The newest observations from the Ulysses spacecraft suggest the heliospheric chaos of solar maximum occupies a unique and rather short-lived portion of the solar cycle; the majority of the cycle is dominated by large polar coronal holes and a relatively simple solar wind structure, according to Southwest Research Institute (SwRI) space scientists. As the spacecraft completes its second pass around the sun in its unique high-latitude orbit over the sun's poles, scientists are learning more about the variable behavior of the solar wind over the 11-year solar cycle. During its solar odyssey already 10 years in duration, Ulysses measures, among other things, the solar wind, the plasma of hot ionized gas that flows out from the sun.

Scientists compare solar wind observations collected by the spacecraft Ulysses during two separate polar orbits of the sun, six years apart, at nearly opposite times in the solar cycle. The left data show the sun when it's least active ‹ activity is focused at low altitudes, high speed solar wind prevails and magnetic fields are dipolar. The data on the right, collected near solar maximum, show that the solar winds are slower and more chaotic with fluctuating magnetic fields. Towards the north, the data show a return to the more ubiquitous faster solar winds with magnetic orientation (blue) opposite to that observed in the northern hemisphere over the previous solar cycle (yellow indicates no magnetic field data available yet). Credit: Southwest Research Institute

"The solar wind behaves quite differently during the most active part of the solar cycle," reported Dr. David J. McComas at the Ulysses/Voyager/ACE Heliospheric Workshop on October 15. McComas, who is the principal investigator of a Ulysses experiment called the Solar Wind Observations Over the Poles of the Sun (SWOOPS) and executive director of Space Science and Engineering at SwRI, collaborated with researchers from Los Alamos National Laboratory, NASA's Jet Propulsion Laboratory, and Imperial College, London.

"The solar maximum data expose a remarkably different and more complicated three-dimensional solar wind structure than the simple structure observed throughout most of the solar cycle," continued McComas. During most of the solar cycle, the solar wind at high latitudes is almost uniformly fast, while lower latitudes emit a wind of varying speeds. The polar solar wind at maximum is slower and gustier than at other times. "Another interesting discovery from the most recent data collected since July is that the heliospheric chaos of solar maximum is apparently very short-lived; soon after solar maximum, within a year, we already see a return to a predominantly fast solar wind at high latitudes."

While the Earth's magnetic field reverses only once every 200,000 years or so, the sun's internal magnet flips every 11 years. As it flips, the magnetic field becomes disordered, and the surface of the sun becomes more active. This period of change is called solar maximum because the sun's entire surface sprouts small, short-lived coronal holes and streamers. In addition, bubbles of gas and energy called coronal mass ejections, which shoot away from the sun in every direction, are far more common around solar maximum.

From 1992 through 1997, Ulysses saw strong and unipolar magnetic fields at both poles of the sun. Large, dark, persistent X-ray coronal holes formed in the solar atmosphere over the polar caps. The solar wind structure is simple at solar minimum, with fast, hot flows from polar coronal holes, which remain open over long periods of time. The reversal of the magnetic field has a particularly dramatic effect on the poles. The sun's polar coronal holes shrink and ultimately disappear around solar maximum because the magnetic field is unevenly spread around the surface of the sun.

"The solar wind we observe at all latitudes around solar maximum looks a lot like previous observations near the solar equator, or the ecliptic plane, throughout the solar cycle," said McComas.

"Recent observations of faster solar winds indicate that the sun's dipole has indeed reversed," said McComas. The most recent fast winds are quantitatively as well as qualitatively different from winds observed earlier in this pass. The new data, when statistically compared with data from the first orbit, are virtually indistinguishable from the polar winds observed at solar minimum. The winds are almost uniformly hot and fast and have relatively constant composition and density, indicating that polar coronal holes are beginning to reform within a year of solar maximum.

"Because solar activity remains high, it is not clear whether the northern polar coronal hole forming will grow and stabilize to become the sort of multi-year polar coronal hole observed throughout much of the previous solar cycle," continued McComas. "However, it is clear that the sun can, and in this case, does start to form large polar holes very soon after solar maximum."

Ulysses, launched in 1990 with a gravity assist from Jupiter to create its unique solar polar orbit, is a joint venture of NASA and the European Space Agency.

SwRI is an independent, nonprofit, applied research and development organization based in San Antonio, Texas, with more than 2,700 employees and an annual research volume of more than $315 million.