more]
"I was amazed that the auroras were so bright to the south of me," agreed Todd Carlson, who enjoyed the spectacle from his home in Ontario, Canada. Indeed, before the storm was done, observers as far south as the Carolinas in the United States had caught a rare glimpse of Northern Lights.
It was a good time to be outside.
Indeed, it may have been the best time: Autumn nights are long and dark, but not yet wintry-cold -- a good combination for sky watching. But there's more to it than that, say researchers. Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoxes -- that is, early Autumn and Spring.
It's a bit puzzling. Solar activity does not depend on Earth's seasons. Why should geomagnetic storms?
"We've known about this seasonal effect for more than 100 years," says Dennis Gallagher, a space physicist at the NASA Marshall Space Flight Center. "Some aspects of it are understood, but not all."
Above: Still frames from a digital movie showing how coronal mass ejections compress Earth's magnetosphere and trigger auroras. Click to view the full 750 kb Quicktime animation created by Digital Radiance, Inc.
Geomagnetic storms erupt when solar wind gusts or coronal mass ejections (CMEs) hit Earth's magnetosphere -- a magnetic bubble around our planet that protects us from the relentless solar wind. The magnetosphere is filled with electrons and protons. Normally these particles are trapped by lines of force (so-called "magnetic bottles") that prevent them from escaping to space or descending to the planet below.
"When a CME hits the magnetosphere," explains Tony Lui, "the impact knocks loose some of those trapped particles. They rain down on Earth's atmosphere and cause the air to glow where they hit." Lui is a space physicist at the Johns Hopkins University Applied Physics Lab.
"Precipitating particles mostly follow magnetic field lines that lead to Earth's poles," he added. "The auroral ovals (circular regions of auroral light around the magnetic poles) expand during magnetic storms." Sometimes they grow so large that people at middle latitudes -- like North Carolinans -- can see the light.
Left: Geomagnetic activity from 1875 to 1927. Based on a histogram that appears in "Semiannual Variation of Geomagnetic Activity" by C.T. Russell and R.L. McPherron, J. Geophys. Res., 78(1), 92, 1973.
Such widespread storms are usually nurtured by what scientists call "Bz" (pronounced "Bee sub Zee") -- in other words, the component of the interplanetary magnetic field (IMF) that lies along Earth's magnetic axis. At the magnetopause, the part of our planet's magnetosphere that fends off the solar wind, Earth's magnetic field points north. If the IMF tilts south (i.e., Bz becomes large and negative) it can partially cancel Earth's magnetic field at the point of contact.
"At such times the two fields (Earth's and the IMF) link up," says Christopher Russell, a Professor of Geophysics and Space Physics at UCLA. "You can then follow a magnetic field line from Earth directly into the solar wind." South-pointing Bz's open a door through which energy from the solar wind can reach Earth's inner magnetosphere.
In the early 1970's Russell and colleague R. L. McPherron recognized a connection between Bz and Earth's changing seasons: The average size of Bz is greatest each year in early Spring and Autumn.
It's a result of geometry, explains Russell. The interplanetary magnetic field comes from the Sun; it's carried outward from our star by the solar wind. Because the Sun rotates (once every 27 days) the IMF has a spiral shape -- named the "Parker spiral" after the scientist who first described it. Earth's magnetic dipole axis is most closely aligned with the Parker spiral in April and October. As a result, southward (and northward) excursions of Bz are greatest then.
Right: Steve Suess (NASA/MSFC) prepared this figure, which shows the Sun's spiraling magnetic field from a vantage point ~100 AU from the Sun.
"We've learned in the last 28 years that the north-south component of the IMF controls the energy flow of the solar wind into our magnetosphere," says Russell. Northward fields have little effect, he added, but southward Bz's can set the stage for substantial geomagnetic activity.
This week was a good example. The widespread auroral storm of Oct. 21st and 22nd was preceded by a 24-hour period of mostly south-pointing Bz. The IMF continued to tilt south after a coronal mass ejection struck Earth's magnetosphere on the 21st; and the ensuing display of Northern Lights was one of the most memorable of the current solar cycle.
The influence of Bz on geomagnetic activity is undeniable, but researchers agree it's not the only influence. For instance: The Sun's rotation axis is tilted 7 degrees with respect to the plane of Earth's orbit. Because the solar wind blows more rapidly from the Sun's poles than from its equator, the average speed of particles buffeting Earth's magnetosphere waxes and wanes every six months. The solar wind speed is greatest -- by about 50 km/s, on average -- around Sept. 5th and March 5th when Earth lies at its highest heliographic latitude.
Left: On October 11, 2001, during another autumnal geomagnetic storm, Jody Majko captured this photo of bright Northern Lights above the city lights of Winnipeg, Manitoba, Canada. [more]
In a recent Geophysical Research Letter (28, 2353-2356, June15) Lyatsky et al argue that neither Bz nor the solar wind can fully explain the seasonal behavior of geomagnetic storms. According to their study, those factors together contribute only about one-third of the observed semiannual variation.
What remains is a puzzle that space scientists are still trying to solve. "This is an area of active research," notes Lui. "We don't have all the answers yet, because it's a complicated problem."
But not too complicated to enjoy: dark nights, bright stars, an occasional meteor -- and the promise of Northern Lights. Perhaps scientists haven't figured out why auroras prefer autumn, but it's easy to understand why sky watchers do....
SpaceWeather.com - find out when the next magnetic storm is about to erupt.
Selected aurora galleries from SpaceWeather.com:
- March 30 - April 1, 2001: Sky watchers spotted Northern Lights as far south as Mexico and Texas during this springtime geomagnetic storm.
- April 17 - 18, 2001: This display was triggered by one of the most powerful solar explosions ever recorded.
- Sept. 29 - Oct 3, 2001: A pair of coronal mass ejections struck Earth's magnetosphere, igniting a 4-day geomagnetic storm.
- Oct. 21 - 23, 2001: The Northern Lights dipped as far south as the Carolinas.
Introducing the Aurora - Earth's Great Light Show - a nice overview of auroras
Semiannual Variation of Geomagnetic Activity - C.T. RUSSELL AND R. L. MCPHERRON, J. Geophys. Res., 78(1), 92, 1973.
Magnetic activity indices - defined: the many ways researchers measure geomagnetic activity
What is the Interplanetary Magnetic Field -- or "IMF"? - From spaceweather.com
The Spiral of the IMF - includes an eye-catching lawn sprinkler animation that illustrates the Sun's spiraling magnetic field.
The Solar Wind -- learn more about the wind from the Sun and how it blows faster at high heliographic latitudes.
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