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.
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MAY |
May 28, 2008
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No Colloquium Today
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SPD Meeting
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May 21, 2008 CG-1 2126
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Steven Saar
Harvard-Smithsonian Astrophysical Observatory
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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.
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May 14, 2008 CG-1 2126
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Peter MacNeice
NASA, Goddard Space Flight Center
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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.
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May 7, 2008
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No Colloquium Today
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IMAGE Theme-of-the-Year Workshop
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APRIL |
April 30, 2008
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No Colloquium Today
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Space Weather Week
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April 23, 2008 CG-1 2126
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TBD
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TBD
:
TBD
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April 16, 2008 CG-1 2126
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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.
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April 9, 2008 CG-1 2139 Captain Mary Room
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Ashley Crouch CoRA
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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.
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April 2, 2008 CG-1 2126 at 2:30
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Yi-Min Huang
University of Wisconsin, Madison
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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.
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MARCH |
March 26, 2008 CG-1 2126
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TBD
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TBD
:
TBD
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March 19, 2008 CG-1 2126
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Steve Tomczyk
High Altitude Observatory
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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.
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March 12, 2008 CG-1 2126
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Yuhong Fan
High Altitude Observatory
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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.
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MONDAY, March 10, 2008 CG-1 2126
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Mausumi Dikpati
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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.
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March 5, 2008 CG-1 2126
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Hunter Waite
Southwest Research Institute
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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.
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TUESDAY, March 4, 2008 CG-1 2139, Captain Mary Room
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Gabor Toth
University of Michigan
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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.
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FEBRUARY |
TUESDAY February 26, 2008 CG-1 2139, Captain Mary Room
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Orlagh Creevey NCAR/High
Altitude Observatory
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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.
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February 20, 2008 CG-1 2126
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Bill Abbett
Space Scieces Laboratory, University of California, Berkeley
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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.
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TUESDAY, February 12, 2008 CG-1 2126
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Paul Cally
Monash University
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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.
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February 6, 2008 CG-1 2139, Captain Mary Room
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Hector Socas-Navarro
High Altitude Observatory
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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.
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JANUARY |
January 30, 2008 CG-1 South Auditorium
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Geoff Vasil
University of Colorado, JILA
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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.
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January 23, 2008 CG-1 2126
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Shane Keating Univ of California, San Diego
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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
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January 16, 2008 CG-1 South Auditorium
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Tom Ayres
University of Colorado, CASA
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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!
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January 9, 2008 CG-1 2503
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Aaron Ridley
University of Michigan
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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.
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January 2, 2008 CG-1 2126
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No Colloquium Today
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Holiday
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DECEMBER |
December 26, 2007 CG-1 2126
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No Colloquium Today
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Christmas Holiday
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December 19, 2007 CG-1 2126
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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.
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December 12, 2007 CG-1 2126
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No Colloquium
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AGU Meeting
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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.
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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
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November 21, 2007 CG-1 2126
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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.
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OCTOBER |
October 31, 2007 CG-1 2126
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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.
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October 10, 2007 CG-1 2126
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Stefan Maus
NOAA
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Electric fields and zonal winds in the equatorial ionosphere inferred from
satellite magnetic measurements
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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.
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October 3, 2007
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No Colloquium Today
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HAO Vision Committee Meeting
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SEPTEMBER |
September 26, 2007 CG-1 2126
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Hugh Hudson
University of California, Berkeley, SSL
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Measuring the shape of the Sun
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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.
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September 19, 2007 CG-1 2139 Captain Mary Room
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Mark Rast
University of Colorado, LASP
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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.
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FRIDAY September 14, 2007 CG-1 2139 Captain Mary Room
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Shravan Hanasoge Stanford University
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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.
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