Author name code: cranmer
ADS astronomy entries on 2022-09-14
author:"Cranmer, Steven R."
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Title: The Mouse that Squeaked: A small flare from Proxima Cen
observed in the millimeter, optical, and soft X-ray with Chandra
and ALMA
Authors: Howard, Ward S.; MacGregor, Meredith A.; Osten, Rachel;
Forbrich, Jan; Cranmer, Steven R.; Tristan, Isaiah; Weinberger,
Alycia J.; Youngblood, Allison; Barclay, Thomas; Loyd, R. O. Parke;
Shkolnik, Evgenya L.; Zic, Andrew; Wilner, David J.
Bibcode: 2022arXiv220905490H
Altcode:
We present millimeter, optical, and soft X-ray observations of a
stellar flare with an energy squarely in the regime of typical X1 solar
flares. The flare was observed from Proxima Cen on 2019 May 6 as part
of a larger multi-wavelength flare monitoring campaign and was captured
by Chandra, LCOGT, du Pont, and ALMA. Millimeter emission appears to be
a common occurrence in small stellar flares that had gone undetected
until recently, making it difficult to interpret these events within
the current multi-wavelength picture of the flaring process. The May
6 event is the smallest stellar millimeter flare detected to date. We
compare the relationship between the soft X-ray and millimeter emission
to that observed in solar flares. The X-ray and optical flare energies
of 10$^{30.3\pm0.2}$ and 10$^{28.9\pm0.1}$ erg, respectively, the
coronal temperature of T=11.0$\pm$2.1 MK, and the emission measure
of 9.5$\pm$2.2 X 10$^{49}$ cm$^{-3}$ are consistent with M-X class
solar flares. We find the soft X-ray and millimeter emission during
quiescence are consistent with the Gudel-Benz Relation, but not during
the flare. The millimeter luminosity is >100X higher than that of an
equivalent X1 solar flare and lasts only seconds instead of minutes
as seen for solar flares.
Title: Exploring Structures and Flows with NASA's under-construction
PUNCH mission
Authors: DeForest, Craig; Gibson, Sarah; Thompson, Barbara;
Malanushenko, Anna; Desai, Mihir; Elliott, Heather; Viall, Nicholeen;
Cranmer, Steven; de Koning, Curt
Bibcode: 2022cosp...44.1077D
Altcode:
The Polarimeter to UNify the Corona and Heliosphere is a NASA Small
Explorer to image the corona and heliosphere as parts of a single
system. PUNCH comprises four ~50kg smallsats, each carrying one imaging
instrument, that work together to form a single "virtual coronagraph"
with a 90° field of view, centered on the Sun. Scheduled for joint
launch with NASA's SPHEREx mission, PUNCH starts its two-year prime
science phase in 2025. PUNCH will generate full polarized image
sequences of Thomson-scattered light from free electrons in the corona
and young solar wind, once every four minutes continuously. This
enables tracking the young solar wind and turbulent structures within
it as they disconnect from the Sun itself, as well as large transients
such as CMEs, CIRs, and other shocks within the young solar wind. A
student-contributed X-ray spectrometer (STEAM) will address questions
of coronal heating and flare physics. We present motivating science,
expected advances, mission status, and how to get involved with PUNCH
science now.
Title: Expected results for the cradle of the Solar Wind with the
Polarimeter to UNify the Corona and Heliosphere (PUNCH)
Authors: DeForest, Craig; Gibson, Sarah; De Koning, Curt A.; Thompson,
Barbara; Malanushenko, Anna; Desai, Mihir; Elliott, Heather; Viall,
Nicholeen; Cranmer, Steven
Bibcode: 2022cosp...44.1324D
Altcode:
The Polarimeter to UNify the Corona and Heliosphere is a NASA Small
Explorer to image the corona and heliosphere as parts of a single
system. Imaging the corona and heliosphere together from a constellation
of four synchronized smallsats, PUNCH will — starting in 2025 —
provide a unique window on global structure and cross-scale processes
in the outer corona and young solar wind. PUNCH science is informed
by, and complements, the results of PSP and Solar Orbiter; and will
synergize with PROBA3/ASPIICS. We present early prototype results from
STEREO/SECCHI and current preparation work to enable PUNCH science
when data arrive, discuss anticipated results from the deeper-field,
higher time resolution imaging that PUNCH will provide, and describe
how to get involved with PUNCH science now.
Title: Electron Heat Flux in the Solar Wind: Generalized Approaches
to Fluid Transport With a Variety of Skewed Velocity Distributions
Authors: Cranmer, Steven R.; Schiff, Avery J.
Bibcode: 2021JGRA..12629666C
Altcode: 2021arXiv210915267C
In the solar corona and solar wind, electron heat conduction is an
important process that transports energy over large distances and
helps determine the spatial variation of temperature. High-density
regions undergoing rapid particle-particle collisions exhibit a heat
flux described well by classical Spitzer-Härm theory. However, much
of the heliosphere is closer to a more collisionless state, and there
is no standard description of heat conduction for fluid-based (e.g.,
magnetohydrodynamic) models that applies generally. Some proposed models
rely on electron velocity distributions that exhibit negative values of
the phase-space density. In this study, we explore how positive-definite
velocity distributions can be used in fluid-based conservation equations
for the electron heat flux along magnetic-field lines in the corona
and solar wind. We study both analytic forms of skewed distributions
(e.g., skew-normal distributions, two-sided bi-Maxwellians, and
constant-collision-time electrostatic solutions) and empirical fits
to measurements of core, halo, and strahl electrons in interplanetary
space. We also present example solutions to a generalized conservation
equation for the heat flux in the solar wind, with some limiting cases
found to resemble known free-streaming approximations. The resulting
values of the electron heat flux vary as a function of radial distance
and Knudsen number in ways that resemble observed data. We note that
this model does not include the effects of kinetic instabilities (which
may impose saturation limits when active), so for now its regime of
applicability is limited to collisionless heat-flux evolution away
from the known instability boundaries in parameter space.
Title: High-frequency Wave Power Observed in the Solar Chromosphere
with IBIS and ALMA
Authors: Molnar, Momchil E.; Reardon, Kevin P.; Cranmer, Steven R.;
Kowalski, Adam F.; Chai, Yi; Gary, Dale
Bibcode: 2021ApJ...920..125M
Altcode: 2021arXiv210708952M
We present observational constraints on the chromospheric heating
contribution from acoustic waves with frequencies between 5 and 50
mHz. We use observations from the Dunn Solar Telescope in New Mexico,
complemented with observations from the Atacama Large Millimeter Array
collected on 2017 April 23. The properties of the power spectra of the
various quantities are derived from the spectral lines of Ca II 854.2
nm, H I 656.3 nm, and the millimeter continuum at 1.25 and 3 mm. At
the observed frequencies, the diagnostics almost all show a power-law
behavior, whose particulars (slope, peak, and white-noise floors)
are correlated with the type of solar feature (internetwork, network,
and plage). In order to disentangle the vertical versus transverse
Alfvénic plasma motions, we examine two different fields of view: one
near disk center, and the other close to the limb. To infer the acoustic
flux in the middle chromosphere, we compare our observations with
synthetic observables from the time-dependent radiative hydrodynamic
RADYN code. Our findings show that acoustic waves carry up to about
1 kW m-2 of energy flux in the middle chromosphere, which
is not enough to maintain the quiet chromosphere. This is in contrast
to previous publications.
Title: Brown Dwarfs are Violet: A New Calculation of Human-eye Colors
of Main-sequence Stars and Substellar Objects
Authors: Cranmer, Steven R.
Bibcode: 2021RNAAS...5..201C
Altcode:
There has always been interest in the perceived colors of stars. They
were key to the development of the Hertzsprung-Russell diagram, and they
are also used widely in educational and public-outreach imagery. Thus,
it is useful to develop tools to compute these colors from spectral
energy distributions. This paper presents a collection of objective (CIE
coordinate) and subjective (RGB triple) colors for main-sequence stars
and brown dwarfs, as well as links to related codes and tables. Using
the proposed conversion from CIE to RGB colors, O and B stars are bluer
than equivalent blackbodies because of Paschen continuum absorption,
and M dwarfs tend to be less red and more beige. Although brown dwarfs
over a wide range of effective temperatures (400-2000 K) emit most
of their flux in the infrared, their visible spectra are dominated by
short wavelengths. Thus, they may appear violet to human eyes.
Title: Turbulent Generation of Magnetic Switchbacks in the Alfvénic
Solar Wind
Authors: Shoda, Munehito; Chandran, Benjamin D. G.; Cranmer, Steven R.
Bibcode: 2021ApJ...915...52S
Altcode: 2021arXiv210109529S
One of the most important early results from the Parker Solar Probe
(PSP) is the ubiquitous presence of magnetic switchbacks, whose origin
is under debate. Using a three-dimensional direct numerical simulation
of the equations of compressible magnetohydrodynamics from the corona
to 40 solar radii, we investigate whether magnetic switchbacks emerge
from granulation-driven Alfvén waves and turbulence in the solar
wind. The simulated solar wind is an Alfvénic slow-solar-wind stream
with a radial profile consistent with various observations, including
observations from PSP. As a natural consequence of Alfvén-wave
turbulence, the simulation reproduced magnetic switchbacks with many
of the same properties as observed switchbacks, including Alfvénic
v-b correlation, spherical polarization (low magnetic compressibility),
and a volume filling fraction that increases with radial distance. The
analysis of propagation speed and scale length shows that the magnetic
switchbacks are large-amplitude (nonlinear) Alfvén waves with
discontinuities in the magnetic-field direction. We directly compare
our simulation with observations using a virtual flyby of PSP in our
simulation domain. We conclude that at least some of the switchbacks
observed by PSP are a natural consequence of the growth in amplitude of
spherically polarized Alfvén waves as they propagate away from the Sun.
Title: STRIA: A new module within FORWARD towards modelling PUNCH
datasets
Authors: Gilly, C. R.; Cranmer, S.; Gibson, S.
Bibcode: 2021AAS...23832802G
Altcode:
A new module is being written within the FORWARD toolkit in SSW which
will help us to interpret future observations from the PUNCH mission
(a new heliosphere imager being launched in 2023). This presentation
will consist of preliminary results from this project. The next
step past this striated model (STRIA) will involve placing radially
outflowing blobs of plasma into the model and discerning expected
detection challenges/limits.
Title: Spectroscopic Study Of Wave Propagation In The Quiet Solar
Chromosphere with IRIS and IBIS
Authors: Molnar, M. E.; Cranmer, S. R.; Reardon, K. P.; Kowalski, A. F.
Bibcode: 2021AAS...23811303M
Altcode:
In this work, we present constraints on the longitudinal (compressive)
and transverse (Alfvenic) wave velocity perturbations observed in the
chromosphere. Better knowledge of the power in these different wave
modes in different regions of the atmosphere are important inputs into
models for the heating of the solar corona. By using observations
at multiple viewing angles (distances from the disc center), the
relative importance of these two components can be evaluated and
the power in the local acoustic flux can be explored. This work is
based on Doppler velocity measurements from IRIS of the ultraviolet
Mg II h & k and the Mn I 280.19 nm lines. These are compared with
co-temporal observations from IBIS of the H-alpha and Ca II 854.2 nm
chromospheric lines in the visible. The observed phase differences
between the velocity diagnostics in these different lines allows us to
estimate a formation height of the Mn I 280.19 nm line and compare it
with recent results from simulations. We can also measure the lowest
observed frequency at which the phase differences indicate the presence
of wave propagation in order to calculate the local acoustic-wave
cutoff. We calculate the coherency of the signals and their phases with
a cross-wavelet analysis. We further combine the IRIS observations
with 1D simulations of the lower solar atmosphere from the RADYN
code to estimate the wave flux inthe upper chromosphere. This study
provides heating constraints for the middle and upper chromospheres and
additional estimates of the transverse wave power in the chromosphere
extending previous work by Molnar et al. (2021).
Title: Proposing new ways to analyze the resolved shape changes of
photospheric bright points as wave drivers
Authors: Van Kooten, S. J.; Cranmer, S. R.
Bibcode: 2021AAS...23832816V
Altcode:
Magnetic bright points on the solar photosphere, prominent in the G
band but also visible in the continuum, mark the footpoints of kilogauss
magnetic flux tubes extending toward the corona. The horizontal motions
of these footpoints, driven by convective buffeting, are believed to
excite MHD waves which propagate to the corona, where they deposit heat
through turbulent dissipation. Measuring these motions observationally
can thus constrain MHD-wave energy transport and provide a key lower
boundary condition in coronal and heliospheric models. At ~100 km
in diameter, most bright points are currently unresolved. Tracking
bright-point centroids has been a mainstay analysis technique, and
it allows the modeling of kink-mode wave excitation in the overlying
flux tubes. First-light images from DKIST have resolved the sizes
and shapes of bright points, and the coming science operations
will reveal the time evolution of these high-resolution details,
which is expected to excite sausage-mode and higher-order flux-tube
waves. We propose two complementary ways to take the "next step"
beyond centroid tracking and account for these additional wave modes,
and we demonstrate these techniques on MURaM simulated images of
DKIST-like resolution as a proof-of-concept. In the first technique,
we describe each bright point with a centroid location as well as
the parameters of an ellipse fitted to the bright point's shape. We
derive expressions for the energy flux of n=0, 1, and 2 wave modes in
terms of the evolution of these parameters. In the second approach,
we use an off-the-shelf algorithm for computing the earth mover's
distance to infer a horizontal velocity field responsible for shifting
a bright point from one shape to the next, under an assumed advective
and planar process. Despite the simplicity of this approach, we find
some agreement with the "ground truth" plasma velocities in the MURaM
simulation. These velocity fields can then be used to estimate energy
fluxes. We present estimated wave energy fluxes from both of these
approaches. These fluxes are non-negligible, suggesting these wave
modes are a worthy target for observational study and motivating
further development of these and other techniques, all of which can
provide new constraints for wave-based models of coronal heating.
Title: Chromospheric and Coronal Heating in Cool Stars: Constraints
on Physical Processes from X-ray and Lyman Alpha Observations
Authors: Cranmer, S. R.; Aarnio, A.; Molnar, M. E.
Bibcode: 2021AAS...23813102C
Altcode:
Nearly all low-mass stars are believed to exhibit subsurface convection,
some level of magnetic dynamo activity, and radiative emission
from chromospheric (T = 10,000 K) and coronal (T > 1 million K)
layers above their photospheres. Linsky et al. (2020) highlighted
the usefulness of comparing X-ray and H I Lyman alpha flux trends
from cool stars as a way of constraining how these atmospheres are
produced and maintained. Here, we seek to simulate chromospheric and
coronal heating for a broad set of F, G, K, and M stars and investigate
whether the observed trends in X-ray and Lyman alpha emission can be
reproduced. We also produce a new conversion of the Sun's observed
time-variable X-ray emission (from the GOES 1-8 Angstrom band) into the
lower-energy ROSAT/PSPC band more commonly used in studies of cool-star
X-rays. Because we have not yet conclusively solved our Sun's own
chromospheric and coronal heating problems, we parameterize the rate
of simulated energy deposition using known expressions for the maximum
available Poynting flux and efficiencies of various proposed mechanisms
(see, e.g., Cranmer & Winebarger 2019). A key input parameter turns
out to be the driving velocity at the photospheric base of the coronal
magnetic field lines. Straightforward extrapolation from mixing-length
convection theory drastically underestimates the velocity required to
explain the emission from M dwarfs. However, empirical trends from
spectroscopically inferred microturbulence velocities seem to do a
better job, and we will explore why this may be an important clue
to the underlying physics. Lastly, we note that understanding the
origins of X-ray and UV emission from cool stars will also help us
better predict the present-day properties and long-term evolution of
exoplanet atmospheres.
Title: Simulating the Solar Minimum Corona in UV Wavelengths with
Forward Modeling II. Doppler Dimming and Microscopic Anisotropy Effect
Authors: Zhao, Jie; Gibson, Sarah E.; Fineschi, Silvano; Susino,
Roberto; Casini, Roberto; Cranmer, Steven R.; Ofman, Leon; Li, Hui
Bibcode: 2021ApJ...912..141Z
Altcode:
In ultraviolet (UV) spectropolarimetric observations of the solar
corona, the existence of a magnetic field, solar wind velocity, and
temperature anisotropies modify the linear polarization associated with
resonant scattering. Unlike previous empirical models or global models,
which present blended results of the above physical effects, in this
work, we forward-model expected signals in the H I Lyα line (121.6 nm)
by adopting an analytic model that can be adjusted to test the roles
of different effects separately. We find that the impact of all three
effects is most evident in the rotation of the linear polarization
direction. In particular, (1) for magnetic fields between ∼10 and
∼100 G, the Hanle effect modifies the linear polarization at low
coronal heights, rotating the linear polarization direction clockwise
(counterclockwise) when the angle between the magnetic field and the
local vertical is greater (less) than the van Vleck angle, which is
consistent with the result of Zhao et al.; (2) solar wind velocity,
which increases with height, has a significant effect through the
Doppler dimming effect at higher coronal heights, rotating the linear
polarization direction in an opposite fashion to the Hanle effect;
and (3) kinetic temperature anisotropies are most significant at
lower heights in open nonradial magnetic field regions, producing
tilt opposite to isotropic Doppler dimming. The fact that the three
effects operate differently in distinct spatial regimes opens up the
possibility for using linear polarization measurements in UV lines to
diagnose these important physical characteristics of the solar corona.
Title: A Refined Model of Convectively Driven Flicker in Kepler
Light Curves
Authors: Van Kooten, Samuel J.; Anders, Evan H.; Cranmer, Steven R.
Bibcode: 2021ApJ...913...69V
Altcode: 2021arXiv210406533V
Light curves produced by the Kepler mission demonstrate stochastic
brightness fluctuations (or flicker) of stellar origin which contribute
to the noise floor, limiting the sensitivity of exoplanet detection
and characterization methods. In stars with surface convection,
the primary driver of these variations on short (sub-eight-hour)
timescales is believed to be convective granulation. In this work,
we improve existing models of this granular flicker amplitude,
or F8, by including the effect of the Kepler bandpass on
measured flicker, by incorporating metallicity in determining convective
Mach numbers, and by using scaling relations from a wider set of
numerical simulations. To motivate and validate these changes, we use
a recent database of convective flicker measurements in Kepler stars,
which allows us to more fully detail the remaining model-prediction
error. Our model improvements reduce the typical misprediction of
flicker amplitude from a factor of 2.5-2. We rule out rotation period
and strong magnetic activity as possible explanations for the remaining
model error, and we show that binary companions may affect convective
flicker. We also introduce an envelope model that predicts a range of
flicker amplitudes for any one star to account for some of the spread
in numerical simulations, and we find that this range covers 78%
of observed stars. We note that the solar granular flicker amplitude
is lower than most Sun-like stars. This improved model of convective
flicker amplitude can better characterize this source of noise in
exoplanet studies as well as better inform models and simulations of
stellar granulation.
Title: Inward-propagating Plasma Parcels in the Solar Corona: Models
with Aerodynamic Drag, Ablation, and Snowplow Accretion
Authors: Cranmer, Steven R.; DeForest, Craig E.; Gibson, Sarah E.
Bibcode: 2021ApJ...913....4C
Altcode: 2021arXiv210312039C
Although the solar wind flows primarily outward from the Sun to
interplanetary space, there are times when small-scale plasma inflows
are observed. Inward-propagating density fluctuations in polar coronal
holes were detected by the COR2 coronagraph on board the STEREO-A
spacecraft at heliocentric distances of 7-12 solar radii, and these
fluctuations appear to undergo substantial deceleration as they move
closer to the Sun. Models of linear magnetohydrodynamic waves have
not been able to explain these deceleration patterns, so they have
been interpreted more recently as jets from coronal sites of magnetic
reconnection. In this paper, we develop a range of dynamical models
of discrete plasma parcels with the goal of better understanding the
observed deceleration trend. We found that parcels with a constant
mass do not behave like the observed flows, and neither do parcels
undergoing ablative mass loss. However, parcels that accrete mass in
a snowplow-like fashion can become decelerated as observed. We also
extrapolated OMNI in situ data down to the so-called Alfvén surface
and found that the initial launch point for the observed parcels may
often be above this critical radius. In other words, in order for the
parcels to flow back down to the Sun, their initial speeds are probably
somewhat nonlinear (i.e., supra-Alfvénic), and thus the parcels may be
associated with structures such as shocks, jets, or shear instabilities.
Title: Code and Data for "A Refined Model of Convectively-Driven
Flicker in Kepler Light Curves"
Authors: Van Kooten, Samuel J.; Anders, Evan H; Cranmer, Steven R.
Bibcode: 2021zndo...4444282V
Altcode:
Code and Data for "A Refined Model of Convectively-Driven
Flicker in Kepler Light Curves" This is the code implementing
our model for stellar flicker in Kepler light curves, as well as
both the data we produce and the data required for reproducing
our results. If any updates are made, they will be available at
https://github.com/svank/modeling-stellar-flicker. See README.txt for
descriptions of the included files.
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: Discovery of an Extremely Short Duration Flare from Proxima
Centauri Using Millimeter through Far-ultraviolet Observations
Authors: MacGregor, Meredith A.; Weinberger, Alycia J.; Loyd,
R. O. Parke; Shkolnik, Evgenya; Barclay, Thomas; Howard, Ward S.;
Zic, Andrew; Osten, Rachel A.; Cranmer, Steven R.; Kowalski, Adam
F.; Lenc, Emil; Youngblood, Allison; Estes, Anna; Wilner, David J.;
Forbrich, Jan; Hughes, Anna; Law, Nicholas M.; Murphy, Tara; Boley,
Aaron; Matthews, Jaymie
Bibcode: 2021ApJ...911L..25M
Altcode: 2021arXiv210409519M
We present the discovery of an extreme flaring event from Proxima
Cen by the Australian Square Kilometre Array Pathfinder (ASKAP),
Atacama Large Millimeter/submillimeter Array (ALMA), Hubble Space
Telescope (HST), Transiting Exoplanet Survey Satellite (TESS), and
the du Pont Telescope that occurred on 2019 May 1. In the millimeter
and FUV, this flare is the brightest ever detected, brightening
by a factor of >1000 and >14,000 as seen by ALMA and HST,
respectively. The millimeter and FUV continuum emission trace each
other closely during the flare, suggesting that millimeter emission
could serve as a proxy for FUV emission from stellar flares and become
a powerful new tool to constrain the high-energy radiation environment
of exoplanets. Surprisingly, optical emission associated with the event
peaks at a much lower level with a time delay. The initial burst has
an extremely short duration, lasting for <10 s. Taken together with
the growing sample of millimeter M dwarf flares, this event suggests
that millimeter emission is actually common during stellar flares and
often originates from short burst-like events.
Title: Discovery of an Extremely Short Duration 'Building Block'
Flare from Proxima Centauri
Authors: MacGregor, M.; Weinberger, A.; Loyd, P.; Shkolnik, E.;
Barclay, T.; Howard, W.; Zic, A.; Osten, R.; Cranmer, S.; Kowalski,
A.; Lenc, E.; Youngblood, A.; Estes, A.; Wilner, D.; Forbrich, J.;
Hughes, A.; Law, N.; Murphy, T.; Boley, A.; Matthews, J.
Bibcode: 2021BAAS...53c1249M
Altcode:
At a distance of only 1.3 pc, Proxima Cen is the closest exoplanetary
system orbiting an M-type flare star, making it a benchmark case to
explore the properties and potential effects of stellar activity on
exoplanet atmospheres. Here, we present the discovery of an extreme
flaring event from Proxima Cen by the the Australian Square Kilometre
Array Pathfinder (ASKAP), the Atacama Large Millimeter/submillimeter
Array (ALMA), the Transiting Exoplanet Survey Satellite (TESS), the
du Pont telescope at Las Campanas, and the Hubble Space Telescope
(HST). In the millimeter and FUV, this flare is the brightest ever
detected, brightening by a factor of >1000 and >14000 as seen by
ALMA and HST, respectively. The millimeter and FUV continuum emission
trace each other closely during the flare, suggesting that millimeter
emission could serve as a proxy for FUV emission from stellar flares
and become a powerful new tool to constrain the high-energy radiation
environment of exoplanets. Optical emission is decoupled, peaking at
a much lower level with a time delay. The extremely short duration
of this event indicates that it could originate from a single flare
loop or 'building block.' These are the first results from a larger
campaign executed in April-July 2019 consisting of roughly 40 hours
of simultaneous observations of Proxima Cen spanning radio to X-ray
wavelengths.
Title: An Updated Formalism for Line-driven Radiative Acceleration
and Implications for Stellar Mass Loss
Authors: Lattimer, Aylecia S.; Cranmer, Steven R.
Bibcode: 2021ApJ...910...48L
Altcode: 2021arXiv210110375L
Radiation contributes to the acceleration of large-scale flows in
various astrophysical environments because of strong opacity in the
spectral lines. Quantification of the associated force is crucial to
understanding these line-driven flows, and a large number of lines
(due to the full set of elements and ionization stages) must be taken
into account. Here we provide new calculations of the dimensionless
line strengths and associated opacity-dependent force multipliers for
an updated list of approximately 4.5 million spectral lines compiled
from the NIST, CHIANTI, CMFGEN, and TOPbase databases. To maintain
generality of application to different environments, we assume local
thermodynamic equilibrium, illumination by a Planck function, and the
Sobolev approximation. We compute the line forces in a two-dimensional
grid of temperatures (i.e., values between 5200 and 70,000 K) and
densities (varying over 11 orders of magnitude). Historically, the
force multiplier function has been described by a power-law function
of optical depth. We revisit this assumption by fitting alternate
functions that include saturation to a constant value (Gayley's
$\bar{Q}$ parameter) in the optically thin limit. This alternate form
is a better fit than the power-law form, and we use it to calculate
example mass-loss rates for massive main-sequence stars. Because the
power-law force multiplier does not continue to arbitrarily small
optical depths, we find a sharp decrease, or quenching, of line-driven
winds for stars with effective temperatures less than about 15,000 K.
Title: A New Model of Granulation-Driven Flicker in Kepler Light
Curves
Authors: Van Kooten, S. J.; Cranmer, S. R.
Bibcode: 2021AAS...23751506V
Altcode:
The light curves produced by the Kepler mission demonstrate real,
stochastic brightness fluctuations (or "flicker") which contribute to
the noise floor limiting the sensitivity of exoplanet detection and
characterization methods. In stars with outer convective envelopes,
the primary driver of these variations on shorter (sub-eight-hour)
timescales is convective granulation. We have improved upon existing
efforts to model this granular flicker by incorporating a wider set
of scaling relations from numerical simulations, adding a correction
factor for the effect of the Kepler bandpass, and incorporating
metallicity in determining Mach numbers. In validating this model,
we draw upon an expanded database of convective flicker measurements
in Kepler stars, allowing us to more fully detail the remaining errors
in model predictions. We introduce an "envelope" model which accounts
for some of the spread in numerical simulations by producing a range of
predicted flicker values for any one star, and we find that nearly 70%
of observed stars fall within this range. We rule out rotation period
and strong magnetic activity as possible explanations for the remaining
model error. We also note that the solar granular flicker amplitude,
measured in SOHO/Virgo data, is lower than most Sun-like stars. This
progress toward an improved understanding of convective flicker can
better characterize this source of noise in exoplanet detection and
characterization as well as better inform models of stellar granulation.
Title: The Relative Emission from Chromospheres and Coronae:
Dependence on Spectral Type and Age
Authors: Linsky, J.; Wood, B.; Youngblood, A.; Brown, A.; France,
K.; Buccino, A.; Froning, C.; Cranmer, S.; Mauas, P.; Miguel, Y.;
Pineda, J.; Rugheimer, S.; Vieytes, M.; Wheatley, P.; Wilson, D.
Bibcode: 2021AAS...23714110L
Altcode:
Extreme-UV and X-ray emissions from stellar coronae drive mass loss
from exoplanet atmospheres, and UV emission from stellar chromospheres
drives photo-chemistry in exoplanet atmospheres. Comparisons of the
spectral energy distributions of host stars are, therefore, essential
for understanding the evolution and habitability of exoplanets. The
large number of stars observed with the MUSCLES, Mega-MUSCLES, and
other recent HST observing programs has provided for the first time
a large sample (79 stars) of reconstructed Lyman-alpha fluxes that
we compare with X-ray fluxes to identify significant patterns in the
relative emission from these two atmospheric regions as a function of
stellar age and effective temperature. We find that as stars age on
the main sequence, a single trend line slope describes the pattern of
X-ray vs. Lyman-alpha emission for F, G and K dwarfs, but the different
trend lines for M dwarf stars show that the Lyman-alpha fluxes of M
stars are significantly smaller than warmer stars with the same X-ray
flux. The X-ray and Lyman-alpha luminosities divided by the stellar
bolometric luminosities show different patterns depending on stellar
age. The L(Lyα)/L(bol) ratios increase smoothly to cooler stars
of all ages, but the L(X)/L(bol) ratios show different trends. For
older stars, the increase in coronal emission with decreasing T(eff)
is much steeper than chromospheric emission. We suggest a fundamental
link between atmospheric properties and trend lines relating coronal
and chromospheric heating. See paper in ApJ volume 902 (2020).
Title: An Updated Formalism for Line-Driven Outflows and Consequences
for Mass Loss
Authors: Lattimer, A. S.; Cranmer, S.
Bibcode: 2021AAS...23711602L
Altcode:
The colloquially-termed "radiation pressure" of line-driven winds
plays an important role in driving outflows in various astrophysical
environments. Quantification of the associated force is crucial to
understanding interactions within these environments. The large
number of spectral lines in ay given ion of the outflow material
must be tabulated in order to specify this force. Here we provide new
calculations of the dimensionless line strength parameter, describing
the ratio of radiative acceleration of bound to free electrons, from an
updated line list comprised of approximately 4.5 million spectral lines,
compiled from four spectral databases. We assume local thermodynamic
equilibrium and compute the line strengths for a range of temperatures
and densities in a 2D grid. These are combined with dimensionless
flux-weighting functions from an assumed Planck-function source to form
the canonical line-force multiplier M(t), where t is a fiducial Sobolev
optical depth. Historically, M(t) has been described by a power-law
function, and we revisit this assumption by fitting alternative
functions that include a saturation to a constant value (Gayley's
Q-bar parameter) at low values of t. We find that this alternate form
is a better fit than the power-law form, and we use it to calculate
mass-loss rates for our density-temperature grid. A sharp drop-off is
present in the mass-loss rates when compared to the power-law form,
representing a previously undescribed quenching of the wind.
Title: Discovery of an Extremely Short Duration 'Building Block'
Flare from Proxima Cen Using Millimeter through FUV Observations
Authors: MacGregor, M. A.; Weinberger, A. J.; Loyd, P.; Shkolnik,
E. L.; Barclay, T.; Osten, R.; Howard, W. S.; Zic, A.; Cranmer, S. R.;
Kowalski, A. F.; Youngblood, A.; Estes, A.; Wilner, D. J.; Forbrich,
J.; Murphy, T.; Law, N.; Hughes, A.; Boley, A.; Tristan, I. I.; Fuson,
J. F.; Matthews, J.
Bibcode: 2021AAS...23751502M
Altcode:
At a distance of only 1.3 pc, Proxima Cen is the closest exoplanetary
system orbiting an M-type flare star, making it a benchmark
case to explore the properties and potential effects of stellar
activity on exoplanet atmospheres. Our previous discovery of a flare
from Proxima Cen at millimeter wavelengths with the Atacama Large
Millimeter/submillimeter Array (ALMA) has opened up an entirely new
observational regime to study stellar flaring mechanisms. These are the
first results from a larger campaign consisting of roughly 40 hours of
simultaneous observations spanning radio to X-ray wavelengths. Here,
we present the discovery of a second flaring event on 1 May 2019 from
Proxima Cen with ALMA, but this time complemented by multi-wavelength
observations with the Hubble Space Telescope (HST) of far-ultraviolet
(FUV) spectroscopy, the Transiting Exoplanet Survey Satellite (TESS)
of optical photometry, and the DuPont telescope at Las Campanas of
optical spectroscopy. In the millimeter and FUV, the May 1 flare is
the brightest ever detected from Proxima Cen, brightening by a factor
of >1000 and >14000 as seen by ALMA and HST, respectively. The
millimeter and FUV continuum emission trace each other very closely
during the flare, exhibiting similar rise and decay times, peaking near
simultaneously, and achieving large enhancements in luminosity. Optical
emission is somewhat decoupled, peaking at a much lower level with
a slight time delay. Given the unique characteristics of this event,
it is possible that we are seeing an entirely new type of flare. The
extremely short duration of this event suggests that it could
originate from a single flare loop or 'building block' instead of an
arcade structure consisting of multiple superimposed loops. The strong
correlation between millimeter and FUV emission allows us to determine
a tentative scaling relation. If this holds for a larger sample of
events, millimeter emission could serve as a proxy for FUV emission
from stellar flares and become a powerful new tool to constrain the
high energy radiation environment of planets orbiting flare stars,
required input for models of planetary atmosphere evolution.
Title: Constraining wave propagation throughout the solar atmosphere
with IBIS, ALMA and IRIS
Authors: Molnar, M.; Reardon, K.; Cranmer, S. R.; Kowalski, A. F.
Bibcode: 2020AGUFMSH0010003M
Altcode:
The heating mechanism of the solar chromosphere is still an open
scientific question. We present observational constraints on the
high-frequency (acoustic) wave contribution to the chromospheric
heating. We utilize a unique combination of observations from NSO's
Dunn Solar Telescope and the Atacama Large Millimeter Array obtained on
April 23rd 2017 to estimate the high-frequency wave flux in the lower
solar atmosphere. We extend this study to the upper chromosphere and
the transition region with archival IRIS data. We infer the wave flux
through comparison of the observations with synthetic observables
from the time-dependent hydrodynamic RADYN code. Our findings are
able to constrain the wave flux at higher altitudes in the solar
atmosphere than previous works using similar approaches. Furthermore,
the different diagnostics we use form at different heights, which
allow us to explore the propagation and dissipation of waves with
height. We will discuss future plans to extend this work with more
advanced modeling and additional observations with the upcoming Innoue
Solar Telescope (DKIST).
Title: Contemporary Analysis Methods for Coronagraph and Heliospheric
Imager Data
Authors: Thompson, B. J.; Attie, R.; Chhiber, R.; Cranmer, S. R.;
DeForest, C.; Gallardo-Lacourt, B.; Gibson, S. E.; Jones, S. I.;
Moraes Filho, V.; Reginald, N. L.; Uritsky, V. M.; Viall, N. M.
Bibcode: 2020AGUFMSH031..05T
Altcode:
Coronagraphs, polarimeters, and heliospheric imagers are providing
new insight into how structures in the solar wind form and develop as
they flow from the inner corona into the heliosphere. With this comes
a whole new frontier of physical observables in 3D, including kinetic
(velocity and acceleration), thermodynamic (density, temperature, and
shock boundary), and magnetic field properties. These measurements
inform and challenge models of global solar wind flow, turbulence,
and CME propagation. We will discuss recent advances in quantifying
physical properties of the corona and solar wind using coronagraph
and heliospheric imager data, and make predictions of what new models
and instrumentation (including the in-development PUNCH mission)
will bring us in the future.
Title: Measuring complex bright-point motion and wave excitation in
high-resolution solar observations
Authors: Van Kooten, S.; Cranmer, S. R.
Bibcode: 2020AGUFMSH0010012V
Altcode:
Magnetic bright points on the solar photosphere, prominent in
the G band but visible in the continuum, indicate footpoints of
kilogauss magnetic flux tubes extending to the corona. The horizontal
motions of these footpoints are believed to excite MHD waves which
propagate to the corona, where they deposit heat through turbulent
dissipation. Analyzing this motion can thus constrain MHD-wave energy
transport and provide a key lower boundary condition in coronal and
heliospheric models. At ~100 km in diameter, most bright points are
unresolved. Traditional tracking of their centroids allows kink-mode
wave excitation to be modeled in the overlying flux tubes. However,
centroid tracking cannot easily handle the merging or splitting of
bright points nor can it adequately track extremely long, thin bright
points. First-light images from DKIST have resolved the sizes and shapes
of bright points, and future science observations will reveal the time
evolution of these high-resolution details, which is expected to excite
higher-order flux-tube waves. But centroid tracking cannot analyze
this more detailed motion, and even in a study limited to kink-mode
waves, Agrawal et al. (2018) and Van Kooten & Cranmer (2017) have
shown that centroid tracking applied to DKIST observations of bright
points is likely to experience a strong, spurious "jitter" signal
due to the high spatial and temporal resolution. We present initial
results from efforts on multiple fronts to overcome these limitations,
including an algorithm to infer the horizontal plasma velocity field
inside bright points at DKIST-like resolution (at which bright points
are resolved but not large enough for traditional correlation-tracking
techniques). These inferred velocity fields assist in the modeling of
higher-order waves generated in the overlying flux tubes. By preparing
our approaches using output data from high-resolution MURaM simulations
now, our new approach will be ready to analyze upcoming DKIST data as it
becomes available. This work will enable estimates of the significance
of the contribution to the coronal heating budget of more complex waves
generated by small-scale motions and so provide a more complete lower
boundary condition for coronal and heliospheric models.
Title: Forward Models of Coronal Nonthermal Line-Widths
Authors: Gilly, C. R.; Cranmer, S. R.
Bibcode: 2020AGUFMSH0300003G
Altcode:
Spectral lines observed from optically-thin sources can be difficult
to interpret. Forward models are an excellent tool for understanding
the connection between line of sight plasma conditions and integrated
observations. We use our GHOSTS model to generate ensembles of realistic
spectral lines from the Sun's corona. In particular we focus on the
effect of (1) polar-plume-like structures along the line of sight, (2)
the presence of outwardly propagating Alfvén waves, and (3) changing
the integration time of the observations. The simulated observables
are also compared to real observations, such as those from Hinode/EIS,
SUMER/SOHO, and CoMP.
Title: How the breakout-limited mass in B-star centrifugal
magnetospheres controls their circumstellar H α emission
Authors: Owocki, Stanley P.; Shultz, Matt E.; ud-Doula, Asif;
Sundqvist, Jon O.; Townsend, Richard H. D.; Cranmer, Steven R.
Bibcode: 2020MNRAS.499.5366O
Altcode: 2020MNRAS.tmp.3094O; 2020arXiv200912359O
Strongly magnetic B-type stars with moderately rapid rotation form
'centrifugal magnetospheres' (CMs) from the magnetic trapping
of stellar wind material in a region above the Kepler co-rotation
radius. A long-standing question is whether the eventual loss of such
trapped material occurs from gradual drift and/or diffusive leakage, or
through sporadic 'centrifugal breakout' (CBO) events, wherein magnetic
tension can no longer contain the built-up mass. We argue here that
recent empirical results for Balmer-α emission from such B-star CMs
strongly favour the CBO mechanism. Most notably, the fact that the
onset of such emission depends mainly on the field strength at the
Kepler radius, and is largely independent of the stellar luminosity,
strongly disfavours any drift/diffusion process, for which the net mass
balance would depend on the luminosity-dependent wind feeding rate. In
contrast, we show that in a CBO model, the maximum confined mass in
the magnetosphere is independent of this wind feeding rate and has a
dependence on field strength and Kepler radius that naturally explains
the empirical scalings for the onset of H α emission, its associated
equivalent width, and even its line profile shapes. However, the general
lack of observed Balmer emission in late-B and A-type stars could still
be attributed to a residual level of diffusive or drift leakage that
does not allow their much weaker winds to fill their CMs to the breakout
level needed for such emission; alternatively, this might result from
a transition to a metal-ion wind that lacks the requisite hydrogen.
Title: New Measurements of Plasma Properties near the Cusps of
Pseudostreamers and Helmet Streamers
Authors: Miralles, M. P.; Cranmer, S. R.
Bibcode: 2020AGUFMSH0280020M
Altcode:
The role of magnetic topology in solar wind acceleration is an
unsolved problem in solar physics. Coronal pseudostreamers differ
from the more traditional helmet streamers in that they separate
open-field lines of the same polarity rather than the opposite
polarity. Helmet streamers and pseudostreamers are believed to be
sources of slow to intermediate speed solar wind streams. Using
multi-spacecraft and ground-based observations, we characterize the
physical parameters of these coronal structures, and focus on their
differences and similarities. Line-of-sight forward modeling (of a range
of visible-light and UV emission diagnostics) is used to investigate
how the differences in magnetic topology affect the plasma properties
of the coronal structures and their wind. This work is supported by
NASA grant NNX17AI27G to the Smithsonian Astrophysical Observatory.
Title: Updated Measurements of Proton, Electron, and Oxygen
Temperatures in the Fast Solar Wind
Authors: Cranmer, Steven R.
Bibcode: 2020RNAAS...4..249C
Altcode: 2020arXiv201210509C
The high-speed solar wind is typically the simplest and least stochastic
type of large-scale plasma flow in the heliosphere. For much of the
solar cycle, it is connected magnetically to large polar coronal holes
on the Sun's surface. Because these features are relatively well-known
(and less complex than the multiple source-regions of the slow wind),
the fast wind is often a useful testing-ground for theoretical models
of coronal heating. In order to provide global empirical constraints
to these models, here we collect together some older and more recent
measurements of the temperatures of protons, electrons, and oxygen
ions as a function of radial distance.
Title: The Relative Emission from Chromospheres and Coronae:
Dependence on Spectral Type and Age
Authors: Linsky, Jeffrey L.; Wood, Brian E.; Youngblood, Allison;
Brown, Alexander; Froning, Cynthia S.; France, Kevin; Buccino,
Andrea P.; Cranmer, Steven R.; Mauas, Pablo; Miguel, Yamila; Pineda,
J. Sebastian; Rugheimer, Sarah; Vieytes, Mariela; Wheatley, Peter J.;
Wilson, David J.
Bibcode: 2020ApJ...902....3L
Altcode: 2020arXiv200901958L
Extreme-ultraviolet and X-ray emission from stellar coronae drives
mass loss from exoplanet atmospheres, and ultraviolet emission
from stellar chromospheres drives photochemistry in exoplanet
atmospheres. Comparisons of the spectral energy distributions of host
stars are, therefore, essential for understanding the evolution and
habitability of exoplanets. The large number of stars observed with
the MUSCLES, Mega-MUSCLES, and other recent Hubble Space Telescope
observing programs has provided for the first time a large sample
(79 stars) of reconstructed Lyα fluxes that we compare with X-ray
fluxes to identify significant patterns in the relative emission
from these two atmospheric regions as a function of stellar age and
effective temperature. We find that as stars age on the main sequence,
the emissions from their chromospheres and coronae follow a pattern
in response to the amount of magnetic heating in these atmospheric
layers. A single trend-line slope describes the pattern of X-ray versus
Lyα emission for G and K dwarfs, but the different trend lines for
M dwarf stars show that the Lyα fluxes of M stars are significantly
smaller than those of warmer stars with the same X-ray flux. The X-ray
and Lyα luminosities divided by the stellar bolometric luminosities
show different patterns depending on stellar age. The L(Lyα)/L(bol)
ratios increase smoothly to cooler stars of all ages, but the
L(X)/L(bol) ratios show different trends. For older stars, the increase
in coronal emission with decreasing ${T}_{\mathrm{eff}}$ is much steeper
than that of chromospheric emission. We suggest a fundamental link
between atmospheric properties and trend lines relating coronal and
chromospheric heating, * Based on observations made with the NASA/ESA
Hubble Space Telescope, obtained from the Mikulski Archive for Space
Telescopes (MAST) at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy,
Inc., under NASA contract NAS AR-09525.01A. These observations are
associated with program Nos. 12475, 12596, 13650, 14640, and 15071.
Title: The Effect of Solar Wind Expansion and Nonequilibrium
Ionization on the Broadening of Coronal Emission Lines
Authors: Gilly, Chris R.; Cranmer, Steven R.
Bibcode: 2020ApJ...901..150G
Altcode: 2020arXiv200809580G
When observing spectral lines in the optically thin corona,
line-of-sight (LOS) effects can strongly affect the interpretation
of the data, especially in regions just above the limb. We present a
semiempirical forward model, called the Global Heliospheric Optically
thin Spectral Transport Simulation (GHOSTS), to characterize these
effects. GHOSTS uses inputs from several other models to compute
nonequilibrium ionization states (which include the solar-wind
freezing-in effect) for many ions. These are used to generate
ensembles of simulated spectral lines that are examined in detail,
with emphasis on (1) relationships between quantities derived from
observables and the radial variation of the observed quantities, (2)
the behavior of thermal and nonthermal components of the line width,
and (3) relative contributions of collisionally excited and radiatively
scattered photons. We find that rapidly changing temperatures in the low
corona can cause ion populations to vary dramatically with height. This
can lead to line width measurements that are constant with height (a
"plateau" effect) even when the temperature is increasing rapidly,
as the plane of sky becomes evacuated and the foreground/background
plasma dominates the observation. We find that LOS effects often
drive the velocity width to be close to the plane-of-sky value of the
wind speed, despite it flowing perpendicularly to the LOS there. The
plateau effect can also cause the nonthermal component of the line
width to greatly exceed the solar wind velocity at the observation
height. Lastly, we study how much of the LOS is significant to the
observation, and the importance of including continuum in the solar
spectrum when computing the radiatively scattered emission.
Title: Heating Rates for Protons and Electrons in Polar Coronal Holes:
Empirical Constraints from the Ultraviolet Coronagraph Spectrometer
Authors: Cranmer, Steven R.
Bibcode: 2020ApJ...900..105C
Altcode: 2020arXiv200713180C
Ultraviolet spectroscopy of the extended solar corona is a powerful tool
for measuring the properties of protons, electrons, and heavy ions in
the accelerating solar wind. The large coronal holes that expand up from
the north and south poles at solar minimum are low-density collisionless
regions in which it is possible to detect departures from one-fluid
thermal equilibrium. An accurate characterization of these departures is
helpful in identifying the kinetic processes ultimately responsible for
coronal heating. In this paper, Ultraviolet Coronagraph Spectrometer
(UVCS) measurements of the H I Lyα line are analyzed to constrain
values for the solar wind speed, electron density, electron temperature,
proton temperature (parallel and perpendicular to the magnetic field),
and Alfvén-wave amplitude. The analysis procedure involves creating a
large, randomized ensemble of empirical models, simulating their Lyα
profiles, and building posterior probability distributions for only
the models that agree with the UVCS data. The resulting temperatures
do not exhibit a great deal of radial variation between heliocentric
distances of 1.4 and 4 solar radii. Typical values for the electron,
parallel proton, and perpendicular proton temperatures are 1.2, 1.8, and
1.9 MK, respectively. Resulting values for the "nonthermal" Alfvén wave
amplitude show evidence for weak dissipation, with a total energy-loss
rate that agrees well with an independently derived total heating rate
for the protons and electrons. The moderate Alfvén-wave amplitudes
appear to resolve some tension in the literature between competing
claims of both higher (undamped) and lower (heavily damped) values.
Title: Solar Wind and Line-of-sight Effects Broaden Coronal Spectral
Lines
Authors: Gilly, C.; Cranmer, S.
Bibcode: 2020SPD....5121010G
Altcode:
When observing spectral lines in the optically-thin corona,
line-of-sight (LOS) effects can strongly affect the interpretation
of the data, especially in regions just above the limb. We present a
semi-empirical forward model, called GHOSTS, to characterize these
effects. GHOSTS uses inputs from several other models to compute
non-equilibrium ionization states (which include the solar-wind
freezing-in effect) for many ions. These are used to generate
ensembles of simulated spectral lines that are examined in detail,
with emphasis on: (1) relationships between quantities derived from
observables and the radial variation of the observed quantities, (2)
the behavior of thermal and non-thermal components of the line width,
and (3) relative contributions of collisionally excited and radiatively
scattered photons. We find that rapidly changing temperatures in the low
corona can cause ion populations to vary dramatically with height. This
can lead to line-width measurements that are constant with height (a
"plateau" effect) even when the temperature is increasing rapidly,
as the plane-of-sky becomes evacuated and the foreground/background
plasma dominates the observation. We find that LOS effects often
drive the velocity width to be close to the plane-of-sky value of the
wind speed, despite it flowing perpendicularly to the LOS there. The
plateau effect can also cause the non-thermal component of the line
width to greatly exceed the solar wind velocity at the observation
height. Lastly, we study how much of the LOS is significant to the
observation, and the importance of including continuum in the solar
spectrum when computing the radiatively scattered emission.
Title: High-frequency Wave Power Observed in the Chromosphere with
IBIS and ALMA
Authors: Molnar, M. E.; Cranmer, S.; Reardon, K.; Kowalski, A.
Bibcode: 2020SPD....5120106M
Altcode:
The heating mechanism of the solar chromosphere is still an open
scientific question. In this work we study observational constraints on
the contribution to chromospheric heating from high-frequency acoustic
waves. We utilize a unique combination of observations from NSO's Dunn
Solar Telescope and from the Atacama Large Millimeter Array obtained
on April 23rd 2017 to estimate the high-frequency wave flux in the
lower solar atmosphere. The wave flux is inferred from comparison of
the observations with synthetic observables from the time-dependent
hydrodynamic RADYN code. Our findings suggest thatacoustic waves may
carry up to a few kW/m2 of flux, which is comparable to
what is required to heat the quiet chromosphere.
Title: Alfvén-wave-driven Magnetic Rotator Winds from Low-mass
Stars. I. Rotation Dependences of Magnetic Braking and Mass-loss Rate
Authors: Shoda, Munehito; Suzuki, Takeru K.; Matt, Sean P.; Cranmer,
Steven R.; Vidotto, Aline A.; Strugarek, Antoine; See, Victor;
Réville, Victor; Finley, Adam J.; Brun, Allan Sacha
Bibcode: 2020ApJ...896..123S
Altcode: 2020arXiv200509817S
Observations of stellar rotation show that low-mass stars lose angular
momentum during the main sequence. We simulate the winds of sunlike
stars with a range of rotation rates, covering the fast and slow
magneto-rotator regimes, including the transition between the two. We
generalize an Alfvén-wave-driven solar wind model that builds on
previous works by including the magneto-centrifugal force explicitly. In
this model, the surface-averaged open magnetic flux is assumed to scale
as ${B}_{* }{f}_{* }^{\mathrm{open}}\propto {\mathrm{Ro}}^{-1.2}$ ,
where ${f}_{* }^{\mathrm{open}}$ and Ro are the surface open-flux
filling factor and Rossby number, respectively. We find that, (1)
the angular-momentum loss rate (torque) of the wind is described
as ${\tau }_{{\rm{w}}}\approx 2.59\times {10}^{30}\ \mathrm{erg}\
{\left({{\rm{\Omega }}}_{* }/{{\rm{\Omega }}}_{\odot }\right)}^{2.82}$
, yielding a spin-down law ${{\rm{\Omega }}}_{* }\propto {t}^{-0.55}$
. (2) The mass-loss rate saturates at ${\dot{M}}_{{\rm{w}}}\sim
3.4\times {10}^{-14}{M}_{\odot }\ {\mathrm{yr}}^{-1}$ , due to
the strong reflection and dissipation of Alfvén waves in the
chromosphere. This indicates that the chromosphere has a strong impact
in connecting the stellar surface and stellar wind. Meanwhile, the
wind ram pressure scales as ${P}_{{\rm{w}}}\propto {{\rm{\Omega }}}_{*
}^{0.57}$ , which is able to explain the lower envelope of the observed
stellar winds by Wood et al. (3) The location of the Alfvén radius
is shown to scale in a way that is consistent with one-dimensional
analytic theory. Additionally, the precise scaling of the Alfvén
radius matches previous works, which used thermally driven winds. Our
results suggest that the Alfvén-wave-driven magnetic rotator wind
plays a dominant role in the stellar spin-down during the main sequence.
Title: Solar physics in the 2020s: DKIST, parker solar probe, and
solar orbiter as a multi-messenger constellation
Authors: Martinez Pillet, V.; Tritschler, A.; Harra, L.; Andretta, V.;
Vourlidas, A.; Raouafi, N.; Alterman, B. L.; Bellot Rubio, L.; Cauzzi,
G.; Cranmer, S. R.; Gibson, S.; Habbal, S.; Ko, Y. K.; Lepri, S. T.;
Linker, J.; Malaspina, D. M.; Matthews, S.; Parenti, S.; Petrie, G.;
Spadaro, D.; Ugarte-Urra, I.; Warren, H.; Winslow, R.
Bibcode: 2020arXiv200408632M
Altcode:
The National Science Foundation (NSF) Daniel K. Inouye Solar Telescope
(DKIST) is about to start operations at the summit of Haleakala
(Hawaii). DKIST will join the early science phases of the NASA
and ESA Parker Solar Probe and Solar Orbiter encounter missions. By
combining in-situ measurements of the near-sun plasma environment and
detail remote observations of multiple layers of the Sun, the three
observatories form an unprecedented multi-messenger constellation to
study the magnetic connectivity inside the solar system. This white
paper outlines the synergistic science that this multi-messenger
suite enables.
Title: Alfven Waves in the Solar Corona and Solar Wind: An Updated
Energy Budget
Authors: Cranmer, S. R.
Bibcode: 2020AAS...23514908C
Altcode:
The Sun's upper atmosphere is heated to temperatures greater than 1
million K, and solar plasma flows out into the heliosphere at supersonic
speeds. There are many different proposed explanations for how the solar
corona is heated and how the solar wind is accelerated, and we still
do not have observations that allow us to distinguish conclusively
between those rival theories. However, we are continuing to collect
data on magnetohydrodynamic (MHD) waves and turbulence, which appear
to exist nearly everywhere above the solar surface, and which are
relevant to constrain a broad class of models that rely on damping
these fluctuations to produce heat. This presentation will review
recent improvements in our observational database of Alfvenic (i.e.,
transverse and incompressible) MHD fluctuations. First, new constraints
on wave amplitudes in polar coronal holes, at heliocentric distances
between 1.5 and 4 solar radii, will be presented from a new analysis of
Ultraviolet Coronagraph Spectrometer (UVCS) Lyman alpha data. Second,
in-situ measurements from the first two perihelia of Parker Solar Probe
(PSP) — at heliocentric distances between 35 and 215 solar radii —
will also be analyzed. Incorporating multiple sources of data provides
a better global picture of how waves from the Sun propagate, dissipate,
and heat the plasma. If I am feeling lucky, I may also make predictions
about the properties of the MHD fluctuations to be seen during future
perihelia of PSP (i.e., from 35 down to 9 solar radii).
Title: A New Atomic Database for Line-Driven Outflows in a Variety
of Astrophysical Environments
Authors: Lattimer, A.; Cranmer, S.
Bibcode: 2020AAS...23511027L
Altcode:
Flows driven by photons have been studied for almost a century,
and a quantitative description of the radiative forces on atoms and
ions is important for understanding a wide variety of systems. These
systems include massive stars, cataclysmic variables, central stars
of planetary nebulae, active galactic nuclei (AGN), and a variety of
other environments with accretion disks. In many of these systems,
line-driving forces (where the opacity that couples the photons
and gas is confined to bound-bound transitions) is dominant, and
our understanding depends on knowing the properties of all spectral
lines that exist in the system. We have assembled atomic data for more
than 4.5 million lines from the NIST, CHIANTI, and CMFGEN databases,
and we have computed dimensionless line-strength parameters for each
line using the formalism developed by Gayley (1995). We compute
the traditional "line-force multiplier" (i.e., the ratio of the
line force to the force on free electrons) for broad ranges of
temperature, density, and central-source SED, and we also make use
of several different assumptions about the ionization balance of the
environment. Historically, the line-force multiplier has been assumed
to be described by a power-law, but we explore alternative fitting
functions and the associated implications on the dynamics of rapidly
outflowing winds.
Title: Toward a Better Understanding of Convectively-Driven Flicker
in Kepler Light Curves
Authors: Van Kooten, S. J.; Cranmer, S.
Bibcode: 2020AAS...23535205V
Altcode:
The light curves produced by the Kepler mission demonstrate real,
stochastic brightness fluctuations (or "flicker") which impose a
limit to the sensitivity of exoplanet detection and characterization
methods. The sources of this brightness variation can include convective
granulation, acoustic oscillations, magnetic activity, and stellar
rotation. In this work we focus on better characterizing the flicker
component due to convective granulation, present in all Kepler stars
with outer convective envelopes. Past work has extracted the convective
flicker component of cool, low-mass Kepler stars, and additional
past work has compared the amplitude of this variability component
to the predictions of theoretical models and derived an empirical
correction factor for these models motivated by the magnetic activity
and shallow convection zones of F-dwarf stars. In this work we draw upon
an expanded database of Kepler star convective flicker measurements,
including a substantive sample of cool, giant stars, and we present an
updated comparison of observations of convectively-driven flicker to
theoretical predictions. A better understanding of convective flicker
will better characterize a source of noise in exoplanet detection and
characterization as well as better inform models of stellar granulation.
Title: University of Colorado Space Weather Technology, Research,
and Education Center (SWx TREC): An academic center of excellence
to accelerate research to operations and operations to research
transitions
Authors: Berger, T. E.; Thayer, J. P.; Baker, D. N.; Knipp, D. J.;
Pankratz, C. K.; Cranmer, S. R.; Sutton, E. K.; Baltzer, T.; Lucas,
G.; Craft, J.; Bosanac, N.; Smith, T. R.
Bibcode: 2019AGUFMSA13A..06B
Altcode:
The University of Colorado at Boulder Space Weather Technology Research
and Education Center (SWx TREC) is a University Chancellor's Grand
Challenge Initiative forming a campus-wide center for research, mission
and technology development, and educational initiatives in the space
weather enterprise. SWx TREC offers a unique open academic environment
with contributions not only from the departments of Astrophysics and
Planetary Sciences, Aerospace Engineering Sciences, and Atmospheric
and Oceanic Sciences, but from campus institutes such as the Laboratory
for Atmospheric and Space Physics (LASP) and the Cooperative Institute
for Research in Environmental Sciences (CIRES) as well. In addition,
SWx TREC serves as a "Front Range" space weather collaboration engine,
reaching out to local government (NOAA/SWPC, USAF/SpaceCommand)
and industry (Ball Aerospace, Lockheed Martin, Raytheon, Google)
elements, commercial space weather providers such as Astra LLC and Space
Environment Technologies (SET), and local FFRDCs such as the National
Solar Observatory (NSO), NCAR's High Altitude Observatory (HAO), the
Southwest Research Institute (SWRI), and Northwest Research Associates
(NWRA) to pursue opportunities to advance space weather forecasting
through innovative research. We discuss how SWx TREC is working with
these partners to develop new satellite drag models for Civil Space
Traffic Management (STM) applications, a Space Weather Data Portal to
ease multi-instrument data display and analysis, and a Space Weather
Testbed that will allow academic and commercial developers to test new
models and forecasting tools in a cloud-based prototyping facility
with student and professional forecaster engagement. SWx TREC is
also developing two innovative mission concepts to fill major gaps in
the current space weather observing system: the Solar Polar Observing
Constellation (SPOC) with Ball Aerospace, and the Operational Radiation
Belts (ORB) mission for the Air Force.
Title: The Effect of Non-Equilibrium Ionization, Resonant Scattering,
and the Solar Wind on the Broadening of Coronal Emission Lines
Authors: Gilbert, C. R.; Cranmer, S. R.
Bibcode: 2019AGUFMSH11C3406G
Altcode:
In this work, a semi-empirical forward model (GHOSTS) is developed
and used to generate simulated spectral observations of the solar
corona. The widths of these spectral lines are often used to infer
thermal and non-thermal velocities in the corona as a function of
height. However, because the corona is optically thin, a variety
of line-of-sight (LOS) effects keep these raw measurements from
representing the values in the plane of the sky (POS). We explore
the dependence of these observations on LOS plasma properties, with
physical data for the model drawn from models (e.g., ZEPHYR, CHIANTI)
and observations (e.g., the SUMER spectral atlas). Non-equilibrium
ionization effects cause different ions to have very different relative
emissivities along the LOS, which leads to some notable effects on the
observations. Line-fit temperatures only seem to match the POS value
when the POS is the densest part of the line of sight, which is often
not the case below heights of a few tenths of a solar radius. The
spectral lines seem to be significantly broadened by the presence
of the solar wind, even when the POS wind velocity is negligible. The
resonantly scattered component of the line is significantly broadened if
the surrounding continuum is included in the incident line profile. Work in preparation will also address the effects of Alfvén waves,
preferential ion heating, and fine magnetic structure on the spectral
profiles.
Title: Stars at High Spatial Resolution
Authors: Carpenter, Kenneth G.; van Belle, Gerard; Brown, Alexander;
Cranmer, Steven R.; Drake, Jeremy; Dupree, Andrea K.; Creech-Eakman,
Michelle; Evans, Nancy R.; Grady, Carol A.; Guinan, Edward F.; Harper,
Graham; Karovska, Margarita; Kolenberg, Katrien; Labeyrie, Antoine;
Linsky, Jeffrey; Peters, Geraldine J.; Rau, Gioia; Ridgway, Stephen;
Roettenbacher, Rachael M.; Saar, Steven H.; Walter, Frederick M.;
Wood, Brian
Bibcode: 2019arXiv190805665C
Altcode:
We summarize some of the compelling new scientific opportunities
for understanding stars and stellar systems that can be enabled by
sub-milliarcsec (sub-mas) angular resolution, UV-Optical spectral
imaging observations, which can reveal the details of the many dynamic
processes (e.g., evolving magnetic fields, accretion, convection,
shocks, pulsations, winds, and jets) that affect stellar formation,
structure, and evolution. These observations can only be provided
by long-baseline interferometers or sparse aperture telescopes in
space, since the aperture diameters required are in excess of 500 m
(a regime in which monolithic or segmented designs are not and will
not be feasible) and since they require observations at wavelengths
(UV) not accessible from the ground. Such observational capabilities
would enable tremendous gains in our understanding of the individual
stars and stellar systems that are the building blocks of our Universe
and which serve as the hosts for life throughout the Cosmos.
Title: Solar Chromospheric Temperature Diagnostics: A Joint ALMA-Hα
Analysis
Authors: Molnar, Momchil E.; Reardon, Kevin P.; Chai, Yi; Gary, Dale;
Uitenbroek, Han; Cauzzi, Gianna; Cranmer, Steven R.
Bibcode: 2019ApJ...881...99M
Altcode: 2019arXiv190608896M
We present the first high-resolution, simultaneous observations of
the solar chromosphere in the optical and millimeter wavelength
ranges, obtained with the Atacama Large Millimeter Array (ALMA)
and the Interferometric Bidimensional Spectrometer at the Dunn Solar
Telescope. In this paper we concentrate on the comparison between the
brightness temperature observed in ALMA Band 3 (3 mm; 100 GHz) and the
core width of the Hα 6563 Å line, previously identified as a possible
diagnostic of the chromospheric temperature. We find that in the area
of plage, network and fibrils covered by our field of view, the two
diagnostics are well correlated, with similar spatial structures
observed in both. The strength of the correlation is remarkable,
given that the source function of the millimeter radiation obeys local
thermodynamic equilibrium, while the Hα line has a source function that
deviates significantly from the local Planck function. The observed
range of ALMA brightness temperatures is sensibly smaller than the
temperature range that was previously invoked to explain the observed
width variations in Hα. We employ analysis from forward modeling
with the Rybicki-Hummer (RH) code to argue that the strong correlation
between Hα width and ALMA brightness temperature is caused by their
shared dependence on the population number n 2 of the first
excited level of hydrogen. This population number drives millimeter
opacity through hydrogen ionization via the Balmer continuum, and
Hα width through a curve-of-growth-like opacity effect. Ultimately,
the n 2 population is regulated by the enhancement or lack
of downward Lyα flux, which coherently shifts the formation height
of both diagnostics to regions with different temperature, respectively.
Title: The Properties of the Solar Corona and Its Connection to the
Solar Wind
Authors: Cranmer, Steven R.; Winebarger, Amy R.
Bibcode: 2019ARA&A..57..157C
Altcode: 2018arXiv181100461C
The corona is a layer of hot plasma that surrounds the Sun, traces out
its complex magnetic field, and ultimately expands into interplanetary
space as the supersonic solar wind. Although much has been learned in
recent decades from advances in observations, theory, and computer
simulations, we still have not identified definitively the physical
processes that heat the corona and accelerate the solar wind. In
this review, we summarize these recent advances and speculate about
what else is required to finally understand the fundamental physics
of this complex system. Specifically: We discuss recent subarcsecond
observations of the corona, some of which appear to provide evidence
for tangled and braided magnetic fields and some of which do not. We
review results from three-dimensional numerical simulations that,
despite limitations in dynamic range, reliably contain sufficient
heating to produce and maintain the corona. We provide a new tabulation
of scaling relations for a number of proposed coronal heating theories
that involve waves, turbulence, braiding, nanoflares, and helicity
conservation. An understanding of these processes is important not
only for improving our ability to forecast hazardous space-weather
events but also for establishing a baseline of knowledge about a
well-resolved star that is relevant to other astrophysical systems.
Title: Element Abundances: A New Diagnostic for the Solar Wind
Authors: Laming, J. Martin; Vourlidas, Angelos; Korendyke, Clarence;
Chua, Damien; Cranmer, Steven R.; Ko, Yuan-Kuen; Kuroda, Natsuha;
Provornikova, Elena; Raymond, John C.; Raouafi, Nour-Eddine; Strachan,
Leonard; Tun-Beltran, Samuel; Weberg, Micah; Wood, Brian E.
Bibcode: 2019ApJ...879..124L
Altcode: 2019arXiv190509319L
We examine the different element abundances exhibited by the closed
loop solar corona and the slow speed solar wind. Both are subject
to the first ionization potential (FIP) effect, the enhancement in
coronal abundance of elements with FIP below 10 eV (e.g., Mg, Si,
Fe) with respect to high-FIP elements (e.g., O, Ne, Ar), but with
subtle differences. Intermediate elements, S, P, and C, with FIP
just above 10 eV, behave as high-FIP elements in closed loops, but
are fractionated more like low-FIP elements in the solar wind. On
the basis of FIP fractionation by the ponderomotive force in the
chromosphere, we discuss fractionation scenarios where this difference
might originate. Fractionation low in the chromosphere where hydrogen
is neutral enhances the S, P, and C abundances. This arises with
nonresonant waves, which are ubiquitous in open field regions, and is
also stronger with torsional Alfvén waves, as opposed to shear (i.e.,
planar) waves. We discuss the bearing these findings have on models
of interchange reconnection as the source of the slow speed solar
wind. The outflowing solar wind must ultimately be a mixture of the
plasma in the originally open and closed fields, and the proportions
and degree of mixing should depend on details of the reconnection
process. We also describe novel diagnostics in ultraviolet and extreme
ultraviolet spectroscopy now available with these new insights, with
the prospect of investigating slow speed solar wind origins and the
contribution of interchange reconnection by remote sensing.
Title: Forward Models of Off-Limb Emission Lines in Solar Coronal
Holes
Authors: Gilly, Chris R.; Cranmer, Steven R.
Bibcode: 2019AAS...23410606G
Altcode:
There is debate in the solar community regarding the mechanism by
which the corona is heated to millions of degrees. Alfvén waves,
driven by granulation in the photosphere and propagating upwards to
dissipate in the corona, are one of several ideas for the source of the
thermal energy. Observations of off-limb spectral lines are in theory
able to constrain some properties of these waves (e.g., amplitudes
and phase speeds) as a function of heliocentric altitude, but in
practice the interpretation of these measurements is difficult due to
the optically-thin nature of the corona. In this work, a forward model
(GHOSTS) is developed and refined so that it can be used to generate
realistic simulated observations of these lines. Recent improvements to
the model include the addition of resonantly scattered light and the
self-consistent calculation of frozen-in non-equilibrium ionization
states. Early results indicate that the non-thermal widths of these
lines seem to be significantly broadened by the presence of the solar
wind, even as close to the Sun as a few tenths of a solar radius. We
also aim to put constraints on the cadence and integration times
needed to resolve individual Alfven-wave oscillations, as have been
seen with CoMP.
Title: Preparing for DKIST: Connecting the High-Resolution Sun to
the Turbulent Corona
Authors: Van Kooten, Samuel; Cranmer, Steven R.
Bibcode: 2019AAS...23430204V
Altcode:
Magnetic bright points on the solar photosphere, visible in both
continuum and G-band images, indicate footpoints of kilogauss magnetic
flux tubes extending to the corona. The horizontal motions of these
footpoints are believed to excite MHD waves which propagate to the
corona, where they deposit heat through turbulent dissipation. Analyzing
this motion can thus provide a power spectrum of MHD wave energy
transport, which is a key lower boundary condition in coronal and
heliospheric models. At 100 km in diameter, most bright points are seen
as unresolved blobs. Tracking their centroids allows the excitation of
kink-mode waves to be modeled in the overlying flux tubes. However,
centroid tracking cannot easily handle the merging or splitting of
bright points nor can it track extremely long bright points, and
current observations cannot reveal more complicated motions, such
as size or shape changes, which are expected to excite higher-order
waves. DKIST promises to resolve the sizes and shapes of bright
points. However, Agrawal et al. (2018) showed that centroid tracking
is likely to experience a spurious "jitter" signal when applied
to high-resolution data, and this limitation is in addition to the
inability of centroid tracking to produce new insights from the new size
and shape information. We present efforts to overcome these limitations
by developing an algorithm to infer the horizontal plasma flow inside
bright points at DKIST-like resolution (at which bright points are
resolved but not large enough for traditional correlation-tracking
techniques) and using these inferred flows to model the higher-order
waves generated in the overlying flux tubes. By using output data from
high-resolution MURaM simulations now, we expect to be prepared to
analyze DKIST images soon after they become available next year. This
work will estimate the significance of the contribution to the coronal
heating budget of these more complex waves and so provide a more
complete lower boundary condition for coronal and heliospheric models.
Title: Simulations of Stellar Winds and Exoplanet Magnetospheres:
Hidden Assumptions in the Boundary Conditions
Authors: Cranmer, Steven R.
Bibcode: 2019AAS...23421501C
Altcode:
For at least a century, the Sun has served as a useful template for
our understanding of stellar atmospheres. This is especially true
for the observational signatures of magnetic fields, hot coronae,
and outflowing winds, all of which have been traditionally difficult
to detect and characterize around other stars. Recently, the solar
community has developed a number of self-consistent multidimensional
simulations of the Sun's coronal heating and wind acceleration,
and these models have been quite successful in reproducing a range
of local measurements. These models are now being applied to other
stars, with conclusions often being drawn from their output about the
habitability of exoplanets circling those stars (i.e., magnetospheric
plasma conditions, magnetic fields, and high-energy radiation). However,
there are sometimes subtle parameters embedded in these models that
have been fine-tuned for the Sun and not varied when applying them to
other stars. For example, models that make use of the dissipation of
MHD turbulence to power the corona rely on lower boundary conditions in
the photospheric granulation. Using the Sun's granular velocity field
for, say, the corona of an M dwarf would not be appropriate. Also,
using the output of convection-driven wave-flux models to adjust
the solar parameters to those of other stars may not be appropriate
either. In the case of nearby M dwarf AU Mic, this kind of extrapolation
ended up drastically under-predicting the amount of coronal heating
observed in X-ray and submillimeter emission. It has been suspected
that - for the coronae of cool stars with different properties than
the Sun - the most important wave/turbulence modes and dissipation
channels may be fundamentally different from those most important in
the solar atmosphere. Thus, conclusions from solar simulation codes
about extrasolar wind mass loss rates, magnetospheres, and EUV/X-ray
irradiances may need to be reevaluated with more appropriate boundary
conditions and coronal heating physics.
Title: Coronal Turbulence Driven from the Photosphere: Opportunities
for DKIST
Authors: Van Kooten, Samuel J.; Cranmer, Steven R.
Bibcode: 2019shin.confE.166V
Altcode:
Magnetic bright points on the solar photosphere, visible in both
continuum and G-band images, indicate footpoints of kilogauss magnetic
flux tubes extending to the corona. The horizontal motions of these
footpoints are believed to excite MHD waves which propagate to the
corona, where they deposit heat through turbulent dissipation. Analyzing
this motion can thus provide a power spectrum of MHD wave energy
transport, which is a key lower boundary condition in coronal and
heliospheric models. At 100 km across, most bright points are seen as
unresolved blobs. Tracking their centroids allows the excitation of
kink-mode waves to be modeled. However, centroid tracking struggles
with the merging or splitting of bright points and with extremely
long bright points. And while DKIST promises to resolve the sizes and
shapes of bright points (with changes in these properties expected to
excite sausage-mode and higher-order waves), centroid tracking ignores
this additional information. Additionally, Agrawal et al. (2018)
showed that centroid tracking is likely to experience a spurious
""jitter"" signal which may dwarf true centroid motion at DKIST's
resolution and cadence. We present progress developing an algorithm
that is resilient to centroid jitter and can treat merging, splitting,
and shape changes properly. It will infer the horizontal plasma flow
inside bright points in a way appropriate for the limited resolution
of bright-point observations, and we will use these inferred flows to
model the waves (kink-mode, sausage-mode, and other modes) generated in
the overlying flux tubes. We are developing and testing this procedure
now with simulated images from high-resolution MURaM simulations in
order to be prepared to analyze DKIST images upon availability. This
work will estimate the significance of the contribution to the coronal
heating budget of these more complex waves and provide a more complete
lower boundary condition for coronal and heliospheric models.
Title: Thermal conduction throughout the solar wind: the eight-moment
approximation
Authors: Schiff, Avery Jean; Cranmer, Steven
Bibcode: 2019shin.confE.118S
Altcode:
A careful treatment of thermal conduction is vital to accurately
describe the corona and solar wind. Typically, the complex physical
process is modeled as a diffusive process using Spitzer-Härm
conductivity. We explore an alternative to this standard: direct
integration of the conservation equations derived from the 8-moment
approximation. Not only does this approach avoid the numerical
strictness imposed by diffusive processes, but it also functions
without the assumption necessary for Spitzer-Härm conductivity that
the plasma is fully collisional. We present the results of integrating
the 8-moment equations for a variety of solar wind conditions and
demonstrate that it robustly captures thermal conduction in collisional
and collisionless regimes.
Title: COHERENT: Studying the corona as a holistic environment
Authors: Caspi, Amir; Seaton, Daniel B.; Case, Traci; Cheung, Mark;
Cranmer, Steven; DeForest, Craig E.; de Toma, Giuliana; Downs, Cooper;
Elliott, Heather; Gold, Anne U.; Longcope, Dana; Savage, Sabrina L.;
Sullivan, Susan; Viall, Nicholeen; Vourlidas, Angelos; West, Matthew J.
Bibcode: 2019shin.confE.241C
Altcode:
The solar corona and the heliosphere must be part of a single
physical system, but because the dominant physical processes change
dramatically from the magnetically-dominated low corona, through the
sparsely-observed middle corona, and into the plasma flow-dominated
outer corona and heliospheric interface, unified frameworks to study
the corona as a whole are essentially nonexistent. Understanding how
physical processes shape and drive the dynamics of the corona as a
global system, on all spatiotemporal scales, is critical for solving
many fundamental problems in solar and heliospheric physics. However,
the lack of unifying observations and models has led to a fragmentation
of the community into distinct regimes of plasma parameter space,
largely clustering around regions where existing instrumentation has
made observations widely available and where models can be sufficiently
self-contained to be tractable. We describe COHERENT, the 'Corona as a
Holistic Environment' Research Network, a focused effort to facilitate
interdisciplinary collaborative research to develop frameworks for
unifying existing and upcoming observations, theory, models, and
analytical tools to study the corona as a holistic system.
Title: Interpreting Off-Limb Emission Lines from Polar Coronal Holes
Authors: Gilly, Chris R.; Cranmer, Steven R.
Bibcode: 2019shin.confE.203G
Altcode:
Spectroscopic line-width measurements taken of the solar corona can
be used to infer temperature and velocity information about it as a
function of height. However, a variety of line-of-sight effects due to
the optically-thin nature of the corona cause these raw measurements
not to match the real values in the plane of the sky. In this work, a
semi-empirical forward model (GHOSTS) is developed and used to generate
simulated off-limb spectral lines from an idealized time-steady polar
coronal hole, using physical data from the ZEPHYR code, CHIANTI, and
the SUMER spectral atlas. Early results of the analysis indicate some
startling results: The widths of these lines seem to be significantly
broadened by the solar wind, even as close to the Sun as a hundredth of
a solar radius; A temperature floor was observed, where non-equilibrium
ionization prevents the line-temperatures from matching the plane-of-sky
value below their density maximum; When simulating resonantly scattered
light, including a continuum component with the incident line causes
significant line broadening. Further work regarding the effect of
dynamic processes on the measurements is ongoing. This work will help us
correctly interpret new data coming from DKIST over the next few years.
Title: Stars at High Spatial Resolution
Authors: Carpenter, Kenneth; van Belle, Gerard; Brown, Alexander;
Cranmer, Steven R.; Drake, Jeremy; Dupree, Andrea K.; Creech-Eakman,
Michelle; Evans, Nancy R.; Grady, Carol A.; Guinan, Edward F.; Harper,
Graham; Karovska, Margarita; Kolenberg, Katrien; Labeyrie, Antoine;
Linsky, Jeffrey; Peters, Geraldine J.; Rau, Gioia; Ridgway, Stephen;
Roettenbacher, Rachael M.; Saar, Steven H.; Walter, Frederick M.;
Wood, Brian
Bibcode: 2019BAAS...51c..56C
Altcode: 2019astro2020T..56C
We summarize compelling new scientific opportunities for understanding
stars and stellar systems that can be enabled by sub-milliarcsec angular
resolution, UV/Optical spectral imaging observations. These can reveal
details of many dynamic processes that affect stellar formation,
structure, and evolution.
Title: Solar Wind Origins: A Survey of Proposed Physical Processes
Authors: Cranmer, Steven R.
Bibcode: 2019shin.confE.138C
Altcode:
Coronal heating and solar wind acceleration are not 'problems' because
of an absence of proposed explanations, but because there are too many
proposed explanations! This session is meant to help theorists and
observers continue the process of testing, validating, and ultimately
winnowing down the list of physical processes that may be responsible
for accelerating the solar wind. In this scene-setting talk I will
attempt to review the current state of the debate, mainly from the
theoretical side, about solar wind origins. The major battle-lines
are often drawn between physical processes that energize the plasma
along open field lines (i.e., 'topology is destiny'), versus those
that involve magnetic reconnection between regions of open and closed
topology. However, it is likely that the real Sun employs both types
of plasma energization, such that our job is then to determine their
relative strengths in the different source regions. Of necessity, this
talk will be peppered by my own biased opinions about which processes
I believe are more important than others, but I will do my best to
label those opinions clearly.
Title: High frequency chromospheric observations with IBIS and ALMA
Authors: Molnar, Momchil Emil; Reardon, Kevin; Cranmer, Steven
Bibcode: 2019shin.confE.148M
Altcode:
The heating mechanism sustaining the quiet chromosphere is still
unknown and is an area of active research (Kalkofen 2007). A possible
explanation for the chromospheric heating conundrum is the dissipation
of high-frequency waves permeating the chromosphere. However, there
are few studies of this frequency regime (above 30 mHz) with full
spectral scanning to derive proper Doppler velocity fields in the
chromosphere. We present observations of the power spectrum of the
Doppler velocities in the chromosphere from the DST at Sunspot, NM. We
used the IBIS instrument with a novel reduction technique to derive
chromospheric Doppler velocities in the spectral lines of H-alpha and Ca
II 8542 with cadences of about 3.5 seconds for a full spectral scan. We
find that the power spectrum of the measured velocities follow a power
law ubiquitous in our field of view up to 60 mHz. The power law index
is coherent for similar chromospheric regions. We find comparable power
law in the power spectrum of the temperature variation in simultaneous
observations with ALMA of the same region. This power law could be a
signature of turbulent cascade leading to the dissipation of energy of
these high frequency perturbations at small scales. However, this is
an unexpected result as the extended formation region of these resonant
chromospheric lines smears out the spectral signatures produced by short
wavelength perturbations in the atmosphere. To explain the presence of
high-frequency energy in the power spectra we study the transmission
function of the wave perturbations through the solar atmosphere with
the RH code. This exploratory study could be greatly improved with the
DKI Solar Telescope, which would enable us to resolve smaller scales
in the solar atmosphere as well as measure velocities at higher cadence
and better precision.
Title: Element Abundances and Solar Wind Acceleration
Authors: Cranmer, Steven R.
Bibcode: 2019shin.confE.137C
Altcode:
When attempting to solve the intertwined problems of coronal
heating and solar wind acceleration, it is important to make use of
observations that allow us to distinguish between the many different
proposed physical processes. This session highlights the usefulness
of elemental abundances in this regard, and this presentation is
meant to complement another scene-setting talk that will focus on
the chromospheric origins of FIP-dependent elemental abundances. In
this talk, I will review how there can be additional imprinting of
elemental abundance signatures throughout the extended acceleration
region of the solar wind. For example, some abundance patterns may be
determined by differential flows between protons, alpha particles, and
heavier ions in the corona. These flows may take several tens of solar
radii to eventually become locked into the values measured at 1 AU,
and they carry signatures of effects such as wave-particle interactions,
collisional friction, and gravitational settling. This talk will attempt
to distinguish between the 'frozen-in' composition diagnostics that can
be used as long-distance probes of origin sites on the solar surface,
and those that are more sensitive to gradual evolution in the corona.
Title: Origins of the Ambient Solar Wind: Implications for Space
Weather
Authors: Cranmer, Steven R.; Gibson, Sarah E.; Riley, Pete
Bibcode: 2019sfsw.book...41C
Altcode:
No abstract at ADS
Title: Some Turbulent Predictions for Parker Solar Probe
Authors: Cranmer, Steven R.
Bibcode: 2018RNAAS...2..158C
Altcode: 2018arXiv180809477C; 2018RNAAS...2c.158C
From the the solar photosphere to the outer heliosphere, the Sun's
plasma properties are fluctuating with a broad range of temporal and
spatial scales. In fact, a turbulent cascade of energy from large
to small scales is a frequently invoked explanation for heating the
corona and accelerating the solar wind. NASA's Parker Solar Probe (PSP)
is expected to revolutionize our understanding of coronal heating and
magnetohydrodynamic (MHD) turbulence by performing in situ sampling
closer to the Sun than any other prior space mission. This research note
presents theoretical predictions for some properties of MHD turbulence
(e.g., spacecraft-frame power spectra and variance anisotropies)
in the regions to be explored by PSP. These results are derived from
a previously published semi-empirical model of coupled Alfvenic and
fast-mode turbulence in the fast solar wind. The primary reason for
this research note is to show how straightforward it can be to extract
useful predictions from existing theoretical models about measurable
quantities that were not even considered when creating the models
initially. The variance anisotropy, for example, may be an important
quantity for distinguishing between different theoretical models for
coronal heating and solar wind acceleration.
Title: Investigating the Complex Motions of Photospheric Bright Points
Authors: Van Kooten, Samuel J.; Cranmer, Steven R.
Bibcode: 2018shin.confE..72V
Altcode:
Magnetic bright points in the solar photosphere, visible in both
continuum and G-band images, indicate footpoints of kilogauss magnetic
flux tubes extending to the corona. The power spectrum of bright-point
motion is thus also the power spectrum of Alfven wave excitation,
and these waves transport energy up flux tubes into the corona. This
spectrum is a key boundary condition in coronal and heliospheric
models. In past work, using photospheric images from a radiative
magnetohydrodynamic simulation with spatial resolution higher than
that of present-day observations, we performed automated tracking of
bright points to produce large data quantities, and we generated a power
spectrum of the bright-point motions. We found slightly higher amounts
of power at all frequencies compared to observation-based spectra,
while confirming the spectrum shape of recent observations. This
serves as a prediction for observations of bright points with DKIST,
which will achieve similar resolution and high sensitivity. However,
our past work was unable to process extremely long bright points, the
merging or splitting of bright points, or internal motions including
size or shape changes of bright points. If bright point motion is to
be used as a complete lower boundary condition for coronal waves, these
additional aspects must be quantitatively understood. Such an approach
must move away from centroid tracking to tracking the inferred motion
of field lines at a scale near the pixel scale. With DKIST promising
to provide high-resolution, high-cadence bright point observations,
now is the time to develop this approach for bright points. In this
poster, we present initial efforts and a variety of approaches that
may help measure the velocity field of bright points and the MHD waves
excited by those motions.
Title: Low-frequency Alfvén Waves Produced by Magnetic Reconnection
in the Sun’s Magnetic Carpet
Authors: Cranmer, Steven R.
Bibcode: 2018ApJ...862....6C
Altcode: 2018arXiv180512184C
The solar corona is a hot, dynamic, and highly magnetized plasma
environment whose source of energy is not yet well understood. One
leading contender for that energy source is the dissipation of
magnetohydrodynamic (MHD) waves or turbulent fluctuations. Many
wave-heating models for the corona and the solar wind presume that these
fluctuations originate at or below the Sun’s photosphere. However,
this paper investigates the idea that magnetic reconnection may
generate an additional source of MHD waves over a gradual range of
heights in the low corona. A time-dependent Monte Carlo simulation of
the mixed-polarity magnetic field is used to predict the properties of
reconnection-driven coronal MHD waves. The total power in these waves
is typically small in comparison to that of photosphere-driven waves,
but their frequencies are much lower. Reconnection-driven waves begin
to dominate the total power spectrum at periods longer than about
30 minutes. Thus, they may need to be taken into account in order to
understand the low-frequency power-law spectra observed by both coronal
spectropolarimetry and in situ particle/field instruments. These
low-frequency Alfvén waves should carry more magnetic energy than
kinetic energy, and thus they may produce less nonthermal Doppler
broadening (in comparison to photosphere-driven high-frequency waves)
in emission lines observed above the solar limb.
Title: Waves and Turbulence in the Solar Corona: A Surplus of Sources
and Sinks
Authors: Cranmer, Steven R.
Bibcode: 2018AAS...23240502C
Altcode:
The Sun's corona is a hot, dynamic, and highly stochastic plasma
environment, and we still do not yet understand how it is heated. Both
the loop-filled coronal base and the extended acceleration region of the
solar wind appear to be filled with waves and turbulent eddies. Models
that invoke the dissipation of these magnetohydrodynamic (MHD)
fluctuations have had some success in explaining the heating. In this
presentation I will review some new insights about the different ways
these waves are thought to be created and destroyed. For example: (1)
Intergranular bright points in the photosphere are believed to extend
upwards as coronal flux tubes, and their transverse oscillations are
driven by the underlying convection. New high-resolution MHD simulations
predict the kinetic energy spectra of the resulting coronal waves and
serve as predictions for upcoming DKIST observations. (2) Magnetic
reconnection in the supergranular network of the low corona can also
generate MHD waves, and new Monte Carlo models of the resulting power
spectra will be presented. The total integrated power in these waves
is typically small in comparison to that of photosphere-driven waves,
but they dominate the total spectrum at periods longer than about 30
minutes. (3) Because each magnetic field line in the corona is tied
to at least one specific chromospheric footpoint (each with its own
base pressure), the corona also plays host to field-aligned "density
striations." These fluctuations vary with the supergranular network on
timescales of roughly a day, but they also act as a spatially varying
background through which the higher-frequency waves propagate. These
multiple sources of space/time variability must be taken into account
to properly understand off-limb measurements from CoMP and EIS/Hinode,
as well as in-situ measurements from Parker Solar Probe.
Title: Diffusion-plus-drift models for the mass leakage from
centrifugal magnetospheres of magnetic hot-stars
Authors: Owocki, Stanley P.; Cranmer, Steven R.
Bibcode: 2018MNRAS.474.3090O
Altcode: 2017arXiv171105414O
In the subset of luminous, early-type stars with strong, large-scale
magnetic fields and moderate to rapid rotation, material from the
star's radiatively driven stellar wind outflow becomes trapped by
closed magnetic loops, forming a centrifugally supported, corotating
magnetosphere. We present here a semi-analytic analysis of how this
quasi-steady accumulation of wind mass can be balanced by losses
associated with a combination of an outward, centrifugally driven
drift in the region beyond the Kepler co-rotation radius, and an
inward/outward diffusion near this radius. We thereby derive scaling
relations for the equilibrium spatial distribution of mass, and the
associated emission measure for observational diagnostics like Balmer
line emission. We discuss the potential application of these relations
for interpreting surveys of the emission line diagnostics for OB stars
with centrifugally supported magnetospheres. For a specific model of
turbulent field-line-wandering rooted in surface motions associated
with the iron opacity bump, we estimate values for the associated
diffusion and drift coefficients.
Title: Detection of a Millimeter Flare from Proxima Centauri
Authors: MacGregor, Meredith A.; Weinberger, Alycia J.; Wilner,
David J.; Kowalski, Adam F.; Cranmer, Steven R.
Bibcode: 2018ApJ...855L...2M
Altcode: 2018arXiv180208257M
We present new analyses of ALMA 12 m and Atacama Compact Array (ACA)
observations at 233 GHz (1.3 mm) of the Proxima Centauri system with
sensitivities of 9.5 and 47 μJy beam-1, respectively,
taken from 2017 January 21 through April 25. These analyses reveal
that the star underwent a significant flaring event during one of
the ACA observations on 2017 March 24. The complete event lasted for
approximately 1 minute and reached a peak flux density of 100 ± 4 mJy,
nearly a factor of 1000 times brighter than the star’s quiescent
emission. At the flare peak, the continuum emission is characterized
by a steeply falling spectral index with frequency F ν
∝ ν α with α = -1.77 ± 0.45, and a lower limit on
the fractional linear polarization of | Q/I| =0.19+/- 0.02. Because
the ACA observations do not show any quiescent excess emission, we
conclude that there is no need to invoke the presence of a dust belt
at 1-4 au. We also posit that the slight excess flux density of 101 ±
9 μJy observed in the 12 m observations, compared to the photospheric
flux density of 74 ± 4 μJy extrapolated from infrared wavelengths,
may be due to coronal heating from continual smaller flares, as is
seen for AU Mic, another nearby well-studied M dwarf flare star. If
this is true, then the need for warm dust at ∼0.4 au is also removed.
Title: Origins of the Ambient Solar Wind: Implications for Space
Weather
Authors: Cranmer, Steven R.; Gibson, Sarah E.; Riley, Pete
Bibcode: 2017SSRv..212.1345C
Altcode: 2017arXiv170807169C; 2017SSRv..tmp..167C
The Sun's outer atmosphere is heated to temperatures of millions
of degrees, and solar plasma flows out into interplanetary space at
supersonic speeds. This paper reviews our current understanding of
these interrelated problems: coronal heating and the acceleration of
the ambient solar wind. We also discuss where the community stands in
its ability to forecast how variations in the solar wind (i.e., fast and
slow wind streams) impact the Earth. Although the last few decades have
seen significant progress in observations and modeling, we still do not
have a complete understanding of the relevant physical processes, nor
do we have a quantitatively precise census of which coronal structures
contribute to specific types of solar wind. Fast streams are known
to be connected to the central regions of large coronal holes. Slow
streams, however, appear to come from a wide range of sources, including
streamers, pseudostreamers, coronal loops, active regions, and coronal
hole boundaries. Complicating our understanding even more is the
fact that processes such as turbulence, stream-stream interactions,
and Coulomb collisions can make it difficult to unambiguously map a
parcel measured at 1 AU back down to its coronal source. We also review
recent progress—in theoretical modeling, observational data analysis,
and forecasting techniques that sit at the interface between data and
theory—that gives us hope that the above problems are indeed solvable.
Title: Characterizing the Motion of Solar Magnetic Bright Points at
High Resolution
Authors: Van Kooten, Samuel J.; Cranmer, Steven R.
Bibcode: 2017ApJ...850...64V
Altcode: 2017arXiv171004738V
Magnetic bright points in the solar photosphere, visible in both
continuum and G-band images, indicate footpoints of kilogauss magnetic
flux tubes extending to the corona. The power spectrum of bright-point
motion is thus also the power spectrum of Alfvén wave excitation,
transporting energy up flux tubes into the corona. This spectrum is
a key input in coronal and heliospheric models. We produce a power
spectrum of bright-point motion using radiative magnetohydrodynamic
simulations, exploiting spatial resolution higher than can be obtained
in present-day observations, while using automated tracking to produce
large data quantities. We find slightly higher amounts of power at all
frequencies compared to observation-based spectra, while confirming the
spectrum shape of recent observations. This also provides a prediction
for observations of bright points with DKIST, which will achieve
similar resolution and high sensitivity. We also find a granule size
distribution in support of an observed two-population distribution,
and we present results from tracking passive tracers, which show a
similar power spectrum to that of bright points. Finally, we introduce
a simplified, laminar model of granulation, with which we explore the
roles of turbulence and of the properties of the granulation pattern
in determining bright-point motion.
Title: Simulations of non-linear mode conversion between Alfvén
waves and compressive modes in the solar corona
Authors: Schiff, Avery Jean; Cranmer, Steven
Bibcode: 2017shin.confE.127S
Altcode:
In the solar corona, Alfvén waves are believed to be responsible for
heating the surrounding plasma through a turbulent cascade. Sufficiently
strong, linearly polarized Alfvén waves are also capable of generating
a strong ponderomotive force that produces compressive fluctuations. The
compressive waves in turn heat the plasma through a dissipative
force. We seek to understand the degree to which this mode conversion is
present and relevant in the Sun's coronal loops. Using the MHD module
of the PLUTO code, we simulate coronal loops with Alfvén fluctuations
having a range of different polarization states driven at the base
of the loop. We characterize the fraction of energy from the initial
Alfvén waves that is converted into compressive fluctuations with some
discussion of the implications for the loop's temperature structure.
Title: Kinetic Effects in Coronal Holes and High-Speed Streams:
A Roundup of Observational Constraints
Authors: Cranmer, Steven R.
Bibcode: 2017shin.confE.105C
Altcode:
Although we have come to understand many links in the chain of events
that produces the hot solar corona and the supersonic solar wind, it
is still the case that the final links - i.e., the actual dissipation
processes that act on the smallest scales - remain elusive. Different
proposed processes act on particles of different charge and mass in
different ways. However, some regions of the corona and heliosphere
are so dense that Coulomb collisions are frequent enough to wipe out
these unique charge/mass signatures. Thus, theorists tend to look to
the lowest-density regions (e.g., coronal holes and fast solar wind
streams) to have the best chance to identify these kinetic clues. In
this poster, I will collect and present a number of remote-sensing and
in-situ measurements that have shown, for example: (1) preferential
bulk acceleration of heavy ions (with respect to proton bulk flows), (2)
ion temperatures in the corona exceeding 100 million K, and (3) extreme
departures from Maxwellian velocity distribution shapes. Hopefully
having these observational constraints all in one place will help prod
theorists (including myself!) to find the best ways to move forward. As
additional fodder for discussion, I will also present a laundry list
of proposed collisionless mechanisms that have been proposed to explain
the observed particle properties.
Title: Characterizing the Motion of Photospheric Magnetic Bright
Points at High Resolution
Authors: Van Kooten, Samuel Jay; Cranmer, Steven R.; Rempel, Matthias
Bibcode: 2017shin.confE..68V
Altcode:
Magnetic bright points on the solar photosphere, visible in both
continuum and G-band images, indicate footpoints of kilogauss magnetic
flux tubes extending to the corona. The power spectrum of transverse
bright point motion is thus also the power spectrum of Alfven wave
excitation, with these waves transporting energy up flux tubes into
the corona. This spectrum is a key input in coronal and heliospheric
models. After briefly reviewing observations of bright point motion, we
present a power spectrum of bright point motion derived from radiative
MHD simulations, exploiting spatial resolution higher than can be
obtained in observations while using automated tracking to produce
large data quantities. We find slightly higher amounts of power at
all frequencies compared to observational spectra while confirming
the spectrum shape of recent observations. This provides a prediction
for DKIST observations of bright points, which will achieve similar
resolution. We also present results from tracing test particles
in the horizontal plasma flow, finding similar power spectra but
differing motion paths. Finally, we introduce a simplified, laminar
model of granulation, with which we explore the roles of turbulence
and of the properties of the granulation pattern in determining bright
point motion.
Title: Mass-loss Rates from Coronal Mass Ejections: A Predictive
Theoretical Model for Solar-type Stars
Authors: Cranmer, Steven R.
Bibcode: 2017ApJ...840..114C
Altcode: 2017arXiv170406689C
Coronal mass ejections (CMEs) are eruptive events that cause
a solar-type star to shed mass and magnetic flux. CMEs tend to
occur together with flares, radio storms, and bursts of energetic
particles. On the Sun, CME-related mass loss is roughly an order of
magnitude less intense than that of the background solar wind. However,
on other types of stars, CMEs have been proposed to carry away much
more mass and energy than the time-steady wind. Earlier papers have
used observed correlations between solar CMEs and flare energies, in
combination with stellar flare observations, to estimate stellar CME
rates. This paper sidesteps flares and attempts to calibrate a more
fundamental correlation between surface-averaged magnetic fluxes and
CME properties. For the Sun, there exists a power-law relationship
between the magnetic filling factor and the CME kinetic energy flux,
and it is generalized for use on other stars. An example prediction of
the time evolution of wind/CME mass-loss rates for a solar-mass star is
given. A key result is that for ages younger than about 1 Gyr (I.e.,
activity levels only slightly higher than the present-day Sun), the
CME mass loss exceeds that of the time-steady wind. At younger ages,
CMEs carry 10-100 times more mass than the wind, and such high rates
may be powerful enough to dispel circumstellar disks and affect the
habitability of nearby planets. The cumulative CME mass lost by the
young Sun may have been as much as 1% of a solar mass.
Title: Imaging the Top of the Solar Corona and the Young Solar Wind
Authors: DeForest, C. E.; Matthaeus, W. H.; Viall, N. M.; Cranmer,
S. R.
Bibcode: 2016AGUFMSH53A..05D
Altcode:
We present the first direct visual evidence of the quasi-stationary
breakup of solar coronal structure and the rise of turbulence in
the young solar wind, directly in the future flight path of Solar
Probe. Although the corona and, more recently, the solar wind have both
been observed directly with Thomson scattered light, the transition from
the corona to the solar wind has remained a mystery. The corona itself
is highly structured by the magnetic field and the outflowing solar
wind, giving rise to radial "striae" - which comprise the familiar
streamers, pseudostreamers, and rays. These striae are not visible
in wide-field heliospheric images, nor are they clearly delineated
with in-situ measurements of the solar wind. Using careful photometric
analysis of the images from STEREO/HI-1, we have, for the first time,
directly observed the breakup of radial coronal structure and the rise
of nearly-isotropic turbulent structure in the outflowing slow solar
wind plasma between 10° (40 Rs) and 20° (80 Rs) from the Sun. These
observations are important not only for their direct science value,
but for predicting and understanding the conditions expected near SPP as
it flies through - and beyond - this final frontier of the heliosphere,
the outer limits of the solar corona.
Title: The FIELDS Instrument Suite for Solar Probe Plus. Measuring
the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence,
and Radio Signatures of Solar Transients
Authors: Bale, S. D.; Goetz, K.; Harvey, P. R.; Turin, P.; Bonnell,
J. W.; Dudok de Wit, T.; Ergun, R. E.; MacDowall, R. J.; Pulupa,
M.; Andre, M.; Bolton, M.; Bougeret, J. -L.; Bowen, T. A.; Burgess,
D.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen,
C. H. K.; Choi, M. K.; Connerney, J. E.; Cranmer, S.; Diaz-Aguado, M.;
Donakowski, W.; Drake, J. F.; Farrell, W. M.; Fergeau, P.; Fermin, J.;
Fischer, J.; Fox, N.; Glaser, D.; Goldstein, M.; Gordon, D.; Hanson,
E.; Harris, S. E.; Hayes, L. M.; Hinze, J. J.; Hollweg, J. V.; Horbury,
T. S.; Howard, R. A.; Hoxie, V.; Jannet, G.; Karlsson, M.; Kasper,
J. C.; Kellogg, P. J.; Kien, M.; Klimchuk, J. A.; Krasnoselskikh,
V. V.; Krucker, S.; Lynch, J. J.; Maksimovic, M.; Malaspina, D. M.;
Marker, S.; Martin, P.; Martinez-Oliveros, J.; McCauley, J.; McComas,
D. J.; McDonald, T.; Meyer-Vernet, N.; Moncuquet, M.; Monson, S. J.;
Mozer, F. S.; Murphy, S. D.; Odom, J.; Oliverson, R.; Olson, J.;
Parker, E. N.; Pankow, D.; Phan, T.; Quataert, E.; Quinn, T.; Ruplin,
S. W.; Salem, C.; Seitz, D.; Sheppard, D. A.; Siy, A.; Stevens, K.;
Summers, D.; Szabo, A.; Timofeeva, M.; Vaivads, A.; Velli, M.; Yehle,
A.; Werthimer, D.; Wygant, J. R.
Bibcode: 2016SSRv..204...49B
Altcode: 2016SSRv..tmp...16B
NASA's Solar Probe Plus (SPP) mission will make the first in situ
measurements of the solar corona and the birthplace of the solar
wind. The FIELDS instrument suite on SPP will make direct measurements
of electric and magnetic fields, the properties of in situ plasma waves,
electron density and temperature profiles, and interplanetary radio
emissions, amongst other things. Here, we describe the scientific
objectives targeted by the SPP/FIELDS instrument, the instrument
design itself, and the instrument concept of operations and planned
data products.
Title: Solar Wind Electrons Alphas and Protons (SWEAP) Investigation:
Design of the Solar Wind and Coronal Plasma Instrument Suite for
Solar Probe Plus
Authors: Kasper, Justin C.; Abiad, Robert; Austin, Gerry;
Balat-Pichelin, Marianne; Bale, Stuart D.; Belcher, John W.; Berg,
Peter; Bergner, Henry; Berthomier, Matthieu; Bookbinder, Jay; Brodu,
Etienne; Caldwell, David; Case, Anthony W.; Chandran, Benjamin D. G.;
Cheimets, Peter; Cirtain, Jonathan W.; Cranmer, Steven R.; Curtis,
David W.; Daigneau, Peter; Dalton, Greg; Dasgupta, Brahmananda;
DeTomaso, David; Diaz-Aguado, Millan; Djordjevic, Blagoje; Donaskowski,
Bill; Effinger, Michael; Florinski, Vladimir; Fox, Nichola; Freeman,
Mark; Gallagher, Dennis; Gary, S. Peter; Gauron, Tom; Gates, Richard;
Goldstein, Melvin; Golub, Leon; Gordon, Dorothy A.; Gurnee, Reid; Guth,
Giora; Halekas, Jasper; Hatch, Ken; Heerikuisen, Jacob; Ho, George; Hu,
Qiang; Johnson, Greg; Jordan, Steven P.; Korreck, Kelly E.; Larson,
Davin; Lazarus, Alan J.; Li, Gang; Livi, Roberto; Ludlam, Michael;
Maksimovic, Milan; McFadden, James P.; Marchant, William; Maruca,
Bennet A.; McComas, David J.; Messina, Luciana; Mercer, Tony; Park,
Sang; Peddie, Andrew M.; Pogorelov, Nikolai; Reinhart, Matthew J.;
Richardson, John D.; Robinson, Miles; Rosen, Irene; Skoug, Ruth M.;
Slagle, Amanda; Steinberg, John T.; Stevens, Michael L.; Szabo, Adam;
Taylor, Ellen R.; Tiu, Chris; Turin, Paul; Velli, Marco; Webb, Gary;
Whittlesey, Phyllis; Wright, Ken; Wu, S. T.; Zank, Gary
Bibcode: 2016SSRv..204..131K
Altcode: 2015SSRv..tmp..119K
The Solar Wind Electrons Alphas and Protons (SWEAP) Investigation
on Solar Probe Plus is a four sensor instrument suite that provides
complete measurements of the electrons and ionized helium and hydrogen
that constitute the bulk of solar wind and coronal plasma. SWEAP
consists of the Solar Probe Cup (SPC) and the Solar Probe Analyzers
(SPAN). SPC is a Faraday Cup that looks directly at the Sun and measures
ion and electron fluxes and flow angles as a function of energy. SPAN
consists of an ion and electron electrostatic analyzer (ESA) on
the ram side of SPP (SPAN-A) and an electron ESA on the anti-ram
side (SPAN-B). The SPAN-A ion ESA has a time of flight section that
enables it to sort particles by their mass/charge ratio, permitting
differentiation of ion species. SPAN-A and -B are rotated relative to
one another so their broad fields of view combine like the seams on a
baseball to view the entire sky except for the region obscured by the
heat shield and covered by SPC. Observations by SPC and SPAN produce
the combined field of view and measurement capabilities required to
fulfill the science objectives of SWEAP and Solar Probe Plus. SWEAP
measurements, in concert with magnetic and electric fields, energetic
particles, and white light contextual imaging will enable discovery
and understanding of solar wind acceleration and formation, coronal and
solar wind heating, and particle acceleration in the inner heliosphere
of the solar system. SPC and SPAN are managed by the SWEAP Electronics
Module (SWEM), which distributes power, formats onboard data products,
and serves as a single electrical interface to the spacecraft. SWEAP
data products include ion and electron velocity distribution functions
with high energy and angular resolution. Full resolution data are stored
within the SWEM, enabling high resolution observations of structures
such as shocks, reconnection events, and other transient structures
to be selected for download after the fact. This paper describes the
implementation of the SWEAP Investigation, the driving requirements
for the suite, expected performance of the instruments, and planned
data products, as of mission preliminary design review.
Title: Explaining Inverted-temperature Loops in the Quiet Solar
Corona with Magnetohydrodynamic Wave-mode Conversion
Authors: Schiff, Avery J.; Cranmer, Steven R.
Bibcode: 2016ApJ...831...10S
Altcode: 2016arXiv160804398S
Coronal loops trace out bipolar, arch-like magnetic fields above the
Sun’s surface. Recent measurements that combine rotational tomography,
extreme-ultraviolet imaging, and potential-field extrapolation have
shown the existence of large loops with inverted-temperature profiles,
I.e., loops for which the apex temperature is a local minimum, not a
maximum. These “down loops” appear to exist primarily in equatorial
quiet regions near solar minimum. We simulate both these and the more
prevalent large-scale “up loops” by modeling coronal heating as a
time-steady superposition of (1) dissipation of incompressible Alfvén
wave turbulence and (2) dissipation of compressive waves formed by
mode conversion from the initial population of Alfvén waves. We found
that when a large percentage (>99%) of the Alfvén waves undergo
this conversion, heating is greatly concentrated at the footpoints and
stable “down loops” are created. In some cases we found loops with
three maxima that are also gravitationally stable. Models that agree
with the tomographic temperature data exhibit higher gas pressures for
“down loops” than for “up loops,” which is consistent with
observations. These models also show a narrow range of Alfvén wave
amplitudes: 3 to 6 km s-1 at the coronal base. This is low
in comparison to typical observed amplitudes of 20-30 km s-1
in bright X-ray loops. However, the large-scale loops we model are
believed to compose a weaker diffuse background that fills much of
the volume of the corona. By constraining the physics of loops that
underlie quiescent streamers, we hope to better understand the formation
of the slow solar wind.
Title: Predictions for Dusty Mass Loss from Asteroids During Close
Encounters with Solar Probe Plus
Authors: Cranmer, Steven R.
Bibcode: 2016EM&P..118...51C
Altcode: 2016arXiv160601785C; 2016EM&P..tmp...11C
The Solar Probe Plus ( SPP) mission will explore the Sun's corona and
innermost solar wind starting in 2018. The spacecraft will also come
close to a number of Mercury-crossing asteroids with perihelia less
than 0.3 AU. At small heliocentric distances, these objects may begin
to lose mass, thus becoming "active asteroids" with comet-like comae
or tails. This paper assembles a database of 97 known Mercury-crossing
asteroids that may be encountered by SPP, and it presents estimates of
their time-dependent visible-light fluxes and mass loss rates. Assuming
a similar efficiency of sky background subtraction as was achieved by
STEREO , we find that approximately 80 % of these asteroids are bright
enough to be observed by the Wide-field Imager for SPP (WISPR). A
model of gas/dust mass loss from these asteroids is developed and
calibrated against existing observations. This model is used to
estimate the visible-light fluxes and spatial extents of spherical
comae. Observable dust clouds occur only when the asteroids approach
the Sun closer than 0.2 AU. The model predicts that during the primary
SPP mission between 2018 and 2025, there should be 113 discrete events
(for 24 unique asteroids) during which the modeled comae have angular
sizes resolvable by WISPR. The largest of these correspond to asteroids
3200 Phaethon, 137924, 155140, and 289227, all with angular sizes of
roughly 15-30 arcminutes. We note that the SPP trajectory may still
change, but no matter the details there should still be multiple
opportunities for fruitful asteroid observations.
Title: Fading Coronal Structure and the Onset of Turbulence in the
Young Solar Wind
Authors: DeForest, C. E.; Matthaeus, W. H.; Viall, N. M.; Cranmer,
S. R.
Bibcode: 2016ApJ...828...66D
Altcode: 2016arXiv160607718D
Above the top of the solar corona, the young, slow solar wind
transitions from low-β, magnetically structured flow dominated
by radial structures to high-β, less structured flow dominated by
hydrodynamics. This transition, long inferred via theory, is readily
apparent in the sky region close to 10° from the Sun in processed,
background-subtracted solar wind images. We present image sequences
collected by the inner Heliospheric Imager instrument on board the
Solar-Terrestrial Relations Observatory (STEREO/HI1) in 2008 December,
covering apparent distances from approximately 4° to 24° from the
center of the Sun and spanning this transition in the large-scale
morphology of the wind. We describe the observation and novel techniques
to extract evolving image structure from the images, and we use those
data and techniques to present and quantify the clear textural shift in
the apparent structure of the corona and solar wind in this altitude
range. We demonstrate that the change in apparent texture is due both
to anomalous fading of the radial striae that characterize the corona
and to anomalous relative brightening of locally dense puffs of solar
wind that we term “flocculae.” We show that these phenomena are
inconsistent with smooth radial flow, but consistent with the onset
of hydrodynamic or magnetohydrodynamic instabilities leading to a
turbulent cascade in the young solar wind.
Title: Magnetic Thresholds on IRIS Network Jet Productivity
Authors: Woolsey, Lauren Nicole; Cranmer, Steven R.
Bibcode: 2016shin.confE..64W
Altcode:
We present an investigation of the small-scale, short-lived network
jets seen with the Interface Region Imaging Spectrograph (IRIS)
spacecraft. We look at the production of network jets in the vicinity
of a coronal hole and at the underlying magnetic field data gathered
by the Helioseismic and Magnetic Imager on Solar Dynamics Observatory
(SDO/HMI). We find that there is an absolute magnetic flux density
that triggers the production of network jets, and that these phenomena
form preferentially in regions of high flux imbalance. This work was
included in the Ph.D. dissertation "Magnetic Influences on the Solar
Wind" completed in May by L. N. Woolsey.
Title: How Important is Alfven Wave Heating? (Scene-setting talk)
Authors: Cranmer, Steven R.
Bibcode: 2016shin.confE..65C
Altcode:
This session (Outstanding Challenges in Understanding the Heating
of the Solar Corona and Solar Wind) is meant to confront the fact
that there are MANY proposed solutions to the intertwined problems of
coronal heating and wind acceleration, but the true problem is how to
choose between them. The main battle-lines are often drawn between
wave and nanoflare paradigms, but it may be the case that a single
unifying language (e.g., turbulence) can describe them both. In this
scene-setting talk I will briefly review the observational limits we
have on the energy present in coronal Alfvenic fluctuations, how they
vary as a function of fundamental coronal properties (magnetic field,
radial distance, frequency, wavenumber), and whether they dissipate
efficiently enough to heat the corona. Of course this talk will be
peppered by my own biased opinions about which processes I believe
dominate over the others, but I will do my best to label those opinions
clearly.
Title: Explaining Inverted Temperature Loops in the Quiet Solar
Corona With Magnetohydrodynamic Wave Mode Conversion
Authors: Schiff, Avery; Cranmer, Steven R.
Bibcode: 2016shin.confE..62S
Altcode:
We simulate the temperature profiles along coronal loops measured
with AIA DEM tomography and field-line extrapolation by Nuevo et al
(2013). By varying the strength and nature of the heating mechanism,
we modeled steady-state, gravitationally stable loops that have
temperature profiles with local maxima below the loop apex. Because
these loops have negative vertical temperature gradients over much of
their length, they have been called 'down loops' and were seen to exist
primarily in equatorial quiet regions near solar minimum. In our models,
the amount of heat deposited in the loop is attributed to two sources:
(1) the dissipation of Alfvén waves in a turbulent cascade, and (2)
the dissipation of compressive waves over a variable length. The
compressive waves are generated in a nonlinear process by which
some fraction of the Alfvén waves undergo mode conversion instead
of contributing directly to the heating process. We found that when
a large percentage (> 99%) of the Alfvén waves underwent this
conversion, the heating was greatly concentrated at the base of the
loop and stable 'down loops' were created. In some cases, we found
loops with three extrema that are gravitationally stable. We map the
full parameter space to explore which conditions lead to which loop
types, and we demonstrate that the simulated characteristics of the
loops - including magnetic field strength, pressure, and temperature -
are consistent with values measured by Nuevo et al. (2013). For more
details see Schiff & Cranmer (2016).
Title: Constraints on Planetesimal Collision Models in Debris Disks
Authors: MacGregor, Meredith A.; Wilner, David J.; Chandler, Claire;
Ricci, Luca; Maddison, Sarah T.; Cranmer, Steven R.; Andrews, Sean M.;
Hughes, A. Meredith; Steele, Amy
Bibcode: 2016ApJ...823...79M
Altcode: 2016arXiv160305644M
Observations of debris disks offer a window into the physical and
dynamical properties of planetesimals in extrasolar systems through the
size distribution of dust grains. In particular, the millimeter spectral
index of thermal dust emission encodes information on the grain size
distribution. We have made new VLA observations of a sample of seven
nearby debris disks at 9 mm, with 3\prime\prime resolution
and ∼5 μJy beam-1rms. We combine these with archival
ATCA observations of eight additional debris disks observed at 7 mm,
together with up-to-date observations of all disks at (sub)millimeter
wavelengths from the literature, to place tight constraints on the
millimeter spectral indices and thus grain size distributions. The
analysis gives a weighted mean for the slope of the power-law grain size
distribution, n(a)\propto {a}-q, of < q> =3.36+/- 0.02,
with a possible trend of decreasing q for later spectral type stars. We
compare our results to a range of theoretical models of collisional
cascades, from the standard self-similar, steady-state size distribution
(q = 3.5) to solutions that incorporate more realistic physics such
as alternative velocity distributions and material strengths, the
possibility of a cutoff at small dust sizes from radiation pressure,
and results from detailed dynamical calculations of specific disks. Such
effects can lead to size distributions consistent with the data, and
plausibly the observed scatter in spectral indices. For the AU Mic
system, the VLA observations show clear evidence of a highly variable
stellar emission component; this stellar activity obviates the need
to invoke the presence of an asteroid belt to explain the previously
reported compact millimeter source in this system.
Title: Stirring Coronal Spaghetti: Exploring Multiple Interactions
Between MHD Waves and Density Fluctuations
Authors: Cranmer, Steven R.
Bibcode: 2016SPD....4720104C
Altcode:
The solar corona has been revealed in the past few decades to be a
highly dynamic nonequilibrium plasma environment. Both the loop-filled
coronal base and the extended acceleration region of the solar wind
appear to be strongly turbulent, and models that invoke the dissipation
of incompressible Alfvenic fluctuations have had some success in
explaining the heating. However, many of these models neglect the
mounting evidence that density and pressure variations may play an
important role in the mass and energy balance of this system. In this
presentation I will briefly review observations of both compressible
and incompressible MHD fluctuations in the corona and solar wind, and
discuss future prospects with DKIST. I will also attempt to outline the
many ways that these different fluctuation modes have been proposed
to interact with one another -- usually with an eye on finding ways
to enhance their dissipation and heating. One under-appreciated type
of interaction is the fact that Alfven waves will undergo multiple
reflections and refractions in a "background plasma" filled with
localized density fluctuations. It is becoming increasingly clear that
models must not only include the effects of longitudinal variability
(e.g., magnetoacoustic waves and pulse-like jets) but also transverse
"striations" that appear naturally in a structured magnetic field
with small-scale footpoint variability. Future off-limb observations,
such as those with DKIST's Cryo-NIRSP instrument, will be crucial
for providing us with a detailed census of MHD waves and their mutual
interactions in the corona.
Title: Erratum: “Driving Solar Spicules and Jets with
Magnetohydrodynamic Turbulence: Testing a Persistent Idea” (2015, ApJ, 812, 71)
Authors: Cranmer, Steven R.; Woolsey, Lauren N.
Bibcode: 2016ApJ...822..119C
Altcode:
No abstract at ADS
Title: Explaining Inverted Temperature Loops in the Quiet Solar
Corona with Magnetohydrodynamic Wave Mode Conversion
Authors: Schiff, Avery; Cranmer, Steven R.
Bibcode: 2016SPD....47.0331S
Altcode:
We simulate the temperature profiles along coronal loops measured
with AIA DEM tomography and field-line extrapolation by Nuevo et al
(2013). By varying the strength and nature of the heating mechanism,
we modeled steady-state, gravitationally stable loops that have
temperature profiles with local maxima below the loop apex. Because
these loops have negative vertical temperature gradients over much of
their length, they have been called "down loops" and were seen to exist
primarily in equatorial quiet regions near solar minimum. In our models,
the amount of heat deposited in the loop is attributed to two sources:
(1) the dissipation of Alfven waves in a turbulent cascade, and (2) the
dissipation of compressive waves over a variable length. The compressive
waves are generated in a nonlinear process by which some fraction of the
Alfven waves undergo mode conversion instead of contributing directly
to the heating process. We found that when a large percentage (>
99%) of the Alfven waves underwent this conversion, the heating was
greatly concentrated at the base of the loop and stable "down loops"
were created. In some cases, we found loops with three extrema that
are gravitationally stable. We map the full parameter space to explore
which conditions lead to which loop types, and we demonstrate that the
simulated characteristics of the loops -- including magnetic field
strength, pressure, and temperature -- are consistent with values
measured by Nuevo et al. (2013).
Title: Statistical Study of Network Jets Observed in the Solar
Transition Region: a Comparison Between Coronal Holes and Quiet-Sun
Regions
Authors: Narang, Nancy; Arbacher, Rebecca T.; Tian, Hui; Banerjee,
Dipankar; Cranmer, Steven R.; DeLuca, Ed E.; McKillop, Sean
Bibcode: 2016SoPh..291.1129N
Altcode: 2016arXiv160406295N; 2016SoPh..tmp...56N
Recent IRIS observations have revealed a prevalence of intermittent
small-scale jets with apparent speeds of 80 -250 kms−1,
emanating from small-scale bright regions inside network boundaries
of coronal holes. We find that these network jets appear not only in
coronal holes but also in quiet-sun regions. Using IRIS 1330 Å (C
II) slit-jaw images, we extracted several parameters of these network
jets, e.g. apparent speed, length, lifetime, and increase in foot-point
brightness. Using several observations, we find that some properties of
the jets are very similar, but others are obviously different between
the quiet Sun and coronal holes. For example, our study shows that
the coronal-hole jets appear to be faster and longer than those in
the quiet Sun. This can be directly attributed to a difference in the
magnetic configuration of the two regions, with open magnetic field
lines rooted in coronal holes and magnetic loops often present in
the quiet Sun. We also detected compact bright loops that are most
likely transition region loops and are mostly located in quiet-Sun
regions. These small loop-like regions are generally devoid of network
jets. In spite of different magnetic structures in the coronal hole and
quiet Sun in the transition region, there appears to be no substantial
difference for the increase in footpoint brightness of the jets, which
suggests that the generation mechanism of these network jets is very
likely the same in both regions.
Title: VizieR Online Data Catalog: Granulation model for 508 KIC stars
(Cranmer+, 2014)
Authors: Cranmer, S. R.; Bastien, F. A.; Stassun, K. G.; Saar, S. H.
Bibcode: 2016yCat..17810124C
Altcode:
A goal of this work is to find self-consistent and accurate ways to
predict the properties of stellar light-curve variability, and to use
this variability to calibrate against other methods of determining
their fundamental parameters. Thus, it may be possible to develop
the analysis of granular flicker measurements in a way that augments
the results of asteroseismology and improves the accuracy of, e.g.,
stellar mass and radius measurements. To assist in this process,
we provide tabulated data for 508 stars with photometric light
curves measured by the Kepler mission, which also includes their
derived masses and predicted values of the turbulent Mach number
(Ma), the root-mean-square (rms) granulation intensity
amplitude σ, and the flicker amplitude F8. These data are
also hosted, with updates as needed, on the first author's Web site
(http://www.cfa.harvard.edu/~scranmer/). With the data is a short code
written in the Interactive Data Language (IDL) that reads the data
and reproduces two of the three panels of Figure4 in the paper. (3 data files).
Title: Magnetic Influences on Turbulent Heating and Jet Production
in Coronal Holes
Authors: Woolsey, L. N.; Cranmer, S. R.
Bibcode: 2015AGUFMSH13C2447W
Altcode:
The heating of the solar wind from open-field regions in the
corona is the subject of an ongoing body of work in the solar
physics community. We present recent progress to understand the
role of Alfvén-wave-driven turbulence in flux tubes open to the
heliosphere. Our models use three-dimensional, time-dependent forms
of the reduced magnetohydrodynamics equations to find the resulting
properties of the solar wind. We use the BRAID model (van Ballegooijen
et al., 2011) on open flux tubes that epitomize the most common magnetic
structures in the corona: a polar coronal hole, an open flux tube on
the boundary of an equatorial streamer, and one that neighbors a strong
active region. Our results agree with prior work using the time-steady,
one-dimensional ZEPHYR model (Cranmer et al., 2007; Woolsey and Cranmer,
2014). In addition, the time dependence in BRAID lets us explore the
bursty, nanoflare-like nature of the heating in these flux tubes. We
find that the transient heating can be captured into separate events
with an average energy of 1022 erg, with a maximum energy of 1025
erg. The bursty heating lead us to pursue a better understanding of
the physical processes responsible for the network jets seen in IRIS
data (see e.g. Tian et al., 2014). We search for correlations between
the supergranular magnetic field properties—using the Helioseismic
and Magnetic Imager aboard SDO—and jet productivity to make better
estimates of the mass and energy budget of these small-scale features
and to find evidence of the mechanisms responsible for the network jets.
Title: Time-dependent Turbulent Heating of Open Flux Tubes in the
Chromosphere, Corona, and Solar Wind
Authors: Woolsey, L. N.; Cranmer, S. R.
Bibcode: 2015ApJ...811..136W
Altcode: 2015arXiv150900377W
We investigate several key questions of plasma heating in open-field
regions of the corona that connect to the solar wind. We present
results for a model of Alfvén-wave-driven turbulence for three
typical open magnetic field structures: a polar coronal hole,
an open flux tube neighboring an equatorial streamer, and an open
flux tube near a strong-field active region. We compare time-steady,
one-dimensional turbulent heating models against fully time-dependent
three-dimensional reduced-magnetohydrodynamic modeling of BRAID. We
find that the time-steady results agree well with time-averaged
results from BRAID. The time dependence allows us to investigate the
variability of the magnetic fluctuations and of the heating in the
corona. The high-frequency tail of the power spectrum of fluctuations
forms a power law whose exponent varies with height, and we discuss
the possible physical explanation for this behavior. The variability
in the heating rate is bursty and nanoflare-like in nature, and we
analyze the amount of energy lost via dissipative heating in transient
events throughout the simulation. The average energy in these events
is 1021.91 erg, within the “picoflare” range, and many
events reach classical “nanoflare” energies. We also estimated
the multithermal distribution of temperatures that would result from
the heating-rate variability, and found good agreement with observed
widths of coronal differential emission measure distributions. The
results of the modeling presented in this paper provide compelling
evidence that turbulent heating in the solar atmosphere by Alfvén
waves accelerates the solar wind in open flux tubes.
Title: Driving Solar Spicules and Jets with Magnetohydrodynamic
Turbulence: Testing a Persistent Idea
Authors: Cranmer, Steven R.; Woolsey, Lauren N.
Bibcode: 2015ApJ...812...71C
Altcode: 2015arXiv150903263C
The solar chromosphere contains thin, highly dynamic strands of
plasma known as spicules. Recently, it has been suggested that the
smallest and fastest (Type II) spicules are identical to intermittent
jets observed by the Interface Region Imaging Spectrograph. These
jets appear to expand out along open magnetic field lines rooted in
unipolar network regions of coronal holes. In this paper we revisit
a thirty-year-old idea that spicules may be caused by upward forces
associated with Alfvén waves. These forces involve the conversion
of transverse Alfvén waves into compressive acoustic-like waves
that steepen into shocks. The repeated buffeting due to upward shock
propagation causes nonthermal expansion of the chromosphere and a
transient levitation of the transition region (TR). Some older models
of wave-driven spicules assumed sinusoidal wave inputs, but the solar
atmosphere is highly turbulent and stochastic. Thus, we model this
process using the output of a time-dependent simulation of reduced
magnetohydrodynamic turbulence. The resulting mode-converted compressive
waves are strongly variable in time, with a higher TR occurring when the
amplitudes are large and a lower TR when the amplitudes are small. In
this picture, the TR bobs up and down by several Mm on timescales less
than a minute. These motions produce narrow, intermittent extensions
of the chromosphere that have similar properties as the observed jets
and Type II spicules.
Title: An Investigation of Magnetic Thresholds for the Production
of IRIS Network Jets
Authors: Woolsey, Lauren Nicole; Cranmer, Steven R.
Bibcode: 2015shin.confE.108W
Altcode:
The enhanced resolution provided by IRIS has allowed for the direct
observation of small-scale features in the chromosphere and transition
region. One feature identified in IRIS observations is jet-like
emission from bright network patches. In this project, we compare the
network jets presented by Tian et al. (2014, Science) with magnetogram
data from Helioseismic and Magnetic Imager (HMI) on Solar Dynamics
Observatory. We search for a correlation between jet productivity and
the magnetic field strength of the local supergranular network. We are
also working to develop new image processing techniques to identify and
track network jets, using machine learning and pattern recognition used
in other disciplines. This multidisciplinary effort will allow us to
accurately identify and characterize small-scale, transient features
in existing IRIS data. With an expanded catalog of features and the
identification of correlations between supergranular magnetic field
strength and jet production, we can make better estimates of the mass
and energy budget contained in these network jets.
Title: IEHI: Ionization Equilibrium for Heavy Ions
Authors: Cranmer, Steven R.
Bibcode: 2015ascl.soft07020C
Altcode:
IEHI, written in Fortran, outputs a simple "coronal" ionization
equilibrium (i.e., collisional ionization and auto-ionization balanced
by radiative and dielectronic recombination) for a plasma at a given
electron temperature.
Title: The Role(s) of Electrons in the Turbulent Corona and Solar Wind
Authors: Cranmer, Steven R.
Bibcode: 2015shin.confE.140C
Altcode:
Even though free electrons are the least massive constituents of the
corona and solar wind, they do more than their share in keeping the
plasma locally neutral and current-free, and they are responsible
for nearly all of the heat conduction. In this scene-setting talk
I will review what we know observationally about electron velocity
distributions (VDFs) in the corona and heliosphere, and also discuss
what we still need to understand theoretically. I hope to make the case
for developing BOTH multi-fluid and kinetic models, since each has
unique strengths that can help us improve our understanding. On the
multi-fluid side, it is possible to compute volumetric heating rates
and heat conductivities for individual particle species (electrons,
protons, heavy ions) and use them in large-scale models of coronal
heating and solar wind acceleration. On the kinetic side, there are
multiple proposed ideas for the origin of the observed non-Maxwellian
features in electron VDFs. To show how the different models can talk
to one another, we will show that simple electron-kinetic corrections
to the fluid models can help us make better sense of some complicated
effects like the freezing-in of ion charge states in the solar wind.
Title: HEATCVB: Coronal heating rate approximations
Authors: Cranmer, Steven R.
Bibcode: 2015ascl.soft06009C
Altcode:
HEATCVB is a stand-alone Fortran 77 subroutine that estimates the local
volumetric coronal heating rate with four required inputs: the radial
distance r, the wind speed u, the mass density ρ, and the magnetic
field strength |B0|. The primary output is the heating rate Qturb at
the location defined by the input parameters. HEATCVB also computes
the local turbulent dissipation rate of the waves, γ = Qturb/(2UA).
Title: Time-dependent modeling of solar wind acceleration from
turbulent heating in open flux tubes
Authors: Woolsey, Lauren Nicole; Cranmer, Steven R.
Bibcode: 2015TESS....131005W
Altcode:
The acceleration of the solar wind, particularly from open flux tubes,
remains an open question in solar physics. Countless physical processes
have been suggested to explain all or parts of the coupled problem of
coronal heating and wind acceleration, but the current generation of
observations have been so far unable to distinguish which mechanism(s)
dominates. In this project, we consider heating by Alfvén waves
in a three-dimensional, time-dependent reduced magnetohydrodynamics
model. This model solves for the heating rate as a function of time due
to the twisting and braiding of magnetic field lines within a flux tube,
which is caused by Alfvén waves generated at the single footpoint of
the flux tube. We investigate three specific structures commonly found
in the corona: 1) an open flux tube in a coronal hole, 2) an open flux
tube on the edge of an equatorial streamer, and 3) an open flux tube
directly neighboring an active region. We present the time-dependent
heating rate, power spectra of fluctuations, and the time-averaged
properties of the solar wind arising from each magnetic structure. We
compare the time-averaged properties from the present modeling with
previous results from a one-dimensional, time-steady code (Cranmer
et al. 2007) to better calibrate the physics in the lower-dimensional
code and get a better understanding of the intricate role that bursty,
transient heating from Alfvén-wave-driven turbulence plays in the
acceleration of the solar wind from different magnetic structures.
Title: The role of turbulence in coronal heating and solar wind
expansion
Authors: Cranmer, S. R.; Asgari-Targhi, M.; Miralles, M. P.; Raymond,
J. C.; Strachan, L.; Tian, H.; Woolsey, L. N.
Bibcode: 2015RSPTA.37340148C
Altcode: 2014arXiv1412.2307C
Plasma in the Sun's hot corona expands into the heliosphere as a
supersonic and highly magnetized solar wind. This paper provides an
overview of our current understanding of how the corona is heated and
how the solar wind is accelerated. Recent models of magnetohydrodynamic
turbulence have progressed to the point of successfully predicting
many observed properties of this complex, multi-scale system. However,
it is not clear whether the heating in open-field regions comes mainly
from the dissipation of turbulent fluctuations that are launched from
the solar surface, or whether the chaotic "magnetic carpet" in the
low corona energizes the system via magnetic reconnection. To help
pin down the physics, we also review some key observational results
from ultraviolet spectroscopy of the collisionless outer corona.
Title: Comparing High-speed Transition Region Jets in Coronal Holes
and Quiet Sun Regions
Authors: Tate Arbacher, Rebecca; Tian, Hui; Cranmer, Steven R.
Bibcode: 2015AAS...22513705T
Altcode:
The complicated energy transfer and plasma motion in the transition
region, between the photosphere and the corona, may play a significant
role in the formation and acceleration of the solar wind. New
observations from the Interface Region Imaging Spectrograph (IRIS)
have revealed unprecedented levels of detail in this less-studied
region. Coronal holes in particular are a likely source of solar
wind material, though the formation and acceleration mechanisms of
the fast solar wind are still largely unknown. In our previous work,
we have reported the prevalence of small-scale high-speed (~80-250
km/s) jets with transition region temperatures from the network
structures of coronal holes. Here we undertake a comparative study
of these short-lived episodic network jets in a coronal hole region
and a quiet sun region using IRIS sit-and-stare slit-jaw imaging in
the 1330 Angstrom (C II) passband. The pointing coordinates, exposure
time, observing cadence, and field of view of both observations are all
identical. Our preliminary study suggests that the speeds and lengths
of the network jets may differ between quiet sun and coronal hole
regions. The quiet sun region exhibits many compact bright regions with
sizes of 5-10 arcseconds which produce very few jets. The jets that do
exist tend to propagate at much slower speeds over smaller distances
than their coronal hole counterparts. Comparatively, in the coronal
hole, such compact regions are almost absent and all network patches
are permeated by the intermittent high-reaching jets. Such a difference
suggests that magnetic loops are much smaller in the coronal hole and
the network jets are produced at low heights. The recurrence frequency
seems to be higher in the coronal hole region, with many of the isolated
quiet sun region jets demonstrating curved trajectories.This work is
supported under contract 8100002705 from Lockheed-Martin to SAO and
by the NSF-REU solar physics program at SAO, grant number AGS-1263241.
Title: Multi-Wavelength Spectroscopy of Two Classical T Tauri Stars
Authors: Dupree, Andrea K.; Brickhouse, Nancy S.; Cranmer, Steven R.
Bibcode: 2015AAS...22534804D
Altcode:
X-ray, optical, and near-infrared spectra of two accreting T Tauri
stars: TW Hya and BP Tau are analysed for a comparison of accretion
properties and effects. The two stars form a valuable pair for
study. While similar in spectral type (K7) and mass (0.8 M_sun), they
differ in other properties. TW Hya is a 10 Myr star, viewed pole-on
thus placing the accretion process in full view. BP Tau, in comparison,
is younger (1 Myr), accreting material at a much higher rate and is
viewed at 45 degrees. Deep CHANDRA spectra (HETG and LETG) of both stars
characterize the corona and accretion parameters. Additional optical
and near-IR spectra at high resolution(Magellan/MIKE, FLWO/TRES,
KPNO/PHOENIX, KECK/NIRSPEC) were taken both simultaneously and
contemporaneously to detail the post-shock material and the stellar
wind.
Title: Comparing High-speed Transition Region Jets in Coronal Holes
and Quiet Sun Regions
Authors: Arbacher, R. T.; Tian, H.; Cranmer, S. R.
Bibcode: 2014AGUFMSH51C4181A
Altcode:
The complicated energy transfer and plasma motion in the transition
region, between the photosphere and the corona, may play a significant
role in the formation and acceleration of the solar wind. New
observations from the Interface Region Imaging Spectrograph (IRIS)
have revealed unprecedented levels of detail in this less-studied
region. Coronal holes in particular are a likely source of solar
wind material, though the formation and acceleration mechanisms of
the fast solar wind are still largely unknown. In our previous work,
we have reported the prevalence of small-scale high-speed (~80-250
km/s) jets with transition region temperatures from the network
structures of coronal holes. Here we undertake a comparative study
of these short-lived episodic network jets in a coronal hole region
and a quiet sun region using IRIS sit-and-stare slit-jaw imaging in
the 1330 Angstrom (C II) passband. The pointing coordinates, exposure
time, observing cadence, and field of view of both observations are
all identical. Our preliminary study suggests that the speeds and
lengths of the network jets may differ between quiet sun and coronal
hole regions. The quiet sun region exhibits many compact bright regions
with sizes of 5-10 arcseconds which produce very few jets. The jets
that do exist tend to propagate at much slower speeds over smaller
distances than their coronal hole counterparts. Comparatively, in the
coronal hole, such compact regions are almost absent and all network
patches are permeated by the intermittent high-reaching jets. Such
a difference suggests that magnetic loops are much smaller in the
coronal hole and the network jets are produced at low heights. The
recurrence frequency seems to be higher in the coronal hole region,
with many of the isolated quiet sun region jets demonstrating curved
trajectories. This work is supported under contract 8100002705 from
Lockheed-Martin to SAO and by the NSF-REU solar physics program at SAO,
grant number AGS-1263241.
Title: Plasma Properties of Pseudostreamers and Associated Solar
Wind Streams
Authors: Miralles, M. P.; Cranmer, S. R.; Stenborg, G.
Bibcode: 2014AGUFMSH31B..02M
Altcode:
We study pseudostreamers (i.e., open-field extensions of plasma from
unipolar footpoints in the corona; distinct from classical helmet
streamers that have opposite-polarity footpoints) that are believed
to be sources of slow to intermediate speed wind streams. We make
use of multi-spacecraft and ground-based observations that extend
from the solar corona to the solar wind at 1 AU. We compare the
physical properties of selected pseudostreamers and helmet streamers
to characterize how the differences in magnetic topology affect the
plasma properties of the coronal structures and their wind. Due to
the large number of pseudostreamers and their long persistence over
multiple solar rotations, their contribution to the solar wind is
likely to be substantial. In order to investigate solar wind heating
and acceleration, we also compare our measurements with predictions from
pseudostreamer and streamer theoretical models. This work is supported
by NASA grant NNX10AQ58G to the Smithsonian Astrophysical Observatory.
Title: Prevalence of Micro-Jets from the Network Structures of the
Solar Transition Region and Chromosphere
Authors: DeLuca, E. E.; Tian, H.; Cranmer, S. R.; Reeves, K.; Miralles,
M. P.; McCauley, P.; McKillop, S.
Bibcode: 2014AGUFMSH51C4180D
Altcode:
IRIS observations in the 1330Å, 1400Å and 2796Å passbands have
revealed numerous small-scale jet-like features with speeds of ~80-250
km/s from the chromospheric network. These network jets occur in
both the quiet Sun and coronal holes. Their widths are often ~300
km or less. Many of these jets show up as elongated features with
enhanced line width in maps obtained with transition region (TR)
lines, suggesting that these jets reach at least TR temperatures and
they constitute an important element of TR structures. The ubiquitous
presence of these high-reaching (often >10 Mm) jets also suggests
that they may play a crucial role in the mass and energy budgets
of the corona and solar wind. The generation of these jets in the
network and the accompanying Alfven waves is also consistent with
the "magnetic furnace model" of solar wind proposed by Axford &
McKenzie (1992). The large speeds (greater than sound speed) suggest
that the Lorentz force (perhaps related to reconnection) must play
an important role in the generation and propagation of the network
jets. We believe that many network jets are the on-disk counterparts
and TR manifestation of type-II spicules.
Title: Prevalence of small-scale jets from the networks of the solar
transition region and chromosphere
Authors: Tian, H.; DeLuca, E. E.; Cranmer, S. R.; De Pontieu, B.;
Peter, H.; Martínez-Sykora, J.; Golub, L.; McKillop, S.; Reeves,
K. K.; Miralles, M. P.; McCauley, P.; Saar, S.; Testa, P.; Weber,
M.; Murphy, N.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
Tarbell, T. D.; Wuelser, J. P.; Kleint, L.; Kankelborg, C.; Jaeggli,
S.; Carlsson, M.; Hansteen, V.; McIntosh, S. W.
Bibcode: 2014Sci...346A.315T
Altcode: 2014arXiv1410.6143T
As the interface between the Sun’s photosphere and corona, the
chromosphere and transition region play a key role in the formation and
acceleration of the solar wind. Observations from the Interface Region
Imaging Spectrograph reveal the prevalence of intermittent small-scale
jets with speeds of 80 to 250 kilometers per second from the narrow
bright network lanes of this interface region. These jets have lifetimes
of 20 to 80 seconds and widths of ≤300 kilometers. They originate from
small-scale bright regions, often preceded by footpoint brightenings
and accompanied by transverse waves with amplitudes of ~20 kilometers
per second. Many jets reach temperatures of at least ~105
kelvin and constitute an important element of the transition region
structures. They are likely an intermittent but persistent source of
mass and energy for the solar wind.
Title: Suprathermal Electrons in the Solar Corona: Can Nonlocal
Transport Explain Heliospheric Charge States?
Authors: Cranmer, Steven R.
Bibcode: 2014ApJ...791L..31C
Altcode: 2014arXiv1407.5941C
There have been several ideas proposed to explain how the Sun's corona
is heated and how the solar wind is accelerated. Some models assume
that open magnetic field lines are heated by Alfvén waves driven
by photospheric motions and dissipated after undergoing a turbulent
cascade. Other models posit that much of the solar wind's mass and
energy is injected via magnetic reconnection from closed coronal
loops. The latter idea is motivated by observations of reconnecting
jets and also by similarities of ion composition between closed
loops and the slow wind. Wave/turbulence models have also succeeded
in reproducing observed trends in ion composition signatures versus
wind speed. However, the absolute values of the charge-state ratios
predicted by those models tended to be too low in comparison with
observations. This Letter refines these predictions by taking better
account of weak Coulomb collisions for coronal electrons, whose
thermodynamic properties determine the ion charge states in the low
corona. A perturbative description of nonlocal electron transport is
applied to an existing set of wave/turbulence models. The resulting
electron velocity distributions in the low corona exhibit mild
suprathermal tails characterized by "kappa" exponents between 10 and
25. These suprathermal electrons are found to be sufficiently energetic
to enhance the charge states of oxygen ions, while maintaining the same
relative trend with wind speed that was found when the distribution
was assumed to be Maxwellian. The updated wave/turbulence models are
in excellent agreement with solar wind ion composition measurements.
Title: Structure and Dynamics of the Accretion Process and Wind in
TW Hya
Authors: Dupree, A. K.; Brickhouse, N. S.; Cranmer, S. R.; Berlind,
P.; Strader, Jay; Smith, Graeme H.
Bibcode: 2014ApJ...789...27D
Altcode: 2014arXiv1405.2935D
Time-domain spectroscopy of the classical accreting T Tauri star, TW
Hya, covering a decade and spanning the far UV to the near-infrared
spectral regions can identify the radiation sources, the atmospheric
structure produced by accretion, and properties of the stellar wind. On
timescales from days to years, substantial changes occur in emission
line profiles and line strengths. Our extensive time-domain spectroscopy
suggests that the broad near-IR, optical, and far-uv emission lines,
centered on the star, originate in a turbulent post-shock region
and can undergo scattering by the overlying stellar wind as well as
some absorption from infalling material. Stable absorption features
appear in Hα, apparently caused by an accreting column silhouetted
in the stellar wind. Inflow of material onto the star is revealed by
the near-IR He I 10830 Å line, and its free-fall velocity correlates
inversely with the strength of the post-shock emission, consistent with
a dipole accretion model. However, the predictions of hydrogen line
profiles based on accretion stream models are not well-matched by these
observations. Evidence of an accelerating warm to hot stellar wind is
shown by the near-IR He I line, and emission profiles of C II, C III,
C IV, N V, and O VI. The outflow of material changes substantially
in both speed and opacity in the yearly sampling of the near-IR He I
line over a decade. Terminal outflow velocities that range from 200
km s-1 to almost 400 km s-1 in He I appear to be
directly related to the amount of post-shock emission, giving evidence
for an accretion-driven stellar wind. Calculations of the emission from
realistic post-shock regions are needed. Data presented herein
were obtained at the W. M. Keck Observatory, which is operated as a
scientific partnership among the California Institute of Technology,
the University of California, and the National Aeronautics and Space
Administration. The Observatory was made possible by the generous
financial support of the W. M. Keck Foundation. Infrared spectra were
taken at the Gemini Observatory, which is operated by the Association
of Universities for Research in Astronomy, Inc., under a cooperative
agreement with the NSF on behalf of the Gemini partnership: the National
Science Foundation (United States), formerly the Science and Technology
Facilities Council (United Kingdom), the National Research Council
(Canada), CONICYT (Chile), the Australian Research Council (Australia),
Ministério da Ciência e Tecnologia (Brazil) and Ministerio de
Ciencia, Tecnología e Innovación Productiva (Argentina). This paper
also includes spectra gathered with the 6.5 m Magellan Telescope/CLAY
located at Las Campanas Observatory, Chile. Additional spectra were
obtained at the 1.5 m Tillinghast Telescope at the Fred Lawrence
Whipple Observatory of the Smithsonian Astrophysical Observatory.
Title: Ensemble Simulations of Proton Heating in the Solar Wind via
Turbulence and Ion Cyclotron Resonance
Authors: Cranmer, Steven R.
Bibcode: 2014ApJS..213...16C
Altcode: 2014arXiv1406.0678C
Protons in the solar corona and heliosphere exhibit anisotropic
velocity distributions, violation of magnetic moment conservation,
and a general lack of thermal equilibrium with the other particle
species. There is no agreement about the identity of the physical
processes that energize non-Maxwellian protons in the solar wind, but a
traditional favorite has been the dissipation of ion cyclotron resonant
Alfvén waves. This paper presents kinetic models of how ion cyclotron
waves heat protons on their journey from the corona to interplanetary
space. It also derives a wide range of new solutions for the relevant
dispersion relations, marginal stability boundaries, and nonresonant
velocity-space diffusion rates. A phenomenological model containing
both cyclotron damping and turbulent cascade is constructed to explain
the suppression of proton heating at low alpha-proton differential flow
speeds. These effects are implemented in a large-scale model of proton
thermal evolution from the corona to 1 AU. A Monte Carlo ensemble of
realistic wind speeds, densities, magnetic field strengths, and heating
rates produces a filled region of parameter space (in a plane described
by the parallel plasma beta and the proton temperature anisotropy ratio)
similar to what is measured. The high-beta edges of this filled region
are governed by plasma instabilities and strong heating rates. The
low-beta edges correspond to weaker proton heating and a range of
relative contributions from cyclotron resonance. On balance, the models
are consistent with other studies that find only a small fraction of
the turbulent power spectrum needs to consist of ion cyclotron waves.
Title: Solar/Stellar Granulation as the Key Lower Boundary Condition
for Coronal Heating and Wind Acceleration
Authors: Cranmer, Steven R.
Bibcode: 2014AAS...22421106C
Altcode:
Much of the hot plasma that eventually becomes the supersonic solar
wind appears to have its origin in small (100 km diameter) magnetic flux
tubes that sit in the downflowing lanes between convective granules in
the Sun's photosphere. Convective overturning motions jostle these flux
tubes and induce kink-mode oscillations that can grow into Alfven waves
in the corona. A great deal of recent work has been done to explore
how these Alfvenic fluctuations may drive a turbulent cascade, heat
the plasma by gradual dissipation, and provide direct acceleration to
a wind via wave pressure gradients. This presentation will outline
this work and show how an accurate description of granulation is a
key input to self-consistent models of coronal heating and solar
wind acceleration. These self-consistent models have also been
applied successfully to predicting: (1) high-energy emission from
accreting T Tauri stars, (2) the mass loss rates of cool dwarfs and
red giants, and (3) the combined X-ray, radio, and submillimeter
emission from a young nearby M dwarf. In addition, a recent analysis
of stellar granulation with Kepler photometry has shown that our
understanding of the shallow convection zones of F-type stars still
requires additional refinement. In all cases, the combination of
multiple types of observational data has been crucial to improving
our understanding. For the Sun, the next-generation capabilities of
ATST/DKIST are expected to provide much more precise knowledge about
this important lower boundary condition to the heliosphere.
Title: Untangling Coronal Streamers from Pseudostreamers
Authors: Miralles, Mari Paz; Cranmer, Steven R.; Stenborg, Guillermo A.
Bibcode: 2014AAS...22432356M
Altcode:
We study the coronal source regions of the solar wind -- in particular
the coronal streamers and pseudostreamers that are believed to be the
sources of slow wind streams -- with multi-spacecraft and ground-based
observations. Due to the large number of both unipolar pseudostreamers
and classical bipolar helmet streamers and their long persistence over
multiple solar rotations, their relative contributions to the solar
wind are likely to be substantial.We compare the physical properties of
selected helmet streamers and pseudostreamers to characterize how the
differences in magnetic topology affect the plasma properties of the
coronal structures. In order to investigate slow solar wind heating and
acceleration, we also compare our measurements with predictions from
streamer and pseudostreamer theoretical models.This work is supported
by NASA grant NNX10AQ58G to the Smithsonian Astrophysical Observatory.
Title: Coronal Density Structure Revealed by Comet Lovejoy (C/2011 W3)
Authors: Raymond, John C.; McCauley, Patrick I; Cranmer, Steven R.;
Downs, Cooper
Bibcode: 2014AAS...22440201R
Altcode:
Images of Comet Lovejoy (C/2011 W3) obtained with AIA are dominated
by emission from moderately ionized oxygen (O III - O VI). The
images show striations due to magnetic field structure. In each
striation,the oxygen emission moves along the magnetic field and
stretches with time. The speed and the rate ofbroadening are related
to the parallel and perpendicular components of the velocities of
the cometary neutrals when they become ionized and behave as pickup
ions. The intensity structure indicates density contrasts of a factor
of 6 between neighboring magnetic flux tubes on scales of around 4000
km. That implies substantial variation in Alfven speeds, which results
in dispersion and dissipation of Alfven waves. This observation imposes
an upper limit in the outer scale of the turbulence spectrum in the
corona and suggests that density structures may affect the heating of
the corona and the driving of the solar wind.
Title: Turbulence-Driven Solar Wind Models in 2014: Filling in
the Gaps
Authors: Cranmer, Steven R.
Bibcode: 2014shin.confE.132C
Altcode:
This poster will be a potpourri of short contributions, each of which
is a part of an ongoing project to model self-consistent coronal
heating and solar wind acceleration from the dissipation of MHD
turbulence. Topics to be presented (may) include the following. (1)
An existing Monte Carlo model of loop-opening in the magnetic carpet
has been used to predict the amplitudes of reconnection-driven waves
in the low corona. (2) A classic description of nonlocal electron
transport (Scudder and Olbert 1979) has been applied to ZEPHYR
results to show how weak nonthermal tails may be maintained in a
turbulence-heated corona and produce the right amount of frozen-in
ionization balance. (3) A time-dependent reduced MHD model that was
previously used for coronal loops has been extended to open-field
regions, and the time-averaged levels of turbulent heating are found
to agree well with previously used phenomenological rates. (4) A new
kinetic model of proton evolution in the inner heliosphere reproduces a
range of in-situ measured trends regarding the temperature anisotropy,
plasma beta, and proton-alpha differential flow speed. (5) Although
the models are still largely incompressible, I will highlight some
new observational results that indicate how density fluctuations are
likely to be an important ingredient in models of open flux tubes.
Title: TIME-DOMAIN SPECTROSCOPY OF A T TAURI STAR
Authors: Dupree, Andrea K.; Brickhouse, Nancy S.; Cranmer, Steven R.;
Berlind, Perry L.; Strader, Jay; Smith, Graeme H.
Bibcode: 2014AAS...22440407D
Altcode:
High resolution optical and near-infrared spectra of TW Hya, the nearest
accreting T Tauri star, cover a decade and reveal the substantial
changes in accretion and wind properties. Our spectra suggest that
the broad near-IR, optical, and far-uv emission lines, centered on
the star, originate in a turbulent post-shock region and can undergo
scattering by the overlying stellar wind as well as absorption from
infalling material. Stable absorption features appear in H-alpha,
apparently caused by an accreting column silhouetted in the stellar
wind. The free-fall velocity of material correlates inversely with
the strength of the post-shock emission, consistent with a dipole
accretion model. Terminal outflow velocities appear to be directly
related to the amount of post-shock emission, giving evidence for an
accretion-driven stellar wind.
Title: Solar Wind Acceleration: Modeling Effects of Turbulent Heating
in Open Flux Tubes
Authors: Woolsey, Lauren N.; Cranmer, Steven R.
Bibcode: 2014AAS...22440205W
Altcode:
We present two self-consistent coronal heating models that determine
the properties of the solar wind generated and accelerated in magnetic
field geometries that are open to the heliosphere. These models require
only the radial magnetic field profile as input. The first code, ZEPHYR
(Cranmer et al. 2007) is a 1D MHD code that includes the effects of
turbulent heating created by counter-propagating Alfven waves rather
than relying on empirical heating functions. We present the analysis
of a large grid of modeled flux tubes (> 400) and the resulting
solar wind properties. From the models and results, we recreate the
observed anti-correlation between wind speed at 1 AU and the so-called
expansion factor, a parameterization of the magnetic field profile. We
also find that our models follow the same observationally-derived
relation between temperature at 1 AU and wind speed at 1 AU. We
continue our analysis with a newly-developed code written in Python
called TEMPEST (The Efficient Modified-Parker-Equation-Solving Tool)
that runs an order of magnitude faster than ZEPHYR due to a set of
simplifying relations between the input magnetic field profile and the
temperature and wave reflection coefficient profiles. We present these
simplifying relations as a useful result in themselves as well as the
anti-correlation between wind speed and expansion factor also found
with TEMPEST. Due to the nature of the algorithm TEMPEST utilizes to
find solar wind solutions, we can effectively separate the two primary
ways in which Alfven waves contribute to solar wind acceleration:
1) heating the surrounding gas through a turbulent cascade and 2)
providing a separate source of wave pressure. We intend to make TEMPEST
easily available to the public and suggest that TEMPEST can be used
as a valuable tool in the forecasting of space weather, either as a
stand-alone code or within an existing modeling framework.
Title: Waves and Turbulence in the Corona and Solar Wind
Authors: Cranmer, Steven R.
Bibcode: 2014shin.confE.149C
Altcode:
The solar corona has been revealed in the past few decades to be a
highly dynamic nonequilibrium plasma environment. Both the loop-filled
coronal base and the extended acceleration region of the solar wind
appear to be strongly turbulent, but direct observational evidence for a
cascade of fluctuation energy from large to small scales is lacking. In
this scene-setting talk I will briefly review the observations of
wavelike fluctuations in the corona and solar wind. I also hope to
summarize some of the active debates about the origins of these waves,
their ability to feed a turbulent cascade, and whether or not they
are intense enough to heat the corona and/or accelerate the wind.
Title: Turbulence-driven Coronal Heating and Improvements to Empirical
Forecasting of the Solar Wind
Authors: Woolsey, Lauren Nicole; Cranmer, Steven R.
Bibcode: 2014shin.confE.136W
Altcode:
Forecasting models of the solar wind often rely on simple
parameterizations of the magnetic field that ignore the effects of the
full magnetic field geometry. In this paper, we present the results of
two solar wind prediction models that consider the full magnetic field
profile and include the effects of Alfvén waves on coronal heating
and wind acceleration. The one-dimensional magnetohydrodynamic code
ZEPHYR self-consistently finds solar wind solutions without the need for
empirical heating functions. Another one-dimensional code, introduced
in this paper (The Efficient Modified-Parker-Equation-Solving Tool,
TEMPEST), can act as a smaller, stand-alone code for use in forecasting
pipelines. TEMPEST is written in Python and will become a publicly
available library of functions that is easy to adapt and expand. We
discuss important relations between the magnetic field profile and
properties of the solar wind that can be used to independently validate
prediction models. ZEPHYR provides the foundation and calibration for
TEMPEST, and ultimately we will use these models to predict observations
and explain space weather created by the bulk solar wind. We are
able to reproduce with both models the general anticorrelation seen
in comparisons of observed wind speed at 1 AU and the flux tube
expansion factor. There is significantly less spread than comparing
the results of the two models than between ZEPHYR and a traditional
flux tube expansion relation. We suggest that the new code, TEMPEST,
will become a valuable tool in the forecasting of space weather.
Title: Turbulence-driven Coronal Heating and Improvements to Empirical
Forecasting of the Solar Wind
Authors: Woolsey, Lauren N.; Cranmer, Steven R.
Bibcode: 2014ApJ...787..160W
Altcode: 2014arXiv1404.5998W
Forecasting models of the solar wind often rely on simple
parameterizations of the magnetic field that ignore the effects of the
full magnetic field geometry. In this paper, we present the results of
two solar wind prediction models that consider the full magnetic field
profile and include the effects of Alfvén waves on coronal heating
and wind acceleration. The one-dimensional magnetohydrodynamic code
ZEPHYR self-consistently finds solar wind solutions without the need for
empirical heating functions. Another one-dimensional code, introduced
in this paper (The Efficient Modified-Parker-Equation-Solving Tool,
TEMPEST), can act as a smaller, stand-alone code for use in forecasting
pipelines. TEMPEST is written in Python and will become a publicly
available library of functions that is easy to adapt and expand. We
discuss important relations between the magnetic field profile and
properties of the solar wind that can be used to independently validate
prediction models. ZEPHYR provides the foundation and calibration for
TEMPEST, and ultimately we will use these models to predict observations
and explain space weather created by the bulk solar wind. We are
able to reproduce with both models the general anticorrelation seen
in comparisons of observed wind speed at 1 AU and the flux tube
expansion factor. There is significantly less spread than comparing
the results of the two models than between ZEPHYR and a traditional
flux tube expansion relation. We suggest that the new code, TEMPEST,
will become a valuable tool in the forecasting of space weather.
Title: The Solar Corona as Probed by Comet Lovejoy (C/2011 W3)
Authors: Raymond, J. C.; McCauley, P. I.; Cranmer, S. R.; Downs, C.
Bibcode: 2014ApJ...788..152R
Altcode: 2014arXiv1405.1639R
Extreme-ultraviolet images of Comet Lovejoy (C/2011 W3) from the
Atmospheric Imaging Assembly show striations related to the magnetic
field structure in both open and closed magnetic regions. The brightness
contrast implies coronal density contrasts of at least a factor of
six between neighboring flux tubes over scales of a few thousand
kilometers. These density structures imply variations in the Alfvén
speed on a similar scale. They will drastically affect the propagation
and dissipation of Alfvén waves, and that should be taken into account
in models of coronal heating and solar wind acceleration. In each
striation, the cometary emission moves along the magnetic field and
broadens with time. The speed and the rate of broadening are related
to the parallel and perpendicular components of the velocities of the
cometary neutrals when they become ionized. We use a magnetohydrodynamic
model of the coronal magnetic field and the theory of pickup ions to
compare the measurements with theoretical predictions, in particular
with the energy lost to Alfvén waves as the cometary ions isotropize.
Title: Stellar Granulation as the Source of High-frequency Flicker
in Kepler Light Curves
Authors: Cranmer, Steven R.; Bastien, Fabienne A.; Stassun, Keivan G.;
Saar, Steven H.
Bibcode: 2014ApJ...781..124C
Altcode: 2013arXiv1312.5928C
A large fraction of cool, low-mass stars exhibit brightness fluctuations
that arise from a combination of convective granulation, acoustic
oscillations, magnetic activity, and stellar rotation. Much of the
short-timescale variability takes the form of stochastic noise, whose
presence may limit the progress of extrasolar planet detection and
characterization. In order to lay the groundwork for extracting useful
information from these quasi-random signals, we focus on the origin of
the granulation-driven component of the variability. We apply existing
theoretical scaling relations to predict the star-integrated variability
amplitudes for 508 stars with photometric light curves measured by the
Kepler mission. We also derive an empirical correction factor that
aims to account for the suppression of convection in F-dwarf stars
with magnetic activity and shallow convection zones. So that we can
make predictions of specific observational quantities, we performed
Monte Carlo simulations of granulation light curves using a Lorentzian
power spectrum. These simulations allowed us to reproduce the so-called
flicker floor (i.e., a lower bound in the relationship between the
full light-curve range and power in short-timescale fluctuations)
that was found in the Kepler data. The Monte Carlo model also
enabled us to convert the modeled fluctuation variance into a flicker
amplitude directly comparable with observations. When the magnetic
suppression factor described above is applied, the model reproduces
the observed correlation between stellar surface gravity and flicker
amplitude. Observationally validated models like these provide new and
complementary evidence for a possible impact of magnetic activity on
the properties of near-surface convection.
Title: Turbulent Dissipation and Kinetic Heating in the Solar Wind:
Benefits of an Ensemble Simulation Approach
Authors: Cranmer, S. R.; Woolsey, L. N.
Bibcode: 2013AGUFMSH41F..04C
Altcode:
Despite many years of study, the basic physical processes responsible
for producing the solar wind are not known -- or at least not
universally agreed upon. There are many proposed solutions to the
intertwined problems of coronal heating, wind acceleration, and
particle energization, and the real problem is how to choose between
them. Confirming that any one proposed mechanism is acting in the
heliosphere is difficult, not only because measurements are limited,
but also because many of the suggested processes act on a huge range of
spatial scales (from centimeters to astronomical units) with complex
feedback effects that are not yet understood. This presentation will
attempt to summarize the outstanding questions in our understanding
of the gradual energization of protons, electrons, and heavy ions in
the solar wind. The focus will be on the collisionless dissipation of
turbulent fluctuations that originate at the solar surface, and how
the Turbulent Dissipation Challenge can help identify the dominant
physical processes that transfer energy to the particles. We will
also discuss the importance of making the best use of in-situ and
remote-sensing measurements that probe the highly variable corona
and heliosphere. There is key information in the variability that
sometimes gets ignored when theorists attempt to model the properties of
well-known "mean states" (i.e., fast wind streams from polar coronal
holes). Instead, it could be a more convincing test for models
to reproduce the full statistical ensemble of plasma/field states
observed at a given place in the heliosphere. As an example of this,
we will present preliminary results of a Monte Carlo model that aims to
reproduce the full distribution of variations in the proton temperature
anisotropy and plasma beta measured in the solar wind at 1 AU.
Title: Turbulent Heating and Wave Pressure in Solar Wind Acceleration
Modeling: New Insights to Empirical Forecasting of the Solar Wind
Authors: Woolsey, L. N.; Cranmer, S. R.
Bibcode: 2013AGUFMSH43A..03W
Altcode:
The study of solar wind acceleration has made several important
advances recently due to improvements in modeling techniques. Existing
code and simulations test the competing theories for coronal
heating, which include reconnection/loop-opening (RLO) models and
wave/turbulence-driven (WTD) models. In order to compare and contrast
the validity of these theories, we need flexible tools that predict
the emergent solar wind properties from a wide range of coronal
magnetic field structures such as coronal holes, pseudostreamers, and
helmet streamers. ZEPHYR (Cranmer et al. 2007) is a one-dimensional
magnetohydrodynamics code that includes Alfven wave generation
and reflection and the resulting turbulent heating to accelerate
solar wind in open flux tubes. We present the ZEPHYR output for a
wide range of magnetic field geometries to show the effect of the
magnetic field profiles on wind properties. We also investigate the
competing acceleration mechanisms found in ZEPHYR to determine the
relative importance of increased gas pressure from turbulent heating
and the separate pressure source from the Alfven waves. To do so, we
developed a code that will become publicly available for solar wind
prediction. This code, TEMPEST, provides an outflow solution based on
only one input: the magnetic field strength as a function of height
above the photosphere. It uses correlations found in ZEPHYR between
the magnetic field strength at the source surface and the temperature
profile of the outflow solution to compute the wind speed profile
based on the increased gas pressure from turbulent heating. With this
initial solution, TEMPEST then adds in the Alfven wave pressure term
to the modified Parker equation and iterates to find a stable solution
for the wind speed. This code, therefore, can make predictions of the
wind speeds that will be observed at 1 AU based on extrapolations from
magnetogram data, providing a useful tool for empirical forecasting
of the sol! ar wind.
Title: Determining the Coronal Origin of the Slow Solar Wind Using
Remote Sensing and In Situ Observations
Authors: Miralles, M. P.; Landi, E.; Cranmer, S. R.; Raymond, J. C.;
Cohen, O.; Oran, R.
Bibcode: 2013AGUFMSH32A0005M
Altcode:
We study the origin of the slow solar wind by characterizing the
physical properties of the slow solar wind plasma with multi-spacecraft
and ground-based observations. We compare the characteristics of
coronal-streamer wind streams obtained during solar cycle 24 with
results from the previous solar cycle. In order to investigate slow
solar wind heating and acceleration, we also compare our measurements
with predictions from theoretical models. We aim to use the empirical
measurements to distinguish between different proposed physical
processes for slow wind acceleration (e.g., waves/turbulence versus
reconnection). This work is supported by NASA grant NNX10AQ58G
to the Smithsonian Astrophysical Observatory.
Title: Improved Models of X-Ray Emission from Accreting Young Stars
with Complex Magnetic Fields
Authors: Cranmer, Steven
Bibcode: 2013cxo..prop.4273C
Altcode:
We propose to construct a new generation of self-consistent models
of magnetospheric accretion, shock formation, and coronal heating
for T Tauri stars. We will produce 3D distributions of circumstellar
plasma parameters and synthesize X-ray spectral diagnostics for direct
comparison with observations. Existing models will be improved by
accounting for more realistic magnetic fields and a more physically
consistent thermal/ionization state. The new models will be highly
computationally efficient in order to enable the production of
thousands of trial cases, each with different accretion and magnetic
properties. This work will further the understanding of Chandra data
by providing versatile tools for the testing of many ideas regarding
low-mass stellar evolution and planet formation.
Title: Constraining a Model of Turbulent Coronal Heating for AU
Microscopii with X-Ray, Radio, and Millimeter Observations
Authors: Cranmer, Steven R.; Wilner, David J.; MacGregor, Meredith A.
Bibcode: 2013ApJ...772..149C
Altcode: 2013arXiv1306.4567C
Many low-mass pre-main-sequence stars exhibit strong magnetic activity
and coronal X-ray emission. Even after the primordial accretion disk
has been cleared out, the star's high-energy radiation continues to
affect the formation and evolution of dust, planetesimals, and large
planets. Young stars with debris disks are thus ideal environments for
studying the earliest stages of non-accretion-driven coronae. In this
paper we simulate the corona of AU Mic, a nearby active M dwarf with an
edge-on debris disk. We apply a self-consistent model of coronal loop
heating that was derived from numerical simulations of solar field-line
tangling and magnetohydrodynamic turbulence. We also synthesize the
modeled star's X-ray luminosity and thermal radio/millimeter continuum
emission. A realistic set of parameter choices for AU Mic produces
simulated observations that agree with all existing measurements
and upper limits. This coronal model thus represents an alternative
explanation for a recently discovered ALMA central emission peak that
was suggested to be the result of an inner "asteroid belt" within
3 AU of the star. However, it is also possible that the central 1.3
mm peak is caused by a combination of active coronal emission and a
bright inner source of dusty debris. Additional observations of this
source's spatial extent and spectral energy distribution at millimeter
and radio wavelengths will better constrain the relative contributions
of the proposed mechanisms.
Title: The Spatial and Temporal Dependence of Coronal Heating by
Alfvén Wave Turbulence
Authors: Asgari-Targhi, M.; van Ballegooijen, A. A.; Cranmer, S. R.;
DeLuca, E. E.
Bibcode: 2013ApJ...773..111A
Altcode: 2013arXiv1306.6038A
The solar atmosphere may be heated by Alfvén waves that propagate up
from the convection zone and dissipate their energy in the chromosphere
and corona. To further test this theory, we consider wave heating in
an active region observed on 2012 March 7. A potential field model of
the region is constructed, and 22 field lines representing observed
coronal loops are traced through the model. Using a three-dimensional
(3D) reduced magnetohydrodynamics code, we simulate the dynamics
of Alfvén waves in and near the observed loops. The results for
different loops are combined into a single formula describing the
average heating rate Q as a function of position within the observed
active region. We suggest this expression may be approximately valid
also for other active regions, and therefore may be used to construct
3D, time-dependent models of the coronal plasma. Such models are needed
to understand the role of thermal non-equilibrium in the structuring
and dynamics of the Sun's corona.
Title: Comparison of Coronal Streamer Properties to Solar Wind Models
For The Last Two Solar Cycle Minima
Authors: Miralles, Mari Paz; Landi, E.; Cranmer, S. R.; Raymond,
J. C.; Cohen, O.; Oran, R.
Bibcode: 2013SPD....44...28M
Altcode:
We characterize the physical properties of two coronal streamers
during Earth/Ulysses quadrature configurations for the previous two
solar minimum periods. Comparisons between coronal remote-sensing
observations and in situ measurements of solar wind plasma properties
are being used to characterize the origin of slow wind streams. In order
to investigate slow solar wind heating and acceleration, we compare
the measurements with predictions from MHD models. We aim to use
the empirical measurements to distinguish between different proposed
physical processes for the slow solar wind. This work is supported by
NASA grant NNX10AQ58G to the Smithsonian Astrophysical Observatory.
Title: The Spatial and Temporal Dependence of Coronal Heating by
Alfven Wave Turbulence
Authors: Asgari-Targhi, Mahboubeh; Van Ballegooijen, A. A.; Cranmer,
S. R.; DeLuca, E. E.
Bibcode: 2013SPD....4430501A
Altcode:
The solar atmosphere may be heated by Alfven waves that propagate up
from the convection zone and dissipate their energy in the chromosphere
and corona. To further test this theory, we consider wave heating in an
active region observed on 2012 March 7. A potential field model of the
region is constructed, and 22 field lines representing observed coronal
loops are traced through the model. Using a three-dimensional (3D)
reduced magneto-hydrodynamics (MHD) code, we simulate the dynamics of
Alfven waves in and near the observed loops. The results for different
loops are combined into a single formula describing the average
heating rate $Q$ as function of position within the observed active
region. We suggest this expression may be approximately valid also
for other active regions, and therefore may be used to construct 3D,
time-dependent models of the coronal plasma. Such models are needed
to understand the role of thermal non-equilibrium in the structuring
and dynamics of the Sun's corona.
Title: X-ray Measurements of Variable Accretion onto the Young Star
TW Hydrae
Authors: Brickhouse, Nancy S.; Cranmer, S. R.; Dupree, A. K.; Wolk,
S. J.; Guenther, H. M.
Bibcode: 2013AAS...22231004B
Altcode:
We report X-ray line ratio diagnostics of the electron temperature,
electron density and hydrogen column density observed from the classical
T Tauri star (CTTS) TW Hydrae using the High Energy Transmission
Grating (HETG) spectrometer onboard Chandra. Applying a classical
model of magnetically channeled flow from an accretion disk onto the
stellar surface, and making the assumption that the absorber of the
X-ray shock is the accreting stream itself, we are able to determine
all the properties of the accretion, namely the mass accretion rate,
stellar magnetic field strength, disk truncation radius, and surface
filling factor. We find that the diagnostic ratios, and thus the
accretion parameters, are variable, lending support to the absorption
assumption. We also report X-ray and optical signatures that respond
to the variable accretion, with timescales suggesting the response of
the stellar atmosphere to the impact of accretion.
Title: Turbulence-Driven Coronal Heating Models: New Insights and
Improvements to Empirical Forecasting of the Solar Wind
Authors: Woolsey, Lauren Nicole; Cranmer, Steven R.
Bibcode: 2013shin.confE.107W
Altcode:
There have been many successful attempts to model the solar corona
and solar wind, but empirical heating functions are often used for
simplicity. Employing these ad hoc prescriptions sweeps under the
rug the increasingly important issue of determining the true heating
mechanisms of the corona. Many theories exist to explain the linked
problems of coronal heating and solar wind acceleration. It is vital to
include the underlying physics of heating in a self-consistent way. The
one-dimensional magnetohydrodynamic model ZEPHYR (Cranmer et al. 2007)
can accurately match observations using real physics instead of an
empirical heating estimate. ZEPHYR inserts Alfven waves at the single
footpoint of an open flux tube as one of two free parameters; the other
is the radial magnetic field strength throughout the flux tube. The
Alfven waves are then free to propagate outward, and as the density of
the solar atmosphere drops, some waves are reflected back towards the
photosphere. Counter-propagating waves interact and create turbulence,
which generates an energy cascade that eventually heats the surrounding
gas. The acceleration of the solar wind by Alfven waves occurs through
two channels: 1) heating of the corona increases the gas pressure,
and 2) the waves themselves provide a separate source of pressure. In
this project, we seek to disentangle these effects to understand the
relative influence they provide to the wind acceleration. We solve
the equations to find a modified Parker (1958) critical point using
the ZEPHYR magnetic field geometry input and temperature structure
output. Thus, we attempt to isolate a key part of the process that
we will develop into a tool for solving for the outflow speed based
on generic input magnetic field models and temperatures either from
a detailed and self-consistent model like ZEPHYR or from observations
near the solar surface and at 1 AU.
Title: Connecting the Sun's High-resolution Magnetic Carpet to the
Turbulent Heliosphere
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.; Woolsey,
Lauren N.
Bibcode: 2013ApJ...767..125C
Altcode: 2013arXiv1303.0563C
The solar wind is connected to the Sun's atmosphere by flux tubes
that are rooted in an ever-changing pattern of positive and negative
magnetic polarities on the surface. Observations indicate that the
magnetic field is filamentary and intermittent across a wide range
of spatial scales. However, we do not know to what extent the complex
flux-tube topology seen near the Sun survives as the wind expands into
interplanetary space. In order to study the possible long-distance
connections between the corona and the heliosphere, we developed new
models of turbulence-driven solar wind acceleration along empirically
constrained field lines. We used a potential field model of the quiet
Sun to trace field lines into the ecliptic plane with unprecedented
spatial resolution at their footpoints. For each flux tube, a
one-dimensional model was created with an existing wave/turbulence code
that solves equations of mass, momentum, and energy conservation from
the photosphere to 4 AU. To take account of stream-stream interactions
between flux tubes, we used those models as inner boundary conditions
for a time-steady magnetohydrodynamic description of radial and
longitudinal structure in the ecliptic. Corotating stream interactions
smear out much of the smallest-scale variability, making it difficult
to see how individual flux tubes on granular or supergranular scales
can survive out to 1 AU. However, our models help clarify the level of
"background" variability with which waves and turbulent eddies should
be expected to interact. Also, the modeled fluctuations in magnetic
field magnitude were seen to match measured power spectra quite well.
Title: Turbulence as a Unifying Principle in Coronal Heating and
Solar Wind Acceleration
Authors: Cranmer, Steven R.
Bibcode: 2013enss.confE..41C
Altcode:
The origins of the hot solar corona and the supersonically expanding
solar wind are still the subject of much debate. Far from there being
a shortage of ideas, there is in fact a surplus of proposed physical
mechanisms, each of which requires testing by comparison with the
right observations. Many of the suggested processes are related to
the dissipation of solar MHD waves, and many involve multiple steps
of energy conversion between waves, turbulence, current sheets,
and other nonlinear plasma features. This presentation will give a
summary of wave/turbulence models that seem to succeed in explaining
the time-steady properties of the corona and the existence of fast and
slow solar wind streams. Models employing turbulent heating have been
found to reproduce many of the observed features of the fast and slow
solar wind without the need for artificial "coronal heating functions"
used by earlier models. The newest versions of these models are also
being used to simulate the development of corotating stream structures
at 1 AU, seeded by small-scale coronal flux tubes resolved at the
sub-arcsecond level at the solar surface. This presentation will also
summarize the results of time-dependent 3D reduced MHD simulations of
turbulence in coronal loops and open field regions. These simulations
largely validate the phenomenological turbulent heating terms used in
larger-scale models, and they shed light on the apparent inability of
slow quasi-static "braiding" to provide sufficient energy to explain
coronal heating.
Title: Self-Consistent Models of the Solar Wind
Authors: Cranmer, Steven R.
Bibcode: 2013mspc.book..145C
Altcode:
No abstract at ADS
Title: X-Ray Determination of the Variable Rate of Mass Accretion
onto TW Hydrae
Authors: Brickhouse, N. S.; Cranmer, S. R.; Dupree, A. K.; Günther,
H. M.; Luna, G. J. M.; Wolk, S. J.
Bibcode: 2012ApJ...760L..21B
Altcode: 2012arXiv1211.1710B
Diagnostics of electron temperature (Te ), electron density
(ne ), and hydrogen column density (N H) from
the Chandra High Energy Transmission Grating spectrum of He-like Ne
IX in TW Hydrae (TW Hya), in conjunction with a classical accretion
model, allow us to infer the accretion rate onto the star directly from
measurements of the accreting material. The new method introduces the
use of the absorption of Ne IX lines as a measure of the column density
of the intervening, accreting material. On average, the derived mass
accretion rate for TW Hya is 1.5 × 10-9 M ⊙
yr-1, for a stellar magnetic field strength of 600 G
and a filling factor of 3.5%. Three individual Chandra exposures
show statistically significant differences in the Ne IX line ratios,
indicating changes in N H, Te , and ne
by factors of 0.28, 1.6, and 1.3, respectively. In exposures separated
by 2.7 days, the observations reported here suggest a five-fold
reduction in the accretion rate. This powerful new technique promises
to substantially improve our understanding of the accretion process
in young stars.
Title: Coronal Streamers and Their Associated Solar Wind Streams
Authors: Miralles, M. P.; Landi, E.; Cranmer, S. R.; Cohen, O.;
Raymond, J. C.
Bibcode: 2012AGUFMSH53A2268M
Altcode:
We use the EUV spectrometers aboard SOHO and Hinode and white-light
coronagraphs to characterize the physical properties of coronal
streamers during Earth/Ulysses quadrature configurations for the
previous two solar minimum periods. In addition, comparisons between
coronal observations and in situ measurements of solar wind plasma
properties are being used to further characterize the origins of slow
wind streams. In order to investigate slow solar wind heating and
acceleration, we also compare with predictions from three-dimensional
MHD models. We aim to use the empirical measurements to distinguish
between different proposed physical processes for slow wind acceleration
(e.g., waves/turbulence versus reconnection). This work is supported
by NASA grant NNX10AQ58G to the Smithsonian Astrophysical Observatory.
Title: A magnetic field parameter study of turbulence-driven
solar wind
Authors: Woolsey, L. N.; Cranmer, S. R.
Bibcode: 2012AGUFMSH33D2252W
Altcode:
The solar wind has traditionally been described by two components
that are separated by wind speed at 1 AU. Coronal holes, which are
characterized by superradial expansion and low densities, are held as
the source of fast solar wind (v > 600 km/s). Slow solar wind (v
< 400 km/s) has often been attributed to sources in the streamer
belt, but recent progress suggests that perhaps pseudostreamers or
the edges of coronal holes may contribute significantly to this slower
population. Further debate pertains to the mechanism that accelerates
the solar wind, and a key question is whether the two populations
arise due to A) two distinct acceleration processes or B) a single
process occurring in regions of differing magnetic geometries. In
this project, we investigate the latter possibility. We use the code
ZEPHYR (Cranmer et al. 2007) to compute solar wind properties from an
input of magnetic field strength as a function of height. This code
produces self-consistent models of the solar wind. Turbulence caused
by the reflection of Alfven waves cascades to the smallest scales and
dissipates heat to the corona. The gas- and wave-pressure gradients
accelerate the wind. We vary the magnetic field geometry input by
specifying the strengths at four to five heights in the chromosphere
and corona and using a spline interpolation to create a smooth radial
dependence of the magnetic field in each flux tube. The range of field
strengths is constrained by potential field source surface models using
harmonic coefficients from the Wilcox Solar Observatory. We seek to
better understand current correlations between solar wind properties
while seeking new relationships. Currently, the anticorrelation between
expansion factor and wind speed takes into account only the magnetic
field strength at the photospheric base and at the source surface
(Wang & Sheeley 1990). However, we look for relations that are
based on different ratios of magnetic field strengths or other aspects
of the magnetic field geometry. We also investigate the observed
anticorrelation between the freezing-in temperature of the O7+/O6+
ion ratio and wind speed, which has been attributed to solar wind
acceleration by reconnection and loop opening. Any relation that we
see between these two properties in our model output must be due to
wave-driven processes.
Title: The Spatial Dependence of Coronal Heating by Alfven Wave
Turbulence
Authors: Asgari-targhi, M.; Van Ballegooijen, A. A.; Cranmer, S. R.;
DeLuca, E. E.
Bibcode: 2012AGUFMSH31B..05A
Altcode:
We consider the wave heating in an active region observed on 7th of
March 2012 (Image). Using a potential field model we choose 22 field
lines and construct 3D MHD models of the Alfven waves along those
field lines. Based on those results we develop a heating formula
for the coronal loops observed. In our calculations, we establish
explicit relationships between the energy deposited and the loop
parameters, such as the length, and the magnetic field strength along
the loop. We also look at the variation of the heating within the loops
and predict the velocity fluctuations seen with future high-resolution
spectrographs.A potential field modeling of an active region observed
on 7th of March 2012.
Title: New Models of Solar Wind Acceleration and Stream Interactions
in the Sun's Topologically Complex Magnetic Field
Authors: Cranmer, S. R.; Van Ballegooijen, A. A.; Woolsey, L. N.
Bibcode: 2012AGUFMSH53A2266C
Altcode:
The last decade has seen significant progress toward identifying and
characterizing the processes that heat the corona and accelerate the
solar wind. It is believed that the low-speed solar wind comes from
a wide range of source regions in the corona, including streamers,
pseudostreamers, active regions, and small coronal holes. These
source regions tend to be associated with the most topologically
complex magnetic fields, and it is unclear how the coronal field lines
connect to the large-scale open heliospheric field. To learn more about
these connections, we present new models of turbulence-driven coronal
heating and solar wind acceleration along empirically constrained field
lines. To begin, we chose a time period during which the footpoints
linked to the ecliptic plane were rooted in Quiet Sun (QS) regions away
from both large coronal holes and strong-field active regions. The
weak and mixed-polarity QS field was observed at high resolution by
the VSM instrument of SOLIS, and we extrapolated this field into the
corona using the potential field source surface method. Time-steady
1D models of individual flux tubes were created with the ZEPHYR code
(Cranmer et al. 2007) that solves the one-fluid equations of mass,
momentum, and energy conservation from the photosphere to 4 AU. Then,
to take account of stream-stream interactions between the flux tubes, we
solved a 2D time-steady set of MHD conservation equations to determine
the corotating longitudinal structure in the ecliptic plane. We aim to
understand the extent to which fine-scale inter-tube plasma structures
in the corona survive to large distances. In other words, we want to
know how much of the coronal flux tube "spaghetti" is either shredded
by turbulence or smeared out by stream interactions. We also plan
to evaluate the level of high-resolution detail that is needed in
coronal flux tube modeling in order to accurately predict the space
weather consequences of various kinds of corotating structures in the
solar wind.
Title: Self-Consistent Models of the Solar Wind
Authors: Cranmer, Steven R.
Bibcode: 2012SSRv..172..145C
Altcode: 2010arXiv1007.0954C; 2010SSRv..tmp..177C
The origins of the hot solar corona and the supersonically expanding
solar wind are still the subject of much debate. This paper summarizes
some of the essential ingredients of realistic and self-consistent
models of solar wind acceleration. It also outlines the major issues
in the recent debate over what physical processes dominate the mass,
momentum, and energy balance in the accelerating wind. A key obstacle
in the way of producing realistic simulations of the Sun-heliosphere
system is the lack of a physically motivated way of specifying the
coronal heating rate. Recent models that assume the energy comes from
Alfvén waves that are partially reflected, and then dissipated by
magnetohydrodynamic turbulence, have been found to reproduce many of
the observed features of the solar wind. This paper discusses results
from these models, including detailed comparisons with measured plasma
properties as a function of solar wind speed. Some suggestions are also
given for future work that could answer the many remaining questions
about coronal heating and solar wind acceleration.
Title: Proton, Electron, and Ion Heating in the Fast Solar Wind from
Nonlinear Coupling between Alfvénic and Fast-mode Turbulence
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.
Bibcode: 2012ApJ...754...92C
Altcode: 2012arXiv1205.4613C
In the parts of the solar corona and solar wind that experience the
fewest Coulomb collisions, the component proton, electron, and heavy ion
populations are not in thermal equilibrium with one another. Observed
differences in temperatures, outflow speeds, and velocity distribution
anisotropies are useful constraints on proposed explanations for how the
plasma is heated and accelerated. This paper presents new predictions of
the rates of collisionless heating for each particle species, in which
the energy input is assumed to come from magnetohydrodynamic (MHD)
turbulence. We first created an empirical description of the radial
evolution of Alfvén, fast-mode, and slow-mode MHD waves. This model
provides the total wave power in each mode as a function of distance
along an expanding flux tube in the high-speed solar wind. Next, we
solved a set of cascade advection-diffusion equations that give the
time-steady wavenumber spectra at each distance. An approximate term
for nonlinear coupling between the Alfvén and fast-mode fluctuations is
included. For reasonable choices of the parameters, our model contains
enough energy transfer from the fast mode to the Alfvén mode to excite
the high-frequency ion cyclotron resonance. This resonance is efficient
at heating protons and other ions in the direction perpendicular to
the background magnetic field, and our model predicts heating rates
for these species that agree well with both spectroscopic and in situ
measurements. Nonetheless, the high-frequency waves comprise only a
small part of the total Alfvénic fluctuation spectrum, which remains
highly two dimensional as is observed in interplanetary space.
Title: Understanding the Origins of the Solar Wind (Thursday plenary)
Authors: Cranmer, Steven R.
Bibcode: 2012shin.confE...2C
Altcode:
The last decade has seen significant progress toward identifying
andcharacterizing the processes that heat the corona and accelerate
thesolar wind. Much of this progress has come about because
newmeasurements are diminishing the traditional gap between solarphysics
(i.e., near-Sun astronomy) and interplanetary space physics.These
two communities are becoming increasingly aware of the value ofeach
other's data and theoretical insights. This presentation willgive an
overview of some of the ways that connections between the Sunand the
heliosphere are leading to new answers to old questions.First, I will
summarize the state of ongoing debate between competingtheoretical camps
that advocate either waves/turbulence or magneticreconnection as the
primary drivers of coronal heating in open fluxtubes. In some areas,
traditional observational diagnostics of MHDplasma properties may not
be sufficient to distinguish between thesecompeting paradigms. Thus,
this presentation will also describewhy it is wise to confront the
truly microscopic (nonlinear,non-Maxwellian, collisionless) nature of
the relevant particles andfields. Theories and measurements that zoom in
to this level ofkinetic detail have the greatest potential for improving
ourunderstanding of the origins of solar wind acceleration. This isthe
natural realm of coronagraphic spectroscopy, so if I have timeI may
also emphasize the need for stringent stray light controlsin instruments
that observe above the solar limb.
Title: Properties of Polar Coronal Jets in the Fast Solar Wind
Authors: Miralles, Mari Paz; Cranmer, S. R.; Raymond, J. C.;
Stenborg, G.
Bibcode: 2012AAS...22020118M
Altcode: 2012AAS...22020118P
We present results of an ongoing observational study of the main
properties of polar coronal jets and how they interact with the
surrounding corona. While magnetic reconnection is considered the prime
driving mechanism of the ejected plasma, the processes at work during
reconnection are not yet completely understood. We use multi-instrument
measurements to probe the jet plasma, and we trace polar jets from
their reconnection sites into the fast solar wind. This study will
put firm constraints on the mechanisms driving the jets and on the
relative contribution of jets to the overall fast solar wind. This work is supported by NASA grant NNX09AH22G to the Smithsonian
Astrophysical Observatory.
Title: TW Hya: Spectral Variability, X-Rays, and Accretion Diagnostics
Authors: Dupree, A. K.; Brickhouse, N. S.; Cranmer, S. R.; Luna,
G. J. M.; Schneider, E. E.; Bessell, M. S.; Bonanos, A.; Crause,
L. A.; Lawson, W. A.; Mallik, S. V.; Schuler, S. C.
Bibcode: 2012ApJ...750...73D
Altcode: 2012arXiv1202.6373D
The nearest accreting T Tauri star, TW Hya was intensively and
continuously observed over ~17 days with spectroscopic and photometric
measurements from four continents simultaneous with a long segmented
exposure using the Chandra satellite. Contemporaneous optical
photometry from WASP-S indicates a 4.74 day period was present during
this time. The absence of a similar periodicity in the Hα flux and
the total X-ray flux which are dominated by accretion processes and
the stellar corona, respectively, points to a different source of
photometric variations. The Hα emission line appears intrinsically
broad and symmetric, and both the profile and its variability suggest an
origin in the post-shock cooling region. An accretion event, signaled by
soft X-rays, is traced spectroscopically for the first time through the
optical emission line profiles. After the accretion event, downflowing
turbulent material observed in the Hα and Hβ lines is followed by He I
(λ5876) broadening near the photosphere. Optical veiling resulting from
the heated photosphere increases with a delay of ~2 hr after the X-ray
accretion event. The response of the stellar coronal emission to an
increase in the veiling follows ~2.4 hr later, giving direct evidence
that the stellar corona is heated in part by accretion. Subsequently,
the stellar wind becomes re-established. We suggest a model that
incorporates the dynamics of this sequential series of events: an
accretion shock, a cooling downflow in a supersonically turbulent
region, followed by photospheric and later, coronal heating. This
model naturally explains the presence of broad optical and ultraviolet
lines, and affects the mass accretion rates determined from emission
line profiles.
Title: Tools for Predicting the Rates of Turbulent Heating for
Protons, Electrons, and Heavy Ions in the Solar Wind
Authors: Cranmer, S. R.; Chandran, B. D.; Van Ballegooijen, A. A.
Bibcode: 2011AGUFMSH41C..04C
Altcode:
In the parts of the solar corona and solar wind that experience
the fewest Coulomb collisions, the various particle species (i.e.,
protons, electrons, and heavy ions) are not in thermal equilibrium
with one another. The particles exhibit a range of different outflow
speeds, temperatures, and velocity distribution anisotropies, and
these differences can be used to probe the kinetic physical processes
that are responsible for depositing energy into the plasma. In this
presentation, we outline a new modeling framework for simulating the
rates of collisionless heating for each species, in which the energy
input is assumed to come from MHD turbulence. We begin by creating a
one-dimensional model of damped wave action conservation for Alfven,
fast-mode, and slow-mode MHD waves. This model provides the total wave
power in each mode as a function of radial distance along an expanding
solar wind flux tube. Next we solve a set of cascade advection-diffusion
equations that give the time-steady Fourier wavenumber spectra at each
distance. An approximate term for nonlinear mode coupling between
the Alfven and fast-mode fluctuations is included. We find that for
sufficiently high amplitudes of the fast-mode waves, there arises
enough Alfven wave energy at high frequencies to excite the proton
and ion cyclotron resonances and heat these particles in the direction
perpendicular to the background magnetic field. Although results will be
shown primarily for the plasma conditions in polar coronal holes that
give rise to high-speed solar wind streams, the tools outlined above
can be applied straightforwardly in other plasma environments as well.
Title: Telescoping in on the Microscopic Origins of the Fast
Solar Wind
Authors: Cranmer, S. R.
Bibcode: 2011AGUFMSH43F..01C
Altcode:
Despite many years of study, the basic physical processes that are
responsible for producing the solar wind are not known (or at least
not universally agreed upon). The fact that we have an overabundance
of proposed ideas for solving the problems of coronal heating and
wind acceleration can be seen as both a blessing and a curse. It
is a blessing because it highlights the insight and creativity of
the community, but it is a curse because we still do not know how to
validate or falsify many of these ideas. Discerning the presence of any
given proposed mechanism is difficult not only because measurements are
limited, but also because many of the suggested processes act on a wide
range of spatial scales (from centimeters to solar radii) with complex
feedback effects that are not yet understood. This presentation will
discuss a few key examples and controversies regarding the importance of
small spatial and temporal scales in the regions where the solar wind is
accelerated. For example, new observations have led to a revived debate
about whether the hot plasma in the solar wind is injected dynamically
from cooler regions below or whether it "evaporates" from the combined
effects of radiation and conduction from above. There is also debate
about how the open field lines are energized: Is the energy input
from waves and turbulent eddies that propagate up from the Sun and
dissipate, or is the constantly evolving magnetic carpet responsible
for heating the plasma via reconnection? In some areas, traditional
observational diagnostics of magnetohydrodynamic plasma properties may
not be sufficient to distinguish between competing predictions. Thus,
this presentation will also describe why it is probably wise to confront
the truly microscopic (nonlinear, non-Maxwellian, collisionless) nature
of the relevant particles and fields. Theories and measurements that
"zoom in" to this level of kinetic detail have the greatest potential
for improving our understanding of the origins of coronal heating and
solar wind acceleration.
Title: UVCS/SOHO Search for Coronal Suprathermal Seed Particles:
2011 Campaign
Authors: Kohl, J. L.; Panasyuk, A. V.; Cranmer, S. R.; Raymond, J. C.;
Rosati, R. E.
Bibcode: 2011AGUFMSH33D..05K
Altcode:
The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and
Heliospheric Observatory is being used to measure precise coronal
H I Ly-alpha spectral line profiles out to several Doppler half
widths. Such observations can be used to reveal the proton velocity
distribution along the line-of-sight. Departures from a Maxwellian
distribution are believed to be needed for the acceleration of solar
energetic particles (SEPs) by coronal mass ejection (CME) shocks. It is
generally believed that the seed particle population needed to produce
the numbers of SEPs observed in large events would have 0.001 to 0.01
of the particles with speeds that exceed 1000 km/s. Assuming a kappa
distribution that is symmetric in the tangential plane and Maxwellian
in the radial direction, this would correspond to a distribution
with kappa = 3.5 or smaller. We have shown that UVCS observations can
distinguish a Gaussian from a kappa exponent of 4 or less. Previously
reported observations near solar minimum have not yielded any such
distributions, but the sampling of the corona was not very systematic. A
new observational campaign was begun on 28 April 2011. Observations
are all at a heliographic height of 2.7 solar radii from Sun-center
and at a solar position angle of 290 degrees. Observations are made
on a continuous basis except during SOHO Keyhole periods when UVCS
observations are not possible, and at a few other times. Results of this
campaign and some earlier observations will be reported. This work is
supported by the National Aeronautics and Space Administration (NASA)
under Grant NNX11AM46G to the Smithsonian Astrophysical Observatory.
Title: Characterization of Slow Solar Wind Sources
Authors: Miralles, M. P.; Landi, E.; Cranmer, S. R.; Raymond, J. C.
Bibcode: 2011AGUFMSH33B2055M
Altcode:
The slow wind is a sizable component of the solar wind and plays a
fundamental role in shaping the interplanetary environment and its
variability. Coronal streamers are the prime source of slow solar
wind plasma. We use the EUV spectrometers aboard SOHO and Hinode to
characterize the physical properties of streamers and other candidate
coronal source regions of slow wind. In addition, comparisons between
coronal observations and in situ measurements of solar wind plasma
properties are being used to further characterize the origins of slow
wind streams. In order to investigate slow solar wind heating and
acceleration, we also compare with predictions from three-dimensional
models. We aim to use the empirical measurements to distinguish between
different proposed physical processes for slow wind acceleration (e.g.,
waves/turbulence versus reconnection). This work is supported by NASA
grant NNX10AQ58G to the Smithsonian Astrophysical Observatory.
Title: Comparison of Velocity, Density, Temperature, and Mass Flux
Results with Solar Coronal Models
Authors: Strachan, L.; Cranmer, S. R.; Panasyuk, A.; Kohl, J. L.;
Lamy, P. L.
Bibcode: 2011AGUFMSH53C..07S
Altcode:
We have recently computed a series of global maps of plasma parameters
in the extended corona using data from the Solar and Heliospheric
Observatory (SOHO). The synoptic maps of velocity, density, temperature,
and mass flux were derived from UV and white light coronal data
obtained from the Ultraviolet Coronagraph Spectrometer (UVCS) and
the Large Angle Spectroscopic Coronagraph (LASCO). The parameters
are defined on a sphere at 2.3~ R⊙ from Sun-center and
are organized by Carrington Rotations during the 1996 -- 1998 solar
minimum for Solar Cycle 23. The data imply that there are large flux
tube expansion factors near the coronal hole/streamer boundaries,
but these factors change significantly as the corona evolves from
minimum to the rising phase. We compare these data to an independently
developed theoretical model that includes damping and acceleration
by Alfven waves in the corona (see Cranmer et al. 2007, ApJS, 171,
520). The data set will be extended in the future and it will be used
for constraining other theoretical models of the corona and solar wind.
Title: Testing a Predictive Theoretical Model for the Mass Loss
Rates of Cool Stars
Authors: Cranmer, Steven R.; Saar, Steven H.
Bibcode: 2011ApJ...741...54C
Altcode: 2011arXiv1108.4369C
The basic mechanisms responsible for producing winds from cool,
late-type stars are still largely unknown. We take inspiration from
recent progress in understanding solar wind acceleration to develop a
physically motivated model of the time-steady mass loss rates of cool
main-sequence stars and evolved giants. This model follows the energy
flux of magnetohydrodynamic turbulence from a subsurface convection
zone to its eventual dissipation and escape through open magnetic
flux tubes. We show how Alfvén waves and turbulence can produce
winds in either a hot corona or a cool extended chromosphere, and we
specify the conditions that determine whether or not coronal heating
occurs. These models do not utilize arbitrary normalization factors, but
instead predict the mass loss rate directly from a star's fundamental
properties. We take account of stellar magnetic activity by extending
standard age-activity-rotation indicators to include the evolution of
the filling factor of strong photospheric magnetic fields. We compared
the predicted mass loss rates with observed values for 47 stars and
found significantly better agreement than was obtained from the popular
scaling laws of Reimers, Schröder, and Cuntz. The algorithm used to
compute cool-star mass loss rates is provided as a self-contained and
efficient computer code. We anticipate that the results from this kind
of model can be incorporated straightforwardly into stellar evolution
calculations and population synthesis techniques.
Title: Propagation of Polar Coronal Jets in the Fast Solar Wind
Authors: Miralles, M. P.; Cranmer, S. R.; Raymond, J. C.; Savcheva,
A. S.; Stenborg, G.; Deluca, E. E.
Bibcode: 2011exas.conf..119M
Altcode:
We present results of an ongoing observational study of the physical
properties and kinematics of polar coronal jets. While magnetic
reconnection is considered the prime driving mechanism of the ejected
plasma, the processes at work during reconnection are not yet completely
understood. We use a combination of X-ray, UV, and visible-light imaging
to probe the jet plasma, and we trace polar jets from their reconnection
sites into the fast solar wind. Multi-instrument measurements of
polar jets will put firm constraints on the mechanisms driving the
jets and on the relative contribution of jets to the overall fast
solar wind. This work is supported by NASA grant NNX09AH22G to the
Smithsonian Astrophysical Observatory.
Title: BOREAS: Mass Loss Rate of a Cool, Late-type Star
Authors: Cranmer, Steven R.; Saar, Steven H.
Bibcode: 2011ascl.soft08019C
Altcode:
The basic mechanisms responsible for producing winds from cool,
late-type stars are still largely unknown. We take inspiration from
recent progress in understanding solar wind acceleration to develop
a physically motivated model of the time-steady mass loss rates
of cool main-sequence stars and evolved giants. This model follows
the energy flux of magnetohydrodynamic turbulence from a subsurface
convection zone to its eventual dissipation and escape through open
magnetic flux tubes. We show how Alfven waves and turbulence can
produce winds in either a hot corona or a cool extended chromosphere,
and we specify the conditions that determine whether or not coronal
heating occurs. These models do not utilize arbitrary normalization
factors, but instead predict the mass loss rate directly from a star's
fundamental properties. We take account of stellar magnetic activity
by extending standard age-activity-rotation indicators to include
the evolution of the filling factor of strong photospheric magnetic
fields. We compared the predicted mass loss rates with observed values
for 47 stars and found significantly better agreement than was obtained
from the popular scaling laws of Reimers, Schroeder, and Cuntz. The
algorithm used to compute cool-star mass loss rates is provided as a
self-contained and efficient IDL computer code. We anticipate that the
results from this kind of model can be incorporated straightforwardly
into stellar evolution calculations and population synthesis techniques.
Title: Heating of the Solar Chromosphere and Corona by Alfvén
Wave Turbulence
Authors: van Ballegooijen, A. A.; Asgari-Targhi, M.; Cranmer, S. R.;
DeLuca, E. E.
Bibcode: 2011ApJ...736....3V
Altcode: 2011arXiv1105.0402V
A three-dimensional magnetohydrodynamic (MHD) model for the propagation
and dissipation of Alfvén waves in a coronal loop is developed. The
model includes the lower atmospheres at the two ends of the loop. The
waves originate on small spatial scales (less than 100 km) inside
the kilogauss flux elements in the photosphere. The model describes
the nonlinear interactions between Alfvén waves using the reduced
MHD approximation. The increase of Alfvén speed with height in
the chromosphere and transition region (TR) causes strong wave
reflection, which leads to counter-propagating waves and turbulence
in the photospheric and chromospheric parts of the flux tube. Part of
the wave energy is transmitted through the TR and produces turbulence
in the corona. We find that the hot coronal loops typically found in
active regions can be explained in terms of Alfvén wave turbulence,
provided that the small-scale footpoint motions have velocities of 1-2
km s-1 and timescales of 60-200 s. The heating rate per
unit volume in the chromosphere is two to three orders of magnitude
larger than that in the corona. We construct a series of models with
different values of the model parameters, and find that the coronal
heating rate increases with coronal field strength and decreases
with loop length. We conclude that coronal loops and the underlying
chromosphere may both be heated by Alfvénic turbulence.
Title: Modeling Iron Abundance Enhancements in the Slow Solar Wind
Authors: Byhring, H. S.; Cranmer, S. R.; Lie-Svendsen, Ø.; Habbal,
S. R.; Esser, R.
Bibcode: 2011ApJ...732..119B
Altcode:
We have studied the behavior of Fe ions in the slow solar wind,
using a fluid model extending from the chromosphere to 1 AU. Emphasis
is on elemental "pileup" in the corona, i.e., a region where the Fe
density increases and has a local maximum. We study the behavior of
individual Fe ions relative to each other in the pileup region, where
Fe+10 and Fe+12 have been used as examples. We
find that elemental pileups can occur for a variety of densities
and temperatures in the corona. We also calculate the ion fractions
and obtain estimates for the freezing-in distance of Fe in the slow
solar wind. We find that the freezing-in distance for iron is high,
between 3 and 11 R sun, and that a high outflow velocity,
of order 50-100 km s-1, in the region above the temperature
maximum is needed to obtain ion fractions for Fe+10 and
Fe+12 that are consistent with observations.
Title: The Coronal Physics Investigator (cpi) Experiment For Iss:
A New Vision For Understanding Solar Wind Acceleration
Authors: Raymond, John C.; Janzen, P. H.; Kohl, J. L.; Reisenfeld,
D. B.; Chandran, B. D. G.; Cranmer, S. R.; Forbes, T. G.; Isenberg,
P. A.; Panasyuk, A. V.; van Ballegooijen, A. A.
Bibcode: 2011SPD....42.2406R
Altcode: 2011BAAS..43S.2406R
We propose an Explorer Mission of Opportunity program to develop and
operate a large-aperture ultraviolet coronagraph spectrometer called
the Coronal Physics Investigator (CPI) as an attached International
Space Station (ISS) payload. The primary goal of this program is
to identify and characterize the physical processes that heat and
accelerate the primary and secondary components of the fast and slow
solar wind. Also, CPI can make key measurements needed to understand
CMEs. CPI is dedicated to high spectral resolution measurements of
the off-limb extended corona with far better stray light suppression
than can be achieved by a conventional instrument. UVCS/SOHO allowed
us to identify what additional measurements need to be made to answer
the fundamental questions about how solar wind streams are produced,
and CPI's next-generation capabilities were designed specifically to
make those measurements. Compared to previous instruments, CPI provides
unprecedented sensitivity, a wavelength range extending from 25.7 to
126 nm, higher temporal resolution, and the capability to measure line
profiles of He II, N V, Ne VII, Ne VIII, Si VIII, S IX, Ar VIII, Ca IX,
and Fe X, never before seen in coronal holes above 1.3 solar radii. CPI
will constrain the properties and effects of coronal MHD waves by (1)
observing many ions over a large range of charge and mass,(2) providing
simultaneous measurements of proton and electron temperatures to probe
turbulent dissipation mechanisms, and (3) measuring amplitudes of
low-frequency compressive fluctuations. CPI is an internally occulted
ultraviolet coronagraph that provides the required high sensitivity
without the need for a deployable boom, and with all technically mature
hardware including an ICCD detector. A highly experienced Explorer and
ISS contractor, L-3 Com Integrated Optical Systems and Com Systems East
will provide the tracking and pointing system as well as the instrument,
and the integration to the ISS.
Title: Testing a Predictive Theoretical Model for the Mass Loss
Rates of Cool Stars
Authors: Cranmer, Steven R.; Saar, S. H.
Bibcode: 2011AAS...21820503C
Altcode: 2011BAAS..43G20503C
All stars are believed to possess expanding outer atmospheres known
as stellar winds. The continual evaporation of gas from stars has a
significant impact on stellar and planetary evolution, and also on
the larger-scale evolution of gas and dust in galaxies. Despite more
than a half-century of study, though, the basic mechanisms responsible
for producing stellar winds are still largely unknown. Fortunately,
there has been a great deal of recent progress toward identifying
and characterizing the processes that produce our own Sun's mass
outflow. Based on this progress, we have developed a new generation
of physically motivated models of stellar wind acceleration for
cool main-sequence stars and evolved giants. These models follow the
production of magnetohydrodynamic turbulent motions from subsurface
convection zones to their eventual dissipation and escape through the
stellar wind. The magnetic activity of these stars is taken into account
by extending standard age/rotation/activity indicators to include the
evolution of the filling factor of strong magnetic fields in stellar
photospheres. We will present tests of these models based on a large
database of observationally determined mass loss rates, in combination
with accurate measurements of the basic properties of these stars
(e.g., masses, radii, luminosities, metallicities, and rotation rates)
on which the mass loss rates must depend. The eventual goal of this
project is to provide a simple stand-alone algorithm for predicting
the mass loss rates of cool stars for use in stellar atmosphere and
population synthesis calculations.
Title: The Coronal Physics Investigator (CPI) Experiment for ISS:
A New Vision for Understanding Solar Wind Acceleration
Authors: Kohl, J. L.; Cranmer, S. R.; Raymond, J. C.; Norton, T. J.;
Cucchiaro, P. J.; Reisenfeld, D. B.; Janzen, P. H.; Chandran, B. D. G.;
Forbes, T. G.; Isenberg, P. A.; Panasyuk, A. V.; van Ballegooijen,
A. A.
Bibcode: 2011arXiv1104.3817K
Altcode:
In February 2011 we proposed a NASA Explorer Mission of Opportunity
program to develop and operate a large-aperture ultraviolet
coronagraph spectrometer called the Coronal Physics Investigator
(CPI) as an attached International Space Station (ISS) payload. The
primary goal of this program is to identify and characterize the
physical processes that heat and accelerate the primary and secondary
components of the fast and slow solar wind. In addition, CPI can make
key measurements needed to understand CMEs. UVCS/SOHO allowed us to
identify what additional measurements need to be made to answer the
fundamental questions about how solar wind streams are produced, and
CPI's next-generation capabilities were designed specifically to make
those measurements. Compared to previous instruments, CPI provides
unprecedented sensitivity, a wavelength range extending from 25.7 to
126 nm, higher temporal resolution, and the capability to measure line
profiles of He II, N V, Ne VII, Ne VIII, Si VIII, S IX, Ar VIII, Ca IX,
and Fe X, never before seen in coronal holes above 1.3 solar radii. CPI
will constrain the properties and effects of coronal MHD waves by
(1) observing many ions over a large range of charge and mass, (2)
providing simultaneous measurements of proton and electron temperatures
to probe turbulent dissipation mechanisms, and (3) measuring amplitudes
of low-frequency compressive fluctuations. CPI is an internally occulted
ultraviolet coronagraph that provides the required high sensitivity
without the need for a deployable boom, and with all technically mature
hardware including an ICCD detector. A highly experienced Explorer and
ISS contractor, L-3 Com Integrated Optical Systems and Com Systems East,
will provide the tracking and pointing system as well as the instrument,
and the integration to the ISS.
Title: Recent Successes of Wave/Turbulence Driven Models of Solar
Wind Acceleration
Authors: Cranmer, S. R.; Hollweg, J. V.; Chandran, B. D.; van
Ballegooijen, A. A.
Bibcode: 2010AGUFMSH41B1786C
Altcode:
A key obstacle in the way of producing realistic simulations of the
Sun-heliosphere system is the lack of a first-principles understanding
of coronal heating. Also, it is still unknown whether the solar wind
is "fed" through flux tubes that remain open (and are energized by
footpoint-driven wavelike fluctuations) or if mass and energy are
input intermittently from closed loops into the open-field regions. In
this presentation, we discuss self-consistent models that assume the
energy comes from solar Alfven waves that are partially reflected,
and then dissipated, by magnetohydrodynamic turbulence. These models
have been found to reproduce many of the observed features of the fast
and slow solar wind without the need for artificial "coronal heating
functions" used by earlier models. For example, the models predict
a variation with wind speed in commonly measured ratios of charge
states and elemental abundances that agrees with observed trends. This
contradicts a commonly held assertion that these ratios can only be
produced by the injection of plasma from closed-field regions on the
Sun. This presentation also reviews two recent comparisons between the
models and empirical measurements: (1) The models successfully predict
the amplitude and radial dependence of Faraday rotation fluctuations
(FRFs) measured by the Helios probes for heliocentric distances between
2 and 15 solar radii. The FRFs are a particularly sensitive test of
turbulence models because they depend not only on the plasma density
and Alfven wave amplitude in the corona, but also on the turbulent
correlation length. (2) The models predict the correct sense and
magnitude of changes seen in the polar high-speed solar wind by Ulysses
from the previous solar minimum (1996-1997) to the more recent peculiar
minimum (2008-2009). By changing only the magnetic field along the polar
magnetic flux tube, consistent with solar and heliospheric observations
at the two epochs, the model correctly predicts that the wind speed
remains relatively unchanged, but the in-situ density and temperature
decrease by approximately 20 percent and 10 percent, respectively.
Title: Incorporating Kinetic Effects into Global Models of the Solar
Wind (Invited)
Authors: Cranmer, S. R.
Bibcode: 2010AGUFMSM33E..02C
Altcode:
The origins of the hot solar corona and the supersonically expanding
solar wind are still the subject of much debate. Far from there
being a shortage of ideas, there is in fact a surplus of proposed
physical mechanisms, each of which requires testing by comparison
with observations. This process is difficult not only because
the empirical measurements are limited, but also because many of
the proposed mechanisms act on a wide range of spatial scales (from
centimeters to solar radii) with feedback effects that are not yet well
understood. Many of these mechanisms are related to the dissipation of
solar MHD waves, and many involve multiple steps of energy conversion
between waves, turbulence, current sheets, and other nonlinear plasma
features. This presentation will give a summary of wave/turbulence
models that seem to succeed in explaining the time-steady properties of
the corona and the existence of fast and slow solar wind streams. A new
global model of nonlinear MHD wave transport and dissipation in polar
coronal holes, which includes both compressible and incompressible
fluctuations, will also be presented. The goal of this model is to
predict the kinetic partitioning of heat deposition between electrons,
protons, and heavy ions, with as few free parameters as possible.
Title: A Unified Model for Chromospheric and Coronal Heating Driven
by Small-Scale Random Footpoint Motions
Authors: van Ballegooijen, A. A.; Cranmer, S. R.; Asgari-Targhi, M.;
Deluca, E. E.
Bibcode: 2010AGUFMSH31C1802V
Altcode:
The solar corona is thought to be heated by dissipation of magnetic
disturbances that propagate up from the Sun's convection zone. We
propose that a major contribution to the heating comes from disturbances
that originate on small spatial scales inside the kilogauss magnetic
flux elements in the photosphere. Interactions of convective flows with
such flux elements produce Alfven waves that travel upward along the
magnetic field lines. When they reach the chromosphere and transition
region, the waves reflect, producing counter-propagating waves in the
chromosphere. Such counter-propagating waves are subject to well-known
nonlinear wave-wave interactions that can lead to the development
of turbulence. We simulate the dynamics of Alfven waves using a 3D
MHD model of a coronal loop (including the lower atmospheres at the
two ends of the loop) and we find that strong turbulence does indeed
develop in the lower parts of the flux tube. Some of the wave energy is
transmitted into the corona and produces turbulence there. We find that
the hot coronal loops typically found in active regions can be explained
in terms of Alfven wave turbulence, provided the photospheric footpoint
motions have a velocity of 1 - 2 km/s and a correlation time of about
60 seconds. The heating rate in the chromosphere is 2 to 3 orders of
magnitude larger than that in the corona, consistent with empirical
models of facular regions. We conclude that coronal loops and the
underlying chromosphere may both be heated by Alfven wave turbulence.
Title: Science Objectives for an X-Ray Microcalorimeter Observing
the Sun
Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M;
Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse,
N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.;
DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra,
A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg,
C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.;
Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska,
J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres,
G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.;
Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz,
J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.;
Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B.
Bibcode: 2010arXiv1011.4052L
Altcode:
We present the science case for a broadband X-ray imager with
high-resolution spectroscopy, including simulations of X-ray spectral
diagnostics of both active regions and solar flares. This is part of
a trilogy of white papers discussing science, instrument (Bandler et
al. 2010), and missions (Bookbinder et al. 2010) to exploit major
advances recently made in transition-edge sensor (TES) detector
technology that enable resolution better than 2 eV in an array that
can handle high count rates. Combined with a modest X-ray mirror, this
instrument would combine arcsecondscale imaging with high-resolution
spectra over a field of view sufficiently large for the study of
active regions and flares, enabling a wide range of studies such as
the detection of microheating in active regions, ion-resolved velocity
flows, and the presence of non-thermal electrons in hot plasmas. It
would also enable more direct comparisons between solar and stellar
soft X-ray spectra, a waveband in which (unusually) we currently have
much better stellar data than we do of the Sun.
Title: Ultraviolet Coronagraph Spectroscopy: A Key Capability for
Understanding the Physics of Solar Wind Acceleration
Authors: Cranmer, S. R.; Kohl, J. L.; Alexander, D.; Bhattacharjee,
A.; Breech, B. A.; Brickhouse, N. S.; Chandran, B. D. G.; Dupree,
A. K.; Esser, R.; Gary, S. P.; Hollweg, J. V.; Isenberg, P. A.; Kahler,
S. W.; Ko, Y. -K.; Laming, J. M.; Landi, E.; Matthaeus, W. H.; Murphy,
N. A.; Oughton, S.; Raymond, J. C.; Reisenfeld, D. B.; Suess, S. T.;
van Ballegooijen, A. A.; Wood, B. E.
Bibcode: 2010arXiv1011.2469C
Altcode:
Understanding the physical processes responsible for accelerating the
solar wind requires detailed measurements of the collisionless plasma
in the extended solar corona. Some key clues about these processes
have come from instruments that combine the power of an ultraviolet
(UV) spectrometer with an occulted telescope. This combination enables
measurements of ion emission lines far from the bright solar disk,
where most of the solar wind acceleration occurs. Although the UVCS
instrument on SOHO made several key discoveries, many questions remain
unanswered because its capabilities were limited. This white paper
summarizes these past achievements and also describes what can be
accomplished with next-generation instrumentation of this kind.
Title: Coronal Faraday Rotation Fluctuations and a
Wave/Turbulence-driven Model of the Solar Wind
Authors: Hollweg, Joseph V.; Cranmer, Steven R.; Chandran, Benjamin
D. G.
Bibcode: 2010ApJ...722.1495H
Altcode:
Some recent models for coronal heating and the origin of the solar wind
postulate that the source of energy and momentum consists of Alfvén
waves of solar origin dissipating via MHD turbulence. We use one of
these models to predict the level of Faraday rotation fluctuations
(FRFs) that should be imposed on radio signals passing through the
corona. This model has the virtue of specifying the correlation length
of the turbulence, knowledge of which is essential for calculating the
FRFs; previous comparisons of observed FRFs with models suffered from
the fact that the correlation length had to be guessed. We compare the
predictions with measurements of FRFs obtained by the Helios radio
experiment during occultations in 1975 through 1977, close to solar
minimum. We show that only a small fraction of the FRFs are produced by
density fluctuations; the bulk of the FRFs must be produced by coronal
magnetic field fluctuations. The observed FRFs have periods of hours,
suggesting that they are related to Alfvén waves which are observed
in situ by spacecraft throughout the solar wind; other evidence also
suggests that the FRFs are due to coronal Alfvén waves. We choose
a model field line in an equatorial streamer which has background
electron concentrations that match those inferred from the Helios
occultation data. The predicted FRFs are found to agree very well
with the Helios data. If the FRFs are in fact produced by Alfvén
waves with the assumed correlation length, our analysis leads us to
conclude that wave-turbulence models should continue to be pursued
with vigor. But since we cannot prove that the FRFs are produced by
Alfvén waves, we state the more conservative conclusion, still subject
to the correctness of the assumed correlation length, that the corona
contains long-period magnetic fluctuations with sufficient energy to
heat the corona and drive the solar wind.
Title: Can the Solar Wind be Driven by Magnetic Reconnection in the
Sun's Magnetic Carpet?
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.
Bibcode: 2010ApJ...720..824C
Altcode: 2010arXiv1007.2383C
The physical processes that heat the solar corona and accelerate the
solar wind remain unknown after many years of study. Some have suggested
that the wind is driven by waves and turbulence in open magnetic flux
tubes, and others have suggested that plasma is injected into the open
tubes by magnetic reconnection with closed loops. In order to test the
latter idea, we developed Monte Carlo simulations of the photospheric
"magnetic carpet" and extrapolated the time-varying coronal field. These
models were constructed for a range of different magnetic flux imbalance
ratios. Completely balanced models represent quiet regions on the Sun
and source regions of slow solar wind streams. Highly imbalanced models
represent coronal holes and source regions of fast wind streams. The
models agree with observed emergence rates, surface flux densities,
and number distributions of magnetic elements. Despite having no
imposed supergranular motions in the models, a realistic network
of magnetic "funnels" appeared spontaneously. We computed the rate
at which closed field lines open up (i.e., recycling times for open
flux), and we estimated the energy flux released in reconnection events
involving the opening up of closed flux tubes. For quiet regions and
mixed-polarity coronal holes, these energy fluxes were found to be much
lower than that which is required to accelerate the solar wind. For the
most imbalanced coronal holes, the energy fluxes may be large enough to
power the solar wind, but the recycling times are far longer than the
time it takes the solar wind to accelerate into the low corona. Thus,
it is unlikely that either the slow or fast solar wind is driven by
reconnection and loop-opening processes in the magnetic carpet.
Title: A Summary of the Evidence in Favor of the Idea that the Solar
Wind is Accelerated by Waves and/or Turbulence
Authors: Cranmer, Steven R.
Bibcode: 2010shin.confE.112C
Altcode:
Despite more than a half-century of study, the basic physical processes
that are responsible for accelerating the solar wind are not known
(or at least not universally agreed upon). The mechanism that has
been studied the most appears to be the dissipation of waves and
turbulent eddies. Roberts (2010) presented a series of arguments
why these processes may not be as effective as has been assumed in
the past. In this presentation, I attempt to counter these arguments
and demonstrate that there may still be hope for the wave/turbulence
explanation. A combination of observational and model-based evidence
will be brought to bear in order to show that the most likely strength
of Alfven waves in coronal holes is sufficient to provide both: (1)
substantial wave-pressure acceleration in high-speed streams, and
(2) sufficient coronal heating, via MHD turbulence seeded by partial
reflection, to heat and accelerate open-field regions of the corona
that connect to the solar wind.
Title: Connections Between the Magnetic Carpet and the Unbalanced
Corona: New Monte Carlo Models
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.
Bibcode: 2010shin.confE...2C
Altcode:
It is clear from observations of the solar magnetic carpet that much
of the heating in closed-field regions is driven by the interplay
between emergence, separation, merging, and cancellation of many small
flux elements. However, we do not yet know to what extent the open flux
tubes are energized by these processes. In order to begin investigating
this, we developed Monte Carlo simulations of the photospheric magnetic
carpet and extrapolated the time-varying magnetic field up into the
corona. These models were constructed for a range of different magnetic
flux imbalance ratios (i.e., for both quiet regions and coronal holes),
and they appear to be the first simulations to utilize newly observed
flux emergence rates that are at least an order of magnitude larger than
those used in earlier models. The results agree with a wide range of
observations, including surface flux densities and number distributions
of magnetic elements. Despite having no imposed supergranular motions
in the models, a realistic network of magnetic funnels appeared
spontaneously. We also computed the rate at which closed field lines
open up (i.e., the recycling times for open flux), and we estimated
the energy flux released in reconnection events involving the opening
up of closed flux tubes. For quiet regions and mixed-polarity coronal
holes, these energy fluxes were found to be much lower than required
to accelerate the solar wind. For the most imbalanced coronal holes,
the energy fluxes may be large enough to power the solar wind, but the
recycling times are far longer than the time it takes the solar wind
to accelerate into the low corona. Thus, reconnection and loop-opening
processes in the magnetic carpet may be responsible for intermittent
events in coronal holes (e.g., polar jets), but probably not for the
majority of bulk solar wind acceleration.
Title: SOHO-23: Understanding a Peculiar Solar Minimum
Authors: Cranmer, S. R.; Hoeksema, J. T.; Kohl, J. L.
Bibcode: 2010ASPC..428.....C
Altcode:
No abstract at ADS
Title: Ion Temperatures in the Low Solar Corona: Polar Coronal Holes
at Solar Minimum
Authors: Landi, E.; Cranmer, S. R.
Bibcode: 2010ASPC..428..197L
Altcode:
We use a SUMER spectrum of a polar coronal hole to measure the
ion temperatures Ti of many ions between 0.03 and 0.17
Rsun above the limb. We find that the measured Ti
are almost always larger than the electron temperatures (Te)
and exhibit a non-monotonic dependence on the charge-to-mass ratio. We
use these measurements to provide empirical constraints to a theoretical
model of ion heating and acceleration based on gradually replenished
ion-cyclotron waves and anisotropic magnetohydrodynamic turbulence. We
find that the empirical heating model and the turbulent cascade model
agree with one another and with observations for charge-to-mass (Z/A)
ratios smaller than about 0.25. Ions with Z/A > 0.25 disagree with
the model.
Title: Extended Coronal Heating and Solar Wind Acceleration over
the Solar Cycle
Authors: Cranmer, S. R.; Kohl, J. L.; Miralles, M. P.; van
Ballegooijen, A. A.
Bibcode: 2010ASPC..428..209C
Altcode: 2010arXiv1002.0297C
This paper reviews our growing understanding of the physics behind
coronal heating (in open-field regions) and the acceleration of the
solar wind. Many new insights have come from the last solar cycle's
worth of observations and theoretical work. Measurements of the
plasma properties in the extended corona, where the primary solar wind
acceleration occurs, have been key to discriminating between competing
theories. We describe how UVCS/SOHO measurements of coronal holes and
streamers over the last 14 years have provided clues about the detailed
kinetic processes that energize both fast and slow wind regions. We also
present a brief survey of current ideas involving the coronal source
regions of fast and slow wind streams, and how these change over the
solar cycle. These source regions are discussed in the context of recent
theoretical models (based on Alfvén waves and MHD turbulence) that have
begun to successfully predict both the heating and acceleration in fast
and slow wind regions with essentially no free parameters. Some new
results regarding these models—including a quantitative prediction
of the lower density and temperature at 1 AU seen during the present
solar minimum in comparison to the prior minimum—are also shown.
Title: Variations in the Absolute Ultraviolet Intensities of Polar
Coronal Holes
Authors: Gardner, L. D.; Kohl, J. L.; Cranmer, S. R.; Lin, M.;
Panasyuk, A. V.; Uzzo, M.
Bibcode: 2010ASPC..428..191G
Altcode:
The well-maintained UVCS/SOHO radiometric calibration is used
to determine the variations in the ultraviolet intensities in
polar coronal holes between the Solar Cycle 22/23 and Cycle 23/24
minima. The radiometric calibration has been carefully monitored
and updated during the mission by observing an ensemble of B stars,
which, as a group, are believed to have a stable mean irradiance. These
observations, along with data from the freshly calibrated Ultraviolet
Coronal Spectrometer on the Spartan 201 satellite in 1998, have been
used to determine the changes in the calibration. The Spartan 201
inter-calibration, together with the original laboratory calibration,
was used to establish the in-flight absolute radiometric calibration
scale. This paper summarizes the in-flight radiometric calibration of
UVCS/SOHO and observed variations in polar coronal hole intensities
as a function of heliographic height above the poles of the Sun.
Title: The Tale of Two Minima and a Solar Cycle in Between: An
Ongoing Fast Solar Wind Investigation
Authors: Miralles, M. P.; Cranmer, S. R.; Panasyuk, A. V.; Uzzo, M.
Bibcode: 2010ASPC..428..237M
Altcode: 2010arXiv1005.3242M
We have measured the physical properties of polar coronal holes from
the minimum activity phase of solar cycle 23 (1996-1997) to the present
minimum of solar cycle 24 (2007-2009) using the UVCS instrument on
SOHO. Observations in H I Lyman alpha (121.6 nm) and O VI (103.2, 103.7
nm) provide spectroscopic diagnostics of proton and O5+
bulk outflow velocities and velocity distributions as a function of
heliocentric distance above the poles of the Sun. These observations
have allowed us to follow the changes in the physical properties of
the polar coronal holes during solar cycle 23 and its approach to
the current minimum. Recent ground- and space-based observations
have reported a variety of phenomena associated with the current
minimum. We present the comparison of observed oxygen line intensities,
line ratios, and profiles for polar coronal holes at both minima and
during solar cycle 23 and show how this new minimum manifests itself in
the ultraviolet corona. The comparison of the physical properties of
these two minima as seen by UVCS in the extended corona, now possible
for the first time, may provide crucial empirical constraints on models
of extended coronal heating and acceleration for the fast solar wind.
Title: A Model for the Stray-Light Contamination of the UVCS
Instrument on SOHO
Authors: Cranmer, S. R.; Gardner, L. D.; Kohl, J. L.
Bibcode: 2010SoPh..263..275C
Altcode: 2010arXiv1001.3843C; 2010SoPh..tmp...31C
We present a detailed model of stray-light suppression in the
spectrometer channels of the Ultraviolet Coronagraph Spectrometer (UVCS)
on the SOHO spacecraft. The control of diffracted and scattered stray
light from the bright solar disk is one of the most important tasks of
a coronagraph. We compute the fractions of light that diffract past the
UVCS external occulter and non-specularly pass into the spectrometer
slit. The diffracted component of the stray light depends on the
finite aperture of the primary mirror and on its figure. The amount
of non-specular scattering depends mainly on the micro-roughness of
the mirror. For reasonable choices of these quantities, the modeled
stray-light fraction agrees well with measurements of stray light
made both in the laboratory and during the UVCS mission. The models
were constructed for the bright H I Lyα emission line, but they are
applicable to other spectral lines as well.
Title: The Impact of Accretion on Young Stellar Atmospheres
Authors: Brickhouse, Nancy S.; Dupree, A.; Luna, J.; Cranmer, S.;
Wolk, S.
Bibcode: 2010HEAD...11.1717B
Altcode: 2010BAAS...42R.685B
The 489 ks Chandra HETG/ACIS-S observation of the classical T Tauri
star TW Hydrae has provided a wealth of spectroscopic diagnostics
not available in lower signal-to-noise ratio observations. Using line
ratios for electron temperature, electron density, and column density
we have found that the shock produced by the accelerating material in
the accretion stream behaves as predicted by standard theory. However,
the properties of the post-shock plasma differ substantially from the
predictions of standard 1D shock models (Brickhouset et al. 2010). The
accretion process apparently heats the stellar atmosphere up to
soft X-ray emitting temperatures, providing hot ions to populate the
magnetic corona, in loops, stellar wind, and/or jets. This gas is highly
turbulent, as evidenced by non-thermal line broadening. The observed
properties of accretion-fed corona should constrain theoretical models
of an accretion-driven dynamo.
Title: Modeling the Preferential Acceleration and Heating of Coronal
Hole O5+ as Measured by UVCS/SOHO
Authors: Isenberg, Philip A.; Vasquez, Bernard J.; Cranmer, Steven R.
Bibcode: 2010AIPC.1216...56I
Altcode:
We have recently presented a mechanism for preferential acceleration
and heating of coronal hole minor ions [1, 2]. The energization is
due to the effect of multiple cyclotron resonances in the presence of
sunward and anti-sunward dispersive ion cyclotron waves, providing a
second-order Fermi interaction. The mechanism is preferential because
coronal hole protons do not experience such multiple resonances. The
detailed model results depend on many parameters, including poorly-known
quantities such as the wave intensities, spectral shapes and radial
profiles. In this paper, we show that reasonable choices for these
quantities can yield excellent agreement with the observations. We find
that only a small fraction of the extrapolated wave power is needed
to provide the observed heating, and there is an indication that the
resonant wave levels are increasing with radial position between r =
2 Rs and 4 Rs.
Title: Tangled Magnetic Fields in Solar Prominences
Authors: van Ballegooijen, A. A.; Cranmer, S. R.
Bibcode: 2010ApJ...711..164V
Altcode: 2010arXiv1001.2757V
Solar prominences are an important tool for studying the structure and
evolution of the coronal magnetic field. Here we consider so-called
hedgerow prominences, which consist of thin vertical threads. We
explore the possibility that such prominences are supported by tangled
magnetic fields. A variety of different approaches are used. First,
the dynamics of plasma within a tangled field is considered. We find
that the contorted shape of the flux tubes significantly reduces
the flow velocity compared to the supersonic free fall that would
occur in a straight vertical tube. Second, linear force-free models
of tangled fields are developed, and the elastic response of such
fields to gravitational forces is considered. We demonstrate that the
prominence plasma can be supported by the magnetic pressure of a tangled
field that pervades not only the observed dense threads but also their
local surroundings. Tangled fields with field strengths of about 10 G
are able to support prominence threads with observed hydrogen density
of the order of 1011 cm-3. Finally, we suggest
that the observed vertical threads are the result of Rayleigh-Taylor
instability. Simulations of the density distribution within a prominence
thread indicate that the peak density is much larger than the average
density. We conclude that tangled fields provide a viable mechanism
for magnetic support of hedgerow prominences.
Title: Direct Evidence for an Accretion-Fed Corona
Authors: Dupree, Andrea K.; Brickhouse, N. S.; Cranmer, S.;
Schneider, E.
Bibcode: 2010HEAD...11.1707D
Altcode: 2010BAAS...42R.683D
High resolution optical spectra of the nearby T Tauri star TW Hya
were obtained with MIKE on Magellan/Clay to be simultaneous with
a long CHANDRA observation. The optical spectra can be used to
evaluate the veiling which is thought to arise from accretion. There
is a correlation between the amount of veiling and the coronal X-ray
emission. The variation in the coronal emission follows variation of
the optical continuum veiling with a significant time delay (about 2.4
hours). This suggests that accretion processes can feed the stellar
corona, and corroborates the HETG spectrum reported by Brickhouse et al
(2009). In addition, measurements of the veiling continuum produced
by the hotspot in the photosphere of TW Hya, indicate temperatures
ranging from 8500 K - 12000 K with small covering fractions.
Title: Heating of the solar wind with electron and proton effects
Authors: Breech, Ben; Cranmer, Steven R.; Matthaeus, William H.;
Kasper, Justin C.; Oughton, Sean
Bibcode: 2010AIPC.1216..214B
Altcode:
We examine the effects of including effects of both protons
and electrons on the heating of the fast solar wind through two
different approaches. In the first approach, we incorporate the
electron temperature in an MHD turbulence transport model for the solar
wind. In the second approach, we adopt more empirically based methods by
analyzing the measured proton and electron temperatures to calculate the
heat deposition rates. Overall, we conclude that incorporating separate
proton and electron temperatures and heat conduction effects provides
an improved and more complete model of the heating of the solar wind.
Title: A Deep Chandra X-Ray Spectrum of the Accreting Young Star
TW Hydrae
Authors: Brickhouse, N. S.; Cranmer, S. R.; Dupree, A. K.; Luna,
G. J. M.; Wolk, S.
Bibcode: 2010ApJ...710.1835B
Altcode: 2010arXiv1001.0750B
We present X-ray spectral analysis of the accreting young star TW Hydrae
from a 489 ks observation using the Chandra High Energy Transmission
Grating. The spectrum provides a rich set of diagnostics for electron
temperature Te , electron density Ne , hydrogen
column density NH , relative elemental abundances, and
velocities, and reveals its source in three distinct regions of the
stellar atmosphere: the stellar corona, the accretion shock, and a
very large extended volume of warm postshock plasma. The presence of
Mg XII, Si XIII, and Si XIV emission lines in the spectrum requires
coronal structures at ~10 MK. Lower temperature lines (e.g., from O
VIII, Ne IX, and Mg XI) formed at 2.5 MK appear more consistent with
emission from an accretion shock. He-like Ne IX line ratio diagnostics
indicate that Te = 2.50 ± 0.25 MK and Ne =
3.0 ± 0.2 × 1012 cm-3 in the shock. These
values agree well with standard magnetic accretion models. However,
the Chandra observations significantly diverge from current model
predictions for the postshock plasma. This gas is expected to cool
radiatively, producing O VII as it flows into an increasingly dense
stellar atmosphere. Surprisingly, O VII indicates Ne =
5.7+4.4 -1.2 × 1011 cm-3,
5 times lower than Ne in the accretion shock itself
and ~7 times lower than the model prediction. We estimate that the
postshock region producing O VII has roughly 300 times larger volume
and 30 times more emitting mass than the shock itself. Apparently,
the shocked plasma heats the surrounding stellar atmosphere to soft
X-ray emitting temperatures and supplies this material to nearby
large magnetic structures—which may be closed magnetic loops or open
magnetic field leading to mass outflow. Our model explains the soft
X-ray excess found in many accreting systems as well as the failure to
observe high Ne signatures in some stars. Such accretion-fed
coronae may be ubiquitous in the atmospheres of accreting young stars.
Title: An Efficient Approximation of the Coronal Heating Rate for
use in Global Sun-Heliosphere Simulations
Authors: Cranmer, Steven R.
Bibcode: 2010ApJ...710..676C
Altcode: 2009arXiv0912.5333C
The origins of the hot solar corona and the supersonically expanding
solar wind are still the subject of debate. A key obstacle in the
way of producing realistic simulations of the Sun-heliosphere system
is the lack of a physically motivated way of specifying the coronal
heating rate. Recent one-dimensional models have been found to reproduce
many observed features of the solar wind by assuming the energy comes
from Alfvén waves that are partially reflected, then dissipated by
magnetohydrodynamic turbulence. However, the nonlocal physics of wave
reflection has made it difficult to apply these processes to more
sophisticated (three-dimensional) models. This paper presents a set
of robust approximations to the solutions of the linear Alfvén wave
reflection equations. A key ingredient of the turbulent heating rate
is the ratio of inward-to-outward wave power, and the approximations
developed here allow this to be written explicitly in terms of local
plasma properties at any given location. The coronal heating also
depends on the frequency spectrum of Alfvén waves in the open-field
corona, which has not yet been measured directly. A model-based
assumption is used here for the spectrum, but the results of future
measurements can be incorporated easily. The resulting expression
for the coronal heating rate is self-contained, computationally
efficient, and applicable directly to global models of the corona
and heliosphere. This paper tests and validates the approximations
by comparing the results to exact solutions of the wave transport
equations in several cases relevant to the fast and slow solar wind.
Title: Discovery of an Accretion-Fed Corona in an Accreting Young Star
Authors: Wolk, Scott J.; Brickhouse, N.; Cranmer, S.; Dupree, A.;
Luna, G. J. M.
Bibcode: 2010AAS...21542905W
Altcode: 2010BAAS...42..350W
A deep (489 ks) Chandra High Energy Transmission Grating spectrum
of the classical T Tauri star TW Hydrae shows a new type of coronal
structure that is produced by the accretion process. In the standard
model for a stellar dipole, the magnetic field truncates the disk and
channels the accreting material onto the star. The He-like diagnostic
lines of Ne IX provide excellent agreement with the shock conditions
predicted by this model, with an electron temperature of 2.5 MK and
electron density of 3 times 1012 cm-3 (see also
Kastner et al. 2002). However, the standard model completely fails to
predict the post-shock conditions, significantly overpredicting both
the density and absorption observed at O VII. Instead the observations
require a second "post-shock" component with 30 times more mass and
1000 times larger volume than found at the shock itself. We note that
in the standard model, the shocked plasma is conveniently located near
both closed (coronal) and open (stellar wind) magnetic structures,
as the magnetic field connecting the star and disk also separates the
open and closed field regions on the stellar surface. The shocked
plasma thus can provide the energy to heat not only the post-shock
plasma, but also adjacent regions (i.e. an "accretion-fed corona")
and drive stellar material into surrounding coronal structures. These
observations provide new clues to the puzzling soft X-ray excess found
in accreting systems, which depends on both the presence of accretion
and the level of coronal activity (Guedel and Telleschi 2007). This
work is partially supported by CXO grant G07-8018X.
Title: TW Hya: A Simultaneous Optical and X-Ray Campaign
Authors: Dupree, Andrea K.; Brickhouse, N. S.; Cranmer, S. R.; Irwin,
J.; Bessell, M. S.; Crause, L. A.; Lawson, W. A.; Luna, J.; Mallik,
S. V.; Pallavicini, R.; Schuler, S. C.
Bibcode: 2010AAS...21542904D
Altcode: 2010BAAS...42..350D
A world-wide campaign of spectroscopy and photometry was carried out
for 17 days in February- March 2007 (JD 2454147 - 2454164) in support
of an extended CHANDRA HETG observation of the nearby accreting T
Tauri star: TW Hya (CD -34 7151).This program included photometry from
Super WASP-South and SAAO. Spectroscopy was obtained from TNG/SARG,
Vainu Bappu Observatory, SAAO, MSSO, Magellan/MIKE, Pico do Dios,
and Gemini-S. The photometric period of the star derived from the
periodogram of WASP-S photometry during this time was 4.76+/-0.01
d. Hα fluxes do not appear to correlate well with the photometric
period nor the total X-ray flux, perhaps influenced by flaring that
occurred in both optical and X-ray sequences during this time. Hα
profiles from TW Hya can change dramatically during a night, with
substantial systematic changes in the wind opacity signaled both in Hα
and the He I 10830 Å transition. Related posters by Schneider et al.,
and Wolk et al. address the optical veiling and X-ray spectrum of TW
Hya from this program. Research supported in part by NASA and
the Smithsonian Astrophysical Observatory.
Title: Testing Models of Accretion-Driven Coronal Heating and Stellar
Wind Acceleration for T Tauri Stars
Authors: Cranmer, Steven R.
Bibcode: 2009ApJ...706..824C
Altcode: 2009arXiv0910.2686C
Classical T Tauri stars are pre-main-sequence objects that undergo
simultaneous accretion, wind outflow, and coronal X-ray emission. The
impact of plasma on the stellar surface from magnetospheric accretion
streams is likely to be a dominant source of energy and momentum
in the upper atmospheres of these stars. This paper presents a set
of models for the dynamics and heating of three distinct regions
on T Tauri stars that are affected by accretion: (1) the shocked
plasmas directly beneath the magnetospheric accretion streams, (2)
stellar winds that are accelerated along open magnetic flux tubes,
and (3) closed magnetic loops that resemble the Sun's coronal active
regions. For the loops, a self-consistent model of coronal heating
was derived from numerical simulations of solar field-line tangling
and turbulent dissipation. Individual models are constructed for the
properties of 14 well-observed stars in the Taurus-Auriga star-forming
region. Predictions for the wind mass-loss rates are, on average,
slightly lower than the observations, which suggests that disk winds
or X-winds may also contribute to the measured outflows. For some of
the stars, however, the modeled stellar winds do appear to contribute
significantly to the measured mass fluxes. Predictions for X-ray
luminosities from the shocks and loops are in general agreement with
existing observations. The stars with the highest accretion rates tend
to have X-ray luminosities dominated by the high-temperature (5-10 MK)
loops. The X-ray luminosities for the stars having lower accretion
rates are dominated by the cooler accretion shocks.
Title: Electron and proton heating by solar wind turbulence
Authors: Breech, B.; Matthaeus, W. H.; Cranmer, S. R.; Kasper, J. C.;
Oughton, S.
Bibcode: 2009JGRA..114.9103B
Altcode: 2009JGRA..11409103B; 2009arXiv0907.4074B
Previous formulations of heating and transport associated with strong
magnetohydrodynamic (MHD) turbulence are generalized to incorporate
separate internal energy equations for electrons and protons. Electron
heat conduction is included. Energy is supplied by turbulent heating
that affects both electrons and protons and is exchanged between
them via collisions. Comparison to available Ulysses data shows that
a reasonable accounting for the data is provided when (1) the energy
exchange timescale is very long and (2) the deposition of heat due to
turbulence is divided, with 60% going to proton heating and 40% into
electron heating. Heat conduction, determined here by an empirical fit,
plays a major role in describing the electron data.
Title: Empirical Constraints on Proton and Electron Heating in the
Fast Solar Wind
Authors: Cranmer, Steven R.; Matthaeus, William H.; Breech, Benjamin
A.; Kasper, Justin C.
Bibcode: 2009ApJ...702.1604C
Altcode: 2009arXiv0907.2650C
We analyze measured proton and electron temperatures in the high-speed
solar wind in order to calculate the separate rates of heat deposition
for protons and electrons. When comparing with other regions of the
heliosphere, the fast solar wind has the lowest density and the least
frequent Coulomb collisions. This makes the fast wind an optimal testing
ground for studies of collisionless kinetic processes associated with
the dissipation of plasma turbulence. Data from the Helios and Ulysses
plasma instruments were collected to determine mean radial trends in the
temperatures and the electron heat conduction flux between 0.29 and 5.4
AU. The derived heating rates apply specifically for these mean plasma
properties and not for the full range of measured values around the
mean. We found that the protons receive about 60% of the total plasma
heating in the inner heliosphere, and that this fraction increases to
approximately 80% by the orbit of Jupiter. A major factor affecting
the uncertainty in this fraction is the uncertainty in the measured
radial gradient of the electron heat conduction flux. The empirically
derived partitioning of heat between protons and electrons is in rough
agreement with theoretical predictions from a model of linear Vlasov
wave damping. For a modeled power spectrum consisting only of Alfvénic
fluctuations, the best agreement was found for a distribution of
wavenumber vectors that evolves toward isotropy as distance increases.
Title: Coronal Holes
Authors: Cranmer, Steven R.
Bibcode: 2009LRSP....6....3C
Altcode: 2009arXiv0909.2847C
Coronal holes are the darkest and least active regions of the Sun,
as observed both on the solar disk and above the solar limb. Coronal
holes are associated with rapidly expanding open magnetic fields and
the acceleration of the high-speed solar wind. This paper reviews
measurements of the plasma properties in coronal holes and how these
measurements are used to reveal details about the physical processes
that heat the solar corona and accelerate the solar wind. It is still
unknown to what extent the solar wind is fed by flux tubes that remain
open (and are energized by footpoint-driven wave-like fluctuations),
and to what extent much of the mass and energy is input intermittently
from closed loops into the open-field regions. Evidence for both
paradigms is summarized in this paper. Special emphasis is also given
to spectroscopic and coronagraphic measurements that allow the highly
dynamic non-equilibrium evolution of the plasma to be followed as
the asymptotic conditions in interplanetary space are established
in the extended corona. For example, the importance of kinetic
plasma physics and turbulence in coronal holes has been affirmed by
surprising measurements from the UVCS instrument on SOHO that heavy
ions are heated to hundreds of times the temperatures of protons and
electrons. These observations point to specific kinds of collisionless
Alfvén wave damping (i.e., ion cyclotron resonance), but complete
theoretical models do not yet exist. Despite our incomplete knowledge
of the complex multi-scale plasma physics, however, much progress has
been made toward the goal of understanding the mechanisms ultimately
responsible for producing the observed properties of coronal holes.
Title: Discovery of an Accretion-Fed Corona in an Accreting Young Star
Authors: Brickhouse, Nancy; Cranmer, S. R.; Dupree, A. K.; Luna,
G. J. M.; Wolk, S.
Bibcode: 2009cfdd.confE...8B
Altcode:
A deep (489 ks) Chandra High Energy Transmission Grating spectrum of the
classical T Tauri star TW Hydrae shows a new type of coronal structure
that is produced by the accretion process. In the standard model for a
stellar dipole, the magnetic field truncates the disk and channels the
accreting material onto the star. The He-like diagnostic lines of Ne
IX provide excellent agreement with the shock conditions predicted by
this model, with an electron temperature of 2.5 MK and electron density
of 3 × 10^{12} cm^{-3} (see also Kastner et al. 2002). However, the
standard model completely fails to predict the post-shock conditions,
significantly overpredicting both the density and absorption observed
at O VII. Instead the observations require a second ``post-shock''
component with 30 times more mass and 1000 times larger volume than
found at the shock itself. We note that in the standard model, the
shocked plasma is conveniently located near both closed (coronal)
and open (stellar wind) magnetic structures, as the magnetic field
connecting the star and disk also separates the open and closed field
regions on the stellar surface. The shocked plasma thus can provide
the energy to heat not only the post-shock plasma, but also adjacent
regions (i.e. an ``accretion-fed corona'') and drive stellar material
into surrounding coronal structures. These observations provide new
clues to the puzzling soft X-ray excess found in accreting systems,
which depends on both the presence of accretion and the level of
coronal activity (Guedel and Telleschi 2007). This Large Program with
Chandra demonstrates the value of high signal-to-noise, high resolution
spectroscopy for understanding the complex interaction of magnetic
and accretion processes in late-type star formation.
Title: A Pulsational Mechanism for Producing Keplerian Disks Around
Be Stars
Authors: Cranmer, Steven R.
Bibcode: 2009ApJ...701..396C
Altcode: 2009arXiv0906.2772C
Classical Be stars are an enigmatic subclass of rapidly rotating hot
stars characterized by dense equatorial disks of gas that have been
inferred to orbit with Keplerian velocities. Although these disks seem
to be ejected from the star and not accreted, there is substantial
observational evidence to show that the stars rotate more slowly than
required for centrifugally driven mass loss. This paper develops an idea
(proposed originally by Hiroyasu Ando and colleagues) that nonradial
stellar pulsations inject enough angular momentum into the upper
atmosphere to spin up a Keplerian disk. The pulsations themselves are
evanescent in the stellar photosphere, but they may be unstable to the
generation of resonant oscillations at the acoustic cutoff frequency. A
detailed theory of the conversion from pulsations to resonant waves
does not yet exist for realistic hot-star atmospheres, so the current
models depend on a parameterized approximation for the efficiency of
wave excitation. Once resonant waves have been formed, however, they
grow in amplitude with increasing height, steepen into shocks, and
exert radial and azimuthal Reynolds stresses on the mean fluid. Using
reasonable assumptions for the stellar parameters, these processes were
found to naturally create the inner boundary conditions required for
dense Keplerian disks, even when the underlying B-star photosphere is
rotating as slowly as 60% of its critical rotation speed. Because there
is evidence for long-term changes in Be-star pulsational properties,
this model may also account for the long-term variability of Be stars,
including transitions between normal, Be, and shell phases.
Title: Modeling the Preferential Acceleration and Heating of Coronal
Hole O5+ as Measuredb byUVCS/SOHO
Authors: Isenberg, Philip A.; Vasquez, Bernard J.; Cranmer, Steven R.
Bibcode: 2009shin.confE.127I
Altcode:
We have recently presented a mechanism for preferential acceleration
and heating of coronal hole minor ions [Isenberg & Vasquez, ApJ,
2009]. The energization is due to the effect of multiple cyclotron
resonances in the presence of sunward and anti-sunward dispersive ion
cyclotron waves, providing a second-order Fermi interaction. The
mechanism is preferential because coronal hole protons do not
experience such multiple resonances. The detailed model results
depend on many parameters, including poorly-known quantities such
as the wave intensities, spectral shapes and radial profiles. For
this reason, Isenberg & Vasquez explored the effects of a range
of assumptions for the waves, to provide the broadest background for
more specific models. In this paper, we show that reasonable choices
for the poorly-known quantities can yield excellent agreement with the
UVCS observations in collisionless coronal holes. Since preferential
heating is also observed in coronal streamers, this mechanism may
also be applicable under the more collisional conditions appropriate
to sources of the slow solar wind.
Title: Testing and Refining Models of Slow Solar Wind Acceleration
Authors: Cranmer, Steven R.
Bibcode: 2009shin.confE.129C
Altcode:
It is now well known that the low-speed solar wind appears to be
connected with a wide range of source regions in the corona (essentially
everything except the largest coronal holes). Evidence is growing for
there being specific, measurable differences in the plasma properties of
slow wind streams that originate in large quiescent streamers versus
those that originate in active regions. These differences are key
diagnostics of the physical processes that heat the open-field corona
and accelerate the slow wind. This talk will focus on describing recent
successes of theoretical models that involve waves and turbulence as the
primary driver. However, it is important not to neglect intermittent
energy addition from closed-field regions as well. Progress will come
both from working out these individual ideas in more detail (i.e.,
pushing them toward greater accuracy, self-consistency, and predictive
power) and from putting multiple ideas together in 'sandbox' models that
allow the relative contributions of these processes to be determined.
Title: Ion Heating in the Solar Corona and Solar Wind
Authors: Cranmer, Steven
Bibcode: 2009APS..APR.D6003C
Altcode:
The solar corona is the hot, ionized outer atmosphere of the
Sun that expands into interplanetary space as a supersonic solar
wind. This tenuous medium is a unique laboratory for the study of
magnetohydrodynamics (MHD) and plasma physics with ranges of parameters
that are inaccessible on Earth. The last decade has seen significant
progress toward identifying and characterizing the processes that
heat the corona and accelerate the solar wind, but the basic physics
is still unclear. Some key clues about the mechanisms responsible for
energizing the plasma have come from UV spectroscopy of the extended
corona (i.e., using a combination of an occulting coronagraph and a
spectrometer). There is evidence for preferential acceleration of heavy
ions in the fast solar wind, ion temperatures exceeding 100 million K,
and marked departures from Maxwellian velocity distributions. These
collisionless departures from thermal equilibrium point to specific
types of kinetic processes. This presentation reviews the measurements
(both telescopic and from `in situ' probes) that constrain theoretical
explanations and provides a current survey of the landscape of proposed
ideas for ion energization. Many of the suggested processes are related
to the dissipation of MHD waves (e.g., ion cyclotron waves), and many
involve multiple steps of energy conversion between waves, turbulence,
current sheets, and other nonlinear plasma features. A discussion
of future measurements that could help to test, refine, and possibly
winnow down the list of competing models will also be presented.
Title: Accretion-driven winds of T Tauri stars: A new generation of
models with self-consistent coronal heating and MHD turbulence
Authors: Cranmer, Steven R.
Bibcode: 2009AIPC.1094..357C
Altcode: 2009csss...15..357C
Classical T Tauri stars are observed to be surrounded by both accretion
flows and some kind of wind or jet-like outflow. There are several
possible explanations of how and where the outflows arise, including
disk winds, X-winds, impulsive (CME-like) ejections, and stellar
winds. Recent work by Matt and Pudritz has suggested that if there is
a stellar wind with a mass loss rate about 0.1 times the accretion
rate, the wind may be able to carry away enough angular momentum to
keep the stars from being spun up unrealistically by accretion. In
this presentation, I show a preliminary set of theoretical models of
accretion-driven winds from the polar regions of T Tauri stars. These
models are based on recently published self-consistent simulations of
the Sun's coronal heating and wind acceleration. In addition to the
convection-driven MHD turbulence (which dominates in the solar case),
I add a source of wave energy at the photosphere that is driven by the
impact of plasma in neighboring flux tubes undergoing magnetospheric
accretion. This added energy, which is determined quantitatively from
the far-field theory of MHD wave generation, seems to be enough to
produce T Tauri-like mass loss rates. It is still uncertain, though,
whether it is enough to solve the T Tauri angular momentum problem.
Title: Star-Planet Interactions
Authors: Shkolnik, Evgenya; Aigrain, Suzanne; Cranmer, Steven; Fares,
Rim; Fridlund, Malcolm; Pont, Frederic; Schmitt, Jürgen; Smith,
Alexis; Suzuki, Takeru
Bibcode: 2009AIPC.1094..275S
Altcode: 2009csss...15..275S; 2008arXiv0809.4482S
Much effort has been invested in recent years, both observationally and
theoretically, to understand the interacting processes taking place
in planetary systems consisting of a hot Jupiter orbiting its star
within 10 stellar radii. Several independent studies have converged
on the same scenario: that a short-period planet can induce activity
on the photosphere and upper atmosphere of its host star. The growing
body of evidence for such magnetic star-planet interactions includes
a diverse array of photometric, spectroscopic and spectropolarimetric
studies. The nature of which is modeled to be strongly affected by both
the stellar and planetary magnetic fields, possibly influencing the
magnetic activity of both bodies, as well as affecting irradiation and
non-thermal and dynamical processes. Tidal interactions are responsible
for the circularization of the planet orbit, for the synchronization of
the planet rotation with the orbital period, and may also synchronize
the outer convective envelope of the star with the planet. Studying such
star-planet interactions (SPI) aids our understanding of the formation,
migration and evolution of hot Jupiters. In this proceeding,
we briefly summarise the observations and theories presented during
the Cool Stars 15 splinter session of this diverse and growing field
of star-planet interactions.
Title: Mass Transport Processes and their Roles in the Formation,
Structure, and Evolution of Stars and Stellar Systems
Authors: Carpenter, Kenneth G.; Karvovska, Margarita; Schrijver,
Carolus J.; Grady, Carol A.; Allen, Ronald J.; Brown, Alexander;
Cranmer, Steven R.; Dupree, Andrea K.; Evans, Nancy R.; Guinan,
Edward F.; Harper, Graham; Labeyrie, Antoine; Linsky, Jeffrey;
Peters, Geraldine J.; Roberge, Aki; Saar, Steven H.; Sonneborn,
George; Walter, Frederick M.
Bibcode: 2009astro2010S..40C
Altcode: 2009arXiv0903.2433C
We summarize some of the compelling new scientific opportunities
for understanding stars and stellar systems that can be enabled
by sub-mas angular resolution, UV/Optical spectral imaging
observations, which can reveal the details of the many dynamic
processes (e.g., variable magnetic fields, accretion, convection,
shocks, pulsations, winds, and jets) that affect their formation,
structure, and evolution. These observations can only be provided
by long-baseline interferometers or sparse aperture telescopes in
space, since the aperture diameters required are in excess of 500 m -
a regime in which monolithic or segmented designs are not and will
not be feasible - and since they require observations at wavelengths
(UV) not accessible from the ground. Two mission concepts which could
provide these invaluable observations are NASA's Stellar Imager (SI;
http://hires.gsfc.nasa.gov/si/) interferometer and ESA's Luciola
sparse aperture hypertelescope, which each could resolve hundreds
of stars and stellar systems. These observatories will also open an
immense new discovery space for astrophysical research in general and,
in particular, for Active Galactic Nuclei (Kraemer et al. Decadal
Survey Science Whitepaper). The technology developments needed for
these missions are challenging, but eminently feasible (Carpenter et
al. Decadal Survey Technology Whitepaper) with a reasonable investment
over the next decade to enable flight in the 2025+ timeframe. That
investment would enable tremendous gains in our understanding of the
individual stars and stellar systems that are the building blocks of our
Universe and which serve as the hosts for life throughout the Cosmos.
Title: Ion Temperatures in the Low Solar Corona: Polar Coronal Holes
at Solar Minimum
Authors: Landi, E.; Cranmer, S. R.
Bibcode: 2009ApJ...691..794L
Altcode: 2008arXiv0810.0018L
In the present work we use a deep-exposure spectrum taken by the
SUMER spectrometer in a polar coronal hole in 1996 to measure the ion
temperatures of a large number of ions at many different heights above
the limb between 0.03 and 0.17 solar radii. We find that the measured
ion temperatures are almost always larger than the electron temperatures
and exhibit a nonmonotonic dependence on the charge-to-mass ratio. We
use these measurements to provide empirical constraints to a theoretical
model of ion heating and acceleration based on gradually replenished
ion-cyclotron waves. We compare the wave power required to heat the ions
to the observed levels to a prediction based on a model of anisotropic
magnetohydrodynamic turbulence. We find that the empirical heating model
and the turbulent cascade model agree with one another, and explain
the measured ion temperatures, for charge-to-mass ratios smaller than
about 0.25. However, ions with charge-to-mass ratios exceeding 0.25
disagree with the model; the wave power that they require to be heated
to the measured ion temperatures shows an increase with charge-to-mass
ratio (i.e., with increasing frequency) that cannot be explained by
a traditional cascade model. We discuss possible additional processes
that might be responsible for the inferred surplus of wave power.
Title: Heating the Solar Wind Through Turbulence and Electron Heat
Conduction Modelling
Authors: Breech, B.; Matthaeus, W.; Cranmer, S.; Kasper, J.;
Oughton, S.
Bibcode: 2008AGUFMSH41C..05B
Altcode:
We employ a turbulence transport model to explore the heating of the
solar wind by turbulent dissipation, including, for the time, separate
equations for heating of electrons and protons. Heating occurs through
the deposition of internal energy from kinetic effects that terminate
the MHD cascade at small scales. We utilize a simple transport model
for three turbulence quantities -- the energy per unit mass, the cross
helicity or Alfvénicity, and a similarity length scale. The model
includes a von Karman -- Taylor phenomenological model for turbulent
dissipation, which modifies the electron and proton temperatures. The
involvement of the electron temperature raises several new and
interesting issues; How should the electron heat flux be modeled? How
long is the collision time between protons and electrons? How much
turbulence dissipation goes into heating the electrons and how much goes
into heating the protons? Using Voyager and Ulysses observational data,
we begin to explore these issues. We find that the inclusion of electron
conduction effects provides a more complete description of the solar
wind plasma and may help explain the observed temperature profiles.
Title: Turbulence-driven Polar Winds from T Tauri Stars Energized
by Magnetospheric Accretion
Authors: Cranmer, Steven R.
Bibcode: 2008ApJ...689..316C
Altcode: 2008arXiv0808.2250C
Pre-main-sequence stars are observed to be surrounded by both accretion
flows and some kind of wind or jetlike outflow. Recent work by Matt and
Pudritz has suggested that if classical T Tauri stars exhibit stellar
winds with mass-loss rates about 0.1 times their accretion rates,
the wind can carry away enough angular momentum to keep the stars
from being spun up unrealistically by accretion. This paper presents
a preliminary set of theoretical models of accretion-driven winds from
the polar regions of T Tauri stars. These models are based on recently
published self-consistent simulations of the Sun's coronal heating and
wind acceleration. In addition to the convection-driven MHD turbulence
(which dominates in the solar case), we add another source of wave
energy at the photosphere that is driven by the impact of plasma
in neighboring flux tubes undergoing magnetospheric accretion. This
added energy, determined quantitatively from the far-field theory of
MHD wave generation, is sufficient to produce T Tauri-like mass-loss
rates of at least 0.01 times the accretion rate. While still about
an order of magnitude below the level required for efficient angular
momentum removal, these are the first self-consistent models of T
Tauri winds that agree reasonably well with a range of observational
mass-loss constraints. The youngest modeled stellar winds are supported
by Alfvén wave pressure, they have low temperatures ("extended
chromospheres"), and they are likely to be unstable to the formation
of counterpropagating shocks and clumps far from the star.
Title: Plasmoids in reconnecting current sheets: Solar and terrestrial
contexts compared
Authors: Lin, J.; Cranmer, S. R.; Farrugia, C. J.
Bibcode: 2008JGRA..11311107L
Altcode: 2008arXiv0809.3755L
Magnetic reconnection plays a crucial role in violent energy conversion
occurring in the environments of high electrical conductivity, such as
the solar atmosphere, magnetosphere, and fusion devices. We focus on
the morphological features of the process in two different environments,
the solar atmosphere and the geomagnetic tail. In addition to indirect
evidence that indicates reconnection in progress or having just
taken place, such as auroral manifestations in the magnetosphere and
the flare loop system in the solar atmosphere, more direct evidence
of reconnection in the solar and terrestrial environments is being
collected. Such evidence includes the reconnection inflow near the
reconnecting current sheet and the outflow along the sheet characterized
by a sequence of plasmoids. Both turbulent and unsteady Petschek-type
reconnection processes could account for the observations. We also
discuss other relevant observational consequences of both mechanisms
in these two settings. While on face value, these are two completely
different physical environments, there emerge many commonalities,
for example, an Alfvén speed of the same order of magnitude, a key
parameter determining the reconnection rate. This comparative study
is meant as a contribution to current efforts aimed at isolating
similarities in processes occurring in very different contexts in the
heliosphere and even in the universe.
Title: Hyperdiffusion as a Mechanism for Solar Coronal Heating
Authors: van Ballegooijen, A. A.; Cranmer, S. R.
Bibcode: 2008ApJ...682..644V
Altcode: 2008arXiv0802.1751V
A theory for the heating of coronal magnetic flux ropes is
developed. The dissipated magnetic energy has two distinct
contributions: (1) energy injected into the corona as a result
of granule-scale, random footpoint motions and (2) energy from the
large-scale, nonpotential magnetic field of the flux rope. The second
type of dissipation can be described in terms of hyperdiffusion, a type
of magnetic diffusion in which the helicity of the mean magnetic field
is conserved. The associated heating rate depends on the gradient of
the torsion parameter of the mean magnetic field. A simple model of
an active region containing a coronal flux rope is constructed. We
find that the temperature and density on the axis of the flux rope are
lower than in the local surroundings, consistent with observations of
coronal cavities. The model requires that the magnetic field in the
flux rope be stochastic in nature, with a perpendicular length scale
of the magnetic fluctuations of the order of 1000 km.
Title: Improved Constraints on the Preferential Heating and
Acceleration of Oxygen Ions in the Extended Solar Corona
Authors: Cranmer, Steven R.; Panasyuk, Alexander V.; Kohl, John L.
Bibcode: 2008ApJ...678.1480C
Altcode: 2008arXiv0802.0144C
We present a detailed analysis of oxygen ion velocity distributions
in the extended solar corona, based on observations made with
the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO
spacecraft. Polar coronal holes exhibit broad line widths and unusual
intensity ratios of the O VI λλ1032, 1037 emission-line doublet. A
traditional interpretation of these features is that oxygen ions have
a strong temperature anisotropy, with the temperature perpendicular
to the magnetic field being much larger than the temperature parallel
to the field. However, recent work by Raouafi and Solanki suggested it
may be possible to model the observations using an isotropic velocity
distribution. In this paper we analyze an expanded data set to show
that the original interpretation of an anisotropic distribution is the
only one fully consistent with the observations. It is necessary to
search the full range of ion plasma parameters to determine the values
with the highest probability of agreement with the UVCS data. The
derived ion outflow speeds and perpendicular kinetic temperatures
are consistent with earlier results, and there continues to be strong
evidence for preferential ion heating and acceleration with respect to
hydrogen. At heliocentric heights above 2.1 solar radii, every UVCS
data point is more consistent with an anisotropic distribution than
with an isotropic distribution. At heights above 3 solar radii, the
exact probability of isotropy depends on the electron density chosen
to simulate the line-of-sight distribution of O VI emissivity. The
most realistic electron densities (which decrease steeply from 3 to
6 solar radii) produce the lowest probabilities of isotropy and most
probable temperature anisotropy ratios that exceed 10.
Title: Turbulence and Waves as Sources for the Solar Wind
Authors: Cranmer, S. R.
Bibcode: 2008AGUSMSH34B..03C
Altcode:
Gene Parker's insights from 50 years ago provided the key causal link
between energy deposition in the solar corona and the acceleration
of solar wind streams. However, the community is still far from
agreement concerning the actual physical processes that give rise to
this energy. It is still unknown whether the solar wind is fed by flux
tubes that remain open (and are energized by footpoint-driven wavelike
fluctuations) or if mass and energy is input more intermittently
from closed loops into the open-field regions. No matter the relative
importance of reconnections and loop-openings, though, we do know that
waves and turbulent motions are present everywhere from the photosphere
to the heliosphere, and it is important to determine how they affect
the mean state of the plasma. In this presentation, I will give a
summary of wave/turbulence models that seem to succeed in explaining
the time-steady properties of the corona (and the fast and slow
solar wind). The coronal heating and solar wind acceleration in these
models comes from anisotropic turbulent cascade, which is driven by
the partial reflection of low-frequency Alfven waves propagating along
the open magnetic flux tubes. Specifically, a 2D model of coronal holes
and streamers at solar minimum reproduces the latitudinal bifurcation
of slow and fast streams seen by Ulysses. The radial gradient of the
Alfven speed affects where the waves are reflected and damped, and thus
whether energy is deposited below or above Parker's critical point. As
predicted by earlier studies, a larger coronal expansion factor gives
rise to a slower and denser wind, higher temperature at the coronal
base, less intense Alfven waves at 1 AU, and correlative trends
for commonly measured ratios of ion charge states and FIP-sensitive
abundances that are in general agreement with observations. Finally,
I will outline the types of future observations that would be most
able to test and refine these ideas.
Title: UVCS/SOHO Search for Coronal Suprathermal Seed Particles:
Results for Solar Minimum
Authors: Kohl, J. L.; Panasyuk, A. V.; Cranmer, S. R.; Gardner, L. D.;
Raymond, J. C.
Bibcode: 2008AGUSMSH41B..05K
Altcode:
The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and
Heliospheric Observatory is being used to measure precise coronal H I
Ly-alpha spectral line profiles out to several Doppler half widths. Such
observations can be used to reveal the proton velocity distribution
along the line-of-sight. Departures from a Maxwellian distribution are
believed to be needed for the acceleration of solar energetic particles
(SEPs) by coronal mass ejection (CME) shocks. We have now completed
all required instrument characterizations and refined the observations
to the point where it is possible to distinguish a Gaussian coronal
velocity distribution from a power law tail of sufficient strength
to accelerate SEPs. It is generally believed that the required seed
particle population needed to produce SEPs of interest with a CME shock
would have a velocity distribution with 0.001 to 0.01 of the particles
with speeds that exceed 1000 km/s. Assuming a kappa distribution that
is symmetric in the tangential plane and Maxwellian in the radial
direction, this would correspond to a distribution with kappa = 3.5
or smaller. We show that UVCS observations can distinguish a Gaussian
from a kappa of 4 or less. This paper will report the results of
examining observations during 2006-2008 and report on any departures
from a Maxwellian distribution. It will also report the CME/flare
predecessor history of each observation. This work is supported by
the National Aeronautics and Space Administration (NASA) under Grant
NNX07AL72G to the Smithsonian Astrophysical Observatory.
Title: Winds of Main-Sequence Stars: Observational Limits and a Path
to Theoretical Prediction
Authors: Cranmer, S. R.
Bibcode: 2008ASPC..384..317C
Altcode: 2008csss...14..317C; 2007astro.ph..1561C
It is notoriously difficult to measure the winds of solar-type
stars. Traditional spectroscopic and radio continuum techniques are
sensitive to mass loss rates at least two to three orders of magnitude
stronger than the Sun's relatively feeble wind. Much has been done with
these methods to probe the more massive outflows of younger (T Tauri)
and older (giant, AGB, supergiant) cool stars, but the main sequence
remains terra incognita. This presentation reviews the limits on
traditional diagnostics and outlines more recent ideas such as Lyman
alpha astrospheres and charge-exchange X-ray emission. In addition,
there are hybrid constraints on mass loss rates that combine existing
observables and theoretical models. The Sackmann/Boothroyd conjecture
of a more massive young Sun (and thus a much stronger ZAMS wind) is
one such idea that needs to be tested further. Another set of ideas
involves a strong proposed coupling between coronal heating and stellar
mass loss rates, where the former is easier to measure in stars down
to solar-like values. The combined modeling of stellar coronae and
stellar winds is developing rapidly, and it seems to be approaching
a level of development where the only remaining ``free parameters''
involve the sub-photospheric convection. This talk will also summarize
these theoretical efforts to predict the properties of solar-type
main-sequence winds.
Title: On Competing Models of Coronal Heating and Solar Wind
Acceleration: The Debate in '08
Authors: Cranmer, Steven R.
Bibcode: 2008arXiv0804.3058C
Altcode:
In preparation for lively debate at the May 2008 SPD/AGU Meeting in
Fort Lauderdale, this document attempts to briefly lay out my own view
of the evolving controversy over how the solar wind is accelerated. It
is still unknown to what extent the solar wind is fed by flux tubes
that remain open (and are energized by footpoint-driven wavelike
fluctuations), and to what extent much of the mass and energy is input
more intermittently from closed loops into the open-field regions. It
may turn out that a combination of the two ideas is needed to explain
the full range of observed solar wind phenomena.
Title: Next generation UV coronagraph instrumentation for solar
cycle-24
Authors: Kohl, John L.; Jain, Rajmal; Cranmer, Steven R.; Gardner,
Larry D.; Pradhan, Anil K.; Raymond, John C.; Strachan, Leonard
Bibcode: 2008JApA...29..321K
Altcode:
No abstract at ADS
Title: Hydrogen Lyman Alpha Spectral Line Profiles in Coronal Holes
from 1.5 - 6.5 Solar Radii
Authors: Suleiman, R. M.; Kohl, J. L.; Cranmer, S. R.
Bibcode: 2007AGUFMSH21A0285S
Altcode:
UVCS has made detailed measurements of H I Lyα spectral line profiles
in a polar coronal hole at projected heliocentric heights from 3.5
to 6.5 R\odot during 1998 January 5 -- 11. Similar polar coronal
hole measurements were made during 1998 June 16 -- 21. Earlier UVCS
observations obtained at 1.5 to 2.5 R\odot are used for comparison. In
addition, new measurements are being made for the current phase of
the solar cycle. From these measurements we derive 1/e half widths of
coronal velocity distributions at the observed heights. The velocity
distribution includes all motions contributing to the Doppler shifts
along the line of sight (LOS). We also measure absolute intensities that
can be used to derive outflow speeds via a Doppler dimming analysis. At
large heights in coronal holes, the outflowing coronal plasma becomes
nearly collisionless and the ionization balance is believed to become
frozen. H I Lyα profile measurements characterize the neutral hydrogen
velocity distribution, which at lower heights can also be used to
describe the proton distribution. However, in the regions above
3~R\odot, the H0 velocity distribution may not be the same as that
for the protons because the characteristic time for charge transfer
between H0 and protons becomes longer than the time it takes for the
plasma to flow through a density scale height. Hence, the H0 velocity
distribution may not be directly affected by transverse wave motion
or wave damping. An indication of an adiabatic radial decrease in the
neutral hydrogen temperature would indicate a decoupling of the protons
and neutral hydrogen, and also indicate the absence of mechanisms that
would heat the neutrals. We will compare the observations with the
predictions of a theoretical model of the combined electron, proton and
neutral hydrogen plasma. Such a comparison could lead to an improved
knowledge of the proton heating at these large heights. This work is
supported by the National Aeronautics and Space Administration (NASA)
under Grant NNX07AL72G to the Smithsonian Astrophysical Observatory.
Title: UVCS Empirical Constraints on Theoretical Models of Solar
Wind Source Regions
Authors: Kohl, J. L.; Cranmer, S. R.; Miralles, M. P.; Panasyuk, A.;
Strachan, L.
Bibcode: 2007AGUFMSH22B..02K
Altcode:
Spectroscopic observations from the Ultraviolet Coronagraph
Spectrometer (UVCS) on the Solar and Heliospheric Observatory
(SOHO) have resulted in empirical models of polar coronal holes,
polar plumes, coronal jets and streamers. These findings have
been used to make significant progress toward identifying and
characterizing the physical processes that produce extended heating
in the corona and accelerate fast and slow solar wind streams. The
UVCS scientific observations, which began in April 1996 and continue
at this writing, have provided determinations of proton and minor ion
temperatures (including evidence for anisotropic microscopic velocity
distributions in coronal holes and quiescent equatorial streamers),
outflow velocities, and elemental abundances. The variations in these
quantities over the solar cycle also have been determined. For example,
observations of large polar coronal holes at different phases of the
solar cycle indicate that line width is positively correlated with
outflow speed and anti-correlated with electron density. This paper
will review these results, and present new results from measurements
taken as the current solar activity cycle approaches solar minimum. The
results regarding preferential ion heating and acceleration of heavy
ions (i.e., O5+) in polar coronal holes have contributed in a major
way to the advances in understanding solar wind acceleration that
have occurred during the past decade. It is important to verify and
confirm the key features of these findings. Hence, the results from a
new analysis of an expanded set of UVCS data from polar coronal holes
at solar minimum by S. R. Cranmer, A. Panasyuk and J. L. Kohl will be
presented. This work has been supported by the National Aeronautics
and Space Administration (NASA) under Grants NNG06G188G and NNX07AL72G
and NNX06AG95G to the Smithsonian Astrophysical Observatory.
Title: Polar Coronal Jets During the 2007 Joint SOHO/Hinode Campaigns
Authors: Miralles, M. P.; Cranmer, S. R.; Raymond, J. C.; Kohl, J. L.
Bibcode: 2007AGUFMSH21B..02M
Altcode:
We will present ultraviolet spectroscopy of polar coronal jets obtained
by the Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) during the
two SOHO/Hinode observing campaigns (9-21 January and 12-20 March,
2007) for the north and south polar coronal holes. The emphasis is on
identifying and tracing polar jets from the solar surface out into the
accelerating solar wind and determining their physical properties as a
function of height and time. UVCS/SOHO observed ultraviolet counterparts
in the extended corona of the hot jets resolved by Hinode/XRT. These
polar jets observed by UVCS have different characteristics in the
acceleration region of the solar wind than the cooler jets identified
at the last solar minimum by LASCO, UVCS, and EIT. Observations
such as these are needed to clarify the relationship between the
episodic jets, the longer-lived polar plumes, and the fast solar
wind. This work is supported by NASA grants NNX06AG95G and NNX07AL72G
to the Smithsonian Astrophysical Observatory. SOHO is a project of
international cooperation between ESA and NASA.
Title: Measurements of Coronal Proton Velocity Distributions
Authors: Kohl, J. L.; Panasyuk, A.; Cranmer, S. R.; Gardner, L. D.;
Raymond, J. C.
Bibcode: 2007AGUFMSH21A0298K
Altcode:
The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and
Heliospheric Observatory is being used to measure precise coronal H I
Ly-alpha spectral line profiles out to several Doppler half widths. Such
observations can be used to reveal the proton velocity distribution
along the line-of-sight. Departures from a Maxwellian distribution are
believed to be needed for the acceleration of solar energetic particles
(SEPs) by coronal mass ejection (CME) shocks. Our initial attempt to
measure suprathermal proton velocity distributions has been described
by Kohl et al. (2006). We have made considerable additional progress
on such measurements since then. Improvements include the following:
a much more accurate instrument spectral line profile, an increase
in the wavelength range used for the observations, an increase in the
statistical accuracy of the observations by increasing the observation
time, and inclusion of a background measurement as part of every set of
observations. We have also investigated the sensitivity to the detector
high voltage, investigated the effects of diffraction in the instrument,
determined the stray light effects and the Thompson scattering
effects, which both turn out to be small except for scattering of Si
III 120.6 nm. That scattered light is out of the primary wavelength
range of interest. We believe that we have now demonstrated that
UVCS has the sensitivity to distinguish between a Gaussian coronal
velocity distribution and a kappa = 4 or smaller distribution. It is
generally believed that the required seed particle population needed
to produce SEPs of interest with a CME shock would have a velocity
distribution with 0.001 to 0.01 of the particles with speeds that
exceed 1000 km/s. Assuming a kappa distribution that is symmetric
in the tangential plane and Maxwellian in the radial direction, this
would correspond to a distribution with kappa = 3.5 or smaller. This
paper will report the results of examining a fairly large body of new
observations obtained with the new procedure and report the departures
from a Maxwellian distribution. It will also report the CME predecessor
history of each observation. This work is supported by the National
Aeronautics and Space Administration (NASA) under Grant NNX07AL72G to
the Smithsonian Astrophysical Observatory. Kohl J. L., Cranmer, S. R.,
Fineschi, S., Gardner, L. D., Phillips, D. H., Raymond, J. C., and Uzzo,
M., Proc. SOHO 17 - 10 Years of SOHO and Beyond (ESA SP-617, July 2006).
Title: Self-consistent Coronal Heating and Solar Wind Acceleration
from Anisotropic Magnetohydrodynamic Turbulence
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.; Edgar,
Richard J.
Bibcode: 2007ApJS..171..520C
Altcode: 2007astro.ph..3333C
We present a series of models for the plasma properties along open
magnetic flux tubes rooted in solar coronal holes, streamers, and active
regions. These models represent the first self-consistent solutions
that combine (1) chromospheric heating driven by an empirically guided
acoustic wave spectrum; (2) coronal heating from Alfvén waves that have
been partially reflected, then damped by anisotropic turbulent cascade;
and (3) solar wind acceleration from gradients of gas pressure, acoustic
wave pressure, and Alfvén wave pressure. The only input parameters
are the photospheric lower boundary conditions for the waves and the
radial dependence of the background magnetic field along the flux
tube. We have not included multifluid or collisionless effects (e.g.,
preferential ion heating), which are not yet fully understood. For a
single choice for the photospheric wave properties, our models produce a
realistic range of slow and fast solar wind conditions by varying only
the coronal magnetic field. Specifically, a two-dimensional model of
coronal holes and streamers at solar minimum reproduces the latitudinal
bifurcation of slow and fast streams seen by Ulysses. The radial
gradient of the Alfvén speed affects where the waves are reflected
and damped, and thus whether energy is deposited below or above the
Parker critical point. As predicted by earlier studies, a larger
coronal ``expansion factor'' gives rise to a slower and denser wind,
higher temperature at the coronal base, less intense Alfvén waves at
1 AU, and correlative trends for commonly measured ratios of ion charge
states and FIP-sensitive abundances that are in general agreement with
observations. These models offer supporting evidence for the idea that
coronal heating and solar wind acceleration (in open magnetic flux
tubes) can occur as a result of wave dissipation and turbulent cascade.
Title: Turbulence in the solar corona
Authors: Cranmer, Steven R.
Bibcode: 2007AIPC..932..327C
Altcode:
The solar corona has been revealed in the past decade to be a highly
dynamic nonequilibrium plasma environment. Both the loop-filled coronal
base and the extended acceleration region of the solar wind appear to
be strongly turbulent, but direct observational evidence for a cascade
of fluctuation energy from large to small scales is lacking. In this
paper I will review the observations of wavelike motions in the corona
over a wide range of scales, as well as the macroscopic effects of
wave-particle interactions such as preferential ion heating. I will
also present a summary of recent theoretical modeling efforts that
seem to explain the time-steady properties of the corona (and the fast
and slow solar wind) in terms of an anisotropic MHD cascade driven
by the partial reflection of low-frequency Alfvén waves propagating
along the superradially expanding solar magnetic field. Complete
theoretical models are difficult to construct, though, because many of
the proposed physical processes act on a multiplicity of spatial scales
(from centimeters to solar radii) with feedback effects not yet well
understood. This paper is thus a progress report on various attempts
to couple these disparate scales.
Title: Multi-Instrument Searches for Polar Jets: Characterizing Jet
Heating and Cooling
Authors: Miralles, M. P.; Cranmer, S. R.; Raymond, J. C.; Kohl, J. L.
Bibcode: 2007IUGG...24..691M
Altcode:
We will present the preliminary results obtained with SOHO, in
particular UVCS, during the SOHO-TRACE-Hinode coordinated observation
campaign (8-21 January 2007) for the north and south polar coronal
holes. The emphasis is on identifying and tracing polar jets from the
solar surface out into the accelerating solar wind and determining
their physical properties as a function of height and time. It is still
unclear whether the hot jets resolved by Hinode are the same phenomenon
as the cooler jets identified at the last solar minimum by EIT, LASCO,
and UVCS. We also aim to clarify the relationship between the episodic
jets and the longer-lived polar plumes.
Title: Towards the UVCS Coronal Hole Atlas for Solar Cycle 23:
The Data
Authors: Miralles, Mari Paz; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2007AAS...210.3005M
Altcode: 2007BAAS...39..143M
We have measured with the SOHO Ultraviolet Coronagraph Spectrometer
(UVCS) the physical properties of over 165 large coronal holes that
produced a variety of high-speed solar wind conditions at 1 AU. This
data set includes observations of coronal holes of different sizes and
latitudes having a range of magnetic field properties. We will
present measured parameters (intensities, line widths) for protons and
oxygen ions, as well as electron density as constrained by polarization
brightness (pB), for the coronal holes observed from the last solar
minimum in 1996 to the present. We will also present selected portions
of in-situ solar wind data that are matched, where possible, to source
regions in the corona, which have been characterized by UVCS. The
UVCS atlas of coronal holes will provide information about the H I
Lyman alpha and O VI line profiles in the extended corona. The atlas
will be made available in electronic format, with machine readable
tables, as a supplement to a leading archival journal. This work
is supported by NASA under Grants NNX06AG95G and NNG06GI88G to the
Smithsonian Astrophysical Observatory, by the Italian Space Agency,
and by PRODEX (Swiss contribution).
Title: Exoplanet-Induced Chromospheric Activity: Realistic Light
Curves from Solar-type Magnetic Fields
Authors: Cranmer, Steven R.; Saar, Steven H.
Bibcode: 2007astro.ph..2530C
Altcode:
There is growing observational evidence for some kind of interaction
between stars and close-in extrasolar giant planets. Shkolnik et
al. reported variability in the chromospheric Ca H and K lines of
HD 179949 and upsilon And that seemed to be phased with the planet's
orbital period, instead of the stellar rotational period. However, the
observations also indicate that the chromospheric light curves do not
repeat exactly, which may be expected for a planet plowing through a
variable stellar magnetic field. Using the complex solar magnetic field
(modeled with the Potential Field Source Surface technique) as a guide,
we simulate the shapes of light curves that would arise from planet-star
interactions that are channeled along magnetic field lines. We also
study the orbit-to-orbit variability of these light curves and how they
vary from solar minimum (i.e., a more or less axisymmetric stretched
dipole) to solar maximum (a superposition of many higher multipole
moments) fields. Considering more complex magnetic fields introduces
new difficulties in the interpretation of observations, but it may
also lead to valuable new diagnostics of exoplanet magnetospheres.
Title: The Structured Chromosphere and Wind of TW Hya
Authors: Dupree, A. K.; Avrett, E. H.; Brickhouse, N. S.; Cranmer,
S. R.; Szalai, T.
Bibcode: 2007astro.ph..2395D
Altcode:
A continuous set of echelle spectra of TW Hya, taken with the MIKE
spectrograph on the Magellan2/Clay telescope at Las Campanas Observatory
in April 2006 reveals systematic variations in the flux, velocity,
and profile of the H-alpha emission line which appear to be consistent
with the photometric period of 2.8 days. Absorption features recur
at high outflow velocities in the wind. This behavior suggests that:
(a) accretion is not uniformly distributed over the stellar hemisphere
in view; (b) stable structures are present in the chromosphere, most
likely due to the stellar magnetic field configuration. Semi-empirical
models of the atmosphere are constructed to reproduce line profiles
of H-alpha and He I, 10830A and to define the wind structure. These
preliminary calculations suggest the mass loss rate is variable and
comparable to H-alpha mass accretion rates in the literature, requiring
a very efficient mechanism if the wind is powered only by accretion.
Title: 2006 LWS TR & T Solar Wind Focused Science Topic Team:
The Beginnings
Authors: Miralles, M. P.; Bhattacharjee, A.; Landi, E.; Markovskii,
S.; Cranmer, S. R.; Doschek, G. A.; Forbes, T. G.; Isenberg, P. A.;
Kohl, J. L.; Ng, C.; Raymond, J. C.; Vasquez, B. J.
Bibcode: 2006AGUFMSH11A0371M
Altcode:
The Solar Wind Focused Science Topic (FST) team was created to
apply a combination of theoretical studies, numerical simulations,
and observations to the understanding of how the fast and slow solar
wind are heated and accelerated. Four proposals were selected for this
FST team. They will investigate the role of energy sources and kinetic
mechanisms responsible for the heating and acceleration of the solar
wind. In particular, the FST team will examine magnetic reconnection
and turbulence as possible heating mechanisms. Plasma properties and
their evolution over the solar cycle, determined from the analysis of
remote and in situ measurements, will be used to put firm constraints
on the models. The work of the Solar Wind FST team is in its initial
stages. The organization, planning, and findings resulting from the
first FST team meeting will be reported.
Title: Coronal Hole Properties During the First Decade of UVCS/SOHO
Authors: Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2006ESASP.617E..15M
Altcode: 2006soho...17E..15M
No abstract at ADS
Title: Progress Toward Measurements of Suprathermal Tails in Coronal
Proton Velocity Distributions
Authors: Kohl, J. L.; Panazyuk, A. V.; Cranmer, S. R.; Fineschi, S.;
Gardner, L. D.; Phillips, D. H.; Raymond, J. C.; Uzzo, M.
Bibcode: 2006ESASP.617E..25K
Altcode: 2006soho...17E..25K
No abstract at ADS
Title: The First Decade of UVCS/SOHO: Coronal Hole Properties
Authors: Miralles, Mari Paz; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2006SPD....37.1003M
Altcode: 2006SPD....37.1003P; 2006BAAS...38R.237M
We have measured with the SOHO Ultraviolet Coronagraph Spectrometer
(UVCS) the physical properties of over 155 large coronal holes that
produced a variety of high-speed solar wind conditions at 1 AU. This
data set includes observations of coronal holes of different sizes
and latitudinal locations present throughout solar cycle 23.In the
spring of 2006, the polar coronal holes have not yet evolved to the
fully quiescent minimum state seen in 1996-1997, although the next
solar minimum is expected to occur during 2006-2007. We will present
the solar cycle dependence of the derived plasma parameters for O5+
and protons from the last solar minimum in 1996 to present and compare
them, where possible, with the in situ solar wind properties.This
work is supported by NASA under Grant NNG05GG38G tothe Smithsonian
Astrophysical Observatory, by the Italian Space Agency, and by PRODEX
(Swiss contribution).
Title: Turbulence And Wave Dissipation In The Chromosphere, Corona,
And Solar Wind
Authors: Cranmer, Steven R.
Bibcode: 2006SPD....37.2101C
Altcode: 2006BAAS...38..249C
The continually evolving convection below the solar photospheregives
rise to a wide spectrum of acoustic and magneticfluctuations
that propagate out into the heliosphere.In this talk I will
review the various ways that waves,shocks, and turbulent eddies
are expected to interact withthe mean plasma conditions of the
outer solar atmosphere.For open magnetic flux tubes, the heating
of the chromosphereand corona, as well as the acceleration of
the solar wind,can be understood from the standpoint of wave
dissipation andturbulent cascade.For example, the importance of
magnetohydrodynamic turbulence inthe extended corona has been affirmed
by the surprisingmeasurements of the UVCS instrument on SOHO that
heavy ions areheated to hundreds of times the temperatures of protons
andelectrons, indicating collisionless Alfven wave dissipation.Complete
theoretical models are difficult to construct, though,because many
of the proposed physical processes act on amultiplicity of spatial
scales (from centimeters to solar radii)with feedback effects not yet
well understood.Despite these difficulties, progress has been made
toward the goalof producing models that predict the plasma properties
everywhereabove the solar surface using only lower boundary conditions
atthe photosphere.
Title: Ultraviolet spectroscopy of the extended solar corona
Authors: Kohl, John L.; Noci, Giancarlo; Cranmer, Steven R.; Raymond,
John C.
Bibcode: 2006A&ARv..13...31K
Altcode: 2006A&ARv.tmp....1K
The first observations of ultraviolet spectral line profiles
and intensities from the extended solar corona (i.e., more than
1.5 solar radii from Sun-center) were obtained on 13 April 1979
when a rocket-borne ultraviolet coronagraph spectrometer of the
Harvard-Smithsonian Center for Astrophysics made direct measurements
of proton kinetic temperatures, and obtained upper limits on outflow
velocities in a quiet coronal region and a polar coronal hole. Following
those observations, ultraviolet coronagraphic spectroscopy has
expanded to include observations of over 60 spectral lines in
coronal holes, streamers, coronal jets, and solar flare/coronal
mass ejection (CME) events. Spectroscopic diagnostic techniques
have been developed to determine proton, electron and ion kinetic
temperatures and velocity distributions, proton and ion bulk flow
speeds and chemical abundances. The observations have been made during
three sounding rocket flights, four Shuttle deployed and retrieved
Spartan 201 flights, and the Solar and Heliospheric Observatory (SOHO)
mission. Ultraviolet spectroscopy of the extended solar corona has led
to fundamentally new views of the acceleration regions of the solar wind
and CMEs. Observations with the Ultraviolet Coronagraph Spectrometer
(UVCS) on SOHO revealed surprisingly large temperatures, outflow speeds,
and velocity distribution anisotropies in coronal holes, especially for
minor ions. Those measurements have guided theorists to discard some
candidate physical processes of solar wind acceleration and to increase
and expand investigations of ion cyclotron resonance and related
processes. Analyses of UVCS observations of CME plasma properties
and the evolution of CMEs have provided the following: temperatures,
inflow velocities and derived values of resistivity and reconnection
rates in CME current sheets, compression ratios and extremely high ion
temperatures behind CME shocks, and three dimensional flow velocities
and magnetic field chirality in CMEs. Ultraviolet spectroscopy has
been used to determine the thermal energy content of CMEs allowing the
total energy budget to be known for the first time. Such spectroscopic
observations are capable of providing detailed empirical descriptions
of solar energetic particle (SEP) source regions that allow theoretical
models of SEP acceleration to be tailored to specific events, thereby
enabling in situ measurements of freshly emitted SEPs to be used for
testing and guiding the evolution of SEP acceleration theory. Here we
review the history of ultraviolet coronagraph spectroscopy, summarize
the physics of spectral line formation in the extended corona, describe
the spectroscopic diagnostic techniques, review the advances in our
understanding of solar wind source regions and flare/CME events provided
by ultraviolet spectroscopy and discuss the scientific potential of
next generation ultraviolet coronagraph spectrometers.
Title: A Solar Energetic Particle Mission (SEPM) for the S3C Great
Observatory
Authors: Strachan, L.; Kohl, J. L.; Cranmer, S. R.; Esser, R.; Gardner,
L. D.; Lin, J.; Raymond, J. C.; van Ballegooijen, A.; Socker, D. G.
Bibcode: 2005AGUFMSH51C1221S
Altcode:
The S3C Great Observatory concept is guided by a systems approach
to understanding the heliosphere. A Solar Energetic Particle Mission
(SEPM) can make valuable contributions to this Great Observatory in
conjunction with upcoming Living with a Star (LWS) missions e.g. Solar
Dynamics Observatory and Sentinels. SEPM can provide the remote sensing
component to a program for better understanding how, when, and where
solar energetic particles (SEPs) are produced. Such a coordinated
approach will include coronagraphic UV spectroscopy and visible light
polarimetery with SEPM, along with in situ particles and fields,
X-ray and gamma-ray measurements from spacecraft close to the Sun
(Inner Heliospheric Sentinels). While SEPM will use plasma diagnostics
that are derived from the UVCS and LASCO coronagaraphs on SOHO, it
will have an order of magnitude improvement in its capabilities.
Title: An Initial Attempt to Measure Suprathermal Tails in Coronal
Proton Velocity Distributions
Authors: Kohl, J. L.; Cranmer, S. R.; Fineschi, S.; Gardner, L. D.;
Panasyuk, A. V.; Raymond, J. C.; Uzzo, M.
Bibcode: 2005AGUFMSH44A..05K
Altcode:
Test observations made with the Ultraviolet Coronagraph Spectrometer
(UVCS) on SOHO in August 2005 are being used to make an initial
assessment of the possibility of measuring suprathermal tails in the
proton velocity distribution functions. Any successful theory of solar
energetic particle (SEP) production by CME shocks must account for the
large observed variations in SEP spectral characteristics and elemental
abundances. Some have proposed that this variability arises from an
inherently variable population of suprathermal seed particles (e.g.,
Mason et al. 2005): some that exist all the time in the solar wind
(with varying properties depending on wind speed) and some that are
associated with prior flares and CME shocks (e.g., Kahler 2004; Tylka
et al. 2005). As yet, though, the suprathermal particle population in
the solar corona has never been measured. The seed particle number
density predicted for typical gradual SEP events is about 0.002 -
0.01 times the thermal population (e.g., Lee 2005), and should, in at
least some cases, correspond to a measurable enhancement in the wing
of the H I Ly-alpha profile. In August 2005, the Ly-alpha channel of
UVCS was recommissioned and used to observe HI Ly-alpha line profiles
at 2.0 solar radii in coronal holes, helmet streamers, above active
regions and after a CME. The holographically ruled diffraction grating
provides the low stray light level needed to observe the tail of the
line profile out to about 0.5 nm from line center. New observations
as well as measurements from the original UVCS laboratory calibration
and in flight measurements from earlier in the mission are being
used to characterize the instrument response to monochromatic light
so such instrument effects can be removed. Initial results will be
reported. This work is supported by NASA Grant NNG05GG38G to the
Smithsonian Astrophysical Observatory Kahler, S. W. 2004, ApJ, 603,
330. Lee, M. A. 2005, ApJ Supp., 158, 38. Mason, G., Desai, M.,
Mazur, J., & Dwyer, J. 2005, COSPAR 35th Scientific Assemly,
p. 1596. Tylka, A. J., et al. 2005, ApJ, 625, 474.
Title: Detailed Modeling of Fast Hot Winds from T Tauri Stars
Authors: Dupree, A. K.; Avrett, E. H.; Cranmer, S. R.
Bibcode: 2005AAS...207.7413D
Altcode: 2005BAAS...37.1286D
Infrared and ultraviolet spectra recently revealed the presence of
hot (300,000 K), fast (450 km s-1) winds from 2 classical
(accreting) T Tauri stars: TW Hya and T Tau. The mass loss rate
attributed to these outflows may be related to the mass accretion
rate, but this and other possible explanations can only be tested by
determining the mass loss rates, wind speeds, and temperature variations
in the outer atmospheres of these stars. In addition, a sufficiently
robust wind may lead to optical jets, could remove accreted angular
momentum from the star, might contribute to the opacity needed for
X-ray absorption, and may influence the diminution of dust in accretion
disks. We are constructing detailed models of wind-sensitive line
profiles for expanding and rotating atmospheres using both: (1) the
Avrett/Loeser PANDORA code for full non-LTE effects, and (2) a simpler
line-specific code that will allow complex three-dimensional mass
flows to be varied with minimal computational expense. These models
will constrain both the atmospheric structure and the mass loss rate
indicated by the observed P Cygni line profiles. This research
is supported in part by NASA and the Smithsonian Institution.
Title: A Statistical Study of Threshold Rotation Rates for the
Formation of Disks around Be Stars
Authors: Cranmer, Steven R.
Bibcode: 2005ApJ...634..585C
Altcode: 2005astro.ph..7718C
This paper presents a detailed statistical determination of the
equatorial rotation rates of classical Be stars. The rapid rotation
of Be stars is likely to be linked to the ejection of gas that forms
dense circumstellar disks. The physical origins of these disks are
not understood, although it is generally believed that the ability to
spin up matter into a Keplerian disk depends on how close the stellar
rotation speed is to the critical speed at which the centrifugal
force cancels gravity. There has been recent disagreement between the
traditional idea that Be stars rotate between 50% and 80% of their
critical speeds and new ideas (inspired by the tendency for gravity
darkening to mask rapid rotation at the equator) that their rotation
may be very nearly critical. This paper utilizes Monte Carlo forward
modeling to simulate distributions of the projected rotation speed
(vsini), taking into account gravity darkening, limb darkening, and
observational uncertainties. A χ2 minimization procedure
was used to find the distribution parameters that best reproduce
observed vsini distributions from R. Yudin's database. Early-type
(O7e-B2e) Be stars were found to exhibit a roughly uniform spread
of intrinsic rotation speed that extends from 40%-60% up to 100%
of critical. Late-type (B3e-A0e) Be stars exhibit progressively
narrower ranges of rotation speed as the effective temperature
decreases; the lower limit rises to reach critical rotation for the
coolest Be stars. The derived lower limits on equatorial rotation
speed represent conservative threshold rotation rates for the onset
of the Be phenomenon. The significantly subcritical speeds found for
early-type Be stars represent strong constraints on physical models
of angular momentum deposition in Be star disks.
Title: Why is the Fast Solar Wind Fast and the Slow Solar Wind
Slow? (Invited) A Survey of Geometrical Models
Authors: Cranmer, S. R.
Bibcode: 2005ESASP.592..159C
Altcode: 2005soho...16E..24C; 2005ESASP.592E..24C; 2005astro.ph..6508C
Four decades have gone by since the discovery that the solar wind
at 1 AU seems to exist in two relatively distinct states: slow and
fast. There is still no universal agreement concerning the primary
physical cause of this apparently bimodal distribution, even in its
simplest manifestation at solar minimum. In this presentation we review
and extend a series of ideas that link the different states of solar
wind to the varying superradial geometry of magnetic flux tubes in the
extended corona. Past researchers have emphasized different aspects of
this relationship, and we attempt to disentangle some of the seemingly
contradictory results. We apply the hypothesis of Wang and Sheeley
(as well as Kovalenko) that Alfven wave fluxes at the base are the
same for all flux tubes to a recent model of non-WKB Alfven wave
reflection and turbulent heating, and we predict coronal heating rates
as a function of flux tube geometry. We compare the feedback of these
heating rates on the locations of Parker-type critical points, and we
discuss the ranges of parameters that yield a realistic bifurcation
of wind solutions into fast and slow. Finally, we discuss the need
for next-generation coronagraph spectroscopy of the extended corona -
especially measurements of the electron temperature above 1.5 solar
radii - in order to confirm and refine these ideas.
Title: Capabilities of UV Coronagraphic Spectroscopy for Studying
the Source Regions of Solar Energetic Particles and the Solar Wind
Authors: Kohl, J. L.; Cranmer, S. R.; Gardner, L. D.; Lin, Jun;
Raymond, John C.; Strachan, Leonard
Bibcode: 2005ESASP.592..677K
Altcode: 2005astro.ph..6509K; 2005ESASP.592E.135K; 2005soho...16E.135K
We summarize the unique capabilities of UV coronagraphic spectroscopy
for determining the detailed plasma properties (e.g., density,
temperature, outflow speed, composition) of the source regions of both
transient phenomena such as CMEs, flares, and solar energetic particles
(SEPs) and more time-steady solar wind streams. UVCS/SOHO observations
have provided the first detailed diagnostics of the physical conditions
of CME plasma in the extended corona. It provided new insights into
the roles of shock waves, reconnection, and magnetic helicity in CME
eruptions. We summarize past observations and discuss the diagnostic
potential of UV coronagraphic spectroscopy for characterizing two
possible sites of SEP production: CME shocks and reconnection current
sheets. UVCS/SOHO has also led to fundamentally new views of the
acceleration region of the solar wind. Understanding the physical
processes in this region, which ranges from the low corona (1.1 to
1.5 Rsun) past the sonic points (greater than 5 Rsun), is key to
linking the results of solar imaging to in situ particle and field
detection. Despite the advances that have resulted from UVCS/SOHO,
more advanced instrumentation could determine properties of additional
ions with a wider sampling of charge/mass combinations. This would
provide much better constraints on the specific kinds of waves that are
present as well as the specific collisionless damping modes. Electron
temperatures and departures from Maxwellian velocity distributions
could also be measured. The instrumentation capable of making the
above observations will be described.
Title: Ultraviolet spectroscopy of solar energetic particle source
regions
Authors: Kohl, J. L.; Cranmer, S.; Esser, R.; Gardner, L. D.; Fineschi,
S.; Lin, J.; Panasyuk, A.; Raymond, J. C.; Strachan, L.
Bibcode: 2005SPIE.5901..262K
Altcode:
A problem of fundamental importance for future space travel to the
Moon and Mars is the determination and prediction of the radiation
environment generated by the Sun. The sources of solar energetic
particles (SEP) and the physical processes associated with their
acceleration and propagation are not well understood. Ultraviolet
coronagraphic spectroscopy uniquely has the capabilities for determining
the detailed plasma properties of the likely source regions of such
particles. This information can be used to develop empirical models
of the source regions for specific events, and it can provide the key
information needed to identify and understand the physical processes
that produce SEP hazards. UVCS/SOHO observations have provided the first
detailed diagnostics of the plasma parameters of coronal mass ejections
(CMEs) in the extended corona. These observations have provided new
insights into the roles of shock waves, reconnection and magnetic
helicity in CME eruptions. Next generation ultraviolet coronagraph
spectrometers could provide additional diagnostic capabilities. This
paper summarizes past observations, and discusses the diagnostic
potential of advanced ultraviolet coronagraphic spectroscopy for
characterizing two possible sites of SEP production: CME shocks and
reconnection current sheets.
Title: Solar Cycle Variations of Coronal Hole Properties
Authors: Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2005AGUSMSP51B..07M
Altcode:
As of early 2005, we have measured with the SOHO Ultraviolet
Coronagraph Spectrometer (UVCS) the physical properties of at least
136 large coronal holes that produced a variety of high-speed solar
wind conditions at 1 AU. UVCS has been used to observe O VI (103.2 and
103.7 nm) and H I Lyman alpha (121.6 nm) emission lines as a function
of heliocentric distance in coronal holes since 1996. The analysis of
their spectroscopic parameters allows us to identify similarities
and differences among coronal holes at different phases of the
solar cycle. From such measurements we can derive plasma parameters
(densities, temperatures, velocity distribution anisotropies, and
outflow speeds) for O5+ and protons as a function of heliocentric
distance in the coronal holes. These properties, combined with other
observed quantities such as white-light polarization brightness and the
more-or-less unipolar magnetic fluxes measured on-disk, let us map out
the "allowed parameter space" of coronal hole plasma properties more
fully than ever before. We will present the solar cycle dependence
of the above plasma parameters from the last solar minimum in 1996
to present and compare them, where possible, with the in situ solar
wind properties. We will also present an update on the pattern that
is beginning to emerge, i.e., coronal holes with lower densities at
a given heliocentric distance tend to exhibit faster ion outflow and
higher ion temperatures. This information will thus be used to set firm
empirical constraints on coronal heating and solar wind acceleration in
coronal holes. In 2005, the polar coronal holes have not yet evolved
to the fully quiescent minimum state seen in 1996-1997, though the
next solar minimum is expected to occur in about 1.5 to 2 years. This
work is supported by NASA under Grant NNG04GE84G to the Smithsonian
Astrophysical Observatory, by the Italian Space Agency, and by PRODEX
(Swiss contribution).
Title: Towards a Universal Physics-based "Coronal Heating Function"
for Electrons, Protons, and Heavy Ions in the Accelerating Solar Wind
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2005AGUSMSH51A..03C
Altcode:
The Sun is often highlighted as a benchmark for the study of other
stars, and as a stepping stone to the study of galaxies and cosmic
distances. Not to be outdone, the solar wind is rapidly becoming a key
baseline for the understanding of basic plasma phenomena such as MHD
turbulence, kinetic wave-particle interactions, and nonlinear wave-mode
coupling. In keeping with the IHY focus on these kinds of universal
processes, we present a distillation of recent modeling efforts to
understand how Alfven waves are generated, reflected, cascaded, and
damped throughout the solar wind. A physical understanding of solar
wind turbulence is crucial to the modeling of energetic particle
transport in the heliosphere and the interaction with interstellar
neutrals. The goal of this work is to derive a useful "recipe" for solar
wind modelers that, given the background zero-order plasma properties,
yields the wave amplitudes, the turbulent cascade rates, and the
kinetic partitioning of the resultant heating into electrons, protons,
and heavy ions (differentiating between parallel and perpendicular
heating as well). We also discuss preliminary ideas concerning how the
collisionless particle heating is modified if the turbulent cascade
ends with the production of small-scale reconnection current sheets.
Title: On the Incompatibility Between UVCS/SOHO Observations of
Polar Coronal Holes and Isotropic Oxygen Velocity Distributions
Authors: Cranmer, S. R.; Panasyuk, A. V.; Kohl, J. L.
Bibcode: 2005AGUSMSP33A..02C
Altcode:
We present a reanalysis of UVCS/SOHO observations of the O VI 1032,
1037 emission line doublet at large heliocentric distances in polar
coronal holes during the last solar minimum (1996-1997). The traditional
interpretation of the broad line widths and unusual intensity ratios
has been that the oxygen ions exhibit a strong temperature anisotropy,
with the temperature perpendicular to the magnetic field being much
larger than the temperature parallel to the field. However, a recent
paper by Raouafi and Solanki suggested that it may be possible to
model the observations using an isotropic velocity distribution of
(still very hot) oxygen ions. In this presentation we show that the
standard interpretation of an anisotropic distribution is the only
one that is fully consistent with the observational data. Using the
same electron density and magnetic field models assumed by Raouafi
and Solanki, we varied the 3 main ion properties (outflow speed and
the 2 bi-Maxwellian temperature components) in a 3D data-cube that
exhaustively treated all possibilities. This data-cube spans the
parameter space of both earlier UVCS/SOHO empirical models and the new
proposal of Raouafi and Solanki. Even so, we find that above about 2.5
solar radii the only points in the data-cube that reproduce the actual
observed line widths and intensity ratios are those with substantial
temperature anisotropies.
Title: New insights into solar wind physics from SOHO
Authors: Cranmer, S. R.
Bibcode: 2005ESASP.560..299C
Altcode: 2004astro.ph..9260C; 2005csss...13..299C
The Solar and Heliospheric Observatory (SOHO) was launched in
December 1995 with a suite of instruments designed to answer
long-standing questions about the Sun's internal structure, its
extensive outer atmosphere, and the solar wind. This paper reviews
the new understanding of the physical processes responsible for the
solar wind that have come from the past 8 years of SOHO observations,
analysis, and theoretical work. For example, the UVCS instrument on
SOHO has revealed the acceleration region of the fast solar wind to
be far from simple thermal equilibrium. Evidence for preferential
acceleration of ions, 100 million K ion temperatures, and marked
departures from Maxwellian velocity distributions all point to specific
types of collisionless heating processes. The slow solar wind, typically
associated with bright helmet streamers, has been found to share some of
the nonthermal characteristics of the fast wind. Abundance measurements
from spectroscopy and visible-light coronagraphic movies from LASCO
have led to a better census of the plasma components making up the slow
wind. The origins of the solar wind in the photosphere and chromosphere
have been better elucidated with disk spectroscopy from the SUMER and
CDS instruments. Finally, the impact of the solar wind on spacecraft
systems, ground-based technology, and astronauts has been greatly aided
by having continuous solar observations at the Earth-Sun L1 point,
and SOHO has set a strong precedent for future studies of space weather.
Title: On the Generation, Propagation, and Reflection of Alfvén
Waves from the Solar Photosphere to the Distant Heliosphere
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2005ApJS..156..265C
Altcode: 2004astro.ph.10639C
We present a comprehensive model of the global properties of Alfvén
waves in the solar atmosphere and the fast solar wind. Linear non-WKB
wave transport equations are solved from the photosphere to a distance
past the orbit of the Earth, and for wave periods ranging from 3 s
to 3 days. We derive a radially varying power spectrum of kinetic and
magnetic energy fluctuations for waves propagating in both directions
along a superradially expanding magnetic flux tube. This work differs
from previous models in three major ways. (1) In the chromosphere
and low corona, the successive merging of flux tubes on granular and
supergranular scales is described using a two-dimensional magnetostatic
model of a network element. Below a critical flux-tube merging height
the waves are modeled as thin-tube kink modes, and we assume that all
of the kink-mode wave energy is transformed into volume-filling Alfvén
waves above the merging height. (2) The frequency power spectrum of
horizontal motions is specified only at the photosphere, based on prior
analyses of G-band bright point kinematics. Everywhere else in the model
the amplitudes of outward and inward propagating waves are computed
with no free parameters. We find that the wave amplitudes in the
corona agree well with off-limb nonthermal line-width constraints. (3)
Nonlinear turbulent damping is applied to the results of the linear
model using a phenomenological energy loss term. A single choice for
the normalization of the turbulent outer-scale length produces both
the right amount of damping at large distances (to agree with in situ
measurements) and the right amount of heating in the extended corona (to
agree with empirically constrained solar wind acceleration models). In
the corona, the modeled heating rate differs by more than an order of
magnitude from a rate based on isotropic Kolmogorov turbulence.
Title: Simultaneous VLA and UVCS/SOHO Observations of the Solar Corona
Authors: Spangler, S. R.; Miralles, M. P.; Cranmer, S. R.; Raymond,
J. C.
Bibcode: 2004AAS...205.1008S
Altcode: 2004BAAS...36.1350S
Measurement of Faraday rotation of radio waves which propagate
through the solar corona is one of the best ways of measuring the
coronal magnetic field. Faraday rotation can provide information
on both the large scale, static component of this field as well as
the fluctuating, turbulent component, but the technique requires
supplementary information on the coronal plasma. On August 16, 2003,
the line of sight to the extended, polarized radio source 3C228
passed through the corona, with a closest heliocentric distance of
7 to 8 solar radii. Polarimetric observations with the Very Large
Array of the National Radio Astronomy Observatory were made at 21
and 18 cm. These data yielded measurements of the rotation measure
(proportional to the path integral of plasma density and line-of-sight
component of the magnetic field) along several, closely-spaced lines of
sight through the corona. Simultaneous observations of the OVI and HI
Lyman alpha emission lines with the Ultraviolet Coronagraph Spectrometer
(UVCS) aboard SOHO were used to determine kinetic temperature, average
densities, and outflow speeds in the corona. On that day, the line
of sight passed close to a coronal streamer. The VLA data show a very
large Faraday rotation event during the eight hour observing session,
in which the rotation measure changed by 62 rad/m2. This
large variation seems to be associated with passage of the coronal
current sheet through the line of sight. We will present models of
the coronal magnetic field consistent with our observations. This
work was supported at the University of Iowa by grant ATM03-54782 from
the Division of Atmospheric Sciences, National Science Foundation. At
Smithsonian Astrophysical Observatory, this work is supported by NASA
under grant NNG04GE84G, by the Italian Space Agency, and by PRODEX
(Swiss contribution).
Title: Coronal Heating Versus Solar Wind Acceleration
Authors: Cranmer, S. R.
Bibcode: 2004ESASP.575..154C
Altcode: 2004astro.ph..9724C; 2004soho...15..154C
Parker's initial insights from 1958 provided a key causal link
between the heating of the solar corona and the acceleration of the
solar wind. However, we still do not know what fraction of the solar
wind's mass, momentum, and energy flux is driven by Parker-type gas
pressure gradients, and what fraction is driven by, e.g., wave-particle
interactions or turbulence. SOHO has been pivotal in bringing these
ideas back to the forefront of coronal and solar wind research. This
paper reviews our current understanding of coronal heating in the
context of the acceleration of the fast and slow solar wind. For the
fast solar wind, a recent model of Alfven wave generation, propagation,
and non-WKB reflection is presented and compared with UVCS, SUMER,
radio, and in-situ observations at the last solar minimum. The derived
fractions of energy and momentum addition from thermal and nonthermal
processes are found to be consistent with various sets of observational
data. For the more chaotic slow solar wind, the relative roles of steady
streamer-edge flows (as emphasized by UVCS abundance analysis) versus
bright blob structures (seen by LASCO) need to be understood before the
relation between streamer heating and and slow-wind acceleration can be
known with certainty. Finally, this presentation summarizes the need for
next-generation remote-sensing observations that can supply the tight
constraints needed to unambiguously characterize the dominant physics.
Title: New views of the solar wind with the Lambert W function
Authors: Cranmer, Steven R.
Bibcode: 2004AmJPh..72.1397C
Altcode: 2004astro.ph..6176C
This paper presents closed-form analytic solutions to two illustrative
problems in solar physics that have been considered not solvable in
this way previously. Both the outflow speed and the mass loss rate
of the solar wind of plasma particles ejected by the Sun are derived
analytically for certain illustrative approximations. The calculated
radial dependence of the flow speed applies to both Parker's isothermal
solar wind equation and Bondi's equation of spherical accretion. These
problems involve the solution of transcendental equations containing
products of variables and their logarithms. Such equations appear
in many fields of physics and are solvable by use of the Lambert W
function, which is briefly described. This paper is an example of how
new functions can be applied to existing problems.
Title: Towards a Physical Characterization of Large Coronal Holes
Authors: Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2004AAS...204.7106M
Altcode: 2004BAAS...36S.797M
The Ultraviolet Coronagraph Spectrometer (UVCS) aboard SOHO was used
to observe O VI (103.2 and 103.7 nm) and H I Lyman alpha (121.6 nm)
emission lines as a function of heliocentric distance in more than 85
coronal holes, in order to characterize the physical properties of
coronal holes at different phases of the solar cycle. Our previous
analyses of UVCS observations have shown that polar and equatorial
coronal holes produce different acceleration profiles and have
different oxygen kinetic temperatures. We examine the variation
in the characteristics of representative large coronal holes
producing a variety of high-speed conditions at 1 AU. This
work is supported by NASA under Grant NAG5-12865 to the Smithsonian
Astrophysical Observatory, by the Italian Space Agency and by PRODEX
(Swiss contribution).
Title: Differences in Plasma Conditions Among 85 Large Coronal Holes
Authors: Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2004AGUSMSH41A..03M
Altcode:
We have measured ultraviolet spectroscopic parameters as a function
of heliocentric distance for more than 85 coronal holes, in order to
characterize the physical properties of coronal holes at different
phases of the solar cycle. The Ultraviolet Coronagraph Spectrometer
(UVCS) aboard SOHO was used to observed O VI (103.2 and 103.7 nm)
and H I Lyman alpha (121.6 nm) emission lines to determine kinetic
temperatures, average densities, and outflow speeds in coronal
holes. UVCS observations provide unique information on the heating and
acceleration processes in the corona. Our previous analyses of UVCS
observations have shown that solar minimum (polar) and solar maximum
(equatorial) coronal holes produce different acceleration profiles
and have different oxygen kinetic temperatures. We also examine the
differences in the characteristics of representative coronal holes
producing a variety of high-speed conditions (550-800 km/s) at 1
AU. These analyses provide limits on the coronal plasma properties
and put constraints on the physical processes that are responsible
for the heating of the extended corona and the acceleration of the
solar wind. This work is supported by NASA under Grant NAG5-12865 to
the Smithsonian Astrophysical Observatory, by the Italian Space Agency
and by PRODEX (Swiss contribution).
Title: An Overview of Alfven Wave Generation, Reflection, and Damping
from the Solar Photosphere to the Distant Heliosphere
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2004AAS...204.0401C
Altcode: 2004BAAS...36..698C
The continually evolving convection below the solar photosphere gives
rise to a wide spectrum of magnetohydrodynamic (MHD) fluctuations in
the magnetic atmosphere and solar wind. The propagation of waves through
the solar atmosphere has been studied for more than a half century, and
the mainly incompressible Alfven mode has been believed to be dominant
in regions that are open to the heliosphere. As a part of an ongoing
study of various aspects of solar MHD waves and turbulence, we present
a comprehensive model of the radially evolving properties of Alfvenic
fluctuations in a representative open magnetic region. This work
differs from previous models in the following ways. (1) The background
plasma density, magnetic field, and flow velocity are constrained
empirically from below the photosphere to distances past 1 AU. The
successive merging of flux tubes on granular and supergranular scales
is described using a two-dimensional magnetostatic model of a magnetic
network element. (2) The frequency power spectrum of horizontal motions
is specified only at the photosphere, based on prior analyses of G-band
bright points. Everywhere else in the model the amplitudes of outward
and inward propagating waves are computed with no free parameters. We
compare the resulting wave properties with observed nonthermal motions
in the chromosphere and corona, radio scintillation measurements,
and in-situ fluctuation spectra. This work is supported by NASA
under grants NAG5-11913, NAG5-12865, and NAG5-10996 to the Smithsonian
Astrophysical Observatory, by Agenzia Spaziale Italiana, and by the
Swiss contribution to the ESA PRODEX program.
Title: Contributions from Ultraviolet Spectroscopy to the Prediction
of High-energy Proton Hazards from CME Shocks
Authors: Lin, J.; Raymond, J. C.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2004AAS...204.7205L
Altcode: 2004BAAS...36Q.799L
A significant potential hazard to astronauts and their equipment
in interplanetary space is the relativistic proton flux produced by
coronal mass ejections (CMEs) and solar flares. The longest-duration
phase of solar energetic particle (SEP) activity is believed to come
from the CME shock as it propagates through the extended corona and
heliosphere. Ultraviolet spectroscopy by SOHO has revealed a means
for: (1) detecting and characterizing CME shocks in the corona, and
(2) determining the plasma conditions in the pre-CME corona which are
needed to understand the formation and evolution of shocks. Such remote
sensing - combined with models of SEP acceleration and transport -
can be used to predict the strength, duration, and production sites of
the radiation. This poster describes the specific means by which
ultraviolet spectroscopy and other remote-sensing data can be used to
determine the inputs and boundary conditions for individual events (such
as the October-November 2003 storms) in existing SEP model codes. We
also discuss an additional potential source of SEP radiation associated
with electric fields in the current sheets that form in flare regions
in the wake of CME. Both observations and model calculations show that
the reconnection-induced electric field can reach a maximum strength
of a few V/cm within tens of minutes after the onset of the eruption,
then decreases gradually over several hours. SEPs produced in these
regions may account for X-rays and γ -rays observed prior to the
formation of CME shocks. Ultraviolet spectroscopy has been shown to
provide constraints on the plasma properties in all of the above CME
features. This work is supported by NASA under grant NAG5-12865 to
the Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana,
and by the Swiss contribution to ESA's PRODEX program.
Title: Low-latitude coronal holes during solar maximum
Authors: Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2004AdSpR..33..696M
Altcode:
The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO has been used
to observe large low-latitude coronal holes during solar maximum that
produced fast solar wind streams. UVCS observations show that large
low-latitude coronal holes at solar maximum, coronal holes of at
least 10° in longitude, have plasma properties that seem to bridge
the gap between solar minimum polar coronal holes and streamers. The
ion kinetic perpendicular temperatures in equatorial coronal holes are
about 2 times larger than those in a solar minimum equatorial streamer,
and about a factor of 2 smaller than those in polar coronal holes
above 2 Rsolar. The outflow speeds for the large equatorial
coronal holes observed by UVCS are 3-4 times lower than those in polar
coronal holes between 2 and 3 Rsolar. The values for high-
and mid-latitude coronal holes are in between those. In all these cases,
the in situ data corresponding to these coronal holes showed high-speed
wind streams with asymptotic speeds of 600-750 km s-1. These
wind speeds approach those observed over polar coronal holes at solar
minimum, but the outflow speeds in these coronal holes between 2 and
3 Rsolar are different. In contrast to the polar coronal
holes, the bulk of the solar wind acceleration must occur above
3 Rsolar for large low-latitude coronal holes at solar
maximum. These observations provide detailed empirical constraints
for theoretical models and may be key to understanding how the various
types of solar wind plasma are heated and accelerated.
Title: Observational Aspects of Wave Acceleration in Open Magnetic
Regions
Authors: Cranmer, S. R.
Bibcode: 2004ESASP.547..353C
Altcode: 2003astro.ph..9676C; 2004soho...13..353C
This paper reviews the latest observational evidence for the existence
of propagating waves in the open magnetic flux tubes of the solar
corona. SOHO measurements have put tentative limits on the fluxes of
various types of magnetohydrodynamic (MHD) waves in the acceleration
region of the solar wind. Also, continually improving measurements
of fluctuations at larger distances (i.e., in situ detection and
radio scintillation) continue to provide significant constraints
on the dominant types of plasma oscillation throughout the corona
and wind. The dissipation of MHD fluctuations of some kind, probably
involving anisotropic turbulent cascade, is believed to dominate the
heating of the extended corona. Spectroscopic observations from the UVCS
instrument on SOHO have helped to narrow the field of possibilities for
the precise modes, generation mechanisms, and damping channels. This
presentation will also review some of the collisionless, kinetic
aspects of wave heating and acceleration that are tied closely to the
observational constraints.
Title: Non-WKB Alfven Wave Reflection from the Solar Photosphere to
the Distant Heliosphere
Authors: Cranmer, S. R.; van Ballegooijen, A.
Bibcode: 2003AGUFMSH21B0115C
Altcode:
Magnetohydrodynamic (MHD) turbulence has been considered for
several decades as a possibly substantial heat source for the solar
chromosphere, corona, and heliosphere. However, it is still not well
understood how the turbulent fluctuations are generated and how they
evolve in frequency and wavenumber. Although the dominant population of
Alfvén waves near the Sun must be propagating outwards, one also needs
waves propagating inwards in order to ``seed'' a turbulent cascade. As
a part of an ongoing study of various aspects of solar MHD turbulence,
we present a model of linear, non-WKB reflection of Alfvén waves that
propagate in both directions along an open magnetic flux tube. Our work
differs from previous models in the following ways. (1) The background
plasma density, magnetic field, and flow velocity are constrained
empirically from below the photosphere to distances past 1 AU. The
successive merging of flux tubes on granular and supergranular scales
is described using a two-dimensional magnetostatic model of a magnetic
network element in the stratified solar atmosphere. (2) The amplitudes
of horizontal wave motions are specified only at the photosphere, based
on previous analyses of G-band bright point motions. Everywhere else in
the model the amplitudes of outward and inward propagating waves are
computed self-consistently. We compare the resulting wave properties
with observed nonthermal motions in the chromosphere and corona, radio
scintillation measurements, and in-situ fluctuation spectra. Quantities
such as the MHD turbulent heating rate and the non-WKB wave pressure
are computed, and the need for other sources of inward waves (e.g.,
nonlinear reflection or scattering off density inhomogeneities) will
also be discussed. This work is supported by the National Aeronautics
and Space Administration under grants NAG5-11913 and NAG5-12865 to the
Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana,
and by the Swiss contribution to the ESA PRODEX program.
Title: Coordinated UVCS/SOHO and VLA Observations of the Solar Corona
Authors: Miralles, M. P.; Cranmer, S. R.; Raymond, J. C.; Spangler,
S. R.; Kohl, J. L.
Bibcode: 2003AGUFMSH22A0188M
Altcode:
Coordinated UVCS/SOHO and VLA coronal observations took place during
August 16--19, 2003. The radio source 3C 228 passed behind a streamer
in the northeast at a heliocentric distance of about 7 solar radii,
and behind the north coronal hole at about 4 solar radii in the latter
part of the radio observation. The goal of this campaign is to combine
the analysis of radio polarimetric sounding measurements of the corona
with ultraviolet spectroscopy of the same regions, in order to obtain
qualitatively new information about the properties of the solar coronal
plasma. The Ultraviolet Coronagraph Spectrometer (UVCS) aboard SOHO
observed O VI (103.2 and 103.7 nm) and H I Lyman alpha (121.6 nm)
emission lines to determine kinetic temperatures, average densities
and outflow speeds in the corona. UVCS observations provide unique
information on the heating and acceleration processes in the corona. The
Very Large Array (VLA) observations reveal the Faraday rotation of
polarized radio waves due to passage through the magnetized plasma of
the corona. These measurements provide limits on the coronal magnetic
field strength and constrain the properties of magnetohydrodynamic (MHD)
waves. Radio propagation techniques are a useful complementary tool
to ultraviolet coronagraphic spectroscopy in determining the physical
processes that are responsible for the heating of the extended corona
and the acceleration of the solar wind. This work is supported by NASA
under Grant NAG5-12865 to the Smithsonian Astrophysical Observatory,
by the Italian Space Agency and by PRODEX (Swiss contribution).
Title: Empirically Determined Anisotropic Velocity Distributions and
Outflows of O5+ Ions in a Coronal Streamer at Solar Minimum
Authors: Frazin, R. A.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2003ApJ...597.1145F
Altcode:
Empirical constraints on the O5+ velocity distributions
and outflow speeds in a solar minimum equatorial streamer between 2.6
and 5.1 Rsolar are determined using a spectral synthesis
code that includes O VI Doppler dimming. These constraints follow
directly from UV spectra taken on 1996 October 12 with the Ultraviolet
Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric
Observatory (SOHO) satellite and three-dimensional electron densities
derived from tomography applied to a time series of polarized
white-light images taken with the Large Angle and Spectrometric
Coronagraph (LASCO) also on SOHO. Four conclusions result from this
work: (1) our analysis shows O5+ velocity distribution
anisotropy in the streamer legs and stalk and gives strong evidence that
the microscopic velocity distribution (which excludes wave motions that
equally affect all charged particles) is anisotropic, where the most
probable speed perpendicular to the magnetic field direction exceeds
that in the parallel direction; (2) there is preferential heating of the
O5+ ions over the protons in the streamer stalk and legs;
(3) there is no evidence for preferential O5+ heating in
the core; and (4) the outflow velocity of the O5+ ions is
determined at heights above 4.6 Rsolar. All results have
a confidence level of at least 70%.
Title: Alfvénic Turbulence in the Extended Solar Corona: Kinetic
Effects and Proton Heating
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2003ApJ...594..573C
Altcode: 2003astro.ph..5134C
We present a model of magnetohydrodynamic (MHD) turbulence in the
extended solar corona that contains the effects of collisionless
dissipation and anisotropic particle heating. Recent observations have
shown that preferential heating and acceleration of positive ions occur
in the first few solar radii of the high-speed solar wind. Measurements
made by the Ultraviolet Coronagraph Spectrometer aboard SOHO have
revived interest in the idea that ions are energized by the dissipation
of ion cyclotron resonant waves, but such high-frequency (i.e.,
small-wavelength) fluctuations have not been observed. A turbulent
cascade is one possible way of generating small-scale fluctuations
from a preexisting population of low-frequency MHD waves. We
model this cascade as a combination of advection and diffusion in
wavenumber space. The dominant spectral transfer occurs in the direction
perpendicular to the background magnetic field. As expected from earlier
models, this leads to a highly anisotropic fluctuation spectrum with a
rapidly decaying tail in the parallel wavenumber direction. The wave
power that decays to high enough frequencies to become ion cyclotron
resonant depends on the relative strengths of advection and diffusion in
the cascade. For the most realistic values of these parameters, however,
there is insufficient power to heat protons and heavy ions. The dominant
oblique fluctuations (with dispersion properties of kinetic Alfvén
waves) undergo Landau damping, which implies strong parallel electron
heating. We discuss the probable nonlinear evolution of the electron
velocity distributions into parallel beams and discrete phase-space
holes (similar to those seen in the terrestrial magnetosphere), which
can possibly heat protons via stochastic interactions.
Title: The Advanced Spectroscopic and Coronagraphic Explorer (ASCE)
Mission Concept Study
Authors: Kohl, J.; Howard, R.; Davila, J.; Noci, G.; Gardner, L.;
Socker, D.; Romoli, M.; Strachan, L.; Floyd, L.; Cranmer, S.; Raymond,
J.; van Ballegooijen, A.
Bibcode: 2002AGUFMSH52A0463K
Altcode:
The ASCE Mission is currently in a Phase A feasibility study as a
candidate for the upcoming MIDEX selection. The ASCE science payload
provides next generation spectroscopic and polarimetric instrumentation
aimed at identifying the physical processes governing solar wind
generation and coronal mass ejections (CMEs). During the current phase,
engineering design and analyses have demonstrated the feasibility of
accomplishing the original mission objectives within the MIDEX mission
constraints. The launch is planned for early 2007 and the operations
and analyses are expected to continue for 5 years. ASCE data along with
data analysis software and calibration data will be unrestricted and
available to the scientific community via an automated web site. A
Guest Investigator program is planned with an average of 15 grants
running concurrently during 2008 to 2012. Grants would be awarded in
response to proposals submitted during the first and subsequent years
of the mission.
Title: UVCS/SOHO Observations of Large Coronal Holes During Solar
Cycle 23
Authors: Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2002AGUFMSH52A0451M
Altcode:
The Ultraviolet Coronagraph Spectrometer (UVCS) aboard SOHO has been
collecting spectroscopic data from numerous coronal holes as part of
an ongoing campaign to determine the plasma properties of the solar
wind acceleration region throughout the current solar cycle. The
UVCS observations show marked variations of ion properties (in the
acceleration region of the high-speed solar wind) in different types of
coronal holes. We present empirical models for the physical properties
of large coronal holes and the acceleration of the associated high-speed
solar wind derived from ultraviolet coronagraphic spectroscopy. We
discuss the role of solar cycle trends and the variation of ambient
coronal-hole properties (e.g., magnetic field, geometry, density). We
use these observations to test phenomenological models of coronal
heating and solar wind acceleration. This work is supported by NASA
under Grant NAG5-11420 to the Smithsonian Astrophysical Observatory,
by the Italian Space Agency and by PRODEX (Swiss contribution).
Title: Proton Heating in the Extended Solar Corona Resulting From
Kinetic Alfven Turbulence
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2002AGUFMSH12A0407C
Altcode:
Spectroscopic observations of the solar corona have made it clear that
the ``coronal heating problem'' comprises not only the local deposition
of heat immediately above the transition region, but also extended
heat deposition throughout the (collisionless) acceleration region of
the solar wind. The dissipation of magnetohydrodynamic (MHD) waves
and/or turbulence has been considered as a likely heating mechanism
in the solar wind for several decades. However, it is still not well
understood how MHD fluctuations are generated, how they evolve in
frequency and wavenumber, or how their damping leads to the observed
proton, electron, and ion properties of the fast wind. We present
a model of MHD turbulence that specifically addresses the issue of
kinetic dissipation and particle heating in the collisionless extended
corona. The nonlinear cascade is modeled as a combination of advection
and diffusion in wavenumber space, with the dominant cascade occurring
in the direction perpendicular to the background magnetic field. This
leads to a highly anisotropic fluctuation spectrum (as expected,
based on many earlier simulations and scaling models) with a rapidly
decreasing power-law tail in the parallel wavenumber direction. In
the low-plasma-beta corona, the dominant oblique fluctuations (with
dispersion properties of kinetic Alfven waves) are dissipated by
electron Landau damping, with only a tiny fraction of the energy going
to high-frequency ion cyclotron waves. This implies strong parallel
electron heating and weak proton and ion heating, which is not what is
observed. We discuss the probable nonlinear evolution of the electron
velocity distributions into parallel beams and discrete phase-space
holes (similar to those seen in the terrestrial magnetosphere) which
can possibly heat protons via stochastic interactions.
Title: Coronal Holes and the High-Speed Solar Wind
Authors: Cranmer, Steven R.
Bibcode: 2002SSRv..101..229C
Altcode:
Coronal holes are the lowest density plasma components of the Sun's
outer atmosphere, and are associated with rapidly expanding magnetic
fields and the acceleration of the high-speed solar wind. Spectroscopic
and polarimetric observations of the extended corona, coupled with
interplanetary particle and radio sounding measurements going back
several decades, have put strong constraints on possible explanations
for how the plasma in coronal holes receives its extreme kinetic
properties. The Ultraviolet Coronagraph Spectrometer (UVCS) aboard the
Solar and Heliospheric Observatory (SOHO) spacecraft has revealed
surprisingly large temperatures, outflow speeds, and velocity
distribution anisotropies for positive ions in coronal holes. We
review recent observations, modeling techniques, and proposed heating
and acceleration processes for protons, electrons, and heavy ions. We
emphasize that an understanding of the acceleration region of the wind
(in the nearly collisionless extended corona) is indispensable for
building a complete picture of the physics of coronal holes.
Title: Cyclical variations in the plasma properties of coronal holes
Authors: Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2002ESASP.508..351M
Altcode: 2002soho...11..351M
UVCS/SOHO has been used to measure the plasma properties of several
types of coronal holes from 1996 to early 2002 at heights from
1.5 to 3.5 Rsolar. UVCS observations show a variation
in ion properties between coronal holes from solar minimum to solar
maximum. Different coronal holes seem to exhibit different heating and
acceleration rates. Polar coronal holes at solar minimum exhibited
the most extreme ion properties with O5+ perpendicular
temperatures in excess of 2×108K, O5+ outflow
speeds of at least 400 km/s by 3 Rsolar, and the lowest
densities. Equatorial coronal holes at solar maximum exhibited the
least extreme ion plasma properties with O5+ perpendicular
temperatures less than 8×107K, O5+ outflow
speeds of only 100 km/s by 3 Rsolar, and the largest
densities. However, large polar and equatorial coronal holes produce
interplanetary wind streams with similar speeds (v ~ 700 km/s). Thus,
most of the solar wind acceleration in large equatorial coronal
holes must occur above 3 Rsolar. The first high-latitude
coronal hole of the new negative magnetic polarity observed at the
north in 2001 exhibited extreme properties similar to those of the
1996 - 1997 polar coronal holes, even though it was 6 years prior to
the next minimum. During 2001 - 2002, we have observed mid-latitude
coronal holes, with properties in between large polar and equatorial
coronal holes.
Title: Solar wind acceleration in coronal holes
Authors: Cranmer, Steven R.
Bibcode: 2002ESASP.508..361C
Altcode: 2002astro.ph..9301C; 2002soho...11..361C
This paper reviews the current state of our understanding of high-speed
solar wind acceleration in coronal holes. Observations by SOHO,
coupled with interplanetary particle measurements going back several
decades, have put strong constraints on possible explanations for how
the protons, electrons, and minor ions receive their extreme kinetic
properties. The asymptotic plasma conditions of the wind depend on
energy and momentum deposition both at the coronal base (where, e.g.,
the mass flux is determined) and in the extended acceleration region
between 2 and 10 solar radii (where the plasma becomes collisionless
and individual particle species begin to exhibit non-Maxwellian
velocity distributions with different moments). The dissipation
of magnetohydrodynamic fluctuations (i.e., waves, turbulence, and
shocks) is believed to dominate the heating in the extended corona,
and spectroscopic observations from the UVCS instrument on SOHO have
helped to narrow the field of possibilities for the precise modes,
generation mechanisms, and damping channels. We will survey recent
theoretical and observational results that have contributed to new
insights, and we will also show how next-generation instruments can be
designed to identify and characterize the dominant physical processes
to an unprecedented degree.
Title: Empirically Determined Anisotropic Velocity Distributions
and Outflows of O5+\ Ions in a Coronal Streamer at
Solar Minimum
Authors: Frazin, R. A.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2002AAS...200.1601F
Altcode: 2002BAAS...34..667F
Empirical constraints on the O5+\ velocity distributions
and outflow speeds in a solar minimum equatorial streamer between 2.6
and 5.1 Rsun\ are determined using a spectral synthesis
code that includes O VI Doppler dimming. These constraints follow
directly from UV spectra taken on 12 October 1996 with the Ultraviolet
Coronagraph Spectrometer (UVCS) on the SOHO satellite and 3D electron
densities derived from tomography applied to a time series of polarized
white-light images taken with the Large Angle Spectrometric Coronagraph
Experiment (LASCO) on SOHO. Four conclusions result from this work: 1)
Our analysis shows O5+\ velocity distribution anisotropy
in the streamer legs and stalk and that the microscopic velocity
distribution (which excludes wave motions that equally affect all
charged particles) is also anisotropic, where the most probable
speed perpendicular to the magnetic field direction exceeds that in
the parallel direction. 2) There is no evidence of anisotropy in the
streamer core. 3) There is preferential heating of the O5+\
ions over the protons in the streamer stalk and legs, but not in the
core. 4) The outflow velocity of the O5+\ ions is determined
at heights above 4.6 Rsun. All results have a confidence
level of at least 70%. The evidence for microscopic anisotropy in the
O5+\ velocity distributions and preferential heating of the
O5+\ ions over the protons presented here is reminiscent
of that provided for coronal holes by Cranmer et al. (1999). One
particularly favorable candidate mechanism to explain these phenomena
is ion cyclotron resonance, in which high frequency Alfvén waves
are absorbed by the heavy ions. Cranmer et al. discuss the relevance
of this process to an empirical model of a polar coronal hole. Our
data suggest that the dominant processes that heat the heavy ions in
coronal holes may also be important in streamers. Reference: Cranmer,
S.R., et al. 1999, ApJ, 511, 481
Title: Empirically Determined Anisotropic Velocity Distributions
and Outflows of O5+\ ions in a Coronal Streamer at
Solar Minimum
Authors: Frazin, R. A.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2002AGUSMSH21B..06F
Altcode:
Empirical constraints on the O5+\ velocity distributions
and outflow speeds in a solar minimum equatorial streamer between 2.6
and 5.1 Rsun\ are determined using a spectral synthesis
code that includes O VI Doppler dimming. These constraints follow
directly from UV spectra taken on 12 October 1996 with the Ultraviolet
Coronagraph Spectrometer (UVCS) on the SOHO satellite and 3D electron
densities derived from tomography applied to a time series of polarized
white-light images taken with the Large Angle Spectrometric Coronagraph
Experiment (LASCO) on SOHO. Four conclusions result from this work:
1) Our analysis shows O5+\ anisotropy in the streamer legs
and stalk. The microscopic velocity distribution (which excludes wave
motions that equally affect all charged particles) is also found to
be anisotropic, where the most probable speed perpendicular to the
magnetic field direction exceeds that in the parallel direction. 2)
There is no evidence of such anisotropy in the streamer core. 3)
There is preferential heating of the O5+ions over the
protons in the streamer stalk and legs, but not in the core. 4) The
outflow velocity of the O5+\ ions is determined at heights
above 4.6 Rsun. This work is supported by NASA under grant
NAG5-10093 to the Smithsonian Astrophysical Observatory.
Title: Polar Coronal Jets at Solar Minimum
Authors: Dobrzycka, D.; Cranmer, S. R.; Raymond, J. C.; Biesecker,
D. A.; Gurman, J. B.
Bibcode: 2002ApJ...565..621D
Altcode:
We present an analysis of six polar coronal jets observed by
the Ultraviolet Coronagraph Spectrometer (UVCS) at solar minimum
(1996). Four of the events were also recorded by the Extreme-Ultraviolet
Imaging Telescope (EIT) and/or the Large Angle Spectrometric Coronagraph
(LASCO) C2 coronagraph. We compared the jets with others recorded in
1997. We modeled the observable properties of the jet from 1996 June 11,
detected at 1.5 Rsolar. It represents a type of polar jet
in which H I Lyα and O VI get brighter at the same time. The model
reproduced the line properties with an electron density enhancement
of a factor of 2 (with a resulting density of 4.8×106
cm-3), an outflow velocity enhancement of a factor of
3 (yielding a velocity of 200 km s-1), and an electron
temperature decrease of a factor of 0.36 (with a resulting temperature
of 5.3×105 K). We derived the jet's electron densities
from the LASCO C2 white-light observations. They are a factor of 1.5
higher than in the interplume corona and comparable to those in plume
regions within the C2 field of view. We developed a model for the origin
of polar jets based on Wang's model for plumes. We envisioned that
jets may be the result of short-lived bursts of base heating, while
plumes may be the result of base-heating events that last longer than
several hours. Models with the base heat flux near 3×105
ergs cm-2 s-1 come closest to matching the
observations, though they are not entirely consistent.
Title: Polar coronal jets
Authors: Dobrzycka, D.; Raymond, J. C.; Cranmer, S. R.
Bibcode: 2002AdSpR..29..337D
Altcode:
We present ultraviolet spectroscopy of polar coronal jets obtained
with the Ultraviolet Coronagraph Spectrometer aboard the Solar and
Heliospheric Observatory. They correlate with the Extreme—Ultraviolet
Imaging Telescope Fe XII (195Å) and Large Angle Spectrometric
Coronagraph white—light jet events. We found that the jets typically
undergo two phases: at the first phase the O VI lines show a brief
intensity enhancement and narrowing, while the H I Lyα line is not
enhanced, and the second phase, about 25 minutes later, when the H I
Lyα line shows maximum intensity enhancement and narrowing, while the
O VI line is relatively unchanged. We modeled the observable properties
of the jets from 1997 August 5, detected at 1.71 R⊙. We
interpret the first phase as the fast, dense centroid of the jet
passing by the slit, and the second phase as a passage of cooler,
lower density material following the centroid. Possible scenarios of
the electron temperature variations needed to account for observed
conditions on 1997 August 5 indicate that some heating is required. We
computed models of the temperature and nonequilibrium ionization state
of an expanding plasma using various forms for the heating rates. We
discuss the model results and estimate the initial electron temperature
and heating rate required to reproduce the observed O VI ionization
state. We also place some constraints on the origin of the jet material
based on the inferred plasma properties.
Title: The Link between Radiation-Driven Winds and Pulsation in
Massive Stars (invited paper)
Authors: Owocki, S. P.; Cranmer, S. R.
Bibcode: 2002ASPC..259..512O
Altcode: 2002rnpp.conf..512O; 2002IAUCo.185..512O
No abstract at ADS
Title: Coronal Holes and the Solar Wind [Invited]
Authors: Cranmer, S. R.
Bibcode: 2002mwoc.conf....3C
Altcode:
Coronal holes are the darkest regions of the ultraviolet and X-ray Sun,
both on the disk and away from the limb. Coronal holes are associated
with rapidly expanding open magnetic fields and the acceleration of
the high-speed solar wind. This presentation will review measurements
of the plasma properties of coronal holes and how these measurements
have been used to put constraints on theoretical models of coronal
heating and solar wind acceleration. Heat deposition at the dense and
collisional coronal base is of comparable importance (in determining,
e.g., temperature gradients and asymptotic outflow speeds) as extended
heating in the collisionless regions above 2 solar radii. Thus,
a complete understanding of the physics requires both observations
of the solar disk and inner corona (Yohkoh, EIT, CDS, SUMER) and
coronagraphic observations of the wind's acceleration region (UVCS,
LASCO). Although strong evidence has been found to suggest that the
high-speed wind is driven mainly by proton pressure, the differences
between proton, electron, and heavy ion velocity distributions are
extremely valuable as probes of the dominant physical processes.
Title: Low-latitude Coronal Holes during Solar Maximum
Authors: Miralles, M.; Cranmer, S.; Kohl, J.
Bibcode: 2002cosp...34E1125M
Altcode: 2002cosp.meetE1125M
Analyses of in situ observations have shown that some small coronal
holes are sources of slow solar wind near solar maximum when
polar coronal holes become smaller and disappear. However, not all
coronal holes at solar maximum produce slow wind. The Ultraviolet
Coronagraph Spectrometer (UVCS) aboard SOHO has been used to observe
large low-latitude coronal holes during solar maximum that produced
fast solar wind. UVCS observations show that large equatorial holes
at solar maximum have plasma properties that seem to bridge the gap
between solar minimum polar coronal holes and streamers. The ion kinetic
temperatures in equatorial holes are about 2 times larger than those in
a solar minimum equatorial streamer, and about a factor of 2 smaller
than those in polar coronal holes above 2 R . The outflow speeds for
the large equatorial holes observed by UVCS are only about 100 km s-1
, a factor of 4 smaller than those in polar holes, at 3 R . However,
in situ data corresponding to these equatorial coronal holes showed
asymptotic wind speeds of 600-700 km s-1 . These wind speeds are similar
to those observed over polar coronal holes at solar minimum. In contrast
to the polar coronal holes, the bulk of the solar wind acceleration
in large equatorial coronal holes at solar maximum must occur above
3 R . Thus, the combination of spectroscopic measurements in the
extended corona, where the primary solar wind acceleration occurs, and
in situ measurements made in the solar wind can be used to obtain the
solar wind acceleration as a function of heliocentric distance. These
observations provide detailed empirical constraints for theoretical
models and may be key to understanding how the various types of solar
wind plasma are heated and accelerated. This work is supported by NASA
under Grant NAG5-11420 to the Smithsonian Astrophysical Observatory,
by the Italian Space Agency and by PRODEX (Swiss contribution).
Title: UV and Soft X-ray Polar Coronal Jets
Authors: Dobrzycka, D.; Raymond, J. C.; Cranmer, S. R.; Li, J.
Bibcode: 2002mwoc.conf...23D
Altcode:
Coronal jets are spectacular dynamic events originating from different
structures in the solar corona. Jetlike phenomena were observed by
various instruments aboard SOHO, and the X--ray jets were discovered
by Yohkoh's soft X--ray telescope (SXT). The relation among the
different types of jets is still not yet clear. We present ultraviolet
spectroscopy of polar coronal jets obtained by the Ultraviolet
Coronagraph Spectrometer (UVCS/SOHO) at heights in the corona ranging
from 1.5 Rodot to 2.5 Rodot. The jets appear to
originate near flaring ultraviolet bright points within polar coronal
holes and were recorded by UVCS as a significant enhancement in the
integrated intensities of the strongest coronal emission lines: mainly
H I Ly alpha and O VI lambda lambda 1032,1037. A number of the detected
jets are correlated with EIT Fe XII 195Å and LASCO C2 white-light
events. Our modeling of the jet's observable properties provided
estimates of the jet plasma conditions, as well as the initial electron
temperature and heating rate required to reproduce the observed O VI
ionization state. We discuss possible relationship between the polar
ultraviolet and X--ray jets based on the results of coordinated SXT
and UVCS observations in December 1996. We compare their properties and
consider the magnetic reconnection models, developed for X--ray jets,
as a model for UV jet formation. This work is supported by the National
Aeronautics and Space Administration under grant NAG5--10093 to the
Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana,
and by the ESA PRODEX program (Swiss contribution).
Title: In-flight Calibration of the UVCS White Light Channel
Authors: Romoli, M.; Frazin, R. A.; Kohl, J. L.; Gardner, L. D.;
Cranmer, S. R.; Reardon, K.; Fineschi, S.
Bibcode: 2002ISSIR...2..181R
Altcode: 2002ESASR...2..181R; 2002rcs..conf..181R
The UVCS White Light Channel (WLC) is designed to measure the linearly
polarized radiance (pB) of the corona, in the wavelength band from 450
nm to 600 nm, in order to derive one of the fundamental parameters of
the solar corona: the electron density. This paper gives a thorough
description of the in-flight radiometric calibration of the WLC, which
uses the star α Leo and the planet Jupiter as transfer standards
and is based on calibrations of ground-based instruments. The
method for computing the polarized radiance from the measurements
is also described, together with the stray light and polarization
characterizations obtained from dedicated, in-flight measurements.
Title: UVCS/SOHO Observations of Coronal Holes from Solar Minimum
to Solar Maximum
Authors: Miralles, M. P.; Cranmer, S. R.; Esser, R.; Kohl, J. L.
Bibcode: 2001AGUFMSH32A0721M
Altcode:
Coronal holes are open field, low-density source regions of the solar
wind. At solar minimum, large coronal holes are present at the poles
and are the dominant source of the solar wind flow for this part of
the solar cycle. At solar maximum, coronal holes of varying sizes and
shapes appear at all latitudes and last for several rotations. During
this stage of the cycle, the dominant component is mainly slow wind,
but fast wind streams are generated by large coronal holes. UVCS/SOHO
has been used to measure the plasma properties in several types
of coronal holes from 1996 to 2001. Spectroscopic diagnostics of
O5+ velocity distributions and outflow velocities are
derived from measurements of intensities and line widths for O~VI
103.2 and 103.7 nm as a function of height. We compare the plasma
properties of coronal holes from solar minimum to solar maximum and
discuss the evolution of coronal holes during the solar cycle. We
also study the compatibility between the growing database of coronal
hole plasma properties and theoretical models of extended coronal
heating via ion cyclotron resonance. This work is supported by NASA
under Grant NAG5-10093 to the Smithsonian Astrophysical Observatory,
by the Italian Space Agency and by PRODEX (Swiss contribution).
Title: The Advanced Spectroscopic and Coronagraphic Explorer Mission
Authors: Kohl, J. L.; Howard, R.; Davila, J.; Noci, G.; Esser, R.;
Ciaravella, A.; Cranmer, S.; Fineschi, S.; Gardner, L.; Raymond, J.;
Romoli, M.; Smith, P.; Socker, D.; Strachan, L.; Van Ballegooijen, A.
Bibcode: 2001AGUFMSH31B0711K
Altcode:
SOHO has provided profound insights into the physics of solar wind
acceleration and coronal mass ejections. Although significant
progress has been made, most of the dominant physical processes
controlling these phenomena are still not identified. The Advanced
Spectroscopic and Coronagraphic Explorer Mission provides next
generation spectroscopic and polarimetric instrumentation aimed at
identifying these processes. The launch is planned for March 2007 with
mission operations and data analysis continuing for 5 years. The data
will be unrestricted and available to the community. The envisioned
program includes a Guest Investigator Program with an average of 15
grants to be awarded in response to proposals submitted during the
first year of the mission. Information about the proposed scientific
goals and instrumentation will be presented.
Title: Ion cyclotron diffusion of velocity distributions in the
extended solar corona
Authors: Cranmer, Steven R.
Bibcode: 2001JGR...10624937C
Altcode:
The Ultraviolet Coronagraph Spectrometer aboard the Solar and
Heliospheric Observatory has revealed strong kinetic anisotropies
and extremely large perpendicular temperatures of heavy ions in the
extended solar corona. These observations have revived interest in
the idea that the high-speed solar wind is heated and accelerated
by the dissipation of ion cyclotron resonant Alfvén waves. This
process naturally produces departures from Maxwellian and bi-Maxwellian
velocity distributions. Here it is argued that these departures must be
taken into account in order to understand the resonant velocity space
diffusion, the wave damping, and the formation of ultraviolet emission
lines. Time-dependent ion velocity distributions are computed for a
fixed spectrum of waves in a homogeneous plasma, and the moments of
the distributions are compared with simple bi-Maxwellian models. The
existence of a boundary, in parallel velocity space, between resonance
and nonresonance produces an effective saturation of the velocity space
diffusion that bi-Maxwellian models could not predict. The damping of
an input wave spectrum is computed for a coronal population of 1000
ion species with the above saturation effect included. For realistic
levels of fluctuation power, it is concluded that waves propagating
solely from the coronal base would not be able to heat and accelerate
the ions that have been observed to exhibit strong energization and that
local wave generation is required. Ultraviolet emission line profiles
are computed for the derived non-Maxwellian distributions, and possible
unique identifiers of the ion cyclotron resonance mechanism are noted.
Title: Ion cyclotron damping in the solar corona and solar wind
Authors: Cranmer, Steven R.
Bibcode: 2001AIPC..595...25C
Altcode:
The solar corona is the hot, ionized outer atmosphere of the
Sun. Coronal plasma expands into interplanetary space as a supersonic
bulk outflow known as the solar wind. This tenuous and unbounded medium
is a unique laboratory for the study of kinetic theory in a nearly
collisionless plasma, as well as magnetohydrodynamic waves, shocks, and
jets. Particle velocity distributions in the solar wind have been probed
directly by spacecraft (outside the orbit of Mercury), and indirectly by
ultraviolet spectroscopy (close to the Sun). Fluctuations in the plasma
properties and in electromagnetic fields have been measured on time
scales ranging from seconds to years. Despite more than a half-century
of study, though, the basic physical processes responsible for heating
the million-degree corona and accelerating the solar wind past the Sun's
escape velocity are still not known with certainty. Understanding the
basic physics of the solar wind is necessary to predict the Sun's
impact on the Earth's climate and local space environment. This
presentation will review the kinetic origins of several physical
processes that are currently believed to be important in depositing
energy and momentum in coronal particle velocity distributions. Because
ions in the solar wind are heated and accelerated more than would be
expected in either thermodynamic equilibrium or via a mass-proportional
process, an ion cyclotron resonance has been suggested as a likely
mechanism. Other evidence for gyroresonant wave dissipation in the
corona will be presented, and possible generation mechanisms for the
(as yet unobserved) high-frequency cyclotron waves will be reviewed. The
mean state of the coronal and heliospheric plasma is intimately coupled
with kinetic fluctuations about that mean, and theories of turbulence,
wave dissipation, and instabilities must continue to be developed
along with steady state descriptions of the solar wind. .
Title: Ultraviolet Coronagraph Spectrometer Observations of a
High-Latitude Coronal Hole with High Oxygen Temperatures and the
Next Solar Cycle Polarity
Authors: Miralles, M. P.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2001ApJ...560L.193M
Altcode:
We announce the resurgence of extreme ion properties in a large,
high-latitude coronal hole observed above the north heliographic pole
in 2001 February at solar maximum. The observations were taken with
the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric
Observatory. These observations are part of an ongoing campaign to
determine the plasma properties of coronal holes during the current
solar cycle. In this Letter, we compare the observations and analysis
of O VI λλ1032, 1037 spectral lines of a high-latitude coronal hole
in 2001 with observations of an equatorial solar maximum hole in 1999
and polar coronal holes observed near solar minimum (1996-1997). These
lines provide spectroscopic diagnostics of O+5 velocity
distributions and outflow velocities. The O VI line profiles show
a narrow core and broad wings. The narrow core is attributed to
foreground and background streamers and, possibly, dense polar plumes
at the lowest observed heights. The broad wings are attributed to the
coronal hole. The comparison of the coronal hole line widths shows
that the O+5 perpendicular kinetic temperatures in the 2001
high-latitude hole are similar to those observed in polar coronal holes
at solar minimum. These observations of extremely high ion kinetic
temperatures (exceeding 108 K) at the north pole in 2001
occurred nearly simultaneously with the polarity change of the Sun's
magnetic field, as seen in recent magnetogram data. This coronal hole
in 2001 may represent the first manifestation of the negative polarity
polar coronal holes that will dominate the Sun's open magnetic flux
tubes at the next solar minimum. The reappearance of broad O VI
profiles at a time when not all of the ``new polarity'' magnetic
flux has migrated to the poles was an interesting development. The
variations in coronal hole parameters with the solar cycle provide
constraints on models of extended coronal heating.
Title: Solar Cycle 23: Variation of the Solar Corona in the
Ultraviolet from Solar Minimum to Solar Maximum
Authors: Miralles, M. P.; Panasyuk, A. V.; Strachan, L.; Gardner,
L. D.; Suleiman, R.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2001iscs.symp...59M
Altcode:
UVCS/SOHO measurements of H I Ly-alpha and O VI (103.2 nm and 103.7 nm)
intensities in the solar corona have been made from solar Cycle 23's
minimum in 1996 to its current maximum. At solar minimum, the corona
consisted of large coronal holes at the poles and quiescent streamers at
the equator. During the approach to solar maximum, equatorial coronal
holes and high latitude streamers became more conspicuous. Recently,
coronal holes at higher latitudes have reappeared, allowing a comparison
to be made of O VI intensities and line widths of coronal holes at
different latitudes. We also characterize the variation of coronal
hole properties with height, and location over the solar cycle. This
work is supported by NASA under Grant NAG5-10093 to the Smithsonian
Astrophysical Observatory, by the Italian Space Agency and by PRODEX
(Swiss contribution)
Title: Ultraviolet Spectroscopy of Coronal Jets Within the Fast
Solar Wind
Authors: Dobrzycka, D.; Cranmer, S. R.; Raymond, J. C.; Biesecker,
D. A.; Gurman, J. B.
Bibcode: 2001AGUSM..SH41B09D
Altcode:
The coronal jets are spectacular dynamic events originating from
different structures in the solar corona. We present UVCS/SOHO
observations of polar coronal jets. They appear to originate near
flaring ultraviolet bright points within polar coronal holes that are
source regions of the fast solar wind. UVCS recorded the jets as a
significant enhancement in the integrated intensities of the strongest
coronal emission lines: mostly H~I Lyα and O~VI λ λ 1032,1037. A
number of detected jets are correlated with the EIT Fe~XII 195~Å
and LASCO C2 white-light events. Typically, the observed H~I Lyα
enhancement was up to a factor of 1.3-1.7 over the ambient corona and
lasted for 20-30 minutes. The narrow profiles of the emission lines
indicate that the material in the jets is cooler than the underlying
corona. We modeled the observable properties of the jets to get
estimates on jet plasma conditions. We discuss the model results, the
initial electron temperature and the heating rate required to reproduce
the observed O~VI ionization state. We also discuss connection of
the polar jets to the fast solar wind. This work is supported by the
National Aeronautics and Space Administration under grant NAG5--7822 to
the Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana,
and by the ESA PRODEX program (Swiss contribution).
Title: Observations of a High-latitude Coronal Hole at Solar Maximum
Authors: Miralles, M.; Cranmer, S. R.; Kohl, J. L.
Bibcode: 2001AGUSM..SH21B07M
Altcode:
A large coronal hole at a latitude of about 60 degrees was observed
above the north pole with the Ultraviolet Coronagraph Spectrometer
(UVCS) aboard SOHO during 10--19 February 2001. These observations
are part of an ongoing campaign to characterize equatorial and
mid-latitude coronal holes during the active phase of the current
solar cycle. Observations in H~I Lyα and O~VI 103.2 and 103.7 nm
provided spectroscopic diagnostics of proton and O5+
velocity distributions. The O~VI line profiles show a narrow core
and broad wings. The narrow core is attributed to the foreground and
background streamers. The broad wings are attributed to the coronal
hole. We compare the observed line intensities and widths of this
high-latitude hole with those of other solar maximum (lower latitude)
holes and solar minimum polar coronal holes. The comparison of the
line widths shows that the O~VI line widths of this solar maximum
hole are similar to those observed in polar coronal holes at solar
minimum. The observation of extremely high ion kinetic temperatures at
the north pole occurs simultaneously with the polarity change of the
Sun's magnetic field, as seen in recent magnetogram data. This coronal
hole may represent the first manifestation of the stable polar coronal
holes that will dominate the Sun's open magnetic flux tubes at the next
solar minimum. The re-appearance of broad O~VI profiles at a time when
not all of the ``new polarity'' magnetic flux has migrated to the poles
is an interesting development that may provide a crucial constraint
on models of extended coronal heating. This work is supported by NASA
under Grant NAG5-10093 to the Smithsonian Astrophysical Observatory,
by the Italian Space Agency and by PRODEX (Swiss contribution).
Title: Observed Variations of O5+ Velocity Distributions
with Electron Density
Authors: Kohl, J. L.; Cranmer, S. R.; Frazin, R. A.; Miralles, M.;
Strachan, L.
Bibcode: 2001AGUSM..SH21B08K
Altcode:
The Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO satellite
has been used to measure the line profiles of O~VI 103.2 and 103.7
nm versus heliographic height in a variety of coronal holes and
streamers during the period from 1996 to 2001. Those observations
have been used to derive velocity distributions in the line-of-sight
direction, which is typically perpendicular to the apparent magnetic
field direction. In the case of polar coronal holes at solar minimum,
the electron density is the smallest observed and the most-probable
speed is the largest observed reaching values as high as 500 km/s at the
largest heights. The O5+ most-probable speed is much larger
than the hydrogen speed in those structures. The ratio of O5+
to hydrogen most-probable speeds increases with height. In contrast,
the O5+ values are much smaller than those of hydrogen at
the base of high-latitude streamers and never reach the hydrogen values
at any observed height. The electron density in those structures is
much greater than in the solar minimum coronal holes. Other structures
have intermediate values of the electron density and O5+
most-probable speeds. In general, the O5+ most-probable speed
and its ratio to the hydrogen value seem to decrease with increasing
density. This apparent observational correlation may be related to
thermalization from higher collision rates or it might be related to the
physical process that causes the extreme O5+ perpendicular
heating. This work is supported by NASA under Grant NAG5-10093 to the
Smithsonian Astrophysical Observatory, by the Italian Space Agency,
and by PRODEX (Swiss Contribution).
Title: New Applications of Ultraviolet Spectroscopy to the
Identification of Coronal Heating and Solar Wind Acceleration
Processes
Authors: Cranmer, S. R.
Bibcode: 2001AGUSM..SH21B09C
Altcode:
The Ultraviolet Coronagraph Spectrometer (UVCS) aboard SOHO has
revealed surprisingly extreme plasma conditions in the extended
solar corona. This presentation reviews several new ways that UVCS
and future spectroscopic instruments can be used to identify the
physical processes responsible for producing the various components
of the solar wind. The most promising mechanism for heating and
accelerating heavy ions remains the dissipation of ion cyclotron waves,
but the origin of these waves---as well as the dominant direction of
propagation relative to the background magnetic field---is not yet
known. Ultraviolet spectroscopy of a sufficient number of ions would be
able to pinpoint the precise magnetohydrodynamic modes and the relative
amounts of damping, turbulent cascade, and local plasma instability
in the corona. (A simple graphical comparison of line-width ratios
will be presented as a first step in this direction.) Spectroscopic
observations with sufficient sensitivity can also detect departures
from Gaussian line shapes that are unique identifiers of non-Maxwellian
velocity distributions arising from cyclotron (or other) processes. Even
without these next-generation diagnostics, UVCS data are continuing
to put constraints on how the heating and acceleration mechanisms
respond to changes in the ``background'' properties of coronal holes
and streamers; i.e., geometry, latitude, and density. These provide
crucial scaling relations in the acceleration region of the fast and
slow solar wind that must be reproduced by any candidate theory. This
work is supported by the National Aeronautics and Space Administration
under grant NAG5-10093 to the Smithsonian Astrophysical Observatory,
by Agenzia Spaziale Italiana, and by the Swiss contribution to the
ESA PRODEX program.
Title: Plasma Conditions in Polar Plumes and Interplume Regions in
Polar Coronal Holes
Authors: Cranmer, S. R.; Kohl, J. L.; Miralles, M.; Panasyuk, A. V.
Bibcode: 2001AGUSM..SH41B08C
Altcode:
During times of low solar activity, large polar coronal holes are
observed to contain bright raylike polar plumes that appear to follow
open magnetic field lines. Plumes are believed to be flux tubes that
are heated impulsively at their base, which leads to a higher density,
a lower outflow speed, and a lower overall temperature in the extended
corona, compared to the surrounding interplume regions. Despite years
of white light and spectroscopic observations, though, the differences
in mass, momentum, and energy flux in plumes and between plumes are not
known precisely. This poster presents an updated survey of data from the
Ultraviolet Coronagraph Spectrometer (UVCS), aboard SOHO, that attempts
to sort out the local plume and interplume conditions. These results
will be compared with previous analyses that characterized the ``mean''
plume/interplume coronal hole, averaged over many lines of sight through
varying concentrations of plumes. Limits on the relative contributions
of plumes and interplume regions to the high-speed solar wind will be
determined, with emphasis on the proton outflow speed in the corona
and at 1 AU. Implications for theoretical models of coronal heating and
solar wind acceleration will be discussed. This work is supported by the
National Aeronautics and Space Administration under grant NAG5-10093 to
the Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana,
and by the Swiss contribution to the ESA PRODEX program.
Title: Comparison of Empirical Models for Polar and Equatorial
Coronal Holes
Authors: Miralles, M. P.; Cranmer, S. R.; Panasyuk, A. V.; Romoli,
M.; Kohl, J. L.
Bibcode: 2001ApJ...549L.257M
Altcode:
We present a self-consistent empirical model for several plasma
parameters of a large equatorial coronal hole observed on 1999 November
12 near solar maximum. The model was derived from observations with
the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric
Observatory. In this Letter, we compare the observations of O VI
λλ1032, 1037 emission lines with previous observations of a polar
coronal hole observed near solar minimum. At the time of the 1999
observations, there was no evidence of large polar coronal holes. The
resulting empirical model for the equatorial coronal hole describes
the outflow velocities and most probable speeds for O5+, and
we compared the derived ion properties with the empirical model for a
solar minimum polar coronal hole. The comparison of the empirical models
shows that the 1999 equatorial hole has lower O5+ outflow
speeds and perpendicular temperatures than its polar counterpart from
1996 to 1997 at heights between 2 and 3 Rsolar. However,
in situ asymptotic speeds of the wind streams coming from the
1996-1997 polar hole and from the 1999 equatorial hole are only ~15%
different. Thus, the bulk of the solar wind acceleration must occur
above 3 Rsolar for the equatorial coronal hole. The
equatorial hole also has a higher density than the polar hole at
similar heights. It is not yet known whether the higher densities
are responsible for the seeming inhibition of the fast ion outflow
speeds and extremely large perpendicular temperatures that occur in
polar coronal holes at solar minimum. We discuss the constraints
and implications on various theoretical models of coronal heating
and acceleration.
Title: Ion Cyclotron Diffusion of Velocity Distributions in the
Extended Solar Corona
Authors: Cranmer, S. R.
Bibcode: 2000AGUFMSH21B..09C
Altcode:
The UVCS instrument aboard SOHO has revealed strong kinetic anisotropies
and extremely large perpendicular temperatures of minor ions in the
extended solar corona. These observations have given rise to several
new theoretical models of particle energization via resonance with
ion cyclotron Alfven waves. The analysis of the UVCS emission line
profiles, though, has only been performed under the assumption of simple
two-temperature bi-Maxwellian velocity distribution functions. This
presentation investigates some of the consequences of departures from
bi-Maxwellians, and treats the problem of ion cyclotron diffusion
in velocity space using the standard quasi-linear approximation. A
case will be made for the possibility that this diffusion acts on
time scales of same order as the solar wind expansion time (and not
on much shorter time scales as is often assumed). The distortion of
ion velocity distributions along curved "shells" in velocity space
will be explored, and the approach to marginal stability in the
linear damping rates is found to be much more accurately portrayed
than when using bi-Maxwellian distributions. The shapes and strengths
of ultraviolet emission line profiles are also affected by departures
from bi-Maxwellian distributions, and the possible impact on UVCS/SOHO
data analysis will be discussed.
Title: Coronal Holes
Authors: Cranmer, S.
Bibcode: 2000eaa..bookE1999C
Altcode:
Coronal holes are regions of low-density plasma on the Sun that have
magnetic fields that open freely into interplanetary space. During
times of low solar activity, coronal holes cover the north and south
polar caps of the Sun. During more active periods, coronal holes can
exist at all solar latitudes, but they may only persist for several
solar rotations before evolving into a different magnetic...
Title: Ultraviolet Spectroscopy of Polar Coronal Jets
Authors: Dobrzycka, D.; Raymond, J. C.; Cranmer, S. R.
Bibcode: 2000ApJ...538..922D
Altcode:
We have observed a total of five UVCS/SOHO polar jets that correlate
with the Extreme-Ultraviolet Imaging Telescope (EIT) and Large Angle
Spectrometric Coronagraph (LASCO) jet events. We analyzed spectroscopic
observations of these jets and found that they typically undergo
two phases: at the first phase the O VI lines show a brief intensity
enhancement (by a factor of 1.4) and narrowing (by a factor of 0.8),
while the H I Lyα line is not enhanced, and the second phase, about
25 minutes later, when the H I Lyα line shows an intensity enhancement
(by a factor of 1.3) and narrowing (by a factor of 0.8), while the O VI
line is relatively unchanged. We modeled the observable properties of
the jets from 1997 August 5, detected at 1.71 Rsolar. We
interpret the first phase as the fast, dense centroid of the jet
passing by the UVCS slit. The empirical jet model was able to reproduce
the observed line properties with electron density enhancement by
a factor of 3.2 (with a resulting density of 4.5×106
cm-3), an electron temperature decrease (change by a factor
of 0.50 to 750,000 K), and the centroid outflow velocity larger than
280 km s-1. During the second phase, the model required a
further decrease in the electron temperature (change by a factor of
0.10, with a jet temperature of only 150,000 K), along with a weaker
electron density (1.7×106 cm-3) and an outflow
velocity of 205 km s-1. Possible scenarios of the electron
temperature variations needed to account for observed conditions on
1997 August 5 indicate that some heating is required. We computed
models of the temperature and nonequilibrium ionization state of an
expanding plasma using various forms for the heating rates. We found
that the jet had to leave the Sun at an electron temperature below
2.5×106 K and that a heating rate of the same order as the
average coronal hole heating is required. Such low initial temperatures
are consistent with the idea that the jets observed by LASCO, EIT,
and UVCS are different than previously observed coronal X-ray jets.
Title: A Multiwavelength Campaign on γ Cassiopeiae. IV. The Case
for Illuminated Disk-enhanced Wind Streams
Authors: Cranmer, Steven R.; Smith, Myron A.; Robinson, Richard D.
Bibcode: 2000ApJ...537..433C
Altcode:
On 1996 March 14-15 we conducted a campaign with the Hubble Space
Telescope GHRS to observe the Si IV λλ1394, 1403 lines of the B0.5e
star γ Cas at high temporal and spectral resolution. As a part of this
~22 hr campaign, the Rossi X-Ray Timing Explorer (RXTE) was also used
to monitor this star's copious and variable X-ray emission. In this
fourth paper of a series we present an analysis of the rapid variations
of the discrete absorption components (DACs) of the Si IV doublet. The
DACs attain a maximum absorption at -1280 km s-1, taper
at higher velocities, and extend to -1800 km s-1. The DACs
in this star's resonance lines have been shown to be correlated with
a >~6 yr cycle in the Balmer line emission V/R ratio, and in 1996
this DAC strength was near its maximum. We derive hydrogen densities of
109-1010 cm-3 in the DAC material using
a curve-of-growth method and find that the plasma becomes marginally
optically thick near -1280 km s-1. The ``mean DAC'' probably
represents a broad ``plateau'' with a volume density intermediate
between the star's midlatitude wind and equatorial disk. We also follow
the blueward evolution of subfeatures in the DACs. These features
appear to emanate primarily from one or two discrete azimuths on the
star and accelerate much more slowly than expected for the background
wind, thereby exhibiting an enhanced opacity spiral stream pattern
embedded within the structure forming the DAC. In the first two papers
in this series, we suggested that active X-ray centers are associated
with at least two major cool clouds forced into corotation. Several
correlations of flickering in the Si IV DACs are found in our data,
which support the idea that changes in X-ray ionizing flux cause
changes in the ionization of material at various sectors along the
spiral pattern. We demonstrate that similar flickering is visible in
archival IUE data from 1982 and may also be responsible for earlier
reports from Copernicus of rapid changes in this star's UV and optical
lines. Finally, we discovered that flickering of the DAC fluxes in the
1982 data is correlated with rotation phase and shows a modulation
with a 7.5 hr cyclical cessation of X-ray flares that was observed
recently by RXTE. This confirms our basic picture that lulls in X-ray
activity close to the star's surface cause both a lower Si V ionization
fraction and an increase in Si IV variability within the DAC structures.
Title: UVCS/SOHO Observations of Equatorial and Polar Coronal Holes
Authors: Kohl, J. L.; Miralles, M. P.; Cranmer, S. R.; Suleiman, R. M.
Bibcode: 2000SPD....31.0232K
Altcode: 2000BAAS...32..816K
A large equatorial coronal hole was observed above the west limb with
the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO from November
1999 to March 2000. Observations in H I Lyα and O VI 103.2, 103.7
nm provided spectroscopic diagnostics of proton and O5+
velocity distributions and outflow velocities. These properties will be
compared to those of the large polar coronal holes observed near solar
minimum. The equatorial coronal hole corresponded to a high-speed
solar wind stream at 1 AU, but there were significant differences
between the interplanetary properties of this stream and the steady
high-speed wind seen over the poles at solar minimum. The several
obvious differences between the two structures in the extended corona
may be associated with the different densities and magnetic field
configurations and flux tube expansion factors. Preliminary results
from a detailed empirical model of the equatorial coronal hole will
be presented. This work is supported by NASA under Grant NAG5-7822 to
the Smithsonian Astrophysical Observatory, by the Italian Space Agency
and by PRODEX (Swiss contribution).
Title: New Diagnostics of Coronal Heating and Solar Wind Acceleration
Processes Achievable With The Advanced Solar Coronal Explorer (ASCE)
Authors: Cranmer, S. R.; Kohl, J. L.; Gardner, L. D.; Raymond, J. C.;
Strachan, L.; Smith, P. L.; Howard, R. A.; Davila, J. M.; Fisher,
R. R.; Noci, G.; Tondello, G.; Socker, D. G.; Moses, D.
Bibcode: 2000SPD....31.0297C
Altcode: 2000BAAS...32..828C
The Advanced Solar Coronal Explorer (ASCE) is a proposed NASA
Medium-class Explorer (MIDEX) mission that underwent a detailed
Concept Study in 1999. The science payload includes large aperture EUV
and visible light coronagraphs. ASCE's unprecedented spectral range,
spatial resolution, and sensitivity (30 to 100 times the EUV sensitivity
of UVCS/SOHO) provide measurements needed to investigate the role of
high-frequency and low-frequency waves in heating and accelerating
the fast and slow speed solar wind. This presentation will outline
the advanced capabilities of ASCE for obtaining detailed empirical
descriptions of solar wind acceleration regions, specifying coronal
temperatures, flow speeds, densities, and elemental abundances. Velocity
distributions for electrons and more than 10 to 20 ion species with
mass-to-charge ratios from 4 to 1 (including singly ionized helium) can
be measured by ASCE in coronal holes and streamers. This information
is sufficient to derive the wavenumber power spectrum of magnetic
fluctuations that affect the primary electron/proton plasma. The main
goal is to identify the physical processes responsible for heating
and acceleration of the primary particles and minor ions in the fast
and slow speed solar wind.
Title: Surf's Still Up: UVCS/SOHO Observations as Strong Constraints
on Coronal Heating Theories
Authors: Cranmer, S. R.; Kohl, J. L.
Bibcode: 2000SPD....31.1502C
Altcode: 2000BAAS...32..848C
In 1996, the Ultraviolet Coronagraph Spectrometer (UVCS) instrument
aboard SOHO observed surprisingly broad line profiles of the O VI 1032,
1037 doublet in polar coronal holes. These measurements indicated
perpendicular ion temperatures of at least 100--200 million K above
two solar radii in the nascent high-speed solar wind. Since then,
these observations have been supplemented by profiles of other ions,
Doppler dimming measurements made possible by Spartan 201, and a
great deal of theoretical work. This talk outlines the current state of
understanding about coronal heating and solar wind acceleration that has
been facilitated by UVCS. The most promising mechanism for heating and
accelerating minor ions remains the dissipation of high-frequency (10 to
10,000 Hz) ion cyclotron waves, but heating the protons is a more open
question. The physics of the ion cyclotron interaction in the corona has
only begun to be explored, and we will discuss recent insights into the
generation and damping of these waves. A self-consistent theory of wave
damping and turbulent cascade ``replenishment'' would allow the question
of proton heating to be answered more definitively. Also, a kinetic
approach to ion cyclotron heating yields non-bi-Maxwellian ``resonant
shell'' velocity distributions that could produce emission line profiles
narrower than expected from their most probable speeds. Thus, the UVCS
measurements of 100--200 million K ion temperatures may only be lower
limits. This work is supported by the National Aeronautics and Space
Administration under grant NAG5-7822 to the Smithsonian Astrophysical
Observatory, by Agenzia Spaziale Italiana, and by the ESA PRODEX program
(Swiss contribution).
Title: Ion Cyclotron Wave Dissipation in the Solar Corona: The Summed
Effect of More than 2000 Ion Species
Authors: Cranmer, Steven R.
Bibcode: 2000ApJ...532.1197C
Altcode:
In this paper the dissipation of ion cyclotron resonant Alfvén waves
in the extended solar corona is examined in detail. For the first
time, the wave damping arising from more than 2000 low-abundance
ion species is taken into account. Useful approximations for the
computation of coronal ionization equilibria for elements heavier than
nickel are presented. Also, the Sobolev approximation from the theory
of hot-star winds is applied to the resonant wave dissipation in the
solar wind, and the surprisingly effective damping ability of ``minor''
ions is explained in simple terms. High-frequency (10-10,000 Hz) waves
propagating up from the base of the corona are damped significantly when
they resonate with ions having charge-to-mass ratios of about 0.1, and
negligible wave power would then be available to resonate with higher
charge-to-mass ratio ions at larger heights. This result confirms
preliminary suggestions from earlier work that the waves that heat
and accelerate the high-speed solar wind must be generated throughout
the extended corona. The competition and eventual equilibrium between
wave damping and wave replenishment may explain observed differences
in coronal O VI and Mg X emission line widths.
Title: On the Generation and Dissipation of Ion Cyclotron Waves in
the Extended Solar Corona
Authors: Cranmer, S. R.
Bibcode: 1999EOSTr..80..800C
Altcode:
The dissipation of high frequency (10 to 10,000 Hz) ion cyclotron
resonant Alfven waves has been proposed as a leading candidate for the
heating of the extended solar corona and the acceleration of the high
speed solar wind. The competition between various wave generation
mechanisms and resonant wave damping is examined in detail, and a
database of more than 2000 low-abundance ion species is taken into
account for completeness. Also, the Sobolev approximation from the
theory of hot star winds is applied to the gyroresonant wave-particle
interaction in the solar wind, and the surprisingly effective damping
ability of ``minor'' ions is explained in simple terms. High frequency
waves (propagating parallel to open magnetic field lines) that originate
at the base of the corona are damped significantly when they resonate
with ions having charge-to-mass ratios of about 0.1. Thus, if the
waves came solely from the coronal base, there would be negligible
wave power available to resonate with higher charge-to-mass ratio ions
at larger heights. This result confirms preliminary suggestions from
earlier work that the waves that heat and accelerate the high speed
solar wind must be generated throughout the extended corona. This work
is supported by the National Aeronautics and Space Administration under
grant NAG5-7822 to the Smithsonian Astrophysical Observatory, by Agenzia
Spaziale Italiana, and by the ESA PRODEX program (Swiss contribution).
Title: The impact of ion-cyclotron wave dissipation on heating and
accelerating the fast solar wind
Authors: Cranmer, Steven R.; Field, George B.; Kohl, John L.
Bibcode: 1999AIPC..471...35C
Altcode: 1999sowi.conf...35C
Using empirical ion velocity distributions derived from UVCS and
SUMER ultraviolet spectroscopy, we construct theoretical models of
the nonequilibrium plasma state of the polar solar corona. The primary
energy deposition mechanism we investigate is the dissipation of high
frequency (10-10000 Hz) ion-cyclotron resonant Alfvén waves which
can heat and accelerate ions differently depending on their charge
and mass. We find that it is possible to explain many of the kinetic
properties of the plasma with relatively small amplitudes for the
resonant waves. There is evidence for steepening of the Alfvén wave
spectrum between the coronal base and the largest heights observed
spectroscopically, and it is important to take Coulomb collisions into
account to understand observations at the lowest heights. Because the
ion-cyclotron wave dissipation is rapid, the extended heating seems to
demand a constantly replenished population of waves over several solar
radii. This indicates that the waves are probably generated throughout
the wind rather than propagated up from the base of the corona.
Title: Spectroscopic Constraints on Models of Ion Cyclotron Resonance
Heating in the Polar Solar Corona and High-Speed Solar Wind
Authors: Cranmer, Steven R.; Field, George B.; Kohl, John L.
Bibcode: 1999ApJ...518..937C
Altcode:
Using empirical ion velocity distributions derived from Ultraviolet
Coronagraph Spectrometer (UVCS) and Solar Ultraviolet Measurements
of Emitted Radiation (SUMER) ultraviolet spectroscopy, we construct
theoretical models of the nonequilibrium plasma state of the polar solar
corona. The primary energy deposition mechanism we investigate is the
dissipation of high-frequency (10-10,000 Hz) ion cyclotron resonant
Alfvén waves which can heat and accelerate ions differently depending
on their charge and mass. We solve the internal energy conservation
equations for the ion temperature components parallel and perpendicular
to the superradially expanding magnetic field lines and use empirical
constraints for the remaining parameters. We find that it is possible
to explain many of the kinetic properties of the plasma (such as high
perpendicular ion temperatures and strong temperature anisotropies)
with relatively small amplitudes for the resonant waves. There is
suggestive evidence for steepening of the Alfvén wave spectrum between
the coronal base and the largest heights observed spectroscopically, and
it is important to take Coulomb collisions into account to understand
observations at the lowest heights. Because the ion cyclotron wave
dissipation is rapid, the extended heating seems to demand a constantly
replenished population of waves over several solar radii. This indicates
that the waves are generated gradually throughout the wind rather than
propagated up from the base of the corona.
Title: Study of the latitudinal dependence of H I Lyman α and O VI
emission in the solar corona: Evidence for the superradial geometry
of the outflow in the polar coronal holes
Authors: Dobrzycka, Danuta; Cranmer, Steven R.; Panasyuk, Alexander
V.; Strachan, Leonard; Kohl, John L.
Bibcode: 1999JGR...104.9791D
Altcode:
We study the latitudinal distribution of the H I Lyman α and O VI
(103.2 nm and 103.7 nm) line emission during the period of the Whole
Sun Month campaign (August 10 to September 8, 1996) when the Sun was
close to the minimum of its activity. The H I Lyman α and O VI line
intensities appeared to be almost constant with latitude within the
polar coronal holes and have abrupt increases toward the streamer
region. We found that both north and south polar coronal holes had
similar line intensities and line-of-sight velocities, as well as
kinetic temperatures of H0 and O5+. The dependence
of these parameters on latitude and radius is provided. We derived
boundaries of the polar coronal holes based on the H I Lyman α and O VI
line intensity distributions for several days during the Whole Sun Month
campaign. We found that the polar coronal hole boundaries clearly have
a superradial geometry with diverging factor fmax ranging
from 6.0 to 7.5, and they are consistent with boundaries previously
derived from the electron density distributions. We also found that,
in general, they are not symmetric with respect to the heliographic
poles, and their size and geometry change over periods of days. The H
I Lyman α, O VI (103.2 nm), and the O VI (103.7 nm) line intensities
showed similar boundaries within the uncertainties of our data. We
modeled the latitudinal distribution of the H I Lyman α and O VI
(103.2 nm and 103.7 nm) line intensities in the south polar coronal
hole on August 17, 1996, assuming the coronal plasma outflow along
either purely radial or nonradial flux tubes. A comparison of model
predictions with the observed distributions shows evidence that the
outflow velocity vectors follow nonradial intensity pattern.
Title: New Insights into Solar Coronal Plasma Kinetics from UVCS/SOHO
Authors: Cranmer, S. R.
Bibcode: 1999AAS...194.3206C
Altcode: 1999BAAS...31Q.871C
The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) has measured
anisotropic temperatures and differential outflow velocities for
hydrogen, oxygen, and magnesium ions in polar coronal holes. Line widths
of the O VI 1032, 1037 doublet indicate perpendicular temperatures of at
least 200 million K above 2 solar radii. We present theoretical models
of the dissipation of high frequency (10 to 10,000 Hz) ion cyclotron
resonant Alfven waves, and we find that it is possible to explain many
of the observed kinetic properties of the plasma with relatively small
wave amplitudes. There is suggestive evidence that such waves should be
generated gradually throughout the wind rather than propagated up from
the base of the corona. We also discuss how additional insight into the
ion cyclotron resonance interaction can be obtained by considering the
process as an analogue of Sobolev-theory radiative transfer. This work
is supported by the National Aeronautics and Space Administration under
grant NAG5-3192 to the Smithsonian Astrophysical Observatory, by Agenzia
Spaziale Italiana, and by the ESA PRODEX program (Swiss contribution).
Title: The Advanced Solar Coronal Explorer Mission (ASCE)
Authors: Kohl, J.; Cranmer, S.; Gardner, L.; Golub, L.; Raymond, J.;
Smith, P. L.; Strachan, L.; Howard, R.; Moses, D.; Socker, D.; Wang,
D.; Fisher, R. R.; Davila, J.; St. Cyr, C.; Noci, G.; Tondello, G.
Bibcode: 1999AAS...194.6506K
Altcode: 1999BAAS...31Q.928K
The Advanced Solar Coronal Explorer (ASCE) mission was selected
for a Phase A Concept Study in the current round of proposed MIDEX
missions. It addresses three fundamental problems: 1) What physical
processes heat coronal holes and drive the fast solar wind? 2) What
physical processes heat streamers and drive the slow solar wind? and 3)
How are coronal mass ejections (CMEs) heated and accelerated, and what
role to they play in the evolution of the solar magnetic field. ASCE
has two instruments, the Spectroscopic and Polarimetric Coronagraph
(SPC) and the Extreme Ultraviolet Imager (EUVI). A deployable boom
supports a distant external occulter that allows large aperture optics
for the SPC coronagraphic channels. SPC's EUV channels will provide
spectroscopy of the extended solar corona with 30 - 200 times the
sensitivity of UVCS/SOHO and the first He II 30.4 nm spectroscopy of
the extended corona. SPC's Large Aperture Spectroscopic Coronagraph
channel will provide two orders of magnitude improvement in stray
light suppression for wide field visible spectroscopy and 2 arcsec
resolution elements for imaging and polarimetry. EUVI provides full
disk imaging with 0.9 arcsec resolution elements and extremely high
cadence. ASCE is designed to determine the thermal, kinetic, and
wave energy densities in coronal structures, determine the rates of
transformation among these forms of energy, their flow in space, and
their loss to radiation, and determine the composition and ionization
state of the corona in static and transient conditions.
Title: Line-driven Ablation and Wind Tilting by External Irradiation
Authors: Gayley, K. G.; Owocki, S. P.; Cranmer, S. R.
Bibcode: 1999ApJ...513..442G
Altcode:
The directional variation of the velocity gradient in a supersonic,
radiatively accelerated flow gives an effectively anisotropic
character to the line-scattering process. This leads to surprising
consequences in source geometries that are more complex than isolated
nonrotating stars. In this paper we explore the wind dynamics from
a planar slab atmosphere that is irradiated by an external oblique
source, within the framework of standard Castor, Abbott, & Klein
(CAK) wind theory. We show that the presence of externally incident
radiation can be surprisingly effective at tilting the flow away
from the vertical. Even more surprising is our conclusion that such
illumination should often enhance the mass loss and can even induce
outflow from a surface with no intrinsic radiation source. We examine
the physical causes of such ``line-driven ablation'' and discuss the
potential implications for modeling line-driven flows in massive-star
binary and accretion-disk systems.
Title: An Empirical Model of a Polar Coronal Hole at Solar Minimum
Authors: Cranmer, S. R.; Kohl, J. L.; Noci, G.; Antonucci, E.;
Tondello, G.; Huber, M. C. E.; Strachan, L.; Panasyuk, A. V.;
Gardner, L. D.; Romoli, M.; Fineschi, S.; Dobrzycka, D.; Raymond,
J. C.; Nicolosi, P.; Siegmund, O. H. W.; Spadaro, D.; Benna, C.;
Ciaravella, A.; Giordano, S.; Habbal, S. R.; Karovska, M.; Li, X.;
Martin, R.; Michels, J. G.; Modigliani, A.; Naletto, G.; O'Neal,
R. H.; Pernechele, C.; Poletto, G.; Smith, P. L.; Suleiman, R. M.
Bibcode: 1999ApJ...511..481C
Altcode:
We present a comprehensive and self-consistent empirical model
for several plasma parameters in the extended solar corona above
a polar coronal hole. The model is derived from observations
with the SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO)
during the period between 1996 November and 1997 April. We compare
observations of H I Lyα and O VI λλ1032, 1037 emission lines
with detailed three-dimensional models of the plasma parameters and
iterate for optimal consistency between measured and synthesized
observable quantities. Empirical constraints are obtained for
the radial and latitudinal distribution of density for electrons,
H0, and O5+, as well as the outflow velocity
and unresolved anisotropic most probable speeds for H0 and
O5+. The electron density measured by UVCS/SOHO is consistent
with previous solar minimum determinations of the white-light coronal
structure; we also perform a statistical analysis of the distribution
of polar plumes using a long time series. From the emission lines we
find that the unexpectedly large line widths of H0 atoms
and O5+ ions at most heights are the result of anisotropic
velocity distributions. These distributions are not consistent with
purely thermal motions or the expected motions from a combination of
thermal and transverse wave velocities. Above 2 Rsolar,
the observed transverse most probable speeds for O5+ are
significantly larger than the corresponding motions for H0,
and the outflow velocities of O5+ are also significantly
larger than the corresponding velocities of H0. Also, the
latitudinal dependence of intensity constrains the geometry of the
wind velocity vectors, and superradial expansion is more consistent
with observations than radial flow. We discuss the constraints and
implications on various theoretical models of coronal heating and
acceleration.
Title: EUV Spectral Line Profiles in Polar Coronal Holes from 1.3
to 3.0 Rsolar
Authors: Kohl, J. L.; Esser, R.; Cranmer, S. R.; Fineschi, S.; Gardner,
L. D.; Panasyuk, A. V.; Strachan, L.; Suleiman, R. M.; Frazin, R. A.;
Noci, G.
Bibcode: 1999ApJ...510L..59K
Altcode:
Spectral line profiles have been measured for H I λ1216, O VI λλ1032,
1037, and Mg X λ625 in a polar coronal hole observed during 1997
September 15-29, at projected heliographic heights ρ between 1.34
and 2.0 Rsolar. Observations of H I λ1216 and the O
VI doublet from 1997 January for ρ=1.5-3.0 Rsolar are
provided for comparison. The O VI lines are well fit to a narrow and
broad component which appear to be associated with regions of higher
and lower spectral radiance, respectively. The narrow components
dominate at low heights and become a small fraction of the lines at
higher heights. Mg X λ625 is observed to have a narrow component at
ρ=1.34 Rsolar which accounts for only a small fraction of
the observed spectral radiance. In the case of the broad components,
the values of v1/e for O VI are only slightly larger than
those for H I at ρ=1.34 Rsolar but are significantly
larger at ρ=1.5 Rsolar and much larger for ρ>1.75
Rsolar. In contrast, the Mg X values are less than those
of H I up to 1.75 and then increase rapidly up to at least ρ=2.0
Rsolar but never reach the values of O VI.
Title: UVCS/SOHO Observations of H I Lyman Alpha Line Profiles in
Coronal Holes at Heliocentric Heights Above 3.0 R⊙
Authors: Suleiman, R. M.; Kohl, J. L.; Panasyuk, A. V.; Ciaravella,
A.; Cranmer, S. R.; Gardner, L. D.; Frazin, R.; Hauck, R.; Smith,
P. L.; Noci, G.
Bibcode: 1999SSRv...87..327S
Altcode:
The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and
Heliospheric Observatory (SOHO) has been used to measure spectral line
profiles for H I Lyα in the south polar coronal hole at projected
heliocentric heights from 3.5 to 6.0 R⊙ during 1998
January 5 11. Observations from 1.5 to 2.5 R⊙ were made
for comparison. The H I Lyα profile is the only one observable with
UVCS above 3.5 R⊙ in coronal holes. Within this region
the outflowing coronal plasma becomes nearly collisionless and the
ionization balance is believed to become frozen.
Title: Coronal Holes and Solar Wind Acceleration, Proceedings of
the SOHO-7 Workshop.
Authors: Kohl, John L.; Cranmer, Steven R.
Bibcode: 1999soho....7.....K
Altcode:
No abstract at ADS
Title: Foreword
Authors: Kohl, John L.; Cranmer, Steven R.
Bibcode: 1999SSRv...87....9K
Altcode:
No abstract at ADS
Title: Line-Driven Ablation by External Irradiation
Authors: Gayley, Kenneth G.; Owocki, Stanley P.; Cranmer, Steven R.
Bibcode: 1999LNP...523..151G
Altcode: 1999vnss.conf..151G; 1999IAUCo.169..151G
The Sobolev approximation for supersonic flows creates an effective
opacity distribution that is nonisotropic, because the line-of-sight
velocity gradient is different in different directions. To better
understand the importance of this phenomenon in a simplified geometry,
we consider line-driven flows in the plane-parallel zero-sound-speed
limit, and solve for the wind driven by radiation with an arbitrary
angular distribution. One conclusion, surprising at first glance, is
that the acceleration component normal to the surface is independent
of both the strength and angular profile of the driving radiation
field. The flow tilt and overall mass-loss rate do depend on the
character of the radiation field. Also interesting is that mass loss
through a surface may be generated or enhanced by irradiation that
originates above the surface.
Title: UVCS/SOHO Observations of Spectral Line Profiles in Polar
Coronal Holes
Authors: Kohl, J. L.; Fineschi, S.; Esser, R.; Ciaravella, A.; Cranmer,
S. R.; Gardner, L. D.; Suleiman, R.; Noci, G.; Modigliani, A.
Bibcode: 1999SSRv...87..233K
Altcode:
Ultraviolet emission line profiles have been measured on 15-29 September
1997 for H I 1216 Å, O VI 1032, 1037 Å and Mg X 625 Å in a polar
coronal hole, at heliographic heights ϱ (in solar radii) between 1.34
and 2.0. Observations of H I 1216 Å and the O VI doublet from January
1997 for ϱ = 1.5 to 3.0 are provided for comparison. Mg X 625 Å is
observed to have a narrow component at ϱ = 1.34 which accounts for
only a small fraction of the observed spectral radiance, and a broad
component that exists at all observed heights. The widths of O VI broad
components are only slightly larger than those for H I at ϱ = 1.34,
but are significantly larger at ϱ = 1.5 and much larger for ϱ >
1.75. In contrast, the Mg X values are less than those of H I up to
1.75 and then increase rapidly up to at least ϱ = 2.0, but never
reach the values of O VI.
Title: Coronal holes and solar wind acceleration. Proceedings. SOHO-7
Workshop, Northeast Harbor, ME (USA), 28 Sep - 1 Oct 1998.
Authors: Kohl, J. L.; Cranmer, S. R.
Bibcode: 1999SSRv...87.....K
Altcode:
The following topics were dealt with: solar coronal holes, solar wind
acceleration, solar wind models, high speed solar wind, compositional
variations, coronal loops, solar magnetic fields, plasma waves, solar
polar region, coronal heating, streamers.
Title: Spectroscopic Constraints on Models of Ion-cyclotron Resonance
Heating in the Polar Solar Corona
Authors: Cranmer, S. R.; Field, G. B.; Kohl, J. L.
Bibcode: 1999SSRv...87..149C
Altcode:
Using empirical velocity distributions derived from UVCS and
SUMER ultraviolet spectroscopy, we construct theoretical models of
anisotropic ion temperatures in the polar solar corona. The primary
energy deposition mechanism we investigate is the dissipation of high
frequency (10-10000 Hz) ion-cyclotron resonant Alfvén waves which
can heat and accelerate ions differently depending on their charge
and mass. We find that it is possible to explain the observed high
perpendicular temperatures and strong anisotropies with relatively small
amplitudes for the resonant waves. There is suggestive evidence for
steepening of the Alfvén wave spectrum between the coronal base and the
largest heights observed spectroscopically. Because the ion-cyclotron
wave dissipation is rapid, even for minor ions like O5+,
the observed extended heating seems to demand a constantly replenished
population of waves over several solar radii. This indicates that
the waves are generated gradually throughout the wind rather than
propagated up from the base of the corona.
Title: Non-Maxwellian Redistribution in Solar Coronal Lyα Emission
Authors: Cranmer, Steven R.
Bibcode: 1998ApJ...508..925C
Altcode:
This paper presents theoretical models of H I Lyα emission from
the extended solar corona, taking into account various plasma
kinetic effects that induce departures from Maxwellian velocity
distributions. Such phenomena as suprathermal tails, strong temperature
anisotropies, and skewed or double-peaked distributions have been
observed in the solar wind, and UV spectroscopy is beginning to
be able to detect their signatures in the corona. For resonantly
scattered lines like H I Lyα, most of the physics is contained in the
frequency-dependent redistribution function. The dependence of this
function on the local plasma parameters is presented analytically for
four different non-Maxwellian distributions, and optically thin line
intensities are computed for a representative model of the fast solar
wind. Isotropic power-law ``κ'' tails in the velocity distribution
should be detectable between 2 and 5 Å from line center. Although
existing observations that appear to have broad tails do not
resemble those arising from κ-distributions, their presence is still
possible. Anisotropic bi-Maxwellian distributions affect line profile
shapes and total intensities via both their parallel and perpendicular
components, and it is important to include an accurate description of
the photon redistribution for large anisotropies. Skewness caused by a
Chapman-Enskog expansion in the conductive heat flux is detectable as
a unique non-Gaussian profile shape, but other types of collisionally
beamed or skewed distributions may not noticeably affect the emission
lines.
Title: Spectroscopic Constraints on Models of Ion-Cyclotron Resonance
Heating in the Polar Solar Corona and Fast Solar Wind
Authors: Cranmer, S. R.; Field, G. B.; Kohl, J. L.
Bibcode: 1998EOSTr..79..722C
Altcode:
We present preliminary results from a theoretical model of the heating
of minor ions in the fast solar wind. We examine the compatibility
between these models and spectroscopic determinations of velocity
distribution functions from the UVCS and SUMER instruments aboard
SOHO. By examining the dependence of line shapes (which probe
the perpendicular velocity distribution) on ion charge and mass,
detailed information can be extracted about the preferential heating
and the Coulomb collisional coupling. The primary momentum and energy
deposition mechanism we investigate is the dissipation of high-frequency
(ion-cyclotron resonant) Alfven waves, which can accelerate and heat
ions differently depending on their charge and mass. Minor ions which
do not appreciably damp the resonant wave amplitudes can be used to
constrain the slope of the fluctuation spectrum. SUMER measurements of
several ions at heliocentric heights between 1.02 and 1.07 solar radii
allow the ``base'' spectrum to be analyzed, and UVCS O VI line widths
measured between 1.5 and 3.5 solar radii provide information about
the radial evolution of the spectrum. This work is supported by the
National Aeronautics and Space Administration under grant NAG5-3192 to
the Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana,
and by the ESA PRODEX program (Swiss contribution).
Title: Mass Loss from Rotating Hot-stars: Inhibition of Wind
Compressed Disks by Nonradial Line-forces
Authors: Owocki, S. P.; Cranmer, S. R.; Gayley, K. G.
Bibcode: 1998Ap&SS.260..149O
Altcode:
We review the dynamics of radiatively driven mass loss from rapidly
rotating hot-stars. We first summarize the angular momentum conservation
process that leads to formation of a Wind Compressed Disk(WCD) when
material from a rapidly rotating star is driven gradually outward
in the radial direction. We next describe how stellar oblateness
and asymmetries in the Sobolev line-resonance generally leads to
nonradialcomponents of the driving force is a line-driven wind,
including an azimuthal spin-down force acting against the sense of
the wind rotation, and a latitudinal force away from the equator. We
summarize results from radiation-hydrodynamical simulations showing
that these nonradial forces can lead to an effective suppressionof
the equatorward flow needed to form a WCD, as well as a modest
(∼ 25%) spin-downof the wind rotation. Furthermore, contrary to
previous expectations that the wind mass flux should be enhanced by the
reduced effective gravity near the equator, we show here that gravity
darkening effects can actually lead to a reducedmass loss, and thus
lower density, in the wind from the equatorial region. Finally, we
examine the equatorial bistability model, and show that a sufficiently
strong jump in wind driving parameters can, in principle, overcome the
effect of reduced radiative driving flux, thus still allowing moderate
enhancements in density in an equatorial, bistability zone wind.
Title: UVCS/SOHO Empirical Determinations of Anisotropic Velocity
Distributions in the Solar Corona
Authors: Kohl, J. L.; Noci, G.; Antonucci, E.; Tondello, G.; Huber,
M. C. E.; Cranmer, S. R.; Strachan, L.; Panasyuk, A. V.; Gardner,
L. D.; Romoli, M.; Fineschi, S.; Dobrzycka, D.; Raymond, J. C.;
Nicolosi, P.; Siegmund, O. H. W.; Spadaro, D.; Benna, C.; Ciaravella,
A.; Giordano, S.; Habbal, S. R.; Karovska, M.; Li, X.; Martin, R.;
Michels, J. G.; Modigliani, A.; Naletto, G.; O'Neal, R. H.; Pernechele,
C.; Poletto, G.; Smith, P. L.; Suleiman, R. M.
Bibcode: 1998ApJ...501L.127K
Altcode:
We present a self-consistent empirical model for several plasma
parameters of a polar coronal hole near solar minimum, derived from
observations with the Solar and Heliospheric Observatory Ultraviolet
Coronagraph Spectrometer. The model describes the radial distribution of
density for electrons, H0, and O5+ and the outflow
velocity and unresolved most probable velocities for H0
and O5+ during the period between 1996 November and 1997
April. In this Letter, we compare observations of H I Lyα and O
VI λλ1032, 1037 emission lines with spatial models of the plasma
parameters, and we iterate for optimal consistency between measured and
synthesized observable quantities. The unexpectedly large line widths
of H0 atoms and O5+ ions at most radii are the
result of anisotropic velocity distributions, which are not consistent
with purely thermal motions or the expected motions from a combination
of thermal and transverse wave velocities. Above 2 Rsolar,
the observed transverse, most probable speeds for O5+ are
significantly larger than the corresponding motions for H0,
and the outflow velocities of O5+ are also significantly
larger than the corresponding velocities of H0. We discuss
the constraints and implications on various theoretical models of
coronal heating and acceleration.
Title: Line-Driven Ablation and Wind Tilting by External Irradiation
Authors: Gayley, K.; Owocki, S.; Cranmer, S.
Bibcode: 1998AAS...192.2603G
Altcode: 1998BAAS...30..850G
Sobolev opacity in a hot-star wind preferentially scatters photons that
are incident along the direction of steepest velocity gradient. This
non-isotropic response can rotate the force vector relative to the
direction of net radiative flux, in a manner analogous to the way a
non-isotropic sail and keel can allow a boat to sail upwind. For hot
star binaries, the curious feedback between the radiative forces and
the flows they drive allows for counter-intuitive self-consistent
solutions. For example, we show that illumination that is purely
external to a reflecting radiative-equilibrium atmosphere can ablate
a highly tilted and fast wind, loosely reminiscent of ``tacking''
in the sailing analogy. The conclusion is that whenever the radiation
source geometry is complicated, the non-isotropic nature of Sobolev
opacity must be carefully accounted for to obtain even a qualitative
understanding of the atmospheric response. Thus CAK theory continues
to reveal new surprises even in its most basic formulation.
Title: UVCS/SOHO: The First Two Years
Authors: Cranmer, S. R.; Kohl, J. L.; Noci, G.
Bibcode: 1998SSRv...85..341C
Altcode:
The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) has observed
the extended solar corona between 1 and 10 R· for more than two
years. We review spectroscopic and polarimetric measurements made
in coronal holes, equatorial streamers, and coronal mass ejections,
as well as selected non-solar targets. UVCS/SOHO has provided a great
amount of empirical information about the physical processes that heat
and accelerate the solar wind, and about detailed coronal structure
and evolution.
Title: Study of the Latitudinal Dependence of HI Lyman alpha and OVI
Emission - Evidence for the Super-Radial Geometry of the Outflow in
the Polar Coronal Holes
Authors: Dobrzycka, D.; Strachan, L.; Panasyuk, A.; Cranmer, S. R.;
Kohl, J. L.; Romoli, M.
Bibcode: 1998EOSTr..79..283D
Altcode:
The observations obtained during the Whole Sun Month (WSM) campaign
(Aug.10 - Sept.08, 1996) provide us with detailed information about
the Sun and solar corona near the minimum of solar activity. This data
set is especially important for analysis of the latitudinal dependence
of the emission from the solar corona, since at solar minimum the
polar coronal holes are large, stable structures and streamers are
long lasting features occupying a narrow region in the equatorial
plane. We present our analysis of the UVCS/SOHO data acquired during
the period of the WSM campaign. We describe the distribution of the
HI Lyman alpha and O VI (1032,1037 A) emission as a function of both
latitude and radius and derive characteristic plasma parameters like
velocities along the line of sight and kinetic temperatures. We put
constraints on the boundaries of the polar coronal holes by modeling
them with flow tubes that expand radially or super-radially and
compare our results with constraints from other instruments. We also
model the latitudinal distribution of the Lyman alpha and O VI line
intensities assuming outflow either along purely radial or non-radial
flux tubes. This work was supported by NASA under Grant NAG5-3192 to
the Smithsonian Astrophysical Observatory, by the Italian Space Agency,
and by Swiss Funding Agencies.
Title: Acceleration and Heating of the Fast Solar Wind: Diagnostics
and Theory
Authors: Cranmer, S. R.; Field, G. B.; Noci, G.; Kohl, J. L.
Bibcode: 1998EOSTr..79..278C
Altcode:
We present empirical models based on UVCS/SOHO observational data
for several plasma parameters in the fast solar wind emerging
from near-solar-minimum polar coronal holes. We also discuss the
constraints on various theoretical processes of coronal heating and
acceleration, and present preliminary kinetic models of the velocity
distributions. UVCS/SOHO has measured hydrogen kinetic temperatures
in polar coronal holes in excess of 3 million K, and O5+
ion kinetic temperatures of at least 200 million K. The velocity
distributions parallel to the open magnetic field are smaller than
those perpendicular to the field, possibly implying temperature
anisotropy ratios of order 100 for minor ions. In addition, Doppler
dimming and pumping of the emission line intensities indicates that the
O5+ ions may have higher outflow velocities than the protons;
the ions may reach 500 km/s by a radius of 4 solar radii. We examine
various features of plasma heating by the dissipation of high-frequency
ion-cyclotron resonance Alfven waves, which may be the most natural
physical mechanism to produce the observed plasma conditions. This
resonant wave damping produces anisotropic velocity-space diffusion and
a significant outward acceleration in addition to that provided to ions
by the magnetic mirror force. Because different ions have different
resonant frequencies, they receive different amounts of heating and
acceleration as a function of radius. Thus, the more ionic species
that are observed, the greater the extent in frequency space the wave
spectrum can be inferred and spatially mapped.
Title: Latitudinal Dependence of Radiatively Driven Mass Loss from
Rapidly Rotating Hot-Stars
Authors: Owocki, S. P.; Cranmer, S. R.; Gayley, K. G.
Bibcode: 1998ASSL..233..205O
Altcode: 1998best.work..205O
No abstract at ADS
Title: Effects of Gravity Darkening on Radiatively Driven Mass Loss
from Rapidly Rotating Stars
Authors: Owocki, S. P.; Gayley, K. G.; Cranmer, S. R.
Bibcode: 1998ASPC..131..237O
Altcode: 1998phls.conf..237O
No abstract at ADS
Title: UVCS/SOHO: The First Two Years
Authors: Cranmer, S. R.; Kohl, J. L.; Noci, G.
Bibcode: 1998sce..conf..341C
Altcode:
No abstract at ADS
Title: The Impact of UVCS/SOHO Observations on Models of Ion-Cyclotron
Resonance Heating of the Solar Corona
Authors: Cranmer, Steven R.; Field, George B.; Kohl, John L.
Bibcode: 1998ASPC..154..592C
Altcode: 1998csss...10..592C
We examine the compatibility between theoretical models and observations
of the temperatures and anisotropic velocity distributions of hydrogen
and minor ions in the solar corona. The UVCS instrument on board SOHO
has measured hydrogen kinetic temperatures along lines of sight in
coronal holes in excess of 3 x 106 K, and O^{+5} ion kinetic
temperatures of at least 2 x 108 K. In addition, the velocity
distributions in the radial direction (mainly perpendicular to the
line of sight) are smaller, possibly implying temperature anisotropies
of order Tperp / Tparallel ~ 100 for the oxygen
ions. These properties can be understood only in terms of a mechanism
which heats and/or accelerates heavier ions more than lighter ones
(possibly proportionally to mionalpha, where
alpha >~ 1), and preferentially in directions perpendicular to the
magnetic field. We examine various features of plasma heating by the
dissipation of high-frequency ion-cyclotron resonance Alfven waves,
which may be the most natural physical mechanism to produce such
plasma conditions. We show that a quantitative model should predict
the spectrum of waves required to reproduce the observations.
Title: Comparison of SPARTAN and UVCS/SOHO observations.
Authors: Dobrzycka, D.; Strachan, L.; Miralles, M. P.; Kohl, J. L.;
Gardner, L. D.; Smith, P. L.; Cranmer, S. R.; Guhathakurta, M.;
Fisher, R.
Bibcode: 1998ASPC..154..601D
Altcode:
The authors present a comparison of the H I Lyα Spartan Ultraviolet
Coronal Spectrometer observations of the north and south polar coronal
holes in 1993 - 1995 with SOHO Ultraviolet Coronograph Spectrometer
data obtained near solar minimum. These data span several years of
the declining phase of the current solar cycle. Detailed analysis of
the data revealed that the average H I Lyα intensities at similar
heights decreased towards solar minimum in both polar coronal hole
regions. In 1993 the authors observed a 15% - 40% scatter in the
intensities measured at the same height but different position angles
within the same coronal hole. Towards solar minimum the scatter was
clearly reduced. Also the shape of the Lyα profile changed over
the last four years. The narrow component present in 1993 data being
attributed to the contribution from streamers along the line-of-sight
disappeared in 1996/97. They interpret these effects as mainly due to
a decrease in the number of high latitude streamers. At solar minimum
streamers occupy mostly the equatorial region and do not contribute
to the line-of-sight intensity as much as in 1993.
Title: Comparison of SPARTAN and UVCS/SOHO Observations
Authors: Dobrzycka, D.; Strachan, L.; Miralles, M. P.; Kohl, J. L.;
Gardner, L. D.; Smith, P. L.; Cranmer, S. R.; Guhathakurta, M.;
Fisher, R.
Bibcode: 1998ASPC..154..607D
Altcode: 1998csss...10..607D
We present a comparison of the H 1 Lyalpha Spartan Ultraviolet Coronal
Spectrometer observations of the north and south polar coronal holes in
1993-1995 with SOHO Ultraviolet Coronograph Spectrometer data obtained
near solar minimum. These data span several years of the declining phase
of the current solar cycle. Detailed analysis of the data revealed that
the average H 1 Lyalpha intensities at similar heights decreased towards
solar minimum in both polar coronal hole regions. In 1993 we observed
a 15% - 40% scatter in the intensities measured at the same height but
different position angles within the same coronal hole. Towards solar
minimum the scatter was clearly reduced. Also the shape of the Lyalpha
profile changed over the last four years. The narrow component present
in 1993 data being attributed to the contribution from streamers along
the line-of-sight disappeared in 1996/97. We interpret these effects as
mainly due to a decrease in the number of high latitude streamers. At
solar minimum streamers occupy mostly the equatorial region and do
not contribute to the line-of-sight intensity as much as in 1993.
Title: A User's Guide to UVCS/SOHO
Authors: Kohl, J. L.; Noci, G.; Cranmer, S. R.; Fineschi, S.; Gardner,
L. D.; Halas, C. D.; Smith, P. L.; Strachan, L.; Suleiman, R. M.
Bibcode: 1997AAS...191.7309K
Altcode: 1997BAAS...29Q1322K
The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is currently
being used to observe the extended solar corona between 1 and 10
heliocentric radii. In its first two years of operation, UVCS/SOHO
has made spectroscopic measurements leading to the determination of
densities, velocities, temperatures, and elemental abundances in coronal
holes, equatorial streamers, and coronal mass ejections. Observations
of selected non-solar targets, such as near-ecliptic stars, planets,
comets, and interplanetary hydrogen and helium, have also produced
interesting astronomical results. This poster presents a brief
review of the UVCS/SOHO spectroscopic and polarimetric diagnostic
capabilities, highlighted by pertinent observational data. Most
importantly, we summarize the procedures that have been designed
to allow UVCS/SOHO data to be used by a wide array of researchers,
and invite participation in this unique investigation. Scientists
interested in UVCS/SOHO observations are encouraged to fill
out the ``Get Involved'' questionnaire located on the WWW at:
http://cfa-www.harvard.edu/uvcs/ This work is supported by the National
Aeronautics and Space Administration under grant NAG5-3192 to the
Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana,
and by Swiss funding agencies.
Title: The Impact of UVCS/SOHO Observations on Models of Ion-Cyclotron
Resonance Heating of the Solar Corona
Authors: Cranmer, S. R.; Field, G. B.; Noci, G.; Kohl, J. L.
Bibcode: 1997ESASP.415...89C
Altcode: 1997cpsh.conf...89C
No abstract at ADS
Title: The Impact of Ion-Cyclotron Wave Dissipation on Minor Ion
Velocity Distributions in the Solar Corona
Authors: Cranmer, S. R.; Field, G. B.; Noci, G.; Kohl, J. L.
Bibcode: 1997AAS...191.7411C
Altcode: 1997BAAS...29.1325C
We present theoretical models of the acceleration and heating
of minor ions in the solar wind, as well as detailed anisotropic
velocity distribution functions computed numerically by solving the
Boltzmann transport equation. We examine the compatibility between
these models and spectroscopic measurements of the velocities
and kinetic temperatures of various particle species in the solar
corona. The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO)
has measured hydrogen kinetic temperatures in polar coronal holes in
excess of 3 million K, and O VI ion kinetic temperatures of at least
200 million K. In addition, the velocity distributions parallel to
the open magnetic field are smaller than those perpendicular to the
field, possibly implying temperature anisotropy ratios of order 100
for minor ions. We examine various features of plasma heating by the
dissipation of high-frequency ion-cyclotron resonance Alfven waves,
which may be the most natural physical mechanism to produce the
observed plasma conditions. The modeled ion velocity distributions
depend sensitively on the assumed amplitudes and frequencies of the
waves, and these computations can be used to accurately predict many
quantitative features of the wave power spectrum. Indeed, the more
ionic species that are observed spectroscopically, the greater the
extent in frequency space the wave spectrum can be inferred. This work
is supported by the National Aeronautics and Space Administration
under grant NAG5-3192 to the Smithsonian Astrophysical Observatory,
by Agenzia Spaziale Italiana, and by Swiss funding agencies.
Title: Wind variability of B supergiants. III. Corotating spiral
structures in the stellar wind of HD 64760.
Authors: Fullerton, A. W.; Massa, D. L.; Prinja, R. K.; Owocki, S. P.;
Cranmer, S. R.
Bibcode: 1997A&A...327..699F
Altcode:
Fourier analysis of two spectroscopic time series obtained with the
IUE observatory confirm that the ultraviolet stellar wind profiles of
HD 64760 (B0.5 Ib) are periodically variable. The periodic component
consists of modulations that extend over most of the P Cygni absorption
trough, and can frequently be traced through the emission lobe. The
modulations coexist with variations due to the propagation of discrete
absorption components, but there does not seem to be a direct link
between these two types of variability. In a long time series obtained
in 1995 January during the IUE MEGA Campaign, the modulations in
the P Cygni profiles of the Si III, Si IV, C IV, and N V resonance
lines were dominated by two sinusoidal variations with semi-amplitudes
between ~5-10% of the continuum flux and periods of 1.202+/-0.004 and
2.44+/-0.04days. The weak emission-lobe variability was predominantly
due to the 2.4-day modulation. In the absorption trough, the ratio
of the amplitude of the 1.2-day modulation to the amplitude of the
2.4-day modulation increased systematically as a function of ionization
potential. For both periods, the distribution of the phase constant with
position in the absorption trough exhibited a maximum near -710km/s,
and decreased symmetrically toward larger and smaller velocities. There
was a systematic decrease in the value of the maximum phase between
Si IV and N V. Only the 2.4-day period was present in a shorter time
series obtained in 1993 March, when its amplitude was nearly twice its
1995 value and it was more concentrated toward smaller velocities in
the absorption trough. There is no clear evidence for phase bowing in
the 1993 data. Since the 2.4- and 1.2-day periods are approximately
a half and a quarter of the estimated rotational period of HD 64760,
respectively, we interpret the modulations in terms of 2 (1993) and 4
(1995) broad, corotating circumstellar structures that modulate the
optical depth of the stellar wind. The bowed distribution of phase
implies that the structures are azimuthally extended, probably
spiral-shaped arms, and we develop a kinematic interpretation of
the projected velocity associated with the phase turnover in terms
of the degree of bending of the spirals. We derive a value for the
exponent governing the radial expansion of the wind of β=~1, which
is in good agreement with the canonical value for smooth, spherically
symmetric winds and suggests that the spiral structures are long-lived
perturbations through which material flows. The systematic phase lag
associated with higher ions suggests that they are preferentially
located along the inner, trailing edge of the spiral, as expected
if the structures are formed by the collision of fast and slow winds
originating from equally-spaced longitudinal sectors of the stellar
surface. Although a photospheric process is implicated in the origin
of these structures, it is not clear that magnetic fields or nonradial
pulsations could readily account for the switch between 2- and 4-equally
spaced surface patches that evidently occurred between 1993 and 1995.
Title: Composition of Coronal Streamers from the SOHO Ultraviolet
Coronagraph Spectrometer
Authors: Raymond, J. C.; Kohl, J. L.; Noci, G.; Antonucci, E.;
Tondello, G.; Huber, M. C. E.; Gardner, L. D.; Nicolosi, P.; Fineschi,
S.; Romoli, M.; Spadaro, D.; Siegmund, O. H. W.; Benna, C.; Ciaravella,
A.; Cranmer, S.; Giordano, S.; Karovska, M.; Martin, R.; Michels, J.;
Modigliani, A.; Naletto, G.; Panasyuk, A.; Pernechele, C.; Poletto,
G.; Smith, Peter L.; Suleiman, R. M.; Strachan, L.
Bibcode: 1997SoPh..175..645R
Altcode:
The Ultraviolet Coronagraph Spectrometer on the SOHO satellite covers
the 940-1350 Å range as well as the 470-630 Å range in second
order. It has detected coronal emission lines of H, N, O, Mg, Al, Si,
S, Ar, Ca, Fe, and Ni, particularly in coronal streamers. Resonance
scattering of emission lines from the solar disk dominates the
intensities of a few lines, but electron collisional excitation produces
most of the lines observed. Resonance, intercombination and forbidden
lines are seen, and their relative line intensities are diagnostics
for the ionization state and elemental abundances of the coronal gas.
Title: First Results from the SOHO Ultraviolet Coronagraph
Spectrometer
Authors: Kohl, J. L.; Noci, G.; Antonucci, E.; Tondello, G.; Huber,
M. C. E.; Gardner, L. D.; Nicolosi, P.; Strachan, L.; Fineschi, S.;
Raymond, J. C.; Romoli, M.; Spadaro, D.; Panasyuk, A.; Siegmund,
O. H. W.; Benna, C.; Ciaravella, A.; Cranmer, S. R.; Giordano, S.;
Karovska, M.; Martin, R.; Michels, J.; Modigliani, A.; Naletto, G.;
Pernechele, C.; Poletto, G.; Smith, P. L.
Bibcode: 1997SoPh..175..613K
Altcode:
The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is being
used to observe the extended solar corona from 1.25 to 10 R⊙ from
Sun center. Initial observations of polar coronal holes and equatorial
streamers are described. The observations include measurements of
spectral line profiles for HI Lα and Lβ, Ovi 1032 Å and 1037 Å,
Mgx 625 Å, Fexii 1242 Å and several others. Intensities for Mgx
610 Å, Sixii 499 Å, and 520 Å, Sx 1196 Å, and 22 others have been
observed. Preliminary results for derived H0, O5+,
Mg9+, and Fe11+ velocity distributions and initial
indications of outflow velocities for O5+ are described. In
streamers, the H0 velocity distribution along the line of
sight (specified by the value at e-1, along the line of
sight) decreases from a maximum value of about 180 km s-1
at 2 R⊙ to about 140 km s-1 at 8 R⊙. The value for
O5+ increases with height reaching a value of 150 km
s-1 at 4.7 R⊙. In polar coronal holes, the O5+
velocity at e-1 is about equal to that of H0
at 1.7 R⊙ and significantly larger at 2.1 R⊙. The O5+
in both streamers and coronal holes were found to have anisotropic
velocity distributions with the smaller values in the radial direction.
Title: The line-profile variable λ Scorpii is a spectroscopic
triple system.
Authors: De Mey, K.; Aerts, C.; Waelkens, C.; Cranmer, S. R.;
Schrijvers, C.; Telting, J. H.; Daems, K.; Meeus, G.
Bibcode: 1997A&A...324.1096D
Altcode:
An analysis of 278 spectra of the line-profile variable λ Scorpii
leads to the following conclusions. λ Sco is the primary of a binary
system. The radial-velocity variations have a peak-to-peak amplitude of
~60km/s and an orbital period of 5.959 days. The orbit is not circular
but has an eccentricity of 0.29. The 5.959^d^-binary system probably
moves in orbit with another distant, as yet unknown third star. By
means of three five-hour time series of high-resolution spectra, the
oscillations of the rapidly rotating β Cephei-type main component
are investigated. Line-profile variations, which reveal travelling
subfeatures across the lines, are discovered. λ Sco is so far one
of the very few βCephei stars in which such a moving-bump phenomenon
is detected. Radial-velocity variations are derived from the data and
analysed to reveal a main oscillation frequency near 4.66cycles/day,
and some more candidate frequencies. λ Sco is a non-radially pulsating
β Cephei star which rotates supersynchronously. The characteristics
of λ Sco and the ζ Oph stars are briefly addressed. Despite the
common line-profile behaviour, spectral type, and vsin(i), we find no
evidence of circumstellar material around λ Sco.
Title: Empirical Models of the Extended Solar Corona
Authors: Kohl, J. L.; Noci, G.; Antonucci, E.; Ciaravella, A.;
Cranmer, S.; Dobrzycka, D.; Fineschi, S.; Gardner, L. D.; Huber,
M. C. E.; Panasyuk, A.; Raymond, J. C.; Strachan, L.
Bibcode: 1997SPD....28.0303K
Altcode: 1997BAAS...29..907K
Ultraviolet spectroscopy is being used to produce self consistent
empirical models of polar coronal holes and equatorial streamers in the
extended solar corona. The models are intended to provide experimental
values for many of the primary plasma parameters of the extended corona,
which can then be used to constrain theoretical coronal and solar
wind models. The empirical models are based on synoptic observations
and other measurements of spectral line profiles and intensities of
H I Lyalpha , O VI 1032 Angstroms and 1037 Angstroms, Fe XII 1242
Angstroms, Mg X 625 Angstroms and several others. Information about
velocity distributions, outflow velocities, densities and elemental
abundances as derived from the observations are specified in the
models. The models used to specify the empirically derived parameters
include a description of well established theoretical processes such
as those controlling ionization balance, collisional excitation,
and resonant scattering. They do not include any descriptions of
less well established processes such as heating functions, transverse
wave motions or direct momentum deposition by waves. The intent is to
provide, to the maximum extent possible, empirical descriptions that
can be used, together with theoretical models, to help identify the
dominant physical processes responsible for coronal heating, solar wind
acceleration and the chemical composition of the solar wind. This work
is supported by NASA Grant NAG5-3192 to the Smithsonian Astrophysical
Observatory, the Italian Space Agency and Swiss funding sources.
Title: Comparison of SPARTAN and UVCS/SOHO Observations
Authors: Dobrzycka, D.; Strachan, L.; Kohl, J. L.; Gardner, L. D.;
Smith, P. L.; Cranmer, S. R.; Guhathakurta, M.; Fisher, R.
Bibcode: 1997EOSTr..78..242D
Altcode:
Three SPARTAN flights in 1993, 1994, and 1995 provided us with
observations of HI Lyman alpha in the south and north polar coronal
hole regions. These data span several years of the declining phase of
the current solar cycle. We analyzed the data using various diagnostic
techniques and obtained strong constraints on the geometry of coronal
holes, electron density distributions, hydrogen velocity distributions
and outflow velocities. We compare SPARTAN HI Lyman alpha observations
with UVCS/SOHO data obtained in solar minimum to characterize changes
of the plasma parameters in the polar coronal holes over a four year
period. This work is supported by NASA under Grant NAG 5-613 to the
Smithsonian Astrophysical Observatory and by NASA under Grant NAG5-3192
to the Smithsonian Astrophysical Observatory, by the Italian Space
Agency, and by Swiss Funding Agencies.
Title: Evidence for Nonradial Solar Wind Acceleration in Magnetostatic
Coronal Holes
Authors: Cranmer, S. R.; Strachan, L.; Romoli, M.; Dobrzycka, D.;
Panasyuk, A. V.; Kohl, J. L.
Bibcode: 1997EOSTr..78..258C
Altcode:
We compare UVCS/SOHO observations of densities and outflow velocities
in polar coronal holes with theoretical mass-conservation models of an
idealized force-free magnetostatic geometry. Open flux tubes bordering
the closed-field streamer belt initially expand much more rapidly than
flux tubes over the poles, then turn nearly radial and expandless
rapidly. In the past, this variation in the super radial expansion
factor has been inversely correlated with solar wind velocity at 1 AU,
but we find an even stronger inverse correlation in the acceleration
region of the solar wind (1 to 10 solar radii). A considerable
reduction in the wind speed of low-latitude flux tubes, compared with
those over the poles, results in a geometry-dependent variation in the
Doppler dimming of UV resonance lines such as H I Lyman alpha. Thus,
we are able to use these emission diagnostics not only to infer the
magnitude of the solar wind velocity, but also to map out the geometry
of the solar wind emerging from large polar coronal holes. This work is
supported by NASA under Grant NAG5-3192 to the Smithsonian Astrophysical
Observatory, by the Italian Space Agency, and by Switzerland.
Title: Sudden Radiative Braking in Colliding Hot-Star Winds
Authors: Gayley, K. G.; Owocki, S. P.; Cranmer, S. R.
Bibcode: 1997ApJ...475..786G
Altcode:
Hot, massive stars have strong stellar winds, and in hot-star binaries
these winds can undergo violent collision. Because such winds are
thought to be radiatively driven, radiative forces may also play an
important role in moderating the wind collision. However, previous
studies have been limited to considering how such forces may inhibit
the initial acceleration of the companion stellar wind. In this paper
we analyze the role of an even stronger radiative braking effect,
whereby the primary wind is rather suddenly decelerated by the radiative
momentum flux it encounters as it approaches a bright companion. We
further show that the braking location and velocity law along the
line of centers between the stars can be approximated analytically
using a simple one-dimensional analysis. The results of this analysis
agree well with a detailed two-dimensional hydrodynamical simulation
of the wind collision in the WR + O binary V444 Cygni and demonstrate
that radiative braking can significantly alter the bow-shock geometry
and reduce the strength of the wind collision. We then apply
the derived analytic theory to a set of 14 hot-star binary systems,
and conclude that radiative braking is likely to be of widespread
importance for wind-wind collisions in WR + O binaries with close
to medium separation, D <~ 100 R⊙. It may also be
important in other types of hot-star binaries that exhibit a large
imbalance between the component wind strengths.
Title: KCorona polarized brightness and electron density measurement
with the visible light polarimeter of UVCS
Authors: Romoli, M.; Benna, C.; Cranmer, S.; Fineschi, S.; Gardner,
L. D.; Strachan, L.; Kohl, J. L.; Noci, G.
Bibcode: 1997ESASP.404..633R
Altcode: 1997cswn.conf..633R
No abstract at ADS
Title: UVCS/SOHO empirical models of solar coronal holes
Authors: Cranmer, S. R.; Kohl, J. L.; Noci, G.; Strachan, L.;
Panasyuk, A. V.; Romoli, M.; Fineschi, S.; Dobrzycka, D.; Raymond,
J. C.; Suleiman, R. M.; O'Neal, R. H.
Bibcode: 1997ESASP.404..295C
Altcode: 1997cswn.conf..295C
No abstract at ADS
Title: Measurements of H I and O VI velocity distributions in the
extended solar corona with UVCS/SOHO and UVCS/Spartan 201
Authors: Kohl, J. H.; Noci, G.; Antonucci, E.; Tondello, G.; Huber,
M. C. E.; Gardner, L. D.; Nicolosi, P.; Fineschi, S.; Raymond, J. C.;
Romoli, M.; Spadaro, D.; Siegmund, O. H. W.; Benna, C.; Ciaravella,
A.; Cranmer, S. R.; Giordano, S.; Karovska, M.; Martin, R.; Michels,
J.; Modigliani, A.; Naletto, G.; Panasyuk, A.; Pernechele, C.; Poletto,
G.; Smith, P. L.; Strachan, L.
Bibcode: 1997AdSpR..20....3K
Altcode:
The Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric
Observatory, UVCS/SOHO, and the Ultraviolet Coronal Spectrometer on
the Spartan 201 satellite, UVCS/Spartan, have been used to measure
H I 1215.67 A˚ line profiles in polar coronal holes of the Sun at
projected heliocentric heights between 1.5 and 3.0 R_solar. UVCS/SOHO
also measured line profiles for H I 1025.72 A˚, O VI 1032/1037 A˚,
and Mg X 625 A˚. The reported UVCS/SOHO observations were made between
5 April and 21 June 1996 and the UVCS/Spartan observations were made
between 11 and 12 April 1993. Both sets of measurements indicate that a
significant fraction of the protons along the line of sight in coronal
holes have velocities larger than those for a Maxwellian velocity
distribution at the expected electron temperature. Most probable
speeds for O^5+ velocity distributions along the lines of sight are
smaller than those of H^0 at 1.5 R_solar, are comparable at about 1.7
R_solar and become significantly larger than the H^0 velocities above
2 R_solar. There is a tendency for the O^5+ line of sight velocity
distribution in concentrations of polar plumes to be more narrow than
those in regions away from such concentrations. UVCS/SOHO has identified
31 spectral lines in the extended solar corona.
Title: Inhibition FO Wind Compressed Disk Formation by Nonradial
Line-Forces in Rotating Hot-Star Winds
Authors: Owocki, S. P.; Cranmer, S. R.; Gayley, K. G.
Bibcode: 1996ApJ...472L.115O
Altcode:
We investigate the effects of nonradial line forces on the formation of
a "wind-compressed disk" (WCD) around a rapidly rotating B star. Such
nonradial forces can arise both from asymmetries in the line resonances
in the rotating wind and from rotational distortion of the stellar
surface. They characteristically include a latitudinal force component
directed away from the equator and an azimuthal force component
acting against the sense of rotation. Here we present results from
radiation-hydrodynamical simulations showing that these nonradial
forces can lead to an effective suppression of the equatorward
flow needed to form a WCD as well as a modest (~20%) spin-down of
the wind rotation. Furthermore, contrary to previous expectations
that the wind mass flux should be enhanced by the reduced effective
gravity near the equator, we show here that gravity darkening effects
can actually lead to a reduced mass loss, and thus lower density,
in the wind from the equatorial region. Overall, the results here
thus imply a flow configuration that is markedly different from that
derived in previous models of winds from rotating early-type stars. In
particular, a major conclusion is that equatorial wind compression
effects should be effectively suppressed in any radiatively driven
stellar wind for which, as in the usual CAK formalism, the driving
includes a significant component from optically thick lines. This
presents a serious challenge to the WCD paradigm as an explanation for
disk formation around Be and other rapidly rotating hot stars thought
to have CAK-type, line-driven winds.
Title: The Incidence and Origin of Rotational Modulation in B
Supergiant Winds
Authors: Massa, D.; Prinja, R. K.; Fullerton, A. W.; Owocki, S. P.;
Cranmer, S. R.
Bibcode: 1996AAS...189.9615M
Altcode: 1996BAAS...28.1401M
We report the results of a 30 day IUE time series (with a mean sampling
of ~ 3 times a day) of wind variability in two B supergiants with
typical projected rotational velocities. The implied rotation periods
for the program stars are <= 18.6 days for one and <= 27 days
for the other. The wind variability in the more rapidly rotating
supergiant clearly shows cyclical behavior with a period of ~ 7.7
days. The modulation is most clearly seen at low velocity in the low
ions (C ii lambda lambda 1335, Al iii lambda lambda 1860, and the Si
iii lambda lambda 1300 triplets), demonstrating a photospheric origin
of the disturbances. Furthermore, since the period of the variability
is roughly half of the most probable rotation period of the star,
we attribute the variability to rotational modulation of its wind
by two distinct, equidistant surface features. We note, however,
that there is also complex substructure to the modulation which is
unresolved at our temporal sampling rate. The more slowly rotating
supergiant does not show distinctly repeating structures in its wind
lines, but there is an indication that a single feature is repeating
on the same time scale as its rotation period. When considered in
context with previous observations of a rapidly rotating supergiant,
the current results indicate that wind variability in B supergiants
is intimately linked to the presence of surface features on these stars.
Title: Sudden radiative braking in colliding hot-star winds.
Authors: Gayley, K. G.; Owocki, S. P.; Cranmer, S. R.
Bibcode: 1996RMxAC...5...68G
Altcode:
When two hot-star winds collide, their interaction centers at the
point where the momentum fluxes balance. However, in WR+O systems, the
imbalance in the corporeal momentum fluxes may be extreme enough to
preclude a standard head-on wind/wind collision. On the other hand,
an important component of the total momentum flux in radiatively
driven winds is carried by photons. Thus, if the wind interaction
region has sufficient scattering opacity, it can reflect stellar
photons and cause important radiative terms to enter the momentum
balance. This radiative input would result in additional braking of
the wind. We use a radiative-hydrodynamics calculation to show that
such radiative braking can be an important effect in many types of
colliding hot-star winds. Characterized by sudden deceleration of the
stronger wind in the vicinity of the weak-wind star, it can allow a wind
ram balance that would otherwise be impossible in many WR+O systems
with separations less than a few hundred solar radii. It also greatly
weakens the shock strength and the encumbent X ray production. We
demonstrate the significant features of this effect using V444 Cygni
as a characteristic example. We also derive a general analytic theory
that applies to a wide class of binaries, yielding simple predictions
for when radiative braking should play an important role.
Title: The Impact of Pulsations and Waves on Hot-Star Wind Variability
Authors: Cranmer, S. R.; Massa, D.; Owocki, S. P.
Bibcode: 1996AAS...188.5907C
Altcode: 1996BAAS...28R.918C
Hot luminous stars (O, B, W-R) are observed to have strong and variable
stellar winds, and many classes of these stars are also inferred
to pulsate radially or nonradially. It has been suspected for some
time that these oscillations can induce periodic modulations in the
surrounding stellar wind and produce observational signatures in, e.g.,
ultraviolet P Cygni line profiles. However, the fact that most low-order
and low-degree oscillation modes are evanescent in the photosphere
(i.e., damping exponentially instead of propagating sinusoidally)
presents a problem to the survival of significant wave amplitude in
the wind. We find, though, that the presence of an accelerating wind
can provide the necessary impetus for evanescent modes to effectively
``tunnel'' their way out of the interior. First, in the subsonic, or
near-static wind, the reference frame of the temporal oscillations is
itself beginning to propagate, and this implies that a small degree
of group velocity is imparted to the evanescent waves. Second, in
the supersonic wind, the density no longer falls off exponentially,
but much more slowly, so the effective scale height grows much
larger. Frequencies previously evanescent here no longer ``see''
as much of an underlying density gradient, and are free to propagate
nearly acoustically. We model the propagation of oscillations into
a hot-star wind via a numerical radiation-hydrodynamics code, and
we find that evanescence is indeed not a hindrance to producing
wind variability correlated with stellar pulsations. Preliminary
models of strong (nonlinear) radial wind oscillations of the beta
Cephei variable BW Vulpeculae show good agreement between observed
and modeled base ``radial velocity curves'' and wind-contaminated UV
profile variability. We are currently applying this general modeling
technique to other systems, especially those which rotate rapidly
and exhibit nonradial oscillations (e.g., zeta Puppis and HD 64760,
extensively observed by the IUE MEGA project).
Title: The impact of pulsations and waves on hot-star wind
variability.
Authors: Cranmer, S. R.; Massa, D.; Owocki, S. P.
Bibcode: 1996BAAS...28Q.918C
Altcode:
No abstract at ADS
Title: Dynamical Models of Winds from Rotating Hot Stars
Authors: Cranmer, Steven R.
Bibcode: 1996PhDT........92C
Altcode:
The hottest and most massive stars (spectral types O, B, Wolf-Rayet)
have strong stellar winds that are believed to be driven by line
scattering of the star's continuum radiation field. The atmospheres
and winds of many hot stars exhibit the effects of rapid rotation,
pulsation, and possibly surface magnetic fields, inferred from
observations of ultraviolet spectral lines and polarization. The complex
time variability in these observations is not yet well understood. The
purpose of this dissertation is to model the dynamics of winds
around rotating hot stars and synthesize theoretical observational
diagnostics to compare with actual data. Before dealing with
rotation, however, we derive the theory of radiative driving of
stellar winds, and uncover several new useful aspects of the theory
for spherical, nonrotating stars. The presence of limb darkening of
the stellar radiation is found to be able to increase the mass flux
by 10-15% over standard models assuming a uniformly-bright star, and
the wind's asymptotic terminal velocity should decrease by the same
amount. We also introduce a new approximation method for estimating
the terminal velocity, which is both conceptually simpler and more
physically transparent than existing approximation algorithms. Finally,
from theoretical line profile modeling we find that observational
determinations of the terminal speed may be underestimated by several
hundred km/s if unsaturated P Cygni lines are used. Rotation
affects a star by introducing centrifugal and Coriolis forces,
decreasing the effective gravity and making the star oblate. This
in turn redistributes the emerging radiative flux to preferentially
heat the stellar poles, an effect known as gravity darkening. Although
previous models have computed the increase in equatorial mass flux due
to the lower effective gravity there, none have incorporated gravity
darkening. We find that the brighter (darker) flux from the poles
(equator) has a much stronger impact on the mass flux, increasing
(decreasing) the mass loss and local wind density. This, in addition
to the existence of nonradial radiation forces from a rotating star,
which tend to point latitudinally away from the equator and azimuthally
opposite the rotation, produces a net poleward deflection of wind
streamlines. This is contrary to the "wind compressed disk" model of
Bjorkman and Cassinelli, and also seems incompatible with observational
inferences of equatorial density enhancements in some systems. This
work is ongoing, and we are endeavoring to include all the relevant
physics in hydrodynamical simulations. We also dynamically model
spectral-line time variability by inducing corotating nonaxisymmetric
structure in the equatorial plane of a hot-star wind. By varying
the radiation force over localized "star spots," the wind develops
fast and slow streams which collide to form corotating interaction
regions (CIRs) similar to those in the solar wind. We synthesize P
Cygni type line profiles for a stationary observer, and find that
"discrete absorption components" (DACs) accelerate slowly through
the profiles as complex nonlinear structures rotate in front of the
star. We also examine the photospheric origin of such variability,
in a preliminary manner, by deriving the theory of stellar pulsations,
waves, and discontinuities. Although most observed low-order pulsation
modes are evanescently damped in the photosphere, we find that the
presence of an accelerating wind can allow waves of all frequencies
to propagate radially. We thus make a first attempt at outlining the
possible "photospheric connection" between interior and wind variability
that observations are beginning to confirm.
Title: Inhibition of Wind Compressed Disk Formation by Nonradial
Line-Forces
Authors: Owocki, S.; Gayley, K.; Cranmer, S.
Bibcode: 1996AAS...188.3801O
Altcode: 1996BAAS...28..881O
We investigate the effects of nonradial line-forces on the formation of
a ``Wind Compressed Disk'' (WCD) around a rapidly rotating B-star. Such
nonradial forces can arise from both asymmetries in the line resonances
in the rotating wind, as well as from rotational distortion of the
stellar surface. They characteristically include an azimuthal force
component acting against the sense of rotation, and a latitudinal force
component directed away from the equator. Here we present results from
radiation-hydrodynamical simulations showing that these nonradial
forces can lead to a significant spin-down of the wind rotation, as
well as an effective suppression of the equatorward flow needed to
form a WCD. The qualitative sense of these effects can be understood
from simple physical arguments and analytic test cases, though further
testing and analysis is still needed to confirm their quantitative
importance. Nonetheless, these results indicate that nonradial force
components can effectively inhibit equatorial wind compression in
a line-driven outflow. If confirmed, these effects would seriously
undermine the WCD paradigm as an explanation for disk formation around
Be and other rapidly rotating hot stars with line-driven stellar winds.
Title: Hydrodynamical Simulations of Corotating Interaction Regions
and Discrete Absorption Components in Rotating O-Star Winds
Authors: Cranmer, Steven R.; Owocki, Stanley P.
Bibcode: 1996ApJ...462..469C
Altcode: 1995astro.ph..8004C
We present two-dimensional hydrodynamical simulations of corotating
interaction regions (CIRs) in the wind from a rotating 0 star,
together with resulting synthetic line profiles showing discrete
absorption components (DACs). For computational tractability, we use a
local, Sobolev treatment of the radiative force, which suppresses the
small-scale instability intrinsic to line driving but still allows us to
model the dynamics of large-scale wind structure. As a first step toward
modeling the wind response to large-scale base perturbations (e.g., from
surface magnetic fields or nonradial pulsations), the structure here is
explicitly induced by localized increases or decreases in the radiative
force, as would result from a bright or dark "star spot" near the star's
equator. We find that bright spots with enhanced driving generate
high-density, low-speed streams, while dark spots generate low-density,
high-speed streams. CIRs form where fast material collides with slow
material; e.g., at the leading (trailing) edge of a stream from a dark
(bright) spot, often steepening into shocks. The unperturbed supersonic
wind obliquely impacts the high-density CIR and sends back a nonlinear
signal that takes the form of a sharp propagating discontinuity ("kink"
or "plateau") in the radial velocity gradient. In the wind's comoving
frame, these features propagate inward at the fast characteristic speed
derived by Abbott for radiatively modified acoustic waves, but because
this is generally only slightly less than the outward wind speed, the
features evolve only slowly outward in the star's frame. We find that
these slow kinks, rather than the CIRs themselves, are more likely to
result in DACs in the absorption troughs of unsaturated P Cygni line
profiles. Because the hydrodynamic structure settles to a steady state
in a frame corotating with the star, the more tightly spiraled kinks
sweep by an observer on a longer timescale than material moving with
the wind itself. This is in general accord with observations showing
slow apparent accelerations for DACs.
Title: Sudden Radiative Braking in Colliding Hot-Star Winds
Authors: Gayley, K.; Owocki, S.; Cranmer, S.
Bibcode: 1996AAS...188.6016G
Altcode: 1996BAAS...28Q.922G
Hot, massive stars have strong stellar winds, and in hot-star binaries
these winds can undergo violent collision. Because such winds are
thought to be radiatively driven, radiative forces may also play an
important role in moderating the wind collision. However, previous
studies have been limited to considering how such forces may inhibit
the initial acceleration of the companion stellar wind. In this
poster we describe the role of an even stronger radiative braking
effect, whereby the primary wind is rather suddenly decelerated by
the radiative momentum flux it encounters as it approaches a bright
companion. We show that the braking location and velocity law along
the line of centers between the stars can be approximated analytically
using a simple one-dimensional analysis. The results of this analysis
agree well with a detailed two-dimensional hydrodynamical simulation of
the wind collision in the WR+O binary V444 Cygni, and demonstrate that
radiative braking can significantly alter the bow-shock geometry and
reduce the strength of the wind collision. We also apply the derived
analytic theory to a set of 14 hot-star binary systems, and conclude
that radiative braking is likely to be of widespread importance for
wind-wind collisions in WR+O binaries with close to medium separation,
D <= 100 Rsun. It may also be important in other types of
hot-star binaries that exhibit a large imbalance between the component
wind strengths.
Title: Sudden radiative braking in colliding hot-star winds.
Authors: Gayley, K.; Owocki, S. P.; Cranmer, S. R.
Bibcode: 1996BAAS...28..922G
Altcode:
No abstract at ADS
Title: Dynamical models of winds from rotating hot stars
Authors: Cranmer, Steven Robert
Bibcode: 1996PhDT.......126C
Altcode:
No abstract at ADS
Title: Periodic Variations in Ultraviolet Spectral Lines of the
B0.5 Ib Star HD 64760: Evidence for Corotating Wind Streams Rooted
in Surface Variations
Authors: Owocki, Stanley P.; Cranmer, Steven R.; Fullerton,
Alexander W.
Bibcode: 1995ApJ...453L..37O
Altcode:
We discuss recently observed periodic modulations in the UV wind lines
of the B-type supergiant HD 64760, with a focus on the peculiar,
upwardly bowed shape seen in isoflux contours of the absorption
variations plotted against velocity and time. We show that this
qualitative impression of bowed contours is quantitatively confirmed by
a peak in the phase for the associated periodic variation at very nearly
the same line position as the apparent bow minimum. The bowed shape
is significant because it indicates that wind variations evolve both
blueward and redward, i.e., toward both larger and smaller line-of-sight
velocities. We show here, however, that these characteristics arise
naturally from absorption by strictly accelerating corotating wind
streams seen in projection against the stellar disk. The quite good
agreement obtained with the observed profile variations provides strong
evidence for corotating stream modulations in this wind.
Title: Newsletters on the WWW
Authors: Cranmer, S.
Bibcode: 1995BeSN...30...25C
Altcode:
No abstract at ADS
Title: Momentum Deposition in Wolf-Rayet Winds: Nonisotropic Diffusion
with Effectively Gray Opacity
Authors: Gayley, Kenneth G.; Owocki, Stanley P.; Cranmer, Steven R.
Bibcode: 1995ApJ...442..296G
Altcode:
We derive the velocity and mass-loss rate of a steady state Wolf-Rayet
(WR) wind, using a nonisotropic diffusion approximation applied to
the transfer between strongly overlapping spectral lines. Following
the approach of Friend & Castor (1983), the line list is assumed
to approximate a statistically parameterized Poisson distribution in
frequency, so that photon transport is controlled by an angle-dependent,
effectively gray opacity. We show the nonisotropic diffusion
approximation yields good agreement with more accurate numerical
treatments of the radiative transfer, while providing analytic insight
into wind driving by multiple scattering. We illustrate, in particular,
that multiple radiative momentum deposition does not require that
photons be repeatedly reflected across substantial distances within
the spherical envelope, but indeed is greatest when photons undergo a
nearly local diffusion, e.g., through scattering by many lines closely
spaced in frequency. Our results reiterate the view that the so-called
'momentum problem' of Wolf-Rayet winds is better characterized as
an 'opacity problem' of simply identifying enough lines. One way of
increasing the number of thick lines in Wolf-Rayet winds is to transfer
opacity from saturated to unsaturated lines, yielding a steeper opacity
distribution than that found in OB winds. We discuss the implications
of this perspective for extending our approach to W-R wind models
that incorporate a more fundamental treatment of the ionization and
excitation processes that determine the line opacity. In particular,
we argue that developing statistical descriptions of the lines to
allow an improved effective opacity for the line ensemble would offer
several advantages for deriving such more fundamental W-R wind models.
Title: The Effect of Oblateness and Gravity Darkening on the Radiation
Driving in Winds from Rapidly Rotating B Stars
Authors: Cranmer, Steven R.; Owocki, Stanley P.
Bibcode: 1995ApJ...440..308C
Altcode:
We calculate the radiative driving force for winds around rapidly
rotating oblate B stars, and we estimate the impact these forces should
have on the production of a wind compressed disk. The effects of limb
darkening, gravity darkening, oblateness, and an arbitrary wind velocity
field are included in the computation of vector 'oblate finite disk'
(OFD) factors, which depend on both radius and colatitude in the
wind. The impact of limb darkening alone, with or without rotation,
can increase the mass loss by as much as 10% over values computed using
the standard uniformly bright spherical finite disk factor. For rapidly
rotating stars, limb darkening makes 'sub-stellar' gravity darkening the
dominant effect in the radial and latitudinal OFD factors, and lessens
the impact of gravity darkening at other visible latitudes (nearer
to the oblate limb). Thus, the radial radiative driving is generally
stronger over the poles and weaker over the equator, following the
gravity darkening at these latitudes. The nonradial radiative driving
is considerably smaller in magnitude than the radial component, but
is directed both away from the equatorial plane and in a retrograde
azimuthal direction, acting to decrease the effective stellar rotation
velocity. These forces thus weaken the equatorward wind compression
compared to wind models computed with nonrotating finite disk factors.
Title: Hydrodynamical Simulations of Co-Rotating Interaction Regions
and Discrete Absorption Components in Rotating O-Star Winds
Authors: Cranmer, S. R.; Owocki, S. P.
Bibcode: 1994AAS...185.8003C
Altcode: 1994BAAS...26.1446C
We present 2D hydrodynamical simulations of co-rotating stream
structure in the winds from rotating O-stars, together with resulting
synthetic line profiles showing discrete absorption components
(DAC's). The azimuthal variation is induced by a local increase
or decrease in the radiative driving force, as would arise from a
``star spot'' in the equatorial plane. Since much of the emergent
wind structure seems independent of the exact method of perturbation,
we expect similar morphology in winds perturbed by localized magnetic
fields or non-radial pulsations. Because the radiative force depends
on the local rate of mass loss, bright spots with enhanced driving
generate high-density, low-velocity streams, while dark spots generate
low-density, high-velocity streams. Co-rotating interaction regions
(CIR's) form where fast material collides with slow material -- e.g. at
the leading (trailing) edge of a stream from a dark (bright) spot,
often steepening into shocks. The asymmetric wind also generates sharp
propagating discontinuities (``kinks'') in the radial velocity gradient,
which travel inward in the co-moving frame at the radiative-acoustic
characteristic speed, and slowly outward in the star's frame. We find
that these slow kinks, rather than the CIR's themselves, are more
likely to result in high-opacity DAC's in the absorption troughs of
unsaturated P Cygni line profiles. Because the hydrodynamic structure
settles to a steady state in a frame co-rotating with the star, the
more tightly-spiraled kinks sweep by an observer on a longer timescale
than material moving with the wind itself. This is in general accord
with observations showing slow apparent accelerations for DAC's.
Title: 2-D Hydrodynamical Simulations of Wind Compressed Disks
(Abstract)
Authors: Owocki, S. P.; Cranmer, S. R.; Blondin, J. M.
Bibcode: 1994Ap&SS.221..455O
Altcode:
We present results of 2-D hydrodynamical simulations of a radiatively
driven stellar wind from a rapidly rotating Be-star. These generally
confirm predictions of the semi-analytic “Wind-Compressed-Disk”
model recently proposed by Bjorkman and Cassinelli to explain the
circumstellar disks inferred observationally to exist around such
rapidly rotating stars. However, our numerical simulations are able to
incorporate several important effects not accounted for in the simple
model, including a dynamical treatment of the outward radiative driving
and gas pressure, as well as a rotationally distorted, oblate stellar
surface. This enables us to model quantitatively the compressed wind
and shock that forms the equatorial disk. The simulation results thus
do differ in several important details from the simple model, showing,
for example, an inner diskinflow not possible in the heuristic approach
of assuming a fixed outward velocity law. There is also no evidence
for the predicted detachment of the disk that arises in the fixed
outflow picture. The peak equatorward velocity in the dynamical models
is furthermore about a factor of two smaller than the analytically
predicted value of ∼ 50% the stellar equatorial rotation speed. As a
result, the dynamical disks are somewhat weaker than predicted, with a
wider opening angle, lower disk/pole density ratio, and smaller shock
velocity jump (each by roughly the same factor of two).
Title: Two-dimensional Hydrodynamical Simulations of Wind-compressed
Disks around Rapidly Rotating B Stars
Authors: Owocki, Stanley P.; Cranmer, Steven R.; Blondin, John M.
Bibcode: 1994ApJ...424..887O
Altcode:
We use a two-dimensional piecewise parabolic method (PPM) code to
simulate numerically the hydrodynamics of a radiation-driven stellar
wind from a rapidly rotating Be star. The results generally confirm
predictions of the semianalytic 'wind-compressed disk' model recently
proposed by Bjorkman and Cassinelli to explain the circumstellar
disks inferred observationally to exist around such rapidly rotating
stars. However, this numerical simulation is able to incorporate
several important effects not accounted for in the simple model,
including a dynamical treatment of the outward radiative driving and
gas pressure, as well as a rotationally distorted, oblate stellar
surface. This enables us to model quantitatively the compressed wind
and shock that forms the equatorial disk. The simulation results thus
do differ in several important details from the simple method, showing,
for example, an inner disk inflow not possible in the heuristic approach
of assuming a fixed outward velocity law. There is also no evidence
for the predicted detachment of the disk that arises in the fixed
outflow picture. The peak equatorward velocity in the dynamical models
is furthermore about a factor of 2 smaller than the lytically predicted
value of approximately 50% of the stellar equatorial rotation speed. As
a result, the dynamical disks are somewhat weaker than predicted,
with a wider opening angle, lower disk/pole density ratio, and smaller
shock velocity jump. The principal cause of these latter differences
appears to be an artificially strong equatorward drift of the subsonic
outflow in the original analytic model. Much better agreement with the
dynamical results can be obtained, however, from a slightly modified,
analytic wind-compression model with a more detailed specification of
the fixed wind outflow and a lower boundary set to the sonic radius
along a rotationally oblate stellar surface. Hence, despite these
detailed differences, the general predicted effect of disk formation
by wind compression toward the equator is substantially confirmed.
Title: Two-dimensional hydrodynamical simulations of wind-compressed
disks around rapidly rotating B-stars
Authors: Owocki, S. P.; Cranmer, S. R.; Blondin, J. M.
Bibcode: 1994IAUS..162..469O
Altcode:
No abstract at ADS
Title: Some aspects of illuminated model atmosphere theory as applied
to close bynary systems.
Authors: Cranmer, S. R.
Bibcode: 1993MNRAS.263..989C
Altcode:
Only recently has the use of irradiated stellar atmosphere models
become practical in the study of close binary systems. Aspects of this
problem, such as geometrical illumination from a finite solid angle,
the transport of polarized radiation, and multiple-iterative heating
between the stars, must be included for a realistic treatment. These
concepts are developed and applied to grids of plane-parallel
stellar atmospheres computed on modified Roche equipotentials. The
`reflection effect' between the stars, actually a radiative heating
of the outer layers of the atmosphere, is iterated to consistency,
and the incident moments of the radiation field are integrated
numerically. Grey model atmospheres, along with light and polarization
curves, are calculated for illustrative purposes for a test system,
V Puppis. Although some approximations are utilized to make the problem
computationally tractable, realistic variations with orbital phase are
produced. Despite the fact that the resulting light and polarization
curves are not substantially different from those calculated with
simpler approximations to the reflection effect, the ab initio nature
of the present model atmosphere approach allows both the prediction of
additional phenomena and a large reduction in the number of arbitrary
adjustable parameters in the least-squares solutions for the absolute
elements of binary systems.
Title: The Effects of Zonal Atmospheric Currents on the Spectra of
Rotating Early-Type Stars
Authors: Cranmer, Steven R.; Collins, George W., II
Bibcode: 1993ApJ...412..720C
Altcode:
We suggest the existence of zonal currents in the atmospheres of rapidly
rotating stars analogous to those found in planetary atmospheres. The
zonal flow is assumed to be characterized by 'thin' atmospheric,
nearly geostrophic flow which does not change the gravity darkening
and stellar shape determined by the underlying uniformly rotating
model. The contribution that such flows make to the continuum spectra
of such stars is investigated. The additional rotationally induced
Doppler displacement resulting from such zonal wind belts can distort
the rotationally broadened stellar lines leading to significant
departures from the line profiles predicted by the classical model
of rotating stars. Our estimates of the zonal flow velocity stem from
the assumption of a relation between it and the latitudinal wavenumber
of the zonal velocity field. It is thus possible to create barotropic
atmosphere models which, in turn, enable the modeling of the stellar
spectrum including important spectral lines. In addition, the radiative
transfer equations for the Stokes parameters I and Q are solved for the
locally plane-parallel atmospheres so that the polarization structure
of the radiation field is determined. We find that the presence of
zonal wind belts leads to significant changes in the photospheric
polarization from those characteristic of a uniformly rotating model.
Title: 2-D Hydrodynamical Simulations of the Wind-Compressed-Disk
Model for Be Stars
Authors: Owocki, S.; Cranmer, S.; Blondin, J.
Bibcode: 1992AAS...181.1903O
Altcode: 1992BAAS...24.1150O
We use a 2-D PPM code to simulate numerically the hydrodynamics
of a radiation-driven stellar wind from a rapidly rotating
B-star. The results generally confirm predictions of a semi-analytic
``Wind-Compressed-Disk" model recently proposed by Bjorkman and
Cassinelli to explain the circumstellar disks inferred observationally
to exist around Be stars. However, this numerical simulation is able to
incorporate several important effects not accounted for in the simple
model, including a dynamical treatment of the outward radiative driving
and gas pressure. This enables us to model quantatively the compressed
wind and shock that forms the equatorial disk. The simulation results
thus do differ in several important details from the simple model,
showing, for example, cases of inner disk inflow not possible in the
heuristic approach of assuming a fixed outward velocity law. This
poster paper will present a detailed comparison of the analytic and
numerical models.
Title: Atmospheric Wind-Belts in Early Type Stars
Authors: Cranmer, S. R.; Collins, G. W., II
Bibcode: 1992AAS...180.3705C
Altcode: 1992BAAS...24Q.787C
In this paper we discuss the possible existence of zonal wind-belts in
the atmospheres of rapidly rotating early type stars analogous to the
type found in planetary atmospheres. Unlike previous investigations
into differential rotation, the velocity field yielded by these wind
belts is assumed to be an atmospheric phenomenon and therefore does
not significantly alter the shape or flux distribution resulting from
gravity darkening. However, the modification to the surface gravity
and velocity field brought about by the presence of such winds can be
expected to affect the shapes of rotationally broadened lines. While
the changes in the line profiles are generally small, they significantly
complicate the interpretation of small departures of such profiles from
classical rotationally broadened line profiles. Our estimates of the
velocity stem from the assumption of a relation between the magnitude
and latitudinal wavenumber of the zonal velocity field, in addition
to the assumption that the flow is close to being ``geostrophic,''
i.e. where the pressure gradient nearly balances the local coriolis
forces. From this basis, we construct a series of barotropic atmosphere
models for the surface velocity field. Following earlier efforts we then
construct line profiles for Mg II (4481), He I (4471), and the first
three Balmer lines appropriate for these models. We compare integrated
line profiles for these models exhibiting varying numbers of zonal
belts with the line profiles of uniformly rotating models. Various
characteristics of the line profiles such as half widths (FWHM) and
equivalent widths are given. Qualitative changes in the shapes of
the profiles can be related to the direction of the zonal winds with
respect to the underlying angular motion of the model. We also obtain
theoretical atmospheric quantities such as Johnson and Stromgren
photometric indices, absolute magnitudes, and net polarization
information, and make comparisons with the uniformly rotating models.
Title: Model Atmospheres for Rotating B Stars
Authors: Collins, George W., II; Truax, Ryland J.; Cranmer, Steven R.
Bibcode: 1991ApJS...77..541C
Altcode:
The results of extensive model atmosphere calculations applicable to
rotating early-type stars are presented. While the results largely
conform with those of earlier work, it is clear that the effects of
rotation on the structure of the interior are likely to play as large
a role in determining the emergent spectra of rotating stars as the
rotational effects of the atmosphere. For lines that tend to weaken with
decreasing temperature, rotation will cause the line to appear similar
in strength to that of later type stars. The reverse is true for lines
that increase in strength with decreasing temperature. Polarization of
the UV Balmer continuum is shown to persist to much later spectral types
than might be expected on the basis of ionization studies alone. In
addition, it is shown that rotation behaves like a spatial filter for
sharp absorption lines resulting in the enhancement of the degree of
polarization in the wings of these lines. It is suggested that the
effect could be quite pronounced in the FUV of the Balmer continuum
complicating the interpretation of the behavior of the polarization
found in the spectra of Be stars.
Title: The [Ne III]-[O II] Spectrum as an Ionization Indicator
in Nebulae
Authors: Ali, B.; Blum, R. D.; Bumgardner, T. E.; Cranmer, S. R.;
Ferland, G. J.; Haefner, R. I.; Tiede, G. P.
Bibcode: 1991PASP..103.1182A
Altcode:
The usefulness of the Ne III-O II forbidden-line spectrum as a helium
ionization correction factor (ICF) indicator in low-density nebulae,
such as H II regions, is examined, and the results of an extensive grid
of photoionization simulations which have a 'blister'-like geometry
are presented. Fully filled constant pressure gas, Orion dust, and
H II region abundances are used, along with a very wide variety of
ionizing continua. As suggested by the ionization potentials, the He
ICF is small when the Ne III 3869/O II 3727 forbidden line ratio, an
easily measured line pair, reaches above 0.20. The 'eta' parameter,
a ratio of ratios of oxygen and sulfur lines, is probably strongly
affected by low-temperature dielectronic recombination, a basic process
which has no reliable rate coefficients for third-row elements.
Title: Rotationally induced polarization in pure absorption spectral
lines
Authors: Collins, George W., II; Cranmer, Steven R.
Bibcode: 1991MNRAS.253..167C
Altcode:
A simple (Struve-Unsold) model with limb darkening is presented to
show the existence of polarization in rotationally broadened pure
absorption lines and provide estimates of its magnitude. Results
for more complicated models appropriate for early-type stars are
considered. While the results in the visible part of the spectrum are
considerably smaller in magnitude than for the simple model, the form
of those results can be readily understood in terms of that model and
may be detectable. Results for the UV spectra of early-type stars, while
still smaller in magnitude than the Struve-Unsold model, are of opposite
sign and larger in magnitude than the visible. It is inferred that it
is much more likely that the effect may be detected in the UV than the
visible. It is suggested that since this effect results from the axial
symmetry of stellar rotation, it could provide a sensitive diagnostic
for rotational broadening, allowing it to be separated from other forms
of broadening that present higher forms of symmetry to the observer.
Title: Hydrodynamic simulations of close triple encounters.
Authors: McMillan, Stephen L. W.; Cranmer, Steven R.; Shorter, Scott
A.; Hernquist, Lars
Bibcode: 1991ASPC...13..418M
Altcode: 1991fesc.book..418M
Tidal binary systems play an important dynamical and observational
role in our understanding of globular clusters. However, their very
nature means that their interactions with other stars cannot be easily
modelled by simple technqies, and hydrodynamical effects are likely to
be critical in determining the outcome of those events. The authors have
performed a series of simulations of collisions between close binaries
and other cluster members, covering a fairly representative range
of initial orbital configurations. They find that most "interesting"
three-body encounters actually lead to mergers of two or all three of
the stars involved, with around 5 - 10% of the total mass ejected from
the triple system (and probably also from the cluster).