14 June 2013 13:50

When it comes to natural phenomena that are right before your eyes, the Sun is pretty hard to miss. Variously revered, feared and studied, humankind has sought to understand the Sun for millennia. But even though astronomy can be said to be one of the oldest scientific disciplines, it has been advances in astrophysics over the last 150 years that have revealed the true nature of our nearest star.

In the late nineteenth century, the mechanisms that generated the Sun’s tremendous output of heat and light were still unclear. It took the development of quantum physics in the early nineteen-hundreds for nuclear fusion to emerge as a viable energy generation mechanism. Likewise, the processes by which solar activity could trigger geomagnetic storms here on Earth were not understood until twentieth century advances in electromagnetism and atomic physics hinted at a link mediated by sub-atomic particles and electromagnetic fields.

Our view of the Sun was further transformed by the advent of the space-age. Lofted above the Earth’s protective atmosphere, telescopes able to image the Sun in wavelengths of light inaccessible from the ground have been snapping away for decades. They have revealed the Sun to be a gloriously complex and dynamic body and not the unchanging yellow-white disk that most of us are familiar with. Early space missions also revealed that space was not empty. Instead, the outer atmosphere of the Sun was found to be continuously expanding, filling the solar system with a “solar wind” of particles and the remnants of the Sun’s magnetic field.

But unlike many other objects in the solar system, we have never sent a probe to explore its surface. Humankind has never visited the Sun. The reasons why are not surprising. Firstly, as a giant ball of gas and plasma, the Sun doesn’t have a “surface” as such. What human eyes perceive as the surface is the outermost visible layer of the Sun, known as the photosphere. Above this lie the chromosphere and corona, outer layers of the solar atmosphere that are invisible to us except during brief solar eclipses. But the main problem of sending a probe to the Sun is the obvious one. The Sun is hot. Really hot. Light coming from the Sun reveals that the photosphere is a toasty 5,500° C, falling to a relatively cool 3,900° C in the chromosphere before rising to around 2,000,000° C in the corona.

This counter-intuitive behaviour, where the temperature in the outer layers of the solar atmosphere first falls but then rises dramatically, is something that solar physicists would love to explain. The Sun’s massive magnetic field is almost certain to play a crucial role in heating the corona, but magnetic fields are hard to study from a distance. Unlike light, which travels through space and can be remotely-sensed by a telescope, magnetic fields in the corona ideally need to be studied using in situ detectors. Needless to say, building a probe that could survive such an environment presents massive technical challenges.

Fortunately, Mother Nature has her own fleet of solar probes.

In December 2011 comet C/2011 W3, also known as “Lovejoy”, flew deep within the solar corona, coming within 140,000 km of the visible solar surface. This took comet Lovejoy more than forty times closer to the surface than NASA’s planned Solar Probe Plus - the most advanced probe due to fly to the Sun in 2018. Although ultimately fatal to this icy wanderer of the solar system, the fly-by offered an unparalleled opportunity for scientists to study the innermost region of the solar corona. A paper published in Science last week (Downs et al., Science, 1196 (2013); DOI: 10.1126/science.1236550) demonstrated that material subliming from the comet’s surface can act as a marker, revealing the geometry of the coronal magnetic field.

Oxygen from water ice that makes up the comet is broken down in the solar atmosphere to form electrically-charged oxygen ions which become attached to the local magnetic field, rather like beads being threaded onto a string. As they move along the field, the oxygen ions emit high-energy UV light that space-based telescopes can detect. Crucially, the oxygen ions can be tracked remotely and serve as tracers, much like the visible smoke particles introduced into a wind tunnel, or the radioactive dyes exploited in medical imaging. Wiggles in the comet’s tail betray the configuration of the solar magnetic field that permeates the corona - information that cannot usually be obtained remotely.

Downs and co-workers used measurements from three space-based telescopes, each with a different view of the encounter, to test state-of-the-art computer models of the solar atmosphere. Such models have radically advanced solar physics, but observational data from inaccessible regions are still required to compare with the models and validate our understanding of the physics. Data from the Lovejoy fly-by enabled the magnetic field and plasma properties in the lower corona during the fly-by to be constrained and assess which of the models was more realistic.

These exciting results are especially impressive since the team responsible were exploiting a “free” solar probe! And the icing on the cake? The frequency of sun-grazing comets is predicted to increase in the coming decade. Further opportunities to unlock some of the Sun’s remaining secrets are just around the corner.

References:

  • “Probing the Solar Magnetic Field with a Sun-Grazing Comet”, Cooper Downs, Jon A. Linker, Zoran Mikić, Pete Riley, Carolus J. Schrijver, and Pascal Saint-Hilaire, Science 7 June 2013: 340 (6137), 1196-1199. [DOI:10.1126/science.1236550]

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Professor Jim Wild teaches on our BSc Physics, Astrophysics and Cosmology programme.

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