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Solar activity slowly restarting after long lull: scientists

Aug. 14, 2010
Courtesy of Boston University 
and World Science staff

Sci­en­tists have an­nounced they have “sub-visual” ev­i­dence that a new cy­cle of so­lar wind ac­ti­vity around Earth is start­ing.

Re­cent ap­pear­ances of bright sky dis­plays known as au­ro­rae, near the North Pole, were ac­com­pa­nied by much dim­mer glows well south of that, say re­search­ers at Bos­ton Uni­vers­ity’s Cen­ter for Space Phys­ics.

All-sky view of the up­per at­mos­phere as pho­tographed in the red light of ox­y­gen from the Mt. John Ob­serv­a­to­ry in New Zea­land. This fish-eye lens im­age dis­plays emis­sions from a height of 400 km above the Earth’s sur­face (fea­tures in up­per left are build­ings ob­struct­ing view to north­west). Just be­low the low­er coast of South Is­land, the faint emis­sion ex­tend­ing west-to-east is called a Sta­ble Au­ro­ral Red (SAR) arc, a man­i­fes­ta­tion of ox­y­gen atoms heat­ed by hot elec­trons in the ion­o­sphere. The bright­ness lev­els (a­bout 300 Ray­leigh units) are ten to twen­ty times faint­er than can be seen by the na­ked eye. Fur­ther to the south, the ar­ea col­ored white is a brighter form of red emis­sion called Dif­fuse Au­ro­ra, pro­duced by in in­flux of elec­trons from the mag­ne­to­sphere; it is still in­vis­i­ble to the un­aid­ed eye, but on­ly by fac­tors of two to three. Strong vis­i­ble au­ro­ra, the fa­mil­iar "cur­tains of red and green emis­sion" would be still fur­ther to­ward the South Pole, be­yond the field of view shown. This Bos­ton Uni­ver­si­ty im­age of sub-visual au­ro­ra at mid-latitudes is the first un­am­big­u­ous case of a SAR arc in the South­ern Hem­i­sphere dur­ing the new cy­cle of so­lar ac­tiv­i­ty. (Cred­it: Bos­ton U. Cen­ter for Space Phys­ics)


“It’s ex­cit­ing to see the re­turn of au­ro­ra to mid-latitudes,” the cen­ter’s Steve Smith said, re­fer­ring to the cy­clic oc­cur­rence of emis­sions in the Earth’s at­mos­phere that have in­trigued peo­ple since an­cient times.

What has fas­ci­nat­ed space sci­en­tists in re­cent years is the de­layed on­set of such ef­fects. Typ­ic­ally, the Sun has an ac­ti­vity cy­cle of about 11 years, with flares and ejec­tions of elec­tric­ally charged par­t­i­cles called the so­lar wind. These cause changes in the Earth’s mag­net­ic field that pro­duce, as a side prod­uct, lu­mi­nous emis­sions in the at­mos­phere.

Such ef­fects are sub­dued dur­ing so-called so­lar min­i­mum years, such as in 1996-1997, and very prom­i­nent in so­lar max­i­mum years, as in 2001-2002. Ac­cord­ing to this cy­cle, a new wave of such ac­ti­vity had been ex­pected to start by last year, but the Sun re­mained qui­et. 

Now there are fi­nally signs of the cy­cle re-appearing, the Bos­ton group says. 

They used an all-sky cam­era at the Mt. John Ob­serv­a­to­ry in Lake Te­ka­po, New Zea­land, “essen­tially a fish-eye lens that is used to view the full sky,” said Jef­frey Baum­gard­ner, al­so a re­search­er at the cen­ter, who de­signed and built the in­stru­ment. “The emis­sions we study come from re­gions rang­ing from 200-400 km [125-250 miles] above the sur­face. These gas­es are caused to glow by en­er­gy in­put from above, en­er­gy that flows down­ward along the Earth’s mag­net­ic field lines.”

The cur­tains of glow­ing gasses vis­i­ble to the un­aided eye are long been called au­ro­ra bo­realis when near north­ern po­lar re­gions, and au­ro­ra aus­tralis in south­ern po­lar re­gions. The newly de­tected emis­sions in­stead are be­low na­ked eye de­tec­tion lim­its, and were de­tected in the skies just south of New Zea­land.

The­re, a faint arc ex­tend­ing from East to West was de­tected in the red glow of ox­y­gen atoms. This emis­sion is due to col­li­sions be­tween hot elec­trons, or charged par­t­i­cles, and ox­y­gen atoms in a part of the at­mos­phere known as the ion­o­sphere, the re­search­ers said. Such fea­tures, called Sta­ble Au­ro­ral Red or SAR arcs, are an ac­tive top­ic of re­search.

This “is per­haps the first-ever case of im­ag­ing an un­am­big­u­ous SAR arc in the south­ern hemi­sphere,” said Bos­ton Uni­vers­ity as­tron­o­mer Mi­chael Men­dil­lo.

Still fur­ther south of that arc the re­search­ers de­tected a far more dif­fuse, steady at­mos­pher­ic emis­sion, al­so how­ev­er caused by an in­flux of elec­trons hit­ting ox­y­gen atoms. This emis­sion is thought to have a slightly dif­fer­ent un­der­ly­ing cause from that of SAR arcs, which oc­cur when elec­trons trapped in so-called Van Al­len Radia­t­ion Belts in plan­et’s mag­net­ic field de­pos­it heat in­to the ion­o­sphere. This en­er­gy in­put is con­fined to a thin band about 60 miles (100 km) wide from north to south, though it can ex­tend all the way around the globe east to west.

“We fully ex­pect that a si­m­i­lar SAR arc oc­curred in the north­ern hem­i­sphere, but it was cloudy at our ob­serv­a­to­ry in Bos­ton that night, and so one was not seen,” Smith ex­plained. “We hope in the years ahead to have many cases of SAR arcs in our da­ta from both hem­i­spheres, and then ex­am­ine the full glob­al dis­tri­bu­tion of such ef­fects,” he added. “Look­ing to see if the en­er­gy in­put is sim­ul­ta­ne­ous­ly the same or dif­fer­ent in each hem­i­sphere is a fore­front top­ic in the study of so­lar-induced storms in our up­per at­mos­phere.”


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Scientists have announced that they have “sub-visual” evidence of the onset of a new cycle of solar wind activity around Earth. Recent appearances of bright sky displays known as aurorae, near the North Pole, were accompanied by much dimmer glows well south of that, say researchers at Boston University’s Center for Space Physics. “It’s exciting to see the return of aurora to mid-latitudes,” the center’s Steve Smith said, referring to the cyclic occurrence of emissions in the Earth’s atmosphere that have intrigued people since ancient times. What has fascinated space scientists in recent years is the delayed onset of such effects. Typically, the Sun has an activity cycle of about 11 years, with flares and ejections of electrically charged particles called the solar wind. These cause changes in the Earth’s magnetic field that produce, as a side product, luminous emissions in the atmosphere. Such effects are subdued during so-called solar minimum years, such as in 1996-1997, and very prominent in solar maximum years, as in 2001-2002. According to this cycle, a new wave of such activity had been expected to have started by last year, but the Sun remained quiet. Now there are finally signs of the cycle re-appearing, the Boston group said. They used an all-sky camera at the Mt. John Observatory in Lake Tekapo, New Zealand, “essentially a fish-eye lens that is used to view the full sky,” said Jeffrey Baumgardner, also a researcher at the center, who designed and built the instrument. “The emissions we study come from regions ranging from 200-400 km [125-250 miles] above the surface. These gases are caused to glow by energy input from above, energy that flows downward along the Earth’s magnetic field lines.” The curtains of glowing gasses visible to the unaided eye are long been called aurora borealis when near northern polar regions, and aurora australis in southern polar regions. The newly detected emissions instead are below naked eye detection limits, and were detected in the skies just south of New Zealand. Here, a faint arc extending from East to West was detected in the red glow of oxygen atoms. This emission is due to collisions between hot electrons, or charged particles, and oxygen atoms in a part of the atmosphere known as the ionosphere, the researchers said. Such features, called Stable Auroral Red or SAR arcs, are an active topic of research. This “is perhaps the first-ever case of imaging an unambiguous SAR arc in the southern hemisphere,” said Boston University astronomer Michael Mendillo. Still further south of that arc the researchers detected a far more diffuse, steady atmospheric emission, also however caused by an influx of electrons hitting oxygen atoms. This emission has a slightly different underlying cause from the SAR arcs, which occur when electrons trapped in so-called Van Allen Radiation Belts in planet’s magnetic field deposit heat into the ionosphere. This energy input is confined to a thin band about 60 miles (100 km) wide north to south, though it can extend all the way around the globe east to west. “We fully expect that a similar SAR arc occurred in the northern hemisphere, but it was cloudy at our observatory in Boston that night, and so one was not seen,” Smith explained. “We hope in the years ahead to have many cases of SAR arcs in our data from both hemispheres, and then examine the full global distribution of such effects,” he added. “Looking to see if the energy input is simultaneously the same or different in each hemisphere is a forefront topic in the study of solar-induced storms in our upper atmosphere.”