Comparative Magnetic Minima: Characterizing Quiet Times in the Sun and Stars

MANDRINI, C.H.; WEBB, D.F.

Cambridge University Press

Año: 2012 p. 460

9781107019867

PrefaceIAU Symposium 286, “Comparative Magnetic Minima: Characterizing Quiet Timesin the Sun and Stars”, was coordinated through Division II, with the strong support ofDivision IV, including several of their associated commissions. It was held in Mendoza,Argentina, from 3 to 7 October 2011, and attracted nearly 100 scientists expert on variouspertinent topics from 23 countries. The goal of the symposium was to consider solar andstellar minima, from generative dynamo mechanisms to in-depth analyses from Sun toEarth for recent well-observed and modeled minima, to a range of stellar cyclic activity, tooutlier “grand minima”. Solar, heliospheric, geospace, atmospheric, stellar, and planetarysciences were included in the meeting’s scope.Solar and stellar minima represent times of low magnetic activity and simple helio/asterospheres. They are, thus, excellent targets for interdisciplinary, system-widestudies of the origins of stellar variability and consequent impacts on planetary systems.The recent solar minimum extended longer and was “quieter” than any we haveobserved in the Space Age, inspiring both scientific and public interest. A rich variety ofsatellite and ground-based observations, in conjunction with theoretical and numericalmodeling advances, have allowed us to probe the peculiarities of this minimum as neverbefore. The implications are far-reaching, connecting Earth to Sun to stars, radio to Xrayto cosmic rays, and the plethora of observations of recent minima to the Sun’s pastbehavior as preserved in cosmogenic isotopes and historical sunspot and auroral records.At the meeting, the keynote talk on “The nature and significance of solar minima” wasgiven by Eric Priest. This was followed by 28 invited, 6 solicited talks and 28 contributedpresentations spread over five sessions: Solar and Stellar Minima, Dynamos and CycleVariability, Comparative Solar Minima from Sun to Earth, Stellar Cycles and GrandMinima, and Historical Records. A closing discussion on whether we are entering a grandminimum was led by Karel Schrijver. Thirty one poster presentations were put up andremained during the entire meeting. A public outreach talk on global warming and solaractivity was given by Pablo Mauas at the end of the symposium.The presentations described how magnetic fields can be cyclically generated in solarand stellar interiors via various dynamo processes. Numerical models have increased incomplexity to the point where many observed aspects of the cycles in the Sun and starsare captured, although mysteries remain such as the origins of extended, or “grand”minima. Both stellar observations and historical and cosmogenic records at the Earthwere presented, forming a basis of understanding of such intervals, and of solar/stellarlong-term variability in general. A simple method to reconcile the Z¨urich Sunspot Numberand the Group Sunspot Number was presented, with important and wide rangingimplications towards an agreed-upon and vetted single sunspot series for use in the future.The recent extended minimum was the lowest and longest minimum in about a century,having weak polar magnetic fields, a complex corona and heliosphere, and recurrent highspeedstreams. Simultaneously, it was found that solar minima do not all look alike, giventhat the Sun can have different magnetic flux configurations even during very quiet times,yielding distinct 3D magnetic flux distributions and, therefore, diverse structure of thecorona and heliosphere. During this recent minimum, the solar magnetic field achieveda solar maximum-like corona and solar wind source situation, but with weak magneticfields and associated weak heating. The discussed results point out the need for textbooksand solar physics educators to revise the way they describe the solar wind and itssources.In addition, the recent minimum provoked discussions on the possibility of a trend inthe Sun’s current magnetic cycles towards a grand minimum and the potential implicationsfor the Earth’s climate. For instance, there is evidence that a strong decrease of solaractivity can lead to a delay of ozone recovery, partially compensating greenhouse warming,and that irradiance variability is the most important forcing for global problems.A combination of the bottom-up and top-down models seems appropriate for radiativesolar forcing of the atmosphere. Although the forcing due to anthropogenic influencesis about seven times larger than the radiative solar forcing, solar activity certainly doesaffect climate, and all relevant observations need to be maintained or extended.The question of the origins and implications of cyclic behavior, for the Sun-Earthsystem and also for other stellar-planetary systems, was the subject of several presentations.For instance, it was shown that induced magnetospheres directly interact withthe solar wind and, therefore, are more prone to atmospheric evolution than intrinsicmagnetospheres.This symposium was undoubtedly unique in the sense that it brought together a diversegroup of scientists that were able to take part in discussions, appreciate the scientific disciplinesof others, and discover the common aspects of the physical processes involved inthe different studied environments from Sun to Earth, and stars to planets. The editorstake this opportunity to thank Germ´an Cristiani and Marcelo L´opez-Fuentes for theirvaluable assistance in preparing this volume. We also are grateful to the following reviewerswho assisted us in improving the papers: Drs. Thomas Ayres, Alisson Dal Lago,Sergio Dasso, Marcelo L´opez-Fuentes, Daniel G´omez, Manuel G¨udel, Gustavo Guerrero,Jeffrey Hall, Margit Haberreiter, Kanya Kusano, Georgeta Maris, Leif Svalgaard, AndreyTlatov, Ilya Usoskin, Adriana Valio, and Alberto V´asquez. Please note that many of thepapers contain color figures, which are printed here in black and white but which can beviewed online in color.Sarah Gibson and Hebe Cremades, co-chairs SOCCristina H. Mandrini, chair LOCCristina H. Mandrini and David F. Webb, Proceedings EditorsBuenos Aires, Argentina, 29 March 2012