On July 25, 2014, Prof. Subramaniam Ananthkrishnan, from the University of Pune, India, gave a lecture at NAOC exploring the topics of sunspots and space weather. He first gave an overview of how sunspots are formed. Because the Sun rotates but it is not a solid body, different parts of the Sun rotate as different rates, a phenomenon called differential rotation. The magnetic field of the Sun then becomes twisted and tangled. Sunspots appear in places where tangles in the magnetic field trap gas that is cooler than its surroundings, thus appearing black against the photosphere. He went on to explain that astronomers measure the strength of the magnetic field by observing Zeeman splitting and that various observatories around the world, as well as a fleet of satellites in space, continuously monitor solar activity. According to Prof. Ananthkrishnan, researchers have been trying to assemble a set of uniformly calibrated data related to sunspot activity. Umbral magnetic fields have been slowly decreasing, and if these umbral magnetic fields fall below 1500 Gauss, it will be difficult to see sunspots. Prof. Ananthkrishnan continued by saying that cycle 23, the most recent solar cycle, had the slowest rise with the deepest, longest lasting minimum. He also said that extrapolating this behavior forward, by 2020, it should be difficult to see sunspots, though no one really knows what will happen after that. Prof. Ananthkrishnan emphasized that solar activity is very difficult to predict, and all agencies that tried to predict what would happen in cycle 23 were wrong.
Prof. Ananthkrishnan went on to explain how his own field of research is related to monitoring the Sun's magnetic field. He was one of the pioneers of using interplanetary scintillation measurements as a proxy for magnetic field measurements. Activity on the Sun's surface ejects charged particles which stream away from the Sun into interplanetary space, forming the solar wind. As plane waves from point sources of radio, such as quasars, pass through this turbulent medium, the point sources scintillate. These radio waves passing through the interplanetary medium are analogous to light passing through turbulence in the Earth's atmosphere that causes stars to twinkle. By monitoring these changes with radio telescopes, Prof. Ananthkrishnan and his colleagues can track changes in space weather connected to activity on the Sun. These observations will lead to advances in understanding the Sun's dynamic magnetic field.
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