KODAI SCHOOL ON SOLAR PHYSICS
919(2007); http://dx.doi.org/10.1063/1.2756781View Description Hide Description
The equations of stellar structure and evolution need to be solved to model the interior of the Sun. Modelling the Sun is, however, slightly more complicated than modelling other stars since we have more observational constraints for the Sun than for other stars. In this article we discuss the basics of modelling the interior structure of the Sun. We also discuss the inputs needed to construct such a model, and how we test whether the resulting solar model is correct.
919(2007); http://dx.doi.org/10.1063/1.2756782View Description Hide Description
Helioseismology is the study of solar interior using frequencies of solar oscillations. Frequencies of nearly half a million resonant modes of oscillations have been measured. Each of these mode is trapped in a different region of the solar interior and hence its frequency is sensitive to structure and dynamics in the corresponding region. Conversely, by combining the information from these large number of independent modes of solar oscillations it is possible to infer the structure and dynamics in most of the solar interior. These seismic data have provided a test for solar models and input physics like the equation of state, opacities and nuclear reaction rates. Some of the dynamical phenomenon inferred from these data are not yet understood. Some of these developments are described.
919(2007); http://dx.doi.org/10.1063/1.2756783View Description Hide Description
The cyclically varying magnetic field of the Sun is believed to be produced by the hydromagnetic dynamo process. We first summarize the relevant observational data pertaining to sunspots and solar cycle. Then we review the basic principles of MHD needed to develop the dynamo theory. This is followed by a discussion how bipolar sunspots form due to magnetic buoyancy of flux tubes formed at the base of the solar convection zone. Following this, we come to the heart of dynamo theory. After summarizing the basic ideas of a turbulent dynamo and the basic principles of its mean field formulation, we present the famous dynamo wave solution, which was supposed to provide a model for the solar cycle. Finally we point out how a flux transport dynamo can circumvent some of the difficulties associated with the older dynamo models.
919(2007); http://dx.doi.org/10.1063/1.2756784View Description Hide Description
New high‐resolution observations reveal that small‐scale magnetic flux concentrations have a delicate substructure on a spatial scale of 0.1″. Their basic structure can be interpreted in terms of a magnetic flux sheet or tube that vertically extends through the ambient weak‐field or field‐free atmosphere with which it is in mechanical equilibrium. A more refined interpretation comes from new three‐dimensional magnetohydrodynamic simulations that are capable of reproducing the corrugated shape of magnetic flux concentrations and their signature in the visible continuum. Faculae are another manifestation of small‐scale magnetic flux concentrations. It is shown that the characteristic asymmetric shape of the contrast profile of faculae is an effect of radiative transfer across the rarefied atmosphere of the magnetic flux concentration. Also discussed are three‐dimensional radiation magnetohydrodynamic simulations of the integral layers from the top of the convection zone to the mid‐chromosphere. They show a highly dynamic chromospheric magnetic field, marked by rapidly moving filaments of stronger than average magnetic field that form in the compression zone downstream and along propagating shock fronts. The simulations confirm the picture of flux concentrations that strongly expand through the photosphere into a more homogeneous, space filling chromospheric field. Future directions in the simulation of small‐scale magnetic fields are indicated with a few examples from recent reports.
The second part of these lecture notes is devoted to a few basic properties of magnetic flux tubes that can be considered to be an abstraction of the more complicated flux concentrations known from observations and numerical simulations. By analytical means we will find that an electrical current flows in a sheet at the surface of a flux‐tube for which location we also derive the mechanical equilibrium condition. The equations for constructing a magnetohydrostatic flux tube embedded in a gravitationally stratified atmosphere are derived. It is shown that the expansion of a flux tube with height sensibly depends on the difference in the thermal structure between the atmosphere of the flux tube and the surrounding atmosphere. Furthermore, we will find that radiative equilibrium produces a smaller temperature gradient within the flux tube compared to that in the surrounding atmosphere. The condition for interchange stability is derived and it is shown that small‐scale magnetic flux concentrations are liable to the interchange instability.
919(2007); http://dx.doi.org/10.1063/1.2756785View Description Hide Description
The heating of solar atmosphere from chromosphere to corona is one of the key fundamental and yet unresolved questions of modern space and plasma physics. In spite of the multi‐fold efforts spanning over half a century including the many superb technological advances and theoretical developments (both analytical and computational) the unveiling of the subtle of coronal heating still remains an exciting job for the 21st century! In the present paper I review the various popular heating mechanisms put forward in the existing extensive literature. The heating processes are, somewhat arbitrarily, classified as hydrodynamic (HD), magnetohydrodynamic (MHD) or kinetic based on the model medium. These mechanisms are further divided based on the time scales of the ultimate dissipation involved (i.e. AC and DC heating, turbulent heating). In particular, attention is paid to discuss shock dissipation, Landau damping, mode coupling, resonant absorption, phase mixing, and, reconnection. Finally, I briefly review the various observational consequences of the many proposed heating mechanisms and confront them with high‐resolution ground‐based and satellite data currently available.
919(2007); http://dx.doi.org/10.1063/1.2756786View Description Hide Description
The solar chromosphere is very dynamic, due to the presence of large amplitude hydrodynamic waves. Their propagation is affected by NLTE radiative transport in strong spectral lines, which can in turn be used to diagnose the dynamics of the chromosphere. We give a basic introduction into the equations of NLTE radiation hydrodynamics and describe how they are solved in current numerical simulations. The comparison with observation shows that one‐dimensional codes can describe strong brightenings quite well, but the overall chromospheric dynamics appears to be governed by three‐dimensional shock propagation.
919(2007); http://dx.doi.org/10.1063/1.2756787View Description Hide Description
Understanding the solar activity is a fundamental problem which has essentially led to create modern solar physics. The Sun’s magnetic field and differential rotation give rise to much complexity, in particular, to solar activity over a large range in both spatial and temporal scales. Explosive transient events occur in the solar atmosphere in shorter time‐scales of minutes to hours, such as, solar flares and coronal mass ejections (CMEs). At the longer scales, the most notable is the solar activity cycle of 11 years, or magnetic cycle of 22 years. Understanding of the solar activity is important as it affects the space weather, i.e., the interplanetary medium and geo‐magnetic environment. We present an account of the recent developments in our understanding of these phenomena, using both space‐borne and ground‐based observations.
919(2007); http://dx.doi.org/10.1063/1.2756788View Description Hide Description
The lectures present the foundation of solar coronal physics with the main emphasis on the MHD theory and on wave and oscillatory phenomena. We discuss major challenges of the modern coronal physics; the main plasma structures observed in the corona and the conditions for their equilibrium; phenomenology of large scale long period oscillatory coronal phenomena and their theoretical modelling as MHD waves. The possibility of the remote diagnostics of coronal plasmas with the use of MHD oscillations is demonstrated.
919(2007); http://dx.doi.org/10.1063/1.2756789View Description Hide Description
We discuss the energy balance of the upper solar atmosphere and the solar wind, showing how the properties of the corona and solar wind are influenced by the three energy loss mechanisms available to the solar atmosphere; the solar wind, heat conduction, and radiation. We show that the Sun must have a corona with temperature of order 106 K irrespective of what heats the upper atmosphere, and that such a hot corona is a prerequisite for the solar wind. Using a simple, isothermal fluid model of the solar wind we demonstrate how the solar wind mass flux depends sensitively on the coronal temperature. Finally we discuss briefly the encounter of the solar wind with the interstellar gas, showing how the requirement of force balance across the termination shock can be used to estimate the distance to the shock.
919(2007); http://dx.doi.org/10.1063/1.2756790View Description Hide Description
Solar flares, coronal mass ejections (CMEs), solar energetic particles (SEPs), and fast solar wind represent the energetic phenomena on the Sun. Flares and CMEs originate from closed magnetic field structures on the Sun typically found in active regions and quiescent filament regions. On the other hand, fast solar wind originates from open field regions on the Sun, identified as coronal holes. Energetic particles are associated with flares, CMEs, and fast solar wind, but the ones associated with CMEs are the most intense. The energetic phenomena have important consequences in the heliosphere and contribute significantly to adverse space weather. This paper provides an over view of the energetic phenomena on the Sun including their origin interplanetary propagation and space weather consequences.
919(2007); http://dx.doi.org/10.1063/1.2756791View Description Hide Description
Solar radio observations from both ground‐based and space‐based have provided a wealth of data on various solar phenomena. In the recent years, particularly, coordinated studies using solar radio measurements and multi‐wavelength data have provided a deeper understanding of solar flares and coronal mass ejections and the physics behind the fundamental processes of the radio emission mechanism. This chapter has two parts. In the first part, an overview of radio observations of quiet and active Sun is presented. The physics of explosive energy release is discussed. In the next part, the disturbances resulting from the solar phenomena and their heliospheric evolution in space and time is reviewed based on radio scintillation technique.