Solar system's cosmic ray shield weakens faster than expected

Posted by Emma Woollacott

The 'force field' shielding our solar system from cosmic rays has declined in intensity by 10 to 15 percent over the last six months.

Observations from NASA's Interstellar Boundary Explorer, or IBEX, spacecraft, indicate that the intensity of energetic neutral atoms of hydrogen, ENAs, in the heliosphere is more changeable than previously realized.

"We thought we might detect small changes occurring gradually throughout the sun’s 11-year-long activity cycle, but not over just six months," says David McComas of the Southwest Research Institute, principal investigator for the IBEX mission.

"These observations show that the interaction of the sun with the interstellar medium is far more dynamic and variable than anyone envisioned."

In October 2009, scientists announced that IBEX's first map data revealed an unexpected bright ribbon of ENAs flowing towards the sun from the edge of the solar system. 

It's believed they are formed when particles flowing from the sun collide with the interstellar medium, releasing the ENAs, which stream back towards the centre of the solar system. In the process, they deflect the majority of cosmic rays.

But the latest set of measurements show that the intensity of ENAs has fallen sharply, and that a previously-detected 'hot-spot' has diminished and spread out along the ribbon.

"These variations are taking place on remarkably short timescales," says McComas.

He says the drop-off in intensity between the two all-sky maps perhaps makes sense, because the Sun is only now emerging from an unusually long period of very low activity and a correspondingly weak solar wind.

The IBEX spacecraft was launched in October 2008. The team is hoping it's original two-year mission may be extended, allowing further observations of the heliosheath as solar activity increases over the next few years.