The three Ulysses spacecraft orbits of the Sun. Figure shows radial solar wind velocity and images of the Sun at varying degrees of activity (McComas et al. GRL, 2008)
Solar wind output is at its lowest since accurate records began 50 years ago. This finding comes from the seasoned ESA/NASA solar probe Ulysses, which completed nearly three polar orbits of the Sun from 1993 to 2008 (it is still functioning today, but at a reduced capacity). Although a weakening of the solar wind may not sound very important, the effects of this reduction will have serious implications, diminishing the natural defences of the heliopause (our Solar System’s invisible barrier) which protects us from high energy cosmic rays blasting through intergalactic space…
The heliosphere (NASA/Feimer)
Ulysses has orbited the Sun four times longer than was originally planned. This tough solar satellite was launched in 1990 on board Space Shuttle Discovery, and in 1992, the probe used Jupiter to slingshot it out of the Solar System’s ecliptic to begin taking in situ measurements of solar wind speed and density at all latitudes from pole-to-pole. This is an unprecedented mission that continues to function today. However, Ulysses’ plutonium fuel in its radioisotope thermoelectric generator (RTG) is dwindling to the point where this landmark mission will die from old age over the coming months.
And yet, the geriatric spaceship still reveals characteristics about our Sun that we could never hope to observe confined to the ecliptic plane. So, in (possibly) one of Ulysses’ biggest discoveries to date, scientists have uncovered the strange phenomenon that the solar wind output has decreased to an all-time low (since accurate records began half a century ago), as the Ulysses Principal Investigator explains:
“The Sun’s 1.5 million km-per-hour solar wind inflates a protective bubble around the Solar System and can influence how things work here on Earth and even out at the boundary of our Solar System, where it meets the galaxy. Ulysses data indicate the solar wind’s global pressure is the lowest we have seen since the beginning of the space age.” – Dave McComas, Principal Investigator for the Ulysses solar wind instrument and senior Executive Director at the Southwest Research Institute in San Antonio, Texas.
This “protective bubble” is also known as the heliosphere, a huge volume of space in which all the planets, asteroids and comets are deep inside. It is the total extent of the Sun’s influence, pushing out into interstellar space, the limit of which is known as the heliopause. The heliopause is formed through a balance between the outward pressure of the solar wind and the inward pressure of the interstellar medium, should one of these pressures fluctuate, the heliopause will expand or contract. Should the solar wind pressure decrease, the heliopause will shrink under the greater interstellar medium pressures. This is exactly what Ulysses has detected: a reduction in solar wind pressure.
So what does this mean to us? The heliopause blocks and deflects the majority of damaging high energy interstellar particles (a.k.a. cosmic rays). Should the solar wind weaken, the heliopause will become a less-effective shield, letting more cosmic rays into the Solar System.
“Galactic cosmic rays carry with them radiation from other parts of our galaxy. With the solar wind at an all-time low, there is an excellent chance that 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.” – Ed Smith, NASA’s Ulysses Project Scientist from the Jet Propulsion Laboratory, California.
Artist impression of Ulysses (ESA)
The effects of this happening will be far-reaching and could severely impact the future of manned exploration of the Solar System.
Solar physicists made this discovery when analysing Ulysses data from the probe’s third scan of the solar wind and interplanetary magnetic field (IMF) from the Sun’s north to south poles. On comparison with previous scans, it was found that the solar wind pressure and the radial component of the magnetic field embedded in the solar wind had decreased by 20%. The magnetic field strength surrounding Ulysses had dropped by a huge 36%.
So what could this be attributed to? Physicists simply do not know. Perhaps it might be related to the extended solar minimum in recent months, as Smith appears to suggest. “The sun cycles between periods of great activity and lesser activity,” Smith said. “Right now, we are in a period of minimal activity that has stretched on longer than anyone anticipated.”
Compelling results from a compelling solar mission…
Source: ESA
