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Page 138 of 851

  • Preface

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp xiii-xiii

  • Summary

    Magnetic fields play an important role in many astrophysical processes. They are difficult to detect and characterize because often their properties have to be inferred through interpreting the polarization of the light. Magnetic fields are also challenging to model and understand. Magnetized plasmas behave following highly non-linear differential equations having no general solution, so that every astrophysical problem represents a special case to be studied independently. Hence, magnetic fields are often an inconvenient subject that is overlooked or simply neglected (the elephant in the room, as they are dubbed on posters in the XXV Canary Islands Winter School of Astrophysics). Such a difficulty burdens the research on magnetic fields, which has evolved to become a very technical subject, with many small disconnected communities studying specific aspects and details. The school tried to amend the situation by providing a unifying view of the subject. The students had a chance to understand the behavior of magnetic fields in all astrophysical contexts, from cosmology to the Sun, and from starbursts to AGNs. The school was planned to present a balanced yet complete review of our knowledge, with excursions into the unknown to point out present and future lines of research.

    The subject of Cosmic Magnetic Fields was split into seven different topics: cosmic magnetic field essentials, solar magnetic fields, stellar magnetic fields, the role of magnetic fields on AGN feedback, magnetic fields in galaxies, magnetic fields in galaxy clusters and at larger scales, and primordial magnetic fields and magnetic fields in the early Universe. The corresponding lectures were delivered by seven well known and experienced scientists that have played key roles in the major advances of the field during the last years: F. Cattaneo, P. Judge, O. Kochukhov, R. Keppens, R. Beck, K. Dolag, and F. Finelli. Their lectures were recorded and are freely available at the IAC website: http://iactalks.iac.es/talks/serie/19. Together with the reviews included in the present volume, they form a unique resource for both students and professional researchers. They provide a global view of this very compartmentalized, yet fundamental, field of research.

  • 5 - Magnetic Fields in Galaxies

    • By Rainer Beck, Max Planck Institut für Radioastronomie, Germany
  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 123-160

  • Summary

    Introduction

    Magnetism is a fundamental force with special properties.

  • • Most baryonic matter is ionized. Magnetic fields are easy to generate.

  • • Magnetic monopoles do not exist or are extremely rare. Magnetic fields are hard to destroy.

  • • Magnetic fields need illumination. Magnetic fields are difficult to observe.

    • The scarceness of data leaves many questions on cosmic magnetic fields.

  • • When and how were the first fields generated?

  • • Did significant fields exist before galaxies formed?

  • • How and how fast were the fields amplified?

  • • How did fields affect the evolution of stars, planets, galaxies and galaxy clusters?

  • • How strongly is intergalactic space magnetized?

    • Magnetic fields are often ignored in astrophysics, in particular in models of galaxies and the interstellar medium (ISM), although they are a major agent in the ISM and in galaxy halos, and important for the structure and evolution of galaxies.

  • • Magnetic fields contribute significantly to the total pressure which balances the gas disk of galaxies against gravitation (Fletcher & Shukurov, 2001).

  • • Magnetic turbulence distributes energy from supernova explosions within the ISM (Subramanian, 1998).

  • • Magnetic reconnection is a possible heating source for the ISM and halo gas (Birk et al., 1998).

  • • Magnetic fields increase angular momentum transport and hence the gas inflow rate in barred galaxies (Beck et al., 2005; Kim & Stone, 2012).

  • • Magnetic fields affect the dynamics of the turbulent ISM (de Avillez & Breitschwerdt, 2005) and the gas flows in spiral arms (Gómez & Cox, 2002).

  • • Magnetic fields make the gaseous spiral arms more patchy and drive gas outflows into the halo (Pakmor & Springel, 2013).

  • • The shock strength in spiral density waves is decreased and structure formation is reduced in the presence of strong fields (Dobbs & Price, 2008; Fletcher et al., 2011).

  • • Magnetic fields stabilize gas clouds and reduce the star-formation efficiency to the observed low values (Vázquez-Semadeni et al., 2005; Price & Bate, 2008).

  • • Magnetic fields are essential for the onset of star formation as they enable the removal of angular momentum from the protostellar cloud via ambipolar diffusion (Heitsch et al., 2004).

  • 6 - Primordial Magnetic Fields in the Early Universe and Cosmic Microwave Background Anisotropies

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 161-190

  • Summary

    Introductory Remarks

    Our aim in the talks given at the XXV Canary Islands Winter School of Astrophysics in 2013 was to present models for the generation of primordial magnetic fields in the early Universe and the imprints that these leave in cosmological observables, with an emphasis on Cosmic Microwave Background (CMB) anisotropies.

    Introduction

    The origin of the large-scale magnetic fields (LSMF) observed in galaxies, clusters of galaxies, with also hints of their presence in cosmic voids and on even larger scales in filaments, is an open issue of great importance in modern astrophysics (seeWidrow, 2002, for reviews). Both large-scale and stochastic components are present in magnetic fields observed in galaxies (usually dependent on the morphology of the host) with magnitudes from a few to several microGauss (μG). In clusters of galaxies, stochastic magnetic fields from a few to several μG strength are present with a correlation scale of the order of magnitude of ten kiloparsec (Clarke et al., 2001; Enslin & Vogt, 2006). More recently, the presence of intergalactic magnetic fields even in cosmic voids of the large-scale structure was proposed as a possible explanation for the gamma-ray observations of a couple of blazars (Neronov & Vovk, 2010; Taylor et al., 2011; Vovk et al., 2012; Tavecchio et al., 2010, 2011; Dolag et al., 2011). As a possible explanation for the lack of TeV photons observed in the high-energy spectrum from these blazars a lower bound of 10−18−10−15 G was derived for such pervasive intergalactic magnetic fields.

    In light of this multitude of observations of LSMF of different magnitudes and different coherence lengths, a primordial hypothesis for generating the seed magnetic fields, which are amplified afterwards by adiabatic compression and dynamo during structure formation, is a viable possibility (Widrow, 2002), also taking in consideration the recent observations of strong magnetic fields in galaxies at high redshift (Bernet et al., 2008; Wolfe et al., 2008). Recently, initial seeds motivated as primordial magnetic fields (PMF) have been used in N-body simulations to reproduce magnetic fields in clusters of galaxies (Govoni et al., 2013; Xu et al., 2012).

  • 4 - The Role of Magnetic Fields in AGN Activity and Feedback

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 87-122

  • Summary

    Abstract

    Active galactic nuclei (AGNs), the luminous, compact core regions of galaxies where accretion occurs onto supermassive black holes, can dramatically influence their entire host galaxy evolution by a process referred to as AGN feedback. Energy feedback to the galaxy is the result of combined radiation fields and directed outflows, and especially radio-loud active galaxies show pronounced jets and lobes. Their synchrotron radio emission indicates that dynamically important magnetic fields are at play in AGN jet collimation, stability, energy transfer to the intergalactic medium and their overall morphological appearance. Current knowledge on the launching mechanisms for such highly energetic relativistic jets, as well as the near black-hole accretion processes themselves, all invoke magnetic fields as active agents in angular momentum, mass and energy redistributions. In this review, we cover aspects of AGN feedback and the role played by magnetic fields, almost necessarily studied at vastly different length and timescales. We emphasize how typical large-scale galaxy interaction studies rely on parametric prescriptions for feedback, while detailed dedicated studies for near black-hole dynamics and relativistic jet propagation exist which take full account of magnetic field influences. We discuss representative hydro to magnetohydrodynamic (MHD) numerical simulations that exploit analogies with less energetic X-ray binary sources or even protostellar accretion-ejection systems, emphasize relativistic MHD descriptions, and point out that magnetic fields in accretion disks yield many linear instability routes to turbulence that have scarcely been recognized in the astrophysical community. In combination, they serve to show that magnetic field influences in AGN accretion, jet launch, energy feedback, and overall evolution are still far from completely understood, although many aspects have been disclosed by advanced analytical and numerical relativistic MHD studies.

    Motivation: Astrophysical Jets

    Radio galaxies confront us with dramatic views on energy redistributions at all scales, as mediated by central massive black holes lurking in their nucleus. A clear example is provided by the elliptical galaxy NGC5532, a nearby (red shift z = 0.0237, type S0) galaxy where the stellar distribution is in sharp contrast with its double-jetted appearance in radio images.

The International Atlas of Mars Exploration
  • The First Five Decades
  • Volume 1, 1953 to 2003
  • Philip J. Stooke
  • Published online:
    23 March 2018
    Print publication:
    24 September 2012
  • Covering the first five decades of the exploration of Mars, this atlas is the most detailed visual reference available. It brings together, for the first time, a wealth of information from diverse sources, featuring annotated maps, photographs, tables and detailed descriptions of every Mars mission in chronological order, from the dawn of the space age to Mars Express. Special attention is given to landing site selection, including reference to some missions that were planned but never flew. Phobos and Deimos, the tiny moons of Mars, are covered in a separate section. Contemporary maps reveal our improving knowledge of the planet's surface through the latter half of the twentieth century. Written in non-technical language, this atlas is a unique resource for anyone interested in planetary sciences, the history of space exploration and cartography, while the detailed bibliography and chart data are especially useful for academic researchers and students.

  • 1 - Space Age Studies of Planetary Rings

  • from I - Introductory Material
  • Edited by Matthew S. Tiscareno, Carl D. Murray, Queen Mary University of London
  • Book:
    Planetary Ring Systems
    Published online:
    26 February 2018
    Print publication:
    22 March 2018, pp 3-29

  • Summary

    INTRODUCTION: THE ALLURE OF THE RINGED PLANETS

    One of the most enduring symbols of space exploration is a planet surrounded by a ring. This symbol inspires a celestial context: nothing on Earth is like it. It has been a wonderful surprise that the ringed planets are just as beautiful and scientifically compelling seen close up. Furthermore, the ringed planets are not just objects of beauty, but complicated physical systems that provide a local laboratory and analogy for other cosmic systems like galaxies and planet-forming disks. For a general review, see Esposito (2014). For more details, see the individual chapters that follow in this book.

    We now know that planetary rings, once thought unique to the planet Saturn, exist around all the giant planets. These rings are not solid objects, but are composed of countless particles with sizes from specks of dust to small moons. For each planet, the rings are quite different. Jupiter's ring is thin and composed of dust-like small particles. Saturn's rings are broad, bright, and opaque. Uranus has narrow, dark rings among broad lanes of dust that are invisible from Earth. Neptune's rings include incomplete arcs restricted to a small range of their circumference. All rings lie predominantly within their planet's Roche limit, where tidal forces would destroy a self-gravitating fluid body. They are also within the planet's magnetosphere and, in the case of Uranus, they are within the upper reaches of the planetary atmosphere.

    The common occurrence of ring material around the outer planets is one of the major scientific findings of the past 40 years. The new ring systems were discovered by both spacecraft and ground-based observers, often surprising us by contradicting our expectations. The rings’ appearance and composition differ among the various planets, and likewise within each ring system. The broadest set of rings and the most identified processes are found around the planet Saturn, which has been scrutinized by the US/European Cassini space mission since 2004.

  • 3 - The Rings of Saturn

  • from II - Ring Systems by Location
    • By J. N. Cuzzi, NASA Ames Research Center Moffett Field, California, USA, G. Filacchione, INAF-IAPS Institute for Space Astrophysics and Planetology Rome, ITALY, E. A. Marouf, San Jose State University San Jose, California, USA
  • Edited by Matthew S. Tiscareno, Carl D. Murray, Queen Mary University of London
  • Book:
    Planetary Ring Systems
    Published online:
    26 February 2018
    Print publication:
    22 March 2018, pp 51-92

  • Summary

    INTRODUCTION

    One could become an expert on Saturn's iconic rings pretty easily in the early 1970s, as very little was known about them beyond the distinction between the A, B, and C rings, and the Cassini Division or “gap” between rings A and B (Alexander, 1962; Bobrov, 1970). Water ice was discovered spectroscopically on the ring particle surfaces, and radar and microwave emission observations proved that the particles must be centimeters to meters in size, consisting primarily, not just superficially, of water ice (Pollack, 1975). While a 2:1 orbital resonance with Mimas had long been suspected of having something to do with the Cassini Division, computers of the time were unable to model the subtle dynamical effects that we now know to dominate ring structure.

    This innocent state of affairs was exploded by the Voyager 1 and 2 encounters in 1980 and 1981. Spectacular images revealed filigree structure and odd regional color variations, and exquisitely detailed radial profiles of fluctuating particle abundance were obtained from the first stellar and radio occultations, having resolution almost at the scale of single particles. Voyager-era understanding was reviewed by Cuzzi et al. (1984) and Esposito et al. (1984). While the Voyager data kept ring scientists busy for decades, planning which led to the monumentally successful NASA-ESA-ASI Cassini mission, which arrived in 2004, had been under way even before Voyager got to Saturn. A review of pre-Cassini knowledge of Saturn's Rings can be found in Orton et al. (2009).

    This chapter will build on recent topical and process-specific reviews that treat the gamut of ring phenomena and its underlying physics in considerable detail (Colwell et al., 2009; Cuzzi et al., 2009; Horányi et al., 2009; Schmidt et al., 2009; Esposito, 2010; Tiscareno, 2013b; Esposito, 2014). We will follow and extend the general organization of Cuzzi et al. (2010), the most recent general discussion of Saturn's rings. For brevity and the benefit of the reader, we will frequently refer to the above review articles instead of directly to the primary literature they discuss. We will focus on new work since 2010, within a general context, and will connect our high-level discussions with more detailed chapters in this volume.

Page 138 of 851