The Sun's solar wind output has fallen to the lowest level since "accurate readings" became available, and the drop may have an effect on the natural shielding which protects the solar system from cosmic rays, NASA reports.
The data comes from the venerable Ulysses spacecraft, the joint NASA-European Space Agency mission launched in 1990* to "to study the sun and its influence on surrounding space". Readings indicate "the solar wind's global pressure is the lowest we have seen since the beginning of the space age", according to Ulysses' solar wind instrument principal investigator, Dave McComas.
Specifically, NASA explains: "In 2007, Ulysses made its third rapid scan of the solar wind and magnetic field from the sun's south to north pole. When the 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 per cent. The field strength near the spacecraft has decreased by 36 per cent."
Ed Smith, NASA's Ulysses project scientist, said: "The Sun cycles between periods of great activity and lesser activity. Right now, we are in a period of minimal activity that has stretched on longer than anyone anticipated."
The upshot of all this is that the the "heliosphere" surrounding our solar system - formed by the charged solar wind particles and which stretches to the "heliopause" where the particles are no longer energetic enough to advance against rival particles from other stars - could shrink.
Smith said: "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."
NASA has a paricular interest in cosmic rays because they're "linked to engineering decisions for unmanned interplanetary spacecraft and exposure limits for astronauts traveling beyond low-Earth orbit". ®
* NASA explains: "The Ulysses spacecraft was carried into Earth orbit aboard space shuttle Discovery on Oct. 6, 1990. From Earth orbit it was propelled toward Jupiter, passing the planet on Feb. 8, 1992. Jupiter's immense gravity bent the spacecraft's flight path downward and away from the plane of the planets' orbits. This placed Ulysses into a final orbit around the sun that would take it over its north and south poles."
Ulysses is finally "succumbing to the harsh environment of space" and is sure to hang up its detectors in the next few months after "four times its expected mission lifetime".