Glowing success for Cluster quartet
23 October 2001People living at high latitudes may have to endure long, icy winters, but Nature has stepped in to offer some compensation in the form of the auroras - the beautiful, shimmering curtains of red and green that illuminate the polar skies. Now the four Cluster spacecraft have begun to shed new light on the processes that make this dazzling display possible.
Every day we use a variety of electrical appliances - TV, CD player, washing machine .... the list is endless. What makes these modern marvels work is a vast migration of electrons (negatively charged particles), flowing through wires and cables.
Planet Earth is also associated with various natural electrical systems. The best known example of atmospheric electricity is the lightning flash, a sudden flow of electrons between a cloud and the ground.
At higher altitudes, between 60 km and 600 km above the Earth, is the ionosphere, a sparse layer of atmosphere filled with billions of electrons and ions (positively charged atomic particles). This is the home of the aurorae - the Northern and Southern Lights.
Higher still is the magnetosphere, a huge invisible bubble populated by free electrons and ions that sweep around the planet, guided by its magnetic field. Some of the particles inside this magnetic bubble originated in the Sun and sneaked in through the Earth's outer defences. Others began life in the ionosphere and rose to a higher existence.
It is these electrons and their effects on their surroundings which are detected by identical instruments on each member of the Cluster quartet as the particles soar up the magnetic field lines from the ionosphere into the magnetosphere.
Direct measurements of the number of upward moving, energetic electrons are made by the PEACE instrument. At the same time, the EFW experiment detects the electric field which accelerates the electrons, while the FGM magnetometer measures the electric current.
By analysing the data from these instruments, scientists are now able to study in detail what happens during a typical aurora.
Electrons and the Aurora
"We have known for a long time that aurorae are caused by fast-moving electrons that collide with atoms in the upper atmosphere," said Göran Marklund of the Alfvén Laboratory in Sweden, a Co-Investigator with the EFW experiment.
"These electrons are accelerated to high energies - several keV (thousand electron-volts) - as they follow the magnetic field lines," he said.
"We see this happening in narrow, U-shaped vertical structures above the aurora," he explained. "These features start at a height of several thousand kilometres and reach far into space. We call them 'electric potential structures' and they can be negatively or positively charged."
The particles in the structures are accelerated upward or downward, depending on the direction of the electric field.
A negatively charged structure, which has an upward electric field component, accelerates the incoming electrons downward to produce auroras.
A positively charged structure has a downward electric field component that accelerates electrons upward away from the ionosphere towards the magnetosphere. The ionospheric signatures of such a structure are a growing hole in the electron density and a "black aurora", which is visible to the eye if embedded in an area of diffuse bright/normal (?) aurora.
Like some giant cosmic battery, these negatively and positively charged structures are often found together. They are linked by an electrical circuit that includes the aurora, the upward auroral current and the downward return current.
Evolution of acceleration structures first seen by Cluster
The existence of positive potential structures in the downward current region was first reported in 1994 by Professor Marklund and his colleagues, based on observations made at low altitude (1700 km) by the Swedish Freja satellite.
However, until now, no one has been able to find out how the huge vertical structures form, how long they last or how they vary with altitude. Only since the arrival of Cluster have scientists been able to carry out a detailed investigation of what goes on inside these mysterious particle accelerators.
The first Cluster observations took place on the morning of 14 January 2001. From an altitude of 21,684 km, the four spacecraft were able to study the conditions associated with the aurora far below. Aligned like a string of pearls, the quartet swept from south to north across the northern aurora at 100 second time intervals.
During this short period, the EFW experiments on the Rumba, Salsa and Samba spacecraft observed an increase in the electric field corresponding to an increase in the positive electric potential of the structure from 500 volts to over 2,000 volts. However, when the Tango spacecraft reached the same position, the electric field had vanished and no evidence of a structure could be seen.
The corresponding PEACE data showed an increase in the energy of the upward electron beam from about 500 electronvolts to 3,000 electronvolts, but the FGM instrument showed no change in the downward electric current.
Similar results were obtained from a crossing of the southern hemisphere auroral zone on 14 February. Once again, the electric field increased steadily in strength, but the electric current stayed constant as the quartet sped through the upper reaches of the U-shaped structure.
"The data show that the potential structures extend to altitudes greater than 20,000 km and that they grow in size and intensify over time scales of a few minutes," said Professor Marklund. " It was like if the "cosmic battery" was getting stronger and stronger and then after about 5 minutes stopped suddenly." "The increase in the electric field is Nature's way of maintaining the electrical current as the supply of electrons from the ionosphere decreases."
"This is the first time that we have been able to follow the evolution of these structures as they accelerate the electrons away from the auroral ionosphere," he added.
"The period of a few minutes during which they grow is comparable to the time it takes to evacuate the ionospheric electrons over the narrow regions where the electrons are sucked away".
"Understanding the development and growth of these dynamic structures associated with the aurora is a major goal of the Cluster mission, and something which cannot be solved by single satellite measurements," he concluded.
Professor Göran Marklund
Division of Plasma Physics Alfvén Laboratory Royal Institute of Technology Stockholm Sweden
Tel: +46 8790-7695