Thursday, October 3, 2013

Six years of observing the Earth's magnetosphere

By Adonai via The Watchers, 27 September 2013



There is still only a handful of spacecraft to watch for solar and magnetic storms but the number of observatories has been growing over the last six years. Today, these spacecraft have begun to provide the first multipoint measurements to better understand space weather events as they move through space, something impossible to track with a single spacecraft.


Helping to anchor that team of spacecraft is a NASA mission called THEMIS (Time History of Events and Macroscale Interactions during Substorms). THEMIS was launched on February 17, 2007, with five nearly identical spacecraft nestled inside a Delta II rocket. Over time, each spacecraft moved into formation to fly around Earth in a highly-elliptical orbit that would have them travelling through all parts of Earth's space weather environment, a giant magnetic bubble called the magnetosphere. With five different observatories, scientists could watch space weather unfold in a way never before possible.



"Scientists have been trying to understand what drives changes in the magnetosphere since the 1958 discovery by James Van Allen that Earth was surrounded by rings of radiation," says David Sibeck, project scientist for THEMIS at NASA's Goddard Space Flight Center in Greenbelt, Md. "Over the last six years, in conjunction with other key missions such as Cluster and the recently launched Van Allen Probes to study the radiation belts, THEMIS has dramatically improved our understanding of the magnetosphere."

Since that 1958 discovery, observations of the radiation belts and near Earth space have shown that in response to different kinds of activity on the Sun, energetic particles can appear almost instantaneously around Earth, while in other cases they can be wiped out completely. Electromagnetic waves course through the area too, kicking particles along, pushing them ever faster, or dumping them into the Earth's atmosphere. The bare bones of how particles and waves interact have been described, but with only one spacecraft traveling through a given area at a time, it's impossible to discern what causes the observed changes during any given event.

"Trying to understand this very complex system over the last 40 years has been quite difficult," says Vassilis Angelopoulos, the principal investigator for THEMIS at the University of California in Los Angeles (UCLA). "But very recently we have learned how even small variations in the solar wind – which buffets Earth's space environment at a million miles an hour - can sometimes cause extreme responses, causing more particles to arrive or to be lost."

An artist's concept of the THEMIS spacecraft orbiting around Earth. Credit: NASA

Understanding the changes energy from the Sun undergoes as it travels away and out into space is crucial for scientists to achieve their goal of some day predicting the onset of space weather that creates effects such as the shimmering lights of the aurora or interruptions in radio communications at Earth.

Taking advantage of an unprecedented alignment of eight satellites through the vast magnetic environment that surrounds Earth in space, including NASA’s ARTEMIS and THEMIS, scientists now have comprehensive details of the energy’s journey through a process that forms the aurora, called a substorm. Their results, published in the journal Science on September 27, 2013, showed that small events unfolding over the course of a millisecond can result in energy flows that last up to half an hour and cover an area 10 times larger than Earth.

Trying to understand how gigantic explosions on the Sun can create space weather effects involves tracking energy from the original event all the way to Earth. It’s not unlike keeping tabs on a character in a play with many costume changes, because the energy changes form frequently along its journey: magnetic energy causes eruptions that lead to kinetic energy as particles hurtle away, or thermal energy as the particles heat up. Near Earth, the energy can change through all these various forms once again.

Most of the large and small features of substorms take place largely in the portion of Earth’s magnetic environment called the magnetotail. Earth sits inside a large magnetic bubble called the magnetosphere. As Earth orbits around the Sun, the solar wind from the Sun streams past the bubble, stretching it outward into a teardrop. The magnetotail is the long point of the teardrop trailing out to more than 1 million miles on the night side of Earth. The Moon orbits Earth much closer, some 386 000 km (240 000 miles) away, crossing in and out of the magnetotail.

On July 3, 2012, eight spacecraft were lined up on the night side of Earth, enabling scientists to track how magnetic energy from the Sun moved around Earth, reconnected at a point about half way to the moon, and then spread through the back end of Earth’s magnetic environment, the magnetotail. Image credit: NASA/SVS


This short video features commentary by David Sibeck, project scientist for the THEMIS mission, discussing a visualization of reconnection fronts.

Magnetic reconnection

“It’s a meticulous accounting job,” says Angelopoulos. “With all these spacecraft measuring what’s going on continuously throughout the system, we can track the total energy and see where and when it’s converted into different kinds of energy. And the effort paid off handsomely!”

Scientists have observed much of the energy’s journey through a substorm before. When the solar wind streams off the Sun it can connect with the front of Earth’s magnetosphere. As the two sets of magnetic fields come together, a process called magnetic reconnection turns the energy of the forward-moving solar wind into an explosion that sends particles and magnetic fields moving around the planet to the far side of Earth. Here, the fields reconnect again creating a burst that turns magnetic energy into acceleration of particles and heating. Just where and how this energy converted to particle movement, however, has been unclear.

The details of what happened next required observations from many spacecraft simultaneously. While the magnetic reconnection event itself happened in a specific place somewhere halfway between Earth and moon’s orbit in a region just a couple hundreds of miles across, this is not the main place where the energy was converted. Regions, labeled as “reconnection fronts” in the paper, surged away from the original reconnection point -- one propagated toward Earth and one moved away, past the moon and down the magnetotail. These fronts are like sheets of current, a wall hurtling in each direction, continuing to convert energy for up to 30 minutes afterward. The energy moving in toward Earth helps to create the aurora and it also funnels into the giant donuts of radiation around Earth called the radiation belts.

“The amount of power being converted is comparable to the electric power generation on Earth from all sources at any moment in time. And it happens over 30 minutes,” says Angelopoulos. “The amount of energy released is equivalent to a 7.1 Richter scale earthquake.”

The fact that this energy can move around so dramatically is not in and of itself surprising. Scientists have certainly previously suggested such things based on computer models. But it is only with a fleet of spacecraft that scientists can confirm the location and exact nature of the process, not to mention learning something new such as how continuous and long term the energy conversion process is after the initial magnetic reconnection event.

In late 2014, NASA will add a new mission to their Heliophysics fleet. The Magnetospheric Multiscale or MMS mission will put spacecraft directly in the magnetic reconnection areas on both the day- and night-sides of Earth.


“Understanding where to look for the energy conversion, opens up a new window for research,” says Sibeck. “MMS will be focusing on tracking just this kind of observation.”

The light from the aurora is caused by charged particles (mostly electrons) that come from inside the magnetosphere and then speed up to very high speeds as they barrel down along magnetic field-lines into the upper atmosphere. As they collide with the gas, they excite the atoms and molecules, which emit light when they relax from their excited state. Credit: UC Regents



The culprit behind aurora is our own Sun and the solar plasma that is ejected during a magnetic event like a flare or a coronal mass ejection.

 

This video is included here just because it has the best aurora images taken from space.

In its sixth year in space, scientific papers using THEMIS data helped highlight a number of crucial details about what causes space weather events in this complex system.

THEMIS has now traveled through more than 50 solar storms that caused particles in the outer radiation belts to either increase or decrease in number. Historically, it has been difficult for scientists to find commonalities between such occurrences and discover what, if anything consistently caused an enhancement or a depletion.

With so many events to study, however, and a more global view of the system from several spacecraft working together – including, in this case, ground based observations and NOAA's GOES (Geostationary Operational Environment Satellites) and POES (Polar Operational Environmental Satellites) data in addition to THEMIS data – a team of scientists led by Drew Turner at UCLA could better characterize what processes caused which results.

Turner's group recently presented evidence linking specific kinds of electromagnetic waves in space – waves that are differentiated based on such things as their frequencies, whether they interact with ions or electrons, and whether they move along or across the background magnetic fields – to different effects. Chorus waves, so called because when played through an amplifier they sound like a chorus of singing birds, consistently sped up particles, leading to an increase in particle density. On the other hand, two types of waves known as hiss and EMIC (Electromagnetic Ion Cyclotron) waves occurred in those storms that showed particle depletion. Turner also observed that when incoming activity from the Sun severely pushed in the boundaries of the magnetosphere this, too, led to particle drop outs, or sudden losses throughout the system. Such information is helpful to those attempting to forecast changes in the radiation belts, which if they swell too much can encompass many of our spacecraft.

Another group has a paper in print in 2013 based on 2008 data from the five THEMIS spacecraft in conjunction with three of NOAA's GOES (Geostationary Operational Environmental Satellites) spacecraft, and the ESA/NASA Cluster mission. Led by Michael Hartinger at the University of Michigan in Ann Arbor, this group compared observations at the bow shock where the supersonic solar wind brakes to flow around the magnetosphere to what happens inside the magnetosphere. They found that instabilities drive perturbations in the solar wind particles streaming towards the bow shock and that these perturbations can be correlated with another type of magnetized wave – ULF (ultra low frequency) waves - inside the magnetosphere. ULF waves, in turn, are thought to be important for changes in the radiation belts.

"The interesting thing about this paper is that it shows how the magnetosphere actually gets quite a bit of energy from the solar wind, even by seemingly innocuous rotations in the magnetic field," says Angelopoulos. "People hadn't realized that you could get waves from these types of events, but there was a one-to-one correspondence. One THEMIS spacecraft saw an instability at the bow shock and another THEMIS spacecraft then saw the waves closer to Earth."

A third interesting science paper from THEMIS's sixth year focused on features originating even further upstream in the solar wind. Led by Galina Korotova at IZMIRAN in Troitsk, Russia, this work made use of THEMIS and GOES data to observe the magnetosphere boundary, the magnetopause. The researchers addressed how seemingly small perturbations in the solar wind can have large effects near Earth. Wave-particle interactions in the solar wind in the turbulent region upstream from the bow shock act as a gate valve, dramatically changing the bow shock orientation and strength directly in front of Earth, an area that depends critically on the magnetic field orientation. The extreme bow shock variations cause undulations throughout the magnetopause, which, launch pressure perturbations that may in turn energize particles in the Van Allen radiation belts.

Understanding Earth's magnetosphere is a work in progress and all of this recent work helps illuminate the nitty gritty details of how seemingly small changes in a system can lead to large variations in the near-Earth space environment.

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