Graduate Student, University of California, Santa Cruz
Solar Magnetic Fields and its Dynamics
Sun in famously known as the Rosetta stone of Astronomy and is one of the most extensively studied astronomical objects. It offers the opportunity to understand the physical processes in the interior of stars in a greater detail. Observational, theoretical and numerical studies over the past few decades have significantly improved our understanding but it is astonishing to note that there stands a wide gap between the observations and theoretical understanding. One of the most important and dynamical characteristics of Sun, and in general Stars, is their magnetic field. Sun is a hot ball of plasma (ionized gas) with ubiquitous presence of magnetic field all through the solar atmosphere. One of the important outstanding problems in the study of Sun (and in general stars) is about the origin and sustenance of magnetic fields against diffusion which tends to decay the magnetic fields over a period of time. It is believed that magnetic field formation in the stars takes place by a dynamo process which comprises of generation, transport, decay by diffusion and further re-generation of magnetic fields. A substantial element of all the current potential paradigms for the operation of the solar dynamo is the rise of magnetic “flux tubes” from their point of amplification in the interior of the Sun to the outer atmosphere. Vast majority of work in this regard has focused on the rise of an isolated magnetic entity in an otherwise field-free atmosphere which are quite unrealistic constraints and do not capture the real picture of transport of magnetic fields. Isolated magnetic structures are unrealistic and artificially impose unnecessary constraints on its dynamics. The topology of realistic magnetic fields is much more complex and may have different dynamics. My research focuses on examining the consequences of relaxing these constraints. Even in this extremely simple situation, the initial results are startling. Extending this model to 3D can potentially alter the way transport of flux tubes is accounted in solar dynamo models. Study of rise of magnetic flux tubes also gives an important insight into the surface magnetic fields of Sun which potentially governs all the observed high energy phenomenon like solar flares etc. Study of such high energy phenomenon on the Sun has profound impact on installation of satellites in outer space, electrical grid networks on Earth and even space travel.
Abstract: Motivated by the problem of the formation of active regions from a deep-seated solar magnetic field, we consider the nonlinear three-dimensional evolution of magnetic buoyancy instabilities resulting from a smoothly stratified horizontal magnetic field. By exploring the case for which the instability is continuously driven we have identified a new mechanism for the formation of concentrations of magnetic flux.
Pub.: 29 Jun '07, Pinned: 30 Jun '17
Abstract: By incorporating a large-scale shear flow into turbulent rotating convection, we show that a sufficiently strong shear can promote dynamo action in flows that are otherwise nondynamos. Our results are consistent with a dynamo driven either by the shear-current effect or by a fluctuating alpha effect interacting with the shear, but not with either a classical alpha(2) or alpha omega dynamo.
Pub.: 05 Mar '09, Pinned: 30 Jun '17
Abstract: This review provides an introduction to the generation and evolution of the Sun's magnetic field, summarising both observational evidence and theoretical models. The eleven year solar cycle, which is well known from a variety of observed quantities, strongly supports the idea of a large-scale solar dynamo. Current theoretical ideas on the location and mechanism of this dynamo are presented. The solar cycle influences the behaviour of the global coronal magnetic field and it is the eruptions of this field that can impact on the Earth's environment. These global coronal variations can be modelled to a surprising degree of accuracy. Recent high resolution observations of the Sun's magnetic field in quiet regions, away from sunspots, show that there is a continual evolution of a small-scale magnetic field, presumably produced by small-scale dynamo action in the solar interior. Sunspots, a natural consequence of the large-scale dynamo, emerge, evolve and disperse over a period of several days. Numerical simulations can help to determine the physical processes governing the emergence of sunspots. We discuss the interaction of these emerging fields with the pre-existing coronal field, resulting in a variety of dynamic phenomena.
Pub.: 05 Jul '11, Pinned: 30 Jun '17
Abstract: Selected topics in solar dynamo theory are being highlighted. The possible relevance of the near-surface shear layer is discussed. The role of turbulent downward pumping is mentioned in connection with earlier concerns that a dynamo-generated magnetic field would be rapidly lost from the convection zone by magnetic buoyancy. It is argued that shear-mediated small-scale magnetic helicity fluxes are responsible for the success of some of the recent large-scale dynamo simulations. These fluxes help in disposing of excess small-scale magnetic helicity. This small-scale magnetic helicity, in turn, is generated in response to the production of an overall tilt in each Parker loop. Some preliminary calculations of this helicity flux are presented for a system with uniform shear. In the Sun the effects of magnetic helicity fluxes may be seen in coronal mass ejections shedding large amounts of magnetic helicity.
Pub.: 09 May '09, Pinned: 30 Jun '17
Abstract: The magnetic activity of the Sun, as manifested in the sunspot cycle, originates deep within its convection zone through a dynamo mechanism which involves non-trivial interactions between the plasma and magnetic field in the solar interior. Recent advances in magnetohydrodynamic dynamo theory have led us closer towards a better understanding of the physics of the solar magnetic cycle. In conjunction, helioseismic observations of large-scale flows in the solar interior has now made it possible to constrain some of the parameters used in models of the solar cycle. In the first part of this review, I briefly describe this current state of understanding of the solar cycle. In the second part, I highlight some of the outstanding issues in solar dynamo theory related to the the nature of the dynamo $\alpha$-effect, magnetic buoyancy and the origin of Maunder-like minima in activity. I also discuss how poor constraints on key physical processes such as turbulent diffusion, meridional circulation and turbulent flux pumping confuse the relative roles of these vis-a-vis magnetic flux transport. I argue that unless some of these issues are addressed, no model of the solar cycle can claim to be ``the standard model'', nor can any predictions from such models be trusted; in other words, we are still not there yet.
Pub.: 25 Jun '09, Pinned: 30 Jun '17
Abstract: Latitude-time (butterfly) diagrams of the large-scale solar magnetic field differ appreciably from the butterfly diagrams for sunspots. Tilted features corresponding to waves propagating from the middle latitudes to the equator are virtually absent from the diagrams for the large-scale magnetic field. The latitude-time diagram of the 22-year solar cycle based on data for the large-scale surface field appears as a checkerboard pattern rather than a traveling wave. Solutions describing similar behavior for the poloidal magnetic field are found for Parker’s solar-dynamo equations. These solutions agree with observations especially well if meridional circulation is added to the two sources generating the magnetic-field in this dynamo-differential rotation and mirror-asymmetric convection.
Pub.: 12 Nov '10, Pinned: 30 Jun '17
Abstract: Presented here is a review of present knowledge of the long-term behavior of solar activity on a multi-millennial timescale, as reconstructed using the indirect proxy method. The concept of solar activity is discussed along with an overview of the special indices used to quantify different aspects of variable solar activity, with special emphasis upon sunspot number.Over long timescales, quantitative information about past solar activity can only be obtained using a method based upon indirect proxies, such as the cosmogenic isotopes 14C and 10Be in natural stratified archives (e.g., tree rings or ice cores). We give an historical overview of the development of the proxy-based method for past solar-activity reconstruction over millennia, as well as a description of the modern state. Special attention is paid to the verification and cross-calibration of reconstructions. It is argued that this method of cosmogenic isotopes makes a solid basis for studies of solar variability in the past on a long timescale (centuries to millennia) during the Holocene.A separate section is devoted to reconstructions of strong solar energetic-particle (SEP) events in the past, that suggest that the present-day average SEP flux is broadly consistent with estimates on longer timescales, and that the occurrence of extra-strong events is unlikely.Finally, the main features of the long-term evolution of solar magnetic activity, including the statistics of grand minima and maxima occurrence, are summarized and their possible implications, especially for solar/stellar dynamo theory, are discussed.
Pub.: 21 Mar '13, Pinned: 30 Jun '17