Author name code: stenflo
ADS astronomy entries on 2022-09-14
author:"Stenflo, Jan Olof"
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Title: Cosmological constant caused by observer-induced boundary
condition
Authors: Stenflo, Jan O.
Bibcode: 2020JPhCo...4j5001S
Altcode: 2020arXiv201007743S
The evolution of the wave function in quantum mechanics is deterministic
like that of classical waves. Only when we bring in observers
the fundamentally different quantum reality emerges. Similarly the
introduction of observers changes the nature of spacetime by causing
a split between past and future, concepts that are not well defined
in the observer-free world. The induced temporal boundary leads to
a resonance condition for the oscillatory vacuum solutions of the
metric in Euclidean time. It corresponds to an exponential de Sitter
evolution in real time, which can be represented by a cosmological
constant ${\rm{\Lambda }}=2{\pi }^{2}/{r}_{u}^{2}$ , where ru
is the radius of the particle horizon at the epoch when the observer
exists. For the present epoch we get a value of Λ that agrees with the
observed value within 2σ of the observational errors. This explanation
resolves the cosmic coincidence problem. Our epoch in cosmic history
does not herald the onset of an inflationary phase driven by some dark
energy. We show that the observed accelerated expansion that is deduced
from the redshifts is an 'edge effect' due to the observer-induced
boundary and not representative of the intrinsic evolution. The new
theory satisfies the BBN (Big Bang nucleosynthesis) and CMB (cosmic
microwave background) observational constraints equally well as the
concordance model of standard cosmology. There is no link between the
dark energy and dark matter problems. Previous conclusions that dark
matter is mainly non-baryonic are not affected.
Title: Importance of Angle-dependent Partial Frequency Redistribution
in Hyperfine Structure Transitions Under the Incomplete Paschen-Back
Effect Regime
Authors: Nagendra, K. N.; Sowmya, K.; Sampoorna, M.; Stenflo, J. O.;
Anusha, L. S.
Bibcode: 2020ApJ...898...49N
Altcode: 2020arXiv200704044N
Angle-frequency coupling in scattering of polarized light on atoms is
represented by the angle-dependent (AD) partial frequency redistribution
(PRD) matrices. There are several lines in the linearly polarized
solar spectrum, for which PRD combined with quantum interference
between hyperfine structure states play a significant role. Here we
present the solution of the polarized line transfer equation including
the AD-PRD matrix for scattering on a two-level atom with hyperfine
structure splitting and an unpolarized lower level. We account for
the effects of arbitrary magnetic fields (including the incomplete
Paschen-Back effect regime) and elastic collisions. For exploratory
purposes we consider a self-emitting isothermal planar atmosphere and
use atomic parameters that represent an isolated Na I D2
line. For this case we show that the AD-PRD effects are significant
for field strengths below about 30 G, but that the computationally
much less demanding approximation of angle-averaged PRD may be used
for stronger fields.
Title: Polarized Line Formation in Arbitrary Strength Magnetic Fields:
The Case of a Two-level Atom with Hyperfine Structure Splitting
Authors: Sampoorna, M.; Nagendra, K. N.; Sowmya, K.; Stenflo, J. O.;
Anusha, L. S.
Bibcode: 2019ApJ...883..188S
Altcode: 2019arXiv191010913S
Quantum interference effects, together with partial frequency
redistribution (PFR) in line scattering, produce subtle signatures in
the so-called Second Solar Spectrum (the linearly polarized spectrum of
the Sun). These signatures are modified in the presence of arbitrary
strength magnetic fields via the Hanle, Zeeman, and Paschen-Back
effects. In the present paper we solve the problem of polarized line
formation in a magnetized atmosphere taking into account scattering in a
two-level atom with hyperfine structure splitting together with PFR. To
this end we incorporate the collisionless PFR matrix derived in Sowmya
et al. in the polarized transfer equation. We apply the scattering
expansion method to solve this transfer equation. We study the combined
effects of PFR and the Paschen-Back effect on polarized line profiles
formed in an isothermal one-dimensional planar atmosphere. For this
purpose, we consider the cases of D2 lines of Li I and Na I.
Title: Origin of the cosmological constant
Authors: Stenflo, J. O.
Bibcode: 2019Ap&SS.364..143S
Altcode: 2019arXiv190901731S
The observed value of the cosmological constant corresponds to a
time scale that is very close to the current conformal age of the
universe. Here we show that this is not a coincidence but is caused by
a periodic boundary condition, which only manifests itself when the
metric is represented in Euclidian spacetime. The circular property
of the metric in Euclidian spacetime becomes an exponential evolution
(de Sitter or Λ term) in ordinary spacetime. The value of Λ then
gets uniquely linked to the period in Euclidian conformal time, which
corresponds to the conformal age of the universe. Without the use of
any free model parameters we predict the value of the dimensionless
parameter Ω_{Λ} to be 67.2%, which is within 2σ of the value derived
from CMB observations.
Title: Polarized Line Transfer in the Incomplete Paschen-Back Effect
Regime with Angle-dependent Partial Frequency Redistribution.
Authors: Nagendra, K. N.; Sowmya, K.; Sampoorna, M.; Stenflo, J. O.;
Anusha, L. S.
Bibcode: 2019spw..confE..13N
Altcode:
No abstract at ADS
Title: Stokes vectors and Minkowski spacetime: Structural parallels.
Authors: Stenflo, J. O.
Bibcode: 2019spw..confE..15S
Altcode: 2019arXiv191208614S
The Stokes formalism of polarization physics has astounding structural
parallels with the formalism used for relativity theory in Minkowski
spacetime. The structure and symmetry properties of the Mueller
matrices are the same as those for the matrix representations of the
electromagnetic tensor and the Lorentz transformation operator. The
absorption terms $\eta_k$ in the Mueller matrix correspond to the
electric field components $E_k$ in the electromagnetic tensor and the
Lorentz boost terms $\gamma_k$ in the Lorentz transformation matrix,
while the anomalous dispersion terms $\rho_k$ correspond to the magnetic
field components $B_k$ and the spatial rotation angles $\phi_k$. In
a Minkowski-type space spanned by the Stokes $I,Q,U,V$ parameters,
the Stokes vector for 100 % polarized light is a null vector living on
the surface of null cones, like the energy-momentum vector of massless
particles in ordinary Minkowski space. Stokes vectors for partially
polarized light live inside the null cones like the momentum vectors
for massive particles. In this description the depolarization of Stokes
vectors appears as a "mass'' term, which has its origin in a symmetry
breaking caused by the incoherent superposition of uncorrelated fields
or wave packets, without the need to refer to a ubiquitous Higgs field
as is done in particle physics. The rotational symmetry of Stokes
vectors and Mueller matrices is that of spin-2 objects, in contrast
to the spin-1 nature of the electromagnetic field. The reason for
this difference is that the Stokes objects have substructure: they
are formed from bilinear tensor products between spin-1 objects, the
Jones vectors and Jones matrices. The governing physics takes place
at the substructure level.
Title: Effects of Angle-Dependent Partial Frequency Redistribution
on Polarized Line Profiles
Authors: Sampoorna, M.; Nagendra, K. N.; Frisch, H.; Stenflo, J. O.
Bibcode: 2019ASPC..519..109S
Altcode:
Scattering of the solar limb-darkened radiation field on atoms and
molecules produces linearly polarized spectrum of the Sun (Second Solar
Spectrum). Partial frequency redistribution (PFR) plays a fundamental
role in shaping the wings of linearly polarized profiles of strong
resonance lines. Here we present the effects of the angle-dependent
(AD) PFR on resonance polarization both in the presence and absence
of magnetic fields. We consider scattering on a two-level atom with
unpolarized lower level, and a one-dimensional isothermal atmosphere.
Title: Polarized Line Formation with Incomplete Paschen-Back Effect
and Partial Frequency Redistribution
Authors: Sampoorna, M.; Nagendra, K. N.; Sowmya, K.; Stenflo, J. O.;
Anusha, L. S.
Bibcode: 2019ASPC..519..113S
Altcode:
Quantum interference between the hyperfine structure states is
known to depolarize the cores of some of the lines in the linearly
polarized spectrum of the Sun (the Second Solar Spectrum). The
presence of external magnetic fields in the line forming regions
modify these signatures through the Hanle, Zeeman, and incomplete/
complete Paschen-Back effects (PBE), depending on the strength of the
magnetic field. In an earlier paper, Sowmya et al. (2014) derived the
relevant collisionless partial frequency redistribution (PFR) matrix
for scattering on a two-level atom with hyperfine structure splitting
(HFS) and in the presence of arbitrary strength magnetic fields
(including the PBE regime). In the present paper we solve the problem
of polarized line transfer in a magnetized atmosphere, including this
PFR matrix. For this purpose, we apply a scattering expansion method
which is based on orders of scattering approach. We present the results
on the combined effects of PBE and PFR on the polarized line profiles
using the atomic parameters relevant to the Na I D2 line.
Title: Summary Talk: Looking Ahead
Authors: Stenflo, J. O.
Bibcode: 2019ASPC..526..369S
Altcode:
Wrapping up this highly successful and pleasant SPW8 meeting,
we first look back briefly over the four decades of the eminent
career of Egidio Landi Degl'Innocenti and see how the subject area of
"Solar Polarization" has evolved to give rise to a vibrant scientific
community. We then look ahead and identify two new directions, which
have been seriously neglected in the past but are judged to have great
future potential: (1) Laboratory experiments for polarized scattering,
to guide the further development and understanding of the theoretical
foundations, and (2) spectro-polarimetry from space below the visible
spectral range, to explore the role of the magnetic fields for the
physics of the outer atmosphere of the Sun.
Title: The Kramers-Heisenberg Coherency Matrix
Authors: Stenflo, J. O.
Bibcode: 2019ASPC..526...49S
Altcode:
Scattering of light is governed by the Kramers-Heisenberg formula,
which is an expression for the scattering probability amplitude. While
it provides a well established foundation for scattering theory,
its application to the derivation of observable quantities in the
case of multi-level atomic systems is not straightforward. One has
to sum over all the possible bilinear products of the scattering
amplitudes for all the combinations of sublevels in the ground state
and the excited state and then do ensemble averaging to construct the
coherency matrix that directly relates to observable quantities like the
Stokes parameters. Previous applications of density matrix theory to
radiative scattering have from the outset excluded valid interference
effects by doing ensemble averaging of the atomic system before the
scattering processes and thereby (in the absence of optical pumping)
prohibited the possibility of any phase relations between the initial
atomic states. However, the concept of partial polarization or of an
unpolarized state always refers to ensembles of individual quantum
entities (like photons or atoms). The ensemble is unpolarized if
its entities are uncorrelated, although each entity is always fully
polarized (i.e., contains definite phase relations). The averaging must
be done over the ensemble of Mueller matrices from the individual
scattering processes. The definite (but random) phase relations
between the initial ground states give non-zero contributions to
the ensemble average when a phase closure condition with the final
substates of the scattering process is satisfied. We show how the
resulting, previously overlooked interference terms, can be included
in a physically consistent way for any quantum system, and how these
new effects provide an explanation of the decade-long D1
enigma from laboratory scattering at potassium gas, at the same time
as explaining how a symmetric polarization peak can exist in the solar
line of sodium D1.
Title: Partial Frequency Redistribution Theory with Paschen-Back
Effect: Application to Li I 6708 Å Lines
Authors: Sowmya, K.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.
Bibcode: 2019ASPC..526...43S
Altcode:
The diagnostically important Li I D lines in the Second Solar Spectrum
are governed by the quantum interference processes that take place among
the magnetic substates belonging to different fine (J) and hyperfine (F)
structure states. This interference gets modified in the presence of
a magnetic field. The signatures of this interference in polarization
contain information on the nature of the vector magnetic field in
the solar atmosphere. With this motivation, we apply the polarized
redistribution matrix including Paschen-Back effect, derived based
on the Kramers-Heisenberg scattering matrix approach, to model the
polarization profiles of the Li lines observed in the quiet Sun. We
make use of the last scattering approximation which is based on the
concept that the polarization of the emergent radiation is generated
in the last scattering event, before the radiation escapes from the
atmosphere. We present a comparison of the quiet Sun observations of
the linear polarization profiles of Li I 6708 Å line system with the
theoretical profiles computed using our simple modeling approach. We
also present theoretical Stokes profiles in the Paschen-Back regime
of magnetic fields and compare them with the single scattered profiles.
Title: Effects of Lower-Level Polarization and Partial Frequency
Redistribution on Stokes Profiles
Authors: Supriya, H. D.; Nagendra, K. N.; Sampoorna, M.; Stenflo,
J. O.; Ravindra, B.
Bibcode: 2019ASPC..526...61S
Altcode:
The theory of polarized radiative transfer including the effects of
partial frequency redistribution (PRD) for a two-level and two-term atom
has been formulated in the scattering matrix approach. However there
exist several enigmatic features in the Second Solar Spectrum which
cannot be explained on the basis of said approach. The reason for this
lies in the approximations made in this approach. One such approximation
is the assumption that the lower level of the atom involved in the
scattering process is unpolarized. There are alternative approaches
based on the density matrix formalism to relax this assumption. It has
been shown that the inclusion of the polarization of all the atomic
levels involved in the scattering process is important. In our recent
studies, the collisionless redistribution matrix including the effects
of both PRD and lower-level polarization (LLP) was derived starting
from the Kramers-Heisenberg scattering formulation. We proposed a
simple numerical technique namely, the correction method, to solve
the problem of polarized radiative transfer with PRD and LLP. Here
we apply this technique to different atomic systems and discuss the
effects of PRD and LLP on the emergent Stokes profiles.
Title: Atlas of the Solar Intensity Spectrum and its Center-to-Limb
Variation
Authors: Ramelli, R.; Bianda, M.; Setzer, M.; Enegelhard, M.; Paglia,
F.; Stenflo, J. O.; Küveler, G.; Plewe, R.
Bibcode: 2019ASPC..526..287R
Altcode:
The solar limb darkening function is well known and is widely employed
in models of the solar atmosphere. However, there has been a lack
of systematic spectrally resolved measurements. Therefore we recently
decided to start an observing campaign with the Gregory-Coudé Telescope
at IRSOL in Locarno in order to produce a spectral atlas obtained at 10
different heliocentric angles θ, chosen so that μ = cos θ covers the
interval from 0.1 to 1.0 in step of 0.1. The measurements carried out
till now include the spectral range from 439 nm to 666 nm. The collected
data provide information about the anisotropy of the emergent radiation
field on the solar surface, allowing a better modeling of the Second
Solar Spectrum. In addition the data give observational constraints
that should be taken into account when modeling the solar atmosphere.
Title: Spatial variations of the Sr I 4607 Å scattering polarization
peak
Authors: Bianda, M.; Berdyugina, S.; Gisler, D.; Ramelli, R.; Belluzzi,
L.; Carlin, E. S.; Stenflo, J. O.; Berkefeld, T.
Bibcode: 2018A&A...614A..89B
Altcode: 2018arXiv180303531B
Context. The scattering polarization signal observed in the
photospheric Sr I 4607 Å line is expected to vary at granular
spatial scales. This variation can be due to changes in the magnetic
field intensity and orientation (Hanle effect), but also to spatial
and temporal variations in the plasma properties. Measuring the
spatial variation of such polarization signal would allow us to
study the properties of the magnetic fields at subgranular scales,
but observations are challenging since both high spatial resolution
and high spectropolarimetric sensitivity are required.
Aims:
We aim to provide observational evidence of the polarization peak
spatial variations, and to analyze the correlation they might have
with granulation.
Methods: Observations conjugating high spatial
resolution and high spectropolarimetric precision were performed with
the Zurich IMaging POLarimeter, ZIMPOL, at the GREGOR solar telescope,
taking advantage of the adaptive optics system and the newly installed
image derotator.
Results: Spatial variations of the scattering
polarization in the Sr I 4607 Å line are clearly observed. The
spatial scale of these variations is comparable with the granular
size. Small correlations between the polarization signal amplitude
and the continuum intensity indicate that the polarization is higher
at the center of granules than in the intergranular lanes.
Title: History of Solar Magnetic Fields Since George Ellery Hale
Authors: Stenflo, J. O.
Bibcode: 2018smf..book....5S
Altcode:
No abstract at ADS
Title: History of Solar Magnetic Fields Since George Ellery Hale
Authors: Stenflo, J. O.
Bibcode: 2017SSRv..210....5S
Altcode: 2015SSRv..tmp...83S; 2015arXiv150803312S
As my own work on the Sun's magnetic field started exactly 50 years ago
at Crimea in the USSR, I have been a participant in the field during
nearly half the time span since Hale's discovery in 1908 of magnetic
fields in sunspots. The present historical account is accompanied
by photos from my personal slide collection, which show a number of
the leading personalities who advanced the field in different areas:
measurement techniques, from photographic to photoelectric and imaging
methods in spectro-polarimetry; theoretical foundations of MHD and the
origin of cosmic magnetic fields (birth of dynamo theory); the quest for
increased angular resolution from national projects to international
consortia (for instruments both on ground and in space); introduction
of the Hanle effect in astrophysics and the Second Solar Spectrum as its
playground; small-scale nature of the field, the fundamental resolution
limit, and transcending it by resolution-independent diagnostics.
Title: Polarized Line Formation in Arbitrary Strength Magnetic Fields
Angle-averaged and Angle-dependent Partial Frequency Redistribution
Authors: Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.
Bibcode: 2017ApJ...844...97S
Altcode:
Magnetic fields in the solar atmosphere leave their fingerprints in the
polarized spectrum of the Sun via the Hanle and Zeeman effects. While
the Hanle and Zeeman effects dominate, respectively, in the weak
and strong field regimes, both these effects jointly operate in the
intermediate field strength regime. Therefore, it is necessary to
solve the polarized line transfer equation, including the combined
influence of Hanle and Zeeman effects. Furthermore, it is required
to take into account the effects of partial frequency redistribution
(PRD) in scattering when dealing with strong chromospheric lines with
broad damping wings. In this paper, we present a numerical method to
solve the problem of polarized PRD line formation in magnetic fields
of arbitrary strength and orientation. This numerical method is based
on the concept of operator perturbation. For our studies, we consider
a two-level atom model without hyperfine structure and lower-level
polarization. We compare the PRD idealization of angle-averaged
Hanle-Zeeman redistribution matrices with the full treatment of
angle-dependent PRD, to indicate when the idealized treatment is
inadequate and what kind of polarization effects are specific to
angle-dependent PRD. Because the angle-dependent treatment is presently
computationally prohibitive when applied to realistic model atmospheres,
we present the computed emergent Stokes profiles for a range of magnetic
fields, with the assumption of an isothermal one-dimensional medium.
Title: Atlas of the solar intensity spectrum and its center to
limb variation
Authors: Ramelli, R.; Setzer, M.; Engelhard, M.; Bianda, M.; Paglia,
F.; Stenflo, J. O.; Küveler, G.; Plewe, R.
Bibcode: 2017arXiv170803284R
Altcode:
The solar limb darkening function is well known and is widely
employed in models of the solar atmosphere. However, there has been
a lack of systematic spectrally resolved measurements. Therefore
we recently decided to start an observing campaign with the Gregory
Coudé Telescope at IRSOL in Locarno in order to produce a spectral
atlas obtained at 10 different heliocentric angles $\theta$, chosen so
that $\mu = \cos\theta$ covers the interval from 0.1 to 1.0 in step of
0.1. The measurements carried out till now include the spectral range
from 439 nm to 666 nm. The collected data provide information about
the anisotropy of the emergent radiation field on the solar surface,
allowing a better modeling of the Second Solar Spectrum. In addition
the data give observational constraints that should be taken into
account when modeling the solar atmosphere.
Title: Transition of the Sunspot Number from Zurich to Brussels in
1980: A Personal Perspective
Authors: Stenflo, J. O.
Bibcode: 2016SoPh..291.2487S
Altcode: 2016SoPh..tmp....5S; 2015arXiv151206229S
The Swiss Federal Observatory, which had been founded in 1863 by
Rudolf Wolf, was dissolved in connection with the retirement of Max
Waldmeier in 1979. The determination of the Zurich sunpot number,
which had been a cornerstone activity of the observatory, was then
discontinued by ETH Zurich. A smooth transition of the responsibility
for the sunspot number from Zurich to Brussels was achieved in 1980,
however, through which it was possible to avoid a discontinuity in this
important time series. Here we describe the circumstances that led to
the termination in Zurich, how Brussels was chosen for the succession,
and how the transfer was accomplished.
Title: Polarized Line Formation with Lower-level Polarization and
Partial Frequency Redistribution
Authors: Supriya, H. D.; Sampoorna, M.; Nagendra, K. N.; Stenflo,
J. O.; Ravindra, B.
Bibcode: 2016ApJ...828...84S
Altcode:
In the well-established theories of polarized line formation with
partial frequency redistribution (PRD) for a two-level and two-term
atom, it is generally assumed that the lower level of the scattering
transition is unpolarized. However, the existence of unexplained
spectral features in some lines of the Second Solar Spectrum points
toward a need to relax this assumption. There exists a density matrix
theory that accounts for the polarization of all the atomic levels,
but it is based on the flat-spectrum approximation (corresponding to
complete frequency redistribution). In the present paper we propose a
numerical algorithm to solve the problem of polarized line formation
in magnetized media, which includes both the effects of PRD and the
lower level polarization (LLP) for a two-level atom. First we derive a
collisionless redistribution matrix that includes the combined effects
of the PRD and the LLP. We then solve the relevant transfer equation
using a two-stage approach. For illustration purposes, we consider
two case studies in the non-magnetic regime, namely, the J
a = 1, J b = 0 and J a = J b
= 1, where J a and J b represent the
total angular momentum quantum numbers of the lower and upper states,
respectively. Our studies show that the effects of LLP are significant
only in the line core. This leads us to propose a simplified numerical
approach to solve the concerned radiative transfer problem.
Title: The Sun's spectrally resolved center-to-limb variation
Authors: Stenflo, Jan Olof
Bibcode: 2016SPD....47.1208S
Altcode:
The center-to-limb variation (CLV) of the Sun's continuous spectrum is
well known and has served as a major observational constraint on models
of the solar atmosphere. The CLV however also varies dramatically
with wavelength inside each spectral line. Here we report on two
new atlases that show the properties of the CLV with high spectral
resolution. One is a fully resolved spectral atlas from 4084 to 9950
Å of the ratio between the near limb spectrum, at 10 arcsec inside
the limb, and the disk center spectrum, both recorded with the FTS
at NSO/Kitt Peak. The other atlas gives the same kind of information
but covers the whole range of limb distances by giving the ratio
spectra for the nine μ positions 0.1, 0.2, …, 0.9. This set of
nine atlases for different μ have been recorded over the last couple
of years with the solar facility at IRSOL (Istituto Ricerche Solari
Locarno) in Switzerland. We find that the CLV is spectrally as richly
structured as the ordinary intensity spectrum, but the structuring is
different and contains diagnostic information that is not contained
in the intensity spectrum. Here we illustrate the properties of the
new spectral structures and discuss what they mean.
Title: Polarized Scattering of Light for Arbitrary Magnetic Fields
with Level-crossings from the Combination of Hyperfine and Fine
Structure Splittings
Authors: Sowmya, K.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.
Bibcode: 2015ApJ...814..127S
Altcode: 2015arXiv151207736S
Interference between magnetic substates of the hyperfine structure
states belonging to different fine structure states of the same term
influences the polarization for some of the diagnostically important
lines of the Sun's spectrum, like the sodium and lithium doublets. The
polarization signatures of this combined interference contain
information on the properties of the solar magnetic fields. Motivated
by this, in the present paper, we study the problem of polarized
scattering on a two-term atom with hyperfine structure by accounting
for the partial redistribution in the photon frequencies arising due
to the Doppler motions of the atoms. We consider the scattering atoms
to be under the influence of a magnetic field of arbitrary strength
and develop a formalism based on the Kramers-Heisenberg approach to
calculate the scattering cross section for this process. We explore the
rich polarization effects that arise from various level-crossings in
the Paschen-Back regime in a single scattering case using the lithium
atomic system as a concrete example that is relevant to the Sun.
Title: Modeling the center-to-limb variation of the Ca i 4227 Å
line using FCHHT models
Authors: Supriya, H. D.; Smitha, H. N.; Nagendra, K. N.; Stenflo,
J. O.; Bianda, M.; Ravindra, B.; Ramelli, R.; Anusha, L. S.
Bibcode: 2015IAUS..305..381S
Altcode:
The Ca i 4227 Å is a chromospheric line exhibiting the largest degree
of linear polarization near the limb, in the visible spectrum of the
Sun. Modeling the observations of the center-to-limb variations (CLV)
of different lines in the Second Solar Spectrum helps to sample the
height dependence of the magnetic field, as the observations made
at different lines of sight sample different heights in the solar
atmosphere. Supriya et al. (2014) attempted to simultaneously model
the CLV of the (I, Q/I) spectra of the Ca i 4227 Å line using the
standard 1-D FAL model atmospheres. They found that the standard FAL
model atmospheres and also any appropriate combination of them, fail
to simultaneously fit the observed Stokes (I, Q/I) profiles at all the
limb distances (μ) satisfying at the same time all the observational
constraints. This failure of 1-D modeling approach can probably be
overcome by using multi-dimensional modeling which is computationally
expensive. To eliminate an even wider choice of 1-D models, we attempt
here to simultaneously model the CLV of the (I, Q/I) spectra using the
FCHHT solar model atmospheres which are updated and recent versions
of the FAL models. The details of our modeling efforts and the results
are presented.
Title: Paschen-Back effect involving atomic fine and hyperfine
structure states
Authors: Sowmya, K.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.
Bibcode: 2015IAUS..305..154S
Altcode:
The linear polarization in spectral lines produced by coherent
scattering is significantly modified by the quantum interference between
the atomic states in the presence of a magnetic field. When magnetic
fields produce a splitting which is of the order of or greater than the
fine or hyperfine structure splittings, we enter the Paschen-Back effect
(PBE) regime, in which the magnetic field dependence of the Zeeman
splittings and transition amplitudes becomes non-linear. In general,
PBE occurs for sufficiently strong fields when the fine structure
states are involved and for weak fields in the case of hyperfine
structure states. In this work, we apply the recently developed theory
of PBE in the atomic fine and hyperfine structure states including the
effects of partial frequency redistribution to the case of Li i 6708
Å doublet. We explore the signatures of PBE in a single scattering
event and their applicability to the solar magnetic field diagnostics.
Title: Coherence structure of D1 scattering
Authors: Stenflo, Jan Olof
Bibcode: 2015IAUS..305..136S
Altcode:
The extensive literature on the physics of polarized scattering may
give the impression that we have a solid theoretical foundation for
the interpretation of spectro-polarimetric data. This theoretical
framework has however not been sufficiently tested by experiments
under controlled conditions. While the solar atmosphere may be viewed
as a physics laboratory, the observed solar polarization depends
on too many environmental factors that are beyond our control. The
existence of a symmetric polarization peak at the center of the solar
Na D1 line has remained an enigma for two decades, in spite
of persistent efforts to explain it with available quantum theory. A
decade ago a laboratory experiment was set up to determine whether this
was a problem for solar physics or quantum physics. The experiment
revealed a rich polarization structure of D1 scattering,
although available quantum theory predicted null results. It has now
finally been possible to formulate a well-defined and self-consistent
extension of the theory of quantum scattering that can reproduce in
great quantitative detail the main polarization structures that were
found in the laboratory experiment. Here we give a brief overview of
the new physical ingredients that were missing before. The extended
theory reveals that multi-level atomic systems have a far richer
coherence structure than previously believed.
Title: A revisit to model the Cr i triplet at 5204-5208 Å and the
Ba ii D2 line at 4554 Å in the Second Solar Spectrum
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.;
Sampoorna, M.; Ramelli, R.
Bibcode: 2015IAUS..305..372S
Altcode:
In our previous attempt to model the Stokes profiles of the Cr i triplet
at 5204-5208 Å and the Ba ii D2 at 4554 Å, we found it
necessary to slightly modify the standard FAL model atmospheres to fit
the observed polarization profiles. In the case of Cr i triplet, this
modification was done to reduce the theoretical continuum polarization,
and in the case of Ba ii D2, it was needed to reproduce the
central peak in Q/I. In this work, we revisit both these cases using
different standard model atmospheres whose temperature structures
closely resemble those of the modified FAL models, and explore the
possibility of synthesizing the line profiles without the need for
small modifications of the model atmosphere.
Title: Synoptic program to measure the evolution of the photospheric
magnetic field during a solar cycle
Authors: Ramelli, Renzo; Bianda, Michele; Berdyugina, Svetlana;
Stenflo, Jan Olof; Belluzzi, Luca
Bibcode: 2015IAUGA..2257074R
Altcode:
The solar photosphere is seething with a vast amount of magnetic flux
tangled on scales much smaller than the resolution scale of solar
telescopes that can be investigated by considering the Hanle effect. In
2007, near a minimum of the solar cycle, we started a synoptic program
to explore possible variations of such hidden magnetic flux with
the solar cycle, through the application of a differential Hanle
effect technique on observations of scattering polarization in C2
molecular lines in the region around 514.0 nm. The observing program
is still ongoing generally with the cadence of about one month. The
observations obtained up to now, which include the recent maximum
of the solar activity, don't show large variations of the turbulent
unresolved magnetic field. If the apparent constancy is confirmed
through the current cycle, than it will have important implications,
since it provides hints on the existence of a local dynamo effect at
granular and sub-granular scale, uncorrelated with the global magnetic
field varying with the solar cycle.
Title: Atlas of the center to limb variation of the solar intensity
spectrum.
Authors: Ramelli, Renzo; Setzer, Martin; Engelhard, Mathis; Bianda,
Michele; Stenflo, Jan Olof; Küveler, Gerd; Plewe, Rouven
Bibcode: 2015IAUGA..2256989R
Altcode:
The limb darkening function of the Sun in the continuous spectrum
is well known and is largely used in the modeling of the solar
atmosphere. There has however been a lack of systematic spectrally
resolved measurements of the center to limb variation in light
intensity. With the Gregory Coudé Telescope at IRSOL in Locarno we have
recently carried out an observing campaign with systematic measurements
in order to produce a spectral atlas showing the ratio between the
light intensity obtained at 9 different heliocentric angles θ and the
light intensity measured at the solar disc center. The 9 different
heliocentric angles have been chosen so that μ = cos θ covers the
interval from 0.1 to 0.9 in step of 0.1. The measurements collected
till now cover the spectral range from 439 nm to 638 nm. The obtained
data can be used to better determine the anisotropy of the emergent
radiation field on the solar surface, allowing a better modeling of
the the so called Second Solar Spectrum that describes the linear
polarization as a function of wavelength measured near the solar limb,
which is produced by scattering in the solar atmosphere. Furthermore
the data provide observational constraints that can be used to test
the validity of the solar atmosphere models.
Title: The Sun
Authors: Stenflo, Jan O.
Bibcode: 2015psps.book..267S
Altcode:
No abstract at ADS
Title: Physics of Polarized Scattering at Multi-level Atomic Systems
Authors: Stenflo, J. O.
Bibcode: 2015ApJ...801...70S
Altcode: 2015arXiv150102259S
The symmetric peak observed in linear polarization in the core of the
solar sodium D1 line at 5896 Å has remained enigmatic
since its discovery nearly two decades ago. One reason is that the
theory of polarized scattering has not been experimentally tested
for multi-level atomic systems in the relevant parameter domains,
although the theory is continually being used for the interpretation of
astrophysical observations. A laboratory experiment that was set up a
decade ago to find out whether the D1 enigma is a problem
of solar physics or quantum physics revealed that the D1
system has a rich polarization structure in situations where standard
scattering theory predicts zero polarization, even when optical
pumping of the m state populations of the hyperfine-split ground
state is accounted for. Here we show that the laboratory results can
be modeled in great quantitative detail if the theory is extended to
include the coherences in both the initial and final states of the
scattering process. Radiative couplings between the allowed dipole
transitions generate coherences in the initial state. Corresponding
coherences in the final state are then demanded by a phase closure
selection rule. The experimental results for the well understood
D2 line are used to constrain the two free parameters of
the experiment, collision rate and optical depth, to suppress the need
for free parameters when fitting the D1 results.
Title: FTS atlas of the Sun's spectrally resolved center-to-limb
variation
Authors: Stenflo, J. O.
Bibcode: 2015A&A...573A..74S
Altcode: 2014arXiv1410.8474S
The Sun's spectrum varies with center-to-limb distance, which is usually
parameterized by μ = cosθ, where θ is the heliocentric angle. This
variation is governed by the underlying temperature-density structure
of the solar atmosphere. While the center-to-limb variation (CLV)
of the continuous spectrum is well known and has been widely used
for atmospheric modeling, there has been no systematic exploration
of the spectrally resolved CLV. Here we make use of two spectral
atlases recorded with the Fourier transform spectrometer (FTS) at the
McMath-Pierce facility at Kitt Peak. One spectral atlas obtained 10
arcsec inside the solar limb was recorded in 1978-79 as part of the
first survey of the Second Solar Spectrum, while the other atlas is the
well used reference NSO/Kitt Peak FTS atlas for the disk center. Both
atlases represent fully resolved spectra without any spectral stray
light. We then construct an atlas of the limb/disk-center ratio
between the two spectra over the wavelength range 4084-9950 Å. This
ratio spectrum, which expresses the CLV amplitude relative to the
continuum, is as richly structured as the intensity spectrum itself,
but the line profiles differ greatly in both shape and amplitude. It
is as if we are dealing with a new, unfamiliar spectrum of the Sun,
distinctly different from both the intensity spectrum (which we here
refer to with the acronym SS1) and the linear polarization of the Second
Solar Spectrum (for which we use acronym SS2). In analogy we refer to
the new ratio spectrum as SS3. While there is hardly any resemblance
between SS3 and SS2, we are able to identify a non-linear mapping
that can translate SS1 to SS3 in the case of weak to medium-strong
spectral lines that are mainly formed in LTE (being directly coupled
to the local temperature-density structure). This non-linear mapping
is successfully modeled in terms of two free parameters that are
found to vary approximately linearly over the entire wavelength range
covered. These parameters and the various SS3 line profiles provide
a novel, rich set of observational constraints, which may be used to
test the validity of model atmospheres or guide the construction of
improved models.
Title: Nature of Quiet-Sun Magnetic Fields
Authors: Stenflo, J. O.
Bibcode: 2014ASPC..489....3S
Altcode:
Since the magnetic structuring continues to scales of order 10-100 m,
far smaller than can possibly be resolved, and since the polarization
signals are weak on the quiet Sun, one needs to apply robust diagnostic
techniques that do not get biased by measurement noise, are independent
of telescope resolution, and have minimal model dependence, in order
to reliably determine the intrinsic properties of quiet-sun magnetic
fields. Such techniques make use of ensemble averages as the observable
signatures of the spatially unresolved domain. Here we show how such
concepts are applied to derive the field strengths, sizes, and angular
distributions from the observed Hanle depolarization and from the
symmetry properties of the transverse Zeeman effect.
Title: Conversion of the 6302 / 6301 Stokes V Line Ratio to the 5250 /
5247 Ratio for the Diagnostics of Quiet-Sun Magnetic Fields
Authors: Demidov, M. L.; Stenflo, J. O.; Bianda, M.; Ramelli, R.
Bibcode: 2014ASPC..489...21D
Altcode:
Observations in the "green" spectral lines Fe I 5247.06 and 5250.22 Å
and in the "red" lines Fe I 6301.50 and 6302.50 Å are widely used to
explore the properties of solar magnetic fields. The green line pair
was introduced in 1973 as part of the line-ratio technique, which has
been a powerful tool for investigations of intrinsic magnetic field
properties at spatially unresolved scales (magnetic flux tubes with
kG strengths). The red line pair has recently played a major role for
magnetic-field diagnostics due to the large amount of high-quality
data provided by the Hinode space observatory. These red lines however
differ not only in the values of their Landé factors, but also in their
line-formation properties, with the consequence that the magnetic-field
information in their line ratio gets tangled up with thermodynamic
effects. In contrast, as the green Fe I 5247.06 and 5250.22 Å lines
differ only in their Landé factors, the magnetic field effects become
cleanly separated from the thermodynamics, which allows the intrinsic
magnetic field parameters to be extracted without ambiguties. The red
and green line-ratio values are however statistically correlated. By
determining the statistical regression function that relates them,
it becomes possible to convert the "contaminated" and ambiguous red
line ratio into the green line ratio, with which a reliable direct
interpretation in terms of intrinsic field strengths is possible. To
determine how the two line ratios are related we have made Stokesmeter
observations in these four spectral lines with two solar instruments
equipped with high-precision spectropolarimeters, ZIMPOL-3 at IRSOL
(Locarno, Switzerland), and the STOP telescope at the Sayan Solar
Observatory (Irkutsk, Russia). Most of the obtained results are based
on IRSOL observations of quiet-sun magnetic fields. In the case of STOP
the full-disk magnetograms of large-scale solar magnetic fields are
analyzed. A major advantage at IRSOL is the possibility to observe the
green and red line pairs simultaneously on the same CCD chip. We have
determined how the two line ratios decrease with increasing Stokes V
amplitude and increase with increasing distance from disk center. The
relation that allows us to convert the observed values for the red
line ratio into the corresponding values for the green line ratio has
been found and given in analytical form.
Title: Solar Polarization 7
Authors: Nagendra, K. N.; Stenflo, J. O.; Qu, Z. Q.; Sampoorna, M.
Bibcode: 2014ASPC..489.....N
Altcode:
No abstract at ADS
Title: The Role of Quantum Interference and Partial Redistribution
in the Solar Ba <font size=2>II D2 4554 Å Line
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M.
Bibcode: 2014ASPC..489..213S
Altcode: 2014arXiv1409.0465S
The Ba <font size=2>II D2 line at 4554 Å is a good
example, where the F-state interference effects due to the odd isotopes
produce polarization profiles, which are very different from those
of the even isotopes that do not exhibit F-state interference. It is
therefore necessary to account for the contributions from the different
isotopes to understand the observed linear polarization profiles of
this line. In this paper we present radiative transfer modeling with
partial frequency redistribution, which is shown to be essential to
model this line. This is because complete frequency redistribution
cannot reproduce the observed wing polarization. We present the observed
and computed Q/I profiles at different limb distances. The theoretical
profiles strongly depend on limb distance (μ) and the model atmosphere
which fits the limb observations fails at other μ positions.
Title: Solar Cycle Variations of the Second Solar Spectrum
Authors: Bianda, M.; Ramelli, R.; Gisler, D.; Stenflo, J. O.
Bibcode: 2014ASPC..489..167B
Altcode:
The average strength of the spatially unresolved turbulent magnetic
field in the Sun's photosphere can be measured with the Hanle
effect. The possible variations of this average value over time scales
of a solar cycle is the topic of an ongoing synoptic program at IRSOL
that was started in 2008. The scattering polarization of C2
molecular lines around 5140 Å is regularly measured, typically once
per month. These lines allow the application of the differential Hanle
effect to determine the turbulent field strength. Here we report about
the behavior of the determined field strength during the beginning
active phase of the solar cycle and about our intention to start a new
synoptic program based on the Hanle effect in the Sr I 4607 Å line,
with which turbulent field strengths are found that are an order of
magnitude larger than those determined with the molecular C2
lines. These synoptic programs allow us to explore the nature of the
magnetic fields at the small-scale end of the magnetic scale spectrum
and to determine the possible role of a local dynamo for the generation
of these fields.
Title: Polarized Light Scattering with the Paschen-Back Effect,
Level-crossing of Fine Structure States, and Partial Frequency
Redistribution
Authors: Sowmya, K.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.
Bibcode: 2014ApJ...793...71S
Altcode: 2014arXiv1407.5457S
The quantum interference between the fine structure states of an atom
modifies the shapes of the emergent Stokes profiles in the second solar
spectrum. This phenomenon has been studied in great detail both in the
presence and absence of magnetic fields. By assuming a flat-spectrum
for the incident radiation, the signatures of this effect have been
explored for arbitrary field strengths. Even though the theory which
takes into account the frequency dependence of the incident radiation
is well developed, it is restricted to the regime in which the magnetic
splitting is much smaller than the fine structure splitting. In the
present paper, we carry out a generalization of our scattering matrix
formalism including the effects of partial frequency redistribution
for arbitrary magnetic fields. We test the formalism using available
benchmarks for special cases. In particular, we apply it to the Li
I 6708 Å D1 and D2 line system, for which
observable effects from the Paschen-Back regime are expected in the
Sun's spectrum.
Title: The Quantum Interference Effects in the Sc II 4247 Å Line
of the Second Solar Spectrum
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.;
Ramelli, R.
Bibcode: 2014ApJ...794...30S
Altcode: 2014arXiv1408.4247S
The Sc II 4247 Å line formed in the chromosphere is one of the lines
well known, like the Na I D2 and Ba II D2,
for its prominent triple-peak structure in Q/I and the underlying
quantum interference effects governing it. In this paper, we try
to study the nature of this triple-peak structure using the theory
of F-state interference including the effects of partial frequency
redistribution (PRD) and radiative transfer (RT). We compare our
results with the observations taken in a quiet region near the
solar limb. In spite of accounting for PRD and RT effects, it has
not been possible to reproduce the observed triple-peak structure in
Q/I. While the two wing PRD peaks (on either side of central peak)
and the near wing continuum can be reproduced, the central peak is
completely suppressed by the enhanced depolarization resulting from
the hyperfine structure splitting. This suppression remains for all
the tested widely different one-dimensional model atmospheres or for
any multi-component combinations of them. While multidimensional RT
effects may improve the fit to the intensity profiles, they do not
appear capable of explaining the enigmatic central Q/I peak. This
leads us to suspect that some aspect of quantum physics is missing.
Title: Center-to-limb Observations and Modeling of the Ca I 4227
Å Line
Authors: Supriya, H. D.; Smitha, H. N.; Nagendra, K. N.; Stenflo,
J. O.; Bianda, M.; Ramelli, R.; Ravindra, B.; Anusha, L. S.
Bibcode: 2014ApJ...793...42S
Altcode: 2014arXiv1407.5461S
The observed center-to-limb variation (CLV) of the scattering
polarization in different lines of the Second Solar Spectrum can be used
to constrain the height variation of various atmospheric parameters, in
particular the magnetic fields, via the Hanle effect. Here we attempt
to model the nonmagnetic CLV observations of the Q/I profiles of the
Ca I 4227 Å line recorded with the Zurich Imaging Polarimeter-3
at IRSOL. For modeling, we use the polarized radiative transfer
with partial frequency redistribution with a number of realistic
one-dimensional (1D) model atmospheres. We find that all the standard
Fontenla-Avrett-Loeser (FAL) model atmospheres, which we used, fail
to simultaneously fit the observed (I, Q/I) at all the limb distances
(μ). However, an attempt is made to find a single model which can
provide a fit to at least the CLV of the observed Q/I instead of a
simultaneous fit to the (I, Q/I) at all μ. To this end we construct a
new 1D model by combining two of the standard models after modifying
their temperature structures in the appropriate height ranges. This
new combined model closely reproduces the observed Q/I at all μ but
fails to reproduce the observed rest intensity at different μ. Hence
we find that no single 1D model atmosphere succeeds in providing a
good representation of the real Sun. This failure of 1D models does
not, however, cause an impediment to the magnetic field diagnostic
potential of the Ca I 4227 Å line. To demonstrate this we deduce the
field strength at various μ positions without invoking the use of
radiative transfer.
Title: Polarized Scattering with Paschen-Back Effect, Hyperfine
Structure, and Partial Frequency Redistribution in Magnetized
Stellar Atmospheres
Authors: Sowmya, K.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M.
Bibcode: 2014ApJ...786..150S
Altcode: 2015arXiv151207731S
F-state interference significantly modifies the polarization
produced by scattering processes in the solar atmosphere. Its
signature in the emergent Stokes spectrum in the absence of magnetic
fields is depolarization in the line core. In the present paper,
we derive the partial frequency redistribution (PRD) matrix that
includes interference between the upper hyperfine structure states
of a two-level atom in the presence of magnetic fields of arbitrary
strengths. The theory is applied to the Na I D2 line that
is produced by the transition between the lower J = 1/2 and upper J =
3/2 states which split into F states because of the coupling with the
nuclear spin Is = 3/2. The properties of the PRD matrix for
the single-scattering case is explored, in particular, the effects of
the magnetic field in the Paschen-Back regime and their usefulness as
a tool for the diagnostics of solar magnetic fields.
Title: Asymmetric oscillatory expansion of a cylindrical plasma
Authors: Karimov, A. R.; Yu, M. Y.; Stenflo, L.; Stenflo
Bibcode: 2013JPlPh..79.1007K
Altcode:
Asymmetric oscillatory expansion of a cylindrical plasma layer into
vacuum is investigated analytically by solving the fluid equations of
the electrons and ions together with the Maxwell's equations. For the
problem considered, it is found that the asymmetrical flow components
are strongly affected by the symmetrical components, but not the
vice versa.
Title: Stimulated Brillouin scattering in magnetized plasmas
Authors: Brodin, G.; Stenflo, L.; Stenflo
Bibcode: 2013JPlPh..79..983B
Altcode:
Previous theory for stimulated Brillouin scattering is reconsidered
and generalized. We introduce an effective ion sound velocity that
turns out to be useful in describing scattering instabilities.
Title: Alfven wave interactions within the Hall-MHD description*
Authors: Brodin, G.; Stenflo, L.; Stenflo
Bibcode: 2013JPlPh..79..909B
Altcode:
We show that comparatively simple expressions for the Alfven
wave coupling coefficients can be deduced from the well-known
Hall-magnetohydrodynamics (MHD) model equations.
Title: Solar magnetic fields as revealed by Stokes polarimetry
Authors: Stenflo, J. O.
Bibcode: 2013A&ARv..21...66S
Altcode: 2013arXiv1309.5454S
Observational astrophysics started when spectroscopy could be applied
to astronomy. Similarly, observational work on stellar magnetic fields
became possible with the application of spectro-polarimetry. In recent
decades there have been dramatic advances in the observational tools for
spectro-polarimetry. The four Stokes parameters that provide a complete
representation of partially polarized light can now be simultaneously
imaged with megapixel array detectors with high polarimetric precision
(10-5 in the degree of polarization). This has led to new
insights about the nature and properties of the magnetic field, and has
helped pave the way for the use of the Hanle effect as a diagnostic
tool beside the Zeeman effect. The magnetic structuring continues
on scales orders of magnitudes smaller than the resolved ones, but
various types of spectro-polarimetric signatures can be identified,
which let us determine the field strengths and angular distributions of
the field vectors in the spatially unresolved domain. Here we review
the observational properties of the magnetic field, from the global
patterns to the smallest scales at the magnetic diffusion limit,
and relate them to the global and local dynamos.
Title: Calibration of the 6302/6301 Stokes V line ratio in terms of
the 5250/5247 ratio
Authors: Stenflo, J. O.; Demidov, M. L.; Bianda, M.; Ramelli, R.
Bibcode: 2013A&A...556A.113S
Altcode: 2013arXiv1307.1117S
Four decades ago the Stokes V line ratio in the Fe i 5247.06 and 5250.22
Å lines was introduced as a powerful means of exploring the intrinsic
field strengths at sub-pixel scales, which led to the discovery that
most of the photospheric flux is in intermittent kG form. The "green"
5247-5250 line pair is unique because it allows the magnetic-field
effects to be isolated from the thermodynamic effects. No other
line pair with this property has since been identified. In recent
years much of the magnetic-field diagnostics has been based on the
"red" Fe i 6301.5 and 6302.5 Å line pair, since it was chosen in the
design of the Hinode space observatory. Although thermodynamic effects
severely contaminate the magnetic-field signatures for this line ratio,
it is still possible to use it to extract information on intrinsic
magnetic fields, but only after it has been "renormalized", since
otherwise it produces fictitious, superstrong fields everywhere. In the
present work we explore the joint behavior of these two line ratios
to determine how the "contaminated" red line ratio can be translated
into the corresponding green line ratio, which then allows for a direct
interpretation in terms of intrinsic magnetic fields. Our observations
are mainly based on recordings with the ZIMPOL-3 spectro-polarimeter
at IRSOL in Locarno, Switzerland, complemented by data from the STOP
telescope at the Sayan solar observatory (Irkutsk, Russia). The IRSOL
observations are unique by allowing both the green and red line pairs
to be recorded simultaneously on the same CCD sensor. We show how
the line ratios depend on both the measured flux densities and on the
heliocentric distance (the μ value on the solar disk), and finally
derive the calibration function that enables the red line ratio to be
translated to the green ratio for each μ value.
Title: Nature of the solar dynamo at small scales
Authors: Stenflo, J. O.
Bibcode: 2013IAUS..294..119S
Altcode: 2012arXiv1211.5529S
It is often claimed that there is not only one, but two different types
of solar dynamos: the one that is responsible for the appearance of
sunspots and the 11-yr cycle, frequently referred to as the ``global
dynamo'', and a statistically time-invariant dynamo, generally referred
to as the ``local dynamo'', which is supposed to be responsible for
the ubiquitous magnetic structuring observed at small scales. Here
we examine the relative contributions of these two qualitatively
different dynamos to the small-scale magnetic flux, with the following
conclusion: The local dynamo does not play a significant role at any
of the spatially resolved scales, nearly all the small-scale flux,
including the flux revealed by Hinode, is supplied by the global
dynamo. This conclusion is reached by careful determination of the Sun's
noise-corrected basal magnetic flux density while making use of a flux
cancellation function determined from Hinode data. The only allowed
range where there may be substantial or even dominating contributions
from a local dynamo seems to be the scales below about 10 km, as
suggested by observations of the Hanle depolarization effect in atomic
spectral lines. To determine the fraction of the Hanle depolarization
that may be due to the action of a local dynamo, a synoptic program
is being initiated at IRSOL (Istituto Ricerche Solari Locarno).
Title: Horizontal or vertical magnetic fields on the quiet
Sun. Angular distributions and their height variations
Authors: Stenflo, J. O.
Bibcode: 2013A&A...555A.132S
Altcode: 2013arXiv1306.3581S
Different analyses of identical Hinode SOT/SP data of quiet-Sun
magnetic fields have in the past led to contradictory answers to
the question of whether the angular distribution of field vectors
is preferentially horizontal or vertical. These answers have been
obtained by combining the measured circular and linear polarizations
in different ways to derive the field inclinations. A problem with
these combinations is that the circular and linear polarizations
scale with field strength in profoundly different ways. Here, we
avoid these problems by using an entirely different approach that
is based exclusively on the fundamental symmetry properties of the
transverse Zeeman effect for observations away from the disk center
without any dependence on the circular polarization. Systematic errors
are suppressed by the application of a doubly differential technique
with the 5247-5250 Å line pair for observations with the ZIMPOL-2
imaging polarimeter on the French THEMIS telescope on Tenerife. For the
weakest, intranetwork-type magnetic fields, the angular distribution
changes sign with the center-to-limb distance, being preferentially
horizontal limbwards of μ (cosine of the heliocentric angle) = 0.2,
while favoring the vertical direction inside this disk position. Since
decreasing μ corresponds to increasing height of line formation, this
finding implies that the intranetwork fields are more peaked around the
vertical direction in the low to middle photosphere, while they are more
horizontal in the upper photosphere. The angular distribution is however
also found to become more vertical with increasing flux density. Thus,
all facular points that we have observed have a strong preference for
the vertical direction for all disk positions, including those all the
way to the extreme limb. In terms of spatial averages weighted by the
intrinsic magnetic energy density, these results are independent of
telescope resolution.
Title: Line-interlocking Effects on Polarization in Spectral Lines
by Rayleigh and Raman Scattering
Authors: Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.
Bibcode: 2013ApJ...770...92S
Altcode:
The polarized spectrum of the Sun and stars is formed from the
scattering of anisotropic radiation on atoms. Interpretation of
this spectrum requires the solution of polarized line transfer in
multilevel atomic systems. While sophisticated quantum theories of
polarized line formation in multilevel atomic systems exist, they are
limited by the approximation of complete frequency redistribution
in scattering. The partial frequency redistribution (PRD) in line
scattering is a necessary component in modeling the polarized spectra
of strong lines. The polarized PRD line scattering theories developed so
far confine themselves to a two-level or a two-term atom model. In this
paper, we present a heuristic approach to the problem of polarized line
formation in multilevel atoms taking into account the effects of PRD
and a weak magnetic field. Starting from the unpolarized PRD multilevel
atom approach of Hubeny et al., we incorporate the polarization state
of the radiation field. However, the lower level polarization is
neglected. Two iterative methods of solving the polarized PRD line
transfer in multilevel atoms are also presented. Taking the example
of a five-level Ca II atom model, we present illustrative results for
an isothermal one-dimensional model atmosphere.
Title: Modeling the Quantum Interference Signatures of the Ba II
D2 4554 Å Line in the Second Solar Spectrum
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Sampoorna, M.
Bibcode: 2013ApJ...768..163S
Altcode: 2013arXiv1303.7304S
Quantum interference effects play a vital role in shaping the linear
polarization profiles of solar spectral lines. The Ba II D2
line at 4554 Å is a prominent example, where the F-state interference
effects due to the odd isotopes produce polarization profiles,
which are very different from those of the even isotopes that have
no F-state interference. It is therefore necessary to account for the
contributions from the different isotopes to understand the observed
linear polarization profiles of this line. Here we do radiative
transfer modeling with partial frequency redistribution (PRD) of such
observations while accounting for the interference effects and isotope
composition. The Ba II D2 polarization profile is found
to be strongly governed by the PRD mechanism. We show how a full PRD
treatment succeeds in reproducing the observations, while complete
frequency redistribution alone fails to produce polarization profiles
that have any resemblance to the observed ones. However, we also find
that the line center polarization is sensitive to the temperature
structure of the model atmosphere. To obtain a good fit to the line
center peak of the observed Stokes Q/I profile, a small modification
of the FALX model atmosphere is needed, by lowering the temperature
in the line-forming layers. Because of the pronounced temperature
sensitivity of the Ba II D2 line it may not be a suitable
tool for Hanle magnetic-field diagnostics of the solar chromosphere,
because there is currently no straightforward way to separate the
temperature and magnetic-field effects from each other.
Title: Padma Kant Shukla 1950-2013
Authors: Bingham, Robert; Eliasson, Bengt; Mendonca, Tito; Stenflo,
Lennart; Stenflo
Bibcode: 2013JPlPh..79..119B
Altcode:
Professor Padma Kant Shukla passed away on the 26th of January in New
Delhi, India, just after receiving the prestigious Hind Rattan (Jewel
of India) Award. He was born in the village Tulapur, Uttar Pradesh
(UP), India and was educated there. After his Ph.D. in Physics from
Banaras Hindu University in Varanasi, he obtained his second doctorate
degree in Theoretical Plasma Physics from Umea University under the
supervision of one of us (Lennart Stenflo). He worked at the Faculty
of Physics & Astronomy, Ruhr-University Bochum, Germany since
January 1973, where he was a permanent faculty member and Professor of
International Affairs, a position that was created for him to honour
his international accomplishments and reputation.
Title: The project of installing a ZIMPOL_3 polarimeter at GREGOR
in Tenerife
Authors: Bianda, M.; Ramelli, R.; Stenflo, J.; Berdyugina, S.; Gisler,
D.; Defilippis, I.; Bello González, N.
Bibcode: 2013MmSAI..84..413B
Altcode:
A project of collaboration between Kiepenheuer Institut für
Sonnenphysik, KIS, and Istituto Ricerche Solari Locarno, IRSOL,
includes the installation of a ZIMPOL_3 high resolution polarimeter at
the 1.5 meter aperture solar telescope GREGOR in Tenerife. Important
scientific topics are expected to be investigated, in particular in the
case of events showing faint amplitude polarization signatures like
scattering polarization effects, and the Hanle effect. This project
has also a technical importance, this combination can be used as test
bench for future polarimeters to be installed on the new generation
solar telescopes.
Title: Polarized line formation with J-state interference in the
presence of magnetic fields: A Heuristic treatment of collisional
frequency redistribution
Authors: Smitha, H. N.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.
Bibcode: 2013JQSRT.115...46S
Altcode: 2012arXiv1209.0243S
An expression for the partial frequency redistribution (PRD) matrix
for line scattering in a two-term atom, which includes the J-state
interference between its fine structure line components is derived. The
influence of collisions (both elastic and inelastic) and an external
magnetic field on the scattering process is taken into account. The
lower term is assumed to be unpolarized and infinitely sharp. The
linear Zeeman regime in which the Zeeman splitting is much smaller than
the fine structure splitting is considered. The inelastic collision
rates between the different levels are included in our treatment. We
account for the depolarization caused by the collisions coupling the
fine structure states of the upper term, but neglect the polarization
transfer between the fine structure states. When the fine structure
splitting goes to zero, we recover the redistribution matrix that
represents the scattering on a two-level atom (which exhibits only
m-state interference—namely the Hanle effect). The way in which the
multipolar index of the scattering atom enters into the expression
for the redistribution matrix through the collisional branching
ratios is discussed. The properties of the redistribution matrix are
explored for a single scattering process for a L=0→1→0 scattering
transition with S=1/2 (a hypothetical doublet centered at 5000 Å and
5001 Å). Further, a method for solving the Hanle radiative transfer
equation for a two-term atom in the presence of collisions, PRD, and
J-state interference is developed. The Stokes profiles emerging from
an isothermal constant property medium are computed.
Title: Basal magnetic flux and the local solar dynamo
Authors: Stenflo, J. O.
Bibcode: 2012A&A...547A..93S
Altcode: 2012arXiv1210.0122S
The average unsigned magnetic flux density in magnetograms of the quiet
Sun is generally dominated by instrumental noise. Due to the entirely
different scaling behavior of the noise and the solar magnetic pattern
it has been possible to determine the standard deviation of the Gaussian
noise distribution and remove the noise contribution from the average
unsigned flux density for the whole 15-yr SOHO/MDI data set and for a
selection of SDO/HMI magnetograms. There is a very close correlation
between the MDI disk-averaged unsigned vertical flux density and the
sunspot number, and regression analysis gives a residual level of 2.7 G
when the sunspot number is zero. The selected set of HMI magnetograms,
which spans the most quiet phase of solar activity, has a lower limit
of 3.0 G to the noise-corrected average flux density. These apparently
cycle-independent levels may be identified as a basal flux density,
which represents an upper limit to the possible flux contribution from
a local dynamo, but not evidence for its existence. The 3.0 G HMI level,
when scaled to the Hinode spatial resolution, translates to 3.5 G, which
means that the much higher average flux densities always found by Hinode
in quiet regions do not originate from a local dynamo. The contributions
to the average unsigned flux density come almost exclusively from
the extended wings of the probability density function, also in the
case of HMI magnetograms with only basal-level magnetic flux. These
wings represent intermittent magnetic flux. As the global dynamo
continually feeds flux into the small scales at a fast rate through
turbulent shredding, a hypothetical local dynamo may only be relevant
to the Sun if its rate of flux build-up can be competitive. While the
global dynamo appears to dominate the magnetic energy spectrum at all
the resolved spatial scales, there are indications from the observed
Hanle depolarization in atomic lines that the local dynamo may dominate
the spectrum at scales of order 1-10 km and below.
Title: Polarized Line Transfer with F-state Interference in a
Non-magnetic Medium: Partial Frequency Redistribution Effects in
the Collisionless Regime
Authors: Smitha, H. N.; Sowmya, K.; Nagendra, K. N.; Sampoorna, M.;
Stenflo, J. O.
Bibcode: 2012ApJ...758..112S
Altcode: 2012arXiv1208.6369S
Quantum interference phenomena manifest themselves in several ways
in the polarized solar spectrum formed due to coherent scattering
processes. One such effect arises due to interference between the fine
structure (J) states giving rise to multiplets. Another effect is that
which arises due to interference between the hyperfine structure (F)
states. We extend the redistribution matrix derived for the J-state
interference to the case of F-state interference. We then incorporate
it into the polarized radiative transfer equation and solve it for
isothermal constant property slab atmospheres. The relevant transfer
equation is solved using a polarized approximate lambda iteration (PALI)
technique based on operator perturbation. An alternative method derived
from the Neumann series expansion is also proposed and is found to be
relatively more efficient than the PALI method. The effects of partial
frequency redistribution and the F-state interference on the shapes
of the linearly polarized Stokes profiles are discussed. The emergent
Stokes profiles are computed for hypothetical line transitions arising
due to hyperfine structure splitting of the upper J = 3/2 and lower J =
1/2 levels of a two-level atom model with nuclear spin Is
= 3/2. We confine our attention to the non-magnetic scattering in the
collisionless regime.
Title: Angular Distribution of Quiet-Sun Magnetic Fields
Authors: Stenflo, J. O.
Bibcode: 2012ASPC..454...33S
Altcode:
Due to the highly non-linear relation between polarization and flux
density the conversion of Stokes data to inclination angles is very
sensitive to the way one deals with magnetic structuring on subpixel
scales. Exploring the complex behavior of the 6301-6302 Å line system
we have identified observables that allow a robust determination of
the field inclination with minimal model dependence. The angular
distribution is found to be strongly peaked around the vertical
direction for large flux densities but widens as the flux density
decreases to smoothly approach the isotropic case for zero flux density.
Title: Forward-scattering Hanle effect in the solar Ca I 4227 Å line
Authors: Frisch, H.; Anusha, L. S.; Bianda, M.; Holzreuter, R.;
Nagendra, K. N.; Ramelli, R.; Sampoorna, M.; Smitha, H. N.; Stenflo,
J. O.
Bibcode: 2012EAS....55...59F
Altcode:
High sensitivity spectropolarimetric observations of the four Stokes
parameters of the solar Ca I 4227 Å line have been performed in
October 2010 at IRSOL with the ZIMPOL polarimeter, near the disk center,
outside an active region (Bianda et al. 2011). They were analyzed in
Anusha et al. 2011 with a combination of detailed radiative transfer
modelling of the Hanle effect for the linear polarization and weak
field Zeeman approximation for the circular polarization. This approach
made possible a unique determination of the magnetic field vector at
various positions along the slit of the spectrograph. A summary of
the observations and of their analysis is presented here.
Title: Scaling laws for magnetic fields on the quiet Sun
Authors: Stenflo, J. O.
Bibcode: 2012A&A...541A..17S
Altcode: 2012arXiv1203.6011S
The Sun's magnetic field is structured over a range of scales that span
approximately seven orders of magnitudes, four of which lie beyond the
resolving power of current telescopes. Here we have used a Hinode SOT/SP
deep mode data set for the quiet-Sun disk center in combination with
constraints from the Hanle effect to derive scaling laws that describe
how the magnetic structuring varies from the resolved scales down to the
magnetic diffusion limit, where the field ceases to be frozen-in. The
focus of the analysis is a derivation of the magnetic energy spectrum,
but we also discuss the scale dependence of the probability density
function for the flux densities and the role of the cancellation
function for the average unsigned flux density. Analysis of the
Hinode data set with the line-ratio method reveals a collapsed flux
population in the form of flux tubes with a size distribution that
is peaked in the 10-100 km range. Magnetic energy is injected into
this scale range by the instability mechanism of flux tube collapse,
which is driven by the external gas pressure in the superadiabatic
region at the top of the convection zone. This elevates the magnetic
energy spectrum just beyond the telescope resolution limit. Flux tube
decay feeds an inertial range that cascades down the scale spectrum to
the magnetic diffusion limit, and which contains the tangled, "hidden"
flux that is known to exist from observations of the Hanle effect. The
observational constraints demand that the total magnetic energy in
the hidden flux must be of the same order as the total energy in the
kG flux tubes. Both the flux tubes and the hidden flux are found to
be preferentially located in the intergranular lanes, which is to be
expected since they are physically related.
Title: J-state interference signatures in the second solar
spectrum. Modeling the Cr i triplet at 5204-5208 Å
Authors: Smitha, H. N.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.;
Sampoorna, M.; Ramelli, R.; Anusha, L. S.
Bibcode: 2012A&A...541A..24S
Altcode: 2012arXiv1203.4934S
The scattering polarization in the solar spectrum is traditionally
modeled with each spectral line treated separately, but this is
generally inadequate for multiplets where J-state interference
plays a significant role. Through simultaneous observations of all
the 3 lines of a Cr i triplet, combined with realistic radiative
transfer modeling of the data, we show that it is necessary to include
J-state interference consistently when modeling lines with partially
interacting fine structure components. Polarized line formation theory
that includes J-state interference effects together with partial
frequency redistribution for a two-term atom is used to model the
observations. Collisional frequency redistribution is also accounted
for. We show that the resonance polarization in the Cr i triplet is
strongly affected by the partial frequency redistribution effects in
the line core and near wing peaks. The Cr i triplet is quite sensitive
to the temperature structure of the photospheric layers. Our complete
frequency redistribution calculations in semi-empirical models of the
solar atmosphere cannot reproduce the observed near wing polarization or
the cross-over of the Stokes Q/I line polarization about the continuum
polarization level that is due to the J-state interference. When
however partial frequency redistribution is included, a good fit to
these features can be achieved. Further, to obtain a good fit to the
far wings, a small temperature enhancement of the FALF model in the
photospheric layers is necessary.
Title: Scaling laws for quiet-sun magnetic fields
Authors: Stenflo, Jan
Bibcode: 2012decs.confE..23S
Altcode:
The structuring of solar magnetic fields continues down to scales
that are several orders of magnitude smaller than the scales that
can currently be resolved. While the kG type flux tubes that can be
explained in terms of the convective collapse mechanism have typical
sizes in the range 10-70 km, the "hidden" flux of tangled fields that is
revealed by the Hanle effect resides at still smaller scales. We have
used a Hinode SOT/SP data set for the disk center of the quiet Sun to
derive the kinetic and magnetic energy spectra in the resolved domain,
and have used Hinode line-ratio data in combination with constraints
from the Hanle effect to derive how the magnetic energy spectrum needs
to be continued all the way down to the magnetic dissipation limit such
that the combined constraints become satisfied. Special attention is
paid to the effects of polarimetric noise and the modulation transfer
function of the telescope. We find an approximate equipartition between
kinetic and magnetic energy with power-law behavior in the range 200 -
1000 km. Below this scale the character of the energy spectrum changes,
because it becomes dominated by the contribution from the kG flux tubes
in the range 10 - 200 km. The spectrum then needs to be continued
downwards to the dissipation limit (between 10 and 100 m) in a way
that can explain the Hanle effect observations. We describe how the
energy spectrum relates to the probability density functions for the
flux densities and field strengths and to the cancellation function
that describes the scaling of the average unsigned flux density.
Title: Bipolar Magnetic Regions on the Sun: Global Analysis of the
SOHO/MDI Data Set
Authors: Stenflo, J. O.; Kosovichev, A. G.
Bibcode: 2012ApJ...745..129S
Altcode: 2011arXiv1112.5226S
The magnetic flux that is generated by dynamo processes inside the
Sun emerges in the form of bipolar magnetic regions. The properties
of these directly observable signatures of the dynamo can be extracted
from full-disk solar magnetograms. The most homogeneous, high-quality
synoptic data set of solar magnetograms has been obtained with the
Michelson Doppler Imager (MDI) instrument on the Solar and Heliospheric
Observatory spacecraft during 1995-2011. We have developed an IDL
program that has, when applied to the 73,838 magnetograms of the MDI
data set, automatically identified 160,079 bipolar magnetic regions that
span a range of scale sizes across nearly four orders of magnitude. The
properties of each region have been extracted and statistically
analyzed, in particular with respect to the polarity orientations of
the bipolar regions, including their tilt-angle distributions and
their violations of Hale's polarity law. The latitude variation of
the average tilt angles (with respect to the E-W direction), which is
known as Joy's law, is found to closely follow the relation 32fdg1 ×
sin (latitude). There is no indication of a dependence on region size
that one may expect if the tilts were produced by the Coriolis force
during the buoyant rise of flux loops from the tachocline region. A few
percent of all regions have orientations that violate Hale's polarity
law. We show explicit examples, from different phases of the solar
cycle, where well-defined medium-size bipolar regions with opposite
polarity orientations occur side by side in the same latitude zone in
the same magnetogram. Such oppositely oriented large bipolar regions
cannot be part of the same toroidal flux system, but different flux
systems must coexist at any given time in the same latitude zones. These
examples are incompatible with the paradigm of coherent, subsurface
toroidal flux ropes as the source of sunspots, and instead show that
fluctuations must play a major role at all scales for the turbulent
dynamo. To confirm the profound role of fluctuations at large scales,
we show explicit examples in which large bipolar regions differ from the
average Joy's law orientation by an amount between 90° and 100°. We
see no observational support for a separation of scales or a division
between a global and a local dynamo, since also the smallest scales in
our sample retain a non-random component that significantly contributes
to the accumulated emergence of a north-south dipole moment that will
lead to the replacement of the old global poloidal field with a new
one that has the opposite orientation.
Title: Radiative transfer with J-state interference in a two-term
atom. Partial frequency redistribution in the non-magnetic case
Authors: Smitha, H. N.; Nagendra, K. N.; Sampoorna, M.; Stenflo, J. O.
Bibcode: 2011A&A...535A..35S
Altcode:
Context. Quantum interference phenomena play a fundamental role in
the formation of linear polarization that arises from scattering
processes in multiplets of the solar spectrum. In particular,
the J-state interference between different line components of a
multiplet (arising from transitions in a two-term atom) produces
significant effects in the linearly polarized spectra.
Aims:
We aim to solve the polarized radiative transfer equation for a
two-term atom with the unpolarized lower term in isothermal slabs,
including the effect of the interference between the upper J-states
and partial frequency redistribution (PRD). We consider only the case
of non-magnetic scattering.
Methods: The PRD matrix for the
J-state interference derived in previous works is incorporated into
the polarized transfer equation. The standard form of the two-level
atom transfer equation is extended to a two-term atom. The transfer
problem is then solved using a traditional polarized approximate lambda
iteration method.
Results: We show how the PRD and the J-state
interference together affect the shapes of the (I,Q/I) profiles. We
present the benchmark solutions for isothermal, constant-property
slabs of a given optical thickness. We consider a hypothetical doublet
produced by an L = 0 → 1 → 0 scattering transition with spin S =
1/2. We present the results in the form of Stokes (I,Q/I) profiles for
different values of (i) the line separation, (ii) optical thickness,
(iii) thermalization parameter, and (iv) the continuum opacity.
Title: Analysis of the Forward-scattering Hanle Effect in the Ca I
4227 Å Line
Authors: Anusha, L. S.; Nagendra, K. N.; Bianda, M.; Stenflo, J. O.;
Holzreuter, R.; Sampoorna, M.; Frisch, H.; Ramelli, R.; Smitha, H. N.
Bibcode: 2011ApJ...737...95A
Altcode:
Coherent scattering of limb-darkened radiation is responsible for the
generation of the linearly polarized spectrum of the Sun (the Second
Solar Spectrum). This Second Solar Spectrum is usually observed near the
limb of the Sun, where the polarization amplitudes are largest. At the
center of the solar disk the linear polarization is zero for an axially
symmetric atmosphere. Any mechanism that breaks the axial symmetry (like
the presence of an oriented magnetic field, or resolved inhomogeneities
in the atmosphere) can generate a non-zero linear polarization. In the
present paper we study the linear polarization near the disk center
in a weakly magnetized region, where the axisymmetry is broken. We
present polarimetric (I, Q/I, U/I, and V/I) observations of the Ca
I 4227 Å line recorded around μ = cos θ = 0.9 (where θ is the
heliocentric angle) and a modeling of these observations. The high
sensitivity of the instrument (ZIMPOL-3) makes it possible to measure
the weak polarimetric signals with great accuracy. The modeling of
these high-quality observations requires the solution of the polarized
radiative transfer equation in the presence of a magnetic field. For
this we use standard one-dimensional model atmospheres. We show that the
linear polarization is mainly produced by the Hanle effect (rather than
by the transverse Zeeman effect), while the circular polarization is due
to the longitudinal Zeeman effect. A unique determination of the full
\bm {B} vector may be achieved when both effects are accounted for. The
field strengths required for the simultaneous fitting of Q/I, U/I, and
V/I are in the range 10-50 G. The shapes and signs of the Q/I and U/I
profiles are highly sensitive to the orientation of the magnetic field.
Title: Observations of the forward scattering Hanle effect in the
Ca I 4227 Å line
Authors: Bianda, M.; Ramelli, R.; Anusha, L. S.; Stenflo, J. O.;
Nagendra, K. N.; Holzreuter, R.; Sampoorna, M.; Frisch, H.; Smitha,
H. N.
Bibcode: 2011A&A...530L..13B
Altcode: 2011arXiv1105.2157B
Chromospheric magnetic fields are notoriously difficult to measure. The
chromospheric lines are broad, while the fields are producing
a minuscule Zeeman-effect polarization. A promising diagnostic
alternative is provided by the forward-scattering Hanle effect, which
can be recorded in chromospheric lines such as the He i 10 830 Å
and the Ca i 4227 Å lines. We present a set of spectropolarimetric
observations of the full Stokes vector obtained near the center of the
solar disk in the Ca i 4227 Å line with the ZIMPOL polarimeter at the
IRSOL observatory. We detect a number of interesting forward-scattering
Hanle effect signatures, which we model successfully using polarized
radiative transfer. Here we focus on the observational aspects, while
a separate companion paper deals with the theoretical modeling.
Title: Polarized Line Formation with J-state Interference in the
Presence of Magnetic Fields. I. Partial Frequency Redistribution in
the Collisionless Regime
Authors: Smitha, H. N.; Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.
Bibcode: 2011ApJ...733....4S
Altcode:
Quantum interference phenomena play a fundamental role in astrophysical
spectra that are formed by coherent scattering processes. Here we
derive a partial frequency redistribution (PRD) matrix that includes
J-state interference in the presence of magnetic fields of arbitrary
strength. The paper focuses on PRD in the collisionless regime, which
in the traditional PRD terminology is referred to as Hummer's type-II
scattering. By limiting the treatment to the linear Zeeman regime,
for which the Zeeman splitting is much smaller than the fine-structure
splitting, it is possible to formulate analytical expressions for
the PRD matrices. In the special case of non-magnetic scattering we
recover the redistribution matrix derived from an independent quantum
electrodynamic formulation based on the metalevel theory.
Title: High-resolution 3D Radiative MHD Simulations Of Turbulent
Convection And Spectro-polarimetric Properties
Authors: Kitiashvili, Irina; Kosovichev, A. G.; Mansour, N. N.;
Stenflo, J. O.; Wray, A. A.
Bibcode: 2011SPD....42.1708K
Altcode: 2011BAAS..43S.1708K
Realistic numerical simulations of solar magnetoconvection play a
key role for our understanding of the basic physical phenomena in
the subsurface convective boundary layer and the atmosphere. For the
accurate modeling of the turbulent processes on the Sun it is important
to perform the simulations with the highest possible resolution. Our
results have revealed significant changes in properties of the
turbulent motions when the resolution is increased. It is particularly
interesting that small-scale vortex motions in the intergranular
lanes become ubiquitous and strong, and play a critical role in the
large-scale organization of the solar dynamics. For the comparison
with observational data it is necessary to investigate relationships
between the physical and spectro-polarimetric properties in various
conditions of the quiet-Sun and magnetic regions, and model the observed
parameters. By using the radiative line formation code, SPINOR/STOPRO,
we have calculated the Stokes profiles and other characteristics
for the spectral line of the Hinode/SOT and SDO/HMI instruments,
and compared the simulation results with the observational data.
Title: Collapsed, uncollapsed, and hidden magnetic flux on the
quiet Sun
Authors: Stenflo, J. O.
Bibcode: 2011A&A...529A..42S
Altcode:
Since the first applications of the Stokes V line ratio in the early
1970s and the Hanle depolarization effect in the early 1980s we have
had a dualistic view of quiet-Sun magnetism: intermittent kG flux
tubes surrounded by an ocean of turbulent fields with strengths of
order 10-100 G. There has been the concern that this dualism could
be an artefact of using two mutually almost exclusive diagnostic
tools, the Zeeman and Hanle effect. We find however that the Hinode
line-ratio data alone, without any reference to the Hanle effect,
reveal the existence of two distinct flux populations, representing
strong (collapsed) and weak (uncollapsed) flux. The collapsed
population is preferentially located in the intergranular lanes,
while the uncollapsed population is most visible in the bright cell
interiors. From a comparison between the intrinsic field strengths,
as derived from the line ratio for the collapsed population, and the
corresponding flux densities, we can deduce the size distribution of
the flux tubes. The majority of them are found to have sizes in the
range 10-70 km. The intrinsic flux tube field strength decreases with
diminishing size to become substantially smaller than kG for sizes
below about 60 km. Comparison between the average of the unsigned flux
density in the Hinode quiet-Sun data set and earlier constraints from
the Hanle depolarization effect shows that most of the flux remains
invisible at the Hinode resolution scale due to cancellation of the
opposite magnetic polarities within the spatial resolution element. We
have derived the cancellation function that describes how the visibility
of the hidden flux improves with increased spatial resolution. It needs
to be extrapolated to extremely small scales before the constraints
imposed by the Hanle effect get satified, which suggests that the bulk
of the hidden flux resides at scales near the end of the magnetic scale
spectrum (of order 10 m). Appendix is only available in electronic
form at http://www.aanda.org
Title: Linear Polarization of the Solar Ca I 4227 Å Line: Modeling
with Radiative Transfer and Last Scattering Approximation
Authors: Anusha, L. S.; Stenflo, J. O.; Frisch, H.; Bianda, M.;
Holzreuter, R.; Nagendra, K. N.; Sampoorna, M.; Ramelli, R.
Bibcode: 2011ASPC..437...57A
Altcode:
To model the Ca I 4227 Å line polarization, radiative transfer
effects with partial frequency redistribution (PRD) must be taken into
account. The numerical solution of the relevant polarized radiative
transfer (RT) equations is computationally very demanding. The
“last scattering approximation” (LSA) is a concept allowing
faster methods to be devised. It is based on the remark that a single
scattering of the radiation field is sufficient for creating most
of the polarization. Its key ingredient is the anisotropy of the
radiation field. If the anisotropy is extracted from the observed
center to limb variation of the intensity profile, only the wings
of the Q/I spectrum can be modeled (Sampoorna et al. 2009). We show
here that the core region may be modeled as well if one takes into
account the depth variation of the anisotropy which is obtained from
an unpolarized multilevel RT (Anusha et al. 2010). After a validation
of the LSA approach by comparison with a polarized RT calculation, we
apply both approaches to model recent observations of the Ca I 4227 Å
line polarization taken on the quiet Sun. Apart from a global scaling
factor, both approaches give a very good fit to the Q/I spectrum for
all the wavelengths. As the LSA is 8 times faster than the RT approach,
we can recommend it as an efficient method to analyze other strong
resonance lines in the second solar spectrum.
Title: Observations of the Solar Ca I 4227 Å Line
Authors: Bianda, M.; Ramelli, R.; Stenflo, J. O.; Anusha, L. S.;
Nagendra, K. N.; Sampoorna, M.; Holzreuter, R.; Frisch, H.
Bibcode: 2011ASPC..437...67B
Altcode:
Our aim is to understand some interesting polarization features
observed in the solar Ca I 4277 Å line. Here we only discuss the
observational aspects. Observations have also been made in other
chromospheric lines within a few hours of those in the Ca I 4227 Å
line, in the same region near the north solar limb, to illustrate the
potential of simultaneous observations in different lines.
Title: Unsolved Problems in Solar Polarization
Authors: Stenflo, J. O.
Bibcode: 2011ASPC..437....3S
Altcode:
The Second Solar Spectrum has presented us with a rich and unfamiliar
world of polarization phenomena. While the many new spectral structures
have great diagnostic potential, they cannot be exploited before we have
identified the underlying physical processes and formulated a theory
for them. This theoretical challenge has led to considerable advances
in our understanding of the interaction between matter and radiation
in magnetized media, but a number of observed polarization phenomena
remain unexplained. Cases like the enigmatic Na I D1 line
indicate serious gaps in our understanding. A problem has been the lack
of benchmarks, against which the quantum theory of polarized scattering
can be tested. Polarized light scattering was a hot experimental topic
in the early years of quantum physics until about 1935, after which
the quantum physicists turned to other topics. A recent laboratory
experiment to explore the physics of the enigmatic D1
scattering transition has exposed the failure of the currently used
theory and prompted intense efforts to search for remedies. Besides
these issues with scattering polarization we discuss other unsolved
problems like the magnetic structuring on spatially unresolved
scales. There are also enigmas for the global magnetic field of the
Sun. In the final section we expose a case where Hale's polarity law
is being violated.
Title: Summary Talk
Authors: Stenflo, J. O.
Bibcode: 2011ASPC..437..501S
Altcode:
I first discuss the role of “Solar Polarization” in the context of
the development of modern astrophysics, and then show how a few failed
projects stimulated the advance that led to the discovery of the Second
Solar Spectrum and the development of ZIMPOL. The failed LEST project
also helped pave the way for major high-resolution telescope projects
like GREGOR, ATST, and EST. I conclude with a small poem in honor of
this successful Workshop.
Title: Magnetic fields on the quiet Sun
Authors: Stenflo, J. O.
Bibcode: 2011CEAB...35....1S
Altcode:
The structuring of quiet-Sun magnetic fields continues down to scales
that are orders of magnitude smaller than the resolution of current
telescopes. This sub-pixel structuring has in the past been explored
with two types of diagnostic tools: the line-ratio technique with
the longitudinal Zeeman effect, and the depolarization by the Hanle
effect. This has given us a ``standard model'' with two distinct types
of flux components: while most of the net magnetic flux is in the form
of highly intermittent, kG type flux tubes with small filling factors,
the intervening space (the main fraction of the photospheric volume)
is filled with a tangled field of intermediate strength. It has been a
concern that such a scenario with two distinct flux populations could
be an artefact of the use of two complementary but mutually almost
exclusive diagnostic filters, the Zeeman and the Hanle effect. Here we
show how recent analysis of Hinode SOT/SP quiet-sun data has revealed a
magnetic dichotomy with one strong-field (collapsed) and one weak-field
(uncollapsed) flux population, by using Stokes V line-ratio data
alone, without any reference to the Hanle effect. This verifies
that the dichotomy is not an artefact but is real. It is noted that
numerical simulations of magneto convection have not predicted any
such dichotomy. We extend the Hinode analysis to derive scaling laws,
which indicate that the sizes of the majority of the strong-field flux
tubes are smaller than 50 km, and that much of the uncollapsed field
is tangled on scales near the 10 m magnetic diffusion limit.
Title: Generalization of the Last Scattering Approximation for the
Second Solar Spectrum Modeling: The Ca I 4227 Å Line as a Case Study
Authors: Anusha, L. S.; Nagendra, K. N.; Stenflo, J. O.; Bianda, M.;
Sampoorna, M.; Frisch, H.; Holzreuter, R.; Ramelli, R.
Bibcode: 2010ApJ...718..988A
Altcode:
To model the second solar spectrum (the linearly polarized spectrum
of the Sun that is due to coherent scattering processes), one needs
to solve the polarized radiative transfer (RT) equation. For strong
resonance lines, partial frequency redistribution (PRD) effects must be
accounted for, which make the problem computationally demanding. The
"last scattering approximation" (LSA) is a concept that has been
introduced to make this highly complex problem more tractable. An
earlier application of a simple LSA version could successfully model
the wings of the strong Ca I 4227 Å resonance line in Stokes Q/I
(fractional linear polarization), but completely failed to reproduce
the observed Q/I peak in the line core. Since the magnetic field
signatures from the Hanle effect only occur in the line core, we need
to generalize the existing LSA approach if it is to be useful for the
diagnostics of chromospheric and turbulent magnetic fields. In this
paper, we explore three different approximation levels for LSA and
compare each of them with the benchmark represented by the solution of
the full polarized RT, including PRD effects. The simplest approximation
level is LSA-1, which uses the observed center-to-limb variation of
the intensity profile to obtain the anisotropy of the radiation field
at the surface, without solving any transfer equation. In contrast,
the next two approximation levels use the solution of the unpolarized
transfer equation to derive the anisotropy of the incident radiation
field and use it as an input. In the case of LSA-2, the anisotropy
at level τλ = μ, the atmospheric level from which an
observed photon is most likely to originate, is used. LSA-3, on the
other hand, makes use of the full depth dependence of the radiation
anisotropy. The Q/I formula for LSA-3 is obtained by keeping the
first term in a series expansion of the Q-source function in powers
of the mean number of scattering events. Computationally, LSA-1 is 21
times faster than LSA-2, which is 5 times faster than the more general
LSA-3, which itself is 8 times faster than the polarized RT approach. A
comparison of the calculated Q/I spectra with the RT benchmark shows
excellent agreement for LSA-3, including good modeling of the Q/I
core region with its PRD effects. In contrast, both LSA-1 and LSA-2
fail to model the core region. The RT and LSA-3 approaches are then
applied to model the recently observed Q/I profile of the Ca I 4227
Å line in quiet regions of the Sun. Apart from a global scale factor
both give a very good fit to the Q/I spectra for all the wavelengths,
including the core peak and blend line depolarizations. We conclude
that LSA-3 is an excellent substitute for the full polarized RT and
can be used to interpret the second solar spectrum, including the
Hanle effect with PRD. It also allows the techniques developed for
unpolarized three-dimensional RT to be applied to the modeling of the
second solar spectrum.
Title: ZIMPOL-3: a powerful solar polarimeter
Authors: Ramelli, Renzo; Balemi, Silvano; Bianda, Michele; Defilippis,
Ivan; Gamma, Luca; Hagenbuch, Stephan; Rogantini, Marco; Steiner,
Peter; Stenflo, Jan O.
Bibcode: 2010SPIE.7735E..1YR
Altcode: 2010SPIE.7735E..66R
The area of high precision solar spectropolarimetry has made
great advances in recent years and the Zurich IMaging POLarimeter
(ZIMPOL) systems have played a major role in that. ZIMPOL reaches
a polarimetric accuracy of 10-5 by using fast (kHz)
polarization modulation/demodulation of the light beam in combination
with large-area array detectors. A new generation of improved cameras
(ZIMPOL-3) are being implemented for the scientific observations at
the solar observatory at Istituto Ricerche Solari Locarno. The new
system is based on a flexible and compact modular design, which easily
adapts to new applications. A faster electronics and new sensors with
higher quantum efficency compared to the previous ZIMPOL versions,
allow to achieve a better overall efficency. Future plans include
observing campaigns at foremost large telescopes and the exploration
of new technologies (e.g. CMOS).
Title: Distribution functions for magnetic fields on the quiet Sun
Authors: Stenflo, J. O.
Bibcode: 2010A&A...517A..37S
Altcode:
The statistical properties of the highly structured magnetic field of
the quiet Sun are best described in terms of distribution functions,
in particular the probability density functions (PDF) for the flux
densities and the angular distribution for the orientations of the field
vector. They are needed to test the validity of various MHD simulations,
but past determinations have led to contradictory results. A main
reason for these difficulties lies in the circumstance that the
magnetic structuring continues on scales that are much smaller than
the telescope resolution, and that this structuring strongly affects
the quantities averaged over each pixel due to the non-linear relation
between polarization and magnetic field. Here we use a Hinode SOT/SP
data set for the disk center of the quiet Sun to explore the complex
behavior of the polarized 6301-6302 Å line system and identify the
observables that allow the most robust determinations of inclination
angles and flux densities. These observables are then used to derive
the empirical distribution functions. Our Stokes V line ratio analysis
leads us to an unexpected discovery: a magnetic dichotomy with two
distinct populations, representing strong (kG) and weak fields. This
can be understood in terms of the convective collapse mechanism,
which makes the Sun's magnetic flux end up in two states: collapsed
and uncollapsed. With the linear-to-circular polarization ratio as a
robust observable for the inclination angles, we find that the angular
distribution is extremely peaked around the vertical direction for the
largest flux densities, but gradually broadens as we go to smaller flux
densities, to become asymptotically isotropic at zero flux density. The
PDF for the vertical flux density, after accounting for the smearing
effect of measurement noise, is found to have an extremely narrow
core peak centered at zero flux density, which can be analytically
represented by a stretched exponential. The PDF wings are extended
and decline quadratically. The PDFs for the horizontal and total flux
densities have a similar behavior. In particular we demonstrate that
earlier claims that the PDF for the total flux density increases from
small values at zero flux density to have a maximum significantly
shifted from zero is an artefact of measurement noise.
Title: Probability distribution functions for the Sun's magnetic field
Authors: Stenflo, J. O.
Bibcode: 2010AN....331..585S
Altcode:
Magnetoconvection structures the Sun's magnetic field cover a vast
range of scales, down to the magnetic diffusion scale that is orders
of magnitude smaller than the resolution of current telescopes. The
statistical properties of this structuring are governed by probability
density functions (PDFs) for the flux densities and by the angular
distribution functions for the orientations of the field vectors. The
magnetic structuring on sub-pixel scales greatly affects the field
properties averaged over the resolution element due to the non-linear
relation between polarization and magnetic field. Here we use a Hinode
SOT/SP data set for the quiet Sun disk center to explore the complex
behavior of the 6301-6302 Å Stokes line profile system and identify the
observables that allow us to determine the distribution functions in the
most robust and least model dependent way. The angular distribution is
found to be strongly peaked around the vertical direction for large flux
densities but widens with decreasing flux density to become isotropic
in the limit of zero flux density. The noise-corrected PDFs for the
vertical, horizontal, and total flux densities all have a narrowly
peaked maximum at zero flux density that can be fitted with a stretched
exponential, while the extended wings decline quadratically.
Title: Probability distribution functions for solar and stellar
magnetic fields
Authors: Stenflo, Jan O.
Bibcode: 2010IAUS..264..191S
Altcode:
Magneto-convection structures the magnetic field in solar and stellar
atmospheres over scales that for the Sun span about 8 orders of
magnitudes, down to the magnetic diffusion scale of order 10 m. The
statistical properties of this structuring are governed by probability
density functions (PDFs), for the vertical and transverse field
components as well as for the field inclination. Due to the fractal
nature of the field pattern these PDFs appear to have a high degree
of scale invariance. There are serious pitfalls in the derivations
of empirical PDFs, pitfalls that are particularly severe in the case
of the field inclination. This explains the fragmentary and rather
unreliable PDF information available in the published literature. Here
we discuss the nature of these pitfalls and indicate how they may be
avoided, using Hinode quiet-sun Stokes vector data to derive PDFs for
the field strength and field inclination.
Title: Stokes polarimetry of the Zeeman and Hanle effects
Authors: Stenflo, Jan O.
Bibcode: 2010ISSIR...9..543S
Altcode:
Magnetic fields are responsible for almost all variability in the
Universe on intermediate time scales. The information on the magnetic
fields is encoded in the polarization of the radiation from the Sun and
stars through the Zeeman and Hanle effects. Stokes polarimetry is the
observational tool that gives us access to this information and allows
us to determine the structure and evolution of the fields. Space-based
observations are needed for two main reasons: (1) To allow high and
stable angular resolution over a large field of view. (2) To get access
to the vacuum ultraviolet (VUV), which contains information on the
magnetic fields in the corona and the chromosphere-corona transition
region. VUV polarimetry has considerable potential but has been much
neglected in the past.
Title: Measuring the Hidden Aspects of Solar Magnetism
Authors: Stenflo, J. O.
Bibcode: 2010ASSP...19..101S
Altcode: 2010mcia.conf..101S; 2009arXiv0903.4935S
2008 marks the 100th anniversary of the discovery of astrophysical
magnetic fields, when George Ellery Hale recorded the Zeeman splitting
of spectral lines in sunspots. With the introduction of Babcock's
photoelectric magnetograph, it soon became clear that the Sun's
magnetic field outside sunspots is extremely structured. The field
strengths that were measured were found to get larger when the spatial
resolutionwas improved. It was therefore necessary to come up with
methods to go beyond the spatial resolution limit and diagnose the
intrinsic magnetic-field properties without dependence on the quality
of the telescope used. The line-ratio technique that was developed
in the early 1970s revealed a picture where most flux that we see in
magnetograms originates in highly bundled, kG fields with a tiny volume
filling factor. This led to interpretations in terms of discrete,
strong-field magnetic flux tubes embedded in a rather field-free
medium, and a whole industry of flux tube models at increasing levels
of sophistication. This magnetic-field paradigm has now been shattered
with the advent of high-precision imaging polarimeters that allow us
to apply the so-called "Second Solar Spectrum" to diagnose aspects
of solar magnetism that have been hidden to Zeeman diagnostics. It
is found that the bulk of the photospheric volume is seething with
intermediately strong, tangled fields. In the new paradigm, the field
behaves like a fractal with a high degree of self-similarity, spanning
about 8 orders of magnitude in scale size, down to scales of order 10m.
Title: Origin of Spatial Variations of Scattering Polarization in
the Wings of the Ca I 4227 Å Line
Authors: Sampoorna, M.; Stenflo, J. O.; Nagendra, K. N.; Bianda, M.;
Ramelli, R.; Anusha, L. S.
Bibcode: 2009ApJ...699.1650S
Altcode: 2009arXiv0906.1184S
Polarization that is produced by coherent scattering can be modified
by magnetic fields via the Hanle effect. This has opened a window to
explorations of solar magnetism in parameter domains not accessible to
the Zeeman effect. According to standard theory the Hanle effect should
only be operating in the Doppler core of spectral lines but not in the
wings. In contrast, our observations of the scattering polarization
in the Ca I 4227 Å line reveal the existence of spatial variations
of the scattering polarization throughout the far line wings. This
raises the question whether the observed spatial variations in wing
polarization have a magnetic or nonmagnetic origin. A magnetic origin
may be possible if elastic collisions are able to cause sufficient
frequency redistribution to make the Hanle effect effective in the wings
without causing excessive collisional depolarization, as suggested by
recent theories for partial frequency redistribution (PRD) with coherent
scattering in magnetic fields. To model the wing polarization we bypass
the problem of solving the full polarized radiative transfer equations
and instead apply an extended version of the technique based on the
"last scattering approximation." It assumes that the polarization
of the emergent radiation is determined by the anisotropy of the
incident radiation field at the last scattering event. We determine
this anisotropy from the observed limb darkening as a function of
wavelength throughout the spectral line. The empirical anisotropy
profile is used together with the single-scattering redistribution
matrix, which contains all the PRD, collisional, and magnetic field
effects. The model further contains a continuum opacity parameter,
which increasingly dilutes the polarized line photons as we move away
from the line center, and a continuum polarization parameter that
represents the observed polarization level far from the line. This
model is highly successful in reproducing the observed Stokes Q/I
polarization (linear polarization parallel to the nearest solar
limb), including the location of the wing polarization maxima and the
minima around the Doppler core, but it fails to reproduce the observed
spatial variations of the wing polarization in terms of magnetic field
effects with frequency redistribution. This null result points in the
direction of a nonmagnetic origin in terms of local inhomogeneities
(varying collisional depolarization, radiation-field anisotropies,
and deviations from a plane-parallel atmospheric stratification).
Title: Theory of Polarized Scattering in the Mixed Hanle-Zeeman Regime
Authors: Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.
Bibcode: 2009ASPC..405...93S
Altcode:
We present the theory of polarized partial frequency redistribution
(PRD) in the presence of arbitrary magnetic fields. Starting from the
classical theory of \citet{sampoorna_bom99} we derive the laboratory
frame PRD matrices, which we call `Hanle-Zeeman redistribution
matrices', since they cover the partially overlapping weak and strong
field regimes. We show that for the simplest case of a triplet (J=0 →
1 → 0) transition, the classical and quantum \citep{sampoorna_bom97}
treatments give identical results.
Title: Magnetic Field Dependence of Polarized Scattering on Potassium
Authors: Thalmann, C.; Stenflo, J. O.; Feller, A.; Cacciani, A.
Bibcode: 2009ASPC..405..113T
Altcode:
The polarization that is generated by scattering of light on atoms
depends on the atomic quantum number structure. According to standard
scattering theory the sodium D_1 (5896 Å) and potassium D_1 (7699
Å) lines should be intrinsically unpolarizable. Since however the
existence of D_1 scattering polarization in the solar spectrum has been
well documented but resisted theoretical explanations, we have carried
out a laboratory experiment to find out if this is a problem of solar
physics or of quantum physics. We find that the polarization matrix
for scattering of the Stokes vector in the potassium D_1 line exhibits
a rich structure that changes dramatically when the field strength is
varied in the sub-gauss regime. In contrast, standard scattering theory
predicts zero polarization, even when optical pumping of the magnetic
sub-levels of the hyperfine structure multiplet is accounted for.
Title: Spectropolarimetry of Ca II 8542: Probing the Chromospheric
Magnetic Field
Authors: Kleint, L.; Reardon, K.; Stenflo, J. O.; Uitenbroek, H.;
Tritschler, A.
Bibcode: 2009ASPC..405..247K
Altcode:
We present spectropolarimetric observations of the chromospheric Ca II
8542 and photospheric Fe I 6302 lines obtained with the Interferometric
Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope. The
high spatial resolution over a large field of view (FOV) allows us to
connect the observed profiles to the overall topology of the target
region. After suitable calibrations we can extract Stokes profiles
for each point in the FOV. The Stokes V profiles observed in the Ca II
line show a large variety of shapes, indicating widely varying vertical
behavior of the field strength, velocity, and temperature. We examine
the center-of-gravity method for determining a representative field
strength from the observed profiles and use it to directly compare
photospheric and chromospheric magnetic fields.
Title: Solar Research Programs at IRSOL, Switzerland
Authors: Ramelli, R.; Bianda, M.; Stenflo, J. O.; Jetzer, P.
Bibcode: 2009arXiv0906.2348R
Altcode:
The Zurich IMaging POLarimeter (ZIMPOL) developed at ETH Zurich and
installed permanently at the Gregory Coude Telescope at Istituto
Ricerche Solari Locarno (IRSOL) allows a polarimetric precision down
to 10^-5 to be reached. This makes it possible to perform several
accurate spectro-polarimetric measurements of scattering polarization
and to investigate solar magnetic fields through the signatures of the
Hanle and Zeeman effects. The research programs are currently being
extended to monochromatic imaging of the Stokes vector with a recently
installed Fabry-Perot rapidly tunable filter system with a narrow pass
band of about 30mA. The spatial resolution is being improved by the
installation of an Adaptive Optics system.
Title: The Sun as a Rosetta Stone for Polarization Physics
Authors: Stenflo, J. O.
Bibcode: 2009ASPC..405....3S
Altcode:
The Sun has often been referred to as a Rosetta stone for astrophysics,
since its proximity allows us to explore in detail the fundamental
processes that govern the physics of objects in distant parts of
the universe. Among these fundamental processes there was little
attention given to polarization phenomena until about a decade ago,
apart from various applications of the standard Zeeman effect for
magnetic field diagnostics. With the implementation of new, highly
sensitive imaging Stokes polarimeters a new and previously unfamiliar
face of the Sun has been revealed, in the form of the richly structured
Second Solar Spectrum. Its spectral structures are exclusively due
to coherent scattering processes, which are modified by the partial
decoherence caused by magnetic fields via the Hanle effect. The observed
polarization phenomena are signatures of a rich variety of quantum-state
superpositions, which in a unique way reconnects astrophysics with
fundamental aspects of quantum theory. Here I choose to address this
topic from a personal perspective, starting with some biographical
notes and ending with some ideas on how current quantum scattering
theory may need to be extended to explain the enigmatic polarization
observed in the D_1 lines of sodium, barium, and potassium.
Title: The second solar spectrum and the hidden magnetism
Authors: Stenflo, Jan O.
Bibcode: 2009IAUS..259..211S
Altcode:
Applications of the Hanle effect have revealed the existence of vast
amounts of “hidden“ magnetic flux in the solar photosphere, which
remains invisible to the Zeeman effect due to cancellations inside
each spatial resolution element of the opposite-polarity contributions
from this small-scale, tangled field. The Hanle effect is a coherency
phenomenon that represents the magnetic modification of the linearly
polarized spectrum of the Sun that is formed by coherent scattering
processes. This so-called “Second Solar Spectrum” is as richly
structured as the ordinary intensity spectrum, but the spectral
structures look completely different and have different physical
origins. One of the new diagnostic uses of this novel spectrum is
to explore the magnetic field in previously inaccessible parameter
domains. The earlier view that most of the magnetic flux in the
photosphere is in the form of intermittent kG flux tubes with tiny
filling factors has thereby been shattered. The whole photospheric
volume instead appears to be seething with intermediately strong fields,
of order 100G, of significance for the overall energy balance of the
solar atmosphere. According to the new paradigm the field behaves like
a fractal with a high degree of self-similarity between the different
scales. The magnetic structuring is expected to continue down to the 10m
scale, 4 orders of magnitude below the current spatial resolution limit.
Title: Small-Scale Solar Magnetic Fields
Authors: de Wijn, A. G.; Stenflo, J. O.; Solanki, S. K.; Tsuneta, S.
Bibcode: 2009SSRv..144..275D
Altcode: 2008SSRv..tmp..190D; 2008SSRv..tmp..191D; 2008arXiv0812.4465D
As we resolve ever smaller structures in the solar atmosphere,
it has become clear that magnetism is an important component of
those small structures. Small-scale magnetism holds the key to many
poorly understood facets of solar magnetism on all scales, such as the
existence of a local dynamo, chromospheric heating, and flux emergence,
to name a few. Here, we review our knowledge of small-scale photospheric
fields, with particular emphasis on quiet-sun field, and discuss the
implications of several results obtained recently using new instruments,
as well as future prospects in this field of research.
Title: The Photosphere as a Laboratory for Quantum Physics
Authors: Stenflo, J. O.
Bibcode: 2009CEAB...33....1S
Altcode:
With the advent of highly sensitive imaging polarimeters an entirely
new ``spectral face'' of the Sun in linear polarization has become
accessible to exploration. It is due to coherent scattering processes,
which produce a spectrum that is as richly structured as the ordinary
intensity spectrum but with spectral structures that look entirely
different and have different physical origins. The work on trying
to identify these previously unfamiliar structures has led to new
insights in atomic and quantum physics. We find spectral signatures
of various types of quantum interference, hyperfine structure, and
optical pumping. The molecular lines, which are very weak and next to
invisible in the intensity spectrum, stand out with high contrast in
this so-called ``Second Solar Spectrum''. There are also structures
that have remained enigmatic for more than a decade, an example being
the observed polarization peak in the D_1 line of sodium. According
to quantum mechanical predictions this line should be intrinsically
unpolarizable. To determine whether this is a problem of solar physics
or of quantum physics we have set up a laboratory experiment to explore
the properties of polarized D_1 scattering under controlled conditions
and in well defined magnetic fields. This experiment has produced
unexpected results that are unequivocally at odds with our current
understanding of quantum scattering. The second solar spectrum has also
given us a diagnostic tool that allows us to explore aspects of solar
magnetism that have been inaccessible to the Zeeman effect. Thereby
vast amounts of ``hidden'' magnetic flux in the photosphere have been
uncovered, which has led to a new view of the nature of solar magnetism.
Title: Magnetic structuring at spatially unresolved scales
Authors: Stenflo, J. O.
Bibcode: 2009MmSAI..80..690S
Altcode:
Magneto-convection structures the Sun's magnetic field down to the
magnetic diffusion scale of order 10 m, where the field ceases to be
frozen-in. This is about four orders of magnitude below the current
resolution limit of solar telescopes. The subpixel structuring has a
dramatic effect on the derived, spatially averaged flux densities in
the resolved domain, in particular on the angular distribution of the
field. Thus we find that the previously reported apparent predominance
of horizontal magnetic flux on the quiet Sun is an artefact of the
subresolution structuring. Here we try to clarify how Stokes profile
data may be used to explore the spatially unresolved domain.
Title: Solar Dynamo and Magnetic Self-Organization
Authors: Kosovichev, A. G.; Arlt, R.; Bonanno, A.; Brandenburg,
A.; Brun, A. S.; Busse, F.; Dikpati, M.; Hill, F.; Gilman, P. A.;
Nordlund, A.; Ruediger, G.; Stein, R. F.; Sekii, T.; Stenflo, J. O.;
Ulrich, R. K.; Zhao, J.
Bibcode: 2009astro2010S.160K
Altcode:
No abstract at ADS
Title: Small-Scale Solar Magnetic Fields
Authors: de Wijn, A. G.; Stenflo, J. O.; Solanki, S. K.; Tsuneta, S.
Bibcode: 2009odsm.book..275D
Altcode:
As we resolve ever smaller structures in the solar atmosphere,
it has become clear that magnetism is an important component of
those small structures. Small-scale magnetism holds the key to many
poorly understood facets of solar magnetism on all scales, such as the
existence of a local dynamo, chromospheric heating, and flux emergence,
to name a few. Here, we review our knowledge of small-scale photospheric
fields, with particular emphasis on quiet-sun field, and discuss the
implications of several results obtained recently using new instruments,
as well as future prospects in this field of research.
Title: Tilt of Emerging Bipolar Magnetic Regions on the Sun
Authors: Kosovichev, A. G.; Stenflo, J. O.
Bibcode: 2008ApJ...688L.115K
Altcode:
Magnetic fields emerging from the Sun's interior carry information about
the physical processes of magnetic field generation and transport in the
convection zone. A statistical analysis of variations of the tilt angle
of bipolar magnetic regions during the emergence, observed from SOHO
MDI, shows that the systematic tilt with respect to the equator (Joy's
law) is established by the middle of the emergence period. This suggests
that the tilt is most likely generated below the surface. However,
the data do not show evidence of a dependence of the tilt angle on
the amount of flux or a relaxation of the bipolar orientation toward
the east-west direction, in contrast to the predictions of the rising
magnetic flux rope theories.
Title: Zeeman line formation in solar magnetic fields. Studies with
empirical probability distribution functions
Authors: Sampoorna, M.; Nagendra, K. N.; Frisch, H.; Stenflo, J. O.
Bibcode: 2008A&A...485..275S
Altcode:
Context: Numerical simulations of magneto-convection and analysis
of solar magnetograms provide probability distribution functions
(PDFs) for the magnetic field strength.
Aims: In the paper,
we explore the effects of these PDFs on Zeeman line formation.
Methods: We calculate the mean Stokes parameters for a Milne-Eddington
atmosphere in the limit of optically thin (micro-turbulent) and thick
(macro-turbulent) magnetic structures and also the dispersion around
the mean profiles in the optically thick limit. Several types of PDFs
are considered: (a) Voigt function and stretched exponential type
PDFs for fields with fixed direction but fluctuating strength; (b)
a cylindrically symmetrical power law for the angular distribution of
magnetic fields with given field strength; (c) composite PDFs accounting
for randomness in both strength and direction obtained by combining a
Voigt function or a stretched exponential with an angular power law. For
optically thin structures, explicit expressions are given for the mean
values of the Zeeman absorption matrix elements. We also describe how
the averaging technique for a normal Zeeman triplet may be generalized
to the more common case of anomalous Zeeman splitting patterns.
Results: We show that, for magnetic field rms fluctuations of
the order of 6 G, consistent with observational data, Stokes I is
essentially independent of the shapes of the PDFs but Stokes Q, U, and
V and also the dispersion around the mean values are quite sensitive
to the tail behavior of the PDF. We confirm a previous result that
Stokes V is less sensitive to the scale of the magnetic structures
than Stokes Q and U. The composite PDF proposed for the fluctuations of
the magnetic field vector has an angular distribution peaked about the
vertical direction for strong fields, and is isotropically distributed
for weak fields; it can be used to mimic solar surface random fields.
Title: Hanle-Zeeman Redistribution Matrix. III. Solution of the
Polarized Line Formation Problem
Authors: Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.
Bibcode: 2008ApJ...679..889S
Altcode:
The polarized radiative transfer equation is solved numerically,
taking into account both the Zeeman absorption matrix and the
Hanle-Zeeman redistribution matrix, to obtain line profiles for
arbitrary magnetic field strengths, partial frequency redistribution,
and scattering-dominated line transitions. The limiting cases of
weak-field Hanle scattering and strong-field Zeeman true absorption
are retrieved. The intermediate regime, where both Zeeman absorption
and scattering effects are important, is studied in some detail. The
numerical method is applied to various test cases to illustrate aspects
of partial frequency redistribution on line scattering in magnetic
fields of arbitrary strength and direction.
Title: Solar magnetic fields
Authors: Stenflo, J. O.
Bibcode: 2008JApA...29...19S
Altcode:
No abstract at ADS
Title: Hanle-Zeeman Redistribution Matrix. II. Comparison of Classical
and Quantum Electrodynamic Approaches
Authors: Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.
Bibcode: 2007ApJ...670.1485S
Altcode:
The Hanle-Zeeman redistribution matrix accounts for the intricately
coupled correlations in frequency, angle, and polarization between
the incoming and outgoing radiation and embodies the physics of
the scattering process. We show explicitly for a J=0-->1-->0
scattering transition the equivalence between the Hanle-Zeeman
redistribution matrix that is derived through quantum electrodynamics
and the one derived through classical, time-dependent oscillator
theory. This equivalence holds for all strengths and directions of the
magnetic field. Several aspects of the Hanle-Zeeman redistribution
matrix are illustrated, and explicit algebraic expressions are
given, which are of practical use for the polarized line transfer
computations. While the efficiency of the Hanle effect is usually
confined to the line core, we show how elastic collisions can produce a
``wing Hanle effect'' as well under favorable conditions in the solar
atmosphere.
Title: Hanle effect in the CN violet system with LTE modeling
Authors: Shapiro, A. I.; Berdyugina, S. V.; Fluri, D. M.; Stenflo,
J. O.
Bibcode: 2007A&A...475..349S
Altcode: 2007arXiv0709.2515S
Context: Weak entangled magnetic fields with mixed polarity occupy
the main part of the quiet Sun. The Zeeman effect diagnostics
fails to measure such fields because of cancellation in circular
polarization. However, the Hanle effect diagnostics, accessible through
the second solar spectrum, provides us with a very sensitive tool for
studying the distribution of weak magnetic fields on the Sun.
Aims: Molecular lines are very strong and even dominate in some
regions of the second solar spectrum. The CN B {}2 Σ -
X {}2 Σ system is one of the richest and most promising
systems for molecular diagnostics and well suited for the application
of the differential Hanle effect method. The aim is to interpret
observations of the CN B {}2 Σ - X {}2 Σ system
using the Hanle effect and to obtain an estimation of the magnetic
field strength.
Methods: We assume that the CN molecular layer
is situated above the region where the continuum radiation is formed
and employ the single-scattering approximation. Together with the
Hanle effect theory this provides us with a model that can diagnose
turbulent magnetic fields.
Results: We have succeeded in fitting
modeled CN lines in several regions of the second solar spectrum to
observations and obtained a magnetic field strength in the range from
10-30 G in the upper solar photosphere depending on the considered
lines. Tables [see full textsee full textsee full textsee full
text]-[see full textsee full textsee full textsee full text] are only
available in electronic form at http://www.aanda.org
Title: A New Mechanism for Polarizing Light from Obscured Stars
Authors: Kuhn, J. R.; Berdyugina, S. V.; Fluri, D. M.; Harrington,
D. M.; Stenflo, J. O.
Bibcode: 2007ApJ...668L..63K
Altcode: 2007arXiv0708.0599K
Recent spectropolarimetric observations of Herbig Ae/Be stellar systems
show linear polarization variability with wavelength and epoch near
their obscured Hα emission. Surprisingly, this polarization is
not coincident with the Hα emission peak but is variable near the
absorptive part of the line profile. With a new and novel model, we
show here that this is evidence of optical pumping-anisotropy of the
incident radiation that leads to a linear polarization-dependent optical
depth within the intervening hydrogen wind or disk cloud. This effect
can yield a larger polarization signal than scattering polarization
in these systems.
Title: Scattering polarization in strong chromospheric
lines. III. Spatial fluctuations of the polarized Ca ii K line
profiles
Authors: Holzreuter, R.; Stenflo, J. O.
Bibcode: 2007A&A...472..919H
Altcode:
Aims:We explore the spatial variations of the chromospheric Ca ii K
scattering polarization on the quiet Sun, in particular the fluctuations
of the linear polarizations Q/I and U/I, their correlations, varying
profile shapes, and center-to-limb variations.
Methods: A
set of high precision polarimetric recordings with ZIMPOL (Zurich
Imaging Polarimeter) at distinct heliographic positions is used.
Results: Two main effects contribute to the observed fluctuations: (i)
Chromospheric magnetic fields (located in the K3 layer), which induce
polarization variations via the Hanle effect. (ii) “Hot spots” due
to small-scale temperature structuring of the chromosphere, which induce
variations of the local scattering geometry. The observed signatures of
both effects are influenced by the turbulent broadening, which steeply
increases with height, by optical depth variations in the K3 layer,
and by the chromospheric temperature stratification. Almost all the
fluctuations that are seen with our spatial resolution (approximately
5 arcsec) originate in the high chromospheric K3 layer, while the
lower chromospheric layers are much more homogeneous at the resolved
scales. The relatively small Q/I fluctuations observed between the K1
and K2 wavelengths constrain the size and lifetime of cold and hot areas
in the low and mid chromosphere.
Conclusions: At present it is not
possible to disentangle the Hanle effect and the “hot-spot” effect
from each other. This would require 2D spatial mapping of the linear
polarization at the K3, K2, and K1 wavelengths with higher spatial
resolution, which is hard to do with existing telescopes, since the
observations are photon starved and require larger telescope apertures.
Title: Hanle-Zeeman Redistribution Matrix. I. Classical Theory
Expressions in the Laboratory Frame
Authors: Sampoorna, M.; Nagendra, K. N.; Stenflo, J. O.
Bibcode: 2007ApJ...663..625S
Altcode:
Polarized scattering in spectral lines is governed by a 4×4 matrix
that describes how the Stokes vector is scattered and redistributed
in frequency and direction. Here we develop the theory for this
redistribution matrix in the presence of magnetic fields of arbitrary
strength and direction. This general magnetic field case is called
the Hanle-Zeeman regime, since it covers both of the partially
overlapping weak- and strong-field regimes in which the Hanle
and Zeeman effects dominate the scattering polarization. In this
general regime, the angle-frequency correlations that describe the
so-called partial frequency redistribution (PRD) are intimately
coupled to the polarization properties. We develop the theory for
the PRD redistribution matrix in this general case and explore its
detailed mathematical properties and symmetries for the case of a
J=0-->1-->0 scattering transition, which can be treated in terms
of time-dependent classical oscillator theory. It is shown how the
redistribution matrix can be expressed as a linear superposition of
coherent and noncoherent parts, each of which contain the magnetic
redistribution functions that resemble the well-known Hummer-type
functions. We also show how the classical theory can be extended to
treat atomic and molecular scattering transitions for any combinations
of quantum numbers.
Title: Science with flare
Authors: Stenflo, Jan
Bibcode: 2007Natur.448...27S
Altcode:
No abstract at ADS
Title: Are ``EIT Waves'' Fast-Mode MHD Waves?
Authors: Wills-Davey, M. J.; DeForest, C. E.; Stenflo, J. O.
Bibcode: 2007ApJ...664..556W
Altcode: 2007arXiv0704.2828W
We examine the nature of large-scale, coronal, propagating wave
fronts (``EIT waves'') and find they are incongruous with solutions
using fast-mode MHD plane-wave theory. Specifically, we consider the
following properties: nondispersive single pulse manifestations,
observed velocities below the local Alfvén speed, and different
pulses which travel at any number of constant velocities, rather than
at the ``predicted'' fast-mode speed. We discuss the possibility of
a soliton-like explanation for these phenomena, and show how it is
consistent with the above-mentioned aspects.
Title: Scattering polarization in strong chromospheric
lines. II. Influence of the temperature curve on the Ca II K line
Authors: Holzreuter, R.; Stenflo, J. O.
Bibcode: 2007A&A...467..695H
Altcode:
In a previous paper we analyzed the polarization profile of the Ca
II K line near the limb at μ = 0.1. We now extend this work to model
the center-to-limb variation of the linear polarization of the Ca II
K line and compare it with calculated spectra based on standard 1D
model atmospheres. Our previous two-component approach with a hot and
a cool atmospheric component is re-examined. We confirm our previous
result that no single model is able to explain the observations. While
self-consistent single atmosphere solutions may fit the polarization
spectra, they fail to simultaneously fit the corresponding intensity
spectra or the polarization spectra at other heliocentric angles. The
mixing of a cool and a hot component is however a good approach for all
disk positions, although the optimum mixing ratio varies strongly with
heliocentric angle. As we approach the limb the hot component gains
in importance, which is consistent with the scenario of diverging
magnetic canopies overlying a cool atmosphere.
Title: Measurement of the Polarization of the Flash Spectrum during
a Total Solar Eclipse
Authors: Feller, A.; Ramelli, R.; Stenflo, J. O.; Gisler, D.
Bibcode: 2007ASPC..368..627F
Altcode:
Total solar eclipses offer the unique opportunity for a clean
observation of the light emitted by different chromospheric layers
without being disturbed by photospheric stray light, since the
moon is operating as a sharp knife edge. During the 29th March 2006
total solar eclipse we thus performed a pioneering measurement of
the polarization of the flash spectrum from UV to the near IR with a
spectral resolution of order 0.6 nm. The measurement has been obtained
with a dedicated instrument composed of an 8-inch Dall-Kirkham type
telescope and a slitless spectropolarimeter. The complete flash phase
at the second contact was observed with a cadence of 25 frames per
second corresponding to a height resolution of about 20 km in the
solar atmosphere. We could nicely register the dramatic transition
from an absorption-line spectrum to an emission spectrum dominated
by the strong chromospheric resonance lines. At the third contact we
recorded the opposite transition with a variable frame rate reaching
up to 75 frames per second.
Title: Investigation of Magnetic Properties of Emerging Active Regions
Authors: Kosovichev, Alexander G.; Stenflo, J. O.
Bibcode: 2007AAS...210.9214K
Altcode: 2007BAAS...39..210K
Magnetic flux emerging from the Sun's interior carries information
about the physical processes of magnetic field generation and transport
in the convection zone. The current paradigm is that solar magnetic
fields are generated in the tachocline region and emerge in the form
of toroidal flux tubes forming bipolar active regions, and that because
of the Coriolis force the flux tubes emerge with a particular tilt with
respect to the equator, observationally known as Joy's law. In order to
test this and other properties of emerging magnetic flux we have carried
out a systematic study of all, more than 500, emerging active regions
observed by SOHO/MDI during 1996-2006, using 96-min cadence full-disk
MDI magnetograms. The results reveal new interesting properties of
emerging magnetic flux. In particular, they indicate that the
initial tilt may be quite different from Joy's law, but that after
emergence the polarities quickly rearrange themselves to better conform
to this law, something that presents a challenge to current theoretical
models. We investigate statistics of the emerging flux properties,
their changes during the solar cycle and discuss implications for
dynamo theories and models of magnetic flux emergence and formation
of active regions.
Title: Magnetic Structures on the Sun
Authors: Stenflo, Jan O.
Bibcode: 2007AIPC..895...99S
Altcode:
The dynamo processes inside the Sun produce magnetic fields that
manifest themselves over a vast range of scales in the solar atmosphere,
from the global scales that govern the evolution over the 11-year
cycle, to the diffusion and reconnection scales, which are orders of
magnitude smaller than the resolution of solar telescopes. Here we
present an overview of the structures on the various scales, their
observed properties, their physical origins, and their roles for the
structure and dynamics of stellar atmospheres.
Title: International Symposium on Solar Physics and Solar Eclipses
(SPSE 2006)
Authors: Ramelli, R.; Shalabiea, O. M.; Saleh, I.; Stenflo, J. O.
Bibcode: 2007spse.conf.....R
Altcode:
The International Symposium on Solar Physics and Solar Eclipses (SPSE
2006) was held at Waw an Namos, Libya on the center line of the total
solar eclipse of 29th March 2006. 94 scientists from 11 countries
took part in the symposium with a series of talks on topics from
history of astronomy, eclipse observations, coronal and space physics,
fundamental physics, instrumentation, and solar energy utilization. A
number of advanced scientific experiments from different countries and
with different scientific aims were carried out with great success,
giving unique new insights about the Sun.
Title: Molecular Hanle effect in the Paschen-Back regime
Authors: Shapiro, A. I.; Fluri, D. M.; Berdyugina, S. V.; Stenflo,
J. O.
Bibcode: 2007A&A...461..339S
Altcode:
Context: The second solar spectrum resulting from coherent scattering
is a main tool for diagnostics of turbulent magnetic fields on the
Sun. Scattering on diatomic molecules plays an important role in
forming this spectrum and even dominates in some spectral regions.
Aims: In a magnetic field electronic states of a molecule are often
perturbed via the Paschen-Back effect. Sometimes this perturbation
can completely change the spectrum, not only quantitatively, but
even qualitatively. Here we calculate molecular scattering properties
taking into account the Paschen-Back effect.
Methods: Starting
with the Hund's case (a) wave functions as a basis we obtain with the
perturbation theory wave functions of the intermediate Hund's case
(a-b) in a magnetic field. Using new, perturbed values of the Landé
factors and transition amplitudes we calculate the Mueller matrix
for coherent scattering at diatomic molecules in the intermediate
Hund's case (a-b) and look for the effects that can be caused by
the Paschen-Back effect.
Results: We have found a considerable
deviation from the Zeeman regime and discuss here the quantitative
and qualitative effects on observed polarization signals for the CN
B {}2 Σ - X {}2 Σ and MgH B' {}2
Σ - X {}2 Σ systems as examples.
Title: Solar polarimetry with ZIMPOL . Plans for the future
Authors: Stenflo, J. O.
Bibcode: 2007MmSAI..78..181S
Altcode:
ZIMPOL (Zurich Imaging Polarimeter) has been developed since the early
1990s, and various versions at increasing levels of sophistication
have been used with great success at different telescopes (IRSOL,
McMath-Pierce, DST, SST, VTT). The main idea behind ZIMPOL is to
overcome the problem of making fast polarization modulation compatible
with the slow read-out of large-scale CCD sensors. This is done by
creating fast, hidden buffer storage areas within the CCD, and by
shifting the photo charges at kHz rates between the illuminated and the
buffer storage areas in synchrony with the modulation. ZIMPOL is not
dependent on the type of modulator used. Although most observations
have been done with piezo-elastic modulators, FLCs and Pockels cells
have also been used. A version of the ZIMPOL sensor with an array of
cylindrical microlenses to optimize the efficiency has recently been
implemented. An overview is given of the present status and the future
plans with the ZIMPOL systems.
Title: Molecular Hanle effect in the Paschen-Back regime: theory
and application
Authors: Shapiro, A. I.; Berdyugina, S. V.; Fluri, D. M.; Stenflo,
J. O.
Bibcode: 2007msfa.conf..317S
Altcode:
The second solar spectrum resulting from coherent scattering is a main
tool for diagnostics of turbulent magnetic fields on the Sun. Scattering
on diatomic molecules plays an important role in forming this spectrum
and even dominates in some spectral regions. In a magnetic field
electronic states of a molecule are often perturbed via the Paschen-Back
effect. Sometimes this perturbation can completely change the spectrum,
not only quantitatively, but even qualitatively. Here we calculate
molecular scattering properties taking into account the Paschen-Back
effect. We calculate the Mueller matrix for coherent scattering at
diatomic molecules in the intermediate Hund's case (a-b) and look for
the effects that can be caused by the Paschen-Back effect. We have
found a considerable deviation from the Zeeman regime and discuss
here the quantitative and qualitative effects on observed polarization
signals for the CN B 2 [Sigma] - X 2 [Sigma] system as an example. We
show an application of the Hanle effect for the interpretation of
observations of
Title: Variation of the second solar spectrum with the solar cycle.
Authors: Bianda, M.; Ramelli, R.; Stenflo, J. O.
Bibcode: 2007MmSAI..78...38B
Altcode:
The three Volumes of the Atlas of the Second Solar Spectrum
that give thelinear polarization measured inside the limb near
the heliographic poles with ZIMPOL (Zurich Imaging Polarimeter)
are based on observationsdone around the maximum of the solar
cycle. Indications about a possible solarcycle dependence of the
Second Solar Spectrum already exist. Wepresent here some measurements
obtained at IRSOL in 2006 duringperiods of low solar activity with
the same instrumentation andtechnique used for the first two volumes
of the Atlas. Our observations show that some spectral lines, in
particularchromospheric lines, display remarkable changes, while the
shapes of most photospheric lines remain unchanged.
Title: Instrumental developments at the Gregory Coudé Telescope
(GCT) at IRSOL
Authors: Bianda, M.; Ramelli, R.; Feller, A.; Stenflo, J. O.;
Küveler, G.
Bibcode: 2007msfa.conf...59B
Altcode:
The research projects carried out with the Gregory Coudé Telescope
(GCT) at Istituto Ricerche Solari Locarno (IRSOL) are mainly focused
on high precision polarimetry. The Zurich IMaging POLarimeter (ZIMPOL)
developed at ETH Zurich and installed permanently at the GCT at IRSOL
allows a polarimetric precision down to 10-5 to be reached. This
makes it possible to perform several accurate spectro-polarimetric
measurements of scattering polarization and to investigate solar
magnetic fields through the signatures of the Hanle and Zeeman
effects. The research programs are currently being extended to
monochromatic imaging of the Stokes vector with a recently installed
Fabry-Perot rapidly tunable filter system with a narrow pass band
of about 30 mÅ. The spatial resolution is being improved by the
installation of an adaptive optics (AO) system.
Title: Solar Active Regions
Authors: Stenflo, Jan Olof
Bibcode: 2007RoAJ...17S..69S
Altcode:
No abstract at ADS
Title: Imaging polarimetry with a tunable narrow-band filter
Authors: Feller, A.; Bianda, M.; Stenflo, J. O.
Bibcode: 2007msfa.conf...63F
Altcode:
We have developed a fully tunable narrow-band filter system to be
used in combination with ZIMPOL for monochromatic imaging vector
polarimetry. It is designed for the whole visible spectrum, from the
UV at about 395 nm, to the red at about 660 nm, with a band width of
3050 mÅ. The main components are two lithium niobate Fabry-Pérot
etalons made of Y-cut crystals, which means that the transmission
spectra and tuning parameters are different for the two orthogonal
states of polarisation. This allows to enhance the finesse with a
double-pass configuration where the polarisation is rotated by 90
degrees between the passes, a possibility that is not available to other
Fabry-Pérot systems. The filter system is available in two different
optical configurations: telecentric, for high spatial resolution,
and collimated, with a large field of view and high throughput. The
telecentric configuration is compact and can be easily used at foreign
telescopes like the SST, whereas the collimated configuration is
permanently installed at IRSOL. We present the instrument and its
operation.
Title: Second Solar Spectrum: A Brief Overview
Authors: Stenflo, J. O.
Bibcode: 2006ASPC..358..215S
Altcode:
A brief overview of the observed properties of the Second Solar Spectrum
is presented, with emphasis on some of the most recent advances. These
include clarification of the physical nature and properties of
the continuum polarization, diagnostics of chromospheric magnetic
fields with the Ca II K line, progress in elucidating the enigmatic
D1 polarization, and molecular scattering in magnetic fields.
Title: Tunable Narrow-Band Filter for Imaging Polarimetry
Authors: Feller, A.; Boller, A.; Stenflo, J. O.
Bibcode: 2006ASPC..358..155F
Altcode:
A fully tunable narrow-band filter system to be used in combination
with ZIMPOL for monochromatic imaging vector polarimetry is being
developed. It may be used over the whole visible spectrum, from the
UV at about 395 nm, to the red at about 660 nm, with a band width of
30-50 mÅ. The main components are two lithium-niobate Fabry-Perot
etalons made of Y-cut crystals, which means that the channel spectra
and tuning parameters are different for the two orthogonal states
of polarization (ordinary and extraordinary rays). This allows the
finesse to be dramatically enhanced by a double-pass configuration, a
possibility that is not available to other Fabry-Perot systems. Tuning
can be achieved in three ways: temperature tuning to center the pass
band within the selected fine tuning range, voltage tuning for rapid
fine tuning, and tilt tuning as an additional, though normally not
used, possibility. The filter system will allow us to explore the
spatial structuring of the polarization signatures in the Second Solar
Spectrum, including vector mapping of the Hanle and Zeeman effects in
any spectral line between the UV and red part of the spectrum.
Title: Laboratory Experiment for Polarized Scattering at Potassium
Vapor
Authors: Thalmann, C.; Stenflo, J. O.; Feller, A.; Cacciani, A.
Bibcode: 2006ASPC..358..323T
Altcode:
The observed solar polarization peaks of the D1 lines
of sodium at 5896 Å and barium at 4934 Å still elude theoretical
explanation, in spite of considerable theoretical efforts over the past
decade. To clarify the underlying physics of D1 scattering we
have set up a laboratory experiment to explore the spectrally resolved
profiles of the Mueller matrix for D1 and D2
scattering at potassium vapor. Here we present initial results of
this experiment.
Title: Spectro-Polarimetry of Solar Prominences
Authors: Ramelli, R.; Bianda, M.; Trujillo Bueno, J.; Merenda, L.;
Stenflo, J. O.
Bibcode: 2006ASPC..358..471R
Altcode: 2009arXiv0906.2342R
A large set of high-precision, full-Stokes spectro-polarimetric
observations of prominences in He I D3, Hα, and Hβ lines
has been recorded with the ZIMPOL polarimeter at the Gregory-Coudé
Telescope in Locarno. The observational technique allows us to
obtain measurements free from seeing-induced spurious effects. The
instrumental polarization is well under control and taken into account
in the data analysis. We present our observational results for each
of the lines mentioned above. Of particular interest is that most
of our Hα measurements show antisymmetric V profiles that are
a characteristic signature of the Zeeman effect in the prominence
plasma. A Stokes inversion technique based on the quantum theory of the
Hanle and Zeeman effects is being applied on observed Stokes profiles
in the He I D3 line in order to obtain information on the
magnetic field vector that confines the prominence plasma.
Title: Spectro-Polarimetric Observations of Filaments in Hα and He
I D3
Authors: Bianda, M.; Ramelli, R.; Trujillo Bueno, J.; Stenflo, J. O.
Bibcode: 2006ASPC..358..454B
Altcode:
Recordings of the full Stokes vector of filaments in the Hα and He I
D3 lines were obtained at the Gregory-Coudé telescope in
Locarno with the polarimeter ZIMPOL. The aim was to perform preliminary
studies to explore the presence of forward scattering polarization. The
observations show linear polarization signatures, whose interpretation
is still being investigated, although it seems likely that forward
scattering is indeed involved.
Title: Polarized Scattering in Strong Chromospheric Lines: Theory
and Its Confrontation with Observations
Authors: Fluri, D. M.; Holzreuter, R.; Stenflo, J. O.
Bibcode: 2006ASPC..358..237F
Altcode:
We present the current status of modeling scattering polarization in
strong lines and compare it to observations. First, we discuss how the
emergent polarization profile depends directly on the source function
gradient and the anisotropy of the radiation field. This explains
naturally the formation of the triplet peak structure often observed
in these strong lines. Then, we investigate the Ca II K line which is
of particular interest since it forms high in the chromosphere. The
degree of its scattering polarization depends sensitively on atmospheric
properties and even provides observational evidence for the presence of
cool components in the chromosphere. Therefore, polarized scattering
in Ca II K might provide a long searched for, second observational
tool (apart from CO molecules) to access the coolest parts of the
chromosphere.
Title: Hanle Effect in the Paschen-Back Regime
Authors: Shapiro, A. I.; Fluri, D. M.; Berdyugina, S. V.; Stenflo,
J. O.
Bibcode: 2006ASPC..358..311S
Altcode:
The second solar spectrum resulting from coherent scattering is an
important tool for the diagnostics of turbulent magnetic fields on
the Sun. Molecular scattering plays an important role in forming this
spectrum, and even dominates in some spectral regions. We present
a theory that allows us to calculate the Mueller matrix for coherent
scattering from diatomic molecules in Hund's intermediate coupling case
(a-b) for arbitrary molecular transitions. We performed the calculation
of the molecular Hanle effect in the Paschen-Back regime. We found
significant differences from the Zeeman regime, and as an example we
discuss here, both qualitatively and quantitatively, the effects on
observed polarization signals for the CN violet system.
Title: First Polarimetric Measurements and Modeling of the
Paschen-Back Effect in CaH Transitions
Authors: Berdyugina, S. V.; Fluri, D. M.; Ramelli, R.; Bianda, M.;
Gisler, D.; Stenflo, J. O.
Bibcode: 2006ApJ...649L..49B
Altcode: 2006astro.ph.10588B
We report the first spectropolarimetric observations and modeling of CaH
transitions in sunspots. We have detected strong polarization signals in
many CaH lines from the A-X system, and we provide the first successful
fit to the observed Stokes profiles using the previously developed
theory of the Paschen-Back effect in arbitrary electronic states of
diatomic molecules and polarized radiative transfer in molecular lines
in stellar atmospheres. We analyze the CaH Stokes profiles together
with quasi-simultaneous observations in TiO bands and conclude that
CaH provides a valuable diagnostic of magnetic fields in sunspots,
starspots, cool stars, and brown dwarfs.
Title: "EIT Waves" as Coronal Solitons: Explaining The Nature of
Large-Scale Coronal Pulse Waves
Authors: Wills-Davey, Meredith; DeForest, C. E.; Stenflo, J. O.
Bibcode: 2006SPD....37.1006W
Altcode: 2006BAAS...38..238W
We find that a soliton solution is applicable in a fully non-linear,
coronal MHD environment, and the compressive component of this
solution results in a pulse wave similar to the large-scale coronal
propagating wavefronts (``EIT waves'') observed by the SOHO-EIT and
TRACE telescopes. Properties of EIT wave waves that have previously
appeared incongruous using plane-wave theory are resolved in the context
of the soliton solution. Specifically, we show: why the observed wave
speed can be expected to be less than the local Alfvén speed, how a
wave pulse can instigate loop oscillations, and how a single pulse can
be channeled to propagate through the lower corona. We also discuss
the nature of likely triggers, and find that coronal mass ejections
are an ideal wave instigator.
Title: Ca II K polarization as a diagnostic of temperature bifurcation
Authors: Holzreuter, R.; Fluri, D. M.; Stenflo, J. O.
Bibcode: 2006A&A...449L..41H
Altcode:
Aims.We compute the linearly polarized spectrum of Ca II K caused by
coherent scattering and exploit the line for chromospheric diagnosis,
with particular attention to temperature bifurcation, by comparing
the theory with solar observations.Methods.We numerically solve
the statistical equilibrium equations and the radiative transfer
equation taking into account polarized coherent scattering with partial
frequency redistribution. All calculations are performed in 1D within
a plane-parallel atmosphere.Results.We find strong evidence of a
chromospheric temperature bifurcation. This suggests that the linearly
polarized spectrum of Ca II K might become a valuable tool to study
cool components and the dynamics of the chromosphere independently of
observations of molecular CO lines and millimeter and sub-millimeter
continua.
Title: Conclusions of the Symposium
Authors: Stenflo, Jan O.
Bibcode: 2006spse.conf.....S
Altcode:
No abstract at ADS
Title: Introductory Remarks by the Chairman of the Scientific
Organizing Committee
Authors: Stenflo, Jan O.
Bibcode: 2006spse.confD...1S
Altcode:
No abstract at ADS
Title: Polarization at the Extreme Limb of the Sun and the Role of
Eclipse Observations
Authors: Stenflo, Jan O.
Bibcode: 2006spse.conf....1S
Altcode:
No abstract at ADS
Title: Eclipse Instrument to Record the Polarization of the Flash
Spectrum
Authors: Feller, A.; Stenflo, J. O.; Gisler, D.; Ramelli, R.
Bibcode: 2006spse.conf...15F
Altcode:
No abstract at ADS
Title: International Symposium on Solar Physics and Solar Eclipses
Authors: Ramelli, Renzo; Shalabiea, Osama; Saleh, Ibrahim; Stenflo,
Jan O.
Bibcode: 2006spse.conf.....R
Altcode:
No abstract at ADS
Title: Solar Research Programs at IRSOL, Switzerland
Authors: Ramelli, R.; Bianda, M.; Stenflo, J. O.; Jetzer, P.
Bibcode: 2006spse.conf..121R
Altcode:
No abstract at ADS
Title: ZIMPOL/CHEOPS: a Polarimetric Imager for the Direct Detection
of Extra-solar Planets
Authors: Schmid, H. M.; Gisler, D.; Joos, F.; Povel, H. P.; Stenflo,
J. O.; Feldt, M.; Lenzen, R.; Brandner, W.; Tinbergen, J.; Quirrenbach,
A.; Stuik, R.; Gratton, R.; Turatto, M.; Neuhäuser, R.
Bibcode: 2005ASPC..343...89S
Altcode:
In a phase A study supported by ESO, we are currently planning a
dedicated VLT instrument for the direct detection of extra-solar
planets: CHEOPS (CHaracterizing Exoplanets by Opto-infrared Polarization
and Spectroscopy). The envisaged instrument consists of an extreme
adaptive optics system with a coronagraph, and two science channels for
differential imaging, which are (1) a spectroscopic integral field
unit in the J- and H-band optimized for the detection of methane bands,
and (2) a polarimetric I-band imager based on the ZIMPOL technique
for the search of reflected (polarized) light. We highlight here
some aspects of the polarimetric part of this project.
Title: Spectropolarimetric Observations of Prominences and Spicules,
and Magnetic Field Diagnostics
Authors: Ramelli, R.; Bianda, M.; Trujillo Bueno, J.; Merenda, L.;
Stenflo, J. O.
Bibcode: 2005ESASP.596E..82R
Altcode: 2005ccmf.confE..82R
No abstract at ADS
Title: Evolution and rotation of large-scale photospheric magnetic
fields of the Sun during cycles 21-23. Periodicities, north-south
asymmetries and r-mode signatures
Authors: Knaack, R.; Stenflo, J. O.; Berdyugina, S. V.
Bibcode: 2005A&A...438.1067K
Altcode:
We present the results of an extensive time series analysis of
longitudinally-averaged synoptic maps, recorded at the National Solar
Observatory (NSO/Kitt Peak) from 1975 to 2003, and provide evidence
for a multitude of quasi-periodic oscillations in the photospheric
magnetic field of the Sun. In the low frequency range, we have located
the sources of the 3.6~yr, 1.8~yr, and 1.5~yr periodicities that
were previously detected in the north-south asymmetry of the unsigned
photospheric flux (Knaack et al. 2004, A&A, 418, L17). In addition,
quasi-periodicities around 2.6~yr and 1.3~yr have been found. The
1.3~yr period is most likely related to large-scale magnetic surges
toward the poles and appeared in both hemispheres at intermediate
latitudes ~30°-55° during the maxima of all three cycles 21-23,
being particularly pronounced during cycle 22. Periods near 1.3~yr
have recently been reported in the rotation rate at the base of
the convection zone (Howe et al. 2000, Science, 287, 2456), in the
interplanetary magnetic field and geomagnetic activity (Lockwood 2001,
J. Geophys. Res., 106, 16021) and in sunspot data (Krivova & Solanki
2002, A&A, 394, 701). In the intermediate frequency range, we have
found a series of quasi-periodicities of 349-307~d, 282±4~d, 249-232~d,
222-209~d, 177±2~d, 158-151~d, 129-124~d and 103-100~d, which are in
good agreement with period estimates for Rossby-type waves and occurred
predominantly in the southern hemisphere. We provide evidence that
the best known of these periodicities, the Rieger period around 155~d,
appeared in the magnetic flux not only during cycle 21 but also during
cycle 22, likely even during cycle 23. The high frequency range, which
covers the solar rotation periods, shows a dominant (synodic) 28.1±
0.1~d periodicity in the southern hemisphere during cycles 21 and 22. A
periodicity around 25.0-25.5~d occurred in the south during all three
cycles. The large-scale magnetic field of the northern hemisphere
showed dominant rotation periods at 26.9±0.1~d during cycle 21,
at 28.3-29.0~d during cycle 22 and at 26.4±0.1~d during cycle 23.
Title: Spherical harmonic decomposition of solar magnetic fields
Authors: Knaack, R.; Stenflo, J. O.
Bibcode: 2005A&A...438..349K
Altcode:
We have investigated the temporal evolution of large-scale magnetic
fields in the solar photosphere during the time interval 1966-2004
by means of spherical harmonic decomposition and subsequent time
series analysis. Two data sets of daily magnetograms recorded at
the Mt. Wilson and Kitt Peak observatories were used to calculate
the spherical harmonic coefficients of the radial magnetic field for
axisymmetric (m=0) and non-axisymmetric (m≠ 0) modes. Time series
analysis was then applied to deduce their temporal variations. A third
data set of synoptic Carrington rotation maps from Kitt Peak was also
analyzed for completeness. Besides the obvious 22 yr magnetic cycle,
we have found evidence for intermittent oscillations with periods of
2.1{-}2.5 yr, 1.5{-}1.8 yr and 1.2{-}1.4 yr. The biennial oscillation
occurred during the solar maxima of cycles 20-22 (and likely also during
the current cycle 23) and was most pronounced for modes that resemble
non-linear dynamo waves (Stix 1972, A&A, 20, 9). The 1.5{-}1.8
yr period was stronger during the odd cycles 21 and 23 than during
the even cycles 20 and 22, whereas the opposite was the case for the
1.2{-}1.4 yr period. Similar variations of 1.5{-}1.8 yr have recently
been detected in the north-south asymmetry of the magnetic flux (Knaack
et al. 2004, A&A, 418, L17), while quasi-periodicities of 1.3 yr
have been observed in the rotation rate near the base of the convection
zone (Howe et al. 2000, Science, 287, 2456), in the heliosphere and
geomagnetic activity (Lockwood 2001, J. Geophys. Res., 106, 16 021)
in sunspot areas (Krivova & Solanki 2002, A&A, 394, 701), and
in the large-scale photospheric magnetic field (Knaack et al. 2005,
A&A, in press). In agreement with the latter study, we have found
additional quasi-periodicities in the range 320{-}100 d and rotational
periods of 29.0±0.1d, 28.2±0.1d, and 26.8±0.1d. Compared to earlier
decompositions by Stenflo & Vogel (1986, Nature, 319, 285) and
Stenflo & Güdel (1988, A&A, 191, 137), we can confirm the
main features of their results, although several modifications need
to be considered.
Title: Scattering polarization in strong chromospheric
lines. I. Explanation of the triplet peak structure
Authors: Holzreuter, R.; Fluri, D. M.; Stenflo, J. O.
Bibcode: 2005A&A...434..713H
Altcode:
Although the triplet polarization structure of the Na I D2 and Ca I 4227
Å lines in the second solar spectrum has been known for more than two
decades, a clear and consistent explanation has been lacking. Here we
show that the qualitative profile shape may be explained in terms of the
anisotropy of the radiation field and partial frequency redistribution
(PRD) effects. The complicated frequency and depth dependence of the
anisotropy can be understood in terms of simple arguments that involve
the source function gradient and boundary effects. We show in particular
that the triplet peak structure of the polarization profile of Na I D2
has basically the same origin as for the Ca I 4227 Å line. Hyperfine
structure and lower-level atomic polarization only modify the core
polarization without altering the overall qualitative features. For our
calculations we adopt a numerical method that combines the advantages
of both the classical formalism with integral source function and the
density-matrix formalism. In a first step, a multi-level, PRD-capable
MALI code, which solves the statistical equilibrium and the radiative
transfer equation self-consistently, computes intensity, opacities
and collision rates. Keeping these quantities fixed, we obtain the
scattering polarization in a second step by solving the radiative
transfer equation for the transitions of interest with the classical
formalism, which assumes a two-level atomic model with unpolarized
lower level. Quantum interferences and lower-level atomic polarization
are included in terms of a wavelength dependent polarizability W_2,
which is independently obtained with the density-matrix formalism.
Title: Absence of linear polarization in Hα emission of solar flares
Authors: Bianda, M.; Benz, A. O.; Stenflo, J. O.; Küveler, G.;
Ramelli, R.
Bibcode: 2005A&A...434.1183B
Altcode: 2005astro.ph..2263B
High sensitivity observations of Hα polarization of 30 flares of
different sizes and disk positions are reported. Both filter and
spectrographic techniques have been used. The ZIMPOL system eliminates
spurious polarizations due to seeing and flat-field effects. We didn't
find any clear linear polarization signature above our sensitivity
level which was usually better than 0.1%. The observations include an
X17.1 flare with gamma-ray lines reported by the RHESSI satellite. These
results cast serious doubts on previous claims of linear polarization at
the one percent level and more, attributed to impact polarization. The
absence of linear polarization limits the anisotropy of energetic
protons in the Hα emitting region. The likely causes are isotropization
by collisions with neutrals in the chromosphere and defocusing by the
converging magnetic field.
Title: Polarization of the Sun's continuous spectrum
Authors: Stenflo, J. O.
Bibcode: 2005A&A...429..713S
Altcode:
The Sun's spectrum is linearly polarized by coherent scattering
processes. Here we develop the theory for the formation of the
polarized continuum, identify the relevant physical mechanisms, and
clarify their relative roles. The polarized photons are produced by
scattering at neutral hydrogen in its ground state (Lyman scattering),
and to a smaller degree by scattering at free electrons (Thomson
scattering). The polarized photons are diluted by the unpolarized
photons from the H- opacity and radiative absorption from
the Balmer bound-bound and bound-free transitions. Due to pressure
broadening of the Balmer lines from the statistical Stark effect the
polarized Balmer jump is shifted from the series limit to substantially
longer wavelengths. In the second part of the paper the Atlas of
the Second Solar Spectrum that covers 3161-6995 Å for disk position
μ =0.1 (where μ is the cosine of the heliocentric angle) is used
to extract the empirical values of the continuum polarization with
the help of a model for the behavior of the depolarizing lines. The
empirically determined continuum polarization lies systematically
lower than the values that have been predicted for λ >4000 Å from
radiative-transfer modelling. The Balmer jump is found to be shifted
as expected from pressure-broadening theory. Through scaling of the
relative center-to-limb variations obtained from radiative-transfer
theory with the empirically determined values (valid for μ=0.1) we
finally obtain the semi-empirical function that describes the variation
of the continuum polarization with both wavelength and disk position
μ. The empirically determined continuum polarization can be used to
constrain model atmospheres as well as to fix the zero point of the
polarization scale in observations of the scattering polarization and
the Hanle effect.
Title: Instrumentation for the Advanced Technology Solar Telescope
Authors: Rimmele, Thomas R.; Hubbard, Robert P.; Balasubramaniam,
K. S.; Berger, Tom; Elmore, David; Gary, G. Allen; Jennings, Don;
Keller, Christoph; Kuhn, Jeff; Lin, Haosheng; Mickey, Don; Moretto,
Gilberto; Socas-Navarro, Hector; Stenflo, Jan O.; Wang, Haimin
Bibcode: 2004SPIE.5492..944R
Altcode:
The 4-m aperture Advanced Technology Solar Telescope (ATST) is the
next generation ground based solar telescope. In this paper we provide
an overview of the ATST post-focus instrumentation. The majority of
ATST instrumentation is located in an instrument Coude lab facility,
where a rotating platform provides image de-rotation. A high order
adaptive optics system delivers a corrected beam to the Coude lab
facility. Alternatively, instruments can be mounted at Nasmyth or
a small Gregorian area. For example, instruments for observing the
faint corona preferably will be mounted at Nasmyth focus where maximum
throughput is achieved. In addition, the Nasmyth focus has minimum
telescope polarization and minimum stray light. We describe the set of
first generation instruments, which include a Visible-Light Broadband
Imager (VLBI), Visible and Near-Infrared (NIR) Spectropolarimeters,
Visible and NIR Tunable Filters, a Thermal-Infrared Polarimeter &
Spectrometer and a UV-Polarimeter. We also discuss unique and efficient
approaches to the ATST instrumentation, which builds on the use of
common components such as detector systems, polarimetry packages and
various opto-mechanical components.
Title: CHEOPS/ZIMPOL: a VLT instrument study for the polarimetric
search of scattered light from extrasolar planets
Authors: Gisler, Daniel; Schmid, Hans Martin; Thalmann, Christian;
Povel, Hans Peter; Stenflo, Jan O.; Joos, Franco; Feldt, Markus;
Lenzen, Rainer; Tinbergen, Jaap; Gratton, Raffaele; Stuik, Remko;
Stam, Daphne M.; Brandner, Wolfgang; Hippler, Stefan; Turatto, Massimo;
Neuhauser, R.; Dominik, Carsten; Hatzes, Artie; Henning, Thomas; Lima,
Jorge; Quirrenbach, Andreas; Waters, L. B. F. M.; Wuchterl, Gunther;
Zinnecker, Hans
Bibcode: 2004SPIE.5492..463G
Altcode:
We present results from a phase A study supported by ESO for
a VLT instrument for the search and investigation of extrasolar
planets. The envisaged CHEOPS (CHaracterizing Extrasolar planets by
Opto-infrared Polarization and Spectroscopy) instrument consists
of an extreme AO system, a spectroscopic integral field unit and
an imaging polarimeter. This paper describes the conceptual design
of the imaging polarimeter which is based on the ZIMPOL (Zurich
IMaging POLarimeter) technique using a fast polarization modulator
combined with a demodulating CCD camera. ZIMPOL is capable of detecting
polarization signals on the order of p=0.001% as demonstrated in solar
applications. We discuss the planned implementation of ZIMPOL within
the CHEOPS instrument, in particular the design of the polarization
modulator. Further we describe strategies to minimize the instrumental
effects and to enhance the overall measuring efficiency in order to
achieve the very demanding science goals.
Title: Solar polarimetry in the near UV with the Zurich Imaging
Polarimeter ZIMPOL II
Authors: Gandorfer, A. M.; Steiner, H. P. Povel P.; Aebersold, F.;
Egger, U.; Feller, A.; Gisler, D.; Hagenbuch, S.; Stenflo, J. O.
Bibcode: 2004A&A...422..703G
Altcode:
We describe an imaging polarimeter for high sensitivity measurements
of solar polarisation signals in the wavelength range from 300 nm to
1 μm. At higher wavelengths the system is limited by the wavelength
cut-off of the silicon CCD sensor used. To the blue the limitation
arises from the atmospheric cut-off around 310 nm. The complete system
is a modified version of the Zurich Imaging Polarimeter ZIMPOL II
which has been equipped with a special CCD sensor. The CCD combines
for the first time a so-called open electrode structure with on-chip
demodulation. The concept as well as the detailed design of the
instrument are presented. Examples of observations are shown and
interpreted in order to experimentally evaluate the performance of
the system. All appendices are only available in electronic form
at http://www.edpsciences.org
Title: Solar physics: Hidden magnetism
Authors: Stenflo, Jan Olof
Bibcode: 2004Natur.430..304S
Altcode:
Observations of the Hanle effect have revealed the existence of
small-scale 'hidden' magnetic flux on the quiet Sun. The magnetic-energy
density of this hidden flux is much larger than previously thought.
Title: A Soliton Solution for the Solar Corona: ``EIT Waves''
Explained
Authors: Wills-Davey, M. J.; DeForest, C. E.; Stenflo, J. O.
Bibcode: 2004AAS...204.2901W
Altcode: 2004BAAS...36..695W
Many aspects of ``EIT wave'' observations have proved incongruous with
linear, plane-wave solutions. We find that solitons, rather than plane
waves, produce a viable non-linear MHD solution for a single-pulse
coronal compression wave. Aspects of the soliton solution describe
properties of EIT waves, in particular: \begin{itemize} coherence
and integrity over large distances, propagation velocities
that can be substantially slower than expected Alfvén or fast-mode
speeds, and displacements that can potentially instigate loop
oscillations. Additionally, this solution's very non-linearity may
explain why EIT waves are so strongly correlated with coronal mass
ejections. This research was funded by a grant from NASA.
Title: Periodic oscillations in the north-south asymmetry of the
solar magnetic field
Authors: Knaack, R.; Stenflo, J. O.; Berdyugina, S. V.
Bibcode: 2004A&A...418L..17K
Altcode:
We report on significant periodic variations of the magnetic activity
between the north and south hemisphere of the Sun. For this purpose,
we have investigated the north-south asymmetry of two solar data
sets, namely the Kitt Peak synoptic Carrington rotation maps of the
photospheric magnetic field (1975-2003) and monthly averaged sunspot
areas (1874-2003). Using Fourier and wavelet analysis, we have found
a regular pattern of pronounced oscillations with periods of 1.50
± 0.04 yr, 1.79 ± 0.06 yr and 3.6 ± 0.3 yr in the magnetic flux
asymmetry. The former two periods are related to a process which leads
to a gradual shift in the excess magnetic flux from north to south or
vice versa. Additional periods of 43.4 ± 7.1 yr (twice the magnetic
cycle) and 320-329 days were detected in the sunspot asymmetry.
Title: Large Scale Solar Magnetic Fields: Temporal Variations
Authors: Knaack, R.; Stenflo, J. O.
Bibcode: 2004IAUS..219..552K
Altcode: 2003IAUS..219E.159K
The temporal evolution of the solar magnetic field during solar cycles
20 21 and 22 has been investigated by means of spherical harmonic
decomposition and subsequent time series analysis. A 33 yr and a 25 yr
time series of daily magnetic maps of the solar photosphere recorded
at the Mount Wilson and NSO/Kitt Peak observatories respectively were
used to calculate the spherical coefficients of the radial magnetic
field. Fourier and wavelet analysis were then applied to deduce
the temporal variations. Besides the obvious ~22 yr magnetic cycle
shorter-lived fluctuations have been detected. However a resonant modal
structure of the solar magnetic field (apart from the 22 yr cycle) has
not been found which is in contrast to earlier studies. We will present
our results for axisymmetric and non-axisymmetric modes and compare them
with recent studies of large scale magnetic structures on cool stars.
Title: The New World of Scattering Physics Seen by High-precision
Imaging Polarimetry (With 14 Figures)
Authors: Stenflo, Jan O.
Bibcode: 2004RvMA...17..269S
Altcode:
No abstract at ADS
Title: Depolarizing lines in the Sun's spectrum
Authors: Fluri, D. M.; Stenflo, J. O.
Bibcode: 2003A&A...398..763F
Altcode:
The majority of all lines in the solar spectrum depolarize the
continuum. Here we present a theoretical analysis of the depolarizing
lines. In a parameter survey where we vary the details of line formation
such as height of formation, we find that absorption and scattering
lines have entirely different behavior. While the depolarization
by absorption lines rapidly decreases with increasing height of
formation, the depolarization by scattering lines increases with
height of formation. We interpret the results in terms of a simple
qualitative model that contains the relevant physics and support it
with quantitative calculations. The main relevant quantities are the
anisotropy of the radiation field, which varies over the line profile,
and the probability that a photon undergoes polarized scattering above
the height where the atmosphere becomes optically thin. For lines with
intrinsic polarization we find that the coupling between continuum
and line polarization can be neglected.
Title: Imaging polarimetry: opportunities and limitations
Authors: Stenflo, Jan O.
Bibcode: 2003SPIE.4843...76S
Altcode:
With a new class of imaging polarimeters it is now possible to
eliminate the previous main limiting factors of seeing and gain-table
noise in the polarization images to allow spectro-polarimetry with
a precision of 5 × 10-6. This has opened the door to a
previously unexplored world of polarization phenomena with promising
diagnostic possibilities not only for the Sun but also for night-time
astronomy. While illustrating examples of what has been achieved,
we present an overview of the new opportunities and quantify the
limitations imposed by the photon flux.
Title: Flux tubes or fractal distributions - on the nature of
photospheric magnetic fields
Authors: Stenflo, J. O.; Holzreuter, R.
Bibcode: 2003AN....324..397S
Altcode:
No abstract at ADS
Title: Radiative Transfer in Na I D2 and D1
Authors: Fluri, D. M.; Holzreuter, R.; Klement, J.; Stenflo, J. O.
Bibcode: 2003ASPC..307..263F
Altcode:
No abstract at ADS
Title: Influence of Collisions and Magnetic Fields on the Polarization
of the Na I D1 and D2 Lines
Authors: Klement, J.; Stenflo, J. O.
Bibcode: 2003ASPC..307..278K
Altcode:
No abstract at ADS
Title: Search for Impact Polarization in Hα Flares
Authors: Bianda, M.; Stenflo, J. O.; Gandorfer, A.; Gisler, D.;
Küveler, G.
Bibcode: 2003ASPC..307..487B
Altcode:
No abstract at ADS
Title: Molecules as Diagnostics of Solar and Stellar Magnetic Fields
Authors: Berdyugina, S. V.; Solanki, S. K.; Stenflo, J. O.
Bibcode: 2003ASPC..307..181B
Altcode:
No abstract at ADS
Title: Enigmatic Magnetic Field Effects in the Scattering Polarization
of the Ca I 4227 Å Line
Authors: Bianda, M.; Stenflo, J. O.; Gandorfer, A.; Gisler, D.
Bibcode: 2003ASPC..286...61B
Altcode: 2003ctmf.conf...61B
No abstract at ADS
Title: Spectro-polarimetry
Authors: Stenflo, J. O.
Bibcode: 2003dysu.book..128S
Altcode:
Spectro-polarimetry is our tool for remotely diagnosing the Sun's
magnetic field. It deals with the wavelength variation of an observable
vector quantity, the Stokes vector. The observational task is to map
the Stokes vector both in the spectral and spatial domain with highest
possible resolutions (spatial, spectral, temporal) and polarimetric
accuracy. The interpretation or inversion of Stokes vector data to
derive the magnetic and thermodynamic structure of the solar atmosphere
must take into account the extreme structuring of the magnetic field,
which extends to scales far smaller than we can resolve with present-day
telescopes. With novel imaging Stokes polarimeters qualitatively new
diagnostic tools like the Hanle effect and optical pumping are now
available to complement the Zeeman effect in the exploration of the
magnetized solar plasma on all scales.
Title: Spectro-polarimetric Observations: What's Next?
Authors: Stenflo, J. O.
Bibcode: 2003ASPC..307..583S
Altcode:
No abstract at ADS
Title: Scattering Polarization in Magnetic Fields: Anomalies,
Surprises and Enigmas
Authors: Stenflo, J. O.
Bibcode: 2003ASPC..307..385S
Altcode:
No abstract at ADS
Title: Distribution of Magnetic Fields at Scales Beyond the Spatial
Resolution Limit
Authors: Stenflo, J. O.; Holzreuter, R.
Bibcode: 2003ASPC..286..169S
Altcode: 2003ctmf.conf..169S
No abstract at ADS
Title: Harmonic analysis of solar magnetic fields
Authors: Knaack, R.; Stenflo, J. O.
Bibcode: 2002ESASP.505..453K
Altcode: 2002IAUCo.188..453K; 2002solm.conf..453K
The temporal variations of the global magnetic field in the Sun's
photosphere have been investigated through a harmonic analysis of the
zonal (m=0) as well as the non-axisymmetric (m≠0) modes. A 25 yr time
series of magnetic maps, recorded at the Kitt Peak Observatory (Tucson,
AZ) on a daily basis, was used to calculate the spherical coefficients
of the radial magnetic field. Subsequently, we applied Fourier analysis
as well as wavelet analysis to deduce the temporal variations. A complex
structure of resonant modes has been revealed. Besides the obvious
22 yr magnetic cycle, shorter-lived fluctuations have been detected,
e.g. with periods of approx. 6-7 yr and 2-3 yr. The latter is most
accentuated during the solar maxima. However, we cannot yet verify a
systematic correlation between the spherical degree of the even zonal
modes and their resonance frequencies, which is in contrast to earlier
analyses of low-resolution synoptic charts. What we can confirm is
the previously found parity selection rule that the 22 yr cycle is the
dominating feature of the odd zonal modes and we can extend this rule
to non-axisymmetric modes.
Title: Empirical view of magnetoconvection
Authors: Stenflo, J. O.; Holzreuter, R.
Bibcode: 2002ESASP.505..101S
Altcode: 2002solm.conf..101S; 2002IAUCo.188..101S
Solar magnetic fields have a fractal-like structure with a considerable
degree of self-similarity over a large dynamic range. The probability
distribution functions (PDF) of the magnetic field on global scales with
active regions and sunspots are compared with the PDF on small scales
in a quiet region at disk center and are found to be remarkably similar
both in shape and quantitative spread on the field strength values. The
shape of the PDF can be well represented by a Voigt function with a
"Doppler core" and extended damping wings. There is no sign that the
self-similarity would disappear at the scales near the diffraction limit
of current telescopes (Which also represents the approximate transition
between the optically thin and thick regimes). The empirical PDFs are
compared with results of numerical simulations of magnetoconvection. We
finally discuss how the PDFs help us to establish a new interpretative
framework for Zeeman and Hanle diagnostics.
Title: Spatial mapping of the Hanle and Zeeman effects on the Sun
Authors: Stenflo, J. O.; Gandorfer, A.; Holzreuter, R.; Gisler, D.;
Keller, C. U.; Bianda, M.
Bibcode: 2002A&A...389..314S
Altcode:
Spatial mapping of the Hanle and Zeeman effects on the Sun has been done
for the first time, through Stokes vector imaging with a narrow-band
(0.2 Å) universal filter. It is shown how the polarization signatures
of the Hanle and Zeeman effects can be cleanly distinguished from each
other by comparing the Stokes images recorded at different, specially
selected wavelengths within the Na I D2-D1
line system. Examples of the polarization signatures of sunspots,
faculae, the supergranulation network, and large-scale canopy fields
are shown. The most striking result of our observations is that the
scattering polarization has an extremely intermittent structure rather
than being a simple function of limb distance. These intermittent
scattering polarization signals are cospatial with the facular
and supergranulation network seen both in intensity and circular
polarization. The observed pattern can be explained in terms of magnetic
enhancement of the scattering polarization in the network and/or Hanle
depolarization of the scattering polarization outside the network. Since
however no magnetic fields are seen in circular polarization outside the
network, the relative absence of linear scattering polarization there
may be explained by Hanle depolarization only if the volume filling,
depolarizing magnetic field has mixed polarities on a subarcsec scale
that is not resolved.
Title: Molecular line scattering and magnetic field effects:
Resolution of an enigma
Authors: Berdyugina, S. V.; Stenflo, J. O.; Gandorfer, A.
Bibcode: 2002A&A...388.1062B
Altcode:
The linearly polarized solar spectrum that is produced by coherent
scattering processes (also called ``the second solar spectrum'') is full
of polarizing features due to molecular transitions, in particular from
MgH and C2. Their greatly different behavior in comparison
with the observed polarization from atomic transitions has presented
us with a new enigma: While the scattering polarization in atomic lines
is very sensitive to magnetic fields via the Hanle effect and therefore
exhibits polarization signatures that vary both spatially and with the
solar cycle, the molecular polarization appears to be immune to the
influence of magnetic fields. To clarify these issues we here develop
a theoretical foundation for polarized molecular scattering, which
can serve as a general tool for interpretations of the structures in
the second solar spectrum. Intrinsic polarizabilities, line strengths,
and effective Landé factors for the different transitions of the P,
Q, and R branches of MgH and C2 are calculated. While the
intrinsic polarizabilities remain significant, the effective Landé
factors are close to zero for the majority of the lines, in contrast
to the behavior of atomic lines. This resolves the enigma and indicates
how the molecular lines may serve as immutable reference lines against
which the atomic lines can be gauged when trying to determine long-term,
solar-cycle variations of the magnetic fields via the Hanle effect.
Title: Scientific Drivers for a Future Polarimetric Mode for the
VLTI: VISPER
Authors: Vakili, F.; Chesneau, O.; Delplancke, F.; Jankov, S.; Perraut,
K.; Stehle, C.; Stenflo, J.
Bibcode: 2002sdef.conf..331V
Altcode:
The scientific drivers and the concept of a polarimetric instrument
for the VLTI, VISPER (Vlti Imaging Spectro-PolarimetER) will be
presented. Many scientific programs in stellar physics as well
as in extra-galactic astronomy would benefit from interferometric
high-resolution imaging combined with polarimetry. For instance,
we can study scattering phenomena in the extended atmospheres of
hot stars, the circumstellar environment of young stellar objects,
or AGN nuclei. In addition, polarimetric observations allow getting
information related to stellar magnetic fields through the detection
of spatially resolved Zeeman effect. Indeed this kind of studies
requires very accurate measurements of the interferometric fringe
phase and visibility as well as good spectral resolution (>
10000) and a very stable instrument. As great care was taken of
instrumental polarization effects during the design and construction
of the VLTI, polarimetry could be straightforwardly implemented in the
interferometric laboratory. Moreover, the phase-reference astrometric
facility of the VLTI (PRIMA) will offer a very stable and accurate
reference to measure small polarization effects on the visibility and
astrometric position of the star at certain spectral lines.
Title: Polarized radiation diagnostics of solar magnetic fields
Authors: Stenflo, Jan Olof
Bibcode: 2002apsp.conf...55S
Altcode:
The Sun is unique as an astrophysics laboratory because we can spatially
resolve its structures in great detail and apply sophisticated
diagnostic techniques that require high spectral resolution. The
magnetic flux in the solar atmosphere occurs in extremely fragmented,
nearly fractal form, with a range of spatial scales that extend well
beyond the angular resolution limit of current telescopes and into
the optically thin regime. The magnetic field leaves various kinds
of "fingerprints" in the polarized spectrum. In the past only the
fingerprints of the Zeeman effect have been used, but more recently
new, highly sensitive imaging polarimeters have given us access to
other physical effects. In particular a wealth of previously unknown
spectral structures due to coherent scattering processes have been
uncovered. These phenomena show up in linear polarization as a new
kind of spectrum (the so-called "second solar spectrum"), which bear
little resemblance to the ordinary intensity spectrum. Magnetic fields
modify the coherent scattering processes and produce polarized spectral
signatures that greatly extend the diagnostic range of the Zeeman
effect. This diagnostic window has just been opened, and we are only
now beginning to develop the needed diagnostic tools and apply them
to learn about previously "invisible" aspects of solar magnetic fields.
Title: Imaging Polarimetry and Spectropolarimetry of Bright Objects
Authors: Schmid, H. M.; Appenzeller, I.; Stenflo, J. O.; Kaufer, A.
Bibcode: 2002sdef.conf..231S
Altcode:
Polarimetric measurements are a universal tool in astronomy from solar
system studies to high redshift objects. Polarized light is produced
through e.g. light scattering, line emission/absorption in magnetic
fields or synchrotron radiation, which are all ubiquitous physical
processes in astronomy. ESO offers polarimetric instruments for the
investigation of faint targets but no device for high signal-to-noise
(S/N) polarimetry of bright targets. In this contribution we describe
a technology which is able to improve the polarimetric precision
for imaging polarimetry and spectropolarimetry for bright objects
by about two orders of magnitudes in S/N from 103 to
105. This improvement in measuring accuracy will open
up many new opportunities of investigation, e.g. for stellar and
interstellar magnetic fields, for scattering gas and dust structures
near stars and active galactic nuclei, or for reflected light from
close-in extra-solar planets and proto-planetary disks.
Title: Solar Magnetic Field: Zeeman and Hanle Effects
Authors: Stenflo, J.
Bibcode: 2001eaa..bookE2236S
Altcode:
An external magnetic field causes the atomic energy levels to split into
different sublevels, and the emitted radiation becomes polarized. This
phenomenon is called the ZEEMAN EFFECT. When atoms in a magnetic field
scatter radiation via bound-bound transitions, the phase relations or
quantum interferences between the Zeeman-split sublevels give rise to
POLARIZATION phenomena that go under the nam...
Title: New possibilities for the diagnostics of solar magnetic fields
Authors: Stenflo, J. O.
Bibcode: 2001A&AT...20..515S
Altcode:
The polarized spectrum that is produced by coherent scattering processes
has a structural richness comparable to that of the intensity spectrum
but different in appearance and physical origin. The amplitudes of the
polarization features are influenced by magnetic fields via the Hanle
effect in a way that is very different different from the ordinary
Zeeman effect. While the main contribution to Zeeman-effect observations
comes from the strong fields of the photospheric magnetic flux tubes,
the Hanle effect is sensitive to weak magnetic fields, turbulent fields
of mixed polarities, and chromospheric fields. As different spectral
lines respond very differently to the Hanle effect, the scattering
polarization offers novel and rich diagnostic opportunities. In the
present overview we illustrate some of these new effects and indicate
what can be learnt from them.
Title: Stokes profile measurements in each sodium D-line using single
and dual band magneto-optical filters on board Solar Orbiter
Authors: Cacciani, A.; Rapex, P.; Dolci, M.; Reale, F.; Landi, E.;
Stenflo, J.; Bianda, M.; Moses, D.
Bibcode: 2001ESASP.493..177C
Altcode: 2001sefs.work..177C
No abstract at ADS
Title: Influence of magnetic fields on the coherence effects in the
Na I D1 and D2 lines
Authors: Stenflo, J. O.; Gandorfer, A.; Wenzler, T.; Keller, C. U.
Bibcode: 2001A&A...367.1033S
Altcode:
To clarify the physical nature of the enigmatic scattering
polarization in the Na i D1 and D2 line cores
we have explored their behavior with full Stokes vector polarimetry
in regions with varying degree of magnetic activity near the solar
limb. These observations represent the first time that ZIMPOL II,
the second generation of our CCD based imaging polarimeter systems,
has been used for a scientific program. With ZIMPOL II the four Stokes
images can be demodulated and recorded with a single CCD sensor such
that the resulting images of the fractional polarization Q/I, U/I,
and V/I are entirely free from spurious features due to seeing or
flat-field effects. The polarization in the cores of the lines, in
particular in D2, exhibits dramatic and unexpected spatial
variations in both Q/I and U/I, including polarization self-reversals
of the D2 Q/I core peak. As the fluctuations in the Q,
U, and V parameters appear to be relatively uncorrelated, we have
parametrized the profiles and made scatter plots of the extracted
parameters. Comparison with synthetic scatter plots based on different
theoretical models suggests that the polarization signals in the cores
of the D2 and D1 lines have different physical
origins: While the D1 core is likely to be governed by
ground-state atomic polarization, the D2 core is dominated by
the alignment of the excited state and by effects of partial frequency
redistribution.
Title: Observation of Scattering Polarization and the Diagnostics
of Solar Magnetic Fields
Authors: Stenflo, J. O.
Bibcode: 2001ASPC..236...97S
Altcode: 2001aspt.conf...97S
No abstract at ADS
Title: Depolarizing Lines in the Solar Spectrum
Authors: Fluri, D. M.; Stenflo, J. O.
Bibcode: 2001ASPC..236..205F
Altcode: 2001aspt.conf..205F
No abstract at ADS
Title: Hanle Effect Observations in the UV with the Mg I Multiplet
at 3829 - 3838 Å
Authors: Bianda, M.; Stenflo, J. O.
Bibcode: 2001ASPC..236..117B
Altcode: 2001aspt.conf..117B
No abstract at ADS
Title: Limitations and Opportunities for the Diagnostics of Solar
and Stellar Magnetic Fields
Authors: Stenflo, J. O.
Bibcode: 2001ASPC..248..639S
Altcode: 2001mfah.conf..639S
No abstract at ADS
Title: Comparison of far-ultraviolet emission lines formed in coronal
holes and the quiet Sun
Authors: Stucki, K.; Solanki, S. K.; Schühle, U.; Rüedi, I.; Wilhelm,
K.; Stenflo, J. O.; Brković, A.; Huber, M. C. E.
Bibcode: 2000A&A...363.1145S
Altcode:
We present an analysis of 26 far-ultraviolet emission lines belonging
to 19 atoms and ions observed on both sides of the boundary of polar
coronal holes as well as other quiet Sun areas along the limb. The
observations were made with the SUMER instrument (Solar Ultraviolet
Measurements of Emitted Radiation) onboard the Solar and Heliospheric
Observatory (SOHO). We compare line intensities, shifts and widths
in coronal holes with the corresponding values obtained in the quiet
Sun. We find that with increasing formation temperature, spectral
lines show on average an increasingly stronger blueshift in coronal
holes relative to the quiet Sun at equal heliospheric angle, with the
coolest lines in our sample (formation temperature ~ 104
K) indicating a small relative redshift. With respect to the rest
wavelength, however, only lines formed above 5 * 105 K
show blueshifts in coronal holes, which is not very different from
the quiet Sun. The width of the lines is generally larger (by a few
kilometers per second) inside the coronal hole. Intensity measurements
clearly show the presence of the coronal hole in Ne VIII lines as well
as in Fe XII, and provide evidence for a slightly enhanced emission
in polar coronal holes for lines formed below 105 K. This
last result is, however, less certain than the rest due to relatively
poor statistics. Intensity histograms also exhibit distinct differences
between coronal hole and quiet-Sun data. For cooler chromospheric lines,
such as Ni II, the coronal holes display a greater spread in intensities
than the quiet Sun. Transition-region lines, e.g. O IV, do not reveal
such differences, while Ne VIII shows characteristics of a coronal line
with lower average intensity and lower intensity spread inside holes.
Title: Solar Photosphere: Intranetwork and Turbulent Magnetic Fields
Authors: Stenflo, J.
Bibcode: 2000eaa..bookE2009S
Altcode:
The names `intranetwork' and `turbulent magnetic fields' are used to
represent the solar magnetic fields of mixed polarities at the smallest
scales of the spatial spectrum. Since the spatial separation of the
opposite polarities is small, and since the magnetic flux of each
small-scale magnetic element is tiny, they can only be made partly
visible in `deep' magnetograms obtained with both high sp...
Title: Summary Lecture
Authors: Stenflo, J. O.
Bibcode: 2000JApA...21..451S
Altcode:
No abstract at ADS
Title: What Can We Learn About the Corona from Polarization
Measurements? (Invited review)
Authors: Stenflo, J. O.
Bibcode: 2000ASPC..205...41S
Altcode: 2000ltse.conf...41S
Polarization is produced by a variety of physical processes, including
anisotropic excitation by radiation or particle beams, magnetic fields,
and macroscopic electric fields. Many of the potential applications
of polarimetry for the diagnostics of coronal physics still lie in
the future, some even in the rather distant future. Magnetic-field
measurements in the corona are notoriously difficult, both because of
the low intensity and because the corona is optically thin. Stokes
inversion of polarization maps to obtain magnetic-field maps are
possible for evolving structures only if stereoscopic viewing is
available. Without such capabilities polarization measurements in
different portions of the spectrum are still of considerable diagnostic
value to place constraints, although incomplete, on different aspects
of coronal physics, which cannot be constrained by other means.
Title: Center-to-limb variation of the enigmatic Na bt I D_1 and
D_2 polarization profiles
Authors: Stenflo, J. O.; Gandorfer, A.; Keller, C. U.
Bibcode: 2000A&A...355..781S
Altcode:
The remarkable polarization structure of the Na i D_1 and D_2 lines
that is due to coherent scattering has remained an enigma, since it has
not yet been possible to find an explanation that is consistent with
both current understanding of quantum mechanics and the astrophysical
properties of the Sun's atmosphere. To guide future theoretical efforts
we have here explored the detailed center-to-limb variation of the
linearly polarized profiles in non-magnetic regions. In particular we
find that the unexplained narrow polarization peaks in the Doppler
cores of the two lines become even more pronounced with respect to
the relative profile shape as we move away from the limb towards the
center of the solar disk.
Title: Anomalous polarization effects due to coherent scattering on
the Sun
Authors: Stenflo, J. O.; Keller, C. U.; Gandorfer, A.
Bibcode: 2000A&A...355..789S
Altcode:
The richly structured linearly polarized spectrum that is produced
by coherent scattering in the Sun's atmosphere contains a number of
spectral features for which no explanation has been found within the
standard scattering theory. According to this quantum-mechanical
framework, the intrinsic polarizability of a given line should be
determined by the total angular momentum quantum numbers of the
atomic levels involved in the scattering transition (which may be
resonant or fluorescent). Well defined polarization peaks have been
observed in many lines, which according to these theoretical concepts
should be intrinsically unpolarizable. A possible explanation for
these anomalous spectral structures could be that the initial ground
state of the scattering transition becomes polarized by an optical
pumping process. However, such an explanation is contradicted by
other observations, since it seems to require that much of the solar
atmosphere must be filled with extremely weak magnetic fields (<~
10 mG). We have searched through the whole visible solar spectrum
for lines with the quantum numbers that should normally make them
unpolarizable, and have carried out a systematic observing program
for the most prominent of these lines. Here we report on the observed
properties of the polarized line profiles of these lines and explain
in what respect their behaviors are anomalous and cannot be understood
within current conceptual frameworks.
Title: EUV brightness variations in the quiet Sun
Authors: Brković, A.; Rüedi, I.; Solanki, S. K.; Fludra, A.;
Harrison, R. A.; Huber, M. C. E.; Stenflo, J. O.; Stucki, K.
Bibcode: 2000A&A...353.1083B
Altcode:
The Coronal Diagnostic Spectrometer (CDS) onboard the SOHO satellite has
been used to obtain movies of quiet Sun regions at disc centre. These
movies were used to study brightness variations of solar features at
three different temperatures sampled simultaneously in the chromospheric
He I 584.3 Ä (2 * 104 K), the transition region O V 629.7
Ä (2.5 * 105 K) and coronal Mg IX 368.1 Ä (106
K) lines. In all parts of the quiet Sun, from darkest intranetwork to
brightest network, we find significant variability in the He I and O
V line, while the variability in the Mg IX line is more marginal. The
relative variability, defined by rms of intensity normalised to the
local intensity, is independent of brightness and strongest in the
transition region line. Thus the relative variability is the same
in the network and the intranetwork. More than half of the points
on the solar surface show a relative variability, determined over a
period of 4 hours, greater than 15.5% for the O V line, but only 5%
of the points exhibit a variability above 25%. Most of the variability
appears to take place on time-scales between 5 and 80 minutes for the
He I and O V lines. Clear signs of ``high variability'' events are
found. For these events the variability as a function of time seen
in the different lines shows a good correlation. The correlation is
higher for more variable events. These events coincide with the (time
averaged) brightest points on the solar surface, i.e. they occur in
the network. The spatial positions of the most variable points are
identical in all the lines.
Title: Hanle effect observations with the CA BT I 4227 Å line
Authors: Bianda, M.; Stenflo, J. O.; Solanki, S. K.
Bibcode: 1999A&A...350.1060B
Altcode:
The Hanle effect in the Ca i 4227 Ä line has been explored through
the analysis of a large number of Stokes profile recordings obtained
on the quiet Sun with the beam-splitter polarimeter system at IRSOL
(Istituto Ricerche Solari Locarno). In contrast to previous Hanle
observations with this line, which were limited to the Stokes I and
Q parameters, we are now in a position to study the combined effects
of Hanle depolarization (via Stokes Q) and rotation of the plane
of linear polarization (via Stokes U) with the same methods that
we recently applied to the Sr ii 4078 Ä line. The Hanle histograms
for the distributions of the depolarization and rotation parameters
are very similar for the two lines and show that there must be mixed
contributions to the Hanle signals from spatially unresolved magnetic
fields with random orientations (which do not contribute to Stokes U)
and partially resolved magnetic fields with a net orientation of the
field vectors. Field strengths in the range 5-10 G are preferred. We
also determine the ``Hanle efficiency profile'', which shows how the
Hanle effect is confined to the Doppler core but vanishes in the line
wings. It is wider than the corresponding profile for the Sr line,
as expected from the difference in atomic weight and wavelength
between the two lines. The Q/I profiles of the Ca i 4227 Ä line
have minima around the Doppler core which turn negative (polarization
perpendicular to the limb) for limb distances mu =cos theta >~ 0.2,
a likely signature of partial redistribution effects.
Title: Partial frequency redistribution with Hanle and Zeeman
effects. Non-perturbative classical theory
Authors: Bommier, V.; Stenflo, J. O.
Bibcode: 1999A&A...350..327B
Altcode:
A theory for the scattering of polarized radiation with partial
frequency redistribution and coherence effects in the presence of
magnetic fields of arbitrary strength and direction is developed within
a classical framework. The time-dependent equation for a classical
oscillator is solved. While the oscillator is being excited, it is
also damped by emission of radiation and subject to phase-destroying
collisions. Fourier transformation of the emitted wave train with
phase-scrambling collisions leads to the partial-redistribution
expressions for the relation between the polarization and frequencies
of the incident and scattered radiation. While previous treatments
of partial redistribution have been based on quantum perburbation
theory, the classical theory has the advantage of being fully
non-perturbative. It is therefore conceptually more transparent
and leads itself to direct physical interpretation. The classical
and quantum theories give identical results for a J=0-> 1->
0 transition.
Title: Solar polarization
Authors: Nagendra, K. N.; Stenflo, J. O.
Bibcode: 1999ASSL..243.....N
Altcode: 1999sopo.conf.....N
No abstract at ADS
Title: Observations of the Hanle effect in the Ca I 4227 and Sr II
4078 Å lines
Authors: Bianda, M.; Stenflo, J. O.; Solanki, S. K.
Bibcode: 1999ASSL..243...31B
Altcode: 1999sopo.conf...31B
No abstract at ADS
Title: Coronal Hole Properties Observed with SUMER
Authors: Stucki, K.; Solanki, S. K.; Rüedi, I.; Stenflo, J. O.;
Brković , A.; Schühle, U.; Wilhelm, K.; Huber, M. C. E.
Bibcode: 1999SSRv...87..315S
Altcode:
We analyze SUMER spectra of 14 lines belonging to 12 ions, obtained
on both sides of the boundary of polar coronal holes as well as at
other locations along the limb. We compare line intensities, shifts
and widths in coronal holes with values obtained in the quiet Sun. We
find that with increasing formation temperature, spectral lines show
an increasingly stronger blueshift in coronal holes relative to the
quiet Sun at an equal heliospheric angle. The width of the lines is
generally larger (by a few km/s) inside the coronal hole. Intensity
measurements show the presence of the coronal hole in Ne VIII lines
as well as in Fe XII, with evidence for a slightly enhanced emission
in polar coronal holes for lines formed below 105 K.
Title: Coronal Holes Versus Normal Quiet Sun Observed with SUMER
Authors: Stucki, K.; Solanki, S. K.; Rüedi, I.; Stenflo, J. O.;
Brković, A.; Schühle, U.; Wilhelm, K.; Huber, M. C. E.
Bibcode: 1999Ap&SS.264...53S
Altcode: 1998Ap&SS.264...53S
We present a preliminary analysis of spectral lines obtained with the
SUMER instrument (Solar Ultraviolet Measurements of Emitted Radiation)
onboard the Solar and Heliospheric Observatory (SOHO), as observed
during three observing campaigns. From the 70 observed spectral lines,
we selected 12, representing 9 ions or atoms, in order to analyse line
intensities, shifts and widths in polar coronal holes as well as in the
normal quiet Sun. We find that coronal lines show a distinct blueshift
in coronal holes relative to the quiet Sun at equal heliospheric
angle, while there is no evidence for such a shift for lines formed at
temperatures below 10^5 K. The widths of lines formed at temperatures
above 3 - 10^4 K are slightly increased inside the coronal hole,
but unaffected for lower temperatures. Intensity measurements clearly
show the center-to-limb variation, as well as an intensity diminution
inside the coronal hole for lines formed above approximately 10^5 K.
Title: Continuum polarization and blend lines in the solar spectrum
Authors: Fluri, D. M.; Stenflo, J. O.
Bibcode: 1999ASSL..243..171F
Altcode: 1999sopo.conf..171F
No abstract at ADS
Title: Continuum polarization in the solar spectrum
Authors: Fluri, D. M.; Stenflo, J. O.
Bibcode: 1999A&A...341..902F
Altcode:
We present a theoretical study of the continuum polarization due to
radiative scattering in the visible solar spectrum. The results from
nine different solar model atmospheres are compared. The center-to-limb
variation (CLV) as well as the wavelength dependence of the continuum
polarization are determined, and the sources of the dependence on the
model atmospheres are identified. The key physical quantities turn out
to be the scattering coefficient and the temperature gradient in the
layer where the polarization is formed. A simple analytical function
that approximates the CLV of the theoretical continuum polarization
for every wavelength in the visible is found. This is based in first
approximation on the assumption that the scattering layer producing the
polarization is optically thin and lies above the layer of formation of
the continuum intensity. Applications of the analytical function range
from determinations of the instrumental zero-level of the polarization
scale to diagnostic work using empirical center-to-limb curves to
constrain the solar model atmospheres.
Title: Brightness Variations in the Solar Atmosphere as Seen by SOHO
Authors: Brkovic, A.; Rüedi, I.; Solanki, S. K.; Huber, M. C. E.;
Stenflo, J. O.; Stucki, K.; Harrison, R.; Fludra, A.
Bibcode: 1999ASSL..239..231B
Altcode: 1999msa..proc..231B
We present preliminary results of a statistical analysis of the
brightness variations of solar features at different levels in
the solar atmosphere. We observed quiet Sun regions at disc centre
using the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and
Heliospheric Observatory (SOHO). We find significant variability at all
time scales in all parts of the quiet Sun, from darkest intranetwork
to brightest network. Such variations are observed simultaneously in
the chromospheric He I 584.33 Angstroms (2 \cdot 10^4 K) line, the
transition region O V 629.74 Angstroms (2.5 \cdot 10^5 K) and coronal
Mg IX 368.06 Angstroms (10^6 K) line. The relative variability is
independent of brightness and most of the variability appears to take
place on time scales longer than 5 minutes for all 3 spectral lines. No
significant differences are observed between the different data sets.
Title: Solar magnetism and the second solar spectrum: Future
directions
Authors: Stenflo, J. O.
Bibcode: 1999ASSL..243....1S
Altcode: 1999sopo.conf....1S
No abstract at ADS
Title: Hanle-Zeeman scattering matrix
Authors: Stenflo, J. O.
Bibcode: 1998A&A...338..301S
Altcode:
A theory is presented that allows the Mueller matrix for coherent
scattering to be calculated for arbitrary magnetic fields,
atomic multiplets, and scattering transitions (Rayleigh or Raman
scattering). For the special case of a normal Zeeman triplet (a
J=0-> 1-> 0 scattering transition) a compact analytical form
for the scattering matrix is given, which allows us to better see how
the various field-strength regimes are connected. A number of limiting
cases are retrieved from the general theory, including the weak-field
Hanle phase matrix, the polarization of forbidden coronal lines
(strong-field limit), ``thermal'' radiation (emission vector in LTE)
and incoherent scattering. The analytical form for the transition of
the Hanle effect from the line core (where it is present) to the line
wings (where it is absent) is given.
Title: Hanle diagnostics of solar magnetic fields: the SR II 4078
Angstrom line
Authors: Bianda, M.; Stenflo, J. O.; Solanki, S. K.
Bibcode: 1998A&A...337..565B
Altcode:
The Hanle depolarization and rotation effects in the Sr ii
4078\tsAngstroms line have been explored with the instrumentation at
IRSOL (Istituto Ricerche Solari Locarno) by recording the Stokes I,
Q, and U line profiles with high spectral resolution and polarimetric
accuracy in a large number of regions across the solar disk. >From
the extracted line parameters we have constructed ``Hanle histograms''
showing the statistical distributions of the Hanle rotation and
depolarization effects. Comparison with theoretical calculations allow
these histograms to be understood in terms of magnetic fields with a
strength of about 5-10\ts G, which is similar to the field strengths
previously found through analysis of Q/I Hanle depolarization in the
Ca i 4227\tsAngstroms line. While small-scale magnetic fields with
spatially unresolved angular distributions contribute to the observed
Hanle depolarization effects, the observed Hanle rotation effects in
Stokes U are due to spatially resolved fields with net large-scale
orientations (e.g. global or canopy-type fields). We have also for the
first time determined empirical ``Hanle efficiency profiles'', derived
independently for the Hanle rotation and depolarization effects. They
show how the Hanle efficiency has its maximum in the Doppler core of
the line and then rapidly decreases to become zero in the line wings.
Title: Oscillations of sunspot magnetic fields
Authors: Rueedi, I.; Solanki, S. K.; Stenflo, J. O.; Tarbell, T.;
Scherrer, P. H.
Bibcode: 1998A&A...335L..97R
Altcode:
We report on velocity and magnetic field oscillations observed
in sunspots using the MDI instrument onboard SOHO. In addition to
the well-known velocity oscillations, the data clearly show highly
localised oscillations of the magnetogram signal in different parts
of the sunspots. We show that only oscillations of the magnetic field
vector can produce the observed magnetogram oscillations, and that the
observed phase relations suggest an origin in terms of magnetoacoustic
gravity waves.
Title: Hanle depolarisation in the solar chromosphere
Authors: Bianda, M.; Solanki, S. K.; Stenflo, J. O.
Bibcode: 1998A&A...331..760B
Altcode:
A new polarimeter and an extension of the polarimetric technique of
Donati et al. (1990) to higher polarisation values are presented. With
this system, which allows low-noise polarimetric observations in the
blue and near-UV part of the solar spectrum, we have recorded more than
200 Q/I profiles of Ca I 4227 Angstroms with an accuracy of 2-3x 10(-4)
. We find that while the Q/I line shape outside the line core is very
stable from one part of the solar surface to the next, the line core
exhibits large variations relative to the rest of the profile. These
variations are best interpreted in terms of partial depolarisation
produced by the Hanle effect in the presence of weak fields. We obtain
a quantitative and model-independent estimate of the depolarisation,
from which we deduce field strengths of 5-15 G at the level of line-core
formation, i.e. in the low to mid chromosphere. We also find evidence
that the field strength is distributed around these average values
with a FWHM that is approximately the same as the average value.
Title: Complex magnetic fields in an active region
Authors: Bernasconi, P. N.; Keller, C. U.; Solanki, S. K.; Stenflo,
J. O.
Bibcode: 1998A&A...329..704B
Altcode:
High-resolution observations of the full Stokes vector in Fe\sc i
spectral lines around 5250 Angstroms obtained at the Swedish Vacuum
Solar Telescope on La Palma with the ZIMPOL I Stokes polarimeter in a
complex active region reveal the presence of anomalously shaped Stokes
profiles indicating the coexistence of at least two magnetic components
within the same spatial resolution element. These Stokes profiles have
been analyzed with an inversion code based on a 3-component atmospheric
model with two magnetic and one field-free component. The fits to
the observations in a magnetic region that resembles a small penumbra
reveal the presence of a horizontal magnetic field component with an
average field strength of /line{B}=840 G, a mean filling factor of
/lineα=0.12, and an average temperature /line{T}=5400 K at log {tau_
{5000}}=-1.5 embedded in the main ``penumbral'' magnetic field that
has /line{B}=1500 G, /lineα=0.56, and /line{T}=4900 K. The horizontal
component exhibits a mean outflow of 2.7 km s(-1) which is mainly due
to the Evershed flow. In a region where there are strong downflows up
to 7 km s(-1) , we infer the possible presence of a shock front whose
height changes along the slit. The height variation can be explained by
a change of the gas pressure at the base of the photosphere below the
shock front as proposed by Thomas & Montesinos (1991). Small plages
with field strengths below 900 G have been observed in the vicinity
of some pores. Finally, we present a puzzling field structure at the
boundary between two adjacent pores. Ambiguous results suggest that
although the inversion code is able to successfully invert even very
complex Stokes profiles, we are far from a complete description of the
field structure in complex magnetic regions. We warn that magnetograms
and fits to data involving only a single magnetic component may hide
the true complexity of the magnetic structure in at least some parts
of active regions.
Title: Kitt Peak, MacMath, the Sun and JOSO
Authors: Stenflo, Jan O.
Bibcode: 1998ASSL..222...82S
Altcode: 1998ream.conf...82S
No abstract at ADS
Title: Differential Hanle effect and the spatial variation of
turbulent magnetic fields on the Sun
Authors: Stenflo, J. O.; Keller, C. U.; Gandorfer, A.
Bibcode: 1998A&A...329..319S
Altcode:
While diagnostic techniques based on the ordinary Zeeman effect
(e.g. magnetograms) are almost ``blind'' to a turbulent magnetic field
with mixed magnetic polarities within the spatial resolution element,
the Hanle effect is sensitive to this domain of solar magnetism. We
present observational evidence that the turbulent magnetic field that
fills the 99\ts%\ of the volume between the kG flux tubes in quiet
solar regions does not have a unique field-strength distribution,
but the rms turbulent field strength can vary by an order of magnitude
from one solar location to the next. The varying Hanle depolarization
in combinations of spectral lines with different sensitivities to the
Hanle effect is conspicuously evident from direct visual inspection
of the spectra. To quantify these variations we have extracted the
polarization amplitudes for a selection of spectral lines observed in 8
different solar regions with different turbulent field strengths, and
then applied an inversion technique to find the field strengths and
calibrate the selected lines. The inversion gives stable solutions
for the turbulent field strengths, in the range 4--40\ts G, but
the field-strength scale is presently very uncertain. The inversion
exercise has helped to expose a number of problem areas which need to be
attended to before the differential Hanle effect can become a standard,
reliable diagnostic tool. One major problem is the extraction of the
line polarization when the contributions from the line and continuum
are of the same order of magnitude, which is the usual case. For
exploratory purposes we have applied a heuristic, statistical approach
to deal with this problem here.
Title: Sunspot Oscillations Observed with MDI
Authors: Rüedi, I.; Solanki, S. K.; Stenflo, J. O.; Scherrer, P. H.
Bibcode: 1998ESASP.417..281R
Altcode: 1998cesh.conf..281R
No abstract at ADS
Title: Quantum interferences, hyperfine structure, and Raman
scattering on the Sun.
Authors: Stenflo, J. O.
Bibcode: 1997A&A...324..344S
Altcode:
Observations of the scattering polarization in the solar spectrum have
opened a new window for diagnostics of the Sun. In the present paper
a theoretical basis for the interpretation of the linearly polarized
spectral structures is developed. It covers the general case of Raman
scattering for entire atomic multiplets and includes the quantum
interferences between the various possible excited states. Although
the formulation allows for the presence of magnetic fields of arbitrary
strength and direction, the theory is expressed in explicit form only
for the case of zero magnetic field. It is applied to identify and
interpret observed spectral signatures of quantum interferences between
fine structure and hyperfine structure components, isotope effects,
and fluorescence within multiplets.
Title: Center-to-limb variation of the second solar spectrum.
Authors: Stenflo, J. O.; Bianda, M.; Keller, C. U.; Solanki, S. K.
Bibcode: 1997A&A...322..985S
Altcode:
The linear polarization that is caused by scattering processes in the
solar atmosphere has been refered to as the "second solar spectrum",
since it is structurally as rich as the ordinary intensity spectrum
but quite different in appearance and information contents. One
of the most used and theoretically best understood lines in the
second solar spectrum is the SrI 4607Å line, which has served as a
diagnostic tool for determinations of spatially unresolved, turbulent
magnetic fields via the Hanle effect. Here we present the detailed
center-to-limb variation of the scattering polarization in this line
for a number of new data sets obtained both with an electrooptical
modulation system (ZIMPOL) and a non-modulating beam splitter system
(at IRSOL, Locarno), to provide improved observational constraints for
theoretical modelling. The amplitude and width of the polarization
profile, the amount of continuum polarization, as well as the depth
and width of the intensity profile have been evaluated and carefully
corrected for spectral broadening and stray light. While there is
generally good agreement between the five data sets, some systematic
differences are shown to be of solar rather than instrumental origin,
most likely due to spatially varying Hanle depolarization across the
solar disk. A number of other spectral lines have been observed with
the ZIMPOL system at two different limb distances (μ=0.1 and 0.2) to
allow us to compare the steepness of the center-to-limb variation of
their polarization amplitudes. The steepest variation is exhibited by
the continuum polarization, which declines by approximately a factor of
6 when going the 15 arcsec distance from μ=0.1 to μ=0.2. The spectral
lines with the steepest center-to-limb variation are molecular lines,
the CaII infrared triplet, and Hα. In contrast the SrI 4607 and BaII
4554Å lines have only moderately steeper center-to-limb variations
than that of an ideal, purely dipole-scattering atmosphere, for
which the polarization ratio between μ=0.1 and μ=0.2 is 1.38. These
center-to-limb variations may be used to constrain temperature-density
models of the upper photosphere and chromosphere.
Title: The second solar spectrum. A new window for diagnostics of
the Sun.
Authors: Stenflo, J. O.; Keller, C. U.
Bibcode: 1997A&A...321..927S
Altcode:
The Sun's radiation becomes linearly polarized by coherent scattering
processes in the solar atmosphere. With a novel polarimetry system
that achieves a precision of 10^-5^ in the degree of polarization, the
previously largely unexplored territory of scattering physics on the Sun
is now fully accessible. The observations reveal a polarized spectrum
that looks very different as compared with the ordinary, unpolarized
solar spectrum but has an astounding wealth of spectral structures. It
is therefore refered to as the "second solar spectrum". In the present
paper we show how the second solar spectrum is governed by different
physical processes, which provide new diagnostic opportunities and tools
that are complementary to those of the ordinary intensity spectrum. We
illustrate the effects of quantum interferences and hyperfine structure,
isotope abundances, partial frequency redistribution, molecular
contributions, and magnetic canopies. Also shown are polarization
features, for which the underlying physics has not yet been identified.
Title: Properties of Brightenings Seen in CDS Movies
Authors: Rüedi, I.; Brkovic, A.; Solanki, S. K.; Harrison, R.;
Fludra, A.; Huber, M. C. E.; Stenflo, J. O.; Stucki, K.
Bibcode: 1997ESASP.404..641R
Altcode: 1997cswn.conf..641R
No abstract at ADS
Title: New window for spectroscopy
Authors: Stenflo, J. O.
Bibcode: 1996Natur.382..588S
Altcode:
No abstract at ADS
Title: The Second Solar Spectrum
Authors: Keller, C. U.; Stenflo, J. O.
Bibcode: 1996AAS...188.5704K
Altcode: 1996BAAS...28..912K
The Sun's radiation becomes linearly polarized by coherent scattering
processes in the solar atmosphere. With a novel imaging polarimeter
(Zurich Imaging Stokes Polarimeter I) that achieves a precision of
10(-5) in the degree of polarization, the previously largely unexplored
territory of scattering physics on the Sun is now fully accessible. The
observations reveal a polarized spectrum that looks very different from
the ordinary, unpolarized solar spectrum but has an astounding wealth
of spectral structures from atoms as well as molecules. This second
solar spectrum will be used as a new source of information to learn
more about atomic physics, abundances and isotope ratios, radiative
transfer physics, solar magnetic fields, and the thermodynamics of
the solar atmosphere.
Title: Scattering Physics
Authors: Stenflo, J. O.
Bibcode: 1996SoPh..164....1S
Altcode:
The theory of polarized scattering in a stellar atmosphere is
formulated, first within the framework of classical physics, then in
terms of quantum mechanics. The expression for the redistribution matrix
that describes partial redistribution in polarization and frequency is
derived for the general case when the magnetic field is of arbitrary
strength. The special cases of weak fields (the "Hanle limit") and
zero fields (non-magnetic scattering) are discussed. Observational
examples of spectral signatures in linear polarization are presented,
which show effects of hyperfine structure, interference between fine
structure components, and molecular scattering.
Title: Solar polarization. Proceedings. International Workshop on
Solar Polarization, St. Petersburg (Russia), 8 - 12 May 1995.
Authors: Stenflo, J. O.; Nagendra, K. N.
Bibcode: 1996SoPh..164.....S
Altcode:
The following topics were dealt with: Solar physics, radiative transfer,
solar spectra, polarization, Hanle effect, coronal refraction, Stokes
profile inversion, solar prominences, solar atmosphere, imaging
polarimetry method, magnetic inclination of plages, field azimuth
disambiguation, solar pore polarimetry, sunspots, solar flares,
magnetographs, VUV polarization measurement.
Title: Solar polarization
Authors: Stenflo, J. O.; Nagendra, K. N.
Bibcode: 1996sopo.conf.....S
Altcode: 1996QB539.M23S66...
No abstract at ADS
Title: Direct measurements of flux tube inclinations in solar plages.
Authors: Bernasconi, P. N.; Keller, C. U.; Povel, H. P.; Stenflo, J. O.
Bibcode: 1995A&A...302..533B
Altcode:
Observations of the full Stokes vector in three spectral lines
indicate that flux tubes in solar plages have an average inclination
in the photosphere of 14^o^ with respect to the local vertical. Most
flux tubes are inclined in the eastwards direction, i.e., opposite
to the solar rotation. We have recorded the Stokes vector of the
FeI 5247.1A, FeI 5250.2A, and FeI 5250.7A lines in nine different
plages with the polarization-free 20cm Zeiss coronagraph at the Arosa
Astrophysical Observatory of ETH Zuerich. The telescope has been
modified for solar disk observations. The chosen spectral lines are
particularly sensitive to magnetic field strength and temperature. To
determine the field strength and geometry of the flux tubes in the
observed plages we use an inversion code that numerically solves the
radiative transfer equations and derives the emergent Stokes profiles
for one-dimensional model atmospheres consisting of a flux tube and
its surrounding non-magnetic atmosphere. Our results confirm earlier
indirect estimates of the inclination of the magnetic fields in plages.
Title: Visible and near-infrared polarimetry with LEST.
Authors: Keller, C. U.; Bernasconi, P. N.; Egger, U.; Povel, H. P.;
Steiner, P.; Stenflo, J. O.
Bibcode: 1995LFTR...59.....K
Altcode:
This document describes the LEST vector polarimeters for the visible
and the near-infrared part of the solar spectrum. After some general
remarks on precise polarimetry with large telescopes, the authors
present the specifications based on scientific reasons and some
general design considerations. The proposed instrument design for the
visible is based on the ZIMPOL II concept. They present two different
concepts for vector polarimetry in the near infrared. One is based
on a beam-splitter system combined with liquid crystal modulators,
while the other is based on the same modulator package as used in the
visible and optical demodulation in the final focus.
Title: Visible and Near Infrared Polarimetry with lest
Authors: Keller, C. U.; Bernasconi, P. N.; Egger, U.; Powel, H. P.;
Steiner, P.; Stenflo, J. O.
Bibcode: 1995lest.rept....1K
Altcode:
No abstract at ADS
Title: Solar Magnetic Fields: Polarized Radiation Diagnostics
Authors: Stenflo, Jan
Bibcode: 1994ASSL..189.....S
Altcode:
Magnetic fields are responsible for much of the variability and
structuring in the universe, but only on the Sun can the basic magnetic
field related processes be explored in detail. While several excellent
textbooks have established a diagnostic foundation for exploring the
physics of unmagnetized stellar atmospheres through spectral analysis,
no corresponding treatise for magnetized stellar atmospheres has
been available. The present monograph fills this gap. The theoretical
foundation for the diagnostics of stellar magnetism is developed from
first principles in a comprehensive way, both within the frameworks of
classical physics and quantum field theory, together with a presentation
of the various solar applications. This textbook can serve as an
introduction to solar and stellar magnetism for astronomers and
physicists at the graduate or advanced undergraduate level and will
also become a resource book for more senior scientists with a general
interest in cosmic magnetic fields.
Title: Cycle patterns of the axisymmetric magnetic field
Authors: Stenflo, J. O.
Bibcode: 1994ASIC..433..365S
Altcode:
No abstract at ADS
Title: Solar magnetic flux at small scales (Invited Review)
Authors: Stenflo, J. O.
Bibcode: 1994smf..conf..301S
Altcode:
No abstract at ADS
Title: Direct Measurements of Fluxtube Inclinations in Plages
Authors: Bernasconi, Pietro N.; Keller, Christoph U.; Stenflo, Jan Olof
Bibcode: 1994ASPC...68..131B
Altcode: 1994sare.conf..131B
No abstract at ADS
Title: Report of IAU Commission 12: Solar radiation and structure
(Radiation et structure solaires).
Authors: Stenflo, J. O.
Bibcode: 1994IAUTA..22...85S
Altcode:
No abstract at ADS
Title: Strong and Weak Magnetic Fields: Nature of the Small-Scale
Flux Elements (Invited)
Authors: Stenflo, J. O.
Bibcode: 1993ASPC...46..205S
Altcode: 1993mvfs.conf..205S; 1993IAUCo.141..205S
No abstract at ADS
Title: Infrared lines as probes of solar magnetic features. III -
Strong and weak magnetic fields in plages
Authors: Rueedi, I.; Solanki, S. K.; Livingston, W.; Stenflo, J. O.
Bibcode: 1992A&A...263..323R
Altcode:
The diagnostics developed by Zayer et al. 1989 are applied to 27
Stokes V spectra of solar plates. The 1.5 micron lines at 15648 A and
15652 A are used to measure magnetic field strengths between 400 G
and 1700 G at z = 0 in active region plages with an accuracy of up
to 2-3 percent. The results confirm the accuracy of the line-ratio
technique of Stenflo (1973). It is estimated that the fraction of net
magnetic flux in strong-field form is close to 90 percent, supporting
the view that the small-scale magnetic fields are concentrated very
efficiently. The data clearly contradict the claims of Zirin and Popp
(1989) based on 12 micron lines that there is at most indirect evidence
for kG fields in solar plages. They also contradict the large fraction
of weak-field flux reported by Del Toro et al. (1990).
Title: Polarimetry with an imaging FTS.
Authors: Stenflo, J. O.; Solanki, S. K.
Bibcode: 1992ESASP.344..197S
Altcode: 1992spai.rept..197S
The concept of an imaging FTS polarimeter based on piezoelastic
modulation and direct demodulation in partially masked CCDs is
outlined. To illustrate its principle the currently operational FTS
polarimeter at the McMath telescope as well as the detection scheme
of the ETH polarimeter, based on partially masked CCD cameras, are
briefly described.
Title: High Spatial Resolution Magnetograms of Solar Active Regions
Authors: Keller, C. U.; Stenflo, J. O.; von der Luhe, O.
Bibcode: 1992A&A...254..355K
Altcode:
Using the Universal Birefringent Filter at the Sacramento Peak Vacuum
Tower Telescope we have obtained simultaneous observations of left and
right circular polarization in various solar magnetic features with a
resulting spatial resolution of 0".7 in the magnetograms. We describe
the data reduction in some detail and discuss the various instrumental
effects. In particular we show that seeing can create features in
magnetograms. A penumbra near disk center shows small-scale features in
the magnetogram which are associated with the bright filaments. Bright
features in the umbra of a small spot exhibit considerable polarization
signals. In a pore region opposite polarities are found within a few
seconds of arc.
Title: Zurich Imaging Stokes Polarimeter Zimpol-I - Design Review
Authors: Keller, C. U.; Aebersold, F.; Egger, U.; Povel, H. P.;
Steiner, P.; Stenflo, J. O.
Bibcode: 1992lest.rept....1K
Altcode:
No abstract at ADS
Title: Comments on the Concept of an "Extended Solar Cycle"
Authors: Stenflo, Jan Olof
Bibcode: 1992ASPC...27..421S
Altcode: 1992socy.work..421S
No abstract at ADS
Title: Weak solar magnetic fields
Authors: Stenflo, J. O.
Bibcode: 1992AIPC..267...40S
Altcode: 1992ecsa.work...40S
Intrinsically weak magnetic fields are difficult to identify, since flux
measurements (magnetograms) cannot by themselves distinguish between
filling-factor and field-strength effects. The first real determinations
of intrinsically weak (less than kG) fields have in fact been made only
this year, using the Stokes V profiles of an infrared line pair near
1.56 μm. Many cases of discrete magnetic elements with field strengths
as low as 0.4 kG have been found, immediately adjacent (within a couple
of arcsec) to the normal strong-field fluxtubes that have strengths in
the range 1.4-1.6 kG and a magnetic polarity that can be both the same
or opposite to that of the adjacent magnetic component. There
appears to be a continuous sequence of bipolar magnetic regions of
various scales, down to the spatial resolution limit, from active
regions to ephemeral regions and inner-network fields. It seems likely
that this sequence continues in the form of a subarcsec mixed-polarity
or ``turbulent'' field that permeates the 99% of the photospheric
volume not occupied by the kG flux tubes in the network. A one-sigma
upper limit of 100 G to the strength of this hitherto ``invisible''
field has been set from line-broadening constraints, which indicates
that this small-scale field is intrinsically weak. Arguments are given
why the spatial spectrum of flux emergence should saturate when scales
approaching the photon mean free path in the photosphere (about 100
km) are approached, which is the range of scales that may be opened
up to exploration by LEST and OSL. It is shown how correlations
(``active longitudes'') in the pattern of small-scale flux emergence
lead to a replenishment of the global or ``background'' magnetic-field
pattern at high heliographic latitudes in a time as short as weeks,
more than two orders of magnitude faster than predicted by numerical
models of the Babcock-Leighton type. There is thus a close link between
the small-scale dynamics and the global solar-cycle evolution.
Title: Zürich Imaging Stokes Polarimeter - ZIMPOL I. Design review.
Authors: Keller, C. U.; Aebersold, F.; Egger, U.; Povel, H. P.;
Steiner, P.; Stenflo, J. O.
Bibcode: 1992LFTR...53.....K
Altcode:
This document describes the design of ZIMPOL I, the first Zürich
Imaging Stokes Polarimeter. This solar vector polarimeter will mainly
be used for observations of the solar magnetic field at high spatial
and/or spectral resolution. A brief overview of ZIMPOL I is given
in the preface. The scientific requirements are then specified in
detail. They lead to an instrument concept which consists of several
parts: the optical system, the camera system, the real-time image
processing system, and the graphical user interface. Data reduction
and analysis of observations recorded with this polarimeter are also
dealt with in detail. Prototypes of the modulator package and the CCD
camera have been tested at various observatories. Results from these
tests are presented and discussed.
Title: Demodulation of all four Stokes parameters with a single CCD -
ZIMPOL II. Conceptual design.
Authors: Stenflo, J. O.; Keller, C. U.; Povel, H. P.
Bibcode: 1992LFTR...54.....S
Altcode:
It is shown how it is possible to simultaneously record images of
all four Stokes parameters with a single CCD detector chip when
fast (50 kHz), piezoelastic modulation of the polarization state
is used. As the four image planes use the identical pixels of the
CCD, all gain-table or flat-field effects vanish when forming the
fractional polarization images. For each group of four pixel rows,
one row collects the photons, while the other three are used for fast
buffer storage. There are no light losses caused by masking of the pixel
rows used for buffer storage, sinced a microlens array collects all the
photons and directs them to the unmasked pixel rows. The efficiency of
the system for simultaneous recording of all four Stokes parameters
is six times greater than that of ZIMPOL I, the first generation of
the Zürich Imaging Stokes Polarimeter, since no beam splitter with
three separate CCD cameras is needed and no significant light losses
occur at the masked pixel rows. The theoretically possible efficiency
limit is thereby practically reached. The system is planned to be
developed as ZIMPOL II, the second generation of the Zürich Imaging
Stokes Polarimeter.
Title: Strong; Weak Solar Magnetic Fields
Authors: Solanki, S. K.; Ruedi, I.; Livingston, W. C.; Stenflo, J. O.
Bibcode: 1992ASPC...26..262S
Altcode: 1992csss....7..262S
No abstract at ADS
Title: On the Validity of the Babcock-Leighton Approach to Modeling
the Solar Cycle
Authors: Stenflo, Jan Olof
Bibcode: 1992ASPC...27...83S
Altcode: 1992socy.work...83S
No abstract at ADS
Title: Magnetograph Observations with the Swedish Solar Telescope
on La-Palma
Authors: Lundstedt, Henrik; Johannesson, Anders; Scharmer, Göran;
Stenflo, Jan Olof; Kusoffsky, Ulf; Larsson, Birgitta
Bibcode: 1991SoPh..132..233L
Altcode:
A high-resolution videomagnetograph that records the images of
opposite circular polarization simultaneously has been constructed
for the Swedish vacuum solar telescope at La Palma. Magnetograms are
obtained by off-line integration of bursts consisting of typically
50 frames of 20 ms exposures, with bad frames rejected, and the
frame-to-frame image motion of the remaining frames compensated for by
cross-correlation techniques. The short exposures combined with frame
selection and elimination of image motion optimizes the resolution
and thereby also the S/N, allowing good magnetograms to be obtained
with an effective exposure time of less than 1 s at an image scale of
0.1″ pixel−1. The advantages and limitations of the
system are discussed and compared with other techniques of making
filter magnetograms are discussed.
Title: Wir und das Weltall - Teil 2.
Authors: Stenflo, J. O.
Bibcode: 1991Orion..49....4S
Altcode:
No abstract at ADS
Title: Two-dimensional polarimeter with CCD image sensors and
piezo-elastic modulators.
Authors: Povel, H. P.; Keller, C. U.; Stenflo, J. O.
Bibcode: 1991sopo.work..100P
Altcode:
A new type of polarimeter for application in solar physics is described,
which combines fast polarization modulation and high spatial resolution
using piezoelastic modulators and charge coupled device (CCD) image
sensors. The problem of incompatibility between the slow read-out of
CCD sensors and fast modulation has been solved. First two-dimensional
Stokes Q and V images, free from gain-table noise, and with an rms
noise level of about 10-3 have been obtained.
Title: Unified classical theory of line formation in a magnetic field.
Authors: Stenflo, J. O.
Bibcode: 1991sopo.work..416S
Altcode:
A unified theory of line formation in a magnetic field is developed
from first principles within a classical framework. Starting from
Maxwell's equations for the electromagnetic field in vacuum and
the Hamiltonian for an electron in a central potential, the theory
of radiative transfer in a magnetic field is developed, including
magneto-optical effects, scattering, coherence effects (the Hanle
effect), and partial redistribution.
Title: Applications of the Hanle Effect in Solar Physics
Authors: Stenflo, Jan Olof
Bibcode: 1991hels.conf..237S
Altcode:
No abstract at ADS
Title: Inversion of Stokes V profiles: The structure of solar magnetic
fluxtubes and its dependence on the filling factor.
Authors: Keller, C. U.; Solanki, S. K.; Stenflo, J. O.; Zayer, I.
Bibcode: 1991sopo.work..387K
Altcode:
The authors present results from an inversion procedure that derive the
temperature stratification, the turbulent velocity, and the magnetic
field strength of the photospheric layers of magnetic fluxtubes from
observed Stokes V spectra near disk center. In a first step the
inversion has been applied to 10 Fe I and Fe II Stokes V profiles
of a plage and a network region to obtain reliable models of the
fluxtubes. In a second step the dependence of the fluxtube structure
on the filling factor has been studied with spectra of 3 Fe I lines
from 23 different regions based on the models derived in the first step.
Title: Diagnostics of the Solar Dynamo Using the Observed Pattern
of Surface Magnetic Fields
Authors: Stenflo, J. O.
Bibcode: 1991LNP...380..193S
Altcode: 1991sacs.coll..193S; 1991IAUCo.130..193S
The solar surface represents for the solar dynamo an outer boundary
that is directly accessible to observations. The evolutionary and
rotational properties of the magnetic fields at this boundary can be
empirically determined using existing synoptic magnetograph data. The
derived properties provide detailed constraints on the underlying
theory, such that an inversion approach to the dynamo problem becomes
feasible. Ambiguities in the interpretation may be removed using the
independent constraints from helioseismology.
Title: Relation between small-scale emergence and global evolution
of the Sun's magnetic field.
Authors: Stenflo, J. O.
Bibcode: 1991joso.proc.....S
Altcode:
The author shows that there is a close interplay between the large and
small scales on the Sun. The small-scale processes provide the key to
an understanding of the global patterns. The large-scale pattern of
"background" surface magnetic fluxes at high latitudes is not due to the
dispersal and surface transport of low-latitude active-region magnetic
fields as in the Babcock-Leighton picture, but to the accumulated
effect of small-scale flux emergence and removal processes. The time
scale for the overall replenishment of the global high-latitude pattern
is as short as a few weeks.
Title: Optimization of the LEST polarization modulation system.
Authors: Stenflo, J. O.
Bibcode: 1991LFTR...44.....S
Altcode:
A complete parameter study for the proposed LEST polarimeter has been
performed. The free parameters are the modulation amplitudes and the
position angels of all optical components in the modulator package
at the LEST secondary focus and of the beam splitter cubes at the
end focus. The objective is to explore what the optimum parameter
combinations are, and whether different trade-offs require different
observing configurations.
Title: Calibration of the instrumental polarization in LEST.
Authors: Stenflo, J. O.
Bibcode: 1991LFTR...45.....S
Altcode:
A method to calibrate the instrumental polarization of LEST (i.e.,
of the optics in front of the polarization and analysis package at
the LEST secondary focus) is described and analysed. A laser sends a
beam from the LEST observing room at ground level backwards through the
telescope. The modulated return beam that has been reflected at the 2.4
m entrance window is received by the detector system at the end focus
in the LEST observing room. Because of a wedge in the entrance window
the reflections at the front and back window surfaces may be examined
separately. The detected laser beam signals at the three AC frequencies
are recorded as functions of the position angle of the modulator package
at the LEST secondary focus. Aided by a theorem that is proven in the
present paper, the information recorded by the detector system allows
all unknowns of the instrumental polarization to be fully determined.
Title: Wir und das Weltall - Teil 1.
Authors: Stenflo, J. O.
Bibcode: 1990Orion..48..212S
Altcode:
No abstract at ADS
Title: Dependence of the properties of solar magnetic flux tubes on
filling factor. II - Results of an inversion approach
Authors: Zayer, I.; Stenflo, J. O.; Keller, C. U.; Solanki, S. K.
Bibcode: 1990A&A...239..356Z
Altcode:
The dependence of the properties of solar magnetic elements on the
magnetic filling factor is studied using Stokes V spectra of three
lines observed near the center of the solar disk. The inversion
technique developed by Keller et al. (1990) is applied to three
neighboring spectral lines, and the average temperature difference,
the magnetic field strength, and the nonstationary velocity in the
relevant line-forming layers are quantitatively determined. Quantitative
evidence is provided for the dependence of the temperature within
flux tubes on the amount of magnetic flux. The flux tubes are found
to become cooler and their field strengths, at a given optical depth,
to become larger as the filling factor increases. The presence of
kilogauss field strengths within flux tubes is reconfirmed.
Title: Solar magnetic field strength determinations from high spatial
resolution filtergrams
Authors: Keller, C. U.; Stenflo, J. O.; Solanki, S. K.; Tarbell,
T. D.; Title, A. M.
Bibcode: 1990A&A...236..250K
Altcode:
Circularly polarized images with high spatial resolution (better than 1
arcsec) of a solar active region, obtained with a tunable filter in the
wings of Fe I 5247.1 A and Fe I 5250.2 A, have been analyzed in terms of
the magnetic line ratio technique introduced by Stenflo (1973). Whenever
a measurable amount of polarization is present, the distribution of
the observed magnetic-line ratio is compatible with a unique value,
which is randomly blurred by noise due to the photon statistics,
the CCD camera, and atmospheric distortions. There is no need for a
distribution of field strengths to explain the observed distribution of
the magnetic line ratio. Consequently, the observations are compatible
with a unique magnetic field strength in solar small-scale magnetic
elements of about 1000 G at the level of line formation. For a thin
flux tube, this corresponds to a field strength of approximately 2000
G at the level of continuum formation, which is in excellent agreement
with previous field strength determinations from low spatial resolution
spectra (4-10 arcsec).
Title: Time invariance of the sun's rotation rate
Authors: Stenflo, J. O.
Bibcode: 1990A&A...233..220S
Altcode:
In this paper, the existence of solar cycle variations in the rotation
rate is investigated by dividing a 26-yr synoptic data base into
21 consecutive time windows and determining the sun's rotation rate
in the 30 latitude zones by a power spectrum analysis. The pattern
phase velocity is found to be time-invariant within time limits set by
small, apparently random fluctuations around the previously determined
quasi-rigid rotation law. The results support the view that flux storage
inside the sun occurs primarily near the bottom of the convection zone
and that the surface magnetic field pattern is replenished from the
interior sources over a time scale that is shorter than 27 days but
longer than a few days.
Title: Structure of solar magnetic fluxtubes from the inversion of
Stokes spectra at disk center
Authors: Keller, C. U.; Steiner, O.; Stenflo, J. O.; Solanki, S. K.
Bibcode: 1990A&A...233..583K
Altcode:
The paper presents an inversion procedure that derives the temperature
stratification, the turbulent velocity, and the magnetic field strength
of the photospheric layers of small-scale magnetic fields from observed
Stokes V spectra and the continuum intensity. The inversion is based
on the determination of a small number of model flux parameters by a
nonlinear least squares fitting algorithm. The minimization of the sum
of the squared differences between observed and synthetic observables
makes it possible to determine the temperature stratification and the
magnetic field strength.
Title: The Sun's Rotation Rate as Inferred from Magnetic Field Data
Authors: Stenflo, J. O.
Bibcode: 1990IAUS..138..309S
Altcode:
No abstract at ADS
Title: Solar photosphere: structure, convection, and magnetic fields:
proceedings of the 138th Symposium of the International Astronomical
Union, held in Kiev, U.S.S.R., May 15-20, 1989.
Authors: Stenflo, Jan Olof
Bibcode: 1990IAUS..138.....S
Altcode:
No abstract at ADS
Title: Model Calculations of the Photospheric Layers of Solar
Magnetic Fluxtubes
Authors: Steiner, O.; Stenflo, J. O.
Bibcode: 1990IAUS..138..181S
Altcode:
No abstract at ADS
Title: Solar photosphere: Structure, convection and magnetic fields
Authors: Stenflo, Jan O.
Bibcode: 1990spsc.conf.....S
Altcode:
No abstract at ADS
Title: The internal magnetic field structure of solar magnetic
elements
Authors: Solanki, S.; Zayer, I.; Stenflo, J. O.
Bibcode: 1989hsrs.conf..409S
Altcode:
No abstract at ADS
Title: LEST update
Authors: Stenflo, J. O.
Bibcode: 1989hsrs.conf...27S
Altcode:
No abstract at ADS
Title: Objectives of high-resolution Stokes polarimetry
Authors: Stenflo, J. O.
Bibcode: 1989hsrs.conf..369S
Altcode:
No abstract at ADS
Title: Small-scale magnetic structures on the Sun
Authors: Stenflo, J. O.
Bibcode: 1989A&ARv...1....3S
Altcode:
The Sun provides us with a unique astrophysics laboratory for
exploring the fundamental processes of interaction between a turbulent,
gravitationally stratified plasma and magnetic fields. Although the
magnetic structures and their evolution can be observed in considerable
detail through the use of the Zeeman effect in photospheric spectral
lines, a major obstacle has been that all magnetic structures on the
Sun, excluding sunspots, are smaller than what can be resolved by
present-day instruments. This has led to the development of indirect,
spectral techniques (combinations of two or more polarized spectral
lines), which overcome the resolution obstacle and have revealed
unexpected properties of the small-scale magnetic structures. Indirect
empirical and theoretical estimates of the sizes of the flux elements
indicate that they may be within reach of planned new telescopes,
and that we are on the verge of a unified understanding of the
diverse phenomena of solar and stellar activity. In the present
review we describe the observational properties of the smallscale
field structures (while indicating the diagnostic methods used),
and relate these properties to the theoretical concepts of formation,
equilibrium structure, and origin of the surface magnetic flux.
Title: The internal magnetic distribution and the diameters of solar
magnetic elements.
Authors: Zayer, I.; Solanki, S. K.; Stenflo, J. O.
Bibcode: 1989A&A...211..463Z
Altcode:
A diagnostic is proposed for the horizontal distribution and vertical
gradient of the magnetic field with spatially unresolved solar magnetic
elements. Radiative transfer calculations are conducted to model the
magnetic field structure, which is fitted to observational data from a
Fourier transform spectrometer. It is found that a vertical gradient
of the magnetic field strength must be present in solar magnetic
fluxtubes and that, in the lower photosphere, the field strength inside
the fluxtube must be horizontally nearly constant. The magnetic field
strengths and gradients derived at the disk center satisfy observations
near 0.6 microns, if the appropriate model fluxtube size is chosen.
Title: Differential rotation of the sun's magnetic field pattern
Authors: Stenflo, J. O.
Bibcode: 1989A&A...210..403S
Altcode:
The sun's differential rotation is determined, using the pattern of
solar magnetic fields as a tracer. An autocorrelation analysis on a
26-yr synoptic data set of magnetic fields sampled at the sun's central
meridian produces a differential rotation law that is well-determined
all the way to the poles of the sun. This law is very different from
that derived from cross-correlation analysis of flux patterns in daily
magnetograms by Snodgrass (1983). It is suggested that the differences
may be explained if the surface magnetic flux is constantly being
replenished over a time scale of weeks by new flux emitted from the
source region, which is probably near the bottom of the convection zone.
Title: CCD image sensor as a demodulator in a 2-D polarimeter with
a piezo-elastic modulator.
Authors: Povel, H.; Aebersold, H.; Stenflo, J. O.
Bibcode: 1989cisa.book.....P
Altcode:
It is shown how demodulation of rapidly modulated light beams can
be achieved within a single CCD. Two interlaced image planes are
created by optically masking every second CCD row, and transferring
the charges back and forth between the two image planes in synchrony
with the modulation. The method has been successfully tested for
modulation frequencies of 50 and 100 kHz, using integration times up
to 1 s. No significant accumulated charge transfer losses are seen
for integration times as long as 105 mudulation cycles
(1 s). This demonstrates the feasibility of a CCD polarimeter using
piezo-elastic modulation of the state of polarization.
Title: Interpretation of broad band circular polarization measurements
using Stokes V spectra
Authors: Muerset, U.; Stenflo, J. O.; Solanki, S. K.
Bibcode: 1988A&A...204..279M
Altcode:
The wavelength dependence and the center to limb variation of the broad
band circular polarization of solar active regions are determined by
integrating over spectra with a large wavelength range obtained with a
Fourier transform spectrometer (FTS). It is shown that the broad band
circular polarization (BBC) is due mainly to the asymmetry of the Stokes
V profiles. The approximate contributions of lines of different depths
to the total broad band signal are analyzed. The diagnostic contents
of BBC observations with low spatial resolution, such as those of
Kemp et al. (1987), are discussed. These observations are reproduced
with the help of simulated broad band polarization data obtained by
integrating FTS spectra. It is shown that the spatial distribution
of the net field on the solar surface (within a single large spatial
resolution element) can affect the measured BBC signal considerably,
and may even change its sign.
Title: Global Wave Patterns in the Sun's Magnetic Field
Authors: Stenflo, J. O.
Bibcode: 1988Ap&SS.144..321S
Altcode:
When the observed pattern of solar magnetic fields is decomposed
in its spherical-harmonic components and a time series analysis is
performed, a resonant global wave pattern is revealed. The power
spectrum indicates modes with discrete frequencies, obeying a strict
parity selection rule in the case of the zonal, rotationally-symmetric
modes (with spherical-harmonic orderm=0). For instance, the 22
yr resonance that dominates for the anti-symmetric modes (with odd
values of the spherical-harmonic degreel) is completely absent for the
symmetric modes, which instead exhibit a number of resonances having
frequencies increasing withl. A more traditional way of looking
at the evolution of the zonal magnetic pattern is in the form of
isocontours in latitude-time space (as in the ‘butterfly diagram’
of sunspots). We show how this pattern can to a good approximation
be represented as a superposition of 14 discrete modes, each with a
purely sinusoidal time variation, one mode for each value ofl (=1,2,
..., 14). This corresponds to the assumption that the true, fully
resolved and noise-free power spectrum consists of δ-function peaks,
one for eachl value. This approach allows us to analyse the roles of
the individual discrete modes in generating the well-known features
in the traditional ‘btterfly diagrams’, e.g., the drift of the
sunspot zones towards the equator and the prominence zones towards
the poles during the course of the 11 yr cycle. It is shown that these
features are accounted for entirely by the odd parity modes with the
single, sinusoidal period of 22 yr. The drifts (and thus the arrow of
time) are caused by the systematic phase relations between the 22 yr
modes. The even modes exhibit an entirely different pattern. Since
they have considerably shorter periods, they cause an undulation
of the odd-mode contour lines when superposed on the anti-symmetric
pattern. The dispersion, amplitude, and phase relations of the discrete
modes are given. It is indicated how they can be used in combination
with spectral inversion techniques to determine the depth variation
of the parameters in the governing global wave equation.
Title: Observational constraints on a `hidden', turbulent magnetic
field of the Sun
Authors: Stenflo, J. O.
Bibcode: 1988SoPh..114....1S
Altcode:
A search for linear polarization due to the transverse Zeeman effect
in quiet regions near the heliographic north pole has been carried
out. The aim is to determine new constraints on the properties of
the `hidden' or `turbulent' magnetic flux of the Sun. As more than
90% of the total flux seen in magnetograms has its source in kG
fluxtubes with an average filling factor of less than 1%, the term
`hidden' magnetic flux refers to the field in the remaining 99% of the
photospheric volume, which remains undetected in ordinary magnetograms
(at available levels of spatial resolution and sensitivity).
Title: Evolution of solar magnetic fields - Modal structure.
Authors: Stenflo, J. O.; Guedel, M.
Bibcode: 1988A&A...191..137S
Altcode:
The properties of the global evolution of solar magnetic fields are
investigated by carrying out a harmonic analysis of the zonal (m = 0)
and the nonaxisymmetric (m not equal to 0) modes, using a 25-yr long
(1959-1984) data set of synoptic magnetic field data. The zonal modes
are shown to be governed by an extremely strict parity selection rule,
the 22-yr resonance being the dominating feature for the odd modes
while it is absent for the even modes. The even-parity modes exhibit
instead a series of other resonant frequencies that increase with the
spherical harmonic degree l, although with power amplitudes 5-10 smaller
than the power of the odd-mode 22-yr peak. The nonaxisymmetric modes
on the other hand show a number of well-defined resonant frequencies,
which are independent of parity and of the values of l and m.
Title: Spectropolarimetry of Magnetic Stars : HD 125248
Authors: Mathys, G.; Stenflo, J. O.
Bibcode: 1988IAUS..132..317M
Altcode:
The authors present preliminary results about the magnetic field of
the Ap star HD 125248, from spectra recorded in RCP and LCP light with
the Zeeman analyzer of the CASPEC at ESO.
Title: Velocity and temperature in solar magnetic fluxtubes from a
statistical centre-to-limb analysis
Authors: Pantellini, F. G. E.; Solanki, S. K.; Stenflo, J. O.
Bibcode: 1988A&A...189..263P
Altcode:
Stokes I and V profiles of a large number of unblended Fe I
lines observed at different positions on the solar disk have
been analyzed. Asymmetries in the Stokes V profile are noted. The
center-to-limb variation of the zero-crossing wavelength of the weak
and medium strong lines is found to be consistent with the absence of
stationary flows in fluxtubes. The stronger lines are shown to be more
blueshifted than the weak ones at all limb distances. The fluxtube
temperature structure is confirmed to be a function of the filling
factor. The rms velocity amplitude is found to increase somewhat when
approaching the limb, in contrast to what would be expected if the
mass motions were mostly vertical.
Title: LEST update.
Authors: Stenflo, J. O.; Engvold, O.
Bibcode: 1988LFTR...34.....S
Altcode:
No abstract at ADS
Title: Objectives of high-resolution Stokes polarimetry.
Authors: Stenflo, J. O.
Bibcode: 1988LFTR...33.....S
Altcode:
No abstract at ADS
Title: Properties of solar magnetic fluxtubes from only two spectral
lines
Authors: Solanki, S. K.; Keller, C.; Stenflo, J. O.
Bibcode: 1987A&A...188..183S
Altcode:
A method for the determination of the magnetic field strength,
velocity, and temperatures inside solar flux tubes, in addition to
their inclinations and filling factors, is presented which requires
only the Stokes V and Q profiles of the Fe I 5250.2 A and Fe I 5247.1
A spectral lines. Application of the procedure to spectra of the two
lines obtained at various distances from the solar limb shows that
considerable velocity broadening is required at all positions on the
disk in order to reproduce the polarimeter data. The center to limb
variation of the 5250/5247 Stokes V and Q line ratios is found to
contain little information on the height variation of the magnetic
field in the context of one-dimensional models.
Title: Polarimetry in the Mg II h and k lines.
Authors: Henze, W.; Stenflo, J. O.
Bibcode: 1987SoPh..111..243H
Altcode:
The Ultraviolet Spectrometer and Polarimeter (UVSP) on the SMM satellite
has been used to record the linear polarization profile across the
MgII h and k lines, including its center-to-limb variation. Linear
polarization with an orientation of the electric vector parallel to
the solar limb is detected in the line wings on the short wavelength
side of the k line and on the long wavelength side of the h line,
in agreement with theoretical predictions of Auer et al. (1980). The
predicted negative polarization (electric vector perpendicular to
the limb) between the h and k lines is however not confirmed by
the observations. Instead values close to zero are indicated there,
although the statistical significance of the results is marginal.
Title: Evolution of the Sun's Magnetic Polarities
Authors: Stenflo, J. O.; Weisenhorn, A. L.
Bibcode: 1987SoPh..108..205S
Altcode:
The global magnetic-field resonances previously found in a modal
analysis of a 25 yr Mt Wilson-Kitt Peak data set of synoptic
magnetic maps are also revealed when only the magnetic polarities are
used, disregarding the magnitude of the flux. Thus the topological
organization of the magnetic polarities alone suffices to bring out the
correct modal structure, although the results are noisier as compared
with the case when the magnetic fluxes are included.
Title: Anomalous Zeeman Effect - Moments and Expansion Coefficients
Authors: Mathys, G.; Stenflo, J. O.
Bibcode: 1987A&AS...70..142M
Altcode:
No abstract at ADS
Title: Diagnostics of solar magnetic fluxtubes with the infrared
line Fe I lambda 15648.54 A
Authors: Stenflo, J. O.; Solanki, S. K.; Harvey, J. W.
Bibcode: 1987A&A...173..167S
Altcode:
Fourier Transform Spectrometer recordings of May 1984 have been used to
explore the center-to-limb variation of the Stokes I, V, and Q profiles
of the solar infrared Fe I line at 15,648.54 A. The aim is to examine
the new possibilities offerred for the diagnostics of the spatially
unresolved magnetic flux tubes on the sun when lines with complete
Zeeman splitting are used. Comparison is made with the line-ratio
method, which must be used at visible wavelengths, where the splitting
is incomplete. The Stokes V asymmetries observed in the infrared line
are small or even of opposite sign as compared with the corresponding
asymmetries observed at visible wavelengths. This suggests that the
time-averaged height gradient of the Doppler velocities inside the
fluxtubes becomes small and may change its sign when moving down to
the bottom of the fluxtube photosphere.
Title: Anomalous Zeeman effect - Moments and expansion coefficients
Authors: Mathys, G.; Stenflo, J. O.
Bibcode: 1987A&AS...67..557M
Altcode:
The sigma and pi absorption and line dispersion coefficients are
expanded in powers of the magnetic field strength for arbitrary
anomalous Zeeman splitting patterns. The coefficients of the expansion
can be expressed either as a function of the various moments of the
splitting pattern, or directly as a function of the quantum numbers
of the atomic transition. Tables allowing the computation of the
moments for any electric dipole transition, up to the eighth order,
are presented, along with tables of the expansion coefficients for
electric dipole transitions in the case of LS-coupling, up to the eighth
order. The convergence of the expansion of the sigma and pi absorption
coefficients as a function of the field strength and splitting pattern
is illustrated.
Title: Center-to-limb variation of Stokes profiles and the diagnostics
of solar magnetic fluxtubes
Authors: Stenflo, J. O.; Solanki, S. K.; Harvey, J. W.
Bibcode: 1987A&A...171..305S
Altcode:
Simultaneous recordings of the Stokes I, Q, and V spectra have been
performed with a Fourier transform spectrometer in 10 solar regions
distributed over various center-to-limb distances, from disk center
to the extreme limb. The observational material and the recording
technique used are presented. The authors then evaluate the Stokes
profile parameters for a small selected set of spectral lines to explore
the potential of this qualitatively new data set for the diagnostics
of spatially unresolved magnetic fluxtubes.
Title: Anomalous Zeeman effect and its influence on the line
absorption and dispersion coefficients
Authors: Mathys, G.; Stenflo, J. O.
Bibcode: 1987A&A...171..368M
Altcode:
The shape of Stokes line profiles formed in the presence of a magnetic
field does not only depend on the magnetic field structure and the
effective Landé factor, but also on the type of anomalous Zeeman
splitting pattern, which is determined by the atomic structure. To
explore this dependence, the authors have first expanded the profiles of
the σ and π absorption coefficients in terms of the various moments
characterizing the Zeeman pattern. Next they have found expressions
for the moments in terms of the quantum numbers and Landé factors
of the atomic levels involved in the transition. These results have
been combined to give an expansion of the profiles of the absorption
coefficients in terms of the atomic parameters directly. It is shown
that the same treatment also applies to the anomalous dispersion
profiles.
Title: The Magnetic Field of the Sun
Authors: Stenflo, J. O.
Bibcode: 1987MitAG..65...25S
Altcode:
No abstract at ADS
Title: Joint Discussion on Topics of Sessions 3 and 4
Authors: Stenflo, J. O.; Mattig, W.
Bibcode: 1987rfsm.conf..116S
Altcode:
No abstract at ADS
Title: Observational constraints on a 'hidden', turbulent magnetic
field of the sun
Authors: Stenflo, J. O.
Bibcode: 1987SoPh..114....1S
Altcode:
A search for linear polarization due to the transverse Zeeman effect
in quiet regions near the heliographic north pole has been carried
out. The aim is to determine new constraints on the properties of the
"hidden" or "turbulent" magnetic flux of the Sun. As more than 90% of
the toatal flux seen in magnetograms has its source in kG fluxtubes with
an average filling factor of less than 1%, the term "hidden" magnetic
flux refers to the field in the remaining 99% of the photospheric
volume, which remains undetected in ordinary magnetograms (at available
levels of spatial resolution and sensitivity). Simultaneous recordings
of the Stokes, I, Q and V profiles of the Fe I 5250.22 and 5247.06
Å lines with 5×5 sec of arc spatial resolution have been made with
the NSO McMath solar telescope. The analysis shows how the observed
Stokes Q amplitudes, as well as the Q/V ratio in combination with
the 5250/5247 Stokes V line ratio, provide constraints on the field
strength and the angular distribution of the field vectors of the
"hidden" magnetic flux. In the author's observations the linear
polarization has been recorded with a precision of 10-4
with good spectral resolution.
Title: Numerical models for solar magnetic fluxtubes
Authors: Steiner, O.; Pneuman, G. W.; Stenflo, J. O.
Bibcode: 1986A&A...170..126S
Altcode:
A fully self-consistent method for constructing magnetostatic solutions
for magnetic fluxtubes is presented, and the method is applied to three
untwisted fluxtube configurations, two in which a sheet current exists
at the surface of the tube, and one in which the the internal magnetic
field varies continuously as a Gaussian. Convergence was obtained in
all three cases, though the rate of convergence and accuracy of the
solution were superior for the continuous field distribution case. For
twisted fluxtubes, a maximum twist compatible with equilibrium is
found for a given configuration, and the merging height is shown to
decrease slightly with increased twist and to increase if the twist
is concentrated more toward the surface of the tube.
Title: Some effects of finite spectral resolution on the Stokes
V profile
Authors: Solanki, S. K.; Stenflo, J. O.
Bibcode: 1986A&A...170..120S
Altcode:
The authors consider some general effects of spectral smearing on the
Stokes V profile of the Fe I 5250.2 Å line, as observed on the sun with
a Fourier transform spectrometer at a very high spectral resolution. The
effects of low spectral resolution are numerically simulated by
convoluting the observed line profiles with model instrumental
profiles. The authors also try to find some Stokes V parameters which
are insensitive to the spectral resolution. Finally, they show that
the large Stokes V zero-crossing wavelength shifts observed by Wiehr
(1985) and by Scholier and Wiehr (1985) from complete profiles, and
by Giovanelli and Slaughter (1978) with the line-centre-magnetogram
technique of Giovanelli and Ramsey (1971) are compatible with the
results of Stenflo and Harvey (1985) and Solanki (1985, 1986).
Title: Spectropolarimetry of magnetic stars. I. Diagnostic contents
of Stokes I and V line profiles.
Authors: Mathys, G.; Stenflo, J. O.
Bibcode: 1986A&A...168..184M
Altcode:
Observations of HD 147010, obtained with the Zeeman analyzer of the
Cassegrain Echelle Spectrograph and the 3.6-m telescope of the La Silla
ESO, are used to investigate the diagnostic contents of the Stokes
I and V spectra for magnetic stars. A statistical study of 16 Fe II
lines and 5 Si II lines is presented, and average values of the surface
magnetic field and its line of sight component are derived. It is found
that the I and V spectra are not weighted in the same way across the
stellar disk, and that large variations of the temperature and density
structure of the atmosphere over the stellar surface must exist.
Title: Book-Review - Solar-Space Observations and Stellar Prospects
Authors: Harvey, J. W.; Stenflo, J. O.; Hudson, H. S.; Noyes, R. W.;
Kotrc, P.
Bibcode: 1986BAICz..37..252H
Altcode:
No abstract at ADS
Title: Lines in the wavelength range λλ 4300 6700 Å with large
stokes V amplitudes outside sunspots
Authors: Solanki, S. K.; Pantellini, F. G. E.; Stenflo, J. O.
Bibcode: 1986SoPh..107...57S
Altcode: 1987SoPh..107...57S
A list of solar spectral lines in the wavelength λλ 4300-6700 å
exhibiting large Stokes V amplitudes in observed spectra of active
region plages and the quiet network is presented.
Title: Stokes Polarimetry
Authors: Stenflo, J. O.
Bibcode: 1986ssmf.conf...59S
Altcode:
Using the Kitt Peak McMath Fourier transform spectrometer as a
polarimeter, a number of Stokes I, Q, and V spectral atlases have
been obtained, representing different magnetic features at various
center-to-limb distances. These data allow to explore the diagnostic
foundations for the determination of solar magnetic fields, i.e.,
the problem how to derive the magnetic field structure when the
flux elements are spatially unresolved. The height variation of
the magnetic field and the internal fluxtube temperature could be
determined. Non-stationary mass motions inside the fluxtubes have also
been revealed, which may be related to fluxtube oscillations.
Title: The Magnetic Field of the Sun
Authors: Stenflo, J. O.
Bibcode: 1986MitAG..65...25S
Altcode:
To understand the nature of the sun's magnetic field one needs to
explore both its small-scale and large-scale (global) properties. On
a small scale, the field is extremely intermittent, the basic units
(the fluxtubes) being much smaller than the resolving power of present
telescopes. Using a Fourier transform spectrometer as a polarimeter the
properties of these basic units are being systematically explored. The
global field pattern can be decomposed in its spherical harmonics
coefficients. Power spectrum analysis of these coefficients has
revealed a resonant structure in the global evolution of the fields. The
resonances are governed by a parity selection rule.
Title: Structure and merging of solar magnetic fluxtubes
Authors: Pneuman, G. W.; Solanki, S. K.; Stenflo, J. O.
Bibcode: 1986A&A...154..231P
Altcode:
In the present, expansion technique-based model of the solar
atmosphere's magnetic flux tubes, account is taken of the effects of
field line curvature, internal structural variations, and the merging
of the tube with its contiguous neighbors as it expands (through the
use of a small 'seed' magnetic field between the tubes that has no
influence on the solution in the limit of vanishing strength). For the
solutions obtained, the internal magnetic structure of the tube evolves
in height in a nonself-similar manner, although the gas pressure can
vary self-similarly for the special case in which internal temperature
is both uniform and equal to the external temperature. In the vicinity
of the merging height, the field approaches uniformity consistent with
a vertical tube with constant cross section.
Title: Global resonances in the evolution of solar magnetic fields
Authors: Stenflo, J. O.; Vogel, M.
Bibcode: 1986Natur.319..285S
Altcode:
Decomposition of the pattern of solar magnetic fields in spherical
harmonics for a data set of 25 years and power spectrum analysis
of the harmonic coefficients reveals a strikingly resonant modal
structure. The resonance frequencies contain information on the
structure of the magnetic fields in the Sun's interior.
Title: Astronomical polarimeter with 2-D detector arrays
Authors: Stenflo, J. O.; Povel, H.
Bibcode: 1985ApOpt..24.3893S
Altcode:
It is shown how fast (50-100 kHz) piezoelastic modulation of the
full Stokes vector can be used in combination with large CCD-type
detector arrays with long integration times. The technique is to use
an optical demodulation system (replacing the lockin amplifiers in
corresponding single-channel detector systems). This allows the CCD
detectors to be used with integration times and readout rates as in
ordinary photometry. Including an optical phase switch in the system,
the effect of the large pixel-to-pixel sensitivity variations can be
removed from the recorded polarization images. The beam splitter that
suppresses atmospheric noise can be located immediately before the
detectors instead of being part of the polarization analyzer.
Title: Measurements of Magnetic Fields and the Analysis of Stokes
Profiles
Authors: Stenflo, J. O.
Bibcode: 1985SoPh..100..189S
Altcode:
Recent advances in polarimetry allowing the recording of polarized
line profiles with high spectral resolution and signal-to-noise ratio
over large portions of the solar spectrum offer rich new diagnostic
possibilities. Thus we can now in a systematic way build models of
the height variation of the magnetic field, temperature, density,
and mass motions in the spatially unresolved subarcsecond magnetic
structures. The analysis of the Stokes spectra also allows us to
build a foundation for proper diagnostics of vector magnetic fields,
a goal that cannot be achieved before the intrinsic properties of the
spatially unresolved magnetic fields have been determined. Another new
diagnostic tool is the Hanle effect. A recent exploratory survey of
coherence effects through the recording of the linear polarization with
high spectral resolution throughout the whole visible solar spectrum
aims at establishing a foundation for the exploitation of the Hanle
effect on the solar disk.
Title: Models of solar magnetic fluxtubes - Constraints imposed by
Fe I and II lines
Authors: Solanki, S. K.; Stenflo, J. O.
Bibcode: 1985A&A...148..123S
Altcode:
The diagnostic contents of the Stokes I and V profiles of about 50
unblended Fe II lines have been explored and used to set new constraints
on the temperature structure of magnetic fluxtubes. The simultaneous use
of Fe I and II lines makes it possible to determine the temperature in
both the upper and lower fluxtube photosphere. The Fe II lines further
make it possible to obtain model-insensitive values of the magnetic
filling factors. Empirically determined effective Lande factors of most
of the unblended iron lines in the visible part of the solar spectrum
are presented and compared with the corresponding LS coupling values.
Title: Small-Scale Plasma Structures
Authors: Southwood, D.; Stenflo, J.
Bibcode: 1985ESASP.235..237S
Altcode: 1985fmsh.work..237S
No abstract at ADS
Title: Diagnostics of vector magnetic fields
Authors: Stenflo, J. O.
Bibcode: 1985svmf.nasa..263S
Altcode:
It is shown that the vector magnetic fields derived from observations
with a filter magnetograph will be severely distorted if the spatially
unresolved magnetic structure is not properly accounted for. Thus the
apparent vector field will appear much more horizontal than it really
is, but this distortion is strongly dependent on the area factor and
the temperature line weakenings. As the available fluxtube models
are not sufficiently well determined, it is not possible to correct
the filter magnetograph observations for these effects in a reliable
way, although a crude correction is of course much better than no
correction at all. The solution to this diagnostic problem is to
observe simultaneously in suitable combinations of spectral lines,
and/or use Stokes line profiles recorded with very high spectral
resolution. The diagnostic power of using a Fourier transform
spectrometer for polarimetry is shown and some results from I and V
spectra are illustrated. The line asymmetries caused by mass motions
inside the fluxtubes adds an extra complication to the diagnostic
problem, in particular as there are indications that the motions are
nonstationary in nature. The temperature structure appears to be a
function of fluxtube diameter, as a clear difference between plage and
network fluxtubes was revealed. The divergence of the magnetic field
with height plays an essential role in the explanation of the Stokes V
asymmetries (in combination with the mass motions). A self consistent
treatment of the subarcsec field geometry may be required to allow an
accurate derivation of the spatially averaged vector magnetic field
from spectrally resolved data.
Title: Solar Magnetic and Velocity-Field Measurements: New Instrument
Concepts
Authors: Stenflo, J. O.
Bibcode: 1985spit.conf.1139S
Altcode:
No abstract at ADS
Title: Diagnostics of vector magnetic fields.
Authors: Stenflo, J. O.
Bibcode: 1985NASCP2374..263S
Altcode:
The author has shown how the vector magnetic fields derived from
observations with a filter magnetograph will be severely distorted
if the spatially unresolved magnetic structure is not properly
accounted for. The solution to this diagnostic problem is to observe
simultaneously in suitable combinations of spectral lines, and/or use
Stokes line profiles recorded with very high spectral resolution. The
author has indicated the diagnostic power of using a Fourier transform
spectrometer for polarimetry, and illustrated some results from I and
V spectra.
Title: Dependence of the Properties of Magnetic Flux Tubes on Area
Factor or amount of Flux
Authors: Stenflo, J. O.; Harvey, J. W.
Bibcode: 1985SoPh...95...99S
Altcode:
Stokes I and V line profiles with high signal-to-noise ratio of the 1
FeI λλ 5247.06 and 5250.22 Å lines have been recorded in a number of
regions with different amount of magnetic flux near disc center, from
`non-magnetic' regions to strong plages. The objective has been to
study how the intrinsic fluxtube properties may depend on the amount
of flux concentration, i.e., on the magnetic area factor. Indirectly,
the area factor should be related to the average fluxtube diameter.
Title: Structure of the Spatially Unresolved Magnetic Fields on
the Sun
Authors: Stenflo, J. O.
Bibcode: 1985spit.conf..189S
Altcode:
No abstract at ADS
Title: LEST polarimetry with large two-dimensional detector arrays.
Authors: Stenflo, J. O.; Povel, H.
Bibcode: 1985LFTR...12.....S
Altcode:
Using an optical demodulation scheme it is possible to combine fast
(50 - 100 kHz) piezoelastic modulation of the Stokes vector with large
(e.g. 1000×1000) CCD arrays with long integration times. The optical
demodulator is a piezoelastic modulator locked in frequency and phase to
the corresponding modulation frequency. In combination with an optical
phase switch the influence of the large pixel-to-pixel sensitivity
variations is eliminated from the polarization images of Q/I, U/I,
and V/I.
Title: Application of Piezoelastic Modulators in Stokes Polarimetry
Authors: Stenflo, J. O.
Bibcode: 1985spit.conf.1275S
Altcode:
No abstract at ADS
Title: Zeeman and Doppler Measurements with a Michelson Interferometer
Authors: Stenflo, J. O.
Bibcode: 1985spit.conf.1272S
Altcode:
No abstract at ADS
Title: Lest - a Large International Solar Telescope for the 1990'S /
Large European Solar Telescope
Authors: Stenflo, J. O.
Bibcode: 1985VA.....28..571S
Altcode:
LEST represents a project for a large international solar telescope
based on next-generation technology, to be established on the Canary
Islands. 6 European and 3 non-European (Australia, China, USA)
countries are participating in this enterprise. A prime objective
of LEST is to investigate the subarcsec fine structure of solar
magnetic fields. The design of the 2.4m aperture telescope is
"polarization-free". Helium-filling and adaptive optics are used to
achieve 0.1 sec of arc spatial resolution.
Title: Properties of solar magnetic fluxtubes as revealed by Fe
I lines
Authors: Solanki, S. K.; Stenflo, J. O.
Bibcode: 1984A&A...140..185S
Altcode:
The information contents in the solar spectrum for modelling of magnetic
fluxtubes is explored by analysing the statistical properties of the
Stokes I and V line profiles of 400 unblended Fe I lines. Methods of
determining the temperature structure, photospheric magnetic field
strength, magnetic filling factor, and microturbulence velocity are
presented and used to provide estimates of these quantities in enhanced
network and plage regions. Analysis of the magnetic line broadening
shows that the magnetic field strength is approximately equal in network
and plage regions, consistent with previous results. On the other hand,
the temperature structures of the plage fluxtubes and the network
elements are observed to be substantially different. The variation
of the Stokes V line asymmetries with line strength are found to be
similar in the different solar regions, indicating similar velocity
structures in plages and network elements.
Title: Erratum: Solar magnetic and velocity-field measurements:
new instrument concepts [Appl. Opt. 23,1267 (1984)]
Authors: Stenflo, Jan O.
Bibcode: 1984ApOpt..23.3267S
Altcode:
No abstract at ADS
Title: Book-Review - Solar and Stellar Magnetic Fields - Origins
and Coronal Effects
Authors: Stenflo, J. O.; Kuperus, M.
Bibcode: 1984SSRv...38..387S
Altcode:
No abstract at ADS
Title: Solar magnetic and velocity-field measurements: New instrument
concepts
Authors: Stenflo, Jan O.
Bibcode: 1984ApOpt..23.1267S
Altcode:
No abstract at ADS
Title: Book-Review - Solar and Stellar Magnetic Fields - Origins
and Effects - I.A.U. Symposium NO.102
Authors: Stenflo, J. O.; Priest, E. R.
Bibcode: 1984Obs...104..102S
Altcode:
No abstract at ADS
Title: Diagnostics of solar magnetic fluxtubes using a Fourier
transform spectrometer
Authors: Stenflo, J. O.; Solanki, S.; Harvey, J. W.; Brault, J. W.
Bibcode: 1984A&A...131..333S
Altcode:
An overview is presented of the diagnostic contents for fluxtube
modelling of Fourier transform spectrometer recordings of the
longitudinal Zeeman effect near the solar disk center made in
April 1979. The observations and data reductions are summarized
and the application of the weak-field model to the Stokes profiles
is examined. The significance of telluric lines and blends and the
validity of LS coupling are considered. The magnetic fluxes, intrinsic
field strengths, and area factors are discussed and the thermodynamic
properties of fluxtubes are addressed. Mass motions inside the fluxtubes
and the height variation of fluxtube parameters are considered.
Title: Book-Review - Solar and Stellar Magnetic Fields
Authors: Stenflo, J. O.
Bibcode: 1984MNSSA..43...22S
Altcode:
No abstract at ADS
Title: Report of ESA's topical team on solar and heliospheric physics.
Authors: Christensen-Dalsgaard, J.; Delache, P.; Hoyng, P.; Priest,
E. R.; Schwenn, R.; Stenflo, J. O.
Bibcode: 1984ESASP1070...26C
Altcode:
No abstract at ADS
Title: Fine-scale structure of solar magnetic fields
Authors: Stenflo, J. O.
Bibcode: 1984AdSpR...4h...5S
Altcode: 1984AdSpR...4....5S
Seeing limitations of the earth's atmosphere have prevented us from
spatially resolving most of the basic magnetic flux elements on the
sun, since their sizes are all well below one sec of arc (excluding
sunspots). No space experiment to overcome this limitation has yet
been performed, but the first step will be taken with Spacelab 2. Direct mapping of the circular polarization in spectral lines
provides us with information on the morphology and evolution of the
partially resolved magnetic structures. In reviewing recent results,
special attention is payed to the question of flux disappearance, since
it is fundamental for understanding the solar cycle, and depends on
a knowledge of the fine-scale structures. The strong-field (kG)
nature of the photospheric flux was revealed more than a decade ago
using polarization recordings in pairs of spectral lines. A breakthrough
in the use of spectral information to deduce the properties of the
spatially unresolved magnetic fluxtubes has recently been achieved
through the conversion of a Fourier transform spectrometer (FTS) into
a polarimeter for Zeeman-effect recordings. We first use the FTS data
to illustrate the diagnostic contents of the line-ratio technique,
and then indicate how a statistical approach with 400 Fe I lines has
recently been applied. In particular we discuss the implications of
the observed Stokes V asymmetries for fluxtube dynamics. Finally
the ongoing search for a small-scale ``turbulent'' magnetic field
of mixed polarities is described. Observational limits derived from
direct magnetograms, spectral line broadening, and the Hanle effect
are illustrated.
Title: Book-Review - Solar and Stellar Magnetic Fields - Origins
and Coronal Effects - I.A.U. SYMP.102
Authors: Stenflo, J. O.
Bibcode: 1984Natur.307R.669S
Altcode:
No abstract at ADS
Title: Book-Review - Solar and Stellar Magnetic Fields - Origins
and Coronal Effects
Authors: Stenflo, J. O.
Bibcode: 1984S&T....67S.527S
Altcode:
No abstract at ADS
Title: Polarimeter package for LEST.
Authors: Stenflo, J. O.
Bibcode: 1984LFTR....4.....S
Altcode:
Various polarimeter schemes for LEST are investigated. To eliminate
the spherical aberration in the polarimeter, a solution has been
found for which the polarimeter package with collimators, in the form
of a "black box" of length about 20 cm, may be inserted in the beam
without introducing any focus changes or beam deviations. Piezoelastic
modulation appears to be superior to other modulation methods. Two
categories of schemes are presented, one for use in systems that have
a fast frequency response (≡50 kHz), one for systems with a slow
response (CCD or CID-type detectors). All four Stokes parameters can
be recorded simultaneously using two modulators, without any beam
splitter or moving parts.
Title: Book-Review - Solar and Stellar Magnetic Fields - Origins
and Coronal Effects
Authors: Stenflo, J. O.; Mewe, R.
Bibcode: 1984SoPh...91..193S
Altcode:
No abstract at ADS
Title: Coherent scattering in the solar spectrum - Survey of linear
polarization in the range 4200-9950 A
Authors: Stenflo, J. O.; Twerenbold, D.; Harvey, J. W.; Brault, J. W.
Bibcode: 1983A&AS...54..505S
Altcode:
Solar-limb linear polarization observations are reported and
discussed. A linearly polarized 4200-9950-A spectrum was obtained using
the Fourier-transform spectrometer on the KPNO McMath telescope with
an entrance aperture of 10 x 17.5 arcsec centered 10 arcsec inside
the solar limb near one of the heliograhic poles, in seven 48-95-min
exposures on October 2-3, 1978, and April 27-28, 1979, and recorded
using the special modulation scheme described by Brault (1978). The
noise level varies from 0.01 to 0.1 percent, and the continuum
polarization decreases from over 0.1 percent below 4200 A to less than
0.01 percent above 6000 A. The polarization amplitudes of the clearly
significant lines are listed in a table, and the polarization profiles
of the most interesting cases are illustrated and discussed. It is
shown that the conventional model of dipole and isotropic scattering
is contradicted by the observations of fluorescent scattering within
and between multiplets and of quantum-mechanical interferences between
atomic states with different combinations of total angular momenta.
Title: Book-Review - Solar and Stellar Magnetic Fields - Origins
and Coronal Effects
Authors: Stenflo, J. O.
Bibcode: 1983JBAA...94R..41S
Altcode:
No abstract at ADS
Title: Coherent scattering in the solar spectrum - Survey of linear
polarization in the range 3165-4230 A
Authors: Stenflo, J. O.; Twerenbold, D.; Harvey, J. W.
Bibcode: 1983A&AS...52..161S
Altcode:
The linear polarization 10 sec of arc inside the solar limb has been
recorded over the wavelength range 3165-4230 Å with the vertical
spectrograph of the Kitt Peak McMath telescope. This polarization is
caused by coherent resonant and fluorescent scattering in the solar
atmosphere. The polarization observed in several multiplets of Fe I, as
well as in 1Ca II, 3Mg I, and 22Co I, is affected by quantum-mechanical
interference between the excited states of different total angular
momenta. The CN molecule shows significant polarization, increasing
to a maximum at each band head. Unexpectedly large polarization is
found among others in 21Ni I, 2Ti II, 1Cu I, as well as in a number
of Fe I lines. The continuum polarization increases steeply with
decreasing wavelength, and is generally larger than the intrinsic line
polarization. Contrary to theoretical expectations, the Balmer jump
does not show up in the continuum polarization.
Title: Solar and stellar magnetic fields: origins and coronal
effects. International Astronomical Union. Symposium No. 102, held
in Zurich, Switzerland, August 2 - 6, 1982.
Authors: Stenflo, J. O.
Bibcode: 1983ssmf.book.....S
Altcode:
No abstract at ADS
Title: Establishment of the LEST (Large European Solar Telescope)
Foundation.
Authors: Stenflo, J. O.
Bibcode: 1983SoPh...87..419S
Altcode:
No abstract at ADS
Title: Solar and stellar magnetic fields: Origins and coronal effects
Authors: Stenflo, J. O.
Bibcode: 1983IAUS..102.....S
Altcode:
Magnetic fields in stellar photospheres are discussed, taking into
account observations of magnetic fields on late-type stars, long-term
coherent periodicities in the mean magnetic field of the sun, methods
and results for detecting magnetic fields on late-type stars, and Zeeman
effect observations in the RS CVn star HR 5110 and in late-type stars
with strong Ca II emission. Other topics explored are related to the
theory of stellar magnetic field generation, the role of magnetic fields
in the structure and energy balance of stellar atmospheres, stellar
winds and spindown in late-type stars, and observations concerned
with the rotation-activity-cycle-age connection. Attention is given to
observational needs for progress in solar/stellar magnetic activity, an
outlook for ground-based and space observations, relationships between
solar and stellar magnetic fields, magnetic braking and angular momenta
of protostars, effects on mass loss from cool giant stars, expulsion
of magnetized plasmas from coronae, and the heating of intense magnetic
flux tubes by magnetohydrodynamic waves.
Title: Solar Magnetic and Velocity-Field Measurements New Instrument
Concepts
Authors: Stenflo, J. O.
Bibcode: 1983ApOpt..23.1267S
Altcode:
No abstract at ADS
Title: The Hanle Effect and the Diagnostics of Turbulent Magnetic
Fields in the Solar Atmosphere
Authors: Stenflo, J. O.
Bibcode: 1982SoPh...80..209S
Altcode:
The theory of the Hanle effect is used to interpret the linear
polarization measured in a number of spectral lines on the solar disk
near the heliographic north and south poles, in search for a turbulent
magnetic field in the solar atmosphere. The Hanle depolarization
is separated from a number of other effects, including collisional
depolarization and scattering geometry. Although the main aim of the
paper is to elucidate the physics of the Hanle effect as applied to
the Sun, our results indicate the existence of hidden or turbulent
magnetic flux near the temperature minimum of the solar atmosphere,
with a field strength between 10 and 100 G. This field is hidden in the
sense that it is not seen in measurements of the longitudinal Zeeman
effect (solar magnetograms). It carries more total magnetic flux than
the kG network fields.
Title: Institut für Astronomie. Jahresbericht für 1981.
Authors: Stenflo, J. O.
Bibcode: 1982MitAG..56..327S
Altcode:
No abstract at ADS
Title: Anfang und Ende: Evolution des Weltalls.
Authors: Stenflo, J. O.
Bibcode: 1982Orion..40..118S
Altcode:
No abstract at ADS
Title: Observations of Coherence Effects in the Solar Spectrum
Authors: Stenflo, J.
Bibcode: 1981siwn.conf..266S
Altcode:
No abstract at ADS
Title: Institut für Astronomie. Jahresbericht für 1980.
Authors: Stenflo, J. O.
Bibcode: 1981MitAG..53..301S
Altcode:
No abstract at ADS
Title: Resonance-Line Polarization - Part Six - Line Wing Transfer
Calculations Including Excited State Interference
Authors: Auer, L. H.; Rees, D. E.; Stenflo, J. O.
Bibcode: 1980A&A....88..302A
Altcode:
A heuristic theory of polarized radiative transfer is developed for
the wings of solar resonance lines. Magnetic fields are neglected. The
theory includes quantum mechanical interference between j = ½ and
3/2 excited states of line transitions sharing a common j = ½ ground
state. Examples of such lines are Ca II and K, Na I D1
and D2, and Mg II h and k. Calculations are made with the
HSRA solar model for these lines as well as the dipole-type transition
Ca I 4227 which is not affected by interference. The results for Ca I
4227, Ca II H and K and Na I D1 and D2 compare
very well with recent observations, lending support to our theory. The
polarization predicted in the Mg H h and k lines is the largest of all
indicating these lines to be prime candidates for linear polarization
observations in the UV spectrum.
Title: Search for Spectral Line Polarization in the Solar Vacuum
Ultraviolet
Authors: Stenflo, J. O.; Dravins, D.; Wihlborg, N.; Bruns, A.;
Prokofev, V. K.; Zhitnik, I. A.; Biverot, H.; Stenmark, L.
Bibcode: 1980SoPh...66...13S
Altcode:
An instrument designed to record polarization in the region 120-150 nm
of the solar spectrum was launched on the satellite Intercosmos-16,
July 27, 1976. The aim was to search for resonance-line polarization
that is caused by coherent scattering. Oblique reflections at gold-
and aluminium-coated mirrors in the instrument were used to analyze
the polarization. The average polarization of the Lα solar limb was
found to be less than 1%. It is indicated how future improved VUV
polarization measurements may be a diagnostic tool for chromospheric
and coronal magnetic fields and for the three-dimensional geometry of
the emitting structures.
Title: Resonance-line polarization. V - Quantum-mechanical
interference between states of different total angular momentum
Authors: Stenflo, J. O.
Bibcode: 1980A&A....84...68S
Altcode:
The paper discusses the observations of nonmagnetic polarization across
the solar Ca II H-K and Na I D1-D2 lines. The event is explained
in terms of quantum-mechanical interference between the scattering
amplitudes of the two transitions: j = 1/2 - 1/2 (H and D1) and 1/2 -
3/2 (K and D2).
Title: Resonance-line polarization: IV. Observations of non-magnetic
line polarization and its center-to-limb variations.
Authors: Stenflo, J. O.; Baur, T. G.; Elmore, D. F.
Bibcode: 1980A&A....84...60S
Altcode:
In the work described, nonmagnetic resonance-line polarization
was studied with the Sacramento Peak Observatory's high-altitude
Stokesmeter. Polarized line profiles and their center-of-limb variations
are determined for ten different lines, along with the upper limits
of polarization for nine potentially interesting resonance lines.
Title: A Polarimeter for a Fourier Transform Spectrometer and Initial
Solar Observations
Authors: Harvey, J.; Brault, J.; Stenflo, J.; Zwaan, C.
Bibcode: 1980BAAS...12R.476H
Altcode:
No abstract at ADS
Title: Solar variability.
Authors: Stenflo, J. O.
Bibcode: 1980vasg.meet..GL1S
Altcode: 1980vasg.meetQ....S
Solar variability is reviewed briefly, after which a more detailed
discussion is given to the fields of solar oscillations, magnetic
activity, and global energy output. The spectrum of solar variability
shows that the sun varies over a wide range of time scales, involving
different physical mechanisms. The main global variables can be
diagnosed by numerous observable phenomena: (1) energy production of
the sun can be measured against the neutrino flux; (2) energy output is
related to the solar constant, spectral variations, and the climate
on the earth; (3) the interior structure of the sun is diagnosed
by solar oscillations, the solar diameter, differential rotations,
magnetic fields, solar activity, as well as the solar wind.
Title: Concentration of axisymmetric magnetic flux by rotational
shearing motions.
Authors: Nakagawa, Y.; Stenflo, J. O.
Bibcode: 1979A&A....72...67N
Altcode:
The possibility of a pinchlike concentration of magnetic flux produced
by the generation of electric currents in the solar atmosphere is
studied. To isolate this effect from other evolutionary effects, the
evolution of axisymmetric force-free fields due to pure rotational
shearing motions is studied. It is shown that concentration of
magnetic flux to the axis can be achieved by such shearing motions,
with density enhancement in higher layers. This may give rise to
localized high-density flux concentrations in the chromosphere and
corona, containing strong magnetic fields and electric currents. The
dissipation of these currents causes localized heating, which may
explain the observed close correlation between flux concentrations
and temperature and density enhancements in the chromosphere and corona.
Title: Discussion
Authors: Landi Degl'Innocenti, M.; Kundu, M. R.; Landman, D. A.;
Smartt, R. N.; Stenflo, J.
Bibcode: 1979phsp.coll...85L
Altcode: 1979IAUCo..44...85D; 1979phsp.coll...85D
No abstract at ADS
Title: Small-scale solar magnetic fields.
Authors: Stenflo, J. O.
Bibcode: 1979psa..conf...75S
Altcode:
No abstract at ADS
Title: Discussion
Authors: Acton, L. W.; Heasley, J. N.; Sahal-Bréchot, S.; Stenflo, J.
Bibcode: 1979phsp.coll...91A
Altcode: 1979IAUCo..44...91A
No abstract at ADS
Title: Discussion
Authors: Anzer, U.; Chiuderi-Drago, F.; Kundu, M. R.; Leroy, J. L.;
Malville, J.; Rompolt, B.; Sheeley, N.; Stenflo, J.; Tandberg-Hanssen,
E.; Öhman, Y.
Bibcode: 1979phsp.coll...77A
Altcode: 1979IAUCo..44...77A
No abstract at ADS
Title: Magnetic, velocity and brightness structure of solar faculae.
Authors: Frazier, E. N.; Stenflo, J. O.
Bibcode: 1978A&A....70..789F
Altcode:
To investigate the magnetic, velocity, and brightness structure of solar
magnetic elements in active region plages, the line-ratio technique
of Stenflo (1973) was applied by making simultaneous magnetograph
recordings in the two Fe I lines at 525.0 and 524.7 nm. Not only the
B(525.0)/B(524.7) ratio but also the brightness-magnetic field and
velocity-magnetic field correlations were studied. The center-to-limb
variations of the parameters were also observed. The data were analyzed
in terms of a Milne-Eddington model atmosphere. The cross section of the
magnetic field was found to be narrower than the associated velocity
profile. The continuum contrast at disk center had an amplitude of
as much as 18%, which indicates that the heating starts at relatively
large optical depths, with temperature excess increasing rapidly with
height to account for the strong intensity enhancement in the line
core. With the M-E model, however, it was not possible to obtain
a fully consistent fit with all data at the same time. One needs a
multidimensional, time-dependent non-LTE model for the transfer of
polarized light in rapidly evolving small-scale magnetic geometries
Title: Multi-dimensional Non-LTE Transfer of Polarized Radiation in
Magnetic Fluxtubes
Authors: Stenholm, L. G.; Stenflo, J. O.
Bibcode: 1978A&A....67...33S
Altcode:
Summary. Calculations of multi-dimensional transfer of the Stokes
vector in magnetic fluxtubes are presented. The Zeeman splitting
caused by the magnetic field modifies the multi-dimensional effects
in different ways. The line opacities are changed, which affects
the number of photons from the hot fluxtube walls that can reach the
fluxtube axis. Similarly the probabilities of photon escape in various
directions and at various frequencies are changed. These effects are
considered in detail for all the Stokes parameters. It is shown how the
multi-dimensional effects influence the relation between the apparent
field strength observed with a solar magnetograph and the true field
strength. The line-ratio method used to derive true field strengths from
simultaneous maguetograph recordings in two spectral lines of different
Lande' factor is tested. It is found that this method is insensitive
to all the various model parameters, like collisional excitation
cross section, fluxtube radius, or Wilson depression, provided that
the line pair is properly chosen, using equally strong lines from the
same multiplet, e. g. Fe I 525.0 and 524.7 um. Observations in single
lines with Babcock-type magnetographs are not suitable for deriving
field strengths, since the results are very model dependent. Key words:
multi-dime'nsional - non-LTE - polarization - maguetic field - Sun
Title: REVIEW: The measurement of solar magnetic fields
Authors: Stenflo, J. O.
Bibcode: 1978RPPh...41..865S
Altcode:
Methods for studying solar magnetic fields are examined, taking into
account Zeeman and Hanle effects, radio observations, the influence
of magnetic fields in solar structures, theoretical extrapolations of
photospheric measurements, in situ measurements in the solar-wind
region, and meteorite records of the primordial solar magnetic
field. Instrumental techniques for optical polarization measurements
are considered and an interpretation of optical polarization data is
provided. Attention is given to aspects of instrumental polarization,
polarization modulation, wavelength-selecting systems, detector systems,
magnetographs in space, general concepts of spectral line formation
in the presence of magnetic fields, coherence effects in scattering,
multilevel model atoms, multidimensional effects, the magnetograph
calibration error, and a summary of various approximations used in
treating line formation in a magnetic field.
Title: Resonance-line polarization. III. The Hanle effect in a
compact non-LTE radiative transfer formulation.
Authors: Stenflo, J. O.
Bibcode: 1978A&A....66..241S
Altcode:
The interference effects collectively referred to as the Hanle effect
were previously incorporated into a non-LTE theory for the transfer
of polarized radiation in the special case of zero magnetic fields,
and that theory was applied to numerical calculations of polarization
in solar UV emission lines. This paper extends the theory to include
interference effects in the radiative-transfer equation in the case
of arbitrary nonzero magnetic fields that are weak in the sense that
Zeeman splitting is small in comparison with the Doppler line width. The
density-matrix results of Omont et al. (1973) are transformed to obtain
the phase matrix and line emission in a Stokes-vector representation
of light and are then combined with the non-LTE formalism previously
developed for the nonmagnetic case. The behavior of the phase matrix
is illustrated by computing the polarization of scattered light for
a number of scattering processes. It is noted that the magnetic-field
vector cannot be determined uniquely from Hanle-effect measurements in a
single spectral line and that the Hanle effect appears to offer a unique
possibility for constructing three-dimensional vector magnetic-field
maps of the upper solar atmosphere.
Title: Solar-cycle variations in the differential rotation of solar
magnetic fields.
Authors: Stenflo, J. O.
Bibcode: 1977A&A....61..797S
Altcode:
Solar-cycle variations in the rotation rate of solar magnetic
fields have been studied by autocorrelation analysis of photospheric
magnetic fields. The observed angular velocity and its latitudinal
shear show large fluctuations correlated in latitude-time space with
the pattern of solar activity. The regions of large latitudinal shear
and low angular velocity drift towards the equator in much the same
way as sunspot and prominence zones. These results as well as earlier
observations of coexisting rotation laws on the sun can be given a
consistent explanation by assuming that the surface rotation rate of
a magnetic tracer represents mainly the rotation rate of the deepest
layers in the convection zone, to which the field line from the surface
penetrates. The depths where the surface fields are rooted vary with
the solar cycle.
Title: Statistical analysis of solar Fe I lines: magnetic line
broadening.
Authors: Stenflo, J. O.; Lindegren, L.
Bibcode: 1977A&A....59..367S
Altcode:
Results are presented for a statistical analysis of 402 unblended Fe
I lines covering the region from 400 to 686 nm in the solar spectrum
at disk center. The analysis was conducted to search for magnetic
line-broadening effects and to obtain information on the magnetic
fields at the extreme small-scale end of the spatial spectrum. Empirical
relations are determined between line width, line shape, line strength,
excitation potential, and wavelength. An upper limit of 9 mT is placed
on the rms magnetic field responsible for line-broadening effects,
model calculations of magnetic line broadening are performed, and it
is suggested that solar magnetic fields are highly intermittent.
Title: Multi-dimensional non-LTE radiative transfer in magnetic flux
tubes on the sun.
Authors: Stenholm, L. G.; Stenflo, J. O.
Bibcode: 1977A&A....58..273S
Altcode:
Summary. The effects of multi-dimensional non-LTE radiative transfer in
magnetic fluxtubes have been explored. It is found that weakenings of
spectral lines are produced by channelling of the radiation from the hot
walls surrounding the fluxtubes when a Wilson depression is present. The
fictitious temperature enhancement obtained when interpreting the
calculated line weakenings according to LTE agrees with the temperature
structure derived in recent LTE facular models. The dependence of
the multi-dimensional effects on various parameters like fluxtube
radius, Wilson depression, collisional excitation cross section,
and oscillator strength has been studied. The results may be used as
guidelines when selecting suitable combinations of spectral lines to
be used in observations for separation of the various parameters in
facular or fluxtube models. Key words: multi-dimensional radiative
transfer non-LTE magnetic field Sun
Title: Multi-dimensional transfer of polarized radiation in magnetic
fluxtubes.
Authors: Stenholm, L. G.; Stenflo, J. O.
Bibcode: 1977ROLun..12...53S
Altcode:
No abstract at ADS
Title: The Hanle effect in a compact non-LTE radiative-transfer
formulation.
Authors: Stenflo, J. O.
Bibcode: 1977ROLun..12...65S
Altcode:
No abstract at ADS
Title: Measurements and interpretation of polarization arising in
the solar chromosphere and corona : proceedings of a workshop held
at Lund Observatory, May 9-13, 1977
Authors: Stenflo, Jan Olof
Bibcode: 1977ROLun..12.....S
Altcode: 1977QB528.M37......
No abstract at ADS
Title: The Influence of Magnetic Fields on Solar Hydrodynamics,
Experimental Results
Authors: Stenflo, J. O.
Bibcode: 1977ebhs.coll..143S
Altcode: 1977IAUCo..36..143S
No abstract at ADS
Title: Observations of resonance-line polarization in the solar EUV.
Authors: Stenflo, J. O.; Dravins, D.; Öhman, Y.; Wihlborg, N.;
Bruns, A.; Prokof'ev, V. K.; Severnyj, A.; Severny, A.; Zhitnik,
I. A.; Biverot, H.; Stenmark, L.
Bibcode: 1977ROLun..12..147S
Altcode:
No abstract at ADS
Title: Ultraviolet polarimeter to record resonance-line polarization
in the solar spectrum around 130-150 nm
Authors: Stenflo, J. O.; Biverot, H.; Stenmark, L.
Bibcode: 1976ApOpt..15.1188S
Altcode:
A Swedish-built UV spectropolarimeter to be launched on a Soviet
satellite in the Intercosmos series is described. The scientific
objective is to record linear polarization across monochromatic
solar images formed in resonance lines in the 130-150-nm wavelength
region. This polarization arises by coherent scattering in the
chromosphere-corona transition region of the sun. The instrument
uses two parallel optical channels with highly different polarizing
properties. The polarization analysis is done by reflection at
gold-coated mirrors. The UV calibrations of the two flight models
are described.
Title: Resonance-line polarization. I. A non-LTE theory for the
transport of polarized radiation in spectral lines in the case of
zero magnetic field.
Authors: Stenflo, J. O.
Bibcode: 1976A&A....46...61S
Altcode:
Sumitiary. A non-LTE theory for resonance-line polarization due to
coherent scattering is developed for the magnetic field4ree case. The
equation of radiative transfer for the S'tokes vector is written in
a form analogous to the formalism normally used for the well-explored
ndn-LTE case of isotropic scattering and complete redistribution. This
makes it possible to apply the powerful numerical methods developed
in the past to solve non-LTE problems and extend them to calculate
the transport of polarized light for multi-level atoms. Cases of
non-axial symmetry can be treated by making a Fourier expansion o'f
the Stokes vector. A separate transfer equati6n can be Written for
each azimuth-independent Fou'rier component, these equations being
uncoupled from each other. An iterative numerical method to calculate
the polarization is indicated. The explicit formulas for the special
cases of a spherically symmetric atmosphere and for a two-level atom are
also given. Key words: non-LTE - spectral line polarization radiative
transfer - coherent scattering
Title: Small-Scale Solar Magnetic Fields
Authors: Stenflo, J. O.
Bibcode: 1976IAUS...71...69S
Altcode:
No abstract at ADS
Title: Resonance-line polarization. II. Calculations of linear
polarization in solar UV emission lines.
Authors: Stenflo, J. O.; Stenholm, L.
Bibcode: 1976A&A....46...69S
Altcode:
`Summary. Resonance-line polarization in a spherically symmetric,
plane-parallel atmosphere has been explored by making numerical
calculations of the center- to- limb variation of the intensity and
polarization line profiles for the two solar UV emission lines Si
iv 139.38nm and CIII 97.70nm. The model of the line- forming layer
in the chromosphere-corona transition region has been specified by
three parameters: the temperature gradient, the electron pressure,
and the line-broadening velocity. As collisional depolarization is
insignificant in the transition region, the polarization turns out to
depend on the model atmosphere only through Td, the optical thickness
of the line-forming layer, regardless of what 6ombination of the three
atmospheric parameters has generated a given value of Td. Optically thin
radiation is ` thermal ' and therelore unpolarized. Multiple scattering
enhances the polariza tion, but as the anisotropy in the radiation
field (limbbrightening) decreases with increased optical thickness,
the initial rise in the polarization will be followed by a decline as
Td increases. When Td is small, the polarization is constant throughout
the core of the line but decreases in the far wings due to the increased
contribution from the unpolarized continuum. For large values of Td the
polarization is reduced in the opaque line core, and maxima develop
in the wings where the line4orming layer becomes transparent. It
is indicated how observations of resonance-line polarization can be
used to determine the structure of the solar atmosphere, including its
small-scale geometry. Key words. polarization solar UV spectral line -
transition region - radiative transfer
Title: A Model of the Supergranulation Network and of Active Region
Plages
Authors: Stenflo, J. O.
Bibcode: 1975SoPh...42...79S
Altcode:
Analysis of magnetograph recordings made simultaneously in
different spectral lines have shown that the quiet-region network
and active-region plages with average field strengths less than
about 100 G are made up by the same type of elementary structures,
each having the same physical properties. Magnetograph data are used
together with continuum, line profile, and EUV data to derive a model
of these subarcsec, spatially unresolved elementary structures. The
field strength at the center of each basic element is about 2 kG. The
temperature enhancement starts at a height of about 180 km (above
the level τ0 = 1 in HSRA), and increases rapidly with
height. The brightness structures are coarser than the magnetic-field
structures.
Title: Summary of report on current solar research at the Lund
Observatory
Authors: Stenflo, J. O.
Bibcode: 1975cesra...5..120S
Altcode:
No abstract at ADS
Title: Summary of report on current solar research at the Lund
Observatory.
Authors: Stenflo, J. O.
Bibcode: 1975MmArc.105..120S
Altcode:
No abstract at ADS
Title: Svenska Astronomiska Sällskapet; Astronomiska Sällskapet
Tycho Brahe; Göteborgs Astronomiska Klubb. Styrelsens berättelse
för år 1974.
Authors: Elvius, T.; Darsenius, G.; Björklund, P. -Å.; Stenflo,
J. O.; Wilhelmsson, H.; Ekelund, L.
Bibcode: 1975sasa.book.....E
Altcode:
No abstract at ADS
Title: Observations of Resonance Polarization in Ca I λ4227
Authors: Stenflo, J. O.
Bibcode: 1974SoPh...37...31S
Altcode:
The resonance polarization in Ca I λ4226.73 has been measured close
to the solar limb for three values of μ = cosθ. The polarization
has maxima in the wings at about −500 and + 400 mÅ from the line
center, the plane of polarization (electric vector) being parallel
to the solar limb. The blue wing is more polarized than the red wing,
contrary to earlier results of Brückner (1963). Local minima in the
polarization curves occur at the position of blends. The polarization
in the wings increases rapidly with decreasing μ. At μ = 0.15 the
maximum polarization in the blue wing is about 2.4%.
Title: Differential Rotation and Sector Structure of Solar Magnetic
Fields
Authors: Stenflo, J. O.
Bibcode: 1974SoPh...36..495S
Altcode:
The differential rotation and sector structure of solar magnetic fields
has been studied using digitized data on photospheric magnetic fields
recorded at the Mount Wilson Observatory during the period August
1959-May 1970.
Title: Magnetic-Field Structure of the Photospheric Network
Authors: Stenflo, J. O.
Bibcode: 1973SoPh...32...41S
Altcode:
A method is developed to determine the physical parameters of the
spatially unresolved photospheric network. The apparent magnetic
fluxes are recorded simultaneously in the two FeI lines 5250 and
5247 Å, which belong to the same multiplet and have practically the
same oscillator strength and excitation potential of the lower level,
but differ in the effective Lande factor. By analysing magnetograph
recordings in this pair of lines together with simultaneous recordings
in the two FeI lines 5250 and 5233 Å, it is possible to separate the
effects on the line profiles due to Zeeman splitting and temperature
enhancement in the network.
Title: Solar chromatograph.
Authors: Stenflo, J. O.
Bibcode: 1973ApOpt..12..805S
Altcode:
No abstract at ADS
Title: On the Small-Scale Structure of Solar Magnetic Fields
Authors: Frazier, E. N.; Stenflo, J. O.
Bibcode: 1972SoPh...27..330F
Altcode:
The small-scale structure of solar magnetic fields has been studied
using simultaneous recordings in the spectral lines Fe I 5250 Å and Fe
I 5233 Å, obtained with the Kitt Peak multi-channel magnetograph. We
find that more than 90% of the magnetic flux in active regions
(excluding the sunspots), observed with a 2.4 by 2.4″ aperture, is
channelled through narrow filaments. This percentage is even higher in
quiet areas. The field lines in a magnetic filament diverge rapidly
with height, and part of the flux returns back to the neighbouring
photosphere. Therefore the strong fields within a magnetic filament are
surrounded by weak fields of the order of a few gauss of the opposite
polarity. The field-strength distribution within a filament, including
the surrounding opposite-polarity fields, seems to be almost the same
for all filaments within a given active or quiet region.
Title: Evolution of Solar Magnetic Fields over an 11-Year Period
Authors: Stenflo, J. O.
Bibcode: 1972SoPh...23..307S
Altcode:
Digitized data on solar magnetic fields recorded at the Mount Wilson
Observatory during the period August 1959-May 1970 have been used to
study the large-scale evolution of the photospheric magnetic fields. The
latitude distribution (butterfly diagram) of the magnetic field is
compared with the distribution of sunspots, faculae, prominences
and the intensity of the green-line corona. The evolution of the
sector structure of the field is calculated. 36 synoptic charts,
each representing an average of four solar rotations, illustrate the
evolution of the magnetic field over the 11-year period.
Title: On Stellar Activity Cycles
Authors: Durney, B. R.; Stenflo, J. O.
Bibcode: 1972Ap&SS..15..307D
Altcode:
The relation between the average magnetic fieldB, the angular velocity
Ω, and the periodP of stellar activity cycles is studied. For the
calculations we have used Leighton's (1969) model for the solar cycle
with the additional assumption that the differential rotation and the
cyclonic turbulence (Parker, 1955) (that is the ‘sunspot tilt’ or
the ‘α-effect’) are both proportional to Ω. We then find thatB is
roughly proportional to Ω and thatP decreases with increasing Ω. The
period of the solar cycle increases therefore with the age of the Sun.
Title: On the Filamentary Nature of Solar Magnetic Fields
Authors: Howard, Robert; Stenflo, J. O.
Bibcode: 1972SoPh...22..402H
Altcode:
A method is presented for obtaining information about the unresolved
filamentary structure of solar magnetic fields. A comparison is made
of pairs of Mount Wilson magnetograph recordings made in the two
spectral lines FeI 5250 Å and FeI 5233 Å obtained on 26 different
days. Due to line weakenings and saturation in the magnetic filaments,
the apparent field strengths measured in the 5250 Å line are too
low, while the 5233 Å line is expected to give essentially correct
results. From a comparison between the apparent field strengths and
fluxes and their center to limb variations, we draw the following
tentative conclusions: (a) More than 90 % of the total flux seen with
a 17 by 17 arc sec magnetograph aperture is channeled through narrow
filaments with very high field strengths in plages and at the boundaries
of supergranular cells. (b) An upper limit for the interfilamentary
field strength integrated over the same aperture seems to be about
3 G. (c) The field lines in a filament are confined in a very small
region in the photosphere but spread out very rapidly higher up in
the atmosphere. (d) All earlier Mount Wilson magnetograph data should
be multiplied by a factor that is about 1.8 at the center of the disk
and decreased toward the limb in order to give the correct value of
the longitudinal magnetic field averaged over the scanning aperture.
Title: The Electromagnetic Structure of Interplanetary Space
Authors: Stenflo, J. O.
Bibcode: 1972NASSP.308..115S
Altcode: 1972sowi.conf..115S
No abstract at ADS
Title: Solar chromatograph.
Authors: Stenflo, J. O.
Bibcode: 1972ApOpt..12..805S
Altcode:
No abstract at ADS
Title: On the Filamentary Nature of Solar Magnetic Fields
Authors: Howard, R.; Stenflo, J. O.
Bibcode: 1972lfpm.conf..251H
Altcode:
No abstract at ADS
Title: On the Filamentary Nature of Active-Region Magnetic Fields
Authors: Stenflo, J. O.; Frazier, E. N.
Bibcode: 1972lfpm.conf..295S
Altcode:
No abstract at ADS
Title: The Coronal and Interplanetary Magnetic Fields at the Time
of the Solar Eclipse of 7 March, 1970 (Papers presented at the
Proceedings of the International Symposium on the 1970 Solar Eclipse,
held in Seattle, U. S. A. , 18-21 June, 1971.)
Authors: Stenflo, J. O.
Bibcode: 1971SoPh...21..263S
Altcode:
The magnetic field in the outer corona and in interplanetary space has
been calculated from the photospheric magnetic fields measured around
the time of the 7 March, 1970 eclipse. The field-line maps are compared
with eclipse photographs showing coronal structures out to about 12
r⊙. The projected field lines as well as the observed
streamers appear straight. This is caused by the rapid expansion of the
outer corona and is not an indication of corotation. The calculations
show that the angular velocity of the coronal plasma decreases rapidly
with distance.
Title: Structure of the interplanetary magnetic field.
Authors: Stenflo, J. O.
Bibcode: 1971CosEl...2..309S
Altcode:
No abstract at ADS
Title: Observation of the Polar Magnetic Fields
Authors: Stenflo, J. O.
Bibcode: 1971IAUS...43..714S
Altcode:
No abstract at ADS
Title: VI och de.
Authors: Ehrensvard, Gosta Carl Henrik; Stenflo, Jan Olof
Bibcode: 1971vod..book.....E
Altcode: 1971QB54.V48.......
No abstract at ADS
Title: The Interpretation of Magnetograph Results: the Formation of
Absorption Lines in a Magnetic Field
Authors: Stenflo, J. O.
Bibcode: 1971IAUS...43..101S
Altcode:
No abstract at ADS
Title: Erratum: "The profiles of Fraunhofer lines in the presence
of Zeeman splitting" [Sol. Phys., Vol. 9, p. 372 - 386 (1969)].
Authors: Stenflo, J. O.; Beckers, J. M.
Bibcode: 1970SoPh...15..507S
Altcode:
No abstract at ADS
Title: A Comparison of Simultaneous Measurements of the Polar Magnetic
Fields Made at the Crimea and Mount Wilson
Authors: Kotov, V. A.; Stenflo, J. O.
Bibcode: 1970SoPh...15..265K
Altcode:
Measurements of the polar magnetic fields of the sun made in August
1968 with the Crimean and Mt Wilson magnetographs are compared. The
agreement between the results obtained at the two observatories is
rather satisfactory. The correlation coefficient between the Crimean
and Mt Wilson values of the observed average field strength at different
latitudes is 0.7 for the north and 0.5 for the south polar region. The
earlier conclusion based on the Mt Wilson material that a polarity
reversal of the field occurred at latitudes +70° and -55° in the north
and south hemispheres (Stenflo, 1970) is confirmed by the Crimean data.
Title: Hale's Attempts to Determine the Sun's General Magnetic Field
Authors: Stenflo, Jan Olof
Bibcode: 1970SoPh...14..263S
Altcode:
Hale's attempts to determine the sun's general magnetic field are
reviewed. The field reported by Hale was an order of magnitude stronger
than that presently measured with photoelectric techniques. The
polarity was opposite to that expected from Babcock's theory of the
solar cycle. Practically all the reduction work had been made by Van
Maanen with a tipping-plate micrometer.
Title: The Polar Magnetic Fields of July and August 1968
Authors: Stenflo, Jan Olof
Bibcode: 1970SoPh...13...42S
Altcode:
Observations of the polar magnetic fields were made during the period
July 3-August 23, 1968, with the Mt. Wilson magnetograph. The scanning
aperture was 5″ × 5″. The magnetic field was found to be ofS
polarity near the heliographic north pole and ofN polarity near the
south pole. At lower latitudes the polarity was the opposite. The
polarity reversal occurred at a latitude of about +70° in the north
and -55° in the south hemisphere. This coincides with the position
of the polar prominence zones at that time. The observations indicate
that the average field strength at the south pole was well above 5 G.
Title: Interpretation of magnetograph results.
Authors: Stenflo, J. O.
Bibcode: 1970ROLun...2.....S
Altcode:
No abstract at ADS
Title: A Fast and Accurate Guiding System
Authors: Pâlsgârd, G.; Stenflo, J. O.
Bibcode: 1970SoPh...11..155P
Altcode:
No abstract at ADS
Title: The Zeeman Effect for Weak Magnetic Fields
Authors: Stenflo, Jan Olof
Bibcode: 1969SoPh....8..260S
Altcode:
The polarization of a normal Zeeman triplet is discussed for the case
in which the lifetime τ of the excited state of the atom is comparable
to or shorter than the period of Larmor precession.
Title: A Mechanism for the Build-Up of Flare Energy
Authors: Stenflo, Jan Olof
Bibcode: 1969SoPh....8..115S
Altcode:
It is shown how the kinetic energy of the rotational motion of a
sunspot can be transferred to electromagnetic energy in filamentary
currents. The time needed for preconditioning the solar atmosphere
for a flare varies within wide limits. For small flares it may be of
the order of minutes; for large flares, of the order of hours or days.
Title: A Mechanism for the Buildup of Flare Energy
Authors: Stenflo, Jan Olof
Bibcode: 1969BAAS....1T.293S
Altcode:
No abstract at ADS
Title: A New Technique to Obtain Solar-Velocity Maps Directly in
One Spectroheliogram
Authors: Bhatnagar, A.; Stenflo, J. O.
Bibcode: 1969BAAS....1S.273B
Altcode:
No abstract at ADS
Title: A Method to obtain a Solar Velocity Map Directly in One
Spectroheliogram
Authors: Bhatnagar, A.; Stenflo, J. O.
Bibcode: 1969SoPh....7..329B
Altcode:
No abstract at ADS
Title: Solar Magnetic-Field Measurements using Babinet Compensators
Authors: Beckers, J. M.; Stenflo, J. O.
Bibcode: 1969SoPh....6..480B
Altcode:
No abstract at ADS
Title: The Magnetoheliograph
Authors: Stenflo, Jan Olof
Bibcode: 1969SoPh....6..476S
Altcode:
A method is described, which makes it possible to obtain a magnetic
photograph directly in one image without the use of photographic
subtraction.
Title: A New Type of Magnetograph
Authors: Stenflo, Jan Olof
Bibcode: 1968SoPh....3..482S
Altcode:
A solar magnetograph without electronics or any moving parts has been
designed. The principle is the use of a subtracting double-dispersion
optical system. The exposure time is of the order of 1 sec.
Title: On the interpretation of measurements with a solar magnetograph
Authors: Stenflo, Jan Olof
Bibcode: 1968MeLuS.153....3S
Altcode:
No abstract at ADS
Title: Measurements of the sun's magnetic field
Authors: Stenflo, Jan Olof
Bibcode: 1968MeLuS.152....5S
Altcode: 1968MeLuS.152....1S
No abstract at ADS
Title: The Balance of Magnetic Fluxes in Active Regions
Authors: Stenflo, Jan Olof
Bibcode: 1968IAUS...35...47S
Altcode:
No abstract at ADS
Title: The sun's magnetic field
Authors: Stenflo, Jan Olof
Bibcode: 1968PhDT.......222S
Altcode:
No abstract at ADS
Title: The sun's magnetic field
Authors: Stenflo, Jan Olof
Bibcode: 1968MeLuF.233....3S
Altcode:
No abstract at ADS
Title: On the Balance of Magnetic Fluxes in Sunspot Groups
Authors: Stenflo, Jan Olof
Bibcode: 1967MeLuS.150....1S
Altcode:
No abstract at ADS
Title: On the influence of the slit size on measurements with a
solar magnetograph
Authors: Stenflo, J. O.
Bibcode: 1966Obs....86...73S
Altcode:
No abstract at ADS
Title: Solar photometry in Ha
Authors: Stenflo, J. O.
Bibcode: 1966ArA.....3..481S
Altcode:
No abstract at ADS
Title: On the influence of the slit size on measurements with a
solar magnetograph
Authors: Stenflo, J. O.
Bibcode: 1966ArA.....4..173S
Altcode: 1968ArA.....4..173S
No abstract at ADS