Voyager 1 Hits Rumble Strips At the Edge of the Solar System

Something strange is happening to the Voyager 1 spacecraft.

After 35 years of travel, NASA’s oldest interstellar spacecraft is now some 20 billion kilometres from the Sun. It’s easy to imagine that conditions in this isolated region of space would be quiet and calm but on 25 August, the spacecraft’s instruments suddenly went haywire, recording changes unlike anything it has seen throughout its long journey.

Today, Bill Webber at New Mexico State University in Las Cruces and a few pals detail the dramatic changes Voyager has experienced and suggest various explanations for the phenomenon.

Voyager 1 is currently investigating the outer reaches of the Sun’s influence, the region where the solar magnetic field ploughs into the galactic field as the Sun moves through the Milky Way. This creates a magnetic bow wave that traps charged particles in various ways.

One of the important tasks for Voyager is to measure these particles, which fall into two types. The first are protons, electrons and nuclei accelerated to high energies inside our galaxy by processes such as supernovas. These are called galactic cosmic rays. 

The second are similar charged particles accelerated to lower energies by a different mechanism–the Sun’s magnetic field. These are called anomalous cosmic rays and, because they are accelerated by a different mechanism, their intensity changes relative to the galactic cosmic rays.

Indeed it was the intensity of anomalous cosmic rays that changed so dramatically on 25 August. Webber and pals say the intensity of anomalous cosmic rays dropped some 500 times in just a few days. In that time, Voyager 1 travelled only 14 million kilometres.

That’s like falling off a cliff. “The magnitude of this intensity change for anomalous cosmic rays has never previously been observed in the 35 year Voyager 1 mission except for the Jupiter encounter,” say Webber and co.

Clearly, the spacecraft has moved into a region of space that is more or less free of anomalous cosmic rays.

That’s hugely important because it has revealed something entirely new: the low energy galactic cosmic rays that are otherwise drowned by the numbers of anomalous cosmic rays. Webber and co describe this discovery as “one of the holy grails of galactic cosmic rays studies”. 

Another interesting feature of this event is that Voyager 1 experienced temporary drops in the intensity of anomalous cosmic ray particles on 28 July and 14 August.

Webber and co say there could be two different explanations for this. The first is that the boundary that Voyager crossed is uneven and moving at roughly the same speed as the spacecraft. So the probe has moved back and forth across it several times.  

The second possibility is that the boundary has a ribbon-like structure, creating a kind of magnetic rumble strip for the spacecraft to pass over.

Either way, Voyager is in a new region of space that will give space scientists new insights into the nature of interstellar space and the galactic medium.

Long may the mission continue. 

Ref: arxiv.org/abs/1212.0883 :At Voyager 1 Starting On About August 25, 2012 At A Distance of 121.7 AU From the Sun, A Sudden Sustained Disappearance of Anomalous Cosmic Rays and an Unusually Large Sudden Sustained Increase of Galactic Cosmic Ray H and He Nuclei And Electrons Occurred