Tuesday, September 4, 2007

Elusive Waves In Sun's Corona




Scientists for the first time have observed elusive oscillations in the Sun's corona, known as Alfvén waves, that transport energy outward from the surface of the Sun. The discovery is expected to give researchers more insight into the fundamental behavior of solar magnetic fields, eventually leading to a fuller understanding of how the Sun affects Earth and the solar system.

Alfvén waves can provide us with a window into processes that are fundamental to the workings of the Sun and its impacts on Earth.

Alfvén waves are fast-moving perturbations that emanate outward from the Sun along magnetic field lines, transporting energy. Although they have been detected in the heliosphere outside the Sun, they have never before been viewed within the corona, which is the outer layer of the Sun's atmosphere. Alfvén waves are difficult to detect partly because, unlike other waves, they do not lead to large-intensity fluctuations in the corona. In addition, their velocity shifts are small and not easily spotted.

The waves are visible all the time and they occur all over the corona, which was initially surprising.

Insights into the Sun

By tracking the speed and direction of the waves, researchers will be able to infer basic properties of the solar atmosphere, such as the density and direction of magnetic fields. The waves may provide answers to questions that have puzzled physicists for generations, such as why the Sun's corona is hundreds of times hotter than its surface.

The research also can help scientists better predict solar storms that spew thousands of tons of magnetized matter into space, sometimes causing geomagnetic storms on Earth that disrupt sensitive telecommunications and power systems. By learning more about solar disruptions, scientists may be able to better protect astronauts from potentially dangerous levels of radiation in space.

Insights into the Sun

By tracking the speed and direction of the waves, researchers will be able to infer basic properties of the solar atmosphere, such as the density and direction of magnetic fields. The waves may provide answers to questions that have puzzled physicists for generations, such as why the Sun's corona is hundreds of times hotter than its surface.

The research also can help scientists better predict solar storms that spew thousands of tons of magnetized matter into space, sometimes causing geomagnetic storms on Earth that disrupt sensitive telecommunications and power systems. By learning more about solar disruptions, scientists may be able to better protect astronauts from potentially dangerous levels of radiation in space.

What Makes Mars Magnetic?



Scientists think that the secret lies beneath the crust, in the slippery asthenosphere. In order for the mantle to convect and the plates to slide they require a lubricated layer. On Mars this lubrication has long since dried up, but on Earth the plates can still glide around with ease.

If you could pick up a rock from the surface of Mars, then the chances are it would be magnetic. And yet, Mars doesn’t have a magnetic field coming from its core. These rocks are clinging to the signal of an ancient magnetic field, dating back billions of years, to the times when Mars had a magnetic field like Earth’s.

So how have these rocks hung onto their magnetic directions and what do they tell us about Mars? Strangely, the answer to these questions might be sitting here on Earth.

Most continental rocks on Earth align their magnetic moments with the current magnetic field – they are said to have ‘induced’ magnetism. “I consider induced rocks to have ‘Alzheimers’. These are the rocks that forgot where they were born and how to get home,” explains Suzanne McEnroe.

However, not all of Earth’s continental rocks have an induced magnetization. Some rocks stubbornly refuse to swing with the latest magnetic field, and instead keep hold of the direction they were born with. These rocks are said to have a remanent magnetization.

McEnroe and her colleagues have been studying some of Earth’s strongest and oldest remanent magnetic rocks, to find out why they have such good memories. Understanding these rocks may give us clues as to what kind of rocks lie on Mars.

These rocks are around 1 billion years old and have a strong magnetic remanence, more than 30 times larger than typically found in basaltic rocks.

The mining company had assumed that the rocks in this strongly magnetic area were holding an induced magnetic field and that there would be magnetite buried down below. However, they were puzzled when a different mineral – hematite, came out of the drill core. Had they missed their target, or were their assumptions wrong?

By studying the samples under a powerful microscope and modelling their magnetic properties, McEnroe was able to show that the hematite was responsible for the strong magnetic field and that it was holding a remanent field from around 1 billion years ago. “We could see that the hematite contained small intergrowths that carried the magnetism”.

And it turns out that the microstructure of the rock is the key to whether it can hold a remanent magnetization or not. Together with Richard Harrison, a mineral physicist at Cambridge University, UK, and Peter Robinson at NGU, McEnroe has been studying strong remanent magnetic rocks from a variety of places including Scandinavia and the USA.

A study on nearly billion-year-old rocks in Norway showed a remanent magnetic anomaly comparable in scale to those observed on Mars. The remanent magnetic anomaly dominates the local magnetic field to such a degree that more than half the Earth's field is cancelled. It is nearly impossible to use a compass in the area, which cannot point correctly north because of the strong remanent magnetization in the rocks.

What they have found is that rocks containing nanometre scale intergrowths of ilmenite and hematite are better able to cling onto their original magnetization than those without such fine-scale features. “Placing a nanoparticle of ilmenite into the hematite host creates a strong and stable magnetic signal that can survive large changes in temperature and magnetic field over billions of years,” explains Harrison.

So can this tell us anything about the rocks on Mars? “These rocks are good analogues for the magnetic rocks we see on Mars because of their strong magnetism and the length of time they have retained this memory,” says McEnroe. Certainly this nano-scale microstructure is a plausible candidate for the magnetic rocks on Mars.

However, the rocks on Earth can’t answer all our questions.