Author name code: schrijver
ADS astronomy entries on 2022-09-14
author:"Schrijver, Carolus J." OR author:"Schrijver, Karel"
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Title: Extreme solar events
Authors: Cliver, Edward W.; Schrijver, Carolus J.; Shibata, Kazunari;
Usoskin, Ilya G.
Bibcode: 2022LRSP...19....2C
Altcode: 2022arXiv220509265C
We trace the evolution of research on extreme solar and
solar-terrestrial events from the 1859 Carrington event to the rapid
development of the last twenty years. Our focus is on the largest
observed/inferred/theoretical cases of sunspot groups, flares on the
Sun and Sun-like stars, coronal mass ejections, solar proton events,
and geomagnetic storms. The reviewed studies are based on modern
observations, historical or long-term data including the auroral and
cosmogenic radionuclide record, and Kepler observations of Sun-like
stars. We compile a table of 100- and 1000-year events based on
occurrence frequency distributions for the space weather phenomena
listed above. Questions considered include the Sun-like nature of
superflare stars and the existence of impactful but unpredictable solar
"black swans" and extreme "dragon king" solar phenomena that can involve
different physics from that operating in events which are merely large.
Title: Coronal Dimming as a Proxy for Solar and Stellar Coronal
Mass Ejections
Authors: Jin, Meng; Nitta, Nariaki; Derosa, Marc; Cheung, Mark; Osten,
Rachel; France, Kevin; Mason, James; Kowalski, Adam; Schrijver, Carolus
Bibcode: 2022cosp...44.1404J
Altcode:
Solar coronal dimmings have been observed extensively in the past two
decades. Due to their close association with coronal mass ejections
(CMEs), there is a critical need to improve our understanding of the
physical processes that cause dimmings as well as their relationship
with CMEs. Recent study (e.g., Veronig et al. 2021) also shows promising
dimming signals from distant stars, which suggest the possibility of
using coronal dimming as a proxy to diagnose stellar CMEs. In this
study, we first conduct a comparative study of solar coronal dimming
using MHD simulations and SDO observations. A detailed analysis of
the simulation and observation data reveals how transient dimming
/ brightening are related to plasma heating processes, while the
long-lasting core and remote dimmings are caused by mass loss process
induced by the CME. Using metrics such as dimming depth and dimming
slope, we uncover a relationship between dimmings and CME properties
(e.g., CME mass, CME speed) in the simulation. We further extend the
model for simulating the stellar CMEs and dimmings and compare with
solar cases. Our result suggests that coronal dimmings encode important
information about the associated CMEs, which provides a physical basis
for detecting stellar CMEs from distant solar-like stars.
Title: Coronal Mass Ejections and Dimmings: A Comparative Study
Using MHD Simulations and SDO Observations
Authors: Jin, Meng; Cheung, Mark C. M.; DeRosa, Marc L.; Nitta,
Nariaki V.; Schrijver, Carolus J.
Bibcode: 2022ApJ...928..154J
Altcode: 2022arXiv220213034J
Solar coronal dimmings have been observed extensively in recent
years. Due to their close association with coronal mass ejections
(CMEs), there is a critical need to improve our understanding of the
physical processes that cause dimmings as well as their relationship
with CMEs. In this study, we investigate coronal dimmings by combining
simulation and observational efforts. By utilizing a data-constrained
global magnetohydrodynamics model (Alfvén-wave solar model), we
simulate coronal dimmings resulting from different CME energetics and
flux rope configurations. We synthesize the emissions of different EUV
spectral bands/lines and compare with SDO/AIA and EVE observations. A
detailed analysis of the simulation and observation data suggests
that the transient dimming/brightening are related to plasma heating
processes, while the long-lasting core and remote dimmings are caused
by mass-loss process induced by the CME. Moreover, the interaction
between the erupting flux rope with different orientations and the
global solar corona could significantly influence the coronal dimming
patterns. Using metrics such as dimming depth and dimming slope,
we investigate the relationship between dimmings and CME properties
(e.g., CME mass, CME speed) in the simulation. Our result suggests
that coronal dimmings encode important information about the associated
CMEs, which provides a physical basis for detecting stellar CMEs from
distant solar-like stars.
Title: Sun-as-a-star Spectral Irradiance Observations of Transiting
Active Regions: a Milestone for Characterization of Stellar Active
Regions
Authors: Toriumi, Shin; Airapetian, Vladimir; Hudson, Hugh; Schrijver,
Karel; Cheung, Chun Ming Mark; DeRosa, Marc
Bibcode: 2021AGUFM.U43B..05T
Altcode:
Recent observations have revealed that solar-type stars can produce
massive "superflares". The strongest flares on the Sun are almost
always associated with large, complex, rapidly-evolving active regions
(ARs) including sunspots. Therefore, to understand why and how stellar
flares and coronal eruptions occur, which may directly determine
the circumstances of exoplanets, it is critically important to gain
information on stellar ARs. One possible way to do so is to monitor the
star in multiple wavelengths. In this study, we perform multi-wavelength
irradiance monitoring of transiting solar ARs by using full-disk
observational (i.e. Sun-as-a-star) data from four satellites. We find
that the near UV light curves show strong correlations with photospheric
total magnetic flux and that there are time lags between the coronal
and photospheric light curves when ARs are close to the limb. Such time
lags result from high-arching, bright coronal loops above stellar ARs
being visible even when the AR is behind the limb. It is also found
that the EUV light curves sensitive to transition-region temperatures
are sometimes dimmed because of a reduction in the emission measure of
0.60.8 MK due to the plasma being heated to higher temperatures over a
wide area around the AR. These results indicate that, by measuring the
stellar light curves in multiple wavelengths, we may obtain information
on the structures and evolution of stellar ARs.
Title: Sun-as-a-star Spectral Irradiance Observations: Milestone
For Characterizing The Stellar Active Regions
Authors: Toriumi, S.; Airapetian, V.; Hudson, H.; Schrijver, C.;
Cheung, M.; DeRosa, M.
Bibcode: 2021AAS...23820503T
Altcode:
For understanding the physical mechanism behind the solar flares, it
is crucial to measure the magnetic fields of active regions (ARs) from
the photosphere to the corona and investigate their scale, complexity,
and evolution. This is true for the stellar flares. However, it is
still difficult to spatially resolve the starspots, and one possible
way to probe their evolution and structure is to monitor the star in
multiple wavelengths. To test this possibility with the solar data,
we perform multi-wavelength irradiance monitoring of transiting solar
ARs by using full-disk observation data from SDO, Hinode, GOES, and
SORCE. As a result, we find, for instance, that the near UV light
curves show strong correlations with photospheric total magnetic flux
and that there are time lags between the coronal and photospheric light
curves when ARs are close to the limb, which together may enable one
to discern how high bright coronal loops extend above stellar ARs. It
is also revealed that the sub-MK (i.e. transition-region temperature)
EUV light curves are sometimes dimmed because the emission measure
is reduced owing to the heating over a wide area around the AR. These
results indicate that, by measuring the stellar light curves in multiple
wavelengths, we may obtain information on the structure and evolution
of stellar ARs.
Title: Sun-as-a-star Multi-wavelength Observations: A Milestone for
Characterization of Stellar Active Regions
Authors: Toriumi, Shin; Airapetian, Vladimir S.; Hudson, Hugh S.;
Schrijver, Carolus J.; Cheung, Mark C. M.; DeRosa, Marc L.
Bibcode: 2021csss.confE..46T
Altcode:
It has been revealed that "superflares" can occur on solar-type
stars. The magnetic energy of the flares is likely to be stored in
active-region atmospheres. Therefore, to explain the energy storage and
occurrence of the flares, it is important to monitor the evolutions of
the active regions, not only in visible light but also in ultraviolet
(UV) and X-rays. To demonstrate this, we perform multi-wavelength
irradiance monitoring of transiting solar active regions by using
full-disk observation data. As a result of this sun-as-a-star spectral
irradiance analysis, we confirm that the visible continuum that
corresponds to the photosphere becomes darkened when the spot is at the
central meridian, whereas most of the UV, EUV and X-rays, which are
sensitive to chromospheric to coronal temperatures, are brightened,
reflecting the bright magnetic features above the starspots. The
time lags between the coronal and photospheric light curves have
the potential to probe the extent of coronal magnetic fields above
the starspots. These results indicate that, by measuring the stellar
light curves in multiple wavelengths, we may obtain information on
the structures and evolution of stellar active regions.
Title: Sun-as-a-star Spectral Irradiance Observations of Transiting
Active Regions
Authors: Toriumi, Shin; Airapetian, Vladimir S.; Hudson, Hugh S.;
Schrijver, Carolus J.; Cheung, Mark C. M.; DeRosa, Marc L.
Bibcode: 2020ApJ...902...36T
Altcode: 2020arXiv200804319T
Major solar flares are prone to occur in active-region (AR) atmospheres
associated with large, complex, dynamically evolving sunspots. This
points to the importance of monitoring the evolution of starspots,
not only in visible but also in ultraviolet (UV) and X-rays, in
understanding the origin and occurrence of stellar flares. To this end,
we perform spectral irradiance analysis on different types of transiting
solar ARs by using a variety of full-disk synoptic observations. The
target events are an isolated sunspot, spotless plage, and emerging flux
in prolonged quiet-Sun conditions selected from the past decade. We find
that the visible continuum and total solar irradiance become darkened
when the spot is at the central meridian, whereas it is bright near
the solar limb; UV bands sensitive to the chromosphere correlate well
with the variation of total unsigned magnetic flux in the photosphere;
amplitudes of extreme ultraviolet (EUV) and soft X-ray increase with
the characteristic temperature, whose light curves are flat-topped
due to their sensitivity to the optically thin corona; the transiting
spotless plage does not show the darkening in the visible irradiance,
while the emerging flux produces an asymmetry in all light curves about
the central meridian. The multiwavelength Sun-as-a-star study described
here indicates that the time lags between the coronal and photospheric
light curves have the potential to probe the extent of coronal magnetic
fields above the starspots. In addition, EUV wavelengths that are
sensitive to temperatures just below 1 MK sometimes show antiphased
variations, which may be used for diagnosing plasmas around starspots.
Title: Testing the Solar Activity Paradigm in the Context of
Exoplanet Transits
Authors: Schrijver, Carolus J.
Bibcode: 2020ApJ...890..121S
Altcode: 2020arXiv200101093S
Transits of exoplanets across cool stars contain blended information
about structures on the stellar surface and about the planetary body
and atmosphere. To advance understanding of how this information is
entangled, a surface-flux transport code, based on observed properties
of the Sun's magnetic field, is used to simulate the appearance of
hypothetical stellar photospheres from the visible near 4000 Å to the
near-IR at 1.6 μm by mapping intensities characteristic of faculae
and spots onto stellar disks. Stellar appearances are computed for
a Sun-like star of solar activity up to a star with a mean magnetic
flux density that is ∼30× higher. Simulated transit signals for a
Jupiter-class planet are compared with observations. This (1) indicates
that the solar paradigm is consistent with transit observations for
stars throughout the activity range explored, provided that infrequent
large active regions with fluxes up to ∼3 × 1023 Mx
are included in the emergence spectrum, (2) quantitatively confirms
that for such a model, faculae brighten relatively inactive stars
while starspots dim more-active stars, and suggests (3) that large
starspots inferred from transits of active stars are consistent
with clusters of more compact spots seen in the model runs, (4) that
wavelength-dependent transit-depth effects caused by stellar magnetic
activity for the range of activity and the planetary diameter studied
here can introduce apparent changes in the inferred exoplanetary radii
across wavelengths from a few hundred to a few thousand kilometers,
increasing with activity, and (5) that activity-modulated distortions
of broadband stellar radiance across the visible to near-IR spectrum
can reach several percent.
Title: Stellar Imager (SI) — A UV/Optical Interferometer to Observe
the Universe in High Definition
Authors: Carpenter, K. G.; Karovska, M.; Rau, G.; Schrijver, C.;
SI Team
Bibcode: 2020AAS...23530102C
Altcode:
The concept for a space based, UV/Optical Interferometer
with over 200x HST's resolution, named "Stellar Imager" (http://hires.gsfc.nasa.gov/si/),
was developed as part of the NASA Vision Mission studies ("NASA
Space Science Vision Missions" 2008, ed. M. Allen). SI was a
"Landmark/Discovery Mission" in the 2005 Heliophysics Roadmap and a
candidate UV-optical interferometer (UVOI) in the 2006 Astrophysics
Strategic Plan. SI would enable 0.1 milli-arcsec spectral imaging of
stellar surfaces, and many sources in the Universe in general, and open
an enormous new "discovery space" for Astrophysics with its combination
of high angular resolution, dynamic imaging, and spectral energy
resolution. SI's goal is to study the role of magnetism in the Universe
and revolutionize our understanding of: 1) Solar/Stellar Magnetic
Activity and their impact on Space Weather, Planetary Climates, and
Life; 2) Magnetic and Accretion Processes and their roles in the Origin
and Evolution of Structure and in the Transport of Matter throughout
the Universe; 3) the close-in structure of Active Galactic Nuclei;
and 4) Exo-Solar Planet Transits and Disks. Significant technology
development is critical to enabling SI and other future space-based,
sparse aperture telescopes and distributed-spacecraft missions. The
key technology needs include: 1) precision formation flying of many
spacecraft, 2) precision metrology over km-scales, 3) closed-loop
control of many-element, sparse optical arrays, 4) staged-control
systems with very high dynamic ranges (nm to km-scale). We describe
the needed technology development, science goals, and feasibility
of interferometry from space, as well as provide detail performance
parameters and simulations of the data that could be acquired by this
space interferometer.
Title: Coronal dimming as a proxy for stellar coronal mass ejections
Authors: Jin, M.; Cheung, M. C. M.; DeRosa, M. L.; Nitta, N. V.;
Schrijver, C. J.; France, K.; Kowalski, A.; Mason, J. P.; Osten, R.
Bibcode: 2020IAUS..354..426J
Altcode: 2020arXiv200206249J
Solar coronal dimmings have been observed extensively in the past
two decades and are believed to have close association with coronal
mass ejections (CMEs). Recent study found that coronal dimming
is the only signature that could differentiate powerful flares
that have CMEs from those that do not. Therefore, dimming might be
one of the best candidates to observe the stellar CMEs on distant
Sun-like stars. In this study, we investigate the possibility of using
coronal dimming as a proxy to diagnose stellar CMEs. By simulating a
realistic solar CME event and corresponding coronal dimming using a
global magnetohydrodynamics model (AWSoM: Alfvén-wave Solar Model),
we first demonstrate the capability of the model to reproduce solar
observations. We then extend the model for simulating stellar CMEs
by modifying the input magnetic flux density as well as the initial
magnetic energy of the CME flux rope. Our result suggests that with
improved instrument sensitivity, it is possible to detect the coronal
dimming signals induced by the stellar CMEs.
Title: Principles Of Heliophysics: a textbook on the universal
processes behind planetary habitability
Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer,
Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David;
Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen,
Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry;
Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George;
Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo;
Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert;
Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp,
Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel;
Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav;
Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George;
Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley,
Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan;
Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian;
Woods, Tom; Zapp, Neal
Bibcode: 2019arXiv191014022S
Altcode:
This textbook gives a perspective of heliophysics in a way that
emphasizes universal processes from a perspective that draws attention
to what provides Earth (and similar (exo-)planets) with a relatively
stable setting in which life as we know it can thrive. The book is
intended for students in physical sciences in later years of their
university training and for beginning graduate students in fields of
solar, stellar, (exo-)planetary, and planetary-system sciences.
Title: Developing a vision for exoplanetary transit spectroscopy:
a shared window on the analysis of planetary atmospheres and of
stellar magnetic structure
Authors: Kowalski, Adam; Schrijver, Karel; Pillet, Valentin; Criscuoli,
Serena
Bibcode: 2019BAAS...51c.149K
Altcode: 2019astro2020T.149K; 2019arXiv190405976K
We describe how accurate exoplanet atmospheres' characterization will
inevitably require taking into consideration stellar inhomogeneities
caused by convection and magnetic fields. Disentangling these two
components requires a multipronged approach with new solar reference
spectra, MHD modeling, and collaborations among astrophysics
communities.
Title: Community Input Solicited for Heliophysics Decadal Survey
Midterm Assessment Committee
Authors: Woods, Thomas; Millan, Robyn; Charo, Art; Bastian, Tim;
Bobra, Monica; Coster, Anthea; DeLuca, Ed; England, Scott; Fuselier,
Stephen; Lopez, Ramon; Luhmann, Janet; Nykyri, Katariina; Oberheide,
Jens; Opher, Merav; Schrijver, Karel; Semeter, Josh; Thayer, Jeff;
Title, Alan
Bibcode: 2019shin.confE...6W
Altcode:
The National Academies of Sciences, Engineering, and Medicine has
convened a committee to review the progress towards implementing the
2013 Heliophysics Decadal Survey, titled Solar and Space Physics: a
Science for a Technological Society. This review serves as a midterm
assessment before the next Heliophysics Decadal Survey committee would
begin its formulation. This committee is interested to receive input
from the heliophysics and space weather communities about the 2013-2018
progress realizing the 15 recommendations and applications specified in
the 2013 Heliophysics Decadal Survey, about any suggested actions to
optimize the science value during 2019-2023, about any suggestions to
improve the process for the next Heliophysics Decadal Survey, and about
any suggested actions to enhance all stages of careers for scientists
and engineers in the solar and space physics community. This poster
outlines the Heliophysics Decadal Survey recommendations and recent
progress, and it also summarizes the tasks for this midterm assessment
committee. There will be an opportunity to discuss your inputs with
a couple of the Committee members during the SHINE meeting.
Title: One of ten billion Earths: How we Learn about our Planet's
Past and Future from Distant Exoplanets
Authors: Schrijver, Karel
Bibcode: 2018otbe.book.....S
Altcode:
Illustrated with breathtaking images of the Solar System and of the
Universe around it, this book explores how the discoveries within the
Solar System and of exoplanets far beyond it come together to help us
understand the habitability of Earth, and how these findings guide the
search for exoplanets that could support life. The author highlights
how, within two decades of the discovery of the first planets outside
the Solar System in the 1990s, scientists concluded that planets
are so common that most stars are orbited by them. The lives
of exoplanets and their stars, as of our Solar System and its Sun,
are inextricably interwoven. Stars are the seeds around which planets
form, and they provide light and warmth for as long as they shine. At
the end of their lives, stars expel massive amounts of newly forged
elements into deep space, and that ejected material is incorporated
into subsequent generations of planets. How do we learn about
these distant worlds? What does the exploration of other planets tell
us about Earth? Can we find out what the distant future may have in
store for us? What do we know about exoworlds and starbirth, and where
do migrating hot Jupiters, polluted white dwarfs, and free-roaming nomad
planets fit in? And what does all that have to do with the habitability
of Earth, the possibility of finding extraterrestrial life, and the
operation of the globe-spanning network of the sciences?
Title: Coronal Mass Ejections and Dimmings: A Comparative Study
using MHD Simulations and SDO Observations
Authors: Jin, M.; Cheung, C. M. M.; DeRosa, M. L.; Nitta, N.;
Schrijver, K.
Bibcode: 2017AGUFMSH41A2758J
Altcode:
Solar coronal dimmings have been observed extensively in the past two
decades. Due to their close association with coronal mass ejections
(CMEs), there is a critical need to improve our understanding of the
physical processes that cause dimmings and determine their relationship
with CMEs. In this study, we investigate coronal dimmings by combining
simulation and observational efforts. By utilizing a data-driven global
magnetohydrodynamics model (AWSoM: Alfven-wave Solar Model), we simulate
coronal dimmings resulting from different CME energetics and flux rope
configurations. We synthesize the emissions of different EUV spectral
bands/lines and compare with SDO/AIA and EVE observations. A detailed
analysis of simulation and observation data suggests that although
the transient dimming / brightening patterns could relate to plasma
heating processes (either by adiabatic compression or reconnection),
the long-lasting "core" and "remote" (also known as "secondary")
dimmings both originate from regions with open/quasi-open fields and
are caused by mass loss process. The mass loss in the remote dimming
region is induced by CME-driven shock. Using metrics such as dimming
depth, dimming slope, and recovery time, we investigate the relationship
between dimmings and CME properties (e.g., CME mass, CME speed) in the
simulation. Our result suggests that coronal dimmings encode important
information about CME energetics, CME-driven shock properties, and
magnetic configuration of erupting flux ropes. We also discuss how our
knowledge about solar coronal dimmings could be extended to the study
of stellar CMEs, which may prove important for exoplanet atmospheres
and habitability but which are currently not observable.
Title: Magnetic Properties of Solar Active Regions that Govern Large
Solar Flares and Eruptions
Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise; Hudson,
Hugh S.; Nagashima, Kaori
Bibcode: 2017SPD....4820001T
Altcode:
Strong flares and CMEs are often produced from active regions (ARs). In
order to better understand the magnetic properties and evolutions of
such ARs, we conducted statistical investigations on the SDO/HMI and
AIA data of all flare events with GOES levels >M5.0 within 45 deg
from the disk center for 6 years from May 2010 (from the beginning to
the declining phase of solar cycle 24). Out of the total of 51 flares
from 29 ARs, more than 80% have delta-sunspots and about 15% violate
Hale’s polarity rule. We obtained several key findings including
(1) the flare duration is linearly proportional to the separation
of the flare ribbons (i.e., scale of reconnecting magnetic fields)
and (2) CME-eruptive events have smaller sunspot areas. Depending on
the magnetic properties, flaring ARs can be categorized into several
groups, such as spot-spot, in which a highly-sheared polarity inversion
line is formed between two large sunspots, and spot-satellite, where a
newly-emerging flux next to a mature sunspot triggers a compact flare
event. These results point to the possibility that magnetic structures
of the ARs determine the characteristics of flares and CMEs. In the
presentation, we will also show new results from the systematic flux
emergence simulations of delta-sunspot formation and discuss the
evolution processes of flaring ARs.
Title: Coronal Mass Ejections and Dimmings: A Comparative Study
using MHD Simulations and SDO Observations
Authors: Jin, Meng; Cheung, Mark; DeRosa, Marc L.; Nitta, Nariaki;
Schrijver, Karel
Bibcode: 2017SPD....4820602J
Altcode:
Solar coronal dimmings have been observed extensively in the past two
decades. Due to their close association with coronal mass ejections
(CMEs), there is a critical need to improve our understanding of the
physical processes that cause dimmings and determine their relationship
with CMEs. In this study, we investigate coronal dimmings by combining
simulation and observational efforts. By utilizing a data-driven
global magnetohydrodynamics model (AWSoM: Alfven-wave Solar Model), we
simulate coronal dimmings resulting from different CME energetics and
flux rope configurations. We synthesize the emissions of different EUV
spectral bands/lines and compare with SDO/AIA and EVE observations. A
detailed analysis of simulation and observation data suggests that the
“core” dimming is mainly caused by the mass loss from the CME, while
the “remote” dimming could have a different origin (e.g., plasma
heating). Moreover, the interaction between the erupting flux rope with
different orientations and the global solar corona could significantly
influence the coronal dimming patterns. Using metrics such as dimming
depth, dimming slope, and recovery time, we investigate the relationship
between dimmings and CME properties (e.g., CME mass, CME speed) in the
simulation. Our result suggests that coronal dimmings encode important
information about CMEs. We also discuss how our knowledge about solar
coronal dimmings could be extended to the study of stellar CMEs.
Title: Magnetic Properties of Solar Active Regions That Govern Large
Solar Flares and Eruptions
Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise K.;
Hudson, Hugh; Nagashima, Kaori
Bibcode: 2017ApJ...834...56T
Altcode: 2016arXiv161105047T
Solar flares and coronal mass ejections (CMEs), especially the larger
ones, emanate from active regions (ARs). With the aim of understanding
the magnetic properties that govern such flares and eruptions, we
systematically survey all flare events with Geostationary Orbiting
Environmental Satellite levels of ≥M5.0 within 45° from disk center
between 2010 May and 2016 April. These criteria lead to a total of 51
flares from 29 ARs, for which we analyze the observational data obtained
by the Solar Dynamics Observatory. More than 80% of the 29 ARs are found
to exhibit δ-sunspots, and at least three ARs violate Hale’s polarity
rule. The flare durations are approximately proportional to the distance
between the two flare ribbons, to the total magnetic flux inside the
ribbons, and to the ribbon area. From our study, one of the parameters
that clearly determine whether a given flare event is CME-eruptive
or not is the ribbon area normalized by the sunspot area, which may
indicate that the structural relationship between the flaring region
and the entire AR controls CME productivity. AR characterization shows
that even X-class events do not require δ-sunspots or strong-field,
high-gradient polarity inversion lines. An investigation of historical
observational data suggests the possibility that the largest solar
ARs, with magnetic flux of 2 × 1023 Mx, might be able to
produce “superflares” with energies of the order of 1034
erg. The proportionality between the flare durations and magnetic
energies is consistent with stellar flare observations, suggesting a
common physical background for solar and stellar flares.
Title: A Comparison of Flare Forecasting Methods. I. Results from
the “All-Clear” Workshop
Authors: Barnes, G.; Leka, K. D.; Schrijver, C. J.; Colak, T.;
Qahwaji, R.; Ashamari, O. W.; Yuan, Y.; Zhang, J.; McAteer, R. T. J.;
Bloomfield, D. S.; Higgins, P. A.; Gallagher, P. T.; Falconer, D. A.;
Georgoulis, M. K.; Wheatland, M. S.; Balch, C.; Dunn, T.; Wagner, E. L.
Bibcode: 2016ApJ...829...89B
Altcode: 2016arXiv160806319B
Solar flares produce radiation that can have an almost immediate effect
on the near-Earth environment, making it crucial to forecast flares
in order to mitigate their negative effects. The number of published
approaches to flare forecasting using photospheric magnetic field
observations has proliferated, with varying claims about how well
each works. Because of the different analysis techniques and data
sets used, it is essentially impossible to compare the results from
the literature. This problem is exacerbated by the low event rates of
large solar flares. The challenges of forecasting rare events have long
been recognized in the meteorology community, but have yet to be fully
acknowledged by the space weather community. During the interagency
workshop on “all clear” forecasts held in Boulder, CO in 2009,
the performance of a number of existing algorithms was compared
on common data sets, specifically line-of-sight magnetic field and
continuum intensity images from the Michelson Doppler Imager, with
consistent definitions of what constitutes an event. We demonstrate
the importance of making such systematic comparisons, and of using
standard verification statistics to determine what constitutes a good
prediction scheme. When a comparison was made in this fashion, no one
method clearly outperformed all others, which may in part be due to the
strong correlations among the parameters used by different methods to
characterize an active region. For M-class flares and above, the set
of methods tends toward a weakly positive skill score (as measured
with several distinct metrics), with no participating method proving
substantially better than climatological forecasts.
Title: Properties and Developments of Flaring Active Regions
Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise K.;
Hudson, Hugh; Nagashima, Kaori
Bibcode: 2016usc..confE..15T
Altcode:
Larger flares and CMEs are often produced from active regions (ARs). In
order to better understand the magnetic properties and evolutions
of such ARs, we picked up all flare events with GOES levels >M5.0
with heliocentric angles of <45 deg in the period of May 2010 to
April 2016, which led to a total of 29 ARs with 51 flares. We analyzed
the observational data obtained by SDO and found that more than 80%
of the 29 ARs have delta-sunspots. Most of them can be classified
depending on their magnetic structures into (1) spot-spot, where a long
sheared polarity inversion line (PIL: characterized by flare ribbons)
is formed between two major sunspots, and (2) spot-satellite, where
a newly-emerging minor bipole next to a pre-existing spot creates a
compact PIL. The remaining minor groups are (3) quadrupole, where two
emerging bipoles produce a PIL in between, and (4) inter-AR, which
produces flares not from delta-spots but from between two separated
ARs. From statistical investigations we found for example that the
spot-spot group generally shows long-duration events due to large
coronal structures, while the spot-satellite has impulsive events
because of their compact magnetic nature. We will also present flux
emergence simulations and discuss their formation processes.
Title: The Characteristics of Solar X-Class Flares and CMEs: A
Paradigm for Stellar Superflares and Eruptions?
Authors: Harra, Louise K.; Schrijver, Carolus J.; Janvier, Miho;
Toriumi, Shin; Hudson, Hugh; Matthews, Sarah; Woods, Magnus M.; Hara,
Hirohisa; Guedel, Manuel; Kowalski, Adam; Osten, Rachel; Kusano,
Kanya; Lueftinger, Theresa
Bibcode: 2016SoPh..291.1761H
Altcode: 2016SoPh..tmp..111H
This paper explores the characteristics of 42 solar X-class flares that
were observed between February 2011 and November 2014, with data from
the Solar Dynamics Observatory (SDO) and other sources. This flare
list includes nine X-class flares that had no associated CMEs. In
particular our aim was to determine whether a clear signature could
be identified to differentiate powerful flares that have coronal
mass ejections (CMEs) from those that do not. Part of the motivation
for this study is the characterization of the solar paradigm for
flare/CME occurrence as a possible guide to the stellar observations;
hence we emphasize spectroscopic signatures. To do this we ask the
following questions: Do all eruptive flares have long durations? Do
CME-related flares stand out in terms of active-region size vs. flare
duration? Do flare magnitudes correlate with sunspot areas, and, if so,
are eruptive events distinguished? Is the occurrence of CMEs related to
the fraction of the active-region area involved? Do X-class flares with
no eruptions have weaker non-thermal signatures? Is the temperature
dependence of evaporation different in eruptive and non-eruptive
flares? Is EUV dimming only seen in eruptive flares? We find only one
feature consistently associated with CME-related flares specifically:
coronal dimming in lines characteristic of the quiet-Sun corona,
i.e. 1 - 2 MK. We do not find a correlation between flare magnitude
and sunspot areas. Although challenging, it will be of importance to
model dimming for stellar cases and make suitable future plans for
observations in the appropriate wavelength range in order to identify
stellar CMEs consistently.
Title: a Numerical Study of Long-Range Magnetic Impacts during
Coronal Mass Ejections
Authors: Jin, Meng; Schrijver, Karel; Cheung, Mark; DeRosa, Marc;
Nitta, Nariaki; Title, Alan
Bibcode: 2016shin.confE..38J
Altcode:
With the global view and high-cadence observations from SDO/AIA and
STEREO, many spatially separated solar eruptive events appear to be
coupled. However, the mechanisms for 'sympathetic' events are still
largely unknown. In this study, we investigate the impact of an erupting
flux rope on surrounding solar structures through large-scale magnetic
coupling. We build a realistic environment of the solar corona on
2011 February 15 using a global magnetohydrodynamics (MHD) model and
initiate coronal mass ejections (CMEs) in active region (AR) 11158
by inserting Gibson-Low analytical flux ropes. We show that a CME's
impact on the surrounding structures depends not only on the magnetic
strength of these structures and their distance to the source region,
but also on the interaction between the CME with the large-scale
magnetic field. Within the CME expansion domain where the flux rope
field directly interacts with the solar structures, expansion-induced
reconnection often modifies the overlying field, thereby increasing
the decay index. This effect may provide a primary coupling mechanism
underlying the sympathetic eruptions. The magnitude of the impact
is found to depend on the orientation of the erupting flux rope,
with the largest impacts occurring when the flux rope is favorably
oriented for reconnecting with the surrounding regions. Outside the
CME expansion domain, the influence of the CME is mainly through field
line compression or post-eruption relaxation. Based on our numerical
experiments, we discuss a way to quantify the eruption impact, which
could be useful for forecasting purposes.
Title: Introduction
Authors: Schrijver, Carolus J.; Bagenal, Frances; Sojka, Jan J.
Bibcode: 2016hasa.book....1S
Altcode:
No abstract at ADS
Title: Heliophysics: Active Stars, their Astrospheres, and Impacts
on Planetary Environments
Authors: Schrijver, Carolus J.; Bagenal, Frances; Sojka, Jan J.
Bibcode: 2016hasa.book.....S
Altcode:
Heliophysics is a fast-developing scientific discipline that integrates
studies of the Sun's variability, the surrounding heliosphere,
and the environment and climate of planets. This volume, the fourth
in the Heliophysics collection, explores what makes the conditions
on Earth 'just right' to sustain life, by comparing Earth to other
solar system planets, by comparing solar magnetic activity to that of
other stars, and by looking at the properties of evolving exoplanet
systems. By taking an interdisciplinary approach and using comparative
heliophysics, the authors illustrate how we can learn about our
local cosmos by looking beyond it, and in doing so, also enable the
converse. Supplementary online resources are provided, including
lecture presentations, problem sets and exercise labs, making this
ideal as a textbook for advanced undergraduate- and graduate-level
courses, as well as a foundational reference for researchers in the
many subdisciplines of helio- and astrophysics. # INGEST NEW
Title: Publication Statistics on the Sun and the Heliosphere
Authors: Schrijver, Carolus J.
Bibcode: 2016SoPh..291.1267S
Altcode: 2016arXiv160308943S; 2016SoPh..tmp...52S
The professional literature provides means to review the evolution
and geographic distribution of the scientific communities engaged in
solar and heliospheric physics. With the help of the Astrophysics Data
System (NASA/ADS), I trace the growth of the research community over
the past century from a few dozen researchers early in the twentieth
century to over 4000 names with more than 2000 refereed publications
in recent years, published in 90 distinct journals. Of these, 90 %
originated in 20 countries. Overall, 45 % of the lead authors of the
publications have affiliations in Europe, 29 % in the Americas, 24 %
in Australasia, and 2 % in Africa and Arab countries. Publications most
frequently appear (in decreasing order) in The Astrophysical Journal,
the Journal of Geophysical Research (Space Physics), Solar Physics,
Astronomy and Astrophysics, and Advances in Space Research (adding up
to 59 % of all publications in 2015).
Title: The Nonpotentiality of Coronae of Solar Active Regions,
the Dynamics of the Surface Magnetic Field, and the Potential for
Large Flares
Authors: Schrijver, Carolus J.
Bibcode: 2016ApJ...820..103S
Altcode: 2016arXiv160207244S
Flares and eruptions from solar active regions (ARs) are associated
with atmospheric electrical currents accompanying distortions of the
coronal field away from a lowest-energy potential state. In order to
better understand the origin of these currents and their role in M-
and X-class flares, I review all AR observations made with Solar
Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager and
SDO/Atmospheric Imaging Assembly from 2010 May through 2014 October
within ≈40° from the disk center. I select the roughly 4% of all
regions that display a distinctly nonpotential coronal configuration
in loops with a length comparable to the scale of the AR, and all
that emit GOES X-class flares. The data for 41 regions confirm,
with a single exception, that strong-field, high-gradient polarity
inversion lines (SHILs) created during emergence of magnetic flux
into, and related displacement within, pre-existing ARs are associated
with X-class flares. Obvious nonpotentiality in the AR-scale loops
occurs in six of ten selected regions with X-class flares, all with
relatively long SHILs along their primary polarity inversion line,
or with a long internal filament there. Nonpotentiality can exist
in ARs well past the flux-emergence phase, often with reduced or
absent flaring. I conclude that the dynamics of the flux involved
in the compact SHILs is of pre-eminent importance for the large-flare
potential of ARs within the next day, but that their associated currents
may not reveal themselves in AR-scale nonpotentiality. In contrast,
AR-scale nonpotentiality, which can persist for many days, may inform
us about the eruption potential other than those from SHILs which is
almost never associated with X-class flaring.
Title: Division E Commission 10: Solar Activity
Authors: Schrijver, Carolus J.; Fletcher, Lyndsay; van Driel-Gesztelyi,
Lidia; Asai, Ayumi; Cally, Paul S.; Charbonneau, Paul; Gibson, Sarah
E.; Gomez, Daniel; Hasan, Siraj S.; Veronig, Astrid M.; Yan, Yihua
Bibcode: 2016IAUTA..29..245S
Altcode: 2015arXiv151003348S
After more than half a century of community support related to the
science of ``solar activity'', IAU's Commission 10 was formally
discontinued in 2015, to be succeeded by C.E2 with the same area
of responsibility. On this occasion, we look back at the growth of
the scientific disciplines involved around the world over almost a
full century. Solar activity and fields of research looking into the
related physics of the heliosphere continue to be vibrant and growing,
with currently over 2,000 refereed publications appearing per year from
over 4,000 unique authors, publishing in dozens of distinct journals
and meeting in dozens of workshops and conferences each year. The
size of the rapidly growing community and of the observational and
computational data volumes, along with the multitude of connections
into other branches of astrophysics, pose significant challenges;
aspects of these challenges are beginning to be addressed through,
among others, the development of new systems of literature reviews,
machine-searchable archives for data and publications, and virtual
observatories. As customary in these reports, we highlight some
of the research topics that have seen particular interest over the
most recent triennium, specifically active-region magnetic fields,
coronal thermal structure, coronal seismology, flares and eruptions,
and the variability of solar activity on long time scales. We close
with a collection of developments, discoveries, and surprises that
illustrate the range and dynamics of the discipline.
Title: A Numerical Study of Long-range Magnetic Impacts during
Coronal Mass Ejections
Authors: Jin, M.; Schrijver, C. J.; Cheung, M. C. M.; DeRosa, M. L.;
Nitta, N. V.; Title, A. M.
Bibcode: 2016ApJ...820...16J
Altcode: 2016arXiv160304900J
With the global view and high-cadence observations from Solar Dynamics
Observatory/Atmospheric Imaging Assembly and Solar TErrestrial RElations
Observatory, many spatially separated solar eruptive events appear
to be coupled. However, the mechanisms for “sympathetic” events
are still largely unknown. In this study, we investigate the impact
of an erupting flux rope on surrounding solar structures through
large-scale magnetic coupling. We build a realistic environment of the
solar corona on 2011 February 15 using a global magnetohydrodynamics
model and initiate coronal mass ejections (CMEs) in active region
11158 by inserting Gibson-Low analytical flux ropes. We show that a
CME’s impact on the surrounding structures depends not only on the
magnetic strength of these structures and their distance to the source
region, but also on the interaction between the CME and the large-scale
magnetic field. Within the CME expansion domain where the flux rope
field directly interacts with the solar structures, expansion-induced
reconnection often modifies the overlying field, thereby increasing
the decay index. This effect may provide a primary coupling mechanism
underlying the sympathetic eruptions. The magnitude of the impact
is found to depend on the orientation of the erupting flux rope,
with the largest impacts occurring when the flux rope is favorably
oriented for reconnecting with the surrounding regions. Outside the
CME expansion domain, the influence of the CME is mainly through field
line compression or post-eruption relaxation. Based on our numerical
experiments, we discuss a way to quantify the eruption impact, which
could be useful for forecasting purposes.
Title: The Role of Large-scale Magnetic Coupling for Solar Corona
Sympathy
Authors: Jin, M.; Schrijver, K.; Cheung, C. M. M.; DeRosa, M. L.;
Nitta, N.; Title, A. M.
Bibcode: 2015AGUFMSH23A2425J
Altcode:
With the comprehensive view and high cadence observations from
SDO/AIA and STEREO in solar cycle 24, a large number of spatially
separated solar eruptive events are found to be coupled. However,
compared with the established initiation mechanisms for "isolated"
events, the mechanisms for "sympathetic" events are still largely
unknown, and nascent theories are untested. In this study, we build
a realistic environment of solar corona on 2011 February 15 using
a global MHD model and investigate how an eruption can impact the
surrounding solar structures. Our result shows that the solar eruption's
impact on the different structures can be quite different. Within
the CME expansion domain, it is possible to trigger an eruption by
overlaying field removal through expansion induced reconnection. The
magnitude of impact is found to be dependent on the orientation
of the erupting flux rope. Outside the CME expansion domain, the
post-eruption reconfiguration could play an important role for solar
sympathy. Based on the modeling results, we discuss the possibility of
using observable/estimable parameters to quantify the eruption impact
therefore providing an useful parameter for forecasting sympathy.
Title: a Roadmap to Advance Understanding of the Science of Space
Weather
Authors: Schrijver, K.; Kauristie, K.; Aylward, A.; De Nardin, C. M.;
Gibson, S. E.; Glover, A.; Gopalswamy, N.; Grande, M.; Hapgood, M. A.;
Heynderickx, D.; Jakowski, N.; Kalegaev, V. V.; Lapenta, G.; Linker,
J.; Liu, S.; Mandrini, C. H.; Mann, I. R.; Nagatsuma, T.; Nandy, D.;
Obara, T.; O'Brien, T. P., III; Onsager, T. G.; Opgenoorth, H. J.;
Terkildsen, M. B.; Valladares, C. E.; Vilmer, N.
Bibcode: 2015AGUFMSH12A..01S
Altcode:
There is a growing appreciation that the environmental conditions that
we call space weather impact the technological infrastructure that
powers the coupled economies around the world. With that comes the need
to better shield society against space weather by improving forecasts,
environmental specifications, and infrastructure design. A COSPAR/ILWS
team recently completed a roadmap that identifies the scientific focus
areas and research infrastructure that are needed to significantly
advance our understanding of space weather of all intensities and of
its implications and costs for society. This presentation provides a
summary of the highest-priority recommendations from that roadmap.
Title: The Influence of Spatial resolution on Nonlinear Force-free
Modeling
Authors: DeRosa, M. L.; Wheatland, M. S.; Leka, K. D.; Barnes, G.;
Amari, T.; Canou, A.; Gilchrist, S. A.; Thalmann, J. K.; Valori,
G.; Wiegelmann, T.; Schrijver, C. J.; Malanushenko, A.; Sun, X.;
Régnier, S.
Bibcode: 2015ApJ...811..107D
Altcode: 2015arXiv150805455D
The nonlinear force-free field (NLFFF) model is often used to
describe the solar coronal magnetic field, however a series of
earlier studies revealed difficulties in the numerical solution of the
model in application to photospheric boundary data. We investigate
the sensitivity of the modeling to the spatial resolution of the
boundary data, by applying multiple codes that numerically solve the
NLFFF model to a sequence of vector magnetogram data at different
resolutions, prepared from a single Hinode/Solar Optical Telescope
Spectro-Polarimeter scan of NOAA Active Region 10978 on 2007 December
13. We analyze the resulting energies and relative magnetic helicities,
employ a Helmholtz decomposition to characterize divergence errors, and
quantify changes made by the codes to the vector magnetogram boundary
data in order to be compatible with the force-free model. This study
shows that NLFFF modeling results depend quantitatively on the spatial
resolution of the input boundary data, and that using more highly
resolved boundary data yields more self-consistent results. The
free energies of the resulting solutions generally trend higher
with increasing resolution, while relative magnetic helicity values
vary significantly between resolutions for all methods. All methods
require changing the horizontal components, and for some methods also
the vertical components, of the vector magnetogram boundary field in
excess of nominal uncertainties in the data. The solutions produced
by the various methods are significantly different at each resolution
level. We continue to recommend verifying agreement between the modeled
field lines and corresponding coronal loop images before any NLFFF
model is used in a scientific setting.
Title: A Statistical Study of Distant Consequences of Large Solar
Energetic Events
Authors: Schrijver, Carolus J.; Higgins, Paul A.
Bibcode: 2015SoPh..290.2943S
Altcode: 2015SoPh..tmp..144S; 2015arXiv150905680S
Large solar flares and eruptions may influence remote regions through
perturbations in the outer-atmospheric magnetic field, leading
to causally related events outside of the primary or triggering
eruptions that are referred to as "sympathetic events". We quantify
the occurrence of sympathetic events using the full-disk observations
by the Atmospheric Imaging Assembly onboard the Solar Dynamics
Observatory associated with flares of GOES class M5 or larger from 01
May 2010 through 31 December 2014. Using a superposed-epoch analysis,
we find an increase in the rate of flares, filament eruptions, and
substantial sprays and surges more than 20∘ away from
the primary flares within the first 4 hours at a significance of 1.8
standard deviations. We also find that the rate of distant events
drops by two standard deviations, or a factor of 1.2, when comparing
intervals between 4 hours and 24 hours before and after the start times
of the primary large flares. We discuss the evidence for the concluding
hypothesis that the gradual evolution leading to the large flare and the
impulsive release of the energy in that flare both contribute to the
destabilization of magnetic configurations in distant active regions
and quiet-Sun areas. These effects appear to leave distant regions,
in an ensemble sense, in a more stable state, so that fewer energetic
events happen for at least a day following large energetic events.
Title: Blind Stereoscopy of the Coronal Magnetic Field
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Malanushenko,
Anna
Bibcode: 2015SoPh..290.2765A
Altcode: 2015SoPh..tmp..147A; 2015arXiv150604713A
We test the feasibility of 3D coronal-loop tracing in stereoscopic
EUV image pairs, with the ultimate goal of enabling efficient 3D
reconstruction of the coronal magnetic field that drives flares and
coronal mass ejections (CMEs). We developed an automated code designed
to perform triangulation of coronal loops in pairs (or triplets) of EUV
images recorded from different perspectives. The automated (or blind)
stereoscopy code includes three major tasks: i) automated pattern
recognition of coronal loops in EUV images, ii) automated pairing of
corresponding loop patterns from two different aspect angles, and iii)
stereoscopic triangulation of 3D loop coordinates. We perform tests
with simulated stereoscopic EUV images and quantify the accuracy of
all three procedures. In addition we test the performance of the
blind-stereoscopy code as a function of the spacecraft-separation
angle and as a function of the spatial resolution. We also test the
sensitivity to magnetic non-potentiality. The automated code developed
here can be used for analysis of existing Solar TErrestrial RElationship
Observatory (STEREO) data, but primarily serves for a design study
of a future mission with dedicated diagnostics of non-potential
magnetic fields. For a pixel size of 0.6″ (corresponding
to the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly
(AIA) spatial resolution of 1.4″), we find an optimum
spacecraft-separation angle of αs≈5∘.
Title: Socio-Economic Hazards and Impacts of Space Weather: The
Important Range Between Mild and Extreme
Authors: Schrijver, Carolus J.
Bibcode: 2015SpWea..13..524S
Altcode: 2015arXiv150708730S
Society needs to prepare for more severe space weather than it
has experienced in the modern technological era. To enable that
we must both quantify extreme-event characteristics and analyze
impacts of lesser events that are frequent yet severe enough to be
informative. Exploratory studies suggest that economic impacts of a
century-level space hurricane and of a century of lesser space weather
"gales" may turn out to be of the same order of magnitude. The economic
benefits of effective mitigation of the impacts of space gales may
substantially exceed the required investments, even as these investments
provide valuable information to prepare for the worst possible storms.
Title: Thermal Diagnostics with the Atmospheric Imaging Assembly
on board the Solar Dynamics Observatory: A Validated Method for
Differential Emission Measure Inversions
Authors: Cheung, Mark C. M.; Boerner, P.; Schrijver, C. J.; Testa,
P.; Chen, F.; Peter, H.; Malanushenko, A.
Bibcode: 2015ApJ...807..143C
Altcode: 2015arXiv150403258C
We present a new method for performing differential emission measure
(DEM) inversions on narrow-band EUV images from the Atmospheric
Imaging Assembly (AIA) on board the Solar Dynamics Observatory. The
method yields positive definite DEM solutions by solving a linear
program. This method has been validated against a diverse set of
thermal models of varying complexity and realism. These include
(1) idealized Gaussian DEM distributions, (2) 3D models of NOAA
Active Region 11158 comprising quasi-steady loop atmospheres in a
nonlinear force-free field, and (3) thermodynamic models from a fully
compressible, 3D MHD simulation of active region (AR) corona formation
following magnetic flux emergence. We then present results from the
application of the method to AIA observations of Active Region 11158,
comparing the region's thermal structure on two successive solar
rotations. Additionally, we show how the DEM inversion method can be
adapted to simultaneously invert AIA and Hinode X-ray Telescope data,
and how supplementing AIA data with the latter improves the inversion
result. The speed of the method allows for routine production of DEM
maps, thus facilitating science studies that require tracking of the
thermal structure of the solar corona in time and space.
Title: Understanding space weather to shield society: A global road
map for 2015-2025 commissioned by COSPAR and ILWS
Authors: Schrijver, Carolus J.; Kauristie, Kirsti; Aylward, Alan D.;
Denardini, Clezio M.; Gibson, Sarah E.; Glover, Alexi; Gopalswamy,
Nat; Grande, Manuel; Hapgood, Mike; Heynderickx, Daniel; Jakowski,
Norbert; Kalegaev, Vladimir V.; Lapenta, Giovanni; Linker, Jon A.;
Liu, Siqing; Mandrini, Cristina H.; Mann, Ian R.; Nagatsuma, Tsutomu;
Nandy, Dibyendu; Obara, Takahiro; Paul O'Brien, T.; Onsager, Terrance;
Opgenoorth, Hermann J.; Terkildsen, Michael; Valladares, Cesar E.;
Vilmer, Nicole
Bibcode: 2015AdSpR..55.2745S
Altcode: 2015arXiv150306135S
There is a growing appreciation that the environmental conditions
that we call space weather impact the technological infrastructure
that powers the coupled economies around the world. With that comes
the need to better shield society against space weather by improving
forecasts, environmental specifications, and infrastructure design. We
recognize that much progress has been made and continues to be made
with a powerful suite of research observatories on the ground and
in space, forming the basis of a Sun-Earth system observatory. But
the domain of space weather is vast - extending from deep within the
Sun to far outside the planetary orbits - and the physics complex
- including couplings between various types of physical processes
that link scales and domains from the microscopic to large parts
of the solar system. Consequently, advanced understanding of space
weather requires a coordinated international approach to effectively
provide awareness of the processes within the Sun-Earth system through
observation-driven models. This roadmap prioritizes the scientific focus
areas and research infrastructure that are needed to significantly
advance our understanding of space weather of all intensities and
of its implications for society. Advancement of the existing system
observatory through the addition of small to moderate state-of-the-art
capabilities designed to fill observational gaps will enable significant
advances. Such a strategy requires urgent action: key instrumentation
needs to be sustained, and action needs to be taken before core
capabilities are lost in the aging ensemble. We recommend advances
through priority focus (1) on observation-based modeling throughout the
Sun-Earth system, (2) on forecasts more than 12 h ahead of the magnetic
structure of incoming coronal mass ejections, (3) on understanding
the geospace response to variable solar-wind stresses that lead to
intense geomagnetically-induced currents and ionospheric and radiation
storms, and (4) on developing a comprehensive specification of space
climate, including the characterization of extreme space storms to guide
resilient and robust engineering of technological infrastructures. The
roadmap clusters its implementation recommendations by formulating
three action pathways, and outlines needed instrumentation and research
programs and infrastructure for each of these. An executive summary
provides an overview of all recommendations.
Title: JD3 - 3D Views of the Cycling Sun in Stellar Context: Overview
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.
Bibcode: 2015HiA....16...81V
Altcode:
We summarise the motivations and main results of the joint discussion
``3D Views of the Cycling Sun in Stellar Context'', and give credit
to contributed talks and poster presentations, as due to the limited
number of pages, this proceedings could only include contributions
from the keynote speakers.
Title: Introduction
Authors: Schrijver, Carolus J.
Bibcode: 2015hsws.book....1S
Altcode:
Space weather is a real and permanent hazard to society that needs
to be, and can be, addressed by combining scientific research
with engineering ingenuity: protecting society from space weather
requires that we adequately understand the physical processes of space
weather, that we characterize the conditions to which technological
infrastructures need to be designed, that we learn to effectively
forecast space weather, and that the consequences of acting on such
forecasts are accepted as necessary for the protection of societal
infrastructure.
Title: Preface
Authors: Schrijver, Carolus J.; Bagenal, Frances; Sojka, Jan J.
Bibcode: 2015hsws.bookD...3S
Altcode:
This volume is being developed over the course of several years
of the Heliophysics Summer School, starting with the first chapter
in 2012. Chapters are being added as they become available from the
authors/lecturers, after which this volume will be completed as the 5th
in the Heliophysics series. This volume will be available as a freely
accessible online volume to complement the four printed Heliophysics
volumes published by Cambridge University Press. We recommend that
the reader occiasionally check the School's website (see below) for
updates. Until the volume is complete, the numbering of chapters,
figures, and tables is subject to change.
Title: Heliophysics: Space Weather and Society
Authors: Schrijver, Carolus J.; Bagenal, Frances; Sojka, Jan J.
Bibcode: 2015hsws.book.....S
Altcode:
No abstract at ADS
Title: Opportunities in Heliophysics
Authors: Austin, M.; Guhathakurta, M.; Schrijver, K.; Sojka, J. J.;
Bagenal, F.
Bibcode: 2014AGUFMSH13B4083A
Altcode:
Heliophysics is a developing scientific discipline integrating studies
of the Sun's variability, the surrounding heliosphere, and climate
environments. Over the past few centuries our understanding of how the
Sun drives space weather and climate on the Earth and other planets has
advanced at an ever-increasing rate. NASA Living With a Star and the
UCAR Visiting Scientist Progams sponsor the annual Heliophysics Summer
Schools to build the next generation of scientists in this emerging
field. The highly successful series of the summer schools (commencing
2007) trains a select group of graduate students, postdoctoral fellows
and university faculty to learn and develop the science of heliophysics
as a broad, coherent discipline that reaches in space from the Earth's
troposphere to the depths of the Sun, and in time from the formation
of the solar system to the distant future. The first three years of
the school resulted in the publication of three textbooks now being
used at universities worldwide. Subsequent years have also developed
the complementary materials that support teaching of heliophysics
at both graduate and undergraduate levels. The textbooks are edited
by Carolus J. Schrijver, Lockheed Martin, and George L. Siscoe,
Boston University. The books provide a foundational reference for
researchers in heliophysics, astrophysics, plasma physics, space
physics, solar physics aeronomy, space weather, planetary science and
climate science. The Jack Eddy Postdoctoral Fellowship Program matches
newly graduated postdoctorates with hosting mentors for the purpose of
training the next generation researchers needed in heliophysics. The
fellowships are for two years, and any U.S. university or research
lab may apply to host a fellow.
Title: Bright Hot Impacts by Erupted Fragments Falling Back on the
Sun: UV Redshifts in Stellar Accretion
Authors: Reale, F.; Orlando, S.; Testa, P.; Landi, E.; Schrijver, C. J.
Bibcode: 2014ApJ...797L...5R
Altcode: 2014arXiv1410.7193R
A solar eruption after a flare on 2011 June 7 produced EUV-bright
impacts of fallbacks far from the eruption site, observed with the
Solar Dynamics Observatory. These impacts can be taken as a template
for the impact of stellar accretion flows. Broad redshifted UV lines
have been commonly observed in young accreting stars. Here we study
the emission from the impacts in the Atmospheric Imaging Assembly's
UV channels and compare the inferred velocity distribution to stellar
observations. We model the impacts with two-dimensional hydrodynamic
simulations. We find that the localized UV 1600 Å emission and its
timing with respect to the EUV emission can be explained by the
impact of a cloud of fragments. The first impacts produce strong
initial upflows. The following fragments are hit and shocked by these
upflows. The UV emission comes mostly from the shocked front shell of
the fragments while they are still falling, and is therefore redshifted
when observed from above. The EUV emission instead continues from the
hot surface layer that is fed by the impacts. Fragmented accretion
can therefore explain broad redshifted UV lines (e.g., C IV 1550 Å)
to speeds around 400 km s-1 observed in accreting young
stellar objects.
Title: How Pre-Eruption Configurations Lead to Bz in the Corona
Authors: Schrijver, K.
Bibcode: 2014AGUFMSH33B..06S
Altcode:
One of the key properties that determines the strength of geomagnetic
disturbances is the pattern of the magnetic field contained in
coronal mass ejections (CME), often summarized in the term 'Bz' that
characterizes the field orientation in the leading segment of the
CME. That Bz has its origin in the field that leaves the solar corona at
the initial phases of the eruption, and is thus set by the properties of
the erupting active region field and the field into which the eruption
initially propagates. Mapping the 3D configuration of these fields
remains a challenge, but as observational and modeling capabilities
advance, we can expect to improve our knowledge of the field leaving the
Sun. I will review the current status of our capabilities to model the
3D field above solar active regions, summarize our growing skills at
using observations of solar surface and corona to drive these models,
and discuss opportunities for the near future.
Title: A COSPAR/ILWS roadmap towards advanced space weather science
to protect society's technological infrastructure
Authors: Schrijver, K.; Kauristie, K.
Bibcode: 2014AGUFMSH21C4135S
Altcode:
With the rapid development of the technological infrastructure upon
which modern society depends comes a growing appreciation of the
hazards presented by the phenomena around our home planet that we
call space weather. The complexity of the coupled Sun-Earth system,
the sparseness by which it can be covered by remote-sensing and
in-situ instrumentation, and the costs of the required observational
and computational infrastructure warrant an international approach with
feasible, affordable solutions. COSPAR and the steering committee of the
International Living With a Star program tasked a multi-disciplinary,
international team with the development of a roadmap with the goal of
demonstrably improving our observational capabilities for, scientific
understanding of, and ability to forecast the various aspects of space
weather. We summarize the roadmap, its top-priority recommendations to
achieve its goals, and their underlying rationale. More information
on the roadmap, including the team's full membership, can be found
at http://www.lmsal.com/~schryver/COSPARrm.
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Kushner, G. D.;
Akin, D. J.; Allard, B.; Berger, T.; Boerner, P.; Cheung, M.; Chou,
C.; Drake, J. F.; Duncan, D. W.; Freeland, S.; Heyman, G. F.; Hoffman,
C.; Hurlburt, N. E.; Lindgren, R. W.; Mathur, D.; Rehse, R.; Sabolish,
D.; Seguin, R.; Schrijver, C. J.; Tarbell, T. D.; Wülser, J. -P.;
Wolfson, C. J.; Yanari, C.; Mudge, J.; Nguyen-Phuc, N.; Timmons,
R.; van Bezooijen, R.; Weingrod, I.; Brookner, R.; Butcher, G.;
Dougherty, B.; Eder, J.; Knagenhjelm, V.; Larsen, S.; Mansir, D.;
Phan, L.; Boyle, P.; Cheimets, P. N.; DeLuca, E. E.; Golub, L.;
Gates, R.; Hertz, E.; McKillop, S.; Park, S.; Perry, T.; Podgorski,
W. A.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Weber, M.; Dunn, C.;
Eccles, S.; Jaeggli, S. A.; Kankelborg, C. C.; Mashburn, K.; Pust, N.;
Springer, L.; Carvalho, R.; Kleint, L.; Marmie, J.; Mazmanian, E.;
Pereira, T. M. D.; Sawyer, S.; Strong, J.; Worden, S. P.; Carlsson,
M.; Hansteen, V. H.; Leenaarts, J.; Wiesmann, M.; Aloise, J.; Chu,
K. -C.; Bush, R. I.; Scherrer, P. H.; Brekke, P.; Martinez-Sykora,
J.; Lites, B. W.; McIntosh, S. W.; Uitenbroek, H.; Okamoto, T. J.;
Gummin, M. A.; Auker, G.; Jerram, P.; Pool, P.; Waltham, N.
Bibcode: 2014SoPh..289.2733D
Altcode: 2014arXiv1401.2491D; 2014SoPh..tmp...25D
The Interface Region Imaging Spectrograph (IRIS) small explorer
spacecraft provides simultaneous spectra and images of the photosphere,
chromosphere, transition region, and corona with 0.33 - 0.4 arcsec
spatial resolution, two-second temporal resolution, and 1 km
s−1 velocity resolution over a field-of-view of up to
175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous
orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a
19-cm UV telescope that feeds a slit-based dual-bandpass imaging
spectrograph. IRIS obtains spectra in passbands from 1332 - 1358 Å,
1389 - 1407 Å, and 2783 - 2834 Å, including bright spectral lines
formed in the chromosphere (Mg II h 2803 Å and Mg II k 2796 Å) and
transition region (C II 1334/1335 Å and Si IV 1394/1403 Å). Slit-jaw
images in four different passbands (C II 1330, Si IV 1400, Mg II k
2796, and Mg II wing 2830 Å) can be taken simultaneously with spectral
rasters that sample regions up to 130 arcsec × 175 arcsec at a variety
of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to
emission from plasma at temperatures between 5000 K and 10 MK and will
advance our understanding of the flow of mass and energy through an
interface region, formed by the chromosphere and transition region,
between the photosphere and corona. This highly structured and dynamic
region not only acts as the conduit of all mass and energy feeding
into the corona and solar wind, it also requires an order of magnitude
more energy to heat than the corona and solar wind combined. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative-MHD codes to facilitate interpretation of
observations of this complex region. Approximately eight Gbytes of data
(after compression) are acquired by IRIS each day and made available
for unrestricted use within a few days of the observation.
Title: Accretion impacts studied on the Sun
Authors: Reale, F.; Orlando, S.; Testa, P.; Peres, G.; Landi, E.;
Schrijver, C.
Bibcode: 2014xru..confE.169R
Altcode:
Accretion in star-forming regions is a hot topic. The Sun has recently
offered an interesting opportunity to study accretion impacts observed
in great detail at high energies (Reale et al. 2013, Science, 341,
6143, 251). After the eruption of a dense filament triggered by an
energetic flare on June 7, 2011 part of the ejected material falls
back onto the solar surface. The impact of the downfalling plasma is
similar to that of accretion flows on young stellar objects, and was
imaged in the EUV by the Atmospheric Imaging Assembly (AIA) on-board
the Solar Dynamics Observatory (SDO). Hydrodynamic simulations confirm
that the high energy emission is produced by the impact of high-density
plasma at the highest free-fall speeds and show the importance of
the absorption in reducing the X-ray emission and of fragmentation
in explaining the line broadenings. Impacts such as these present
a laboratory for stellar astronomers to study the impact of dense
(accreting) circumstellar material in unique detail.
Title: A study of sympathetic eruptions using the Heliophysics
Events Knowledgebase
Authors: Higgins, Paul A.; Schrijver, Carolus J.; Title, Alan M.;
Bloomfield, D. Shaun; Gallagher, Peter T
Bibcode: 2014AAS...22412316H
Altcode:
Over the past few decades there have been a number of papers
investigating the connection between flares occurring in
succession. Statistically, any connection that affects the timing of
successive flares that exists is found to be weak. However, the majority
of previous investigations has been limited by only considering the
causal connection between soft X-ray flares. More recent case studies
have shown convincing evidence that large eruptions cause a global
reorganization of overlying magnetic fields that can result in the
eruption of both flares and filaments at large distances from the
original event. In this work, the connection between GOES X-ray flares
(C-, M-, and X-class) and filament eruptions occurring in succession in
two different active regions is considered statistically. The filament
eruptions are recorded in the Heliophysics Events Knowledgebase
by observers using SDO/AIA data. A significant causal connection is
found between the two event types, such that large flares are followed
by filament eruptions within 24 hours much more often than they are
preceded by filament eruptions. This stipulates that the flares either
cause the filaments to erupt or affect the eruption timing such that
the filament eruptions follow the flares more closely in time.
Title: Space Weather From Explosions on the Sun: How Bad Could It Be?
Authors: Schrijver, Carolus J.; Beer, Jürg
Bibcode: 2014EOSTr..95Q.201S
Altcode:
The variable conditions in geospace driven by the Sun's magnetic
activity, known as space weather, pose an increasing threat to society
[National Research Council, 2008]. Of particular concern are the
infrequent and poorly known extremes.
Title: Photometric and Thermal Cross-calibration of Solar EUV
Instruments
Authors: Boerner, P. F.; Testa, P.; Warren, H.; Weber, M. A.;
Schrijver, C. J.
Bibcode: 2014SoPh..289.2377B
Altcode: 2013arXiv1307.8045B
We present an assessment of the accuracy of the calibration measurements
and atomic physics models that go into calculating the SDO/AIA response
as a function of wavelength and temperature. The wavelength response
is tested by convolving SDO/EVE and Hinode/EIS spectral data with the
AIA effective area functions and by comparing the predictions with
AIA observations. For most channels, the AIA intensities summed over
the disk agree with the corresponding measurements derived from the
current version (V2) of the EVE data to within the estimated 25 %
calibration error. This agreement indicates that the AIA effective
areas are generally stable in time. The AIA 304 Å channel, however,
does show degradation by a factor of almost 3 from May 2010 through
September 2011, when the throughput apparently reached a minimum. We
also found some inconsistencies in the 335 Å passband, possibly due to
higher-order contamination of the EVE data. The intensities in the AIA
193 Å channel agree to within the uncertainties with the corresponding
measurements from EIS full CCD observations. Analysis of high-resolution
X-ray spectra of the solar-like corona of Procyon and of EVE spectra
allowed us to investigate the accuracy and completeness of the CHIANTI
database in the AIA shorter wavelength passbands. We found that in
the 94 Å channel, the spectral model significantly underestimates the
plasma emission owing to a multitude of missing lines. We derived an
empirical correction for the AIA temperature responses by performing
differential emission measure (DEM) inversion on a broad set of EVE
spectra and adjusting the AIA response functions so that the count
rates predicted by the full-disk DEMs match the observations.
Title: IRIS Observations of Coronal Rain and Prominences: Return
Flows of the Chromosphere-Corona Mass Cycle
Authors: Liu, Wei; Berger, Thomas; Antolin, Patrick; Schrijver, Karel
Bibcode: 2014AAS...22431303L
Altcode:
It has recently been recognized that a mass cycle (e.g., Berger
et al. 2011; McIntosh et al. 2012) between the hot, tenuous solar
corona and the cool, dense chromosphere underneath it plays an
important role in the mass budget and dynamic evolution of the solar
atmosphere. Although the corona ultimately loses mass through the solar
wind and coronal mass ejections, a fraction of its mass returns to the
chromosphere in coronal rain, downflows of prominences, and other as-yet
unidentified processes. We present here analysis of joint observations
of IRIS, SDO/AIA, and Hinode/SOT of such phenomena. By utilizing the
wide temperature coverage (logT: 4 - 7) provided by these instruments
combined, we track the coronal cooling sequence (e.g., Schrijver 2001;
Liu et al. 2012; Berger et al. 2012) leading to the formation of such
material at transition region or chromospheric temperatures (logT: 4 -
5) in the million-degree corona. We compare the cooling times with those
expected from the radiative cooling instability. We also measure the
kinematics and densities of such downflows and infer their mass fluxes,
which are compared to the upward mass fluxes into the corona, e.g.,
those associated with spicules and flux emergence. Special attention is
paid to coronal rain formed near cusp-shaped portions of coronal loops,
funnel-shaped prominences at dips of coronal loops, and their respective
magnetic environments. With the information about where and when such
catastrophic cooling events take place, we discuss the implications for
the enigmatic coronal heating mechanisms (e.g., Antolin et al. 2010).
Title: Forward Modeling of Coronal Emission
Authors: Malanushenko, Anna; Schrijver, Carolus J.; Van Ballegooijen,
Adriaan A.
Bibcode: 2014AAS...22432102M
Altcode:
In this work, we present simulations of the coronal emission in
Extreme Ultraviolet wavelengths, subject to the possible physical
models of how the solar corona is heated. In order to maximize the
match of the simulations with the observations, we also use models
of coronal magnetic field which are constructed to match the observed
coronal features (see Malanushenko et al, 2014). While we utilize the 1D
quasi-steady atmosphere approach (as in Schrijver & van Ballegoijen,
2005), we take a step away from the commonly used assumption about
circular cross-sections of magnetic flux tubes, as our previous research
(Malanushenko & Schrijver, 2013) suggests that this assumption might
lead to substantial artefacts when comparing the simulations to the
observations. In this work, we explore how such treatment of magnetic
flux tubes is capable of producing realistic coronal features. Using
these two major advances, the realistic field model and the realistic
treatment of the cross-section of flux tubes, we test a wide range
of possible heating scenarios, ruling out possibilities by comparing
the simulations with data from a wide range of EUV channels onboard
SDO/AIA spacecraft.
Title: Height-dependent Refraction of A Global EUV Wave and Its
Associated Sympathetic Eruptions
Authors: Liu, Wei; Ofman, Leon; Downs, Cooper; Schrijver, Karel
Bibcode: 2014AAS...22421814L
Altcode:
The height dependence of global extreme-ultraviolet (EUV) waves in
the solar corona, especially of their wave-like behaviors such as
transmission and reflection, is critical to understanding their physical
nature. Prior observations of such behaviors, when detected on the solar
disk, were compromised because height-dependent information is lost
due to the line-of-sight projection from a top-down view. We report a
global EUV wave on the limb observed by SDO/AIA from a side-view that
evidently shows height-dependent transmission and refraction. As the
wave travels through an active region, the orientation of the low-corona
wave front changes from a forward inclination toward the solar surface
to a backward inclination. This indicates that the EUV wave speed
is lower at higher altitudes, which is expected because of the rapid
drop with height of the Alfven and fast-mode speeds in active regions,
as predicted by MHD models. When traveling into the active region,
the EUV wave speed in the low corona increases from ~600 km/s to ~900
km/s. In addition, in the neighborhood of the active region, sympathetic
eruptions of local coronal structures take place sequentially upon
the wave impact and may appear as wave reflection. Understanding
propagation behaviors of global EUV waves brings us one step closer
to fully utilizing them for seismological diagnostics of the global
corona, such as mapping the spatial distribution of the Alfven speed
and magnetic field strength.
Title: Active Region Magnetic Field Modeling Guided by Coronal Loops
and Surface Fields
Authors: DeRosa, Marc L.; Malanushenko, Anna; Schrijver, Carolus J.;
Wheatland, Michael S
Bibcode: 2014AAS...22432319D
Altcode:
Dynamic events such as solar flares, filament eruptions, and mass
ejections are powered by the evolving coronal magnetic field. However,
the ways in which energy is stored in, and released from, the coronal
magnetic field are poorly understood, in large part because the field
configuration cannot be determined directly from observations and has
eluded the successful application of routine modeling based on surface
magnetograms. Recently, we have demonstrated that the Quasi-Grad-Rubin
(QGR) method for modeling the current-carrying field associated with
active regions shows promise. In Malanushenko et al. (2014, ApJ 783:102)
we have used the QGR method to construct the magnetic field at several
times during the evolution of AR11158 during February 2011. The QGR
method does not require vector magnetograms, and instead uses the
trajectories of observed coronal loops to constrain the locations
of electric currents within the modeling domain. In this study,
we continue to assess the utility of QGR by applying this method to
additional active regions from the current activity cycle, making use
of SDO/HMI line-of-sight magnetograms and imagery from the extreme
ultraviolet channels of SDO/AIA.
Title: CME Mass Estimates via EVE Coronal Dimmings for X-class Flares
Authors: Hudson, Hugh S.; Hannah, Iain; Schrijver, Karel
Bibcode: 2014AAS...22421810H
Altcode:
The EVE instrument on SDO detects post-flare dimmings, mainly in the
spectral regions of Fe IX-XII in its MEGS-A range, which is available
for most of the 29 X-class flares that have occurred between SDO launch
and the end of April 2014. Based upon earlier X-ray observations
we interpret these dimmings as the result of CME mass ejection from
the low corona. We estimate the masses involved in these dimmings by
deriving a best pre-event temperature and emission measure in the dimmed
region from EVE, and a source volume from AIA images. The dimming for
SOL2011-02-15, the first of these events, "peaked"at -3.4% in Fe IX
in terms of the pre-event emission from the whole Sun, with smaller
relative depletions in higher ionization states of Fe. The "maximum"
occurred more than one hour after GOES peak. The dimming signature is
generally cleanly measurable in the EVE/MEGS-A spectral samples at10
s cadence, with the dominant source of uncertainty stemming from the
"sun-as-a-star" integrations; for example flare-related excess emission
at a given wavelength tends to compensate for the dimming,and in this
sense the mass estimate must be considered a lower limit. We address
these uncertainties for the solar case by appealing to the AIA images,
but for analogous processes in stellar flares one would not have
this luxury.
Title: Using Coronal Loops to Reconstruct the Magnetic Field of an
Active Region before and after a Major Flare
Authors: Malanushenko, A.; Schrijver, C. J.; DeRosa, M. L.; Wheatland,
M. S.
Bibcode: 2014ApJ...783..102M
Altcode: 2013arXiv1312.5389M
The shapes of solar coronal loops are sensitive to the presence
of electrical currents that are the carriers of the non-potential
energy available for impulsive activity. We use this information in
a new method for modeling the coronal magnetic field of active region
(AR) 11158 as a nonlinear force-free field (NLFFF). The observations
used are coronal images around the time of major flare activity on
2011 February 15, together with the surface line-of-sight magnetic
field measurements. The data are from the Helioseismic and Magnetic
Imager and Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory. The model fields are constrained to approximate the coronal
loop configurations as closely as possible, while also being subject
to the force-free constraints. The method does not use transverse
photospheric magnetic field components as input and is thereby
distinct from methods for modeling NLFFFs based on photospheric vector
magnetograms. We validate the method using observations of AR 11158
at a time well before major flaring and subsequently review the field
evolution just prior to and following an X2.2 flare and associated
eruption. The models indicate that the energy released during the
instability is about 1 × 1032 erg, consistent with what
is needed to power such a large eruptive flare. Immediately prior to
the eruption, the model field contains a compact sigmoid bundle of
twisted flux that is not present in the post-eruption models, which
is consistent with the observations. The core of that model structure
is twisted by ≈0.9 full turns about its axis.
Title: Extreme solar events
Authors: Schrijver, Carolus
Bibcode: 2014cosp...40E2935S
Altcode:
Space-based measurements of the most energetic solar events reveal
a continuous distribution of ever lower frequency with increasing
total energy. These measurements are limited to the past half
century, however, so they provide little information on the rarest,
most energetic events. The properties of those can only be obtained
indirectly by the analysis of radionuclides on Earth and on the Moon,
comparison with observations of Sun-like stars, and inferences from
four centuries of sunspot observations. These combined records help
constrain the frequencies and energies of the most powerful solar
storms, but the sparsenes of the data and some inconsistencies in the
available information leave us with considerable uncertainties about
the most extreme space weather that the Sun can generate.
Title: Using coronal loops to model the coronal magnetic field before
and after major eruptive events
Authors: Malanushenko, Anna; Schrijver, Carolus; Wheatland, M. S.;
DeRosa, Marc
Bibcode: 2014cosp...40E1960M
Altcode:
Solar flares are believed to be a manifestation of major release of
magnetic energy stored in active region field. Modeling the coronal
magnetic field may enable us to evaluate the energy available for
release, as well as possible sites of the reconnection and other
relevant properties of the field. We use a new method to aid this
problem by including the observed structure of the field (manifested
in coronal loops) as additional constraints. We verify that the method
(previously shown to work on synthetic data in Malanushenko et. al.,
ApJ, 756, 153, 2012) is generally acceptable for the solar data, as
it gives self-consistent, slowly changing results for slowly evolving
structures. We further develop the potential of this method to access
changes in the coronal magnetic field triggered by major eruptive
events, and compare the results with observations.
Title: A roadmap towards advanced space weather science to protect
society's technological infrastructure: Panel Discussion 3
Authors: Schrijver, Carolus; Kauristie, Kirsti
Bibcode: 2014cosp...40E2940S
Altcode:
This single 90minute slot will follow on from the morning plenary
presentation of the roadmap, providing an opportunity for further
discussion of the panel’s findings with an invited panel of key
stakeholders. --- As mankind’s technological capabilities grow,
society constructs a rapidly deepening insight into the workings
of the universe at large, being guided by exploring space near to
our home. But at the same time our societal dependence on technology
increases and with that comes a growing appreciation of the challenges
presented by the phenomena that occur in that space around our home
planet: Magnetic explosions on the Sun and their counterparts in the
geomagnetic field can in extreme cases endanger our all-pervasive
electrical infrastructure. Powerful space storms occasionally lower
the reliability of the globe-spanning satellite navigation systems
and interrupt radio communications. Energetic particle storms lead to
malfunctions and even failures in satellites that are critical to the
flow of information in the globally connected economies. These and other
Sun-driven effects on Earth’s environment, collectively known as space
weather, resemble some other natural hazards in the sense that they pose
a risk for the safe and efficient functioning of society that needs to
be understood, quantified, and - ultimately - mitigated against. The
complexity of the coupled Sun-Earth system, the sparseness by which it
can be covered by remote-sensing and in-situ instrumentation, and the
costs of the required observational and computational infrastructure
warrant a well-planned and well-coordinated approach with cost-efficient
solutions. Our team is tasked with the development of a roadmap with
the goal of demonstrably improving our observational capabilities,
scientific understanding, and the ability to forecast. This paper
summarizes the accomplishments of the roadmap team in identifying the
highest-priority challenges to achieve these goals.
Title: A roadmap towards advanced space weather science to protect
society's technological infrastructure
Authors: Schrijver, Carolus
Bibcode: 2014cosp...40E2937S
Altcode:
As mankind’s technological capabilities grow, society constructs
a rapidly deepening insight into the workings of the universe at
large, being guided by exploring space near to our home. But at the
same time our societal dependence on technology increases and with
that comes a growing appreciation of the challenges presented by the
phenomena that occur in that space around our home planet: Magnetic
explosions on the Sun and their counterparts in the geomagnetic
field can in extreme cases endanger our all-pervasive electrical
infrastructure. Powerful space storms occasionally lower the reliability
of the globe-spanning satellite navigation systems and interrupt radio
communications. Energetic particle storms lead to malfunctions and even
failures in satellites that are critical to the flow of information in
the globally connected economies. These and other Sun-driven effects on
Earth’s environment, collectively known as space weather, resemble
some other natural hazards in the sense that they pose a risk for the
safe and efficient functioning of society that needs to be understood,
quantified, and - ultimately - mitigated against. The complexity of the
coupled Sun-Earth system, the sparseness by which it can be covered
by remote-sensing and in-situ instrumentation, and the costs of the
required observational and computational infrastructure warrant
a well-planned and well-coordinated approach with cost-efficient
solutions. Our team is tasked with the development of a roadmap with
the goal of demonstrably improving our observational capabilities,
scientific understanding, and the ability to forecast. This paper
summarizes the accomplishments of the roadmap team in identifying the
highest-priority challenges to achieve these goals.
Title: Energy transfer from the photosphere to the corona:
observational aspects
Authors: Schrijver, Carolus
Bibcode: 2014cosp...40E2936S
Altcode:
The outer solar atmosphere, and indeed the entire heliosphere, is
powered by the reservoir of non-radiative energy that exists in the
forms of electromagnetic field and kinetic energy below the solar
surface. A small fraction of that reservoir leaks away into the solar
corona where it powers the EUV and X-ray glow, the solar wind, and
on occasion large flares. I will review observational constraints
on the physical pathways that are most important in this coupling,
guided by theoretical consideration. I will focusing in particular on
recent space-based observations by IRIS, Hinode, and SDO that together
cover the domain from below the solar surface into the the high corona.
Title: A roadmap towards advanced space weather science to protect
society's technological infrastructure: Panel Discussion 1
Authors: Schrijver, Carolus; Kauristie, Kirsti
Bibcode: 2014cosp...40E2938S
Altcode:
This single 90minute slot will follow on from the morning plenary
presentation of the roadmap, providing an opportunity for further
discussion of the panel’s findings with an invited panel of key
stakeholders. --- As mankind’s technological capabilities grow,
society constructs a rapidly deepening insight into the workings
of the universe at large, being guided by exploring space near to
our home. But at the same time our societal dependence on technology
increases and with that comes a growing appreciation of the challenges
presented by the phenomena that occur in that space around our home
planet: Magnetic explosions on the Sun and their counterparts in the
geomagnetic field can in extreme cases endanger our all-pervasive
electrical infrastructure. Powerful space storms occasionally lower
the reliability of the globe-spanning satellite navigation systems
and interrupt radio communications. Energetic particle storms lead to
malfunctions and even failures in satellites that are critical to the
flow of information in the globally connected economies. These and other
Sun-driven effects on Earth’s environment, collectively known as space
weather, resemble some other natural hazards in the sense that they pose
a risk for the safe and efficient functioning of society that needs to
be understood, quantified, and - ultimately - mitigated against. The
complexity of the coupled Sun-Earth system, the sparseness by which it
can be covered by remote-sensing and in-situ instrumentation, and the
costs of the required observational and computational infrastructure
warrant a well-planned and well-coordinated approach with cost-efficient
solutions. Our team is tasked with the development of a roadmap with
the goal of demonstrably improving our observational capabilities,
scientific understanding, and the ability to forecast. This paper
summarizes the accomplishments of the roadmap team in identifying the
highest-priority challenges to achieve these goals.
Title: A roadmap towards advanced space weather science to protect
society's technological infrastructure: Panel Discussion 2
Authors: Schrijver, Carolus; Kauristie, Kirsti
Bibcode: 2014cosp...40E2939S
Altcode:
This single 90minute slot will follow on from the morning plenary
presentation of the roadmap, providing an opportunity for further
discussion of the panel’s findings with an invited panel of key
stakeholders. --- As mankind’s technological capabilities grow,
society constructs a rapidly deepening insight into the workings
of the universe at large, being guided by exploring space near to
our home. But at the same time our societal dependence on technology
increases and with that comes a growing appreciation of the challenges
presented by the phenomena that occur in that space around our home
planet: Magnetic explosions on the Sun and their counterparts in the
geomagnetic field can in extreme cases endanger our all-pervasive
electrical infrastructure. Powerful space storms occasionally lower
the reliability of the globe-spanning satellite navigation systems
and interrupt radio communications. Energetic particle storms lead to
malfunctions and even failures in satellites that are critical to the
flow of information in the globally connected economies. These and other
Sun-driven effects on Earth’s environment, collectively known as space
weather, resemble some other natural hazards in the sense that they pose
a risk for the safe and efficient functioning of society that needs to
be understood, quantified, and - ultimately - mitigated against. The
complexity of the coupled Sun-Earth system, the sparseness by which it
can be covered by remote-sensing and in-situ instrumentation, and the
costs of the required observational and computational infrastructure
warrant a well-planned and well-coordinated approach with cost-efficient
solutions. Our team is tasked with the development of a roadmap with
the goal of demonstrably improving our observational capabilities,
scientific understanding, and the ability to forecast. This paper
summarizes the accomplishments of the roadmap team in identifying the
highest-priority challenges to achieve these goals.
Title: The Perihelion Passage of Comet ISON as seen by SDO
Authors: Pesnell, W. D.; Schrijver, C. J.; Boerner, P.; DeRosa, M. L.;
Liu, W.; Thompson, B. J.
Bibcode: 2013AGUFM.P24A..10P
Altcode:
Comet ISON will fly through perihelion on November 28, 2013. It is one
of the largest sungrazing comets to be seen in the Space Age. The Solar
Dynamics Observatory (SDO) has seen two previous sungrazing comets in
the extreme ultraviolet channels of the Atmospheric Imaging Assembly
(AIA). Comet ISON will fly farther from the Sun (perihelion distance
of 2.7 Rsun compared to 1.15 for Comet Lovejoy), meaning it probes
a different part of the solar corona, but its larger size should
provide enough mass to illuminate the path of the nucleus. Based on
the latest ephemeris, SDO will be able to track Comet ISON through
the entire perihelion passage by a series of off-point maneuvers. We
will present the AIA data obtained from the Comet ISON perihelion,
discussing the differences between Comets ISON and Lovejoy. We will
then summarize what we have learned from the observations and offer
some thoughts on what sungrazing comets may reveal about comets,
the Sun, and their interaction.
Title: A survey of of uses and value of space weather information
Authors: Schrijver, C. J.; Rabanal, J.
Bibcode: 2013AGUFMSM53D2238S
Altcode:
We analyze some 2,800 responses to a survey among subscribers of NOAA's
Space Weather Prediction Center email services. Interest in, anticipated
impacts from, and responses to solar flares, energetic particle
events, and geomagnetic storms are quite uniform across societal
sectors. Approximately 40% of the respondents expect serious to very
serious impacts from space weather events if no action were taken to
mitigate or in the absence of adequate space weather information. The
impacts of space weather are deemed to be substantially reduced
because of the availability of, and the response to, space-weather
forecasts and alerts. Space weather information is primarily used
as aid to understand anomalies, to implement mitigating strategies
designed to avoid impacts on operations, and to prepare for potential
contingencies related directly or indirectly to space weather. Current
and near-future space-weather conditions are generally highly valued,
considered useful, and generally, though not fully, adequate to avoid
or mitigate societal impacts (related most frequently to human safety
and reliability of operations). We conclude that even among those
receiving space weather information, there is considerable uncertainty
about how to act on the information provided.
Title: On the Anisotropy in Expansion of Magnetic Flux Tubes in the
Solar Corona
Authors: Malanushenko, A.; Schrijver, C. J.
Bibcode: 2013ApJ...775..120M
Altcode: 2013arXiv1307.3440M
Most one-dimensional hydrodynamic models of plasma confined to magnetic
flux tubes assume circular tube cross sections. We use potential field
models to show that flux tubes in circumstances relevant to the solar
corona do not, in general, maintain the same cross-sectional shape
through their length and therefore the assumption of a circular cross
section is rarely true. We support our hypothesis with mathematical
reasoning and numerical experiments. We demonstrate that lifting this
assumption in favor of realistic, non-circular loops makes the apparent
expansion of magnetic flux tubes consistent with that of observed
coronal loops. We propose that in a bundle of ribbon-like loops, those
that are viewed along the wide direction would stand out against those
that are viewed across the wide direction due to the difference in
their column depths. That result would impose a bias toward selecting
loops that appear not to be expanding, seen projected in the plane of
sky. An implication of this selection bias is that the preferentially
selected non-circular loops would appear to have increased pressure
scale heights even if they are resolved by current instruments.
Title: Large-scale Coronal Propagating Fronts in Solar Eruptions
as Observed by the Atmospheric Imaging Assembly on Board the Solar
Dynamics Observatory—an Ensemble Study
Authors: Nitta, Nariaki V.; Schrijver, Carolus J.; Title, Alan M.;
Liu, Wei
Bibcode: 2013ApJ...776...58N
Altcode: 2013arXiv1308.3544N
This paper presents a study of a large sample of global disturbances
in the solar corona with characteristic propagating fronts as
intensity enhancement, similar to the phenomena that have often
been referred to as Extreme Ultraviolet Imaging Telescope (EIT)
waves or extreme-ultraviolet (EUV) waves. Now EUV images obtained by
the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory provide a significantly improved view of these large-scale
coronal propagating fronts (LCPFs). Between 2010 April and 2013 January,
a total of 171 LCPFs have been identified through visual inspection of
AIA images in the 193 Å channel. Here we focus on the 138 LCPFs that
are seen to propagate across the solar disk, first studying how they
are associated with flares, coronal mass ejections (CMEs), and type II
radio bursts. We measure the speed of the LCPF in various directions
until it is clearly altered by active regions or coronal holes. The
highest speed is extracted for each LCPF. It is often considerably
higher than EIT waves. We do not find a pattern where faster LCPFs
decelerate and slow LCPFs accelerate. Furthermore, the speeds are
not strongly correlated with the flare intensity or CME magnitude,
nor do they show an association with type II bursts. We do not find
a good correlation either between the speeds of LCPFs and CMEs in a
subset of 86 LCPFs observed by one or both of the Solar and Terrestrial
Relations Observatory spacecraft as limb events.
Title: Pathways of Large-scale Magnetic Couplings between Solar
Coronal Events
Authors: Schrijver, Carolus J.; Title, Alan M.; Yeates, Anthony R.;
DeRosa, Marc L.
Bibcode: 2013ApJ...773...93S
Altcode: 2013arXiv1305.0801S
The high-cadence, comprehensive view of the solar corona by SDO/AIA
shows many events that are widely separated in space while occurring
close together in time. In some cases, sets of coronal events are
evidently causally related, while in many other instances indirect
evidence can be found. We present case studies to highlight a variety
of coupling processes involved in coronal events. We find that physical
linkages between events do occur, but concur with earlier studies that
these couplings appear to be crucial to understanding the initiation
of major eruptive or explosive phenomena relatively infrequently. We
note that the post-eruption reconfiguration timescale of the large-scale
corona, estimated from the extreme-ultraviolet afterglow, is on average
longer than the mean time between coronal mass ejections (CMEs), so
that many CMEs originate from a corona that is still adjusting from a
previous event. We argue that the coronal field is intrinsically global:
current systems build up over days to months, the relaxation after
eruptions continues over many hours, and evolving connections easily
span much of a hemisphere. This needs to be reflected in our modeling
of the connections from the solar surface into the heliosphere to
properly model the solar wind, its perturbations, and the generation and
propagation of solar energetic particles. However, the large-scale field
cannot be constructed reliably by currently available observational
resources. We assess the potential of high-quality observations from
beyond Earth's perspective and advanced global modeling to understand
the couplings between coronal events in the context of CMEs and solar
energetic particle events. .
Title: On Cross-Sectional Properties of Coronal Loops
Authors: Malanushenko, Anna; Schrijver, C. J.
Bibcode: 2013SPD....4420105M
Altcode:
Coronal loops have been observed for several decades, yet some of their
properties remain a mystery. These in particular include the lack of
apparent expansion of coronal loops and the increased pressure scale
height in loops compared to the diffuse background. We approach these
problems in an entirely new way. We demonstrate that solely lifting
the assumption about circular cross-sectional shape of flux tubes is
alone sufficient to explain lack of expansion and increased pressure
scale height. While magnetic flux tubes expand in the corona, they do
so in a highly anisotropic manner, which we examine in details for
several model fields and quantify for a potential field model based
on HMI data. We demonstrate how, and why, this leads towards (1)
selection bias which might make some loops stand out if they expand
mostly along the line of sight, due to their increased column depth;
(2) principal limitations on measuring expansion of coronal loops,
even if they are resolved and (3) the apparent increased pressure scale
height. We also address the existing studies which seemingly concluded
the opposite. The latter was based on several properties of the loops'
emission which, as we show, are also reproduced when loops are oblate
in cross-section.
Title: Probing the Solar Magnetic Field With a Sun-Grazing Comet
Authors: Downs, Cooper; Linker, J. A.; Mikic, Z.; Riley, P.; Schrijver,
C. J.; Saint-Hilaire, P.
Bibcode: 2013SPD....4430503D
Altcode:
Observations of comets occupy a rich history within Solar and
Heliospheric science. Cometary plasma tails probe the solar wind in
the inner solar system 0.5-3 AU) and their observations led to its
discovery more than half a century ago. Fast forwarding to today,
recent observations of sun-grazing comets within the solar corona
have opened up a whole new avenue to study the Sun with these striking
celestial bodies. Here we present our recent study of the perihelion
passage of comet C/2011 W3 (Lovejoy), which came within 140Mm of
the solar surface. Imaged from multiple perspectives by SDO/AIA and
the STEREO/EUVI, extreme ultraviolet (EUV) observations of Lovejoy's
tail showed substantial changes in direction, intensity, magnitude,
and persistence. To understand this unique signature, we combine a
state-of-the-art magnetohydrodynamic (MHD) model of the solar corona
and a prescription for the motion of emitting cometary tail ions in
an embedded plasma. We show how the observed tail motions reveal the
inhomogeneous magnetic field of the solar corona, and demonstrate
how they constrain field and plasma properties in a region where the
coronal plasma is normally not easily observed in EUV. We will also
discuss our results in context of the upcoming perihelion passage of
comet C/2012 S1 (ISON), expected by many to be a spectacular probe of
the near-sun environment. Work supported by NASA and NSF.
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, Bart; Title, A. M.; Lemen, J.; Wuelser, J.;
Tarbell, T. D.; Schrijver, C. J.; Golub, L.; Kankelborg, C.; Carlsson,
M.; Hansteen, V. H.; Worden, S.; IRIS Team
Bibcode: 2013SPD....44...03D
Altcode:
The solar chromosphere and transition region (TR) form a highly
structured and dynamic interface region between the photosphere and
the corona. This region not only acts as the conduit of all mass and
energy feeding into the corona and solar wind, it also requires an
order of magnitude more energy to heat than the corona. Nevertheless,
the chromosphere remains poorly understood, because of the complexity
of the required observational and analytical tools: the interface
region is highly complex with transitions from optically thick to
optically thin radiation, from pressure to magnetic field domination,
and large density and temperature contrasts on small spatial scales. The
Interface Region Imaging Spectrograph (IRIS) was selected for a NASA
SMEX mission in 2009 and is scheduled to launch on 26-June-2013 (with
first light scheduled for mid July). IRIS addresses critical questions:
(1) Which types of non-thermal energy dominate in the chromosphere and
beyond? (2) How does the chromosphere regulate mass and energy supply
to the corona and heliosphere? (3) How do magnetic flux and matter
rise through the lower atmosphere, and what role does flux emergence
play in flares and mass ejections? These questions are addressed with
a high-resolution near and far UV imaging spectrometer sensitive to
emission from plasma at temperatures between 5,000 K and 10 MK. IRIS
has a field-of-view of 120 arcsec, a spatial resolution of 0.4 arcsec,
and velocity resolution of 0.5 km/s. The IRIS investigation includes
a strong numerical modeling component based on advanced radiative MHD
codes to facilitate interpretation of observations. We describe the
IRIS instrumentation and numerical modeling, and present the plans for
observations, calibration and data distribution. We will highlight some
of the issues that IRIS observations can help resolve. More information
can be found at http://iris.lmsal.com
Title: Bright Hot Impacts by Erupted Fragments Falling Back on the
Sun: A Template for Stellar Accretion
Authors: Reale, Fabio; Orlando, Salvatore; Testa, Paola; Peres,
Giovanni; Landi, Enrico; Schrijver, Carolus J.
Bibcode: 2013Sci...341..251R
Altcode:
Impacts of falling fragments observed after the eruption of a filament
in a solar flare on 7 June 2011 are similar to those inferred for
accretion flows on young stellar objects. As imaged in the ultraviolet
(UV)-extreme UV range by the Atmospheric Imaging Assembly onboard
the Solar Dynamics Observatory, many impacts of dark, dense matter
display uncommonly intense, compact brightenings. High-resolution
hydrodynamic simulations show that such bright spots, with plasma
temperatures increasing from ~104 to ~106
kelvin, occur when high-density plasma (>>1010
particles per cubic centimeter) hits the solar surface at several
hundred kilometers per second, producing high-energy emission as in
stellar accretion. The high-energy emission comes from the original
fragment material and is heavily absorbed by optically thick plasma,
possibly explaining the lower mass accretion rates inferred from x-rays
relative to UV-optical-near infrared observations of young stars.
Title: SDO AIA Observations of Large-Scale Coronal Propagating Fronts
Authors: Nitta, Nariaki; Schrijver, C. J.; Title, A. M.; Liu, W.
Bibcode: 2013SPD....44...40N
Altcode:
The discovery of "EIT waves" rekindled interests in what used to be
called flare waves, which had been typically observed in H-alpha. In
addition to Moreton waves, first observed at the Lockheed Solar
Observatory, other manifestations of shock waves propagating in the
corona include type II radio bursts and filament oscillations away from
flare sites. Identification of EIT waves with the postulated fast-mode
MHD shock waves in the corona has been questioned, however, largely
because of their low speeds (e.g., 200-400 km/s). EIT's 10-20 minute
cadence could be a contributing factor for this, and we need to find how
fast large-scale coronal propagating fronts are in higher-cadence EUV
images. It is clear that AIA on SDO is the best instrument at the moment
for this type of work. With the availability of high-cadence full-disk
images, we now can compare propagating fronts in different directions,
and determine the highest speed of each event on AIA images more
objectively and accurately than on EIT (and STEREO EUVI) images. In a
large number of EIT wave events, we have measured speeds of propagating
fronts using AIA's 193 A images. Before the fronts are deflected by the
discontinuities, e.g., active regions and coronal holes, the mean and
median speeds are 620 km/s and 600 km/s, respectively, and many exceed
800 km/s. Higher speeds are often seen in events that accompany a type
II burst, strong flare or energetic CME, but the distribution of the
speed with these attributes is broad. We also find that the speeds of
the large-scale coronal propagating fronts are not well correlated
with those of the associated CMEs. Given that large-scale coronal
propagating fronts at large distances represent freely propagating MHD
waves, we discuss how to understand their nature close to their origins.
Title: Probing the Solar Magnetic Field with a Sun-Grazing Comet
Authors: Downs, Cooper; Linker, Jon A.; Mikić, Zoran; Riley, Pete;
Schrijver, Carolus J.; Saint-Hilaire, Pascal
Bibcode: 2013Sci...340.1196D
Altcode:
On 15 and 16 December 2011, Sun-grazing comet C/2011 W3 (Lovejoy)
passed deep within the solar corona, effectively probing a region
that has never been visited by spacecraft. Imaged from multiple
perspectives, extreme ultraviolet observations of Lovejoy's tail
showed substantial changes in direction, intensity, magnitude,
and persistence. To understand this unique signature, we combined a
state-of-the-art magnetohydrodynamic model of the solar corona and
a model for the motion of emitting cometary tail ions in an embedded
plasma. The observed tail motions reveal the inhomogeneous magnetic
field of the solar corona. We show how these motions constrain field
and plasma properties along the trajectory, and how they can be used to
meaningfully distinguish between two classes of magnetic field models.
Title: Disturbances in the US electric grid associated with
geomagnetic activity
Authors: Schrijver, Carolus J.; Mitchell, Sarah D.
Bibcode: 2013JSWSC...3A..19S
Altcode: 2013arXiv1304.5489S
Large solar explosions are responsible for space weather that can
impact technological infrastructure on and around Earth. Here, we
apply a retrospective cohort exposure analysis to quantify the impacts
of geomagnetic activity on the US electric power grid for the period
from 1992 through 2010. We find, with more than 3σ significance, that
approximately 4% of the disturbances in the US power grid reported
to the US Department of Energy are attributable to strong geomagnetic
activity and its associated geomagnetically induced currents.
Title: Some Difficulties in Determining Causality of Sympathetic
Solar Events
Authors: DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Yeates, A. R.
Bibcode: 2013enss.confE..91D
Altcode:
Much has been made regarding the occurrence of synchronous eruptive
events occurring in the solar corona. Determining the frequencies at
which they occur and understanding the causal linkages that may connect
such events (making them sympathetic in addition to synchronous) are an
area of active research. Causal linkages are observed to take the form
of (1) disturbances in magnetic fields that connect active regions,
(2) disturbances in the magnetic field configuration overlying active
regions, and/or (3) triggering by disturbances propagating from one
region to another. Here we display two types of synchronous events:
those where, using a combination of image sequences from SDO and STEREO
as well as coronal-field modeling, evidence for sympathy seems solid,
and those where evidence of sympathy is more ambiguous. We use these two
types of cases to illustrate some difficulties in establishing whether
synchronous events are in fact sympathetic. This has implications
for determining the frequency and importance of sympathetic events,
and thus for understanding of coronal field evolution and the origins
of space weather.
Title: Automated Temperature and Emission Measure Analysis of
Coronal Loops and Active Regions Observed with the Atmospheric
Imaging Assembly on the Solar Dynamics Observatory (SDO/AIA)
Authors: Aschwanden, Markus J.; Boerner, Paul; Schrijver, Carolus J.;
Malanushenko, Anna
Bibcode: 2013SoPh..283....5A
Altcode: 2011SoPh..tmp..384A
We developed numerical codes designed for automated analysis of
SDO/AIA image datasets in the six coronal filters, including: i)
coalignment test between different wavelengths with measurements of
the altitude of the EUV-absorbing chromosphere, ii) self-calibration by
empirical correction of instrumental response functions, iii) automated
generation of differential emission measure [DEM] distributions
with peak-temperature maps [Tp(x,y)] and emission measure
maps [EMp(x,y)] of the full Sun or active region areas,
iv) composite DEM distributions [dEM(T)/dT] of active regions or
subareas, v) automated detection of coronal loops, and vi) automated
background subtraction and thermal analysis of coronal loops, which
yields statistics of loop temperatures [Te], temperature
widths [σT], emission measures [EM], electron densities
[ne], and loop widths [w]. The combination of these
numerical codes allows for automated and objective processing of
numerous coronal loops. As an example, we present the results of an
application to the active region NOAA 11158, observed on 15 February
2011, shortly before it produced the largest (X2.2) flare during the
current solar cycle. We detect 570 loop segments at temperatures in the
entire range of log(Te)=5.7 - 7.0 K and corroborate previous
TRACE and AIA results on their near-isothermality and the validity of
the Rosner-Tucker-Vaiana (RTV) law at soft X-ray temperatures (T≳2
MK) and its failure at lower EUV temperatures.
Title: Heliophysics 3 Volume Paperback Set
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2013heli.book.....S
Altcode:
Volume 1: Preface; 1. Prologue Carolus J. Schrijver and George
L. Siscoe; 2. Introduction to heliophysics Thomas J. Bogdan; 3. Creation
and destruction of magnetic field Matthias Rempel; 4. Magnetic field
topology Dana W. Longcope; 5. Magnetic reconnection Terry G. Forbes;
6. Structures of the magnetic field Mark B. Moldwin, George L. Siscoe
and Carolus J. Schrijver; 7. Turbulence in space plasmas Charles
W. Smith; 8. The solar atmosphere Viggo H. Hansteen; 9. Stellar winds
and magnetic fields Viggo H. Hansteen; 10. Fundamentals of planetary
magnetospheres Vytenis M. Vasyliunas; 11. Solar-wind magnetosphere
coupling: an MHD perspective Frank R. Toffoletto and George L. Siscoe;
12. On the ionosphere and chromosphere Tim Fuller-Rowell and Carolus
J. Schrijver; 13. Comparative planetary environments Frances Bagenal;
Bibliography; Index. Volume 2: Preface; 1. Perspective on heliophysics
George L. Siscoe and Carolus J. Schrijver; 2. Introduction to
space storms and radiation Sten Odenwald; 3. In-situ detection of
energetic particles George Gloeckler; 4. Radiative signatures of
energetic particles Tim Bastian; 5. Observations of solar and stellar
eruptions, flares, and jets Hugh Hudson; 6. Models of coronal mass
ejections and flares Terry Forbes; 7. Shocks in heliophysics Merav
Opher; 8. Particle acceleration in shocks Dietmar Krauss-Varban;
9. Energetic particle transport Joe Giacalone; 10. Energy conversion in
planetary magnetospheres Vytenis Vasyliunas; 11. Energization of trapped
particles Janet Green; 12. Flares, CMEs, and atmospheric responses Tim
Fuller-Rowell and Stanley C. Solomon; 13. Energetic particles and manned
spaceflight Stephen Guetersloh and Neal Zapp; 14. Energetic particles
and technology Alan Tribble; Appendix I. Authors and editors; List of
illustrations; List of tables; Bibliography; Index. Volume 3: Preface;
1. Interconnectedness in heliophysics Carolus J. Schrijver and George
L. Siscoe; 2. Long-term evolution of magnetic activity of Sun-like
stars Carolus J. Schrijver; 3. Formation and early evolution of stars
and proto-planetary disks Lee W. Hartmann; 4. Planetary habitability
on astronomical time scales Donald E. Brownlee; 5. Solar internal
flows and dynamo action Mark S. Miesch; 6. Modeling solar and stellar
dynamos Paul Charbonneau; 7. Planetary fields and dynamos Ulrich
R. Christensen; 8. The structure and evolution of the 3D solar wind
John T. Gosling; 9. The heliosphere and cosmic rays J. Randy Jokipii;
10. Solar spectral irradiance: measurements and models Judith L. Lean
and Thomas N. Woods; 11. Astrophysical influences on planetary climate
systems Juerg Beer; 12. Evaluating the drivers of Earth's climate
system Thomas J. Crowley; 13. Ionospheres of the terrestrial planets
Stanley C. Solomon; 14. Long-term evolution of the geospace climate
Jan J. Sojka; 15. Waves and transport processes in atmospheres and
oceans Richard L. Walterscheid; 16. Solar variability, climate, and
atmospheric photochemistry Guy P. Brasseur, Daniel Marsch and Hauke
Schmidt; Appendix I. Authors and editors; List of illustrations;
List of tables; Bibliography; Index.
Title: SDO AIA Observations of Large-Scale Coronal Disturbances in
the Form of Propagating Fronts
Authors: Nitta, Nariaki V.; Schrijver, Carolus J.; Title, Alan M.;
Liu, Wei
Bibcode: 2013enss.confE.111N
Altcode:
One of the most spectacular phenomena detected by SOHO EIT
was the large-scale propagating fronts associated with solar
eruptions. Initially these 'EIT' waves were thought to be coronal
counterparts of chromospheric Moreton waves. However, different
spatial and kinematic properties of the fronts seen in H-alpha and
EUV images, and far more frequent occurrences of the latter have
led to various interpretations that are still actively debated by
a number of researchers. A major factor for the lack of closure was
the various limitation in EIT data, including the cadence that was
typically every 12 minutes. Now we have significantly improved data
from SDO AIA, which have revealed some very interesting phenomena
associated with EIT waves. However, the studies so far conducted
using AIA data have primarily dealt with single or a small number of
events, where selection bias and particular observational conditions
may prevent us from discovering the general and true nature of EIT
waves. Although automated detection of EIT waves was promised for
AIA images some time ago, it is still not actually implemented in the
data pipeline. Therefore we have manually found nearly 200 examples
of large-scale propagating fronts, going through movies of difference
images from the AIA 193 A channel up to January 2013. We present our
study of the kinematic properties of the fronts in a subset of about
150 well-observed events in relation with other phenomena that can
accompany EIT waves. Our emphasis is on the relation of the fronts
with the associated coronal eruptions often but not always taking
the form of full-blown CMEs, utilizing STEREO data for a subset of
more than 80 events that have occurred near the limb as viewed from
one of the STEREO spacecraft. In these events, the availability of
data from the STEREO inner coronagraph (COR1) as well as from the EUVI
allows us to trace eruptions off the solar disk during the times of our
propagating fronts. The representative relations between the fronts and
CMEs will be discussed in terms of the evolution of EIT waves observed
in different channels of AIA, which provide information of the thermal
properties of the fronts. Our study will further clarify the variety
of solar eruptions and their associated manifestations in the corona.
Title: Collective Solar Behavior
Authors: Title, Alan; Schrijver, Karel; Derosa, MArc
Bibcode: 2013enss.confE.120T
Altcode:
The Atmospheric Imaging Assembly (AIA) on the Solar Dynamic Observatory
(SDO) together with the Helioseismic and Magnetic Imager (HMI) and the
Extreme Ultraviolet Variability Experiment (EVE) allow observations of
the entire Sun from 6000 K to 20,000,000 K with arcsecond resolution
and a 12 second cadence (AIA), obtain doppler and continuum images at
a 45 second cadence and Line of Sight and vector magnetograms (HMI)
every few minutes, and integrated solar spectra from 1 to 100 nm on a 2
second cadence (EVE) 24/7. Because of the enhanced thermal and temporal
coverage and the high dynamic range available with AIA, it has been
able to discovery associated behavior associated with extreme solar
events that are apparently driven by the rapid expansion of magnetic
structures. The extent of the events are recognized by using co-temporal
STEREO data. The rapidly expanding magnetic structures, speeds between
500 and 2000 km/s, can apparently trigger filament eruptions, CME's,
and other flares. These "triggered" events are sometimes larger that
the initial disturbance. The remote triggering makes flare prediction
based upon ONLY local energy build up models less valuable, but suggests
that with proper coverage prediction of solar events with potential
for Earth impact may be made more reliable. Movies of sample events
discovered in AIA together with STEREO data will be shown.
Title: The standard flare model in three dimensions. II. Upper limit
on solar flare energy
Authors: Aulanier, G.; Démoulin, P.; Schrijver, C. J.; Janvier, M.;
Pariat, E.; Schmieder, B.
Bibcode: 2013A&A...549A..66A
Altcode: 2012arXiv1212.2086A
Context. Solar flares strongly affect the Sun's atmosphere as well as
the Earth's environment. Quantifying the maximum possible energy of
solar flares of the present-day Sun, if any, is thus a key question in
heliophysics.
Aims: The largest solar flares observed over the
past few decades have reached energies of a few times 1032
erg, possibly up to 1033 erg. Flares in active Sun-like
stars reach up to about 1036 erg. In the absence of direct
observations of solar flares within this range, complementary methods
of investigation are needed to assess the probability of solar flares
beyond those in the observational record.
Methods: Using
historical reports for sunspot and solar active region properties
in the photosphere, we scaled to observed solar values a realistic
dimensionless 3D MHD simulation for eruptive flares, which originate
from a highly sheared bipole. This enabled us to calculate the magnetic
fluxes and flare energies in the model in a wide paramater space.
Results: Firstly, commonly observed solar conditions lead to modeled
magnetic fluxes and flare energies that are comparable to those
estimated from observations. Secondly, we evaluate from observations
that 30% of the area of sunspot groups are typically involved in
flares. This is related to the strong fragmentation of these groups,
which naturally results from sub-photospheric convection. When the
model is scaled to 30% of the area of the largest sunspot group ever
reported, with its peak magnetic field being set to the strongest value
ever measured in a sunspot, it produces a flare with a maximum energy of
~6 × 1033 erg.
Conclusions: The results of the model
suggest that the Sun is able to produce flares up to about six times as
energetic in total solar irradiance fluence as the strongest directly
observed flare of Nov. 4, 2003. Sunspot groups larger than historically
reported would yield superflares for spot pairs that would exceed tens
of degrees in extent. We thus conjecture that superflare-productive
Sun-like stars should have a much stronger dynamo than in the Sun.
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Wuelser, J.;
Tarbell, T. D.; Schrijver, C.; Golub, L.; Kankelborg, C. C.; Hansteen,
V. H.; Carlsson, M.
Bibcode: 2012AGUFMSH33D2256D
Altcode:
The solar chromosphere and transition region (TR) form a highly
structured and dynamic interface region between the photosphere and
the corona. This region not only acts as the conduit of all mass and
energy feeding into the corona and solar wind, it also requires an
order of magnitude more energy to heat than the corona. Nevertheless,
the chromosphere remains poorly understood, because of the complexity
of the required observational and analytical tools: the interface
region is highly complex with transitions from optically thick to
optically thin radiation, from pressure to magnetic field domination,
and large density and temperature contrasts on small spatial scales. The
Interface Region Imaging Spectrograph (IRIS) was selected for a NASA
SMEX mission in 2009 and is scheduled to launch in early 2013. IRIS
addresses critical questions: (1) Which types of non-thermal energy
dominate in the chromosphere and beyond? (2) How does the chromosphere
regulate mass and energy supply to the corona and heliosphere? (3)
How do magnetic flux and matter rise through the lower atmosphere, and
what role does flux emergence play in flares and mass ejections? These
questions are addressed with a high-resolution near and far UV imaging
spectrometer sensitive to emission from plasma at temperatures between
5,000 K and 10 MK. IRIS has a field-of-view of 120 arcsec, a spatial
resolution of 0.4 arcsec, and velocity resolution of 0.5 km/s. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative MHD codes to facilitate interpretation of
observations. We will describe the IRIS instrumentation and numerical
modeling, and present the status of the IRIS observatory development. We
will highlight some of the issues that IRIS observations can help
resolve.
Title: Quantifying Coronal Dimming as Observed in EUV and X-ray
Images in Eruptive Events
Authors: Nitta, N. V.; Aschwanden, M. J.; Boerner, P.; Hill, S. M.;
Lemen, J. R.; Liu, W.; Schrijver, C.; Wuelser, J.
Bibcode: 2012AGUFMSH41A2097N
Altcode:
Data from SOHO have shown that coronal dimming is closely related
with coronal mass ejections (CMEs). In particular, dimming areas in
EIT 195 A images often match the lateral extension of the associated
CMEs. In this presentation, we summarize how CMEs compare with dimming
as identified at different wavelengths and by other instruments, such as
Yohkoh SXT, TRACE, GOES (12-15) SXI, STEREO EUVI and SDO AIA. Emphasis
is placed on recent data, since the combination of AIA and STEREO
data can lead us to better characterize CMEs and to more accurately
estimate how much mass is ejected. We discuss technical issues that
arise when quantifying dimming as a proxy for a CME. The issues include
instrument calibration, effects of heating and cooling and integration
along the line of sight. We also touch on the relation of dimming with
globally propagating coronal fronts, which are routinely isolated in
running difference images, and its implications on the magnitudes of
the associated CMEs.
Title: The Interaction of Comet C/2011 W3 (Lovejoy) with the Global
Solar Corona: Insight and Perspectives from Realistic Thermodynamic
MHD Simulations
Authors: Downs, C.; Mikic, Z.; Linker, J. A.; Lionello, R.;
Schrijver, C.
Bibcode: 2012AGUFMSH13B2255D
Altcode:
On December 15-16 2011 the perihelion passage of sun-grazing comet
C-2011 W3 (Lovejoy) took it perilously close to the sun. Its voyage
through the low solar corona was readily observed by the EUV imagers
onboard the Solar Dynamics Observatory and the STEREO spacecraft
pair. During this time, EUV emission from the cometary tail underwent
several significant changes in both intensity and overall inclination
and shape, possibly an indication of rapidly changing ambient plasma
or magnetic field conditions. To study this phenomenon, we employ
a realistic 3D thermodynamic MHD model to obtain a self-consistent
picture of the global thermodynamic and magnetic structure of
the corona at this time. We use these simulations to examine the
ambient plasma parameters along the trajectory and place them in the
context of the multi-viewpoint observations, directly connecting the
important influence of the global magnetic field structure to comet
tail appearance and dynamics. By employing multiple simulations,
we demonstrate how such a unique set of observations can be used to
probe not only the comet-corona interaction, but also to constrain
properties of the coronal plasma and magnetic field.
Title: The impact of geomagnetic storms on the US electric power grid
Authors: Schrijver, C.; Mitchell, S.; Title, A. M.
Bibcode: 2012AGUFMSM23B2303S
Altcode:
Large solar explosions are responsible for space weather that can
impact technological infrastructure on and around Earth. We study
the impacts of geomagnetic activity on the U.S. electric power
grid for the period from 1992 through 2010. We find, with more than
3-sigma significance, that approximately 4% of the disturbances in
the U.S. power grid reported to the U.S. Department of Energy are
attributable to geomagnetic activity. The combination of our results
with an economic assessment study by the electric power industry
suggests that the average cost to the U.S. economy of non-catastrophic
grid disturbances in which space weather conditions are a contributing
factor exceeds $3 billion per year. The magnitude of this apparent
economic impact warrants extensive follow-up studies to validate,
understand, and mitigate against the weak but significant contribution
of space weather in power grid disturbances.
Title: Mining Solar Data: the experience with SDO, Hinode, and TRACE
Authors: Schrijver, C.
Bibcode: 2012IAUSS...6E.402S
Altcode:
Center Space-based telescopes that observe the Sun collect close
to a terabyte of data each day. The magnitude of these observatory
archives finding data of relevance to particular research problems,
efficient and fast access to the stored data, and the analysis of
the observations. In this talk I will describe experiences with
the 'live' SDO and Hinode archives as well with the archive of the
recently retired TRACE mission that - as a precursor to SDO's AIA -
offers a 12-year extension of the coverage by AIA of solar coronal
activity. Topics include data access, summaries, meta-data extraction,
autonomous data monitoring and mining, and remote access and analysis.
Title: Sun-grazing comets as probes of the physics of the solar corona
Authors: Schrijver, C.
Bibcode: 2012AGUFMSH21D..02S
Altcode:
In 2011, two Sun-grazing comets were observed at EUV wavelengths as
they descended into the inner solar corona. The first, C/2011 N3 (SoHO),
was observed by the Atmospheric Imaging Assembly of the Solar Dynamics
Observatory up to the point at which it terminated its existence
very near to its orbital perihelion. Its tail emission was visible
to within 0.146 solar radii of the solar surface. The second comet in
the inner corona, C/2011 W3, was observed by a fleet of observatories,
including the STEREO spacecraft which saw it during its approach to,
and subsequent voyage away from, its orbital perihelion behind the
Sun as seen from Earth. C2011 W3 survied for only about 1.6 more days
when the last of its material sublimated. The variable cometary tails,
observed in a multiple EUV passbands, were seen to be deflected by the
interaction with the solar magnetic field as the comets flew through
the corona. In this talk, I will discuss lessons about the solar corona
learned from the cometary emission and absorption features and from
the interaction of the ionizing cometary material with the magnetized
corona through which it traveled.
Title: Evolution of the solar luminosity during solar cycle 23
Authors: Vieira, L. A.; Schrijver, C.; DeRosa, M. L.; Norton, A.;
Dudok de Wit, T.; Da Silva, L.; Vuets, A.
Bibcode: 2012AGUFMSH12A..04V
Altcode:
The effect of the solar activity on the solar luminosity, which is
the total electromagnetic solar output, is one of the fundamental
questions in solar physics. Changes of the solar luminosity can arise
from changes of the energy flux in the convection zone that can also
affects other solar parameters such as the surface temperature, the
apparent radius and shape, and the symmetry of the radiative field
itself. Additionally, understanding the latitudinal distribution of the
flux density is needed to compare the solar variability and its stellar
analogues. Nevertheless, our observations of the solar flux density
are limited to a region near the ecliptic plane, which have provided
just a raw estimate of the variability of the solar luminosity. Here
we present a reconstruction of the solar flux density and solar
luminosity for the solar cycle 23 and ascending phase of cycle 24. The
reconstruction is based on a combination of a state-of-art solar surface
magnetic flux transport model and a semi-empirical total and spectral
irradiance model. The flux transport model is based on assimilation
of MDI/SOHO and HMI/SDO magnetograms. The irradiance model's free
parameters are estimated by minimizing the difference between
the model's output and the PMOD Composite of TSI measurements. We
have obtained a good agreement between the model's output and the
measurements. The distribution of active regions leads to a clear
low latitude brightening during the solar maximum. This brightening
results from the balance of the contributions from bright (faculae and
network) and dark features (sunspots) located in the solar surface,
which peaks near the solar equator. As the effects of dark features
are limited to a narrower region, the variability of the flux density
at the poles is dominated by the evolution of faculae and network. The
preliminary results indicate that the heat flux blocked by sunspots
is lower than the flux leaked by bright features. Consequently, an
increase of the luminosity through the cycle is observed as previously
estimated based on near ecliptic measurements. This work also enables
an assessment of the properties of solar variability when viewed from
out of the ecliptic, i.e., such as we might be viewing other stars of
solar activity level. Finally, the limitations of the model and future
strategies to extend the reconstruction of the flux density and solar
luminosity will be presented.
Title: Interaction of Cometary Material With the Solar Corona:
EUV Observations and MHD Simulations
Authors: Liu, W.; Jia, Y.; Downs, C.; Schrijver, C.; Saint-Hilaire,
P.; Battams, K.; Tarbell, T. D.; Shine, R. A.
Bibcode: 2012AGUFMSH13B2254L
Altcode:
Extreme ultraviolet (EUV) emission from two recent sun-grazing comets,
C/2011 N3 and C/2011 W3 (Lovejoy), has been observed in the solar corona
for the first time by the SDO/AIA and STEREO/EUVI instruments (Schrijver
et al. 2011). These observations provided a unique opportunity to
investigate the interaction of the cometary material with the solar
corona and probe their physical conditions. We present here EUV
observations and MHD simulations on this subject, focusing on the
deceleration of the cometary tail material within the corona. We found
that despite their different local coronal environments, the two comets
exhibited quite similar characteristics. The initial EUV emitting tail
had a projected velocity of 100-200 km/s, which was much lower than the
orbital velocity of 500-600 km/s in the plane-of-sky. This indicates
that significant deceleration had taken place while the tail material
was heated to coronal temperatures on the order of 1 MK before it
started to emit in EUV (Bryans & Pesnell 2012). After its initial
appearance, the tail further experienced a projected deceleration
of ~1 km/s^2 (or 4 g_Sun). In particular, in the Lovejoy case, the
tail appeared as clusters of bright parallel striations roughly at
right angles to the orbit direction, suggestive of magnetic field
lines illuminated by the plasma frozen onto them. These striations
came to a stop and then accelerated in an opposite direction (seen
in projection), approaching a constant velocity of ~50 km/s. These
observations suggest that a Lorentz force from the coronal magnetic
field was operating on the newly ionized cometary plasma. To test this
hypothesis and understand tail deceleration mechanisms, we adopted a
multi-fluid MHD model (Jia et al. 2012) to simulate the interaction
between charged particles and the magnetized coronal plasma. We used
potential extrapolation (Schrijver & DeRosa 2003) and a more
sophisticated global MHD model (Lionello et al. 2009) to infer the
magnetic field and plasma conditions of the corona along the comet's
orbit as inputs to the simulations. We will compare the observations and
simulation results, and discuss the implications for using sun-grazing
comets as probes to the solar corona in the context of NASA's future
Solar Probe Plus mission.
Title: The Thermal Structure of the Corona as observed with SDO/AIA,
With and Without Image Deconvolution
Authors: Boerner, P.; Cheung, C.; Schrijver, C.; Testa, P.; Weber, M.
Bibcode: 2012AGUFMSH33B2240B
Altcode:
We describe a fast, robust method for deriving a differential emission
measure function DEM(T) that matches the observations in the six coronal
channels of SDO/AIA pixel-by-pixel over the full image, and present
some of the results obtained by applying this technique to AIA movies
of a variety of phenomena including "AIA waves," post-flare loops, and
coronal holes. In addition, we discuss the effect of deconvolving the
AIA point-spread function from the images on the inferred temperature
structures.;
Title: Guiding Nonlinear Force-free Modeling Using Coronal
Observations: First Results Using a Quasi-Grad-Rubin Scheme
Authors: Malanushenko, A.; Schrijver, C. J.; DeRosa, M. L.; Wheatland,
M. S.; Gilchrist, S. A.
Bibcode: 2012ApJ...756..153M
Altcode: 2012arXiv1202.5420M
At present, many models of the coronal magnetic field rely on
photospheric vector magnetograms, but these data have been shown
to be problematic as the sole boundary information for nonlinear
force-free field extrapolations. Magnetic fields in the corona
manifest themselves in high-energy images (X-rays and EUV) in the
shapes of coronal loops, providing an additional constraint that
is not at present used as constraints in the computational domain,
directly influencing the evolution of the model. This is in part due
to the mathematical complications of incorporating such input into
numerical models. Projection effects, confusion due to overlapping
loops (the coronal plasma is optically thin), and the limited number
of usable loops further complicate the use of information from
coronal images. We develop and test a new algorithm to use images of
coronal loops in the modeling of the solar coronal magnetic field. We
first fit projected field lines with those of constant-α force-free
fields to approximate the three-dimensional distribution of currents
in the corona along a sparse set of trajectories. We then apply a
Grad-Rubin-like iterative technique, which uses these trajectories as
volume constraints on the values of α, to obtain a volume-filling
nonlinear force-free model of the magnetic field, modifying a code
and method presented by Wheatland. We thoroughly test the technique
on known analytical and solar-like model magnetic fields previously
used for comparing different extrapolation techniques and compare the
results with those obtained by currently available methods relying
only on the photospheric data. We conclude that we have developed a
functioning method of modeling the coronal magnetic field by combining
the line-of-sight component of the photospheric magnetic field with
information from coronal images. Whereas we focus on the use of coronal
loop information in combination with line-of-sight magnetograms, the
method is readily extended to incorporate vector-magnetic data over
any part of the photospheric boundary.
Title: Estimating the frequency of extremely energetic solar events,
based on solar, stellar, lunar, and terrestrial records
Authors: Schrijver, C. J.; Beer, J.; Baltensperger, U.; Cliver,
E. W.; Güdel, M.; Hudson, H. S.; McCracken, K. G.; Osten, R. A.;
Peter, T.; Soderblom, D. R.; Usoskin, I. G.; Wolff, E. W.
Bibcode: 2012JGRA..117.8103S
Altcode: 2012arXiv1206.4889S; 2012JGRA..11708103S
The most powerful explosions on the Sun - in the form of bright
flares, intense storms of solar energetic particles (SEPs), and fast
coronal mass ejections (CMEs) - drive the most severe space-weather
storms. Proxy records of flare energies based on SEPs in principle
may offer the longest time base to study infrequent large events. We
conclude that one suggested proxy, nitrate concentrations in polar
ice cores, does not map reliably to SEP events. Concentrations of
select radionuclides measured in natural archives may prove useful in
extending the time interval of direct observations up to ten millennia,
but as their calibration to solar flare fluences depends on multiple
poorly known properties and processes, these proxies cannot presently be
used to help determine the flare energy frequency distribution. Being
thus limited to the use of direct flare observations, we evaluate the
probabilities of large-energy solar events by combining solar flare
observations with an ensemble of stellar flare observations. We conclude
that solar flare energies form a relatively smooth distribution from
small events to large flares, while flares on magnetically active,
young Sun-like stars have energies and frequencies markedly in excess
of strong solar flares, even after an empirical scaling with the mean
coronal activity level of these stars. In order to empirically quantify
the frequency of uncommonly large solar flares extensive surveys
of stars of near-solar age need to be obtained, such as is feasible
with the Kepler satellite. Because the likelihood of flares larger
than approximately X30 remains empirically unconstrained, we present
indirect arguments, based on records of sunspots and on statistical
arguments, that solar flares in the past four centuries have likely
not substantially exceeded the level of the largest flares observed
in the space era, and that there is at most about a 10% chance of a
flare larger than about X30 in the next 30 years.
Title: Extreme solar eruptions and their impacts in geospace and
on society
Authors: Schrijver, Carolus
Bibcode: 2012cosp...39.1727S
Altcode: 2012cosp.meet.1727S
The most extreme solar eruptive events drive major space
weather in geospace which can couple into mankind's technological
infrastructure. Observations of Sun-like stars show that explosions
on such stars can be orders of magnitude more energetic than the solar
flares observed in the modern instrumental era which spans only a few
decades in time. Can the Sun surprise us with flares and coronal mass
ejections that are significantly more powerful than those observed in
recent decades? If so, how large can these be and how frequently can
they happen? How could these impact mankind's technology in space and
on Earth? Solar, stellar, lunar, and terrestrial records together can
teach us about the extremes of space weather to be expected from the
star we live with. In this talk, I summarize our current knowledge on
the most extreme solar events, discuss how further information may be
obtained, and explore gaps in our knowledge.
Title: Dynamics of the global Sun from interior to outer atmosphere
Authors: Schrijver, Carolus
Bibcode: 2012cosp...39.1728S
Altcode: 2012cosp.meet.1728S
The Sun is the only star whose magnetic activity can be resolved
in stunning detail. Current observational capabilities range from
full-sphere coverage to measurements of details more than 10,000 times
smaller than that. Acoustic waves enable us to probe the dynamics of
the deep interior, while heliospheric imagers reveal the evolution
of coronal mass ejections to beyond the orbit of the Earth. This
comprehensive view of a magnetically active star, complemented by rapid
advances in numerical capabilities, are revealing how the coupled system
of interior, atmosphere, and heliosphere evolves dynamically through
the sunspot cycle, punctuated by flux emergence, field eruptions,
and irradiance variations. The Sun is not only a touchstone for the
interpretation of many astrophysical observations, but its variability
affects our society in more ways than we routinely appreciate; this
drives a need to understand it well enough that forecasts of its
electromagnetic weather can be made. This lecture, starting from the
very different perspectives of astrophysical curiosity and societal
need, focuses on trends near the frontier of our knowledge about the
Sun's functioning as a global system.
Title: Quasi-periodic Fast-mode Wave Trains within a Global EUV Wave
and Sequential Transverse Oscillations Detected by SDO/AIA
Authors: Liu, Wei; Ofman, Leon; Nitta, Nariaki V.; Aschwanden, Markus
J.; Schrijver, Carolus J.; Title, Alan M.; Tarbell, Theodore D.
Bibcode: 2012ApJ...753...52L
Altcode: 2012arXiv1204.5470L
We present the first unambiguous detection of quasi-periodic wave
trains within the broad pulse of a global EUV wave (so-called EIT wave)
occurring on the limb. These wave trains, running ahead of the lateral
coronal mass ejection (CME) front of 2-4 times slower, coherently
travel to distances >~ R ⊙/2 along the solar surface,
with initial velocities up to 1400 km s-1 decelerating to
~650 km s-1. The rapid expansion of the CME initiated at
an elevated height of 110 Mm produces a strong downward and lateral
compression, which may play an important role in driving the primary
EUV wave and shaping its front forwardly inclined toward the solar
surface. The wave trains have a dominant 2 minute periodicity that
matches the X-ray flare pulsations, suggesting a causal connection. The
arrival of the leading EUV wave front at increasing distances produces
an uninterrupted chain sequence of deflections and/or transverse (likely
fast kink mode) oscillations of local structures, including a flux-rope
coronal cavity and its embedded filament with delayed onsets consistent
with the wave travel time at an elevated (by ~50%) velocity within
it. This suggests that the EUV wave penetrates through a topological
separatrix surface into the cavity, unexpected from CME-caused magnetic
reconfiguration. These observations, when taken together, provide
compelling evidence of the fast-mode MHD wave nature of the primary
(outer) fast component of a global EUV wave, running ahead of the
secondary (inner) slow component of CME-caused restructuring.
Title: Large-Scale Coronal Disturbances as Observed by SDO AIA
Authors: Nitta, Nariaki; Schrijver, Carolus; Title, Alan; Lemen,
James; Liu, Wei
Bibcode: 2012cosp...39.1378N
Altcode: 2012cosp.meet.1378N
With increasing solar activity, the Atmospheric Imaging Assembly
(AIA) on the Solar Dynamics Observatory (SDO) has observed a number
of large-scale coronal disturbances, which may correspond to what we
have generally known as "EIT waves." Although their nature is still
actively debated, these disturbances usually accompany CMEs. In certain
cases, the fronts of the disturbances may signify CME-related shock
waves important for particle acceleration. Using the unprecedented
temporal resolution and broad temperature coverage of the AIA, we have
studied more than 100 such events. Here we discuss their kinematics
characterized by faster fronts than EIT waves in Solar Cycle 23, and
spatial relations with CMEs using STEREO data that provide triangulation
of the fronts. We also try plasma diagnostic using images in different
filters. Association of these disturbances with CMEs, flares and type
II bursts is discussed on a statistical basis. Lastly, we explore the
possible relation of the larger-scale coronal disturbances with SEP
events observed at widely separate longitudes and their onset times.
Title: The Journey of Sungrazing Comet Lovejoy
Authors: Bryans, Paul; A'Hearn, M.; Battams, K.; Biesecker, D.;
Bodewits, D.; Boice, D.; Brown, J.; Caspi, A.; Chodas, P.; Hudson,
H.; Jia, Y.; Jones, G.; Keller, H. U.; Knight, M.; Linker, J.; Lisse,
C.; Liu, W.; McIntosh, S.; Pesnell, W. D.; Raymond, J.; Saar, S.;
Saint-Hilaire, P.; Schrijver, C.; Snow, M.; Tarbell, T.; Thompson,
W.; Weissman, P.; Comet Lovejoy Collaboration Team
Bibcode: 2012AAS...22052507B
Altcode:
Comet Lovejoy (C/2011 W3) was the first sungrazing comet, observed
by space-based instruments, to survive perihelion passage. First
observed by ground-based telescopes several weeks prior to perihelion,
its journey towards the Sun was subsequently recorded by several solar
observatories, before being observed in the weeks after perihelion by
a further array of space- and ground-based instruments. Such a surfeit
of wide-ranging observations provides an unprecedented insight into
both sungrazing comets themselves, and the solar atmosphere through
which they pass. This paper will summarize what we have learnt from the
observations thus far and offer some thoughts on what future sungrazing
comets may reveal about comets, the Sun, and their interaction.
Title: SDO/AIA Detection of Quasi-periodic Wave Trains Within Global
EUV ("EIT") Waves and Their Coronal Seismology Implications
Authors: Liu, Wei; Ofman, L.; Aschwanden, M. J.; Nitta, N.; Schrijver,
C. J.; Title, A. M.; Tarbell, T. D.
Bibcode: 2012AAS...22051501L
Altcode:
The nature of global EUV waves (so-called "EIT waves") has long
been under debate because of instrumental limitations and projection
effects when viewed on the solar disk. We present here high cadence
SDO/AIA observations of global EUV waves occurring on the limb. We
report newly discovered quasi-periodic wave trains located in the low
corona within a broad, diffuse pulse of the global EUV wave ahead of
the lateral CME front/flank. These waves coherently travel to large
distances on the order of 1 solar radii with initial velocities up
to 1400 km/s. They have dominant 1-3 minute periodicities that often
match the X-ray pulsations of the accompanying flare, suggestive of
a causal connection. In addition, recently discovered quasi-periodic
fast propagating (QFP) waves of 1000-2000 km/s (Liu, Title, Zhao et
al. 2011 ApJL) are found in the funnel of coronal loops rooted at the
flare kernel. These waves are spatially confined within the CME bubble
and rapidly disappear while approaching the CME front, suggestive
of strong damping and/or dispersion. These observations provide new
evidence of the fast-mode wave nature of the primary, fast component
of a global EUV wave, running ahead of a secondary, slow component
of CME-caused restructuring of the coronal magnetic field. We suggest
that the two types of quasi-periodic waves are both integral parts of
global coronal dynamics manifested as a CME/flare eruption, and they
have important implications for global and local coronal seismology.
Title: Estimate of Energy Release In a Major Flare Using Coronal
Loops Data
Authors: Malanushenko, Anna; Schrijver, C. J.; DeRosa, M. L.
Bibcode: 2012AAS...22052115M
Altcode:
Coronal loops provide with valuable source of information about coronal
magnetic field. In particular, they allow one to observe reconfiguration
of the coronal magnetic field during eruptive episodes. The changes
in the coronal field, as observed in X-rays and extreme ultraviolet,
are often dramatic in even minor eruptions. Therefore, models of
magnetic field which take coronal loops into account might provide
for new insight at changes of the field during eruptions. We use
coronal loops data (gathered from SDO/AIA images) along with the
line-of-sight magnetograms (by SDO/HMI) to model magnetic field in
AR 11158 before and after the so-called Valentine's Flare, an X-class
flare in Feb 15, 2011. This is done using the recently developed Quasi
Grad-Rubin algorithm (QGR), which allows a reconstruction of non-linear
force-free field based on information about electric currents along
a set of arbitrary tracks in the computational domain. Tests of QGR
on solar-like fields demonstrate its ability to recover over 50% of
the free energy, as well as the large-scale structure of currents and
overall shape of field lines. We analyze model magnetic fields of AR
11158 before and after the flare, demonstrate their resemblance with
the observed structure of coronal loops and analyze the changes in
the structure of currents caused by the flare, and compare our results
with existing studies of the same event.
Title: Large-scale Coronal Disturbances As Observed By SDO AIA
Authors: Nitta, Nariaki; Schrijver, C.; title, A.; Liu, W.; Lemen, J.
Bibcode: 2012AAS...22051502N
Altcode:
With increasing solar activity, the Atmospheric Imaging Assembly
(AIA) on the Solar Dynamics Observatory (SDO) has observed a number
of large-scale coronal disturbances, which may correspond to what
we have generally known as "EIT waves." Their nature is still
actively debated. In certain cases, the fronts of the disturbances
may signify CME-related shock waves that are important for particle
acceleration. Using the unprecedented temporal resolution and broad
temperature coverage of the AIA, we have studied more than 100 such
events. Here we discuss their kinematics characterized by faster fronts
than EIT waves in Solar Cycle 23, and spatial relations with CMEs
using STEREO data that provide triangulation of the fronts. We also
try plasma diagnostic using images in different filters. Association
of these disturbances with other phenomena such as CMEs, flares and
type II bursts, is discussed on a statistical basis.
Title: A First Look at Magnetic Field Data Products from SDO/HMI
Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai,
T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat,
S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares,
C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.;
DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk,
S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka,
K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.;
Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić,
Z.; Riley, P.; Wu, S. T.
Bibcode: 2012ASPC..455..337L
Altcode:
The Helioseismic and Magnetic Imager (HMI; Scherrer & Schou 2011)
is one of the three instruments aboard the Solar Dynamics Observatory
(SDO) that was launched on February 11, 2010 from Cape Canaveral,
Florida. The instrument began to acquire science data on March 24. The
regular operations started on May 1. HMI measures the Doppler velocity
and line-of-sight magnetic field in the photosphere at a cadence of
45 seconds, and the vector magnetic field at a 135-second cadence,
with a 4096× 4096 pixels full disk coverage. The vector magnetic
field data is usually averaged over 720 seconds to suppress the p-modes
and increase the signal-to-noise ratio. The spatial sampling is about
0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which
has a Landé factor of 2.5. These data are further used to produce
higher level data products through the pipeline at the HMI-AIA Joint
Science Operations Center (JSOC) - Science Data Processing (Scherrer et
al. 2011) at Stanford University. In this paper, we briefly describe the
data products, and demonstrate the performance of the HMI instrument. We
conclude that the HMI is working extremely well.
Title: Division II: Sun and Heliosphere
Authors: Martínez Pillet, Valentín; Klimchuk, James A.; Melrose,
Donald B.; Cauzzi, Gianna; van Driel-Gesztelyi, Lidia; Gopalswamy,
Natchimuthuk; Kosovichev, Alexander; Mann, Ingrid; Schrijver,
Carolus J.
Bibcode: 2012IAUTA..28...61M
Altcode: 2012IAUTA..28...61P
The solar activity cycle entered a prolonged quiet phase that started
in 2008 and ended in 2010. This minimum lasted for a year longer
than expected and all activity proxies, as measured from Earth and
from Space, reached minimum values never observed before (de Toma,
2012). The number of spotless days from 2006 to 2009 totals 800, the
largest ever recorded in modern times. Solar irradiance was at historic
minimums. The interplanetary magnetic field was measured at values as
low as 2.9 nT and the cosmic rays were observed at records-high. While
rumors spread that the Sun could be entering a grand minimum quiet
phase (such as the Maunder minimum of the XVII century), activity
took over in 2010 and we are now well into Solar Cycle 24 (albeit,
probably, a low intensity cycle), approaching towards a maximum due
by mid 2013. In addition to bringing us the possibility to observe
a quiet state of the Sun and of the Heliosphere that was previously
not recorded with modern instruments, the Sun has also shown us how
little we know about the dynamo mechanism that drives its activity as
all solar cycle predictions failed to see this extended minimum coming.
Title: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
Bibcode: 2012IAUTA..28...69V
Altcode:
Commission 10 of the International Astronomical Union has more than
650 members who study a wide range of activity phenomena produced by
our nearest star, the Sun. Solar activity is intrinsically related
to solar magnetic fields and encompasses events from the smallest
energy releases (nano- or even picoflares) to the largest eruptions
in the Solar System, coronal mass ejections (CMEs), which propagate
into the Heliosphere reaching the Earth and beyond. Solar activity is
manifested in the appearance of sunspot groups or active regions, which
are the principal sources of activity phenomena from the emergence of
their magnetic flux through their dispersion and decay. The period
2008-2009 saw an unanticipated extended solar cycle minimum and
unprecedentedly weak polar-cap and heliospheric field. Associated with
that was the 2009 historical maximum in galactic cosmic rays flux since
measurements begun in the middle of the 20th Century. Since then Cycle
24 has re-started solar activity producing some spectacular eruptions
observed with a fleet of spacecraft and ground-based facilities. In
the last triennium major advances in our knowledge and understanding
of solar activity were due to continuing success of space missions as
SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched
by the breathtaking images of the solar atmosphere produced by the
Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the
framework of NASA's Living with a Star program. In August 2012, at the
time of the IAU General Assembly in Beijing when the mandate of this
Commission ends, we will be in the unique position to have for the
first time a full 3-D view of the Sun and solar activity phenomena
provided by the twin STEREO missions about 120 degrees behind and
ahead of Earth and other spacecraft around the Earth and ground-based
observatories. These new observational insights are continuously
posing new questions, inspiring and advancing theoretical analysis
and modelling, improving our understanding of the physics underlying
magnetic activity phenomena. Commission 10 reports on a vigorously
evolving field of research produced by a large community. The number
of refereed publications containing `Sun', `heliosphere', or a synonym
in their abstracts continued the steady growth seen over the preceding
decades, reaching about 2000 in the years 2008-2010, with a total of
close to 4000 unique authors. This report, however, has its limitations
and it is inherently incomplete, as it was prepared jointly by the
members of the Organising Committee of Commission 10 (see the names
of the primary contributors to the sections indicated in parentheses)
reflecting their fields of expertise and interest. Nevertheless, we
believe that it is a representative sample of significant new results
obtained during the last triennium in the field of solar activity.
Title: Kinematics and helicity evolution of a loop-like eruptive
prominence
Authors: Koleva, K.; Madjarska, M. S.; Duchlev, P.; Schrijver, C. J.;
Vial, J. -C.; Buchlin, E.; Dechev, M.
Bibcode: 2012A&A...540A.127K
Altcode: 2012arXiv1202.4541K
Aims: We aim at investigating the morphology as well as
kinematic and helicity evolution of a loop-like prominence during its
eruption.
Methods: We used multi-instrument observations from
AIA/SDO, EUVI/STEREO and LASCO/SoHO. The kinematic, morphological,
geometrical, and helicity evolution of a loop-like eruptive
prominence were studied in the context of the magnetic flux rope
model of solar prominences.
Results: The prominence eruption
evolved as a height-expanding twisted loop with both legs anchored
in the chromosphere of a plage area. The eruption process consisted
of a prominence activation, acceleration, and a phase of constant
velocity. The prominence body was composed of counter-clockwise twisted
threads around the main prominence axis. The twist during the eruption
was estimated at 6π (3 turns). The prominence reached a maximum height
of 526 Mm before contracting to its primary location and was partially
reformed in the same place two days after the eruption. This ejection,
however, triggered a coronal mass ejection (CME) observed in LASCO
C2. The prominence was located in the northern periphery of the CME
magnetic field configuration and, therefore, the background magnetic
field was asymmetric with respect to the filament position. The physical
conditions of the falling plasma blobs were analysed with respect to
the prominence kinematics.
Conclusions: The same sign of the
prominence body twist and writhe, as well as the amount of twisting
above the critical value of 2π after the activation phase indicate that
possibly conditions for kink instability were present. No signature
of magnetic reconnection was observed anywhere in the prominence body
and its surroundings. The filament/prominence descent following the
eruption and its partial reformation at the same place two days later
suggest a confined type of eruption. The asymmetric background magnetic
field possibly played an important role in the failed eruption. Movies showing the temporal evolution are available in electronic
form at http://www.aanda.org
Title: Force-free Magnetic Fields and Electric Currents inferred
from Coronal Loops and Stereoscopy
Authors: Aschwanden, Markus J.; Boerner, P.; Schrijver, C. J.;
Malanushenko, A.
Bibcode: 2012decs.confE.105A
Altcode:
Force-free magnetic fields are considered to be a natural state of the
low plasma-beta corona. There exist about a dozen of numerical nonlinear
force-free field (NLFFF) computation codes that are able to caclulate
a divergence-free and force-free solution of the magnetic field, by
extrapolation from a lower boundary condition that is specified with
3D vector magnetograph data. However, significant differences in the
solutions have been found among the different NLFFF codes, as well as in
comparison with stereoscopically triangulated 3D coordinates of coronal
loops, exhibiting field misalignment angles of 20-40 degrees. Each
calculation of a NLFFF solution is computing-intensive and no code is
fast enough to enable forward-fitting to observations. Here we derive
an analytical approximation of NLFFF solutions that is accurate to
second order and can efficiently be used for forward-fitting to coronal
loops. We demonstrate the accurcay of the NLFFF forward-fitting code by
reproducing the Low and Lou (1990) analytical model withg an accuracy
of <5 degres. Further, we show examples of fitted NLFFF solutions to
STEREO observations of coronal loops. Future NLFFF fits are expected
based on line-of-sight magnetograms and automated loop tracings only,
without requiring vector field and STEREO data.
Title: Non-Linear Force-Free Modeling of Solar Corona With The Aid
of Coronal Loops
Authors: Malanushenko, A.; DeRosa, M.; Schrijver, C.; Wheatland,
M. S.; Gilchrist, S.
Bibcode: 2012decs.confE.113M
Altcode:
Accurate models of the coronal magnetic field are vital for
understanding and predicting solar activity and are therefore of the
greatest interest for solar physics. As no reliable measurements of the
coronal magnetic field exists at present, the problem of constructing
field models is typically viewed as a boundary value problem. The
construction of realistic field models requires knowledge of the full
vector of magnetic field at the boundaries of the model domain; vector
magnetograms are, however, measured in the non force-free photosphere
and their horizontal components are subject to large uncertainties. Even
if an uncertainty-free vector magnetogram at the top layer of the
chromosphere was known, the problem remains an extremely challenging
non-linear problem. There are various methods for pre-processing
vector magnetograms and using them to construct models of the coronal
field. The success of these models is often judged based on how close
its field lines correspond to the observed coronal loops, which are
believed to follow lines of the coronal magnetic field. At present,
the correspondence between coronal loops and magnetic field lines
of many models based on the vector magnetograms is far from perfect
(DeRosa et. al., 2009). The estimates of free energy in the field as
well as distribution of the magnetic currents through the volume could
be dramatically different for different models used (Schrijver et. al.,
2008). This testifies to the need of a completely new approach to this
problem. We present such an approach and demonstrate its results based
on AIA and HMI data. We have developed a way to use coronal loops as a
constraint for magnetic modelling; the field is therefore constructed to
match coronal loops. We found that when tested on known magnetic fields
the new method is able to reproduce overall shape of the field lines,
large-scale spatial distribution of the electric currents and measure
up to 60% of the free energy stored in the field. This was achieved
with as little as line-of-sight magnetogram and less than hundred of
synthetic "loops", that is, lines of magnetic fields projected onto
a plane of the sky. We found that line-of-sight HMI magnetograms and
spatial resolution of the AIA instrument combined with the amount of
filters available are more than sufficient for obtaining such data. We
briefly describe this new method and demonstrate reconstructions of the
coronal magnetic field obtained using AIA and HMI data. We evaluate how
well it reproduces coronal features and how much energy and helicity
estimates fluctuate with time for a stable non-flaring active region,
thus establishing the reliability of the new method.
Title: The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics
Observatory (SDO)
Authors: Lemen, James R.; Title, Alan M.; Akin, David J.; Boerner,
Paul F.; Chou, Catherine; Drake, Jerry F.; Duncan, Dexter W.; Edwards,
Christopher G.; Friedlaender, Frank M.; Heyman, Gary F.; Hurlburt, Neal
E.; Katz, Noah L.; Kushner, Gary D.; Levay, Michael; Lindgren, Russell
W.; Mathur, Dnyanesh P.; McFeaters, Edward L.; Mitchell, Sarah; Rehse,
Roger A.; Schrijver, Carolus J.; Springer, Larry A.; Stern, Robert A.;
Tarbell, Theodore D.; Wuelser, Jean-Pierre; Wolfson, C. Jacob; Yanari,
Carl; Bookbinder, Jay A.; Cheimets, Peter N.; Caldwell, David; Deluca,
Edward E.; Gates, Richard; Golub, Leon; Park, Sang; Podgorski, William
A.; Bush, Rock I.; Scherrer, Philip H.; Gummin, Mark A.; Smith, Peter;
Auker, Gary; Jerram, Paul; Pool, Peter; Soufli, Regina; Windt, David
L.; Beardsley, Sarah; Clapp, Matthew; Lang, James; Waltham, Nicholas
Bibcode: 2012SoPh..275...17L
Altcode: 2011SoPh..tmp..106L; 2011SoPh..tmp..172L; 2011SoPh..tmp..241L;
2011SoPh..tmp..115L
The Atmospheric Imaging Assembly (AIA) provides multiple simultaneous
high-resolution full-disk images of the corona and transition region
up to 0.5 R⊙ above the solar limb with 1.5-arcsec spatial
resolution and 12-second temporal resolution. The AIA consists of four
telescopes that employ normal-incidence, multilayer-coated optics to
provide narrow-band imaging of seven extreme ultraviolet (EUV) band
passes centered on specific lines: Fe XVIII (94 Å), Fe XVII, XXI
(131 Å), Fe IX (171 Å), Fe XII, XXIV (193 Å), Fe XIV (211 Å),
He II (304 Å), and Fe XVI (335 Å). One telescope observes C IV
(near 1600 Å) and the nearby continuum (1700 Å) and has a filter
that observes in the visible to enable coalignment with images from
other telescopes. The temperature diagnostics of the EUV emissions
cover the range from 6×104 K to 2×107 K. The
AIA was launched as a part of NASA's Solar Dynamics Observatory (SDO)
mission on 11 February 2010. AIA will advance our understanding of the
mechanisms of solar variability and of how the Sun's energy is stored
and released into the heliosphere and geospace.
Title: Initial Calibration of the Atmospheric Imaging Assembly (AIA)
on the Solar Dynamics Observatory (SDO)
Authors: Boerner, Paul; Edwards, Christopher; Lemen, James; Rausch,
Adam; Schrijver, Carolus; Shine, Richard; Shing, Lawrence; Stern,
Robert; Tarbell, Theodore; Title, Alan; Wolfson, C. Jacob; Soufli,
Regina; Spiller, Eberhard; Gullikson, Eric; McKenzie, David; Windt,
David; Golub, Leon; Podgorski, William; Testa, Paola; Weber, Mark
Bibcode: 2012SoPh..275...41B
Altcode:
The Atmospheric Imaging Assembly (AIA) instrument onboard the Solar
Dynamics Observatory (SDO) is an array of four normal-incidence
reflecting telescopes that image the Sun in ten EUV and UV wavelength
channels. We present the initial photometric calibration of AIA,
based on preflight measurements of the response of the telescope
components. The estimated accuracy is of order 25%, which is consistent
with the results of comparisons with full-disk irradiance measurements
and spectral models. We also describe the characterization of the
instrument performance, including image resolution, alignment,
camera-system gain, flat-fielding, and data compression.
Title: Heliophysics Event Knowledgebase for the Solar Dynamics
Observatory (SDO) and Beyond
Authors: Hurlburt, N.; Cheung, M.; Schrijver, C.; Chang, L.; Freeland,
S.; Green, S.; Heck, C.; Jaffey, A.; Kobashi, A.; Schiff, D.; Serafin,
J.; Seguin, R.; Slater, G.; Somani, A.; Timmons, R.
Bibcode: 2012SoPh..275...67H
Altcode: 2010arXiv1008.1291H
The immense volume of data generated by the suite of instruments
on the Solar Dynamics Observatory (SDO) requires new tools for
efficient identifying and accessing data that is most relevant for
research. We have developed the Heliophysics Events Knowledgebase
(HEK) to fill this need. The HEK system combines automated data mining
using feature-detection methods and high-performance visualization
systems for data markup. In addition, web services and clients are
provided for searching the resulting metadata, reviewing results,
and efficiently accessing the data. We review these components and
present examples of their use with SDO data.
Title: Heliophysics: Evolving Solar Activity and the Climates of
Space and Earth
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2012hesa.book.....S
Altcode:
Preface; 1. Interconnectedness in heliophysics Carolus J. Schrijver
and George L. Siscoe; 2. Long-term evolution of magnetic activity of
Sun-like stars Carolus J. Schrijver; 3. Formation and early evolution
of stars and proto-planetary disks Lee W. Hartmann; 4. Planetary
habitability on astronomical time scales Donald E. Brownlee; 5. Solar
internal flows and dynamo action Mark S. Miesch; 6. Modeling solar and
stellar dynamos Paul Charbonneau; 7. Planetary fields and dynamos Ulrich
R. Christensen; 8. The structure and evolution of the 3D solar wind
John T. Gosling; 9. The heliosphere and cosmic rays J. Randy Jokipii;
10. Solar spectral irradiance: measurements and models Judith L. Lean
and Thomas N. Woods; 11. Astrophysical influences on planetary climate
systems Juerg Beer; 12. Evaluating the drivers of Earth's climate
system Thomas J. Crowley; 13. Ionospheres of the terrestrial planets
Stanley C. Solomon; 14. Long-term evolution of the geospace climate
Jan J. Sojka; 15. Waves and transport processes in atmospheres and
oceans Richard L. Walterscheid; 16. Solar variability, climate, and
atmospheric photochemistry Guy P. Brasseur, Daniel Marsch and Hauke
Schmidt; Appendix I. Authors and editors; List of illustrations;
List of tables; Bibliography; Index.
Title: Heliophysics: Space Storms and Radiation: Causes and Effects
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2012hssr.book.....S
Altcode:
Preface; 1. Perspective on heliophysics George L. Siscoe and
Carolus J. Schrijver; 2. Introduction to space storms and radiation
Sten Odenwald; 3. In-situ detection of energetic particles George
Gloeckler; 4. Radiative signatures of energetic particles Tim Bastian;
5. Observations of solar and stellar eruptions, flares, and jets Hugh
Hudson; 6. Models of coronal mass ejections and flares Terry Forbes;
7. Shocks in heliophysics Merav Opher; 8. Particle acceleration in
shocks Dietmar Krauss-Varban; 9. Energetic particle transport Joe
Giacalone; 10. Energy conversion in planetary magnetospheres Vytenis
Vasyliūnas; 11. Energization of trapped particles Janet Green;
12. Flares, CMEs, and atmospheric responses Tim Fuller-Rowell and
Stanley C. Solomon; 13. Energetic particles and manned spaceflight 358
Stephen Guetersloh and Neal Zapp; 14. Energetic particles and technology
Alan Tribble; Appendix I. Authors and editors; List of illustrations;
List of tables; Bibliography; Index.
Title: The Helioseismic and Magnetic Imager (HMI) Investigation for
the Solar Dynamics Observatory (SDO)
Authors: Scherrer, P. H.; Schou, J.; Bush, R. I.; Kosovichev, A. G.;
Bogart, R. S.; Hoeksema, J. T.; Liu, Y.; Duvall, T. L.; Zhao, J.;
Title, A. M.; Schrijver, C. J.; Tarbell, T. D.; Tomczyk, S.
Bibcode: 2012SoPh..275..207S
Altcode:
The Helioseismic and Magnetic Imager (HMI) instrument and investigation
as a part of the NASA Solar Dynamics Observatory (SDO) is designed
to study convection-zone dynamics and the solar dynamo, the origin
and evolution of sunspots, active regions, and complexes of activity,
the sources and drivers of solar magnetic activity and disturbances,
links between the internal processes and dynamics of the corona and
heliosphere, and precursors of solar disturbances for space-weather
forecasts. A brief overview of the instrument, investigation objectives,
and standard data products is presented.
Title: Destruction of Sun-Grazing Comet C/2011 N3 (SOHO) Within the
Low Solar Corona
Authors: Schrijver, C. J.; Brown, J. C.; Battams, K.; Saint-Hilaire,
P.; Liu, W.; Hudson, H.; Pesnell, W. D.
Bibcode: 2012Sci...335..324S
Altcode:
Observations of comets in Sun-grazing orbits that survive solar
insolation long enough to penetrate into the Sun's inner corona provide
information on the solar atmosphere and magnetic field as well as on
the makeup of the comet. On 6 July 2011, the Solar Dynamics Observatory
(SDO) observed the demise of comet C/2011 N3 (SOHO) within the low solar
corona in five wavelength bands in the extreme ultraviolet (EUV). The
comet penetrated to within 0.146 solar radius (~100,000 kilometers)
of the solar surface before its EUV signal disappeared. Before that,
material released into the coma - at first seen in absorption - formed
a variable EUV-bright tail. During the final 10 minutes of observation
by SDO's Atmospheric Imaging Assembly, ~6 × 10^8 to 6 × 10^10 grams
of total mass was lost (corresponding to an effective nucleus diameter
of ~10 to 50 meters), as estimated from the tail's deceleration due to
interaction with the surrounding coronal material; the EUV absorption
by the comet and the brightness of the tail suggest that the mass was
at the high end of this range. These observations provide evidence
that the nucleus had broken up into a family of fragments, resulting
in accelerated sublimation in the Sun's intense radiation field.
Title: Data Discovery and Access via the Heliophysics Events
Knowledgebase (HEK)
Authors: Somani, A.; Hurlburt, N. E.; Schrijver, C. J.; Cheung, M.;
Freeland, S.; Slater, G. L.; Seguin, R.; Timmons, R.; Green, S.;
Chang, L.; Kobashi, A.; Jaffey, A.
Bibcode: 2011AGUFMSM21A1989S
Altcode:
The HEK is a integrated system which helps direct scientists to solar
events and data from a variety of providers. The system is fully
operational and adoption of HEK has been growing since the launch of
NASA's SDO mission. In this presentation we describe the different
components that comprise HEK. The Heliophysics Events Registry (HER)
and Heliophysics Coverage Registry (HCR) form the two major databases
behind the system. The HCR allows the user to search on coverage event
metadata for a variety of instruments. The HER allows the user to
search on annotated event metadata for a variety of instruments. Both
the HCR and HER are accessible via a web API which can return search
results in machine readable formats (e.g. XML and JSON). A variety
of SolarSoft services are also provided to allow users to search the
HEK as well as obtain and manipulate data. Other components include
- the Event Detection System (EDS) continually runs feature finding
algorithms on SDO data to populate the HER with relevant events, -
A web form for users to request SDO data cutouts for multiple AIA
channels as well as HMI line-of-sight magnetograms, - iSolSearch,
which allows a user to browse events in the HER and search for specific
events over a specific time interval, all within a graphical web page,
- Panorama, which is the software tool used for rapid visualization of
large volumes of solar image data in multiple channels/wavelengths. The
user can also easily create WYSIWYG movies and launch the Annotator
tool to describe events and features. - EVACS, which provides a JOGL
powered client for the HER and HCR. EVACS displays the searched for
events on a full disk magnetogram of the sun while displaying more
detailed information for events.
Title: The EUV Emission in Comet-Solar Corona Interactions
Authors: Bryans, P.; Pesnell, W. D.; Schrijver, C. J.; Brown, J. C.;
Battams, K.; Saint-Hilaire, P.; Liu, W.; Hudson, H. S.
Bibcode: 2011AGUFMSH34B..05B
Altcode:
The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory
(SDO) viewed a comet as it passed through the solar corona on 2011 July
5. This was the first sighting of a comet by a EUV telescope. For 20
minutes, enhanced emission in several of the AIA wavelength bands marked
the path of the comet. We explain this EUV emission by considering
the evolution of the cometary atmosphere as it interacts with the
ambient solar atmosphere. Water ice in the comet rapidly sublimates
as it approaches the Sun. This water vapor is then photodissociated,
primarily by Ly-α, by the solar radiation field to create atomic H and
O. Other molecules present in the comet also evaporate and dissociate
to give atomic Fe and other metals. Subsequent ionization of these
atoms can be achieved by a number of means, including photoionization,
electron impact, and charge exchange with coronal protons and other
highly-charged species. Finally, particles from the cometary atmosphere
are thermalized to the background temperature of the corona. Each step
could cause emission in the AIA bandpasses. We will report here on
their relative contribution to the emission seen in the AIA telescopes.
Title: Using the EUV to Weigh a Sun-grazing Comet as it Disappears
in the Solar Corona
Authors: Pesnell, W. D.; Schrijver, C. J.; Brown, J. C.; Battams,
K.; Saint-Hilaire, P.; Hudson, H. S.; Lui, W.
Bibcode: 2011AGUFMSH33A2040P
Altcode:
On July 6, 2011, the Atmospheric Imaging Assembly (AIA) on the
Solar Dynamics Observatory (SDO) observed a comet in most of its
EUV passbands. The comet disappeared while moving through the solar
corona. The comet penetrated to 0.146 solar radii ( ∼~100,000
km) above the photosphere before its EUV faded. Before then, the
comet's coma and a tail were observed in absorption and emission,
respectively. The material in the variable tail quickly fell behind
the nucleus. An estimate of the comet's mass based on this effect,
one derived from insolation, and one using the tail's EUV brightness,
all yield ∼ 50 giga-grams some 10 minutes prior to the end of
its visibility. These unique first observations herald a new era in
the study of Sun-grazing comets close to their perihelia and of the
conditions in the solar corona and solar wind. We will discuss the
observations and interpretation of the comet by SDO as well as the
coronagraph observations from SOHO and STEREO. A search of the SOHO
comet archive for other comets that could be observed in the SDO/AIA
EUV channels will be described.
Title: Heliophysics
Authors: Austin, M.; Guhathakurta, M.; Bhattacharjee, A.; Longcope,
D. W.; Sojka, J. J.; Schrijver, C. J.; Siscoe, G. L.
Bibcode: 2011AGUFMSH13B1968A
Altcode:
Heliophysics is a developing scientific discipline integrating
studies of the Sun's variability, the surrounding heliopsphere, and
climate environments. Over the past few centuries, our understanding
of how the Sun drives space weather and climate on the Earth and
other planets has advanced at an ever-increasing rate. NASA Living
With a Star and the UCAR Visiting Scientist Programs, sponsor the
annual Heliophysics Summer Schools to build the next generation of
scientists in this emerging field. The highly successful series of
summer schools (commencing 2007) trains a select group of graduate
students, postdoctoral fellows and university faculty to learn and
develop the science of heliophysics as a broad, coherent discipline
that reaches in space from the Earth's troposphere to the depths
of the Sun, and in time from the formation of the solar system to
the distant future. The first three years of the school resulted in
the publication of three textbooks now being used at universities
worldwide. Subsequent years have also developed the complementary
materials that support teaching of heliophysics at both graduate and
undergraduate levels. The textbooks are edited by Carolus J. Schrijver,
Lockheed Martin, and George L. Siscoe, Boston University. The books
provide a foundational reference for researchers in heliophysics,
astrophysics, plasma physics, space physics, solar physics, aeronomy,
space weather, planetary science and climate science. The Jack Eddy
Postdoctoral Fellowship program matches newly graduated postdoctorates
with hosting mentors for the purpose of training the next generation of
researchers needed in heliophysics. The fellowships are for two years,
and any U.S university or research lab may apply to host a fellow. Two
major topics of focus for the program are the science of space weather
and of the Sun-climate connection. Since the goal of this fellowship
program is to train Sun-Earth system researchers, preference is also
given to research projects that cross the traditional heliophysics
subdomains of the Sun, heliosphere, magnetosphere, and ionosphere/upper
atmosphere, as well as Sun-climate investigations. Host mentors play
critical roles. Potential hosts may enter information about their
research on a central database. Application deadline: January 11, 2012
Title: Large-scale Coronal Propagating Fronts During the Rising
Phase of Solar Cycle 24
Authors: Nitta, N. V.; Liu, W.; Schrijver, C. J.; Title, A. M.; Lemen,
J. R.
Bibcode: 2011AGUFMSH23A1941N
Altcode:
With increasing solar activity, the AIA on SDO has observed a number
of large-scale coronal propagating fronts, which are often called
"EIT waves." Although their nature is still actively debated,
these propagating fronts usually accompany CMEs, and, in certain
cases, may signify CME-related shock waves important for particle
acceleration. Using the unprecedented temporal resolution and broad
temperature coverage of the AIA, it is possible to characterize
the propagating fronts in the corona far better than before, as
demonstrated in the literature for a yet small number of cases. We
study the properties of more than 40 propagating fronts as observed
by AIA, and discuss the key properties for them to be associated with
other phenomena such as type II radio bursts, flares, CMEs, ICMEs,
and SEP events. We make use of data, both remote-sensing and in-situ,
from STEREO which provides two additional vantage points, to make the
associations more solid. For the associated phenomena, their basic
properties are correlated with those of the propagating fronts. We
also revisit the association of EIT waves with other phenomena during
the similar phase of Solar Cycle 23 and discuss possible differences
in terms of global magnetic field. Understanding their relation with
other phenomena, we can have a more complete picture of the coronal
propagating fronts in the context of CME acceleration and deceleration.
Title: Solar Energetic Events, the Solar-Stellar Connection, and
Statistics of Extreme Space Weather
Authors: Schrijver, C. J.
Bibcode: 2011ASPC..448..231S
Altcode: 2011csss...16..231S; 2010arXiv1012.2791S
Observations of the Sun and of Sun-like stars provide access to
different aspects of stellar magnetic activity that, when combined,
help us piece together a more comprehensive picture than can be achieved
from only the solar or the stellar perspective. Where the Sun provides
us with decent spatial resolution of, e.g., magnetic bipoles and the
overlying dynamic, hot atmosphere, the ensemble of stars enables us to
see rare events on at least some occasions. Where the Sun shows us how
flux emergence, dispersal, and disappearance occur in the complex mix
of polarities on the surface, only stellar observations can show us
the activity of the ancient or future Sun. In this review, I focus on
a comparison of statistical properties, from bipolar-region emergence
to flare energies, and from heliospheric events to solar energetic
particle impacts on Earth. In doing so, I point out some intriguing
correspondences as well as areas where our knowledge falls short of
reaching unambiguous conclusions on, for example, the most extreme
space-weather events that we can expect from the present-day Sun. The
difficulties of interpreting stellar coronal light curves in terms of
energetic events are illustrated with some examples provided by the SDO,
STEREO, and GOES spacecraft.
Title: Non-Linear Force-Free Modeling With The Aid of Coronal
Observations
Authors: Malanushenko, A. V.; DeRosa, M. L.; Schrijver, C. J.;
Gilchrist, S. A.; Wheatland, M. S.
Bibcode: 2011AGUFMSH43B1956M
Altcode:
Currently many models of coronal magnetic field rely on vector
magnetograms and other kinds of information drawn from the
photosphere. Magnetic fields in the corona, however, manifest themselves
in the shapes of coronal loops, providing a constraint that at the
present stage receives little use due to mathematical complications of
incorporating such input into the numeric models. Projection effects
and the limited number of usable loops further complicate their
use. We present a possible way to account for coronal loops in the
models of magnetic field. We first fit the observed loops with lines
of constant-alpha fields and thus approximate three-dimensional
distribution of currents in the corona along a sparse set of
trajectories. We then apply a Grad-Rubin-like averaging technique
to obtain a volume-filling non-linear force-free model of magnetic
field, modified from the method presented in Wheatland & Regnier
(2009). We present thorough tests of this technique on several known
magnetic fields that were previously used for comparing different
extrapolation techniques (Schrijver et. al., 2006; Metcalf et. al.,
2008; Schrijver et. al., 2008; DeRosa et. al., 2009), as well as on
solar data and compare the results with those obtained by the currently
developed methods that rely completely on the photospheric data.
Title: Cross-Calibration and Thermal Analysis with SDO/AIA
Authors: Boerner, P.; Warren, H. P.; Testa, P.; Weber, M.; Schrijver,
C. J.
Bibcode: 2011AGUFMSH13B1955B
Altcode:
The measured intensity in each pixel of the images from SDO/AIA and
similar narrowband EUV imagers can be used to perform quantitative
analysis of the temperature and density of the plasma along the
line of sight. This type of analysis depends very sensitively on the
accuracy of the instrument calibration and the atomic physics models
used to estimate the plasma emissivity. Unfortunately, it is difficult
to assess the accuracy of these parameters, other than by verifying
the consistency of datasets from different instruments and analysis
techniques. Here we use differential emission measure models of the
plasma temperature structure constrained by spectroscopic observations
from SDO/EVE and Hinode/EIS to assess the AIA temperature response
functions. The response functions generated using the CHIANTI database
underestimate the emission from the non-flaring corona in the 94 and 131
Å channels. We find empirical corrections to the temperature response
functions for these channels that are internally consistent and provide
good agreement with DEMs obtained from other instruments. We present an
assessment of the reliability of thermal analysis using AIA data based
on the current state of the instrument calibration and spectral models.
Title: New Solar Extreme-ultraviolet Irradiance Observations during
Flares
Authors: Woods, Thomas N.; Hock, Rachel; Eparvier, Frank; Jones,
Andrew R.; Chamberlin, Phillip C.; Klimchuk, James A.; Didkovsky,
Leonid; Judge, Darrell; Mariska, John; Warren, Harry; Schrijver,
Carolus J.; Webb, David F.; Bailey, Scott; Tobiska, W. Kent
Bibcode: 2011ApJ...739...59W
Altcode:
New solar extreme-ultraviolet (EUV) irradiance observations from the
NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide
full coverage in the EUV range from 0.1 to 106 nm and continuously at
a cadence of 10 s for spectra at 0.1 nm resolution and even faster,
0.25 s, for six EUV bands. These observations can be decomposed into
four distinct characteristics during flares. First, the emissions
that dominate during the flare's impulsive phase are the transition
region emissions, such as the He II 30.4 nm. Second, the hot coronal
emissions above 5 MK dominate during the gradual phase and are highly
correlated with the GOES X-ray. A third flare characteristic in the
EUV is coronal dimming, seen best in the cool corona, such as the
Fe IX 17.1 nm. As the post-flare loops reconnect and cool, many of
the EUV coronal emissions peak a few minutes after the GOES X-ray
peak. One interesting variation of the post-eruptive loop reconnection
is that warm coronal emissions (e.g., Fe XVI 33.5 nm) sometimes exhibit
a second large peak separated from the primary flare event by many
minutes to hours, with EUV emission originating not from the original
flare site and its immediate vicinity, but rather from a volume of
higher loops. We refer to this second peak as the EUV late phase. The
characterization of many flares during the SDO mission is provided,
including quantification of the spectral irradiance from the EUV late
phase that cannot be inferred from GOES X-ray diagnostics.
Title: The 2011 February 15 X2 Flare, Ribbons, Coronal Front, and
Mass Ejection: Interpreting the Three-dimensional Views from the
Solar Dynamics Observatory and STEREO Guided by Magnetohydrodynamic
Flux-rope Modeling
Authors: Schrijver, Carolus J.; Aulanier, Guillaume; Title, Alan M.;
Pariat, Etienne; Delannée, Cecile
Bibcode: 2011ApJ...738..167S
Altcode:
The 2011 February 15 X2.2 flare and associated Earth-directed halo
coronal mass ejection were observed in unprecedented detail with
high resolution in spatial, temporal, and thermal dimensions by the
Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory,
as well as by instruments on the two STEREO spacecraft, then at
near-quadrature relative to the Sun-Earth line. These observations
enable us to see expanding loops from a flux-rope-like structure over
the shearing polarity-inversion line between the central δ-spot groups
of AR 11158, developing a propagating coronal front ("EIT wave"),
and eventually forming the coronal mass ejection moving into the inner
heliosphere. The observations support the interpretation that all of
these features, including the "EIT wave," are signatures of an expanding
volume traced by loops (much larger than the flux rope only), surrounded
by a moving front rather than predominantly wave-like perturbations;
this interpretation is supported by previously published MHD models
for active-region and global scales. The lateral expansion of the
eruption is limited to the local helmet-streamer structure and halts
at the edges of a large-scale domain of connectivity (in the process
exciting loop oscillations at the edge of the southern polar coronal
hole). The AIA observations reveal that plasma warming occurs within
the expansion front as it propagates over quiet Sun areas. This warming
causes dimming in the 171 Å (Fe IX and Fe X) channel and brightening
in the 193 and 211 Å (Fe XII-XIV) channels along the entire front,
while there is weak 131 Å (Fe VIII and Fe XXI) emission in some
directions. An analysis of the AIA response functions shows that
sections of the front running over the quiet Sun are consistent with
adiabatic warming; other sections may require additional heating which
MHD modeling suggests could be caused by Joule dissipation. Although
for the events studied here the effects of volumetric expansion are
much more obvious than true wave phenomena, we discuss how different
magnetic environments within and around the erupting region can lead
to the signatures of either or both of these aspects.
Title: Coronal Loop Oscillations Observed with Atmospheric Imaging
Assembly—Kink Mode with Cross-sectional and Density Oscillations
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.
Bibcode: 2011ApJ...736..102A
Altcode: 2011arXiv1105.2191A
A detailed analysis of a coronal loop oscillation event is presented,
using data from the Atmospheric Imaging Assembly on board the Solar
Dynamics Observatory (SDO) for the first time. The loop oscillation
event occurred on 2010 October 16, 19:05-19:35 UT and was triggered
by an M2.9 GOES-class flare, located inside a highly inclined cone
of a narrow-angle coronal mass ejection. This oscillation event had a
number of unusual features: (1) excitation of kink-mode oscillations in
vertical polarization (in the loop plane), (2) coupled cross-sectional
and density oscillations with identical periods, (3) no detectable
kink amplitude damping over the observed duration of four kink-mode
periods (P=6.3 minutes), (4) multi-loop oscillations with slightly
(≈10%) different periods, and (5) a relatively cool loop temperature
of T ≈ 0.5 MK. We employ a novel method of deriving the electron
density ratio external and internal to the oscillating loop from the
ratio of Alfvénic speeds deduced from the flare trigger delay and the
kink-mode period, i.e., ne /ni = (vA
/v Ae)2 = 0.08 ± 0.01. The coupling of the
kink mode and cross-sectional oscillations can be explained as a
consequence of the loop length variation in the vertical polarization
mode. We determine the exact footpoint locations and loop length
with stereoscopic triangulation using STEREO/EUVI/A data. We model
the magnetic field in the oscillating loop using Helioseismic and
Magnetic Imager/SDO magnetogram data and a potential-field model and
find agreement with the seismological value of the magnetic field,
B kink = 4.0 ± 0.7 G, within a factor of two.
Title: Heliophysics: Plasma Physics of the Local Cosmos
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2011hppl.book.....S
Altcode:
Preface; 1. Prologue Carolus J. Schrijver and George L. Siscoe;
2. Introduction to heliophysics Thomas J. Bogdan; 3. Creation and
destruction of magnetic field Matthias Rempel; 4. Magnetic field
topology Dana W. Longcope; 5. Magnetic reconnection Terry G. Forbes;
6. Structures of the magnetic field Mark B. Moldwin, George L. Siscoe
and Carolus J. Schrijver; 7. Turbulence in space plasmas Charles
W. Smith; 8. The solar atmosphere Viggo H. Hansteen; 9. Stellar winds
and magnetic fields Viggo H. Hansteen; 10. Fundamentals of planetary
magnetospheres Vytenis M. Vasyliūnas; 11. Solar-wind magnetosphere
coupling: an MHD perspective Frank R. Toffoletto and George L. Siscoe;
12. On the ionosphere and chromosphere Tim Fuller-Rowell and Carolus
J. Schrijver; 13. Comparative planetary environments Frances Bagenal;
Bibliography; Index.
Title: Direct Imaging of Quasi-periodic Fast Propagating Waves of
~2000 km s-1 in the Low Solar Corona by the Solar Dynamics
Observatory Atmospheric Imaging Assembly
Authors: Liu, Wei; Title, Alan M.; Zhao, Junwei; Ofman, Leon;
Schrijver, Carolus J.; Aschwanden, Markus J.; De Pontieu, Bart;
Tarbell, Theodore D.
Bibcode: 2011ApJ...736L..13L
Altcode: 2011arXiv1106.3150L
Quasi-periodic propagating fast mode magnetosonic waves in the solar
corona were difficult to observe in the past due to relatively low
instrument cadences. We report here evidence of such waves directly
imaged in EUV by the new Atmospheric Imaging Assembly instrument
on board the Solar Dynamics Observatory. In the 2010 August 1 C3.2
flare/coronal mass ejection event, we find arc-shaped wave trains of
1%-5% intensity variations (lifetime ~200 s) that emanate near the
flare kernel and propagate outward up to ~400 Mm along a funnel of
coronal loops. Sinusoidal fits to a typical wave train indicate a phase
velocity of 2200 ± 130 km s-1. Similar waves propagating
in opposite directions are observed in closed loops between two flare
ribbons. In the k-ω diagram of the Fourier wave power, we find a
bright ridge that represents the dispersion relation and can be well
fitted with a straight line passing through the origin. This k-ω
ridge shows a broad frequency distribution with power peaks at 5.5,
14.5, and 25.1 mHz. The strongest signal at 5.5 mHz (period 181 s)
temporally coincides with quasi-periodic pulsations of the flare,
suggesting a common origin. The instantaneous wave energy flux
of (0.1-2.6) × 107 erg cm-2 s-1
estimated at the coronal base is comparable to the steady-state heating
requirement of active region loops.
Title: The Interface Region Imaging Spectrograph (IRIS) NASA SMEX
Authors: Lemen, James; Title, A.; De Pontieu, B.; Schrijver, C.;
Tarbell, T.; Wuelser, J.; Golub, L.; Kankelborg, C.
Bibcode: 2011SPD....42.1512L
Altcode: 2011BAAS..43S.1512L
The solar chromosphere and transition region (TR) is highly structured,
dynamic, and intimately connected to the corona. It requires more
than ten times the energy required to heat the corona, and yet it
has received far less interest because of the complexity of the
required observational and analytical tools. In the TR the density
drops by six orders of magnitude and the temperature increases by
three orders of magnitude. Hinode observations reveal the importance
the magnetic field has on this region of the solar atmosphere that
acts as the interface between the photosphere and the corona. The
Interface Region Imaging Spectrograph (IRIS) was selected for a NASA
SMEX mission in 2009 and is scheduled to launch in December 2012. IRIS
addresses critical questions in order to understand the flow of energy
and mass through the chromosphere and TR, namely: (1) Which types of
non-thermal energy dominate in the chromosphere and beyond? (2) How
does the chromosphere regulate mass and energy supply to the corona
and heliosphere? (3) How do magnetic flux and matter rise through the
lower atmosphere, and what roles dos flux emergence play in flares and
mass ejections? These questions are addressed with a high-resolution
imaging spectrometer that observes Near- and Far-VU emissions that
are formed at temperatures between 5,000K and 1.5 x 106
K. IRIS has a field-of-view of 120 arcsec, a spatial resolution
of 0.4 arcsec, and velocity resolution of 0.5 km/s. Members of the
IRIS investigation team are developing advanced radiative MHD codes
to facilitate comparison with and interpretation of observations. We
present the status of the IRIS observatory development, which completed
its Critical Design Review in December 2010.
Title: Direct Imaging by SDO/AIA of Quasi-periodic Propagating Fast
Mode Magnetosonic Waves of 2000 km/s in the Solar Corona
Authors: Liu, Wei; Title, A. M.; Zhao, J.; Ofman, L.; Schrijver,
C. J.; Aschwanden, M. J.; De Pontieu, B.; Tarbell, T. D.
Bibcode: 2011SPD....42.2114L
Altcode: 2011BAAS..43S.2114L
Quasi-periodic, propagating fast mode magnetosonic waves in the
corona were difficult to observe in the past due to relatively low
instrument cadences. We report here unprecedented evidence of such
waves directly imaged in EUV by the new SDO/AIA instrument. In the 2010
August 1 C3.2 flare/CME event, we find arc-shaped wave trains of 1-5%
intensity variations emanating near the flare kernel and propagating
outward along a funnel of coronal loops. Sinusoidal fits to a typical
wave train indicate a phase velocity of 2350 +/- 210 km/s. Similar
waves propagating in opposite directions are observed in closed loops
between two flare ribbons. In the k-omega diagram of the Fourier wave
power, we find a bright ridge that represents the dispersion relation
and can be well fitted with a straight line passing through the
origin, giving an equal phase and group velocity of 1630 +/- 760 km/s
averaged over the event. This k-omega ridge shows a broad frequency
distribution with prominent power at four non-harmonic frequencies,
5.5, 14.5, 25.1, and 37.9 mHz, among which the 14.5 mHz (period:
69 s) signal is the strongest. The signal at 5.5 mHz (period: 181 s,
same as chromospheric 3-minute oscillations) temporally coincides with
flare pulsations, suggesting a common origin of possibly quasi-periodic
magnetic reconnection. The instantaneous wave energy flux of (0.1-2.6)e7
ergs/cm2/s estimated at the coronal base is comparable to
the steady-state heating requirement of active region loops.
Title: Topology of Coronal Fields from Potential Field Models
Authors: DeRosa, Marc L.; Schrijver, C. J.; Barnes, G.
Bibcode: 2011SPD....42.1810D
Altcode: 2011BAAS..43S.1810D
The topology of the solar coronal magnetic field has been the subject of
much recent interest, due to its apparent importance in determining (for
example) the sources of the solar wind, the evolution of coronal hole
boundaries, and whether the configurations of coronae overlying active
regions are unstable and thus possibly eruption-prone. We identify
the topological skeleton (null points, spline lines, separators, and
separatrix surfaces) for a selection of dates of interest from the
database of potential-field source-surface models available through
the ``PFSS'' SolarSoft package. Several features of interest have been
identified by recent studies (e.g., Antiochos et al. 2007, Parnell et
al. 2010, Titov et al. 2011), including exceedingly narrow channels of
open field or separators associated with inferred reconnection sites. We
find that these features of interest occur frequently in the topologies
of even potential-field models of the magnetic corona. The actual solar
corona is of course likely to involve even more complex topologies,
especially as its dynamics and evolution are taken into account.
Title: Simulating Coronal Emission in Six AIA Channels Using
Quasi-Static Atmosphere Models and Non-Linear Magnetic Field Models
Authors: Malanushenko, Anna; Schrijver, C.; DeRosa, M.; Aschwanden,
M.; Wheatland, M. S.; van Ballegooijen, A. A.
Bibcode: 2011SPD....42.2116M
Altcode: 2011BAAS..43S.2116M
We present the results of simulations of the EUV coronal emission in
AIA channels. We use a non-linear force-free model of magnetic field
constructed in such a way that its field lines resemble the observed
coronal loops in EUV. We then solve one-dimensional quasi-steady
atmosphere model along the magnetic field lines (Schrijver &
Ballegooijen, 2005). Using coronal abundances from CHIANTI and AIA
response functions we then simulate the emission that would be observed
in AIA EUV channels. The resulting intensities are compared against the
real observations in a manner similar to that in Aschwanden et. al.,
2011. The study is similar to those by Lindquist et. al., 2008, with a
few important differences. We use a model of the coronal magnetic field
that resembles the topology observed in EUV, we study EUV emission of
cool loops (rather than SXR) and we make use of high resolution and
cadence AIA and HMI data.
Title: Long-range magnetic couplings between solar flares and coronal
mass ejections observed by SDO and STEREO
Authors: Schrijver, C. J.; Title, A. M.
Bibcode: 2011JGRA..116.4108S
Altcode:
The combination of SDO and STEREO observations enables us to
view much of the solar surface and atmosphere simultaneously and
continuously. These near-global observations often show near-synchronous
long-distance interactions between magnetic domains that exhibit flares,
eruptions, and frequent minor forms of activity. Here we analyze a
series of flares, filament eruptions, coronal mass ejections, and
related events which occurred on 1-2 August 2010. These events extend
over a full hemisphere of the Sun, only two-thirds of which is visible
from the Earth's perspective. The combination of coronal observations
and global field modeling reveals the many connections between these
events by magnetic field lines, particularly those at topological
divides. We find that all events of substantial coronal activity,
including those where flares and eruptions initiate, are connected by
a system of separatrices, separators, and quasi-separatrix layers, with
little activity within the deep interiors of domains of connectivity. We
conclude that for this sequence of events the evolution of field on
the hemisphere invisible from Earth's perspective is essential to
the evolution, and possibly even to the initiation, of the flares and
eruptions over an area that spans at least 180 degrees in longitude. Our
findings emphasize that the search for the factors that play a role
in the initiation and evolution of eruptive and explosive phenomena,
sought after for improved space weather forecasting, requires knowledge
of much, if not all, of the solar surface field.
Title: Magneto-thermal convection in solar prominences
Authors: Berger, Thomas; Testa, Paola; Hillier, Andrew; Boerner, Paul;
Low, Boon Chye; Shibata, Kazunari; Schrijver, Carolus; Tarbell, Ted;
Title, Alan
Bibcode: 2011Natur.472..197B
Altcode:
Coronal cavities are large low-density regions formed by
hemispheric-scale magnetic flux ropes suspended in the Sun's outer
atmosphere. They evolve over time, eventually erupting as the dark
cores of coronal mass ejections. Although coronal mass ejections are
common and can significantly affect planetary magnetospheres, the
mechanisms by which cavities evolve to an eruptive state remain poorly
understood. Recent optical observations of high-latitude `polar crown'
prominences within coronal cavities reveal dark, low-density `bubbles'
that undergo Rayleigh-Taylor instabilities to form dark plumes rising
into overlying coronal cavities. These observations offered a possible
mechanism for coronal cavity evolution, although the nature of the
bubbles, particularly their buoyancy, was hitherto unclear. Here we
report simultaneous optical and extreme-ultraviolet observations of
polar crown prominences that show that these bubbles contain plasma at
temperatures in the range (2.5-12)×105 kelvin, which is
25-120 times hotter than the overlying prominence. This identifies a
source of the buoyancy, and suggests that the coronal cavity-prominence
system supports a novel form of magneto-thermal convection in the solar
atmosphere, challenging current hydromagnetic concepts of prominences
and their relation to coronal cavities.
Title: The minimal solar activity in 2008-2009 and its implications
for long-term climate modeling
Authors: Schrijver, C. J.; Livingston, W. C.; Woods, T. N.; Mewaldt,
R. A.
Bibcode: 2011GeoRL..38.6701S
Altcode: 2011GeoRL..3806701S
Variations in the total solar irradiance (TSI) associated with solar
activity have been argued to influence the Earth's climate system,
in particular when solar activity deviates from the average for a
substantial period. One such example is the 17th Century Maunder Minimum
during which sunspot numbers were extremely low, as Earth experienced
the Little Ice Age. Estimation of the TSI during that period has relied
on extrapolations of correlations with sunspot numbers or even more
indirectly with modulations of galactic cosmic rays. We argue that
there is a minimum state of solar magnetic activity associated with
a population of relatively small magnetic bipoles which persists
even when sunspots are absent, and that consequently estimates of
TSI for the Little Ice Age that are based on scalings with sunspot
numbers are generally too low. The minimal solar activity, which
measurements show to be frequently observable between active-region
decay products regardless of the phase of the sunspot cycle, was
approached globally after an unusually long lull in sunspot activity
in 2008-2009. Therefore, the best estimate of magnetic activity, and
presumably TSI, for the least-active Maunder Minimum phases appears to
be provided by direct measurement in 2008-2009. The implied marginally
significant decrease in TSI during the least active phases of the
Maunder Minimum by 140 to 360 ppm relative to 1996 suggests that
drivers other than TSI dominate Earth's long-term climate change.
Title: The Stellar Imager (SI) - A Mission to Resolve Stellar
Surfaces, Interiors, and Magnetic Activity
Authors: Christensen-Dalsgaard, Jørgen; Carpenter, Kenneth G.;
Schrijver, Carolus J.; Karovska, Margarita; Si Team
Bibcode: 2011JPhCS.271a2085C
Altcode:
The Stellar Imager (SI) is a space-based, UV/Optical Interferometer
(UVOI) designed to enable 0.1 milli-arcsecond (mas) spectral imaging
of stellar surfaces and of the Universe in general. It will also probe
via asteroseismology flows and structures in stellar interiors. SI
will enable the development and testing of a predictive dynamo model
for the Sun, by observing patterns of surface activity and imaging
of the structure and differential rotation of stellar interiors in
a population study of Sun-like stars to determine the dependence of
dynamo action on mass, internal structure and flows, and time. SI's
science focuses on the role of magnetism in the Universe and will
revolutionize our understanding of the formation of planetary systems,
of the habitability and climatology of distant planets, and of many
magneto-hydrodynamically controlled processes in the Universe. SI
is a "Landmark/Discovery Mission" in the 2005 Heliophysics Roadmap,
an implementation of the UVOI in the 2006 Astrophysics Strategic Plan,
and a NASA Vision Mission ("NASA Space Science Vision Missions" (2008),
ed. M. Allen). We present here the science goals of the SI Mission,
a mission architecture that could meet those goals, and the technology
development needed to enable this mission. Additional information on
SI can be found at: http://hires.gsfc.nasa.gov/si/.
Title: The Origins of Hot Plasma in the Solar Corona
Authors: De Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Boerner, P.; Martinez-Sykora, J.; Schrijver,
C. J.; Title, A. M.
Bibcode: 2011Sci...331...55D
Altcode:
The Sun's outer atmosphere, or corona, is heated to millions of degrees,
considerably hotter than its surface or photosphere. Explanations for
this enigma typically invoke the deposition in the corona of nonthermal
energy generated by magnetoconvection. However, the coronal heating
mechanism remains unknown. We used observations from the Solar Dynamics
Observatory and the Hinode solar physics mission to reveal a ubiquitous
coronal mass supply in which chromospheric plasma in fountainlike jets
or spicules is accelerated upward into the corona, with much of the
plasma heated to temperatures between ~0.02 and 0.1 million kelvin (MK)
and a small but sufficient fraction to temperatures above 1 MK. These
observations provide constraints on the coronal heating mechanism(s)
and highlight the importance of the interface region between photosphere
and corona.
Title: SDO Data Access And Analysis
Authors: Somani, A.; Hurlburt, N. E.; Schrijver, C. J.; Cheung, C.;
Freeland, S. L.; Slater, G. L.; Seguin, R.; Timmons, R.; Green, S.;
Chang, L.; Kobashi, A.; Jaffey, A.
Bibcode: 2010AGUFMSH23C1870S
Altcode:
The Heliophysics Event Knowledgebase (HEK), Event Detection System
(EDS), iSolSearch, Panorama, Event Viewer and Control Software (EVACS),
and a variety of SolarSoft routines all work together to provide a
suite of tools to facilitate access and analysis of SDO data. The
HEK, which consists of the Heliophysics Event Registry (HER) and
the Heliophysics Coverage Registry (HCR), uses XML formats built
upon the IVOA VOEvent specification to ingest, store, and search
events. Web services and SolarSoft routines are available to make
use of these functions. The EDS is one of the sources that provides
events for ingest into the HEK. The EDS continuously runs feature
finding modules on SDO data. It's a distributed system that allows
it to keep up with SDO's data rate. iSolSearch allows the user to
browse the events in the HER and search for events given a specific
time interval and other constraints. Panorama is the software tool
used for rapid visualization of large volumes of solar image data
in multiple channels/wavelengths. With the EVACS front-end GUI tool,
Panorama allows the user to, in real-time, change channel pixel scaling,
weights, alignment, blending and colorization of data. The user can
also easily create WYSIWYG movies and launch the Annotator tool to
describe events and features the user observes in the data. Panorama
can also be used to drive clustered HiperSpace walls using the CGLX
toolkit. Panorama harnesses the power of the GPU and OpenGL fragment
shaders to enable stunning visualization. EVACS provides a JOGL powered
GUI that the user can search both the HER and HCR with. EVACS displays
the searched for events on a full disk magnetogram of the sun while
displaying more detailed information for events. EVACS can also be used
to launch Panorama with a selected set of FITS or PRGB files, as well
as control many aspects of Panorama. A host of SolarSoft routines are
available to not only access functions of the HEK, but to also access
metadata and/or image data from the Joint Science Operations Center
(JSOC). Routines are also provided to create cutout images and movies
of SDO data. A data order web page is provided to allow a casual user
to order cutouts or full disk images. This page will make use of both
the SSW cutout service as well as the JSOC cutout service.
Title: Simulation of Flux Emergence in Solar Active Regions
Authors: Fang, F.; Manchester, W. B.; Abbett, W. P.; van der Holst,
B.; Schrijver, C. J.
Bibcode: 2010AGUFMSH31A1781F
Altcode:
We present results of magnetohydrodynamic (MHD) simulations of
magnetic flux emergence from the convection zone into the solar
corona using BATSRUS. The MHD equations are modified to take account
of the radiative terms, coronal heating and heat conduction. The
implementation of non-ideal equation of state describes the partially
ionized plasma in the convection zone. The simulations are carried out
on a domain of active-region size of 30×30×40 Mm3, extending 20 Mm
down into the convection zone. The magnetic fields are coupled with
the convective motion during the emerging process, and concentrates
in the downflow regions. A coherent shear pattern is formed in the
lower corona during the rising. We also compare our model results
at the photosphere with SDO/HMI vector magnetograms and illustrate
the mechanism of flux emergence that give rise to complexity of the
structures in active regions.
Title: Multi-thermal observations of flares and eruptions
with the Atmospheric Imaging Assembly on the Solar Dynamics
Observatory. (Invited)
Authors: Schrijver, C. J.; Aia Science Team
Bibcode: 2010AGUFMSH13A..02S
Altcode:
The revolutionary advance in observational capabilities offered by SDO's
AIA offers new views of solar flares and eruptions. The high cadence and
spatial resolution, the full-Sun coverage, and the variety of thermal
responses of the AIA channels from thousands to millions of degrees
enable the study the source regions of solar explosions, as well as the
responses of the solar corona from their immediate vicinity to regions
over a solar radius away. These observations emphasize the importance
of magnetic connectivity and topology, the frequent occurrence of fast
wave-like perturbations, and the contrasts between impulsive compact
X-ray-bright flares and long-duration EUV-bright phenomena.
Title: Coordinated observations of solar prominences with Hinode/SOT
and SDO/AIA
Authors: Berger, T. E.; Tarbell, T. D.; Schrijver, C. J.; Title,
A. M.; Boerner, P.; Shine, R. A.
Bibcode: 2010AGUFMSH21C..04B
Altcode:
We show the first detailed study of a solar quiescent prominence
using simultaneous observations from the Hinode/SOT and SDO/AIA
instruments. The prominence studied is a polar crown prominence
located at the base of a large coronal cavity on the NW solar limb on
22-June-2010. Hinode observed the prominence for 2.75 hours running
the HOP 73 prominence observation program to acquire Ca II H-line
filtergrams and H-alpha doppler observations at a 20-second cadence. SOT
observations in Ca II H-line and H-alpha spectral lines reveal the
common dynamics of filamentary downflows and large-scale oscillations
of the prominence body. In addition a dark cavity is observed to
rise into the prominence and stagnate before going unstable to form
Rayleigh-Taylor plume upflows. AIA observations in the 304, 171, 193,
and 211 channels with 14 second cadence reveal that both the cavity
and the plume upflows are bright in these hotter passbands. Filter
ratio measurements as well as preliminary EM estimates imply that
the cavity and plume plasma temperature is at least 10^6 K. Plasma at
this temperature has never been detected or theorized in a confined
configuration in the lower chromosphere below a prominence. Assuming
an electron number density of 3e09 cm-3, the balance between thermal
pressure in the cavity and magnetic pressure in the overlying prominence
implies a magnetic flux density of order 10 gauss, in line with earlier
measurements of prominence magnetic fields. However the cavity likely
contains a significant magnetic energy density of its own implying that
the prominence magnetic fields may need to be significantly higher to
balance the cavity buoyancy. The existence of 10^6 K plasma confined
below a quiescent prominence and the subsequent onset of buoyancy
instabilities present new challenges to theories of prominence and
coronal cavity formation and suggest new avenues for supply of mass
and magnetic flux to the associated coronal cavity systems that make
up the bulk of CMEs. Hinode/SOT Ca II H-line image overlain on SDO/AIA
304A image of a quiescent solar prominence.
Title: First SDO/AIA Observations of Global Coronal EUV "Waves":
Multiple Components and "Ripples"
Authors: Liu, W.; Nitta, N. V.; Schrijver, C. J.; Title, A. M.;
Tarbell, T. D.
Bibcode: 2010AGUFMSH13A..07L
Altcode:
Global coronal EUV disturbances (so-called "EIT waves") are useful
diagnostics for physical conditions on the Sun. Major drawbacks that
hindered our understanding of this phenomenon were previous instruments'
single view point, low cadence (e.g., 12 minutes of EIT), and limited
wavelength coverage. The Atmospheric Imaging Assembly (AIA) on board
the Solar Dynamics Observatory (SDO) observes the full-sun corona
at 10 EUV and UV wavelengths, covering a wide range of temperatures,
with high resolution (1.4") and cadence (up to 12 s). It thus offers a
great chance to end the decade long debate on the nature of global EUV
"waves". We present here the first AIA observations of such phenomenon
on 2010 April 8 revealed in unprecedented detail and discuss their
physical implications. The disturbance exhibits two components: one
weak, diffuse pulse superimposed by multiple strong, sharp fronts,
which again have slow and fast components. The disturbance originates
in front of erupting coronal loops and the slow sharp front undergoes
acceleration, both implying the disturbance being driven by the
coronal mass ejection (CME). Even at a 20 s cadence, the diffuse pulse
propagates at a surprisingly constant velocity of ~200 km/s, weakly
dependent on direction. The fast sharp front overtakes the slow front,
producing multiple "ripples" and steepening of the local pulse, and both
fronts propagate independently afterwards. These resemble the nature
of real waves. Unexpectedly, the amplitude and FWHM of the diffuse
pulse decreases linearly with distance. The diffuse pulse appears as
emission enhancement at hotter 193 Å but reduction at cooler 171 Å,
suggestive of heating, while the sharp fronts appear as enhancement at
both wavelengths, indicating density increase. As evidence for both
wave and non-wave models of "EIT waves" has been found, we suggest
that a hybrid model combining both mechanisms (e.g., Cohen et al. 2010)
may best explaine the data. In addition to the global EUV disturbance,
we found fast (600-1100 km/s) features repeated at 100 s intervals
as tentative evidence of fast mode MHD waves. Discoveries of the fast
features, multiple ripples, and two-component fronts were made possible
for the first time thanks to AIA's high cadences and sensitivities.
Title: Solar flare impulsive phase observations from SDO and other
observatories
Authors: Chamberlin, P. C.; Woods, T. N.; Schrijver, C. J.; Warren,
H. P.; Milligan, R. O.; Christe, S.; Brosius, J. W.
Bibcode: 2010AGUFMSH23A1832C
Altcode:
With the start of normal operations of the Solar Dynamics Observatory
in May 2010, the Extreme ultraviolet Variability Experiment (EVE)
and the Atmospheric Imaging Assembly (AIA) have been returning the
most accurate solar XUV and EUV measurements every 10 and 12 seconds,
respectively, at almost 100% duty cycle. The focus of the presentation
will be the solar flare impulsive phase observations provided by EVE
and AIA and what these observations can tell us about the evolution
of the initial phase of solar flares. Also emphasized throughout
is how simultaneous observations with other instruments, such as
RHESSI, SOHO-CDS, and HINODE-EIS, will help provide a more complete
characterization of the solar flares and the evolution and energetics
during the impulsive phase. These co-temporal observations from the
other solar instruments can provide information such as extending
the high temperature range spectra and images beyond that provided
by the EUV and XUV wavelengths, provide electron density input into
the lower atmosphere at the footpoints, and provide plasma flows of
chromospheric evaporation, among other characteristics.
Title: The role of the chromosphere in filling the corona with hot
plasma (Invited)
Authors: de Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Boerner, P.; Martinez-Sykora, J.; Schrijver,
C. J.; Title, A. M.
Bibcode: 2010AGUFMSH21C..03D
Altcode:
We use coordinated observations from the Solar Dynamics Observatory
(SDO), Hinode and the Swedish Solar Telescope (SST) to show how
plasma is heated to coronal temperatures from its source in the
chromosphere. Our observations reveal a ubiquitous mass supply
for the solar corona in which chromospheric plasma is accelerated
upward into the corona with much of the plasma heated to transition
region temperatures, and a small, but significant fraction heated
to temperatures in excess of 1 million K. Our observations show,
for the first time, how chromospheric spicules, fountain-like jets
that have long been considered potential candidates for coronal
heating, are directly associated with heating of plasma to coronal
temperatures. These results provide strong physical constraints on
the mechanism(s) responsible for coronal heating and do not seem
compatible with current models. The association with chromospheric
spicules highlights the importance of the interface region between
the photosphere and corona to gain a full understanding of the coronal
heating problem.
Title: Guided searches to SDO Data using the Heliophysics Events
Knowledgebase (Invited)
Authors: Hurlburt, N. E.; Cheung, C.; Schrijver, C. J.; Hek Team
Bibcode: 2010AGUFMSH22A..02H
Altcode:
The immense volume of data generated by the suite of instruments on
SDO requires new tools for efficiently identifying and accessing data
that are most relevant to research investigations. We have developed
a set of tools and web services to fill this need. The central element
to these is the Heliophysics Events Knowledgebase (HEK). We present an
overview of the HEK and describe how our tools and services can be used
to guide you to the most useful data for your research. With guidance
from the HEK, you can access data by specifying cutouts, wavelengths,
and sampling rates in space and time. It also encourages reuse of the
extract data and can suggest datasets that are immediately available
that might satisfy your requirements, or pass the information on
to the VSO, Helioviewer and other services to search for associated
data products.
Title: Heliophysics 3 Volume Set
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2010heli.book.....S
Altcode:
Volume 1: Preface; 1. Prologue Carolus J. Schrijver and George
L. Siscoe; 2. Introduction to heliophysics Thomas J. Bogdan; 3. Creation
and destruction of magnetic field Matthias Rempel; 4. Magnetic field
topology Dana W. Longcope; 5. Magnetic reconnection Terry G. Forbes;
6. Structures of the magnetic field Mark B. Moldwin, George L. Siscoe
and Carolus J. Schrijver; 7. Turbulence in space plasmas Charles
W. Smith; 8. The solar atmosphere Viggo H. Hansteen; 9. Stellar winds
and magnetic fields Viggo H. Hansteen; 10. Fundamentals of planetary
magnetospheres Vytenis M. Vasyliūnas; 11. Solar-wind magnetosphere
coupling: an MHD perspective Frank R. Toffoletto and George L. Siscoe;
12. On the ionosphere and chromosphere Tim Fuller-Rowell and Carolus
J. Schrijver; 13. Comparative planetary environments Frances Bagenal;
Bibliography; Index. Volume 2: Preface; 1. Perspective on heliophysics
George L. Siscoe and Carolus J. Schrijver; 2. Introduction to space
storms and radiation Sten Odenwald; 3. In-situ detection of energetic
particles George Gloeckler; 4. Radiative signatures of energetic
particles Tim Bastian; 5. Observations of solar and stellar eruptions,
flares, and jets Hugh Hudson; 6. Models of coronal mass ejections
and flares Terry Forbes; 7. Shocks in heliophysics Merav Opher;
8. Particle acceleration in shocks Dietmar Krauss-Varban; 9. Energetic
particle transport Joe Giacalone; 10. Energy conversion in planetary
magnetospheres Vytenis Vasyliūnas; 11. Energization of trapped
particles Janet Green; 12. Flares, CMEs, and atmospheric responses
Tim Fuller-Rowell and Stanley C. Solomon; 13. Energetic particles and
manned spaceflight 358 Stephen Guetersloh and Neal Zapp; 14. Energetic
particles and technology Alan Tribble; Appendix I. Authors and editors;
List of illustrations; List of tables; Bibliography; Index. Volume 3:
Preface; 1. Interconnectedness in heliophysics Carolus J. Schrijver
and George L. Siscoe; 2. Long-term evolution of magnetic activity of
Sun-like stars Carolus J. Schrijver; 3. Formation and early evolution
of stars and proto-planetary disks Lee W. Hartmann; 4. Planetary
habitability on astronomical time scales Donald E. Brownlee; 5. Solar
internal flows and dynamo action Mark S. Miesch; 6. Modeling solar and
stellar dynamos Paul Charbonneau; 7. Planetary fields and dynamos Ulrich
R. Christensen; 8. The structure and evolution of the 3D solar wind
John T. Gosling; 9. The heliosphere and cosmic rays J. Randy Jokipii;
10. Solar spectral irradiance: measurements and models Judith L. Lean
and Thomas N. Woods; 11. Astrophysical influences on planetary climate
systems Juerg Beer; 12. Evaluating the drivers of Earth's climate
system Thomas J. Crowley; 13. Ionospheres of the terrestrial planets
Stanley C. Solomon; 14. Long-term evolution of the geospace climate
Jan J. Sojka; 15. Waves and transport processes in atmospheres and
oceans Richard L. Walterscheid; 16. Solar variability, climate, and
atmospheric photochemistry Guy P. Brasseur, Daniel Marsch and Hauke
Schmidt; Appendix I. Authors and editors; List of illustrations;
List of tables; Bibliography; Index.
Title: First SDO AIA Observations of a Global Coronal EUV "Wave":
Multiple Components and "Ripples"
Authors: Liu, Wei; Nitta, Nariaki V.; Schrijver, Carolus J.; Title,
Alan M.; Tarbell, Theodore D.
Bibcode: 2010ApJ...723L..53L
Altcode: 2012arXiv1201.0815L
We present the first Solar Dynamics Observatory Atmospheric Imaging
Assembly (AIA) observations of a global coronal EUV disturbance
(so-called "EIT wave") revealed in unprecedented detail. The disturbance
observed on 2010 April 8 exhibits two components: one diffuse pulse
superimposed, on which are multiple sharp fronts that have slow and
fast components. The disturbance originates in front of erupting coronal
loops and some sharp fronts undergo accelerations, both effects implying
that the disturbance is driven by a coronal mass ejection. The diffuse
pulse, propagating at a uniform velocity of 204-238 km s-1
with very little angular dependence within its extent in the south,
maintains its coherence and stable profile for ~30 minutes. Its arrival
at increasing distances coincides with the onsets of loop expansions and
the slow sharp front. The fast sharp front overtakes the slow front,
producing multiple "ripples" and steepening the local pulse, and both
fronts propagate independently afterward. This behavior resembles
the nature of real waves. Unexpectedly, the amplitude and FWHM of
the diffuse pulse decrease linearly with distance. A hybrid model,
combining both wave and non-wave components, can explain many, but
not all, of the observations. Discoveries of the two-component fronts
and multiple ripples were made possible for the first time thanks to
AIA's high cadences (<=20 s) and high signal-to-noise ratio.
Title: Interconnectedness in heliophysics
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2010hesa.book....1S
Altcode:
No abstract at ADS
Title: Stellar Imager (SI): developing and testing a predictive
dynamo model for the Sun by imaging other stars
Authors: Carpenter, Kenneth G.; Schrijver, Carolus J.; Karovska,
Margarita; Kraemer, Steve; Lyon, Richard; Mozurkewich, David;
Airapetian, Vladimir; Adams, John C.; Allen, Ronald J.; Brown, Alex;
Bruhweiler, Fred; Conti, Alberto; Christensen-Dalsgaard, Joergen;
Cranmer, Steve; Cuntz, Manfred; Danchi, William; Dupree, Andrea; Elvis,
Martin; Evans, Nancy; Giampapa, Mark; Harper, Graham; Hartman, Kathy;
Labeyrie, Antoine; Leitner, Jesse; Lillie, Chuck; Linsky, Jeffrey L.;
Lo, Amy; Mighell, Ken; Miller, David; Noecker, Charlie; Parrish, Joe;
Phillips, Jim; Rimmele, Thomas; Saar, Steve; Sasselov, Dimitar; Stahl,
H. Philip; Stoneking, Eric; Strassmeier, Klaus; Walter, Frederick;
Windhorst, Rogier; Woodgate, Bruce; Woodruff, Robert
Bibcode: 2010arXiv1011.5214C
Altcode:
The Stellar Imager mission concept is a space-based UV/Optical
interferometer designed to resolve surface magnetic activity and
subsurface structure and flows of a population of Sun-like stars,
in order to accelerate the development and validation of a predictive
dynamo model for the Sun and enable accurate long-term forecasting of
solar/stellar magnetic activity.
Title: Heliophysics: Evolving Solar Activity and the Climates of
Space and Earth
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2010hesa.book.....S
Altcode:
Heliophysics is a fast-developing scientific discipline that integrates
studies of the Sun's variability, the surrounding heliosphere, and the
environment and climate of planets. Over the past few centuries, our
understanding of how the Sun drives space weather and climate on the
Earth and other planets has advanced at an ever increasing rate. This
volume, the last in a series of three heliophysics texts, focuses on
long-term variability from the Sun's decade-long sunspot cycle and
considers the evolution of the planetary system over ten billion years
from a climatological perspective. Topics covered range from the dynamo
action of stars and planets to processes in the Earth's troposphere,
ionosphere, and magnetosphere and their effects on planetary climate and
habitability. Supplemented by online teaching materials, it can be used
as a textbook for courses or as a foundational reference for researchers
in fields from astrophysics and plasma physics to planetary and climate
science. Other volumes in this series: Heliophysics: Plasma Physics of
the Local Cosmos (Volume I) Heliophysics: Space Storms and Radiation:
Causes and Effects (Volume II)
Title: Long-term evolution of magnetic activity of Sun-like stars
Authors: Schrijver, Carolus J.
Bibcode: 2010hesa.book...11S
Altcode:
No abstract at ADS
Title: Magnetic Field Topology and the Thermal Structure of the
Corona over Solar Active Regions
Authors: Schrijver, Carolus J.; DeRosa, Marc L.; Title, Alan M.
Bibcode: 2010ApJ...719.1083S
Altcode:
Solar extreme ultraviolet (EUV) images of quiescent active-region
coronae are characterized by ensembles of bright 1-2 MK loops that fan
out from select locations. We investigate the conditions associated
with the formation of these persistent, relatively cool, loop fans
within and surrounding the otherwise 3-5 MK coronal environment by
combining EUV observations of active regions made with TRACE with
global source-surface potential-field models based on the full-sphere
photospheric field from the assimilation of magnetograms that are
obtained by the Michelson Doppler Imager (MDI) on SOHO. We find that in
the selected active regions with largely potential-field configurations
these fans are associated with (quasi-)separatrix layers (QSLs) within
the strong-field regions of magnetic plage. Based on the empirical
evidence, we argue that persistent active-region cool-loop fans are
primarily related to the pronounced change in connectivity across a QSL
to widely separated clusters of magnetic flux, and confirm earlier work
that suggested that neither a change in loop length nor in base field
strengths across such topological features are of prime importance to
the formation of the cool-loop fans. We discuss the hypothesis that
a change in the distribution of coronal heating with height may be
involved in the phenomenon of relatively cool coronal loop fans in
quiescent active regions.
Title: Perspective on Heliophysics
Authors: Siscoe, George L.; Schrijver, Carolus J.
Bibcode: 2010hssr.book....1S
Altcode:
No abstract at ADS
Title: Erratum: "The Dependence of Ephemeral Region Emergence on
Local Flux Imbalance" (2008, ApJ,
678, 541)
Authors: Hagenaar, Hermance J.; DeRosa, Marc L.; Schrijver, Carolus J.
Bibcode: 2010ApJ...715..696H
Altcode:
We have discovered an error in the labeling of Figure 5. The importance
of the figure is to indicate the dependence of flux emergence on local
flux (im-) balance. However, the scales of the figures were incorrect,
causing a discrepancy between Table 2 and Figure 5(a). The corrected
Figure 5 appears below. The change does not affect the conclusion.
Title: An Introduction to the Heliophysics Event Knowledgebase
Authors: Hurlburt, Neal E.; Cheung, M.; Schrijver, C.; Chang, L.;
Freeland, S.; Green, S.; Heck, C.; Jaffey, A.; Kobashi, A.; Schiff,
D.; Serafin, J.; Seguin, R.; Slater, G.; Somani, A.; Timmons, R.
Bibcode: 2010AAS...21640222H
Altcode: 2010BAAS...41T.876H
The immense volume of data generated by the suite of instruments on
SDO requires new tools for efficiently identifying and accessing data
that are most relevant to research investigations. We have developed
the Heliophysics Events Knowledgebase (HEK) to fill this need. The
system developed to support the HEK combines automated datamining using
feature detection methods; high-performance visualization systems for
data markup; and web-services and clients for searching the resulting
metadata, reviewing results and efficient access to the data. We will
review these components and present examples of their use with SDO data.
Title: Magnetic Field Topology and the Thermal Structure of the
Corona over Solar Active Regions
Authors: Schrijver, Carolus J.; DeRosa, M. L.; Title, A. M.
Bibcode: 2010AAS...21631201S
Altcode:
Solar extreme-ultraviolet images of active-region coronae are
characterized by ensembles of bright 1-2 MK loops that fan out from
select locations. We investigate the conditions associated with the
formation of these relatively cool loop fans within the otherwise
3-5 MK coronal environment by combining EUV observations of active
regions made with the Transition Region and Coronal Explorer (TRACE)
with global source-surface potential-field models based on the
full-sphere photospheric field from the assimilation of magnetograms
that are obtained by MDI on SOHO. We find that in the selected active
regions with largely potential field configurations these fans are
associated with (quasi-)separatrices within the strong-field regions
of magnetic plage and vice versa. We argue that the divergence of the
field lines across a (quasi-)separatrix may cause heating to happen
relatively low in the corona, resulting in a lower loop temperature
and flatter thermal profile of relatively dense (and thus EUV-bright)
loops that are surrounded by warmer, thermally more stratified loops
in field that does not straddle such topological divides.
Title: Heliophysics: Space Storms and Radiation: Causes and Effects
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2010hssr.book.....S
Altcode:
Heliophysics is a fast-developing scientific discipline that integrates
studies of the Sun's variability, the surrounding heliosphere, and
the environment and climate of planets. The Sun is a magnetically
variable star and, for planets with intrinsic magnetic fields,
planets with atmospheres, or planets like Earth with both, there are
profound consequences. This volume, the second in a series of three
heliophysics texts, integrates the many aspects of space storms and
the energetic radiation associated with them - from causes on the Sun
to effects in planetary environments. It reviews the physical processes
in solar flares and coronal mass ejections, interplanetary shocks, and
particle acceleration and transport, and considers many space weather
responses in geospace. In addition to its utility as a textbook, it
also constitutes a foundational reference for researchers in fields
from heliophysics to climate science. Additional online resources,
including lecture presentations and other teaching materials, are
available at www.cambridge.org/9780521760515. Other volumes in this
series: Heliophysics: Plasma Physics of the Local Cosmos (Volume I)
Heliophysics: Evolving Solar Activity and the Climates of Space and
Earth (Volume III)
Title: New Insights to Global Coronal EUV Waves: First Double
Quadrature Observations by SDO/AIA and STEREO/EUVI
Authors: Liu, Wei; Nitta, N. V.; Schrijver, C. J.; Title, A. M.
Bibcode: 2010AAS...21640230L
Altcode:
Global coronal EUV waves are useful diagnostic tools for physical
conditions on the Sun. Major drawbacks that hindered our understanding
of EUV waves were previous instruments' low cadence (e.g., 12
minutes for SoHO/EIT) and limited spatial resolution and wavelength
coverage. The Atmospheric Imaging Assembly (AIA) on board the recently
launched Solar Dynamics Observatory (SDO), joined by the STEREO EUV
Imager (EUVI), offers a great chance to end the decade long debate
on the nature of global EUV waves. AIA observes the corona at 10 EUV
and UV wavelengths, covering a wide range of temperatures. It has high
resolution (0.6") and cadence (20 s, 7 times faster than EUVI). These
capabilities allow us to study the thermal structure and kinematics
of EUV waves in unprecedented detail. We present here the first AIA
observations of an EUV wave occurring on 2010 April 8. AIA observed
this event on the solar disk, while the STEREO Ahead (A) and Behind
(B) spacecraft, which were 67 degree ahead and 72 degree behind the
Earth, respectively, provided side views of both the EUV wave and the
halo coronal mass ejection (seen by SoHO/LASCO) near the limb. This
formed a double quadrature configuration with great advantages to
infer the 3D structure. Initial analysis indicates that this wave
exhibited strong anisotropy, propagating primarily toward the south,
on the same side of the erupting loop system. We will examine its
spatial and temporal relationship with the erupting loop and CME and
discuss physical implications.
Title: The Heliophysics Event Knowledgebase for the Solar Dynamics
Observatory - A User's Perspective
Authors: Slater, Gregory L.; Cheung, M.; Hurlburt, N.; Schrijver,
C.; Somani, A.; Freeland, S. L.; Timmons, R.; Kobashi, A.; Serafin,
J.; Schiff, D.; Seguin, R.
Bibcode: 2010AAS...21641505S
Altcode: 2010BAAS...41S.825S
The recently launched Solar Dynamics Observatory (SDO) will
generated over 2 petabytes of imagery in its 5 year mission. The
Heliophysics Events Knowledgebase (HEK) system has been developed to
continuously build a database of solar features and events contributed
by a combination of machine recognition algorithms run on every single
image, and human interactive data exploration. Access to this growing
database is provided through a set of currently existing tools as well
as an open source API. We present an overview of the user interface
tools including illustrative examples of their use.
Title: Solar Observation Target Identification Convention for use
in Solar Physics
Authors: Leibacher, John; Sakurai, Takashi; Schrijver, Carolus J.;
van Driel-Gesztelyi, Lidia
Bibcode: 2010SoPh..263....1L
Altcode: 2010SoPh..263....1.; 2010SoPh..tmp...71.
We strongly encourage the use of a standardized target identification
to be included in pub- lications on solar events. The primary purpose
is to enable the automated identification of publications on the same
event, or on other related events, in the on-line literature by search
engines such as the Astrophysics Data System (ADS). The convention does
not aim to categorize or classify events, but is limited specifically
to the identification of regions in space and intervals in time within
which events occur.
Title: Eruptions from Solar Ephemeral Regions as an Extension of
the Size Distribution of Coronal Mass Ejections
Authors: Schrijver, Carolus J.
Bibcode: 2010ApJ...710.1480S
Altcode: 2009arXiv0912.0969S
Observations of the quiet solar corona in the 171 Å (~1 MK) passband
of the Transition Region and Coronal Explorer (TRACE) often show
disruptions of the coronal part of small-scale ephemeral bipolar
regions that resemble the phenomena associated with coronal mass
ejections (CMEs) on much larger scales: ephemeral regions exhibit
flare-like brightenings, rapidly rising filaments carrying absorbing
material at chromospheric temperatures, or the temporary dimming of the
surrounding corona. I analyze all available TRACE observing sequences
between 1998 April 1 and 2009 September 30 with full-resolution 171
Å image sequences spanning a day or more within 500 arcsec of disk
center, observing essentially the quiet Sun with good exposures and
relatively low background. Ten such data sets are identified between
2000 and 2008, spanning 570 hr of observing with a total of 17,133
exposures. Eighty small-scale coronal eruptions are identified. Their
size distribution forms a smooth extension of the distribution of
angular widths of CMEs, suggesting that the eruption frequency for
bipolar magnetic regions is essentially scale free over at least 2
orders of magnitude, from eruptions near the arcsecond resolution limit
of TRACE to the largest CMEs observed in the inner heliosphere. This
scale range may be associated with the properties of the nested set
of ranges of connectivity in the magnetic field in which increasingly
large and energetic events can reach higher and higher into the corona
until the heliosphere is reached.
Title: An Introduction to the Heliophysics Event Knowledgebase for SDO
Authors: Hurlburt, Neal; Schrijver, Carolus; Cheung, Mark
Bibcode: 2010cosp...38.2879H
Altcode: 2010cosp.meet.2879H
The immense volume of data generated by the suite of instruments on
SDO requires new tools for efficient identifying and accessing data
that is most relevant to research investigations. We have developed the
Heliophysics Events Knowledgebase (HEK) to fill this need. The system
developed in support of the HEK combines automated datamining using
feature detection methods; high-performance visualization systems for
data markup; and web-services and clients for searching the resulting
metadata, reviewing results and efficient access to the data. We will
review these components and present examples of their use with SDO data.
Title: The Stellar Imager (SI) - A Mission to Resolve Stellar
Surfaces, Interiors, and Magnetic Activity
Authors: Carpenter, K. G.; Schrijver, C. J.; Karovska, M.; Si Vision
Mission Team
Bibcode: 2009ASPC..412...91C
Altcode:
The Stellar Imager (SI) is a UV/Optical, Space-Based Interferometer
designed to enable 0.1 milli-arcsecond (mas) spectral imaging of
stellar surfaces and, via asteroseismology, stellar interiors and of
the Universe in general. The ultra-sharp images of the Stellar Imager
will revolutionize our view of many dynamic astrophysical processes
by transforming point sources into extended sources, and snapshots
into evolving views. SI's science focuses on the role of magnetism
in the Universe, particularly on magnetic activity on the surfaces of
stars like the Sun. SI's prime goal is to enable long-term forecasting
of solar activity and the space weather that it drives. SI will also
revolutionize our understanding of the formation of planetary systems,
of the habitability and climatology of distant planets, and of many
magneto-hydrodynamically controlled processes in the Universe. SI is
included as a ``Flagship and Landmark Discovery Mission'' in the 2005
NASA Sun Solar System Connection (SSSC) Roadmap and as a candidate
for a ``Pathways to Life Observatory'' in the NASA Exploration of the
Universe Division (EUD) Roadmap (May, 2005). In this paper we discuss
the science goals and technology needs of, and the baseline design
for, the SI Mission (http://hires.gsfc.nasa.gov/si/) and its ability
to image the Biggest, Baddest, Coolest Stars.
Title: Prologue
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2009hppl.book....1S
Altcode:
The place that we call home, the surface of the planet Earth, presents
us with an environment in which temperatures range over perhaps 80
kelvins from the cool arctic regions or mountain tops to the hottest
deserts or jungles. We are composed largely of liquid water with a
density of 1 gram per cubic centimeter; we walk on solid rock with a
density that is about five times higher than this and breathe a gas
with a density that is 1000 times lower. These conditions are such
that chemical reactions and phase transitions between solids, liquids,
and gases are the processes that dominate our everyday experience.
Title: Observing the Roots of Solar Coronal Heating—in the
Chromosphere
Authors: De Pontieu, Bart; McIntosh, Scott W.; Hansteen, Viggo H.;
Schrijver, Carolus J.
Bibcode: 2009ApJ...701L...1D
Altcode: 2009arXiv0906.5434D
The Sun's corona is millions of degrees hotter than its 5000 K
photosphere. This heating enigma is typically addressed by invoking
the deposition at coronal heights of nonthermal energy generated
by the interplay between convection and magnetic field near the
photosphere. However, it remains unclear how and where coronal heating
occurs and how the corona is filled with hot plasma. We show that energy
deposition at coronal heights cannot be the only source of coronal
heating by revealing a significant coronal mass supply mechanism that
is driven from below, in the chromosphere. We quantify the asymmetry
of spectral lines observed with Hinode and SOHO and identify faint
but ubiquitous upflows with velocities that are similar (50-100 km
s-1) across a wide range of magnetic field configurations and
for temperatures from 100,000 to several million degrees. These upflows
are spatiotemporally correlated with and have similar upward velocities
as recently discovered, cool (10,000 K) chromospheric jets or (type II)
spicules. We find these upflows to be pervasive and universal. Order
of magnitude estimates constrained by conservation of mass and observed
emission measures indicate that the mass supplied by these spicules can
play a significant role in supplying the corona with hot plasma. The
properties of these events are incompatible with coronal loop models
that include only nanoflares at coronal heights. Our results suggest
that a significant part of the heating and energizing of the corona
occurs at chromospheric heights, in association with chromospheric jets.
Title: Structures of the magnetic field
Authors: Moldwin, Mark B.; Siscoe, George L.; Schrijver, Carolus J.
Bibcode: 2009hppl.book..139M
Altcode:
No abstract at ADS
Title: On the ionosphere and chromosphere
Authors: Fuller-Rowell, Tim J.; Schrijver, Carolus J.
Bibcode: 2009hppl.book..324F
Altcode:
No abstract at ADS
Title: Heliophysics: Plasma Physics of the Local Cosmos
Authors: Schrijver, Carolus J.; Siscoe, George L.
Bibcode: 2009hppl.book.....S
Altcode:
Heliophysics is a developing scientific discipline integrating studies
of the Sun's variability, the surrounding heliosphere, and climatic
environments. Over the past few centuries, our understanding of how the
Sun drives space weather and climate on the Earth and other planets
has advanced at an ever-increasing rate. This volume, the first in a
series of three heliophysics texts, integrates such diverse topics for
the first time as a coherent intellectual discipline. It emphasizes
the physical processes coupling the Sun and Earth, allowing insights
into the interaction of the solar wind and radiation with the Earth's
magnetic field, atmosphere and climate system. It provides a core
resource for advanced undergraduates and graduates, and also constitutes
a foundational reference for researchers in heliophysics, astrophysics,
plasma physics, space physics, solar physics, aeronomy, space weather,
planetary science and climate science. Additional online resources,
including lecture presentations and other teaching materials, are
accessible at www.cambridge.org/9781107403222.
Title: Stellar imager (SI): enhancements to the mission enabled by
the constellation architecture (Ares I/Ares V)
Authors: Carpenter, Kenneth G.; Karovska, Margarita; Lyon, Richard G.;
Mozurkewich, D.; Schrijver, Carolus
Bibcode: 2009SPIE.7436E..0AC
Altcode: 2009SPIE.7436E...9C
Stellar Imager (SI) is a space-based, UV/Optical Interferometer (UVOI)
with over 200x the resolution of HST. It will enable 0.1 milli-arcsec
spectral imaging of stellar surfaces and the Universe in general
and open an enormous new "discovery space" for astrophysics with its
combination of high angular resolution, dynamic imaging, and spectral
energy resolution. SI's goal is to study the role of magnetism in
the Universe and revolutionize our understanding of: 1) Solar/Stellar
Magnetic Activity and their impact on Space Weather, Planetary Climates,
and Life, 2) Magnetic and Accretion Processes and their roles in the
Origin & Evolution of Structure and in the Transport of Matter
throughout the Universe, 3) the close-in structure of Active Galactic
Nuclei and their winds, and 4) Exo-Solar Planet Transits and Disks. SI
is a "Landmark/Discovery Mission" in 2005 Heliophysics Roadmap and a
candidate UVOI in the 2006 Astrophysics Strategic Plan and is targeted
for launch in the mid-2020's. It is a NASA Vision Mission and has
been recommended for further study in a 2008 NRC report on missions
potentially enabled/enhanced by an Ares V launch. In this paper,
we discuss the science goals and required capabilities of SI, the
baseline architecture of the mission assuming launch on one or more
Delta rockets, and then the potential significant enhancements to the
SI science and mission architecture that would be made possible by a
launch in the larger volume Ares V payload fairing, and by servicing
options under consideration in the Constellation program.
Title: The Nature of Flare Ribbons in Coronal Null-Point Topology
Authors: Masson, S.; Pariat, E.; Aulanier, G.; Schrijver, C. J.
Bibcode: 2009ApJ...700..559M
Altcode:
Flare ribbons are commonly attributed to the low-altitude impact, along
the footprints of separatrices or quasi-separatrix layers (QSLs),
of particle beams accelerated through magnetic reconnection. If
reconnection occurs at a three-dimensional coronal magnetic null
point, the footprint of the dome-shaped fan surface would map a closed
circular ribbon. This paper addresses the following issues: does the
entire circular ribbon brighten simultaneously, as expected because
all fan field lines pass through the null point? And since the spine
separatrices are singular field lines, do spine-related ribbons look
like compact kernels? What can we learn from these observations about
current sheet formation and magnetic reconnection in a null-point
topology? The present study addresses these questions by analyzing
Transition Region and Coronal Explorer and Solar and Heliospheric
Observatory/Michelson Doppler Imager observations of a confined flare
presenting a circular ribbon. Using a potential field extrapolation,
we linked the circular shape of the ribbon with the photospheric
mapping of the fan field lines originating from a coronal null
point. Observations show that the flare ribbon outlining the fan lines
brightens sequentially along the counterclockwise direction and that
the spine-related ribbons are elongated. Using the potential field
extrapolation as initial condition, we conduct a low-β resistive
magnetohydrodynamics simulation of this observed event. We drive the
coronal evolution by line-tied diverging boundary motions, so as to
emulate the observed photospheric flow pattern associated with some
magnetic flux emergence. The numerical analysis allows us to explain
several observed features of the confined flare. The vorticity induced
in the fan by the prescribed motions causes the spines to tear apart
along the fan. This leads to formation of a thin current sheet and
induces null-point reconnection. We also find that the null point
and its associated topological structure is embedded within QSLs,
already present in the asymmetric potential field configuration. We
find that the QSL footprints correspond to the observed elongated
spine ribbons. Finally, we observe that before and after reconnecting
at the null point, all field lines undergo slipping and slip-running
reconnection within the QSLs. Field lines, and therefore particle
impacts, slip or slip-run according to their distance from the spine,
in directions and over distances that are compatible with the observed
dynamics of the ribbons.
Title: On a Transition from Solar-Like Coronae to Rotation-Dominated
Jovian-Like Magnetospheres in Ultracool Main-Sequence Stars
Authors: Schrijver, Carolus J.
Bibcode: 2009ApJ...699L.148S
Altcode: 2009arXiv0905.1354S
For main-sequence stars beyond spectral type M5, the characteristics of
magnetic activity common to warmer solar-like stars change into the
brown-dwarf domain: the surface magnetic field becomes more dipolar
and the evolution of the field patterns slows, the photospheric
plasma is increasingly neutral and decoupled from the magnetic
field, chromospheric and coronal emissions weaken markedly, and the
efficiency of rotational braking rapidly decreases. Yet, radio emission
persists, and has been argued to be dominated by electron-cyclotron
maser emission instead of the gyrosynchrotron emission from warmer
stars. These properties may signal a transition in the stellar extended
atmosphere. Stars warmer than about M5 have a solar-like corona and
wind-sustained heliosphere in which the atmospheric activity is powered
by convective motions that move the magnetic field. Stars cooler
than early-L, in contrast, may have a Jovian-like rotation-dominated
magnetosphere powered by the star's rotation in a scaled-up analog
of the magnetospheres of Jupiter and Saturn. A dimensional scaling
relationship for rotation-dominated magnetospheres by Fan et al. is
consistent with this hypothesis.
Title: Observing the Roots of Solar Coronal Heating in the
Chromosphere
Authors: McIntosh, Scott W.; De Pontieu, B.; Hansteen, V.; Schrijver,
C. J.
Bibcode: 2009SPD....40.2602M
Altcode:
The Sun's atmosphere or corona is millions of degrees hotter than
its 5,000 K surface or photosphere. This heating enigma is typically
addressed by invoking the deposition at coronal heights of non-thermal
energy generated by the interplay between convection and magnetic field
near the photosphere. However, it remains unclear how and where coronal
heating occurs and how the corona is filled with hot plasma. Here,
we show that energy deposition at coronal heights cannot be the only
source of coronal heating, by revealing a significant coronal mass
supply mechanism that is driven from below, in the chromosphere, the
interface between photosphere and corona. We quantify the asymmetry
of spectral lines observed with Hinode and SOHO and identify faint
but ubiquitous upflows with velocities that are similar (50-100
km/s) across a wide range of magnetic field configurations and for
temperatures from 100,000 to several million degrees. These upflows
are correlated with and have similar upward velocities as the very fine
and dynamic chromospheric jets, or spicules, discovered by Hinode. As
these phenomena are incompatible with models of coronal loops that
only include nanoflare heating at coronal heights, we conclude that
a significant fraction of the energy needed to heat coronal plasma is
deposited at chromospheric heights in association with spicular jets
driven from below.
Title: Nonlinear Force-Free Magnetic Field Modeling of AR 10953:
A Critical Assessment
Authors: De Rosa, Marc L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; Amari, T.; Canou, A.; McTiernan,
J. M.; Régnier, S.; Thalmann, J. K.; Valori, G.; Wheatland, M. S.;
Wiegelmann, T.; Cheung, M. C. M.; Conlon, P. A.; Fuhrmann, M.;
Inhester, B.; Tadesse, T.
Bibcode: 2009SPD....40.3102D
Altcode:
Nonlinear force-free field (NLFFF) modeling seeks to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have failed to arrive at consistent solutions when
applied to (thus far, two) cases using the highest-available-resolution
vector magnetogram data from Hinode/SOT-SP (in the region of the
modeling area of interest) and line-of-sight magnetograms from
SOHO/MDI (where vector data were not available). One issue is that
NLFFF models require consistent, force-free vector magnetic boundary
data, and vector magnetogram data sampling the photosphere do not
satisfy this requirement. Consequently, several problems have arisen
that are believed to affect such modeling efforts. We use AR 10953
to illustrate these problems, namely: (1) some of the far-reaching,
current-carrying connections are exterior to the observational field
of view, (2) the solution algorithms do not (yet) incorporate the
measurement uncertainties in the vector magnetogram data, and/or (3)
a better way is needed to account for the Lorentz forces within the
layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
Title: A Critical Assessment of Nonlinear Force-Free Field Modeling
of the Solar Corona for Active Region 10953
Authors: De Rosa, Marc L.; Schrijver, Carolus J.; Barnes, Graham;
Leka, K. D.; Lites, Bruce W.; Aschwanden, Markus J.; Amari, Tahar;
Canou, Aurélien; McTiernan, James M.; Régnier, Stéphane; Thalmann,
Julia K.; Valori, Gherardo; Wheatland, Michael S.; Wiegelmann, Thomas;
Cheung, Mark C. M.; Conlon, Paul A.; Fuhrmann, Marcel; Inhester,
Bernd; Tadesse, Tilaye
Bibcode: 2009ApJ...696.1780D
Altcode: 2009arXiv0902.1007D
Nonlinear force-free field (NLFFF) models are thought to be viable
tools for investigating the structure, dynamics, and evolution of
the coronae of solar active regions. In a series of NLFFF modeling
studies, we have found that NLFFF models are successful in application
to analytic test cases, and relatively successful when applied
to numerically constructed Sun-like test cases, but they are less
successful in application to real solar data. Different NLFFF models
have been found to have markedly different field line configurations
and to provide widely varying estimates of the magnetic free energy in
the coronal volume, when applied to solar data. NLFFF models require
consistent, force-free vector magnetic boundary data. However,
vector magnetogram observations sampling the photosphere, which is
dynamic and contains significant Lorentz and buoyancy forces, do not
satisfy this requirement, thus creating several major problems for
force-free coronal modeling efforts. In this paper, we discuss NLFFF
modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT,
STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process
illustrate three such issues we judge to be critical to the success of
NLFFF modeling: (1) vector magnetic field data covering larger areas
are needed so that more electric currents associated with the full
active regions of interest are measured, (2) the modeling algorithms
need a way to accommodate the various uncertainties in the boundary
data, and (3) a more realistic physical model is needed to approximate
the photosphere-to-corona interface in order to better transform the
forced photospheric magnetograms into adequate approximations of nearly
force-free fields at the base of the corona. We make recommendations
for future modeling efforts to overcome these as yet unsolved problems.
Title: The Heliophysics Event Knowledgebase for the Solar Dynamics
Observatory
Authors: Hurlburt, Neal E.; Cheung, M.; Schrijver, K.; HEK development
Team
Bibcode: 2009SPD....40.1511H
Altcode:
The Solar Dynamics Observatory will generated over 2 petabytes
of imagery in its 5 year mission. In order to improve scientific
productivity and to reduce system requirements , we have developed
a system for data markup to identify "interesting” datasets and
direct scientists to them through an event-based querying system. The
SDO Heliophysics Event Knowledgebase (HEK) will enable caching of
commonly accessed datasets within the Joint Science Operations Center
(JSOC) and reduces the (human) time spent searching for and downloading
relevant data. We present an overview of our HEK including the ingestion
of images, automated and manual tools for identifying and annotation
features within the images, and interfaces and web tools for querying
and accessing events and their associated data.
Title: The AIA for SDO: Plans for Flight Operations
Authors: Lemen, James; Title, A. M.; Schrijver, K.; Boerner, P.;
Wolfson, C. J.; Nightingale, R.
Bibcode: 2009SPD....40.1703L
Altcode:
The Atmospheric Imaging Array (AIA) has been integrated to the NASA
Solar Dynamics Observatory for over a year and is being prepared for
launch in the fourth quarter of 2009. AIA will provide unprecedented
full-disk images of the solar corona simultaneously covering a
wide range of temperatures. Four normal incidence telescopes with
multilayer-coated optics obtain images with 0.6 arcsec pixels of
EUV-emitting iron lines (covering temperatures between 0.6 and 15 x
106 K), He II 304Å, and C IV in broadband UV. In normal
operations, AIA acquires eight 4k x 4k pixel images every 10s that are
compressed for transmission to the ground, and require approximately
1 TByte of ground storage per day if compressed by a factor of 2. The
instrument calibration has been completed and comprehensive performance
predictions are available (see Wolfson et al). The observing program is
configurable by tables that can be uploaded from the ground. AIA data
is processed to Level 1 in the JSOC pipeline and made available for
export to scientific users. Additional data products, such as movies
of active regions, light curves, and DEM maps, have been developed
for distribution. We present an update of the predicted instrument
performance, discuss the concept of operations, and describe the data
processing for Level 1 data products. Exported FITs data files conform
to SolarSoft standards and the data flow design enables a seamless
connection to the Heliosphysics Event Knowledgebase (see Hurlburt
et al).
Title: Observing and Modeling the Effects of the Solar Magnetic
Field in the Corona
Authors: Schrijver, Carolus J.
Bibcode: 2009SPD....40.3502S
Altcode:
The AIA on SDO will offer arcsecond-resolution coronal imaging at a
wide range of temperatures, at 10s cadence, with continuous full-disk
coverage, supported by HMI's (vector-)magnetography. These properties
can be expected to stimulate advances in particular where large
fields of view, continuous coverage, large samples, comprehensive
yet differentiating thermal coverage, coronal field modeling, or high
cadence are required. In this context, I plan to look forward to the
SDO era by focusing on some of the key scientific questions about the
solar corona that can be addressed particularly well with SDO, and
on their associated problems. These topics include flux emergence and
reconnection with pre-existing field, the powering of flares and CMEs,
the thermal structure of the active-region corona, quiet-Sun filaments
and their field configurations, and coronal seismology.
Title: Designing A Quicklook Data Product For The Multithermal
Capabilities Of AIA/SDO
Authors: Weber, Mark A.; Boerner, P.; Schrijver, K.
Bibcode: 2009SPD....40.1709W
Altcode:
The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory
(SDO) is a set of normal-incidence imaging telescopes with a variety of
passbands in the UV and EUV. The instrument will observe solar plasmas
from the chromosphere to the corona. Six of the passbands are dominated
by iron lines and observe the emission from the optically thin corona,
which is known to be a highly structured and multithermal region. This
sort of imager data lends itself to differential emission measure
analysis, which reconstructs the thermal structure of the observed
plasma (with spatial resolution) across the field of view. The AIA
Science Team will provide a data product that visually represents this
thermal structure as a color mapped image. In this poster we discuss
the analysis and visualization techniques we will employ to produce
this data product. This work is supported by the AIA GSFC/NASA contract
to Lockheed-Martin.
Title: The Stellar Imager (SI) project: a deep space UV/Optical
Interferometer (UVOI) to observe the Universe at 0.1 milli-arcsec
angular resolution
Authors: Carpenter, Kenneth G.; Schrijver, Carolus J.; Karovska,
Margarita
Bibcode: 2009Ap&SS.320..217C
Altcode: 2008Ap&SS.tmp...99C
The Stellar Imager (SI) is a space-based, UV/Optical Interferometer
(UVOI) designed to enable 0.1 milli-arcsecond (mas) spectral imaging
of stellar surfaces and of the Universe in general. It will also probe
via asteroseismology flows and structures in stellar interiors. SI’s
science focuses on the role of magnetism in the Universe and will
revolutionize our understanding of the formation of planetary systems,
of the habitability and climatology of distant planets, and of many
magneto-hydrodynamically controlled processes, such as accretion, in
the Universe. The ultra-sharp images of SI will revolutionize our view
of many dynamic astrophysical processes by transforming point sources
into extended sources, and snapshots into evolving views. SI is a
“Flagship and Landmark Discovery Mission” in the 2005 Heliophysics
Roadmap and a potential implementation of the UVOI in the 2006 Science
Program for NASA’s Astronomy and Physics Division. We present here
the science goals of the SI Mission, a mission architecture that
could meet those goals, and the technology development needed to
enable this mission. Additional information on SI can be found at:
http://hires.gsfc.nasa.gov/si/
Title: Technology Development for Future Sparse Aperture Telescopes
and Interferometers in Space
Authors: Carpenter, Kenneth G.; Gendreau, Keith; Leitner, Jesse; Lyon,
Richard; Stoneking, Eric; Stahl, H. Philip; Parrish, Joe; Schrijver,
Carolus J.; Woodruff, Robert; Lillie, Chuck; Lo, Amy; Mozurkewich,
David; Labeyrie, Antoine; Miller, David; Mighell, Ken; Karovska,
Margarita; Phillips, James; Allen, Ronald J.; Cash, Webster
Bibcode: 2009astro2010T..47C
Altcode:
No abstract at ADS
Title: Driving major solar flares and eruptions: A review
Authors: Schrijver, Carolus J.
Bibcode: 2009AdSpR..43..739S
Altcode: 2008arXiv0811.0787S
This review focuses on the processes that energize and
trigger M- and X-class solar flares and associated flux-rope
destabilizations. Numerical modeling of specific solar regions is
hampered by uncertain coronal-field reconstructions and by poorly
understood magnetic reconnection; these limitations result in uncertain
estimates of field topology, energy, and helicity. The primary
advances in understanding field destabilizations therefore come from
the combination of generic numerical experiments with interpretation of
sets of observations. These suggest a critical role for the emergence
of twisted flux ropes into pre-existing strong field for many, if not
all, of the active regions that produce M- or X-class flares. The flux
and internal twist of the emerging ropes appear to play as important
a role in determining whether an eruption will develop predominantly
as flare, confined eruption, or CME, as do the properties of the
embedding field. Based on reviewed literature, I outline a scenario
for major flares and eruptions that combines flux-rope emergence,
mass draining, near-surface reconnection, and the interaction with the
surrounding field. Whether deterministic forecasting is in principle
possible remains to be seen: to date no reliable such forecasts can
be made. Large-sample studies based on long-duration, comprehensive
observations of active regions from their emergence through their
flaring phase are needed to help us better understand these complex
phenomena.
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: Commission 10: Solar Activity
Authors: Klimchuk, James A.; van Driel-Gesztelyi, Lidia; Schrijver,
Carolus J.; Melrose, Donald B.; Fletcher, Lyndsay; Gopalswamy,
Natchimuthuk; Harrison, Richard A.; Mandrini, Cristina H.; Peter,
Hardi; Tsuneta, Saku; Vršnak, Bojan; Wang, Jing-Xiu
Bibcode: 2009IAUTA..27...79K
Altcode: 2008arXiv0809.1444K
Commission 10 deals with solar activity in all of its forms,
ranging from the smallest nanoflares to the largest coronal mass
ejections. This report reviews scientific progress over the roughly
two-year period ending in the middle of 2008. This has been an exciting
time in solar physics, highlighted by the launches of the Hinode and
STEREO missions late in 2006. The report is reasonably comprehensive,
though it is far from exhaustive. Limited space prevents the inclusion
of many significant results. The report is divided into the following
sections: Photosphere and chromosphere; Transition region; Corona and
coronal heating; Coronal jets; flares; Coronal mass ejection initiation;
Global coronal waves and shocks; Coronal dimming; The link between low
coronal CME signatures and magnetic clouds; Coronal mass ejections in
the heliosphere; and Coronal mass ejections and space weather. Primary
authorship is indicated at the beginning of each section.
Title: The Solar Chromosphere: Old Challenges, New Frontiers
Authors: Ayres, T.; Uitenbroek, H.; Cauzzi, G.; Reardon, K.; Berger,
T.; Schrijver, C.; de Pontieu, B.; Judge, P.; McIntosh, S.; White,
S.; Solanki, S.
Bibcode: 2009astro2010S...9A
Altcode:
No abstract at ADS
Title: Stellar Imager (SI) - Observing the Universe in High Definition
Authors: Carpenter, Kenneth G.; Karovska, M.; Schrijver, C. J.;
SI Development Team
Bibcode: 2009AAS...21345113C
Altcode: 2009BAAS...41..346C
Stellar Imager (http://hires.gsfc.nasa.gov/si/) is a space-based,
UV/Optical Interferometer (UVOI) with over 200x HST's resolution. It
will enable 0.1 milli-arcsec spectral imaging of stellar surfaces and
the Universe in general and open an enormous new "discovery space"
for Astrophysics with its combination of high angular resolution,
dynamic imaging, and spectral energy resolution. SI's goal is to
study the role of magnetism in the Universe and revolutionize our
understanding of: 1) Solar/Stellar Magnetic Activity and their
impact on Space Weather, Planetary Climates, and Life, 2) Magnetic
and Accretion Processes and their roles in the Origin and Evolution
of Structure and in the Transport of Matter throughout the Universe,
3) the close-in structure of Active Galactic Nuclei, and 4) Exo-Solar
Planet Transits and Disks. The SI mission is targeted for the mid
2020's - thus significant technology development in the upcoming decade
is critical to enabling it and future space-based sparse aperture
telescope and distributed spacecraft missions. The key technology
needs include: 1) precision formation flying of many spacecraft,
2) precision metrology over km-scales, 3) closed-loop control of
many-element, sparse optical arrays, 4) staged-control systems with
very high dynamic ranges (nm to km-scale). It is critical that the
importance of timely development of these capabilities is called
out in the upcoming Astrophysics and Heliophysics Decadal Surveys,
to enable the flight of such missions in the following decade. SI
is a "Landmark/Discovery Mission" in the 2005 Heliophysics Roadmap
and a candidate UVOI in the 2006 Astrophysics Strategic Plan. It is
a NASA Vision Mission ("NASA Space Science Vision Missions" (2008),
ed. M. Allen) and has also been recommended for further study in the
2008 NRC interim report on missions potentially enabled or enhanced
by an Ares V launch, although an incrementally-deployed version could
be launched using smaller rockets.
Title: Dynamos and magnetic fields of the Sun and other cool stars,
and their role in the formation and evolution of stars and in the
habitability of planets
Authors: Schrijver, Karel; Carpenter, Ken; Karovska, Margarita; Ayres,
Tom; Basri, Gibor; Brown, Benjamin; Christensen-Dalsgaard, Joergen;
Dupree, Andrea; Guinan, Ed; Jardine, Moira; Miesch, Mark; Pevtsov,
Alexei; Rempel, Matthias; Scherrer, Phil; Solanki, Sami; Strassmeier,
Klaus; Walter, Fred
Bibcode: 2009astro2010S.262S
Altcode:
No abstract at ADS
Title: Nonlinear Force-Free Magnetic Field Modeling of the Solar
Corona: A Critical Assessment
Authors: De Rosa, M. L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; McTiernan, J. M.; Régnier, S.;
Thalmann, J.; Valori, G.; Wheatland, M. S.; Wiegelmann, T.; Cheung,
M.; Conlon, P. A.; Fuhrmann, M.; Inhester, B.; Tadesse, T.
Bibcode: 2008AGUFMSH41A1604D
Altcode:
Nonlinear force-free field (NLFFF) modeling promises to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have so far failed to arrive at consistent solutions
when applied to cases using the highest-available-resolution vector
magnetogram data from Hinode/SOT-SP (in the region of the modeling
area of interest) and line-of-sight magnetograms from SOHO/MDI (where
vector data were not been available). It is our view that the lack of
robust results indicates an endemic problem with the NLFFF modeling
process, and that this process will likely continue to fail until (1)
more of the far-reaching, current-carrying connections are within the
observational field of view, (2) the solution algorithms incorporate
the measurement uncertainties in the vector magnetogram data, and/or
(3) a better way is found to account for the Lorentz forces within
the layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
Title: Developing a Heliophysics Event Knowledgebase for Solar
Dynamics Observatory
Authors: Schrijver, K.; Hurlburt, N.; Mark, C.; Freeland, S.; Green,
S.; Jaffey, A.; Kobashi, A.; Schiff, D.; Seguin, R.; Slater, G.;
Somani, A.; Timmons, R.
Bibcode: 2008AGUFMSM11B1619S
Altcode:
The Solar Dynamics Observatory will generated over 2 petabytes
of imagery in its 5 year mission. In order to improve scientific
productivity and to reduce system requirements , we have developed a
system for data markup to identify -interesting" datasets and direct
scientists to them through an event-based querying system. The SDO
Heliophysics Event Knowledgebase (HEK) will enable caching of commonly
accessed datasets within the Joint Science Operations Center (JSOC) and
reduces the (human) time spent searching for and downloading relevant
data. We present an overview of our HEK including the ingestion of
images, automated and manual tools for identifying and annotation
features within the images, and interfaces and webtools for querying
and accessing events and their associated data. informatcs/hpkb
Title: On the Solar Origins of Open Magnetic Fields in the Heliosphere
Authors: Rust, David M.; Haggerty, Dennis K.; Georgoulis, Manolis K.;
Sheeley, Neil R.; Wang, Yi-Ming; DeRosa, Marc L.; Schrijver, Carolus J.
Bibcode: 2008ApJ...687..635R
Altcode:
A combination of heliospheric and solar data was used to identify open
magnetic fields stretching from the lower corona to Earth orbit. 35
near-relativistic electron beams detected at the ACE spacecraft
"labeled" the heliospheric segments of the open fields. An X-ray
flare occurred <20 minutes before injection of the electrons
in 25 events. These flares labeled the solar segment of the open
fields. The flares occurred in western-hemisphere active regions (ARs)
with coronal holes whose polarity agreed with the polarity of the
beam-carrying interplanetary fields in 23 of the 25 events. We conclude
that electron beams reach 1 AU from open AR fields adjacent to flare
sites. The Wang & Sheeley implementation of the potential-field
source-surface model successfully identified the open fields in
36% of cases. Success meant that the open fields reached the source
surface within 3 heliographic deg of the interplanetary magnetic field
connected to ACE at 1 AU. Inclusion of five near misses improves
the success rate to 56%. The success rate for the Schrijver &
DeRosa PFSS implementation was 50%. Our results suggest that, even
if the input magnetic data are updated frequently, the PFSS models
succeed in only ~50% of cases to identify the coronal segment of open
fields. Development of other techniques is in its infancy.
Title: Magnetic reconnection and particle accelerationinitiated by
flux emergence
Authors: Masson, S.; Aulanier, G.; Pariat, E.; Klein, K. -L.;
Schrijver, C. J.
Bibcode: 2008sf2a.conf..555M
Altcode:
So as to perform an MHD simulation of the evolution of the corona driven
by the evolution of the photosphere, a key aspect is the definition of
the boundary conditions for reaching a good compromise between physical
conditions and numerical constraints. In this work, we focused on the
simulation of a confined flare observed on Nov 16, 2002. As initial
configuration, we considered a uniform temperature corona, with a
magnetic field resulting from a 3D potential field extrapolation
from a SOHO/MDI magnetogram. We prescribed a velocity field at the
photospheric boundary of the domain, so as to mimic the observed flow
pattern associated to a flux emergence. This resulted in a combination
of ``slipping reconnection'' in a halo of QSLs surrounding a 3D null
point, through which a ``fan reconnection'' regime took place. This
simplified approach of flux emergence has successfully reproduced
the main characteristics of the observed flare: the flare ribbons
observed in the EUV with TRACE being due to the chromospheric impact
of particles accelerated along reconnecting field lines, this bimodal
regime could explain both the shapes and dynamics of these ribbons. We
foresee that this kind of modeling should be able to simulate the
evolution of slipping magnetic flux tubes in open configurations,
allowing to predict the spatio-temporal evolution of particle beams
injected into the heliosphere.
Title: The Global Solar Magnetic Field Through a Full Sunspot Cycle:
Observations and Model Results
Authors: Schrijver, Carolus J.; Liu, Yang
Bibcode: 2008SoPh..252...19S
Altcode: 2008SoPh..tmp..141S
Based on 11 years of SOHO/MDI observations from the cycle minimum in
1997 to the next minimum around 2008, we compare observed and modeled
axial dipole moments to better understand the large-scale transport
properties of magnetic flux in the solar photosphere. The absolute
value of the axial dipole moment in 2008 is less than half that in the
corresponding cycle-minimum phase in early 1997, both as measured from
synoptic maps and as computed from an assimilation model based only on
magnetogram data equatorward of 60° in latitude. This is incompatible
with the statistical fluctuations expected from flux-dispersal modeling
developed in earlier work at the level of 7 - 10 σ. We show how this
decreased axial dipole moment can result from an increased strength of
the diverging meridional flow near the Equator, which more effectively
separates the two hemispheres for dispersing magnetic flux. Based on
the combination of this work with earlier long-term simulations of the
solar surface field, we conclude that the flux-transport properties
across the solar surface have changed from preceding cycles to the most
recent one. A plausible candidate for such a change is an increase
of the gradient of the meridional-flow pattern near the Equator so
that the two hemispheres are more effectively separated. The required
profile as a function of latitude is consistent with helioseismic and
cross-correlation measurements made over the past decade.
Title: Solar and Stellar Magnetic Activity
Authors: Schrijver, C. J.; Zwaan, C.
Bibcode: 2008ssma.book.....S
Altcode:
1. Introduction: solar features and terminology; 2. Stellar structure;
3. Solar rotation and meridional flow; 4. Solar magnetic structure;
5. Solar magnetic configurations; 6. Global properties of the solar
magnetic field; 7. The solar dynamo; 8. The solar outer atmosphere;
9. Stellar outer atmospheres; 10. Mechanisms of outer-atmospheric
heating; 11. Activity and stellar properties; 12. Stellar magnetic
phenomena; 13. Activity and rotation on evolutionary time scales;
14. Activity in binary stars; 15. Propositions on stellar dynamos;
Appendix I: unit conversions; Index.
Title: Solar Magnetism and the Solar-stellar Connection
Authors: Schrijver, C.
Bibcode: 2008ESPM...12..1.2S
Altcode:
The solar-stellar connection, the interdisciplinary research of the
properties of stellar magnetic fields and their atmospheric consequences
in Sun and Sun-like stars, has helped us deepen our understanding of
processes as diverse as astrophysical dynamo action, starspots, large
flares and prominences, coronal abundances and magnetic braking. What
are the key questions for which solar physics needs to look towards
the stars? What kinds of developments may we look forward to over the
next decade or two in stellar research that will support understanding
of the Sun's evolving activity?
Title: Preprocessing of Hinode/SOT Vector Magnetograms for Nonlinear
Force-Free Coronal Magnetic Field Modeling
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
Bibcode: 2008ASPC..397..198W
Altcode: 2008arXiv0801.2884W
The solar magnetic field is key to understanding the physical processes
in the solar atmosphere. Nonlinear force-free codes have been shown
to be useful in extrapolating the coronal field from underlying vector
boundary data (for an overview see Schrijver et al. (2006)). However,
we can only measure the magnetic field vector routinely with high
accuracy in the photosphere with, e.g., Hinode/SOT, and unfortunately
these data do not fulfill the force-free consistency condition as
defined by Aly (1989). We must therefore apply some transformations
to these data before nonlinear force-free extrapolation codes can be
legitimately applied. To this end, we have developed a minimization
procedure that uses the measured photospheric field vectors as input
to approximate a more chromospheric like field (The method was dubbed
preprocessing. See Wiegelmann et al. (2006) for details). The procedure
includes force-free consistency integrals and spatial smoothing. The
method has been intensively tested with model active regions (see
Metcalf et al. 2008) and been applied to several ground based vector
magnetogram data before. Here we apply the preprocessing program to
photospheric magnetic field measurements with the Hinode/SOT instrument.
Title: The Dependence of Ephemeral Region Emergence on Local Flux
Imbalance
Authors: Hagenaar, Hermance J.; DeRosa, Marc L.; Schrijver, Carolus J.
Bibcode: 2008ApJ...678..541H
Altcode:
We investigate the distribution and evolution of existing and emerging
magnetic network elements in the quiet-Sun photosphere. The ephemeral
region emergence rate is found to depend primarily on the imbalance of
magnetic flux in the area surrounding its emergence location, such that
the rate of flux emergence is lower within strongly unipolar regions by
at least a factor of 3 relative to flux-balanced quiet Sun. As coronal
holes occur over unipolar regions, this also means that ephemeral
regions occur less frequently there, but we show that this is an
indirect effect—independent of whether the region is located within
an open-field coronal hole or a closed-field quiet region. We discuss
the implications of this finding for near-photospheric dynamo action and
for the coupling between closed coronal and open heliospheric fields.
Title: Non-Linear Force-Free Field Modeling of a Solar Active Region
Around the Time of a Major Flare and Coronal Mass Ejection
Authors: De Rosa, M. L.; Schrijver, C. J.; Metcalf, T. R.; Barnes,
G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann,
T.; Wheatland, M.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
M.; Kusano, K.; Régnier, S.; Thalmann, J.
Bibcode: 2008AGUSMSP31A..06D
Altcode:
Solar flares and coronal mass ejections are associated with rapid
changes in coronal magnetic field connectivity and are powered by
the partial dissipation of electrical currents that run through
the solar corona. A critical unanswered question is whether the
currents involved are induced by the advection along the photosphere
of pre-existing atmospheric magnetic flux, or whether these currents
are associated with newly emergent flux. We address this problem by
applying nonlinear force-free field (NLFFF) modeling to the highest
resolution and quality vector-magnetographic data observed by the
recently launched Hinode satellite on NOAA Active Region 10930 around
the time of a powerful X3.4 flare in December 2006. We compute 14
NLFFF models using 4 different codes having a variety of boundary
conditions. We find that the model fields differ markedly in geometry,
energy content, and force-freeness. We do find agreement of the best-fit
model field with the observed coronal configuration, and argue (1)
that strong electrical currents emerge together with magnetic flux
preceding the flare, (2) that these currents are carried in an ensemble
of thin strands, (3) that the global pattern of these currents and
of field lines are compatible with a large-scale twisted flux rope
topology, and (4) that the ~1032~erg change in energy associated with
the coronal electrical currents suffices to power the flare and its
associated coronal mass ejection. We discuss the relative merits of
these models in a general critique of our present abilities to model
the coronal magnetic field based on surface vector field measurements.
Title: The Atmospheric Imaging Array Feature and Event System (AFES)
for SDO
Authors: Hurlburt, N.; Freeland, S.; Cheung, M.; Schrijver, C.
Bibcode: 2008AGUSMSM21A..07H
Altcode:
The great data volumes involved in Solar Dynamics Observatory impose
the need to have efficient means to access, process and transport
data products that goes beyond basic data discovery. In order to
reduce system requirements and to improve scientific productivity,
we pre-package Ðinterestingî datasets and direct scientists to them
through an event-based querying system. This will enable caching of
commonly accessed datasets within the Joint Science Operations Center
(JSOC) and reduces the (human) time spent searching for and downloading
relevant data. This system leverages the infrastructure developed
for the Hinode Observation System (http://sot.lmsal.com/sot-data)
and incorporates elements of the evolving heliophysics knowledgebase
(http://www.lmsal.com/helio-informatics/hpkb). We present the details
of the AFES including the ingestion of images, automated and manual
tools for identifying and annotation features within the images, and
interfaces and webtools for querying and accessing events and their
associated data. This work has been supported by NASA through contract
NNG04AE00C and Lockheed Martin Research Funds.
Title: Nonlinear Force-free Field Modeling of a Solar Active Region
around the Time of a Major Flare and Coronal Mass Ejection
Authors: Schrijver, C. J.; DeRosa, M. L.; Metcalf, T.; Barnes, G.;
Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.;
Wheatland, M. S.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
M.; Kusano, K.; Régnier, S.; Thalmann, J. K.
Bibcode: 2008ApJ...675.1637S
Altcode: 2007arXiv0712.0023S
Solar flares and coronal mass ejections are associated with rapid
changes in field connectivity and are powered by the partial dissipation
of electrical currents in the solar atmosphere. A critical unanswered
question is whether the currents involved are induced by the motion of
preexisting atmospheric magnetic flux subject to surface plasma flows or
whether these currents are associated with the emergence of flux from
within the solar convective zone. We address this problem by applying
state-of-the-art nonlinear force-free field (NLFFF) modeling to the
highest resolution and quality vector-magnetographic data observed
by the recently launched Hinode satellite on NOAA AR 10930 around
the time of a powerful X3.4 flare. We compute 14 NLFFF models with
four different codes and a variety of boundary conditions. We find
that the model fields differ markedly in geometry, energy content,
and force-freeness. We discuss the relative merits of these models in
a general critique of present abilities to model the coronal magnetic
field based on surface vector field measurements. For our application
in particular, we find a fair agreement of the best-fit model field
with the observed coronal configuration, and argue (1) that strong
electrical currents emerge together with magnetic flux preceding the
flare, (2) that these currents are carried in an ensemble of thin
strands, (3) that the global pattern of these currents and of field
lines are compatible with a large-scale twisted flux rope topology,
and (4) that the ~1032 erg change in energy associated with
the coronal electrical currents suffices to power the flare and its
associated coronal mass ejection.
Title: Can We Improve the Preprocessing of Photospheric Vector
Magnetograms by the Inclusion of Chromospheric Observations?
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
Bibcode: 2008SoPh..247..249W
Altcode: 2008arXiv0801.2707W; 2008SoPh..tmp...27W
The solar magnetic field is key to understanding the physical processes
in the solar atmosphere. Nonlinear force-free codes have been shown to
be useful in extrapolating the coronal field upward from underlying
vector boundary data. However, we can only measure the magnetic
field vector routinely with high accuracy in the photosphere, and
unfortunately these data do not fulfill the force-free condition. We
must therefore apply some transformations to these data before nonlinear
force-free extrapolation codes can be self-consistently applied. To
this end, we have developed a minimization procedure that yields a more
chromosphere-like field, using the measured photospheric field vectors
as input. The procedure includes force-free consistency integrals,
spatial smoothing, and - newly included in the version presented here
- an improved match to the field direction as inferred from fibrils
as can be observed in, for example, chromospheric Hα images. We test
the procedure using a model active-region field that included buoyancy
forces at the photospheric level. The proposed preprocessing method
allows us to approximate the chromospheric vector field to within a few
degrees and the free energy in the coronal field to within one percent.
Title: Nonlinear Force-Free Modeling of Coronal Magnetic
Fields. II. Modeling a Filament Arcade and Simulated Chromospheric
and Photospheric Vector Fields
Authors: Metcalf, Thomas R.; De Rosa, Marc L.; Schrijver, Carolus J.;
Barnes, Graham; van Ballegooijen, Adriaan A.; Wiegelmann, Thomas;
Wheatland, Michael S.; Valori, Gherardo; McTtiernan, James M.
Bibcode: 2008SoPh..247..269M
Altcode: 2008SoPh..tmp...17M
We compare a variety of nonlinear force-free field (NLFFF) extrapolation
algorithms, including optimization, magneto-frictional, and Grad -
Rubin-like codes, applied to a solar-like reference model. The model
used to test the algorithms includes realistic photospheric Lorentz
forces and a complex field including a weakly twisted, right helical
flux bundle. The codes were applied to both forced "photospheric" and
more force-free "chromospheric" vector magnetic field boundary data
derived from the model. When applied to the chromospheric boundary data,
the codes are able to recover the presence of the flux bundle and the
field's free energy, though some details of the field connectivity are
lost. When the codes are applied to the forced photospheric boundary
data, the reference model field is not well recovered, indicating
that the combination of Lorentz forces and small spatial scale
structure at the photosphere severely impact the extrapolation of the
field. Preprocessing of the forced photospheric boundary does improve
the extrapolations considerably for the layers above the chromosphere,
but the extrapolations are sensitive to the details of the numerical
codes and neither the field connectivity nor the free magnetic energy in
the full volume are well recovered. The magnetic virial theorem gives
a rapid measure of the total magnetic energy without extrapolation
though, like the NLFFF codes, it is sensitive to the Lorentz forces in
the coronal volume. Both the magnetic virial theorem and the Wiegelmann
extrapolation, when applied to the preprocessed photospheric boundary,
give a magnetic energy which is nearly equivalent to the value derived
from the chromospheric boundary, but both underestimate the free
energy above the photosphere by at least a factor of two. We discuss
the interpretation of the preprocessed field in this context. When
applying the NLFFF codes to solar data, the problems associated with
Lorentz forces present in the low solar atmosphere must be recognized:
the various codes will not necessarily converge to the correct, or
even the same, solution.
Title: Observations and Modeling of the Early Acceleration Phase of
Erupting Filaments Involved in Coronal Mass Ejections
Authors: Schrijver, Carolus J.; Elmore, Christopher; Kliem, Bernhard;
Török, Tibor; Title, Alan M.
Bibcode: 2008ApJ...674..586S
Altcode: 2007arXiv0710.1609S
We examine the early phases of two near-limb filament destabilizations
involved in coronal mass ejections (CMEs) on 2005 June 16 and July
27, using high-resolution, high-cadence observations made with the
Transition Region and Coronal Explorer (TRACE), complemented by
coronagraphic observations by the Mauna Loa Solar Observatory (MLSO)
and the Solar and Heliospheric Observatory (SOHO). The filaments'
heights above the solar limb in their rapid-acceleration phases are
best characterized by a height dependence h(t) propto tm
with m near, or slightly above, 3 for both events. Such profiles are
incompatible with published results for breakout, MHD-instability,
and catastrophe models. We show numerical simulations of the
torus instability that approximate this height evolution in case a
substantial initial velocity perturbation is applied to the developing
instability. We argue that the sensitivity of magnetic instabilities
to initial and boundary conditions requires higher fidelity modeling of
all proposed mechanisms if observations of rise profiles are to be used
to differentiate between them. The observations show no significant
delays between the motions of the filament and of overlying loops:
the filaments seem to move as part of the overall coronal field until
several minutes after the onset of the rapid-acceleration phase.
Title: Ephemeral Bipolar Regions in Coronal Holes
Authors: Hagenaar, H.; Schrijver, C.; De Rosa, M.
Bibcode: 2008ASPC..383..343H
Altcode:
We investigate the distribution and evolution of magnetic network
elements in quiet Sun with or without coronal holes. Ephemeral region
emergence rates are found to depend on the degree of imbalance of
magnetic flux, but independent of whether there is a coronal hole or
not. We discuss the implications of this finding for near-photospheric
dynamo action and for the coupling between closed coronal and open
heliospheric fields.
Title: The Stellar Imager
Authors: Carpenter, Kenneth G.; Schrijver, Carolus J.; Karovska,
Margarita
Bibcode: 2008nssv.book..191C
Altcode:
No abstract at ADS
Title: Magnetic Field Patterns In Major Flares And Flare Forecasting
Authors: Schrijver, Carolus
Bibcode: 2008cosp...37.2784S
Altcode: 2008cosp.meet.2784S
Solar flares are powered by magnetic reconnection in the solar
atmosphere. The processes by which the energy that is released in
these flares is injected into the solar atmosphere continue to be
debated and researched. Ensemble studies of hundreds of major solar
flares that have been observed over the years with instruments like
SOHO and TRACE suggest that much of that free energy is injected in the
form of electrical currents that emerge from within the Sun. Detailed
studies of high-resolution vector magnetograms observed by, e.g.,
Hinode's Solar Optical Telescope, support that finding. This talk will
review these and other ideas concerning the powering of solar flares,
and test our growing understanding against our ability to forecast
the times of occurrence and the magnitudes of flares: can we learn to
specify flare magnitudes by metrics based on polarity-inversion lines,
field gradients and geometry, or other magnetogrambased indicators?
Title: Chromospheric Alfvénic Waves Strong Enough to Power the
Solar Wind
Authors: De Pontieu, B.; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Tarbell, T. D.; Schrijver, C. J.; Title, A. M.; Shine, R. A.;
Tsuneta, S.; Katsukawa, Y.; Ichimoto, K.; Suematsu, Y.; Shimizu, T.;
Nagata, S.
Bibcode: 2007Sci...318.1574D
Altcode:
Alfvén waves have been invoked as a possible mechanism for the heating
of the Sun's outer atmosphere, or corona, to millions of degrees and
for the acceleration of the solar wind to hundreds of kilometers per
second. However, Alfvén waves of sufficient strength have not been
unambiguously observed in the solar atmosphere. We used images of
high temporal and spatial resolution obtained with the Solar Optical
Telescope onboard the Japanese Hinode satellite to reveal that the
chromosphere, the region sandwiched between the solar surface and
the corona, is permeated by Alfvén waves with strong amplitudes on
the order of 10 to 25 kilometers per second and periods of 100 to
500 seconds. Estimates of the energy flux carried by these waves and
comparisons with advanced radiative magnetohydrodynamic simulations
indicate that such Alfvén waves are energetic enough to accelerate
the solar wind and possibly to heat the quiet corona.
Title: Can we Improve the Preprocessing of Photospheric
Vectormagnetograms by the Inclusion of Chromospheric Observations?
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
Bibcode: 2007AGUFMSH51C..02W
Altcode:
The solar magnetic field is key to understanding the physical
processes in the solar atmosphere. Unfortunately, we can measure
the magnetic field vector routinely with high accuracy only in the
photosphere with, e.g., Hinode/SOT and in future with SDO/HMI. These
measurements are extrapolated into the corona under the assumption
that the field is force-free. That condition is not fulfilled in the
photosphere, but is in the chromosphere and corona. In order to make
the observed boundary data consistent with the force-free assumption,
we therefore have to apply some transformations before nonlinear
force-free extrapolation codes can be legitimately applied. We develop
a minimization procedure that uses the measured photospheric field
vectors as input to approximate a more chromospheric-like field. The
procedure includes force-free consistency integrals, spatial smoothing,
and - newly included in the version presented here - an improved match
to the field direction as inferred from fibrils as can be observed in,
e.g., chromospheric H-alpha images. We test the procedure using a model
active-region field that included buoyancy forces at the photospheric
level. We apply the combined preprocessing and nonlinear force-free
extrapolation method to compute the coronal magnetic field in an active
region measured with the Hinode/SOT instrument.
Title: On Connecting the Dynamics of the Chromosphere and Transition
Region with Hinode SOT and EIS
Authors: Hansteen, Viggo H.; de Pontieu, Bart; Carlsson, Mats;
McIntosh, Scott; Watanabe, Tetsuya; Warren, Harry P.; Harra, Louise K.;
Hara, Hirohisa; Tarbell, Theodore D.; Shine, Dick; Title, Alan M.;
Schrijver, Carolus J.; Tsuneta, Saku; Katsukawa, Yukio; Ichimoto,
Kiyoshi; Suematsu, Yoshinori; Shimizu, Toshifumi
Bibcode: 2007PASJ...59S.699H
Altcode: 2007arXiv0711.0487H
We use coordinated Hinode SOT/EIS observations that include
high-resolution magnetograms, chromospheric, and transition region
(TR) imaging, and TR/coronal spectra in a first test to study how
the dynamics of the TR are driven by the highly dynamic photospheric
magnetic fields and the ubiquitous chromospheric waves. Initial
analysis shows that these connections are quite subtle and require a
combination of techniques including magnetic field extrapolations,
frequency-filtered time-series, and comparisons with synthetic
chromospheric and TR images from advanced 3D numerical simulations. As a
first result, we find signatures of magnetic flux emergence as well as
3 and 5mHz wave power above regions of enhanced photospheric magnetic
field in both chromospheric, transition region, and coronal emission.
Title: A Tale of Two Spicules: The Impact of Spicules on the Magnetic
Chromosphere
Authors: de Pontieu, Bart; McIntosh, Scott; Hansteen, Viggo H.;
Carlsson, Mats; Schrijver, Carolus J.; Tarbell, Theodore D.; Title,
Alan M.; Shine, Richard A.; Suematsu, Yoshinori; Tsuneta, Saku;
Katsukawa, Yukio; Ichimoto, Kiyoshi; Shimizu, Toshifumi; Nagata,
Shin'ichi
Bibcode: 2007PASJ...59S.655D
Altcode: 2007arXiv0710.2934D
We use high-resolution observations of the Sun in CaIIH (3968Å)
from the Solar Optical Telescope on Hinode to show that there are
at least two types of spicules that dominate the structure of the
magnetic solar chromosphere. Both types are tied to the relentless
magnetoconvective driving in the photosphere, but have very different
dynamic properties. ``Type-I'' spicules are driven by shock waves
that form when global oscillations and convective flows leak into
the upper atmosphere along magnetic field lines on 3--7minute
timescales. ``Type-II'' spicules are much more dynamic: they form
rapidly (in ∼ 10s), are very thin (≤ 200 km wide), have lifetimes
of 10-150s (at any one height), and seem to be rapidly heated to
(at least) transition region temperatures, sending material through
the chromosphere at speeds of order 50--150kms-1. The
properties of Type II spicules suggest a formation process that is
a consequence of magnetic reconnection, typically in the vicinity
of magnetic flux concentrations in plage and network. Both types of
spicules are observed to carry Alfvén waves with significant amplitudes
of order 20kms-1.
Title: Direct UV/optical imaging of stellar surfaces: the Stellar
Imager Vision Mission
Authors: Carpenter, Kenneth G.; Lyon, Richard G.; Schrijver, Carolus;
Karovska, Margarita; Mozurkewich, David
Bibcode: 2007SPIE.6687E..0GC
Altcode: 2007SPIE.6687E..15C
The Stellar Imager (SI) is a UV/optical, space-based interferometer
designed to enable 0.1 milli-arcsecond (mas) spectral imaging of
stellar surfaces and, via asteroseismology, stellar interiors and of
the Universe in general. SI's science focuses on the role of magnetism
in the Universe, particularly on magnetic activity on the surfaces of
stars like the Sun. SI's prime goal is to enable long-term forecasting
of solar activity and the space weather that it drives, in support of
the Living with a Star program in the Exploration Era. SI will also
revolutionize our understanding of the formation of planetary systems,
of the habitability and climatology of distant planets, and of many
magneto-hydrodynamically controlled processes in the Universe. SI
is a "Flagship and Landmark Discovery Mission" in the 2005 Sun Solar
System Connection (SSSC) Roadmap and a candidate for a "Pathways to
Life Observatory" in the Exploration of the Universe Division (EUD)
Roadmap (May, 2005). We discuss herein the science goals of the SI
Mission, a mission architecture that could meet those goals, and the
technologies needed to enable this mission. Additional information on
SI can be found at: http://hires.gsfc.nasa.gov/si/.
Title: Dynamos, Asteroseismology, and the Stellar Imager
Authors: Schrijver, C. J.; Carpenter, K. G.; Karovska, M.
Bibcode: 2007CoAst.150..364S
Altcode:
The ultra-sharp images of the Stellar Imager (SI) will revolutionize
our view of many dynamic astrophysical processes: The 0.1 milli-arcsec
resolution of this deep-space telescope will transform point sources
into extended sources, and simple snapshots into spellbinding evolving
views. SI's science focuses on the role of magnetism in the Universe,
particularly on magnetic activity on the surfaces of stars like the Sun
and on the subsurface flows that drive this activity. SI's prime goal
is to image magnetically active stars with enough resolution to map
their evolving dynamo patterns and their internal flows. By exploring
the Universe at ultra-high resolution, SI will also revolutionize our
understanding of the formation of planetary systems, of the habitability
and climatology of Earth as well as distant exoplanets, and of many
magneto-hydrodynamically controlled structures and processes in the
Universe. See http://hires.gsfc.nasa.gov/si/ for details on the
Stellar Imager mission.
Title: Braiding-induced Interchange Reconnection of the Magnetic
Field and the Width of Solar Coronal Loops
Authors: Schrijver, Carolus J.
Bibcode: 2007ApJ...662L.119S
Altcode:
The random walk of the footpoints of coronal loops in the rapidly
evolving solar granulation is expected to cause braiding of the field,
which in turn should lead to a multitude of coronal reconnection
events. Consequently, plasma volumes in coronal loops should repeatedly
be spliced onto parts of other such loop structures within their
lifetime in the corona. Both heat and plasma can thereby in effect
be dispersed perpendicular to the field's guide direction, leading to
an increase of loop width with time. I show that observed loop cross
sections are consistent with this process for a reconnection-induced
dispersal coefficient of 93+/-9 km2 s-1, which
equals the dispersal coefficient that characterizes the granular random
walk up to several hours. Loop width observations thus offer support
for the hypothesis that granular braiding is countered statistically
by frequent coronal reconnections, which in turn explains the general
absence of entangled coronal field structures in high-resolution
observations of the quiescent solar corona. This finding suggests that
reconnection-enabled cross-field plasma dispersal needs to be included
in models of coronal loop atmospheres.
Title: Helio-informatics: Preparing For The Future Of Heliophysics
Research.
Authors: Schrijver, Carolus J.; Hurlburt, N. E.; Cheung, M. C.; Title,
A. M.; Delouille, V.; Hochedez, J.; Berghmans, D.
Bibcode: 2007AAS...210.2514S
Altcode: 2007BAAS...39..133S
The rapidly growing data volumes for space- and ground-based
observatories for Sun and heliosphere will soon make it impractical,
costly, and perhaps effectively impossible for researchers to download
and locally inspect substantial portions of the data archives. By
the end of 2008, for example, the Solar Dynamics Observatory will
downlink over 2TB/day of compressed data; such a large volume would
readily saturate internet connections to the archive site if it were
exported to a handful of researchers around the world. We envision a
revolution in research methodology towards a mode in which researchers
run autonomous event-finding algorithms at a primary data archive in
order to pre-select relatively small subsets of the data that can
subsequently be inspected and analyzed in detail at a researcher's
home institution. Teams from the SDO, Hinode, STEREO, and TRACE
missions are developing the infrastructure that is needed to make this
into a useful research tool: we are (1) defining standardized event
attributes compatible with the Virtual Observatory and EGSO concepts,
(2) developing a knowledge base supported by a web-based tool for
compound queries based on the contents of solar and heliospheric
observations, and (3) assembling a group of researchers who are
interested in helping us develop a prototype system while beta-testing
it in real scientific studies. We invite you to contact us (a) if you
have feature-finding algorithms that you would like to see applied to
existing data archives, (b) if you would like to contribute expertise
in developing the knowledge-base system, or (c) if you would like
to participate in the testing of the system for scientific use. More
information on our plans, target dates, and contact information can
be found at http://www.lmsal.com/helio-informatics/hpkb/. The
helio-informatics project is being developed with support from
the HINODE/SOT (NNM07AA01C), SDO/AIA (NNG04EA00C), STEREO/SECCHI
(N00173-02-C-2035), and TRACE (NAS5-38099) science investigations.
Title: Emerging And Rotating Magnetic Flux Structures Associated
With Solar Eruptions In AR10930 As Observed By Hinode/SOT, TRACE,
And SOHO/MDI
Authors: Nightingale, Richard W.; Schrijver, C. J.; Frank, Z. A.
Bibcode: 2007AAS...210.9423N
Altcode: 2007BAAS...39..221N
During the period of December 6-14, 2006 AR10930 produced 4 X-flares
in what was otherwise a quiet solar minimum. The multiple flaring
appears to be associated with small emerging and rotating magnetic flux
structures around a larger stable sunspot. The eruptions were observed
by several solar instruments, including the Solar Optical Telescope
(SOT) on the recently launched Hinode Spacecraft, as well as TRACE
and SOHO/MDI. The high resolution of SOT highlights the rotation of
the small emerging flux regions, which can carry magnetic energy and
helicity up into the solar atmosphere from the interior of the sun
to help facilatate the eruptions. Images at several wavelengths from
the suite of observational instrumentation mentioned above will be
utilized to illustrate these events. This work was supported by
NASA in part under the TRACE contract NAS5-38099 and in part under
the Hinode/SOT contract NNM07AA01C.
Title: Observational Evidence For The Ubiquity Of Strong Alfven
Waves In The Magnetized Chromosphere
Authors: De Pontieu, Bart; McIntosh, S. W.; Carlsson, M.; Hansteen,
V. H.; Schrijver, C. J.; Tarbell, T. D.; Title, A.; SOT Team
Bibcode: 2007AAS...210.9415D
Altcode: 2007BAAS...39R.219D
Hinode/SOT Ca II broadband observations show that Alfven waves with
significant amplitudes of order 10-20 km/s and periods of 150-300 s are
extremely ubiquitous in the magnetized middle to upper chromosphere. Our
observations focus on spicules at the limb, and straw-like features
associated with network and plage on the disk. We find that the
weak straw-like features and most spicules all undergo significant
transverse motions that are driven by Alfven waves. These waves are
seen to propagate both up- and downward, and may carry an energy flux
that is significant compared to both the local, coronal and solar wind
energy balance. We will provide estimates of the energy flux carried
by these waves, and will compare our observations with Alfven waves
that are observed in 3D numerical simulations that include advanced
radiative transfer treatment for the chromosphere. This work was
supported by NASA contract NNM07AA01C.
Title: A Tale of Two Spicules
Authors: McIntosh, Scott W.; De Pontieu, B.; Carlsson, M.; Hansteen,
V. H.; Schrijver, C. J.; Tarbell, T. D.; Title, A. M.; SOT Team
Bibcode: 2007AAS...210.9414M
Altcode: 2007BAAS...39..219M
Hinode/SOT Ca II broadband images and movies show that there are
several different types of spicules at the limb. These different
types are distinguished by dynamics on different timescales. The
first type involves up- and downward motion on timescales of 3-5
minutes. The dynamics of these spicules are very similar to those of
fibrils and mottles as observed on the disk. Recent work suggests that
these are driven by slow-mode magnetoacoustic shocks that form when
convective flows and global oscillations leak into the chromosphere
along magnetic flux tubes. The second type is much more dynamic with
typical lifetimes of 10-60 s. These spicules are characterized by
sudden appearance and disappearance that may be indicative of rapid
heating to TR temperatures. We will describe the properties of these
spicules in various magnetic environments (coronal hole, quiet Sun,
active region) and study the possible role of reconnection in driving
the second type of spicules. In addition, we will perform detailed
comparisons of these different types of jets with synthetic Ca images
derived from advanced 3D numerical simulations that encompass the
convection zone up through the corona.
Title: Non-linear Force-free Modeling Of Coronal Magnetic Fields
Authors: Metcalf, Thomas R.; De Rosa, M. L.; Schrijver, C. J.; Barnes,
G.; van Ballegooijen, A.; Wiegelmann, T.; Wheatland, M. S.; Valori,
G.; McTiernan, J. M.
Bibcode: 2007AAS...210.9102M
Altcode: 2007BAAS...39..204M
We compare a variety of nonlinear force-free field (NLFFF)
extrapolation algorithms, including optimization, magneto-frictional,
and Grad-Rubin-like codes, applied to a solar-like reference
model. The model used to test the algorithms includes realistic
photospheric Lorentz forces and a complex field including a weakly
twisted, right helical flux bundle. The codes were applied to both
forced "photospheric'' and more force-free "chromospheric'' vector
magnetic field boundary data derived from the model. When applied to
the chromospheric boundary data, the codes are able to recover
the presence of the flux bundle and the field's free energy, though
some details of the field connectivity are lost. When the codes are
applied to the forced photospheric boundary data, the reference
model field is not well recovered, indicating that the Lorentz
forces on the photosphere severely impact the extrapolation of the
field. Preprocessing of the photospheric boundary does improve the
extrapolations considerably, although the results depend sensitively
on the details of the numerical codes. When applying the NLFFF codes
to solar data, the problems associated with Lorentz forces present in
the low solar atmosphere must be recognized: the various codes will
not necessarily converge to the correct, or even the same, solution.
Title: Commission 10: Solar Activity
Authors: Melrose, Donald B.; Klimchuk, James A.; Benz, A. O.; Craig,
I. J. D.; Gopalswamy, N.; Harrison, R. A.; Kozlovsky, B. Z.; Poletto,
G.; Schrijver, K. J.; van Driel-Gesztelyi, L.; Wang, J. -X.
Bibcode: 2007IAUTA..26...75M
Altcode:
Commission 10 aims at the study of various forms of solar activity,
including networks, plages, pores, spots, fibrils, surges, jets,
filaments/prominences, coronal loops, flares, coronal mass ejections
(CMEs), solar cycle, microflares, nanoflares, coronal heating etc.,
which are all manifestation of the interplay of magnetic fields and
solar plasma. Increasingly important is the study of solar activities
as sources of various disturbances in the interplanetary space
and near-Earth "space weather".Over the past three years a major
component of research on the active Sun has involved data from the
RHESSI spacecraft. This review starts with an update on current and
planned solar observations from spacecraft. The discussion of solar
flares gives emphasis to new results from RHESSI, along with updates on
other aspects of flares. Recent progress on two theoretical concepts,
magnetic reconnection and magnetic helicity is then summarized, followed
by discussions of coronal loops and heating, the magnetic carpet
and filaments. The final topic discussed is coronal mass ejections
and space weather.The discussions on each topic is relatively brief,
and intended as an outline to put the extensive list of references
in context.The review was prepared jointly by the members of the
Organizing Committee, and the names of the primary contributors to
the various sections are indicated in parentheses.
Title: A Characteristic Magnetic Field Pattern Associated with All
Major Solar Flares and Its Use in Flare Forecasting
Authors: Schrijver, Carolus J.
Bibcode: 2007ApJ...655L.117S
Altcode:
Solar flares result from some electromagnetic instability that occurs
within regions of relatively strong magnetic field in the Sun's
atmosphere. The processes that enable and trigger these flares remain
topics of intense study and debate. I analyze observations of 289 X-
and M-class flares and over 2500 active region magnetograms to discover
(1) that large flares, without exception, are associated with pronounced
high-gradient polarity-separation lines, while (2) the free energy
that emerges with these fibrils is converted into flare energy in a
broad spectrum of flare magnitudes that may well be selected at random
from a power-law distribution up to a maximum value. This maximum is
proportional to the total unsigned flux R within ~15 Mm of strong-field,
high-gradient polarity-separation lines, which are a characteristic
appearance of magnetic fibrils carrying electrical currents as they
emerge through the photosphere. Measurement of R is readily automated,
and R can therefore be used effectively for flare forecasting. The
probability for major flares to occur within 24 hr of the measurement
of R approaches unity for active regions with the highest values of R
around 2×1021 Mx. For regions with R<~1019
Mx, no M- or X-class flares occur within a day.
Title: Consequences of large-scale flows around active regions on
the dispersal of magnetic field across the solar surface
Authors: De Rosa, M. L.; Schrijver, C. J.
Bibcode: 2006ESASP.624E..12D
Altcode: 2006soho...18E..12D
No abstract at ADS
Title: Gamma Rays and the Evolving, Compact Structures of the 2003
October 28 X17 Flare
Authors: Schrijver, C. J.; Hudson, H. S.; Murphy, R. J.; Share, G. H.;
Tarbell, T. D.
Bibcode: 2006ApJ...650.1184S
Altcode:
The X17 flare on 2003 October 28 was observed by high-resolution
imaging or spectroscopic instruments on CORONAS, GOES, INTEGRAL,
RHESSI, SOHO, and TRACE. These spacecraft observed the temporal
evolution of the γ-ray positron-annihilation and nuclear de-excitation
line spectra, imaged the hard X-ray bremsstrahlung and EUV and UV
emission, and measured the surface magnetic field and subphotospheric
pressure perturbations. In the usual pattern, the onset of the flare
is dominated by particle acceleration and interaction, and by the
filling of coronal magnetic structures with hot plasma. The associated
positron-annihilation signatures early in the impulsive phase from
11:06 to 11:16 UT have a line-broadening temperature characteristic of
a few hundred thousand kelvins. The most intense precipitation sites
within the extended flare ribbons are very compact, with diameters
of less than 1400 km, and a 195 Å TRACE intensity that can exceed
7500 times the quiescent active-region value. These regions appear to
move at speeds of up to 60 km s-1. The associated rapidly
evolving, compact perturbations of the photosphere below these sites
excite acoustic pulses that propagate into the solar interior. Less
intense precipitation sites typically persist for several minutes
behind the advancing flare ribbons. After ~1 ks, the flare enters
a second phase, dominated by coronal plasma cooling and downflows
and by annihilation-line radiation characteristic of a photospheric
environment. We point out (1) that these detailed observations
underscore that flare models need to explicitly incorporate the
multitude of successively excited environments whose evolving signals
differ at least in their temporal offsets and energy budgets, if not
also in the exciting particle populations and penetration depths, and
(2) that the spectral signatures of the positron annihilation do not
fit conventional model assumptions.
Title: On the Long-Term Future of Solar and Heliospheric Physics
Authors: Schrijver, C. J.
Bibcode: 2006ESASP.617E..33S
Altcode: 2006soho...17E..33S
No abstract at ADS
Title: Non-linear Force-free Modeling: Applications To Solar Data
Authors: De Rosa, Marc L.; Schrijver, C. J.; Metcalf, T. R.; NLFFF Team
Bibcode: 2006SPD....37.1805D
Altcode: 2006BAAS...38..247D
Understanding the conditions under which solar magnetic fields can
destabilizeto cause flares and other eruptive events requires a
quantitativeunderstanding of the coronal magnetic field and of the
currents that itcarries. Because no direct measurements of magnetic
fields and current withincoronal volumes exist, the coronal field is
typically modeled usinginformation contained in photospheric vector
magnetograms, to be compared toH-alpha images of the chromosphere
and EUV and X-ray imagery of the corona.We report on recent results
of a team effort to further understand theintricacies of non-linear
force-free extrapolations of the coronal magneticfield, presenting
results from several solar and solar-like test cases. Wealso consider
the use of such coronal field modeling in the upcoming Solar-Band
SDO missions.
Title: The Stellar Imager (SI) vision mission
Authors: Carpenter, Kenneth G.; Schrijver, Carolus J.; Karovska,
Margarita
Bibcode: 2006SPIE.6268E..21C
Altcode: 2006astro.ph..6411C; 2006SPIE.6268E..63C
The Stellar Imager (SI) is a UV-Optical, Space-Based Interferometer
designed to enable 0.1 milli-arcsecond (mas) spectral imaging of
stellar surfaces and of the Universe in general and asteroseismic
imaging of stellar interiors. SI is identified as a "Flagship and
Landmark Discovery Mission" in the 2005 Sun Solar System Connection
(SSSC) Roadmap and as a candidate for a "Pathways to Life Observatory"
in the Exploration of the Universe Division (EUD) Roadmap (May,
2005). SI will revolutionize our view of many dynamic astrophysical
processes: its resolution will transform point sources into extended
sources, and snapshots into evolving views. SI's science focuses
on the role of magnetism in the Universe, particularly on magnetic
activity on the surfaces of stars like the Sun. SI's prime goal is to
enable long-term forecasting of solar activity and the space weather
that it drives. SI will also revolutionize our understanding of the
formation of planetary systems, of the habitability and climatology
of distant planets, and of many magneto-hydrodynamically controlled
processes in the Universe. The results of the SI "Vision Mission"
Study are presented in this paper. Additional information on the SI
mission concept and related technology development can be found at URL:
http://hires.gsfc.nasa.gov/si/.
Title: The Consequences Of Active-region Inflows On The Large-scale
Dispersal Of Magnetic Field Across The Solar Surface.
Authors: Schrijver, Carolus J.; De Rosa, M. L.; Hurlburt, N. E.
Bibcode: 2006SPD....37.0716S
Altcode: 2006BAAS...38..230S
Helioseismic analysis of near-surface modes recently revealed horizontal
flows near the solar surface towards regions with enhanced magnetic
activity. The magnitude of these flows appears to increase with the
magnetic flux contained within them. Such flows help to confine magnetic
flux to the activity belt and perhaps even to theactive regions within
which the field emerges, and will likely slow the random-walk dispersal
of the field. We report on experiments witha surface flux dispersal
model to study the consequences of such inflows towards strong-flux
regions. We constrain the flow magnitudeby comparing results of a flux
assimilation model to solar observations over six-month intervals
throughout the last solar cycle. The best-fit model is then used to
quantify the effects of these flows on the Sun's global dipole and
quadrupole fields on time scales of multiple centuries.
Title: Nonlinear Force-Free Modeling of Coronal Magnetic Fields Part
I: A Quantitative Comparison of Methods
Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Metcalf, Thomas R.;
Liu, Yang; McTiernan, Jim; Régnier, Stéphane; Valori, Gherardo;
Wheatland, Michael S.; Wiegelmann, Thomas
Bibcode: 2006SoPh..235..161S
Altcode:
We compare six algorithms for the computation of nonlinear force-free
(NLFF) magnetic fields (including optimization, magnetofrictional,
Grad-Rubin based, and Green's function-based methods) by evaluating
their performance in blind tests on analytical force-free-field models
for which boundary conditions are specified either for the entire
surface area of a cubic volume or for an extended lower boundary
only. Figures of merit are used to compare the input vector field to
the resulting model fields. Based on these merit functions, we argue
that all algorithms yield NLFF fields that agree best with the input
field in the lower central region of the volume, where the field and
electrical currents are strongest and the effects of boundary conditions
weakest. The NLFF vector fields in the outer domains of the volume
depend sensitively on the details of the specified boundary conditions;
best agreement is found if the field outside of the model volume is
incorporated as part of the model boundary, either as potential field
boundaries on the side and top surfaces, or as a potential field in
a skirt around the main volume of interest. For input field (B) and
modeled field (b), the best method included in our study yields an
average relative vector error En = « |B−b|»/« |B|» of
only 0.02 when all sides are specified and 0.14 for the case where only
the lower boundary is specified, while the total energy in the magnetic
field is approximated to within 2%. The models converge towards the
central, strong input field at speeds that differ by a factor of one
million per iteration step. The fastest-converging, best-performing
model for these analytical test cases is the Wheatland, Sturrock, and
Roumeliotis (2000) optimization algorithm as implemented by Wiegelmann
(2004).
Title: The Science Behind the Solar Corona
Authors: Schrijver, Carolus J.
Bibcode: 2006S&T...111d..28S
Altcode:
No abstract at ADS
Title: The Atmospheric Imaging Assembly on the Solar Dynamics
Observatory
Authors: Title, A. M.; Hoeksema, J. T.; Schrijver, C. J.; Aia Team
Bibcode: 2006cosp...36.2600T
Altcode: 2006cosp.meet.2600T
The Atmospheric Imaging Assembly AIA on SDO will provide revolutionary
coverage of the entire visible solar hemisphere observed from
photospheric to coronal temperatures at 1-arcsecond resolution with a
characteristic cadence of 10 seconds for each channel The AIA comprises
four dual normal-incidence telescopes that enable it to cycle through
a set of EUV channels centered on strong emission lines of iron ranging
from Fe IX through XXIII and helium 304A plus two UV channels near 1600A
and a broad band visible channel Combined with the vector- magnetic
imagery from SDO HMI the AIA observations will significantly further
our understanding of the dynamics of the magnetic field in the solar
atmosphere and heliosphere both in quiescent and eruptive stages The
comprehensive thermal coverage of the corona will open new avenues of
study for coronal energetics and seismology which will benefit from
the excellent calibration against the SDO EVE spectral irradiance
measurements The AIA data will be easily accessible on the web with
a time delay that is expected to be of the order of 15 minutes to 1
hour Users will be able to browse the data through summary web pages
that are complemented by a comprehensive metadata catalog Data analysis
will be supported through the freely available SolarSoft libraries and
through modules in a flexible evolving pipeline data-analysis system
to be operated at the AIA-HMI Joint Science Operations Center We plan
to incorporate feature recognition software automated movie making
coronal field modeling
Title: SI - The Stellar Imager: Results from the Vision Mission Study
Authors: Carpenter, K. G.; Schrijver, C. J.; Karovska, M.; SI Vision
Mission Study Team
Bibcode: 2005AAS...207.2307C
Altcode: 2005BAAS...37R1196C
The Stellar Imager (SI) is a UV-Optical, Space-Based Interferometer
designed to enable 0.1 milli-arcsecond (mas) spectral imaging of
stellar surfaces and stellar interiors (via asteroseismology) and of
the Universe in general. SI was included as a "far horizon" or "Vision
Mission" in the 2000 and 2003 SEC Roadmaps and is now identified as
a "Flagship and Landmark Discovery Mission" in the 2005 Sun Solar
System Connection (SSSC) Roadmap and as a candidate for a "Pathways
to Life Observatory" in the Exploration of the Universe Division (EUD)
Roadmap (May, 2005). The ultra-sharp images of the Stellar Imager will
revolutionize our view of many dynamic astrophysical processes: The
0.1 mas resolution of this deep-space telescope will transform point
sources into extended sources, and snapshots into evolving views. SI's
science focuses on the role of magnetism in the Universe, particularly
on magnetic activity on the surfaces of stars like the Sun. SI's prime
goal is to enable long-term forecasting of solar activity and the space
weather that it drives in support of the Living With a Star program in
the Exploration Era. SI will also revolutionize our understanding of
the formation of planetary systems, of the habitability and climatology
of distant planets, and of many magneto-hydrodynamically controlled
processes in the Universe. The results of a just-concluded "Vision
Mission" Study of Stellar Imager will be presented in this paper. This work was supported, in part, by Vision Mission Study grants
from NASA HQ to NASA-GSFC and from GSFC to Smithsonian Astrophysical
Observatory, Seabrook Engineering, SUNY/Stonybrook, U. Colorado/Boulder,
and STScI. Substantial complementary internal institutional support
is gratefully acknowledged from all of the participating institutions.
Title: Small-Scale Magnetic Activity
Authors: Schrijver, C. J.
Bibcode: 2005ESASP.596E..32S
Altcode: 2005ccmf.confE..32S
No abstract at ADS
Title: Is the Quiet-Sun Corona a Quasi-steady, Force-free Environment?
Authors: Schrijver, Carolus J.; van Ballegooijen, Adriaan A.
Bibcode: 2005ApJ...630..552S
Altcode:
We model a coronal volume over a quiet, mixed-polarity solar network as
an ensemble of quasi-steady loop atmospheres. These are contained by an
assumed potential field, including the associated variations in the loop
cross section through the coronal volume and the loop flows induced by
such asymmetries. The average temperature and density stratifications
are close to those of the quiet-Sun corona for a coronal heating flux
density into the corona of FH=8×1014B/L (ergs
cm-2 s-1) for loop-base field strengths B (G)
and loop half-lengths L (cm). Earlier, that heating parameterization
was shown to be consistent with the appearance and radiative losses of a
solar corona in which active regions dominated the emission. This study
thus supports the hypothesis that the same, likely braiding-driven,
heating dominates throughout the quiescent corona. The average ratio
β of gas to magnetic pressure lies close to unity throughout the
modeled coronal height range of 22 Mm, with β>1 in ~30% of the
volume and β>0.4 in ~90% of the volume, perhaps indicating that
the quiet-Sun corona is driven to near its maximum heating capacity by
the random walk of its footpoints. Our findings that the solar corona
has β close to unity, and that our model corona exhibits insufficient
fine structure and no significant spatially averaged Doppler shifts,
imply that the quiet-Sun corona is often neither quasi-steady nor
force free and thus that dynamic magnetohydrodynamics (MHD) models
are essential to furthering our understanding of the quiet solar corona.
Title: The Magnetic Connection between the Sun and the Heliosphere
(Tutorial Talk)
Authors: Schrijver, C. J.
Bibcode: 2005ESASP.592..213S
Altcode: 2005soho...16E..33S; 2005ESASP.592E..33S
No abstract at ADS
Title: The Nonpotentiality of Active-Region Coronae and the Dynamics
of the Photospheric Magnetic Field
Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Title, Alan M.;
Metcalf, Thomas R.
Bibcode: 2005ApJ...628..501S
Altcode:
The magnetic field in the solar photosphere frequently carries strong
electric currents, even though the global coronal configuration often
resembles a potential field ringed by the heliospheric current sheet. To
understand this, we compare TRACE EUV images of active-region coronae
and potential-field source-surface extrapolations based on SOHO MDI
magnetograms for 95 active regions. We conclude that significant
nonpotentiality of the overall active-region coronal field occurs
(1) when new flux has emerged within or very near a region within
the last ~30 hr, resulting in complex polarity separation lines, or
(2) when rapidly evolving, opposite-polarity concentrations are in
contact at 4" resolution. If these criteria are met by more than 15%
of the region's flux, they correctly identify the (non) potentiality of
active-region coronae in 88% of the cases. Flares are found to occur
2.4 times more frequently in active regions with nonpotential coronae
than in near-potential regions, while their average X-ray peak flare
brightness is 3.3 times higher. We suggest that the currents associated
with coronal nonpotentiality have a characteristic growth and decay
timescale of ~10-30 hr. We find that shear flows drive enhanced flaring
or coronal nonpotentiality only if associated with complex and dynamic
flux emergence within the above timescale. We discuss the implications
of this finding for the modeling of the coronal-heliospheric coupling.
Title: VITRUV - Science Cases
Authors: Garcia, Paulo J. V.; Berger, Jean-Phillipe; Corradi, Romano;
Forveille, Thierry; Harries, Tim; Henri, Gilles; Malbet, Fabien;
Marconi, Alessandro; Perraut, Karine; Petrucci, Pierre-Olivier;
Schrijver, Karel; Testi, Leonardo; Thiébaut, Eric; Wolf, Sebastian
Bibcode: 2005astro.ph..7580G
Altcode:
VITRUV is a second generation spectro-imager for the PRIMA enabled
Very Large Telescope Interferometer. By combining simultaneously up to
8 telescopes VITRUV makes the VLTI up to 6 times more efficient. This
operational gain allows two novel scientific methodologies: 1) massive
surveys of sizes; 2) routine interferometric imaging. The science cases
presented concentrate on the qualitatively new routine interferometric
imaging methodology. The science cases are not exhaustive but
complementary to the PRIMA reference mission. The focus is on: a) the
close environment of young stars probing for the initial conditions of
planet formation and disk evolution; b) the surfaces of stars tackling
dynamos, activity, pulsation, mass-loss and evolution; c) revealing
the origin of the extraordinary morphologies of Planetary Nebulae and
related stars; d) studying the accretion-ejection structures of stellar
black-holes (microquasars) in our galaxy; e) unveiling the different
interacting components (torus, jets, BLRs) of Active Galactic Nuclei;
and f) probing the environment of nearby supermassive black-holes and
relativistic effects in the Galactic Center black-hole.
Title: Non-linear force-free field modeling: model techniques,
boundary conditions, hares, and hounds
Authors: Schrijver, C. J.; De Rosa, M. L.; Metcalf, T.
Bibcode: 2005AGUSMSH31A..05S
Altcode:
Understanding the conditions under which solar magnetic fields can
destabilize to erupt in flares and coronal mass ejections requires
a quantitative understanding of the coronal magnetic field and of
the currents that it carries. The increased availability of vector
magnetograms, together with EUV and X-ray coronal images, should
provide adequate constraints to model the coronal field, and thus to
visualize its 3D geometry and to measure the available free energy
and helicity. Non-linear force-free fields (NLFFF) are likely a useful
model to use when extrapolating the solar surface field upward into the
coronal volume. It may even be possible to use the observed trajectories
of coronal loops, evident in EUV images of the corona, as a further
constraint. We present initial results of a team effort to understand
the intricacies of NLFFF modeling: we discuss and evaluate comparisons
of NLFFF models computed with different models and applications of
boundary conditions, and look ahead to full coronal field modeling
for the upcoming Solar-B and SDO missions.
Title: The Stellar Imager: a deep-space telescope to image stellar
surfaces
Authors: Schrijver, C. J.; Carpenter, K. G.; Karovska, M.; Si Vision
Mission Team
Bibcode: 2005ESASP.560..951S
Altcode: 2005csss...13..951S
No abstract at ADS
Title: Coronal heating and the appearance of solar and stellar coronae
Authors: Schrijver, C. J.; Sandman, A. W.; Aschwanden, M. J.; De Rosa,
M. L.
Bibcode: 2005ESASP.560...65S
Altcode: 2005csss...13...65S
No abstract at ADS
Title: The Heating of Cool-Star Coronae: From Individual Loops to
Global Flux-Flux Scalings
Authors: Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2005ApJ...619.1077S
Altcode:
We simulate surface field patterns for cool dwarf stars of widely
different levels of activity and estimate the associated global
coronal radiative losses. We find that the integrated X-ray
brightness of coronae of cool stars is relatively insensitive to
the patterns of their surface magnetic fields, and that the X-ray
flux is determined almost entirely by the magnetic flux through the
stellar surfaces. This reconciles potentially inconsistent findings
in the literature for the heating of solar and stellar coronae: solar
studies suggested that the heating flux density entering coronal loops
scales as FH=ɛ0Bβ/Lλ
(for a magnetic flux density B at the base of a loop of length L,
with β=1.0+/-0.3 and λ=1.0+/-0.5), whereas combined solar and
stellar data suggested that the X-ray flux density FX
from entire cool-star coronae depends only on the average magnetic
flux density <|ϕ|> through the stellar photospheres. We find
that the above two scalings are compatible because loop lengths and
base magnetic flux densities are essentially uncorrelated for the
global coronal loop ensemble and because the average loop lengths
differ much less from star to star for Sun-like stars of different
activity than the average base field strengths. We also explore the
scaling properties of the constant of proportionality ɛ0
for stars of significantly different surface gravity.
Title: Solar and stellar variability
Authors: Schrijver, C. J.
Bibcode: 2005MmSAI..76..766S
Altcode:
The magnetic dynamo that is responsible for the variability of the
solar atmosphere has functioned throughout the history of the Sun, as
it does in the multitude of Sun-like stars. Combined solar and stellar
observations provide insight into the variability of the Sun's energy
output on time scales up to billions of years. This brief overview
references select areas of current research.
Title: Stellar Imager (SI) Vision Mission Science Drivers
Authors: Karovska, M.; Carpenter, K. G.; Schrijver, C. J.; SI Mission
Concept Development
Bibcode: 2004AAS...20518005K
Altcode: 2005BAAS...37..384K
We describe the key scientific drivers for the Stellar Imager (SI)
Vision Mission - a UV-optical 0.1 milli-arcsecond (mas) imaging space
interferometer. Ultra-high sub-mas angular resolution imaging at UV and
optical wavelengths is key to studies of magnetic field structures that
govern the formation of stars and planetary systems, the habitability of
planets, long-term space weather in the Exploration era, and transport
processes on many scales in the Universe. Magnetic fields affect the
evolution of structure in the Universe and drive solar and stellar
activity which is a key to life's origin and survival. However,
our understanding of how magnetic fields form, operate, and evolve
is currently very limited, even for the nearest star, our Sun. The key science goal of the Stellar Imager Mission is to obtain an
understanding of stellar dynamos, and in particular the solar dynamo,
through a population study of other stars representing a broad range
of stellar parameters and activity. The end goal is to understand the
variable impact of stellar magnetic activity on planetary climates and
the origin and maintenance of life. Improved knowledge of solar-type
dynamo activity and the interaction of global fields and flows
is also crucial for developing models for long-term space weather
forcasting. Furthermore, SI's high-angular resolution capabilities, over
two magnitudes better then the HST, will enable unprecedented studies
of dynamo and accretion-driven processes, and mass exchange and mass
flows in a wide variety of galactic and extragalactic sources, including
young star/disk systems, solar-type and evolved stars and astrospheres,
numerous interacting binaries, and SN, AGN, and black hole environments.
Title: The Stellar Imager (SI): Preliminary Results from the Ongoing
Vision Mission Study
Authors: Carpenter, K. G.; Schrijver, C. J.; SI Mission Concept
Development Team
Bibcode: 2004AAS...205.0509C
Altcode: 2004BAAS...36.1342C; 2004AAS...205..509C
The Stellar Imager (SI) is a "Vision Mission" in the far-horizon
NASA Roadmap, conceived for the purpose of understanding the
effects of stellar magnetic fields, the dynamos that generate them,
and the internal structure and dynamics of the stars in which they
exist. The ultimate goal is to achieve the best possible forecasting
of solar/stellar activity and to understand the impact of that
activity on planetary climates and the origin and maintenance of
life in the Universe. The science goals of SI require an ultra-high
angular resolution, at ultraviolet wavelengths, on the order of 100
micro-arcsec and baselines on the order of 0.5 km. SI's resolution will
make it an invaluable resource for many other areas of astrophysics,
including studies of AGN's, supernovae, cataclysmic variables, young
stellar objects, QSO's, and stellar black holes. These requirements
call for a large, multi-spacecraft (>20) imaging interferometer,
utilizing precision formation flying in a stable environment, such as
in a Lissajous orbit around the Sun-Earth L2 point. In this paper,
we present an update on the ongoing SI Vision Mission concept and
technology development studies. The SI Mission Concept Study is
supported, in part, by grants from NASA-HQ through the 2003 Vision
Mission NRA.
Title: The Coronal Heating Mechanism as Identified by Full-Sun
Visualizations
Authors: Schrijver, Carolus J.; Sandman, Anne W.; Aschwanden, Markus
J.; De Rosa, Marc L.
Bibcode: 2004ApJ...615..512S
Altcode:
We constrain the properties of the mechanism(s) responsible for the
bulk of the heating of the corona of the Sun by simulating, for the
first time, the appearance of the entire solar corona. Starting from
full-sphere magnetic field maps for 2000 December 1 and 8, when
the Sun was moderately active, we populate nearly 50,000 coronal
field lines with quasi-static loop atmospheres. These atmospheres
are based on heating flux densities FH that depend in
different ways on the loop half-length L, the field strength B at
the chromospheric base, the loop expansion with height, and the
heating scale height. The best match to X-ray and EUV observations
of the corona over active regions and their environs is found for
FH~4×1014B1.0+/-0.3/L1.0+/-0.5
(in ergs cm-2 s-1 for B in Mx cm-2 and
L in cm), while allowing for substantial loop expansion with increasing
height, and for a heating scale height that is at least a sizeable
fraction of the loop length. This scaling for coronal heating points
to DC reconnection at tangential discontinuities as the most likely
coronal heating mechanism, provided that the reconnection progresses
proportional to the Alfvén velocity. The best-fit coronal filling
factor equals unity, suggesting that most of the corona is heated most
of the time. We find evidence that loops with half-lengths exceeding
~100,000 km are heated significantly more than suggested by the above
scaling, possibly commensurate with the power deposited in the open
field of coronal holes.
Title: Stream structure and coronal sources of the solar wind during
the May 12th, 1997 CME
Authors: Arge, C. N.; Luhmann, J. G.; Odstrcil, D.; Schrijver, C. J.;
Li, Y.
Bibcode: 2004JASTP..66.1295A
Altcode: 2004JATP...66.1295A
We report on our efforts to model the ambient solar wind out to 1 AU
around the time of the May 12, 1997 halo coronal mass ejection (CME) and
to identify its coronal source regions. We use the simple physics and
empirical based Wang-Sheeley-Arge (WSA) model driven by two different
sets of updated photospheric field synoptic maps to accomplish this:
daily updated maps from Mount Wilson Solar Observatory and updated
SOHO/MDI maps constructed with the Schrijver et al. flux transport
data assimilation algorithm. The results generated by the WSA model
are then compared with the WIND satellite observations near Earth, as
well as with each other. We find that the model describes the observed
ambient solar wind stream structure around the time of the May 12,
1997 CME generally well, except for the ejecta itself. Our results
suggest that the source of the high-speed stream that followed the
CME is a coronal hole extension located south of the Sun's equator. We
conclude that the northern active region associated with the May 12th
CME did not play a role in the formation of the small southern coronal
hole extension that produced the high-speed stream, which followed and
eventually compressed the ICME from behind. Overall, this analysis
suggests how the solar wind context of CME-related events can be
analyzed and understood using coronal and solar wind models.
Title: The Stellar Imager (SI): a revolutionary large-baseline
imaging interferometer at the Sun-Earth L2 point
Authors: Carpenter, Kenneth G.; Schrijver, Carolus J.; Allen, Ronald
J.; Brown, Alexander; Chenette, David; Danchi, William C.; Karovska,
Margarita; Kilston, Steven; Lyon, Richard G.; Marzouk, Joe; Mazzuca,
Lisa M.; Moe, Rud V.; Walter, Frederick; Murphy, Neil
Bibcode: 2004SPIE.5491..243C
Altcode:
The Stellar Imager (SI) is a far-horizon or "Vision" mission in the
NASA Sun-Earth Connection (SEC) Roadmap, conceived for the purpose
of understanding the effects of stellar magnetic fields, the dynamos
that generate them, and the internal structure and dynamics of the
stars in which they exist. The ultimate goal is to achieve the best
possible forecasting of solar/stellar activity and its impact on life
in the Universe. The science goals of SI require an ultra-high angular
resolution, at ultraviolet wavelengths, on the order of 0.1 milliarcsec
and thus baselines on the order of 500 meters. These requirements
call for a large, multi-spacecraft (>20) imaging interferometer,
utilizing precision formation flying in a stable environment, such as
in a Lissajous orbit around the Sun-Earth L2 point. SI's resolution
(several 100 times that of HST) will make it an invaluable resource
for many other areas of astrophysics, including studies of AGN's,
supernovae, cataclysmic variables, young stellar objects, QSO's,
and stellar black holes. In this paper, we present an update on
the ongoing mission concept and technology development studies for
SI. These studies are designed to refine the mission requirements
for the science goals, define a Design Reference Mission, perform
trade studies of selected major technical and architectural issues,
improve the existing technology roadmap, and explore the details of
deployment and operations, as well as the possible roles of astronauts
and/or robots in construction and servicing of the facility.
Title: Solar Coronal Heating Inferred from Full-disk Models of
Coronal Emission
Authors: Schrijver, C. J.; Sandman, A. W.; De Rosa, M. L.; Aschwanden,
M. J.
Bibcode: 2004AAS...204.9501S
Altcode: 2004BAAS...36Q.826S
The appearance of the corona as viewed by different instruments, as
well as its global spectral irradiance, sensitively depends on how
coronal heating scales with the properties of the coronal magnetic
field. We explore a variety of scaling dependences by simulating the
appearance of the full-disk solar corona as viewed by SOHO/EIT and by
YOHKOH/SXT, based on observed photospheric magnetic fields combined with
a potential-field source-surface model. This leads us to conclude that
the best match to X-ray and EUV observations of the corona over active
regions and their environments is found for a heating flux density going
into the corona that scales linearly with the field strength at the
coronal base and roughly inversely with loop length. This scaling points
to DC reconnection at tangential discontinuities as the most likely
coronal heating mechanism, provided that the reconnection progresses
at a rate proportional to the Alfven velocity. We also find that the
best-fit coronal filling factor equals unity, suggesting that most of
the corona is heated most of the time. We find evidence that loops with
half lengths exceeding approximately 100,000 km are heated significantly
more than suggested by the above scaling, possibly commensurate with
the power deposited in the open field of coronal holes.
Title: Stellar activity and its impact on life in the universe
Authors: Schrijver, C. J.
Bibcode: 2004AAS...204.2501S
Altcode: 2004BAAS...36..691S
Magnetic fields play a key role in the formation of stars and, likely,
their planetary systems. The strong coronal emission and stellar wind
that are powered by the intense field characteristic of a star's early
years impact the planetary magnetospheres and atmospheres, and may
play a role in the formation of molecules essential to life. Magnetic
fields continue to play a role even in a star as old as our Sun,
as we experience when space weather fronts affect our communication
and navigation technologies or endanger astronauts. Longer term
variations continue to influence planetary climates, leading on the
earth to periods of above average temperature, as in the Grand Maxima,
and to periods of cooler mean global temperatures, as in the Maunder
Minimum. The stellar magnetic field is generated in the interior of
a star like the Sun by a process that we refer to as the dynamo. The
functioning of a nonlinear astrophysical dynamo remains shrouded in
mystery, even though we know that rotation and convection are the
most important ingredients. This is in part because at present we can
study only one stellar dynamo in adequate detail, namely that of our
Sun. In this talk, I explore: 1) the impacts of magnetic fields on
stars and their environments and 2) how we can expect to achieve an
understanding of the dynamo process by observing its functioning in
distant stars of different physical properties.
Title: The Stellar Imager (SI) ``Vision Mission"
Authors: Carpenter, K.; Danchi, W.; Leitner, J.; Liu, A.; Lyon,
R.; Mazzuca, L.; Moe, R.; Chenette, D.; Schrijver, C.; Kilston, S.;
Karovska, M.; Allen, R.; Brown, A.; Marzouk, J.; Murphy, N.; Walter, F.
Bibcode: 2004AAS...204.0810C
Altcode: 2004BAAS...36..791C
The Stellar Imager (SI) is a `Vision' mission in the Sun-Earth
Connection (SEC) Roadmap, conceived for the purpose of understanding
the effects of stellar magnetic fields, the dynamos that generate
them, and the internal structure and dynamics of the stars in
which they exist. The ultimate goal is to achieve the best possible
forecasting of solar/stellar magnetic activity and its impact on life
in the Universe. The science goals of SI require an ultra-high angular
resolution, at ultraviolet wavelengths, on the order of 100 micro-arcsec
and thus baselines on the order of 0.5 km. These requirements call for
a large, multi-spacecraft (>20) imaging interferometer, utilizing
precision formation flying in a stable environment, such as in a
Lissajous orbit around the Sun-Earth L2 point. SI's resolution will
make it an invaluable resource for many other areas of astrophysics,
including studies of AGN's, supernovae, cataclysmic variables, young
stellar objects, QSO's, and stellar black holes. We present here
an overview of the ongoing mission concept and technology development
studies for SI. These studies are designed to refine the mission
requirements for the science goals, define a Design Reference Mission,
perform trade studies of selected major technical and architectural
issues, improve the existing technology roadmap, and explore the
details of deployment and operations, as well as the possible roles
of astronauts and/or robots in construction and servicing of the
facility. Additional information on SI mission concepts and technology
can be found at URL: http://hires.gsfc.nasa.gov/ ∼si . The SI
Mission Concept Study is supported, in part, by grants from NASA-HQ
through the 2003 Vision Mission NRA.
Title: Toward understanding the sun's magnetic fields
Authors: Title, A.; Schrijver, K.
Bibcode: 2004cosp...35.1292T
Altcode: 2004cosp.meet.1292T
The magnetic field of the Sun is responsible for heating of the outer
solar atmosphere. Thus it controls the EUV, x-Ray, and gamma-ray
emission of the Sun. Recent observations with the TRACE satellite
show that the outer atmosphere is highly structured, dynamic, and
multi-thermal. The combination of convection cells and large scale
flows moves, mixes, and inserts magnetic feature. In this talk origin
and scales of resulting magnetic configurations are examined. It now
appears that small-scale localized fields interact with large-scale
flux systems to feed energy from scales of 1000 km to 100,000 km
and more. How scales of convection and magnetic fields interact my
provide clues on the fundamental processes responsible for heating in
astrophysical systems. The talk will be illustrated by visualizations
of numerical simulations and observations from observatories on the
ground and in space.
Title: Living with an active star
Authors: Schrijver, C. J.
Bibcode: 2004cosp...35.3240S
Altcode: 2004cosp.meet.3240S
The Sun's dynamic magnetic field drives space weather throughout
the heliosphere, and impacts planetary atmospheres by subtle effects
linked by complex pathways. The magnetic field in the solar atmosphere
is continually adjusting to the large-scale flows on the surface,
to flux emergence and subduction through that surface, and to the
forces that open up the field into interplanetary space. Whereas many
of the processes involved in the evolution of the field continue to
be elusive in their details, the International Living With a Star
program requires us to take a systems engineering point of view,
that is, to assemble our piece-wise knowledge into a comprehensive
model. In this talk, I plan to show early results of this effort,
with emphasis on the surface magnetic field, on coronal heating and
irradiance, and on the corona-heliosphere connection.
Title: Observations of small scale reconnection in the solar
atmosphere
Authors: Schrijver, C. J.
Bibcode: 2004cosp...35.3239S
Altcode: 2004cosp.meet.3239S
The magnetic field in the solar atmosphere is continually adjusting
to the large-scale flows on the surface, and to flux emergence and
subduction through that surface. The resulting coronal configuration
as seen by EUV and X-ray telescopes reveals, with few exceptions,
a remarkably simple structure, with little sign of twists or
braids. Consequently, reconnection must occur frequently, allowing the
coronal field to relax apparently smoothly, except during rare flares
and eruptions. High-resolution movies of the solar corona as taken by
the Transition Region and Coronal Explorer offer some insights into how
this process generally occurs. In this talk, I contrast expectations
and observations of the the solar corona on scales of tens of thousands
of kilometers and below.
Title: TRACE and SOHO/MDI Observations of 3 Rotating Sunspots in
AR9002 and AR9004, Along With Modeled Coronal Magnetic Fields
Authors: Nightingale, R. W.; Schrijver, C. J.; De Rosa, M. L.
Bibcode: 2003AGUFMSH42B0511N
Altcode:
The TRACE data set provides a view of the solar atmosphere from the
photosphere in white light, through the transition region in ultraviolet
wavelengths, and into the corona in extreme ultraviolet wavelengths
(EUV). From May 16-23, 2000 TRACE and MDI/SOHO observed at least 3
rotating sunspots in AR9002 and AR9004 at several wavelengths. Over this
time period several small flares, along with a CME on May 23, occurred
in these regions. In addition we have potential-field renderings of the
coronal magnetic fields for the TRACE pointings, extrapolated based on
the Virtual Starlab forecaster data, which in turn has been generated
from the MDI/SOHO observations. An analysis of the rotating sunspots,
together with images and movies, will be provided for these active
regions, accompanied by images of the extrapolated coronal magnetic
fields for comparison with images of the TRACE 1-1.5 MK EUV loops. This
work was supported by NASA, in part under the TRACE contract NAS5-38099
and in part under the MDI/SOHO contract NAG5-13261.
Title: Coronal heating and the appearance of the solar corona
Authors: Schrijver, C. J.; Sandman, A.; De Rosa, M. L.; Aschwanden,
M. J.
Bibcode: 2003AGUFMSH32A1104S
Altcode:
The details of the dependence of coronal heating on the conditions
within the corona determine the appearance of the corona as viewed
by different instruments. For example, strong fields at the base of
short loops cause relatively hot, X-ray bright loops, whereas the
much weaker fields over the quiet Sun result in cooler, EUV bright
loops. Any dependence of the volume heating rates on local conditions
(such as height or field strength) has a signature in the thermal
profiles along the loops, translating into an appearance that depends
on the instrumental pass band. In this preliminary study, we explore
how such dependences of coronal heating on coronal conditions affect
the appearance of the solar corona, and investigate the consequences
for the global EUV and X-ray spectral irradiance. These results will
eventually be used to compute the solar spectral irradiance in the
EUV and X-rays for quiescent conditions throughout the solar cycle.
Title: The Magnetic Connection between the Solar Photosphere and
the Corona
Authors: Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2003ApJ...597L.165S
Altcode:
The solar magnetic field that extends through the chromosphere into
the corona is envisioned to fan out from strong flux concentrations
located within the supergranular downflow lanes. That so-called network
field appears to be surrounded by a mixed-polarity magnetic field with
a scale comparable to that of the granulation. We argue that for an
internetwork field with a magnitude of a few tens of Mx cm-2,
as suggested by both observations and models, the commonly held notion
of a wineglass-shaped magnetic canopy of network flux that fully
encloses weakly magnetic regions below it is fundamentally wrong. We
estimate that in the presence of such a relatively strong internetwork
field, as much as half of the coronal field over very quiet Sun may
be rooted in that mixed-polarity internetwork field throughout the
supergranules rather than in the network flux concentrations, as
assumed until now. A corresponding amount of flux forms collars of
closed loops around the network concentrations, connecting network
flux back down onto the internetwork field over distances of several
thousand kilometers. Within such a geometry, the rapid evolution of
the internetwork field may substantially affect coronal heating and the
acceleration of the solar wind. We discuss the potential consequences
of these interacting network and internetwork fields for atmospheric
heating, for wave propagation and the formation of acoustic shadows,
and for the appearance of the near-surface solar outer atmosphere.
Title: Modeling Atmospheric Activity of Cool Stars
Authors: Schrijver, C. J.
Bibcode: 2003csss...12..240S
Altcode:
This review discusses a set of simple models for cool-star activity
with which we compute (1) photospheric field patterns on stars of
different activity levels, (2) the associated outer-atmospheric field
configurations, and (3) the soft X-ray emission that is expected to
result from the ensemble of loop atmospheres in the coronae of these
stars. The model is based on empirically-determined properties of
solar activity. It allows us to extrapolate to stars of significantly
higher and lower activity than seen on the present-day Sun through
its cycle. With it, we can, for example, gain insight into stellar
field patterns (including a possible formation mechanism for polar
starspots), as well as in the properties of coronal heating (helpful in
the identification of the quiescent coronal heating mechanism). Lacking
comprehensive theoretical understanding, the model's reliance on
empirical solar data means that the multitude of processes involved
are approximated to be independent of rotation rate, activity level,
and fundamental stellar parameters, or -- where unavoidably necessary
-- assumed to simply scale with activity. An evaluation of the most
important processes involved guides a discussion of the limits of the
model, of the limitations in our knowledge, and of future needs. "I propose to adopt such rules as will ensure the testability of
scientific statements; which is to say, their falsifiability." Karl
Popper (1902-1994)
Title: Imaging the Surfaces and Interiors of Other Stars: The Stellar
Imager (SI) Mission Concept
Authors: Carpenter, K. G.; Lyon, R. G.; Schrijver, C. J.; Mundy,
L. J.; Allen, R. J.; Rajagopal, J.
Bibcode: 2003csss...12.1091C
Altcode:
The Stellar Imager (SI) is envisioned as a large (0.5 km diameter)
space-based, UV-optical interferometer. It is designed to image surface
features and, through asteroseismology, sub-surface structures of
other stars and measure their spatial and temporal variations. These
observations are needed to improve our understanding of the underlying
dynamo process(es) and enable improved forecasting of solar/stellar
activity and its impact on planetary climates and life. Schrijver and
Carpenter (this volume) discuss the science goals of the mission in
detail, while in this paper we discuss the performance requirements
implied by the science goals and how these translate into specific
design requirements on the mission architecture, and we present some
preliminary visions for how the required observations (e.g., 1000 pixel,
100 micro-arcsec resolution, UV-optical images of the surface of nearby
dwarf stars) for this ambitious project might be obtained.
Title: Observations of Rotating Sunspots from TRACE
Authors: Brown, D. S.; Nightingale, R. W.; Alexander, D.; Schrijver,
C. J.; Metcalf, T. R.; Shine, R. A.; Title, A. M.; Wolfson, C. J.
Bibcode: 2003SoPh..216...79B
Altcode:
Recent observations from TRACE in the photospheric white-light channel
have shown sunspots that rotate up to 200° about their umbral centre
over a period of 3-5 days. The corresponding loops in the coronal fan
are often seen to twist and can erupt as flares. In an ongoing study,
seven cases of rotating sunspots have been identified, two of which
can be associated with sigmoid structures appearing in Yohkoh/SXT and
six with events seen by GOES. This paper analyzes the rotation rates
of the sunspots using TRACE white-light data. Observations from AR
9114 are presented in detail in the main text and a summary of the
results for the remaining six sunspots is presented in Appendixes
A-F. Discussion of the key results, particularly common features,
are presented, as well as possible mechanisms for sunspot rotation.
Title: Asterospheric Magnetic Fields and Winds of Cool Stars
Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Title, Alan M.
Bibcode: 2003ApJ...590..493S
Altcode:
This study addresses the winds and magnetic fields in the inner
asterospheres of Sun-like magnetically active stars by combining
empirical relationships between rotation rate and mass loss,
angular-momentum loss, and radiative losses with models of the magnetic
fields at the surfaces of cool stars and in their inner asterospheres
based on the solar example. Our models, for mean magnetic flux densities
up to 10 times solar, suggest that the asterospheric fields of such
stars are dominated by the large-scale dipole component of the surface
field, as is the case for the Sun. Hence, most of the time a single
current sheet is expected to separate domains of opposite magnetic
polarity; the current sheets of more active stars generally have smaller
latitudinal ripples. Magnetic braking requires that the total unsigned
asterospheric magnetic flux increase linearly with the stellar angular
velocity, which is a very much weaker increase than seen for the flux at
the stellar surface. We show that this can be achieved by an increase
in the radial distance at which the coronal field is forced open as
surface activity increases. Combined with measured mass-loss rates
and the assumption that the wind velocity is largely independent of
activity, this requires the wind's Alfvén radius to be nearly constant,
decreasing with surface activity with a power of only -0.16+/-0.13. We
point out that the surface flux density of energy needed to drive a
cool-star wind scales linearly with the unsigned surface magnetic flux
density, as does that needed to heat the corona.
Title: A New Method to Constrain the Iron Abundance from Cooling
Delays in Coronal Loops
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Winebarger,
Amy R.; Warren, Harry P.
Bibcode: 2003ApJ...588L..49A
Altcode: 2003astro.ph..9506A
Recent observations with the Transition Region and Coronal Explorer
reveal that the time delay between the appearance of a cooling loop in
different EUV temperature filters is proportional to the loop length,
Δt12~L. We model this cooling delay in terms of radiative
loss and confirm this linear relationship theoretically. We derive an
expression that can be used to constrain the coronal iron enhancement
αFe=AcorFe/AphFe
relative to the photospheric value as function of the cooling delay
Δt12, flux F2, loop width w, and filling factor
qw<=1. With this relation, we find upper limits on
the iron abundance enhancement of αFe<=4.8+/-1.7 for
10 small-scale nanoflare loops, and αFe<=1.4+/-0.4
for five large-scale loops, in the temperature range of T~1.0-1.4
MK. This result supports the previous finding that low first ionization
potential elements, including Fe, are enhanced in the corona. The
same relation constitutes also a lower limit for the filling factor,
which is qw>=0.2+/-0.1 and qw>=0.8+/-0.2
for the two groups of coronal loops.
Title: A New Method to Constrain the Iron Abundance from Cooling
Delays in Coronal Loops
Authors: Aschwanden, M. J.; Schrijver, C. J.; Winebarger, A. R.;
Warren, H. P.
Bibcode: 2003SPD....34.1701A
Altcode: 2003BAAS...35..837A
Recent observations with TRACE reveal that the time delay between
the appearance of a cooling loop in different EUV temperature
filters is proportional to the loop length, dt12
∼ L . We model this cooling delay in terms of radiative loss
and confirm this linear relationship theoretically. We derive an
expression that can be used to constrain the coronal iron enhancement
AFe=AFecor/AFePh
relative to the photospheric value as function of the cooling delay
dt12, flux F2, loop width w, and filling factor
qw < 1. With this relation we find upper limits on
the iron abundance enhancement of AFe < 4.8 +/- 1.7
for 10 small-scale nanoflare loops, and AFe < 1.4 +/-
0.4 for 5 large-scale loops, in the temperature range of T ∼ 1.0-1.4
MK. This result supports the previous finding that low-FIP elements,
including Fe, are enhanced in the corona. The same relation constitutes
also a lower limit for the filling factor, which is qw >
0.2 +/- 0.1 and qw > 0.8 +/- 0.2 for the two groups of
coronal loops.
Title: Stellar Imager (SI) mission concept
Authors: Carpenter, Kenneth G.; Schrijver, Carolus J.; Lyon, Richard
G.; Mundy, Lee G.; Allen, Ronald J.; Armstrong, J. Thomas; Danchi,
William C.; Karovska, Margarita; Marzouk, Joseph; Mazzuca, Lisa M.;
Mozurkewich, David; Neff, Susan G.; Pauls, Thomas A.; Rajagopal,
Jayadev K.; Solyar, Gregory; Zhang, Xiaolei
Bibcode: 2003SPIE.4854..293C
Altcode:
The Stellar Imager (SI) is envisioned as a space-based, UV-optical
interferometer composed of 10 or more one-meter class elements
distributed with a maximum baseline of 0.5 km. It is designed to
image stars and binaries with sufficient resolution to enable long-term
studies of stellar magnetic activity patterns, for comparison with those
on the sun. It will also support asteroseismology (acoustic imaging) to
probe stellar internal structure, differential rotation, and large-scale
circulations. SI will enable us to understand the various effects of
the magnetic fields of stars, the dynamos that generate these fields,
and the internal structure and dynamics of the stars. The ultimate
goal of the mission is to achieve the best-possible forecasting
of solar activity as a driver of climate and space weather on time
scales ranging from months up to decades, and an understanding of the
impact of stellar magnetic activity on life in the Universe. In this
paper we describe the scientific goals of the mission, the performance
requirements needed to address these goals, the "enabling technology"
development efforts being pursued, and the design concepts now under
study for the full mission and a possible pathfinder mission.
Title: The Properties of Small Magnetic Regions on the Solar Surface
and the Implications for the Solar Dynamo(s)
Authors: Hagenaar, Hermance J.; Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2003ApJ...584.1107H
Altcode:
We find that bipolar active regions that emerge onto the Sun's surface
are part of a smoothly decreasing frequency distribution that spans
almost 4 orders of magnitude in flux and 8 orders of magnitude in
frequency. Distributions of emergence latitude and dipole orientation
narrow from nearly uniform for the smallest observed ephemeral regions
(~5×1018 Mx) up to narrowly distributed about the mean for
the largest active regions (close to 1022 Mx), while the
emergence frequency increases smoothly and rapidly with decreasing
flux. At the low end of the flux spectrum, the cycle variation in
emergence frequency is at most a factor of 1.5, in antiphase with
the cycle variation of close to an order of magnitude for the large
active regions. We discuss a scenario in which the ephemeral regions
with fluxes below ~30×1018 Mx have their origin in a
turbulent dynamo, largely independent of the global sunspot cycle. Our
empirical findings are based on a combination of previously published
work on active regions and large ephemeral regions, complemented
here with an analysis of the photospheric magnetic field outside
active regions, as observed in SOHO/MDI full-disk magnetograms taken
from the most recent sunspot minimum in 1996 to about 1 yr after
sunspot maximum in 2001. We find that the spectrum of the emerging
bipoles with fluxes (6-30)×1018 Mx can be approximated
throughout this period by a fixed exponential distribution with
an e-folding scale of (5.3+/-0.1)×1018 Mx. We confirm
that the ephemeral regions are an important source of flux for the
quiet magnetic network, in particular for the smallest scales; the
larger scale patterns are dominated by flux dispersing from decaying
active regions. As the variation of these two sources is nearly in
antiphase, the flux contained in the quiet-Sun network shows little
overall variation: the flux spectrum and the total absolute flux for
network concentrations with fluxes <~20×1018 Mx are
essentially independent of cycle phase. For network concentrations with
fluxes >~30×1018 Mx, mostly found in regions populated
substantially by decayed active regions, the network flux distribution
approaches an exponential for which the e-folding scale increases with
sunspot activity from ~20×1018 Mx to ~33×1018
Mx, as the total flux in this component varies in phase with the sunspot
cycle. A comparison of the flux-emergence rate with the network flux
implies an overall mean replacement time for flux in quiet Sun of
8-19 hr.
Title: The Stellar Imager (SI): An Ultra-High Angular Resolution
Ultraviolet/Optical Observatory
Authors: Carpenter, K. G.; Lyon, R. G.; Schrijver, C. J.; Mundy, L.;
Allen, R. J.; Rajagopal, J.
Bibcode: 2003ASPC..291..355C
Altcode: 2003hslf.conf..355C
No abstract at ADS
Title: Photospheric and heliospheric magnetic fields
Authors: Schrijver, Carolus J.; De Rosa, Marc L.
Bibcode: 2003SoPh..212..165S
Altcode:
The magnetic field in the heliosphere evolves in response to the
photospheric field at its base. This evolution, together with the
rotation of the Sun, drives space weather through the continually
changing conditions of the solar wind and the magnetic field embedded
within it. We combine observations and simulations to investigate the
sources of the heliospheric field from 1996 to 2001. Our algorithms
assimilate SOHO/MDI magnetograms into a flux-dispersal model,
showing the evolving field on the full sphere with an unprecedented
duration of 5.5 yr and temporal resolution of 6 hr. We demonstrate
that acoustic far-side imaging can be successfully used to estimate
the location and magnitude of large active regions well before they
become visible on the solar disk. The results from our assimilation
model, complemented with a potential-field source-surface model for the
coronal and inner-heliospheric magnetic fields, match Yohkoh/SXT and
KPNO/He 10830 Å coronal hole boundaries quite well. Even subject to the
simplification of a uniform, steady solar wind from the source surface
outward, our model matches the polarity of the interplanetary magnetic
field (IMF) at Earth ∼3% of the time during the period 1997-2001
(independent of whether far-side acoustic data are incorporated into
the simulation). We find that around cycle maximum, the IMF originates
typically in a dozen disjoint regions. Whereas active regions are
often ignored as a source for the IMF, the fraction of the IMF that
connects to magnetic plage with absolute flux densities exceeding 50 Mx
cm−2 increases from ≲10% at cycle minimum up to 30-50%
at cycle maximum, with even direct connections between sunspots and the
heliosphere. For the overall heliospheric field, these fractions are
≲1% to 20-30%, respectively. Two case studies based on high-resolution
TRACE observations support the direct connection of the IMF to magnetic
plage, and even to sunspots. Parallel to the data assimilation,
we run a pure simulation in which active regions are injected based
on random selection from parent distribution functions derived from
solar data. The global properties inferred for the photospheric and
heliospheric fields for these two models are in remarkable agreement,
confirming earlier studies that no subtle flux-emergence patterns or
field-dispersal properties are required of the solar dynamo beyond those
that are included in the model in order to understand the large-scale
solar and heliospheric fields.
Title: Active regions as sources of the heliospheric field
Authors: Schrijver, C. J.; De Rosa, M. L.; Title, A. M.
Bibcode: 2002AGUFMSH52A0436S
Altcode:
The magnetic field in the heliosphere originates from a variety
of sources on the surface of the Sun, including mature, decaying,
and decayed active regions, as well as sunspots. The emergence of new
active regions together with the dispersal of flux from older active
regions causes the coronal magnetic field topology to continually
evolve, allowing previously closed-field regions to open into the
heliosphere and previously open-field regions to close. Such evolution
of the coronal field, together with the rotation of the Sun, drive
space weather through the continually changing conditions of the solar
wind and the magnetic field embedded within it. We combine observations
and numerical simulations by assimilating SOHO/MDI magnetograms into a
surface flux transport model, in order to investigate the origins of
the heliospheric field on the solar surface through the rising phase
of the current activity cycle. We find that around cycle maximum,
the interplanetary magnetic field (IMF) is typically rooted in a
dozen disjoint regions on the solar surface. Whereas active regions
are sometimes ignored as a source for the IMF, the fraction of the
IMF that connects directly to magnetic plage is found to reach up to
30-50%\ at cycle maximum, with even direct connections between sunspots
and the heliosphere. We further compare this data assimilation model
with a pure simulation model, in which the properties of the emergent
active regions were chosen at random from parent distribution functions
measured for the sun. The two models show remarkable agreement in the
temporal behavior of the sector structure of the IMF, in the magnitude
and time-behavior of the heliospheric field, and even in such global
properties as the tilt angle of the Sun's large scale dipole. We thus
conclude that no additional flux-emergence patterns or field-dispersal
properties are required of the solar dynamo beyond those that are
included in the model in order to understand the large-scale solar
and heliospheric fields.
Title: High Speed Reconnection in the Low Corona
Authors: Title, A. M.; Shine, R. A.; Schrijver, C. J.
Bibcode: 2002AGUFMSH52A0470T
Altcode:
High cadence observations taken with the Transition Region and Corona
Explorer (TRACE) instrument in the 1600Å\ band (with ≈~2 second
cadence) and in the Fe~IX/X 171Å\ band (≈~8 seconds cadence)
reveal fast reconnection events of several types. The most common
is a newly emerging magnetic loop that reconnects with an overlying
fan of loops. As the loops intersect, material is injected into the
overlying loops. A newly formed small bright condensation travels in
a helical path with a pitch angle of about 45 degrees and a speed of
700 to 1000 km/s. Movies of example events in both spectral bands will
be shown. This work was supported by NASA contract NAS5-38099.
Title: The properties of small magnetic regions on the solar surface
and the implications for the solar dynamo(s)
Authors: Hagenaar, M.; Schrijver, C. J.; Title, A. M.
Bibcode: 2002AGUFMSH52A0447H
Altcode:
We study a combination of previously published work on active regions
and large ephemeral regions, complemented with an analysis of the
photospheric magnetic field outside active regions, as observed in
SOHO/MDI full-disk magnetograms taken from the most recent sunspot
minimum in 1996 to about a year after sunspot maximum in 2001. We
find that bipolar active regions that emerge onto the Sun's surface
are part of a smoothly decreasing frequency distribution that spans
almost 4 orders of magnitude in flux and 8 orders of magnitude in
frequency. Distributions of emergence latitude and dipole orientation
narrow from nearly uniform for the smallest observed ephemeral regions
(~ 5x 1018 Mx) up to narrowly distributed about the mean
for the largest active regions (close to 1022 Mx), while
the emergence frequency increases smoothly and rapidly with decreasing
flux. At the low end of the flux spectrum, the cycle variation in
emergence frequency is at most a factor of 1.5, in antiphase with the
cycle variation of close to an order of magnitude for the large active
regions. We discuss a scenario in which the ephemeral regions with
fluxes below ~ 30x 1018 Mx have their origin in a turbulent
dynamo, largely independent of the global sunspot cycle. We confirm
that the ephemeral regions are an important source of flux for the
quiet magnetic network, in particular for the smallest scales; the
larger scale patterns are dominated by flux dispersing from decaying
active regions. A comparison of the flux-emergence rate with the
network flux implies an overall mean replacement time for flux in
quiet Sun of 8-19 hrs.
Title: The long-term variations of the solar and heliospheric fields
Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Title, Alan M.
Bibcode: 2002ESASP.505..253S
Altcode: 2002IAUCo.188..253S; 2002solm.conf..253S
The heliospheric field is determined by the largest-scale patterns of
magnetism at the solar surface, dominated by the lower-latitude active
regions during cycle maximum, and by the circumpolar fields during
cycle minimum. To study these patterns, we simulate the evolution of
the magnetic field at the solar surface and in the heliosphere during
the last 340 years. We conclude that, contrary to current thinking,
the observed magnetic flux in the polar regions of the Sun cannot be
understood as merely a long-term accumulation of active-region decay
products from a dynamo that modulates only the rate at which flux
emerges from cycle to cycle. We suggest that simulation and observation
may be reconciled if the high-latitude solar field decays on a time
scale comparable to that of the sunspot cycle.
Title: Analytical Approximations to Hydrostatic Solutions and Scaling
Laws of Coronal Loops
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.
Bibcode: 2002ApJS..142..269A
Altcode:
We derive accurate analytical approximations to hydrostatic solutions
of coronal loop atmospheres, applicable to uniform and nonuniform
heating in a large parameter space. The hydrostatic solutions of the
temperature T(s), density ne(s), and pressure profile p(s)
as a function of the loop coordinate s are explicitly expressed in
terms of three independent parameters: the loop half-length L, the
heating scale length sH, and either the loop-top temperature
Tmax or the base heating rate EH0. The analytical
functions match the numerical solutions with a relative accuracy of
<~10-2-10-3. The absolute accuracy of the
scaling laws for loop base pressure p0(L, sH,
Tmax) and base heating rate EH0(L, sH,
Tmax), previously derived for uniform heating by Rosner
et al., and for nonuniform heating by Serio et al., is improved to a
level of a few percent. We generalize also our analytical approximations
for tilted loop planes (equivalent to reduced surface gravity) and for
loops with varying cross sections. There are many applications for such
analytical approximations: (1) the improved scaling laws speed up the
convergence of numeric hydrostatic codes as they start from better
initial values, (2) the multitemperature structure of coronal loops
can be modeled with multithread concepts, (3) line-of-sight integrated
fluxes in the inhomogeneous corona can be modeled with proper correction
of the hydrostatic weighting bias, (4) the coronal heating function
can be determined by forward-fitting of soft X-ray and EUV fluxes, or
(5) global differential emission measure distributions dEM/dT of solar
and stellar coronae can be simulated for a variety of heating functions.
Title: What Is Missing from Our Understanding of Long-Term Solar
and Heliospheric Activity?
Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Title, Alan M.
Bibcode: 2002ApJ...577.1006S
Altcode:
The heliospheric magnetic field is associated with changes in space
weather, cosmic-ray flux, and likely climate. This field is determined
by the largest scale patterns of magnetism at the solar surface,
dominated by the lower latitude active regions during cycle maximum and
by the circumpolar fields during cycle minimum. Whereas the magnetic
field in the activity belt is readily studied, the high-latitude
field is much less accessible, and its study requires a combination of
modeling and observation. Current models hold that the high-latitude
magnetic field on the Sun is determined solely by the accumulation of
field transported poleward from lower latitude active regions. We test
this hypothesis by simulating the evolution of the magnetic field at
the solar surface and in the heliosphere during the last 340 yr using a
state-of-the-art model that incorporates all processes that are known to
contribute significantly to the evolution of the large-scale patterns
in the solar field. We find that if only the emergence frequency of
magnetic bipoles is varied in accordance with observed sunspot records,
the polar-cap field reservoir does not match measurements during past
years. Based on comparisons of our simulations with observed polar
fluxes over the last few decades and with the proxy for the heliospheric
flux formed by 340 yr of 10Be ice-core data, we suggest that
the high-latitude field may be subject to decay on a timescale of 5-10
yr. We discuss the consequences of this finding for our understanding
of the Sun-Earth connection and explore inferences for the coupling
of the Sun's internal magnetic field to the heliospheric field.
Title: Dedication (SOLMAG 2002): Karen L. Harvey 1942 - 2002
Authors: Schrijver, Karel; van Driel-Gesztelyi, Lidia
Bibcode: 2002ESASP.505D..15S
Altcode: 2002IAUCo.188D..15S; 2002solm.confD..15S
No abstract at ADS
Title: Observations of rotating sunspots and their effect in the
corona
Authors: Brown, D. S.; Nightingale, R. W.; Alexander, D.; Schrijver,
C. J.; Metcalf, T. R.; Shine, R. A.; Title, A. M.; Wolfson, C. J.
Bibcode: 2002ESASP.505..261B
Altcode: 2002IAUCo.188..261B; 2002solm.conf..261B
Recent observations from TRACE have seen sunspots, in the photospheric
white light filter, rotate up to 180 degrees about their umbral
centre. The corresponding loops in the coronal fan are seen to twist
and can erupt. In an ongoing study, five cases of rotating sunspots
have been identified, three of which can be identified with sigmoid
structures appearing in Yohkoh/SXT. This paper will present images from
one of these events, showing the coupling between the photosphere and
the corona, and observational analysis deducing the rotation speeds
and how they change through time and with radius of the sunspot. In
particular, the paper will focus on the best example of a rotating
sunspot observed so far, that of AR 9114 which occurred over 8-10 August
2000 and was observed by TRACE, SoHO/MDI and Yohkoh/SXT. The sunspot
rotated 150 degrees within this time and is associated with a sigmoid.
Title: Solar spots as prototypes for stellar spots
Authors: Schrijver, C. J.
Bibcode: 2002AN....323..157S
Altcode:
What is the nature of what we commonly refer to as starspots? The
answer to that question determines what we can learn from the study
of starspots about stars, their internal dynamics, and their magnetic
activity. Observations of the Sun, the only cool star that we can
observe in detail, naturally led to the hypothesis that starspots
in other cool stars are magneto-convective phenomena in which a
magnetic field substantially affects the structure of a stellar
atmosphere. Spectral line features, eclipse light curves, and intensity
modulations are consistent with that hypothesis. Yet for almost all
stars for which starspots are reported, the surface coverage is very
much higher than for the Sun, while many stars manifest spots at high
latitudes where the Sun has never been seen to do so. In this review, I
address how well starspots compare to sunspots, discuss some differences
expected when moving away from the Sun in the HR diagram or in level
of activity, and identify resources to further deepen our understanding.
Title: The topology of a mixed-polarity potential field, and
inferences for the heating of the quiet solar corona
Authors: Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2002SoPh..207..223S
Altcode:
We study the statistical properties of the connectivity of the corona
over the quiet Sun by analyzing the potential magnetic field above
the central area of source planes sprinkled randomly with some 300
magnetic monopoles each. We find that the field is generally more
complex than one might infer from a study of the field within the
source plane alone, or from a study of the 3D field around a small
number of sources. Whereas a given source most commonly connects
to only its nearest neighbors, it may connect to up to several dozen
sources; only a weak trend relates the source strength and the number of
connections. The connections between pairs of sources define volumes,
or domains, of connectivity. Domains that have a finite cross section
with the source plane are enclosed by surfaces that contain a pair
of null points. In contrast, most of the bounding surfaces of domains
that lie above the source plane appear not to contain null points. We
argue that the above findings imply (i) that we should expect at
best a weak correlation between coronal brightness and the flux in an
underlying flux concentration, and (ii) that the low-lying chromospheric
field lines (such as are observable in Hα) provide information on
source connections that are largely complementary to those traced by
the higher-reaching coronal field lines (observable in the extreme
ultraviolet). We compare sample TRACE and SOHO/MDI observations of the
quiet corona and photosphere with our finding that the number density
of null points within the source plane closely matches that of the
sources; because we find essentially no foci of coronal brightening
away from significant photospheric magnetic flux concentrations, we
conclude that coronal heating at such null points does not contribute
significantly to the overall heating. We argue that the divergence of
field lines towards multiple sources restricts the propagation of braids
and twists, so that any coronal heating that is associated with the
dissipation of braids induced by footpoint shuffling in mixed-polarity
network is likely (a) to occur predominantly low in the corona, and
(b) to be relatively more efficient in quiet Sun than in active regions
for a given field strength and loop length.
Title: The Stellar Imager (SI): An UV-Optical Interferometer in Space
Authors: Zhang, X.; Carpenter, K.; Schrijver, C.
Bibcode: 2002AAS...200.6102Z
Altcode: 2002BAAS...34..744Z
The Stellar Imager (SI) is envisioned as a space-based, UV-optical
interferometer composed of 10 or more one-meter class mirrors
distributed with a maximum baseline of 0.5 km and providing a resolution
of 60 micro-arcseconds at 1550 A. It will image stars and binaries
with one hundred to one thousand resolution elements on their surfaces
and enable long-term studies of stellar magnetic activity patterns and
their evolution with time, for comparison with those on the sun. It will
also sound their interiors through asteroseismology to image internal
structure, differential rotation, and large-scale circulations. SI will
enable us to understand the various effects of magnetic fields of stars,
the dynamos that generate them, and the internal structure and dynamics
of the stars in which they exist. The ultimate goal is to achieve the
best-possible forecasting of solar activity on times scales ranging
up to decades, and an understanding of the impact of stellar magnetic
activity on life in the Universe. With substantial improvements in
normal-incidence mirror coatings for the EUV, the concept could be
extended into that shorter wavelength regime as well. Fitting naturally
within the NASA long-term time line, SI complements defined missions,
and with them will show us entire other solar systems, from the central
star to their orbiting planets.
Title: Erratum: ``On the Formation of Polar Spots in Sun-like Stars''
(ApJ, 551, 1099 [2001])
Authors: Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2002ApJ...568.1100S
Altcode:
Equation (3) and the subsequent expression for the
tapering function were misprinted and should appear as
follows: ``We find an acceptable fit to solar observations
forvM=13sin(2θ)f(θ')f(π-θ')ms-
1,(3)with a tapering function of colatitude θ' (in
radians) of f(θ')=1-exp[-(1.45θ')3]
that is effective only above ~40°.''
Title: Transverse oscillations in coronal loops observed with
TRACE I. An Overview of Events, Movies, and a Discussion of Common
Properties and Required Conditions
Authors: Schrijver, Carolus J.; Aschwanden, Markus J.; Title, Alan M.
Bibcode: 2002SoPh..206...69S
Altcode:
We study transverse loop oscillations triggered by 17flares and filament
destabilizations; only 2 such cases have been reported in the literature
until now. Oscillation periods are estimated to range over a factor
of ∼15, with most values between 2 and 7 min. The oscillations are
excited by filament destabilizations or flares (in 6% of the 255 flares
inspected, ranging from about C3 to X2). There is no clear dependence
of oscillation amplitude on flare magnitude. Oscillations occur in
loops that close within an active region, or in loops that connect
an active region to a neighboring region or to a patch of strong
flux in the quiet Sun. Some magnetic configurations are particularly
prone to exhibit oscillations: two active regions showed two, and one
region even three, distinct intervals with loop oscillations. The
loop oscillations are not a resonance that builds up: oscillations
in loops that are excited along their entire length are likely to be
near the fundamental resonance mode because of that excitation profile,
but asymmetrically excited oscillations clearly show propagating waves
that are damped too quickly to build up a resonance, and some cases show
multiple frequencies. We discuss evidence that all oscillating loops lie
near magnetic separatrices that outline the large-scale topology of the
field. All magnetic configurations are more complicated than a simple
bipolar region, involving mixed-polarities in the interior or vicinity
of the region; this may reflect that the exciting eruptions occur only
in such environments, but this polarity mixing likely also introduces
the large-scale separatrices that are involved. Often the oscillations
occur in conjunction with gradual adjustments in loop positions in
response to the triggering event. We discuss the observations in
the context of two models: (a) transverse waves in coronal loops
that act as wave guides and (b) strong sensitivity to changes in
the field sources for field lines near separatrices. Properties
that favor model b are (1) the involvement of loops at or near
separatrices that outline the large-scale topology of the field,
(2) the combined occurrence of oscillations and loop translations,
(3) the small period spread and similar decay time scale in a set of
oscillating loops in one well-observed event, and (4) the existence
of loops oscillating in antiphase with footpoints close together in
two cases. All other properties are compatible with either model,
except the fact that almost all of the oscillations start away from
the triggering event, suggestive of an outward-pushing exciting wave
more in line with model a. The spread in periods from event to event
suggests that the oscillations may reflect the properties of some
driver mechanism that is related to the flare or mass ejection.
Title: Transverse Oscillations in Coronal Loops Observed with TRACE
II. Measurements of Geometric and Physical Parameters
Authors: Aschwanden, Markus J.; De Pontieu, Bart; Schrijver, Carolus
J.; Title, Alan M.
Bibcode: 2002SoPh..206...99A
Altcode:
We measure geometric and physical parameters oftransverse oscillations
in 26 coronal loops, out of the 17 events described in Paper I by
Schrijver, Aschwanden, and Title (2002). These events, lasting
from 7 to 90 min, have been recorded with the Transition Region
and Coronal Explorer (TRACE) in the 171 and 195 Å wavelength
bands with a characteristic angular resolution of 1", with time
cadences of 15-75 seconds. We estimate the unprojected loop (half)
length L and orientation of the loop plane, based on a best-fit of a
circular geometry. Then we measure the amplitude A(t) of transverse
oscillations at the loop position with the largest amplitude. We
decompose the time series of the transverse loop motion into an
oscillating component Aosc(t) and a slowly-varying trend
Atrend(t). We find oscillation periods in the range of
P=2-33 min, transverse amplitudes of A=100-8800 km, loop half lengths
of L=37 000-291 000 km, and decay times of td=3.2-21 min. We
estimate a lower limit of the loop densities to be in the range of
nloop=0.13-1.7×109 cm−3. The
oscillations show (1) strong deviations from periodic pulses, (2)
spatially asymmetric oscillation amplitudes along the loops, and
(3) nonlinear transverse motions of the centroid of the oscillation
amplitude. From these properties we conclude that most of the
oscillating loops do not fit the simple model of kink eigen-mode
oscillations, but rather manifest flare-induced impulsively generated
MHD waves, which propagate forth and back in the loops and decay
quickly by wave leakage or damping. In contrast to earlier work we
find that the observed damping times are compatible with estimates of
wave leakage through the footpoints, for chromospheric density scale
heights of ≈400-2400 km. We conclude that transverse oscillations
are most likely excited in loops that (1) are located near magnetic
nullpoints or separator lines, and (2) are hit by a sufficiently
fast exciter. These two conditions may explain the relative rarity of
detected loop oscillations. We show that coronal seismology based on
measurements of oscillating loop properties is challenging due to the
uncertainties in estimating various loop parameters. We find that a
more accurate determination of loop densities and magnetic fields,
as well as advanced numerical modeling of oscillating loops, are
necessary conditions for true coronal seismology.
Title: Constraining the Properties of Nonradiative Heating of the
Coronae of Cool Stars and the Sun
Authors: Schrijver, Carolus J.; Aschwanden, Markus J.
Bibcode: 2002ApJ...566.1147S
Altcode:
The dominant mechanism that heats the coronae of the Sun and of other
cool stars remains to be identified, despite numerous solar and stellar
studies. We address the problem from a statistical point of view,
by approximating the emission expected from the ensemble of loops
in stellar coronae. We develop a prototype of a global atmospheric,
empirical model that employs (1) simulations of the surface magnetic
field of the Sun and active stars throughout sunspot cycles, (2)
potential field computations of the corresponding coronal field, and
(3) an approximation of atmospheres for 2000 coronal loops for randomly
selected field lines in each flux configuration, representative of all
environments from very quiet to the interior of active regions. The
latter requires specification of the flux density PH that
passes through the base of the loops to heat the corona. We parameterize
PH as a function of the base field strength Bbase
(in G), loop half-length l (in Mm), and footpoint velocity v (in
km s-1). We find a best fit for a heating flux density
of PH~2×107(Bbase/100)1.0+/-
0.5(l/24)-0.7+/-0.3(v/0.4)0.0+/-0.5 ergs
cm-2 s-1 (the allowed ranges of the exponents
are shown). This parameterization matches the observed soft X-ray
losses from the coronae of the Sun and more active stars with rotation
periods down to 5 days, throughout their activity cycles, as well as
the characteristic coronal temperatures, and the relationships between
disk-averaged radiative and magnetic flux densities. We compare this
parameterization to models previously published in the literature
and find that dissipation of current layers and turbulence are the
most likely candidate heating mechanisms, for which both low-frequency
driving and high-frequency driving meet the criteria comparably well. We
find, moreover, that the heating scale length of ~20 Mm inferred from
solar observations matches the characteristic e-folding height of the
field strength over solar active regions, which suggests that coronal
heating depends on the local field strength. Our modeling suggests
that there is no need for a strong selection mechanism to determine
which loops are heated and which are not, but that the sensitive
dependence of the heating on the base field strength causes the
appearance of a corona that consists of bright loops embedded in less
bright environments. We compare the differential emission measures for
the simulated coronae to those of the Sun and more active cool stars,
and we also discuss the apparently weak velocity dependence of the
best-fit parameterization for PH.
Title: Concurrent Rotating Sunspots, Twisted Coronal Fans, Simgoid
Structures and Coronal Mass Ejections
Authors: Nightingale, R. W.; Brown, D. S.; Metcalf, T. R.; Schrijver,
C. J.; Shine, R. A.; Title, A. M.; Wolfson, C. J.
Bibcode: 2002mwoc.conf..149N
Altcode:
In an on-going study, several sunspots, in apparent rotation, have
been identified in TRACE photospheric white light (WL) images with
accompanying twisting of coronal fans in the corresponding EUV (171,
195 AA) images. These observations can also be temporally and spatially
associated with S or inverse-S shaped regions (sigmoid structures)
appearing in Yohkoh SXT images and with concurrent coronal mass
ejections (CMEs) and/or flares. We have determined the rotational
speed of the apparently rotating sunspot in AR 9114 over 8-10 August
2000, established the inverse S shape observed in the SXT data, and
viewed a rapid, bright flash of possible reconnection in a TRACE
EUV movie. A CME was observed during the 15-18 August 1999 event,
which also included an inverse S shaped region in the SXT data, and
a rotating sunspot and twisting coronal fans in the TRACE data. The
large Bastille Day CME event of 14 July 2000 was accompanied by one
or more apparently rotating sunspots as observed in TRACE WL and by
an inverse S shaped region as seen in a difference SXT image. Movies
and plots of some of these data will be shown along with flow maps and
a list of the pertinent parameters for several rotating sunspots. We
will report on our attempt to determine the vertical electric current
flowing through the 8 August 2000 sunspot utilizing the Mees vector
magnetograph data in order to better understand the apparent rotation
"driver". These observations display the coupling of the solar magnetic
field from the photosphere into the corona.
Title: Transverse oscillations in coronal loops observed with TRACE
Authors: Schrijver, C. J.; Aschwanden, M. J.; De Pontieu, B.; Title,
A. M.
Bibcode: 2001AGUFMSH11A0703S
Altcode:
TRACE discovered transverse oscillations in coronal loops associated
with a flare three years ago, and until recently only two such events
were known. We have now identified a total of 17 events that trigger
some form of loop oscillations. Oscillation periods are estimated to
range over a factor of ~ 15, with most values between 2 and 7 min. The
oscillations are excited by filament destabilizations or flares (in 6%\
of the 255 flares inspected, ranging from about C3 to X2). Oscillations
occur in loops that close within an active region, or in loops that
connect an active region to a neighboring region or to a patch of strong
flux in the quiet Sun. Some magnetic configurations are particularly
prone to exhibit oscillations: two active regions showed two, and
one region even three, distinct intervals with loop oscillations. The
loop oscillations are not a resonance that builds up: oscillations in
loops that are excited along their entire length are likely to be near
the fundamental resonance mode because of that excitation profile, but
asymmetrically excited oscillations clearly show propagating waves that
are damped too quickly to build up a resonance, and some cases show
multiple frequencies. We discuss evidence that all oscillating loops
lie near magnetic separatrices that outline the large-scale topology
of the field. Often the oscillations occur in conjunction with gradual
adjustments in loop positions in response to the triggering event. We
discuss the observations in the context of two models, and evaluate
the contraints on coronal properties that can be deduced from them.
>http://vestige.lmsal.com/TRACE/POD/TRACEoscillations.html</a>
Title: On the Formation of Polar Spots in Sun-like Stars
Authors: Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2001ApJ...551.1099S
Altcode:
We simulate the photospheric magnetic field on very active cool stars
using a flux injection and surface distribution model with strictly
solar parameters, including an 11 yr sunspot cycle and associated
butterfly pattern. The only free parameter is the magnitude of the
cycle. We demonstrate that the combination of supergranulation-driven
dispersal and meridional advection of the field on a very active
Sun-like star leads to a strong polar cap field. The polar caps contain
so much flux that in comparable environments on the Sun convection is
suppressed; it appears inevitable that this leads to the formation
of (clusters of) starspots within large unipolar areas that form at
high latitudes. The model exhibits a peak total flux in the polar cap
regions that is roughly proportional to the cycle amplitude for active
regions, A0, whereas the total absolute flux covering the
stellar surface has a significantly weaker than linear dependence on
A0. We find, for example, that for a star with a rotation
period of P~6 days, the total absolute flux on the star is ~10 times
that characteristic of the active Sun, while the peak polar cap flux is
stronger by a factor of ~30. Within the polar caps of such active stars,
flux concentrations coagulate to form relatively large clusters. The
tendency for larger flux concentrations to disperse more slowly causes
the polar cap flux to be concentrated in a more restricted latitude
range with increasing activity; the corresponding longitudinally
averaged flux density in the polar caps of the simulated star with P~6
days reaches ~300-500 Mx cm-2. The polar cap field in these
simulations displays a ring of one polarity at high latitudes around a
polar patch of the opposite polarity during much of the spot cycle. This
bipolar pattern forms a persistent flux reservoir, so that the cycle
modulation of the polar cap flux of an active star is relatively weak,
consistent with observations of polar cap spot coverage on active
cool stars. Based on these model results, we propose that polar spots
are consistent with a dynamo like that of the Sun for a sufficiently
enhanced emergence frequency of active regions.
Title: Modeling of Coronal EUV Loops Observed with
TRACE. I. Hydrostatic Solutions with Nonuniform Heating
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Alexander, David
Bibcode: 2001ApJ...550.1036A
Altcode:
Recent observations of coronal loops in EUV wavelengths with
the Transition Region and Coronal Explorer (TRACE) and the
Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and
Heliospheric Observatory (SOHO) demonstrated three new results that
cannot be explained by most of the existing loop models: (1) EUV loops
are near-isothermal along their coronal segments, (2) they show an
overpressure or overdensity compared with the requirements of steady
state loops with uniform heating, and (3) the brightest EUV loops
exhibit extended scale heights up to 4 times the hydrostatic scale
height. These observations cannot be reconciled with the classical RTV
(Rosner, Tucker, & Vaiana) model, they do not support models with
uniform heating, and they even partially violate the requirements
of hydrostatic equilibrium. In this study we are fitting for the
first time steady state solutions of the hydrodynamic equations to
observed intensity profiles, permitting a detailed consistency test of
the observed temperature T(s) and density profiles ne(s)
with steady state models, which was not possible in previous studies
based on scaling laws. We calculate some 500 hydrostatic solutions,
which cover a large parameter space of loop lengths (L~4-300 Mm), of
nonuniform heating functions (with heating scale heights in the range
of λH~1-300 Mm), approaching also the limit of uniform
heating (λH>>L). The parameter space can be subdivided
into three regimes, which contain (1) solutions of stably stratified
loops, (2) solutions of unstably stratified loops (in the case of
short heating scale heights, λH,Mm~LMm),
and (3) a regime in which we find no numerical solutions (when
λH,Mm<~LMm). Fitting the hydrostatic
solutions to 41 EUV loops observed with TRACE (selected by the
criterion of detectability over their entire length), we find that
only 30% of the loops are consistent with hydrostatic steady state
solutions. None of the observed EUV loops is consistent with a uniform
heating function while in quasi-steady state. Those loops compatible
with a steady state are found to be heated near the footpoints, with a
heating scale height of λH=12+/-5 Mm, covering a fraction
λH/L=0.2+/-0.1 of the loop length. These results support
coronal heating mechanisms operating in or near the chromosphere and
transition region.
Title: Today's Science of the Sun -- Part 2
Authors: Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2001S&T...101c..34S
Altcode:
No abstract at ADS
Title: On the Evolution of the Solar Photospheric and Coronal
Magnetic Field
Authors: Handy, Brian N.; Schrijver, Carolus J.
Bibcode: 2001ApJ...547.1100H
Altcode:
The Extreme Ultraviolet Imaging Telescope (EIT) and the Michelsen
Doppler Imager (MDI) instruments on the Solar and Heliospheric
Observatory (SOHO) ran a coordinated observing campaign over the
dates 1997 August 10-14. MDI generated 1.2" resolution magnetograms
(0.6" pixels) of the solar photosphere at a nominal cadence of
one minute while EIT observed Fe XII (195 Å) (5" resolution; 2.5"
pixels) in the MDI high-resolution field of view at a cadence of
~17 minutes. We investigate the relationship between the quiet-Sun
photospheric magnetic field and the quiet solar corona by first removing
instrumental effects from the EIT data, time-averaging the MDI data for
improved statistics, and coaligning the two data sets and applying solar
rotation tracking. At the time of the observation, this was the longest
continuous run of its kind. We conduct a detailed investigation of the
processes of evolution in the photospheric magnetic field and look for
sympathetic effects in the solar corona. We measure the lifetimes,
dimensions, and orientations of small-scale coronal brightenings
and reconcile these processes against the evolution of the underlying
photospheric magnetic field. Using statistics collected from this study,
we find that emerging bipoles in quiet Sun reach a typical length of ~14
Mm before fading or reconnecting to other flux concentrations in a time
period of ~5-12 hr, and the quiet solar corona completely decorrelates
in approximately 15 hr. We find that the majority of large coronal
loops in quiet Sun are the products of numerous smaller magnetic flux
concentrations coalescing in the photosphere rather than the product of
a single large emerging bipole. This continuous process of emergence
and coalescence leads to the observation that there is no preferred
orientation to small-scale coronal brightenings in the solar corona.
Title: Catastrophic cooling and high-speed downflow in quiescent
solar coronal loops observed with TRACE
Authors: Schrijver, Carolus J.
Bibcode: 2001SoPh..198..325S
Altcode:
Observations with the Transition Region and Coronal Explorer, TRACE,
show frequent catastrophic cooling and evacuation of quiescent solar
coronal loops over active regions. We analyze this process using
image sequences taken in passbands showing plasma from a few million
degrees down to less than 100 000 K, taken at a cadence of 90 s. The
loop evacuation often occurs after plasma high in the corona has
cooled to transition-region or even chromospheric temperatures. The
cooling loops frequently show Lyman-α and C iv emission developing
initially near the loop tops; later, that cool plasma usually slides
down on both sides of the loop. The relatively cool material often
forms clumps that move at speeds of up to 100 km s−1. The
downward acceleration is no more than 80 m s−2, less than
of the surface gravity. Cooling appears to progress with delays of
the order of up to 103 s between thin, neighboring strands
within flux bundles with cross-sections of at least 1-2 Mm, so that
hot and cool loops are transiently outlined at essentially the same
location. The falling material at temperatures of ≲ 0.1 MK shows no
evidence of loop braiding on scales above the resolution of ∼1 Mm;
loop cross-sections appear independent of height. Existing numerical
models suggest that the observed catastrophic loop-top cooling in
non-flaring conditions can occur if the loop heating precipitously
drops by 1.5 orders of magnitude or more, first and most strongly
high in the corona. Using order-of-magnitude geometrical arguments,
we estimate that loop bundles in the interior of an active region
undergo catastrophic cooling on average once every 2 days, while in
a decayed bipolar region that time interval is approximately a week.
Title: Today's Science of the Sun -- Part 1
Authors: Schrijver, Carolus J.; Title, Alan M.
Bibcode: 2001S&T...101b..34S
Altcode:
No abstract at ADS
Title: The Stellar Imager (SI) mission concept
Authors: Carpenter, Kenneth G.; Neff, Susan G.; Schrijver, Carolus J.;
Allen, Ronald J.; Rajagopal, Jay
Bibcode: 2001LIACo..36..177C
Altcode: 2001fomi.conf..177C
The Stellar Imager (SI) is envisioned as a space-based, UV-optical
interferometer composed of 10 or more one-meter class elements
distributed with a maximum baseline of 0.5 km. It will image stars
and binaries with one hundred to one thousand resolution elements on
their surface and enable long-term studies of stellar magnetic activity
patterns and their evolution with time, for comparison with those on
the sun. It will also sound their interiors through asteroseismology
to image internal structure, differential rotation, and large-scale
circulations. SI will enable us to understand the various effects
of magnetic fields of stars, the dynamos that generate them, and the
internal structure and dynamics of the stars in which they exist. The
ultimate goal is to achieve the best-possible forecasting of solar
activity on times scales ranging up to decades, and an understanding
of the impact of stellar magnetic activity on astrobiology and
life in the Universe. The road to that goal will revolutionize our
understanding of stars and stellar systems, the building blocks of the
Universe. Fitting naturally within the NASA and ESA long-term time
lines, SI complements defined missions, and with them will show us
entire other solar systems, from the central star to their orbiting
planets. In this paper we will describe the scientific goals of the
mission, the performance requirements needed to address those goals,
and the design concepts now under study.
Title: The Coronae of the Sun and Solar-type Stars (CD-ROM Directory:
contribs/schrijv)
Authors: Schrijver, C. J.
Bibcode: 2001ASPC..223..131S
Altcode: 2001csss...11..131S
No abstract at ADS
Title: Simulations of the Photospheric Magnetic Activity and Outer
Atmospheric Radiative Losses of Cool Stars Based on Characteristics
of the Solar Magnetic Field
Authors: Schrijver, Carolus J.
Bibcode: 2001ApJ...547..475S
Altcode:
The observed disk-integrated radiative losses from the outer atmospheres
of stars with convective envelopes are determined by the distribution of
magnetic field over their surfaces. Earlier modeling of the random walk
transport of the solar photospheric magnetic field with the classical
Leighton model has given us insight into how field patterns form and
evolve on large scales. This paper presents the first comprehensive
simulations of the dynamic photospheric magnetic field of the Sun
down to the scale of the mixed polarity network, thus incorporating
all flux involved in outer atmospheric heating. The algorithm
incorporates the classical diffusion model but includes ephemeral
regions (which populate the network that contributes significantly to
the disk-integrated chromospheric emission) and the early phase of
decay of active regions (which is important for the field patterns
in very active stars). Moreover, individual flux concentrations are
tracked and subjected to collisions and fragmentation, and the flux
dispersal is made dependent on the flux contained in the concentrations,
as observed on the Sun. The latter modification causes the model to be
nonlinear. Tests demonstrate that the new model successfully describes
the solar magnetic field. The model is then used to simulate the field
on other cool stars covering several orders of magnitude in activity
and to estimate the surface-averaged radiative losses associated with
that field. The stellar extrapolations are based on the statistical
properties of solar bipolar regions throughout the cycle. Simulations
in which only the frequency of flux emergence is changed to simulate
stars of different activity are shown to be consistent with the observed
nonlinear relationships between disk-averaged radiative losses from
chromospheres and coronae of cool stars. Consequently, the properties of
the solar magnetic field from small ephemeral regions up to large active
regions are compatible with stellar observations. Stellar observations
suggest that those field properties are not the only ones that can
explain the flux-flux relationships, however, because also stars with
polar spots or persistent active longitudes obey these same flux-flux
relationships. The model is also used to understand how rapidly flux is
processed in stellar photospheres in stars with activity patterns like
the Sun: the average total absolute magnetic flux <Φ*>
(Mx) at the stellar surface is found to be proportional to the mean
rate of flux emergence and cancellation <E*> (Mx
s-1) within the range from 1/10<Esolar>
up to 10<Esolar>, where <Esolar>
is the flux injection rate for the active Sun. This linearity is
primarily a consequence of an activity-dependent change in the shape
of the flux histogram for emerging bipoles. This change reflects that
active regions and ephemeral regions have a different dependence on
dynamo strength. The implications of the results of the simulations
for the dynamo and for the relationship between activity and stellar
rotation are discussed.
Title: Chromospheric Activity in Metal-Poor Dwarfs (CD-ROM Directory:
contribs/peterson)
Authors: Peterson, R. C.; Schrijver, C. J.
Bibcode: 2001ASPC..223..300P
Altcode: 2001csss...11..300P
No abstract at ADS
Title: The Stellar Imager (SI) Mission Concept
Authors: Carpenter, K. G.; Schrijver, C. J.
Bibcode: 2000AAS...197.1410C
Altcode: 2000BAAS...32.1426C
The Stellar Imager (SI) is envisioned as a space-based, UV-optical
interferometer composed of 10 or more one-meter class elements
distributed with a maximum baseline of 0.5 km. It will image stars
and binaries with one hundred to one thousand resolution elements on
their surface and enable long-term studies of stellar magnetic activity
patterns and their evolution with time, for comparison with those on
the sun. It will also sound their interiors through asteroseismology
to image internal structure, differential rotation, and large-scale
circulations. SI will enable us to understand the various effects
of magnetic fields of stars, the dynamos that generate them, and the
internal structure and dynamics of the stars in which they exist. The
ultimate goal is to achieve the best-possible forecasting of solar
activity on times scales ranging up to decades, and an understanding of
the impact of stellar magnetic activity on astrobiology and life in the
Universe. The road to that goal will revolutionize our understanding of
stars and stellar systems, the building blocks of the Universe. Fitting
naturally within the NASA long-term time line, SI complements defined
missions, and with them will show us entire other solar systems,
from the central star to their orbiting planets.
Title: Solar-Stellar Connection
Authors: Schrijver, C.
Bibcode: 2000eaa..bookE2084S
Altcode:
The solar-stellar connection refers to the complementarity of solar and
stellar studies in general, and to those concerning dynamic phenomena
in stellar outer atmospheres in particular. The most important agent
in these phenomena is the stellar magnetic field, although wave motions
associated with convection also play a role....
Title: Oscillations in the Magnetic Field of the Solar Corona in
Response to Flares near the Photosphere
Authors: Schrijver, Carolus J.; Brown, Daniel S.
Bibcode: 2000ApJ...537L..69S
Altcode:
The magnetic field in the outer solar atmosphere is frequently distorted
by flares. In some cases, a fraction of the field exhibits a rapidly
damped oscillation (Schrijver et al.; Aschwanden et al.). If this
is a resonating wave trapped in the field, then the rapid damping
requires a viscosity or resistivity that is at least 108
times larger than expected (Nakariakov et al.). We propose instead that
some of the field lines are so sensitive to the source positions that
rocking motions of the photospheric plasma associated with some solar
flares (Kosovichev & Zharkova) cause a few loops to oscillate in
(anti)phase in the fundamental mode, with a period and decay rate
that are determined largely by the characteristics of the photosphere,
saying little about the high atmosphere.
Title: Time Variability of the ``Quiet'' Sun Observed with
TRACE. II. Physical Parameters, Temperature Evolution, and Energetics
of Extreme-Ultraviolet Nanoflares
Authors: Aschwanden, Markus J.; Tarbell, Ted D.; Nightingale, Richard
W.; Schrijver, Carolus J.; Title, Alan; Kankelborg, Charles C.;
Martens, Piet; Warren, Harry P.
Bibcode: 2000ApJ...535.1047A
Altcode:
We present a detailed analysis of the geometric and physical
parameters of 281 EUV nanoflares, simultaneously detected with the
TRACE telescope in the 171 and 195 Å wavelengths. The detection and
discrimination of these flarelike events is detailed in the first paper
in this series. We determine the loop length l, loop width w, emission
measure EM, the evolution of the electron density ne(t) and
temperature Te(t), the flare decay time τdecay,
and calculate the radiative loss time τloss, the conductive
loss time τcond, and the thermal energy Eth. The
findings are as follows: (1) EUV nanoflares in the energy range of
1024-1026 ergs represent miniature versions
of larger flares observed in soft X-rays (SXR) and hard X-rays
(HXR), scaled to lower temperatures (Te<~2 MK),
lower densities (ne<~109 cm-3),
and somewhat smaller spatial scales (l~2-20 Mm). (2) The cooling
time τdecay is compatible with the radiative cooling
time τrad, but the conductive cooling timescale
τcond is about an order of magnitude shorter, suggesting
repetitive heating cycles in time intervals of a few minutes. (3)
The frequency distribution of thermal energies of EUV nanoflares,
N(E)~10-46(E/1024)-1.8 (s-1
cm-2 ergs-1) matches that of SXR microflares
in the energy range of 1026-1029, and exceeds
that of nonthermal energies of larger flares observed in HXR by a
factor of 3-10 (in the energy range of 1029-1032
ergs). Discrepancies of the power-law slope with other studies, which
report higher values in the range of a=2.0-2.6 (Krucker & Benz;
Parnell & Jupp), are attributed to methodical differences in the
detection and discrimination of EUV microflares, as well as to different
model assumptions in the calculation of the electron density. Besides
the insufficient power of nanoflares to heat the corona, we find also
other physical limits for nanoflares at energies <~1024
ergs, such as the area coverage limit, the heating temperature limit,
the lower coronal density limit, and the chromospheric loop height
limit. Based on these quantitative physical limitations, it appears
that coronal heating requires other energy carriers that are not
luminous in EUV, SXR, and HXR.
Title: A Dream of a Mission: Stellar Imager and Seismic Probe
Authors: Carpenter, K. G.; Schrijver, C. J.
Bibcode: 2000AAS...196.3207C
Altcode: 2000BAAS...32..721C
The Stellar Imager and Seismic Probe (SISP) is a mission to understand
the various effects of magnetic fields of stars, the dynamos that
generate them, and the internal structure and dynamics of the stars in
which they exist. The ultimate goal is to achieve the best-possible
forecasting of solar activity on times scales ranging up to decades,
and an understanding of the impact of stellar magnetic activity on
astrobiology and life in the Universe. The road to that goal will
revolutionize our understanding of stars and stellar systems, the
building blocks of the Universe. SISP will zoom in on what today
- with few exceptions - we only know as point sources, revealing
processes never before seen, thus providing a tool to astrophysics as
fundamental as the microscope is to the study of life on Earth. SISP
is an ultraviolet aperture-synthesis imager with 8-10 telescopes
with meter-class apertures, and a central hub with focal-plane
instrumentation that allows spectrophotometry in passbands as narrow
as a few Angstroms up to hundreds of Angstroms. SISP will image stars
and binaries with one hundred to one thousand resolution elements
on their surface, and sound their interiors through asteroseismology
to image internal structure, differential rotation, and large-scale
circulations; this will provide accurate knowledge of stellar structure
and evolution and complex transport processes, and will impact numerous
branches of (astro)physics ranging from the Big Bang to the future of
the Universe. Fitting naturally within the NASA long-term time line,
SISP complements defined missions, and with them will show us entire
other solar systems, from the central star to their orbiting planets.
Title: A dream of a mission: the stellar imager and seismic probe.
Authors: Schrijver, C. J.; Carpenter, K. G.
Bibcode: 2000BAAS...32R.828S
Altcode:
No abstract at ADS
Title: A construction kit for atmospheric activity of cool stars
based on solar characteristics
Authors: Schrijver, C. J.
Bibcode: 2000SPD....31.1002S
Altcode: 2000BAAS...32R.842S
Stars other than the Sun can, at present, be studied only as point
sources. The Sun, in contrast, can be studied in great detail, but
it shows us the magnetic workings of only a single star. This talk
presents numerical simulations of model stars of widely different
levels of magnetic activity, based on the solar recipes for emergence,
dispersal, and disappearance of magnetic field. These simulations
are used to simulated stellar observations. The Sun is found to be
typical of its class: the solar properties are compatible with the
disk-integrated properties of other stars with convective envelopes that
exhibit magnetic activity. This conclusion requires three new processes
to be included in a long-familiar description of the surface field: the
emergence and evolution of small and ephemeral regions, the collision
and fragmentation of concentrations of flux, and the magnetoconvective
coupling that slows the motion of large concentrations. The model allows
us to draw conclusions on ephemeral-region populations in other stars,
on rotation-activity relationships, and on radiative losses from the
outer atmospheres (from chromosphere to corona) of all cool stars:
the solar-stellar connection is a strong component of our quest to
understand what makes the Sun tick. With this model, the solar/stellar
outer atmosphere can be modeled in a laboratory setting, in which the
experimenter has control over all parameters.
Title: Chromospheric Activity in Metal-Poor Dwarfs
Authors: Peterson, Ruth C.; Schrijver, Carolus J.
Bibcode: 2000astro.ph..5367P
Altcode:
We have obtained echelle spectra with the Hubble Space Telescope (HST)
of the MgII 2800A region of ten stars whose metallicities range from
1/300 to 1/3 that of the Sun, and whose space velocities suggest a
halo or old thick-disk origin. Spectra of all ten show double-peaked
emission in the MgII core, very much like the quiet Sun. A half-dozen
non-rotating stars were observed more than once, and show at most a
low level of variability in the emission flux, comparable to that of
quiet stars of solar metallicity. For four stars, we have obtained
Lyman-alpha spectra at 0.2A resolution; all four show emission. The
data thus strongly suggest that chromospheric activity at a minimum
level is present in all stars of near-solar temperature, regardless of
age or metallicity. While this points to non-magnetic sources such as
acoustic waves, a contribution from globally-organized magnetic fields
is possible at a low level. We plan a longer series of MgII and Lyman
alpha observations to constrain this.
Title: A dream of a mission: the Stellar Imager and Seismic Probe
Authors: Schrijver, C. J.; Carpenter, K. G.
Bibcode: 2000SPD....31.0298S
Altcode: 2000BAAS...32..828S
The Stellar Imager and Seismic Probe (SISP) is a mission to understand
the various effects of magnetic fields of stars, the dynamos that
generate them, and the internal structure and dynamics of the stars in
which they exist. The ultimate goal is to achieve the best-possible
forecasting of solar activity on times scales ranging up to decades,
and an understanding of the impact of stellar magnetic activity on
astrobiology and life in the Universe. The road to that goal will
revolutionize our understanding of stars and stellar systems, the
building blocks of the Universe. SISP represents an advance in image
detail of several hundred times over the Hubble Space Telescope. SISP
will zoom in on what today - with few exceptions - we only know as
point sources, revealing processes never before seen, thus providing
a tool to astrophysics as fundamental as the microscope is to the
study of life on Earth. SISP is an ultraviolet aperture-synthesis
imager with 8-10 telescopes with meter-class apertures, and a central
hub with focal-plane instrumentation that allows spectrophotometry in
passbands as narrow as a few Angstroms up to hundreds of Angstroms. SISP
will image stars and binaries with one hundred to one thousand
resolution elements on their surface, and sound their interiors through
asteroseismology to image internal structure, differential rotation,
and large-scale circulations; this will provide accurate knowledge
of stellar structure and evolution and complex transport processes,
and will impact numerous branches of (astro)physics ranging from the
Big Bang to the future of the Universe. Fitting naturally within the
NASA long-term time line, SISP complements defined missions, and with
them will show us entire other solar systems, from the central star
to their orbiting planets.
Title: Solar and Stellar Magnetic Activity
Authors: Schrijver, Carolus J.; Zwaan, Cornelis
Bibcode: 2000ssma.book.....S
Altcode:
Magnetic activity in the sun and similar stars results in a wealth of
phenomena--including starspots, nonradiatively heated outer atmospheres,
activity cycles, deceleration of rotation rates, and even, in close
binaries, stellar cannibalism. This volume uniquely combines studies of
the sun with those of other stars to provide a comprehensive picture of
stellar magnetic activity. Coverage brings together recent results in
solar and stellar studies to provide an illuminating, new view of the
subject. Key topics include radiative transfer, convective simulations,
dynamo theory, outer-atmospheric heating, stellar winds and angular
momentum loss. Researchers are provided with a state-of-the-art review
of this exciting field. Graduate students in astrophysics and plasma
physics will find the volume an ideal introduction to the subject. The
book will also interest researchers studying the connection of solar
Sctivity with the Earth's climate change, such as geologists, planetary
scientists and atmospheric scientists.
Title: A Case for Resonant Scattering in the Quiet Solar Corona in
Extreme-Ultraviolet Lines with High Oscillator Strengths
Authors: Schrijver, Carolus J.; McMullen, Rebecca A.
Bibcode: 2000ApJ...531.1121S
Altcode:
Observations of the quiet off-limb corona in most EUV lines and in
white light are compatible with a (nearly) hydrostatic stratification of
the density at a temperature of 1.2-1.5 MK. We show that, in contrast,
the quiet solar corona in the 171 and 195 Å passbands of the SOHO/EIT
has an effective emission scale height that significantly exceeds
that for hydrostatic stratification at the observed characteristic
temperature. We conclude that this different behavior is caused
primarily by resonant scattering that occurs in the EUV lines of Fe IX,
Fe X, and Fe XII as a result of their exceptionally high oscillator
strengths. A fit to an approximate model suggests that roughly 1/10 to
possibly more than half of the detected photons in EIT (and in TRACE)
images in the 195 and 171 Å passbands, respectively, of the quiet solar
corona have been subjected to scattering. This scattering is responsible
for much of the background haze that is seen in high-resolution TRACE
images of the quiet corona; the hotter active-region corona is subject
to much less scattering. We discuss these results in view of stellar
coronal studies and future solar instrumentation.
Title: Physics of the Solar Corona and Transition Region
Authors: Schrijver, C. J.; Hurlburt, N. E.
Bibcode: 2000PASP..112..427S
Altcode:
Conference was held in Monterey, CA, on 1999 August 24-27. Proceedings
are published in the topical issues of Solar Physics of 1999 December
and 2000 April.
Title: Solar and stellar magnetic activity.
Authors: Schrijver, C. J.; Zwaan, C.
Bibcode: 2000CAS....34.....S
Altcode:
This timely volume provides the first comprehensive review and synthesis
of the current understanding of the origin, evolution, and effects
of magnetic fields in the Sun and other cool stars. Magnetic activity
results in a wealth of phenomena - including starspots, nonradiatively
heated outer atmospheres, activity cycles, deceleration of rotation
rates, and even, in close binaries, stellar cannibalism - all of which
are covered clearly and authoritatively. This book brings together
for the first time recent results in solar studies, with their wealth
of observational detail, and stellar studies, which allow the study
of how activity evolves and depends on the mass, age, and chemical
composition of stars. The result is an illuminating and comprehensive
view of stellar magnetic activity. Observational data are interpreted
by using the latest models in convective simulations, dynamo theory,
outer-atmospheric heating, stellar winds, and angular momentum loss.
Title: Dynamics of Transition Region `Moss' at high time resolution
Authors: De Pontieu, B.; Berger, T. E.; Schrijver, C. J.; Title, A. M.
Bibcode: 1999SoPh..190..419D
Altcode:
Recent observations of solar active regions made with the Transition
Region and Coronal Explorer (TRACE) have revealed finely textured,
low-lying extreme ultraviolet (EUV) emission, called the moss. It
appears as a bright, dynamic pattern with dark inclusions, structured
on spatial scales of 1 to 3 Mm. The moss has been interpreted as the
upper transition region above active region plage and below relatively
hot loops. Here we study the temporal variability of the morphology of
the moss using a 2-hr time sequence of high-cadence TRACE 171 Å images
and G-band, Ca ii K-line and Hα filtergrams from the Swedish Vacuum
Solar Telescope (SVST, La Palma) on 1 June 1999. The data provide a
unique view of the connections between the photosphere, chromosphere,
transition region and corona in an active region. We find that the
moss is dynamic on time scales of 10-30 s due to intrinsic changes
in brightness, obscuration by chromospheric jets and motion caused
by physical interaction with these jets. The temporal variations of
the bright moss elements occur on shorter time scales than those of
the Ca ii K-line bright points. The bright moss elements generally do
not occur directly above the G-band or Ca ii K-line bright points in
the photosphere or lower chromosphere. This suggests that the upper
transition region emission often occurs at the interface of neighboring
flux tubes. The temporal variability of the moss brightness on 30
s time scales may suggest that the energy source of these intensity
changes occurs relatively locally (height <10 000 km).
Title: Aspects of Three-Dimensional Magnetic Reconnection -
(Invited Review)
Authors: Priest, E. R.; Schrijver, C. J.
Bibcode: 1999SoPh..190....1P
Altcode: 1999SoPh..190....1S
In this review paper we discuss several aspects of magnetic reconnection
theory, focusing on the field-line motions that are associated with
reconnection. A new exact solution of the nonlinear MHD equations for
reconnective annihilation is presented which represents a two-fold
generalization of the previous solutions. Magnetic reconnection at
null points by several mechanisms is summarized, including spine
reconnection, fan reconnection and separator reconnection, where it
is pointed out that two common features of separator reconnection are
the rapid flipping of magnetic field lines and the collapse of the
separator to a current sheet. In addition, a formula for the rate of
reconnection between two flux tubes is derived. The magnetic field of
the corona is highly complex, since the magnetic carpet consists of
a multitude of sources in the photosphere. Progress in understanding
this complexity may, however, be made by constructing the skeleton of
the field and developing a theory for the local and global bifurcations
between the different topologies. The eruption of flux from the Sun may
even sometimes be due to a change of topology caused by emerging flux
break-out. A CD-ROM attached to this paper presents the results of a
toy model of vacuum reconnection, which suggests that rapid flipping
of field lines in fan and separator reconnection is an essential
ingredient also in real non-vacuum conditions. In addition, it gives
an example of binary reconnection between a pair of unbalanced sources
as they move around, which may contribute significantly to coronal
heating. Finally, we present examples in TRACE movies of geometrical
changes of the coronal magnetic field that are a likely result of
large-scale magnetic reconnection.
Title: Does Magnetic Flux Submerge at Flux Cancelation Sites?
Authors: Harvey, Karen L.; Jones, Harrison P.; Schrijver, Carolus J.;
Penn, Matthew J.
Bibcode: 1999SoPh..190...35H
Altcode:
Simultaneous measurements of the magnetic fields in the photosphere and
chromosphere were used to investigate if magnetic flux is submerging
at sites between adjacent opposite polarity magnetic network elements
in which the flux is observed to decrease or `cancel'. These data
were compared with chromospheric and coronal intensity images to
establish the timing of the emission structures associated with these
magnetic structures as a function of height. We found that most of
the cancelation sites show either that the bipole is observed longer
in the photosphere than in the chromosphere and corona (44%) or that
the timing difference of the disappearance of the bipole between these
levels of the atmosphere is unresolved. The magnetic axis lengths of the
structures associated with the cancelation sites are on average slightly
smaller in the chromosphere than the photosphere. These observations
suggest that magnetic flux is retracting below the surface for most,
if not all, of the cancelation sites studied.
Title: What is Moss?
Authors: Berger, T. E.; De Pontieu, B.; Fletcher, L.; Schrijver,
C. J.; Tarbell, T. D.; Title, A. M.
Bibcode: 1999SoPh..190..409B
Altcode:
TRACE observations of active regions show a peculiar extreme ultraviolet
(EUV) emission over certain plage areas. Termed `moss' for its spongy,
low-lying, appearance, observations and modeling imply that the
phenomenon is caused by thermal conduction from 3-5 MKcoronal loops
overlying the plage: moss is the upper transition region emission of
hot coronal loops. The spongy appearance is due to the presence of
chromospheric jets or `spicules' interspersed with the EUV emission
elements. High cadence TRACE observations show that the moss EUV
elements interact with the chromospheric jets on 10 s time scales. The
location of EUV emission in the moss does not correlate well to the
locations of underlying magnetic elements in the chromosphere and
photosphere, implying a complex magnetic topology for coronal loop
footpoint regions. We summarize here the key observations leading to
these conclusions and discuss new implications for understanding the
structuring of the outer solar atmosphere.
Title: Active regions losing their moorings by subsurface reconnection
Authors: Schrijver, Carolus J.; Title, Alan M.
Bibcode: 1999SoPh..188..331S
Altcode:
The properties of emerging active regions suggest that they
originate from deep flux bundles with a field strength well above the
equipartition value, so that they can resist strong deformation by
convection as they surface. Yet upon flux emergence, the field appears
in a multitude of bundles with a field strength near to equipartition
with the pressure component that is associated with the convective
motions. During the subsequent decay of active regions, the flux
disperses in a random walk that seems to be caused uniquely by the
flows in the near-surface convection. We propose that this apparently
untethered random walk is the consequence of subsurface reconnection,
which leads to the formation of flexible weak-field connections between
the strong, buoyant top segments, cut loose from the deep source
region. The frequent reconnection between flux tubes underneath each of
the polarities in the active region acts to maintain an approximately
vertical organization of the subsurface field. We concur with earlier
studies that the distribution of the field can be described by the
horizontal dispersal of a scalar at least in these layers near the
surface, because in addition to this combing of the field, mixing-length
models suggest that the horizontal diffusion coefficient is almost
constant down to a depth of some 10 000 km. Our model predicts that the
sub-surface reconnection between fields of opposite polarity causes
the surface field to be disconnected from the deep source region on
a time scale that is in fair agreement with the observed lifetimes
of active regions of a range of sizes. We explore whether branching
of flux bundles into ever smaller bundles between the bottom of the
convective envelope and the photosphere allows the limited bending of
flux tubes by convection that is required to induce reconnection.
Title: C. Zwaan (1928 - 16 June 1999).
Authors: Rutten, R. J.; Schrijver, C. J.
Bibcode: 1999SoPh..188.....R
Altcode: 1999SoPh..188....0R
No abstract at ADS
Title: Coronal Loop Oscillations Observed with the Transition Region
and Coronal Explorer
Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Schrijver, Carolus
J.; Alexander, David
Bibcode: 1999ApJ...520..880A
Altcode:
We report here, for the first time, on spatial oscillations of coronal
loops, which were detected in extreme-ultraviolet wavelengths (171 Å)
with the Transition Region and Coronal Explorer, in the temperature
range of Te~1.0-1.5 MK. The observed loop oscillations
occurred during a flare that began at 1998 July 14, 12:55 UT and are
most prominent during the first 20 minutes. The oscillating loops
connect the penumbra of the leading sunspot to the flare site in
the trailing portion. We identified five oscillating loops with an
average length of L=130,000+/-30,000 km. The transverse amplitude
of the oscillations is A=4100+/-1300 km, and the mean period
is T=280+/-30 s. The oscillation mode appears to be a standing
wave mode (with fixed nodes at the footpoints). We investigate
different MHD wave modes and find that the fast kink mode with a
period τ=205(L/1010 cm)(ne/109
cm-3)1/2(B/10 G)-1 s provides the
best agreement with the observed period. We propose that the onset
of loop oscillations in distant locations is triggered by a signal or
disturbance that propagates from the central flare site with a radial
speed of ~700 km s-1. Because the observed loop oscillation
periods are comparable to photospheric 5 minute oscillations, a resonant
coupling between the two systems is possible. We further find evidence
for global extreme-UV dimming in the entire active region possibly
associated with a coronal mass ejection.
Title: High-resolution Imaging of the Solar Chromosphere/Corona
Transition Region
Authors: Berger, T. E.; De Pontieu, B.; Schrijver, C. J.; Title, A. M.
Bibcode: 1999ApJ...519L..97B
Altcode:
The properties of a previously unresolved extreme-ultraviolet (EUV)
emission in solar active regions are examined using coordinated data
sets from the Transition Region and Coronal Explorer (TRACE) satellite,
the Michelson Doppler Imager on the Solar and Heliospheric Observatory
satellite, the Soft X-Ray Telescope (SXT) on the Yohkoh satellite, and
the ground-based Swedish Vacuum Solar Telescope (SVST) on La Palma. The
emission appears most prominently in TRACE Fe IX/Fe X 171 Å images
as a bright dynamic network surrounding dark inclusions on scales of
2-3 Mm, confined to layers approximately 1-3 Mm thick with base heights
approximately 2-4 Mm above the photosphere. It is seen only above plage
regions that underlie (3-5)×106 K coronal loops visible
in SXT images. The bright EUV elements emit at temperatures of about
106 K. Fine-scale motions and brightness variations of the
emission occur on timescales of 1 minute or less. The dark inclusions
correspond to jets of chromospheric plasma seen in simultaneous SVST
filtergrams in the wings of Hα. The combined characteristics imply
that we are at least partially resolving the structure and dynamics
of the conductively heated upper transition region between the solar
chromosphere and corona.
Title: A new view of the solar outer atmosphere by the Transition
Region and Coronal Explorer
Authors: Schrijver, C. J.; Title, A. M.; Berger, T. E.; Fletcher, L.;
Hurlburt, N. E.; Nightingale, R. W.; Shine, R. A.; Tarbell, T. D.;
Wolfson, J.; Golub, L.; Bookbinder, J. A.; DeLuca, E. E.; McMullen,
R. A.; Warren, H. P.; Kankelborg, C. C.; Handy, B. N.; De Pontieu, B.
Bibcode: 1999SoPh..187..261S
Altcode:
The Transition Region and Coronal Explorer (TRACE) - described in the
companion paper by Handy et al. (1999) - provides an unprecedented
view of the solar outer atmosphere. In this overview, we discuss the
initial impressions gained from, and interpretations of, the first
million images taken with TRACE. We address, among other topics,
the fine structure of the corona, the larger-scale thermal trends,
the evolution of the corona over quiet and active regions, the high
incidence of chromospheric material dynamically embedded in the coronal
environment, the dynamics and structure of the conductively dominated
transition region between chromosphere and corona, loop oscillations
and flows, and sunspot coronal loops. With TRACE we observe a corona
that is extremely dynamic and full of flows and wave phenomena, in
which loops evolve rapidly in temperature, with associated changes in
density. This dynamic nature points to a high degree of spatio-temporal
variability even under conditions that traditionally have been referred
to as quiescent. This variability requires that coronal heating can
turn on and off on a time scale of minutes or less along field-line
bundles with cross sections at or below the instrumental resolution
of 700 km. Loops seen at 171 Å (∼1 MK) appear to meander through
the coronal volume, but it is unclear whether this is caused by the
evolution of the field or by the weaving of the heating through the
coronal volume, shifting around for periods of up to a few tens of
minutes and lighting up subsequent field lines. We discuss evidence
that the heating occurs predominantly within the first 10 to 20 Mm
from the loop footpoints. This causes the inner parts of active-region
coronae to have a higher average temperature than the outer domains.
Title: The transition region and coronal explorer
Authors: Handy, B. N.; Acton, L. W.; Kankelborg, C. C.; Wolfson, C. J.;
Akin, D. J.; Bruner, M. E.; Caravalho, R.; Catura, R. C.; Chevalier,
R.; Duncan, D. W.; Edwards, C. G.; Feinstein, C. N.; Freeland, S. L.;
Friedlaender, F. M.; Hoffmann, C. H.; Hurlburt, N. E.; Jurcevich,
B. K.; Katz, N. L.; Kelly, G. A.; Lemen, J. R.; Levay, M.; Lindgren,
R. W.; Mathur, D. P.; Meyer, S. B.; Morrison, S. J.; Morrison, M. D.;
Nightingale, R. W.; Pope, T. P.; Rehse, R. A.; Schrijver, C. J.;
Shine, R. A.; Shing, L.; Strong, K. T.; Tarbell, T. D.; Title, A. M.;
Torgerson, D. D.; Golub, L.; Bookbinder, J. A.; Caldwell, D.; Cheimets,
P. N.; Davis, W. N.; Deluca, E. E.; McMullen, R. A.; Warren, H. P.;
Amato, D.; Fisher, R.; Maldonado, H.; Parkinson, C.
Bibcode: 1999SoPh..187..229H
Altcode:
The Transition Region and Coronal Explorer (TRACE) satellite, launched
2 April 1998, is a NASA Small Explorer (SMEX) that images the solar
photosphere, transition region and corona with unprecedented spatial
resolution and temporal continuity. To provide continuous coverage
of solar phenomena, TRACE is located in a sun-synchronous polar
orbit. The ∼700 Mbytes of data which are collected daily are made
available for unrestricted use within a few days of observation. The
instrument features a 30-cm Cassegrain telescope with a field of view
of 8.5×.5 arc min and a spatial resolution of 1 arc sec (0.5 arc sec
pixels). TRACE contains multilayer optics and a lumogen-coated CCD
detector to record three EUV wavelengths and several UV wavelengths. It
observes plasmas at selected temperatures from 6000 K to 10 MK with
a typical temporal resolution of less than 1 min.
Title: TRACE Observations of the Birth and Evolution of Emerging
Flux Regions
Authors: Wolfson, C. J.; Schrijver, C. J.; Title, A. M.
Bibcode: 1999AAS...194.7805W
Altcode: 1999BAAS...31..961W
TRACE is uniquely able to observe the birth and evolution of emerging
flux regions, X-ray ``bright points" and bona-fide active regions due to
its high spatial resolution, wide temperature coverage, and continual
solar viewing. On board memory limitations and photon statistics,
however, do force compromises. In this poster we discuss several cases
of emerging flux and subsequent region evolution. These include events
at different aspect angles (on disk and near the limb), near and not
near existing active regions with which there can be interconnection,
and regions that disappear in a couple of days as well as those which
develop more fully. Some of the observations include several channels
(temperature regimes) and some concentrate on higher temporal resolution
with less spectral coverage. Analysis of some of the latter include
MDI magnetograms at a one minute cadence. This work was supported by
NASA contract NAS5-38099.
Title: Dynamics of Transition Region Moss
Authors: Berger, T. E.; de Pontieu, B.; Schrijver, C. J.; Title, A. M.
Bibcode: 1999AAS...194.7901B
Altcode: 1999BAAS...31..963B
We examine the dynamics of solar transition region "moss", the 10(6)
K EUV emission at the footpoint regions of 2--3 MK active region coronal
loops. Comparisons of TRACE 171 Angstroms movies with SVST (La Palma) Ca
II K-line, Hα , and G-band movies are made. Local Correlation Tracking
(LCT) flowmapping techniques are used to establish the photospheric
flowfield in plage regions with and without associated moss. The
relation of moss emission to chromospheric spicules or fibrils is
examined in detail using Hα movies and dopplergrams. In addition,
several microflare events occuring in plage regions are analyzed using
TRACE and SVST movies. This research was supported by NASA contract
NAS5-38099 (TRACE) and NASA SR&T grant NASW-98008.
Title: Effects of extinction in the quiet solar corona observed
with TRACE
Authors: Schrijver, C. J.; McMullen, R. A.
Bibcode: 1999AAS...194.7908S
Altcode: 1999BAAS...31..964S
We use full-disk composite images of the solar corona as observed in
the 171 Angstroms ( ~ 1 MK) and 195 Angstroms ( ~ 1.5 MK) pass bands
of the Transition Region and Coronal Explorer to demonstrate that the
emission from the quiet solar corona must necessarily be subjected
to substantial extinction. We discuss the potential causes for these
effects, including resonant scattering in the high, hot corona, and
absorption by material at chromospheric temperatures in a multitude
of dynamic structures embedded within the low corona. This work was
supported by contract NAS5-38099 with NASA Goddard Space Flight Center.
Title: SONAR - Solar Near-surface Active Region Rendering
Authors: Scherrer, P. H.; Hoeksema, J. T.; Kosovichev, A. G.; Duvall,
T. L.; Schrijver, K. J.; Title, A. M.
Bibcode: 1999AAS...194.7606S
Altcode: 1999BAAS...31Q.957S
The processes in the top 20,000-km of the Sun's convection zone govern
the growth and decay of active regions and provide the magnetic flux and
energy for the active phenomena of the upper solar atmosphere. The MDI
experiment on SOHO has demonstrated that this region is now accessible
to study by means of local helioseismology. However, SOHO provides
neither the temporal nor spatial resolution and coverage necessary
to exploit these techniques to study the eruption and evolution of
active region magnetic structures. The SONAR mission with moderate
resolution full disk Doppler and vector magnetic field observations,
and atmospheric magnetic connectivity observations via EUV imaging
can provide the necessary data. The science motivation and general
instrumentation requirements for the mission are presented.
Title: A new view of the solar corona from the transition region
and coronal explorer (TRACE)
Authors: Golub, L.; Bookbinder, J.; Deluca, E.; Karovska, M.; Warren,
H.; Schrijver, C. J.; Shine, R.; Tarbell, T.; Title, A.; Wolfson,
J.; Handy, B.; Kankelborg, C.
Bibcode: 1999PhPl....6.2205G
Altcode:
The TRACE Observatory is the first solar-observing satellite in the
National Aeronautics and Space Administration's (NASA) Small Explorer
series. Launched April 2, 1998, it is providing views of the solar
transition region and low corona with unprecedented spatial and
temporal resolution. The corona is now seen to be highly filamented,
and filled with flows and other dynamic processes. Structure is seen
down to the resolution limit of the instrument, while variability and
motions are observed at all spatial locations in the solar atmosphere,
and on very short time scales. Flares and shock waves are observed,
and the formation of long-lived coronal structures, with consequent
implications for coronal heating models, has been seen. This overview
describes the instrument and presents some preliminary results from
the first six months of operation.
Title: Dynamics and Plasma Diagnostics of Transition Region ``Moss''
using SOHO/CDS, TRACE and SVST (La Palma)
Authors: de Pontieu, B.; Berger, T. E.; Fletcher, L.; Schrijver,
C. J.; Title, A. M.
Bibcode: 1999AAS...194.7804D
Altcode: 1999BAAS...31..961D
Recent observations of solar active regions with the Transition
Region And Coronal Explorer (TRACE) have revealed finely textured,
low-lying extreme ultraviolet (EUV) emission, called the ``moss'',
appearing as a bright dynamic pattern with dark inclusions. The moss
has been interpreted as the upper transition region by Berger et al.,
(1999). In this poster we study the physical conditions in the moss
plasma, as well as its dynamics and connections to photosphere and
chromosphere. Using simultaneous SOHO Coronal Diagnostic Spectrometer
(CDS) and TRACE observations of Active Region (AR) 8227 on 30-May-1998
we determine the physical parameters of the moss material. We find T_e =
0.6-1.5 10(6) K and n_e = 2-5 10(9) cm(-3) at a temperature of 1.3 10(6)
K. The pressure in the moss plasma is higher than that in coronal loops
observed in the TRACE Fe IX/X 171 Angstroms passband, and moss emission
is associated with high temperature loops, observed by SXT and by CDS
in lines of T_max > 2.5 10(6) K. The volume filling factor of the
moss plasma is of the order 0.1 and the path along which the emission
originates is of the order 1,000 km long. We examine the dynamics of
the moss plasma, by making comparisons of TRACE 171 Angstroms movies
with SVST (La Palma) Ca II K-line, Hα , and G-band movies. Local
Correlation Tracking (LCT) flowmapping techniques are used to establish
the photospheric flowfield in plage regions with and without associated
moss. The relation of moss emission to chromospheric spicules or fibrils
is examined in detail using Hα movies and dopplergrams. In addition,
several miniflare events occuring in plage regions are analyzed using
TRACE and SVST movies. This research was supported by NASA contract
NAS5-38099 (TRACE) and NASA SR&T grant NASW-98008.
Title: Strategic Plans for the Future of Solar Physics: a community
discussion of the NASA Sun-Earth Connection Program Roadmap and the
NAS Decadal Survey of Astronomy and Astrophysics (Solar Astronomy
section)
Authors: Schrijver, K.; Knoelker, M.
Bibcode: 1999AAS...194.6101S
Altcode: 1999BAAS...31..920S
The NASA Sun-Earth Connections Program is currently revising its
Roadmap, the long-range plan for science goals, technology development,
and missions between 2000 and 2040. From the interior dynamics of
the Sun, to the interactions of plasma, fields, and radiation in the
photosphere and solar atmosphere, to the heating and structure of the
corona, to the acceleration, structure, and evolution of the solar wind,
to the interactions of the heliosphere with the interstellar medium,
to the processes of solar, stellar, and solar system evolution -
progress in each of these domains will help us understand how the Sun
impacts our home in space. The Roadmap Committee is seeking to refine
and extend the SEC's vision and identify the milestone missions for
the future. During this session, an outline of the current draft
Roadmap will be presented, and further community involvement will
be solicited to ensure the strongest possible concensus on the
revised Roadmap. The National Academy of Sciences' Space Science
Board has appointed a committee to perform a Decadal Survey of
Astronomy and Astrophysics, which is surveying the field of space-
and ground-based astronomy and astrophysics, recommending priorities
for the most important new initiatives of the decade 2000-2010. The
prioritization delivered by the earlier Decadal Surveys has played
an important role in guiding the funding agencies in setting their
priorities for astronomy and astrophysics. Therefore it will be of
crucial importance for solar physics to contribute a strong case
for its own set of future projects to be incorpoprated into the
survey. The solar physics of the next decade will be characterized by
its increasing societal relevance in the context of the National Space
Weather Program and related issues, as well as its classical importance
as a ``base" for many astrophysical questions. The presentation and
subsequent discussion at the Chicago meeting is intended to solicit
further community input, to achieve optimal representation for solar
physics in the Decadal Survey. The Roadmap Committee and the Decadal
Survey's solar panel encourage the whole solar physics community to
contact them prior to the meeting. The list of the committee/panel
members and their e-mail addresses, as well as related information,
can be accessed via their websites at http://www.lmsal.com/sec/ and
http://www.nas.edu/bpa/projects/astrosurvey/solar/ , respectively.
Title: Observations of Filaments
Authors: Title, A.; Schrijver, C.; Tarbell, T.; Shine, R.
Bibcode: 1999AAS...194.7905T
Altcode: 1999BAAS...31..963T
Filaments are clearly seen as absorption features in the TRACE FE
IX and XII images. Because of the 24 hour coverage of TRACE many
filaments have been observed on the disk and near the limb, and as
quiescent and active structures. A quiescent filament consists of many
parallel strands the run nearly parallel to the surface. The strands
are often at the TRACE resolution and are always in motion. Adjacent
strands often exhibit flows in opposite directions with speeds of
10 to 40 km/second. Filaments occasionally erupt explosive from the
surface. The initial accelerations have not been observed with a 30
second cadence. Velocities in the initial phase range between 200
and 400 km/s. Filaments are observed to erupt and travel out of the
TRACE field of view, erupt and fall back to the solar surface, and
erupt and travel a short distance before being constraint by overlying
fields. Examples of the dynamic structure of filaments and their modes
of eruption will be demonstrated in a video presentation.
Title: Dispersal of Magnetic Flux in the Quiet Solar Photosphere
Authors: Hagenaar, H. J.; Schrijver, C. J.; Title, A. M.; Shine, R. A.
Bibcode: 1999ApJ...511..932H
Altcode:
We study the random walk of magnetic flux concentrations on two
sequences of high-resolution magnetograms, observed with the Michelson
Doppler Imager on board SOHO. The flux contained in the concentrations
ranges from |Φ|=1018 Mx to |Φ|=1019 Mx, with
an average of |Φ|=2.5×1018 Mx. Larger concentrations tend
to move slower and live longer than smaller ones. On short timescales,
the observed mean-square displacements are consistent with a random
walk, characterized by a diffusion coefficient D(t<10 ks)=70-90
km2 s-1. On longer timescales, the diffusion
coefficient increases to D(t>30 ks)=200-250 km2
s-1, approaching the measurements for a five-day set of Big
Bear magnetograms, D~=250 km2 s-1. The transition
between the low and large diffusion coefficients is explained with
a model and simulations of the motions of test particles, subject to
random displacements on both the granular and supergranular scales,
simultaneously. In this model, the supergranular flow acts as a
negligible drift on short timescale, but dominates the granular
diffusion on longer timescales. We also investigate the possibility
that concentrations are temporarily confined, as if they were caught
in supergranular vertices, that form short-lived, relatively stable
environments. The best agreement of model and data is found for step
lengths of 0.5 and 8.5 Mm, associated evolution times of 14 minutes
and 24 hr, and a confinement time of no more than a few hours. On
our longest timescale, DSim(t>105)-->285
km2 s-1, which is the sum of the small- and
large-scale diffusion coefficients. Models of random walk diffusion on
the solar surface require a larger value: DWang=600+/-200
km2 s-1. One possible explanation for the
difference is a bias in our measurements to the longest lived, and
therefore slower concentrations in our data sets. Another possibility
is the presence of an additional, much larger diffusive scale.
Title: Coordinated Observations of Transition Region Dynamics using
TRACE and the SVST
Authors: Berger, T.; de Pontieu, B.; Schrijver, C.; Title, A.;
Scharmer, G.
Bibcode: 1999ASPC..183..365B
Altcode: 1999hrsp.conf..365B
No abstract at ADS
Title: The Dynamic Nature of the Solar Magnetic Field
Authors: Schrijver, C. J.; Title, A. M.
Bibcode: 1999ASPC..158...15S
Altcode: 1999ssa..conf...15S
No abstract at ADS
Title: Obituary: Cornelis Zwaan, 1928-1999
Authors: Rutten, Rob; Schrijver, Karel
Bibcode: 1999BAAS...31.1612R
Altcode:
No abstract at ADS
Title: The Transition Region and Coronal Explorer
Authors: Handy, B. N.; Deluca, E. E.; McMullen, R. A.; Schrijver,
C. J.; Tarbell, T. D.; Title, A. M.; Wolfson, C. J.
Bibcode: 1998AAS...193.1207H
Altcode: 1998BAAS...30R1269H
The Transition Region and Coronal Explorer (TRACE), launched 1 April
1998, will have at the time of this meeting been in orbit for just over
8 months. In that time, the instrument will have taken over 500,000
exposures of the sun in ultraviolet and extreme ultraviolet wavelengths,
will have completed three-forths of the nominal mission and will be
approaching the end of the first eclipse season. The TRACE telescope
is unique in its ability to observe in UV and EUV wavelengths at high
cadence with unprecedented resolution. We present a review of the TRACE
instrument and show current observations and results. We discuss the
performance of the instrument in terms of observational capabilities,
sensitivity, calibration, effects of aging on the instrument, CCD
effects, and contamination effects.
Title: The TRACE Mission: Initial Scientific Results
Authors: Title, A. M.; Tarbell, T. D.; Wolfson, J.; Schrijver, K.;
Fisher, R. R.; Gang, Th.; Golub, L.; McMullen, R. A.; Kankelborg,
C.; TRACE Collaboration
Bibcode: 1998AAS...19310008T
Altcode: 1998BAAS...30R1398T
TRACE (Transition Region and Coronal Explorer) is a Small Explorer
Mission (SMEX) devoted to studying the evolution and propagation
of fine-scale magnetic fields and plasma structures throughout the
solar atmosphere. The instrument consists of a telescope with a 30 cm
primary mirror, normal incidence coatings for three EUV bands (171,
195 and 284 Angstroms), and interference filters for UV bands (1216
to 1700 Angstroms) as well as white light (allowing the selection of
temperature ranges from ~ 6 000 to ~ 2 500 000 degrees K). The 1024
x 1024 CCD camera has a field of view of 8.5 arcmin with a spatial
resolution of 1 arcsec and exposure times of 0.002 to 260 sec with
a cadence as short as two seconds. The spacecraft was launched on
April 1, 1998, and first light for the telescope occurred on April
20. Observations have been collected nearly 24 hours per day since
then, with no significant problems in any segment of the spacecraft,
instrument, or mission operations. TRACE transmits about 3--4 GB of
data per week which gets automatically reformatted and becomes available
for the scientific community within approx. 24 hours. It is accessible
without restrictions (only guidelines) together with other informations
(technical details, educational material, movies, images,...) at:
http://vestige.lmsal.com/TRACE. The images reveal activity in the
solar atmosphere in stunning detail and include the first detailed
observations of a magnetic energy release. This magnetic reconnection
was observed on May 8, 1998, in a region of the solar atmosphere where
two sets of perpendicular magnetic loops expanded into each other (see
NASA Press Release 98-92). The TRACE mission has been developed and
operated by NASA's Goddard Space Flight Center, Lockheed Martin Solar
& Astrophysics Laboratory, Smithsonian Astrophysical Observatory,
and Montana State University.
Title: Coronal heating: an overview of observations and models
Authors: Schrijver, Carolus
Bibcode: 1998APS..DPP.C2M01S
Altcode:
The list of potential mechanisms that contribute to the heating of
the hottest domain in the outer atmosphere of the Sun (and of other
stars like the Sun) is steadily growing. The last few years have
seen both a rapid increase in computational power and in angular and
temporal resolution of space--based instruments. This is gradually
leading to a deeper understanding of coronal heating, but has also
taught us that the corona is a very dynamic place with interrelated
processes on the scale of the large bipolar active regions down to
very small spatio-temporal scales. In this overview, I discuss some
of the recent developments in our understanding of stellar coronae,
primarily focusing on those processes that result in relatively
long-lived bright coronal structures. Recent high-resolution movies
taken with the Transition--Region and Coronal Explorer TRACE will be
shown to illustrate the complex dynamic character of the solar corona
on scales that were unexplored until this year.
Title: Alpha Centauri: coronal temperature structure and abundances
from ASCA observations
Authors: Mewe, R.; Drake, S. A.; Kaastra, J. S.; Schrijver, C. J.;
Drake, J. J.; Guedel, M.; Schmitt, J. H. M. M.; Singh, K. P.; White,
N. E.
Bibcode: 1998A&A...339..545M
Altcode:
We have analyzed the X-ray spectrum of the nearby binary alpha Cen AB
(G2V + K1V) that has been obtained from observations with ASCA. The
coronal temperature structure and abundances have been derived from
multi-temperature fitting and confirmed by a differential emission
measure analysis. The corona as seen by ASCA is essentially isothermal
with a temperature around 0.3 keV, consistent with the evolutionary
picture of coronae of aging solar-type stars. A comparison between the
measurements from various instruments indicates a source variability
in the coronal flux (which precludes the joint fitting of data from
different instruments taken at different epochs) and temperature
structure consistent with that discovered in a series of ROSAT
observations. The elemental abundances agree with solar photospheric
abundances for Ne, Si, and Fe at 1hbox {\sigma^2 CrB}ma level, while O
appears to be underabundant by a factor of about 3 relative to solar
photospheric values, and Mg overabundant by a factor of a few. The
abundance ratios with respect to Fe are better determined: [O/Fe] =
0.4+/-0.14 (x solar, etc.), [Mg/Fe] = 4+/-1, [Ne/Fe] = 1+/-0.3, and
[Si/Fe] = 6+/-4.
Title: Large-scale coronal heating by the small-scale magnetic field
of the Sun
Authors: Schrijver, C. J.; Title, A. M.; Harvey, K. L.; Sheeley,
N. R.; Wang, Y. -M.; van den Oord, G. H. J.; Shine, R. A.; Tarbell,
T. D.; Hurlburt, N. E.
Bibcode: 1998Natur.394..152S
Altcode:
Magnetic fields play a crucial role in heating the outer atmospheres
of the Sun and Sun-like stars, but the mechanisms by which magnetic
energy in the photosphere is converted to thermal energy in the corona
remain unclear. Observations show that magnetic fields emerge onto
the solar surface as bipolar regions with a broad range of length
scales. On large scales, the bipolar regions survive for months before
dispersing diffusively. On the smaller scales, individual bipolar
regions disappear within days but are continuously replenished by new
small flux concentrations, resulting in a sustained state of mixed
polarity. Here we determine the rate of emergence of these small
bipolar regions and we argue that the frequent magnetic reconnections
associated with these regions (an unavoidable consequence of continued
flux replacement) will heat the solar atmosphere. The model that
describes the details of these mixed-polarity regions is complementary
to the traditional diffusion model for large-scale flux dispersal and
a combination of the two should lead to a more complete understanding
of the role of magnetic fields in stellar atmospheres.
Title: First Results from the TRACE Mission
Authors: Title, A.; Tarbell, T.; Schrijver, C.; Wolfson, J.; Shine,
R.; Hurlburt, N.; Golub, L.; Deluca, E.; Bookbinder, J.; Handy, B.;
Acton, L.; Harrison, R.; Delaboudinere, J. -P.
Bibcode: 1998AAS...192.1507T
Altcode: 1998BAAS...30..841T
The TRACE spacecraft was launched on 1 April and all systems are
functioning as designed. The initial outgassing period will conclude
on 20 April and the science program will then begin. TRACE is a UV-EUV
imager with one arc second spatial resolution and is capable of taking
images with a cadence as high as two seconds. We will present images
and image sequences. We hope to present initial comparisons of magnetic
evolution and transition region and coronal brightenings.
Title: The Sun's Magnetic Carpet
Authors: Title, A. M.; Schrijver, C. J.
Bibcode: 1998ASPC..154..345T
Altcode: 1998csss...10..345T
Recent observations by the Michelson Doppler Imager on the Solar and
Heliospheric Observatory show that the magnetic field in the quiet
sun is replaced in about 40 hours. Flux emergence is accompanied by an
equivalent disappearance that results in a mean field in the quiet Sun
of about 2 Gauss. The kinetic balance of emergence, fragmentation,
merging, and cancellation is responsible for the magnetic network
observed in the quiet Sun. The constantly changing mixed polarity
fields in the network are potential sources for the energy release
necessary for heating the corona.
Title: Modeling the distribution of magnetic fluxes in field
concentrations in a solar active region
Authors: Schrijver, Carolus J.; Title, Alan M.; Hagenaar, Hermance J.;
Shine, Richard A.
Bibcode: 1997SoPh..175..329S
Altcode:
Much of the magnetic field in solar and stellar photospheres is
arranged into clusters of `flux tubes', i.e., clustered into compact
areas in which the intrinsic field strength is approximately a
kilogauss. The flux concentrations are constantly evolving as they
merge with or annihilate against other concentrations, or fragment
into smaller concentrations. These processes result in the formation
of concentrations containing widely different fluxes. Schrijver et
al. (1997, Paper I) developed a statistical model for this distribution
of fluxes, and tested it on data for the quiet Sun. In this paper we
apply that model to a magnetic plage with an average absolute flux
density that is 25 times higher than that of the quiet network studied
in Paper I. The model result matches the observed distribution for the
plage region quite accurately. The model parameter that determines the
functional form of the distribution is the ratio of the fragmentation
and collision parameters. We conclude that this ratio is the same in the
magnetic plage and in quiet network. We discuss the implications of this
for (near-)surface convection, and the applicability of the model to
stars other than the Sun and as input to the study of coronal heating.
Title: EUV spectroscopy of cool stars. III. Interpretation of EUVE
spectra in terms of quasi-static loops.
Authors: van den Oord, G. H. J.; Schrijver, C. J.; Camphens, M.;
Mewe, R.; Kaastra, J. S.
Bibcode: 1997A&A...326.1090V
Altcode:
We discuss the limitations of coronal spectroscopy to derive physical
parameters of stellar magnetic loops. We distinguish between the
intrinsic non-uniqueness of emitted spectra for models of quasi-static
coronal loops, and the supplemental ambiguity introduced by both
instrumental effects and spectral line formation. We demonstrate
that the spectrum emitted by loops with constant cross-sections is
the same for a large range of values of the conductive flux at the
base when the apex temperature is fixed. Because it is impossible to
estimate the conductive flux at the base from observations, it is also
impossible to determine the volume heating rate and the loop length
uniquely. For geometrically expanding (tapered) loops, the emitted
spectrum depends on the expansion and on the conductive flux at the
base, and there is a trade off between them without significant changes
in the spectrum. We show that loop length and heating rate can only be
derived if the density is known, but that even then a large intrinsic
uncertainty remains for these loop parameters. We conclude that there
is no unambiguous relationship between loop parameters and emitted
spectra: modeling the spectra as the sum of spectra from discrete loops
cannot result in a unique determination of coronal structure. Based
on spectra observed with the Extreme Ultra Violet Explorer (EUVE) we
find that quasi-static loop models allow adequate modeling of stellar
coronal spectra. We show that coronal loops on active cool stars must
expand with height. The minimum required areal expansion between base
and apex is not very large, lying between 2 and 5. For three stars
(α Cen, Capella and ξ UMa) the observations suggest the presence of
two distinct, dominant loop populations, while for χ^1^ Ori a single
population, characterized by a single apex temperature, suffices. The
high electron densities (10^12^-10^13^cm^-3^) for coronal components
on Capella and ξ UMa require abnormally large heating rates. It is
likely that these high densities are related to a multitude of small
volumes that are temporarily excited.
Title: Sustaining the Quiet Photospheric Network: The Balance of
Flux Emergence, Fragmentation, Merging, and Cancellation
Authors: Schrijver, Carolus J.; Title, Alan M.; van Ballegooijen,
Adriaan A.; Hagenaar, Hermance J.; Shine, Richard A.
Bibcode: 1997ApJ...487..424S
Altcode:
The magnetic field in the solar photosphere evolves as flux
concentrations fragment in response to sheared flows, merge when they
collide with others of equal polarity, or (partially) cancel against
concentrations of opposite polarity. Newly emerging flux replaces the
canceled flux. We present a quantitative statistical model that is
consistent with the histogram of fluxes contained in concentrations
of magnetic flux in the quiet network for fluxes exceeding ~2 ×
1018 Mx, as well as with estimated collision frequencies
and fragmentation rates. This model holds for any region with weak
gradients in the magnetic flux density at scales of more than a few
supergranules. We discuss the role of this dynamic flux balance (i)
in the dispersal of flux in the photosphere, (ii) in sustaining the
network-like pattern and mixed-polarity character of the network, (iii)
in the formation of unipolar areas covering the polar caps, and (iv) on
the potential formation of large numbers of very small concentrations
by incomplete cancellation. Based on the model, we estimate that as
much flux is cancelled as is present in quiet-network elements with
fluxes exceeding ~2 × 1018 Mx in 1.5 to 3 days, which is
compatible with earlier observational estimates. This timescale is
close to the timescale for flux replacement by emergence in ephemeral
regions, so that this appears to be the most important source of flux
for the quiet-Sun network; based on the model, we cannot put significant
constraints on the amount of flux that is injected on scales that are
substantially smaller than that of the ephemeral regions. We establish
that ephemeral regions originate in the convection zone and are not
merely the result of the reemergence of previously cancelled network
flux. We also point out that the quiet, mixed-polarity network is
generated locally and that only any relatively small polarity excess
is the result of flux dispersal from active regions.
Title: Coronal and chromospheric emission from cool stars in
near-simultaneous ROSAT all-sky survey and Mount Wilson data.
Authors: Piters, A. J. M.; Schrijver, C. J.; Schmitt, J. H. M. M.;
Rosso, C.; Baliunas, S. L.; van Paradijs, J.; Zwaan, C.
Bibcode: 1997A&A...325.1115P
Altcode:
Mt. Wilson Ca II H&K line-core emission fluxes for 215 F-,
G- and K-type stars were obtained within at most a few days of the
corresponding ROSAT All-Sky Survey observations. These stars cover wide
ranges of stellar activity, spectral type and luminosity class. In
this paper we study the well-known relationship between the Ca II
H&K line-core emission in excess of the minimum emission and the
soft X-ray emission. We find that flux densities normalised with the
bolometric flux densities are the best quantity in which to express
activity when comparing radiative emission in different temperature
regimes. We find a power-law relationship, in which the X-ray normalised
emission varies approximately quadratically with the normalised excess
Ca II H&K line-core emission. This relationship does not depend on
luminosity class at least up to luminosity class III, and it does not
depend on effective temperature. The scatter around this relationship
is consistent with the measurement errors. The X-ray spectral hardness
ratios of main-sequence stars increase with the X-ray flux densities;
a similar trend, but with substantially larger scatter, is also present
for evolved stars. A comparison between values from different passbands
of the Mt. Wilson HK spectrophotometer shows that relatively hot stars
((B-V)<=0.50) appear to have a Ca II line core emission peak about
a factor 2 to 3 wider than cooler stars.
Title: VizieR Online Data Catalog: Near-simultaneous ROSAT and Mt
Wilson data (Piters+ 1997)
Authors: Piters, A. J. M.; Schrijver, C. J.; Schmitt, J. H. M. M.;
Rosso, C.; Baliunas, S. L.; van Paradijs, J.; Zwaan, C.
Bibcode: 1997yCat..33251115P
Altcode:
Table 1 lists near-simultaneous X-ray data and Ca II H&K line-core
emission data from the ROSAT All-Sky Survey and from the Mt. Wilson
H&K spectrometer, respectively. The stars in this sample are 215
bright F-, G-, and K-type stars. Table 2 lists the derived excess Ca II
H&K line-core and the X-ray flux densities for the same stars. (2
data files).
Title: Chromospheric Emission in Metal-poor Solar-Type Stars
Authors: Peterson, Ruth C.; Schrijver, Carolus J.
Bibcode: 1997ApJ...480L..47P
Altcode:
Cool stars like the Sun show chromospheric activity, which is
largely attributed to nonradiative sources of energy linked to
rotationally driven magnetic fields. However, a nonmagnetic process,
such as dissipation of acoustic waves, may operate side by side with
the first process. We report detections of chromospheric emission
reversals in Mg II 2800 Å spectra for nine metal-poor solar-type
stars, some believed to be among the oldest stars known, whose great
age and slow rotation argue against strong magnetic activity. Although
interstellar absorption is always present, in seven stars with high
radial velocities each reversal profile is seen to be doubly peaked,
with the blue peak stronger than the red, much like such profiles
from the quiet Sun. Despite having as little as 1/300th as much metal
as the Sun and being 3 times as old, these stars have chromospheric
activity comparable to that in solar quiet regions. While these data
do not rule out magnetic fields, they support an acoustic origin of
chromospheric emission and show that relatively inactive solar-type
stars of all ages have chromospheres whose characteristics are largely
independent of metallicity.
Title: On the Dynamics of Magnetic Flux Concentrations in Quiet
Photospheric Network.
Authors: Sakai, J. I.; Ryutova, M.; Schrijver, K.; Shine, R.; Tarbell,
T.; Berger, T.; Title, A.; Hagenaar, H.
Bibcode: 1997SPD....28.0260S
Altcode: 1997BAAS...29..904S
Magnetic flux concentrations in the quiet photospheric network show
a complex dynamics which includes merging of colliding fluxes, the
"total" or partial cancellation of neighboring fluxes, fragmentation
and others. We propose a mechanism to explain the observed phenomena
based on the idea that magnetic flux concentrations in the photospheric
network are essentially non-collinear. We show that non-collinearity
of colliding fluxes leads to the whole new class of effects which are
observed; for example, the apparent cancellation of opposite polarity
fluxes turns into the formation of horizontal magnetic fluxes (which
later may appear as a new weaker bipoles) and is accompanied by the
shock formation and mini-flares. In the case of shock formation
the reconnection area becomes a source of the acoustic emission;
mini-flares may be seen as bright points. The energetics of these
processes strongly depends on geometry of "collision" and physical
parameters of colliding fluxes. For example, if colliding fluxes have
comparable and "small" cross sections, the reconnection results in
complete reorganization of their magnetic fields; if merging fluxes
are large enough or considerably different, magnetic flux may be
only partially reconnected and partially survived. Reconnection of
non-collinear equal polarity fluxes leads to the "scattering" processes
which include the fragmentation into several smaller fluxes if initially
colliding concentrations carried different amount of magnetic flux. We
give the example of numerical simulation for the case of merging and
fragmentation process occurring during the collision of collinear
"strong" and "weak" magnetic flux concentrations. The calculation
results shown to be consistent with observational data from both
the SOHO/MDI instrument and the Swedish Vacuum Solar Telescope on
La Palma. This research is supported by NASA contract NAG5-3077 at
Stanford University and the MDI contract PR 9162 at Lockheed.
Title: The Distribution of Cell Sizes of the Solar Chromospheric
Network
Authors: Hagenaar, Hermance J.; Schrijver, Carolus J.; Title, Alan M.
Bibcode: 1997ApJ...481..988H
Altcode:
This paper studies the cellular pattern of the supergranular network. We
present an algorithm to draw a surface-filling cell pattern on an
uninterrupted two-day sequence of Ca II K filtergrams with a 1 nm
bandpass. The 60° × 40° field of view contains both quiet and
enhanced network and plages. The algorithm uses a threshold-independent
method of steepest descent on spatially smoothed and time-averaged
images. We determine the distribution function of cell areas and
find a broad, asymmetric spectrum of areas. The distribution is
found to be invariant for different spatial smoothings if the cell
areas are normalized to a unit mean. It is this invariance that
leads us to believe we have determined the intrinsic distribution of
cell areas. Extrapolation of the average cell size to zero spatial
smoothing yields a characteristic cell diameter of L = 13-18 Mm. This
is roughly half the generally quoted supergranular length scale L ~
32 Mm as determined with autocorrelation methods. The difference
in characteristic cell size reflects the application of a different
measurement method: the autocorrelation method as used by Simon &
Leighton and others is preferentially weighted towards relatively
large cells. We find no significant dependence of cell size on local
magnetic flux density.
Title: Preliminary SoHO/MDI Observations of Supergranular Evolution
Authors: Simon, G. W.; Strous, L. H.; Matt, S.; Title, A. M.;
Schrijver, C. J.
Bibcode: 1997SPD....28.0264S
Altcode: 1997BAAS...29R.904S
We present preliminary results of a study into the evolution of
supergranules, using data from SoHO/MDI. We discuss the supergranular
size spectrum, lifetimes, and topological evolution. We compare
structures of supergranular size visible in high-resolution SoHO/MDI
dopplergrams and in divergence maps derived from tracking of features
in dopplergrams. This work was supported by NASA Grant NAG5-3077 at
Stanford and Lockheed Martin, and by AFOSR and the Fellows Program of
AF Phillips Lab at NSO/SP.
Title: A search for interaction between magnetic fields and
supergranular flows in the network based on MDI observations
Authors: Schrijver, C. J.; Shine, R. A.; Title, A. M.; Hagenaar,
H. J.; Hurlburt, N. E.; Tarbell, T. D.; Simon, G. W.
Bibcode: 1997SPD....28.0243S
Altcode: 1997BAAS...29..901S
We study the supergranular flow field and its temporal evolution in
the quiet Sun as observed with the Michelson Doppler Imager on board
SOHO. We use the intensity images to derive the flow fields using
local correlation tracking. The data sets span one to two days with a
one--minute cadence. We separate areas with a relatively high filling
factor for magnetic concentrations from areas with a low magnetic
filling factor in order to study to what extent the flows influence
the magnetic network in the quiet Sun and vice versa. This work is
supported by NASA Grant NAG5-3077 at Stanford and Lockheed Martin,
and by AFOSR and the Fellows Program of AF Phillips Lab at NSO/SP
Title: Dispersal of magnetic flux in the quiet network as observed
on a day-long magnetogram sequences observed with MDI on SOHO
Authors: Hagenaar, H. J.; Schrijver, C. J.; Shine, R. A.; Title, A. M.
Bibcode: 1997SPD....28.0244H
Altcode: 1997BAAS...29..901H
We study the dynamic behavior of magnetic flux elements in the quiet
solar network using high--resolution magnetograms observed with the
Michelson Doppler Imager on board SOHO. We track concentrations of
magnetic flux in uninterrupted time sequences spanning 20 to 45 hours
in order to study the dispersal of magnetic elements in the turbulent
photospheric flows. We measure the displacements and derive average
speeds as a function of time. The displacements are compared to a
random walk model. This work is supported by NASA Grant NAG5-3077 at
Stanford and Lockheed.
Title: The TRACE Mission
Authors: Wolfson, J.; Bruner, M.; Jurcevich, B.; Lemen, J.; Schrijver,
K.; Shine, R.; Strong, K.; Tarbell, T.; Title, A.; Golub, L.;
Bookbinder, J.; Deluca, E.; Acton, L.; Handy, B.; Kankelborg, C.;
Fisher, R.
Bibcode: 1997SPD....28.0143W
Altcode: 1997BAAS...29..887W
The TRACE (Transition Region and Coronal Explorer) mission will explore
the connections between fine-scale magnetic fields and plasma structures
in the coronal, transition zone and temperature minimum regions of the
sun. TRACE will collect images of solar plasmas at temperatures from
10(4) to 10(7) K, with one arc second spatial resolution and excellent
temporal resolution and continuity. With a scheduled launch date of 15
December 1997, the mission will emphasize collaborative observations
with SoHO, enabling simultaneous observations of high-resolution images,
spectra, and magnetograms. The 30 cm aperture TRACE telescope uses four
normal-incidence coatings for the EUV and UV on quadrants of the primary
and secondary mirrors. Interference filters further isolate 5 different
UV bands. The images are co-aligned and internally stabilized against
spacecraft jitter. A 1024 x 1024 lumigen-coated CCD detector collects
images over an 8.5 x 8.5 arc minute field-of-view. LMATC, SAO, and GSFC
built the TRACE instrument, which was integrated with the GSFC-produced
SMEX spacecraft on 28 February (just over two years from the start
of its development). It will be put into a Sun-synchronous orbit and
operated in coordination with the SoHO Experiment Operations Facility at
GSFC. We are committed to maintaining a publicly accessible data base
for TRACE data. Browsing and data set requesting capabilities will be
provided at Web site www.space.lockheed.com/TRACE/TRACElinks.html. This
site already contains a large volume of information on the mission
including preliminary scientific observing programs and directions
as to how to participate in the mission now and in the future. This
project is supported by NASA contract NAS5-38099.
Title: The dynamic nature of the supergranular network
Authors: Title, A. M.; Schrijver, C. J.; van Ballegooijen, A. A.;
Hagenaar, H. J.; Shine, R. A.
Bibcode: 1997SPD....28.0242T
Altcode: 1997BAAS...29..900T
The magnetic field in the quiet solar photosphere evolves as flux
concentrations fragment in response to sheared flows, merge when they
collide with others of equal polarity, or (partially) cancel against
concentrations of opposite polarity. Newly emerging flux, mostly in
ephemeral regions, replaces the canceled flux in a matter of a few
days. We present a quantitative statistical model to describe the
resulting histogram of fluxes contained in concentrations of magnetic
flux in the quiet network. We discuss this dynamic flux balance with
respect to (i) the potential dispersal of flux in the photosphere
as a function of ephemeral-region properties, (ii) sustaining the
network--like pattern and mixed--polarity character of the network,
and (iii) the formation of unipolar areas covering the polar caps. We
establish that ephemeral regions are not the result of the re-emergence
of previously cancelled network flux. Moreover, their emergence cannot
be correlated to the emergence of active regions but must instead
be relatively homogeneous. We also point out that the bulk of the
quiet, mixed-polarity network is generated locally, and that only any
relatively small polarity excess is the result of flux dispersal from
active regions.
Title: On the Patterns of the Solar Granulation and Supergranulation
Authors: Schrijver, Carolus J.; Hagenaar, Hermance J.; Title, Alan M.
Bibcode: 1997ApJ...475..328S
Altcode:
We study the cellular patterns of the white light granulation and of the
chromospheric Ca II K supergranular network. We apply a gradient-based
tessellation algorithm to define the cell outlines. The geometry of
the patterns formed by the associated granular and supergranular
flows are very similar, in spite of the substantial difference in
length scale. We compare these patterns to generalized Voronoi foams
and conclude that both convective patterns are very nearly compatible
with an essentially random distribution of upflow centers, with the
downflow boundaries determined by the competing strengths of outflows
of neighboring upwellings. There appears to be a slight clustering
in upflow positions for the granulation, consistent with the granular
evolution. This slight preference for large granules to be surrounded
by somewhat smaller ones makes the granular and supergranular patterns
differ enough to allow a correct identification in three out of four
cases by eye. The model analogy suggests that the range in outflow
strengths is remarkably small. The patterns appear to be rather
insensitive to the details of the competing forces that establish the
pattern of the downflow network: similar patterns result under very
different conditions, so that little can be learned about the details
of the forces involved by studying the geometry of these patterns only.
Title: On the dynamics of magnetic flux concentrations in quiet
photospheric network.
Authors: Sakai, J. I.; Ryutova, M.; Schrijver, K.; Shine, R. A.;
Tarbell, T. D.; Berger, T. E.; Title, A. M.; Hagenaar, H. J.
Bibcode: 1997BAAS...29T.904S
Altcode:
No abstract at ADS
Title: The Dynamic Quiet Solar Corona: 4 Days of Joint Observing
with MDI and EIT
Authors: Schrijver, C. J.; Shine, R. A.; Hurlburt, N. E.; Tarbell,
T. D.; Lemen, J. R.
Bibcode: 1997ESASP.404..669S
Altcode: 1997cswn.conf..669S
No abstract at ADS
Title: Angular Momentum Evolution in Late-Type Stars
Authors: Charbonneau, P.; Schrijver, C. J.; MacGregor, K. B.
Bibcode: 1997cwh..conf..677C
Altcode: 2006mslp.conf..677C
No abstract at ADS
Title: Working Group 6: Magnetic Fields, Coronal Structure and
Phenomena
Authors: Schrijver, K. J.
Bibcode: 1997ESASP.404..149S
Altcode: 1997cswn.conf..149S
No abstract at ADS
Title: Dynamics of the Chromospheric Network: Mobility, Dispersal,
and Diffusion Coefficients
Authors: Schrijver, Carolus J.; Shine, Richard A.; Hagenaar, Hermance
J.; Hurlburt, Neal E.; Title, Alan M.; Strous, Louis H.; Jefferies,
Stuart M.; Jones, Andrew R.; Harvey, John W.; Duvall, Thomas L., Jr.
Bibcode: 1996ApJ...468..921S
Altcode:
Understanding the physics behind the dispersal of photo spheric magnetic
flux is crucial to studies of magnetoconvection, dynamos, and stellar
atmospheric activity. The rate of flux dispersal is often quantified by
a diffusion coefficient, D. Published values of D differ by more than a
factor of 2, which is more than the uncertainties allow. We propose that
the discrepancies between the published values for D are the result of
a correlation between the mobility and flux content of concentrations of
magnetic flux. This conclusion is based on measurements of displacement
velocities of Ca II K mottles using an uninterrupted 2 day sequence
of filtergrams obtained at the South Pole near cycle minimum. We
transform the Ca II K intensity to an equivalent magnetic flux density
through a power-law relationship defined by a comparison with a nearly
simultaneously observed magnetogram. One result is that, wherever the
network is clearly defined in the filtergrams, the displacement vectors
of the mottles are preferentially aligned with the network, suggesting
that network-aligned motions are more important to field dispersal than
deformation of the network pattern by cell evolution. The rms value
of the inferred velocities, R = <|v|2>½,
decreases with increasing flux, Φ, contained in the mottles, from R
≍ 240 m s-1 down to 140 s-1. The value of R(Φ)
appears to be independent of the flux surrounding the concentration,
to the extreme that it does not matter whether the concentration is
in a plage or in the network. The determination of a proper effective
diffusion coefficient requires that the function R(Φ) be weighted
by the number density n(Φ) of mottles that contain a total flux. We
find that n(Φ) decreases exponentially with Φ and propose a model
of continual random splitting and merging of concentrations of flux to
explain this dependence. Traditional methods used to measure D tend to
be biased toward the larger, more sluggish flux concentrations. Such
methods neglect or underestimate the significant effects of the
relatively large number of the more mobile, smaller concentrations. We
argue that the effective diffusion coefficient for the dispersal of
photo spheric magnetic flux is ∼600 km2 s-1.
Title: TRACE: the Transition Region and Coronal Explorer
Authors: Schrijver, C.; Title, A.; Acton, L.; Bruner, M.; Fischer,
R.; Golub, L.; Harrison, R.; Lemen, J.; Rosner, R.; Scharmer, G.;
Scherrer, P.; Strong, K.; Tarbell, T.; Wolfson, J.
Bibcode: 1996AAS...188.6704S
Altcode: 1996BAAS...28..934S
The TRACE mission is designed to obtain images of the solar
transition region and corona of unprecedented quality. With these
images we will be able to explore quantitatively the connections
between the photospheric magnetic field and the associated hot and
tenuous structures in the outer atmosphere. The TRACE telescope has
an aperture of 30 cm, and will observe an 8.5 x 8.5 arcminute field of
view with a resolution of one arcsecond. Finely tuned coatings on four
quadrants on the primary and secondary normal--incidence mirrors will
allow observations in narrow EUV and UV spectral bands. The passbands
are set to Fe IX, XII, and XV lines in the EUV band, while filters
allow observations in C IV, Ly alpha , and the UV continuum using
the UV mirror quadrant. The data thus cover temperatures from 10(4)
K up to 10(7) K. The Sun--synchronous orbit allows long intervals of
uninterrupted viewing. Observations at different wavelengths can be
made in rapid succession with an alignment of 0.1 arcsec. Coordinated
observing with TRACE, SoHO and YOHKOH will give us the first opportunity
to observe all temperature regimes in the solar atmosphere, including
magnetograms, simultaneously from space. TRACE is currently scheduled
to be launched in October 1997. More information can be found on the
web at ``http://pore1.space.lockheed.com/TRACE/welcome.html''.
Title: What is the size scale of the solar supergranular network?
Authors: Hagenaar, H.; Schrijver, C.; Title, A.
Bibcode: 1996AAS...188.0201H
Altcode: 1996BAAS...28..820H
We developed an algorithm to outline the chromospheric network on a
2-day sequence of Ca II K observations made from the South Pole, in
order to study the sizes of supergranulation cells. We find an average
cell diameter that is substantially smaller than the generally quoted
value of 30--35 Mm, as first determined by Simon and Leighton (1964)
from autocorrelation curves of the line--of--sight velocities. We
argue that the autocorrelation method is preferentially weighted
towards large cells, which results in an estimated size that is
approximately 1.5 to 2 times larger than the true average cell
diameter. A comparable difference should occur in studies of the size
scale of the chromospheric network. In addition, we find that secondary
maxima to the autocorrelation peaks of the Doppler signal imply that
the correlation between cell size and flow velocity is weak at best. If
such a correlation should exist, it would be too weak to affect the
spacing of the secondary maxima of the autocorrelation function.
Title: Initial Results from SOI/MDI High Resolution Magnetograms
Authors: Title, A.; Tarbell, T.; Frank, Z.; Schrijver, C.; Shine,
R.; Wolfson, J.; Zayer, I.; Scherrer, P.; Bush, R.; Deforest, C.;
Hoeksema, T.
Bibcode: 1996AAS...188.6915T
Altcode: 1996BAAS...28..938T
The Michelson Doppler Imager (MDI) on SoHO takes magnetogram
s with resolutions of 1.2 (high resolution) and 4 (full disk)
arcseconds. Movies of 16 hour duration have been constructed in full
disk and high resolution mode. High resolution movies of the south
polar region also have been obtained. In sums of nine high resolution
magnetograms it is possible to detect fields as low as 5 gauss and
total fluxes as low as 5 10(1) 6 Mx. In mid latitude regions new flux
is observed to emerge everywhere. At all latitudes below 60 degrees
flux is mixed on the scale of supergranulation. In the polar region
above 60 degrees only fields of a single polarity are observed above
the detection limit.
Title: Preliminary SOI/MDI Observations of Surface Flows by
Correlation Tracking in the Quiet Solar Photosphere and an Emerging
Active Region
Authors: Tarbell, T.; Frank, Z.; Hurlburt, N.; Saba, J.; Schrijver,
C.; Shine, R.; Title, A.; Simon, G.; Strous, L.
Bibcode: 1996AAS...188.6914T
Altcode: 1996BAAS...28..937T
The extended observation of the solar surface with frequent sampling
provided by MDI on SoHO offers the chance to observe the evolution of
supergranules and to measure surface flows associated with active
regions and perhaps larger scale zonal and meridonal flows. We
have used local correlation tracking of the granulation pattern for
measuring surface flows from MDI high resolution continuum images. The
datasets consist of 1024 x 1024 pixel images collected with a cadence
of one minute and extending many hours each. The images are typically
centered upon the central meridian of the sun and offset to the north
of sun center, spanning roughly 40 degrees of solar longitude and from
approximately -10 to +30 degrees of solar latitude. The latitude
dependence of the differential rotation is evident. We present
preliminary results of our search for signatures of mesogranules,
supergranules and giant cells. On 23 Feb. 1996, we obtained a 12-hour
continuous sequence including quiet sun near disk center and NOAA
region 7946 at about N08 E30. The active region grew rapidly over this
interval, forming several sunpots. We show preliminary comparisons of
the measured flow fields with coaligned SOI/MDI magnetograms taken
at 15-minute intervals. The SOI/MDI program is supported by NASA
grant NAG5-3077.
Title: SOI/MDI Measurements of Horizontal Flows in the South Polar
Region of the Sun by Correlation Tracking and Doppler Shifts
Authors: Simon, G.; Frank, Z.; Hurlburt, N.; Schrijver, C.; Shine,
R.; Tarbell, T.; Title, A.; Deforest, C.
Bibcode: 1996AAS...188.6913S
Altcode: 1996BAAS...28R.937S
On 7 March 1996, the SOHO spacecraft was offset from its usual
disk center pointing for an 11-hour observation of the South
Polar region. MDI took a continuous time series of high resolution
longitudinal magnetograms during this period, in support of the
SOHO-wide Joint Observing Program on polar plumes. It also ran several
hours each of two other programs: one to map the horizontal flows near
the pole by correlation tracking and Doppler shifts, and another to
study wave propagation (e.g., by time-distance helioseismology) at
these high latitudes. In this poster we present preliminary results
from the first program. Both techniques yield measurements of the
differential rotation profile near the pole and of horizontal flows of
supergranulation. These results are compared with each other and with
corresponding measurements in low latitudes. The location of magnetic
features in the horizontal flows is also shown. The SOI/MDI program
is supported by NASA grant NAG5-3077.
Title: DEM Analyses with the Utrecht Codes
Authors: Mewe, R.; van den Oord, G. H. J.; Schrijver, C. J.; Kaastra,
J. S.
Bibcode: 1996aeu..conf..553M
Altcode: 1996IAUCo.152..553M
No abstract at ADS
Title: On the Coronal Field Topology in Warm Stars: Is Procyon a
Warm Hybrid
Authors: Schrijver, Carolus J.; Haisch, Bernhard
Bibcode: 1996ApJ...456L..55S
Altcode:
The height up to which a coronal magnetic field line can remain
closed is determined principally by the balance between the gas
pressure, pg, and the magnetic pressure, pB. At an on-axis height,
z, above an ideal dipole, the magnetic pressure falls off as z-6 in
the far-field regime, where z exceeds the separation dAR between the
poles of the stellar active region. Thus, for a given exponentially
decreasing gas pressure, the smaller the dipole separation, dAR,
the lower the transition point at which pg exceeds pB, where closed
magnetic loops cannot be sustained. Because the maximum size of bipolar
stellar active regions is likely to be limited by the depth of the
convective envelope, this suggests that an increasing fraction of the
field should open up for stars with progressively shallower convective
envelopes. We discuss the available empirical evidence for this effect
for F stars on or near the main sequence. This has direct implications
for the mechanism for the coronal dividing line for cool K-type giants
proposed by Rosner and coworkers, in that a similar---albeit more
gradual---transition should take place for warm F-type main-sequence
and subgiant stars. The explanation of the Linsky-Haisch dividing line
proposed by Rosner and coworkers depends on a change in the dynamo
mode and a consequent change in the surface-level size scale of active
regions. For F-type stars we expect a change in size scale but not
a change in dynamo mode, which allows a separation of the effects of
these two changes on hot coronae. We also suggest that Procyon may be
a "warm hybrid" star, an interpretation consistent with recent Hubble
Space Telescope observations of an excess emission in the blue wings
of transition-region lines and with Extreme-Ultraviolet Explorer
measurements of its coronal temperature, structure, and density.
Title: The magnetic field of the nearest cool star (review)
Authors: Schrijver, C. J.
Bibcode: 1996IAUS..176....1S
Altcode:
No abstract at ADS
Title: EUV spectroscopy and coronal loop models
Authors: van den Oord, G. H. J.; Schrijver, C. J.; Mewe, R.; Kaastra,
J. S.
Bibcode: 1996ASPC..109..231V
Altcode: 1996csss....9..231V
No abstract at ADS
Title: Are Some Stellar Coronae Optically Thick?
Authors: Schrijver, C. J.; van den Oord, G. H. J.; Mewe, R.; Kaastra,
J. S.
Bibcode: 1996aeu..conf..121S
Altcode: 1996IAUCo.152..121S
No abstract at ADS
Title: Is the optical depth of stellar coronae really negligible?
Authors: Schrijver, C. J.; Mewe, R.; Kaastra, J. S.; van den Oord,
G. H. J.; Bruls, J. H. M. J.
Bibcode: 1996ASPC..109..289S
Altcode: 1996csss....9..289S
No abstract at ADS
Title: A U.S.--Russian Industrial Partnership to Develop a Low-Cost
IUE-2
Authors: Haisch, B.; Robb, P.; Strong, K.; Stern, R.; Schrijver,
C. J.; Lemen, J.
Bibcode: 1995AAS...187.7204H
Altcode: 1995BAAS...27.1388H
In 18 years as a NASA observatory IUE has generated more than 10(5)
spectra and 3000 articles, hosted over 2000 guest observers and launched
more than 200 doctoral dissertations. On 1 October 1995 science
operations were transferred entirely to ESA. IUE has been a central
facility in many multiwavelength programs. It has also supported
HST by carrying out projects that require more dedicated time than
HST can accomodate, including the ability to carry out uninterrupted
observations. Ready access to the UV spectrum has become a routine part
of modern astronomical capability, especially with respect to surveying
classes of objects and monitoring for variability and cycles. A
feasibility study has been initiated in the Solar and Astrophysics
Laboratory and the Optical Sciences Laboratory to examine an upgraded
IUE-2 to be developed in partnership with the Vavilov State Optical
Institute of St. Petersburg, Russia. The Vavilov Institute is the
premier space optics facility in the former Soviet Union. The recent
``swords into plowshares'' industrial partnership with Vavilov and
cost-effective capabilities involving commercial boosters such as LLV-2
and a version of the Commercial Remote Sensing Satellite (CRSS) bus
open innovative new opportunities for developing scientific facilities
in space. We are also investigating the economics of a shared launch on
the large capacity Proton rocket now operated jointly at the Baikonur
complex in Kazakhstan by Lockheed Martin, Khrunichev Enterprises and
NPO Energia. The centerpiece of IUE-2 would be a lightweight, advanced
techology silicon carbide mirror up to 1.2 m in diameter coupled to
modern imaging detectors. The Vavilov Institute has developed a robust
ceramic material of remarkable specific rigidity and thermal stability
that shows no distortion or hysteresis when thermally cycled between
cryogenic and room temperatures. Mirrors are routinely polished
to 0.03 waves in the visible. Spectroscopic capabilities would be
similar to the current IUE. The CRSS spacecraft provides 10 GBytes
of onboard data storage. In the interest of cost-savings, a highly
eccentric Exosat-like elliptical orbit is under consideration to
provide uninterrupted viewing times of at least 12 hours. This study,
now underway, will examine innovative ways in which a science-driven
program can be successfully developed and provided to NASA as a
purchased commercial product. Inputs are solicited, especially from
IUE guest observers and potentially interested IUE-2 users.
Title: Motion and Evolution of Solar Magnetic Elements
Authors: Berger, T. E.; Schrijver, C. J.; Shine, R. S.; Tarbell,
T. D.; Title, A. M.; Scharmer, G.
Bibcode: 1995AAS...18710104B
Altcode: 1995BAAS...27.1426B
The dynamics of sub-arcsecond solar magnetic flux tubes are analyzed
based on very-high resolution movies of photospheric bright points
obtained in 1994 at the 50-cm Swedish Solar Vacuum Telescope (SVST)
on the island of La Palma, Spain. The bright points are imaged using
a 12 Angstroms bandpass interference filter centered at 4305 Angstroms
in the ``G Band'' molecular bandhead of the CH molecule. The image sets
typically consist of up to 4 hours of consecutive images taken at a 10
to 20 second cadence. Spatial resolution throughout the movies averages
less than 0\arcsec.5 and many frames in the sets exhibit resolution
down to 0\arcsec.25. Magnetic flux elements in the photosphere are
shown to move continually along the intergranular lanes at speeds
of up to 5 km/sec and ranges up to several thousand km. Evolution of
individual magnetic elements is dominated by the local evolution of
surrounding granules. Fragmentation and merging is the fundamental
mode of evolution of the majority of magnetic elements seen in our
data. Rotation and folding of chains or groups of elements is also
frequently observed. The time scale for the fragmentation/merging
evolution of the elements is on the order of the lifetime of granulation
(6--8 minutes), but significant morphological changes are seen to
occur on time scales as short as 100 seconds. The concept of a stable,
isolated, sub-arcsecond magnetic flux element in the solar photosphere
is inconsistent with the observations presented here.
Title: New Observations of Subarcsecond Photospheric Bright Points
Authors: Berger, T. E.; Schrijver, C. J.; Shine, R. A.; Tarbell,
T. D.; Title, A. M.; Scharmer, G.
Bibcode: 1995ApJ...454..531B
Altcode:
We have used an interference filter centered at 4305 Å within the
bandhead of the CH radical (the "G band") and real-time image selection
at the Swedish Vacuum Solar Telescope on La Palma to produce very
high contrast images of subarcsecond photospheric bright points at all
locations on the solar disk. During the 6 day period of 1993 September
15-20 we observed active region NOAA 7581 from its appearance on the
East limb to a near disk-center position on September 20. A total of
1804 bright points were selected for analysis from the disk center image
using feature extraction image processing techniques. The measured FWHM
distribution of the bright points in the image is subnormal with a modal
value of 220 km (0".30) and an average value of 250 km (0".35). The
smallest measured bright point diameter is 120 km (0".17) and the
largest is 600 km (0".69). Approximately 60% of the measured bright
points are circular (eccentricity ∼1.0), the average eccentricity
is 1.5, and the maximum eccentricity corresponding to filigree in
the image is 6.5. The peak contrast of the measured bright points is
normally distributed. The contrast distribution variance is much greater
than the measurement accuracy, indicating a large spread in intrinsic
bright-point contrast. When referenced to an averaged "quiet-Sun area 1n
the image, the modal contrast is 29% and the maximum value is 75%; when
referenced to an average intergranular lane brightness in the image,
the distribution has a modal value of 61 % and a maximum of 119%. The
bin-averaged contrast of G-band bright points is constant across the
entire measured size range. The measured area of the bright points,
corrected for population and selection effects, covers about 1.8% of
the total image area. Large pores and micropores occupy an additional
2% of the image area, implying a total area fraction of magnetic
proxy features in the image of 3.8%. We discuss the implications of
this area fraction measurement in the context of previously published
measurements which show that typical active region plage has a magnetic
filling factor on the order of 10% or greater. The results suggest that
in the active region analyzed here, less than 50% of the small-scale
magnetic flux tubes are demarcated by visible proxies such as bright
points or pores.
Title: EUV spectroscopy of cool stars. II. Coronal structure of
selected cool stars observed with the EUVE.
Authors: Schrijver, C. J.; Mewe, R.; van den Oord, G. H. J.; Kaastra,
J. S.
Bibcode: 1995A&A...302..438S
Altcode:
We analyze the coronal EUV spectra of seven cool stars, solar-like
single stars and components of RS CVn-like binaries, as observed with
the Spectrometers of the Extreme Ultraviolet Explorer (EUVE). The
observations cover the wavelength range of 60A up to 800A with a
resolution of λ/{DELTA}λ=~160-320. The data constrain the coronal
temperature structure between several hundred thousand Kelvin up
to roughly 10 million Kelvin through a differential emission measure
analysis. The resulting differential emission measure distributions show
distinct features from source to source, but the common properties are
a) a relatively weak emission from coronal plasma below about 1MK, b)
a dominant component somewhere between 2MK and about 10MK, often peaking
at solar-like coronal temperatures of 2 to 4MK, and c) in all cases but
χ^1^ Ori a very hot component in the formal solution with a temperature
exceeding several tens of million of Kelvin. This hot tail in the
differential emission measure distribution may reflect, as discussed
in this paper, one or several of the following sources or processes:
a real hot component, a reduced coronal abundance of heavy elements,
or scattering in some of the strongest coronal lines with subsequent
photon destruction upon impact on the lower, dense atmosphere. Coronal
electron densities of brightly emitting regions are constrained by an
analysis of ratios of density-sensitive iron lines. Strengths of Fe
XIX-Fe XXII lines (corresponding to a temperature range of T=6-11MK) for
α Aur, AU Mic (of which the spectrum is dominated by a large flare),
ξ UMa, and σ Gem suggest typical electron densities in the range
n_e_~10^12^-10^13^cm^-3^. Cooler Fe X and Fe XII-Fe XIV lines (T=1-2MK)
in the case of α CMi suggest n_e_~10^9^-10^10^cm^-3^. In general, the
electron densities of the hot 5-15MK components are some three orders
of magnitude larger than typical of the solar-like component around
2MK; the volume filling factors of the hot components are therefore
expected to be substantially smaller than those of the cooler component.
Title: Book-Review - Isolated Pulsars
Authors: Vauclair, G.; Achterberg, A.; Narlikar, Jayant; Lub, J.;
van der Laan, H.; Sakai, J. I.; Schrijver, C. J.; de Jager, Cornelis;
Léna, P.; Vanbeveren, D.; Audouze, J.; van den Heuvel, E. P. J.
Bibcode: 1995SSRv...73..435V
Altcode:
No abstract at ADS
Title: Book Review: Physics of solar and stellar coronae: G. S. Vaiana
memorial symposium / Kluwer, 1993
Authors: Schrijver, C. J.
Bibcode: 1995SSRv...73..440S
Altcode: 1995SSRv...73..440L
No abstract at ADS
Title: Simulated MDI Observations of Convection
Authors: Hurlburt, N. E.; Schrijver, C. J.; Shine, R. A.; Title, A. M.
Bibcode: 1995ESASP.376b.239H
Altcode: 1995soho....2..239H; 1995help.confP.239H
No abstract at ADS
Title: EUV spectroscopy of cool stars. I. The corona of α Centauri
observed with EUVE. R
Authors: Mewe, R.; Kaastra, J. S.; Schrijver, C. J.; van den Oord,
G. H. J.; Alkemade, F. J. M.
Bibcode: 1995A&A...296..477M
Altcode:
We perform an emission measure analysis of spectra of the coronae of
the cool star binary α Cen (A: HD 128620 (G2 V) + B: HD 128621 (K1 V))
as observed with the Spectrometers of the Extreme UltraViolet Explorer
(EUVE). These observations, covering the wavelength range of 60A up
to 800A with a resolution of λ/{DELTA}λ=~160-320, constrain the
coronal temperature structure between about 10^5^K up to roughly 10MK,
while some additional line and continuum information is available to
extend that coverage from several tens of thousands of Kelvin up to
several tens of millions of Kelvin, although the solution is poorly
constrained in the extended range. We performed a re-calibration of
the wavelength scale for each of the three instrumental pass bands by
comparing the model list of emission lines with the spectra of α Cen
and eight other cool stars. The high signal-to-noise ratio of the α
Cen spectrum yielded indications for a Lorentzian rather than a Gaussian
line profile for the MW passband. The observed spectrum was decomposed
into a linear combination of spectra emitted by isothermal plasmas in
thermal equilibrium, using the SPEX plasma emission code developed in
Utrecht. The resulting differential emission measure distribution shows:
a) emission from plasma below ~5MK with a broad peak around ~3MK and
with a tail extending down to about 0.5MK, b) very little emission from
plasma between 0.1MK and 0.5MK, c) emission from plasma below 0.1MK,
and d) possibly a hot component exceeding several tens of million of
Kelvin which, however, may (in part) have another origin than a hot
coronal plasma component. We propose that the hot component may be
an artifact of the corona being close to unit optical thickness for
resonant scattering in the strongest spectral lines, so that part
of the line photons can be destroyed by being scattered towards the
stellar surface while the plasma remains optically thin for continuum
photons. Diagnostics for the electron density n_e_, using Fe X, XII,
XIII, and XIV lines, yields values in the range 2-20x10^8^cm^-3^
at T=1-2MK.
Title: Properties of Sub-Arcsecond Facular Bright Points
Authors: Berger, T.; Schrijver, C.; Shine, R.; Tarbell, T.; Title,
A.; Scharmer, G.
Bibcode: 1995SPD....26..505B
Altcode: 1995BAAS...27..957B
No abstract at ADS
Title: STARS: A Proposal for a Dedicated Space Mission to Study
Stellar Structure and Evolution
Authors: Fridlund, M.; Gough, D. O.; Jones, A.; Appourchaux, T.;
Badiali, M.; Catala, C.; Frandsen, S.; Grec, G.; Roca Cortes, T.;
Schrijver, K.
Bibcode: 1995ASPC...76..416F
Altcode: 1995gong.conf..416F
No abstract at ADS
Title: Observations of Convection
Authors: Title, A. M.; Hurlburt, N.; Schrijver, C.; Shine, R.;
Tarbell, T.
Bibcode: 1995ESASP.376a.113T
Altcode: 1995heli.conf..113T; 1995soho....1..113T
The primary goal of the Solar Oscillations Investigation is
to understand the interior of the Sun using the techniques of
helioseismology. In addition the Michelson Doppler Imager produces
images of the solar surface with sufficient resolution to measure
surface flows via the technique of local correlation tracking and
magnetograms which allow feature tracking of magnetic fields. It will
be possible to measure the evolution of meso and supergranulation, the
evolution of the meso and supergranulation patterns, and the motion of
magnetic elements in the flow field. With observing periods of 8 hours
one should be able to detect large scale flow fields of 10 m/s second
or less. The magnetograms will provide the data to understand how the
cell patterns evolve as a function of magnetic field configuration.
Title: (Erratum) Rotating, magnetic braking, and dynamos in cool
giants and subgiants.
Authors: Schrijver, C. J.; Pols, O. R.
Bibcode: 1995A&A...293..640S
Altcode:
Erratum to Astron. Astrophys. 278, 51-67 (1993).
Title: Basal heating in the atmospheres of cool stars
Authors: Schrijver, Carolus J.
Bibcode: 1995A&ARv...6..181S
Altcode:
This paper reviews observational evidence concerning the existence
of so-called basal heating that occurs in the outer atmospheres of
all stars with convective envelopes. Effects of basal heating depend
primarily on the effective temperature, with little sensitivity
to surface gravity or elemental abundances. Basal heating occurs
predominantly in the chromosphere, possibly in the (lower) transition
region, but not at an observable level in coronae (except perhaps in
early F-type and in M-type dwarf stars). Basal fluxes are observed in
the slowest rotators where it shows no significant modulation. The
basal flux level is observed directly on the Sun only over regions
void of intrinsically strong photospheric fields. There is substantial
quantitative observational and theoretical evidence that the basal
emission from stellar outer atmospheres is caused by the dissipation of
acoustic waves generated by turbulent convection. The magnetic canopy
turns out to be of little consequence, but effects of intrinsically
weak fields on the basal mechanism cannot be entirely ruled out. Solar
observations constrain the spatio-temporal character of the basal
atmosphere and the acoustic flux levels as a function of height,
resulting in a model in which intermittent wave dissipation causes
emission characteristic of both cool and warm atmospheric areas, in
which — at least in the solar case — a time-averaged chromospheric
temperature rise may not even exist.
Title: The optical thickness of stellar coronae in the EUV lines.
Authors: Schrijver, C. J.; van den Oord, G. H. J.; Mewe, R.
Bibcode: 1994A&A...289L..23S
Altcode:
Stellar coronae are commonly assumed to be optically thin. Recent
spectroscopic observations in the EUV, however, lead us to question
the validity of this approximation for strong lines. We argue that
scattering may significantly affect the strongest coronal lines in
coronae composed of magnetic loops, possibly embedded in a hot stellar
wind. Even if the average number of scatterings per photon in some
coronal lines is only of order unity, the relative line strenghts
and the line-to-continuum ratio can be significantly affected in
non-symmetric inhomogeneous atmospheres: photons in weak lines and
in the optically thin continuum escape without any scattering, but
strong lines can be weakened or enhanced depending on the balance
between outward traveling line photons that are scattered back toward
the stellar surface (if not lost by branching), there destroyed by
absorption, and downward traveling line photons that are scattered
upward and escape. We draw attention to the fact that line scattering
due to the non-negligible optical thickness in strong coronal lines
can have severe implications for differential emission measure models
and for abundance and density determinations, while it may serve as
a diagnostic for the existence of tenuous hot winds.
Title: Books-Received - Solar Surface Magnetism
Authors: Rutten, R. J.; Schrijver, C. J.
Bibcode: 1994Sci...265.1902R
Altcode:
No abstract at ADS
Title: On scintillation obfuscation
Authors: Badiali, M.; Catala, C.; Fossat, E.; Fransden, S.; Gough,
D. O.; Rocca-Cortes, T.; Schrijver, K.
Bibcode: 1994Obs...114...53B
Altcode:
No abstract at ADS
Title: The Photospheric Magnetic Flux Budget
Authors: Schrijver, C. J.; Harvey, K. L.
Bibcode: 1994SoPh..150....1S
Altcode:
The ensemble of bipolar regions and the magnetic network both contain
a substantial and strongly variable part of the photospheric magnetic
flux at any phase in the solar cycle. The time-dependent distribution
of the magnetic flux over and within these components reflects the
action of the dynamo operating in the solar interior. We perform a
quantitative comparison of the flux emerging in the ensemble of magnetic
bipoles with the observed flux content of the solar photosphere. We
discuss the photospheric flux budget in terms of flux appearance and
disappearance, and argue that a nonlinear dependence exists between
the flux present in the photosphere and the rate of flux appearance
and disappearance. In this context, we discuss the problem of making
quantitative statements about dynamos in cool stars other than the Sun.
Title: Activity and Braking of Evolving Cool Stars (Invited Review)
Authors: Schrijver, C. J.
Bibcode: 1994ASPC...64..328S
Altcode: 1994csss....8..328S
No abstract at ADS
Title: EUVE Spectroscopy of XI UMa; sigma Gem; and Chi {1} ORI
Authors: Mewe, R.; Schrijver, C. J.; Kaastra, J. S.; Alkemade,
F. J. M.; Haisch, B. M.
Bibcode: 1994ASPC...64...41M
Altcode: 1994csss....8...41M
No abstract at ADS
Title: Solar magnetic fields and percolation theory
Authors: Schrijver, C. J.
Bibcode: 1994ASIC..433..271S
Altcode:
No abstract at ADS
Title: Solar Surface Magnetism
Authors: Rutten, Robert J.; Schrijver, Carolus J.
Bibcode: 1994ASIC..433.....R
Altcode: 1994ssm..work.....R
No abstract at ADS
Title: EUVE observations of NGC 5548.
Authors: Kaastra, J. S.; Mewe, R.; Heise, J.; Alkemade, F. J. M.;
Schrijver, C. J.; Carone, T.
Bibcode: 1994IAUS..159..325K
Altcode:
No abstract at ADS
Title: Rotation, magnetic braking, and dynamos in cool giants and
subgiants
Authors: Schrijver, C. J.; Pols, O. R.
Bibcode: 1993A&A...278...51S
Altcode:
We analyze the rotation rates of giants and subgiants evolving from the
main sequence to the red giant branch. A comparison of observational
data with rotational velocities expected from evolutionary models
incorporating stellar magnetic braking results in the following
conclusions: (a) subgiants have lost most of their total angular
momentum prior to reaching the red giant branch, while luminosity class
III giants have lost typically 50% of their total angular momentum
by that time; (b) stellar magnetic braking, and hence probably the
stellar dynamo, does not switch on at its full strength at the onset
of envelope convection, but rapidly increases in strength over an
interval of at least Delta(B - V) approximately equal to 0.2-0.3;
(c) the radiative-equilibrium interior and the convective envelope
appear to exchange angular momentum on a time scale short compared to
the time scale associated with the loss of angular momentum through
a magnetized wind so that the rotational shear between envelope and
interior is limited to at most about a factor of four and probably
less; (d) magnetic braking is strongly enhanced in giants as compared
to dwarf stars: the parametrization of the angular momentum loss used
in the present simulations supports a proportionality with the sixth
power of the radius.
Title: Book reviews
Authors: Schrijver, C.; Kotrč, Pavel
Bibcode: 1993SoPh..146..409S
Altcode: 1993SoPh..146..409T
No abstract at ADS
Title: Het wisselende aangezicht van zon en sterren.
Authors: Schrijver, K.; Zwaan, K.
Bibcode: 1993Zenit..20..292S
Altcode:
No abstract at ADS
Title: Anomalous Diffusion of Magnetic Elements across the Solar
Surface
Authors: Lawrence, J. K.; Schrijver, C. J.
Bibcode: 1993ApJ...411..402L
Altcode:
Results are presented of observations of the random walks of
concentrations of magnetic flux on the solar surface, giving a natural,
macroscopic realization of anomalous diffusion with fractal dimension
D = 1.56 +/- 0.08 and exponent of anomalous diffusion theta = 0.25
+/- 0.40. The results indicate that the random walk of magnetic flux
in the solar photosphere is non-Euclidian and not two-dimensional
diffusion. The results are entirely consistent with results from
percolation theory for diffusion on clusters at a density below the
percolation threshold.
Title: Magnetic activity in dwarf stars with shallow convective
envelopes.
Authors: Schrijver, C. J.
Bibcode: 1993A&A...269..446S
Altcode:
The magnetic activity in dwarf stars with shallow convective envelopes
is addressed by studying the C II and soft X-ray surface flux densities
in relation to the stellar rotation velocity. The C II emission from
the acoustically heated basal atmospheric component increases with
increasing T(eff) up to at least B-V about 0.25, reaching a level
of about 8 x 10 exp 4 erg/sq cm/s for early F-type dwarf stars. A
relationship between rotation rate and level of magnetic activity
is found to extend down to at least B-V about 0.25, although the
C II emission associated with the magnetic activity at B-V = 0.25
is suppressed by a factor of about 100 as compared to stars cooler
than B-V about 0.60 with the same rotation rate. The suppression
of activity is strongly color-dependent below B-V about 0.60, with
the dynamo efficiency leveling off for cooler stars. The radiative
losses associated with magnetic activity do not show a noticeable
dependence on color in diagrams relating fluxes from chromosphere
and corona for F-, G-, and K-type dwarf stars, suggesting comparable
atmospheric structures for the magnetic component. The observed mean
rotational velocities as a function of spectral type are shown to be
consistent with the assumptions of (1) a reduced dynamo efficiency and
(2) a comparable mechanism for angular momentum loss through a stellar
wind for all magnetically active cool stars.
Title: Relations between the photospheric magnetic field and
the emission from the outer atmosphere of cool stars. III - The
chromospheric emission from individual flux tubes
Authors: Schrijver, C. J.
Bibcode: 1993A&A...269..395S
Altcode:
The two-step explanation of the observed nonlinearity of relationships
between photospheric magnetic and chromospheric radiative flux
densities of cool stars starts with the sensitivity of the emission of
individual flux tubes on the mean magnetic flux density. I argue that
the relationship between flux densities for individual flux tubes is
virtually the same as that found empirically from solar data with a
spatial resolution of a few arcsec. For this I use a simple model in
which the chromospheric radiative output of a flux tube is assumed to
be a function only of the area a into which the flux tube can expand
above the canopy, determined by the positions of the surrounding flux
tubes. The scatter about the mean relationship that is expected from
the random positioning of flux tubes within a resolution element is
small compared to the observed scatter. The latter must therefore
be caused largely by time variability, by effects of the detailed
geometry, and possibly by a range in flux-tube diameters. The second
step in explaining the nonlinearity of stellar flux-flux relationships
involves effects of nonrandom positioning on larger scales on the
averaging over the entire solar or stellar disk.
Title: Percolation theory and the geometry of photospheric magnetic
flux concentrations
Authors: Balke, A. C.; Schrijver, C. J.; Zwaan, C.; Tarbell, T. D.
Bibcode: 1993SoPh..143..215B
Altcode:
The magnetic field in solar active regions forms a highly structured
pattern without an apparent length scale. We study this pattern in
detail for a plage and its surroundings observed with the Swedish Solar
Observatory on La Palma. The magnetogram has a resolution of about
1/3″, after image optimisation. We analysed the geometric properties
of isolated patches of magnetic flux. Patches with a linear size up to
3″ appear to be statistically self-similar, with a fractal dimension
ofDf = 1.54 ± 0.05 for the relation between area and linear
size. This value agrees very well with the dimensionDf
= 1.56 which is found in percolation theory for clusters of tracers
placed randomly on a lattice with a tracer density below a critical
threshold. The distribution of observed cluster areas also agrees
with that of clusters on such a random lattice. The correspondence
between properties of observations and of clusters on randomly filled
lattices suggests that- well after emergence - the magnetic flux on
the Sun is randomly distributed at least up to sizes of about 3″
and possibly larger.
Title: X-ray/Optical Survey of Late-Type Stars
Authors: Piters, A. J. M.; Schmitt, J. H. M. M.; Schrijver, C. J.;
Baliunas, S.; Zwaan, C.; van Paradijs, J.
Bibcode: 1993ASSL..183..377P
Altcode: 1993pssc.symp..377P
No abstract at ADS
Title: Observational constraints on dynamos in cool stars.
Authors: Schrijver, C. J.
Bibcode: 1993ASPC...40..591S
Altcode: 1993ist..proc..591S; 1993IAUCo.137..591S
Non-radiatively heated outer atmospheres are common among stars on
the cool side of the HR diagram. These atmospheres resemble that of
the Sun in which the bulk of the heating is associated with magnetic
fields. Consequently it is assumed that other cool stars also generate
and maintain an internal magnetic field through a dynamo action. This
field gives rise to a wide spectrum of phenomena in the stellar
atmosphere. The dynamo process that sustains the field thrives on the
interaction of rotation and turbulent convection. In this review the
author formulates a set of propositions outlining the present knowledge
of this interaction as distilled from stellar observations. In doing so,
he outlines the effects of stellar evolution on rotation rate, discuss
some of the proposed "dividing lines" in the HR-diagram, and comments on
the possible role of the convective turnover time in dynamo efficiency
and on dynamo activity for extremely slowly and rapidly rotating stars.
Title: The basal and strong-field components of the solar atmosphere
Authors: Schrijver, C. J.
Bibcode: 1992A&A...258..507S
Altcode:
Spectroheliograms of quiet and active solar regions observed in spectral
lines originating in the upper chromosphere and transition region are
studied. Relationships between line intensities originating at different
temperatures in the solar atmosphere are quantified presupposing a
two-component model, comprising (1) a background basal emission and (2)
a magnetically controlled emission which shows power-law dependences
between emissions in different spectral lines. The spatial extent of
coronal structures and substantial projection effects inhibit derivation
of point-by-point intensity relationships for coronal emissions. The
consistency of the results of the modeling yields strong evidence in
favor of a basal emission component that is most likely nonmagnetic in
origin. The basal component dominates the emission from outside the
magnetic network, but is also present in pixels of at least moderate
activity in network and plage, at a resolution of 5 x 5 arcsec. The
inferred solar basal flux density in the C II (1335 A) line equals
the basal flux found for solar-like dwarf stars. The distribution of
intensities associated with the basal component is asymmetric, with
a relatively strong high-intensity tail.
Title: Nearly simultaneous observations of chromospheric and coronal
radiative losses of cool stars.
Authors: Schrijver, C. J.; Dobson, A. K.; Radick, R. R.
Bibcode: 1992A&A...258..432S
Altcode:
The flux-flux relationships of cool stars are studied on the basis
of nearly simultaneous measurements of Ca II H+K, Mg II h+k, and soft
X-ray fluxes. A linear relationship is derived between IUE Mg II h+k
fluxes and Mount Wilson Ca II H+K fluxes which were obtained within 36
hr of each other for a sample of 26 F5-K3 main-sequence stars. Nearly
simultaneous EXOSAT soft X-ray fluxes are compared with Ca II H+K fluxes
for a sample of 20 dwarfs and gaints with spectral types ranging from
F6 to K2, and 72 additional cool stars for which noncontemporaneous Ca
II H+K and EINSTEIN soft X-ray fluxes are available are compared. It is
confirmed that a nonradiatively heated chromosphere exists on even the
least active main-sequence stars. This basal chromosphere is probably
independent of stellar magnetic activity.
Title: Patterns in the photospheric magnetic field and percolation
theory
Authors: Schrijver, C. J.; Zwaan, C.; Balke, A. C.; Tarbell, T. D.;
Lawrence, J. K.
Bibcode: 1992A&A...253L...1S
Altcode:
The magnetic field in solar plages forms a highly structured pattern
with no apparent characteristic length scale. This pattern appears
to be a fractal with a dimension between 1.45 and 1.60. Small-scale
displacements of concentrations of magnetic flux in the network
are consistent with a random walk on a fractal with a similar
dimension. Percolation theory offers an effective explanation for
observed geometric properties of small-scale flux concentrations
in the solar photosphere, by demonstrating the close correspondence
with clusters formed by randomly placed tracers on a 2D (irregular)
lattice. Percolation theory also offers a model for the subdiffusive
behavior of tracers performing a random walk on clusters formed
by bonded sites. The geometry of flux concentrations and of the
displacement of magnetic flux as a function of time are equivalent
to situations in percolation theory below a critical value, called
'the percolation threshold'.
Title: Activity in Tidally Interacting Binaries
Authors: Schrijver, C. J.; Zwaan, C.
Bibcode: 1992ASPC...26..370S
Altcode: 1992csss....7..370S
No abstract at ADS
Title: Fractals in Magnetograms
Authors: Schrijver, C. J.; Zwaan, C.; Balke, A. C.; Tarbell, T. D.;
Lawrence, J. K.
Bibcode: 1992ASPC...27...67S
Altcode: 1992socy.work...67S
No abstract at ADS
Title: The Effect of Fractal Distribution on the Evolution of Solar
Surface Magnetic Fields
Authors: Lawrence, J. K.; Schrijver, C. J.
Bibcode: 1992ASPC...27...48L
Altcode: 1992socy.work...48L
No abstract at ADS
Title: Activity in Tidally Interacting Binaries
Authors: Schrijver, Carolus J.
Bibcode: 1992euve.prop....3S
Altcode:
In tially interacting binaries containing at least one magnetically
active cool star, the outer-atmosphere emission is enhanced by up to
a factor of hundred as compared to single stars of the same rotation
period. These overactive binaries cannot be distinguished from single
stars with current diagnostics which suggests a similar atmospheric
structure, and consequently a strengthened dynamo. We propose
to observe RS CVn-like binaries to substantiate that suggestion,
and to study the role of alternatives such as accretion, or wind
or magnetic interaction. EUVE spectra enable determination of the
differential emission-measure distribution at temperatures where a)
fits to quasi-static loop models constrain loop geometry, and b)
the alternative processes are expected to show up if they contribute
substantially.
Title: Magnetic structure in cool stars. XVII. Minimum radiative
losses from theouter atmosphere.
Authors: Rutten, R. G. M.; Schrijver, C. J.; Lemmens, A. F. P.;
Zwaan, C.
Bibcode: 1991A&A...252..203R
Altcode:
The emissions in several chromospheric and transition region lines and
in coronal soft X-rays are analyzed for a sample of cool stars. The
nature of the lower-limit flux densities is explored, and evidence is
given for the possibility of a basal, nonmagnetic heating mechanism
being responsible for these emission fluxes up to, and perhaps
including, the upper transition region. It is argued that the excess
flux density, derived by subtraction of the basal flux density from the
observed stellar flux, is the proper measure of magnetic activity. The
level of the basal flux density as a function of color is determined
to be 2 x 10 exp 6 erg/sq cm/s for F-type stars and 2 x 10 exp 5 erg/sq
cm/s for K-type stars.
Title: Observations of the phase differences between irradiances
and velocity for low-degree solar acoustic modes
Authors: Schrijver, C. J.; Dappen, W.; Jimenez, A.
Bibcode: 1991A&A...251..655S
Altcode:
Irradiance measurements from the IPHIR instrument flown on the
Phobos 2 mission to Mars are used to derive phase differences and
gains between broadband relative irradiance signals and full-disk
velocity variations for l = 0, 1, and 2 mode with order ranging from
12 to 32. These data are compared with contemporaneous measurements of
photospheric velocities obtained during four intervals within the 155 d
observation period of Phobos 2. It is found that the phase difference
between irradiance at 500 nm and velocity is roughly constant with an
average value of -119 +/- 3 deg from 2.5 mHz up to about 4.3 mHz with a
slight increase suggested by data up to 4.6 mHz. The phase differences
between the green and red channels do not differ significantly from zero
below 2.8 mHz, while a small difference of about 13 +/- 5 deg exists
at higher frequencies, with the red signal ahead of the green. This
jump appears to coincide with a jump in the separation between the l =
0 and l = 2 eigenfrequencies.
Title: Activity in tidally interacting binaries.
Authors: Schrijver, C. J.; Zwaan, C.
Bibcode: 1991A&A...251..183S
Altcode:
A data set encompassing 23 binaries for which essential properties
of components and orbits are known is presently used to study the
relationship between activity, rotation, and revolution in comparatively
close binaries, by comparison to the relationship defined by single
stars. Confirmation is obtained for previous reports that the
relationships between emissions from the outer stellar atmospheres
of the binaries are indistinguishable from those defined by single
stars. While the rotation-activity relationship for single stars can be
described in terms of fundamental stellar properties, properties of the
secondary are important in determining the activity of close binaries.
Title: The AXAF Low-Energy Transmission Grating Spectrometer /
LETGS / Diagnostic Capabilities for the Study of Stellar Coronae
Authors: Mewe, R.; Lemen, J. R.; Schrijver, C. J.
Bibcode: 1991Ap&SS.182...35M
Altcode:
We study the diagnostic capabilities of the high-resolution,
Low-Energy Transmission Grating Spectrometer, LETGS, of NASA's planned
Advanced X-ray Astrophysics Facility, AXAF, for optically thin stellar
coronae. Spectra are simulated on the basis of isothermal and source
loop models and are analyzed with particular emphasis on the extraction
of the differential emission measure distribution. The AXAF-LETGS is
shown to be particularly sensitive for plasma at temperatures between
0.5 and 15 MK. Emission from temperatures in excess of 20 MK can
be observed, but the lack of strong spectral lines hampers accurate
temperature determinations. We simulate spectra of close binaries to
demonstrate the observability of the Doppler effects associated with
orbital motions. We present lists of spectral lines that can be used
for density diagnostics, and we simulate and compare various spectra
at different electron densities.
Title: The Sun as a Prototype in the Study of Stellar Magnetic
Activity.
Authors: Schrijver, C. J.
Bibcode: 1991RvMA....4...18S
Altcode:
No abstract at ADS
Title: Phase difference between irradiance and velocity in low degree
solar p-modes
Authors: Schrijver, C. J.; Jiménez, A.; Domingo, V.; Fröhlich, C.
Bibcode: 1991AdSpR..11d..77S
Altcode: 1991AdSpR..11Q..77S
We derive phase differences between irradiance and full-disk velocity
variations for l=0, 1, and 2 modes with order ranging from 11 up to 30
(1.8-4.3mHz). We use irradiance measurements from the IPHIR instrument
flown on the PHOBOS mission to Mars during the second half of 1988,
and simultaneous velocity measurements obtained at Tenerife. The
IPHIR instrument measures broad-band irradiance fluctuations and the
derived phase differences are therefore typical of the deep layers of
the photosphere. We select three one week intervals from the 155 day
observing interval of PHOBOS 2 for which simultaneous good quality
velocity data are available. We find a smooth variation of the phase
difference between irradiance at 500nm (5nm FWHM) and velocity from
about 70° at 1.8mHz to 145° at 2.5mHz, while it remains roughly
constant at 145° degrees up to at least 3.5mHz, and possibly up to
4.3mHz. We also show that the phase differences between the green
(500nm) and red (865nm) channels does not differ significantly from
zero below 3mHz, while a small difference of about 10° may exist at
higher frequencies.
Title: Relations Between Activity and Magnetic Fields (With 6 Figures)
Authors: Schrijver, C. J.
Bibcode: 1991mcch.conf..257S
Altcode:
No abstract at ADS
Title: Activity of Relatively Close Binaries
Authors: Schrijver, Carolus J.; Zwaan, Cornelis
Bibcode: 1991LNP...380..435S
Altcode: 1991sacs.coll..435S; 1991IAUCo.130..435S
Whereas the rotation-activity relationship for single cool stars can
be described in terms of fundamental stellar properties, the activity
in relatively close binaries is enhanced with respect to that of
single stars by an amount which depends strongly on the properties of
the companion. We consider mechanisms which could cause an excess in
activity of binaries, and conclude that it is likely that the presence
of a companion affects the interior structure of the stars in such
a way that either the efficiency of the dynamo or of the atmospheric
heating is enhanced.
Title: Helioseismology with the IPHIR instrument on the
U.S.S.R. Phobos mission
Authors: Froehlich, C.; Toutain, Th.; Schrijver, C. J.
Bibcode: 1991AdSpR..11d..69F
Altcode: 1991AdSpR..11...69F
We analyze the solar irradiance data gathered by the IPHIR experiment
on the USSR planetary mission to Phobos during 160 days of the cruise
phase to Mars of PHOBOS II, launched on 12 July 1988. We use two methods
to `clean' the data from the unexpected effects caused by the variable
pointing. Both methods yield spectra of the green and red passbands in
the range of the solar 5-minute p-mode oscillations with a very high
signal-to-noise ratio. The consistency of the four spectra demonstrates
the effectiveness of the `cleaning' and allows to determine highly
accurate p-mode frequencies. For the low-frequency analysis another,
independent method is applied which produces reasonable power spectra in
the range from 0.1 to 200 μHz. Interesting features of the medium-term
variability of the solar spectral irradiance are revealed. No evidence
was found for solar g modes, possibly because the solar or the pointing
noise prevents their disclosure.
Title: Properties of the Largescale and Smallscale Flow Patterns in
and around AR:19824
Authors: Schrijver, C. J.; Martin, S. F.
Bibcode: 1990SoPh..129...95S
Altcode:
We trace the photospheric motions of 170 concentrations of magnetic
flux tubes in and around the decaying active region No. 19824 (CMP 23
October 1986), using a series of magnetograms obtained at the Big Bear
Solar Observatory. The magnetograms span an interval of just over five
days and cover an area of about 4 × 5 arc min centered on the active
region. We find a persistent large-scale flow pattern that is superposed
on the small-scale random motions of both polarities. Correction
for differential rotation unveils the systematic, large-scale flow
surrounding the core region of the magnetic plage. The flow (with a
mean velocity of 30 m s−1) is faster and more pronounced
around the southern side of the core region than around the northern
side, and it accelerates towards the western side of the active
region. The northern and southern branches of the large-scale flow
converge westward of the core region, dragging along the westernmost
sunspot and some of the magnetic flux near it. The overall pattern
of the large-scale flow resembles the flow of a river around a sand
bar. The long-term evolution of the active region suggests that the
flow persists for several months. We discuss the possible association
of the large-scale flow with the torsional oscillation.
Title: Magnetische aktiviteit van zon en koele sterren.
Authors: Schrijver, C. J.
Bibcode: 1990Zenit..17..317S
Altcode:
No abstract at ADS
Title: Relations between the photospheric magnetic field and the
emission from the outer atmospheres of cool stars.
Authors: Schrijver, C. J.
Bibcode: 1990A&A...234..315S
Altcode:
Solar and stellar data are used to determine the relationship
between C IV 1548 + 1551 A and magnetic flux densities. C IV
SMM-UVSP spectroheliograms of solar quiet regions and plages are
analyzed. Typical distribution functions of C IV flux densities over
both quiet and active solar regions are presented. The C IV emission
is shown to be dependent on magnetic flux density.
Title: Quiescent X-ray emission of cool stars
Authors: Schrijver, C. J.
Bibcode: 1990nwus.book..233S
Altcode:
No abstract at ADS
Title: The Relationship Between EXOSAT Soft X-Ray and MT. Wilson
CAII H+K Flux Densities
Authors: Schrijver, C. J.; Dobson, A. K.; Radick, R. R.; Giommi, P.
Bibcode: 1990ASPC....9..136S
Altcode: 1990csss....6..136S
Data of Exosat soft X-ray fluxes, F(X), are compared to Mount Wilson
Ca II H+K flux measurements, F(Ca), to assess the probability of
nonradiatively heated atmospheres when coronal emissions cannot be
observed. F(X) and F(Ca) have been observed within three days of
each other in 13 stars and within 90 days in 8 stars, and the average
interval for all but three stars is about 3 days. The Exosat data and
the Einstein data are employed to relate F(X) and F(Ca) by means of a
power law, which is consistent with the notion that X-ray fluxes are
very low at the Ca II H+K lower limit flux.
Title: Temperatures of coronae of cool stars, derived from EXOSAT
observations
Authors: Mewe, R.; Lemen, J. R.; Schrijver, C. J.
Bibcode: 1990AdSpR..10b.129M
Altcode: 1990AdSpR..10..129M
We observed the late-type stars Capella, σ2 CrB, λ And,
AD Leo, GL494, GL569 and Procyon with different combinations of three
instruments aboard EXOSAT : the low-energy channel multiplier array
(LE), the medium-energy proportional counters (ME), and the transmission
grating spectrometer (TGS). We derive crude temperature information on
the coronae of these stars from the photometric data, using one- and
two-temperature models of optically thin, thermal plasmas. We compare
the results with 2-T fits and differential emission measure analyses
of three grating observations and with simulations for mixtures of two
plasma components with varying emission measure ratios. But for Procyon,
all sources have a strong, hot component between 10 and 25 MK. The TGS
spectra of Capella and σ2 CrB reveal a cooler component
with comparable emission measure around 5 MK. The emission from the
corona of Procyon is dominated by a very cool (0.6 MK) component,
with an additional contribution from a 2-3 MK component.
Title: The Effect of an Interaction of Magnetic Flux and
Supergranulation on the Decay of Magnetic Plages
Authors: Schrijver, C. J.
Bibcode: 1989SoPh..122..193S
Altcode:
This paper studies how the properties of large-scale convection affect
the decay of plages. The plage decay, caused by the random-walk
dispersion of flux tubes, is suggested to be severely affected by
differences between the mean size of cellular openings within and
around plages. The smaller cell size within a plage largely explains
the smaller diffusion coefficient within plages as compared to that
of the surrounding regions. Moreover, the exchange of flux tubes
between the inner regions of the plage and the surrounding network
is suggested to be modified by this difference in cell size, and the
concept of a partially transmitting plage periphery is introduced:
this periphery preferentially turns back flux parcels that are moving
out of the plage and preferentially lets through flux parcels that are
moving into the plage, thus confining the flux tubes to within the
plage. This semi-permeability of the plage periphery, together with
the dependence of the diffusion coefficient on the flux-tube density,
can explain the observed slow decay of plages (predicting a typical
life time of about a month for a medium-sized plage), the existence
of a well-defined plage periphery, and the observed characteristic
mean magnetic flux density of about 100 G. One effect of the slowed
decay of the plage by the semi-permeability of the plage periphery is
the increase of the fraction of the magnetic flux that can cancel with
flux of the opposite polarity along the neutral line to as much as 80%,
as compared to at most 50% in the case of non-uniform diffusion. This
may explain why only a small fraction of the magnetic flux is observed
to escape from the plage into the surrounding network.
Title: The Distribution of Solar Magnetic Fluxes and the Nonlinearity
of Stellar Flux-Flux Relations
Authors: Schrijver, C. J.; Harvey, K. L.
Bibcode: 1989ApJ...343..481S
Altcode:
Synoptic maps for the 1975-1984 period are used to determine the
time-dependent distribution function of magnetic flux densities
in the solar atmosphere. The distribution function depends only
on the global level of magnetic activity, and it is used to study
how relations between magnetic flux densities and radiative flux
densities from different temperature regimes in the outer atmosphere
(derived from spatially resolved solar observations) transform into
relations between surface-averaged flux densities. It is found that
the transformation to surface-averaged fluxes preserves the power-law
character of relations between radiative and magnetic flux densities
for spatially resolved data.
Title: Magnetic structure in cool stars. XVI. Emissions from the
outer atmospheres of M-type dwarfs.
Authors: Rutten, R. G. M.; Schrijver, C. J.; Zwaan, C.; Duncan, D. K.;
Mewe, R.
Bibcode: 1989A&A...219..239R
Altcode:
Consideration is given to emission from the outer atmospheres of M-type
dwarfs in several spectral lines originating from the chromosphere, the
transition-region, and the soft X-ray emission from the corona. It is
shown that M-type dwarfs systematically deviate from relations between
flux densities in soft X-rays and chromospheric and transition-region
emission lines. The quantitative relation between the equivalent width
of H-alpha and the Ca II, H, and K emission index is determined. It
is suggested that the emission in the Balmer spectrum may result from
back heating by coronal soft X-rays.
Title: Coronal Activity in F-, G-, and K-Type Stars. III. The
Coronal Differential Emission Measure Distribution of Capella,
sigma 2 Coronae Borealis, and Procyon
Authors: Lemen, J. R.; Mewe, R.; Schrijver, C. J.; Fludra, A.
Bibcode: 1989ApJ...341..474L
Altcode:
EXOSAT soft X-ray spectra of three binary systems of cool stars
are analyzed: Capella (G6 III + F9 III), Sigma-squared CrB (F8 V +
G1 V), and Procyon (F5 IV-V + DF). The EXOSAT transmission grating
spectrometer permits the study of individual spectral lines and line
complexes between 10 and 200 A with approximately 3 A resolution. First
it is demonstrated that the spectra can be described reasonably well
by a two-temperature model corona. Then the assumption that only two
temperatures exist in the stellar coronas is relaxed and differential
emission measure distributions are derived from the three spectra. The
results from the multithermal modeling are consistent with those of
the two-temperature models: emission from the coronas of each of the
three stars is dominated by plasma in two relative narrow temperature
intervals. These intervals are centered on 5 MK and 25 MK in the cases
of Capella and Sigma-squared CrB, and 0.6 MK and 3 MK in the case of
Procyon. The implications of the results for the structure of stellar
coronas are briefly discussed.
Title: Coronal Activity in F-, G-, and K-Type Stars. IV. Evidence
for Expanding Loop Geometries in Stellar Coronae
Authors: Schrijver, C. J.; Lemen, J. R.; Mewe, R.
Bibcode: 1989ApJ...341..484S
Altcode:
A detailed analysis is presented of X-ray spectra of Capella and of
Sigma2 CrB. The spectra of both stars are compatible with coronae
consisting of two different ensembles of static loops with different
maximum temperatures and ratios of the cross sectional areas at the
loop top and at the footpoint. The cool (5 MK) loop components in both
stars show evidence of relatively strong expansion with height. The hot
(30 MK) components appear to expand much less.
Title: The Magnetic, Basal, and Radiative Equilibrium Components in
Mount Wilson CA II H+K Fluxes
Authors: Schrijver, C. J.; Dobson, Andrea K.; Radick, Richard R.
Bibcode: 1989ApJ...341.1035S
Altcode:
Mount Wilson Ca II H + K flux measurements of cool dwarf stars are
analyzed and compared with stellar Mg II h + k fluxes, variability
amplitudes, rotation rates, and solar data. It is concluded that the
Mount Wilson Ca II H + K fluxes comprise three principal parts: (1) a
photospheric contribution in the line wings, (2) a basal chromospheric
component that appears to be unrelated to stellar magnetic activity and
is, therefore, possibly nonmagnetic in origin, and (3) a chromospheric
component which is associated with magnetically active regions and the
(quiet and active) network. The basal chromosphere appears to cover
the entire surface of magnetically inactive stars. The basal Ca II
H + K flux density for solar-type stars equals the average emission
observed in the centers of solar supergranulation cells, where the
magnetic flux density is small.
Title: Relations between the Photospheric Magnetic Field and the
Emission from the Outer Atmospheres of Cool Stars. I. The Solar CA
II K Line Core Emission
Authors: Schrijver, C. J.; Cote, J.; Zwaan, C.; Saar, S. H.
Bibcode: 1989ApJ...337..964S
Altcode:
Observations of a solar active region complex and its surroundings are
used to establish a quantitative relation between the Ca II K line core
intensity and magnetic flux density. The Ca II K line core intensity
is transformed to a Ca II H + K line core flux density to facilitate
a comparison of solar and stellar data. A new absolute calibration for
the Mount Wilson Ca II H + K fluxes for G-type dwarfs is derived. The
minimum Ca II K flux, found in the centers of supergranulation cells
in quiet regions on the sun, is identical to the minimum flux that
is observed for solar-type stars. An expression is presented for the
nonlinear trend between the Ca II H + K line core excess flux density
and the absolute value of the magnetic flux density. Models that
explain the nonlinearity of the mean trend and the large intrinsic
scatter about it are discussed. The solar data define a relation that
is similar to the relation between stellar hemisphere-average magnetic
flux densities and Ca II H + K excess flux densities.
Title: Active region evolution in the chromosphere and transition
region
Authors: Shine, R. A.; Schrijver, C. J.
Bibcode: 1988fnsm.work...29S
Altcode:
Images in the C IV 1548 A and the Si II 1526 S lines taken with the
ultraviolet spectrometer polarimeter (UVSP) instrument on board the
Solar Maximum Mission (SMM) satellite were combined into movies showing
the evolution of active regions and the neighboring supergranulation
over several days. The data sets generally consist of 240 by 240 arc
second rasters with 3 arc second pixels taken one per orbit (about every
90 minutes). The images are projected on a latitude/longitude grid to
remove the forshortening as the region rotates across the solar disk
and further processed to remove jitter and gain variations. Movies
were made with and without differential rotation. Although there are
occasional missing orbits, these series do not suffer from the long
nighttime gaps that occur in observations taken at a single groundbased
observatory and are excellent for studying changes on time scales of
several hours. The longest sequence processed to date runs from 20
Oct. 1980 to 25 Oct. 1980. This was taken during an SMM flare buildup
study on AR 2744. Several shorter sequences taken in 1980 and 1984
will also be shown. The results will be presented on a video disk
which can be interactively controlled to view the movies.
Title: C IV fluxes from the Sun as a star, and the correlation with
magnetic flux
Authors: Schrijver, C. J.; Linsky, J. L.; Bennett, J.; Brown, A.;
Saar, S. H.
Bibcode: 1988jila.reptR....S
Altcode:
A total of 144 C IV wavelength 1548 Solar Maximum Mission (SMM)-UVSP
spectroheliograms of solar plages were analyzed, some of which are
series of exposures of the same region on the same day. Also analyzed
were the C IV wavelength 1551 rasters of plages and C IV wavelength 1548
rasters of the quiet sun. The sample contained data on 17 different
plages, observed on 50 different days. The center-to-limb variations
of the active regions show that the optical thickness effects in the
C IV wavelength 1548 line can be neglected in the conversion from
intensity to flux density. As expected for the nearly optically thin
situation, the C IV wavelength 1548 line is twice as bright as the C
IV 1551 line. The average C IV wavelength 1548 flux density for a quiet
region is 2700 ergs/cm/s and, with surprisingly little scatter, 18,000
erg/cm/s for plages. The intensity histograms of rasters obtained at
disk center can be separated into characteristic plage and quiet sun
contributions with variable relative filling factors. The relationship
between the C IV and magnetic flux densities for spatially resolved
data is inferred to be almost the same, with only an additional factor
of order unity in the constant of proportionality.
Title: C 4 fluxes from the sun as a star and the correlation with
magnetic flux
Authors: Schrijver, C. J.; Linsky, J. L.; Bennett, J.; Brown, A.;
Saar, S. H.
Bibcode: 1988jila.reptQ....S
Altcode:
A total of 144 C 4 wavelength 1548 SMM-UVSP spectroheliograms of
solar plages were analyzed, some of which are series of exposures of
the same region on the same day. Also analyzed were C 4 wavelength
1551 rasters of plages and C 4 1548 rasters of the quiet sun. The
sample contains data on 17 different plages, observed on 50 different
days. The center-to-limb variations of the active regions show that
the optical thickness effects in the C 4 wavelength 1548 line can be
neglected in the conversion from intensity to flux density. As expected
for the nearly optically thin situation, the C 4 1548 line is twice as
bright as the C 4 wavelength 1551 line. The average C 4 wavelength 1548
flux density for a quiet is 2700 erg/cm/s and, with surprisingly little
scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters
obtained at disk centers can be separated into characteristic plage and
quiet-sun contributions with variable relative filling factors. The
disk-averaged flux density in the C 4 doublet and the disk-averaged
magnitude of the magnetic flux density are related. The relationship
between the C 4 and magnetic flux densities for spatially resolved data
is inferred to be almost the same, with only an additional factor of
order unity in the constant of proportionality.
Title: Transition region fluxes in A-F dwarfs: basal fluxes and
dynamo activity
Authors: Walter, Frederick M.; Schrijver, Carolus J.; Boyd, William
Bibcode: 1988ESASP.281a.323W
Altcode: 1988uvai....1..323W; 1988IUE88...1..323W
The transition region spectra of 87 late A and early F dwarfs and
subgiants were analyzed. The emission line fluxes are uniformly strong
in the early F stars, and drop off rapidly among the late A stars. The
basal flux level in the F stars is consistent with an extrapolation of
that observed among the G stars, while the magnetic component displays
the same flux-flux relations seen among solar-like stars. Despite the
steep decrease in transition region emission flux for B-V less than
0.28, C II emission is detected in alpha Aql (B-V = 0.22). The dropoff
in emission is inconsistent with models of the mechanically generated
acoustic flux available. It is concluded that, although the nonmagnetic
basal heating is an increasingly important source of atmospheric heating
among the early F stars, magnetic heating occurs in any star which
has a sufficiently thick convective zone to generate acoustic heating.
Title: Active Region Evolution in the Chromosphere and Transition
Region
Authors: Shine, R. A.; Schrijver, C.
Bibcode: 1988BAAS...20..744S
Altcode:
No abstract at ADS
Title: Radiative fluxes from the outer atmosphere of a star like
the Sun - A construction kit
Authors: Schrijver, C. J.
Bibcode: 1988A&A...189..163S
Altcode:
Chromospheric and coronal radiative flux densities of a star like the
Sun are simulated as a function of stellar magnetic activity. The
required limb-darkening curves for active regions are derived for
ultraviolet chromospheric, transition-region and coronal emission
lines. The normalized distribution of areas of active regions appears
to be nearly independent of the phase in the solar cycle. The synthesis
of the emission of the Sun-as-a-star from emissions of active and quiet
regions shows that the surface-averaged solar flux densities, averaged
over the solar cycle, are compatible with flux-flux relations defined by
other late-type dwarfs and giants. Some aspects of rotational modulation
are discussed; one result is that chromospheric and coronal flux levels
may be out of phase. The simulated flux level of the Sun-as-a-star
in different phases of the activity cycle can be made to change
along the stellar flux-flux relations (as indicated by observations)
by raising the emission from the quiet disk as the number of active
regions increases.
Title: Solar active regions - Radiative intensities and large-scale
parameters of the magnetic field
Authors: Schrijver, C. J.
Bibcode: 1987A&A...180..241S
Altcode:
Tight relations are established here between parameters characterizing
the large-scale properties of solar active regions. The averaged
magnetic flux density appears nearly constant at 100 + or - 20 G. Hence,
with a typical mean flux density of 1-2 kG in photospheric flux tubes,
the filling factor for the photospheric magnetic field in active regions
lies between 5 and 10 percent. The total intensity of active regions in
the chromospheric C II or in the coronal Mg X line is tightly related
with the total magnetic flux in the active region. A model is presented
to explain the relations between the projected surface areas of active
regions as determined from magnetograms and from spectroheliograms
at different wavelengths. The total coronal heating depends nearly
linearly on the total magnetic flux in the active region or on the
area of the magnetic plage. The mean magnetic flux density in quiet
regions suggests a linear dependence of the coronal heating on the
mean magnetic flux density in active and quiet regions.
Title: Magnetic structure in cool stars. XIII - Appropriate units
for the rotation-activity relation
Authors: Rutten, R. G. M.; Schrijver, C. J.
Bibcode: 1987A&A...177..155R
Altcode:
The paper compares relations between coronal and chromospheric
emissions, and between activity and stellar rotation, in which the
radiative emissions that measure the magnetic activity of cool stars are
expressed in different units. The surface flux density appears to be the
most suitable unit in which to express the radiative emission measuring
stellar magnetic activity: the luminosity introduces an extra dependence
on stellar radius, whereas the flux density normalized by the bolometric
flux density introduces a dependence on color. The dependence on color
in the relation between rotation period and radiative flux density
cannot be repaired by a simple color-dependent scaling of the rotation
period. For example, dividing the rotation period by the turnover time
of convective eddies cannot yield a single, color-independent relation
between rotation and activity, and this scaling disrupts the similarity
in behavior of dwarfs and giants in the rotation-activity relation.
Title: Magnetic structure in cool stars. XIV. Deficiency in
chromospheric fluxes from M-type dwarfs.
Authors: Schrijver, C. J.; Rutten, R. G. M.
Bibcode: 1987A&A...177..143S
Altcode:
Chromospheric, transition-region and coronal emissions of M-type dwarfs
and of dwarfs of earlier spectral type are compared, and related to
the rotation period. M-type dwarfs depart from the relation between
chromopsheric Ca II H+K excess flux density ΔFCa II and
the coronal soft X-ray flux density FX as defined by stars
of earlier spectral type. The M-type dwarfs also show low chromospheric
emissions in comparison to stars of earlier spectral type and the same
rotation period P. It is shown that these deviations are caused by a
deficiency in the chromopsheric emission, rather than by an increase
in the coronal emission.
Title: Evidence for Expanding Loop Geometries in Stellar Coronae
Authors: Mewe, R.; Lemen, J. R.; Schrijver, C. J.; Fludra, A.
Bibcode: 1987LNP...291...60M
Altcode: 1987csss....5...60M; 1987LNP87.291...60M
No abstract at ADS
Title: Empirical Relations Between Magnetic Fluxes and Atmospheric
Radiative Losses for Cool Dwarf Stars
Authors: Saar, Steven H.; Schrijver, C. J.
Bibcode: 1987LNP...291...38S
Altcode: 1987csss....5...38S
We derive and discuss relations between magnetic, X-ray, and Ca II
H+K flux densities for a sample of G, K, and M dwarfs.
Title: The Relation Between the Ca II K Line-Core Flux Density and
the Magnetic Flux Density on the Sun
Authors: Schrijver, C. J.; Coté, J.
Bibcode: 1987LNP...291...51S
Altcode: 1987csss....5...51S
No abstract at ADS
Title: Heating of Stellar Chromospheres and Coronae: Evidence for
Non-Magnetic Heating (Invited review)
Authors: Schrijver, C. J.
Bibcode: 1987LNP...291..135S
Altcode: 1987csss....5..135S
A detailed analysis of the radiative losses from the outer atmospheres
of magnetically active stars suggests that two emission components are
present. These two components may correspond to two distinct mechanisms
of non-radiative heating that operate in the outer atmospheres of cool
stars. One of these mechanisms depends on the stellar rotation rate,
and is associated with stellar magnetic activity. The other component
varies with stellar effective temperature, appears to be insensitive
to surface gravity, and independent of the stellar rotation rate. The
radiative energy loss of this second component results in the observed
lower boundaries in diagrams of flux density versus colour, hence
the name "basal chromosphere." The associated heating mechanism may
be dissipation of acoustic waves. The temperatures of the basal part
of the atmosphere are limited to a few tens of thousands of degrees
or less in G- and K-type stars. The basal atmosphere in F-type dwarfs
may reach up to transition-region temperatures, or higher.
Title: Magnetic structure in cool stars. XI. Relations between
radiative fluxes mesuring stellar activity, and evidence for two
components in stellar chromospheres.
Authors: Schrijver, C. J.
Bibcode: 1987A&A...172..111S
Altcode:
Tight power-law relations between chromospheric, transition-region,
and coronal surface flux densities of late-type stars are obtained,
provided that a lower-limit flux φ is subtracted from radiative
flux densities originating in the lower chromosphere. The author
determines lower-limit, or basal, flux densities φi that
optimize correlation coefficients for power-law relations between
the soft X-ray flux density and the chromospheric excess flux density
(Fi-φi) in the Ca II H and K, Mg II h and k,
and Si II resonance lines.
Title: Activity in Warm Stars
Authors: Walter, F. M.; Schrijver, C. J.
Bibcode: 1987LNP...291..262W
Altcode: 1987csss....5..262W; 1987LNP87.291..262W
No abstract at ADS
Title: A two-component model for the corona of lambda Andromedae.
Authors: Mewe, R.; Schrijver, C. J.
Bibcode: 1986A&A...169..178M
Altcode:
The binary Lambda Andromedae (HD 222107) was studied with the low-energy
(LE) and medium-energy (ME) X-ray detectors of Exosat on December 5,
1983 and on January 8, 1984, half a rotation period later. The ME data
for the observations are compatible with emission from a plasma with
a temperature of about 20 MK. If the strong variability in the hard
X-ray flux is interpreted as due to rotational modulation the scale
height of the hot plasma should be significantly smaller than the
stellar radius. If the ME data are interpreted in terms of a model for
quasi-static loops, the footpoints of these loops cover at most a few
percent of the stellar surface, and the pressure of the hot plasma may
exceed about 25 dyn/sq cm. The LE data require an additional, weaker
contribution by a cooler component that does not vary significantly
with time. A later observation of the object on August 6, 1984 shows
no significant change in the two plasma components as inferred from
the observations on January 8, 1984, when the same hemisphere of the
primary star was observed.
Title: X-Ray Emission of λ Andromedae: Correlation with Spot Activity
Authors: Mewe, R.; Schrijver, C. J.
Bibcode: 1986LNP...254..297M
Altcode: 1986csss....4..297M
No abstract at ADS
Title: Differential emission measure distributions of Capella and
σ2 CrB
Authors: Mewe, R.; Schrijver, C. J.; Lemen, J. R.; Bentley, R. D.
Bibcode: 1986AdSpR...6h.133M
Altcode: 1986AdSpR...6..133M
The active late-type stars Capella (α Aur G6III+F9III) and
σ2 CrB (F6V+GV) (X-ray candidate emitters underlined)
were observed with the EXOSAT Transmission Grating Spectrometer
(TGS) in the region 10-200 Å. We have made an analysis of the
differential emission measure (DEM) distribution. The derived DEM
peaks between 3 and 7 MK and above 10 MK. The results disagree with
models for static loops as developed by Rosner, Tucker and Vaiana [1,
hereafter referred to as RTV]: the contrast in emission between the
maximal and lower temperatures in the loop is larger than predicted by
the RTV model. Other models which predict DEM distributions falling
off more steeply towards lower temperatures are briefly discussed:
e.g. quasi-static loops with varying cross-sectional area or dynamic
loops with strong downward flows.
Title: Stellar magnetic activity: Complementing conclusions based
on solar and stellar observations
Authors: Schrijver, Carolus Josephus Karel
Bibcode: 1986PhDT.......203S
Altcode:
No abstract at ADS
Title: On Stellar Activity, Rotation, and the Reality of the
Vaughan-Preston Gap
Authors: Rutten, R. G. M.; Schrijver, C. J.
Bibcode: 1986LNP...254..120R
Altcode: 1986csss....4..120R
No abstract at ADS
Title: X-Ray Spectroscopy of Capella and σ2 Coronae
Borealis
Authors: Schrijver, C. J.; Mewe, R.
Bibcode: 1986LNP...254..300S
Altcode: 1986csss....4..300S
No abstract at ADS
Title: Relations Between Fluxes Measuring Stellar Activity
Authors: Schrijver, C. J.
Bibcode: 1986LNP...254..112S
Altcode: 1986csss....4..112S
No abstract at ADS
Title: Dispersive X-ray spectroscopy with transmission gratings.
Authors: Paerels, F.; Bleeker, J. A. M.; Brinkman, A. C.; Dijkstra,
J. H.; Mewe, R.; Schrijver, C. J.
Bibcode: 1985ESASP.239..205P
Altcode: 1985cxrs.work..205P
The possibilities for plasma diagnostics with high throughput grating
spectrometers are illustrated by presenting spectra of a coronal
X-ray source (Capella), a Seyfert Galaxy (NGC 4151) and a BL Lac
object (PKS 2155-304). Einstein and EXOSAT spectra are contrasted with
computer-simulated AXAF or XMM spectra, to illustrate the increase of
insight expected from grating spectroscopy with these instruments.
Title: A study of ultraviolet and X-ray emissions of selected
solar regions
Authors: Schrijver, C. J.; Zwaan, C.; Maxson, C. W.; Noyes, R. W.
Bibcode: 1985A&A...149..123S
Altcode:
Apollo Telescope Mount-based Skylab observations of quiet regions,
coronal holes, and active regions of various sizes and in various stages
of evolution are studied. Total intensities in coronal emissions are
noted to increase more rapidly than linearly with total intensities in
chromospheric emissions, and areas of active regions determined from
chromospheric emissions are found to be systematically larger than
areas determined from transition region emissions. The coronal X-ray
temperature, as well as the average intensities of active regions,
depend on the area of the active region, on Ca II intensity, and on
relative coverage by sunspots.
Title: Magnetic Activity in Cool Stars
Authors: Schrijver, C. J.
Bibcode: 1985SSRv...40....3S
Altcode:
Aspects of magnetic activity of late-type stars are reviewed: first,
results obtained through the comparison of emissions from various
temperature regimes in the outer atmospheres of cool stars, then results
from stellar soft X-ray spectroscopy. Spectra obtained with different
instruments (Imaging Proportional Counter and Solid State Spectrometer
onboard the EINSTEIN Satellite, and the Objective Grating Instruments
onboard EINSTEIN and EXOSAT) are interpreted through fits with model
spectra consisting of one or two temperature components. Particular
attention is given to the analysis of EXOSAT spectra of Procyon,
Capella, and σ2 CrB. Finally various relations between
rotation and different measures of activity are compared. The effective
X-ray temperature, derived from single-temperature fits to soft X-ray
IPC spectra, is shown to play a part in the relation between activity
and rotation.
Title: Structural Changes in the Chromospheres of M Dwarfs
Authors: Schrijver, C.
Bibcode: 1985iue..prop.2353S
Altcode:
No abstract at ADS
Title: Coronal activity in F-, G- and K-type stars. II. Coronal
structure and rotation.
Authors: Schrijver, C. J.; Mewe, R.; Walter, F. M.
Bibcode: 1984A&A...138..258S
Altcode:
Einstein Observatory soft X-ray spectra for a sample of 34 late-type
stars are analyzed in order to determine coronal temperatures and
emission features per unit area at the stellar surface. No obvious
relation between coronal temperature and (B-V) color index could be
found. Coronal temperature is dependent on stellar luminosity class. The
data are discussed in terms of a model for static magnetic loops, and
indications are found for a variety of coronae in which the sizes of
the dominating loops range from more compact than loops in solar-active
regions to loops that are comparable to the pressure scale height
(or even greater). Despite observed structural differences, a single,
strict relation holds between X-ray surface flux density and the Ca
II H + K excess flux density of these stars. Coronal activity and
structure are noted to be related to the stellar rotation rate.
Title: Book reviews
Authors: Doom, C.; Schrijver, C. J.; Icke, V.; Rakos, Karl D.; Kleczek,
J.; Forbes, E. G.; Seboldt, W.; Linssen, P. F. J.; McKenna-Lawlor,
S. M. P.; van der Kruit, P. C.; Namba, O.; van Beek, H. F.
Bibcode: 1984SSRv...39..375D
Altcode:
No abstract at ADS
Title: Chromospheric Transition Region and Coronal Diagnostics for
Stellar Magnetic Activity
Authors: Schrijver, C. J.; Zwaan, C.
Bibcode: 1984srps.conf..291S
Altcode:
Using various diagnostics magnetic activity has been inferred for the
vast majority of stars with convective envelopes. The authors discuss
relations between emissions from different temperature regimes, and
their dependence on various stellar parameters. They suggest several
practical indicators for stellar magnetic structure and acitivity,
and outline a few topics for which space observations would be useful.
Title: Book-Review - Cool Stars Stellar Systems and the Sun
Authors: Baliunas, S. L.; Hartmann, L.; Schrijver, C. J.
Bibcode: 1984SSRv...39..375B
Altcode:
No abstract at ADS
Title: Coronal Structure and Rotation in Late-type Stars
Authors: Schrijver, C. J.; Mewe, R.; Walter, F. M.
Bibcode: 1984LNP...193..166S
Altcode: 1984csss....3..166S
No abstract at ADS
Title: Coronal activity in F-, G-, and K-type stars. I. Relations
between parameters characterizing stellar and activity.
Authors: Schrijver, C. J.
Bibcode: 1983A&A...127..289S
Altcode:
A sample of 66 stars, containing dwarfs and giants, is subjected to
a multidimensional common-factor analysis. The parameters used are
the soft X-ray flux density at the stellar surface Fx, the
Ca II H + K line-core flux density FH+K, and parameters
determining the stellar interior. We find a high correlation
between Fx and the Ca II H+K excess flux ΔFH+K,
obtained by subtracting an observational lower limit from FM +
K We argue, that the Ca ii lower-limit flux is uncorrelated with
hot, X-ray emitting, magnetic structures. The common-factor
analysis shows that, for the stars in the present sample, the
relation between Fx and ΔFH+K does not
depend on the stellar mass or radius. All stars included in the
analysis - single dwarfs and giants, and components of short- period
binaries - follow the relation over four decades in Fx:
Fx(:)ΔF1.67H+K. The Sun follows this
relation during its activity cycle, which suggests that structural
changes occur on the Sun (as required by the nonlinearity of the above
relation) similar to the structural differences between stars with
different average levels of activity.
Title: Coronal activity in F-, G-, and K-type stars - Empirical
relations between stellar parameters
Authors: Mewe, R.; Schrijver, C. J.; Gronenschild, E. H. B. M.;
Zwaan, C.
Bibcode: 1983IAUS..102..205M
Altcode:
For a sample of 58 late type stars the authors analyse the relations
between the soft X-ray flux density Fx, the Ca II H and
K line-core flux density FH+K, and parameters determining
the global stellar structure. By analysing the soft X-ray spectra from
15 stars the authors determine the coronal temperatures T and specific
emission measures per unit area ζ. They discuss the dependence of T on
B-V, Fx and stellar radius R. The diagram of the specific
emission measure ζ against the temperature T is interpreted in terms
of a coronal model consisting of static loops. Also, a search for time
variations in the X-ray flux has been performed.
Title: Book reviews
Authors: Machado, M. E.; Duijveman, A.; Schrijver, C. J.
Bibcode: 1982SoPh...79..399M
Altcode:
No abstract at ADS
Title: Book-Review - Solar Phenomena in Stars and Stellar Systems
Authors: Bonett, Rm.; Dupree, A. K.; Schrijver, C. J.
Bibcode: 1982SoPh...79..400B
Altcode:
No abstract at ADS
Title: Coronal activity in F-, G-, and K-type stars; relations between
parameters characterizing stellar structures and X-ray emission
Authors: Schrijver, C. J.; Mewe, R.; Zwaan, C.
Bibcode: 1982AdSpR...2i.243S
Altcode: 1982AdSpR...2..243S
A sample of 52 stars containing dwarfs and giants is subjected to
a multidimensional factor analysis. The parameters used are the
soft X-ray flux at the stellar surface Fx, the Ca II H+K
line-core flux FH+K, the stellar radius and mass. We find
a high correlation between Fx and the Ca II H+K excess flux
ΔFH+K obtained by subtracting an observational lower-limit
flux from FH+K. We conclude that the lower-limit Ca II flux
is uncorrelated with the stellar X-ray emission. The common-factor
analysis shows that, for the present sample, Fx depends
only on ΔFH+K, and not on the stellar radius or mass. All
stars included in our analysis follow the relation Fx
~ Δ1.4H+K over almost four decades in
Fx.
Title: Coronal Activity an F-Type G-Type and K-Type Stars
Authors: Mewe, R.; Schrijver, C. J.; Zwaan, C.
Bibcode: 1981SSRv...30..191M
Altcode:
Soft X-ray (0.3 3.5 keV) observations with the Imaging Proportional
Counter (IPC) onboard Einstein Observatory are presented for a sample
of some 20 cool stars of luminosity classes III V. The results are
compared with the Ca II H and K emission, which had served as a
selection criterion. The specific X-ray flux FX is an
increasing function of the specific Ca II H and K line-core flux
FH+K. This correlation can be considerably improved by
replacing FH+K by the excess flux (ΔFH+K)
above a certain lower limit which varies with B-V. This relation
holds with little scatter over the two decades in FX in
our sample. The FX-ΔFH+K relation shows no
significant dependence on spectral type or luminosity class, it suits
close binaries as well as single stars. However, the coronal X-ray
temperature Tc strongly depends on the luminosity class:
Tc≈ 3 106 K for dwarfs and ≈ 107
K for giants. The results are interpreted in the framework of magnetic
activity. The X-ray emission and the excess Ca II H and K flux are
attributed to magnetic structure in the corona and chromosphere,
the magnetic features emerging from the stellar convective envelope,
where they are generated by dynamo action.