Auroral arcs
Science & Exploration

Cluster – new insights into the electric circuits of polar lights

09/02/2007 1157 views 1 likes
ESA / Science & Exploration / Space Science / Cluster

Giant electrical circuits power the magical open-air light show of the auroras, forming arcs in high-latitude regions like Scandinavia. New results obtained thanks to ESA's Cluster satellites provide a new insight into the source of the difference between the two types of electrical circuits currently known to be associated to the auroral arcs.

The deep mechanisms that rule the creation of the beautiful auroras, or polar lights, have been the subject of studies that are keeping solar and plasma scientists busy since years. While early rockets and ground-observations have already provided a few important clues for the understanding of these phenomena, the real break-throughs in our knowledge have started with dedicated auroral satellites, such as S3-3, Dynamics Explorer, Viking, Freja and FAST, and have now come to full fruition with ESA's multi-point mission Cluster.

Formation of auroras (artist's view)
Formation of auroras (artist's view)

The basic process generating auroras is similar to what happens in an old TV tube. In the TV tube, accelerated electrons hit the screen and make its phosphore glow; electrons in the atmosphere get accelerated in an 'acceleration region' situated between about 5000 and 8000 kilometres altitude, and rush down to the Earth's ionosphere – a region of the upper atmosphere. Here, they crash into ionospheric atoms and molecules, transfer to them some of their energy and so cause them to glow, creating aurorae.

Spiralling-down electrons create auroras
Spiralling-down electrons create auroras

It is today well established that almost-static electric fields, parallel to the Earth's magnetic fields, play an important role in the acceleration of the electrons that cause the auroras to shine. The auroral electric circuits in the near-Earth space involve almost-static 'electric potential' structures through which electrons and ions are accelerated in opposite directions - towards and away from Earth's atmosphere -at high latitudes.

It had been observed that these electric potential structures are mainly of two types - symmetric (U-shaped) or asymmetric (S-shaped). In 2004, Prof. Göran Marklund from the Alfvén Laboratory, at the Royal Institute of Technology, Stockholm (Sweden), noted that the U-shaped and the S-shaped structures typically occurred at the boundaries between magnetospheric regions with different properties.

The former type (U-shaped) was found at a plasma boundary between the so-called ‘central plasma sheet’, situated in the magnetotail at equatorial latitudes, and the ‘plasma sheet boundary layer’, an adjacent area located at higher latitudes. The latter type (S-shaped) was found at the boundary between the ‘plasma sheet boundary layer’ and the polar cap, further up in latitude.

Auroral ovals as seen from space
Auroral ovals as seen from space

Marklund was then in the condition to propose a model to explain this difference. The model suggested that both the asymmetric and symmetric shape of the potential structures, observed at the different plasma boundaries, depended on the specific conditions of the plasma (such as differences in plasma density) in the two regions surrounding the boundary. According to the 2001 observations, he concluded that the plasma conditions at the lower-latitude boundary (where U-shaped structures were observed) are in general more symmetric, while the ones at the polar cap boundary (where the S-shaped structures were observed) are more asymmetric.

However, new Cluster measurements did not seem to be consistent with this picture. On 1 May 2003, one of the Cluster spacecraft crossed the auroral arc at high altitude in the Earth’s magnetotail. As expected, it detected the presence of a U-shaped, symmetric potential structure when crossing the boundary between the 'central plasma sheet' and the 'plasma sheet boundary layer'. Only 16 minutes later a second Cluster spacecraft, moving roughly along the same orbit and crossing the same boundary, detected an asymmetric, S-shaped potential structure, 'typical' of the polar cap boundary and therefore unexpected in that region.

However, within the 16-minute time frame between the crossing of the two spacecraft, the plasma density and the associated currents and fluxes of particles decreased significantly in the plasma sheet boundary layer. In this way this boundary ended up in resmbling the asymmetric conditions typical of the polar cap boundary.

So, the scientists interpreted that the 'reconfiguration' from a U-shaped to a S-shaped potential structure, and of the associated electric circuits that sustain the auroral arcs, reveal the change in the plasma conditions on the two sides of the boundary.

The results represent a major step forward in understanding the auroral electrical circuits, but important questions still remain open, such as: how do the process that accelerate the electrons to form auroras get triggered and maintained? Cluster measurements in the 'acceleration' area to be performed in 2008 and 2009 should help us to find out.

Note for editors

The results, by Marklund et al., were published in the 13 January 2007 issue of the Journal of Geophysical Research.

For more information

Göran Marklund, Royal Institute of Technology, Stockholm, Sweden
Email: goran.marklund @ ee.kth.se

Philippe Escoubet, ESA Cluster Project Scientist
Email: philippe.escoubet @ esa.int

Related Links

Science & Exploration

Double Star overview

01/01/1970 3569 views
Open item
Science & Exploration

Cluster overview

01/01/1970 11055 views
Open item
Double Star, an artist's impression
Science & Exploration

Double Star mission extended

16/11/2006 434 views 2 likes
Open item
Double Star Programme (DSP)
Science & Exploration

Double Star mission extension approved by ESA

12/05/2005 406 views 0 likes
Open item
Cluster and Double Star orbits on 8 May 2004
Science & Exploration

Details of solar particles penetrating the Earth’s environm…

03/10/2006 2108 views 2 likes
Open item
Earth's magnetosphere- an artist's impression
Science & Exploration

Cluster makes an effervescent discovery

20/06/2006 883 views 1 likes
Open item
Artist's impression of the electrical and magnetic energy  event  of August 2004
Science & Exploration

Cluster and Double Star witness a new facet of Earth’s magn…

30/03/2006 894 views 0 likes
Open item
Artist's impression of cracks on a neutron star's surface
Science & Exploration

Cluster and Double Star see star crack during massive ‘star…

21/09/2005 1063 views 1 likes
Open item
Double Star  'Tan Ce 2' satellite successfully launched
Science & Exploration

Second Double Star satellite successfully launched

26/07/2004 484 views 0 likes
Open item
Sun-Earth day events
Science & Exploration

How the Sun affects us on Earth

26/06/2003 30250 views 203 likes
Open item
Cluster
Science & Exploration

Surfing and diving in the Earth's magnetosphere, Cluster ce…

17/07/2001 1322 views 0 likes
Open item
The magnetosphere - a natural protective bubble
Science & Exploration

Cluster's new view of near-Earth space

16/02/2001 947 views 0 likes
Open item
On 4 November captured by SOHO/LASCO
Science & Exploration

Solar storm blasts Cluster

13/11/2001 491 views 0 likes
Open item
Large waves rock regions of the magnetosphere
Science & Exploration

ESA's Cluster sees 'squashed' magnetosphere

31/10/2003 548 views 0 likes
Open item