Model Tracks Storms from the Sun.

Hell hath no fury like a solar storm.

When the sun blows its top, it hurls billions of tons of electrically charged gas into space at speeds up to 2,000 kilometers per second.

Now, a group of astronomers has developed a reliable method for predicting how long it will take these storms to hit Earth. There, they can disrupt satellites, hamper radio communications, and knock out power grids.

Another team has gathered observations that confirm a model of how the sun's outer atmosphere, or corona, manages to store up enough magnetic energy to induce these upheavals.

Both teams reported their findings this week at a meeting of the American Astronomical Society in Stateline, Nev.

The new calculations of solar storms' arrival times relied on observations by two spacecraft. SOHO (Solar and Hellospheric Observatory) stares directly at the sun and measures the speed of massive clouds of electrified gas known as coronal mass ejections. Those clouds that head toward Earth are detected by another spacecraft, called Wind, about an hour before they arrive.

Predicting the travel time for a coronal mass ejection is a tricky business. Once launched from the sun, these clouds must make their way through the solar wind, the vast stream of ions that continuously blows out from the sun. Like the current of a great river acting on a tossed twig, the solar wind tends to pull slower-moving material up to its own speed and hold back material that sets off at a higher speed.

By analyzing the initial and final speeds of 23 coronal mass ejections, as well as their transit times to Earth, Natchimuthuk Gopalswamy of the Catholic University of America in Washington, D.C., and NASA's Goddard Space Flight Center in Greenbelt, Md., and his colleagues determined the amount by which the solar wind sped up or slowed down each storm.

Knowing the speed with which the sun has ejected the particles, astronomers can now estimate the acceleration or deceleration due to the solar wind, Gopalswamy says. That information will allow better predictions of when a coronal mass ejection will reach Earth. There has been no "standard model" for predicting the arrival of solar storms, he notes. "We have established a benchmark."

The original model, which Gopalswamy and his collaborators published in the Jan. 15 GEOPHYSICAL RESEARCH LETTERS, could only calculate arrival times within a window of 18 hours. Its precision was limited by the data from the SOHO spacecraft. Because SOHO lies directly along the line between the sun and Earth, it can only measure one component of the velocity of ejected solar gas.

To improve the model, the researchers reviewed velocity measurements of coronal mass ejections made by earlier spacecraft with a variety of viewing angles. The combination of measurements enabled the team to better estimate the effect of the solar wind on coronal mass ejections both near and far from the sun. As a result, the team has now increased the precision of its predictions to a window of 12 hours, Gopalswamy reported at the meeting.

Ernest Hildner, director of the National Oceanic and Atmospheric Administration's Space Environment Center in Boulder, Colo., says that forecasters in his office hope to use the model to predict magnetic storms "with better precision than we have in the past." He notes that the model could be further improved if follow-up studies determine how variations in the density and speed of the solar wind during the 11-year solar cycle change the arrival time of storms.

Another study described at the meeting focused on the creation of solar eruptions. Researchers have long suspected that the rotation of the sun, combined with the roiling motions of its hot gases, can twist the magnetic fields rooted in the solar surface and impart tremendous energy to them. Unless the power is stored and suddenly released, however, the energy will leak away and an eruption might never occur, notes Spiro K. Antiochos of the Naval Research Laboratory in Washington, D.C.

Antiochos, Guillaume Aulanier, also of the Naval lab, and other researchers have proposed that an overlying magnetic field in the corona, acting like the lid of a pressure cooker, permits the energy to build up. For an explosion to take place, the lid must develop a weak point. The researchers suggested that this may occur if the overlying magnetic field lies adjacent to a field of opposite polarity and somewhere along the border, the magnetic field drops to zero.

Observations with the TRACE (Transition Region and Coronal Explorer) satellite have now confirmed this model, says Edward E. DeLuca of the Smithsonian Astrophysical Observatory in Cambridge, Mass. In 1998, TRACE detected a flow of ionized gas along a region of high-altitude magnetic activity in the corona, exactly where Aulanier's team predicted a weak point should occur.

The flow of gas "is a telltale signature" that the lid has weakened, DeLuca says. About 2 minutes after the flow began, a flare erupted, indicating that the lid had indeed been pushed aside, he notes.

Antiochos says the same magnetic process may induce the flares that have been observed on other stars.