Recent appearances of bright sky displays known as aurorae, near the North Pole, were accompanied by much dimmer glows well south of that, say researchers at Boston University’s Center for Space Physics.
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| All-sky view of the upper atmosphere as photographed in the red light of oxygen from the Mt. John Observatory in New Zealand. This fish-eye lens image displays emissions from a height of 400 km above the Earth’s surface (features in upper left are buildings obstructing view to northwest). Just below the lower coast of South Island, the faint emission extending west-to-east is called a Stable Auroral Red (SAR) arc, a manifestation of oxygen atoms heated by hot electrons in the ionosphere. The brightness levels (about 300 Rayleigh units) are ten to twenty times fainter than can be seen by the naked eye. Further to the south, the area colored white is a brighter form of red emission called Diffuse Aurora, produced by in influx of electrons from the magnetosphere; it is still invisible to the unaided eye, but only by factors of two to three. Strong visible aurora, the familiar "curtains of red and green emission" would be still further toward the South Pole, beyond the field of view shown. This Boston University image of sub-visual aurora at mid-latitudes is the first unambiguous case of a SAR arc in the Southern Hemisphere during the new cycle of solar activity. (Credit: Boston U. Center for Space Physics) |
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. These cause changes in the Earth’s magnetic field that produce, as a side product, luminous emissions in the atmosphere.
Such effects are subdued during so-called solar minimum years, such as in 1996-1997, and very prominent in solar maximum years, as in 2001-2002. According to this cycle, a new wave of such activity had been expected to start by last year, but the Sun remained quiet.
Now there are finally signs of the cycle re-appearing, the Boston group says.
They used an all-sky camera at the Mt. John Observatory in Lake Tekapo, New Zealand, “essentially a fish-eye lens that is used to view the full sky,” said Jeffrey Baumgardner, also a researcher at the center, who designed and built the instrument. “The emissions we study 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.”
The curtains of glowing gasses visible to the unaided eye are long been called aurora borealis when near northern polar regions, and aurora australis in southern polar regions. The newly detected emissions instead are below naked eye detection limits, and were detected in the skies just south of New Zealand.
There, a faint arc extending from East to West was detected in the red glow of oxygen atoms. This emission is due to collisions between hot electrons, or charged particles, and oxygen atoms in a part of the atmosphere known as the ionosphere, the researchers said. Such features, called Stable Auroral Red or SAR arcs, are an active topic of research.
This “is perhaps the first-ever case of imaging an unambiguous SAR arc in the southern hemisphere,” said Boston University astronomer Michael Mendillo.
Still further south of that arc the researchers detected a far more diffuse, steady atmospheric emission, also however caused by an influx of electrons hitting oxygen atoms. This emission is thought to have a slightly different underlying cause from that of SAR arcs, which occur when electrons trapped in so-called Van Allen Radiation Belts in planet’s magnetic field deposit heat into the ionosphere. This energy input is confined to a thin band about 60 miles (100 km) wide from north to south, though it can extend all the way around the globe east to west.
“We fully expect that a similar SAR arc occurred in the northern hemisphere, but it was cloudy at our observatory in Boston that night, and so one was not seen,” Smith explained. “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,” he added. “Looking to see if the energy input is simultaneously the same or different in each hemisphere is a forefront topic in the study of solar-induced storms in our upper atmosphere
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