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Author name code: pietarila
ADS astronomy entries on 2022-09-14
author:"Pietarila, Anna"
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Title: Understanding the Chromospheric Magnetic Field
Authors: Jin, C. L.; Harvey, J. W.; Pietarila, A.
2014ASPC..489...77J Altcode:
The chromospheric magnetic field is an important and essential component
for understanding solar atmospheric fields. Due to the problems
of polarization radiation transfer in the chromosphere and the low
detective sensitivity of chromospheric spectrum lines, observations
of chromospheric magnetic fields are very difficult, so studies
of chromospheric fields are infrequent. However, the understanding
of chromospheric fields is evolving. In this report, we summarize
our current empirical knowledge and basic physical understanding of
chromospheric fields. We concentrate on the comparison of magnetic
fields in the photosphere and chromosphere, and then display their
difference.
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Title: Migration of Ca II H bright points in the internetwork
Authors: Jafarzadeh, S.; Cameron, R. H.; Solanki, S. K.; Pietarila,
A.; Feller, A.; Lagg, A.; Gandorfer, A.
2014A&A...563A.101J Altcode: 2014arXiv1401.7522J
Context. The migration of magnetic bright point-like features (MBP)
in the lower solar atmosphere reflects the dispersal of magnetic
flux as well as the horizontal flows of the atmospheric layer they
are embedded in. <BR /> Aims: We analyse trajectories of the proper
motion of intrinsically magnetic, isolated internetwork Ca ii H MBPs
(mean lifetime 461 ± 9 s) to obtain their diffusivity behaviour. <BR
/> Methods: We use seeing-free high spatial and temporal resolution
image sequences of quiet-Sun, disc-centre observations obtained in
the Ca ii H 3968 Å passband of the Sunrise Filter Imager (SuFI)
onboard the Sunrise balloon-borne solar observatory. Small MBPs in
the internetwork are automatically tracked. The trajectory of each
MBP is then calculated and described by a diffusion index (γ) and
a diffusion coefficient (D). We also explore the distribution of the
diffusion indices with the help of a Monte Carlo simulation. <BR />
Results: We find γ = 1.69 ± 0.08 and D = 257 ± 32 km<SUP>2</SUP>
s<SUP>-1</SUP> averaged over all MBPs. Trajectories of most MBPs are
classified as super-diffusive, i.e. γ > 1, with the determined γ
being the largest obtained so far to our knowledge. A direct correlation
between D and timescale (τ) determined from trajectories of all MBPs is
also obtained. We discuss a simple scenario to explain the diffusivity
of the observed, relatively short-lived MBPs while they migrate within
a small area in a supergranule (i.e. an internetwork area). We show
that the scatter in the γ values obtained for individual MBPs is due
to their limited lifetimes. <BR /> Conclusions: The super-diffusive
MBPs can be described as random walkers (due to granular evolution and
intergranular turbulence) superposed on a large systematic (background)
velocity, caused by granular, mesogranular, and supergranular flows.
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Title: Synoptic Mapping of Chromospheric Magnetic Flux
Authors: Jin, C. L.; Harvey, J. W.; Pietarila, A.
2013ApJ...765...79J Altcode:
We used daily full-disk Ca II 854.2 nm magnetograms from the Synoptic
Optical Long Term Investigations of the Sun (SOLIS) facility to
study the chromospheric magnetic field from 2006 April through 2009
November. We determined and corrected previously unidentified zero
offsets in the SOLIS magnetograms. By tracking the disk passages of
stable unipolar regions, the measured net flux densities were found to
systematically decrease from the disk center to the limb by a factor
of about two. This decrease was modeled using a thin flux tube model
with a difference in signal formation height between the center and
limb sides. Comparison of photospheric and chromospheric observations
shows that their differences are largely due to horizontal spreading of
magnetic flux with increasing height. The north polar magnetic field
decreased nearly linearly with time during our study period while
the south polar field was nearly constant. We used the annual change
in the viewing angle of the polar regions to estimate the radial and
meridional components of the polar fields and found that the south
polar fields were tilted away from the pole. Synoptic maps of the
chromospheric radial flux density distribution were used as boundary
conditions for extrapolation of the field from the chromosphere into the
corona. A comparison of modeled and observed coronal hole boundaries
and coronal streamer positions showed better agreement when using the
chromospheric rather than the photospheric synoptic maps.
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Title: Instrumental and Observational Artifacts in Quiet Sun Magnetic
Flux Cancellation Functions
Authors: Pietarila, A.; Pietarila Graham, J.
2013SoPh..282..389P Altcode: 2012arXiv1209.6388P
Under the assumption that the photospheric quiet Sun magnetic
field is turbulent, the cancellation function has previously been
used to estimate the true, resolution-independent mean, unsigned
vertical flux «|B<SUB>z</SUB>|»<SUB>true</SUB>. We show that
the presence of network elements, noise, and seeing complicate the
measurement of accurate cancellation functions and their power law
exponents κ. Failure to exclude network elements previously led
to estimates that were too low for both the cancellation exponent
κ and «|B<SUB>z</SUB>|»<SUB>true</SUB>. However, both κ and
«|B<SUB>z</SUB>|»<SUB>true</SUB> are overestimated due to noise
in magnetograms. While no conclusive value can be derived with
data from current instruments, our Hinode/SP results of κ⪅0.38
and «|B<SUB>z</SUB>|»<SUB>true</SUB>⪅270 gauss can be taken as
upper bounds.
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Title: Ca II 854.2 nm Bisectors and Circumfacular Regions
Authors: Pietarila, A.; Harvey, J. W.
2013ApJ...764..153P Altcode:
Active regions appear bright in Ca II 854.2 nm line core intensity
while the surrounding areas, referred to as circumfacular regions, are
darker than the active region or the quiet Sun. We use Synoptic Optical
Long-term Investigations of the Sun Vector Spectromagnetograph Ca II
854.2 nm data (photospheric and chromospheric full disk magnetograms
as well as high spectral resolution Stokes I and V profiles) to study
the connection between magnetic canopies, circumfacular regions, and Ca
II 854.2 nm bisector amplitudes (spans). The line bisector amplitude
is reduced in circumfacular regions, where the 3 minute period power
in chromospheric Hα intensity oscillations is also reduced relative
to the surrounding quiet Sun. The latter is consistent with magnetic
canopies in circumfacular regions suppressing upward propagating
steepening acoustic waves. Our results provide further strong evidence
for shock waves as the cause of the inverse C-shaped bisector and
explain the observed solar cycle variation of the shape and amplitude
of Sun-as-a-star Ca II 854.2 nm bisectors.
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Title: Structure and dynamics of isolated internetwork Ca II H bright
points observed by SUNRISE
Authors: Jafarzadeh, S.; Solanki, S. K.; Feller, A.; Lagg, A.;
Pietarila, A.; Danilovic, S.; Riethmüller, T. L.; Martínez Pillet, V.
2013A&A...549A.116J Altcode: 2012arXiv1211.4836J
<BR /> Aims: We aim to improve our picture of the low chromosphere in
the quiet-Sun internetwork by investigating the intensity, horizontal
velocity, size and lifetime variations of small bright points (BPs;
diameter smaller than 0.3 arcsec) observed in the Ca II H 3968 Å
passband along with their magnetic field parameters, derived from
photospheric magnetograms. <BR /> Methods: Several high-quality
time series of disc-centre, quiet-Sun observations from the Sunrise
balloon-borne solar telescope, with spatial resolution of around 100
km on the solar surface, have been analysed to study the dynamics
of BPs observed in the Ca II H passband and their dependence on the
photospheric vector magnetogram signal. <BR /> Results: Parameters such
as horizontal velocity, diameter, intensity and lifetime histograms of
the isolated internetwork and magnetic Ca II H BPs were determined. Mean
values were found to be 2.2 km s<SUP>-1</SUP>, 0.2 arcsec (≈150 km),
1.48 ⟨ I<SUB>Ca</SUB> ⟩ and 673 s, respectively. Interestingly, the
brightness and the horizontal velocity of BPs are anti-correlated. Large
excursions (pulses) in horizontal velocity, up to 15 km s<SUP>-1</SUP>,
are present in the trajectories of most BPs. These could excite kink
waves travelling into the chromosphere and possibly the corona, which we
estimate to carry an energy flux of 310 W m<SUP>-2</SUP>, sufficient to
heat the upper layers, although only marginally. <BR /> Conclusions:
The stable observing conditions of Sunrise and our technique for
identifying and tracking BPs have allowed us to determine reliable
parameters of these features in the internetwork. Thus we find, e.g.,
that they are considerably longer lived than previously thought. The
large velocities are also reliable, and may excite kink waves. Although
these wave are (marginally) energetic enough to heat the quiet corona,
we expect a large additional contribution from larger magnetic elements
populating the network and partly also the internetwork.
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Title: Comparison of Ground-Based and Space-Based Longitudinal
Magnetograms
Authors: Pietarila, A.; Bertello, L.; Harvey, J. W.; Pevtsov, A. A.
2013SoPh..282...91P Altcode: 2012arXiv1209.6390P; 2012SoPh..tmp..274P
We compare photospheric line-of-sight magnetograms from the
Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector
Spectro-Magnetograph (VSM) instrument with observations from the
150-foot Solar Tower at Mt. Wilson Observatory (MWO), the Helioseismic
and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO),
and the Michelson Doppler Imager (MDI) on the Solar and Heliospheric
Observatory (SOHO). We find very good agreement between VSM and
the other data sources for both disk-averaged flux densities and
pixel-by-pixel measurements. We show that the VSM mean flux density
time series is of consistently high signal-to-noise ratio with no
significant zero offsets. We discuss in detail some of the factors -
spatial resolution, flux dependence, and position on the solar disk -
affecting the determination of scaling between VSM and SOHO/MDI or
SDO/HMI magnetograms. The VSM flux densities agree well with spatially
smoothed data from MDI and HMI, although the scaling factors show a
clear dependence on flux density. The factor to convert VSM to HMI
increases with increasing flux density (from ≈1 to ≈1.5). The
nonlinearity is smaller for the VSM vs. SOHO/MDI scaling factor (from
≈1 to ≈1.2).
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Title: Signature of Differential Rotation in Sun-as-a-star Ca II
K Measurements
Authors: Bertello, L.; Pevtsov, A. A.; Pietarila, A.
2012ApJ...761...11B Altcode: 2012arXiv1210.5556B
The characterization of solar surface differential rotation (SDR) from
disk-integrated chromospheric measurements has important implications
for the study of differential rotation and dynamo processes in other
stars. Some chromospheric lines, such as Ca II K, are very sensitive
to the presence of activity on the disk and are an ideal choice
for investigating SDR in Sun-as-a-star observations. Past studies
indicate that when the activity is low, the determination of Sun's
differential rotation from integrated-sunlight measurements becomes
uncertain. However, our study shows that using the proper technique, SDR
can be detected from these type of measurements even during periods of
extended solar minima. This paper describes results from the analysis
of the temporal variations of Ca II K line profiles observed by the
Integrated Sunlight Spectrometer during the declining phase of Cycle 23
and the rising phase of Cycle 24, and discusses the signature of SDR
in the power spectra computed from time series of parameters derived
from these profiles. The methodology described is quite general, and
could be applied to photometric time series of other main-sequence
stars for detecting differential rotation.
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Title: Magnetometry with Large Solar Telescopes: Beyond the
Photospheric Boundaries
Authors: Pietarila, A.
2012ASPC..463....3P Altcode:
Measuring the magnetic fields throughout the solar atmosphere, from the
photosphere through the chromosphere to the corona, is one of the main
goals of the upcoming large aperture solar telescopes. The observational
requirements and the need for compromises will be discussed followed by
a brief review of some of the available diagnostics for chromospheric
and coronal field measurements. The need for a wide range of temporal
and spatial scales as well as wavelength ranges in future solar
observations will also be considered.
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Title: Nonlinear Force-free Field Modeling of a Solar Active Region
Using SDO/HMI and SOLIS/VSM Data
Authors: Thalmann, J. K.; Pietarila, A.; Sun, X.; Wiegelmann, T.
2012AJ....144...33T Altcode: 2012arXiv1206.1141T
We use SDO/HMI and SOLIS/VSM photospheric magnetic field measurements
to model the force-free coronal field above a solar active region,
assuming magnetic forces dominate. We take measurement uncertainties
caused by, e.g., noise and the particular inversion technique, into
account. After searching for the optimum modeling parameters for the
particular data sets, we compare the resulting nonlinear force-free
model fields. We show the degree of agreement of the coronal field
reconstructions from the different data sources by comparing the
relative free energy content, the vertical distribution of the magnetic
pressure, and the vertically integrated current density. Though the
longitudinal and transverse magnetic flux measured by the VSM and
HMI is clearly different, we find considerable similarities in the
modeled fields. This indicates the robustness of the algorithm we use
to calculate the nonlinear force-free fields against differences and
deficiencies of the photospheric vector maps used as an input. We also
depict how much the absolute values of the total force-free, virial,
and the free magnetic energy differ and how the orientation of the
longitudinal and transverse components of the HMI- and VSM-based model
volumes compare to each other.
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Title: Circumfacular Regions in Ca II 854.2 nm
Authors: Pietarila, Anna; Harvey, J.
2012AAS...22011005P Altcode:
Active regions appear bright in Ca II 854.2 nm line core intensity
while the surrounding areas, circumfacular regions, are darker than
the active region or the quiet Sun. We use SOLIS VSM Ca II 854.2
nm data (high spectral resolution Stokes I and V profiles as well
as photospheric and chromospheric LOS magnetograms) to study the
relationship between the atmospheric dynamics, LOS magnetic field
stratification and detailed spectral line properties, e.g., line
bisectors and Stokes V asymmetries. The presence of circumfacular
regions, magnetic canopies and flows may explain the solar cycle
variation of Sun-as-star Ca II 854.2 nm bisectors.
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Title: Detection of Solar Differential Rotation in Disk-Integrated
Ca II K Measurements
Authors: Bertello, Luca; Pietarila, A.; Pevtsov, A. A.
2012AAS...22020311B Altcode:
The characterization of solar differential rotation (SDR) from
disk-integrated chromospheric measurements has important implications
for the study of differential rotation and dynamo processes in other
stars. Chromospheric lines, such as Ca II K, are very sensitive
to the presence of activity on the disk and are an ideal choice for
investigating SDR in Sun-as-a star <P />obervations. Here we use daily
observations from the SOLIS Integrated Sunlight Spectrometer (ISS) to
study the temporal variations of the Ca II K line profiles from 2006
to 2012. <P />We discuss the signature of SDR in the power spectra
computed from time series of parameters derived from these profiles,
and the implications for detecting differential rotation in other
Main-Sequence stars.
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Title: Coronal Rain Observed On-disk with He I Spectropolarimetry
from DST/FIRS
Authors: Schad, Thomas A.; Penn, M. J.; Pietarila, A.
2012AAS...22031005S Altcode:
Coronal rain refers to cool, dense blobs of plasma that presumably
condense near the apex of hot coronal loops and then "rain" down upon
the chromosphere traveling along curved loop-like paths. Considered
to be the result of a thermal instability known as "catastrophic
cooling", coronal rain places constraints on heating mechanisms for
coronal loops. Nearly all observational studies of coronal rain,
however, have been limited to the solar limb where cooler material
within hot coronal loops is more readily identified. Here, we report
observations of what we interpret to be the on-disk counterpart of
coronal rain. Scanned spectropolarimetric observations in the He I
triplet (1083 nm) from the Facility Infrared Spectropolarimeter (FIRS)
reveal highly-redshifted material displaying an acceleration along
curved trajectories terminating within a large sunspot (located at
N17W21). Line-of-sight velocities in the He I triplet peak near 190
km/s, which can be consider high in relation to most coronal rain
observations. This is also the largest redshift ever reported in the
He I triplet. These curved loops correspond to an overarching loop
structure seen in SDO/AIA anchored at its ends by a large sunspot and
a group of pores. The loops observed with SDO/AIA display significant
cooling as dark (EUV absorptive) blobs begin to form near the loop
apex and then traverse along the same trajectories observed in the He
I FIRS observations. Although the EUVI instrument of STEREO-A/SECCHI
has a reduced temporal resolution compared to SDO/AIA, we are able
to confidently match rain features in both spacecraft and thus
stereoscopically reconstruct the three-dimensional trajectory to
confirm the material is raining upon the solar surface.
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Title: Solar magnetism eXplorer (SolmeX). Exploring the magnetic
field in the upper atmosphere of our closest star
Authors: Peter, Hardi; Abbo, L.; Andretta, V.; Auchère, F.; Bemporad,
A.; Berrilli, F.; Bommier, V.; Braukhane, A.; Casini, R.; Curdt,
W.; Davila, J.; Dittus, H.; Fineschi, S.; Fludra, A.; Gandorfer, A.;
Griffin, D.; Inhester, B.; Lagg, A.; Landi Degl'Innocenti, E.; Maiwald,
V.; Sainz, R. Manso; Martínez Pillet, V; Matthews, S.; Moses, D.;
Parenti, S.; Pietarila, A.; Quantius, D.; Raouafi, N. -E.; Raymond, J.;
Rochus, P.; Romberg, O.; Schlotterer, M.; Schühle, U.; Solanki, S.;
Spadaro, D.; Teriaca, L.; Tomczyk, S.; Trujillo Bueno, J.; Vial, J. -C.
2012ExA....33..271P Altcode: 2011arXiv1108.5304P; 2011ExA...tmp..134P
The magnetic field plays a pivotal role in many fields of
Astrophysics. This is especially true for the physics of the solar
atmosphere. Measuring the magnetic field in the upper solar atmosphere
is crucial to understand the nature of the underlying physical
processes that drive the violent dynamics of the solar corona—that
can also affect life on Earth. SolmeX, a fully equipped solar space
observatory for remote-sensing observations, will provide the first
comprehensive measurements of the strength and direction of the
magnetic field in the upper solar atmosphere. The mission consists
of two spacecraft, one carrying the instruments, and another one in
formation flight at a distance of about 200 m carrying the occulter to
provide an artificial total solar eclipse. This will ensure high-quality
coronagraphic observations above the solar limb. SolmeX integrates two
spectro-polarimetric coronagraphs for off-limb observations, one in
the EUV and one in the IR, and three instruments for observations on
the disk. The latter comprises one imaging polarimeter in the EUV for
coronal studies, a spectro-polarimeter in the EUV to investigate the low
corona, and an imaging spectro-polarimeter in the UV for chromospheric
studies. SOHO and other existing missions have investigated the emission
of the upper atmosphere in detail (not considering polarization),
and as this will be the case also for missions planned for the near
future. Therefore it is timely that SolmeX provides the final piece of
the observational quest by measuring the magnetic field in the upper
atmosphere through polarimetric observations.
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Title: Circumfacular regions and magnetic canopies as seen in Ca II
8542 Å
Authors: Pietarila, Anna; Harvey, Jack
2012decs.confE...8P Altcode:
Active regions appear bright in Ca II 8542 Å line core intensity
while the surrounding areas are darker than the active region or the
quiet Sun. These areas are referred to as circumfacular regions. We
use SOLIS VSM Ca II 8542 Å data (photospheric and chromospheric
full disk magnetograms as well as high spectral resolution Stokes I
and V profiles) to study the relationship between photospheric and
chromospheric LOS magnetic fields and detailed properties (e.g., line
bisectors, Stokes V asymmetries) of the spectral profiles. There is
a connection between magnetic canopies, circumfacular regions and Ca
II 8542 Å bisector spans which may explain the observed solar cycle
variation of the Sun-as-a-star Ca II 8542 Å bisectors.
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Title: Diffusivity of Isolated Internetwork Ca II H Bright Points
Observed by SuFI/SUNRISE
Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Feller, A.;
Pietarila, A.; Lagg, A.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
Knoelker, M.; Martinez Pillet, V.; Schmidt, W.; Title, A.
2012decs.confE..99J Altcode:
We analyze trajectories of the proper motion of intrinsically magnetic,
isolated internetwork Ca II H BPs (with mean lifetime of 461 sec) to
obtain their diffusivity behaviors. We use high spatial and temporal
resolution image sequences of quiet-Sun, disc-centre observations
obtained in the Ca II H 397 nm passband of the Sunrise Filter Imager
(SuFI) on board the SUNRISE balloon-borne solar observatory. In
order to avoid misidentification, the BPs are semi-manually selected
and then automatically tracked. The trajectory of each BP is then
calculated and its diffusion index is described by a power law
exponent, using which we classify the BPs' trajectories into sub-,
normal and super- diffusive. In addition, the corresponding diffusion
coefficients (D) based on the observed displacements are consequently
computed. We find a strong super-diffusivity at a height sampled by the
SuFI/SUNRISE Ca II H passband (i.e. a height corresponding roughly to
the temperature minimum). We find that 74% of the identified tiny BPs
are super-diffusive, 18% move randomly (i.e. their motion corresponds
to normal diffusion) and only 8% belong to the sub-diffusion regime. In
addition, we find that 53% of the super-diffusion regime (i.e. 39% of
all BPs) have the diffusivity index of 2 which are termed as "Ballistic
BPs". Finally, we explore the distribution of diffusion index with the
help of a simple simulation. The results suggest that the BPs are random
walkers superposed by a systematic (background) velocity in which the
magnitude of each component (and hence their ratio) depends on the time
and spatial scales. We further discuss a simple sketch to explain the
diffusivity of observed BPs while they migrate within a supergranule
(i.e. internetwork areas) or close to the network regions.
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Title: Chromospheric Observations of a Kink Wave in an On-disk Active
Region Fibril
Authors: Pietarila, A. M.; Aznar Cuadrado, R.; Hirzberger, J.;
Solanki, S.
2011AGUFMSH13B1951P Altcode:
Most observations of kink and Alfven waves in the chromosphere are
made in off-limb spicules. Here we present observations of a kink wave
in high spatial and temporal resolution Ca II 8542 data of an active
region fibril on the solar disk. The properties of the observed wave
are similar to kink waves in spicules. From the inferred wave phase
and period we estimate the lower limit for the field strength in the
chromospheric fibril to be a few hundred Gauss. The observations
indicate that the event may have been triggered by a small-scale
reconnection event higher up in the atmosphere.
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Title: The Longitudinal Solar Magnetic Field Measured by SOLIS
Authors: Bertello, L.; Pevtsov, A. A.; Harvey, J. W.; Pietarila, A.
2011AGUFMSH13B1934B Altcode:
The SOLIS (Synoptic Optical Long-term Investigations of the Sun) Vector
SpectroMagnetograph (VSM) instrument operating at the National Solar
Observatory at Kitt Peak (Arizona) measures the full-disk longitudinal
solar magnetic field in the neutral iron spectral lines at 630.15-630.25
nm (photosphere) and ionized calcium spectral line at 854.2 nm (lower
chromosphere). Over the past two years a considerable effort has
been made to improve the quality of the computed magnetograms. Major
modifications to the original data reduction pipeline include a
new determination of the instrumental magnetic bias and an improved
calibration of the computed solar magnetic flux density. As a result of
these changes SOLIS-VSM is now able to provide, among other products,
more reliable values of the full-disk mean magnetic flux measured in
the photosphere and low chromosphere. In addition, these improvements
open the possibility to extend the current SOLIS-VSM catalog with new
products of interest to the solar and heliospheric communities. We
describe some of these changes and their impact on the quality of the
derived SOLIS-VSM data.
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Title: Erratum:"Convective Nature of Sunspot Penumbral
Filaments: Discovery of Downflows in the Deep Photosphere" <A
href="/abs/2011ApJ...734L..18J">(2011, ApJ, 734, L18)</A>
Authors: Joshi, Jayant; Pietarila, A.; Hirzberger, J.; Solanki, S. K.;
Aznar Cuadrado, R.; Merenda, L.
2011ApJ...740L..55J Altcode:
No abstract at ADS
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Title: Kink Waves in an Active Region Dynamic Fibril
Authors: Pietarila, A.; Aznar Cuadrado, R.; Hirzberger, J.; Solanki,
S. K.
2011ApJ...739...92P Altcode: 2011arXiv1107.3113P
We present high spatial and temporal resolution Ca II 8542 Å
observations of a kink wave in an on-disk chromospheric active region
fibril. The properties of the wave are similar to those observed in
off-limb spicules. From the observed phase and period of the wave we
determine a lower limit for the field strength in the chromospheric
active region fibril located at the edge of a sunspot to be a few
hundred gauss. We find indications that the event was triggered by a
small-scale reconnection event higher up in the atmosphere.
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Title: Solar Cycle Variation in Sun-as-a-star Ca II 854.2 nm Bisectors
Authors: Pietarila, A.; Livingston, W.
2011ApJ...736..114P Altcode: 2011arXiv1105.3437P
The bisector of the strong chromospheric Ca II 854.2 nm line has an
inverse-C shape, the cause of which is not yet fully understood. We
show that the amplitude of the bisector in Sun-as-a-star observations
exhibits a solar cycle variation with smaller amplitudes during highest
activity. The line core intensity is lower during solar minima while
the part of the bisector most sensitive to the line core shows no
systematic change with activity. Our results support the use of Ca
II 854.2 nm bisectors in studying the relationship between convection
and magnetic fields, not only in the Sun but in other stars as well.
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Title: Convective Nature of Sunspot Penumbral Filaments: Discovery
of Downflows in the Deep Photosphere
Authors: Joshi, Jayant; Pietarila, A.; Hirzberger, J.; Solanki, S. K.;
Aznar Cuadrado, R.; Merenda, L.
2011ApJ...734L..18J Altcode: 2011arXiv1105.1877J
We study the velocity structure of penumbral filaments in the deep
photosphere to obtain direct evidence for the convective nature of
sunspot penumbrae. A sunspot was observed at high spatial resolution
with the 1 m Swedish Solar Telescope in the deep photospheric C I 5380
Å absorption line. The Multi-Object Multi-Frame Blind Deconvolution
method is used for image restoration and straylight is filtered out. We
report here the discovery of clear redshifts in the C I 5380 Å line
at multiple locations in sunspot penumbral filaments. For example,
bright head of filaments show larger concentrated blueshift and are
surrounded by darker, redshifted regions, suggestive of overturning
convection. Elongated downflow lanes are also located beside bright
penumbral fibrils. Our results provide the strongest evidence yet
for the presence of overturning convection in penumbral filaments and
highlight the need to observe the deepest layers of the penumbra in
order to uncover the energy transport processes taking place there.
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Title: Ca Ii K and H Measurements from the SOLIS ISS Instrument
Authors: Bertello, Luca; Pevtsov, A. A.; Pietarila, A.; Harvey, J. W.;
Toussaint, R. M.; SOLIS Team
2011SPD....42.1744B Altcode: 2011BAAS..43S.1744B
Precise measurements of the disk integrated solar Ca II K and H
lines play a critical role in many investigations of solar activity
and studies related to solar irradiance variability. The Integrated
Sunlight Spectrometer(ISS) operating at the National Solar Observatory
at Kitt Peak (Arizona) since December 2006 is designed to obtain
high spectral resolution (R = 300,000) observations of the Sun as
a star in a broad range of wavelengths (350 nm -1100 nm). The ISS
is one of three instruments, with the Vector Spectro-Magnetograph
(VSM) and the Full Disk Patrol (FDP), comprising the Synoptic Optical
Long-term Investigations of the Sun (SOLIS) - a synoptic facility for
solar observations operating at NSO/Kitt Peak. The ISS takes daily
observations of solar spectra in nine spectral bands, including the Ca
II K and H lines. We describe recent improvements in data reduction of
Ca II K and H observations, and present time variations of parameters
derived from the profiles of these spectral lines. Some properties of
these time series are also discussed.
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Title: Solar Cycle Variation of the Ca 854.2 nm Line Bisectors
Authors: Pietarila, Anna; Livingston, W.
2011SPD....42.1703P Altcode: 2011BAAS..43S.1703P
The bisector of the strong chromospheric Ca II 854.2 nm line has an
inverse-C shape the cause of which is not yet fully understood. We
show that the amplitude of the bisector in Sun-as-a-star observations
exhibits a solar cycle variation with smaller amplitudes during highest
activity. The line core intensity is lower during solar minima while
the part of the bisector most sensitive to the line core shows no
systematic change with activity. We also show preliminary results of
a connection between magnetic shadows and bisector amplitudes. Our
results support the use of Ca II 854.2 nm bisectors in studying the
relationship between convection and magnetic fields, not only in the
Sun but in other stars as well.
---------------------------------------------------------
Title: Solar Magnetic Fields As Observed By Solis
Authors: Pietarila, Anna; Bertello, L.; Callahan, L.; Harker, B.;
Harvey, J.; Marble, A.; Pevtsov, A.; Toussaint, R.
2011SPD....42.1746P Altcode: 2011BAAS..43S.1746P
The Vector Spectromagnetograph (VSM), part of the Synoptic Optical
Long-term Investigations of the Sun (SOLIS), makes spectropolarimetric
observations of the full-disk of the Sun in the photospheric Fe I lines
around 630 nm (Stokes I, Q, U and V) and the chromospheric Ca II 854.2
nm (Stokes I and V) line. We present some of the updated SOLIS VSM data
products and show how they compare with data from other instruments,
e.g., SDO/HMI and Hinode/SP. We also illustrate some of the differences
between the photospheric and chromospheric magnetograms, and how they
can be used to study the height variation of the magnetic field.
---------------------------------------------------------
Title: Status of Synoptic Optical Long-term Investigation of the Sun
(SOLIS) Project.
Authors: Pevtsov, Alexei A.; Streander, K.; Harvey, J.; Bertello,
L.; Branston, D.; Britanik, J.; Callahan, L.; Cole, L.; Galayda, E.;
Harker, B.; Hauth, D.; Jaksha, D.; Leiker, C.; Marble, A.; Mills, N.;
Pietarila, A.; Schramm, K.; Stover, E.; Suarez-Sola, I.; Toussaint, R.
2011SPD....42.1747P Altcode: 2011BAAS..43S.1747P
Synoptic Optical Long-term Investigation of the Sun (SOLIS) is a suite
of three instruments to study various aspects of solar activity. Vector
Stokes Magnetograph (VSM) takes full disk longitudinal magnetograms in
the photosphere (Fe I 6301 A-6302 A) and the chromosphere (Ca II 8542A),
the photospheric vector magnetograms, and the full disk images of
equivalent width for He I 10830A. The VSM data set goes back to August
2003. Integrated Sunlight Spectrometer (ISS) provides high-resolution
spectra of sun-as-a-star for nine selected spectral bands (starting
from December 2006). Full-Disk Patrol (FDP) observes full disk images
of the Sun at high cadence in several selected wavelengths (starting
from June 2011). In the last two years, the SOLIS instruments and
data reduction went through a series of upgrades resulting in higher
data quality. These upgrades include new CCD cameras, photo guider,
and improved magbias calculations and data reduction of ISS line
profiles. We will present the current status of SOLIS, and show the
comparison between SOLIS observations with other instruments.
---------------------------------------------------------
Title: Transport of Magnetic Flux from the Canopy to the Internetwork
Authors: Pietarila, A.; Cameron, R. H.; Danilovic, S.; Solanki, S. K.
2011ApJ...729..136P Altcode: 2011arXiv1102.1397P
Recent observations have revealed that 8% of linear polarization
patches in the internetwork (INW) quiet Sun are fully embedded in
downflows. These are not easily explained with the typical scenarios for
the source of INW fields which rely on flux emergence from below. Using
radiative MHD simulations, we explore a scenario where magnetic flux
is transported from the magnetic canopy overlying the INW into the
photosphere by means of downward plumes associated with convective
overshoot. We find that if a canopy-like magnetic field is present in
the simulation, the transport of flux from the canopy is an important
process for seeding the photospheric layers of the INW with magnetic
field. We propose that this mechanism is relevant for the Sun as well,
and it could naturally explain the observed INW linear polarization
patches entirely embedded in downflows.
---------------------------------------------------------
Title: Constructing Semi-Empirical Sunspot Models for Helioseismology
Authors: Cameron, R. H.; Gizon, L.; Schunker, H.; Pietarila, A.
2011SoPh..268..293C Altcode: 2010arXiv1003.0528C; 2010SoPh..tmp..167C
One goal of helioseismology is to determine the subsurface structure
of sunspots. In order to do so, it is important to understand
first the near-surface effects of sunspots on solar waves, which are
dominant. Here we construct simplified, cylindrically-symmetric sunspot
models that are designed to capture the magnetic and thermodynamics
effects coming from about 500 km below the quiet-Sun τ<SUB>5000</SUB>=1
level to the lower chromosphere. We use a combination of existing
semi-empirical models of sunspot thermodynamic structure (density,
temperature, pressure): the umbral model of Maltby et al. (1986,
Astrophys. J. 306, 284) and the penumbral model of Ding and Fang (1989,
Astron. Astrophys. 225, 204). The OPAL equation-of-state tables are used
to derive the sound-speed profile. We smoothly merge the near-surface
properties to the quiet-Sun values about 1 Mm below the surface. The
umbral and penumbral radii are free parameters. The magnetic field is
added to the thermodynamic structure, without requiring magnetostatic
equilibrium. The vertical component of the magnetic field is assumed
to have a Gaussian horizontal profile, with a maximum surface field
strength fixed by surface observations. The full magnetic-field vector
is solenoidal and determined by the on-axis vertical field, which,
at the surface, is chosen such that the field inclination is 45° at
the umbral - penumbral boundary. We construct a particular sunspot
model based on SOHO/MDI observations of the sunspot in active region
NOAA 9787. The helioseismic signature of the model sunspot is studied
using numerical simulations of the propagation of f, p<SUB>1</SUB>,
and p<SUB>2</SUB> wave packets. These simulations are compared
against cross-covariances of the observed wave field. We find that
the sunspot model gives a helioseismic signature that is similar to
the observations.
---------------------------------------------------------
Title: Transverse Component of the Magnetic Field in the Solar
Photosphere Observed by SUNRISE
Authors: Danilovic, S.; Beeck, B.; Pietarila, A.; Schüssler, M.;
Solanki, S. K.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta,
J. C.; Domingo, V.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
Knölker, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.149D Altcode: 2010arXiv1008.1535D
We present the first observations of the transverse component of
a photospheric magnetic field acquired by the imaging magnetograph
SUNRISE/IMaX. Using an automated detection method, we obtain statistical
properties of 4536 features with significant linear polarization
signal. We obtain a rate of occurrence of 7 × 10<SUP>-4</SUP>
s<SUP>-1</SUP> arcsec<SUP>-2</SUP>, which is 1-2 orders of magnitude
larger than the values reported by previous studies. We show that
these features have no characteristic size or lifetime. They appear
preferentially at granule boundaries with most of them being caught
in downflow lanes at some point. Only a small percentage are entirely
and constantly embedded in upflows (16%) or downflows (8%).
---------------------------------------------------------
Title: Expansion of magnetic flux concentrations: a comparison of
Hinode SOT data and models
Authors: Pietarila, A.; Cameron, R.; Solanki, S. K.
2010A&A...518A..50P Altcode: 2010arXiv1005.3405P
Context. The expansion of network magnetic fields with height is a
fundamental property of flux tube models. A rapid expansion is required
to form a magnetic canopy. <BR /> Aims: We characterize the observed
expansion properties of magnetic network elements and compare them
with the thin flux tube and sheet approximations, as well as with
magnetoconvection simulations. <BR /> Methods: We used data from
the Hinode SOT NFI NaD<SUB>1</SUB> channel and spectropolarimeter to
study the appearance of magnetic flux concentrations seen in circular
polarization as a function of position on the solar disk. We compared
the observations with synthetic observables from models based on the
thin flux tube approximation and magnetoconvection simulations with two
different upper boundary conditions for the magnetic field (potential
and vertical). <BR /> Results: The observed circular polarization signal
of magnetic flux concentrations changes from unipolar at disk center to
bipolar near the limb, which implies an expanding magnetic field. The
observed expansion agrees with expansion properties derived from the
thin flux sheet and tube approximations. Magnetoconvection simulations
with a potential field as the upper boundary condition for the magnetic
field also produce bipolar features near the limb while a simulation
with a vertical field boundary condition does not. <BR /> Conclusions:
The near-limb apparent bipolar magnetic features seen in high-resolution
Hinode observations can be interpreted using a simple flux sheet
or tube model. This lends further support to the idea that magnetic
features with vastly varying sizes have similar relative expansion
rates. The numerical simulations presented here are less useful in
interpreting the expansion since the diagnostics we are interested in
are strongly influenced by the choice of the upper boundary condition
for the magnetic field in the purely photospheric simulations.
---------------------------------------------------------
Title: STEREO quadrature observations of coronal dimming at the
onset of mini-CMEs
Authors: Innes, D. E.; McIntosh, S. W.; Pietarila, A.
2010A&A...517L...7I Altcode: 2010arXiv1005.2097I
Context. Using unique quadrature observations with the two STEREO
spacecraft, we investigate coronal dimmings at the onset of small-scale
eruptions. In CMEs they are believed to indicate the opening up of
the coronal magnetic fields at the start of the eruption. <BR />
Aims: It is to determine whether coronal dimming seen in small-scale
eruptions starts before or after chromospheric plasma ejection. <BR />
Methods: One STEREO spacecraft obtained high cadence, 75 s, images in
the He II 304 Å channel, and the other simultaneous images in the
Fe IX/Fe X 171 Å channel. We concentrate on two well-positioned
chromospheric eruptions that occurred at disk center in the 171
Å images, and on the limb in 304 Å. One was in the quiet Sun and
the other was in an equatorial coronal hole. We compare the timing
of chromospheric eruption seen in the 304 Å limb images with the
brightenings and dimmings seen on disk in the 171 Å images. Further
we use off-limb images of the low frequency 171 Å power to infer
the coronal structure near the eruptions. <BR /> Results: In both
the quiet Sun and the coronal hole eruption, on disk 171 Å dimming
was seen before the chromospheric eruption, and in both cases it
extends beyond the site of the chromospheric eruption. The quiet
Sun eruption occurred on the outer edge of the enclosing magnetic
field of a prominence and may be related to a small disruption of
the prominence just before the 171 Å dimming. <BR /> Conclusions:
These small-scale chromospheric eruptions started with a dimming in
coronal emission just like their larger counterparts. We therefore
suggest that a fundamental step in triggering them was the removal of
overlying coronal field. <P />Movies are only available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: STEREO quadrature observations of mass flows in prominences
Authors: Innes, Davina; McIntosh, Scott; Pietarila, Anna
2010cosp...38.2917I Altcode: 2010cosp.meet.2917I
Understanding the structure and dynamics of prominences is much
easier when both the promi-nence on the limb and the filament on
the disk are seen together. In February 2009, we obtained STEREO
quadrature observations with a cadence of 75 s and simultaneous images
of promi-nences in 304 A at the limb, and 171 A at disk center. We show
how the observed flows in the prominence are associated with microflares
seen in 171 at disk center for a couple of representative cases.
---------------------------------------------------------
Title: Relation between the Sunrise photospheric magnetic field and
the Ca II H bright features
Authors: Jafarzadeh, Shahin; Hirzberger, J.; Feller, A.; Lagg, A.;
Solanki, S. K.; Pietarila, A.; Danilovic, S.; Riethmueller, T.;
Barthol, P.; Berkefeld, T.; Gandorfer, A.; Knülker, M.; Martínez
Pillet, V.; Schmidt, W.; Schüssler, M.; Title, A.
2010cosp...38.2856J Altcode: 2010cosp.meet.2856J
Recent observations from the Sunrise balloon-borne solar telescope
have enabled us to reach an unprecedented high spatial resolution
on the solar surface with the near-ultraviolet photo-spheric and
chromospheric images as well as the magnetograms. We use these high
resolution observations to investigate the structure of the solar
upper photosphere and lower chromosphere as well as their temporal
evolutions. We study the relation between the inter-granular Ca II
397 nm bright structures in images obtained by the Sunrise Filter
Imager (SuFI) and their corresponding photospheric vector magnetic
field computed from the Imaging Magnetogram eXperiment (IMaX)
observations. The targets under study are in a quiet Sun region and
close to disc-centre.
---------------------------------------------------------
Title: Expansion of Magnetic Flux Concentrations with Height:
A Comparison of Hinode SOT Data and MHD Simulations
Authors: Pietarila, A.; Cameron, R.; Solanki, S.
2009ASPC..415...91P Altcode:
The Hinode SOT (Tsuneta et al. 2008) NFI Na I D<SUB>1</SUB> and SP Fe I
data sampled at different positions on the solar disk provide a unique
diagnostic for studying the expansion of magnetic flux concentrations
with height. We make a comparative study of SOT observations and
2-dimensional (2D) radiative MHD-simulations to see how well the
simulations capture the expansion properties. The expansion of flux
concentrations is clearly seen in the SOT Na I D<SUB>1</SUB> data,
where most of the magnetic features appear unipolar at disk center while
close to the limb bipolar appearance strongly dominates. This trend,
albeit not as strong, is seen in the SP data as well. Some aspects of
the observations are qualitatively reproduced by simulations with a
potential (as opposed to vertical) upper boundary condition for the
magnetic field.
---------------------------------------------------------
Title: Bright fibrils in Ca II K
Authors: Pietarila, A.; Hirzberger, J.; Zakharov, V.; Solanki, S. K.
2009A&A...502..647P Altcode: 2009arXiv0905.3124P
Context: Except for the Ca II resonance lines, fibrils are ubiquitously
present in most high-resolution observations of chromospheric lines. <BR
/>Aims: We show that fibrils are also a prevailing feature in Ca II K,
provided the spatial-resolution is sufficiently high. <BR />Methods:
We present high spatial resolution observations of an active region in
the Ca II K line from the Swedish Solar Telescope. Through a comparison
between photospheric intensity and magnetic field data, we study the
connection between bright chromospheric fibrils and photospheric
structures. Additionally, using Fourier analysis we study how the
fibrils are linked to the observed dynamics. <BR />Results: We find
that very narrow, bright fibrils are a prevailing feature over large
portions of the observed field. We also find a clear connection between
the fibril footpoints and photospheric magnetic features. We show that
the fibrils play two distinct roles in the observed dynamics: depending
on their location they can act as a canopy suppressing oscillations or
they can channel low-frequency oscillations into the chromosphere. <BR
/>Conclusions: The Ca II K fibrils share many characteristics with
fibrils observed in other chromospheric lines, but some features,
such as the very small widths, are unique to these observations.
---------------------------------------------------------
Title: Fibrils in Ca II K
Authors: Pietarila, A.; Solanki, S.; Hirzberger, J.; Zakharov, V.
2008ESPM...12.2.51P Altcode:
High spatial resolution observations have revealed that fibril-like
structures are a ubiquitous feature of the solar chromosphere. They are
observed in most chromospheric lines, e.g., H-? and Ca II IRT. Until
recently, there have been no reports of these structures in the Ca
II H and K lines. Instead, these lines have revealed a hazy, uniform
chromosphere and only in the highest resolution images have there been
any indications of fibril structures. <P />We present high spatial
resolution Ca II K observations from the Swedish Solar Telescope
which show that fibrils are a prevailing feature in regions where
the bulk of the signal is chromospheric. Based on the cotemporal
continuum and nearly cotemporal magnetic field observations it is
clear that the fibril foot points originate from photospheric magnetic
concentrations. The fibrils share many characteristics, e.g. lifetime
and dynamics, with fibrils observed in other spectral lines. They are
also found to play an important role in the dynamics: in the plage
they channel low frequency waves into the chromosphere while in the
more quiet regions the highly inclined fibrils form a multi-layer
canopy that suppresses oscillations from below.
---------------------------------------------------------
Title: Spectropolarimetric Inversions of the Ca II 8498 and 8542 Å
Lines in the Quiet Sun
Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.
2007ApJ...670..885P Altcode:
We study non-LTE inversions of the Ca II infrared triplet lines
as a tool for inferring physical properties of the quiet Sun. The
inversion code is successful in recovering the temperature, velocity,
and longitudinal magnetic flux density in the photosphere and
chromosphere, but the height range where the inversions are sensitive
is limited, especially in the chromosphere. We present results of
inverting spectropolarimetric observations of the lines in a quiet-Sun
region. We find three distinct ranges in chromospheric temperature: low
temperatures in the internetwork, high temperatures in the enhanced
magnetic network, and intermediate temperatures associated with
low magnetic flux regions in the network. The differences between
these regions become more pronounced with height as the plasma-β
decreases. These inversions support the picture of the chromosphere,
especially close to the magnetic network, being highly inhomogeneous
in both the vertical and horizontal directions.
---------------------------------------------------------
Title: Spectropolarimetric Observations of the Ca II λ8498 and
λ8542 in the Quiet Sun
Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.
2007ApJ...663.1386P Altcode: 2007arXiv0707.1310P
The Ca II infrared triplet is one of the few magnetically sensitive
chromospheric lines available for ground-based observations. We present
spectropolarimetric observations of the 8498 and 8542 Å lines in a
quiet Sun region near a decaying active region and compare the results
with a simulation of the lines in a high plasma-β regime. Cluster
analysis of Stokes V profile pairs shows that the two lines,
despite arguably being formed fairly close, often do not have similar
shapes. In the network, the local magnetic topology is more important in
determining the shapes of the Stokes V profiles than the phase of the
wave, contrary to what our simulations show. We also find that Stokes
V asymmetries are very common in the network, and the histograms of
the observed amplitude and area asymmetries differ significantly from
the simulation. Both the network and internetwork show oscillatory
behavior in the Ca II lines. It is stronger in the network, where
shocking waves, similar to those in the high-β simulation, are seen
and large self-reversals in the intensity profiles are common.
---------------------------------------------------------
Title: The Ca II Infrared Triplet Lines as Diagnostics of
Chromospheric Magnetism
Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.
2007ASPC..368..139P Altcode:
The Ca II infrared (IR) triplet lines are a promising candidate for
studying chromospheric magnetism and dynamics. To study how good
of a diagnostic the lines are for chromospheric magnetism in the
quiet Sun we have constructed a MHD simulation in the high plasma-β
regime, analyzed quiet Sun spectropolarimetric data of the lines and
used a non-local thermodynamic equilibrium (nLTE) inversion code on
the observations. In the simulation, where shocking acoustic waves
dominate the dynamics, the Ca lines show a time-varying pattern of
disappearing and reappearing Stokes V lobes. Waves are seen in the
observations as well, but the dynamics are more complex. Unlike in
the simulation, the observed Ca lines do not have similar shapes
and the Stokes V asymmetries are determined by the local magnetic
topology, not the phase of the wave. The fundamental differences
between the observations and the simulation lead one to conclude that
a 1D plane parallel atmosphere is not a valid approximation for the
chromosphere. Nor can the effects of magnetic fields on the dynamics
be neglected. This is further supported by the inversions failure to
reproduce line profile asymmetries caused by gradients in the velocity
and/or magnetic field. To explain the asymmetries, 3D structures and
strongly localized gradients need to be included. The work presented
here will be published in more detail elsewhere.
---------------------------------------------------------
Title: Spectropolarimetric observations of the Ca II 8498 A and 8542
A lines in the quiet Sun
Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.
2007arXiv0704.0617P Altcode:
The Ca II infrared triplet is one of the few magnetically sensitive
chromospheric lines available for ground-based observations. We present
spectropolarimetric observations of the 8498 A and 8542 A lines in a
quiet Sun region near a decaying active region and compare the results
with a simulation of the lines in a high plasma-beta regime. Cluster
analysis of Stokes V profile pairs shows that the two lines,
despite arguably being formed fairly close, often do not have similar
shapes. In the network, the local magnetic topology is more important in
determining the shapes of the Stokes V profiles than the phase of the
wave, contrary to what our simulations show. We also find that Stokes
V asymmetries are very common in the network, and the histograms of
the observed amplitude and area asymmetries differ significantly from
the simulation. Both the network and internetwork show oscillatory
behavior in the Ca II lines. It is stronger in the network, where
shocking waves, similar to those in the high-beta simulation, are seen
and large self-reversals in the intensity profiles are common.
---------------------------------------------------------
Title: The Ca Ir Triplet As A Diagnostic For Chromospheric Magnetism
Authors: Pietarila, Anna M.; Socas-Navarro, H.; Bogdan, T. J.
2006SPD....37.1902P Altcode: 2006BAAS...38..247P
The Ca IR triplet lines are a good candidate for observing chromospheric
magnetic fields. Simulations of the lines in the high-beta regime,
spectropolarimetric observations and inversions of observations give
insight to their usability\suitability in observing QS magnetic fields
and dynamics. Propagating waves are clearly visible in the simulations,
both in Stokes V and I. Observed line profiles are found to be more
complicated, though they do indicate the presence of waves. We present
both simulated and observed profiles and analyze the ability of the
NLTE inversion algorithm to retrieve the underlying physics in the
solar atmosphere.
---------------------------------------------------------
Title: Spectro-Polarimetric Observations and Non-Lte Modeling of
Ellerman Bombs
Authors: Socas-Navarro, Hector; Pillet, Valentín Martínez; Elmore,
David; Pietarila, Anna; Lites, Bruce W.; Sainz, Rafael Manso
2006SoPh..235...75S Altcode: 2005astro.ph..8667S
Ellerman bombs are bright emission features observed in the wings
of Hα, usually in the vicinity of magnetic concentrations. Here we
show that they can also be detected in the Ca II infrared triplet
lines, which are easier to interpret and therefore allow for more
detailed diagnostics. We present full Stokes observations of the
849.8 and 854.2 nm lines acquired with the new spectro-polarimeter
SPINOR. The data show no significant linear polarization at the level
of 3 × 10<SUP>−4</SUP>. The circular polarization profiles exhibit
measureable signals with a very intricate pattern of peaks. A non-LTE
analysis of the spectral profiles emerging from these features reveals
the presence of strong downflows (∼10 {km s<SUP>−1</SUP>}) in a
hot layer between the upper photosphere and the lower chromosphere.
---------------------------------------------------------
Title: Spinor: Visible and Infrared Spectro-Polarimetry at the
National Solar Observatory
Authors: Socas-Navarro, Hector; Elmore, David; Pietarila, Anna;
Darnell, Anthony; Lites, Bruce W.; Tomczyk, Steven; Hegwer, Steven
2006SoPh..235...55S Altcode: 2005astro.ph..8685S
The Spectro-Polarimeter for Infrared and Optical Regions (SPINOR)
is a new spectro-polarimeter that will serve as a facility instrument
for the Dunn Solar Telescope at the National Solar Observatory. This
instrument is capable of achromatic polarimetry over a very broad range
of wavelengths, from 430 to 1600 nm, allowing for the simultaneous
observation of several visible and infrared spectral regions with full
Stokes polarimetry. Another key feature of the design is its flexibility
to observe virtually any combination of spectral lines, limited only
by practical considerations (e.g., the number of detectors available,
space on the optical bench, etc.).
---------------------------------------------------------
Title: Simulation of Quiet-Sun Waves in the Ca II Infrared Triplet
Authors: Pietarila, A.; Socas-Navarro, H.; Bogdan, T.; Carlsson, M.;
Stein, R. F.
2006ApJ...640.1142P Altcode: 2005astro.ph.10744P
The Ca II infrared triplet lines around 8540 Å are good candidates
for observing chromospheric magnetism. Model spectra of these lines
are obtained by combining a radiation hydrodynamic simulation with a
Stokes synthesis code. The simulation shows interesting time-varying
behavior of the Stokes V profiles as waves propagate through the
formation region of the lines. Disappearing and reappearing lobes
in the Stokes V profiles as well as profile asymmetries are closely
related to the atmospheric velocity gradients.
---------------------------------------------------------
Title: On the Formation of Extreme-Ultraviolet Helium Lines in the
Sun: Analysis of SOHO Data
Authors: Judge, Philip G.; Pietarila, Anna
2004ApJ...606.1258J Altcode:
The resonance lines of helium in the Sun are several times stronger than
expected, relative to lines of other ions. To explore the origins of
this “helium enhancement,” we have studied data from the SUMER, CDS,
MDI, and EIT instruments on the Solar and Heliospheric Observatory
(SOHO). Time series data obtained in a quiet region and a coronal
hole indicate that the spatio-temporal properties of the He I 584
Å, and He II 304 and 1084 Å lines are qualitatively unlike other
chromospheric and transition region lines. Helium line intensities
vary slowly compared to chromospheric oscillations and to transient
transition region brightenings seen in other lines, yet they vary
rapidly (both spatially and temporally) compared to the coronal
radiation. This suggests that photoionization/recombination plays a
minor excitation role in these lines. The Doppler shift of the 584 Å
line shows a remarkably clear 4-8 mHz oscillation, with no accompanying
line intensity oscillation. Transient brightenings are used to show
that nonradial photon scattering might explain the previously found
diffuse appearance of the network but cannot account for the reduced
network/internetwork intensity contrast in the 584 Å line. We propose
a new enhancement mechanism, based on arguments in a companion paper,
in which predominantly neutral species such as helium diffuse across
magnetic field lines into regions of hot coronal plasma, but charged
ions do not. The enhanced internetwork helium emission may arise
from the diffusion of helium atoms across the chromospheric canopy;
the enhanced network emission may have contributions from diffusion
from spicules. This mechanism may naturally explain why the 584 Å
line shows Doppler shift, but not intensity, oscillations. Finally,
if a mechanism such as “velocity redistribution” dominates helium
emission, spectroheliograms of helium lines will be bright in regions of
large temperature gradients parallel to the magnetic field. Differences
between helium and other spectroheliograms may then reveal the relative
contributions of “classical” and “cool loop” models to the observed
emission.
---------------------------------------------------------
Title: On the Formation of the Resonance Lines of Helium in the Sun
Authors: Pietarila, Anna; Judge, Philip G.
2004ApJ...606.1239P Altcode:
To investigate the cause of anomalously bright resonance lines of
helium in the Sun, we have studied the magnitude of the enhancements and
some proposals made to explain them. Calculations in new semiempirical
solar models indicate that the resonance lines of helium are enhanced
by factors of 2-5 for He I and between 2 and 9 for He II, depending on
the elemental abundances assumed. These enhancements are substantially
less than earlier work has suggested, with the differences arising
from radiative transfer not only in helium lines but also in lines of
less abundant elements. Photon scattering, even with small line center
optical depths (of order unity or less) throughout the transition
region, is shown to significantly modify line intensities and their
center-to-limb variation. This effect has important consequences
both for our analysis and also for the analysis of solar EUV lines in
general, including emission measure analyses. We have re-examined some
proposals to explain the enhancements based on the “ionizing plasma”
picture. The proposals include explicit heating (the “burst” picture)
and heating via advection (diffusive and/or flowing models and the
“velocity redistribution” [VR] proposal). We argue that the original
VR mechanism must be modified to include kinetic effects for helium
atoms, which reduce the effects of VR. The VR mechanism also naturally
predicts helium lines that are blue-shifted relative to lines less
sensitive to VR, which contradicts observations for the He I 584 Å
line. Ionizing plasma models also potentially lead to enhancement of
Δn>=1 transitions (n is the principal quantum number) in other
atoms and ions. Existing observational evidence for enhanced Δn>=1
transitions in other ions is weak. We conclude that the ionizing plasma
scenarios are not the sole cause of the helium enhancement. Instead,
in a companion paper, we propose that the thermodynamic properties
of noble gases in the upper chromosphere will lead to enhancement of
their spectral lines if cross-field diffusion into coronal plasma is
important. Finally, we show that, surprisingly, multifluid flows of
the kind computed by Fontenla and colleagues can reproduce observed
intensities of several lines of C and Si and may contain a resolution
to the helium enhancement problem.
---------------------------------------------------------
Title: He I 584 A Line in the Solar Spectrum
Authors: Pietarila, A.
2002PhDT.........2P Altcode:
This thesis tries to look into the formation of the neutral helium
resonance line in the solar EUV spectrum. A time dependent approach
is taken when dealing with the helium emission. In the beginning,
the idea was to analyze some observations and put helium into context
with other lines. In the end, it proved to be a difficult task, and
the scope of this thesis broadened. First a brief review of the Sun
and its atmosphere is given in chapter 1. The existing observational
facts and models for the helium line formation will are discussed in
chapters 2 and 3. Chapter 4 focuses on time series observations done
with the SUMER instrument. In chapter 5 two of the models for helium
line formation reviewed earlier are explored. Finally, chapter 6 draws
conclusions from the observations and calculations.
