Solar Cycle Update

Updated predictions from NASA scientists place the solar maximum in mid-2000.

October 14, 1999: Beginning in late September, a continuing series of minor to moderate geomagnetic storms have triggered aurorae along the northern tier of U.S states and may have affected some public power systems operating at high northern latitudes. These events were caused by high-speed material streaming out from areas on the sun known as coronal holes. This week we can expect more of the same as another coronal hole rotates into position to send high-speed solar wind particles toward our planet. Space weather forecasters expect moderate levels of aurorae, shortwave radio disruptions, and power grid fluctuations at high latitudes for at least the next three days.

Right: On Sept. 14 1999, the space-based SOHO observatory photographed a huge eruptive prominence escaping the Sun, seen in the upper right corner of the image. Prominences are loops of magnetic fields with hot gas trapped inside. Sometimes, as the fields become unstable, the prominence will erupt as this one did and become part of a coronal mass ejection. Beautiful prominences like these become more common as we approach solar maximum.

The recent increase in geomagnetic activity offers a taste of things to come as the Sun approaches the maximum of its 11-year sunspot cycle. As sunspot numbers mount, coronal mass ejections and solar wind disturbances will trigger more and more geomagnetic storms. In extreme cases, these storms can induce electric currents in the earth and oceans that interfere with electric power transmission equipment.

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The extra ultraviolet (UV) and X-ray radiation created by magnetic fields around sunspots also cause the Earth's atmosphere to heat up and expand. This creates added drag in the area where low-earth orbit satellites and the Space Shuttle orbit. Solar flares and energetic particle events further complicate matters by interrupting satellite and shortwave radio communications.

It's not all bad news, though. The increased levels of solar activity will likely trigger dazzling auroral displays at mid-latitudes. Sky watchers in the continental U.S. will be treated to sights normally reserved for residents of higher latitudes. Another piece of good news is that increased atmospheric drag at solar maximum acts as a scavenger and helps clean out space debris from low-Earth orbit.

When will the solar maximum actually take place? Recent work by David Hathaway, a solar physicist at the Marshall Space Flight Center, and his collaborators indicate that the solar activity will peak around the middle of the year 2000.

"Our predictions have consistently targeted 2000 as the beginning of solar maximum," says Hathaway, "but the latest numbers suggest that the peak sunspot count in 2000 will be a bit lower than expected. The projected peak is comparable to, but lower than the peaks of the last two maxima (in 1989 and 1978). That would put all three of the recent sunspot maxima in the same class -- above average compared to all the sunspot cycles since the mid 1700's."

Above: By combining data about geomagnetic activity during the previous solar cycle with sunspot counts for the current cycle, David Hathaway and collaborators are able to predict when the next sunspot maximum will occur. [Click here for details]. According to their results, the sunspot number -- and other forms of solar activity -- will peak beginning in mid-2000. The dotted lines above and below the solid curve line indicate the prediction curve's range of error.

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A common misconception about the solar maximum is that it is a brief event that happens on a particular day, as reported in one recent news story that warned of a solar catastrophe on January 1, 2000. Not so, says Hathaway.

"The sunspot maximum is usually a broad peak. There is a two or three year period when activity is quite high. I wouldn't say that we're really in the solar maximum yet. I expect solar activity to be highest in 2000 and 2001, and then in 2002 it may decline back to where we are now in October 1999."

"The effects we've been experiencing for the past few weeks are relatively minor geomagnetic disturbances caused by the solar wind flowing out of coronal holes. A coronal hole is something you see in x-ray pictures of the sun where the corona appears dark. In contrast, bright spots in the x-ray image tend to overlie sunspot groups where hot gas is bottled up in magnetic fields that rise up out of one sunspot and bend back to reconnect at another spot nearby."

Right: This x-ray image of the sun, captured on Oct 12, 1999, by the Japanese Yohkoh X-ray Observatory shows the coronal hole that has rotated into a favorable position to send high-speed solar wind particles toward Earth. The resulting gust of solar wind is expected to buffet Earth's magnetic field and trigger moderate geomagnetic disturbances over the next few days.

"The magnetic fields around coronal holes are different," he continued. "Instead of looping back to reconnect on the sun's surface, these magnetic fields are essentially open. They go way out into the solar system and no one knows exactly where they reconnect. These open field lines allow high-speed solar wind particles to escape."

In fact, the solar wind streams off of the Sun in all directions, not just from coronal holes. But the wind speed is high (800 km/s) over coronal holes and much lower (300 to 400 km/s) elsewhere. The higher pressure streams from coronal holes buffet the Earth's magnetic field and can produce geomagnetic activity.

For more information about space weather and current solar activity, including official alerts, warnings, and forecasts, please see NOAA's Space Environment Center web site at http://www.sec.noaa.gov/.

Left: This movie, which can span from 2 to 6 hours of actual time, shows the most recent images of the Earth's auroral region taken from space by the Ultraviolet Imager (UVI) Instrument on board the POLAR spacecraft. New movies are generated every 6 hours. Hit reload for the latest animation or visit the UVI aurora home page for new pictures every 7 minutes.

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Author: Dr. Tony Phillips Curator: Bryan Walls NASA Official: Frank M. Rose