UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS
MULLARD SPACE SCIENCE LABORATORY
UCL

PHAS4314/SPCEG012(SS6): Solar Physics

 

The Sun is our nearest star. Its proximity provides heat and light to maintain life on Earth, as well as a unique laboratory to test our theories of stellar evolution and galaxy formation. It is crucial for our survival on Earth to understand the Sun's effect on the near-earth environment and on the climate of the earth.

The course begins with a discussion of the Sun's interior. While it has long been clear that nuclear fusion reactions were essential to supply the observed luminous output, direct observation of these reactions in operation was impossible. The detection of neutrinos allowed the confirmation of these reactions but posed a major problem; the observed flux of neutrinos was three times smaller than predicted by the best solar models. Resolution of this paradox is leading us to a new understanding of the fundamentals of elementary particle physics. In the last decade, major advances in our knowledge of the interior have come from Helioseismology or studies of sound wave propagation throughout the Sun. Sophisticated helioseismology instruments on the ESA/NASA SOHO spacecraft are playing a major role in resolving the neutrino paradox, which is increasingly requiring modifications to particle physics theories. The current understanding of the Sun's interior based on results of the neutrino and sound wave propagation studies will be fully developed in the first part of the course.

The Sun's surface or photosphere has a temperature of around 5700K. However the existence of ionized gas at temperatures of up to 5 Million K (MK) or as high as 50 MK during solar flares, poses another serious question for Solar Physics - namely how an extended atmosphere or corona can exist at temperatures that are so much higher than that of the photosphere? Observations of X-ray and Extreme UV (EUV) emission by space borne instruments are allowing advances towards understanding this difficult problem and we now know that the emergence of magnetic fields generated deep in the interior plays a major part. In addition the corona is subject to violent outbursts of energy and mass in Solar Flares and Coronal Mass Ejections (CMEs). CMEs expel some 10^13 kg of material at speeds of up to 2000 km/s and when directed towards earth can damage near-earth spacecraft and in extreme cases, disrupt power distribution systems on the earth's surface. The Sun also emits a constant stream of gas in all directions which is known as the Solar Wind. This outflow fills the Solar System with plasma, interacts with planets and other solar system bodies and eventually with the gas associated with the rest of our galaxy. It thus forms a giant cavity - the Heliosphere, which extends up to 150 AU from the Sun. Our understanding of the Corona, CMEs and Solar Wind is being enormously advanced by currently operating spacecraft - Yohkoh, SOHO and TRACE, while Ulysses and other spacecraft are continuing to probe the distant Heliosphere. In the second part of the course we explore our current understanding of the physics of the Sun's outer atmosphere and its impact on the Earth and the rest of the solar system.

 

Movie caption - Click here to see movie

This movie shows a journey through the solar atmosphere, from the photosphere seen in white light through the chromosphere/transition region and the corona seen in EUV and X-rays. The bright, complex EUV and X-ray emission seen in the chromosphere and corona can be seen to overlie the dark sunspots - regions of strong magnetic field seen in the photosphere. One of the challenges of solar physics is to understand the connections between these different atmospheric components and how energy is transmitted from the convection zone up through the atmosphere to heat the chromosphere and corona above.

 

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