In 2009 it was announced that a mysterious band of charged particles had been found at the edge of the solar system – and finally a convincing theory has been put forward to explain it.
Four years ago in October 2009, NASA’s IBEX (Interstellar Boundary EXplorer) revealed that there was a narrow region of charged particles at the edge of the solar system that came as a shock to scientists – this narrow region had never been theorised to exist and the two Voyager probes that had left the solar system managed to miss this high concentration band entirely, sailing on through “normal density” space. (If you click the image to the left, you can see two dots marked “V1” and “V2” on either side of the central band.)
The Interstellar Boundary of our solar system is the “skin” of the bubble (heliosphere) produced by the solar wind racing away from the Sun and its interaction with the flowing particles of the interstellar medium.
The key to this new theory is a series of complex interactions between particles emanating from the Sun, particles from the interstellar medium and interactions between the magnetic field of the Sun and that of the galaxy that causes the charged particles from the Sun to orbit the magnetic field line in a cloud that then reflects particles back towards the solar system.
I might have to make a new tag for posts like this – IANAPP (for “I Am Not A Particle Physicist). Bear with me as I dredge up A level physics from the back of my memory.
The Solar Wind streaming from the Sun is made up primarily of Hydrogen Ions. Ions are atoms that have had one or more electrons ripped from them, creating a charged particle. On their way out of the Solar System, these hot ions collide with cold atoms, stealing their electrons and forming an Energetic Neutral Atom.
At the edge of the Solar System, most of these ENAs fly off into space never to return. In the region identified by IBEX in 2009, ENAs have their electrons stripped from them again by collisions with other ENA’s, becoming ions again. These ions, now charged, are susceptible to the magnetic field found at the edge of the Solar System, which causes them to bunch up and move randomly.
Some of these charged particles collide with the original ENAs streaming from the solar system, causing the charged particles to steal the electron from the ENA, becoming an ENA for the second time that then re-enter the solar system, causing the bright band seen by IBEX and also causing me a headache.
In 2010 scientists ran a numerical study that got close to this mechanism, modelling the electromagnetic field as a simple mirror that simply reflected the ENA’s straight back into the Solar System, mainly because the particles coming back in to the solar system have about the same energy as the particles being flung out of the solar system. The new 2013 model builds on the 2010 work by adding a holding effect to the mirror model caused by the charged particles essentially orbiting the magnetic field lines.
So this is all very interesting (and by interesting read “headache producing”) but what does this news actually mean?
Well for one thing, it means that the galaxy’s magnetic field is a lot stronger than we previously thought, and that the Sun’s heliosphere protects us from this strong magnetic field in much the same way the Earth’s magnetosphere protects us from the worst of the Sun’s weather. It also suggests that IBEX is detecting particles both inside and outside of the Sun’s sphere of influence, which means that this is something that needs to be looked at again.
Future work on the ribbon could also lead to discoveries about the properties of the local galactic magnetic field as well as its strength and direction.
The new study, “Spatial Retention of Ions Producing the IBEX Ribbon,” by Schwadron and McComas, was published in the Astrophysical Journal on the 4th of February.
Thanks guys for giving me the worst headache yet when posting an article.