Author name code: centeno
ADS astronomy entries on 2022-09-14
author:"Centeno, Rebecca"
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Title: Effects of spectral resolution on simple magnetic field
diagnostics of the Mg II h & k lines
Authors: Centeno, Rebecca; Rempel, Matthias; Casini, Roberto; del
Pino Aleman, Tanausu
Bibcode: 2022arXiv220807507C
Altcode:
We study the effects of finite spectral resolution on the magnetic
field values retrieved through the weak field approximation (WFA)
from the cores of the Mg II h & k lines. The retrieval of the
line-of-sight (LOS) component of the magnetic field, $B_{\rm LOS}$,
from synthetic spectra generated in a uniformly magnetized FAL-C
atmosphere are accurate when restricted to the inner lobes of Stokes
V. As we degrade the spectral resolution, partial redistribution (PRD)
effects, that more prominently affect the outer lobes of Stokes V,
are brought into the line core through spectral smearing, degrading the
accuracy of the WFA and resulting in an inference bias, which is more
pronounced the poorer the resolution. When applied to a diverse set of
spectra emerging from a sunspot simulation, we find a good accuracy
in the retrieved $B_{\rm LOS}$ when comparing it to the model value
at the height where the optical depth in the line core is unity. The
accuracy is preserved up to field strengths of B~1500 G. Limited
spectral resolution results in a small bias toward weaker retrieved
fields. The WFA for the transverse component of the magnetic field is
also evaluated. Reduced spectral resolution degrades the accuracy of
the inferences because spectral mixing results in the line effectively
probing deeper layers of the atmosphere.
Title: Convolutional Neural Networks and Stokes Response Functions
Authors: Centeno, Rebecca; Flyer, Natasha; Mukherjee, Lipi; Egeland,
Ricky; Casini, Roberto; del Pino Alemán, Tanausú; Rempel, Matthias
Bibcode: 2022ApJ...925..176C
Altcode: 2021arXiv211203802C
In this work, we study the information content learned by
a convolutional neural network (CNN) when trained to carry out the
inverse mapping between a database of synthetic Ca II intensity spectra
and the vertical stratification of the temperature of the atmospheres
used to generate such spectra. In particular, we evaluate the ability
of the neural network to extract information about the sensitivity of
the spectral line to temperature as a function of height. By training
the CNN on sufficiently narrow wavelength intervals across the Ca
II spectral profiles, we find that the error in the temperature
prediction shows an inverse relationship to the response function
of the spectral line to temperature, that is, different regions of
the spectrum yield a better temperature prediction at their expected
regions of formation. This work shows that the function that the CNN
learns during the training process contains a physically meaningful
mapping between wavelength and atmospheric height.
Title: On the (Mis)Interpretation of the Scattering Polarization
Signatures in the Ca II 8542 Å Line through Spectral Line Inversions
Authors: Centeno, Rebecca; de la Cruz Rodríguez, Jaime; del Pino
Alemán, Tanausú
Bibcode: 2021ApJ...918...15C
Altcode: 2021arXiv210604478C
Scattering polarization tends to dominate the linear polarization
signals of the Ca II 8542 Å line in weakly magnetized areas (B ≲ 100
G), especially when the observing geometry is close to the limb. In
this paper, we evaluate the degree of applicability of existing
non-LTE spectral line inversion codes (which assume that the spectral
line polarization is due to the Zeeman effect only) at inferring the
magnetic field vector and, particularly, its transverse component. To
this end, we use the inversion code STiC to extract the strength and
orientation of the magnetic field from synthetic spectropolarimetric
data generated with the Hanle-RT code. The latter accounts for the
generation of polarization through scattering processes as well as
the joint actions of the Hanle and the Zeeman effects. We find that,
when the transverse component of the field is stronger than ~80 G,
the inversion code is able to retrieve accurate estimates of the
transverse field strength as well as its azimuth in the plane of the
sky. Below this threshold, the scattering polarization signatures
become the major contributors to the linear polarization signals and
often mislead the inversion code into severely over- or underestimating
the field strength. Since the line-of-sight component of the field is
derived from the circular polarization signal, which is not affected
by atomic alignment, the corresponding inferences are always good.
Title: Measuring the Magnetic Origins of Solar Flares, Coronal Mass
Ejections, and Space Weather
Authors: Judge, Philip; Rempel, Matthias; Ezzeddine, Rana; Kleint,
Lucia; Egeland, Ricky; Berdyugina, Svetlana V.; Berger, Thomas; Bryans,
Paul; Burkepile, Joan; Centeno, Rebecca; de Toma, Giuliana; Dikpati,
Mausumi; Fan, Yuhong; Gilbert, Holly; Lacatus, Daniela A.
Bibcode: 2021ApJ...917...27J
Altcode: 2021arXiv210607786J
We take a broad look at the problem of identifying the magnetic
solar causes of space weather. With the lackluster performance
of extrapolations based upon magnetic field measurements in the
photosphere, we identify a region in the near-UV (NUV) part of the
spectrum as optimal for studying the development of magnetic free energy
over active regions. Using data from SORCE, the Hubble Space Telescope,
and SKYLAB, along with 1D computations of the NUV spectrum and numerical
experiments based on the MURaM radiation-magnetohydrodynamic and
HanleRT radiative transfer codes, we address multiple challenges. These
challenges are best met through a combination of NUV lines of bright Mg
II, and lines of Fe II and Fe I (mostly within the 4s-4p transition
array) which form in the chromosphere up to 2 × 104
K. Both Hanle and Zeeman effects can in principle be used to derive
vector magnetic fields. However, for any given spectral line the τ
= 1 surfaces are generally geometrically corrugated owing to fine
structure such as fibrils and spicules. By using multiple spectral
lines spanning different optical depths, magnetic fields across nearly
horizontal surfaces can be inferred in regions of low plasma β, from
which free energies, magnetic topology, and other quantities can be
derived. Based upon the recently reported successful sub-orbital space
measurements of magnetic fields with the CLASP2 instrument, we argue
that a modest space-borne telescope will be able to make significant
advances in the attempts to predict solar eruptions. Difficulties
associated with blended lines are shown to be minor in an Appendix.
Title: Enhancements to Hinode/SOT-SP Vector Magnetic Field Data
Products
Authors: DeRosa, M. L.; Leka, K. D.; Barnes, G.; Wagner, E.; Centeno,
R.; De Wijn, A.; Bethge, C.
Bibcode: 2021AAS...23821305D
Altcode:
The Solar Optical Telescope Spectro-Polarimeter (SOT-SP), on board the
Hinode spacecraft (launched in 2006), is a scanning-slit spectrograph
that continues to provide polarization spectra useful for inferring the
vector (three-component) magnetic field at the solar photosphere. SOT-SP
achieves this goal by obtaining line profiles of two magnetically
sensitive lines, namely the Fe I 6302 Angstrom doublet, using a
0.16"×164" slit as it scans a region of interest. Once the data are
merged, a Milne-Eddington based spectropolarimetric inversion scheme is
used to infer multiple physical parameters in the solar photosphere,
including the vector magnetic field, from the calibrated polarization
spectra. All of these data are publicly available once the processing
has occurred. As of this year, the Hinode/SOT team is also making
available the disambiguated vector magnetic field and the re-projected
heliographic components of the field. In making the disambiguated vector
field data product, the 180° ambiguity in the plane-of-sky component
of the vector magnetic field inherent in the spectropolarimetric
inversion process has been resolved. This ambiguity is resolved
using the Minimum-Energy algorithm, which is the same algorithm used
within the pipeline producing the vector-magnetogram data product
for the Helioseismic and Magnetic Imager aboard the Solar Dynamics
Observatory. The heliographic field components (Bphi,
Btheta, Br) on the same grid as the inverted data
are also now provided. This poster provides more details about these
data product enhancements, and some examples on how the scientific
community may readily obtain these data.
Title: Hinode and IRIS Synoptic Observations of Solar Cycle Transition
at Mid-Latitudes
Authors: Egeland, R.; Centeno, R.; Lacatus, D.; de Toma, G.; Bryans,
P.; McIntosh, S.
Bibcode: 2021AAS...23811324E
Altcode:
Recent observations by McIntosh et al. using SDO/AIA coronal bright
point density and the magnetogram-derived large scale open magnetic
flux "g-nodes" have traced out extended solar cycle activity bands
that originate at ~55 degrees latitude and propagate toward the
equator. When the opposite hemisphere bands "terminate" at the equator,
this event corresponds with the rapid rise of new cycle flux and active
regions at mid-latitudes, ~35 degrees. We present weekly synoptic
high-resolution observations of mid-latitude (35-40 degrees) magnetic
flux and chromospheric emission from Hinode SOT/SP and IRIS starting
from March 2017 until today, covering the end of cycle 24 and the
beginnings of cycle 25. After carefully correcting for instrumental
shifts, noise, and solar B-angle effects, we analyze time series
of mean magnetic flux, chromospheric line intensity, and statistical
properties of magnetic regions for signals of a developing cycle 25 and
the passage of the activity bands in these typically quiet mid-latitude
regions. Initial results show that the statistical properties of our
activity metrics are roughly constant over the four year period, with
a step function increase in activity that corresponds to a change to a
lower observation latitude. Finally, we investigate the correspondence
of SDO/AIA coronal bright points in our observational field of view
to chromospheric emission and photospheric magnetic features.
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
Bibcode: 2021SoPh..296...70R
Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
Title: Long-term evolution of three light bridges developed on the
same sunspot
Authors: Griñón-Marín, A. B.; Pastor Yabar, A.; Centeno, R.;
Socas-Navarro, H.
Bibcode: 2021A&A...647A.148G
Altcode: 2021arXiv210204392B; 2021arXiv210204392G
One important feature of sunspots is the presence of light
bridges. These structures are elongated and bright (as compared
to the umbra) features that seem to be related to the formation
and evolution of sunspots. In this work, we studied the long-term
evolution and the stratification of different atmospheric parameters
of three light bridges formed in the same host sunspot by different
mechanisms. To accomplish this, we used data taken with the GREGOR
Infrared Spectrograph installed at the GREGOR telescope. These data
were inverted to infer the physical parameters of the atmosphere
where the observed spectral profiles were formed of the three light
bridges. We find that, in general, the behaviour of the three light
bridges is typical of this kind of structure with the magnetic
field strength, inclination, and temperature values between the
values at the umbra and the penumbra. We also find that they are of a
significantly non-magnetic character (particularly at the axis of the
light bridges) as it is deduced from the filling factor. In addition,
within the common behaviour of the physical properties of light bridges,
we observe that each one exhibits a particular behaviour. Another
interesting result is that the light bridge cools down, the magnetic
field decreases, and the magnetic field lines get more inclined higher
in the atmosphere. Finally, we studied the magnetic and non-magnetic
line-of-sight velocities of the light bridges. The former shows that
the magnetic component is at rest and, interestingly, its variation
with optical depth shows a bi-modal behaviour. For the line-of-sight
velocity of the non-magnetic component, we see that the core of the
light bridge is at rest or with shallow upflows and clear downflows
sinking through the edges.
Title: Discovery of long-period magnetic field oscillations and
motions in isolated sunspots
Authors: Griñón-Marín, A. B.; Pastor Yabar, A.; Socas-Navarro,
H.; Centeno, R.
Bibcode: 2020A&A...635A..64G
Altcode: 2020arXiv200106030G
We analyse the temporal evolution of the inclination component of
the magnetic field vector for the penumbral area of 25 isolated
sunspots. Compared to previous works, the use of data from the
HMI instrument aboard the SDO observatory facilitates the study
of a very long time series (≈1 week) with a good spatial and
temporal resolution. We used the wavelet technique and we found some
filamentary-shaped events with large wavelet power. Their distribution
of periods is broad, ranging from the lower limit for this study of 48
min up to 63 h. An interesting property of these events is that they
do not appear homogeneously all around the penumbra but they seem to
concentrate at particular locations. The cross-comparison of these
wavelet maps with AIA data shows that the regions where these events
appear are visually related to the coronal loops that connect the outer
penumbra to one or more neighbouring opposite polarity flux patches.
Title: Achievements of Hinode in the first eleven years
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick;
Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks,
David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio;
Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra,
Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.;
Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao,
Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota,
Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu,
Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin;
Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young,
Peter R.
Bibcode: 2019PASJ...71R...1H
Altcode:
Hinode is Japan's third solar mission following Hinotori (1981-1982)
and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in
operation currently. Hinode carries three instruments: the Solar Optical
Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These
instruments were built under international collaboration with the
National Aeronautics and Space Administration and the UK Science and
Technology Facilities Council, and its operation has been contributed
to by the European Space Agency and the Norwegian Space Center. After
describing the satellite operations and giving a performance evaluation
of the three instruments, reviews are presented on major scientific
discoveries by Hinode in the first eleven years (one solar cycle long)
of its operation. This review article concludes with future prospects
for solar physics research based on the achievements of Hinode.
Title: On the Weak Field Approximation for Ca 8542 Å
Authors: Centeno, Rebecca
Bibcode: 2018ApJ...866...89C
Altcode: 2018arXiv181103168C
The weak field approximation (WFA) is a conceptually simple and
computationally light method for inferring the magnetic field strength
and its orientation in the Sun’s atmosphere. In this work, we study
the validity and limitations of this tool when applied to full Stokes
Ca II 8542 Å profiles to extract information about the chromospheric
magnetic field. We find that the range of validity of the WFA depends,
among other things, on the component of the magnetic field that one
is trying to infer. The retrieval of the line-of-sight component of
the chromospheric magnetic field from the core of the spectral line
is reliable for field strengths up to ∼1200 G, even when moderate
velocity gradients are present. The horizontal component, on the
other hand, is suitably derived using the wing-core boundary of the
spectral line, but typically yields systematic errors of ≥slant
10 % . The effects of scattering polarization further compound the
problem by rendering the transverse field inference problematic in
quiet Sun areas, and for observing geometries within 30◦
of the limb. Magneto-optical effects disproportionately challenge
the determination of the magnetic field azimuth in the transverse
plane, leading to errors of ∼ 10^\circ . Typical noise levels of {σ
}{{n}}={10}-3 relative to the continuum intensity
preclude the accurate retrieval of the transverse field strength and its
azimuth below a threshold of a few hundred Gauss. Striving for a noise
level of {σ }{{n}}={10}-4 significantly improves
the diagnostic capability of the WFA with this spectral line, at which
point the magnetic field inference becomes limited by systematic errors.
Title: Measurements of Photospheric and Chromospheric Magnetic Fields
Authors: Lagg, Andreas; Lites, Bruce; Harvey, Jack; Gosain, Sanjay;
Centeno, Rebecca
Bibcode: 2018smf..book...37L
Altcode:
No abstract at ADS
Title: Measurements of Photospheric and Chromospheric Magnetic Fields
Authors: Lagg, Andreas; Lites, Bruce; Harvey, Jack; Gosain, Sanjay;
Centeno, Rebecca
Bibcode: 2017SSRv..210...37L
Altcode: 2015arXiv151006865L; 2015SSRv..tmp..115L
The Sun is replete with magnetic fields, with sunspots, pores
and plage regions being their most prominent representatives on
the solar surface. But even far away from these active regions,
magnetic fields are ubiquitous. To a large extent, their importance
for the thermodynamics in the solar photosphere is determined by the
total magnetic flux. Whereas in low-flux quiet Sun regions, magnetic
structures are shuffled around by the motion of granules, the high-flux
areas like sunspots or pores effectively suppress convection, leading
to a temperature decrease of up to 3000 K. The importance of magnetic
fields to the conditions in higher atmospheric layers, the chromosphere
and corona, is indisputable. Magnetic fields in both active and
quiet regions are the main coupling agent between the outer layers
of the solar atmosphere, and are therefore not only involved in the
structuring of these layers, but also for the transport of energy from
the solar surface through the corona to the interplanetary space. Consequently, inference of magnetic fields in the photosphere, and
especially in the chromosphere, is crucial to deepen our understanding
not only for solar phenomena such as chromospheric and coronal
heating, flares or coronal mass ejections, but also for fundamental
physical topics like dynamo theory or atomic physics. In this review,
we present an overview of significant advances during the last decades
in measurement techniques, analysis methods, and the availability of
observatories, together with some selected results. We discuss the
problems of determining magnetic fields at smallest spatial scales,
connected with increasing demands on polarimetric sensitivity and
temporal resolution, and highlight some promising future developments
for their solution.
Title: Search for torsional oscillations in isolated sunspots
Authors: Griñón-Marín, A. B.; Socas-Navarro, H.; Centeno, R.
Bibcode: 2017A&A...604A..36G
Altcode: 2017arXiv170601952G; 2017arXiv170601952B
In this work we seek evidence for global torsional oscillations in
alpha sunspots. We have used long time series of continuum intensity
and magnetic field vector maps from the Helioseismic and Magnetic
Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO)
spacecraft. The time series analysed here span the total disk passage
of 25 isolated sunspots. We found no evidence of global long-term
periodic oscillations in the azimuthal angle of the sunspot magnetic
field within 1 degree. This study could help us to understand the
sunspot dynamics and its internal structure.
Title: The Second Flight of the Sunrise Balloon-borne Solar
Observatory: Overview of Instrument Updates, the Flight, the Data,
and First Results
Authors: Solanki, S. K.; Riethmüller, T. L.; Barthol, P.; Danilovic,
S.; Deutsch, W.; Doerr, H. -P.; Feller, A.; Gandorfer, A.; Germerott,
D.; Gizon, L.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.;
Lagg, A.; Meller, R.; Tomasch, G.; van Noort, M.; Blanco Rodríguez,
J.; Gasent Blesa, J. L.; Balaguer Jiménez, M.; Del Toro Iniesta,
J. C.; López Jiménez, A. C.; Orozco Suarez, D.; Berkefeld, T.;
Halbgewachs, C.; Schmidt, W.; Álvarez-Herrero, A.; Sabau-Graziati,
L.; Pérez Grande, I.; Martínez Pillet, V.; Card, G.; Centeno, R.;
Knölker, M.; Lecinski, A.
Bibcode: 2017ApJS..229....2S
Altcode: 2017arXiv170101555S
The Sunrise balloon-borne solar observatory, consisting of a 1 m
aperture telescope that provides a stabilized image to a UV filter
imager and an imaging vector polarimeter, carried out its second science
flight in 2013 June. It provided observations of parts of active regions
at high spatial resolution, including the first high-resolution images
in the Mg II k line. The obtained data are of very high quality, with
the best UV images reaching the diffraction limit of the telescope
at 3000 Å after Multi-Frame Blind Deconvolution reconstruction
accounting for phase-diversity information. Here a brief update is
given of the instruments and the data reduction techniques, which
includes an inversion of the polarimetric data. Mainly those aspects
that evolved compared with the first flight are described. A tabular
overview of the observations is given. In addition, an example time
series of a part of the emerging active region NOAA AR 11768 observed
relatively close to disk center is described and discussed in some
detail. The observations cover the pores in the trailing polarity of
the active region, as well as the polarity inversion line where flux
emergence was ongoing and a small flare-like brightening occurred in
the course of the time series. The pores are found to contain magnetic
field strengths ranging up to 2500 G, and while large pores are clearly
darker and cooler than the quiet Sun in all layers of the photosphere,
the temperature and brightness of small pores approach or even exceed
those of the quiet Sun in the upper photosphere.
Title: A Tale of Two Emergences: Sunrise II Observations of Emergence
Sites in a Solar Active Region
Authors: Centeno, R.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.;
Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger,
J.; Riethmüller, T. L.; van Noort, M.; Orozco Suárez, D.; Berkefeld,
T.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229....3C
Altcode: 2016arXiv161003531C
In 2013 June, the two scientific instruments on board the second Sunrise
mission witnessed, in detail, a small-scale magnetic flux emergence
event as part of the birth of an active region. The Imaging Magnetograph
Experiment (IMaX) recorded two small (∼ 5\prime\prime )
emerging flux patches in the polarized filtergrams of a photospheric Fe
I spectral line. Meanwhile, the Sunrise Filter Imager (SuFI) captured
the highly dynamic chromospheric response to the magnetic fields pushing
their way through the lower solar atmosphere. The serendipitous capture
of this event offers a closer look at the inner workings of active
region emergence sites. In particular, it reveals in meticulous detail
how the rising magnetic fields interact with the granulation as they
push through the Sun’s surface, dragging photospheric plasma in
their upward travel. The plasma that is burdening the rising field
slides along the field lines, creating fast downflowing channels at
the footpoints. The weight of this material anchors this field to the
surface at semi-regular spatial intervals, shaping it in an undulatory
fashion. Finally, magnetic reconnection enables the field to release
itself from its photospheric anchors, allowing it to continue its
voyage up to higher layers. This process releases energy that lights
up the arch-filament systems and heats the surrounding chromosphere.
Title: Filamentary Oscillations in the Penumbra of Sunspots
Authors: Griñon, Ana Bélen; Pastor Yabar, A.; Socas-Navarro, H.;
Centeno, R.
Bibcode: 2017psio.confE.111G
Altcode: 2017psio.confE.111A
No abstract at ADS
Title: Sunrise II Observations of emergence sites in a solar active
region
Authors: Centeno, Rebecca
Bibcode: 2017psio.confE..21C
Altcode:
No abstract at ADS
Title: Filamentary Oscillations in the Penumbra of Sunspots
Authors: Grión-Marín, A. B.; Socas-Navarro, H.; Centeno, R.
Bibcode: 2016usc..confE..66G
Altcode: 2016usc..confE..66B
The issue of long-term (on scales of several hours to days)
morphological changes in sunspots, and particularly the possible
existence of apparent rotational motions and oscillations, has drawn
attention since the early 20th century. This kind of study requires
data with high spatial resolution and good temporal sampling and
coverage. The HMI instrument on board the Solar Dynamics Observatory
routinely measures the full magnetic field vector in sunspots and allows
us to track them with consistent image quality and high cadence during
their entire disk passage. It is the ideal instrument to analyze the
evolution of sunspots, and in particular the azimuthal component of the
penumbral magnetic field. We carried out an analysis (Griñón-Marín et
al. 2016 -Submitted-) looking for torsional oscillations in the penumbra
of sunspots that led to no evidence of this kind of oscillation in the
25 sunspots analyzed. However, we detected filamentary-like oscillations
in some areas of the penumbra with periods of several hours. In this
contribution I will show their morphological analysis and discuss the
possible sources for such oscillations.
Title: Vortex waves in sunspots
Authors: López Ariste, A.; Centeno, R.; Khomenko, E.
Bibcode: 2016A&A...591A..63L
Altcode:
Context. Waves in the magnetized solar atmosphere are one of the
favourite means of transferring and depositing energy into the solar
corona. The study of waves brings information not just on the dynamics
of the magnetized plasma, but also on the possible ways in which the
corona is heated.
Aims: The identification and analysis of the
phase singularities or dislocations provide us with a complementary
approach to the magnetoacoustic and Aflvén waves propagating in the
solar atmosphere. They allow us to identify individual wave modes,
shedding light on the probability of excitation or the nature of the
triggering mechanism.
Methods: We use a time series of Doppler
shifts measured in two spectral lines, filtered around the three-minute
period region. The data show a propagating magnetoacoustic slow
mode with several dislocations and, in particular, a vortex line. We
study under what conditions the different wave modes propagating in
the umbra can generate the observed dislocations.
Results:
The observed dislocations can be fully interpreted as a sequence
of sausage and kink modes excited sequentially on average during
15 min. Kink and sausage modes appear to be excited independently
and sequentially. The transition from one to the other lasts less
than three minutes. During the transition we observe and model the
appearance of superoscillations inducing large phase gradients and
phase mixing.
Conclusions: The analysis of the observed wave
dislocations leads us to the identification of the propagating wave
modes in umbrae. The identification in the data of superoscillatory
regions during the transition from one mode to the other may be an
important indicator of the location of wave dissipation.
Title: Polarimetry
Authors: Nagendra, K. N.; Bagnulo, Stefano; Centeno, Rebecca; Jesús
Martínez González, María.
Bibcode: 2015IAUS..305.....N
Altcode:
Preface; 1. Solar and stellar surface magnetic fields; 2. Future
directions in astrophysical polarimetry; 3. Physical processes;
4. Instrumentation for astronomical polarimetry; 5. Data analysis
techniques for polarization observations; 6. Polarization diagnostics
of atmospheres and circumstellar environments; 7. Polarimetry as a tool
for discovery science; 8. Numerical modeling of polarized emission;
Author index.
Title: The solar cycle dependence of the weak internetwork flux
Authors: Lites, Bruce W.; Centeno, Rebecca; McIntosh, Scott W.
Bibcode: 2014PASJ...66S...4L
Altcode: 2014PASJ..tmp..109L
We examine data from the Hinode Observing Program 79 (the "HOP 79"
irradiance program) as observed using the Hinode Solar Optical
Telescope Spectro-Polarimeter for systematic changes in the weakest
observable magnetic flux during the period 2008-2013. At moderate
latitudes we find no evidence for systematic changes as a function
of time and solar latitude in either the unsigned line-of-sight flux
or in the measures of the transverse flux. However, in the polar
regions, changes are apparent in the measure of signed magnetic flux
corresponding to reversal of the polarity of the poles, changes that
persist even for the weakest observed flux. Also evident in measures
of the weakest signed flux are preferences for positive (negative)
polarity at mid-north (mid-south) latitudes (20°-60°). Center-limb
variations in various measures of the weak flux appear to be independent
of the solar cycle. The results are consistent with the operation of
a small-scale solar dynamo operating within and just below the solar
photosphere, but the measures of the weakest signed flux still contain
small signatures of the global solar cycle.
Title: The Helioseismic and Magnetic Imager (HMI) Vector Magnetic
Field Pipeline: Overview and Performance
Authors: Hoeksema, J. Todd; Liu, Yang; Hayashi, Keiji; Sun, Xudong;
Schou, Jesper; Couvidat, Sebastien; Norton, Aimee; Bobra, Monica;
Centeno, Rebecca; Leka, K. D.; Barnes, Graham; Turmon, Michael
Bibcode: 2014SoPh..289.3483H
Altcode: 2014SoPh..tmp...57H; 2014arXiv1404.1881H
The Helioseismic and Magnetic Imager (HMI) began near-continuous
full-disk solar measurements on 1 May 2010 from the Solar Dynamics
Observatory (SDO). An automated processing pipeline keeps pace
with observations to produce observable quantities, including the
photospheric vector magnetic field, from sequences of filtergrams. The
basic vector-field frame list cadence is 135 seconds, but to reduce
noise the filtergrams are combined to derive data products every 720
seconds. The primary 720 s observables were released in mid-2010,
including Stokes polarization parameters measured at six wavelengths,
as well as intensity, Doppler velocity, and the line-of-sight magnetic
field. More advanced products, including the full vector magnetic field,
are now available. Automatically identified HMI Active Region Patches
(HARPs) track the location and shape of magnetic regions throughout
their lifetime.
Title: The Helioseismic and Magnetic Imager (HMI) Vector Magnetic
Field Pipeline: Optimization of the Spectral Line Inversion Code
Authors: Centeno, R.; Schou, J.; Hayashi, K.; Norton, A.; Hoeksema,
J. T.; Liu, Y.; Leka, K. D.; Barnes, G.
Bibcode: 2014SoPh..289.3531C
Altcode: 2014SoPh..tmp...44C; 2014arXiv1403.3677C
The Very Fast Inversion of the Stokes Vector (VFISV) is a
Milne-Eddington spectral line inversion code used to determine the
magnetic and thermodynamic parameters of the solar photosphere from
observations of the Stokes vector in the 6173 Å Fe I line by the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO). We report on the modifications made to the original
VFISV inversion code in order to optimize its operation within
the HMI data pipeline and provide the smoothest solution in active
regions. The changes either sped up the computation or reduced the
frequency with which the algorithm failed to converge to a satisfactory
solution. Additionally, coding bugs which were detected and fixed in
the original VFISV release are reported here.
Title: Making global map of the solar surface Br from
the HMI vector magnetic field observations
Authors: Hayashi, K.; Liu, Y.; Sun, X.; Hoeksema, J. T.; Centeno,
R.; Barnes, G.; Leka, K. D.
Bibcode: 2013JPhCS.440a2036H
Altcode:
The Helioseismic Magnetic Imager (HMI) has made full-disk vector
magnetic field measurements of the Sun with cadence of 12 minutes. The
three-component solar surface magnetic field vector data are from
the HMI observations with the data process pipeline modules, VFISV
(Very Fast Inversion of the Stokes Vector, Borrero et al., 2011)
for Milne-Eddington inversion and the minimum-energy disambiguation
algorithm (Metcalf 1994, Leka et al, 2009). The models of the global
corona and solar wind, such as the PFSS (potential-field source-surface)
model and the MHD simulations, often use the maps of solar surface
magnetic field, especially the radial component (Br) as the
boundary condition. The HMI observation can provide new Br
data for these model. Because of weak magnetic signals at the quiet
regions of the Sun, the limb darkening, and geometric effects near solar
poles, we need to apply an assumption to make a whole-surface map. In
this paper, we tested two assumptions for determining Br
at weak-field regions. The coronal structures calculated by the PFSS
model with the vector-based Br are compared with those with
the magnetogram-based Br and the corona observed by the
SDO/AIA (Atmospheric Imaging Assembly). In the tested period, CR 2098,
the vector-based Br map gives better agreements than the
line-of-sight magnetogram data, though we need further investigation
for evaluation.
Title: On the Effects of the SDO Orbital Motion on the HMI Vector
Magnetic Field Measurements
Authors: Fleck, B.; Centeno, R.; Cheung, M.; Couvidat, S.; Hayashi,
K.; Rezaei, R.; Steiner, O.; Straus, T.
Bibcode: 2013enss.confE.145F
Altcode:
In a previous study we have investigated the magnetic field diagnostics
potential of SDO/HMI. We have used the output of high-resolution
3D, time-dependent, radiative magneto-hydrodynamics simulations to
calculate Stokes profiles for the Fe I 6173 Å line. From these we
constructed Stokes filtergrams using a representative set of HMI filter
response functions. The magnetic field vector (x,y) and line-of-sight
Doppler velocities V(x,y) were determined from these filtergrams using
a simplified version of the HMI magnetic field processing pipeline,
and the reconstructed magnetic field (x,y) and line-of-sight velocity
V(x,y) were compared to the actual magnetic field (x,y,z) and vertical
velocity V0(x,y,z) in the simulations. The present investigation expands
this analysis to include the effects of the significant orbital motions
of SDO, which, given the limited wavelength range of the HMI filter
profiles, affects the outer wing measurements and therefore might impact
the magnetic field measurements. We find that the effects of the orbital
movement of SDO are noticeable, in particular for the strongest fields
(B > 3 kG) and the maximum wavelength shift of 5.5 km/s (3.5 km/s
orbital movement + 2 km/s solar rotation). Saturation effects for strong
fields (B > 3 kG) are already visible for wavelength shifts of 3.2
km/s (orbital movement, disk center). The measurements of inclination
and vertical velocity are more robust. Compared to other factors of
uncertainty in the inversion of HMI Stokes measurements the orbital
movement is not a major concern or source of error.
Title: The Naked Emergence of Solar Active Regions Observed with
SDO/HMI
Authors: Centeno, Rebecca
Bibcode: 2012ApJ...759...72C
Altcode: 2012arXiv1209.4378C
We take advantage of the HMI/SDO instrument to study the naked emergence
of active regions (ARs) from the first imprints of the magnetic field
on the solar surface. To this end, we followed the first 24 hr in
the life of two rather isolated ARs that appeared on the surface
when they were about to cross the central meridian. We analyze the
correlations between Doppler velocities and the orientation of the
vector magnetic field, consistent finding that the horizontal fields
connecting the main polarities are dragged to the surface by relatively
strong upflows and are associated with elongated granulation that
is, on average, brighter than its surroundings. The main magnetic
footpoints, on the other hand, are dominated by vertical fields and
downflowing plasma. The appearance of moving dipolar features (MDFs,
of opposite polarity to that of the AR) in between the main footpoints
is a rather common occurrence once the AR reaches a certain size. The
buoyancy of the fields is insufficient to lift up the magnetic arcade
as a whole. Instead, weighted by the plasma that it carries, the
field is pinned down to the photosphere at several places in between
the main footpoints, giving life to the MDFs and enabling channels of
downflowing plasma. MDF poles tend to drift toward each other, merge
and disappear. This is likely to be the signature of a reconnection
process in the dipped field lines, which relieves some of the weight
allowing the magnetic arcade to finally rise beyond the detection
layer of the Helioseismic and Magnetic Imager spectral line.
Title: An active region filament studied simultaneously in the
chromosphere and photosphere. II. Doppler velocities
Authors: Kuckein, C.; Martínez Pillet, V.; Centeno, R.
Bibcode: 2012A&A...542A.112K
Altcode: 2012arXiv1204.5090K
Context. Paper I presents the magnetic structure, inferred for the
photosphere and the chromosphere, of a filament that developed in active
region (AR) NOAA 10781, observed on 2005 July 3 and July 5.
Aims: In this paper we complement those results with the velocities
retrieved from Doppler shifts measured at the chromosphere and the
photosphere in the AR filament area.
Methods: The velocities
and magnetic field parameters were inferred from full Stokes inversions
of the photospheric Si I 10 827 Å line and the chromospheric He i 10
830 Å triplet. Various inversion methods with different numbers of
atmospheric components and different weighting schemes of the Stokes
profiles were used. The velocities were calibrated on an absolute
scale.
Results: A ubiquitous chromospheric downflow is found
in the faculae surrounding the filament, with an average velocity of
1.6 km s-1. The filament region, however, displays upflows
in the photosphere on both days, when the linear polarization (which
samples the transverse component of the fields) is given more weight
in the inversions. The upflow speeds of the transverse fields in the
filament region average -0.15 km s-1. In the chromosphere,
the situation is different for the two days of observation. On July 3,
the chromospheric portion of the filament is moving upward as a whole
with a mean speed of -0.24 km s-1. However, on July 5 only
the section above an orphan penumbra shows localized upflow patches,
while the rest of the filament is dominated by the same downflows
observed elsewhere in the facular region. Photospheric supersonic
downflows that last for tens of minutes are detected below the filament,
close to the PIL.
Conclusions: The observed velocity pattern
in this AR filament strongly suggests a scenario where the transverse
fields are mostly dominated by upflows. The filament flux rope is seen
to be emerging at all places and both heights, with a few exceptions
in the chromosphere. This happens within a surrounding facular
region that displays a generalized downflow in the chromosphere and
localized downflows of supersonic character at the photosphere. No
large scale downflow of transverse field lines is observed at the
photosphere. Appendices A and B are available in electronic form
at http://www.aanda.org
Title: On The Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, Bernard; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno,
R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.;
Viticchie, B.
Bibcode: 2012AAS...22020701F
Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk
in the Fe I absorption line at 6173 Å. We use the output of three
high-resolution 3D, time-dependent, radiative magneto-hydrodynamics
simulations (two based on the MURaM code, one on the CO5BOLD
code) to calculate Stokes profiles for the Fe I 6173 Å line
for snapshots of a sunspot, a plage area and an enhanced network
region. Stokes filtergrams are constructed for the 6 nominal HMI
wavelengths by multiplying the Stokes profiles with a representative
set of HMI filter response functions. The magnetic field vector B(x,y)
and line-of-sight Doppler velocities V(x,y) are determined from these
filtergrams using a simplified version of the HMI magnetic field
processing pipeline. Finally, the reconstructed magnetic field B(x,y)
and line-of-sight velocity V(x,y) are compared to the actual magnetic
field B0(x,y,z) and vertical velocity V0(x,y,z)
in the simulations.
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: Opportunities and challenges in determining the chromospheric
magnetic field using He I 10830
Authors: Centeno, Rebecca
Bibcode: 2012decs.confE..33C
Altcode:
Determining the magnetic structure of the Solar Chromosphere
entails all sorts of observational, theoretical and computational
challenges. Within the observational approach, the first step in the
inference of Chromospheric magnetic fields is to measure the imprints
that these fields leave on the spectral line radiation. This already
poses a problem due to the intrinsic weakness of the polarization
signatures and the highly dynamic nature of the Chromosphere. The
second hurdle is to interpret the spectral line radiation. This
usually involves using a spectral line inversion code, i.e., a
non-linear iterative algorithm that requires solving the radiative
transfer equation coupled to the statistical equilibrium equations
(a problem that has a non-linear iterative nature in itself!) at each
iterative step. Due to its peculiar formation mechanism, the He I
10830 A multiplet offers a series of advantages that allow us to skip
some of these obstacles. I will show some of the findings of the past
decade that prove the enormous diagnostic potential of this triplet
and I will argue why it should be considered as one of the Rosetta
Stones for understanding Chromospheric magnetism.
Title: Active region emergence sites observed with HMI
Authors: Centeno, Rebecca
Bibcode: 2012decs.confE..60C
Altcode:
One of the advantages of SDO/HMI is the consistent quality and
uninterrupted nature of the data that it provides. We take advantage of
this to study the emergence of active regions from the moment that the
magnetic fields show their first imprints on the solar surface. Flux
emergence sites are characterized by moving dipolar features (MDFs)
that appear in between the main footpoints of an AR. In longitudinal
magnetograms, MDFs show an inverse polarity configuration with
respect to that of the active region. The vector magnetic field and
line-of-sight velocity measurements of HMI allow us to track the dynamic
and magnetic properties of these features and understand their movements
in terms of the bending and straightening of magnetic field lines.
Title: Doppler velocities studied simultaneously in the chromosphere
and photosphere of an active region filament
Authors: Kuckein, C.; Martinez Pillet, V.; Centeno, R.
Bibcode: 2012decs.confE..42K
Altcode:
We present line-of-sight velocities retrieved simultaneously at
two different heights (chromosphere and photosphere) on two days
in an active region (AR) filament. The velocities, as well as the
magnetic field parameters, were inferred from full Stokes inversions
of the photospheric Si I 10827A line and the chromospheric He I 10830A
triplet. Various inversion methods with different number of components
and different weights of the Stokes parameters were used. Moreover,
the velocities were calibrated on an absolute scale. We found a
ubiquitous chromospheric downflow in the faculae surrounding the
AR filament with an average velocity of 1.6 km/s. However, in the
filament region, upflows in the photosphere were detected, when the
Stokes signals from the transverse fields are given more weight in the
inversions. In the chromosphere, the filament is also moving upward
as a whole with a mean speed of -0.24 km/s as deduced from the He I
inversions. However, on the second day the chromospheric portion above
an orphan penumbra shows localized upflow patches while the rest of the
filament is dominated by the same downflows observed elsewhere in the
plage region. Photospheric supersonic downflows are detected below the
filament, close to the PIL, that last for tens of minutes. The observed
velocities in this AR filament strongly suggest a scenario where the
transverse fields are mostly dominated by upflows. The filament flux
rope is seen to be emerging at all heights with a few exceptions in
the chromosphere. No large scale downflow of transverse field lines
is observed in the photosphere.
Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno,
R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.;
Viticchie, B.
Bibcode: 2012decs.confE.104F
Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk
in the Fe I absorption line at 6173 Å. We use the output of three
high-resolution 3D, time-dependent, radiative magneto-hydrodynamics
simulations (two based on the MURaM code, one on the CO5BOLD code)
to calculate Stokes profiles Fi(λ,x,y; i=I, V, Q, U) for the Fe I
6173 Å line for snapshots of a sunspot, a plage area and an enhanced
network region. Stokes filtergrams are constructed for the 6 nominal
HMI wavelengths by multiplying the Stokes profiles with a representative
set of HMI filter response functions. The magnetic field vector B(x,y)
and line-of-sight Doppler velocities V(x,y) are determined from these
filtergrams using a simplified version of the HMI magnetic field
processing pipeline. Finally, the reconstructed magnetic field B(x,y)
and line-of-sight velocity V(x,y) are compared to the actual magnetic
field B0(x,y,z) and vertical velocity V0(x,y,z) in the simulations.
Title: An active region filament studied simultaneously in the
chromosphere and photosphere. I. Magnetic structure
Authors: Kuckein, C.; Martínez Pillet, V.; Centeno, R.
Bibcode: 2012A&A...539A.131K
Altcode: 2011arXiv1112.1672K
Aims: A thorough multiwavelength, multiheight study of the
vector magnetic field in a compact active region filament (NOAA 10781)
on 2005 July 3 and 5 is presented. We suggest an evolutionary scenario
for this filament.
Methods: Two different inversion codes were
used to analyze the full Stokes vectors acquired with the Tenerife
Infrared Polarimeter (TIP-II) in a spectral range that comprises the
chromospheric He i 10 830 Å multiplet and the photospheric Si i 10
827 Å line. In addition, we used SOHO/MDI magnetograms, as well as
BBSO and TRACE images, to study the evolution of the filament and its
active region (AR). High-resolution images of the Dutch Open Telescope
were also used.
Results: An active region filament (formed
before our observing run) was detected in the chromospheric helium
absorption images on July 3. The chromospheric vector magnetic field
in this portion of the filament was strongly sheared (parallel to the
filament axis), whereas the photospheric field lines underneath had
an inverse polarity configuration. From July 3 to July 5, an opening
and closing of the polarities on either side of the polarity inversion
line (PIL) was recorded, resembling the recently discovered process
of the sliding door effect seen by Hinode. This is confirmed with both
TIP-II and SOHO/MDI data. During this time, a newly created region that
contained pores and orphan penumbrae at the PIL was observed. On July
5, a normal polarity configuration was inferred from the chromospheric
spectra, while strongly sheared field lines aligned with the PIL were
found in the photosphere. In this same data set, the spine of the
filament is also observed in a different portion of the field of view
and is clearly mapped by the silicon line core.
Conclusions:
The inferred vector magnetic fields of the filament suggest a flux rope
topology. Furthermore, the observations indicate that the filament is
divided in two parts, one which lies in the chromosphere and another
one that stays trapped in the photosphere. Therefore, only the top
of the helical structure is seen by the helium lines. The pores and
orphan penumbrae at the PIL appear to be the photospheric counterpart
of the extremely low-lying filament. We suggest that orphan penumbrae
are formed in very narrow PILs of compact ARs and are the photospheric
manifestation of flux ropes in the photosphere.
Title: The synoptic maps of Br from HMI observations
Authors: Hayashi, Keiji; Hoeksema, J. Todd; Liu, Sun; Yang, Xudong;
Centeno, Rebecca; Leka, K. D.; Barnes, Graham
Bibcode: 2012decs.confE..69H
Altcode:
The vector magnetic field measurement can, in principal, give the
"true" radial component of the magnetic field. We prepare 4 types
of synoptic maps of the radial photospheric magnetic field, from
the vector magnetic field data disambiguated by means of the minimum
energy method developed at NWRA/CoRA, the vector data determined under
the potential-field acute assumption, and the vector data determined
under the radial-acute assumption, and the standard line-of-sight
magnetogram. The models of the global corona, the MHD and the PFSS,
are applied to different types of maps. Although the three-dimensional
structures of the global coronal magnetic field with different maps are
similar and overall agreeing well the AIA full-disk images, noticeable
differences among the model outputs are found especially in the high
latitude regions. We will show details of these test maps and discuss
the issues in determining the radial component of the photospheric
magnetic field near the poles and limb.
Title: Solar-cycle variation of oscillation frequencies and surface
magnetic field
Authors: Tan, S.; Thompson, M. J.; Centeno, R.
Bibcode: 2011AGUFMSH13B1933T
Altcode:
We investigate the relationship between solar oscillation frequencies
and surface magnetic fields over the course of the last solar
cycle. Using MDI and GONG data, we study the variation in the even
frequency-splitting coefficients ak (describing solar
asphericity and effects of the magnetic field), and the variation in the
coefficients Bk of the latitudinal Lengendre decomposition
of the surface magnetic field, during the period 1996 - 2010. We find
a strong linear correlation between the a and B coefficients, during
both the rising and declining phases of the solar cycle, consistent
with results published in 2001 (Antia et al.). We also investigated
different ways to handle the magnetic field decomposition at the poles,
and find that the linear correlation persists, though with varying
intercepts. The variation of slope with coefficient index that we find
is non-monotonic, which disagrees with the previous study by Antia et
al. (2001).
Title: Spectropolarimetric Comparison Between SDO/HMI and
Hinode-SOT/SP Through THEMIS/MTR
Authors: Sainz Dalda, A.; Lopez Ariste, A.; Gelly, B.; Tarbell, T. D.;
Centeno, R.; DeRosa, M. L.; Hoeksema, J. T.
Bibcode: 2011AGUFMSH31A1986S
Altcode:
In the golden age of solar spacecraft observatories, the use of similar
instruments observing same targets offers us the possibility to get
more accurate information of the physical processes taking place on
them. We present a comparison between the vector magnetic field and
thermodynamic quantities obtained by three different spectropolarimetric
instruments. We have used the simultaneous multi-wavelength capabilities
of THEMIS/MTR as bridge between the observations at Fe I 6173 Å
provided by SDO/HMI and at Fe I 6301 & 6302 Å by Hinode-SOT/SP
observations. The official inversion codes for these instruments (PCA
based-on, VFISV and MERLIN respectively) have been used with the data
properly arranged for them. Therefore, we compare the final products
usually offered to the community, i.e. after the inversion, using
different codes and these different wavelengths. The cross-calibration
of these products shall allow us to go forward from one instrument
result to other one in an easy, convenient way.
Title: HMI vector magnetic field products: the long-awaited release
has come! Now what?
Authors: Centeno, R.; Barnes, G.; Borrero, J.; Couvidat, S. P.;
Hayashi, K.; Hoeksema, J. T.; Leka, K. D.; Liu, Y.; Schou, J.; Schuck,
P. W.; Sun, X.; Tomczyk, S.
Bibcode: 2011AGUFMSH31A1985C
Altcode:
HMI vector magnetic field test products will be released, alongside
with the corresponding documentation, soon after the submission of this
abstract. These data represent a stage of the project at which the HMI
vector team has a large degree of confidence in the results. However,
longer-term research topics on how to improve certain aspects of the
data pipeline in general -and the spectral line inversion code in
particular- are being pursued as we get valuable input from the user
community. I will give a brief summary of the characteristics of the
released inversion data products and an update of where we stand now.
Title: Continuous upflow of material in an active region filament
from thephotosphere to the corona
Authors: Kuckein, C.; Centeno, R.; Martinez Pillet, V.
Bibcode: 2011hsa6.conf..636K
Altcode:
Using spectropolarimetric data of an Active Region (AR) filament
we have carried out inversions in order to infer vector magnetic
fields in the photosphere (Si I line) and in the chromosphere
(He I line). Our filament lies above the polarity inversion line
(PIL) situated close to disk center and presents strong Zeeman-like
signatures in both photospheric and chromospheric lines. Pore-like
formations with both polarities are identified in the continuum under
the PIL. The azimuth ambiguity is solved at both heights using the
AZAM code. A comparison between the photospheric and chromospheric
vector magnetic fields revealed that they are well aligned in some
areas of the filament. However, especially at chromospheric heights,
the magnetic field is mostly aligned with the dark threads of the
filament. Velocity signatures indicating upflows of field lines are
found at both heights. The combination of all these findings strongly
suggests an emerging flux rope scenario.
Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno, R.;
Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.; Viticchie, B.
Bibcode: 2011sdmi.confE..74F
Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk in the
Fe I 6173 absorption line. We use the output of two high-resolution 3D,
time-dependent, radiative magneto-hydrodynamics simulations (one based
on the MURAM code, the other one on the COBOLD code) to calculate
Stokes profiles for the Fe I 6173 line for a snapshot of a plage
region and a snapshot of an enhanced network region. After spatially
degrading the Stokes profiles to HMI resolution, they are multiplied
by a representative set of HMI filter response functions and Stokes
filtergrams are constructed for the 6 nominal HMI wavelengths. The
magnetic field vector and line-of-sight Doppler velocities are
determined from these filtergrams using a simplified version of the HMI
magnetic field processing pipeline. Finally, the reconstructed magnetic
field is compared to the actual magnetic field in the simulation.
Title: Evolution of an active region filament as observed in the
photosphere and chromosphere simultaneously
Authors: Kuckein, Christoph; Pillet, Martinez; Valentin; Centeno;
Rebeca
Bibcode: 2011sdmi.confE..99K
Altcode:
A multiwavelength study and comparison of the vector magnetic field in
a compact active region filament (NOAA 10781) for 2005 July 3rd and
5th is presented. Different inversion codes were used to analyze the
full Stokes vectors acquired with the Tenerife Infrared Polarimeter
(TIP-II) in a spectral range which comprises the chromospheric He
I 10830 A multiplet and the photospheric Si I 10827 A line. Other
data from ground- and space-based telescopes has been used to have a
complete view of the evolution of the active region (AR). We found
that the filament was clearly observed for the first time, on July
3rd, after a "sliding-door" effect a-la Okamoto et al. (2008) of
the polarity inversion line (PIL). The chromospheric vector magnetic
field in the filament was strongly sheared (parallel to the filament
axis) whereas the photospheric field lines had an inverse polarity
configuration. For July 5th we had a different field of view but still
half of it remained the same. We now observed pores and orphan-penumbral
features that emerged along the PIL. A normal polarity configuration
is inferred in the filament above these features and strongly sheared
field lines along the PIL are found below, in the photosphere. The
inferred vector magnetic fields of the filament suggest a flux
rope topology. Furthermore, the observations indicate that the
filament is divided in two parts, one of it seems to be trapped in
the photosphere. Inferred magnetic field strengths and velocity
measurements inside and below the filament will be presented. An
evolutionary scenario for this AR filament is suggested.
Title: HMI: First Results
Authors: Centeno, R.; Tomczyk, S.; Borrero, J. M.; Couvidat,
S. Hayashi, K.; Hoeksema, T.; Liu, Y.; Schou, J.
Bibcode: 2011ASPC..437..147C
Altcode: 2010arXiv1012.3796C
The Helioseismic and Magnetic Imager (HMI) has just started producing
data that will help determine what the sources and mechanisms of
variability in the Sun's interior are. The instrument measures the
Doppler shift and the polarization of the Fe I 6173 Å line, on the
entire solar disk at a relatively-high cadence, in order to study
the oscillations and the evolution of the full vector magnetic field
of the solar Photosphere. After the data are properly calibrated,
they are given to a Milne-Eddington inversion code (VFISV, Borrero et
al. 2010) whose purpose is to infer certain aspects of the physical
conditions in the Sun's Photosphere, such as the full 3-D topology
of the magnetic field and the line-of-sight velocity at the solar
surface. We will briefly describe the characteristics of the inversion
code, its advantages and limitations -both in the context of the model
atmosphere and the actual nature of the data-, and other aspects of its
performance on such a remarkable data load. Also, a cross-comparison
with near-simultaneous maps from the Spectro-Polarimeter (SP) onboard
Hinode will be made.
Title: Continuous Upflow of Material in an Active Region Filament
from the Photosphere to the Corona
Authors: Kuckein, C.; Centeno, R.; Martínez Pillet, V.
Bibcode: 2011ASPC..437..275K
Altcode: 2010arXiv1010.4260K
Using spectropolarimetric data of an Active Region (AR) filament
we have carried out inversions in order to infer vector magnetic
fields in the photosphere (Si I line) and in the chromosphere
(He I line). Our filament lies above the polarity inversion line
(PIL) situated close to disk center and presents strong Zeeman-like
signatures in both photospheric and chromospheric lines. Pore-like
formations with both polarities are identified in the continuum under
the PIL. The azimuth ambiguity is solved at both heights using the
AZAM code. A comparison between the photospheric and chromospheric
vector magnetic fields revealed that they are well aligned in some
areas of the filament. However, especially at chromospheric heights,
the magnetic field is mostly aligned with the dark threads of the
filament. Velocity signatures indicating upflows of field lines are
found at both heights. The combination of all these findings strongly
suggests an emerging flux rope scenario.
Title: Magnetic Field Measurements at the Photosphere and Coronal Base
Authors: Judge, P. G.; Centeno, R.; Tritschler, A.; Uitenbroek, H.;
Jaeggli, S.; Lin, H.
Bibcode: 2010AGUFMSH31A1783J
Altcode:
We have obtained vector polarimetric measurements in lines of Fe I
(630nm), Ca II (854nm) and He I (1083nm) of several active regions
during 3-14 June 2010. The measurements were made at the Dunn Solar
Telescope at Sacramento Peak Observatory, using the FIRS and IBIS
instruments simultaneously. We discuss these and SDO data for NOAA
11076. The seeing was very good or excellent and the adaptive
optics system functioned well. In this preliminary analysis we
compare extrapolations of photospheric fields with the constraints
available from Stokes polarimetry, including the morphology and
kinematic properties of fibrils. Connections to the corona will also be
discussed. The implications for field extrapolations from photospheric
measurements will be discussed. We will make the reduced data freely
available on the web for interested researchers.
Title: Magnetic field measurements at the photosphere and coronal base
Authors: Judge, Philip; Centeno, R.; Tritschler, A.; Uitenbroek, H.;
Jaeggli, S.; Lin, H.
Bibcode: 2010shin.confE..56J
Altcode:
We have obtained vector polarimetric measurements in lines of Fe I
(630nm), Ca II (854nm) and He I (1083) of several active regions during
3-14 June 2010. The measurements were made at the Dunn Solar Telescope
at Sacramento Peak Observatory, using the FIRS and IBIS instruments
simultaneously. We discuss data for NOAA 11076 observed on 4 June
2010. The seeing was very good or excellent and the adaptive optics
system functioned well. In this preliminary analysis we compare linear
extrapolations of photospheric fields with the constraints available
from Stokes polarimetry, including the morphology and kinematic
properties of fibrils. The implications for field extrapolations from
photospheric measurements will be discussed. We will make the reduced
data freely available on the web for interested researchers.
Title: Developing Physics-Based Procedures for Local Helioseismic
Probing of Sunspots and Magnetic Regions
Authors: Birch, Aaron; Braun, D. C.; Crouch, A.; Rempel, M.; Fan,
Y.; Centeno, R.; Toomre, J.; Haber, D.; Hindman, B.; Featherstone,
N.; Duvall, T., Jr.; Jackiewicz, J.; Thompson, M.; Stein, R.; Gizon,
L.; Cameron, R.; Saidi, Y.; Hanasoge, S.; Burston, R.; Schunker, H.;
Moradi, H.
Bibcode: 2010AAS...21630805B
Altcode:
We have initiated a project to test and improve the local helioseismic
techniques of time-distance and ring-diagram analysis. Our goals are
to develop and implement physics-based methods that will (1) enable the
reliable determinations of subsurface flow, magnetic field, and thermal
structure in regions of strong magnetic fields and (2) be quantitatively
tested with realistic solar magnetoconvection simulations in the
presence of sunspot-like magnetic fields. We are proceeding through a
combination of improvements in local helioseismic measurements, forward
modeling of the helioseismic wavefield, kernel computations, inversions,
and validation through numerical simulations. As improvements over
existing techniques are made they will be applied to the SDO/HMI
observations. This work is funded through the the NASA Heliophysics
Science Division through the Solar Dynamics Observatory (SDO) Science
Center program.
Title: Spectropolarimetric inversions of the He I 10830 Å multiplet
in an active region filament.
Authors: Kuckein, C.; Centeno, R.; Martínez Pillet, V.
Bibcode: 2010MmSAI..81..668K
Altcode: 2010arXiv1001.2434K
Full-Stokes spectropolarimetric data (in the 10830 Å region)
of an active region filament were obtained in July 2005 using
the Tenerife Infrared Polarimeter instrument. The polarization
profiles in the filament show Zeeman-like signatures. Milne-Eddington
inversions were performed to infer the chromospheric magnetic field,
inclination, azimuth, velocity and Doppler width from the He I 10830
Å multiplet. Field strengths of the order of 600-800 G were found in
the filament. Strong transverse fields at chromospheric levels were
detected near the polarity inversion line. To our knowledge, these are
the highest field strengths reliably measured in these structures. Our
findings suggest the possible presence of a flux rope.
Title: On the Magnetic Field of Off-limb Spicules
Authors: Centeno, Rebecca; Trujillo Bueno, Javier; Asensio Ramos,
Andrés
Bibcode: 2010ApJ...708.1579C
Altcode: 2009arXiv0911.3149C
Determining the magnetic field related to solar spicules is vital for
developing adequate models of these plasma jets, which are thought to
play a key role in the thermal, dynamic, and magnetic structure of the
chromosphere. Here we report on the magnetic properties of off-limb
spicules in a very quiet region of the solar atmosphere, as inferred
from new spectropolarimetric observations in the He I 10830 Å triplet
obtained with the Tenerife Infrared Polarimeter. We have used a novel
inversion code for Stokes profiles caused by the joint action of atomic
level polarization and the Hanle and Zeeman effects to interpret the
observations (HAZEL, from HAnle and ZEeman Light). Magnetic fields as
strong as ~50 G were detected in a very localized area of the slit,
which could represent a possible lower value of the field strength of
organized network spicules.
Title: The Magnetic Field of Solar Spicules
Authors: Centeno, R.; Trujillo Bueno, J.; Asensio Ramos, A.
Bibcode: 2010ASSP...19..255C
Altcode: 2010mcia.conf..255C; 2009arXiv0903.0002C
Determining the magnetic field of solar spicules is vital for
developing adequate models of these plasma jets, which are thought to
play a key role in the thermal, dynamic, and magnetic structure of the
chromosphere. Here we report on magnetic spicule properties in a very
quiet region of the off-limb solar atmosphere, as inferred from new
spectropolarimetric observations in the He I 10830Å triplet. We have
used a novel inversion code for Stokes profiles caused by the joint
action of atomic level polarization and the Hanle and Zeeman effects
(HAZEL) to interpret the observations. Magnetic fields as strong as
40G were unambiguously detected in a very localized area of the slit,
which may represent a possible lower value of the field strength of
organized network spicules.
Title: Hinode's SP and G-band Co-Alignment
Authors: Centeno, R.; Lites, B.; de Wijn, A. G.; Elmore, D.
Bibcode: 2009ASPC..415..323C
Altcode: 2009arXiv0902.0027C
We analyze the co-alignment between Hinode's BFI-Gband images and
simultaneous SP maps with the aim of characterizing the general off-sets
between them and the second order non-linear effects in SP's slit
scanning mechanism. We provide calibration functions and parameters
to correct for the nominal pixel scales and positioning
Title: Statistics of Convective Collapse Events in the Photosphere
and Chromosphere Observed with the HINODE SOT
Authors: Fischer, C. E.; de Wijn, A. G.; Centeno, R.; Lites, B. W.;
Keller, C. U.
Bibcode: 2009ASPC..415..127F
Altcode:
Convective collapse, a theoretically predicted process that
intensifies existing weak magnetic fields in the solar atmosphere,
was first directly observed in a single event by Nagata et al. (2008)
using the high resolution Solar Optical Telescope (SOT) of the Hinode
satellite. Using the same space telescope, we observed 49 such events
and present a statistical analysis of convective collapse events. Our
data sets consist of high resolution time series of polarimetric
spectral scans of two iron lines formed in the lower photosphere and
filter images in Mg I b2 and Ca II H. We were thus able
to study the implication of convective collapse events on the high
photospheric and the chromospheric layers. The physical parameters from
the full Stokes profiles were obtained with the MERLIN Milne-Eddington
inversion code. For each of the 49 events we determined the duration,
maximum photospheric downflow, and field strength increase. We found
event durations of about 10 minutes and field strengths of up to
1.65 kG.
Title: Statistics of convective collapse events in the photosphere
and chromosphere observed with the Hinode SOT
Authors: Fischer, C. E.; de Wijn, A. G.; Centeno, R.; Lites, B. W.;
Keller, C. U.
Bibcode: 2009A&A...504..583F
Altcode: 2009arXiv0906.2308F
Convective collapse, a theoretically predicted process that intensifies
existing weak magnetic fields in the solar atmosphere, was first
directly observed in a single event by Nagata et al. (2008, ApJ,
677, L145) using the high resolution Solar Optical Telescope (SOT)
of the Hinode satellite. Using the same space telescope, we observed
49 such events and present a statistical analysis of convective
collapse events. Our data sets consist of high resolution time series
of polarimetric spectral scans of two iron lines formed in the lower
photosphere and filter images in Mg I b{2} and Ca II H, spectral lines
that are formed in the high photosphere and the lower chromosphere,
respectively. We were thus able to study the implication of convective
collapse events on the high photospheric and the chromospheric
layers. We found that in all cases, the event was accompanied by a
continuum bright point and nearly always by a brightening in the Ca
II H images. The magnesium dopplergram exhibits a strong downflow in
about three quarters of the events that took place within the field
of view of the magnesium dopplergram. The physical parameters from
the full Stokes profiles were obtained with the MERLIN Milne-Eddington
inversion code. For each of the 49 events we determined the duration,
maximum photospheric downflow, field strength increase and size. We
found event durations of about 10 min, magnetic element radii of about
0.43 arcsec and 0.35 arcsec, before and after the event, respectively,
and field strengths of up to 1.65 kG.
Title: Magnetic field strength of active region filaments
Authors: Kuckein, C.; Centeno, R.; Martínez Pillet, V.; Casini, R.;
Manso Sainz, R.; Shimizu, T.
Bibcode: 2009A&A...501.1113K
Altcode: 2009arXiv0904.4876K
Aims: We study the vector magnetic field of a filament observed over a
compact active region neutral line.
Methods: Spectropolarimetric
data acquired with TIP-II (VTT, Tenerife, Spain) of the 10 830
Å spectral region provide full Stokes vectors that were analyzed
using three different methods: magnetograph analysis, Milne-Eddington
inversions, and PCA-based atomic polarization inversions.
Results:
The inferred magnetic field strengths in the filament are around 600-700
G by all these three methods. Longitudinal fields are found in the
range of 100-200 G whereas the transverse components become dominant,
with fields as high as 500-600 G. We find strong transverse fields
near the neutral line also at photospheric levels.
Conclusions:
Our analysis indicates that strong (higher than 500 G, but below kG)
transverse magnetic fields are present in active region filaments. This
corresponds to the highest field strengths reliably measured in these
structures. The profiles of the helium 10 830 Å lines observed in
this active region filament are dominated by the Zeeman effect.
Title: Influence of Coronal EUV Irradiance on the Stokes Profiles
of the He I 10830 Å Multiplet
Authors: Centeno, R.; Trujillo Bueno, J.; Uitenbroek, H.; Collados, M.
Bibcode: 2009ASPC..405..297C
Altcode:
One of the most useful spectral windows for spectropolarimetric
investigations of the solar chromosphere is the one provided by the
spectral lines of the He I 10830 Å multiplet, whose polarization
signals are sensitive to the Hanle and Zeeman effects. However, in
order to be able to carry out reliable diagnostics of the dynamic
and magnetic properties of the solar outer atmosphere it is crucial
to have a good physical understanding of the sensitivity of the
observed spectral line radiation to the various competing triggering
mechanisms. Here we report a series of on-disk and off-the-limb
non-LTE calculations of the 10830 Å absorption and emission profiles,
focusing our investigation on their sensitivity to the EUV coronal
irradiation and the model atmosphere used in the calculations. We show
in what respects the on-disk case sensitivity of the polarization
signals induced by the Zeeman effect to the EUV coronal irradiance,
and investigate whether or not inversions based on the Milne-Eddington
model are reliable. Concerning the off-the-limb case we demonstrate
that the intensity ratio of the blue to the red components of the
He I 10830 Å multiplet is a sensitive function of the amount of EUV
coronal illumination. Therefore, measurements of this observable as
a function of the distance to the limb and its confrontation with
radiative transfer modeling might give us valuable information on the
physical properties of the solar atmosphere and on the amount of EUV
radiation penetrating the chromosphere from above.
Title: Direct Imaging of Fine Structure in the Chromosphere of a
Sunspot Umbra
Authors: Socas-Navarro, H.; McIntosh, S. W.; Centeno, R.; de Wijn,
A. G.; Lites, B. W.
Bibcode: 2009ApJ...696.1683S
Altcode: 2008arXiv0810.0597S
High-resolution imaging observations from the Hinode spacecraft in the
Ca II H line are employed to study the dynamics of the chromosphere
above a sunspot. We find that umbral flashes and other brightenings
produced by the oscillation are extremely rich in fine structure,
even beyond the resolving limit of our observations (0farcs22). The
umbra is tremendously dynamic to the point that our time cadence of
20 s does not suffice to resolve the fast lateral (probably apparent)
motion of the emission source. Some bright elements in our data set
move with horizontal propagation speeds of 30 km s-1. We have
detected filamentary structures inside the umbra (some of which have a
horizontal extension of ~1500 km) which, to our best knowledge, had not
been reported before. The power spectra of the intensity fluctuations
reveal a few distinct areas with different properties within the umbra
that seem to correspond with the umbral cores that form it. Inside
each one of these areas the dominant frequencies of the oscillation
are coherent, but they vary considerably from one core to another.
Title: Wave Propagation and Shock Formation in Different Magnetic
Structures
Authors: Centeno, R.; Collados, M.; Trujillo Bueno, J.
Bibcode: 2009ApJ...692.1211C
Altcode: 2008arXiv0810.3613C
Velocity oscillations "measured" simultaneously at the photosphere and
the chromosphere—from time series of spectropolarimetric data in the
10830 Å region—of different solar magnetic features allow us to study
the properties of wave propagation as a function of the magnetic flux
of the structure (i.e., two different-sized sunspots, a tiny pore,
and a facular region). While photospheric oscillations have similar
characteristics everywhere, oscillations measured at chromospheric
heights show different amplitudes, frequencies, and stages of shock
development depending on the observed magnetic feature. The analysis
of the power and the phase spectra, together with simple theoretical
modeling, lead to a series of results concerning wave propagation
within the range of heights of this study. We find that, while the
atmospheric cutoff frequency and the propagation properties of different
oscillating modes depend on the magnetic feature, in all the cases the
power that reaches the high chromosphere above the atmospheric cutoff
comes directly from the photosphere by means of linear vertical wave
propagation rather than from nonlinear interaction of modes.
Title: On the Magnetic Structure of the Solar Transition Region
Authors: Judge, Philip; Centeno, Rebecca
Bibcode: 2008ApJ...687.1388J
Altcode: 2008arXiv0805.1436J
We examine the hypothesis that cool loops dominate emission from solar
transition region plasma below temperatures of 2 × 105
K. We compare published VAULT images of H Lyα, a lower transition
region line, with nearly contemporaneous magnetograms from Kitt Peak,
obtained during the second flight (VAULT-2) on 2002 June 14. The
measured surface fields and potential extrapolations suggest that there
are too few short loops and that Lyα emission is associated with
the base regions of longer, coronal loops. VAULT-2 data of network
boundaries have an asymmetry on scales larger than supergranules,
also indicating an association with long loops. We complement the
Kitt Peak data with very sensitive vector polarimetric data from the
spectropolarimeter on board Hinode to determine the influence of very
small magnetic concentrations on our analysis. From these data, two
classes of behavior are found. Within the cores of strong magnetic
flux concentrations (>5 × 1018 Mx) associated with
active network and plage, small-scale mixed fields are absent, and
any short loops can connect just the peripheries of the flux to cell
interiors. Core fields return to the surface via longer, most likely
coronal, loops. In weaker concentrations, short loops can connect
concentrations and produce mixed fields within network boundaries,
as suggested by Dowdy and colleagues. The VAULT-2 data that we
examined are associated with strong concentrations. We conclude that
the cool-loop model applies only to a small fraction of the VAULT-2
emission, but we cannot discount a significant role for cool loops in
quieter regions. We suggest a physical picture for how network Lyα
emission may occur through the cross-field diffusion of neutral atoms
from chromospheric into coronal plasma.
Title: A New Approach to the Solar Oxygen Abundance Problem
Authors: Centeno, R.; Socas-Navarro, H.
Bibcode: 2008ApJ...682L..61C
Altcode: 2008arXiv0803.0990S; 2008arXiv0803.0990C
In this work we present new data that sets strong constraints on
the solar oxygen abundance. Our approach, based on the analysis of
spectropolarimetric observations, is almost model-independent and
therefore extremely robust. The asymmetry of the Stokes V profile
of the 6300 Å [O I] and Ni I blend is used as an indicator of the
relative abundances of these two elements. The peculiar shape of the
profile requires a value of epsilonO = 730 ± 100 ppm
(parts per million), or log epsilonO = 8.86 ± 0.07 in
the logarithmic scale commonly used in astrophysics. The uncertainty
range includes the model dependence as well as uncertainties in the
oscillator strengths of the lines. We emphasize that the very low
degree of model dependence in our analysis makes it very reliable
compared to traditional determinations.
Title: The Influence of Coronal EUV Irradiance on the Emission in
the He I 10830 Å and D3 Multiplets
Authors: Centeno, R.; Trujillo Bueno, J.; Uitenbroek, H.; Collados, M.
Bibcode: 2008ApJ...677..742C
Altcode: 2007arXiv0712.2203C
Two of the most attractive spectral windows for spectropolarimetric
investigations of the physical properties of the plasma structures
in the solar chromosphere and corona are the ones provided by the
spectral lines of the He I 10830 and 5876 Å (or D3)
multiplets, whose polarization signals are sensitive to the Hanle and
Zeeman effects. However, in order to be able to carry out reliable
diagnostics, it is crucial to have a good physical understanding
of the sensitivity of the observed spectral line radiation to the
various competing driving mechanisms. Here we report a series of
off-the-limb non-LTE calculations of the He I D3 and 10830
Å emission profiles, focusing our investigation on their sensitivity
to the EUV coronal irradiation and the model atmosphere used in the
calculations. We show in particular that the intensity ratio of the
blue to the red components in the emission profiles of the He I 10830
Å multiplet turns out to be a good candidate as a diagnostic tool for
the coronal irradiance. Measurements of this observable as a function of
the distance to the limb and its confrontation with radiative transfer
modeling might give us valuable information on the physical properties
of the solar atmosphere and on the amount of EUV radiation at relevant
wavelengths penetrating the chromosphere from above.
Title: Channeling 5 Minute Photospheric Oscillations into the Solar
Outer Atmosphere through Small-Scale Vertical Magnetic Flux Tubes
Authors: Khomenko, E.; Centeno, R.; Collados, M.; Trujillo Bueno, J.
Bibcode: 2008ApJ...676L..85K
Altcode: 2008arXiv0802.0938K
We report two-dimensional MHD simulations which demonstrate that
photospheric 5 minute oscillations can leak into the chromosphere
inside small-scale vertical magnetic flux tubes. The results of
our numerical experiments are compatible with those inferred from
simultaneous spectropolarimetric observations of the photosphere and
chromosphere obtained with the Tenerife Infrared Polarimeter (TIP)
at 10830 Å. We conclude that the efficiency of energy exchange by
radiation in the solar photosphere can lead to a significant reduction
of the cutoff frequency and may allow for the propagation of the 5
minute waves vertically into the chromosphere.
Title: Hinode Observations of Flux Emergence in Quiet and Active
Regions
Authors: Lites, B. W.; Centeno, R.; Kubo, M.; Socas-Navarro, H. Berger,
T.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.;
Katsukawa, Y.; Tsuneta, S.; Suematsu, Y.; Shimizu, T.; Nagata, S.
Bibcode: 2008ASPC..383...71L
Altcode:
We review briefly the observational understanding of emergence of
flux in both the quiet Sun and active regions in the light of first
results from the joint Japan/US/UK Hinode mission. That spacecraft
is now providing us with our first continuous, high resolution
measurements of the photospheric vector magnetic field, along with
high resolution observations of the thermal and dynamic properties
of the chromosphere and corona. This review is intended to present a
few very early results and to highlight the potential for discovery
offered by this extraordinary new mission. The discovery of ubiquitous
horizontal magnetic flux in the quiet internetwork regions is presented.
Title: Supersonic Downflows in the Photosphere Discovered in Sunspot
Moat Regions
Authors: Shimizu, T.; Martinez-Pillet, V.; Collados, M.; Ruiz-Cobo,
B.; Centeno, R.; Beck, C.; Katsukawa, Y.
Bibcode: 2007ASPC..369..113S
Altcode:
This paper reports on our new findings from the International
Time Program observations at the Canaries islands, Spain, in July
2005. We have found small-scale photospheric events with extremely
red-shifted Stokes V signals in sunspot moat regions. A preliminary
estimate of the physical conditions for an observed Stokes V profile
indicates the presence of a downward motion with a supersonic speed
in the order of 10 km/s. With the currently evaluated observational
information, we interprete the supersonic flows as downward motion from
magnetic reconnection occurring at the upper chromosphere or lower
photosphere. With coordinated observations of the Solar-B onboard
telescopes, Stokes measurements by the SOT spectro-polarimeter would
give new information for further understanding the nature of these
events with strongly red-shifted Stokes V, and for discussing the
physical conditions involving in possible magnetic reconnections in
the lower solar atmosphere.
Title: Emergence of Small-Scale Magnetic Loops in the Quiet-Sun
Internetwork
Authors: Centeno, R.; Socas-Navarro, H.; Lites, B.; Kubo, M.; Frank,
Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Tsuneta, S.;
Katsukawa, Y.; Suematsu, Y.; Shimizu, T.; Nagata, S.
Bibcode: 2007ApJ...666L.137C
Altcode: 2007arXiv0708.0844C
We study the emergence of magnetic flux at very small spatial
scales (less than 2") in the quiet-Sun internetwork. To this aim,
a time series of spectropolarimetric maps was taken at disk center
using the instrument SP/SOT on board Hinode. The LTE inversion of
the full Stokes vector measured in the Fe I 6301 and 6302 Å lines
allows us to retrieve the magnetic flux and topology in the region
of study. In the example presented here, the magnetic flux emerges
within a granular structure. The horizontal magnetic field appears
prior to any significant amount of vertical field. As time goes on,
the traces of the horizontal field disappear, while the vertical dipoles
drift-carried by the plasma motions-toward the surrounding intergranular
lanes. These events take place within typical granulation timescales.
Title: Spicule emission profiles observed in He i 10 830 Å
Authors: Sánchez-Andrade Nuño, B.; Centeno, R.; Puschmann, K. G.;
Trujillo Bueno, J.; Blanco Rodríguez, J.; Kneer, F.
Bibcode: 2007A&A...472L..51S
Altcode: 2007arXiv0707.4421S
Aims:Off-the-limb observations with high spatial and spectral resolution
will help us understand the physical properties of spicules in the
solar chromosphere.
Methods: Spectropolarimetric observations
of spicules in the He i 10 830 Å multiplet were obtained with the
Tenerife Infrared Polarimeter on the German Vacuum Tower Telescope
at the Observatorio del Teide (Tenerife, Spain). The analysis shows
the variation of the off-limb emission profiles as a function of the
distance to the visible solar limb. The ratio between the intensities
of the blue and the red components of this triplet (R=I_blue/I_red)
is an observational signature of the optical thickness along the light
path, which is related to the intensity of the coronal irradiation.
Results: We present observations of the intensity profiles of spicules
above a quiet Sun region. The observable R as a function of the distance
to the visible limb is also given. We have compared our observational
results to the intensity ratio obtained from detailed radiative transfer
calculations in semi-empirical models of the solar atmosphere assuming
spherical geometry. The agreement is purely qualitative. We argue that
future models of the solar chromosphere and transition region should
account for the observational constraints presented here.
Title: Magnetic Flux Emergence In The Quiet Sun Photosphere
Authors: Centeno, Rebecca; Lites, B.; Socas-Navarro, H.; Frank,
Z.; Shine, R.; Tarbell, T.; Title, A.; Ichimoto, K.; Tsuneta, S.;
Katsukawa, Y.; Suematsu, Y.; Kubo, M.; Shimizu, T.
Bibcode: 2007AAS...210.9406C
Altcode: 2007BAAS...39..218C
We study the emergence of magnetic flux at very small spacial scales
(less than 1 arcsec) in the quiet Sun internetwork. To this aim, several
time series of spectropolarimetric maps were taken at disk center using
the instrument SP/SOT on board Hinode. The LTE inversion of the full
Stokes vector measured in the Fe I 6301 and 6302 lines will allow us
to retrieve the magnetic flux and topology in the region of study. We
find that the magnetic flux emerges typically within the granular
structures. In many cases, the horizontal magnetic field appears
prior to any significant amount of vertical field. As time goes on,
the traces of the horizontal field dissapear while the the vertical
dipoles drift -carried by the plasma motions- towards the surrounding
intergranular lanes. Sometimes they stay trapped there for a while
but they eventually either disappear by disgregation/cancelation
or agregate to other magnetic field concentrations giving rise to
larger flux elements. The time scale of these events is of the order
of 10-20 minutes.
Title: Off-limb spectroscopy of the He I 10830 Å multiplet:
observations vs. modelling
Authors: Sánchez-Andrade Nuño, B.; Centeno, R.; Puschmann, K. G.;
Trujillo Bueno, J.; Kneer, F.
Bibcode: 2007msfa.conf..177S
Altcode:
Spectropolarimetric observations of spicules were carried out with
the new optical setup of the Tenerife Infrared Polarimeter (TIP 2)
at the VTT, showing the variation of the He I 10830Å multiplet
off-limb emission profiles with increasing distance to the solar
visible limb. The ratio between the intensities of the blue and the
red components of this triplet (R = Iblue/Ired) is an observational
signature of the optical thickness along the light path, which, at the
same time, is proportional to the population of the lower (metastable)
level that takes part in these transitions. Our observational results
show a variation of R as a function of the distance to the limb. In
agreement with recent theoretical results we conclude that R could be
used as a diagnostic tool for downward UV coronal irradiance, which is
believed to be responsible for the population of the metastable level of
the He I 10830 Å triplet. We have compared our observational results
with the ratio obtained from detailed radiative transfer calculations
in semi-empirical models of the solar atmosphere (assuming spherical
geometry) finding a fairly poor agreement. We argue that future models
of the solar chromosphere and transition region should account for
the observational constraints presented here.
Title: Wave propagation and shock formation in diverse magnetic
structures
Authors: Centeno, R.; Collados, M.; Trujillo Bueno, J.
Bibcode: 2007msfa.conf..245C
Altcode:
Velocity oscillations measured simultaneously at the photosphere
and the chromosphere of different solar magnetic features (sunspots,
pores and facular regions) allow us to study the properties of wave
propagation as a function of the magnetic flux of the structure. While
photospheric oscillations are similar everywhere, oscillations measured
at chromospheric heights show different amplitudes, frequencies
and stages of shock development depending on the observed magnetic
feature. The analysis via power and phase spectra, together with simple
theoretical modeling, lead to a series of results concerning wave
propagation within the range of heights of this study. We find that,
while the atmospheric cut-off frequency and the propagation properties
of the different oscillating modes depend on the magnetic feature,
in all the cases the power that reaches the high chromosphere comes
directly from the photosphere by means of linear wave propagation
rather than from non-linear interaction of modes.
Title: Oscillations and Wave Propagation in Different Solar Magnetic
Features
Authors: Centeno, R.; Collados, M.; Trujillo Bueno, J.
Bibcode: 2006ASPC..358..465C
Altcode:
We present the results of the analysis of temporal series of
spectro-polarimetric data measured simultaneously in the photospheric
Si I 10827 Å line and the chromospheric He I 10830 Å triplet, on
top of two different targets: a facular region and the umbra of a
sunspot. The full Stokes inversion of both spectral features gives us
the temporal variability of the physical conditions at two different
regions in the solar atmosphere, allowing us to compare the LOS velocity
oscillations at the photosphere and the chromosphere, and infer the
main characteristics of wave propagation in both magnetic structures.
Title: Spectropolarimetric Investigation of the Propagation of
Magnetoacoustic Waves and Shock Formation in Sunspot Atmospheres
Authors: Centeno, Rebecca; Collados, Manuel; Trujillo Bueno, Javier
Bibcode: 2006ApJ...640.1153C
Altcode: 2005astro.ph.12096C
Velocity oscillations in sunspot umbrae have been measured
simultaneously in two spectral lines: the photospheric Si I λ10827
line and the chromospheric He I λ10830 multiplet. From the full Stokes
inversion of temporal series of spectropolarimetric observations, we
retrieved, among other parameters, the line-of-sight velocity temporal
variations at photospheric and chromospheric heights. Chromospheric
velocity oscillations show a 3 minute period with a clear sawtooth
shape typical of propagating shock wave fronts. Photospheric velocity
oscillations have basically a 5 minute period, although the power
spectrum also shows a secondary peak in the 3 minute band that has
been proven to be a predecessor for its chromospheric counterpart. The
derived phase spectra yield a value of the atmospheric cutoff frequency
around 4 mHz and give evidence for the upward propagation of higher
frequency oscillation modes. The phase spectrum has been reproduced with
a simple model of linear vertical propagation of slow magnetoacoustic
waves in a stratified magnetized atmosphere that accounts for radiative
losses through Newton's cooling law. The model explains the main
features in the phase spectrum and allows us to compute the theoretical
time delay between the photospheric and chromospheric signals, which
happens to have a strong dependence on frequency. We find a very good
agreement between this and the time delay obtained directly from the
cross-correlation of photospheric and chromospheric velocity maps
filtered around the 6 mHz band. This allows us to infer that the 3
minute power observed at chromospheric heights comes directly from
the photosphere by means of linear wave propagation, rather than from
nonlinear interaction of 5 minute (and/or higher frequency) modes.
Title: Evidence for Fine Structure in the Chromospheric Umbral
Oscillation
Authors: Centeno, R.; Socas-Navarro, H.; Collados, M.; Trujillo
Bueno, J.
Bibcode: 2005ApJ...635..670C
Altcode: 2005astro.ph.10740C
Novel spectropolarimetric observations of the He I multiplet are
used to explore the dynamics of the chromospheric oscillation above
sunspot umbrae. The results presented here provide strong evidence
in support of the two-component model proposed by Socas-Navarro
and coauthors. According to this model, the waves propagate only
inside channels of subarcsecond width (the ``active'' component),
whereas the rest of the umbra remains nearly at rest (the ``quiet''
component). Although the observations support the fundamental elements
of that model, there is one particular aspect that is not compatible
with our data. We find that, contrary to the scenario as originally
proposed, the active component remains through the entire oscillation
cycle and harbors both the upflowing and the downflowing phase of
the oscillation.
Title: The Hanle and Zeeman Effects in Solar Spicules: A Novel
Diagnostic Window on Chromospheric Magnetism
Authors: Trujillo Bueno, J.; Merenda, L.; Centeno, R.; Collados, M.;
Landi Degl'Innocenti, E.
Bibcode: 2005ApJ...619L.191T
Altcode: 2005astro.ph..1533T; 2005astro.ph..1533B
An attractive diagnostic tool for investigating the magnetism of the
solar chromosphere is the observation and theoretical modeling of
the Hanle and Zeeman effects in spicules, as shown in this Letter for
the first time. Here we report on spectropolarimetric observations of
solar chromospheric spicules in the He I λ10830 multiplet and on their
theoretical modeling accounting for radiative transfer effects. We find
that the magnetic field in the observed (quiet-Sun) spicular material at
a height of about 2000 km above the visible solar surface has a strength
of the order of 10 G and is inclined by approximately 35deg
with respect to the local vertical direction. Our empirical finding
based on full Stokes vector spectropolarimetry should be taken into
account in future magnetohydrodynamical simulations of spicules.