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“Space tsunamis” investigated

April 12, 2007
Courtesy ESA
and World Science staff

New find­ings are shed­ding light on “space tsu­na­mis” that help cre­ate dra­mat­ic light shows in the sky when they dis­rupt the nor­mal­ly calm, beau­ti­ful glow of the au­ro­ra, or north­ern lights.

Click to enlarge

An au­ro­ra over Alas­ka, a di­git­ally en­hanced pho­to vot­ed Im­age of the Year at Wi­ki­pe­dia.org. (Cour­te­sy Josh­ua Strang/US­AF; Wi­ki­pe­dia)
Gen­er­al­ly seen in high-latitude re­gions such as Scan­di­na­vi­a or Can­a­da, au­ro­rae are co­lour­ful cur­tains of light in the sky. Caused by in­ter­ac­tions be­tween the Earth’s mag­net­ic field and high-en­er­gy par­t­i­cles stream­ing from the Sun, they take a va­ri­e­ty of shapes.

In the ear­ly eve­ning, the au­ro­ra of­ten forms a mo­tion­less green arc sweep­ing from east to west. This state­ly dis­play can be­come a col­or­ful dance as the re­sult of dis­tur­bances in the mag­net­ic field, called sub­storms or “space tsu­na­mis.”

These are “a hot top­ic of re­search,” said the Eu­ro­pe­an Space Agen­cy’s Philippe Es­cou­bet, a sci­ent­ist stu­dy­ing the sub­storms. Sci­en­tists seek to un­der­stand them in part be­cause they can af­fect dai­ly life by dis­rupting sat­el­lite sig­nals. The events typ­i­cal­ly last one to two hours and spread over al­ti­tudes from about 100 to 150,000 kilo­me­tres (60 to 100,000 miles).

Try­ing to un­der­stand such com­plex pro­cesses with one space­craft is like try­ing to pre­dict a tsu­na­mi’s be­hav­iour with a sin­gle bu­oy, sci­ent­ists say. Thus, the agen­cy is si­mul­ta­ne­ous­ly us­ing four sat­el­lites in a proj­ect called Clus­ter.

Us­ing the re­sulting da­ta, sci­ent­ists said they con­firmed that at least some sub­storms act in ac­cord­ance with a the­o­ry called the cur­rent-dis­ruption mod­el. This holds that the sub­storms re­sult when dis­tur­bances in the mag­net­ic field dis­rupt a flow of elec­tri­cal­ly charged par­t­i­cles. 

A sub­storm builds up in stages. In a lat­er stage, au­ro­ral dis­tur­bances move to­wards the poles, sug­gest­ing the en­er­gy driv­ing the events moves away from Earth, re­search­ers say.

Previous sat­el­lite studies found that, dur­ing this late stage, charged par­t­i­cles switch di­rec­tion in a part of the Earth’s mag­net­ic field that’s pushed in the di­rec­tion of the so­lar wind, the stream of par­t­i­cles from the Sun. In re­cent years it was gen­er­al­ly thought that this re­gion of flow re­ver­sal al­so wit­nesses a pro­cess called mag­net­ic re­con­nec­tion, in which the mag­net­ic field’s en­er­gy is con­vert­ed in­to par­t­i­cles. This re­sults in gas flows that hurt­le to­wards Earth.

Clus­ter sci­ent­ists ex­am­ined re­sults for sat­el­lites cross­ing this re­gion of the mag­net­ic field. They found that the flow re­ver­sal re­sults from both mag­net­ic en­er­gy be­ing con­vert­ed in­to par­t­i­cles, and the op­po­site. This is con­sist­ent with dis­ruption the lo­cal elec­tric cur­rent by tur­bu­lence in a sea of elec­tri­cal­ly charged par­t­i­cles pop­u­lat­ing the mag­net­ic field, they said.

It’s un­clear how wide­ly ap­pli­ca­ble these find­ings are; fu­ture stud­ies will look in­to that, said Clus­ter team mem­ber Tony Lui of the Ap­plied Phys­ics Lab­o­r­a­to­ry at the John Hop­kins Uni­ver­si­ty in Bal­ti­more, Md. The find­ings ap­peared in last Aug­ust’s is­sue of the re­search jour­nal An­na­les Ge­o­phy­si­cae.

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New findings are shedding light on “space tsunamis” that help create dramatic light shows in the sky when they disrupt the normally calm, beautiful glow of the aurora, or northern lights. Generally seen in high-latitude regions such as Scandinavia or Canada, aurorae are colourful curtains of light in the sky. Caused by interactions between the Earth’s magnetic field and high-energy particles streaming from the Sun, they take a variety of shapes. In the early evening, the aurora often forms a motionless green arc sweeping from east to west. This stately display can become a colorful dance as the result of disturbances in the magnetic field, called substorms or “space tsunamis.” These are “a hot topic of research,” said the European Space Agency’s Philippe Escoubet, a scientist studying the substorms. Scientists seek to understand them in part because they can affect daily life, by disrupting satellite signals. The events typically last one to two hours and spread over altitudes from about 100 to 150,000 kilometres (60 to 100,000 miles). Trying to understand such complex processes with one spacecraft is like trying to predict a tsunami’s behaviour with a single buoy, scientists say. Thus, the agency is simultaneously using four satellites in a project called Cluster. Using the resulting data, scientists said they confirmed that at least some substorms act in accordance with a theory called the current-disruption model. This holds that the substorms result when disturbances in the magnetic field disrupt a flow of electrically charged particles. A substorm builds up in stages. In a later stage, auroral disturbances move towards the poles, suggesting the energy driving the events moves away from Earth, researchers say. Older satellite observations found that, during this late stage, charged particles switch direction in a part of the Earth’s magnetic field that’s pushed in the direction of the solar wind, the stream of particles from the Sun. In recent years it was generally thought that this region of flow reversal also witnesses a process called magnetic reconnection, in which the magnetic field’s energy is converted into particles. This results in gas flows that hurtle towards Earth. Cluster scientists examined results for satellites crossing this region of the magnetic field. They found that the flow reversal results from both magnetic energy being converted into particles, and the opposite. This is consistent with disruption the local electric current by turbulence in a sea of electrically charged particles populating the magnetic field, they said. It’s unclear how widely applicable these findings are; future studies will look into that, said Cluster team member Tony Lui of the Applied Physics Labora tory at the John Hopkins University in Baltimore, Md.