Vainu Bappu Memorial Lecture: What is a sunspot?

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 two thousand years. They appear as relatively dark patches on the surface of the Sun, and are caused by concentrations of magnetism which impede the flow of heat from deep inside the Sun up to its othewise brilliant surface. The spots are not permanent: the total number of spots on the Sun varies cyclically in time, with a period of about eleven 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 observing the Sun from Kodaikanal a hundred years ago. I shall give you a glimpse of how we are striving to deepen our understanding of these fascinating features, and of some of the phenomena that appear to be associated with them.Comment: Lecture delivered at the Indian Institute of Astrophysics, December 2008 Typing errors corrected To appear in Magnetic Coupling between the Interior and the Atmosphere of the Sun, ed. S.S. Hasan & R.J. Rutten, Astr. Sp. Sci. Pro

Progress report on solar age calibration

We report on an ongoing investigation into a seismic calibration of solar models designed for estimating the main-sequence age and a measure of the chemical abundances of the Sun. Only modes of low degree are employed, so that with appropriate modification the procedure could be applied to other stars. We have found that, as has been anticipated, a separation of the contributions to the seismic frequencies arising from the relatively smooth, glitch-free, background structure of the star and from glitches produced by helium ionization and the abrupt gradient change at the base of the convection zone renders the procedure more robust than earlier calibrations that fitted only raw frequencies to glitch-free asymptotics. As in the past, we use asymptotic analysis to design seismic signatures that are, to the best of our ability, contaminated as little as possible by those uncertain properties of the star that are not directly associated with age and chemical composition. The calibration itself, however, employs only numerically computed eigenfrequencies. It is based on a linear perturbation from a reference model. Two reference models have been used, one somewhat younger, the other somewhat older than the Sun. The two calibrations, which use BiSON data, are more-or-less consistent, and yield a main-sequence age $t_\odot=4.68\pm0.02$Gy, coupled with a formal initial heavy-element abundance $Z=0.0169\pm0.0005$. The error analysis has not yet been completed, so the estimated precision must be taken with a pinch of salt.Comment: 8 pages, 3 figures, in L. Deng, K.L. Chan, C. Chiosi, eds, The Art of Modelling Stars in the 21st Century, Proc. IAU Symp. No. 252, invited contributed pape

On electrostatic screening of ions in astrophysical plasmas

There has been some controversy over the expression for the so-called `interaction energy' due to screening of charged particles in a plasma. Even in the relatively simple case of weak screening, first discussed in the context of astrophysical plasmas by Salpeter (1954), there is disagreement. In particular, Shaviv and Shaviv (1996) have claimed recently that by not considering explicitly in his calculation the complete screening cloud, Salpeter obtained a result for the interaction energy between two nuclei separated by a distance $r$ which in the limit $r \to 0$ is only 2/3 the correct value. It appears that this claim has arisen from a fundamental misconception concerning the dynamics of the interaction. We rectify this misconception, and show that Salpeter's formula is indeed correct.Comment: 17 pages, no figures, AAS Latex, to appear in The Astrophysical Journa