High Altitude Observatory

Colloquia - 2007-2008

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.

MAY
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
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
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
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 Alfvén speeds coincide. I will also discuss recent work on conversion to Alfvén 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
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.
January 2, 2008 CG-1 2126	No Colloquium Today	Holiday
DECEMBER
December 26, 2007 CG-1 2126	No Colloquium Today	Christmas Holiday
December 19, 2007 CG-1 2126	Aimee Norton National Solar Observatory	The Tilted Solar Magnetic Dipole as Observed and Modeled During the 1996 Solar Minimum : We examine the tilt of the solar magnetic dipole away from the rotational axis near the 1996 solar minimum. A persistent tilted dipole may result from an MHD instability acting upon the toroidal magnetic bands in the solar interior. Non-axisymmetric eruption of sunspots, the decay of the follower spots, and migration of this flux poleward could create polar caps that are slightly misaligned with the N-S rotational axis. To investigate this, we analyze the coronal streamer geometry as observed with LASCO-C2 at 5 R and the center-of-gravity of the polar caps as defined by coronal hole boundaries in EIT images and the unipolar magnetic regions in Kitt Peak Vacuum Tower magnetogram data. We model the coronal hole boundaries and the neutral line locations by potential field source surface (PFSS) modeling using Kitt Peak magnetograms.
December 12, 2007 CG-1 2126	No Colloquium	AGU Meeting
December 5, 2007 CG-1 2126	Rekha Jain University of Sheffield	The Generation of Coronal Loop Waves Below the Photosphere by p-Mode Forcing : Recent observations of coronal loop waves by TRACE (e.g., De Moortel & Rosner 2007) and within the corona as a whole by CoMP (Tomczyk et al. 2007) clearly indicate that the dominate oscillation period is 5 minutes, thus implicating the solar p modes as the root source. We investigate the generation of tube waves within the solar convection zone by the buffeting of p modes (Bogdan et al. 1996). The tube waves---in the form of longitudinal sausage waves and transverse kink waves---are generated on the many magnetic fibrils that lace the convection zone and pierce the solar photosphere. Once generated by p-mode forcing, the tube waves freely propagate up and down the tubes, since the tubes act like light fibers and form a waveguide for these magnetoacoustic waves. Those waves that propagate upwards pass through the photosphere entering the upper atmosphere where they can be measured as loop oscillations and other forms of propagating coronal waves. We treat the magnetic fibrils as vertically-aligned, thin flux tubes and compute the energy flux of tube waves that can be generated and driven into the upper atmosphere. We demonstrate that a flux in excess of $10^5$ erg/cm$^2$/s can be produced, easily supplying enough wave energy to explain the observations. Furthermore, we compute the associated damping rate of the driving p-modes and find that the damping is significant compared to observed line widths only for the lowest order p modes.
NOVEMBER
November 28, 2007 CG-1 2126	B.C. Low High Altitude Observatory	Untwisted Magnetic Fields and Parker Spontaneous Current-Sheets : The spontaneous formation of electric current sheets in a magnetic-field dominated, electrically highly conducting plasma is a physically attractive process to explain the heating of the solar corona, in particular, for the magnetically closed regions. Current sheets by their thinness must dissipate by the resistive reconnection of magnetic fields in spite of the high but finite conductivity. Thus highly conducting plasmas are necessarily also resistively dissipative under astrophysical circumstances. The theory of this process due to E. N. Parker shows that the process is three dimensional in its fundamental nature - the basic properties of this process are non-trivially restricted in two dimensional systems. This talk is on a recent development in the study of the three-dimensional topology of magnetic fields that has led to a direct theoretical demonstration of the Parker current-sheet formation
November 21, 2007 CG-1 2126	No Colloquium Today	Thanksgiving Holiday
November 14, 2007 CG-1 2126	Knut Waagan High Altitude Observatory	Relaxation-based approximate Riemann solvers for compressible flows : I will present new numerical methods for the Euler equations and the ideal MHD (magnetohydrodynamics) equations. These methods satisfy two important stability criteria: They preserve positivity of density and pressure, and satisfy a discrete entropy inequality. The talk will focus on (i) A numerical study for compressible high Mach number flows, including isotropic turbulence, and (ii) The extension of the approach to MHD. The results are from joint work with Francois Bouchut, Christian Klingenberg and Wolfram Schmidt.
November 7, 2007 CG-1 2126	Agnes Kim University of Texas at Austin	Pulsating white dwarfs as dark matter detectors : 5 billion years from now, the Sun will shed its outer envelope and leave behind its inert, degenerate core - a white dwarf. At first very hot (~150,000K), the white dwarf proceeds to cool. This is the way 98% of all stars end their lives. A subset of white dwarfs undergo non-radial oscillations much like the Sun and much for the same reason. The study of these oscillations, white dwarf asteroseismology, allows us to peer into the interior of white dwarfs and derive important stellar parameters such as their mass and surface temperature. Some pulsating white dwarfs, such as G117-B15A, show extremely stable pulsations, losing only 1 second every 10 million years. Such slow rates of period change ("Pdots") are a measure of the cooling rate of the star. With pulsating white dwarfs, we have the unique opportunity to measure a total energy loss rate. White dwarfs cool not only by radiating photons, but also through the emission of weakly interacting particles, source of an "unseen" energy loss. By being able to measure a total energy loss rate, we can constrain the latter. These weakly interacting particles include neutrinos, but also perhaps axions, which are among favored dark matter particle candidates. I will present results of the asteroseismological study of G117-B15A and correspondings limits on axion emission rates.
OCTOBER
October 31, 2007 CG-1 2126	Colloquium CANCELLED
October 24, 2007 CG-1 2126	Nathalie Toque University of St. Mary's, Halifax, Nova Scotia	Rapid internal differential rotation in intermediate mass stars : We first restrict the use of a 2D stellar evolution code to simulate ZAMS models with an internal differential rotation law. This law is conservative and depends on parameters which rule the departure from the solid body rotation. We will show selected ZAMS models with masses equal to 3, 6.5 and 10 solar masses and compare some of these with models of (Jackson et al. 2005). Now, we are working on the evolution of these models on the main sequence. We will show that the conservative initial rotation law can mutate to a looking like “shellular” one on a timescale which is shorter for higher mass.
October 17, 2007 CG-1 2126	Aaron Birch CoRA	Local Helioseismology of Active Regions and Quiet-Sun Granulation : Local helioseismology is a powerful tool for probing the solar interior. I will present recent local helioseismic measurements of solar active regions and results from supporting modeling efforts. Some of these measurements are apparently inconsistent with predictions obtained from standard models. In addition, I will show local helioseismic measurements of granulation and preliminary modeling work aimed at confronting convection simulations with these measurements.
October 10, 2007 CG-1 2126	Stefan Maus NOAA	Electric fields and zonal winds in the equatorial ionosphere inferred from satellite magnetic measurements : Electric currents in the equatorial ionosphere create strong signatures in magnetic measurements of low-orbiting satellites. Eight years of data from CHAMP, Oersted and SAC-C with almost 100,000 equator crossings allow for a detailed characterization of the spatial and temporal structure of these currents. Particularly prominent is the effect of the equatorial electrojet. A climatological model of the EEJ strength and variability, parametrized by longitude, local time, season and solar flux is now available online at http://geomag.org/models/EEJ.html In addition to scalar measurements used in previous studies, we have also inverted vector magnetic field measurements to gain accurate meridional profiles of the eastward current in the equatorial region. By solving the relevant differential equation, the CHAMP-derived current profiles can be inverted for the driving zonal electric fields and zonal winds. After sun-set, CHAMP vector magnetic profiles show interesting F-region dynamo and gravity-driven currents which will also be discussed in this presentation.
October 3, 2007	No Colloquium Today	HAO Vision Committee Meeting
SEPTEMBER
September 26, 2007 CG-1 2126	Hugh Hudson University of California, Berkeley, SSL	Measuring the shape of the Sun : The astrometry of the solar limb potentially provides information about the solar interior. I discuss our new RHESSI observations and ask where the results fit within the context of helioseismology and studies of the total solar irradiance, especially with respect to signatures of solar activity.
September 19, 2007 CG-1 2139 Captain Mary Room	Mark Rast University of Colorado, LASP	Measuring two degrees on the Sun: Precision photometry with the PSPT : The Precision Solar Photometric Telescope was designed for photometric observations of the Sun with the aim of achieving images with an unprecedented 0.1% pixel-to-pixel relative photometric precision. In order to achieve in data images what was promised by the instrument design, spatial variations in the detector gain must be determined to an equivalent accuracy. This talk will present a critical assessment of the instrumental capabilities of the PSPT and the data processing techniques employed by the project in light of recent, not so recent, and future uses of the data to address some long-standing questions: the thermal structure of sunspots, the convective nature of supergranulation, the latitudinal variation of the solar intensity, and the temporal variation of the mean thermodynamic structure of the Sun.
FRIDAY September 14, 2007 CG-1 2139 Captain Mary Room	Shravan Hanasoge Stanford University	Theoretical Studies of Wave Interactions in The Sun : In recent decades, imaging of the solar interior has been made possible through the development and application of techniques of helioseismology which combine mathematical rigour and sophisticated guesswork. Analyses of the high quality observations made by the Michelson Doppler Imager (MDI) instrument onboard the Solar and Heliospheric Observatory (SOHO) satellite have led to continuous progress in our ability to infer subtle aspects of the recondite solar interior. This rush of discoveries has brought with it some skepticism and a need to determine whether the diagnostic agents, namely the waves, indeed behave as we expect them to. Moreover, it is instructive to develop an appreciation for the sensitivities of these waves to anomalies at various depths for it tells us what is detectable and how easily the detection can be realized. Towards this goal, modeling wave behavior in the Sun using either numerical or analytical techniques is a useful way to proceed. Numerical methods were developed to simulate linear wave propagation in a solar-like stratified medium. Calculations are performed in spherical and cartesian geometry, where the former takes into account global, large wavelength, long lived waves and the latter, near- surface short wavelength, short lifetime waves. My talk will focus on several interesting results obtained through time-distance analyses of wave behaviour in the presence of anomalies such as sound-speed, source-strength, flow, magnetic field, and damping perturbations. Broadly dividing these perturbations based on their sizes: "large" and "small" as compared to the wavelength of the imaging agent, I will discuss the (mis)interpretations derived from these analyses. I will also describe the method of noise subtraction, whereby a realistic multiple source picture may be utilized while still preserving the ability to "subtract" a large fraction of the realization noise associated with stochastic excitation.