DHAKA: Three European spacecraft will launch from northern Russia later on a mission to make the most detailed global maps yet of Earth`s magnetic field.

Known as Swarm, the trio of satellites should help scientists understand better how the field is generated, and why it appears to be weakening.

The European Space Agency (Esa) mission is going up from the Plesetsk Cosmodrome on a modified intercontinental ballistic missile.

The satellites are expected to separate from the Breeze upper-stage of their Rockot launcher some 91 minutes later, at an altitude of 490km.

A period of commissioning and testing will then follow before the Swarm starts their four-year measurement campaign.

The major part of Earth`s global magnetic field is generated by convection of molten iron within the planet`s outer liquid core, but there are other components that contribute to the overall signal.

These include the magnetism retained in rocks, and there is even an effect derived from the movement of salt water ocean currents.

Swarm will attempt to tease apart these various factors, to get a clearer picture of the field`s true origins and its changing behaviour.

There are a number of questions researchers want to answer - not least, why the field appears to have lost about 15% of its strength in the past two centuries.

There has been speculation that Earth may be on the cusp of a polarity reversal, which would see the direction of the field flip end to end. North would become south, and vice versa.

This has not happened for 780,000 years, but the phenomenon has nonetheless been a regular occurrence through geological time.

Other uses of the data will include investigations of the electrical environment of the high atmosphere and the way this interacts with the solar wind - the continuous stream of charged particles billowing away from the Sun.

The wind carries its own magnetic field which clashes with Earth`s, producing "storms" that can on occasion disrupt satellites, radio communications and even electricity grids at the planet`s surface.

All three satellites in the Swarm constellation are identical. Their super-sensitive instrumentation acts rather like a 3D compass, enabling the precise strength and direction of the magnetic field to be determined all around the globe.

The trio`s construction was led by the Astrium Company, predominantly in Germany and the UK.

Engineers have had to ensure the magnetism generated by the satellites` own internal electronics do not swamp the scientific data.

This has meant putting the instruments on the end of a long boom to keep them separate from the main spacecraft body. It has given Swarm a very distinctive look - like giant mechanical rats with long tails.

A key moment on Friday will be the deployment of the booms. Stowed for launch, the instrument tails must be opened for there to be an operational mission.

"The first boom will open a few hours after launch," explained Andy Jones, the Astrium UK Swarm project manager.

"Each one is held in place by a nut, which will release the boom and allow it to swing open. Springs will initially pull the boom well past the hinge line, and it will waggle back and forth until eventually settling down. The waggling lasts about a minute and a half."

The mission plan is to make two of the satellites circle the planet in tandem, while shifting the plane of the third spacecraft to an offset of 90 degrees after three years.

This approach is expected to make it much easier for Swarm to separate out all the different components of the global magnetic field.

The most difficult to discern will undoubtedly be the contribution from ocean currents. If one stands at the surface of Earth, the combined field measures some 50,000 nanoTesla (the unit of magnetic field strength). Of this, ocean currents probably account for just 2nT.

"It will be a challenge for sure," said Rune Floberghagen, Esa`s Swarm mission manager, "but we are pretty sure Swarm has the necessary sensitivity, and if we can model precisely the other components in the field then we should be able to dig out this `needle in a haystack`.

"And it would be really exciting because we would then be able to measure the complete water circulation right down to the bottom of the ocean. That`s not something anyone has been able to do before," he told media.

Source: BBC
BDST: 1729 HRS, NOV 22, 2013