Author name code: rast
ADS astronomy entries on 2022-09-14
author:"Rast, Mark P."
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Title: Exploring the cradle of the Solar Wind with the Daniel
K. Inouye Solar Telescope (DKIST)
Authors: Rast, Mark
Bibcode: 2022cosp...44.1318R
Altcode:
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) is in its operations-commissioning phase, a transition from
construction to operations during which there will be a gradual
ramping up of operational and data center capabilities. This phase
of activity will included a series observing-proposal calls with
instrument configurations of increasing complexity. The first of these
calls has closed and proposals have been selected. Observations are
ongoing. Here we will describe the capabilities of the current and
future operations-commissioning phase configurations, and the final
capabilities of the fully commissioned facility. In particular, we
will focus on how the DKIST will contribute to studies of the inner
solar corona. The DKIST's unique high spatial and temporal resolution
high-precision spectropolarimetric capabilities will allow detailed
simultaneous measurements at multiple heights in the solar atmosphere,
unraveling its intricate connectivity and clarifying processes that
span the solar atmosphere.
Title: Identifying Acoustic Wave Sources on the
Sun. I. Two-dimensional Waves in a Simulated Photosphere
Authors: Bahauddin, Shah Mohammad; Rast, Mark Peter
Bibcode: 2021ApJ...915...36B
Altcode: 2021arXiv210110465B
The solar acoustic oscillations are likely stochastically excited
by convective dynamics in the solar photosphere, though few direct
observations of individual source events have been made and their
detailed characteristics are still unknown. Wave source identification
requires measurements that can reliably discriminate the local wave
signal from the background convective motions and resonant modal
power. This is quite challenging as these noise contributions have
amplitudes several orders of magnitude greater than the sources and
the propagating wave fields they induce. In this paper, we employ a
high-temporal-frequency filter to identify sites of acoustic emission
in a radiative magnetohydrodynamic simulation. The properties of the
filter were determined from a convolutional neural network trained
to identify the two-dimensional acoustic Green's function response
of the atmosphere, but once defined, it can be directly applied to
an image time series to extract the signal of local wave excitation,
bypassing the need for the original neural network. Using the filter
developed, we have uncovered previously unknown properties of the
acoustic emission process. In the simulation, acoustic events are
found to be clustered at mesogranular scales, with peak emission quite
deep, about 500 km below the photosphere, and sites of very strong
emission can result from the interaction of two supersonic downflows
that merge at that depth. We suggest that the method developed, when
applied to high-resolution high-cadence observations, such as those
forthcoming with the Daniel K. Inouye Solar Telescope, will have
important applications in chromospheric wave studies and may lead to
new investigations in high-resolution local helioseismology.
Title: The National Science Foundation's Daniel K. Inouye Solar
Telescope — Status Update
Authors: Rimmele, T.; Woeger, F.; Tritschler, A.; Casini, R.; de Wijn,
A.; Fehlmann, A.; Harrington, D.; Jaeggli, S.; Anan, T.; Beck, C.;
Cauzzi, G.; Schad, T.; Criscuoli, S.; Davey, A.; Lin, H.; Kuhn, J.;
Rast, M.; Goode, P.; Knoelker, M.; Rosner, R.; von der Luehe, O.;
Mathioudakis, M.; Dkist Team
Bibcode: 2021AAS...23810601R
Altcode:
The National Science Foundation's 4m Daniel K. Inouye Solar Telescope
(DKIST) on Haleakala, Maui is now the largest solar telescope in the
world. DKIST's superb resolution and polarimetric sensitivity will
enable astronomers to unravel many of the mysteries the Sun presents,
including the origin of solar magnetism, the mechanisms of coronal
heating and drivers of flares and coronal mass ejections. Five
instruments, four of which provide highly sensitive measurements
of solar magnetic fields, including the illusive magnetic field of
the faint solar corona. The DKIST instruments will produce large and
complex data sets, which will be distributed through the NSO/DKIST Data
Center. DKIST has achieved first engineering solar light in December
of 2019. Due to COVID the start of the operations commissioning phase
is delayed and is now expected for fall of 2021. We present a status
update for the construction effort and progress with the operations
commissioning phase.
Title: CHIME's hyperspectral imaging spectrometer design result from
phase A/B1
Authors: Buschkamp, P.; Sang, B.; Peacocke, P.; Pieraccini, S.;
Geiss, M. J.; Roth, C.; Moreau, V.; Borguet, B.; Maresi, L.; Rast,
M.; Nieke, J.
Bibcode: 2021SPIE11852E..2KB
Altcode:
CHIME, the Copernicus Hyperspectral Imaging Mission for the Environment,
is one of the six High Priority Candidate Missions (HPCM) of the
evolution in the Copernicus Space Component (CSC) foreseen in the
mid-2020s that is proposed for further analysis. In this paper we
summarize the results as retrieved by OHB (D) as part of the Phase
A/B1. The contract was kicked off in 2018 and concluded in 2020 after
finalisation of the Pre-development activities. The proposed instrument
is a hyperspectral imager instrument with reflective telescope and
grating-based spectrometer. The selected orbit is in the range of 625
± 30 km, LTDN 10:45 - 11:15 am with a repeat cycle of 20 to 25 days
for a single satellite and 10-12.5 days revisit for 2 satellites. The
payload of each satellite records at a Spatial Sampling Distance
(SSD) of 30m the full spectral range from 400 to 2500nm at a Spectral
Sampling interval < 10nm with Low Keystone/Smile. On the front end
a high performance TMA with wide-band coated optics collects the light
from ground and feeds it to a highly linear almost distortion free
spectrometer assembly attaining very good spectral stability. All units
are integrated in an optical bench structure that offers excellent AIT
access and provides a highly stable LOS. The electro-optical backend
contains low-noise cold MCT detectors creating margin in the predicted
NEDL performance. The instrument can be calibrated via on-board devices
or using reference targets outside the spacecraft. We present the
functional decomposition and the physical instrument architecture:
the optical design and opto-mechanical layout, the electro-optical
imaging chain ant thermal control system.
Title: Identifying Acoustic Wave Sources In A Simulated Solar
Photosphere
Authors: Bahauddin, S.; Rast, M.
Bibcode: 2021AAS...23820507B
Altcode:
The solar acoustic oscillations are likely stochastically excited
by convective dynamics in the solar photosphere, though few direct
observations of individual source events have been made and their
detailed characteristics are still unknown. Wave source identification
requires measurements that can reliably discriminate the local wave
signal from the background convective motions and resonant modal
power. This is quite challenging as these 'noise' contributions have
amplitudes several orders of magnitude greater than the sources and
the propagating wave fields they induce. In this paper, we report on a
new robust method for the unambiguous identification of acoustic source
sites in the photosphere of a MPS/University of Chicago Radiative MHD
(MURaM) magnetohydrodynamic simulation of the upper solar convection
zone. The method was developed by first utilizing a deep learning
algorithm to reliably identify the weak residual high-frequency
signature of local acoustic sources, the two-dimensional acoustic
Green's function response of the atmosphere, in Doppler velocity maps
and then deciphering what underlies its success. We have diagnosed what
the learning algorithm is detecting, mimicked the filter it is applying,
and applied the filter directly to the simulated photospheric time
series, bypassing the dependence on deep-learning and allowing direct
visualization of the local wave pulses that propagate outward from
the acoustic source sites. To be effective, the acoustic-source filter
thus derived requires high cadence (< 3 seconds) and high spatial
resolution (< 50 km) timeseries. Fortuitously, the observational
capabilities required to apply the filter to real solar data are
just now becoming available with the commissioning of the National
Science Foundation's Daniel K. Inouye Solar Telescope (DKIST). Using
the filter developed, we have uncovered previously unknown properties
of the acoustic emission process. In the simulation, acoustic events
are found to be clustered at mesogranular scales, with peak emission
quite deep, about 500 km below the photosphere, and sites of very strong
emission can result from the interaction of two supersonic downflows
that merge at that depth. We suggest that the method developed, when
applied to high-resolution high-cadence observations, such as those
forthcoming with Daniel K. Inouye Solar Telescope (DKIST), will have
important applications in chromospheric wave-studies and may lead to
new investigations in high-resolution local-helioseismology.
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
Bibcode: 2021SoPh..296...70R
Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
Title: Deciphering Solar Convection
Authors: Rast, Mark Peter
Bibcode: 2020ASSP...57..149R
Altcode:
Numerical modeling of solar and stellar convection, and by extension
modeling of solar and stellar dynamos faces a surprising challenge. No
hydrodynamic, magnetohydrodynamic, or radiative magnetohydrodynamic
model of solar convection, if conducted in a sufficiently deep domain,
achieves the velocity power spectrum implied by observations of the
Sun. The horizontal velocity at low wavenumbers in the upper layers of
the simulation domains is much too high, monotonically increasing to low
wavenumber rather than rolling over at supergranular scales, as on the
Sun. This reflects convective amplitudes at depth that are similarly
too large, and results in equatorial differential rotation profiles
in simulations of rotating spherical shells of opposite sign to those
observed. The problem worsens in models with decreasing diffusivities,
as the amplitudes of the convective motions increase. This has come
to be known as the convective conundrum. Solving it is critical to
understanding dynamo behavior on stars, which in turn is central to
the assessment of the structure of the asterospheres in which their
planetary companions are embedded. This paper examines what is known
about solar convection in light of one possible underlying cause of
the convective conundrum, that the deep interior of the Sun is even
more nearly adiabatically stratified than our models suggest or can
achieve. Correcting this in models will likely be difficult, but we
point in some potentially fruitful directions.
Title: The Daniel K. Inouye Solar Telescope - Observatory Overview
Authors: Rimmele, Thomas R.; Warner, Mark; Keil, Stephen L.; Goode,
Philip R.; Knölker, Michael; Kuhn, Jeffrey R.; Rosner, Robert R.;
McMullin, Joseph P.; Casini, Roberto; Lin, Haosheng; Wöger, Friedrich;
von der Lühe, Oskar; Tritschler, Alexandra; Davey, Alisdair; de Wijn,
Alfred; Elmore, David F.; Fehlmann, André; Harrington, David M.;
Jaeggli, Sarah A.; Rast, Mark P.; Schad, Thomas A.; Schmidt, Wolfgang;
Mathioudakis, Mihalis; Mickey, Donald L.; Anan, Tetsu; Beck, Christian;
Marshall, Heather K.; Jeffers, Paul F.; Oschmann, Jacobus M.; Beard,
Andrew; Berst, David C.; Cowan, Bruce A.; Craig, Simon C.; Cross,
Eric; Cummings, Bryan K.; Donnelly, Colleen; de Vanssay, Jean-Benoit;
Eigenbrot, Arthur D.; Ferayorni, Andrew; Foster, Christopher; Galapon,
Chriselle Ann; Gedrites, Christopher; Gonzales, Kerry; Goodrich, Bret
D.; Gregory, Brian S.; Guzman, Stephanie S.; Guzzo, Stephen; Hegwer,
Steve; Hubbard, Robert P.; Hubbard, John R.; Johansson, Erik M.;
Johnson, Luke C.; Liang, Chen; Liang, Mary; McQuillen, Isaac; Mayer,
Christopher; Newman, Karl; Onodera, Brialyn; Phelps, LeEllen; Puentes,
Myles M.; Richards, Christopher; Rimmele, Lukas M.; Sekulic, Predrag;
Shimko, Stephan R.; Simison, Brett E.; Smith, Brett; Starman, Erik;
Sueoka, Stacey R.; Summers, Richard T.; Szabo, Aimee; Szabo, Louis;
Wampler, Stephen B.; Williams, Timothy R.; White, Charles
Bibcode: 2020SoPh..295..172R
Altcode:
We present an overview of the National Science Foundation's Daniel
K. Inouye Solar Telescope (DKIST), its instruments, and support
facilities. The 4 m aperture DKIST provides the highest-resolution
observations of the Sun ever achieved. The large aperture of
DKIST combined with state-of-the-art instrumentation provide the
sensitivity to measure the vector magnetic field in the chromosphere
and in the faint corona, i.e. for the first time with DKIST we will
be able to measure and study the most important free-energy source
in the outer solar atmosphere - the coronal magnetic field. Over its
operational lifetime DKIST will advance our knowledge of fundamental
astronomical processes, including highly dynamic solar eruptions
that are at the source of space-weather events that impact our
technological society. Design and construction of DKIST took over two
decades. DKIST implements a fast (f/2), off-axis Gregorian optical
design. The maximum available field-of-view is 5 arcmin. A complex
thermal-control system was implemented in order to remove at prime
focus the majority of the 13 kW collected by the primary mirror and
to keep optical surfaces and structures at ambient temperature, thus
avoiding self-induced local seeing. A high-order adaptive-optics
system with 1600 actuators corrects atmospheric seeing enabling
diffraction limited imaging and spectroscopy. Five instruments, four
of which are polarimeters, provide powerful diagnostic capability
over a broad wavelength range covering the visible, near-infrared,
and mid-infrared spectrum. New polarization-calibration strategies
were developed to achieve the stringent polarization accuracy
requirement of 5×10−4. Instruments can be combined and
operated simultaneously in order to obtain a maximum of observational
information. Observing time on DKIST is allocated through an open,
merit-based proposal process. DKIST will be operated primarily in
"service mode" and is expected to on average produce 3 PB of raw
data per year. A newly developed data center located at the NSO
Headquarters in Boulder will initially serve fully calibrated data to
the international users community. Higher-level data products, such as
physical parameters obtained from inversions of spectro-polarimetric
data will be added as resources allow.
Title: Supergranulation on the Sun and stars: A simple model for
its length scale
Authors: Rast, Mark; Trampedach, Regner
Bibcode: 2019AAS...23412205R
Altcode:
Turbulent convection in stellar envelopes is critical to heat transport
and dynamo activity. Modeling it well it has proven surprisingly
difficult, and recent solar and stellar observations have raised
questions about our understanding of the dynamics of both the deep solar
convection and the mean structure of the upper layers of convective
stellar envelopes. In particular, the amplitude of low wavenumber
convective motions in both local area radiative magnetohydrodynamic
and global spherical shell magnetohydrodynamic simulations of the Sun
appear to be too high. In global simulations this results in weaker
than needed rotational constraint of the motions and consequent
non solar-like differential rotation profiles. In deep local area
simulations it yields strong horizontal flows in the photosphere
on scales much larger than the observed supergranulation, leaving
the origin of the solar supergranular scale enigmatic. The problems
are not confined to the Sun. Models of stellar convection show too
sharp a transition to the interior adiabatic gradient, leading to a
mismatch between computed and observed oscillation frequencies. We
suggest that there is a common solution to these problems: convective
motions in stellar envelopes are even more nonlocal than numerical
models suggest. Small scale photospherically driven motions dominate
convective transport even at depth, descending through a very nearly
adiabatic interior (more nearly adiabatic in the mean than numerical
models achieve). To test this, we develop a simple model that reproduces
the mean thermodynamic stratification of three dimensional hydrodynamic
stellar envelope models. It can recover the mean thermodynmaic states of
the full models knowing only the filling factor and entropy fluctuations
of the granular downflows in their photospheres. The supergranular scale
of convection is then determined by the depth to which the presence
of granular downflows alters the otherwise adiabatically stratified
background. The supergranular scale of convection is then determined
by the depth to which the presence of granular downflows alters the
otherwise adiabatically stratified background.
Title: Helioseismic Inversion method applied to Stokes data
Authors: Agrawal, Piyush; Rast, Mark; Ruiz Cobo, Basilio
Bibcode: 2019shin.confE.132A
Altcode:
As light travels through an atmosphere, it interacts with the medium
through absorption, emission and scattering processes. Given a light
spectra, inferring the physical properties (for example T, Pg, velocity)
of the atmosphere it traversed, is called an inversion problem. To infer
the unknown atmosphere, one usually starts with a depth-dependent guess
atmospheric model and perturbs it until the synthesized spectra through
this model match the observed spectra. The desired perturbations are
computed using response functions which is a measure of the sensitivity
of spectra to changes in atmospheric variables. Due to the ill-posed
nature of inverse problems, the solutions are non-unique and highly
oscillatory. Thus, nodes are used to obtain a smooth solution. These
nodes are a small number of evenly spaced depth locations where the
perturbations are calculated. Perturbations at remaining depth points
are interpolated using these nodal values. The final model has a depth
resolution set by the number of nodes, independent of the information
content of the spectra. The solution thus obtained, most likely, does
not have the optimal depth resolution. The OLA inversion method
used in helioseismology does not suffer from the limited resolution
issues with nodes. In this method, the response functions are linearly
combined in order to obtain a highly localized, average response kernel
at a given target depth. The width of the kernel corresponds to the
vertical resolution at that depth, and its limit mostly depends on the
amount of spectral information. The inverted physical parameter then
corresponds to this kernel averaged quantity. The process is repeated
for all depths and a smooth inverted solution is obtained. In
this work, we aim to apply the OLA method to spectroscopic data. To
facilitate this, we used SIR code to synthesize spectra through the
1D smooth temperature profiles from MURaM. To this 1D model, we added
a Gaussian perturbation. The goal of the project is how well can we
invert for this perturbed atmosphere using OLA method and how do the
results compare to the SIR inversion code.
Title: Doppler Events in the Solar Photosphere: The Coincident
Superposition of Fast Granular Flows and p-Mode Coherence Patches
Authors: McClure, R. Lee; Rast, Mark P.; Martínez Pillet, Valentin
Bibcode: 2019SoPh..294...18M
Altcode: 2018arXiv181108944M
Observations of the solar photosphere show spatially compact
large-amplitude Doppler velocity events with short lifetimes. In data
from the Imaging Magnetograph eXperiment (IMaX) on the first flight of
the SUNRISE balloon in 2009, events with velocities in excess of 4σ
from the mean can be identified in both intergranular downflow lanes
and granular upflows. We show that the statistics of such events are
consistent with the random superposition of strong convective flows
and p-mode coherence patches. Such coincident superposition complicates
the identification of acoustic wave sources in the solar photosphere,
and may be important in the interpretation of spectral line profiles
formed in solar photosphere.
Title: The Solar Photospheric Continuum Brightness as a Function of
Mean Magnetic Flux Density. I. The Role of the Magnetic Structure
Size Distribution
Authors: Peck, C. L.; Rast, M. P.; Criscuoli, S.; Rempel, M.
Bibcode: 2019ApJ...870...89P
Altcode:
Solar irradiance models indicate that irradiance variations are
dominated by changes in the disk-coverage of magnetic structures,
whose brightness is thought to be determined by their size and average
magnetic flux density. Recent results suggest that the brightness of
small-scale magnetic structures also depends on the mean magnetic flux
of the extended region surrounding them due to reduced convective
vigor. Low spatial resolution, however, may limit the ability to
distinguish the role of magnetic structure size distributions from that
of the mean magnetic flux. Using high-resolution 3D MHD simulations,
we investigate the brightness of magnetic structures embedded in
regions characterized by different mean magnetic flux. In agreement
with previous results, we find reduced brightness with increasing
mean magnetic flux when comparing the pixel-by-pixel continuum
brightness versus magnetic field strength. Evaluating equivalently
sized magnetic structures, however, we find no significant dependence
of the magnetic structure brightness on the mean magnetic flux of the
region in which they are embedded. Rather, we find that simulations
with larger mean magnetic flux generate larger, and therefore darker,
magnetic structures whose contributions result in an overall darkening
of the region. The differences in magnetic structure size distributions
alone can explain the reduced brightness of regions with larger mean
magnetic flux. This implies that, for the range of mean magnetic flux
of the simulations, convective suppression plays at most a secondary
role in determining radiative output of magnetized regions. Quantifying
the role of convective transport over a wider range of mean magnetic
flux is the subject of the second paper in this series.
Title: The Critical Science Plan for DKIST
Authors: Rast, M.; Cauzzi, G.; Martinez Pillet, V.
Bibcode: 2019NCimC..42....7R
Altcode:
The 4-meter Daniel K. Inouye Solar Telescope is nearing completion on
Haleakala, Maui, with first light expected in 2020. In preparation
for early science, the National Solar Observatory is reaching out
to the solar community in order to define the critical science goals
for the first two years of DKIST operations. The overall aim of this
"Critical Science Plan" is to be ready, by start of operations, to
execute a set of observations that take full advantage of the DKIST
capabilities to address critical compelling science.
Title: Status of the Daniel K. Inouye Solar Telescope: unraveling
the mysteries the Sun.
Authors: Rimmele, Thomas R.; Martinez Pillet, Valentin; Goode, Philip
R.; Knoelker, Michael; Kuhn, Jeffrey Richard; Rosner, Robert; Casini,
Roberto; Lin, Haosheng; von der Luehe, Oskar; Woeger, Friedrich;
Tritschler, Alexandra; Fehlmann, Andre; Jaeggli, Sarah A.; Schmidt,
Wolfgang; De Wijn, Alfred; Rast, Mark; Harrington, David M.; Sueoka,
Stacey R.; Beck, Christian; Schad, Thomas A.; Warner, Mark; McMullin,
Joseph P.; Berukoff, Steven J.; Mathioudakis, Mihalis; DKIST Team
Bibcode: 2018AAS...23231601R
Altcode:
The 4m Daniel K. Inouye Solar Telescope (DKIST) currently under
construction on Haleakala, Maui will be the world’s largest solar
telescope. Designed to meet the needs of critical high resolution and
high sensitivity spectral and polarimetric observations of the sun,
this facility will perform key observations of our nearest star that
matters most to humankind. DKIST’s superb resolution and sensitivity
will enable astronomers to address many of the fundamental problems
in solar and stellar astrophysics, including the origin of stellar
magnetism, the mechanisms of coronal heating and drivers of the
solar wind, flares, coronal mass ejections and variability in solar
and stellar output. DKIST will also address basic research aspects of
Space Weather and help improve predictive capabilities. In combination
with synoptic observations and theoretical modeling DKIST will unravel
the many remaining mysteries of the Sun.The construction of DKIST is
progressing on schedule with 80% of the facility complete. Operations
are scheduled to begin early 2020. DKIST will replace the NSO
facilities on Kitt Peak and Sac Peak with a national facility with
worldwide unique capabilities. The design allows DKIST to operate as
a coronagraph. Taking advantage of its large aperture and infrared
polarimeters DKIST will be capable to routinely measure the currently
illusive coronal magnetic fields. The state-of-the-art adaptive optics
system provides diffraction limited imaging and the ability to resolve
features approximately 20 km on the Sun. Achieving this resolution
is critical for the ability to observe magnetic structures at their
intrinsic, fundamental scales. Five instruments will be available at
the start of operations, four of which will provide highly sensitive
measurements of solar magnetic fields throughout the solar atmosphere
- from the photosphere to the corona. The data from these instruments
will be distributed to the world wide community via the NSO/DKIST data
center located in Boulder. We present examples of science objectives
and provide an overview of the facility and project status, including
the ongoing efforts of the community to develop the critical science
plan for the first 2-3 years of operations.
Title: Transport of Internetwork Magnetic Flux Elements in the Solar
Photosphere : Signatures of Large-Scale Flows and their Effect on
Transport Statistics
Authors: Agrawal, Piyush; Rast, Mark; Gosic, Milan; Rempel, Matthias;
Bellot Rubio, Luis
Bibcode: 2018tess.conf21704A
Altcode:
The motions of small-scale magnetic <span class="s1" flux elements
in the solar photosphere can provide some measure of the Lagrangian
properties of the convective <span class="s1" flow. Measurements of
these motions have been critical in estimating the turbulent diffusion
coef<span class="s1" ficient in <span class="s1" flux-transport
dynamo models and in determining the Alfvén wave excitation spectrum
for coronal heating models. We examine the motions of internetwork
<span class="s1" flux elements in Hinode<span class="s1"
/Narrowband Filter Imager magnetograms and study the scaling of
their mean squared displacement and the shape of their displacement
probability distribution as a function of time. We <span class="s1"
find that the mean squared displacement scales super-diffusively with
a slope of about 1.48. Super-diffusive scaling has been observed in
other studies for temporal increments as small as 5 s, increments over
which ballistic scaling would be expected. Using high-cadence MURaM
simulations, we show that the observed super-diffusive scaling at short
increments is a consequence of random changes in barycenter positions
due to <span class="s1" flux evolution. We also <span class="s1"
find that for long temporal increments, beyond granular lifetimes,
the observed displacement distribution deviates from that expected
for a diffusive process, evolving from Rayleigh to Gaussian. This
change in distribution can be modeled analytically by accounting for
supergranular advection along with granular motions. These results
complicate the interpretation of magnetic element motions as strictly
advective or diffusive on short and long timescales and suggest that
measurements of magnetic element motions must be used with caution
in turbulent diffusion or wave excitation models. We propose that
passive tracer motions in measured photospheric <span class="s1"
flows may yield more robust transport statistics.
Title: Transport of Internetwork Magnetic Flux Elements in the
Solar Photosphere
Authors: Agrawal, Piyush; Rast, Mark P.; Gošić, Milan; Bellot Rubio,
Luis R.; Rempel, Matthias
Bibcode: 2018ApJ...854..118A
Altcode: 2017arXiv171101290A
The motions of small-scale magnetic flux elements in the solar
photosphere can provide some measure of the Lagrangian properties of
the convective flow. Measurements of these motions have been critical
in estimating the turbulent diffusion coefficient in flux-transport
dynamo models and in determining the Alfvén wave excitation spectrum
for coronal heating models. We examine the motions of internetwork
flux elements in Hinode/Narrowband Filter Imager magnetograms and
study the scaling of their mean squared displacement and the shape of
their displacement probability distribution as a function of time. We
find that the mean squared displacement scales super-diffusively with
a slope of about 1.48. Super-diffusive scaling has been observed in
other studies for temporal increments as small as 5 s, increments
over which ballistic scaling would be expected. Using high-cadence
MURaM simulations, we show that the observed super-diffusive scaling
at short increments is a consequence of random changes in barycenter
positions due to flux evolution. We also find that for long temporal
increments, beyond granular lifetimes, the observed displacement
distribution deviates from that expected for a diffusive process,
evolving from Rayleigh to Gaussian. This change in distribution can be
modeled analytically by accounting for supergranular advection along
with granular motions. These results complicate the interpretation
of magnetic element motions as strictly advective or diffusive on
short and long timescales and suggest that measurements of magnetic
element motions must be used with caution in turbulent diffusion or
wave excitation models. We propose that passive tracer motions in
measured photospheric flows may yield more robust transport statistics.
Title: An Assessment of and Solution to the Intensity Diffusion
Error Intrinsic to Short-characteristic Radiative Transfer Methods
Authors: Peck, C. L.; Criscuoli, S.; Rast, M. P.
Bibcode: 2017ApJ...850....9P
Altcode: 2017arXiv170809362P
Radiative transfer coupled with highly realistic simulations of the
solar atmosphere is routinely used to infer the physical properties
underlying solar observations. Due to its computational efficiency,
the method of short-characteristics is often employed, despite it
introducing numerical diffusion as an interpolation artifact. In this
paper, we quantify the effect of the numerical diffusion on the spatial
resolution of synthesized emergent intensity images, and derive a
closed form analytical model of the diffusive error made as a function
of viewing angle when using linear interpolation. We demonstrate that
the consequent image degradation adversely affects the comparison
between simulated data and observations away from disk center, unless
the simulations are computed at much higher intrinsic resolutions than
the observations. We also show that the diffusive error is readily
avoided by interpolating the simulation solution on a viewing angle
aligned grid prior to computing the radiative transfer. Doing this
will be critical for comparisons with observations using the upcoming
large aperture telescopes—the Daniel K. Inouye Solar Telescope and
the European Solar Telescope.
Title: The amplitude of the deep solar convection and the origin of
the solar supergranulation
Authors: Rast, Mark
Bibcode: 2017usc..confE...1R
Altcode:
Recent observations and models have raised questions about our
understanding of the dynamics of the deep solar convection. In
particular, the amplitude of low wavenumber convective motions appears
to be too high in both local area radiative magnetohydrodynamic and
global spherical shell magnetohydrodynamic simulations. In global
simulations this results in weaker than needed rotational constraints
and consequent non solar-like differential rotation profiles. In
deep local area simulations it yields strong horizontal flows in the
photosphere on scales much larger than the observed supergranulation. We
have undertaken numerical studies that suggest that solution to this
problem is closely related to the long standing question of the origin
of the solar supergranulation. Two possibilities have emerged. One
suggests that small scale photospherically driven motions dominate
convecive transport even at depth, descending through a very nearly
adiabatic interior (more more nearly adiabatic than current convection
models achieve). Convection of this form can meet Rossby number
constraints set by global scale motions and implies that the solar
supergranulation is the largest buoyantly driven scale of motion in the
Sun. The other possibility is that large scale convection driven deeep
in the Sun dynamically couples to the near surface shear layer, perhaps
as its origin. In this case supergranulation would be the largest
non-coupled convective mode, or only weakly coupled and thus potentially
explaining the observed excess power in the prograde direction. Recent
helioseismic results lend some support to this. We examind both of
these possibilities using carefully designed numerical experiments,
and weigh thier plausibilities in light of recent observations.
Title: Assessing the Impact of Small-Scale Magnetic Morphology on
Solar Variability
Authors: Peck, Courtney; Rast, Mark; Criscuoli, Serena
Bibcode: 2017SPD....48.0503P
Altcode:
Spectral solar irradiance (SSI), the radiant energy flux per wavelength
of the Sun received at Earth, is an important driver of chemical
reactions in the Earth’s atmosphere. Accurate measurements of SSI are
therefore necessary as an input for global climate models. While models
and observations of the spectrally-integrated total solar irradiance
(TSI) variations agree within ∼ 95%, they can disagree on the sign
and magnitude of the SSI variations. In this work, we examine the
contribution of currently-unresolved small-scale magnetic structures
to SSI variations in the photosphere. We examine the emergent spectra
of two atmospheres with differing imposed-field conditions — one
with a small-scale dynamo and the other with a predominantly vertical
magnetic field — with similar mean field strengths at wavelengths
spanning from visible to infrared. Comparing the radiative output at
various viewing angles of pixels of equal vertical magnetic field
strength between the two simulations, we find that the small-scale
dynamo simulations produce higher radiative output than those in the
predominantly vertical field simulation. This implies that the radiative
output of a small magnetic structure depends on the magnetic morphology
of the environment in which it is embedded, which is currently not
included in SSI models. We deduce the effect on inferred irradiance
by comparing the disk-integrated irradiance of these two atmospheres
with standard 1D model atmospheres used in SSI modeling.
Title: Assessment of and a Solution to the Intensity Diffusion Error
Intrinsic in Short-Characteristic Radiative Transfer
Authors: Peck, Courtney; Rast, Mark; Criscuoli, Serena
Bibcode: 2017SPD....4820701P
Altcode:
Short characteristic radiative transfer coupled with 3D MHD simulations
are routinely used to compare simulations with observations of the
solar atmosphere. While it has been known that the method of short
characteristics radiative transfer results in intensity diffusion,
it has been routinely employed to solve radiative transfer due to
its computational expediency. In this talk, we discuss the effect
of spatial smearing due to short characteristics radiative transfer
under both linear and high-order interpolation. We then demonstrate
that linear interpolation results in an effective spatial smearing
related to the number of grid heights above the τ = 1 surface and
conserves intensity. Additionally, we show that the use of high-order
strict monotonic interpolation reduces the amount of smearing, but at
the expense of error in the integrated emergent intensity. Finally,
we demonstrate that these issues can be easily avoided at no
added computational expense by interpolating the atmosphere onto a
ray-directed grid and computing the radiative transfer for vertical
rays through the grid.
Title: Magnetically Modulated Heat Transport in a Global Simulation
of Solar Magneto-convection
Authors: Cossette, Jean-Francois; Charbonneau, Paul; Smolarkiewicz,
Piotr K.; Rast, Mark P.
Bibcode: 2017ApJ...841...65C
Altcode:
We present results from a global MHD simulation of solar convection
in which the heat transported by convective flows varies in-phase
with the total magnetic energy. The purely random initial magnetic
field specified in this experiment develops into a well-organized
large-scale antisymmetric component undergoing hemispherically
synchronized polarity reversals on a 40 year period. A key feature of
the simulation is the use of a Newtonian cooling term in the entropy
equation to maintain a convectively unstable stratification and drive
convection, as opposed to the specification of heating and cooling terms
at the bottom and top boundaries. When taken together, the solar-like
magnetic cycle and the convective heat flux signature suggest that a
cyclic modulation of the large-scale heat-carrying convective flows
could be operating inside the real Sun. We carry out an analysis of
the entropy and momentum equations to uncover the physical mechanism
responsible for the enhanced heat transport. The analysis suggests
that the modulation is caused by a magnetic tension imbalance inside
upflows and downflows, which perturbs their respective contributions to
heat transport in such a way as to enhance the total convective heat
flux at cycle maximum. Potential consequences of the heat transport
modulation for solar irradiance variability are briefly discussed.
Title: Daniel K. Inouye Solar Telescope: High-resolution observing
of the dynamic Sun
Authors: Tritschler, A.; Rimmele, T. R.; Berukoff, S.; Casini, R.;
Kuhn, J. R.; Lin, H.; Rast, M. P.; McMullin, J. P.; Schmidt, W.;
Wöger, F.; DKIST Team
Bibcode: 2016AN....337.1064T
Altcode:
The 4-m aperture Daniel K. Inouye Solar Telescope (DKIST) formerly
known as the Advanced Technology Solar Telescope (ATST) is currently
under construction on Haleakalā (Maui, Hawai'i) projected to
start operations in 2019. At the time of completion, DKIST will be
the largest ground-based solar telescope providing unprecedented
resolution and photon collecting power. The DKIST will be equipped
with a set of first-light facility-class instruments offering unique
imaging, spectroscopic and spectropolarimetric observing opportunities
covering the visible to infrared wavelength range. This first-light
instrumentation suite will include: a Visible Broadband Imager (VBI) for
high-spatial and -temporal resolution imaging of the solar atmosphere; a
Visible Spectro-Polarimeter (ViSP) for sensitive and accurate multi-line
spectropolarimetry; a Fabry-Pérot based Visible Tunable Filter
(VTF) for high-spatial resolution spectropolarimetry; a fiber-fed
Diffraction-Limited Near Infra-Red Spectro-Polarimeter (DL-NIRSP)
for two-dimensional high-spatial resolution spectropolarimetry
(simultaneous spatial and spectral information); and a Cryogenic Near
Infra-Red Spectro-Polarimeter (Cryo-NIRSP) for coronal magnetic field
measurements and on-disk observations of, e.g., the CO lines at 4.7
μm. We will provide an overview of the DKIST's unique capabilities
with strong focus on the first-light instrumentation suite, highlight
some of the additional properties supporting observations of transient
and dynamic solar phenomena, and touch on some operational strategies
and the DKIST critical science plan.
Title: The amplitude of the deep solar convection and the origin of
the solar supergranulation
Authors: Rast, Mark
Bibcode: 2016usc..confE..91R
Altcode:
Recent observations and models have raised questions about our
understanding of the dynamics of the deep solar convection. In
particular, the amplitude of low wavenumber convective motions appears
to be too high in both local area radiative magnetohydrodynamic and
global spherical shell magnetohydrodynamic simulations. In global
simulations this results in weaker than needed rotational constraints
and consequent non solar-like differential rotation profiles. In
deep local area simulations it yields strong horizontal flows in the
photosphere on scales much larger than the observed supergranulation. We
have undertaken numerical studies that suggest that solution to this
problem is closely related to the long standing question of the origin
of the solar supergranulation. Two possibilities have emerged. One
suggests that small scale photospherically driven motions dominate
convecive transport even at depth, descending through a very nearly
adiabatic interior (more more nearly adiabatic than current convection
models achieve). Convection of this form can meet Rossby number
constraints set by global scale motions and implies that the solar
supergranulation is the largest buoyantly driven scale of motion in the
Sun. The other possibility is that large scale convection driven deeep
in the Sun dynamically couples to the near surface shear layer, perhaps
as its origin. In this case supergranulation would be the largest
non-coupled convective mode, or only weakly coupled and thus potentially
explaining the observed excess power in the prograde direction. Recent
helioseismic results lend some support to this. We examind both of
these possibilities using carefully designed numerical experiments,
and weigh thier plausibilities in light of recent observations.
Title: Supergranulation as the Largest Buoyantly Driven Convective
Scale of the Sun
Authors: Cossette, Jean-Francois; Rast, Mark P.
Bibcode: 2016ApJ...829L..17C
Altcode: 2016arXiv160604041C
The origin of solar supergranulation remains a mystery. Unlike
granulation, the size of which is comparable to both the thickness of
the radiative boundary layer and local scale-height in the photosphere,
supergranulation does not reflect any obvious length scale of the
solar convection zone. Moreover, recent observations of flows in the
photosphere using Doppler imaging or correlation or feature tracking
show a monotonic decrease in horizontal flow power at scales larger
than supergranulation. Both local area and global spherical shell
simulations of solar convection by contrast show the opposite, an
increase in horizontal flow amplitudes to a low wavenumber. We examine
these disparities and investigate how the solar supergranulation
may arise as a consequence of nonlocal heat transport by cool diving
plumes. Using three-dimensional anelastic simulations with surface
driving, we show that the kinetic energy of the largest convective
scales in the upper layers of a stratified domain reflects the depth
of transition from strong buoyant driving to adiabatic stratification
below caused by the dilution of the granular downflows. This depth
is quite shallow because of the rapid increase of the mean density
below the photosphere. We interpret the observed monotonic decrease in
solar convective power at scales larger than supergranulation to be
a consequence of this rapid transition, with the supergranular scale
the largest buoyantly driven mode of convection in the Sun.
Title: Turbulent transport of Small-scale magnetic flux elements on
Solar Photosphere
Authors: Agrawal, Piyush; Rempel, Matthias; Bellot Rubio, Luis;
Rast, Mark
Bibcode: 2016SPD....47.1201A
Altcode:
We study the transport of small-scale magnetic elements on the solar
photosphere using both observations and simulations. Observational
data was obtained from Hinode - Solar Optical Telescope (SOT/SP)
instrument and simulations from MURaM code. The magnetic flux elements
were tracked in both data sets and statistics were obtained. We compute
the probability density of the Eulerian distances traveled by the flux
elements along Lagrangian trajectories. For a two-dimensional random
walk process this distribution should be Rayleigh. Preliminary results
show that the measured probability distribution in both the observed
and simulated data approximates a random walk, on time scale close to
the lifetime of granules, but deviates from it for longer times. This
implies that diffusion may not be an appropriate framework for transport
in the solar photosphere. We explore the roles of flux cancelation
and element trapping in producing this result. Work is ongoing.
Title: The structure and evolution of boundary layers in stratified
convection
Authors: Anders, Evan H.; Brown, Benjamin; Brandenburg, Axel;
Rast, Mark
Bibcode: 2016SPD....47.0712A
Altcode:
Solar convection is highly stratified, and the density in the Sun
increases by many orders of magnitude from the photosphere to the
base of the convection zone. The photosphere is an important boundary
layer, and interactions between the surface convection and deep
convection may lie at the root of the solar convection conundrum,
where observed large-scale velocities are much lower than predicted
by full numerical simulations. Here, we study the structure and time
evolution of boundary layers in numerical stratified convection. We
study fully compressible convection within plane-parallel layers using
the Dedalus pseudospectral framework. Within the context of polytropic
stratification, we study flows from low (1e-3) to moderately high (0.1)
Mach number, and at moderate to high Rayleigh number to study both
laminar and turbulent convective transport. We aim to characterize
the thickness and time variation of velocity and thermal (entropy)
boundary layers at the top and bottom boundaries of the domain.
Title: Supergranulation as the Sun's largest buoyantly driven mode
of convection
Authors: Cossette, Jean-Francois; Rast, Mark
Bibcode: 2016SPD....4720305C
Altcode:
Solar supergranulation has been characterized as horizontally divergent
flow motions having a typical scale of 32 Mm using Doppler imaging,
granule tracking and helioseismology. Unlike granules, the size of
which is comparable to both the thickness of the radiative boundary
layer and local scale height at the photosphere, supergranules
do not appear to correspond to any particular length scale of the
flow. Possible explanations ranging from convection theories involving
Helium ionization to spatial correlation or self-organization of
granular flows have been proposed as physical mechanisms to explain
solar supergranulation. However, its existence remains largely a
mystery. Remarkably, horizontal velocity power spectra obtained
from Doppler imaging and correlation tracking of flow features at
the solar surface reveal the presence of peaks corresponding to
granular and supergranular scales, followed by a monotonic decrease
in power at scales larger than supergranulation, which suggests that
large-scale modes in the deep layers of the convection zone may be
suppressed. Using 3D anelastic simulations of solar convection we
investigate whether supergranulation may reflect the largest buoyantly
driven mode of convection inside the Sun. Results show that the amount
of kinetic energy contained in the largest flow scales relative to that
associated with supergranular motions is a function of the depth of
the transition from a convectively unstable to convectively stable mean
stratification inside the simulation. This suggests that the observed
monotonic decrease in power at scales larger than supergranulation
may be explained by rapid cooling in the subphotospheric layers and
an essentially isentropic solar interior, wherein convective driving
is effectively suppressed.
Title: Resolving the source of the solar acoustic oscillations:
What will be possible with DKIST?
Authors: Rast, Mark; Martinez Pillet, Valentin
Bibcode: 2016SPD....4720105R
Altcode:
The solar p-modes are likely excited by small-scale convective dynamics
in the solar photosphere, but the detailed source properties are not
known. Theoretical models differ and observations are yet unable to
differentiate between them. Resolving the underlying source events
is more than a curiosity. It is important to the veracity of global
helioseismic measurements (including local spectral methods such
as ring diagram analysis) because global p-mode line shapes and thus
accurate frequency determinations depend critically on the relationship
between intensity and velocity during the excitation events. It is
also fundamental to improving the accuracy of the local time-distance
measurements because in these kernel calculations depend on knowledge
of the source profile and the properties of the excitation noise. The
Daniel K. Inouye Solar Telescope (DKIST) will have the spatial
resolution and spectral range needed to resolve the solar acoustic
excitation events in both time and space (horizontally and with
height) using multi-wavelength observations. Inversions to determine
the dynamic and thermodynamic evolution of the discrete small-scale
convective events that serve as acoustic sources may also be possible,
though determination of the pressure fluctuations associated with the
sources is a challenge. We describe the DKIST capabilities anticipated
and the preliminary work needed to prepare for them.
Title: Interpreting Irradiance Distributions Using High-Resolution
3D MHD Simulations
Authors: Peck, Courtney; Rast, Mark; Criscuoli, Serena; Uitenbroek,
Han; Rempel, Matthias D.
Bibcode: 2016SPD....4730302P
Altcode:
We present initial results of studies aimed at understanding the
impact of the unresolved magnetic field distribution on solar spectral
irradiance. Using high-resolution 3D MHD simulations (from MURaM code)
and spectral synthesis (with the RH code), we examine the emergent
spectra of two atmospheres with similar mean field strengths but
differing imposed-field conditions at wavelengths spanning from
visible to infrared. Comparing the contrast against the magnetic
field strength for the two magnetic simulations, we find differences
in the distributions of contrasts versus field strength. We repeat
the analysis after convolving the images with the PSF of a typical
solar telescope (1-meter) and discuss the potential implications for
irradiance modeling and future steps.
Title: ESA's Report to the 41st COSPAR Meeting
Authors: Rast, M.
Bibcode: 2016ESASP1333.....R
Altcode:
No abstract at ADS
Title: The Effects of Magnetic Field Morphology on the Determination
of Oxygen and Iron Abundances in the Solar Photosphere
Authors: Moore, Christopher S.; Uitenbroek, Han; Rempel, Matthias;
Criscuoli, Serena; Rast, Mark
Bibcode: 2016AAS...22712501M
Altcode:
The solar chemical abundance (or a scaled version of it) is
implemented in numerous astrophysical analyses. Thus, an accurate
and precise estimation of the solar elemental abundance is crucial
in astrophysics.We have explored the impact of magnetic fields
on the determination of the solar photospheric oxygen andiron
abundances using 3D radiation-magnetohydrodynamic (MHD) simulations
of convection. Specifically, weexamined differences in abundance
deduced from three classes of atmospheres simulated with the MURaM
code: apure hydrodynamic (HD) simulation, an MHD simulation with
a local dynamo magnetic field that has saturated withan unsigned
vertical field strength of 80 G at the optical depth unity surface,
and an MHD simulation with an initially imposed vertical mean field
of 80 G. We use differential equivalent width analysis for diagnosing
abundances derived from five oxygen and four iron spectral lines of
differing wavelength, oscillator strength, excitation potential, and
Lande g-factor, and find that the morphology of the magnetic field
is important to the outcome of abundance determinations. The largest
deduced abundance differences are found in the vertical mean field
simulations and small scale unresolved field resulting from the local
dynamo has a smaller impact on abundance determinations.
Title: Photometric Trends in the Visible Solar Continuum and Their
Sensitivity to the Center-to-Limb Profile
Authors: Peck, C. L.; Rast, M. P.
Bibcode: 2015ApJ...808..192P
Altcode: 2015arXiv150206308P
Solar irradiance variations over solar rotational timescales are
largely determined by the passage of magnetic structures across
the visible solar disk. Variations on solar cycle timescales are
thought to be similarly due to changes in surface magnetism with
activity. Understanding the contribution of magnetic structures
to total solar irradiance and solar spectral irradiance requires
assessing their contributions as a function of disk position. Since
only relative photometry is possible from the ground, the contrasts of
image pixels are measured with respect to a center-to-limb intensity
profile. Using nine years of full-disk red and blue continuum images
from the Precision Solar Photometric Telescope at the Mauna Loa
Solar Observatory, we examine the sensitivity of continuum contrast
measurements to the center-to-limb profile definition. Profiles which
differ only by the amount of magnetic activity allowed in the pixels
used to determine them yield oppositely signed solar cycle length
continuum contrast trends, either agreeing with previous results
and showing negative correlation with solar cycle or disagreeing
and showing positive correlation with solar cycle. Changes in the
center-to-limb profile shape over the solar cycle are responsible for
the contradictory contrast results, and we demonstrate that the lowest
contrast structures, internetwork and network, are most sensitive to
these. Thus the strengths of the full-disk, internetwork, and network
photometric trends depend critically on the magnetic flux density used
in the quiet-Sun definition. We conclude that the contributions of
low contrast magnetic structures to variations in the solar continuum
output, particularly to long-term variations, are difficult, if not
impossible, to determine without the use of radiometric imaging.
Title: Daniel K. Inouye Solar Telescope (DKIST) Critical Science Plan
Authors: Rast, Mark
Bibcode: 2015IAUGA..2257167R
Altcode:
The Daniel K. Inouye Solar Telescope (DKIST), formerly the Advanced
Technology Solar Telescope (ATST), is under construction on Haleakala,
Maui HI, with expected instrument integration in 2018 and start of
operations during the summer of 2019. In preparation, the National
Solar Observatory (NSO) is working with the Science Working Group to
formulate a critical science plan for early operations and is calling
for community involvement in all stages of its development. The first
step in this process is the definition of a set of critical science
themes and, under each of these, use-cases that outline the scientific
motivation along with the instrument suite and high level observing
strategies to be employed. The use-cases will later be refined into
observing proposals, which will guide the development of efficient
operations tools and procedures and provide the framework for some of
the first science observations to be made with the telescope. A web
interface has been established to facilitate community engagement.
Title: Sensitivity of Long-term Photometric Trends to Center-to-Limb
Profile Variations
Authors: Rast, Mark; Peck, Courtney
Bibcode: 2015IAUGA..2257070R
Altcode:
It has been reported (Preminger et al. 2011) that the disk-integrated
contrast of visible solar continuum images varies out of phase with
the solar cycle, in contrast to faculae dominated models of total
solar irradiance and SOHO/VIRGO measurements of the visible continuum
but in qualitative agreement with SIM measurements in some spectral
bands. Since only relative photometry is possible from the ground,
contrast measurements are made with respect to a center-to-limb
intensity profile. Using nine years of full-disk red and blue continuum
images from the Precision Solar Photometric Telescope (PSPT) at the
Mauna Loa Solar Observatory (MLSO), we examine the sensitivity of
deduced cycle related irradiance trends to the center-to-limb profile
definition employed. We find that the disk integrated continuum
contrast, and the integrated contrasts of the internetwork, network,
and active network separately, are very sensitive to the center-to-limb
definition employed. The sensitivity of the center-to-limb profile
itself to changes in the Sun's surface magnetism in turn depends on
how the profile is constructed, and different center-to-limb algorithms
yield contradictory cycle related contrast trends. Radiometric imaging
is required to determine the true center-to-limb variation of magnetic
structures and unambiguously measure their contributions to solar
spectral irradiance variations.
Title: The Effects of Magnetic Field Morphology on the Determination
of Oxygen and Iron Abundances in the Solar Photosphere
Authors: Moore, Christopher S.; Uitenbroek, Han; Rempel, Matthias;
Criscuoli, Serena; Rast, Mark P.
Bibcode: 2015ApJ...799..150M
Altcode:
We have explored the impact of magnetic fields on the determination
of the solar photospheric oxygen and iron abundances using
three-dimensional radiation-magnetohydrodynamic (MHD) simulations
of convection. Specifically, we examined differences in abundance
deduced from three classes of atmospheres simulated with the MURaM
code: a pure hydrodynamic (HD) simulation, an MHD simulation with
a local dynamo magnetic field that has saturated with an unsigned
vertical field strength of 80 G at τ = 1, and an MHD simulation with
an initially imposed vertical mean field of 80 G. We use differential
equivalent width analysis for diagnosing abundances derived from
five oxygen and four iron lines of differing wavelength, oscillator
strength, excitation potential, and Landé g-factor, and find that
the morphology of the magnetic field is important to the outcome of
abundance determinations. The largest deduced abundance differences are
found in the vertical mean field simulations, where the O I and Fe I
abundance corrections compared to the pure HD case are ~+0.011 dex and
+0.065 dex respectively. Small scale unresolved field resulting from
the local dynamo has a smaller impact on abundance determinations,
with corrections of -0.0001 dex and +0.0044 dex in the magnetized
compared to the pure HD simulations. While the overall influence of
magnetic field on abundance estimates is found to be small, we stress
that such estimates are sensitive not only to the magnitude of magnetic
field but also to its morphology.
Title: The Importance of Solar Spectral Irradiance to the Sun-Earth
Connection: Lessons-learned from SORCE and Their Relevance to
Future Missions
Authors: Harder, J. W.; Snow, M. A.; Richard, E. C.; Rast, M.; Merkel,
A. W.; Woods, T. N.
Bibcode: 2014AGUFMSH33B..04H
Altcode:
The Solar Radiation and Climate Experiment (SORCE) mission has provided
for the first time solar spectral irradiance (SSI) observations over
a full solar cycle time period with wavelength coverage from the X-ray
through the near infrared. This paper will discuss the lessons-learned
from SORCE including the need to develop more effective methods to
track on-orbit spectroscopic response and sensitivity degradation. This
is especially important in using these data products as input to
modern day chemistry-climate models that require very broad spectral
coverage with moderate-to-high spectral and temporal resolution to
constrain the solar component to the atmospheric response. A basic
requirement to obtain this essential climate record is to 1) perform
preflight radiometric calibrations that are traceable SI standards
along with a complete specification of the instruments spectroscopic
response, and 2) design the instrument to have the ability to perform
instrument-only sensitivity corrections to objectively account for
on-orbit degradation. The development of the NIST SIRCUS (National
Institute of Science and Technology, Sources for Irradiance and Radiance
Calibration with Uniform Sources) now permits the full characterization
of the spectral radiometer's response, and on-orbit degradation
characterization through comparisons of redundant detectors and
spectrometers appears to be the most practical method to perform these
corrections for the near ultraviolet through the near infrared. Going
forward, we discuss a compact spectral radiometer development that
will couple with advances in CubeSat technology to allow for shorter
mission lengths, relatively inexpensive development and launch costs,
and reduce the risk of data gaps between successive missions without
compromising measurement accuracy. We also discuss the development
of a radiometric solar imager that will both greatly improve the
interpretation of existing Sun-as-a-star irradiance observations and
provide a bridge from our current irradiance capabilities to future
high spatial/temporal resolution solar physics assets such as the
Daniel K. Inouye Solar Telescope (DKIST).
Title: The Role of Subsurface Flows in Solar Surface Convection:
Modeling the Spectrum of Supergranular and Larger Scale Flows
Authors: Lord, J. W.; Cameron, R. H.; Rast, M. P.; Rempel, M.;
Roudier, T.
Bibcode: 2014ApJ...793...24L
Altcode: 2014arXiv1407.2209L
We model the solar horizontal velocity power spectrum at scales
larger than granulation using a two-component approximation to the
mass continuity equation. The model takes four times the density
scale height as the integral (driving) scale of the vertical motions
at each depth. Scales larger than this decay with height from the
deeper layers. Those smaller are assumed to follow a Kolmogorov
turbulent cascade, with the total power in the vertical convective
motions matching that required to transport the solar luminosity in a
mixing length formulation. These model components are validated using
large-scale radiative hydrodynamic simulations. We reach two primary
conclusions. (1) The model predicts significantly more power at low
wavenumbers than is observed in the solar photospheric horizontal
velocity spectrum. (2) Ionization plays a minor role in shaping the
observed solar velocity spectrum by reducing convective amplitudes in
the regions of partial helium ionization. The excess low wavenumber
power is also seen in the fully nonlinear three-dimensional radiative
hydrodynamic simulations employing a realistic equation of state. This
adds to other recent evidence suggesting that the amplitudes of
large-scale convective motions in the Sun are significantly lower
than expected. Employing the same feature tracking algorithm used
with observational data on the simulation output, we show that the
observed low wavenumber power can be reproduced in hydrodynamic
models if the amplitudes of large-scale modes in the deep layers
are artificially reduced. Since the large-scale modes have reduced
amplitudes, modes on the scale of supergranulation and smaller remain
important to convective heat flux even in the deep layers, suggesting
that small-scale convective correlations are maintained through the
bulk of the solar convection zone.
Title: The Earth's Hydrological Cycle
Authors: Bengtsson, Lennart; Bonnet, R. -M.; Calisto, M.; Destouni,
G.; Gurney, R.; Johannessen, J.; Kerr, Y.; Lahoz, W. A.; Rast, M.
Bibcode: 2014ehc..book.....B
Altcode:
No abstract at ADS
Title: Implications of high-resolution ATST observations for global
dynamo and irradiance models
Authors: Rast, Mark
Bibcode: 2013SPD....4440005R
Altcode:
The ATST will provide unprecedented measurements of small-scale
fields and flows in the solar photosphere and chromosphere, and what
we learn at those scales will have implications for models of global
solar behavior. We will discuss these connections in the context of
two important problems: the operation of the global solar dynamo and
the variability of the solar spectral irradiance. For both of these,
measuring the statistical properties of small-scale magnetic flux
elements and their dynamics is critical. ATST will allow exploration
of the small-scale magnetohydrodynamics that underlies the turbulent
diffusion processes central to dynamo behavior. ATST will also allow
characterization of the magnetic substructure that underlies variation
in spectral irradiance. In both cases what we learn about the small
scales will have global impacts that can be studied only by including
their contributions in global models statistically. Arriving at such
statistical descriptions poses a compelling challenge, which we have
only begun to address.
Title: Understanding the Role of Small-Scale Flux in Solar Spectral
Irradiance Variation
Authors: Rast, M. P.; Harder, J. W.
Bibcode: 2012ASPC..463...65R
Altcode:
Global solar spectral irradiance variations depend on changes
in magnetic flux concentrations at the smallest scales. Modeling
has focused on the contributions of magnetic structures in full
disk images as those contributions have strong center-to-limb
dependencies, but these dependencies have never been determined
radiometrically; only the photometric intensity relative to some
reference ‘quiet-sun’,1 the magnetic structure contrast,
is measurable with ground based imagery. This is problematic because
unresolved inhomogeneities influence not only the full-disk structure
intensities themselves, but also the quiet-sun background against
which their contrast is measured. We thus argue that, to understand
the physical causes underlying solar spectral irradiance variations,
two fundamental questions must be addressed: What is the real
Iλ (μ) as a function of B in full-disk images? This can
only be answered by imaging the Sun radiometrically from space, and
we propose a Radiometric Solar Imager design. What governs spectral
irradiance changes at sub arc-second scales? This can be addressed
by a combination of high resolution ground based imaging (ATST-VBI)
and three dimensional radiative magnetohydrodynamic modeling, and
we propose a synoptic approach. Finally, a way to account for the
variance introduced by unresolved substructure in spectral irradiance
modeling must be devised. This is critical, as imaging and modeling
at the highest resolutions but over the full solar disk will likely
remain unattainable for some time.
Title: Measured and modeled trends in the solar spectral irradiance
variability using the SORCE SIM and SOLSTICE instruments
Authors: Harder, J. W.; Fontenla, J. M.; Rast, M. P.; Snow, M. A.;
Woods, T. N.
Bibcode: 2011AGUFMGC22A..06H
Altcode:
The Solar Radiation and Climate Experiment (SORCE) Spectral Irradiance
Monitor (SIM) measures solar spectral variability in the 200-2400 nm
range accounting for about 97% of the total solar irradiance (TSI). SIM
concurrently measures ultraviolet variability from 200-310 nm with
the higher spectral resolution Solar-Stellar Irradiance Comparison
Experiment (SOLSTICE). These instruments monitored the descending phase
of solar cycle 23 and are now continuing these observations in the
rising phase of cycle 24. SIM and SOLSTICE observations clearly show
rotational modulation of spectral irradiance due to the evolution of
dark sunspots and bright faculae that respectively deplete and enhance
solar radiation. In addition to this well-known phenomenon, SORCE
observations indicate a slower evolutionary trend in solar spectral
irradiance (SSI) over solar cycle time-scales that are both in and
out of phase with the TSI, with the ultraviolet component indicating
significantly larger UV variability than reported from the UARS era
instruments. Wavelengths where the brightness temperature is less than
Teff = 5770 K are in phase, and where the brightness temperature >
Teff in the visible and infrared, the time series show an anti-solar
cycle trend. This observation is discussed in terms of the Solar
Radiation Physical Modeling (SRPM) program employing solar images from
Precision Solar Photometric Telescope (PSPT) that provides the areas
of active regions on the solar disk as function of time to generate a
modeled SSI time series that is concurrent with the SORCE observations
but extending back to solar maximum conditions. Comparative studies
of the SIM and SOLSTICE will be presented along with analysis of solar
variability derived from SRPM and PSPT.
Title: The role of magnetic field in supergranular scale selection
Authors: Lord, J. W.; Rast, M. P.; Rempel, M.
Bibcode: 2011AGUFMSH53C..03L
Altcode:
We examine the role of the magnetic field in solar surface convection
using the MURaM radiative MHD code. Using two 74x74x16 Mm simulations,
one without magnetic field and one with an initially uniform and
vertical 10 Gauss field, we investigate the role of magnetic field in
supergranular scale selection. We find that the simulation with magnetic
field has two peaks in the photospheric kinetic energy spectrum, one
corresponding to granular size scales and a second peak near 24 Mm,
while the purely hydrodynamic simulation has a single peak near the
size scale of granulation (Figure 1). We examine two possible physical
mechanisms which may underlie this increased power at low wavenumbers:
the decreased opacity in magnetic elements near the photosphere which
increases the radiative cooling there and the coupling, by regions
of high magnetic flux density in convective downflows, of deeper
larger scale motions to the photosphere. These mechanisms imply two
very different processes. The first suggests that supergranulation
is organized in the photosphere where radiation escapes the system
(top down) and the second suggests that the large scale convection
deep in the sun influences the scales observed in the photosphere
(bottom up). Temporal cross correlation is used to examine which
direction information is moving during pattern formation across
scales. Additionally, a series of experiments were conducted to
isolate individual physical effects, artificially increasing and
decreasing the radiative losses in regions of strong magnetic flux,
reducing the importance of magnetic tension, and constraining the box
depth to understand the sensitivity of the size scales observed to
the boundary conditions imposed.
Title: Observing Evolution in the Supergranular Network Length Scale
During Periods of Low Solar Activity
Authors: McIntosh, Scott W.; Leamon, Robert J.; Hock, Rachel A.;
Rast, Mark P.; Ulrich, Roger K.
Bibcode: 2011ApJ...730L...3M
Altcode: 2011arXiv1102.0303M
We present the initial results of an observational study into the
variation of the dominant length scale of quiet solar emission:
supergranulation. The distribution of magnetic elements in the lanes
that from the network affects, and reflects, the radiative energy in
the plasma of the upper solar chromosphere and transition region at
the magnetic network boundaries forming as a result of the relentless
interaction of magnetic fields and convective motions of the Suns'
interior. We demonstrate that a net difference of ~0.5 Mm in the
supergranular emission length scale occurs when comparing observation
cycle 22/23 and cycle 23/24 minima. This variation in scale is
reproduced in the data sets of multiple space- and ground-based
instruments and using different diagnostic measures. By means of
extension, we consider the variation of the supergranular length
scale over multiple solar minima by analyzing a subset of the Mount
Wilson Solar Observatory Ca II K image record. The observations and
analysis presented provide a tantalizing look at solar activity in
the absence of large-scale flux emergence, offering insight into
times of "extreme" solar minimum and general behavior such as the
phasing and cross-dependence of different components of the spectral
irradiance. Given that the modulation of the supergranular scale
imprints itself in variations of the Suns' spectral irradiance, as well
as in the mass and energy transport into the entire outer atmosphere,
this preliminary investigation is an important step in understanding
the impact of the quiet Sun on the heliospheric system.
Title: Modeling the Near-Surface Shear Layer: Diffusion Schemes
Studied With CSS
Authors: Augustson, Kyle; Rast, Mark; Trampedach, Regner; Toomre, Juri
Bibcode: 2011JPhCS.271a2070A
Altcode: 2010arXiv1012.4781A
As we approach solar convection simulations that seek to model the
interaction of small-scale granulation and supergranulation and even
larger scales of convection within the near-surface shear layer (NSSL),
the treatment of the boundary conditions and minimization of sub-grid
scale diffusive processes become increasingly crucial. We here assess
changes in the dynamics and the energy flux balance of the flows
established in rotating spherical shell segments that capture much
of the NSSL with the Curved Spherical Segment (CSS) code using two
different diffusion schemes. The CSS code is a new massively parallel
modeling tool capable of simulating 3-D compressible MHD convection with
a realistic solar stratification in rotating spherical shell segments.
Title: Radiative emission of solar features in the Ca II K line:
comparison of measurements and models
Authors: Ermolli, I.; Criscuoli, S.; Uitenbroek, H.; Giorgi, F.;
Rast, M. P.; Solanki, S. K.
Bibcode: 2010A&A...523A..55E
Altcode: 2010arXiv1009.0227E
Context. The intensity of the Ca II K resonance line observed with
spectrographs and Lyot-type filters has long served as a diagnostic of
the solar chromosphere. However, the literature contains a relative
lack of photometric measurements of solar features observed at this
spectral range.
Aims: We study the radiative emission of various
types of solar features, such as quiet Sun, enhanced network, plage,
and bright plage regions, identified on filtergrams taken in the Ca II
K line.
Methods: We analysed full-disk images obtained with the
PSPT, by using three interference filters that sample the Ca II K line
with different bandpasses. We studied the dependence of the radiative
emission of disk features on the filter bandpass. We also performed a
non-local thermal equilibrium (NLTE) spectral synthesis of the Ca II
K line integrated over the bandpass of PSPT filters. The synthesis
was carried out by utilizing the partial frequency redistribution
(PRD) with the most recent set of semi-empirical atmosphere models
in the literature and some earlier atmosphere models. As the studied
models were computed by assuming the complete redistribution formalism
(CRD), we also performed simulations with this approximation for
comparison.
Results: We measured the center-to-limb variation
of intensity values for various solar features identified on PSPT
images and compared the results obtained with those derived from the
synthesis. We find that CRD calculations derived using the most recent
quiet Sun model, on average, reproduce the measured values of the
quiet Sun regions slightly more accurately than PRD computations with
the same model. This may reflect that the utilized atmospheric model
was computed assuming CRD. Calculations with PRD on earlier quiet Sun
model atmospheres reproduce measured quantities with a similar accuracy
as to that achieved here by applying CRD to the recent model. We
also find that the median contrast values measured for most of the
identified bright features, disk positions, and filter bandpasses
are, on average, a factor ≈1.9 lower than those derived from PRD
simulations performed using the recent bright feature models. The
discrepancy between measured and modeled values decreases by ≈12%
after taking into account straylight effects on PSPT images. When
moving towards the limb, PRD computations display closer agreement with
the data than performed in CRD. Moreover, PRD computations on either
the most recent or the earlier atmosphere models of bright features
reproduce measurements from plage and bright plage regions with a
similar accuracy. Appendix A is only available in electronic form
at http://www.aanda.org
Title: VAPOR: Visual, Statistical, and Structural Analysis of
Astrophysical Flows
Authors: Clyne, J.; Gruchalla, K.; Rast, M.
Bibcode: 2010ASPC..429..323C
Altcode:
In this paper we discuss recent developments in the capabilities of
VAPOR: a desktop application that leverages today's powerful CPUs and
GPUs to enable visualization and analysis of terascale data sets using
only a commodity PC or laptop. We review VAPOR's current capabilities,
highlighting support for Adaptive Mesh Refinement (AMR) grids, and
present new developments in interactive feature-based visualization
and statistical analysis.
Title: Modeling the solar irradiance background via numerical
simulation
Authors: Viticchié, B.; Vantaggiato, M.; Berrilli, F.; Del Moro,
D.; Penza, V.; Pietropaolo, E.; Rast, M.
Bibcode: 2010Ap&SS.328...39V
Altcode: 2010Ap&SS.tmp...58V
Various small scale photospheric processes are responsible for spatial
and temporal variations of solar emergent intensity. The contribution
to total irradiance fluctuations of such small scale features is
the solar irradiance background. Here we examine the statistical
properties of irradiance background computed via a n-body numerical
scheme mimicking photospheric space-time correlations and calibrated by
means of IBIS/DST spectro-polarimetric data. Such computed properties
are compared with experimental results derived from the analysis of a
VIRGO/SPM data. A future application of the model here presented could
be the interpretation of stellar irradiance power spectra observed by
new missions such as Kepler.
Title: Is There Such a Thing as Quiet Sun?
Authors: Rast, M. P.
Bibcode: 2010ASPC..428...87R
Altcode:
The Cycle 23-Cycle 24 minimum was deep and prolonged, similar to
minima of the late 19th and early 20th centuries but quite different
from those between the overlapping cycles of the early space age. This
provides a unique opportunity to study the Sun at very low levels of
magnetic activity. Here we examine the quiet Sun, defining it to be
those portions of the Sun for which continuum intensity variations are
dominated by thermal perturbations as opposed to opacity fluctuations
due to the presence of magnetic fields. We briefly present evidence
that: (1) The expected thermal signature of the solar supergranulation
can not be separated from magnetic contributions without masking the
contribution of at least 95% of the pixels. By this measure, at most 5%
of the Sun is truly quiet. (2) There was a rapid decay of active network
magnetic fields entering this solar minimum, a consequent increase in
the internetwork area, but a nearly constant fractional area covered by
network fields. This suggests the continuous fragmentation and decay of
active region fields into weaker field components, but also, possibly,
an underlying continuous flux concentration mechanism maintaining
the network field. (3) One of the first flux emergence episodes of
Cycle 24 did not occur as a coherent active region, but instead in
the form of disorganized spatially-dispersed small-scale magnetic
elements. Under the paradigm of a deep-rooted dynamo, this suggests
an episode of incoherent field loss from the generation region or a
failed/shredded omega loop rise through the convection zone.
Title: Supergranule variability in Mt. Wilson Ca II K images
Authors: Hock, Rachel; Eparvier, F. G.; McIntosh, S. W.; Rast, M. P.
Bibcode: 2010AAS...21640107H
Altcode: 2010BAAS...41Q.858H
We examined the Mt. Wilson Ca II K archive to quantify the long-term
changes in the average size of supergranules over five solar cycles
from 1930 to 1985. We determined that, although the Mt. Wilson Ca II K
images are limited by atmospheric seeing, there is sufficient contrast
in the images to identify supergranules. In general, we found that
supergranule size increases during the rising phase of a solar cycle,
reaching a peak at solar maximum. In the declining phase of a solar
cycle, supergranule size has a larger second peak, becoming out of
phase with the solar cycle for several years.
Title: Radiative emission of solar features in Ca II K
Authors: Criscuoli, S.; Ermolli, I.; Fontenla, J.; Giorgi, F.; Rast,
M.; Solanki, S. K.; Uitenbroek, H.
Bibcode: 2010MmSAI..81..773C
Altcode: 2010arXiv1002.0244C
We investigated the radiative emission of different types of solar
features in the spectral range of the Ca II K line. We analyzed
full-disk 2k × 2k observations from the Precision Solar Photometric
Telescope (PSPT). The data were obtained by using three narrow-band
interference filters that sample the Ca II K line with different pass
bands. Two filters are centered in the line core, the other in the red
wing of the line. We measured the intensity and contrast of various
solar features, specifically quiet Sun (inter-network), network,
enhanced network, plage, and bright plage (facula) regions. Moreover,
we compared the results obtained with those derived from the numerical
synthesis performed for the three PSPT filters with a widely used
radiative code on a set of reference semi-empirical atmosphere models.
Title: NLTE spectral synthesis based on 3D MHD convection simulations
-understanding the role of the magnetic field in intensity variations
Authors: Haberreiter, Margit; Wedemeyer-Boehm, Sven; Rast, Mark
Bibcode: 2010cosp...38..132H
Altcode: 2010cosp.meet..132H
While the magnetic field is considered to be the main driver for Solar
Spectral Irradiance (SSI) variations, the detailed physical mechanisms
that explain this relation are not yet fully understood. In this paper
we analyze the effect of small scale magnetic field on the intensity in
Ca II 393.4 nm and various continuum wavelengths calculated with the
NLTE radiative tranfer code SolMod3D. The code calculates NLTE level
populations and line spectra based on 3D MHD simulations carried out
with CO5BOLD. This enables us to study in great detail the effect of
the varying small scale magnetic field on intensity variations. The
results are important for a better understanding of the role of
small-scale magnetic field in irradiance variations.
Title: Measured and modeled trends in solar spectral irradiance
variability in the visible and infrared
Authors: Harder, Jerald; Fontenla, Juan; Rast, Mark; Pilewskie, Peter;
Woods, Thomas
Bibcode: 2010cosp...38...16H
Altcode: 2010cosp.meet...16H
The Spectral Irradiance Monitor (SIM) measures solar spectral
variability in the 200-2400 nm range, accounting for about 97%
of the total solar irradiance (TSI). This instrument monitored
the descending phase of solar cycle 23 and is now continuing these
observations in the rising phase of cycle 24. The SIM observations
clearly show rotational modulation of spectral irra-diance due to
the evolution of dark sunspots and bright faculae that respectively
deplete and enhance solar radiation. In addition to this well-known
phenomenon, SIM observations indicate a slower evolutionary trend in
solar spectral irradiance (SSI) over solar cycle times periods that are
both in and out of phase with the TSI. Wavelengths where the brightness
temperature is less than Teff = 5770 K are in phase, and where the
brightness temperature ¿ Teff in the vis-ible and infrared, the time
series show an anti-solar cycle trend. This observation is discussed
in terms of the Solar Radiation Physical Modeling (SRPM) program and
solar images from Precision Solar Photometric Telescope (PSPT) that
provides the areas of active regions on the solar disk as function of
time to generate a modeled SSI time series that is concurrent with the
SIM observations. The findings from SIM also suggest a very different
solar forcing for the stratosphere than has been previously considered.
Title: The Intensity Profile of the Solar Supergranulation
Authors: Goldbaum, Nathan; Rast, Mark P.; Ermolli, Ilaria; Sands,
J. Summer; Berrilli, Francesco
Bibcode: 2009ApJ...707...67G
Altcode: 2009arXiv0909.3310G
We have measured the average radial (cell center to network
boundary) profile of the continuum intensity contrast associated with
supergranular flows using data from the Precision Solar Photometric
Telescope at the Mauna Loa Solar Observatory. After removing the
contribution of the network flux elements by the application of masks
based on Ca II K intensity and averaging over more than 105
supergranular cells, we find a ~0.1% decrease in red and blue
continuum intensity from the supergranular cell centers outward,
corresponding to a ~1.0 K decrease in brightness temperature across
the cells. The radial intensity profile may be caused either by the
thermal signal associated with the supergranular flows or a variation
in the packing density of unresolved magnetic flux elements. These are
not unambiguously distinguished by the observations, and we raise the
possibility that the network magnetic fields play an active role in
supergranular scale selection by enhancing the radiative cooling of
the deep photosphere at the cell boundaries.
Title: The Convective Signature of the Solar Supergranulation
Authors: Goldbaum, Nathan Jonathan; Rast, M. P.
Bibcode: 2009SPD....40.0932G
Altcode:
The solar supergranulation is an elusive, yet well-observed,
surface-filling network of roughly polygonal cells made up of
horizontally diverging material. Cells have diameters of 30 Mm, flow
speeds of 500 m s-1, and lifetimes of 1 day. Theoretical
models for the supergranulation abound but can be separated into two
classes: convective (Simon and Leighton 1964; van der Borght 1979)
and non-convective (Rieutord et al. 2000; Rast 2003b; Rieutord et
al. 2008). If supergranulation is convective, then cells should be
warmer at their centers than at their borders, on average. However,
the sign and magnitude of the supergranular temperature gradient
is poorly constrained. The Precision Solar Photometric Telescope
(PSPT), operated by the High Altitude Observatory at the Mauna
Loa Solar Observatory, off ers 0.1% relative photometric accuracy,
good enough to resolve the expected low-amplitude thermal intensity
modulation. For this work we have used a library of 3174 PSPT images
to measure the mean azimuthally averaged thermal intensity profile in
supergranules. Using a morphological algorithm (Berrilli et al. 1998;
Rast 2003a), we have produced maps of the chromospheric network present
in Ca II K images. After carefully aligning concurrent continuum images
with these maps, we find that cell borders are on average 0.30 - 0.25%
brighter. This difference, due to the presence of the magnetic network
on supergranule borders, is consistent with previous measurements
(Lin and Kuhn 1992). Once the magnetic contribution is removed from
the intensity signal, we find that cell borders are on average 0.10%
dimmer than cell centers. This corresponds to a temperature drop of
1.0K at the borders of supergranules. This measurement is in good
agreement with the only other values for this quantity available in
the literature (Rast 2003a; Meunier et al. 2007b, 2008).
Title: Photometric properties of resolved and unresolved magnetic
elements
Authors: Criscuoli, S.; Rast, M. P.
Bibcode: 2009A&A...495..621C
Altcode: 2008arXiv0812.1727C
Aims: We investigate the photometric signature of magnetic flux tubes
in the solar photosphere.
Methods: We developed two-dimensional,
static numerical models of isolated and clustered magnetic flux
tubes. We investigated the emergent intensity profiles at different
lines-of-sight for various spatial resolutions and opacity models.
Results: We found that both geometric and photometric properties
of bright magnetic features are determined not only by the physical
properties of the tube and its surroundings, but also by the
particularities of the observations, including the line/continuum
formation height, the spatial resolution, and the image analysis
techniques applied. We show that some observational results presented
in the literature can be interpreted by considering bright magnetic
features to be clusters of smaller elements, rather than a monolithic
flux tube.
Title: Coupled Analysis and Visualization of High Resolution
Astrophysical Simulations
Authors: Rast, M.; Clyne, J.
Bibcode: 2008ASPC..385..299R
Altcode:
Computational physics has benefited from on-going microprocessor
innovations, which have enabled larger and larger numerical
simulations. One consequence of these technological advancements has
been an explosion in the amount of data generated. For many modelers,
available software tools and computing resources are proving inadequate
for investigation of high-resolution numerical outputs. In this paper
we discuss the general problems associated with very large data
visualization and analysis and our work on a particular solution
to those through the development of VAPOR (open source, available
at http://www.vapor.ucar.edu): a desktop application that leverages
today's powerful CPUs and GPUs to enable visualization and analysis
of terascale data sets using only a commodity PC or laptop. We briefly
illustrate VAPOR's utility through the exploration of a high-resolution
simulation aimed at understanding the effects of hydrogen ionization
on convective dynamics in stellar envelopes.
Title: Latitudinal Variation of the Solar Photospheric Intensity
Authors: Rast, Mark P.; Ortiz, Ada; Meisner, Randle W.
Bibcode: 2008ApJ...673.1209R
Altcode: 2007arXiv0710.3121R
We have examined images from the Precision Solar Photometric
Telescope (PSPT) at the Mauna Loa Solar Observatory (MLSO) in search
of latitudinal variation in the solar photospheric intensity. Along
with the expected brightening of the solar activity belts, we have
found a weak enhancement of the mean continuum intensity at polar
latitudes (continuum intensity enhancement ~0.1%-0.2%, corresponding
to a brightness temperature enhancement of ~2.5 K). This appears to be
thermal in origin and not due to a polar accumulation of weak magnetic
elements, with both the continuum and Ca II K intensity distributions
shifted toward higher values with little change in shape from their
midlatitude distributions. Since the enhancement is of low spatial
frequency and of very small amplitude, it is difficult to separate from
systematic instrumental and processing errors. We provide a thorough
discussion of these and conclude that the measurement captures real
solar latitudinal intensity variations.
Title: On the reliability of the fractal dimension measure of solar
magnetic features and on its variation with solar activity
Authors: Criscuoli, S.; Rast, M. P.; Ermolli, I.; Centrone, M.
Bibcode: 2007A&A...461..331C
Altcode: 2006astro.ph..9748C
Context: Several studies have investigated the fractal and multifractal
nature of magnetic features in the solar photosphere and its variation
with the solar magnetic activity cycle.
Aims: Here we extend
those studies by examining the fractal geometry of bright magnetic
features at higher atmospheric levels, specifically in the solar
chromosphere. We analyze structures identified in CaIIK images obtained
with the Precision Solar Photometric Telescopes (PSPTs) at Osservatorio
Astronomico di Roma (OAR) and Mauna Loa Solar Observatory (MLSO).
Methods: Fractal dimension estimates depend on the estimator employed,
the quality of the images, and the structure identification techniques
used. We examine both real and simulated data and employ two different
perimeter-area estimators in order to understand the sensitivity of
the deduced fractal properties to pixelization and image quality.
Results: The fractal dimension of bright “magnetic” features
in CaIIK images ranges between values of 1.2 and 1.7 for small and
large structures respectively. This size dependency largely reflects
the importance of image pixelization in the measurement of small
objects. The fractal dimension of chromospheric features does not show
any clear systematic variation with time over the period examined,
the descending phase of solar cycle 23.
Conclusions: .These
conclusions, and the analysis of both real and synthetic images on
which they are based, are important in the interpretation of previously
reported results.
Title: Expansion of the Supergranular Magnetic Network through the
Solar Atmosphere
Authors: Aiouaz, T.; Rast, M. P.
Bibcode: 2006ApJ...647L.183A
Altcode:
The solar magnetic field has its footpoints in the photosphere,
extends through the chromosphere, and is thought to expand through
the transition region and into the corona. It is organized by fluid
motions to form strong flux concentrations within the boundaries of
the supergranular convection cells. These boundaries are the network
lanes observed in line emission, and they display increasing width with
height through the solar atmosphere. The network field concentrations
are surrounded by a mixed-polarity internetwork magnetic field on the
scale of granulation. We use a potential magnetic field extrapolation
of synthetic photospheric magnetograms to study the magnetic network
topology and the effects of a mixed-polarity background field on
the network expansion with height through the solar atmosphere. We
find that the expansion of the network boundary with height deviates
significantly from the funnel expansion model. Moreover, we find that
the background magnetic field has a considerable effect on the filling
factor of the network area with height, even though the background
flux is strictly equal to zero.
Title: Supergranulation: Self-organization In The Surface Shear
Authors: Rast, Mark
Bibcode: 2006SPD....37.3301R
Altcode: 2006BAAS...38..258R
Supergranular motions in the solar photosphere are implicated in
phemomena as diverse as the global solar dynamo and the heating
of the solar chromosphere, yet the origin of these motions is only
poorly understood. Moreover, recent observations have revealed several
startling properties of the flows: a wavelike spectrum, longitudinal
alignment of the pattern, and very weak thermal contrast. This talk
will review our current understanding of the solar supergranulation,
the possible origin of its scale, and a proposed connection between
its newly discovered properties and the helioseismically observed
surface shear layer.
Title: Expansion of the supergranular magnetic network through the
solar atmosphere
Authors: Aiouaz, T.; Rast, M. P.
Bibcode: 2006IAUS..233..161A
Altcode:
The solar magnetic field that extends through the chromosphere is
thought to expand through the transition region into the corona. The
strong flux concentrations are located within the boundaries
of supergranular convection cells. These boundaries form network
lanes, observed in emission lines as bright lanes with varying width
throughout the solar atmosphere. These network field concentrations
are surrounded by mixed-polarity magnetic field with a scale of the
granule diameter, as suggested by observations. We use potential
magnetic field extrapolations on synthetic magnetograms to study
the magnetic network topology and the effects of background magnetic
field on the network expansion through the solar atmosphere. We find
that the background magnetic field has a considerable effect on the
ratio of network area over field of view. Furthermore we find that the
expansion of the network boundaries with height deviate significantly
from well-assumed funnel model expansion.
Title: Modeling Seismic Emission in the Quiet Sun
Authors: Lindsey, C. A.; Birch, A. C.; Donea, A.; Rast, M. P.
Bibcode: 2005AGUSMSP13A..06L
Altcode:
A major issue in the physics of seismic emission in the quiet Sun
is the degree to which the emission from any particular location is
episodic. Given our present understanding, this question is equivalent
to that of how localized the sources of emission are at any particular
moment. A variety of statistical tools are available to address
this issue. For example, if seismic emission can be characterized
in terms of relatively infrequent episodes sparsely distributed,
then the distribution in amplitude of the source terms over space and
time should be non-Gaussian. If the episodes of emission are densely
disseminenated in space and time such that many phase-independent
episodes would be expected in a space-time resolution element,then
the distribution in amplitude approaches Gaussian statistics, and
the distribution in power becomes exponential. Computational seismic
holography focused at the solar surface from a subjacent vantage
makes it possible to image acoustic sources and do statistics on the
seismic source term. Earlier work by Donea, Lindsey and Braun, based on
holographic imaging of acoustic sources, failed to detect a departure of
source amplitudes from Gaussian statistics. This suggests that seismic
sources are relatively dense on a spatial scale of 3~Mm and a temporal
scale of 10~min. What this means in terms of the physics of acoustic
excitation requires modeling. We will describe beginning efforts to
model seismic emission in a standard model of the solar subphotosphere
in terms of randomly distributed dipoles located close to the solar
surface. A significant departute of the source amplitude distribution
from Gaussian statistics is of fundamental importance to the utility
of local helioseismic diagnostics to seismic emission in the quiet Sun.
Title: Solar variability: a brief review
Authors: Rast, M. P.
Bibcode: 2005MmSAI..76..719R
Altcode:
This paper provides a brief review of solar particulate and radiative
variability focusing on four topics: solar energetic particle events,
cosmic ray modulation, total solar irradiance, and solar spectral
irradiance. Magnetized plasma variability is discussed only in the
context of energetic particle fluxes. Emphasis throughout is on the
current understanding of the physical mechanisms responsible for the
observed variability. References are representative, not comprehensive.
Title: A study of the photometrical properties of solar magnetic
features by numerical simulation
Authors: Criscuoli, S.; Rast, M. P.
Bibcode: 2005MmSAI..76..945C
Altcode:
Existing numerical simulations reproduce many of the observed
geometrical and photometrical characteristics of solar magnetic
structures. Nonetheless, some quite fundemental properties, such as
network center-to-limb variation and facular contrast, which depend on
both the structure's size and magnetic field intensity, are still only
partially understood. In order to investigate these problems, we have
developed a radiative transfer code, based on the short characteristics
method, that enables detailed study of the radiative properties of
individual magnetic flux tubes and unresolved aggregates of them.
Title: How good is the Ca II K as a proxy for the magnetic flux?
Authors: Ortiz, A.; Rast, M.
Bibcode: 2005MmSAI..76.1018O
Altcode:
We have coaligned near-simultanous full disk PSPT images and SOLIS
longitudinal magnetograms of the solar photosphere in order to determine
the relationship between the Ca II K intensity and the magnetic flux
(delta IK vs |B/mu |). We obtain a power-law relationship
with an exponent of 0.66. This relationship allows us to use the Ca
II K intensity as a proxy for the magnetic flux density for those
periods when it is difficult to find both good quality magnetograms
and photometric images of the Sun. Finally, we discuss the physics
behind the behaviour of the contrast as a function of magnetic flux
for the three PSPT wavelengths.
Title: The Spectrum of the Solar Supergranulation: Multiple Nonwave
Components
Authors: Rast, Mark P.; Lisle, Jason P.; Toomre, Juri
Bibcode: 2004ApJ...608.1156R
Altcode:
It has recently been suggested that the solar supergranulation undergoes
oscillations, with a spectrum of superposed traveling waves of unknown
origin showing excess prograde power to yield superrotation. We show
here that the observed supergranular spectrum does not necessarily
imply a wave origin but is instead consistent with two components of
nonoscillatory bulk motions having differing rotation rates and somewhat
asymmetrically distributed in space. The two components are identified
with solar mesogranulation and supergranulation, and the spatial
asymmetry is shown to be caused by a weak north-south alignment of the
supergranular flows. The source of both the supergranular alignment and
its enhanced rotation is likely underlying giant cell motions. Because
no single rotation rate characterizes all components of a solar image,
the spectral properties, including the Fourier dispersion relation, are
extremely sensitive to the rate at which the solar disk is tracked when
making up the time series. A spuriously wavelike spectrum is obtained
when the image tracking rate falls between the actual mesogranular
and supergranular rotation rates.
Title: Persistent North-South Alignment of the Solar Supergranulation
Authors: Lisle, Jason P.; Rast, Mark P.; Toomre, Juri
Bibcode: 2004ApJ...608.1167L
Altcode:
We have found evidence of an alignment of the solar supergranulation
in the direction parallel to the Sun's rotation axis. Signatures
of the alignment are apparent in both time-averaged images and in
three-dimensional power spectra. The north-south organization is
persistent in time, extending over many supergranular lifetimes. It
occurs over a wide latitudinal extent, to +/-60°, and shows variation
on a 10°-30° scale. These properties, as well as the rotation rate
of the pattern, suggest a underlying larger scale dynamical cause. We
examine a mechanism by which giant cell motions may contribute to
such alignment.
Title: The Scales of Granulation, Mesogranulation, and
Supergranulation
Authors: Rast, Mark Peter
Bibcode: 2003ApJ...597.1200R
Altcode:
Solar granulation is described as an advection-fragmentation process in
the upper layers of the convection zone. The fundamental hydrodynamic
unit is the downflow plume, and from its structure the granular
scale follows. Moreover, through the collective advective interaction
of many small-scale and short-lived granular plumes, large spatial
and long temporal mesogranular and supergranular scales naturally
arise. We illustrate and examine this process of scale selection
using a simplified n-body advective-interaction model. For parameters
set by granulation observations and numerical plume simulations,
clustering scales remarkably close to observed mesogranulation and
supergranulation result.
Title: Supergranulation: new observation, possible explanation
Authors: Rast, Mark Peter
Bibcode: 2003ESASP.517..163R
Altcode: 2003soho...12..163R
We briefly review the main observational properties of the solar
supergranulation: divergent horizontal flow, weak thermal signature,
oscillatory power, and super-rotation. We present new photometric
measurements which attempt to disentangle the magnetic network and
convective contributions to the supergranular continuum intensity
contrast, and suggest that the convective signal has now been detected
with some confidence. We propose and examine a purely advective model
for the supergranular flow and show that large-spatial and long-temporal
supergranular (and mesogranular) scales naturally arise through the
collective interaction of many small-scale and short-lived granular
downflow plumes. We show that dynamically unsteady behavior in such a
model can yield oscillatory power and speculate that super-rotation may
be achieved given an underlying size dependent rotation rate. Finally,
we discuss the uncertainties and observational predictions of this
highly simplified model.
Title: A comment on ``Regular structures of the solar photosphere"
Authors: Rast, M. P.
Bibcode: 2002A&A...392L..13R
Altcode:
A recent Letter to the Editor (Getling & Brandt \cite{getling02})
suggests that solar granulation is not entirely random, instead showing
large scale spatial and long term temporal coherence. The authors
cite as evidence the persistence of bright granular size objects in
images even after long term temporal averaging, the reoccurrence of
bright granules in time series at locations of local maxima in the
average image, and the presence of large scale regular structures in
time-average images. This paper demonstrates that all three of these
observations are consistent with a completely random and changing flow
pattern and do not require self organization of the granular flows.
Title: ESA's activities in the area of land surface processes
observations from Space
Authors: Rast, M.
Bibcode: 2002ESASP.474E...1R
Altcode: 2001spec.work....1R
No abstract at ADS
Title: The ESA DAISEX Imaging Spectroscopy campaigns in support of
SPECTRA - first results
Authors: Berger, M.; Rast, M.; Wursteisen, P.; Attema, E.; Moreno,
J.; Müller, A.; Beisl, U.; Richter, R.; Schaepman, M.; Strub, G.;
Stoll, M. P.; Nerry, F.; Leroy, M.
Bibcode: 2002ESASP.474E..14B
Altcode: 2001spec.work...14B
Part of ESA's Earth Observation Envelope Programme (EOEP) is intended
to advance our understanding of the various processes occurring in
the Earth's biosphere/geosphere, and their interactions with the
atmosphere. Thus, the Programme's `Theme 3: Geosphere/Biosphere'
focusses on the modelling and monitoring of land-surface processes,
the study of interactions, and the analysis of climate impacts on the
biosphere, with the objective of enhancing our skills in predicting the
evolution of the Earth system. Real-life applications such as weather
forecasting, crop-yield estimation, precision farming, the management of
renewable and non-renewable resources, as well as environmental-hazard
monitoring/forecasting, would all benefit from improved process model
descriptions. A spaceborne scientific mission addressing the provision
of information on geospheric/biospheric processes and their interactions
with the atmosphere is currently being formulated. This mission implies
the need for an instrument with high spectral and angular resolution
and a very high radiometric performance, operating in the reflective
and thermal parts of the spectrum, not only to identify but also to
quantify the key variables driving the processes.
Title: High Precision Orthogonal Decomposition of the Solar Limb
Darkening
Authors: Meisner, R. W.; Rast, M. P.
Bibcode: 2002AAS...200.5513M
Altcode: 2002BAAS...34..734M
The Precision Solar Photometric Telescope (PSPT) at Mauna Loa Solar
Observatory (MLSO) in Hawaii yields full disk solar images with high
(approximately 0.1%) photometric precision. Determination of the solar
limb darkening function and known image defects to the same level of
precision presents fundamental conceptual and practical difficulties,
but promises synoptic measurement of small variations in the mean
thermodynamic stratification of the solar atmosphere. We describe a
procedure which carefully identifies "quiet sun" in a PSPT image and
uniquely determines a limb darkening function describing its radial
intensity variation. The procedure performs a simultaneous least-squares
fit to a truncated series of Legendre polynomials in radius and
Fourier sine and cosine terms in central angle. The orthogonality
of the functions allows capture, without mixing, of both the solar
center to limb variation and any residual linear gradient present as
an artifact. We demonstrate the accuracy and speed of the method on
both simulated and real data. (The National Center for Atmospheric
Research is operated by the University Corporation for Atmospheric
Research under sponsorship of the National Science Foundation.)
Title: Numerical Simulations of Convective Overshoot
Authors: Rempel, M.; Rast, M. P.
Bibcode: 2002AAS...200.0417R
Altcode: 2002BAAS...34..646R
The structure of the overshoot region at the base of solar convection
zone is crucial to the storage of strong toroidal magnetic field
produced there by the solar dynamo. Both the mean thermodynamic
stratification and the statistical properties of the convective
fluctuations affect the storage capabilities of the region. Overshoot
models of the past, based on the non local mixing-length theory,
generally produce a shallow weakly subadiabatic region with a steep
transition to the radiative interior. A more recent estimation by
Xiong & Deng (Mon. Not. R. Astron. Soc. 327, 1137) suggests a
larger subadiabaticity and a smoother transition to the radiative
gradient. Numerical studies have to date contributed little to
constraining these simpler models, largely because they are unable
to match the very low values of radiative conductivity found in the
solar interior. The abnormally high values of conductivity generally
employed lead to much more vigorous convection and much deeper
convective penetration than anticipated. To address this deficiency
directly we adopt a formulation which explicitly separates of the
thermal conductivity into a turbulent and a radiative component,
and employ a novel thermal relaxation scheme which accelerates the
approach to equilibrium in the deep radiative layers even at very low
values of the latter. This separation also enables adjustment of the
convective properties apart from the radiative ones in the lower half
of the convection zone. Preliminary results suggest that the structure
of the overshoot region is highly sensitive to the properties of the
convection in the lower half of the convection zone. NCAR is sponsored
by the National Since Foundation.
Title: Welcome &Introduction
Authors: Rast, M.; Berger, M.
Bibcode: 2002ESASP.527E...1R
Altcode: 2002rssi.conf....1R
No abstract at ADS
Title: Understanding vegetation response to climate variability
from space the scientific objectives, the approach and the concept
of the Spectra Mission
Authors: Menenti, M.; Rast, M.; Baret, F.; Hurk, B.; Knorr, W.;
Mauser, W.; Miller, J.; Schaepman, M.; Schimel, D.; Verstraete, M.
Bibcode: 2002cosp...34E3029M
Altcode: 2002cosp.meetE3029M
The response of vegetation to climate variability is a major
scientific question. The monitoring of the carbon stock in
terrestrial environments, as well as the improved understanding of the
surface-atmosphere interactions controlling the exchange of matter,
energy and momentum, is of immediate interest for an improved assessment
of the various components of the global carbon cycle. Studies of the
Earth System processes at the global scale rely on models that require
an advanced understanding and proper characterization of processes
at smaller scales. The goal of the SPECTRA mission is to improve the
description of those processes by means of better constraints on and
parameterizations of the associated models. Many vegetation properties
are related to features of reflectance spectra in the region 400 nm -
2500 nm. Detailed observations of spectral reflectance reveal subtle
features related to biochemical components of leaves such as chlorophyll
and water. The architecture of vegetation canopies determines complex
changes of observed reflectance spectra with view and illumination
angle. Quantitative analysis of reflectance spectra requires,
therefore, an accurate characterization of the anisotropy of reflected
radiance. This can be achieved with nearly - simultaneous observations
at different view angles. Exchange of energy between the biosphere
and the atmosphere is an important mechanism determining the response
of vegetation to climate variability. This requires measurements of
the component t mperature ofe foliage and soil. The prime objective of
SPECTRA is to determine the amount, assess the conditions and understand
the response of terrestrial vegetation to climate variability and its
role in the coupled cycles of energy, water and carbon. The amount and
state of vegetation will be determined by the combination of observed
vegetation properties and data assimilation. Specifically, the mission
will characterize the amount and state of vegetation with observations
of the following variables: 1) Fractional vegetation cover; 2) Fraction
Absorbed Photosynthetically Active Radiation (FAPAR); 3) Albedo; 4) Leaf
Area Index (LAI); 5) Leaf chlorophyll content; 6)Leaf water content;
7) Foliage temperature; 8) Soil temperature; 9) Fractional cover of
living and dead biomass. SPECTRA will provide spatially distributed
observations (maps) of the key vegetation properties at the spatial
resolution of one image pixel and a temporal frequency of one week
or lower. Each map will cover an area of 50 km x 50 km. The SPECTRA
mission is being studied by the European Space Agency to address
these scientific issues. The mission comprises the following elements:
A. Space segment consisting of an imaging spectrometer covering the
region 400nm - 2400 nm with a nominal spectral resolution of 10 nm and
of an agileplatform to perform subsequent, along track observations at
seven view anglesbetween -70° and + 70°. B. Ground segment consisting
of a core data processing facility and specializedCenters of Excellence
to guarantee to a wide and diverse community access tohigher level data
products and to specialized data assimilation systems. C. Field segment
consisting of 50 to 100 dedicated sites where teams ofinvestigators
evaluate the observations and assimilate them in models describingthe
functio ning of terrestrial ecosystems.
Title: Measuring cycle variations in the solar limb darkening
Authors: Rast, M.; Meisner, R.
Bibcode: 2002cosp...34E1107R
Altcode: 2002cosp.meetE1107R
The Precision Solar Photometric Telescope (PSPT) at Mauna Loa Solar
Observatory (MLSO) in Hawaii yields full disk solar images with high
(approximately 0.1%) photometric precision. Determination of the solar
limb darkening function and known image defects to the same level of
precision presents fundamental conceptual and practical difficulties,
but promises synoptic measurement of small variations in the mean
thermodynamic stratification of the solar atmosphere. We describe a
procedure which identifies "quiet sun" in PSPT images and uniquely
determines a limb darkening function describing its radial intensity
variation. A series of activity masks, based on CaIIK intensity
variations, are applied to the continuum images in order to understand
the magnetic contribution to the center to limb variation. We present
preliminary results and discuss future plans.
Title: The supergranular intensity contrast
Authors: Rast, M.; Ermolli, I.; Sands, J.; Berrilli, F.
Bibcode: 2002cosp...34E1110R
Altcode: 2002cosp.meetE1110R
The Precision Solar Photometric Telescope (PSPT) at Mauna Loa Solar
Observatory (MLSO) in Hawaii yields full disk solar images with high
(approximately 0.1%/pixel) photometric precision in three wavelength
bands: continuum blue, continuum red, and line-center CaIIK. We use
this data to measure the intensity contrast associated with network
cells. We attempt to disentangle the continuum contributions of the
magnetic network from that of the presumed underlying supergranular
flow by applying a series of network masks. Granular fluctuations
are removed by both temporal averaging of aligned images and spatial
averaging of individual cells after resizing. Preliminary results
indicate that the network contribution to the continuum is small but
measurable in the PSPT data, while the convective contribution lies
very near or below detection limits.
Title: Nonlinear Instability of Compressible Starting Plumes
Authors: Rast, Mark; Hurlburt, Neal
Bibcode: 2001APS..DFD.DG010R
Altcode:
The structure and dynamics of stellar convective envelopes and giant
planet atmospheres is thought to be controlled by narrow buoyantly
driven plumes spanning the convectively unstable region and penetrating
the over or underlying stably stratified layers. The stability and
entrainment properties of such plumes are poorly understood. When
the background state is significantly stratified, downward directed
thermal starting plumes are subject to a nonlinear pinch instability
not realized in their incompressible counterparts. It results from
finite amplitude pressure perturbations dynamically induced in
the wake of the plume head. We examine this instability mechanism
utilizing two-dimensional planar and axisymmetric as well as fully
three-dimensional numerical simulations, and discuss its sensitivity
to geometry and dissipation.
Title: A Thermodynamically Induced Finite-Amplitude Convective
Instability in Stellar Envelopes
Authors: Rast, Mark Peter
Bibcode: 2001ApJ...561L.191R
Altcode:
Stellar envelopes are subject to a finite-amplitude convective
instability that originates with the reduction in the adiabatic
exponent Γ1=(dlnP/dlnρ)ad accompanying partial
ionization of the principle plasma constituents, notably hydrogen. The
instability is one-sided low-Γ1 perturbations are unstable,
while high-Γ1 perturbations are stable. Since a partially
ionized fluid has a lower adiabatic exponent than either a fully
recombined or fully ionized one, convective downflows are stabilized
in the upper regions of a convective envelope where the nearly fully
recombined fluid is embedded in a partially ionized background. They
are significantly destabilized at a depth, however, where the
partially ionized downflowing fluid has a lower Γ1 than
does the highly ionized mean state. Convective upflows, by contrast,
are stabilized at a depth where their fully ionized state contrasts
with the partially ionized background and are destabilized only in
the very upper layers where the mean state of the fluid is nearly
fully recombined and the upflows are partially ionized. This Letter
illustrates the instability mechanism, its finite-amplitude character,
and its possible significance to both idealized compressible convection
simulations and the solar convective envelope.
Title: Sunspot Bright Rings: Evidence from Case Studies
Authors: Rast, M. P.; Meisner, R. W.; Lites, B. W.; Fox, P. A.; White,
O. R.
Bibcode: 2001ApJ...557..864R
Altcode:
We present evidence, from both the Precision Solar Photometric Telescope
and the Advanced Stokes Polarimeter, for a ring of enhanced continuum
intensity surrounding large isolated sunspots. We do not attempt to
evaluate the frequency of the phenomenon based on a large sample of
spots but instead concentrate on illustrative best-case examples. The
rings are about 0.5%-1.0% brighter in red and blue continuum (10 K
warmer) than the surrounding photosphere and extend about one sunspot
radius outward from the outer penumbral boundary. Most of the excess
radiation is not directly associated with the strongest regions of Ca II
K emission surrounding the spots or with measurable vertical magnetic
field when such measurements are available. Moreover, the temporal
evolution of the Ca II K and continuum emission in the ring differ,
with the continuum intensity evolving on a shorter timescale. This
suggests a convective origin for the bright ring, although a role
for weaker, more diffuse magnetic fields cannot be ruled out. While
we have inferred that only about 10% of the radiant energy missing
from the sunspot is emitted through the bright ring, even this small
enhancement may be significant to our understanding of subsurface
sunspot structure and energy transport.
Title: The optical imaging instruments and their applications:
AATSR and MERIS
Authors: Huot, J. -P.; Tait, H.; Rast, M.; Delwart, S.; Bézy, J. -L.;
Levrini, G.
Bibcode: 2001ESABu.106...56H
Altcode:
MERIS is primarily dedicated to observing oceanic biology and marine
water quality through observations of water colour. However, it
will also make contributions to atmospheric and land-surface-related
studies. Similarly, the main role of AATSR is to provide detailed Sea
Surface Temperature maps, and yet it also provides the capability to
measure a range of parameters for cloud microphysics, plus surface
temperatures and various vegetation indices over land. Data from these
instruments are therefore applicable to a wide range of environmental
application.
Title: The Navier-Stokes Equations and their Solution: Convection
and Oscillation Excitation
Authors: Rast, Mark Peter
Bibcode: 2001ASSL..259..155R
Altcode: 2001dysu.conf..155R
These lectures address only select topics in solar convection and the
excitation of solar acoustic oscillations. We thus invoke the von der
Lühe (1999) disclaimer, ``I am presenting a very personal view and
so am excused from any incompleteness,'' and proceed with caution. The
topics to be discussed include the Navier-Stokes equations describing
nonmagnetized fluid motion and their numerical solution, the effects
of hydrogen ionization on compressible convective flow-dynamics and
heat transport, and the role of downflow plumes in acoustic excitation
and their signature in helioseismic spectra.
Title: The Zigzag Path of Buoyant Magnetic Tubes and the Generation
of Vorticity along Their Periphery
Authors: Emonet, T.; Moreno-Insertis, F.; Rast, M. P.
Bibcode: 2001ApJ...549.1212E
Altcode:
We study the generation of vorticity in the magnetic boundary layer
of buoyant magnetic tubes and its consequences for the trajectory of
magnetic structures rising in the solar convection zone. When the
Reynolds number is well above 1, the wake trailing the tube sheds
vortex rolls, producing a von Kármán vortex street, similar to the
case of flows around rigid cylinders. The shedding of a vortex roll
causes an imbalance of vorticity in the tube. The ensuing vortex force
excites a transverse oscillation of the flux tube as a whole so that
it follows a zigzag upward path instead of rising along a straight
vertical line. In this paper, the physics of vorticity generation in
the boundary layer is discussed and scaling laws for the relevant terms
are presented. We then solve the two-dimensional magnetohydrodynamic
equations numerically, measure the vorticity production, and show the
formation of a vortex street and the consequent sinusoidal path of the
magnetic flux tube. For high values of the plasma beta, the trajectory
of the tubes is found to be independent of β but varying with the
Reynolds number. The Strouhal number, which measures the frequency
of vortex shedding, shows in our rising tubes only a weak dependence
with the Reynolds numbers, a result also obtained in the rigid-tube
laboratory experiments. In fact, the actual values measured in the
latter are also close to those of our numerical calculations. As
the Reynolds numbers are increased, the amplitude of the lift force
grows and the trajectory becomes increasingly complicated. It is
shown how a simple analytical equation (which includes buoyancy,
drag, and vortex forces) can satisfactorily reproduce the computed
trajectories. The different regimes of rise can be best understood in
terms of a dimensionless parameter, χ, which measures the importance
of the vortex force as compared with the buoyancy and drag forces. For
χ2<<1, the rise is drag dominated and the trajectory
is mainly vertical with a small lateral oscillation superposed. When
χ becomes larger than 1, there is a transition toward a drag-free
regime and epicycles are added to the trajectory.
Title: The DAISEX campaigns in support of a future
land-surface-processes mission
Authors: Berger, M.; Rast, M.; Wursteisen, P.; Attema, E.; Moreno,
J.; Müller, A.; Beisl, U.; Richter, R.; Schaepman, M.; Strub, G.;
Stoll, M. P.; Nerry, F.; Leroy, M.
Bibcode: 2001ESABu.105..101B
Altcode:
ESA has conducted an airborne imaging-spectrometer campaign called "The
Digital Airborne Imaging Spectrometer Experiment (DAISEX)" in support of
a possible future spaceborne mission. This article describes the state
of the art in retrieving variables relevant to land-surface processes
from hyperspectral data cubes, outlines the scientific objectives,
and demonstrates the first results of the DAISEX campaigns.
Title: Data From the Precision Solar Photometric Telescope (Pspt)
in Hawaii From March 1998 to March 1999
Authors: White, Oran R.; Fox, Peter A.; Meisner, Randy; Rast, Mark
P.; Yasukawa, Eric; Koon, Darryl; Rice, Crystal; Lin, Haosheng; Kuhn,
Jeff; Coulter, Roy
Bibcode: 2000SSRv...94...75W
Altcode:
Two Precision Solar Photometric Telescopes (PSPT) designed and built at
the U.S. National Solar Observatory (NSO) are in operation in Rome and
Hawaii. A third PSPT is now in operation the NSO at Sunspot, NM. The
PSPT system records full disk solar images at three wavelengths:
K line at 393.3 nm and two continua at 409 nm and 607 nm throughout
the observing day. We currently study properties of limb darkening,
sunspots, and network in these images with particular emphasis on data
taken in July and September 1998. During this period, the number of
observations per month was high enough to show directional properties
of the radiation field surrounding sunspots. We show examples of our
PSPT images and describe our study of bright rings around sunspots.
Title: Solar GranulationL A Surface Phenomenon
Authors: Rast, Mark Peter
Bibcode: 2000gac..conf..199R
Altcode:
No abstract at ADS
Title: The Dynamics of Buoyant Magnetic Ropes and the Generation of
Vorticity in their Periphery
Authors: Emonet, T.; Moreno-Insertis, F.; Rast, M. P.
Bibcode: 2000SPD....31.0133E
Altcode: 2000BAAS...32..807E
When the Reynolds number is not small, the wake trailing a buoyant
magnetic flux tube sheds vortex rolls therefore producing a Von Karman
vortex street and an imbalance of vorticity in the tube which results
in a transverse oscillation of the tube as a whole. The actual path
followed by the magnetic structure is therefore directly affected by
the amount of vorticity being produced in its boundary. Analytical
expressions for the magnetic generation and viscous dissipation of
vorticity in the boundary layer of buoyant magnetic flux tubes are
obtained. Corresponding scaling laws are deduced and checked using a
full compressible 2D MHD code. Interestingly, the observed trajectories
can be satisfactorily reproduced by a simple analytical equation (which
includes buoyancy, drag and vortex forces). I will conclude with some
comparisons with classical results from the hydrodynamical literature
(Strouhal number), and some comments about the rise time of buoyant
magnetic structures through the solar convection zone.
Title: p-Mode Intensity-Velocity Phase Differences and Convective
Sources
Authors: Skartlien, R.; Rast, M. P.
Bibcode: 2000ApJ...535..464S
Altcode:
We study the origin of the solar p-mode intensity-velocity phase
differences at high degree (l>100). Observations show phase
differences that are very different from those derived from linear
theory alone. The theory predicts a smooth variation with frequency,
dependent only on atmospheric parameters, while observations show large
fluctuations across modal frequencies. We support previous suggestions
that fluctuations in the intensity-velocity phase differences and line
asymmetries in the intensity and velocity power spectra are produced by
``contamination'' of the p-mode signal with noise correlated with the
excitation sources. It is demonstrated that the qualitative shapes of
the observed phase-difference and power spectra can be realized only if
both temperature (intensity) and velocity (Doppler shift) observations
contain correlated noise. Moreover, the details of the observed spectra
allow only a limited choice of noise parameters and constrain well
the convective process responsible for p-mode excitation. The inferred
correlated noise signals are consistent with the (visible) formation
of convective downflows accompanied by darkening (lowered emergent
intensity) and subsequent acoustic excitation. An upward velocity
pulse follows after the wave excitation, which suggests overshoot of
inflowing material that fills in the evacuated volume in the wake of
the new downflow.
Title: Data from the Precision Solar Photometric Telescope (PSPT)
in Hawaii from March 1998 to March 1999
Authors: White, Oran R.; Fox, Peter A.; Meisner, Randy; Rast, Mark
P.; Yasukawa, Eric; Koon, Darryl; Rice, Crystal; Lin, Haosheng; Kuhn,
Jeff; Coulter, Roy
Bibcode: 2000svc..book...75W
Altcode:
No abstract at ADS
Title: Bright rings around sunspots
Authors: Rast, M. P.; Fox, P. A.; Lin, H.; Lites, B. W.; Meisner,
R. W.; White, O. R.
Bibcode: 1999Natur.401..678R
Altcode:
There are two possible explanations for why sunspots are dark: the
partial suppression by the sunspot magnetic fields of convective energy
transport from the underlying layers, or the removal of energy from
the sunspot by enhanced hydromagnetic wave radiation. Both processes
would reduce the energy emitted radiatively. The first explanation
is currently favoured, and predicts that the blocked energy should
show up as a bright ring around the spot, with the actual brightness
of the ring sensitive to details of solar convective transport and
sunspot structure. Previous searches for these bright rings were
inconclusive because of the presence of bright, vertical magnetic
flux tubes near the spots, and a lack of sufficient precision in the
observations. Here we report high-photometric-precision observations
of bright rings around eight sunspots. The rings are about 10K warmer
than the surrounding photosphere and extend at least one sunspot
radius out from the penumbra. About 10% of the radiative energy
missing from the sunspots is emitted through the bright rings. We
also report observations of a second set of sunspots, for which
simultaneous magnetic field measurements demonstrate that the rings
are not associated with vertical flux tubes.
Title: The Thermal Starting Plume as an Acoustic Source
Authors: Rast, Mark Peter
Bibcode: 1999ApJ...524..462R
Altcode:
We propose that solar acoustic oscillations are excited by
localized cooling events and new downflow-plume formation at the
solar surface. The excitation process involves, in successive stages,
radiative cooling, buoyant acceleration, and advective inflow. Pressure
fluctuations induced at each stage result in monopolar, dipolar, and
quadrupolar acoustic emission. We examine this excitation mechanism in
detail, measure the acoustic energy output by such events, and discuss
possible observational implications for helioseismic spectra.
Title: The four candidate Earth Explorer core missions. Report for
mission selection. 2. Land-Surface Processes and Interactions Mission.
Authors: Rast, M.
Bibcode: 1999fce2.book.....R
Altcode:
The primary goal of this mission is the provision of bio-geophysical
variables to increase the understanding of bio-geophysical processes
and land/atmosphere interactions at the local scale and advance
the understanding of these processes and interactions on a global
scale. This report describes the objectives and scientific requirements
of the LSPIM mission.
Title: Thermal Starting Plumes, Solar Granulation, and the Excitation
of Solar Acoustic Oscillations
Authors: Rast, M.
Bibcode: 1999ASPC..183..443R
Altcode: 1999hrsp.conf..443R
No abstract at ADS
Title: On the Asymmetry of Solar Acoustic Line Profiles
Authors: Rast, Mark P.; Bogdan, Thomas J.
Bibcode: 1998ApJ...496..527R
Altcode:
We study a simplified model of solar acoustic oscillations and show how
asymmetries in spectral lines depend both on the acoustic source depth,
as previously recognized, and on the acoustic source type. We provide
a unified description of modal line asymmetries and high-frequency
pseudomode locations, suggesting an inversion on power spectra minima to
determine source properties and a correction to Lorentzian line shapes
based upon the relative locations of spectral peaks and valleys. We
also consider nonadiabatic effects due to Newtonian cooling and
demonstrate that these do not lead to notable differences between
velocity and intensity power spectral line shapes. We argue more
generally that it is unlikely that any nonadiabatic effect can be
responsible for the observed differences. Finally, we discuss the
importance of both multiplicative and additive background power to
the spectra and show how additive noise can reduce the apparent line
asymmetry of a mode. We note that information on solar convective
motions can be potentially extracted from three components of the
acoustic power spectra: the additive background yielding information on
the spectrum of nonoscillatory motions at the height of observation,
the multiplicative background reflecting the source spectrum, and
the power minima providing the source depth and physical nature. For
stochastically excited linear waves only the first of these contributes
significantly to spectral differences between observed variables.
Title: Photospheric Downflows: How deep, how coherent, how important?
Authors: Rast, M. P.
Bibcode: 1997ASSL..225..135R
Altcode: 1997scor.proc..135R
While convection in the solar envelope globally transports the incident
radiative flux from the interior, the local dynamics of granulation as
seen at the photosphere is dominated by radiative cooling and downflow
formation. Here we examine the stability of such downflows with depth
and their importance to acoustic excitation. We find that downflow
plumes, even in a quiescent adiabatic environment, are subject to
vigorous secondary instabilies causing detrainment of fluid from the
plume region. It seems unlikely that they are coherent to the bottom
of the solar convection zone. We also find that plume initiation by
rapidly localized cooling results in both monopolar and dipolar acoustic
emission. The mechanism is distinct in that pressure fluctuations are
induced thermodynamically by radiative loss as well as dynamically by
fluid motion.
Title: On the Nature of ``Exploding'' Granules and Granule
Fragmentation
Authors: Rast, Mark Peter
Bibcode: 1995ApJ...443..863R
Altcode:
The morphological evolution of solar granulation is dominated by
granule expansion and fragmentation. 'Exploding' granules undergo these
processes in a particularly vigorous manner, rapidly expanding to a
large size, darkening in the center, and splitting by the formation of
dark interior radially directed lanes. We argue that such events can
be better understood if granulation is viewed as downflow-dominated
surface-driven convection rather than as a collection of more deeply
driven upflowing thermal plumes. Regions of maximum granular upflow lie
not in the centers of the granules but along their sides, immediately
adjacent to the intergranular downflow lanes. These upflows occur
primarily in response to the buoyancy and pressure gradient forces
induced in proximity to the strongly driven downflow plumes. The
upflows are thus dynamically linked to the downflow sites, and granular
expansion results in a weakening to the central flow. Radiative losses
can then exceed the advected heat supply in the granule center, with the
fluid cooling until buoyancy forces becomes sufficient to trigger the
formation of a new downflow plume there. Lateral propagation proceeds
as neighboring flows are distributed, with propagation preferentially
occurring in directions predisposed to weak upflow by the strength and
shape of the downflows defining the granule boundary. Thus the radially
oriented structures seen in observations of some fragmenting granules
may be formed. Finally, the strong downflow plumes initiated in the
solar photosphere entrain surrounding material as they descend. With
depth this more weakly downflowing material establishes a connectivity
which is strikingly of mesogranular scale. This may help to explain
the observed correlation between the spatial distribution of exploding
granules and mesogranular flows, and suggests that both mesogranulation
and supergranulation are secondary manifestations of granulation itself.
Title: High-Frequency Oscillations of a Polytropic Layer
Authors: Rast, M. P.; Gough, D. O.
Bibcode: 1995ASPC...76..322R
Altcode: 1995gong.conf..322R
No abstract at ADS
Title: MERIS - The Medium resolution imaging spectrometer. Part A
and B
Authors: Rast, M.
Bibcode: 1995mmri.book.....R
Altcode:
No abstract at ADS
Title: Compressible Convection with Ionization. II. Thermal
Boundary-Layer Instability
Authors: Rast, Mark P.; Toomre, Juri
Bibcode: 1993ApJ...419..240R
Altcode:
Rast & Toomre (1993, Paper I) examined the effects of
ionization-state changes on the stability, flow asymmetry,
and flux transport properties of two-dimensional compressible
convection. Here we employ the same single-atomic-level hydrogen model
and analyze vigorously time-dependent nonlinear solutions. Ionization-
state-dependent variations in thermal diffusivity of the fluid can
result in thermal boundary-layer instability and plume formation. The
interval between pluming events depends on the growth rate of the
instability and both the scale and the velocity of the underlying
convective motions. Such instabilities can occur at either boundary,
depending on the positioning of the partially ionized region within the
domain. Here we concentrate on simulations in which the instability is
manifest in the upper thermal boundary layer, and results in cool plume
formation. Temperature fluctuations and associated buoyancy forces in
the plumes are maintained as long as heat exchange and compressional
heating result primarily in ionization of the fluid rather than in
temperature equilibration, and this can lead to supersonic vertical
flows in an otherwise subsonic flow field. These flows serve to excite
acoustic oscillations, the phase of which can be abruptly altered
by subsequent plume events. For high rates of plume initiation, the
fundamental acoustic period of the domain is greater than the time
span between two descents. Such ionization effects are expected to
influence the dynamics of granulation and acoustic mode excitation
in the Sun and other stars, and likewise the coupling of convection
with pulsations that occurs in stars such as white dwarfs and Cepheid
variables. Additionally, it is possible that thermal instabilities
analogous to those seen in these simulations occur not only in the
photosphere but also at the base of stellar convective envelopes owing
to temperature-sensitive variations in the radiative conductivity of
fluid there.
Title: Compressible Convection with Ionization. I. Stability, Flow
Asymmetries, and Energy Transport
Authors: Rast, Mark P.; Toomre, Juri
Bibcode: 1993ApJ...419..224R
Altcode:
The influence of nonideal effects associated with ionization
upon the dynamics and thermodynamics of compressible convection
is studied. Linear and finite-amplitude analyses and fully
nonlinear two-dimensional simulations of a plane-parallel layer
of single-atomic-level hydrogen fluid are undertaken. Ionization
significantly influences both the global transport properties and
the local dynamics of convective flows by modifying the particle
number density, specific heat, and internal energy content of the
fluid. Strong temperature fluctuations and corresponding buoyancy
forces develop locally in the fluid wherever rapid changes in
ionization state occur. These can result in narrow regions of
intense vertical flow. The flow asymmetries seen in simulations of
compressible ideal-gas convection can either be enhanced or diminished
depending on the vertical positioning of the partially ionized
region within the domain. Additionally, the enthalpy flux achieved
by ionizing convection is dominated in regions of partial ionization
by latent-heat transport. The enthalpy carried by downflow plumes can
be considerably elevated, and the cancellation between kinetic energy
and enthalpy fluxes observed in the downflows in some simulations of
ideal gas turbulence may thus be offset by partial ionization of the
fluid. Such ionization effects are likely to influence the character
of convective motions within stellar envelopes. Convective transport
properties may differ substantially between the partially ionized and
the deeper fully ionized regions of a star, and since ionization zone
placement also varies with respect to both the photosphere and the
lower thermal boundary, between stellar types and during the course
of stellar evolution.
Title: Ionization Effects in Three-dimensional Solar Granulation
Simulations
Authors: Rast, Mark P.; Nordlund, Ake; Stein, Robert F.; Toomre, Juri
Bibcode: 1993ApJ...408L..53R
Altcode:
These numerical studies show that ionization influences both the
transport and dynamical properties of compressible convection
near the surface of the Sun. About two-thirds of the enthalpy
transported by convective motions in the region of partial hydrogen
ionization is carried as latent heat. The role of fast downflow
plumes in total convective transport is substantially elevated
by this contribution. Instability of the thermal boundary layer
is strongly enhanced by temperature sensitive variations in the
radiative properties of the fluid, and this provides a mechanism for
plume initiation and cell fragmentation in the surface layers. As
the plumes descend, temperature fluctuations and associated buoyancy
forces are maintained because of the increased specific heat of the
partially ionized material. This can result is supersonic vertical
flows. At greater depths, ionization effects diminish, and the plumes
are decelerated by significant entrainment of surrounding fluid.
Title: Acoustic Excitation by Thermal Boundary Layer Instability in
a Partially Ionized Convecting Fluid
Authors: Rast, M. P.; Toomre, J.
Bibcode: 1993ASPC...42...41R
Altcode: 1993gong.conf...41R
No abstract at ADS
Title: Ionization Effects on Solar Granulation Dynamics
Authors: Rast, M. P.; Nordlund, A.; Stein, R. F.; Toomre, J.
Bibcode: 1993ASPC...42...57R
Altcode: 1993gong.conf...57R
No abstract at ADS
Title: Compressible Convection with Ionization.
Authors: Rast, Mark Peter
Bibcode: 1992PhDT........21R
Altcode:
The influence of nonideal effects associated with ionization upon
the dynamics and thermodynamics of compressible convection is
studied. Ionization causes changes in the particle number density,
internal energy, specific heat and opacity of a fluid. The effects of
the first three of these are studied with a simplified model involving
pure hydrogen fluid. Linear and finite-amplitude analyses and fully
nonlinear two-dimensional simulations are undertaken, with the numerical
simulations performed on the massively parallel Connection Machine
CM-2. Opacity effects are examined by analyzing solutions obtained
from more realistic three-dimensional simulations of solar granulation
carried out by Nordlund and Stein. Ionization effects are expected to
influence the dynamics of granulation and acoustic mode excitation
in the Sun and other stars, and likewise the coupling of convection
with pulsations that occurs in stars such as white dwarfs and Cepheid
variables. Both the global transport properties and the local dynamical
properties of convective flows are affected by ionization. In regions
of partial ionization, the enthalpy flux is dominated by latent heat
transport. Strong temperature fluctuations and corresponding buoyancy
forces develop wherever rapid changes in ionization state occur. These
can result in narrow regions of intense vertical flow. The flow
asymmetries reported in compressible ideal gas convection can either
be enhanced or diminished depending on the vertical positioning of
the partially ionized region within the domain. Ionization-induced
variations in the radiative properties of a convecting fluid can
result in thermal boundary layer instability and plume formation. The
interval between plume formation events depends on the growth rate of
the instability, the scale of the underlying convective motions and
the phase speed of the perturbation. As plumes formed in the upper
boundary layer descend, buoyancy forces remain significant as long as
heat exchange and compressional heating result primarily in ionization
of the fluid rather than in temperature equilibration. This can lead
to supersonic vertical flows in an otherwise subsonic flow field, and
can serve to excite acoustic oscillations, the phase of which can be
abruptly altered by subsequent plume events. In the three-dimensional
simulations, significant entrainment of surrounding fluid with depth
gradually weakens the sheets and plumes of fast downflow.
Title: Imaging spectroscopy and its application in spaceborne
systems. The development of a coherent strategy for scientific and
application oriented use ...
Authors: Rast, M.
Bibcode: 1991isia.book.....R
Altcode:
No abstract at ADS
Title: High Wavenumber Thermal Convection Enhanced in Regions of
Partial Ionization
Authors: Rast, Mark Peter
Bibcode: 1991LNP...388..179R
Altcode: 1991ctsm.conf..179R
The linear stability of a compressible hydrogen fluid undergoing thermal
ionization is examined. The ionization formulation consistently includes
particle number, latent heat and specific heat effects. It is found
that the eigenfunctions of the convective modes are strongly peaked
in the region of partial ionization. This tendency increases with
increasing horizontal wavenumber. In addition, thermal diffusion is
least effective in this region due to the increased specific heat of
the fluid. Both these effects combine to yield elevated growth rates
at high wavenumbers. This work implies the possible existence of shells
of very high wavenumber convection embedded in stellar envelopes. Such
shells may determine the depth of the peak source of acoustic emission
and may mask large scale coherent flows below. Two-dimensional nonlinear
simulations are in progress and are briefly discussed as well.
Title: Comparative Geological Evaluation of Different Remote Sensing
Data of the Hoggar Mountains (algeria)
Authors: Jaskolla, F.; Rast, M.
Bibcode: 1988ESASP.287..443J
Altcode: 1988ssoe.conf..443J
No abstract at ADS