The seeds of solar storms (continued)

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The life of a CME

About once per week when the Sun is relatively quiet, and two or three times a day at the peak of the 11-year solar cycle, a great bundle of plasma—far bigger than Earth—bursts from the Sun’s surface. This coronal mass ejection, or CME, accelerates through the corona in only a few hours. If it’s pointed at Earth, it can irradiate astronauts, disable the circuitry in satellites, knock out surface power grids, degrade the accuracy of the Global Positioning System by up to a factor of five, and paint the high-latitude sky with auroras. Even a CME emerging on the right-hand side of the Sun can affect our atmosphere, as some of the energetic particles loop around the spiral-shaped magnetic field embedded in the solar wind.

NCAR scientist Stanley Solomon took this unique photo of an aurora unfolding above NCAR's Mesa Laboratory on the evening of 20 November 2003. The aurora lasted over an hour with only minor variations. "It was an amazingly stable feature," says Solomon. The event capped a month-long string of unusually intense solar activity (see sidebar).

When CMEs were discovered by NASA’s pioneering human-staffed satellite Skylab in 1973, they immediately caught the interest of researchers. Like great clouds of smoke, they billowed far from the Sun’s surface into the outer corona, a region largely invisible to ground-based observing systems at that time. It was easy to imagine CMEs ­traveling further into space—even as far as Earth. However, it took until the 1990s to conclusively link CMEs to auroras and other Earth-based impacts.

“CMEs are at the heart of the really important plasma interactions we call space weather, and they’re so inherently interesting as well,” says Thomas Holzer. He’s a 30-year NCAR veteran and the leader of HAO’s Coronal Studies Group, part of the Solar Atmosphere and Heliosphere Section, led by Boon Chye Low.

Like tornadoes, CMEs tend to defy measurement. A CME unfolds too quickly for most instruments to do more than capture a few points in its violent life cycle. “They really challenge the cadence of our systems,” says Holzer. Satellites began to track CMEs in the 1980s on a regular basis, but they could see only CMEs that emerged sideways from the sun’s limb (i.e., perpendicular to Earth). It was impossible to spot and warn for the CMEs ejected Earthward from the front face of the Sun—the very ones that could cause major solar storms.

Things changed in the 1990s, as space- and ground-based systems began to infer the presence of CMEs across the entire Sun, by examining how they affect more intense radiation emitted below the corona, closer to the Sun’s surface. One of thos pioneering systems is still at work at Mauna Loa: NCAR’s chromospheric helium imaging photometer. Every three minutes, CHIP measures the absorption of helium light at a charactericstic frequency; changes in that absorption are linked to changes in the corona.

In tandem with other instruments elsewhere, CHIP helps give quick notice when a CME embarks from the Sun. If a solar storm appears to be in the offing, the NOAA Space Environment Center issues the celestial equivalent of a hurricane warning, giving people a day or two to batten down the hatches.

Along with serving as a first-alert system, CHIP is a fertile source of data for Holzer and other coronal analysts at NCAR. Holly Gilbert, for example, has been using CHIP data to connect several different kinds of waves that zip across the Sun. Right now, it appears that CMEs can trigger a fast-moving wave in the corona that, in turn, compresses the chromosphere below and stimulates a more slow-moving wave.

Meanwhile, Giuliana de Toma has been studying dim regions that appear in X-ray coronal images for a day or two after a CME. They’re interpreted as signs of transient coronal holes, temporary zones of lower density left in a CME’s wake. According to de Toma, the frequent images from CHIP will help to understand the evolution of these holes and, ultimately the CMEs that cause them. “We still don’t fully understand the origin and early development of CMEs,” says de Toma.

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Were last year's solar storms a fluke?

Scientists and laypeople alike were astounded by a string of intense storms that emerged from the Sun last October and November. These storms—including one on 4 November that produced the strongest signal ever recorded in the soft X-ray range by NOAA satellites—occurred several years after the peak of this solar cycle. They produced a widely seen set of auroras, including one visible behind NCAR’s Mesa Laboratory (see photo above).

While the post-peak storms were some of the most intense solar and geomagnetic events on record, “in some respects they weren’t unusual at all,” says Howard Singer (NOAA Space Environment Center). Solar scientists have long noticed a lag of several years between the peak of each solar cycle and its most intense geomagnetic storms. “Many times you see large storms occur on the falling side of the sunspot cycle,” says Singer. SEC’s Bill Murtagh discussed the phenomenon in January while reporting on the current solar cycle at the American Meteorological Society’s annual meeting.

Earth escaped the worst of last November’s storms thanks to a lucky break. A CME’s impact depends on the orientation of the interplanetary magnetic field. Despite its often-orderly structure, the interplanetary field can also switch polarity from minute to minute in unpredictable ways. If the interplanetary field is pointing in an opposite direction from Earth’s own magnetic field, “that’s like a key unlocking a door to let energy in,” says Singer. If both fields are pointing the same way—as was the case when the 4 November storm arrived—the geomagnetic effects are much weaker.