Origin of long-standing space mystery revealed: Origin of the 'theta aurora'

The night side of the terrestrial magnetosphere forms a structured magnetotail, consisting of a plasma sheet at low latitudes that is sandwiched between two regions called the magnetotail lobes. The lobes consist of the regions in which Earth's magnetic field lines are directly connected to the magnetic field carried by the solar wind. Different plasma populations are observed in these regions -- plasma in the lobes is very cool, whereas the plasma sheet is more energetic. The diagram labels by two red dots the location of an ESA Cluster satellite and NASA's IMAGE satellite on 15 September 2005, when particular conditions of the magnetic field configuration gave rise to a phenomenon known as 'theta aurora.'

A University of Southampton researcher has helped solve a long-standing space mystery -- the origin of the 'theta aurora'.

Auroras are the most visible manifestation of the Sun's effect on Earth. They are seen as colourful displays in the night sky, known as the Northern or Southern Lights. They are caused by the solar wind, a stream of plasma -- electrically charged atomic particles -- carrying its own magnetic field, interacting with Earth's magnetic field.

Normally, the main region for this impressive display is the 'auroral oval', which lies at around 65-70 degrees north or south of the equator, encircling the polar caps.

However, auroras can occur at even higher latitudes. One type is known as a 'theta aurora' because seen from above it looks like the Greek letter theta -- an oval with a line crossing through the centre.

While the cause of the auroral oval emissions is reasonably well understood, the origin of the theta aurora was unclear until now.

Researchers observed particles in the two 'lobe' regions of the magnetosphere. The plasma in the lobes is normally cold, but previous observations suggested that theta auroras are linked with unusually hot lobe plasma.

Dr Robert Fear from the University of Southampton (formerly at the University of Leicester, where much of the research took place), and lead author of the paper published in Science this week, says: "Previously it was unclear whether this hot plasma was a result of direct solar wind entry through the lobes of the magnetosphere, or if the plasma is somehow related to the plasma sheet on the night side of Earth.

"One idea is that the process of magnetic reconnection on the night side of Earth causes a build-up of 'trapped' hot plasma in the higher latitude lobes."

The mystery was finally solved by studying data collected simultaneously by the European Space Agency's (ESA) Cluster and NASA's IMAGE satellites on 15 September 2005. While the four Cluster satellites were located in the southern hemisphere magnetic lobe, IMAGE had a wide-field view of the southern hemisphere aurora. As one Cluster satellite observed uncharacteristically energetic plasma in the lobe, IMAGE saw the 'arc' of the theta aurora cross the magnetic footprint of Cluster.

"We found that the energetic plasma signatures occur on high-latitude magnetic field lines that have been 'closed' by the process of magnetic reconnection, which then causes the plasma to become relatively hot," says Dr Fear.

"Because the field lines are closed, the observations are incompatible with direct entry from the solar wind. By testing this and other predictions about the behaviour of the theta aurora, our observations provide strong evidence that the plasma trapping mechanism is responsible for the theta aurora," he adds.

"The study highlights the intriguing process that can occur in the magnetosphere when the interplanetary magnetic field of the solar wind points northwards," adds Philippe Escoubet, ESA's Cluster project scientist.

"This is the first time that the origin of the theta aurora phenomenon has been revealed, and it is thanks to localised measurements from Cluster combined with the wide-field view of IMAGE that we can better understand another aspect of the Sun-Earth connection," he adds.

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Cost of cloud brightening for cooler planet revealed

University of Manchester scientists have identified the most energy-efficient way to make clouds more reflective to the sun in a bid to combat climate change.

Marine Cloud Brightening is a reversible geoengineering method proposed to mitigate rising global temperatures. It relies on propelling a fine mist of salt particles high into the atmosphere to increase the albedo of clouds -- the amount of sunlight they reflect back into space. This would then reduce temperatures on the surface, as less sunlight reaches Earth.

Clouds form when water droplets gather on dust or other particles in the air. Increasing the amount of salt particles in the atmosphere allows more of these water droplets to form, making the clouds denser and therefore more reflective.

A new paper, published in the journal Philosophical Transactions of the Royal Society A, has looked at four different ways of getting the particles into the sky, to compare how effective they may be. The researchers found that a technique called the 'Rayleigh Jet' proved to be best.

Named after Lord Rayleigh, who provided the theory, the technique relies on spraying a fine jet of water that breaks down into small droplets into the sky. The liquid droplets evaporate quickly, leaving behind just the salt particles.

These particles, say the paper's authors, could be generated from specially built ships that could travel the world's oceans spraying salt particles into the air where they then hang in the atmosphere for several days until they return to Earth as rain.

Previous studies have optimised the size of the salt particles needed to produce the best increase in cloud reflectance but haven't taken into account how much energy the technique would need and how much it would cost to operate. This new paper, by teams at the universities of Manchester, Washington and Edinburgh, tackled this question. The researchers tested each technique so there was an increase in reflection of 5%, a figure that would combat the predicted effects of increased carbon dioxide levels over the rest of this century. They then looked at how much energy each would consume.

The scientists say that the Rayleigh jet method could produce the desired effect using 30 megawatts of energy, about the same energy that two large ships produce.

Dr Paul Connolly, based in the School of Earth, Atmospheric and Environmental Sciences at The University of Manchester, said: "It can be incredibly energy intensive to propel water high into the atmosphere and the energy required had never really been tested before. Our paper optimises the salt particle sizes to produce the required change in cloud reflectance for the least energy cost. It is an important finding if these techniques should be needed in the future.

"I am not recommending that we use any of these techniques now, but it is important to know how best to use them should they become necessary. Should no progress be made to reduce CO₂ levels, then geoengineering techniques, similar to this, might become necessary to avoid dangerous rises in global temperatures."

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