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Monday 31 December 2012

The Sun's Bubble

This guest post is by Roger Duthie, Doctoral Candidate in Space Plasma Physics at the Mullard Space Science Laboratory.

In the late 1970s, Nasa’s Voyager probes began an epic journey through the solar system to explore the outer planets (Jupiter, Saturn, Neptune & Uranus). The “in-situ” measurements sent back from Neptune and Uranus by Voyager 2 are still the only local observations of the magnetospheres of these ice giants. But what is so very special and unique about the Voyager missions is that they are still operational over 35 years on and they are heading out of the solar system into new territory within our galaxy (the Milky Way) at large.

Image courtesy NASA/JPL-Caltech
NASA's Voyager probes are the furthest man-made items from the Earth.
Image Courtesy NASA/JPL-Caltech.
Image copyright: ESA
The Earth's magnetosphere is also important for keeping
ahold of the planet's atmosphere. Image credit: ESA
The solar system of eight planets, plus many minor planets, asteroids and comets, is bathed in plasma ("solar wind") streaming outwards from our single, central star (the Sun). The Sun also provides a magnetic bubble in the form of the heliosphere, much like the magnetosphere which Earth possesses. The Earth's magnetosphere is the region around the planet where its' magnetic field dominates, and to some extent it protects the Earth from the impinging solar wind; the heliosphere plays the same role for the entire solar system, diverting the plasma of our galaxy's interstellar medium (the ISM). Where we are able to extensively probe and measure the structure of the Earth's magnetosphere and interaction with the solar wind with satellites and even observations made from the ground, the outer reaches of the heliosphere are the sole domain of the Voyager spacecraft. It happens that they are both heading out towards the 'nose' of the heliosphere, one into the northern part and one to the southern. The nose is the direction facing upstream within the flow of the ISM; in the downstream direction is the heliosphere's tail.

Image courtesy NASA/JPL-Caltech
The Heliosphere is a magnetic bubble created by our Sun. Image courtesy NASA/JPL-Caltech.
Image Credit NASA/Goddard Space Flight Center Conceptual Image Lab
The Interstallar Boundary Explorer, or IBEX.
Image courtesy NASA/Goddard Space Flight Center.
Well, it isn't strictly true that we need to stick a probe somewhere to see what's going on. In the same way that we can produce images of Neptune with the Hubble Space Telescope (and other telescopes), we can use remote sensing techniques to observe structure at the heliosphere's edge (the "heliopause"). One mission which was designed to do just this is Nasa's IBEX spacecraft, which orbits Earth and detects electrically neutral material originating from the heliopause. Such material has a clear path from these distant regions; due to its electrical neutrality it is not affected by the magnetic field of the heliosphere. IBEX discovered what looks like a 'ribbon' around the heliosphere. This data helps scientists say something about the magnetic field and plasma of the regions outside the heliopause without having to make direct in-situ measurements. The ribbon is thought to indicate the direction of the magnetic field in the ISM and also tell us that the magnetic field dominates over the effect of plasma flow. These have big implications for the predicted structure of the heliosphere and the local ISM. However, predictions are all that can be made until direct observations of the structure can be taken. The Voyager spacecraft are currently thought to be in a region called the heliosheath, which is a transition between the solar wind and the ISM and new things are being learnt about this environment as more data is taken by the probes. What we currently know of the structure of the ISM environment beyond the heliosheath is down to the data taken by remote sensing and predictive modelling.

Image courtesy NASA/Goddard Space Flight Center
The "ribbon" detected by IBEX is believed to be where solar wind particles heading out into interstellar space are reflected back into the solar system by a galactic magnetic field. Image courtesy NASA/Goddard Space Flight Center.
With only two spacecraft in the vicinity of the nose of the heliopause and what we have of remote sensing observations, only a few pieces of the puzzle are presented. We also have many observations of the Earth's magnetosphere, some from Mercury, Saturn & Jupiter, and scant few from Uranus and Neptune. The planets Mars and Venus do not have internally generated magnetic fields, so don't possess proper ('intrinsic') magnetospheres, though study of the interaction of magnetic fields and plasma in their ionospheres, and also around objects such as comets, can help answer bigger questions. One thing we can say is that every magnetosphere has its own characteristics and idiosyncrasies; what they might be able to do is help fill in the gaps in our data coverage for the outer heliosphere.

At some point the Voyager probes will exit into the ISM proper. Only time will tell what secrets the galaxy has for us. There are currently no other probes which will be able to give us direct measurements of the heliosphere away from the nose to confirm predictions of the structure of (for example) the tail region. There certainly is much we don't know about the small part of the universe in which we abide - understanding which will become more important once we attempt to exploit more of our solar system in the future.

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