The Sun is minimally active right now, but this quiet state of affairs won't last for long. Over the next few years, the number of solar flares and eruptions known as coronal mass ejections will increase until reaching solar maximum in 2011 or 2012. Such eruptions can impact Earth, disrupting satellites, communications, and even power grids. Some predict the next solar cycle will be the most intense in 50 years. As a result, scientists are striving to understand the mechanism behind solar eruptions in hopes of eventually being able to predict them in a space "weather forecast."
The latest advance in understanding the Sun comes via the Hinode spacecraft and its X-ray Telescope (XRT), an instrument designed and built by engineers at the Smithsonian Astrophysical Observatory. The XRT has provided a new look at x-ray jets – mini-flares that are less energetic, but also more frequent, than their more powerful cousins.
The new research is the cover article in the December 7 issue of the journal Science.
X-ray jets and solar flares both result from a process known as magnetic reconnection, where magnetic field lines of opposing alignment (like the north and south poles on bar magnets) meet and annihilate, releasing energy. That energy propels electrically charged gas, or plasma, outward into the solar system. By studying the less complex X-ray jets, astronomers can get new insights into the stronger, more complex flares and CMEs.
"It’s almost like storm chasing. You see mostly F1 tornadoes, but occasionally a really destructive F5 pops up," said astronomer Leon Golub of the Harvard-Smithsonian Center for Astrophysics. "By understanding the physics behind the smaller events, we hope to predict the larger and more powerful ones."
"X-ray jets are good test cases for evaluating our understanding of magnetic reconnection," he added.
Small x-ray jets release as much energy as thousands of atomic bombs and send material out into space at speeds of nearly 2 million miles per hour. In contrast, CMEs release 1,000 times more energy and blast plasma outward at speeds as high as 6 million miles per hour (fast enough to reach Earth in only 15 hours).
Hinode’s observations indicate that x-ray jets may contribute to the high-speed solar wind – a stream of plasma that is propelled away from the sun in all directions at all times. Moreover, the new research provides the first direct observations that magnetic waves, called Alfvén (al-FEEN) waves, play a critical role in driving the solar wind into space.
In the solar atmosphere, Alfvén waves are created when convective motions and sound waves push magnetic fields around, or when dynamic processes create currents that allow the magnetic fields to change shape or reconnect.
"Until now, Alfvén waves have been impossible to observe because of limited resolution of available instruments," said Alexei Pevtsov, Hinode program scientist at NASA headquarters. "With the help of Hinode, we are now able to see direct evidence of Alfvén waves, which will help us unravel the mystery of how the solar wind is powered."
Using the XRT, a team led by Dr. Jonathan Cirtain, a solar physicist at NASA's Marshall Space Flight Center (and formerly of Harvard-Smithsonian Center for Astrophysics) peered at the sun's poles and observed record numbers of x-ray jets. Previous research detected only a few jets daily. With Hinode’s higher sensitivity, Cirtain's team observed an average of 240 jets per day. They conclude that magnetic reconnection is frequently occurring, forming both Alfvén waves and the energized plasma in x-ray jets.
"These observations show a clear relationship between magnetic reconnection and Alfvén wave formation in the x-ray jets," said Cirtain. "The large number of jets, coupled with the high speeds of the outflowing plasma, lends further credence to the idea that x-ray jets are a driving force in the creation of the fast solar wind."
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.