linkages between the atmosphere on the Earth and sun Phần 2

Solar Physics at the Kodaikanal Observatory: A Historical Perspective 35 Other research areas of study include the following: – Oscillation in the chromospheric network – Solar cycle variations and synoptic observations of solar activity – Dynamics of the solar corona and coronal holes – Sunspots and local helioseismology – Solar interior – Coronal mass ejections 7 Future Programmes 7.1 National Large Solar Telescope The National LargeSolar Telescope (NLST) will be a state-of-the-art2m class tele-scope for carrying out high-resolution studies of the solar atmosphere. Sites in the Himalayanregionataltitudesgreaterthan4,000mthathaveextremelylowwaterva-por contentandare unaffectedby monsoonsare underevaluation.This projectis led by the Indian Institute of Astrophysics and has national and international partners. Its geographical location will fill the longitudinal gap between Japan and Europe and is expected to be the largest solar telescope with an aperture larger than 1.5m till the 4m class Advanced Technology Solar Telescope (ATST) and the European Solar Telescope (EST) come into operation. NLST is an on-axis alt-azimuth Gregorian multi-purpose open telescope with the provision of carrying out night time stellar observations using a spectrograph at the Nasmyth focus. The telescope utilizes an innovative design with low number of reflections to achieve a high throughput and low polarization. High order adaptive optics is integrated into the design that works with a modest Fried’s parameter of 7cm to give diffraction limited performance. The telescope will be equipped with a suite of post-focus instruments, including a high-resolution spectrograph and a polarimeter. A small (20cm) auxiliary telescope will provide full disk images. The detailed concept design of the telescope is presently being finalized. First light is expected in 2013. 7.2 Space Coronagraph Avisibleemissionlinecoronagraphthatusesaninnovativedesigntosimultaneously obtain images of the solar corona in the Fe XIV green emission line at 530.3nm and the Fe X red line at 637.4nm is under development. The mission is capable of taking images in the visible wavelength range covering the coronal region between 1.05 and 3 solar radii with a frequency of 4Hz using an efficient detector. High cadence observations in the inner corona are important to understand the rapidly 36 S.S. Hasan et al. varying dynamics of the corona as well as to study the origin and acceleration of CMEs. There are currently no such payloads planned for the near future. This 20cm space coronagraph, which will be executed under the leadership of the Indian Institute of Astrophysics, is planned for launch in 2012. It will obtain simultaneous images of the solar corona in the green and red emission lines simul-taneously with a field of view between 1.05 and 1.60 solar radii to (1) study the dynamics of coronal structures; (2) map the linear polarization of the inner corona; and (3) monitor the development of CME’s in the inner corona by taking coronal images with high cadence up to 3 solar radii. The large telemetry capability of the dedicated mission will permit a monitoring of CMEs for about 18h a day. This project with several national partners has been accepted in principle by the Indian Space Research Organization. Acknowledgment This article draws heavily on unpublished material from the IIA archives. We are grateful to Dr. Christina Birdie for her help in making the above material available to us and to Dr. Baba Varghese for his help with the figures. References Penn, et al. 2003, ApJ, 590, L119 St. John, C. E. 1913, ApJ, 37, 322 Vainu Bappu Memorial Lecture: What is a Sunspot? D.O. Gough Abstract Sunspots have been known in the West since Galileo Galilei and Thomas Harriot first used telescopes to observe the Sun nearly four centuries ago; they have been known to the Chinese for more than 2,000 years. They appear as relatively dark patches on the surface of the Sun, and are caused by concentrations of mag-netism, which impede the flow of heat from deep inside the Sun up to its otherwise brilliant surface. The spots are not permanent: the total number of spots on the Sun varies cyclically in time, with a period of about 11 years, associated with which there appear to be variations in our climate. When there are many spots, it is more dangerous for spacecraft to operate. The cause of the spots is not well understood; nor is it known for sure how they die. Their structure beneath the surface of the Sun is in some dispute, although much is known about their properties at the surface, including an outward material flow, which was discovered by John Evershed ob-serving the Sun from Kodaikanal a 100 years ago. I shall give you a glimpse of how we are striving to deepen our understanding of these fascinating features, and some of the phenomena that appear to be associated with them. 1 Introduction Sunspots are dark blotches apparent on the surface of the Sun which, under suitable conditions, such as when the Sun is seen through a suitably thin cloud, can some-times be seen with the naked eye. Reports from China date back more than 2,000 years, but in the West the history is less clear. It is likely that the pre-Socratic Greek philosopherAnaxagoras observed sunspots with the naked eye, and there have been scattered reports of sightings in the literature since. In 1607, Johannes Kepler tried to observe with a camera obscura a transit of Mercurythat he had predicted,and did D.O. Gough () Institute of Astronomy, University of Cambridge, UK and Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior 37 and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-02859-5 4, Springer-Verlag Berlin Heidelberg 2010 38 D.O. Gough Fig.1 OntheleftisHarriot’s sunspot drawing ofDecember 1610. Ontheright isoneofasequence of drawings by Galileo, which demonstrates the rotation of the Sun; the rotation is very clearly displayed when the drawings are projected in quick succession, as in a movie. It is then evident that the axis of rotation is diagonal in the image: from bottom left to top right. It is also evident that the sunspots lie in two latitudinal bands roughly equidistant from the equator indeed see a dark spot that he believed to be Mercury, but it is likely that what he saw was actually a sunspot (Fig.1). The scientific study of sunspots began when Thomas Harriot and Galileo Galilei independentlyobservedtheSunthroughtelescopeslate in1610.Thefollowingyear, DavidFabricius,whohadmadethefirst discoveryofaperiodicvariablestar,namely Mira, together with his son Johannes, also observed spots with a telescope, and published about them in the autumn of that year. They had tracked the passage of the spots across the solar disc, and noticed their reappearance on the eastern limb a dozen or so days after they had disappeared to the west, and inferred that the Sun was rotating, a notion that had already been entertained by Giordano Bruno and Kepler. Christoph Scheiner began a serious study at that time: believing the Sun to be perfect, he attributed the spots to solar satellites, which appeared dark when they passed in front of the disc. In contrast, with the help of his prote´ge´ Benedetto Castelli, who developed the method of projecting the Sun’s image onto a screen where it could be studied in great detail, Galileo inferred that the cloud-like spots were actually on the surface of the Sun, blemishes on what others believed to be a perfectobject, therebycriticizing Scheiner’spremise.The spots were notpermanent features on the surface, nor were their lifetimes all the same. A large spot might last a rotation period or two, after which it disappears, perhaps to be replaced by a spot at a different location. Smaller spots are shorter-lived. Galileo also disagreed with Vainu Bappu Memorial Lecture: What is a Sunspot? 39 Scheiner’s adherence to a geocentric cosmology, having been rightly convinced by Copernicus’s cogent arguments. The two men, though civil at first, subsequently became enemies. Scheiner published a massive book, Rosa Ursina, which became the standard work on sunspots for a century or more. By that time he had at least shed his belief in an unblemished Sun, accepting that the spots were on the Sun’s surface, and by careful measurement of the motion of the spots he was able to ascertain that the axis of the Sun’s rotation was inclined by about 7o to the normal to the plane of the ecliptic. But he continued to uphold his Ptolemaic viewpoint. Further productive work was hampered by a dearth of sunspots throughout the second half of the seventeenth century, an epoch now known as the Maunder Min-imum. Perhaps the most important discovery immediately after that period was by Alexander Wilson in 1769, who realized from the changing appearance of a spot as it approaches the solar limb that the central dark umbra is lower than its surround-ings, a phenomenonnow known as the Wilson depression. 2 Subsequent Milestones of Discovery An extremely important milestone for the whole of astronomy is Joseph von Fraunhofer’s introduction of spectroscopy, which has enabled astronomers to draw conclusionsabout the physical conditionsand chemical compositionof celestial ob-jects,mostnotablytheSun,andtorecognizeandmeasureDopplerwavelengthshifts to determine line-of-sight velocity. We now know from spectroscopy that sunspots are cooler than the surrounding photosphere, more of which I shall discuss later. Fig. 2 Landmarks in sunspot discovery ... - tailieumienphi.vn