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There are 123 abstracts


Key- or tomb- stones in the bridge from photosphere to corona?

Author(s): Philip G Judge

Institution(s): HAO, NCAR

Abstract:

The Sun's atmosphere must get from photospheric pressures of $10^{5}$ dyn cm$^{-2}$, where gas pressures and Reynolds stresses dominate, to coronal pressures of $10^{-1}$ dyne cm$^{-2}$, where magnetic stresses become dominant, within a mere 2000km. Outside of sunspots, the photospheric boundary layer exhibits hydrodynamic turbulent structure. The low pressure chromosphere- corona transition, while poorly understood, is clearly ordered by magnetic fields. Across the intervening scale heights, the richness of coupled magneto- hydrodynamics in a partially ionized atmosphere out of LTE, within which ions become magnetized, somehow leads to what we observe as the "magnetic chromosphere". We understand only the overall {\em thermal} structure of the chromosphere, it behaves as a thermostat: in response to heating energy is stored in latent heat of ionization and lost to radiation. But in terms of the {\em magnetic} structure, we must deal with: interaction with the plasma including multi-fluid effects, especially ion-neutral damping; effects of stratification on coupling wave modes; the existence of weak, discontinuous solutions to the MHD equations (current sheets). To generate discussion, I will argue that we have little idea what the chromosphere does to the incoming flux of EM energy from beneath, and that current generations of MHD models are far from providing this understanding. We must not let apparent "successes" of, e.g., potential field models on large scales seduce us into thinking we understand how the Sun makes the photosphere-corona transition.




Obtaining Line Intensities and Profiles From MOSES Sounding Rocket Data

Author(s): Kankelborg, Charles C. (1), Atwood, Shane M. (1), Courrier, Hans T. (1), Plovanic, Jacob T. (1), Rust, Thomas L. (1)

Institution(s): Montana State University, Bozeman, MT

Abstract:

The Multi-Order Solar EUV Spectrograph (MOSES) obtains images dispersed at three spectral orders from an objective grating, with the goal of reconstructing EUV spectra of He II (30.38 nm) and Si XI (30.33 nm) simultaneously over a large 2D field of view. We present preliminary results from a new data inversion code, estimating the spectrum in every pixel. This capability opens a new window on the solar atmosphere.




The generation of shock waves traveling from the photosphere to the transition region within network magnetic elements

Author(s): Kato, Y. (1), Hansteen, V. (2), Steiner, O. (3), Carlsson, M. (2)

Institution(s): (1) NAOJ, (2) ITA, Univ. of Oslo, (3) KIS

Abstract:

We investigate the generation of shock waves near the photosphere by convective downdrafts in the immediate surroundings of the magnetic flux concentration, using radiation magnetohydrodynamic (RMHD) 2D simulations of the solar atmosphere. The simulations comprise the layers from the upper convection zone to the lower corona.  We call this the "magnetic pumping process".  We find that the generated slow modes via magnetic pumping travel upward along the magnetic flux concentration, developing into a shock wave in chromospheric heights. The waves continue to propagate further up through the transition region and into the corona. In the course of propagation through the transition layer, a small fraction of the longitudinal slow mode is converted into a transverse wave mode. We report on how much energy is deposited by propagating shock waves through the transition region and we discuss the the dissipation process above the photosphere within the magnetic flux concentration..




Determining electric fields from vector magnetograms.

Author(s): Kazachenko, M; Fisher, G. H; Welsch, B.T.

Institution(s): Space Sciences Lab, UC BERKELEY

Abstract:

Existence of systematic measurements of vector magnetic fields allows us to estimate electric field in the photosphere, using Poloidal-Toroidal Decomposition of the magnetic field and its partial time derivative (Fisher et l. 2011). The PTD method is based on solving a set of Poisson equations which in the past has been done using Fast Fourier Transform techniques. We modify the existing PTD method by improving the poisson solver using a package of solvers for elliptic partial differential equations called Fishpack. We apply the PTD with a new Poisson equation solver to several test cases with a known electric field. We find that for the ANMHD simulation test case application of the new poisson solver yields a more accurate values of electric field than before. We further investigate the applicability of our method to other test cases using simulations of M. Cheung and Y. Fan.




Beyond single fluid MHD: multi-fluid modeling of the coupled solar atmosphere

Author(s): Elena Khomenko

Institution(s): Instituto de Astrofisica de Canarias

Abstract:

The particular temperature and density conditions in the magnetized photosphere and chromosphere of the Sun usually lead to a very small degree of atomic ionization. In addition, at particular heights, the magnetic field may be strong enough to give rise to a cyclotron frequency larger than the collisional frequency for some species, while for others the opposite may happen. These circumstances can influence the collective behaviour of the particles and some of the hypotheses of magnetohydrodynamics may be relaxed, giving rise to non-ideal MHD effects. These effects are potentially important for the dynamics and energy exchange in the solar photosphere and, especially, chromosphere. In particular, there are evidences that such phenomena as wave propagation and damping, magnetic reconnection, formation of stable magnetic field concentrations, magnetic flux emergence, etc. can be affected. In this contribution, I will discuss the current state-of-the-art of multi-fluid MHD modelling of the coupled solar atmosphere. I will revise the major issues, physical mechanisms and assumptions of the MHD approach, and discuss future simulations that would be required to address some unresolved topics. I will present the first results of numerical simulations using the modified MHD equations, showing that the chromosphere can be effectively heated due to non-ideal MHD effects.




Effects of vortex tube dynamics in the chromosphere

Author(s): Kitiashvili I.N. (1), Kosovichev A.G. (1), Lele S.K. (1), Mansour N.N. (2), Wray A.A. (2)

Institution(s): (1) Stanford University, California, USA, (2) NASA Ames Research Center, California, USA

Abstract:

Investigation of the solar atmosphere dynamics cannot be complete without understanding coupling, and mass and energy exchange between the strongly-turbulent subphotosphere and the chromosphere. Modern computational capabilities allow us to construct realistic dynamical models, which take into account dynamical, chemical and radiative properties of the solar plasma. Such simulations based on first physical principles and accurate modeling of effects of magnetic field and small-scale turbulence, coupled with spectro-polarimetric line formation calculations, provide synthetic multi-wavelength observables, and are very important for interpretation of observational data. The simulations allow us to study physical processes and phenomena that have not been resolved in observations. In this talk we will present our recent results of high-resolution 3D radiative MHD numerical simulations of top layers of the convective zone and the chromosphere. The simulations reveal ubiquitous distribution of small-scale swirling motions in quiet-Sun and magnetic regions, forming vortex tubes extending from the subphosphere into the chromosphere. Our results show that these small-scale vortex tubes that originally formed in subsurface layer and penetrate into the chromosphere provide an efficient coupling of the turbulent convective layers with the atmosphere. They play important role in various processes, such as shearing instabilities, wave excitation, formation of magnetic flux tubes and transport of energy, mass, momentum and also turbulent properties from the convection zone into the chromosphere. In the presentation, we will focus on the physical aspects of the vortex tube formation, penetration into the atmosphere, interaction with magnetic fields, their role in the energy exchange, and on observational diagnostics and comparison with observational data.




Spectropolarimetry of the photosphere and the chromosphere with IBIS

Author(s): L. Kleint (1), A. Sainz Dalda (2)

Institution(s): (1) HAO/NCAR, Boulder CO 80301 (2) Stanford-Lockheed Institute for Space Research, Stanford, CA 94305

Abstract:

We have obtained quasi-simultaneous spectropolarimetric imaging observations of various chromospheric and photospheric features in the lines Fe I 6302 A, Ca II 8542 A, H-alpha 6563 A and Na I 5896 A with the IBIS instrument at Sac Peak. Our targets include the quiet Sun, pores, sunspots, and flaring regions and our goal is to analyze the 3D magnetic field structure of the solar atmosphere. We carry out NTLE inversions with the NICOLE code to investigate interpretation techniques for chromospheric spectropolarimetric observations. The very faint polarization signatures make chromospheric inversions of the quiet Sun challenging. On the other hand, they are quite pronounced during flares and show us that the chromospheric magnetic structure is seemingly unrelated to the photosphere during these events.




The Pros and Cons of 1D vs. 3D Modeling

Author(s): James A. Klimchuk

Institution(s): NASA Goddard Space Flight Center

Abstract:

Advances in computing capability have led to tremendous improvements in 3D modeling. Entire active regions are being simulated in what might be described as a first principles way, in which plasma heating is treated self consistently rather than through the specification of heating functions. There are limitations to this approach, however, as actual heating mechanisms on the Sun involve spatial scales orders of magnitude smaller than what these simulations can resolve. Other simulations begin to resolve these scales, but they only treat a tiny volume and do not include the all important coupling with larger scales or with other parts of the atmosphere, and so cannot be readily compared with observations. Finally, 1D hydrodynamic models capture the field-aligned evolution of the plasma extremely well and are ideally suited for data comparison, but they treat the heating in a totally ad hoc manner. All of these approaches have important contributions to make, but we must be aware of their limitations. I will highlight some of the strengths and weaknesses of each.




Links between photospheric and chromospheric oscillations

Author(s): A.G. Kosovichev (1), I.N. Kitiashvili (1), U. Mitra-Kraev (2), T. Sekii (3)

Institution(s): (1) Stanford University, Stanford, CA, USA; University of Cambridge, Cambridge, UK;National Astronimcal Observatory, Mitaka, Japan

Abstract:

Oscillations excited by turbulent convection play important in the dynamics and energetics of the solar atmosphere. Oscillations below the acoustic cut-off frequency form photospheric resonant modes trapped in the interior but also penetrating into the chromosphere. Above the frequency cut-off, the oscillations represent traveling waves in the chromosphere that form pseudo-modes due to interference with waves coming from the interior. The physics of the chromospheric oscillations, their coupling to the photospheric oscillations, and their role in the chromospheric dynamics and energetics are not fully understood. The observed oscillation properties strongly depend on the excitation mechanism, interaction with turbulence and radiation, and local structure and dynamics of the chromosphere. Significant advances can be made through multi-wavelength observations of atmospheric oscillations and realistic numerical radiative hydrodynamics simulations. Using Hinode/SOT data we investigate the basic properties of solar oscillations observed at two levels in the solar atmosphere, in the G-band (formed in the photosphere) and in the CaII H line (chromospheric emission). We analyzed the data by calculating the individual power spectra as well as the cross-spectral properties, i.e., coherence and phase shift. The observational properties are compared with theoretical models and numerical simulations. The results reveal significant frequency shifts between the CaII H and G-band spectra, in particular above the acoustic cutoff frequency for pseudo-modes. The cross-spectrum phase shows peaks associated with the acoustic oscillation (p-mode) lines, and begins to increase with frequency around the acoustic cut-off. However, we find no phase shift for the (surface gravity wave) f-mode. The observed properties for the p-modes are qualitatively reproduced in a model that includes a correlated background due to radiative effects. Our results show that multi-wavelength observations of solar oscillations, in combination with radiative hydrodynamics modeling, help to understand the coupling between photospheric and chromospheric oscillations.




Using Kepler Data to Characterize the Flare Properties of GK Stars

Author(s): Kowalski, Adam F. (1), Deitrick, Russell J. (1), Brown, Alex (2), Davenport, Jim R. A. (1), Hawley, Suzanne L. (1), Hilton, Eric J. (3), Ayres, Thomas R. (2), Berdyugina, Svetlana V. (4), Harper, Graham M. (5), Korhonen, Heidi (6), Walkowicz, Lucianne M. (7)

Institution(s): (1) University of Washington, (2) University of Colorado, (3) Institute for Astronomy, University of Hawaii, (4) Kiepenheuer Instut feur Sonnenphysik, Germany, (5) Trinity College, Dublin, Ireland, (6) Niels-Bohr Institute, University of Copenhagen, (7) Princeton University

Abstract:

Due to their high occurrence rate and large contrast against the background stellar emission, white-light flares on a handful of very active low-mass M stars have been the primary source for our understanding of optical flare emission. Kepler’s high-precision, long baseline light curves have opened up the characterization of white-light emission to new domains of stars, including active G dwarfs. We present the properties of white-light flares on GALEX-selected solar-type stars from GO data in Q1-Q7. The flares are discussed in relation to intrinsic stellar properties, which are constrained by a vast amount of follow-up characterization of the sample. We compare the flare properties to large white-light flares observed on the Sun. These high-precision state-of-the-art observations will provide important constraints for models of internal magnetic dynamos and NLTE radiative-hydrodynamic simulations of energy deposition in the lower atmospheric layers.





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Last Updated on Tuesday, 24 January 2012 13:45