High Altitude Observatory

Colloquia (2008-2009) Schedule is subject to change. Please check back often.

Colloquia are held Wednesdays from 1:30pm-2:30pm in Center Green 1, Room 2126 (3080 Center Green) unless otherwise noted.
Refreshments are served fifteen minutes before the talk.
To receive colloquium announcements via email, send message: subscribe seminar to majordomo@hao.ucar.edu.
The HAO Colloquium Program is managed by Mark Miesch and Amy Knack.

June 2009	May 2009	April 2009	March 2009	February 2009	January 2009
December 2008	November 2008	October 2008	September 2008	May 2008	April 2008

MAY 2009
May 27, 2009 CG-1 2126	Makenzie Lystrup Laboratory for Atmospheric and Space Physics, University of Colorado	Infrared studies of giant planet aurorae : The upper atmosphere of a gas giant planet is the interface between the planet itself and its magnetosphere and is the locus of large energy inputs (several hundred TW) and of dynamic and variable electromagnetic coupling. However, despite decades of study, fundamental questions remain about these upper atmospheres and their connection to magnetospheres. Chemistry in the upper atmospheres of the gas giants results in production of the H3+ molecular ion, which plays an important role for thermospheric energy balance and ionospheric conductivities. Observations of near infrared auroral H3+ emissions have been successfully used to probe the physics of the auroral atmospheres of Jupiter and Saturn and to investigate ionosphere-magnetosphere coupling. This talk will discuss the role of H3+ in giant planet atmospheres, the range of H3+ infrared observations, and the future of H3+ infrared observations including possible exoplanet studies.
May 20, 2009 CG-1 2126	Keith Julien Applied Math Department, University of Colorado	Reduced Modeling and Numerical Investigation of the Magnetorotational Instability : The magnetorotational instability is a linear instability in the presence of an imposed magnetic field and shear (or differential rotation) in an electrically conducting fluid. Presently, this instability serves as the leading mechanism for the efficient transport of angular momentum necessary for accretion in astrophysical disks. The level of angular momentum transport is determined by the saturated stated of sustained turbulence generated by the instability. However, the mechanism of nonlinear saturation of this instability is not well understood. Many recent numerical investigations of this problem are performed in a local domain, where the global cylindrical background state is projected onto a Cartesian domain. The resulting system is then numerically modeled within a "shearing box" framework. In this talk, I present a reduced model for the magnetorotational instability that allows a large scale feedback of local stresses (Reynolds, Maxwell, and mixed) onto the projected background state. This system is investigated both analytically and numerically to determine the impact of allowing this feedback on the saturated level of angular momentum transport.
May 13, 2009 CG-1 2126	Benjamin Brown JILA/University of Colorado	Dynamo Action and Wreathes of Magnetism in Young, Rapidly Rotating Suns : When stars like our Sun are young they rotate much more rapidly than the Sun's current one month rotation period. These young stars are observed to have strong magnetic fields, but at present it is unknown whether this reflects changes in the global-scale magnetic dynamo. Here we explore 3D MHD simulations of stellar convection and dynamo action in these stars, looking at stars rotating a few times faster than the Sun. These dynamos build striking organized toroidal and poloidal fields in the midst of their convection zones. They do so without using a tachocline of penetration and shear. The wreath-like magnetic structures can persist for thousands of simulated days. In some cases the dynamos become time-varying and even undergo quasi-regular reversals of global-scale magnetic polarity. We will look at how these dynamos build their magnetic fields and how their character changes with increasing levels of turbulence and rotation.
May 6, 2009 CG-1 2126	Steve Cowley Imperial College/United Kingdom Atomic Energy Authority(UKAEA)	How Fast Can Magnetic Fields Be Amplified By Turbulence? Magnetic fields are observed very early in the evolution of structure of the universe. It is not known how or when these fields were created. I will discuss the various theories of the field origin. These theories challenge our present understanding of Magnetohydrodynamic turbulence. A central theoretical question is: how fast can magnetic field be amplified by dynamo action. This question is of interest in many astrophysical situations -- from the sun to galaxy clusters. Small scale fields of observable amplitudes may be created on short time-scales by turbulent flows or compact objects. The central issue is the creation of the observed long scale coherence in the field. I will discuss how dissipation scale dynamics cannot be ignored. Indeed, it is still not clear which aspects of MHD turbulence are universal and which are problem specific.
APRIL 2009
April 29, 2009 CG-1 2126	Marco Velli Jet Propulsion Laboratory	Magnetohydrodynamic turbulence in heliospheric physics: from coronal and solar wind heating to kinetic effects : We have recently studied the Parker field line tangling problem for coronal heating comprehensively via longtime high-resolution simulations of the dynamics of a coronal loop in cartesian geometry within the framework of reduced magnetohydrodynamics (RMHD). Although the turbulent cascade prevents the magnetic field lines from becoming strongly entangled, current sheets are continuously formed and dissipated. Current sheets are the result of the nonlinear cascade that transfer energy from the scale of convective motions down to the dissipative scales, where it is finally converted to heat and/or particle acceleration. We have derived scaling laws for this process which depend only on the ratio betweent he photospheric eddy turnover time and the loop Alfvén wave crossing time. Along open field lines in the solar corona, nonlinear couplings are driven by reflection of Alfvén waves off the solar wind density gradients. I will also discuss simulations of this process using a simplified shell-model, starting from the coronal base up to 17 solar radii, well beyond the Alfvénnic critical point. Turbulent dissipation is found to account for at least half of the heating required to sustain the background imposed solar wind and its shape is found to be determined by the reflection-determined turbulent heating below 1.5 solar radii. Therefore reflection and reflection-driven turbulence are shown to play a key role in the acceleration of the fast solar wind and origin of the turbulent spectrum found at 0.3 AU in the heliosphere. At smaller scales, Alfvénnic turbulence folds into wave-particle interactions. If time permits I will discuss interactions occurring through ion-cyclotron resonace and nonlinear trapping due to the growth of parametric instabilities. Cyclotron interactions control the evolution of the temperature anisotropy providing a perpendicular heating which contrasts the adiabatic cooling caused by the expansion of the solar wind. Ion-acoustic modes driven by parametric effects produce a velocity beam in the particle distribution function, and the resulting proton distribution functions are in reasonable agreement with Helios data.
April 22, 2009 CG-1 2126	Stanislav Sazykin Rice University	Auroral dynamics and magnetospheric processes: First-principles modeling with the Rice Convection Model : In this talk we will investigate how some of the observed features in the auroral region of Earth's ionosphere are related to elementary plasma physics processes in the terrestrial magnetosphere. We have developed a first-principles numerical model that simultaneously treats kinetic physics of energetic plasmas in the magnetospheric domain on closed magnetic field lines and electrodynamic interactions between the magnetosphere and the ionosphere, known as the Rice Convection Model. We will use numerical simulations with this model to explain how and to what extent such widely-observed auroral features as subauroral ion drift (SAID) structures and substorm current wedge can be reproduced. At the same time, we will highlight recent advances and challenges in attempting to model large-scale magnetosphere-ionosphere interactions i n a self-consistent way.
April 15, 2009 CG-1 2126	Brad Hindman JILA/University of Colorado	Subsurface Circulations within Active Regions : Using the local helioseismic technique of ring analysis we deduce subsurface flows within and surrounding magnetic active regions. We apply this technique to data from MDI and analyze the resulting flow fields for durations of several months from each of three consecutive years. We compute the mean motion of magnetic plage, mean inflow rates into magnetic complexes and mean circulation speeds around active regions. Further we compute probability distribution functions (PDFs) of the flow properties in quiet and active sun. We find that the plage within active regions rotates more rapidly than quiet sun by roughly 20 m/s, yet advects poleward at the same rate as quiet sun. We also find that almost all active regions possess a mean inflow (20-30 m/s) and a cyclonic circulation (~5 m/s) at their peripheries; whereas their cores, where the sunspots are located, are zones of strong anticyclonic outflow (~50 m/s). The PDFs indicate that active regions modify the structure of large-scale convection. Quiet sun possesses an asymmetry between inflows and outflows, with a larger percentage of the surface occupied by outflows. Active regions, on the other hand, possess a symmetric distribution.
April 8, 2009 CG-1 2126	Harry Warren Naval Research Laboratory	The Structure of the Solar Corona: New Results from the EIS Spectrometer on Hinode : How the solar corona is heated to millions of degrees is one of the fundamental problems in solar physics. Over the past decade extensive observational and theoretical work has been done trying to understand the properties of individual coronal loops, the building blocks of the solar corona. There is still no consensus, however, on some of the basic properties of the coronal heating mechanism in coronal loops, such as the spatial and temporal scales for energy release. The EUV Imaging Spectrometer (EIS) on Hinode has an unprecedented combination of high spatial, spectral, and temporal resolution and is providing new insights into the temperature and density structure of coronal loops. In this talk I will give an overview of recent EIS results and I will discuss how they relate to current theories of coronal heating.
April 1, 2009 CG-1 2126		No Colloquium: Radiation Hydrodynamics Meeting
MARCH 2009
March 25, 2009 CG-1 2126	Michael Wiltberger NCAR, High Altitude Observatory	ARG Talk: Modeling Magnetosphere-Ionosphere Coupling : The interaction of the magnetosphere-ionosphere system with the solar wind can be broken down into two basic aspects. An electrodynamical coupling occurs as a result of the closure of current systems formed in magnetosphere in the upper levels of the ionosphere via field-aligned currents. This coupling is modulated by the ionospheric conductance that is in turn controlled by solar illumination and auroral particle fluxes. A mechanical coupling occurs via the transfer of plasma from the ionosphere into the magnetosphere. The factors controlling this interaction are not completely understood, but it is likely to regulated by the wave and particle flux from the magnetosphere and the structure of the ionospheric density. In this presentation we discuss how these two coupling modes are included in global scale models of the solar wind-magnetosphere-ionosphere system. The discussion of the electrodynamical coupling focuses on how observed seasonal variations in the particle flux are modeled by introducing a simple modification to traditional method for determine the auroral particle flux. We show that this model improves the agreement between the modeled particle fluxes and observations. In order to study the mechanical coupling we need to use a multifluid version of the global magnetospheric model so we can include outflowing oxygen ion from the ionosphere into the magnetosphere. We show that depending on the parameters of the outflow the oxygen can have a dramatic impact on the evolution of the magnetosphere. The presentation concludes with a few remarks on next steps for improving both the electrodynamical and mechanical coupling in global scale models.
March 18, 2009 CG-1 2126	Delores Knipp US Air Force Academy Dept. of Physics (USAFA/DF)	Enhanced Thermospheric Density: The Roles of Conjoined Magnetic Clouds and Northward Interplanetary Field : During 2005 solar EUV energy input to the thermosphere waned as Solar Cycle 23 declined. The reduction allowed a clearer delineation of episodic density disturbances caused by geomagnetic storms. We show new views of these disturbances based on Poynting flux calculations from the Defense Meteorological Satellite Program (DMSP) F-series satellites, as well as from accelerometer data from polar orbiting satellites, and from the Thermospheric Ionospheric Electrodynamic General Circulation Model (TIEGCM). The new Poynting flux estimates and TIEGCM results allow us to trace the origins of disturbances that are poorly specified by ground indices. In particular we find that intervals of enhanced northward Interplanetary Magnetic Field (IMF) accompanying solar wind shocks allow significant electromagnetic energy input into localized regions of the high-latitude thermosphere. This energy deposition is consistent with IMF-geomagnetic field merging tailward of the Earth's magnetic cusps. We illustrate the results of this type of energy input with a case-study of conjoined magnetic clouds arriving at Earth on 15 May 2005 that produce an extended interval of northward IMF. Additionally we discuss characteristics of storms for which thermospheric density perturbations were/were not well specified.
March 11, 2009 CG-1 2126		No Colloquium this week
March 4, 2009 CG-1 2126	Steven Cranmer Harvard-Smithsonian Center for Astrophysics	Plasma Unbound: New Insights into Coronal Heating and Solar/Stellar Winds : This presentation will review the dramatic new understanding of coronal heating and solar wind acceleration that has come from the past decade of observations and theoretical work. Progress has come from a combination of measurement techniques, including high-resolution imaging of the solar disk, spectroscopy of the extended corona, and "in situ" sampling of particles and fields in the heliosphere. On small scales, it seems increasingly clear that the brightest magnetic loops in the low corona are heated by some kind of intermittent magnetic reconnection that is driven by the continual stressing of their foot-points by convective motions. However, open field lines that reach into interplanetary space appear to be energized by the dissipation of waves and turbulent motions (also driven ultimately by convection). Recent models of anisotropic turbulence have progressed to the point of successfully predicting both the heating of the corona and the acceleration of fast and slow wind streams with no free parameters (other than the photospheric lower boundary conditions and the magnetic field). These models have also been extended to simulate outflows from other stars, including young accreting T Tauri stars. More cross-fertilization between the solar, astrophysics, and plasma physics communities is leading to more fundamental insights about the physics governing the Sun and stars.
FEBRUARY 2009
February 25, 2009 CG-1 2126	Barbara Emery High Altitude Observatory, NCAR	Solar Forcing of Auroral Inputs : by Barbara A. Emery (NCAR), Ian G. Richardson (GSFC), David S. Evans (NOAA), Frederick J. Rich (LL/MIT), Gordon Wilson (AFRL), Sarah Gibson (NCAR), Giuliana deToma (NCAR), and Terry Onsager (NOAA) We assess the contribution of solar forcing from the interplanetary magnetic field (IMF) and solar wind velocity (Vsw) on the auroral inputs. The auroral total (HPt) or electron hemispheric power (HPe) is calculated from 1978 using intercalibrated NOAA and DMSP satellite-track in-situ particle measurements up to 20 keV, while the auroral ion hemispheric power (HPi) is calculated from the NOAA SEM-2 satellites since 1998 as the difference between the total and electron only HP (HPi=HPt-HPe). Hourly global auroral inputs of the electron (Pe) and ion power (Pi) are found as the sum of HP estimated in both hemispheres. Cross-correlations and fits are made between the auroral inputs and solar parameters. Periodicities in the hourly Vsw, IMF, Pe and Pi are calculated using Lomb-Scargle (L-S) on a yearly or longer basis for the entire period of observations. Pe and Pi exhibit solar rotational periodicities similar to those for Vsw, where the 9-day periodicity is particularly strong in 2005-2008, and is present also in the Kp index, in neutral densities ~400 km, and in infrared cooling by [NO] and [CO2] between 100-200 km. This 9-d periodicity is not found in solar UV radiation or the 10.7 cm solar flux. The effects of different solar wind structures (transients (CMEs), high-speed streams (HSS) and slow-speed wind are also investigated. We examine two different solar minimum periods in a broader context, including radiation belt electrons >2 MeV. The first Whole Sun Month (WSM) interval (96223-96252) had a strong solar magnetic dipole. A strong "semiannual" periodicity of ~20% variation in Vsw maximizing in equinoxes was found, which enhanced the equinoctial maxima found in Pe (and Kp) due to the preferred solar wind and magnetospheric reconnection during equinoxes. Equinoctial peaks in Vsw at 1 AU are possible because the Earth's orbit reaches the highest solar latitudes (+7.2 deg) on 6 March and lowest (-7.2 deg) on 6 September. The average equinoctial variation is an increase of ~15% in Pe in equinoxes compared to solstices, but the equinoctial increase in 1996 of Pe was ~40%. In the present solar minimum, the solar magnetic field is weaker with larger quadrupole components during the Whole Heliospheric Interval (WHI, 08080-08107). No semiannual periodicity was found in the auroral inputs or Kp, but a very strong 9-d amplitude of ~40% variation was found in Pe and Pi, which was related to a 9-d amplitude of ~30% variation in Vsw. This 9-d periodicity was also found in the outer radiation belt electrons >2 MeV. The radiation belt electrons were low in 1996 and elevated in 2008, possibly because of the 35% drop in the solar wind density which may control the loss processes.
February 18, 2009 CG-1 2126	Vic Pizzo NOAA Space Weather Prediction Center	STEREO Observations of CMEs : At the Space Weather Prediction Center (SWPC), we have been investigating two different methods for determining from STEREO coronagraph observations the location, extent, and motion of coronal mass ejections (CMEs) in 3-space. Since this work is aimed at space weather applications, the emphasis is on use of the COR2 beacon data, which are available in near-real-time. The spatial coverage of the COR2 images ranges from about 2.3-15 Rsun, and COR2 beacon data is transmitted every 15 min, alternating between total intensity only images and full polarization sets. These data are processed in two different ways to determine CME locations. One is a simple geometric localization, where lines of sight from the two spacecraft are used to localize the CME position to within a small volume. The other involves using the polarization data to infer the mean distance from the plane of the sky, as seen from each spacecraft. Together, these analyses enable us to pin down the locus and motion of CMEs quite accurately. We will show results from some recent examples to illustrate the capabilities. We will also discuss how observations with the new wide-angle coronagraphs (called the Heliospheric Imagers) may also be used to infer properties of solar wind structures in the inner heliosphere.
February 11, 2009 CG-1 2126	David Fritts Colorado Research Associates	Nonlinear Gravity Waves in the Middle and Upper Atmosphere: Interactions, Instabilities, Turbulence, and Bores : Gravity waves (GWs) exhibit a range of interactions and instabilities that span all wave amplitudes and intrinsic frequencies. These processes play increasingly important roles at higher altitudes due to wave amplitude increases with altitude. Linear and nonlinear theories provide useful guides to initial instability structures, growth rates, and evolution time scales, but typically fail to define finite-amplitude effects or responses for realistic environments. Numerical instability studies indicate a competition between 2D and 3D dynamics across the GW amplitude spectrum. Primary instabilities at small amplitudes and high frequencies manifest as 2D interactions transferring initial wave energy to GWs having smaller vertical scales and lower frequencies. 3D dynamics predominate at larger amplitudes, though both 2D and 3D dynamics contribute to rapid energy transfers. Wave breaking at larger amplitudes yields large amplitude reductions, a broad inertial range of turbulence, and a large turbulent Prandtl number. The turbulence generation and cascade follow specific pathways, yielding turbulence that is strongly correlated with the gravity wave phase, highly variable in intensity, and remains highly anisotropic throughout the evolution. Significant GW amplitudes at higher altitudes also provide the environment, and the seed conditions, for the generation and sustained propagation of "mesospheric" bores that closely resemble similar large-amplitude responses observed at much lower altitudes.
February 4, 2009 CG-1 2126	Margarita Ryutova Institute of Geophysics and Planetary Physics at Lawrence Livermore National Laboratory (IGPP/LLNL)	Sunspot Penumbrae: Formation, Dynamics and Impact on the Overlying Chromosphere : I'll present the observations of sunspot penumbrae obtained with the 1 m Swedish Telescope on La Palma and the SOT instrument on Hinode, and discuss a mechanism that explains the fine structure of penumbral filaments, their observed dynamics, and their formation process associated with the evolution of sunspot. The mechanism is based on the fact that the umbra itself is a dense conglomerate of non-collinear interlaced flux tubes. On-going reconnection processes in such a system lead to branching out of the peripheral filaments from the "trunk'' with different inclinations and at different heights. More importantly, each elemental act of reconnection facilitates the onset of a screw pinch instability in reconnected flux tubes, which in turn determines the distribution of electric currents, helical magnetic fields, and temperature inside the filaments. On the other hand, due to special conditions in the photosphere, e.g. finite plasma beta, sharp stratification, etc., reconnection and post-reconnection processes lead to multi-step events that manifest themselves in the chromospheric microjets and bow shocks. Most of the parameters in theory are directly observable and are in good agreement with the observation.
JANUARY 2009
January 28, 2009 CG-1 2126	Chia-Hsien Lin Trinity College	CME kinematics and dynamics : Coronal Mass Ejections (CMEs) are ejections are ejections of large amounts of mass and magnetic field from the Sun. Observations have shown that a CME often begins as a de-stablized flux rope rising slowly, which suddenly erupts upward, and finally slows down and propagates through the interplanetary space at a speed comparable to the ambient solar wind speed. The goal of this study is to investigate the possible driving mechanism of CMEs and to infer the properties of magnetic fields at the onset of the instability. We use the EIT 195\AA passband images and LASCO white-light coronagraph data of a CME event that occurred on 2006 December 17 for this study. The event lasted over seven hours, from approximately 11:00 UT to after 18:00 UT, and was associated with an occulted C2.1 class flare. To determine the driving mechanism, we compared the observational data with three CME models: Breakout model (BO: Antiochos et al, 1999), Catastrophe model (CM: Priest & Forbes, 2000), and Toroidal Instability model (TI: Chen, 1989). The results of our examinations indicate that this CME is best represented by the CM model. The EIT data show a sudden increase in velocity between 14:00 and 15:00 UT, which could be an indication of reconnection. The fitting of EIT data shows that, at the onset of the instability, the Alfvén speed ($V_{\rm A0}$) is approximately 120 km/s, the height of the flux rope ($\lambda_0$) is roughly 100 -- 200 Mm. Our next step is to infer the magnetic condition at the onset of the eruption by using our fitting results.
January 21, 2009 CG-1 2126	Arthur Richmond NCAR, High Altitude Observatory	Magnetospheric energy and momentum inputs to the thermosphere : The high-latitude thermosphere is strongly influenced by electromagnetic coupling with the magnetosphere. Joule heating is an important energy source, while collisional coupling between convecting ions and neutrals accelerates high-speed winds. The effects vary strongly with geomagnetic activity and with the direction and strength of the interplanetary magnetic field (IMF). Our group has been developing empirical models of the fields and currents as functions of magnetic latitude, magnetic local time, season, and the IMF or other indices of geomagnetic activity, for use in upper-atmosphere simulation models like the NCAR Thermosphere-Ionosphere-Electrodynamics General-Circulation Model (TIEGCM). In addition to representing the large-scale features of the electric and perturbation magnetic fields, the empirical models also quantify the magnitudes and correlation distances of the small-scale fields. Comparable contributions to the downward Poynting vector and energy input to the thermosphere come from both the large- and small-scale fields. TIEGCM simulations show that high-latitude winds are often in approximate gradient-wind balance, in which the acceleration due to ion drag is generally smaller than that due to horizontal pressure gradients and Coriolis and centrifugal effects. TIEGCM simulations also illustrate how thermospheric temperatures and densities respond to different vertical distributions of Joule heating by small-scale fields.
January 14, 2009 CG-1 2126	John Bally Univeristy of Colorado, CASA	Disks, Interactions, and Eruptive Outflows in Massive Star Formation : Massive stars tend to form in groups at the centers of high-pressure molecular cloud cores that give birth to clusters where they undergo a rich variety of dynamical interactions with sibling stars and dense gas. The resulting non-hierarchical multiples tend to decay into tight binaries plus ejected high-velocity stars which are surprisingly common among massive stars. I will show evidence for a pulsed, precessing jet emerging from Cepheus A where the circumstellar disk of a moderate-mass star may have assisted in the capture of a sibling star into an eccentric, non-coplanar orbit. I will show first results of a campaign to use the Gemini North telescope and its laser-guide star adaptive-optics to image the explosive Orion Molecular Core 1 outflow powered by massive stars dynamically ejected from their parent core only 500 years ago. These results illustrate some of the complexities of massive star formation and cluster birth. They provide insights into the astrophysics of starbursts, super-star clusters, active galactic nuclei, and the birth of planetary systems.
January 7, 2009 CG-1 2126	Cora E. Randall University of Colorado, LASP	Polar Mesospheric Cloud Morphology and its Dependence on Water Vapor and Temperature: Highlights from the AIM Mission : The Aeronomy of Ice in the Mesosphere (AIM) is a NASA Small Explorer mission with the overall goal to resolve why Polar Mesospheric Clouds (PMCs) form and why they vary. By measuring PMCs and the thermal, chemical and dynamical environment in which they form, AIM will quantify the connection between these clouds and the meteorology of the polar mesosphere. AIM was launched on April 25, 2007 and has provided nearly two years of unprecedented data on PMCs. This talk will provide an overview of AIM contributions to improving our understanding of PMC morphology and the dependence of PMCs on water vapor and temperature. Highlights from recent and ongoing investigations using measurements from the AIM Cloud Imaging and Particle Size (CIPS) experiment and Solar Occultation For Ice Experiment (SOFIE) will be described. We will show that comparisons of AIM measurements of PMCs, water vapor, and temperature compare well with correlative data, and we will present determinations of SOFIE and CIPS particle sizes. Correlations between SOFIE measurements of water vapor, temperature, and PMCs, as well as between CIPS measurements of PMCs over the polar region and ancillary (global) measurements of water vapor and temperature, suggest mechanistic connections between the forcing variables and PMCs. These connections are explored further with two different models, the Navy Operational Global Atmospheric Prediction System (NOGAPS) and the National Center for Atmospheric Research Whole Atmosphere Community Climate Model (WACCM).
DECEMBER 2008
December 31, 2008, 2008	No Colloquium Today	Happy New Year!
December 24, 2008, 2008	No Colloquium Today	Happy Holidays!
December 17, 2008, 2008	No Colloquium Today	Fall AGU Meeting
December 10, 2008, 2008 CG-1 2126	Tomoko Matsuo CIRES/NOAA	Modeling of high-latitude ionospheric electric field variability and its impacts on global themospheric Joule heating and mechanical energy transfer rate : One of the outstanding problems in modeling of the magnetosphere-ionosphere-thermosphere system is the quantitative bias systematically seen in simulated thermosphere and ionosphere responses to magnetospheric forcing. This systematic bias is considered to be attributed largely to insufficient Joule heating or specifically inadequate treatment of electric field variability in general circulation models. Challenges associated with accurate specification of this major forcing of the upper atmosphere remain obstinate in spite of recent improvements of monitoring and modeling of ionospheric convection electric fields. In this talk we will present Gaussian random fields modeling of the electric field variability with consideration of its space-time correlation and variance based on Dynamic Explorer-2 plasma drift measurements (1981-1983). Properties of the electric field variability can be made adaptive through a maximum-likelihood method in a similar manner that the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure makes optimal adjustments to the (prior) model of the mean electric field using observations according to the Bayesian rule. Impacts of the electric field variability on global thermospheric Joule heating and mechanical energy transfer rate are assessed for the storm period of January 10-11, 1997 by using ensembles of NCAR/TIEGCM simulations. Considering that the upper atmosphere is a strongly forced and dissipative system in comparison to the lower atmosphere, it is ever more important that we take account of realistic stochasticity of the forcing in general circulation modeling of the upper atmosphere so that the observed variability can be well reproduced.
December 3, 2008, 2008 CG-1 2126	Michael Mendillo Boston University	The Power of Photons: Exospheres in the Solar System : Surface-Boundary-Exospheres (SBEs) represent a new type of environment discovered at planets and moons that have insufficient gravity to hold on to a permanent atmosphere. These /transient atmospheres /are continuously produced by the vaporization of surface materials and continuously lost by neutral and plasma outflows driven by solar photons or the solar wind. Sodium atoms are a convenient tracer of processes acting at these sites due to emissions readily observable using low-light-level, groundbased instrumentation at visible wavelengths. In this seminar, emphasis will be placed on the broad scope of spatial and temporal scales involved, and on the many mechanisms in space physics that govern their origin and evolution. These will be contrasted with the new paradigm of Heliophysics that fosters a narrow and intellectually less comprehensive approach to solar-planetary-relations.
NOVEMBER 2008
November 26, 2008, 2008	No Colloquium Today	Happy Thanksgiving!
November 19, 2008, 2008 CG-1 2126	Masahito Kubo High Altitude Observatory	Sunspot Decay Process as Revealed by HINODE/SOT : How and where is magnetic flux in sunspots removed from the photosphere in the sun? A time series of spectropolarimetric measurements with the Solar Optical Telescope (SOT) aboard the Hinode satellite allows us, for the first time, to estimate an accurate magnetic flux budget in a decaying sunspot and its surrounding moat region without any effects of atmospheric seeing. The SOT observations reveal that most of the magnetic flux disappeared in the sunspot is transported to the outer boundary of the moat region, and then removed from the photosphere by "magnetic flux cancellation" around the outer boundary of the moat region. We also found that convective motions around the penumbral outer boundary are related to the disintegration of magnetic flux from the sunspot.
November 12, 2008, 2008 CG-1 2126	Leonid Malyshkin University of Chicago	An analytical model of Hall reconnection : The rate of quasi-stationary, two-dimensional magnetic reconnection is calculated in the framework of incompressible Hall magnetohydrodynamics (MHD), which includes the Hall and electron pressure terms in the Ohm's law. The Hall-MHD equations are solved in a local region across the reconnection electron layer, including only the upstream region and the layer center. In the case when the ion inertial length d_i is larger than the Sweet-Parker reconnection layer thickness, the dimensionless reconnection rate is found to be independent of the electrical resistivity and equal to d_i/L, where L is the scale length of the external magnetic field in the upstream region outside the electron layer, and the ion layer thickness is found to be d_i.
November 5, 2008, 2008 CG-1 2126	A. Sacha Brun CEA Saclay	Global Stellar Models of Turbulent Convection and Magnetism Across the H-R diagram : We present recent progress made in modelling in spherical shells the turbulent convection zone of various spectral type of stars with the ASH code. Using the Sun as a reference star to validate the numerical model we then extend our numerical simulations to stars as diverse as young Suns, massive core convective star or evolved low mass RGB stars. We discuss how in turbulent rotating convection zones the heat, energy and angular momentum are redistributed both in radius and latitude. We identify the processes at the origin of the differential rotation and of the meridional circulation observed in such system and how sensible there are to a change of the stellar parameters. We will also discuss how dynamo generated magnetic fields can modify those subtle nonlinear balances with a special emphasis on the solar global dynamo and the organization of its large scale magnetic field.
OCTOBER 2008
October 29, 2008 CG-1 2139 (Captain Mary Room)	Jean-Paul Zahn Observatoire Paris-Site de Meudon	Magnetic instabilities in stellar radiation zones : Certain magnetic configurations in stellar interiors are known to be unstable, as was shown in particular by Roger Tayler and his collaborators. It is now possible to study these instabilities in their non-linear regime, thanks to high resolution numerical simulations. I will present some results obtained with Sacha Brun when examining whether the solar tachocline could be confined by a fossil magnetic field, and I shall discuss the possibility that such instabilities could lead to mixing and operate a dynamo, as was suggested recently by Henk Spruit and Jonathan Braithwaite.
October 22, 2008 CG-1 2126	Jay Johnson Princeton University	Plasma Heating and Transport at the Magnetopause due to ULF Waves : Understanding the origin of the magnetospheric and magnetotail plasma is one of the major unresolved questions of magnetospheric physics. Under northward interplanetary magnetic field conditions, massive transport occurs at the magnetopause leading to the densification and cooling of the plasma sheet. Wave-particle interactions can be a significant source of plasma entry and transport because satellites nearly always detect large amplitude, transverse ULF waves during magnetopause crossings. The wave properties at the magnetopause are consistent with a mode conversion process that couples large-scale compressional MHD fluctuations (commonly observed in the magnetosheath) with kinetic Alfvén waves due to the large increase in Alfven velocity at the magnetopause boundary. We model the mode conversion process with analytic theory that includes ion Larmor radius effects and hybrid simulations. Because heavy ions of terrestrial origin are commonly observed at the magnetopause, we also discuss how they affect the efficiency of the mode conversion process through the introduction of an ion-ion hybrid resonance in addition to the Alfven resonance. We find that ions and electrons can interact nonlinearly with these mode converted waves leading to perpendicular heating of ions, parallel heating of electrons, and plasma transport across the magnetopause boundary. We compare these theoretical findings a with in situ GEOTAIL particle observations and remote observations of plasma along the magnetotail flanks inferred using DMSP. In particular, we consider how observations of (1) dawn-dusk asymmetries in plasma properties, (2) entry rate, and (3) entropy profiles can be explained as the signatures of the magnetopause transport process.
October 15, 2008 CG-1 2139 (Captain Mary Room)	Ramit Bhattacharyya NCAR, High Altitude Observatory	Spontaneous current sheets and break-up of magnetic flux surfaces : Spontaneous current sheet formation during the relaxation of a three dimensional magnetic field in a viscous, perfectly conducting incompressible magnetofluid is demonstrated. The current sheet manifests itself in the form of magnetic tangential discontinuity created when different parts of the fluid press each other as it relaxes to the lowest magnetic energy state. One novel feature of the numerical scheme used for this purpose is the description of the magnetic field in terms of evolving flux surfaces which are possible sites of tangential discontinuity formation. The computation follows initial global flux surfaces of simple geometry as they evolve in time to more complex forms creating magnetic tangential discontinuities in the process.
October 8, 2008 CG-1 2126	Jon Braithwaite Candadia Institute for Theoretical Astrophysics (CITA)	Magnetic fields of non-convective stars : I shall give an overview of what we know from the observations about the magnetic fields in intermediate- and high-mass stars, white dwarfs and neutron stars and compare this to what we expect from first principles. All non-convective stars are born out of convective or otherwise disordered phases and presumably begin their lives with correspondingly disordered fields - I shall present results from numerical simulations which predict how this disordered field relaxes into a stable equilibrium when the convection dies away. In addition, I shall briefly outline the mechanisms in neutron stars by which such an equilibrium can evolve on longer timescales of 10^4 - 10^7 years, as well as some other manifestations of a large-scale magnetic field such as torque-free precession and emission of gravitational radiation.
October 6, 2008 CG-1 2139, Captain Mary Room	Saku Tsuneta Natianal Astronomical Observatory of Japan	Hinode Observations of the polar region, transient horizontal field, emerging flux ropes, and MHD waves : The combination of high spatial resolution with precision spectro-polarimetry was made possible by Hinode, and has been providing us with incredible new information on the magnetism of the Sun. A fundamental issue throughout this talk is how the Sun is producing magnetic fields with cooperation of global and local dynamo processes, and I will concentrate on a few selected Hinode observations directly related to the issue. First, magnetic properties of the solar polar regions are summarized, and are compared with those of the quiet sun. Then, ubiquitous transient horizontal magnetic field (THMFs) are described in detail. An episode on the emergence of large-scale flux rope is reported, and possible connection with THMF is discussed. Finally, if there is time, discovery of the propagating transverse MHD waves, both Alfvenic and magneto-sonic modes on the photosphere will be reported.
October 1, 2008	No Colloquium Today	2nd HINODE Workshop
SEPTEMBER 2008
September 24, 2008 CG-1 2126	Douglas Gough University of Cambridge	Ionization for a seismic diagnostic of solar structure : Most of the seismic investigations of the structure and kinematics of the Sun have concentrated on the direct inferences of what is happening now. There have been attempts to relate those inferences to existing general theory, such as dynamo theory and simulations of large-scale convection, but on the whole progress in that arena has been sparce - although it must be said that the discovery of the tachocline and the realization that most of the radiative interior of the Sun rotates more-or-less uniformly has spawned substantial theoretical activity, as indeed have the seismic observations of solar-cycle variations in the angular velocity. Most of the progress that interests astrophysicists outside our field, however, relates to the basic structure of the Sun, and has been made principally, either directly or indirectly, by calibrating the theory of stellar evolution and investigating the physical properties of stellar material. From such studies it has been possible to make broad statements about the chemical composition of the Sun, and to estimate the Sun's main-sequence age - the latter, if ever we could "measure" it sufficiently precisely, having potential implications in studies of the formation of the solar system. In this talk I shall summarize some of the recent findings, discuss how deductions from them depend on the often unstated assumptions of the theory, and look forward a little to what we might learn in the immediately foreseeable future.
September 17, 2008 CG-1 2139, Captain Mary Room	Janet Kozyra University of Michigan	How Geospace responds to solar cycle changes in high speed solar wind streams: Studies in progress and new information : During the recent IHY Whole Heliosphere Interval (WHI), which took place 20 March - 16 April 2008, observations showed that the nature and distribution of coronal holes had important differences from last solar minimum - isolated, trans-equatorial coronal holes persisted throughout and, in fact, are characteristic features of the last four years in the long approach to solar minimum. The associated recurrent high-speed solar wind streams have been exceptionally fast and long-lived in comparison to last solar minimum. Since the speed and duration of coronal hole winds have been shown to control the nature and severity of resulting geospace and atmospheric disturbances, the geospace response to solar minimum conditions must vary considerably between solar cycles 22 and 23. All solar minima are not the same. The purpose of this overview is to: (1) summarize ongoing studies and new information about the upper atmosphere and geospace response to the low F10.7 and persistent high speed solar wind driving during the current solar minimum; (2) compare to conditions during previous solar minima; and (3) identify interesting Sun-to-Earth coupling processes.
September 10, 2008 CG-1 2126	Scott McIntosh NCAR, High Altitude Observatory	What New "Eyes" Can Tell Us About Quiet Coronal Heating : Using new observational tools like Hinode/SOT and HAO's CoMP we are gaining a better understanding of the transport of magneto- convective energy through the quiet solar atmosphere. We will present and discuss some recent advances made with these instruments and discuss the consistency of the physical picture that is rapidly developing.
September 3, 2008 CG-1 2126	Tom Woods University of Colorado, Laboratory for Atmospheric and Space Physics (LASP)	Solar Irradiance Research at LASP: Recent Results and Applications for Climate Change and Space Weather Studies : The Laboratory for Atmospheric and Space Physics (LASP) has a robust program to observe the solar irradiance from a variety of NASA and NOAA satellites. LASP is providing measurements and models of the total solar irradiance (TSI) and solar spectral irradiance (SSI) from 0.1 nm to 2400 nm. Some of the recent results relate to the current solar cycle minimum condition and variations observed since the last solar cycle maximum. The primary applications are studying (1) what part of climate change is related to natural causes such as solar variability versus what is human-induced changes such as by greenhouse gases and (2) how large solar storms can suddenly alter Earth's ionosphere and thermosphere and affect our satellite technology, such as degrading or even disrupting our communication and navigation systems. NCAR research is a vital part of these climate change and space weather studies.
MAY 2008
May 28, 2008	No Colloquium Today	SPD Meeting
May 21, 2008 CG-1 2126	Steven Saar Harvard-Smithsonian Astrophysical Observatory	The Role of Differential Rotation in Solar/Stellar Magnetic Activity : Differential rotation (DR) is a key component in many dynamo models, but is difficult to measure in other stars. Most of the sparse available data come from three types of observations: measurements of rotational period drift, detailed line profile modeling, and tracking using sequences of Doppler images. Each method has its strengths and limitations. I have assembled a database of stellar surface DR measurements, focusing on dwarf stars which are single or in wide binaries. By restricting the dataset in this way, trends of DR with rotation and activity are much clearer than if binaries and/or evolved stars are included. Surface DR in single dwarfs increases with rotation in slower rotators, but the relationship reverses in the most rapid rotators. A sharp break is seen in the relationship between DR and coronal X-ray emission. One apparent implication is that at above some critical rotation rate, the level of DR becomes less important for the generation of magnetic fields. Some implications for dynamos and stellar activity are discussed.
May 14, 2008 CG-1 2126	Peter MacNeice NASA, Goddard Space Flight Center	Solar and Heliospheric Modeling at the CCMC : The Community Coordinated Modeling Center (CCMC) has three principal functions, to test and validate models in use in the research community that are destined for a future Space Weather forecasting role, to make them available for wider use in the research community, and to assist in transitioning these models to Rapid Prototyping Centers. Beginning in 2000 with a small model inventory, the CCMC now hosts more than 40 models or model combinations, covering the solar atmosphere, heliosphere, magnetosphere and ionosphere. Over this period it has completed more than 2000 run requests for customers, and the run count continues to maintain a near exponential growth rate. In this talk I will describe the solar and heliospheric modeling component, its history, its current status and our future plans. I am particularly interested in feedback from potential users and potential model contributors.
May 7, 2008	No Colloquium Today	IMAGE Theme-of-the-Year Workshop
APRIL 2008
April 30, 2008	No Colloquium Today	Space Weather Week
April 23, 2008 CG-1 2126	TBD	TBD : TBD
April 16, 2008 CG-1 2126	Jiuhou Lei Aerospace Engineering Sciences Department, University of Colorado	Ionospheric and Thermospheric Response to Geomagnetic Storms Simulated by the Coupled Magnetosphere-Ionosphere-Thermosphere Model : Ionospheric storms are extreme space weather phenomena involving complex interactions of several processes within the magnetosphere-ionosphere-thermosphere system. Both observations and theoretical simulations are important to help us understand the ionospheric response to geomagnetic storms and the mechanisms that drive these ionospheric storm effects. A Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model has been used to investigate the thermospheric and ionospheric response to geomagnetic storms. This coupled model can self-consistently simulate the effects of neutral composition, neutral winds, and dynamo and penetration electric fields on the ionosphere, so it can also be used to understand the underlying physical and chemical causes of the observed changes if the model output is in agreement with the data. In this talk I will present model simulations from CMIT for several geomagnetic storms in April 2004, November 2004 and December 2006. These model runs were compared with observations, which include data from ground-based GPS receivers, COSMIC, TIMED GUVI, as well as ionosondes and incoherent scatter radar. Term analysis of the ion continuity equation have been used to demonstrate the relative importance of electric fields, neutral winds and neutral composition in producing ionospheric storm effects. Finally, the advantages and limitations of the CMIT 2.0 will be discussed.
April 9, 2008 CG-1 2139 Captain Mary Room	Ashley Crouch CoRA	A Model for the Total Solar Irradiance Based on Active Region Decay : We present a model for the total solar irradiance that takes the observed location, timing, and area of emerging active regions as input and produces a time-evolving size distribution of magnetic structures over the solar surface. We assume that the bright magnetic structures (faculae), which counteract the irradiance deficit caused by sunspots, consist of the products of active region decay. We simulate the decay process as a combination of fragmentation and boundary erosion of large-scale magnetic structures. The model has several adjustable parameters that control the decay processes and the irradiance contribution from the quiet Sun and the small-scale magnetic elements that are produced during the decay process. We use a genetic algorithm to estimate these parameters by fitting to the observed irradiance and daily sunspot area time series over the 1978--2007 time interval. Given the simplifications associated with the model, the irradiance and daily sunspot area time series produced by the best-fit models agree very well with the observations. However, there are some important differences that will be discussed.
April 2, 2008 CG-1 2126 at 2:30	Yi-Min Huang University of Wisconsin, Madison	Energy Release in Magnetized Corona Driven by Continuous Footpoint Motions : The solar corona is a highly conducting plasma (Lundquist number $S \sim 10^{10-13}$). As such, Ohmic dissipation is negligible except within thin current filaments. In his coronal heating model, Parker suggests that thin current filaments can be induced in a magnetized corona via the shuffling of the field lines driven by continuous footpoint motions. One of the major difficulties in assessing the feasibility of Parker's mechanism as the coronal heating source is that the realistic parameters are way beyond the reach of current computer simulations. One possible approach is to establish the parametric dependence of the dissipation rate with simulations of attainable parameters. We study the Parker's model in three different settings: (1) time independent footpoint twisting, (2) time independent footpoint shearing, and (3) footpoint shearing in alternating directions with random phases, with a reduced MHD code. In all three settings, the system finally settles to a statistical steady state. Thin current filaments are created and dissipated away in a sporadic manner, much like the way solar flares release the stored magnetic energy. Averaged over a long period of time, the Poynting power influx balances the viscous and resistive dissipation. Each configuration can be represented with a few relevant dimensionless parameters, e.g. Reynolds number, magnetic Reynolds number, aspect ratio, the ratio between Alfven transit time and eddy turnover time, etc. We discuss the parametric dependence of the dissipation power, as well as the similarities and differences between the three settings. When realistic parameters of solar coronal loops are used, the dissipation power from our models approximately agrees with the observed power needed to maintain the coronal temperature.
MARCH 2008
March 26, 2008 CG-1 2126	TBD	TBD : TBD
March 19, 2008 CG-1 2126	Steve Tomczyk High Altitude Observatory	Observations of Alfven Waves in the Solar Corona, ARG Talk : Why the temperature of the solar atmosphere rises from 5000 to 2 million K from the photosphere outward to the corona is one of the most important outstanding questions in solar physics. Alfven waves, transverse incompressible magnetic oscillations, have been proposed as a possible mechanism to heat the corona by transporting mechanical energy from the turbulent photosphere into the corona. We present observations of the coronal intensity, line-of-sight velocity, and linear polarization obtained in the FeXIII 1074.7 nm coronal emission line with the Coronal Multi-channel Polarimeter (CoMP) instrument. Analysis of these observations reveal ubiquitous upward propagating waves with phase speeds of 1-4 Mm/s and trajectories consistent with the direction of the magnetic field inferred from the linear polarization measurements. We can definitively identify these as Alfvén waves. An estimate of the energy carried by the waves that we spatially resolve indicates that they are unable to heat the solar corona, however, unresolved waves may carry sufficient energy. We also present prospects for using these waves to probe the magnetic structure of the corona.
March 12, 2008 CG-1 2126	Yuhong Fan High Altitude Observatory	Evolution of coronal flux ropes and onset of coronal mass ejections, ARG Talk : Coronal mass ejections (CMEs) are large-scale, spontaneous ejections of plasma and magnetic flux from the lower solar corona into interplanetary space and are the major driver of space weather near earth. CMEs and associated solar eruptive activities (such as flares and prominence eruptions) are believed to be driven by the free magnetic energy stored in the current carrying (twisted) coronal magnetic fields. However, the detailed underlying magnetic field structure for CME precursors and the initiation mechanisms for their sudden eruption remain fundamental unanswered questions under investigation. In this talk, I present 3D MHD simulations of the loss of confinement and eruption of a twisted magnetic flux rope emerging quasi-statically into a pre-existing coronal arcade field. It is found that the flux rope can erupt through either the onset of the torus instability or the kink instability. In the former case the erupting flux rope primarily shows an outward expansion at the onset of eruption while in the latter case the flux rope develops large writhe or rotation. I will discuss the model results and show how they provide insights into the magnetic nature of a range of CME-related observations.
MONDAY, March 10, 2008 CG-1 2126	Mausumi Dikpati	In Search of the Solar Cycle, ARG Talk : Appearance and variation in sunspots in a cyclic fashion, reversal of the Sun's polar fields after every 11 years, coronal variations -- all are manifestations of the solar activity cycle. A magnetohydrodynamic dynamo is most likely responsible for producing this activity cycle. Solar dynamo models have evolved greatly over the past half a century. The most successful current models are flux-transport dynamos in which meridional circulation works like a conveyor-belt that carries the memory of the Sun's past magnetic fields and provides the potential for predicting certain future solar cycle features. In this talk I will describe the story of my 12 years' research that led to finding the answers to the following questions: (i) how does the Sun tick? (ii) Why is the Sun's global magnetic field antisymmetric about its equator? (iii) Can a solar dynamo model be calibrated? (iv) What solar cycle features are predictable and what are not yet? I will close by presenting my own views about where we are now in solar cycle modeling research and what the future goals should be.
March 5, 2008 CG-1 2126	Hunter Waite Southwest Research Institute	Organic Chemistry at Titan : The Cassini-Huygens mission has unveiled a world at Titan that geologically resembles Earth in many ways. The 1.5 bar surface pressure coupled with the 93K surface temperature put the surface very near the triple point of methane. Methane outgassed from the interior can form clouds and rain leaving dry riverbeds and lakes over much of the surface in the current season. This methane hydrology operates on a seasonal basis, but on a much longer timescale (tens of millions of years) methane and the dominant atmospheric gas - molecular nitrogen - can be converted into complex organics in the upper atmosphere using the free energy from solar ultraviolet light or energetic particles from Saturn's magnetosphere. These complex hydrocarbons form a high level organic haze that persists throughout the atmosphere. On the surface over time this haze precipitates out forming extensive organic dunes covering wide regions of the surface. Some suggest that elements of this process may reflect an earlier time period on Earth before life led to the rise of oxygen and that the chemistry may tell us something about the formation of organics in interstellar clouds. The intrigue of understanding how the building blocks of life can be produced makes the story of Titan captivating. The Ion Neutral Mass Spectrometer has collected rich ion and neutral mass spectra from 1 to 100 Daltons 1000 km above the surface. The complexity and interplay of the ion and neutral species indicate that ion-neutral reactions play a major role in the initial formation of aromatics, such as benzene and toluene. When combined with the Ion Beam Spectrometer and the Electron Spectrometer data of the Cassini Plasma Spectrometer investigation we see not only an extension of the positive ion spectra to over 350 Daltons, but we see indications of the onset of PAH condensation and the apparent formation of large negatively charged organic ions with masses up to 8000 Daltons. We propose that these are the nascent "tholins" from which the organic hazes of Titan are born. In this talk we present both an overview of the long-term methane cycle and a detailed accounting of the formation of the nascent "tholins" based on mass spectra from the INMS and CAPS data sets.
TUESDAY, March 4, 2008 CG-1 2139, Captain Mary Room	Gabor Toth University of Michigan	Developing the Space Weather Modeling Framework : Space weather describes the interactions in the Sun-Earth system that affect human technologies and health, including damage to satellites, loss of communication, degraded accuracy of global positioning systems, safety of astronauts, air plane pilots and passengers flying along polar routes. Modeling and eventually predicting space weather is therefore a practically important challenge. The Center for Space Environment Modeling (CSEM) at the University of Michigan has been at the forefront of physics-based space weather modeling. Our group has developed the Space Weather Modeling Framework (SWMF) that integrates independently developed models into a high performance simulation tool. The SWMF models physics domains spanning from the solar corona and heliosphere to the magnetosphere, ionosphere and thermosphere of the Earth. The SWMF can perform a realistic Sun-to-thermosphere simulation faster than real time on today's supercomputers. I will describe the SWMF and some of the numerical techniques that enable it to achieve the required performance. In particular I plan to talk about a new scheme for modeling Hall MHD on block adaptive grids.
FEBRUARY 2008
TUESDAY February 26, 2008 CG-1 2139, Captain Mary Room	Orlagh Creevey NCAR/High Altitude Observatory	Asteroseismology: the dependence of stellar parameter estimation on observational errors : Asteroseismology promises major advances in our understanding of stellar physics and interiors. For example, it offers the opportunity to understand the Sun in the context of other stars, and will help us to ensure that the solar model is not fine-tuned to fit the Sun. However, there are some important issues that may limit its success, such as observing a relatively small number of oscillation frequencies. In this work I investigate whether combining information from different sources allows us to overcome some of the barriers. I discuss the results for both solar-like stars and delta Scuti stars.
February 20, 2008 CG-1 2126	Bill Abbett Space Scieces Laboratory, University of California, Berkeley	The Magnetic Connection Between the Quiet Sun Convection Zone and Corona : All solar activity --- variations in energy released by the Sun, as electromagnetic radiation and energetic particles --- is mediated by the Sun's magnetic field. Solar activity, in the form of irradiance variations, modulation of the solar wind, acceleration of solar energetic particles, and flares and coronal mass ejections, has important consequences to Earth's geomagnetic environment. Although observed at and above the photosphere, this activity arises as a result of the coupling of the solar magnetic field to the rotating, turbulent plasma of the Sun's convective interior. Therefore, to understand and better predict space weather, we must be able to describe in a quantitative way the physics of the magnetic and energetic coupling between the Sun's convective envelope and its atmosphere. I will present a brief review of recent progress toward this goal, and will present the latest results from a series of three-dimensional MHD simulations of the Quiet Sun magnetic field in a computational domain extends from the upper convection zone out into the corona.
TUESDAY, February 12, 2008 CG-1 2126	Paul Cally Monash University	Coupling Helioseismic Waves to Atmospheric Oscillations : Magnetic field concentrations in the Sun's photosphere open "magnetic portals" through which internal waves may pass into the overlying atmosphere, for example in network (Jefferies et al 2006) and coronal loops (many papers by de Moortel and co-workers). In this colloquium I shall review some of these observations, and discuss the modelling developed over recent years to try to understand the processes involved. Of particular note are the "ramp effect", whereby the acoustic cutoff frequency is reduced in strong inclined magnetic field, and fast/slow mode conversion/transmission, which occurs at the level where the sound and Alfv�n speeds coincide. I will also discuss recent work on conversion to Alfv�n waves, which occurs in 3D.
February 6, 2008 CG-1 2139, Captain Mary Room	Hector Socas-Navarro High Altitude Observatory	But really. How much Oxygen is there in the Sun? : Oxygen is the third most abundant chemical element in the Universe, after Hydrogen and Helium, and the one that is most frequently produced by nuclear fusion in stellar interiors. Its abundance in the Sun was thought to be well established since the 1980s (710 parts per million particles, ppm). However, the recent work of Asplund et al using a new 3D hydrodynamical model of the solar atmosphere recommends a revision of the O abundance to a lower value of 455 ppm. The revised solar composition creates a serious problem because it creates a serious conflict (inexistent with the previous composition) between solar interior models and what is inferred from helioseismology. Since chemical abundances are not directly measureable and imply a model-dependent inferrence, the observations are not conclusive and arguments exist both in favor and against the revision. The controversy on whether the proposed revision should be adopted and the doubts that it would cast on stellar structure and evolution models is serious enough that it is often referred to as the solar oxygen crisis. Only two weeks ago, Tom Ayres gave an excellent colloquium with provocative arguments on why we should stick with the traditional high O abundance. In this presentation I will try to convince the audience to do just the opposite thing and adopt the revised composition.
JANUARY 2008
January 30, 2008 CG-1 South Auditorium	Geoff Vasil University of Colorado, JILA	Results on shear-generated magnetic buoyancy : We address a series of high-resolution fully-nonlinear MHD numerical simulations relevant to processes in the solar interior and tachocline. We focus on the generation of a strong toroidal magnetic field by the stretching action of velocity shear on a weak background poloidal magnetic field and examine the buoyancy properties of the the resulting magnetic configurations. While magnetic layers can indeed be generated spontaneously, and magnetic buoyancy instabilities of layers can exist and lead to rising magnetic structures that resemble arching tubes, the conditions for the instabilities to occur are much more demanding than might be anticipated from previous results where a magnetic layer was arbitrarily introduced rather than spontaneously generated. Furthermore, the magnetic flux transport by the buoyancy instabilities in this case is decidedly inefficient. These points, stemming from the feedback of strong magnetic field on the generating shear, raise serious questions for the efficacy of this process in the current solar dynamo paradigm.
January 23, 2008 CG-1 2126	Shane Keating Univ of California, San Diego	Waves, Irreversibility, and turbulent diffusion in magnetohydrodynamic turbulence : Accurate modeling of the processes of dissipation and transport in a turbulent magnetofluid represents a major challenge to our understanding of magnetic fields in the Sun. The usual intuition about turbulent diffusion of a passively advected field in hydrodynamic flows is of limited utility, however, because the magnetic field can influence the turbulence itself via the Lorentz force. This magnetic "back-reaction" is a crucial feature of hydromagnetic turbulence, and can profoundly influence the nature of the turbulent diffusion of magnetic fields. In particular, nonlinear closure models and numerical simulations of the 2D MHD equations predict that turbulent resistivity in high magnetic Reynolds number flows will be strongly suppressed or "quenched" below the value predicted by simple kinematic models. We explore the theory of quenching of turbulent resistivity in a regime for which the mean field theory can be rigorously constructed at large magnetic Reynolds number. This is achieved by extending the simple two-dimensional problem of 2D MHD turbulence to include body forces, such as buoyancy or Coriolis force, which convert large scale eddies into weakly interacting dispersive waves. We calculate the correction, due to nonlinear wave-wave interactions, to the Zel'dovich theorem to fourth order in the wave-slope. The significance of this correction is that, unlike the lowest-order Zel'dovich balance, it is independent of the molecular resistivity and so will not vanish in the limit of a large magnetic Reynolds number. Thus, we are led to the counterintuitive result that the presence of additional restoring forces such as buoyancy can actually increase the turbulent dissipation of magnetic fields relative to that in regular 2D MHD turbulence. Relevant papers and the PowerPoint file are available in advance of the talk on my website: www.srkeating.com
January 16, 2008 CG-1 South Auditorium	Tom Ayres University of Colorado, CASA	Solar Twins in Crisis : I describe two crises affecting the well known solar twins: the Sun and Alpha Centauri A. For the Sun, the issue is the oxygen abundance: recent recommended low values (~450 ppm relative to hydrogen) have been met with some dismay by helioseismologists, whose interior models require solar oxygen to be in the very narrow range 640-680 ppm. The new low-O values stem largely from applications of 3D convection models to tracers like forbidden [O I] 6300 A and infrared bands of molecules such as OH and CO. I show that contemporary 3D models are too cool in the middle photosphere, resulting in artificially low-O from oxygen bearing molecules. I also demonstrate that the key [O I] 6300 transition is consistent with seismic oxygen, if one carefully calibrates the velocity scale of the feature (important owing to a close blend with a Ni I line), and utilizes an alternative class of 3D models that reproduces absolute continuum intensities and center-limb behavior in the visible (unlike the 3D models used in the original forbidden oxygen work). In the second half of the talk, I will describe recent soft X-ray studies of Alpha Cen A, which seemed to show an abrupt disappearance of the solar twin in early 2005, completely at odds with the previous two decades of X-ray monitoring, and seemingly contrary to the behavior of the solar corona. A series of measurements by Chandra, including a deep LETGS spectrum, resolved the mystery straightforwardly, and have pointed to a deeper understanding of coronal activity cycles among solar-type stars. In the third half of the talk....well, I'm not likely to get that far!
January 9, 2008 CG-1 2503	Aaron Ridley University of Michigan	Adventures in Modeling the Thermosphere, Ionosphere and Magnetosphere : In this talk, I will discuss recent validation and science studies that we have been conducting with the Space Weather Modeling Framework and the models within the framework. One study focuses on validation of the magnetosphere and ionosphere through the comparison with data and with numerical sensitivity studies to show what effect different code features have on the solution. Another study examines how we are investigating the dependence of the thermospheric structure on eddy and molecular conductivities, solar heating efficiency, and other quantities to match data during solar maximum, medium and minimum conditions.