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BU Astronomers Record Evidence of New Solar Activity

August 17, 2010

Scientists from Boston University’s Center for Space Physics (CSP) announced today that they have sub-visual evidence of the onset of a new cycle of solar-terrestrial activity. The key results being reported deal with the fact that recent auroral displays at high latitudes (ones visible to the naked eye) were accompanied by far less luminous glows in the atmosphere at lower latitudes.

“It’s exciting to see the return of aurora to mid-latitudes,” says CSP senior research associate Steven Smith, referring to the periodic occurrence of emissions in the Earth’s atmosphere that have intrigued observers from ancient to modern times.

What has fascinated space scientists in recent years is the delayed onset of such effects. Typically, the sun has an activity cycle of about 11 years, with flares and ejections of electrically charged particles (called the solar wind) that cause changes in the Earth’s magnetic field that produce luminous emissions in the atmosphere. Such effects are subdued during so-called solar minimum years (e.g., in 1996-1997) and very prominent in solar maximum years (e.g., 2001-2002). Thus, the onset of a new wave of such activity had been expected to be well underway by 2009, but the Sun remained surprisingly quiet. Now, in 2010 there are finally signs of the cycle re-appearing.

The observations made by the BU team used an all-sky camera located at the Mt. John Observatory in Lake Tekapo, New Zealand. “This is essentially a fish-eye lens that is used to view the full sky, and the images are captured with a very sensitive digital CCD camera,” says CSP senior research associate Jeffrey Baumgardner, who designed and built the instrument.

Smith, a native New Zealander, explains that the emissions come from regions ranging from 200-400 km (125-250 miles) above the surface. “These gases are caused to glow by energy input from above, energy that flows downward along the Earth’s magnetic field lines,” says Smith.

The curtains of glowing gasses visible to the human eye are called aurora borealis when near polar regions in the northern hemisphere, and aurora australis in southern polar regions. The emissions captured by the CCD camera in the accompanying image are below naked-eye detection limits and were observed at latitudes more distant from the poles than where typical auroras occur. They are caused by a steady influx of electrons that hit oxygen atoms and excite them to glow in a typically red light. Also visible is an even fainter arc that extends from east to west just south of New Zealand. This arc is caused collisions between hot electrons and oxygen atoms in the Earth’s ionosphere. Such features are called Stable Auroral Red (SAR) arcs and are an active topic of research in space physics. “This image of a SAR arc from New Zealand is perhaps the first-ever case of imaging an unambiguous SAR arc in the southern hemisphere,” says Michael Mendillo, BU professor of astronomy.

SAR arcs show where energy from the Van Allen Radiation Belts of electrically charged particles trapped in the Earth’s magnetosphere deposit heat into the ionosphere. The narrow dimension of a SAR arc shows that the energy input is confined to a narrow distance (100 km or 60 miles) in latitude, but in bands that can extend completely around the globe in longitude.

“We fully expect that a similar SAR arc has occurred in the northern hemisphere, but it was cloudy at our observatory in Boston that night, and so one was not seen,” says Smith. “We hope in the years ahead to have many cases of SAR arcs in our data from both hemispheres, and then examine the full global distribution of such effects. To see if energy input is simultaneously the same or different in each hemisphere is a prime topic in the study of solar-induced storms in our upper atmosphere.”

This work was sponsored by the U. S. National Science Foundation, in collaboration with colleagues at the Dept. of Physics and Astronomy at the University of Canterbury, Christchurch, in New Zealand.

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