Posted: September 24, 2008
The joint NASA/ESA Ulysses spacecraft has confirmed that the Sun’s solar wind output is at its lowest levels since recordings began, an effect that could see our natural shielding to cosmic rays diminish.
The solar wind, a continuous outflow of plasma and magnetic fields from the Sun's atmosphere (corona) into interplanetary space, interacts with every planet in our Solar System and defines the bubble-shaped boundary between the outer edge of the Solar System and interstellar space, known as the heliopause. This boundary marks the point at which the solar wind's strength is no longer great enough to push back the wind of other stars. The region around the heliopause, the heliosphere, acts as a protective shield against a significant portion of the cosmic rays outside the Galaxy.
"The Sun's million mile-per-hour solar wind inflates a protective bubble, or heliosphere, around the Solar System. It influences how things work here on Earth and even out at the boundary of our Solar System where it meets the Galaxy," says Dave McComas, principal investigator of the Solar Wind Observations Over the Poles of the Sun (SWOOPS) experiment. "Ulysses data indicate the solar wind's global pressure is the lowest we have seen since the beginning of the space age."
The heliosphere is a big magentic 'bubble' in space carved out by the solar wind. It defines the sphere of influence of the Sun and extends well beyond the furthest fringes of the Solar System. Image: ESA (C. Carreau).
Since its launch in 1990, Ulysses has completed nearly three polar orbits around the Sun, enabling researchers to observe the three-dimensional structure of the solar wind and heliosphere for the first time. When the latest results were compared with observations from the previous solar cycle, the strength of the solar wind pressure and the magnetic field embedded in the solar wind were found to have decreased by 20 percent, and the field strength near the spacecraft had decreased by 36 percent.
"During the third orbit, we weren't surprised to see a return to a solar minimum configuration," says McComas, "but we were surprised to find that the solar wind is much less powerful than it had been in the previous solar minimum. The wind speed is almost the same, but the density and pressure are significantly lower, and the wind is blowing out about a quarter less hard. To see such a significant and consistent long-term reduction in the solar wind output is really remarkable."
Ulysses scientists say that the strength of the solar wind appears to be determined to a significant degree by where in the solar corona the energy that drives the solar wind is inserted. Energy inserted at low altitudes results in lower pressures and densities, without affecting solar wind speed. Energy inserted at higher coronal altitudes is expected to produce a change in the solar wind speed, which was not observed.
To determine whether the weaker solar wind is a whole-Sun phenomenon, the team compared Ulysses' high-latitude observations with measurements made by the Advanced Composition Explorer spacecraft at low solar latitudes. The data show a close correlation in the dynamic pressure of the wind measured at all latitudes, confirming that the unusually weak outflow is a global phenomenon.
These radial plots of the solar wind speed combine data from all three of Ulysses’ polar orbits of the Sun, each of which take six years to complete. The blue coloured lines represent the outward interplanetary magnetic field (IMF); the red coloured lines the inward IMF. The first orbit occurred during solar minimum and showed slow wind over the equator and a fast wind over the poles. The second orbit showed fast and slow winds at all latitudes, consistent with solar maximum activity. Ulysses has completed more than three quarters of the third orbit, occurring around the current solar minimum cycle. While much of the data gathered thus far is consistent with typical solar minimum activity, surprisingly, it also indicates that the solar wind is about 25 percent less powerful than it was in the previous solar minimum cycle. The Sun’s magnetic field flips approximately every 11 years, which explains the reversal of the red and blue IMF lines in the third orbit. A timeline and line graphs showing sunspot frequency are shown at the bottom for comparison. Image: NASA/SOHO/High-Altitude Observatory.
The effects of the weaker wind will be felt several billion miles beyond the orbit of Pluto, at the farthest limits of the Sun's influence. “If the solar wind is blowing out a quarter less hard, that means the outer boundaries of the heliosphere must be shrinking,” says McComas. “The entire heliosphere must be getting smaller."
If this is the case, then galactic cosmic rays may find it easier to penetrate the inner Solar System. Cosmic rays are of great interest to NASA since they are linked to engineering decisions for unmanned interplanetary spacecraft and exposure limits for astronauts travelling beyond low-Earth orbit.
"Galactic cosmic rays carry with them radiation from other parts of our Galaxy," says Ed Smith, NASA's Ulysses project scientist at the Jet Propulsion Laboratory in Pasadena. "With the solar wind at an all-time low, there is an excellent chance the heliosphere will diminish in size and strength. If that occurs, more galactic cosmic rays will make it into the inner part of our Solar System."
Although this low is unusual, it is not uncommon to see subtle variations in the strength of the solar wind. In the early 1990s, the solar wind weakened for some time and regained strength over the following two years. Further observations will determine whether the solar wind continues to lose strength as the Sun moves from the solar minimum back toward maximum. If the strength of the solar wind stays weak and the heliopause is receeding, Voyager 1, which is headed outside of our Solar System, should reach this boundary earlier than expected and will become the first spacecraft to enter interstellar space.
"The Sun cycles between periods of great activity and lesser activity," says Smith. "Right now, we are in a period of minimal activity that has stretched on longer than anyone anticipated."
Future NASA missions will help resolve some of the intriguing questions brought forth by Ulysses, which survived four years longer than the mission originally intended, and is still hanging on past its nominal end of mission date of 1 July 2008. NASA is scheduled to launch the Interstellar Boundary Explorer (IBEX) spacecraft on 5 October to make the first global images of the outer boundaries of the Solar System. In addition, NASA is about to begin development of the Solar Probe mission to fly in close to the Sun and determine what heats its corona and accelerates the solar wind.
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