Author name code: hofmann
ADS astronomy entries on 2022-09-14
=author:"Hofmann, A." OR =author:"Hofmann, Axel" -aff:"Johannesburg" -aff:"South Africa" -title:"Europa" -title:"Spherule" -title:"Enceladus" -title:"acid" -title:"isotope" -title:"Mars"
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Title: Organic Carbon Concentrations in 3-Billion-Year-Old Lacustrine
Mudstones
Authors: Stern, J. C.; Malespin, C.; Eigenbrode, J. L.; Webster, C. R.;
Flesch, G.; House, C. H.; Archer, P. D.; Graham, H. V.; Hofmann, A.;
McAdam, A. C.; Steele, A.; Sutter, B.; Wong, G. M.; Mahaffy, P. R.
Bibcode: 2022LPICo2678.1219S
Altcode:
Organic carbon / Locked in ancient martian rocks / How did it get there?
Title: Addressing Mental Health in Planetary Science
Authors: Vance, Steven; Elder, C.; Hofmann, A.; Howell, S.; Milazzo,
M.; Pappalardo, R. T.; Noviello, J.; Patthoff, D. A.; Khan, Z.;
Rathbun, J.; Vertesi, J.
Bibcode: 2021BAAS...53d.429V
Altcode: 2021psad.rept..429V
As NASA strives to be more inclusive, it must also work to address
a compounding crisis of mental health. We summarize the available
evidence for a mental health crisis among academics. We describe
how this problem intersects with and amplifies problems of equity,
diversity, and inclusion.
Title: Addressing Mental Health in Planetary Science
Authors: Vance, S. D.; Elder, C.; Hofmann, A.; Howell, S.; Milazzo,
M.; Pappalardo, R. T.; Noviello, J. L.; Patthoff, D. A.; Khan, Z.;
Rathbun, J.; Vertesi, J.
Bibcode: 2021LPI....52.2552V
Altcode:
We summarize the available evidence for a mental health crisis among
academics. We describe how this problem intersects with issues of
EDI. Addressing the problems is a necessary step toward creating a
more creative and effective workforce.
Title: Characterization of the High Velocity Impact Phenomena of
Small Water-Ice Particles
Authors: Burke, S.; Miller, M. E. C.; Continetti, R.; Waller, S. E.;
Jaramillo-Botero, A.; Hodyss, R. P.; Malaska, M.; Hofmann, A.; Abel,
B.; Postberg, F.; Lunine, J. I.; Cable, M. L.
Bibcode: 2020AGUFMA181.0009B
Altcode:
For the success of future life finding missions to ocean worlds
like Enceladus, terrestrial characterization of mission-relevant
instrumentation is key. The Cassini mission previously observed small
(~1 μm) ice particles in the plume of Enceladus and characterized these
particles using impact ionization mass spectrometry. The particles
and the molecules entrained in them were subjected to impacts at
hypervelocity speeds. The objective of our work is to investigate
the effect of hypervelocity impact on grain fragmentation, and the
potential for subsequent molecular fragmentation. The Aerosol Impact
Spectrometer (AIS) is an instrument that can produce analogues to
particles previously observed from Cassini, accelerate a single particle
to the high velocities relevant for a fly-by spacecraft (> 3 km/s),
and analyze their impact phenomena on metal targets. Electrospray
ionization (ESI) is used for the generation of 0.5-2 μm, highly charged
water-ice particles. Individual grains are accelerated under high vacuum
(10-8 Torr) with a 41-stage linear accelerator (LINAC)
to a desired final velocity from 0.01 to 5 km/s. The implementation
of this LINAC with ice particle source and its capabilities will
be overviewed. The impact phenomena, including rebound, sticking,
and fragmentation, of the accelerated particles on a metal target
are characterized with a tapered image charge detector (TICD). We
found that 850 nm-sized grains at lower velocities (< 880 m/s)
demonstrate velocity-dependence in their post impact behavior,
including sticking and fragmentation of the grain. Ongoing studies
will further elucidate impact phenomena at higher velocities. Future
studies using ice particles containing organic and inorganic solutes
will examine the post-impact mass spectra using a time-of-flight mass
spectrometer to allow examination of molecular fragmentation of the
solutes at hypervelocity.
Title: DAVINCI+: Opening the History Book of Venus and Connecting
Analog Exoplanets
Authors: Arney, G. N.; Garvin, J. B.; Getty, S.; Johnson, N.; Amato,
M.; Atkinson, D. H.; Atreya, S. K.; Brinckerhoff, W. B.; Campbell,
B. A.; Cottini, V.; Crisp, D.; Filiberto, J.; Forget, F.; Gilmore,
M. S.; Grinspoon, D. H.; Hofmann, A.; Izenberg, N.; Kane, S.; Kiefer,
W. S.; Lebonnois, S.; Lorenz, R. D.; Mahaffy, P. R.; Malespin, C.;
Pavlov, A.; Ravine, M. A.; Schwer, K.; Sekerak, M.; Trainer, M. G.;
Webster, C. R.; Zahnle, K. J.
Bibcode: 2020AGUFMP029...05A
Altcode:
The Deep Atmosphere Venus Investigation of Noble Gases, Chemistry,
and Imaging Plus (DAVINCI+) is a proposed Discovery mission that
would be the first U.S. spacecraft to enter the Venus atmosphere
since 1978. DAVINCI+ will make detailed measurements of the Venus
surface and atmosphere, revealing its current state and history and
helping to place our sister planet into a larger cosmic context of
exoplanets. Venus and Exo-Venus Habitability: Recent analyses
(Way et al. 2016; 2020) have suggested that Venus may have enjoyed
a prolonged period of clement conditions. Intriguingly, the same
processes that might have enabled early Venus habitability may also
enable habitability on the ubiquitous hot, slowly rotating exoplanets
that will be observed with the James Webb Space Telescope. DAVINCI+
will search for clues of early Venus habitability, providing insight
into the conditions of young Venus and analog exoplanet climates. The most important evidence of possible early habitability on Venus
is its D/H ratio. The DAVINCI+ descent probe will directly measure
D/H below ~70 km. Other isotopes, e.g. 4He, will further
constrain atmospheric evolution timescales. Clues to early habitability
may also be encoded in Venus' tessera features, which may be older
than the surrounding terrain and could show mineralogical clues of
ancient water. DAVINCI+'s descent probe will observe the tessera Alpha
Regio at meter-scales. Near-infrared images from orbit will provide
global context for localized descent probe imaging and constrain
global surface composition. Connection to exoplanet observations:
The Venusian environment is a natural laboratory to study the complex
processes that operate on analog hot exoplanets. DAVINCI's quantitative
measurements of the Venus atmosphere will provide needed constraints
for modeling exo-Venus atmospheres and predicting their spectral
observables. Cloud-top UV imaging, including of the mysterious "unknown
UV absorber", will help to constrain the possible spectral appearance of
exo-Venus analogs for possible direct imaging with future astrophysics
flagships, which may be biased towards observing UV wavelengths for
Venus-like planets due to coronagraph inner working angle constraints.
Title: Understanding hypervelocity sampling of ice-borne biosignatures
in space missions
Authors: Jaramillo-Botero, A.; Hofmann, A.; Malaska, M.; Hodyss,
R. P.; Lunine, J. I.; Waller, S. E.; Miller, M. E. C.; Burke, S.;
Continetti, R.; Abel, B.; Postberg, F.; Cable, M. L.
Bibcode: 2020AGUFMP003.0012J
Altcode:
In space missions meant to study the potential for life on other
planetary bodies, current sampling strategies include analysis of
planetary atmospheres and plumes during hypervelocity flyby encounters
at different altitudes. This is conventionally done through the use
of mass spectrometers and other instruments capable of measuring the
composition of gases, ices, and organic compounds. During collection,
fragmentation of these materials can occur following their initial
impact, prior to actual measurement, or from radiolytic processes on
the surface, prior to or during surface sputtering. Although this can be
mitigated by reducing the flyby speed of the spacecraft relative to the
atmosphere, this compromises sampling signal intensities, collisional
ionization strategies for analyzing dust, and may not be practical
for certain missions. We will present results to address some of
the unknowns of hypervelocity impacts on the mass spectra of single
impacting molecules, specifically for neutral molecules, in order to
determine if existing space mission data obtained at hypervelocity
from mass spectrometers and other instruments are compromised by
impact-induced fragmentation. The results provide key insights into the
design criteria for the optimization of spacecraft instruments meant
to measure low concentrations of neutrals in low-density atmospheres
during hypervelocity flybys, and enable establishing parameter bounds
for future hypervelocity sampling missions, considering the trade-off
between potential induced ionization and high encounter velocities to
increase the effective mass flow and accuracy in signal resolution.
Title: DAVINCI+ Answers Long-Standing and Emerging Questions About
the Venus Atmosphere
Authors: Getty, S.; Garvin, J. B.; Arney, G. N.; Johnson, N.; Amato,
M.; Atkinson, D. H.; Atreya, S. K.; Brinckerhoff, W. B.; Campbell,
B. A.; Cottini, V.; Crisp, D.; Filiberto, J.; Forget, F.; Gilmore,
M. S.; Grinspoon, D. H.; Hofmann, A.; Izenberg, N.; Kane, S.; Kiefer,
W. S.; Lebonnois, S.; Lorenz, R. D.; Mahaffy, P. R.; Malespin, C.;
Pavlov, A.; Ravine, M. A.; Schwer, K.; Sekerak, M.; Trainer, M. G.;
Webster, C. R.; Zahnle, K. J.
Bibcode: 2020AGUFMP022...01G
Altcode:
Fundamental questions remain about the origin, evolution, and
composition of Venus, a planet that may have once hosted Earth-like
water oceans. Signatures of Venus' past and present state can be
found in its stratified atmosphere, and particularly its poorly
understood lower atmosphere. The Deep Atmosphere Venus Investigation
of Noble gases, Chemistry, and Imaging plus (DAVINCI+) mission
is designed to answer hypotheses about the climate and evolution
of the potentially once-habitable Venus over time. DAVINCI+
carries a descent probe that will reveal fundamental details of the
Venus atmosphere and surface and camera suite to provide spatial
context from orbit. If selected for implementation in the Discovery
program, DAVINCI+ will build significantly upon the legacy of prior
missions to provide the first detailed in situ study of bulk and
trace gases of the lower atmosphere. The descent probe will conduct
altitude-resolved measurements of composition, isotopes, and winds,
pressure, and temperature down to very near the surface of Alpha Regio
tessera. The profile of sulfur-, oxygen-, and chlorine-bearing species,
coupled with UV imagery from orbit, will elucidate the atmospheric
cycles at play and provide new constraints on the composition of
the unknown UV absorber and exchange processes between the surface
and atmosphere. Bulk measurements of atmospheric noble gases,
particularly xenon, will definitively establish the volatile inventory
of primordial Venus. These measurements are critical to constraining
fundamental knowledge and timing of processes (e.g., volcanism) that
have determined the evolutionary pathways of this runaway greenhouse
planet. Venus has emerged as an ideal target for investigations of
habitability across time. Recent studies suggesting an ancient Venus
with liquid water at its surface, possibly persisting for billions of
years, are strongly motivating our next visit to and through Venus'
atmosphere. The astrobiological context of Venus is emerging as an
area of community excitement and may be revealed through definitive
atmospheric measurements. DAVINCI+ is poised to pave the way for future
missions, including future orbiters, aerial platforms, and landers that
might target the potential for present-day or ancient biosignatures
at Venus.
Title: Spectropolarimetric Observations of an Arch Filament System
with GREGOR
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
Kuckein, C.; Kučera, A.; Schwartz, P.; Berkefeld, T.; Collados, M.;
Denker, C.; Feller, A.; Hofmann, A.; Schlichenmaier, R.; Schmidt,
D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau,
D.; Staude, J.; Strassmeier, K. G.; von der Lühe, O.
Bibcode: 2019ASPC..526..217B
Altcode: 2018arXiv180401789B
We observed an arch filament system (AFS) in a sunspot group with the
GREGOR Infrared Spectrograph attached to the GREGOR solar telescope. The
AFS was located between the leading sunspot of negative polarity and
several pores of positive polarity forming the following part of the
sunspot group. We recorded five spectro-polarimetric scans of this
region. The spectral range included the spectral lines Si I 1082.7
nm, He I 1083.0 nm, and Ca I 1083.9 nm. In this work we concentrate
on the silicon line which is formed in the upper photosphere. The
line profiles are inverted with the code 'Stokes Inversion based
on Response functions' to obtain the magnetic field vector. The
line-of-sight velocities are determined independently with a Fourier
phase method. Maximum velocities are found close to the ends of AFS
fibrils. These maximum values amount to 2.4 km s-1 next
to the pores and to 4 km s-1 at the sunspot side. Between
the following pores, we encounter an area of negative polarity that
is decreasing during the five scans. We interpret this by new emerging
positive flux in this area canceling out the negative flux. In summary,
our findings confirm the scenario that rising magnetic flux tubes
cause the AFS.
Title: Photospheric Magnetic Fields of the Trailing Sunspots in
Active Region NOAA 12396
Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer,
C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello
González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.;
Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar,
A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer,
R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2019ASPC..526..291V
Altcode: 2018arXiv180507752V
The solar magnetic field is responsible for all aspects of solar
activity. Sunspots are the main manifestation of the ensuing solar
activity. Combining high-resolution and synoptic observations has
the ambition to provide a comprehensive description of the sunspot
growth and decay processes. Active region NOAA 12396 emerged on 2015
August 3 and was observed three days later with the 1.5-meter GREGOR
solar telescope on 2015 August 6. High-resolution spectropolarimetric
data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the
photospheric lines Si I λ1082.7 nm and Ca I λ1083.9 nm, together
with the chromospheric He I λ1083.0 nm triplet. These near-infrared
spectropolarimetric observations were complemented by synoptic
line-of-sight magnetograms and continuum images of the Helioseismic
and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO).
Title: Sea Ice as a Sink for CO2 and Biogeochemical
Material: a Novel Sampling Method and Astrobiological Applications
Authors: Wilner, J.; Hofmann, A.; Hand, K. P.
Bibcode: 2017AGUFM.P43C2897W
Altcode:
Accurately modelling the intensification of greenhouse gas effects
in the polar regions ("polar amplification") necessitates a thorough
understanding of the geochemical balance between atmospheric, sea ice,
and oceanic layers. Sea ice is highly permeable to CO2 and therefore
represents a major sink of oceanic CO2 in winter and of atmospheric
CO2 in summer, sinks that are typically either poorly constrained in
or fully absent from global climate models. We present a novel method
for sampling both trapped and dissolved gases (CO2, CH4 and δ13CH4)
in sea ice with a Picarro 2132-i Methane Analyzer, taking the following
sampling considerations into account: minimization of water and air
contamination, full headspace sampling, prevention of inadvertent
sample bag double-puncturing, and ease of use. This method involves
melting of vacuum-sealed ice cores to evacuate trapped gases to the
headspace and sampling the headspace gas with a blunt needle sheathed
by a beveled puncturing needle. A gravity catchment tube prevents input
of dangerous levels of liquid water to the Picarro cavity. Subsequent
ultrasonic degassing allows for dissolved gas measurement. We are in
the process of using this method to sample gases trapped and dissolved
in Arctic autumn sea ice cores and atmospheric samples collected
during the 2016 Polarstern Expedition and during a May 2017 field
campaign north of Barrow, Alaska. We additionally employ this method,
together with inductively coupled plasma mass spectrometry (ICP-MS),
to analyze the transfer of potential biogeochemical signatures
of underlying hydrothermal plumes to sea ice. This has particular
relevance to Europa and Enceladus, where hypothetical hydrothermal
plumes may deliver seafloor chemicals to the overlying ice shell. Hence,
we are presently investigating the entrainment of methane and other
hydrothermal material in sea ice cores collected along the Gakkel Ridge
that may serve as biosignatures of methanogenic organisms in seafloor
oases analogous to icy ocean worlds.
Title: Flows along arch filaments observed in the GRIS `very fast
spectroscopic mode'
Authors: González Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor
Yabar, A.; Collados, M.; Verma, M.; Balthasar, H.; Diercke, A.;
Fischer, C. E.; Gömöry, P.; Bello González, N.; Schlichenmaier,
R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt,
W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude,
J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2017IAUS..327...28G
Altcode: 2017arXiv170102206G
A new generation of solar instruments provides improved spectral,
spatial, and temporal resolution, thus facilitating a better
understanding of dynamic processes on the Sun. High-resolution
observations often reveal multiple-component spectral line profiles,
e.g., in the near-infrared He i 10830 Å triplet, which provides
information about the chromospheric velocity and magnetic fine
structure. We observed an emerging flux region, including two small
pores and an arch filament system, on 2015 April 17 with the `very
fast spectroscopic mode' of the GREGOR Infrared Spectrograph (GRIS)
situated at the 1.5-meter GREGOR solar telescope at Observatorio del
Teide, Tenerife, Spain. We discuss this method of obtaining fast (one
per minute) spectral scans of the solar surface and its potential to
follow dynamic processes on the Sun. We demonstrate the performance
of the `very fast spectroscopic mode' by tracking chromospheric
high-velocity features in the arch filament system.
Title: Slipping reconnection in a solar flare observed in high
resolution with the GREGOR solar telescope
Authors: Sobotka, M.; Dudík, J.; Denker, C.; Balthasar, H.; Jurčák,
J.; Liu, W.; Berkefeld, T.; Collados Vera, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Kuckein, C.; Lagg, A.; Louis, R. E.; von der Lühe, O.;
Nicklas, H.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.;
Volkmer, R.; Waldmann, T.
Bibcode: 2016A&A...596A...1S
Altcode: 2016arXiv160500464S
A small flare ribbon above a sunspot umbra in active region 12205 was
observed on November 7, 2014, at 12:00 UT in the blue imaging channel
of the 1.5 m GREGOR telescope, using a 1 Å Ca II H interference
filter. Context observations from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO), the Solar Optical
Telescope (SOT) onboard Hinode, and the Interface Region Imaging
Spectrograph (IRIS) show that this ribbon is part of a larger one
that extends through the neighboring positive polarities and also
participates in several other flares within the active region. We
reconstructed a time series of 140 s of Ca II H images by means of the
multiframe blind deconvolution method, which resulted in spatial and
temporal resolutions of 0.1″ and 1 s. Light curves and horizontal
velocities of small-scale bright knots in the observed flare ribbon
were measured. Some knots are stationary, but three move along the
ribbon with speeds of 7-11 km s-1. Two of them move in the
opposite direction and exhibit highly correlated intensity changes,
which provides evidence of a slipping reconnection at small spatial
scales. Movies associated to Figs. 1 and 2 are available at http://www.aanda.org
Title: Deep probing of the photospheric sunspot penumbra: no evidence
of field-free gaps
Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.;
Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.;
Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe,
O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.;
Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.;
Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H.
Bibcode: 2016A&A...596A...2B
Altcode: 2016arXiv160708165B
Context. Some models for the topology of the magnetic field in
sunspot penumbrae predict regions free of magnetic fields or with
only dynamically weak fields in the deep photosphere.
Aims:
We aim to confirm or refute the existence of weak-field regions in
the deepest photospheric layers of the penumbra.
Methods:
We investigated the magnetic field at log τ5 = 0 is
by inverting spectropolarimetric data of two different sunspots
located very close to disk center with a spatial resolution of
approximately 0.4-0.45''. The data have been recorded using the GRIS
instrument attached to the 1.5-m solar telescope GREGOR at the El
Teide observatory. The data include three Fe I lines around 1565 nm,
whose sensitivity to the magnetic field peaks half a pressure scale
height deeper than the sensitivity of the widely used Fe I spectral
line pair at 630 nm. Before the inversion, the data were corrected
for the effects of scattered light using a deconvolution method with
several point spread functions.
Results: At log τ5
= 0 we find no evidence of regions with dynamically weak (B<
500 Gauss) magnetic fields in sunspot penumbrae. This result is much
more reliable than previous investigations made on Fe I lines at 630
nm. Moreover, the result is independent of the number of nodes employed
in the inversion, is independent of the point spread function used to
deconvolve the data, and does not depend on the amount of stray light
(I.e., wide-angle scattered light) considered.
Title: Spectropolarimetric observations of an arch filament system
with the GREGOR solar telescope
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
Kuckein, C.; Kavka, J.; Kučera, A.; Schwartz, P.; Vašková, R.;
Berkefeld, T.; Collados Vera, M.; Denker, C.; Feller, A.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.;
Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1050B
Altcode: 2016arXiv160901514B
Arch filament systems occur in active sunspot groups, where a fibril
structure connects areas of opposite magnetic polarity, in contrast to
active region filaments that follow the polarity inversion line. We
used the GREGOR Infrared Spectrograph (GRIS) to obtain the full
Stokes vector in the spectral lines Si I λ1082.7 nm, He I λ1083.0
nm, and Ca I λ1083.9 nm. We focus on the near-infrared calcium line
to investigate the photospheric magnetic field and velocities, and
use the line core intensities and velocities of the helium line to
study the chromospheric plasma. The individual fibrils of the arch
filament system connect the sunspot with patches of magnetic polarity
opposite to that of the spot. These patches do not necessarily coincide
with pores, where the magnetic field is strongest. Instead, areas are
preferred not far from the polarity inversion line. These areas exhibit
photospheric downflows of moderate velocity, but significantly higher
downflows of up to 30 km s-1 in the chromospheric helium
line. Our findings can be explained with new emerging flux where the
matter flows downward along the field lines of rising flux tubes,
in agreement with earlier results.
Title: Magnetic fields of opposite polarity in sunspot penumbrae
Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.;
Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld,
T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.;
Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar,
A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M.
Bibcode: 2016A&A...596A...4F
Altcode: 2016arXiv160800513F
Context. A significant part of the penumbral magnetic field returns
below the surface in the very deep photosphere. For lines in the
visible, a large portion of this return field can only be detected
indirectly by studying its imprints on strongly asymmetric and
three-lobed Stokes V profiles. Infrared lines probe a narrow layer
in the very deep photosphere, providing the possibility of directly
measuring the orientation of magnetic fields close to the solar
surface.
Aims: We study the topology of the penumbral magnetic
field in the lower photosphere, focusing on regions where it returns
below the surface.
Methods: We analyzed 71 spectropolarimetric
datasets from Hinode and from the GREGOR infrared spectrograph. We
inferred the quality and polarimetric accuracy of the infrared data
after applying several reduction steps. Techniques of spectral
inversion and forward synthesis were used to test the detection
algorithm. We compared the morphology and the fractional penumbral
area covered by reversed-polarity and three-lobed Stokes V profiles for
sunspots at disk center. We determined the amount of reversed-polarity
and three-lobed Stokes V profiles in visible and infrared data of
sunspots at various heliocentric angles. From the results, we computed
center-to-limb variation curves, which were interpreted in the context
of existing penumbral models.
Results: Observations in visible
and near-infrared spectral lines yield a significant difference in the
penumbral area covered by magnetic fields of opposite polarity. In
the infrared, the number of reversed-polarity Stokes V profiles is
smaller by a factor of two than in the visible. For three-lobed Stokes
V profiles the numbers differ by up to an order of magnitude.
Title: Horizontal flow fields in and around a small active region. The
transition period between flux emergence and decay
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016A&A...596A...3V
Altcode: 2016arXiv160507462V
Context. The solar magnetic field is responsible for all aspects
of solar activity. Thus, emergence of magnetic flux at the surface
is the first manifestation of the ensuing solar activity.
Aims: Combining high-resolution and synoptic observations aims to
provide a comprehensive description of flux emergence at photospheric
level and of the growth process that eventually leads to a mature
active region.
Methods: The small active region NOAA 12118
emerged on 2014 July 17 and was observed one day later with the 1.5-m
GREGOR solar telescope on 2014 July 18. High-resolution time-series
of blue continuum and G-band images acquired in the blue imaging
channel (BIC) of the GREGOR Fabry-Pérot Interferometer (GFPI) were
complemented by synoptic line-of-sight magnetograms and continuum
images obtained with the Helioseismic and Magnetic Imager (HMI) onboard
the Solar Dynamics Observatory (SDO). Horizontal proper motions and
horizontal plasma velocities were computed with local correlation
tracking (LCT) and the differential affine velocity estimator (DAVE),
respectively. Morphological image processing was employed to measure
the photometric and magnetic area, magnetic flux, and the separation
profile of the emerging flux region during its evolution.
Results: The computed growth rates for photometric area, magnetic
area, and magnetic flux are about twice as high as the respective
decay rates. The space-time diagram using HMI magnetograms of five days
provides a comprehensive view of growth and decay. It traces a leaf-like
structure, which is determined by the initial separation of the two
polarities, a rapid expansion phase, a time when the spread stalls,
and a period when the region slowly shrinks again. The separation
rate of 0.26 km s-1 is highest in the initial stage, and
it decreases when the separation comes to a halt. Horizontal plasma
velocities computed at four evolutionary stages indicate a changing
pattern of inflows. In LCT maps we find persistent flow patterns such
as outward motions in the outer part of the two major pores, a diverging
feature near the trailing pore marking the site of upwelling plasma and
flux emergence, and low velocities in the interior of dark pores. We
detected many elongated rapidly expanding granules between the two
major polarities, with dimensions twice as large as the normal granules.
Title: Solar physics at the Einstein Tower
Authors: Denker, C.; Heibel, C.; Rendtel, J.; Arlt, K.; Balthasar,
Juergen H.; Diercke, A.; González Manrique, S. J.; Hofmann, A.;
Kuckein, C.; Önel, H.; Senthamizh Pavai, V.; Staude, J.; Verman, M.
Bibcode: 2016AN....337.1105D
Altcode: 2016arXiv160906949D
The solar observatory Einstein Tower ({Einsteinturm}) at the
Telegrafenberg in Potsdam is both a landmark of modern architecture
and an important place for solar physics. Originally built for
high-resolution spectroscopy and measuring the gravitational redshift,
research shifted over the years to understanding the active Sun and
its magnetic field. Nowadays, telescope and spectrographs are used for
research and development, i.e., testing instruments and in particular
polarization optics for advanced instrumentation deployed at major
European and international astronomical and solar telescopes. In
addition, the Einstein Tower is used for educating and training of the
next generation astrophysicists as well as for education and public
outreach activities directed at the general public. This article
comments on the observatory's unique architecture and the challenges
of maintaining and conserving the building. It describes in detail the
characteristics of telescope, spectrographs, and imagers; it portrays
some of the research and development activities.
Title: Upper chromospheric magnetic field of a sunspot penumbra:
observations of fine structure
Authors: Joshi, J.; Lagg, A.; Solanki, S. K.; Feller, A.; Collados,
M.; Orozco Suárez, D.; Schlichenmaier, R.; Franz, M.; Balthasar,
H.; Denker, C.; Berkefeld, T.; Hofmann, A.; Kiess, C.; Nicklas, H.;
Pastor Yabar, A.; Rezaei, R.; Schmidt, D.; Schmidt, W.; Sobotka, M.;
Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe,
O.; Waldmann, T.
Bibcode: 2016A&A...596A...8J
Altcode: 2016arXiv160801988J
Aims: The fine-structure of the magnetic field in a sunspot
penumbra in the upper chromosphere is to be explored and compared
to that in the photosphere.
Methods: Spectropolarimetric
observations with high spatial resolution were recorded with the 1.5-m
GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The
observed spectral domain includes the upper chromospheric Hei triplet
at 10 830 Å and the photospheric Sii 10 827.1 Å and Cai 10 833.4 Å
spectral lines. The upper chromospheric magnetic field is obtained
by inverting the Hei triplet assuming a Milne-Eddington-type model
atmosphere. A height-dependent inversion was applied to the Sii 10
827.1 Å and Cai 10 833.4 Å lines to obtain the photospheric magnetic
field.
Results: We find that the inclination of the magnetic
field varies in the azimuthal direction in the photosphere and in the
upper chromosphere. The chromospheric variations coincide remarkably
well with the variations in the inclination of the photospheric field
and resemble the well-known spine and interspine structure in the
photospheric layers of penumbrae. The typical peak-to-peak variations
in the inclination of the magnetic field in the upper chromosphere
are found to be 10°-15°, which is roughly half the variation in
the photosphere. In contrast, the magnetic field strength of the
observed penumbra does not vary on small spatial scales in the upper
chromosphere.
Conclusions: Thanks to the high spatial resolution
of the observations that is possible with the GREGOR telescope at 1.08
microns, we find that the prominent small-scale fluctuations in the
magnetic field inclination, which are a salient part of the property
of sunspot penumbral photospheres, also persist in the chromosphere,
although at somewhat reduced amplitudes. Such a complex magnetic
configuration may facilitate penumbral chromospheric dynamic phenomena,
such as penumbral micro-jets or transient bright dots.
Title: Active region fine structure observed at 0.08 arcsec resolution
Authors: Schlichenmaier, R.; von der Lühe, O.; Hoch, S.; Soltau, D.;
Berkefeld, T.; Schmidt, D.; Schmidt, W.; Denker, C.; Balthasar, H.;
Hofmann, A.; Strassmeier, K. G.; Staude, J.; Feller, A.; Lagg, A.;
Solanki, S. K.; Collados, M.; Sigwarth, M.; Volkmer, R.; Waldmann,
T.; Kneer, F.; Nicklas, H.; Sobotka, M.
Bibcode: 2016A&A...596A...7S
Altcode: 2016arXiv160707094S
Context. The various mechanisms of magneto-convective energy transport
determine the structure of sunspots and active regions.
Aims: We characterise the appearance of light bridges and other
fine-structure details and elaborate on their magneto-convective
nature.
Methods: We present speckle-reconstructed images taken
with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the
486 nm and 589 nm bands. We estimate the spatial resolution from the
noise characteristics of the image bursts and obtain 0.08″ at 589
nm. We describe structure details in individual best images as well
as the temporal evolution of selected features.
Results: We
find branched dark lanes extending along thin (≈1″) light bridges
in sunspots at various heliocentric angles. In thick (≳ 2″) light
bridges the branches are disconnected from the central lane and have a Y
shape with a bright grain toward the umbra. The images reveal that light
bridges exist on varying intensity levels and that their small-scale
features evolve on timescales of minutes. Faint light bridges show
dark lanes outlined by the surrounding bright features. Dark lanes are
very common and are also found in the boundary of pores. They have a
characteristic width of 0.1″ or smaller. Intergranular dark lanes of
that width are seen in active region granulation.
Conclusions: We
interpret our images in the context of magneto-convective simulations
and findings: while central dark lanes in thin light bridges are
elevated and associated with a density increase above upflows, the dark
lane branches correspond to locations of downflows and are depressed
relative to the adjacent bright plasma. Thick light bridges with central
dark lanes show no projection effect. They have a flat elevated plateau
that falls off steeply at the umbral boundary. There, Y-shaped filaments
form as they do in the inner penumbra. This indicates the presence of
inclined magnetic fields, meaning that the umbral magnetic field is
wrapped around the convective light bridge.
Title: Probing deep photospheric layers of the quiet Sun with high
magnetic sensitivity
Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González,
M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.;
Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.;
Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.;
Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.;
Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.;
Schmidt, D.; Kiess, C.; Nicklas, H.
Bibcode: 2016A&A...596A...6L
Altcode: 2016arXiv160506324L
Context. Investigations of the magnetism of the quiet Sun are hindered
by extremely weak polarization signals in Fraunhofer spectral
lines. Photon noise, straylight, and the systematically different
sensitivity of the Zeeman effect to longitudinal and transversal
magnetic fields result in controversial results in terms of the strength
and angular distribution of the magnetic field vector.
Aims:
The information content of Stokes measurements close to the diffraction
limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of
spatial straylight and photon noise into account.
Methods: Highly
sensitive full Stokes measurements of a quiet-Sun region at disk center
in the deep photospheric Fe I lines in the 1.56 μm region were obtained
with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise
statistics and Stokes V asymmetries were analyzed and compared to a
similar data set of the Hinode spectropolarimeter (SOT/SP). Simple
diagnostics based directly on the shape and strength of the profiles
were applied to the GRIS data. We made use of the magnetic line ratio
technique, which was tested against realistic magneto-hydrodynamic
simulations (MURaM).
Results: About 80% of the GRIS spectra
of a very quiet solar region show polarimetric signals above a 3σ
level. Area and amplitude asymmetries agree well with small-scale
surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio
analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss
range with some concentrations of kilo-Gauss fields.
Conclusions:
The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4
are so far unique in the combination of high spatial resolution scans
and high magnetic field sensitivity. Nevertheless, the unavoidable
effect of spatial straylight and the resulting dilution of the weak
Stokes profiles means that inversion techniques still bear a high risk
of misinterpretating the data.
Title: Flow and magnetic field properties in the trailing sunspots
of active region NOAA 12396
Authors: Verma, M.; Denker, C.; Böhm, F.; Balthasar, H.; Fischer,
C. E.; Kuckein, C.; Bello González, N.; Berkefeld, T.; Collados,
M.; Diercke, A.; Feller, A.; González Manrique, S. J.; Hofmann, A.;
Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pator Yabar, A.; Rezaei,
R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.;
Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1090V
Altcode:
Improved measurements of the photospheric and chromospheric
three-dimensional magnetic and flow fields are crucial for a precise
determination of the origin and evolution of active regions. We present
an illustrative sample of multi-instrument data acquired during a
two-week coordinated observing campaign in August 2015 involving,
among others, the GREGOR solar telescope (imaging and near-infrared
spectroscopy) and the space missions Solar Dynamics Observatory (SDO)
and Interface Region Imaging Spectrograph (IRIS). The observations
focused on the trailing part of active region NOAA 12396 with complex
polarity inversion lines and strong intrusions of opposite polarity
flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV
spectral profiles in the photospheric Si I λ1082.7 nm line, the
chromospheric He I λ1083.0 nm triplet, and the photospheric Ca I
λ1083.9 nm line. Carefully calibrated GRIS scans of the active region
provided maps of Doppler velocity and magnetic field at different
atmospheric heights. We compare quick-look maps with those obtained
with the ``Stokes Inversions based on Response functions'' (SIR)
code, which furnishes deeper insight into the magnetic properties
of the region. We find supporting evidence that newly emerging flux
and intruding opposite polarity flux are hampering the formation
of penumbrae, i.e., a penumbra fully surrounding a sunspot is only
expected after cessation of flux emergence in proximity to the sunspots.
Title: Three-dimensional structure of a sunspot light bridge
Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio
Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.;
Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier,
R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki,
S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.;
von der Lühe, O.; Waldmann, T.
Bibcode: 2016A&A...596A..59F
Altcode: 2016arXiv161104803F
Context. Active regions are the most prominent manifestations of solar
magnetic fields; their generation and dissipation are fundamental
problems in solar physics. Light bridges are commonly present during
sunspot decay, but a comprehensive picture of their role in the
removal of the photospheric magnetic field is still lacking.
Aims: We study the three-dimensional configuration of a sunspot,
and in particular, its light bridge, during one of the last stages of
its decay.
Methods: We present the magnetic and thermodynamical
stratification inferred from full Stokes inversions of the photospheric
Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR
Infrared Spectrograph of the GREGOR telescope at the Observatorio del
Teide, Tenerife, Spain. The analysis is complemented by a study of
continuum images covering the disk passage of the active region, which
are provided by the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory.
Results: The sunspot shows a light bridge
with penumbral continuum intensity that separates the central umbra from
a smaller umbra. We find that in this region the magnetic field lines
form a canopy with lower magnetic field strength in the inner part. The
photospheric light bridge is dominated by gas pressure (high-β),
as opposed to the surrounding umbra, where the magnetic pressure
is higher. A convective flow is observed in the light bridge. This
flow is able to bend the magnetic field lines and to produce field
reversals. The field lines merge above the light bridge and become
as vertical and strong as in the surrounding umbra. We conclude that
this occurs because two highly magnetized regions approach each other
during the sunspot evolution. Movies associated to Figs. 2 and 13
are available at http://www.aanda.org
Title: Inference of magnetic fields in the very quiet Sun
Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.;
Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.;
Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.;
González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.;
Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.;
Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman,
T.; Volkmer, R.
Bibcode: 2016A&A...596A...5M
Altcode: 2018arXiv180410089M
Context. Over the past 20 yr, the quietest areas of the solar surface
have revealed a weak but extremely dynamic magnetism occurring
at small scales (<500 km), which may provide an important
contribution to the dynamics and energetics of the outer layers of
the atmosphere. Understanding this magnetism requires the inference
of physical quantities from high-sensitivity spectro-polarimetric
data with high spatio-temporal resolution.
Aims: We present
high-precision spectro-polarimetric data with high spatial resolution
(0.4'') of the very quiet Sun at 1.56 μm obtained with the GREGOR
telescope to shed some light on this complex magnetism.
Methods:
We used inversion techniques in two main approaches. First, we assumed
that the observed profiles can be reproduced with a constant magnetic
field atmosphere embedded in a field-free medium. Second, we assumed
that the resolution element has a substructure with either two constant
magnetic atmospheres or a single magnetic atmosphere with gradients of
the physical quantities along the optical depth, both coexisting with
a global stray-light component.
Results: Half of our observed
quiet-Sun region is better explained by magnetic substructure within
the resolution element. However, we cannot distinguish whether this
substructure comes from gradients of the physical parameters along the
line of sight or from horizontal gradients (across the surface). In
these pixels, a model with two magnetic components is preferred, and
we find two distinct magnetic field populations. The population with
the larger filling factor has very weak ( 150 G) horizontal fields
similar to those obtained in previous works. We demonstrate that the
field vector of this population is not constrained by the observations,
given the spatial resolution and polarimetric accuracy of our data. The
topology of the other component with the smaller filling factor is
constrained by the observations for field strengths above 250 G:
we infer hG fields with inclinations and azimuth values compatible
with an isotropic distribution. The filling factors are typically
below 30%. We also find that the flux of the two polarities is not
balanced. From the other half of the observed quiet-Sun area 50% are
two-lobed Stokes V profiles, meaning that 23% of the field of view
can be adequately explained with a single constant magnetic field
embedded in a non-magnetic atmosphere. The magnetic field vector and
filling factor are reliable inferred in only 50% based on the regular
profiles. Therefore, 12% of the field of view harbour hG fields with
filling factors typically below 30%. At our present spatial resolution,
70% of the pixels apparently are non-magnetised.
Title: Fitting peculiar spectral profiles in He I 10830Å absorption
features
Authors: González Manrique, S. J.; Kuckein, C.; Pastor Yabar, A.;
Collados, M.; Denker, C.; Fischer, C. E.; Gömöry, P.; Diercke, A.;
Bello González, N.; Schlichenmaier, R.; Balthasar, H.; Berkefeld, T.;
Feller, A.; Hoch, S.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma,
M.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1057G
Altcode: 2016arXiv160300679G
The new generation of solar instruments provides better
spectral, spatial, and temporal resolution for a better
understanding of the physical processes that take place on the
Sun. Multiple-component profiles are more commonly observed with these
instruments. Particularly, the He I 10830 Å triplet presents such
peculiar spectral profiles, which give information on the velocity
and magnetic fine structure of the upper chromosphere. The purpose
of this investigation is to describe a technique to efficiently fit
the two blended components of the He I 10830 Å triplet, which are
commonly observed when two atmospheric components are located within
the same resolution element. The observations used in this study were
taken on 2015 April 17 with the very fast spectroscopic mode of the
GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-m GREGOR solar
telescope, located at the Observatorio del Teide, Tenerife, Spain. We
apply a double-Lorentzian fitting technique using Levenberg-Marquardt
least-squares minimization. This technique is very simple and much
faster than inversion codes. Line-of-sight Doppler velocities can
be inferred for a whole map of pixels within just a few minutes. Our
results show sub- and supersonic downflow velocities of up to 32 km
s-1 for the fast component in the vicinity of footpoints of
filamentary structures. The slow component presents velocities close
to rest.
Title: Flows in and around Active Region NOAA12118 Observed with
the GREGOR Solar Telescope and SDO/HMI
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016ASPC..504...29V
Altcode: 2016arXiv160301109V
Accurate measurements of magnetic and velocity fields in and around
solar active regions are key to unlocking the mysteries of the
formation and the decay of sunspots. High spatial resolution images
and spectral sequences with a high cadence obtained with the GREGOR
solar telescope give us an opportunity to scrutinize 3-D flow fields
with local correlation tracking and imaging spectroscopy. We present
GREGOR early science data acquired in 2014 July - August with the GREGOR
Fabry-Pérot Interferometer and the Blue Imaging Channel. Time-series
of blue continuum (λ 450.6 nm) images of the small active region
NOAA 12118 were restored with the speckle masking technique to derive
horizontal proper motions and to track the evolution of morphological
changes. In addition, high-resolution observations are discussed in
the context of synoptic data from the Solar Dynamics Observatory.
Title: PEPSI: The high-resolution échelle spectrograph and
polarimeter for the Large Binocular Telescope
Authors: Strassmeier, K. G.; Ilyin, I.; Järvinen, A.; Weber, M.;
Woche, M.; Barnes, S. I.; Bauer, S. -M.; Beckert, E.; Bittner, W.;
Bredthauer, R.; Carroll, T. A.; Denker, C.; Dionies, F.; DiVarano,
I.; Döscher, D.; Fechner, T.; Feuerstein, D.; Granzer, T.; Hahn,
T.; Harnisch, G.; Hofmann, A.; Lesser, M.; Paschke, J.; Pankratow,
S.; Plank, V.; Plüschke, D.; Popow, E.; Sablowski, D.
Bibcode: 2015AN....336..324S
Altcode: 2015arXiv150506492S
PEPSI is the bench-mounted, two-arm, fibre-fed and stabilized Potsdam
Echelle Polarimetric and Spectroscopic Instrument for the 2×8.4 m
Large Binocular Telescope (LBT). Three spectral resolutions of either
43 000, 120 000 or 270 000 can cover the entire optical/red wavelength
range from 383 to 907 nm in three exposures. Two 10.3k×10.3k CCDs with
9-μm pixels and peak quantum efficiencies of 94-96 % record a total of
92 échelle orders. We introduce a new variant of a wave-guide image
slicer with 3, 5, and 7 slices and peak efficiencies between 92-96
%. A total of six cross dispersers cover the six wavelength settings
of the spectrograph, two of them always simultaneously. These are made
of a VPH-grating sandwiched by two prisms. The peak efficiency of the
system, including the telescope, is 15 % at 650 nm, and still 11 %
and 10 % at 390 nm and 900 nm, respectively. In combination with
the 110 m2 light-collecting capability of the LBT, we
expect a limiting magnitude of ≈ 20th mag in V in the low-resolution
mode. The R = 120 000 mode can also be used with two, dual-beam Stokes
IQUV polarimeters. The 270 000-mode is made possible with the 7-slice
image slicer and a 100-μm fibre through a projected sky aperture of
0.74 arcsec, comparable to the median seeing of the LBT site. The 43
000-mode with 12-pixel sampling per resolution element is our bad
seeing or faint-object mode. Any of the three resolution modes can
either be used with sky fibers for simultaneous sky exposures or with
light from a stabilized Fabry-Pérot étalon for ultra-precise radial
velocities. CCD-image processing is performed with the dedicated
data-reduction and analysis package PEPSI-S4S. Its full error
propagation through all image-processing steps allows an adaptive
selection of parameters by using statistical inferences and robust
estimators. A solar feed makes use of PEPSI during day time and a 500-m
feed from the 1.8 m VATT can be used when the LBT is busy otherwise. In
this paper, we present the basic instrument design, its realization,
and its characteristics. Some pre-commissioning first-light spectra
shall demonstrate the basic functionality.
Title: Evolution of volatile species in the earth’s mantle: A view
from xenology
Authors: Tolstikhin, I.; Marty, B.; Porcelli, D.; Hofmann, A.
Bibcode: 2014GeCoA.136..229T
Altcode:
To understand the scales and chronology of processes governing
the evolution of terrestrial gas species, the constraints from
244Pu-238U-129I-Xe systematics are
crucial and should be included in any model related to gas loss/gain by
the Earth and to gas redistribution among terrestrial reservoirs. Reliable constraints can be derived from meteoritic and terrestrial
abundances of the highly refractory lithophile incompatible parent
isotopes 244Pu (half life τ244 = 80.0 Myr) and
238U (τ238 = 4468 Myr). Both isotopes produce
heavy Xe isotopes by fission; different relative yields for the Xe
isotopes allow contributions of Xe(Pu) and Xe(U) to be distinguished. It
is also useful to consider the 129I-129Xe(I)
systematics (τ129 = 15.6 Myr) even though iodine is a
highly volatile element and its terrestrial abundance is less well
known. The parent isotopes, for which the initial (at the time of
formation of the solar system, 4.567 Gyr ago) abundances are known
from investigations of meteorites and ancient terrestrial zircons,
yield the closed-system (subscript CLOS) present-day ratios of
[136Xe(Pu)/136Xe(U)]CLOS = 28 and
[129Xe(I)/136Xe(Pu)]CLOS = 110, much
exceeding values observed in the depleted heterogeneous mantle reservoir
(DMR): [136Xe(Pu)/136Xe(U)]DMR
⩽ 3 and [129Xe (I)/136Xe(Pu)]DMR
⩽ 60 (Pepin and Porcelli, 2006). Also, the present-day amounts
of 129Xe(I) in the mantle (∼0.01 Tmol) and in the
atmosphere (0.278 Tmol) are well below the total value produced by
decay of 129I (129Xe (I) = 35 Tmol). These
relationships between the closed system and the observed values show
loss of early-produced Xe isotopes occurred not only from the DMR,
but also from the Earth-atmosphere system as a whole. Using
abundances of the parent and daughter isotopes within the framework
of a simple one-mantle-reservoir degassing model we conclude: (1)
the present day mantle is a severely degassed reservoir, so that only
<10-3 of the initially available amount of stable Xe atoms
(e.g., 130Xe) has survived 4.567 Gyr of degassing. This low
retention parameter is practically model-independent, as any solution is
governed by the requirement of almost total 136Xe(Pu) loss
from the mantle. (2) The degassing rate as a function of time appears
to be the most reliable constraint on mantle convection in the past. To
ensure intense early degassing, the rate of mantle convection during
the Hadean era must exceed the present day value by up to a factor
of ∼100. These two issues also follow from the study of two mantle
reservoir models: if primordial and early-produced species were added
into the convecting mantle from a hypothetical early-formed reservoir,
the mantle itself must be degassed to a higher rate than that predicted
by the one-mantle-reservoir model. This is in contrast to the very model
dependent issue (3): the one-mantle-reservoir degassing model predicts
a rather late time for atmosphere closure to Xe loss, between 3.5 and
4 Gyr ago (possibly even extending to the Archean). Several recent
mantle evolution models (based on U-Th-He and K-Ar isotope systematics)
postulate a low overall degassing of the Earth and are inconsistent
with these results from mantle xenology.
Title: The GREGOR Solar Telescope on Tenerife
Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.;
Solanki, S. K.; Balthasar, H.; Bello González, N.; Berkefeld, T.;
Collados Vera, M.; Hofmann, A.; Kneer, F.; Lagg, A.; Puschmann, K. G.;
Schmidt, D.; Sobotka, M.; Soltau, D.; Strassmeier, K. G.
Bibcode: 2012ASPC..463..365S
Altcode: 2012arXiv1202.4289S
2011 was a successful year for the GREGOR project. The telescope was
finally completed in May with the installation of the 1.5-meter primary
mirror. The installation of the first-light focal plane instruments was
completed by the end of the year. At the same time, the preparations
for the installation of the high-order adaptive optics were finished,
its integration to the telescope is scheduled for early 2012. This
paper describes the telescope and its instrumentation in their present
first-light configuration, and provides a brief overview of the science
goals of GREGOR.
Title: The GREGOR Solar Telescope
Authors: Denker, C.; Lagg, A.; Puschmann, K. G.; Schmidt, D.; Schmidt,
W.; Sobotka, M.; Soltau, D.; Strassmeier, K. G.; Volkmer, R.; von
der Luehe, O.; Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.;
Berkefeld, T.; Collados Vera, M.; Hofmann, A.; Kneer, F.
Bibcode: 2012IAUSS...6E.203D
Altcode:
The 1.5-meter GREGOR solar telescope is a new facility for
high-resolution observations of the Sun. The telescope is located at the
Spanish Observatorio del Teide on Tenerife. The telescope incorporates
advanced designs for a foldable-tent dome, an open steel-truss telescope
structure, and active and passive means to minimize telescope and mirror
seeing. Solar fine structure can be observed with a dedicated suite
of instruments: a broad-band imaging system, the "GREGOR Fabry-Perot
Interferometer", and the "Grating Infrared Spectrograph". All post-focus
instruments benefit from a high-order (multi-conjugate) adaptive optics
system, which enables observations close to the diffraction limit of
the telescope. The inclusion of a spectrograph for stellar activity
studies and the search for solar twins expands the scientific usage
of the GREGOR to the nighttime domain. We report on the successful
commissioning of the telescope until the end of 2011 and the first
steps towards science verification in 2012.
Title: A retrospective of the GREGOR solar telescope in scientific
literature
Authors: Denker, C.; von der Lühe, O.; Feller, A.; Arlt, K.;
Balthasar, H.; Bauer, S. -M.; Bello González, N.; Berkefeld, Th.;
Caligari, P.; Collados, M.; Fischer, A.; Granzer, T.; Hahn, T.;
Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Kentischer, T.; Klva{ňa,
M.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.;
Rendtel, J.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Solanki, S. K.;
Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; Waldmann,
T.; Wiehr, E.; Wittmann, A. D.; Woche, M.
Bibcode: 2012AN....333..810D
Altcode: 2012arXiv1210.3167D
In this review, we look back upon the literature, which had the
GREGOR solar telescope project as its subject including science cases,
telescope subsystems, and post-focus instruments. The articles date
back to the year 2000, when the initial concepts for a new solar
telescope on Tenerife were first presented at scientific meetings. This
comprehensive bibliography contains literature until the year 2012,
i.e., the final stages of commissioning and science verification. Taking
stock of the various publications in peer-reviewed journals and
conference proceedings also provides the ``historical'' context
for the reference articles in this special issue of Astronomische
Nachrichten/Astronomical Notes.
Title: The 1.5 meter solar telescope GREGOR
Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.;
Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.; Berkefeld, Th.;
Collados, M.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Hofmann,
A.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.;
Schmidt, D.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.;
Strassmeier, K. G.; Waldmann , T. A.
Bibcode: 2012AN....333..796S
Altcode:
The 1.5 m telescope GREGOR opens a new window to the understanding
of solar small-scale magnetism. The first light instrumentation
includes the Gregor Fabry Pérot Interferometer (GFPI), a filter
spectro-polarimeter for the visible wavelength range, the GRating
Infrared Spectro-polarimeter (GRIS) and the Broad-Band Imager (BBI). The
excellent performance of the first two instruments has already been
demonstrated at the Vacuum Tower Telescope. GREGOR is Europe's largest
solar telescope and number 3 in the world. Its all-reflective Gregory
design provides a large wavelength coverage from the near UV up to at
least 5 microns. The field of view has a diameter of 150 arcsec. GREGOR
is equipped with a high-order adaptive optics system, with a subaperture
size of 10 cm, and a deformable mirror with 256 actuators. The science
goals are focused on, but not limited to, solar magnetism. GREGOR
allows us to measure the emergence and disappearance of magnetic flux
at the solar surface at spatial scales well below 100 km. Thanks to its
spectro-polarimetric capabilities, GREGOR will measure the interaction
between the plasma flows, different kinds of waves, and the magnetic
field. This will foster our understanding of the processes that heat the
chromosphere and the outer layers of the solar atmosphere. Observations
of the surface magnetic field at very small spatial scales will shed
light on the variability of the solar brightness.
Title: The GREGOR polarimetric calibration unit
Authors: Hofmann, A.; Arlt, K.; Balthasar, H.; Bauer, S. M.; Bittner,
W.; Paschke, J.; Popow, E.; Rendtel, J.; Soltau, D.; Waldmann, T.
Bibcode: 2012AN....333..854H
Altcode:
The new Solar telescope GREGOR is designed to observe small-scale
dynamic magnetic structures below a size of 70 km on the Sun
with high spectral resolution and polarimetric accuracy. For
this purpose, the polarimetric concept of GREGOR is based on a
combination of post-focus polarimeters with pre-focus equipment for
high precision calibration. The Leibniz-Institute for Astrophysics
Potsdam developed the GREGOR calibration unit which is an integral
part of the telescope. We give an overview of the function and design
of the calibration unit and present the results of extensive testing
series done in the Solar Observatory ``Einsteinturm'' and at GREGOR.
Title: The GREGOR Fabry-Pérot Interferometer
Authors: Puschmann, K. G.; Denker, C.; Kneer, F.; Al Erdogan, N.;
Balthasar, H.; Bauer, S. M.; Beck, C.; Bello González, N.; Collados,
M.; Hahn, T.; Hirzberger, J.; Hofmann, A.; Louis, R. E.; Nicklas, H.;
Okunev, O.; Martínez Pillet, V.; Popow, E.; Seelemann, T.; Volkmer,
R.; Wittmann, A. D.; Woche, M.
Bibcode: 2012AN....333..880P
Altcode: 2012arXiv1210.2921P
The GREGOR Fabry-Pérot Interferometer (GFPI) is one of three
first-light instruments of the German 1.5-meter GREGOR solar telescope
at the Observatorio del Teide, Tenerife, Spain. The GFPI uses two
tunable etalons in collimated mounting. Thanks to its large-format,
high-cadence CCD detectors with sophisticated computer hard- and
software it is capable of scanning spectral lines with a cadence
that is sufficient to capture the dynamic evolution of the solar
atmosphere. The field-of-view (FOV) of 50 arcsec × 38 arcsec is well
suited for quiet Sun and sunspot observations. However, in the vector
spectropolarimetric mode the FOV reduces to 25 arcsec × 38 arcsec. The
spectral coverage in the spectroscopic mode extends from 530-860 nm
with a theoretical spectral resolution of R ≈ 250,000, whereas in
the vector spectropolarimetric mode the wavelength range is at present
limited to 580-660 nm. The combination of fast narrow-band imaging and
post-factum image restoration has the potential for discovery science
concerning the dynamic Sun and its magnetic field at spatial scales
down to ∼50 km on the solar surface.
Title: Polarimetry with GREGOR
Authors: Balthasar, H.; Bello González, N.; Collados, M.; Denker,
C.; Feller, A.; Hofmann, A.; Lagg, A.; Nagaruju, L.; Puschmann, K. G.;
Soltau, D.; Volkmer, R.
Bibcode: 2011ASPC..437..351B
Altcode:
A brief description of the new 1.5-meter solar telescope GREGOR located
at the Observatorio del Teide in Tenerife will be given. GREGOR will
provide a spatial resolution of about 75 km on the Sun, and with its
light collecting capability we will be able to study the development
of small magnetic features with high cadence. From the beginning,
it will be equipped with the GREGOR Fabry-Pérot Interferometer
(GFPI) for the visible spectral range and with a GRating Infrared
Spectrograph (GRIS). Both postfocus instruments can be combined
with a polarimeter, and in both cases the light is modulated by two
ferro-electric liquid crystals. A calibration unit can be inserted to
determine the instrumental polarization. Because of the altazimuthal
mount, time-dependent rotation of the polarimetric reference plane
is introduced, and we have to develop a polarization model of the
telescope. Measurements to verify this model are in preparation.
Title: GREGOR telescope: start of commissioning
Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S.;
Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
C.; Heidecke, F.; Hofmann, A.; Klvana, M.; Kneer, F.; Lagg, A.; Popow,
E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K.
Bibcode: 2010SPIE.7733E..0KV
Altcode: 2010SPIE.7733E..18V
With the integration of a 1-meter Cesic primary mirror the GREGOR
telescope pre-commissioning started. This is the first time, that
the entire light path has seen sunlight. The pre-commissioning period
includes testing of the main optics, adaptive optics, cooling system,
and pointing system. This time was also used to install a near-infrared
grating spectro-polarimeter and a 2D-spectropolarimeter for the visible
range as first-light science instruments. As soon as the final 1.5
meter primary mirror is installed, commissioning will be completed,
and an extended phase of science verification will follow. In the near
future, GREGOR will be equipped with a multi-conjugate adaptive optics
system that is presently under development at KIS.
Title: The GREGOR Fabry-Perot interferometer: a new instrument for
high-resolution solar observations
Authors: Denker, Carsten; Balthasar, Horst; Hofmann, Axel; Bello
González, Nazaret; Volkmer, Reiner
Bibcode: 2010SPIE.7735E..6MD
Altcode: 2010SPIE.7735E.217D
The GREGOR Fabry-Ṕerot Interferometer (GFPI) is one of the first-light
instruments of the 1.5-meter GREGOR solar telescope currently being
commissioned at Observatorio del Teide (OT), Tenerife, Spain. A
spectral resolution of R ~ 250, 000 over the wavelength range from
530-860 nm can be achieved using a tunable dual etalon system. A high
spectral resolving power is needed to extract physical parameters
(e.g., temperature, plasma velocity and the magnetic field vector) from
inversions of photospheric and chromospheric spectral lines. The GFPI
is outfitted with a polarimeter, which accurately measures the full
Stokes vector. Precision polarimetry is facilitated by a calibration
unit in the immediate vicinity of GREGOR's secondary focus. The GFPI
operates close to the diffraction limit of GREGOR, thus providing
access to fine structures as small as 60 km on the solar surface. The
field-of-view (FOV) of 52" × 40" is sufficiently large to cover
significant portions of active regions. Large-format, high-cadence
CCD detectors are an integral part of the instrument to ensure that
scans of spectral lines can be obtained in time spans corresponding
to the evolution time scale of solar phenomena such as granulation,
evolving magnetic fields or dynamic chromospheric features. Besides
describing the technical features of the GFPI and providing a status
report on commissioning the instrument, we will use two-dimensional
spectropolarimetric data obtained with the Vacuum Tower Telescope
(VTT) at OT to illustrate GFPI's science capabilities.
Title: GREGOR solar telescope: Design and status
Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S. K.;
Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Fischer, A.;
Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Kneer, F.;
Lagg, A.; Popow, E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau,
D.; Strassmeier, K. G.
Bibcode: 2010AN....331..624V
Altcode:
The integration and verification phase of the GREGOR telescope
reached an important milestone with the installation of the
interim 1 m SolarLite primary mirror. This was the first time
that the entire light path had seen sunlight. Since then extensive
testing of the telescope and its subsystems has been carried out. The
integration and verification phase will culminate with the delivery and
installation of the final 1.5 m Zerodur primary mirror in the summer of
2010. Observatory level tests and science verification will commence in
the second half of 2010 and in 2011. This phase includes testing of the
main optics, adaptive optics, cooling and pointing systems. In addition,
assuming the viewpoint of a typical user, various observational modes
of the GREGOR Fabry-Pérot Interferometer (GFPI), the Grating Infrared
Spectrograph (GRIS), and high-speed camera systems will be tested to
evaluate if they match the expectations and science requirements. This
ensures that GREGOR will provide high-quality observations with its
combination of (multi-conjugate) adaptive optics and advanced post-focus
instruments. Routine observations are expected for 2012.
Title: 300 m.y. of komatiite evolution in the Barberton Greenstone
Belt
Authors: Robin, C.; Arndt, N.; Chauvel, C.; Byerly, G.; Kareem, K.;
Hofmann, A.; Wilson, A.
Bibcode: 2009GeCAS..73Q1108R
Altcode:
No abstract at ADS
Title: Banded iron formations of the ∼ 2. 7 Ga Manjeri Formation,
Belingwe greenstone belt, Zimbabwe: An anoxic carbonate- and
sulphide-rich depositional environment
Authors: Bah, M. S., II; Orberger, B.; Hofmann, A.; Trudyn, A.
Bibcode: 2009GeCAS..73Q..71B
Altcode:
No abstract at ADS
Title: The response of sedimentary 231Pa/230Th
on particle flux-findings from the African margin
Authors: Lippold, J.; Christl, M.; Hofmann, A.; Mollenhauer, G.;
Mulitza, S.; Weyer, S.; Mangini, A.
Bibcode: 2009GeCAS..73R.772L
Altcode:
No abstract at ADS
Title: Hydrous ferric oxide nanoparticles - Structural reordering
in response to surface sorption of Al-hydroxy species
Authors: Hofmann, A.; Vantelon, D.; Villain, F.; Montarges-Pelletier,
E.
Bibcode: 2009GeCAS..73Q.543H
Altcode:
No abstract at ADS
Title: A full-Stokes polarimeter for the GREGOR Fabry-Perot
interferometer
Authors: Balthasar, Horst; Bello González, N.; Collados, M.; Denker,
C.; Hofmann, A.; Kneer, F.; Puschmann, K. G.
Bibcode: 2009IAUS..259..665B
Altcode:
One of the first post-focus instruments of the new solar telescope
GREGOR will be a Fabry-Perot spectrometer, which is an upgrade of the
Göttingen Fabry-Perot interferometer at the Vacuum Tower Telescope
(VTT) on Tenerife. This spectrometer is equipped with a full-Stokes
polarimeter. The modulation is performed with two ferroelectric liquid
crystals, one acting nominally as quarter-wave plate, and the other as
half-wave plate. A modified Savart plate serves as polarimetric beam
splitter. With the present liquid crystals, the optimum wavelength range
of this polarimeter is between 580 and 660 nm. The spectro-polarimeter
will benefit from the capabilities of the new telescope GREGOR which
will provide a spatial resolution of about 0″.1 (75 km on the solar
surface). Thus we will be able to investigate small magnetic features,
and we will study their development with high cadence.
Title: Spectropolarimetry with PEPSI at the LBT: accuracy
vs. precision in magnetic field measurements
Authors: Ilyin, Ilya; Strassmeier, Klaus G.; Woche, Manfred; Hofmann,
Axel
Bibcode: 2009IAUS..259..663I
Altcode:
We present the design of the new PEPSI spectropolarimeter to be
installed at the Large Binocular Telescope (LBT) in Arizona to measure
the full set of Stokes parameters in spectral lines and outline its
precision and the accuracy limiting factors.
Title: Toward Polarimetry with GREGOR -- Testing the GREGOR
Polarimetric Unit
Authors: Hofmann, A.; Rendtel, J.; Arlt, K.
Bibcode: 2009CEAB...33..317H
Altcode:
The new 1.5 m solar telescope GREGOR with modern scientific
instrumentation will become operational in the near future. The GREGOR
Polarimetric Unit (GPU) for the calibration of polarimetric measurements
with any post-focal device has been developed at the Astrophysical
Institute Potsdam (AIP). Here we describe details of the extensive
test measurements made in the laboratory of the Solar Observatory
``Einsteinturm''.
Title: Radio Bursts and Magnetic Field Structure During Microflares
Authors: Aurass, H.; Rausche, G.; Hofmann, A.; Berkebile-Stoiser,
S.; Veronig, A.
Bibcode: 2009CEAB...33..159A
Altcode:
This paper presents an analysis of two selected microflares seen
in dynamic radio spectra recorded by the Astrophysical Institute
Potsdam and imaged by the Nançay Multifrequency Radioheliograph. The
microflares were observed in hard X-rays by the Ramaty High Energy
Solar Spectroscopic Imager in AR 10465 on September 26, 2003. Magnetic
field connections are selected compatible with the spatial extent of
the HXR sources and the Transition Region and Coronal Explorer-detected
chromospheric UV continuum brightenings. The selected field lines are a
small subset of the potential extrapolation of a SOHO--Michelson Doppler
Imager high resolution magnetogram. For a type III-associated microflare
a loop-like magnetic field structure is found with a spatial extent
of ≈35 arcsec. A type II-precursor-associated (stronger) microflare
brightened in a smaller-scale circularly arranged arcade formed by
field lines of about 12 arcsec foot point distance. All microflare
site-compatible field lines have turning points in heights of 5--20
arcsec and are situated underneath larger-scale arcades joining the
main leading and trailing field concentrations. For understanding type
III-burst source positions, an electron path must exist between the
HXR source site and large-scale open field lines in the trailing part
of AR 10465. This demands for reconnection of the selected small-scale
closed field lines with overlying field systems, electron propagation
to the trailing spot, and diffusion from closed to open field lines
leading out into the solar wind. The type II burst precursor sources
are formed in a magnetic hole of the potential field. This supports
the view of a growing large amplitude flare wave which is not intense
enough to raise to a spatially extended type II-exciting shock front but
to smaller-scale sub-shocks forming the radio precursor burst sources.
Title: GREGOR - The Calibration Unit
Authors: Hofmann, A.; Rendtel, J.; Arlt, K.; Balthasar, H.
Bibcode: 2008ESPM...12..6.9H
Altcode:
In 2010 we expect "first light" for GREGOR - a new solar telescope
with large aperture and modern scientific instruments. It is designed
to observe small-scale dynamic magnetic structures below a size
of 70 km on the Sun with high spectral resolution and polarimetric
accuracy. For this purpose, the polarimetric concept of GREGOR is based
on a combination of post-focus polarimeters with pre-focus equipment
for high precision calibration. The Astrophysical Institute Potsdam
developed the GREGOR calibration unit which is an integral part of the
telescope. We give an overview on function and design of the calibration
unit and present the results of extensive testing series in the Solar
Observatory "Einsteinturm".
Title: Sulfidic organic-rich shales in the Archean low-sulfate ocean:
Evidence for transient oxygenated conditions, enhanced volcanism,
or low sedimentation rates?
Authors: Bekker, A.; Hofmann, A.; Rumble, D.; Rouxel, O.
Bibcode: 2008GeCAS..72Q..69B
Altcode:
No abstract at ADS
Title: Hydrothermal activity and stromatolite formation in the
pre-2.95 Ga record of South Africa
Authors: Hofmann, Axel
Bibcode: 2008GeCAS..72R.385H
Altcode:
No abstract at ADS
Title: PEPSI: the Potsdam Echelle Polarimetric and Spectroscopic
Instrument for the LBT
Authors: Strassmeier, K. G.; Woche, M.; Ilyin, I.; Popow, E.; Bauer,
S. -M.; Dionies, F.; Fechner, T.; Weber, M.; Hofmann, A.; Storm,
J.; Materne, R.; Bittner, W.; Bartus, J.; Granzer, T.; Denker, C.;
Carroll, T.; Kopf, M.; DiVarano, I.; Beckert, E.; Lesser, M.
Bibcode: 2008SPIE.7014E..0NS
Altcode: 2008SPIE.7014E..21S
We present the status of PEPSI, the bench-mounted fibre-fed and
stabilized "Potsdam Echelle Polarimetric and Spectroscopic Instrument"
for the 2×8.4m Large Binocular Telescope in southern Arizona. PEPSI
is under construction at AIP and is scheduled for first light in
2009/10. Its ultra-high-resolution mode will deliver an unprecedented
spectral resolution of approximately R=310,000 at high efficiency
throughout the entire optical/red wavelength range 390-1050nm without
the need for adaptive optics. Besides its polarimetric Stokes IQUV mode,
the capability to cover the entire optical range in three exposures at
resolutions of 40,000, 130,000 and 310,000 will surpass all existing
facilities in terms of light-gathering-power times spectral-coverage
product. A solar feed will make use of the spectrograph also during day
time. As such, we hope that PEPSI will be the most powerful spectrometer
of its kind for the years to come.
Title: Rare earth element sorption by basaltic rock: Experimental data
and modeling results using the “Generalised Composite approach”
Authors: Tertre, E.; Hofmann, A.; Berger, G.
Bibcode: 2008GeCoA..72.1043T
Altcode:
Sorption of the 14 rare earth elements (REE) by basaltic rock is
investigated as a function of pH, ionic strength and aqueous REE
concentrations. The rock sample, originating from a terrestrial basalt
flow (Rio Grande do Sul State, Brazil), is composed of plagioclase,
pyroxene and cryptocrystalline phases. Small amounts of clay minerals
are present, due to rock weathering. Batch sorption experiments are
carried out under controlled temperature conditions of 20 °C with
the <125 μm fraction of the ground rock in solutions of 0.025
M and 0.5 M NaCl and at pH ranging from 2.7 to 8. All 14 REEs are
investigated simultaneously with initial concentrations varying from
10 -7 to 10 -4 mol/L. Some experiments are
repeated with only europium present to evaluate possible competitive
effects between REE. Experimental results show the preferential
retention of the heavy REEs at high ionic strength and circumneutral pH
conditions. Moreover, results show that REE sorption increases strongly
with decreasing ionic strength, indicating two types of sorption
sites: exchange and specific sites. Sorption data are described by
a Generalised Composite (GC) non-electrostatic model: two kinds of
surface reactions are treated, i.e. cation exchange at >XNa sites,
and surface complexation at >SOH sites. Total site density (>XNa +
>SOH) is determined by measuring the cation exchange capacity (CEC
= 52 μmol/m 2). Specific concentrations of exchange sites
and complexation sites are determined by fitting the Langmuir equation
to sorption isotherms of REE and phosphate ions. Site densities of 22
± 5 and 30 ± 5 μmol/m 2 are obtained for [>XNa] and
[>SOH], respectively. The entire set of REE experimental data is
modeled using a single exchange constant (log Kex = 9.7) and
a surface complexation constant that progressively increases from log K
= -1.15 for La(III) to -0.4 for Lu(III). The model proves to be fairly
robust in describing other aluminosilicate systems. Maintaining the
same set of sorption constants and only adjusting the site densities,
we obtain good agreement with the literature data on REE/kaolinite and
REE/smectite sorption. The Generalised Composite non-electrostatic
model appears as an easy and efficient tool for describing sorption
by complex aluminosilicate mineral assemblages.
Title: Polarimetric Projects with GREGOR
Authors: Hofmann, A.
Bibcode: 2008CEAB...32...17H
Altcode:
In 2009 GREGOR -- at that time the most effective solar telescope
worldwide -- will come into operation for solar research. It is designed
to support high-precision spectro-polarimetry for studying basic
physical processes of the dynamic solar photosphere and chromosphere. To
observe magnetic features on small scales in time and space a small
intrinsic instrumental polarization is needed. For this purpose the
polarimetric concept of GREGOR is based on a combination of post-focus
polarimeters with pre-focus equipment for calibration. Both necessitate
new ways for polarization optics and the measuring methods.
Title: Polarimetry with GREGOR - An Ongoing Project
Authors: Hofmann, A.
Bibcode: 2007SunGe...2....9H
Altcode:
GREGOR is the project of a high-resolution solar telescope with an
aperture of 1.5 m and an effective focal length of about 55 m. It
is designed to support accurate, high sensitive spectro-polarimetry
of the solar photosphere and chromosphere for studying the dynamics
of the solar atmosphere and the underlying physical processes. The
Astrophysical Institute Potsdam is currently developing the polarimetric
unit (GPU) of GREGOR. The GPU is an integral part of the telescope
and allows to calibrate polarimeters in any post-focal device, like
the Tenerife Infrared Polarimeter (TIP) or the POlarimetric LIttrow
Spectrometer (POLIS). The new telescope and its polarimetric equipment
will allow high-precision measurements of magnetic fields and plasma
motions in the solar atmosphere down to scales of 70 km on the Sun.
Title: GREGOR: the New German Solar Telescope
Authors: Balthasar, H.; von der Lühe, O.; Kneer, F.; Staude, J.;
Volkmer, R.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Nicklas, H.; Popow, E.;
Puschmann, K.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier,
K.; Wittmann, A.
Bibcode: 2007ASPC..368..605B
Altcode: 2007arXiv0704.2693B
GREGOR is a new open solar telescope with an aperture of 1.5 m. It
replaces the former 45-cm Gregory Coudé telescope on the Canary island
Tenerife. The optical concept is that of a double Gregory system. The
main and the elliptical mirrors are made from a silicon-carbide material
with high thermal conductivity. This is important to keep the mirrors
on the ambient temperature avoiding local turbulence. GREGOR will be
equipped with an adaptive optics system. The new telescope will be ready
for operation in 2008. Post-focus instruments in the first stage will be
a spectrograph for polarimetry in the near infrared and a 2-dimensional
spectrometer based on Fabry-Pérot interferometers for the visible.
Title: Type III Radio Burst Prolific Magnetic Field Configurations
Authors: Hofmann, A.; Ruždjak, V.
Bibcode: 2007CEAB...31..209H
Altcode:
The occurrence of metre-wave type III radio bursts was investigated in
relation to the location of the associated flare in the magnetic field
configuration for two periods of low solar activity in 1975--1977 and
1985--1987. In a statistical analysis it was found that for subflares
the probability to produce a type III burst is higher by about one
order of magnitude if the flare occurs at the boundary compared to a
position elsewhere inside the general bipolar pattern of the related
active region. The 3-D-topology of the magnetic field was calculated
by extrapolation of the observed magnetic field for selected active
regions and events. The locations at the border where the Hα-patches
of flares associated with type III bursts were observed are close to
open field lines extending high into the corona.
Title: Favourable Magnetic Field Configurations for Generation of
Flare-Associated Meter-Wave Type III Radio Bursts
Authors: Hofmann, A.; Ruždjak, V.
Bibcode: 2007SoPh..240..107H
Altcode:
Magnetic field structures of Hα flares associated with meter-wave
type III bursts during periods of low solar activity in 1975 - 1977
and 1985 - 1987 were investigated. In a statistical analysis it was
confirmed that the association rate depends less on flare importance
than on brightness. For subflares (95% of the sample), the location
of the Hα flare in the bipolar pattern turned out to be crucial
for the association rate. It is almost one order of magnitude larger
for flares occurring at the border of the active regions, compared
to flares located inside the general bipolar pattern. For selected
typical examples of flares, extrapolations of the measured magnetic
fields were performed. By matching Hα filtergrams and calculated 3-D
structures it was found that the positions at the border where the
flares associated with type III bursts occurred were close to open
field lines extending into the corona. In most investigated cases
intrusions of parasitic polarity were found in the vicinity of the
flare locations. The extrapolations showed that subflares located
inside the bipolar pattern but have not been associated with type III
bursts were covered by dense arcades of magnetic loops.
Title: New high resolution solar telescope GREGOR
Authors: Volkmer, R.; von der Lühe, O.; Kneer, F.; Staude, J.;
Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
C.; Heidecke, F.; Hofmann, A.; Klvaña, M.; Sobotka, M.; Nicklas, H.;
Popow, E.; Puschmann, K. G.; Schmidt, W.; Soltau, D.; Strassmeier,
K.; Wittmann, A. D.
Bibcode: 2007msfa.conf...39V
Altcode:
The 1.5m solar telescope GREGOR is being constructed at Tenerife,
Spain. Its purpose is to observe with high spatial and spectral
resolution small-scale dynamic magnetic features on the Sun. The
telescope is completely open with retractable dome and actively cooled
primary mirror made of silicon carbide to minimize thermal effects
on the image quality. After completion it will be one of the most
powerful solar telescopes. This paper presents a general overview of
the telescope characteristics and the current status.
Title: The new 1.5m solar telescope GREGOR: first light and start
of commissioning
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
Jürgen; Berkefeld, Thomas; Caligari, Peter; Halbgewachs, Clemens;
Heidecke, Frank; Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald;
Wittmann, Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus;
Sobotka, Michal; Klvana, Miroslav; Collados, Manuel
Bibcode: 2006SPIE.6267E..0WV
Altcode: 2006SPIE.6267E..29V
The integration of the three main silicon carbide mirrors into the new
1.5 m solar telescope GREGOR at Izana on Tenerife, Spain is planned
during 2006. We expect first light at the end of 2006. A progress
report about integration of the optics and mechanics and planning of
the commissioning phase of the telescope and post focus instruments
will be presented at the meeting. The GREGOR telescope is build by a
consortium of the Kiepenheuer Institut fur Sonnenphysik in Freiburg,
the Astrophysikalische Institut Potsdam, the Institut fur Astronomie
Gottingen and additional national and international Partners.
Title: Fiber bursts as 3D coronal magnetic field probe in postflare
loops
Authors: Aurass, H.; Rausche, G.; Mann, G.; Hofmann, A.
Bibcode: 2005A&A...438..741A
Altcode:
No abstract at ADS
Title: The new 1.5 solar telescope GREGOR: progress report and
results of performance tests
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
Jürgen; Berkefeld, Thomas; Caligari, Peter; Halbgewachs, Clemens;
Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald; Wittmann, Axel;
Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka, Michal;
Klvana, Miroslav; Collados, Manuel
Bibcode: 2005SPIE.5901...75V
Altcode:
The telescope structure including control system and the complete
retractable dome of the new 1.5 m solar telescope GREGOR were
assembled during 2004 at Izana on Tenerife, Spain. The GREGOR
telescope is build by a consortium of the Kiepenheuer Institut fuer
Sonnenphysik, the Astrophysikalische Institut Potsdam, the Institut
fuer Astrophysik Goettingen and additional national and international
Partners. Pointing, tracking and thermal tests were made to verify
the proposed performance. The results of these tests and a progress
report of the project will be presented.
Title: Progress Towards Task-Level Collaboration between Astronauts
and their Robotic Assistants
Authors: Effinger, R.; Hofmann, A.; Williams, B.
Bibcode: 2005ESASP.603E..63E
Altcode: 2005aira.confE..63E
No abstract at ADS
Title: Dynamics of solar active regions. II. Oscillations observed
with MDI and their relation to the magnetic field topology
Authors: Muglach, K.; Hofmann, A.; Staude, J.
Bibcode: 2005A&A...437.1055M
Altcode:
We present new results of an international joint observing campaign,
which was carried out in September 2000 to study the oscillatory
behaviour of solar active regions. We concentrate on oscillations
in the lower layers of the solar atmosphere as observed with the
Michelson Doppler Imager (MDI, Scherrer et al. 1995, Sol. Phys., 162,
129). The resulting photospheric oscillation power maps are compared
with chromospheric maps from simultaneous UV continuum data (taken with
the TRACE instrument). From a magnetic field extrapolation of the MDI
magnetograms we infer that the reduction in chromospheric high frequency
power and enhancement of photospheric high frequency power may be
explained by the interaction of acoustic waves with the magnetic canopy.
Title: Fiber bursts as 3D coronal magnetic field probe in postflare
loops
Authors: Aurass, H.; Rausche, G.; Mann, G.; Hofmann, A.
Bibcode: 2005A&A...435.1137A
Altcode:
Fiber bursts appear in some complex solar radio bursts as a continuum
fine structure in the frequency range of 150\cdots3000 MHz. We present
and test a new method to use fiber bursts as a probe of the magnetic
field strength and the 3D field structure in postflare loops. Thereby
we assume that fiber bursts are driven by whistler waves ascending
in the postflare loops which act as magnetic traps for nonthermal
flare electrons. For a selected event (1997 April 07) we derive from
dynamic radio spectra (Potsdam) and Nançay Radio Heliograph imaging
data of fiber bursts the coronal magnetic field strength within the
fiber burst source. We compare the fiber burst source positions and
field strength estimates with the extrapolated potential magnetic field
above the flaring active region NOAA 8027 using SOHO-MDI photospheric
field data. The field strength from fiber bursts are within a factor
of 0.6 to 1.4 of the field strength of the selected subset of potential
field lines and give preference to a 3.5 times Newkirk (1961, ApJ, 133,
983) coronal density model within the evolving postflare loops. We find
independent proof of the physical significance of considering selected
potential field lines as postflare loop field information regarding
topology and field strength over a time interval of one hour after
the impulsive flare phase. We conclude that radio decimeter and meter
wave spectra and radio imaging at two representative frequencies are
sufficient for a reliable estimate of the (otherwise not measurable)
coronal magnetic field strength in postflare loops. This can be an
important field sounding method using the forthcoming FASR (Frequency
Agile Solar Radiotelescope) instrument.
Title: A Donnan diffusion model for the description of Sr adsorption
kinetics to hydrous ferric oxide
Authors: Hofmann, A.; van Beinum, W.; Meeussen, J. C. L.; Kretzschmar,
R.
Bibcode: 2005GeCAS..69..180H
Altcode:
No abstract at ADS
Title: Active Region Oscillations and Their Relation to the Magnetic
Field Topology
Authors: Muglach, K.; Hofmann, A.; Staude, J.
Bibcode: 2005AGUSMSH13C..03M
Altcode:
In this contribution we present an analysis of time sequences of MDI
intensity and Doppler velocity together with simultaneous filtergrams
taken by TRACE at 1700A in an active region. The high frequency
halos found in MDI velocity and the deficiency of high frequency
power sampled at the height of the TRACE UV filter around the active
region can be explained by an interaction of the acoustic wave field
with the magnetic field of the active region. From a magnetic field
extrapolation we calculate the plasma β for the complete TRACE FOV
up to the base of the corona. The contours giving the location of β
approximately 1 where h=500 km agree very well with the decrease in
1700A high frequency power.
Title: Progress report of the 1.5 m solar telescope GREGOR
Authors: Volkmer, Reiner; von der Lühe, Oskar F.; Kneer, Franz;
Staude, Jürgen; Berkefeld, Thomas; Caligari, Peter; Schmidt,
Wolfgang; Soltau, Dirk; Nicklas, Harald; Wiehr, Eberhardt; Wittmann,
Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka,
Michal; Klvana, Miroslav; Collados, Manuel
Bibcode: 2004SPIE.5489..693V
Altcode:
GREGOR is the new 1.5 m solar telescope assembled on Tenerife, Spain, by
the German consortium of the Kiepenheuer Institut fur Sonnenphysik, the
Astronomischen Institut Potsdam, the Universitats-Sternwarte Gottingen
and other national and international Partners. The refurbishment of
the building is almost finished. The manufacturing of the telescope
structure and the optics is still in progress. After the integration of
the new complete retractable dome in July 2004 the telescope structure,
optic and post focus instruments will be assembled during the rest of
the year. First light is planned during May 2005.
Title: Auto-Calibration of SOL-ACES in the EUV Spectral Region
Authors: Schmidtke, G.; Brunner, R.; Eberhard, D.; Hofmann, A.;
Klocke, U.; Knothe, M.; Konz, W.; Riedel, W. -J.; Wolf, H.
Bibcode: 2004cosp...35.2557S
Altcode: 2004cosp.meet.2557S
The Sol-ACES (SOLAR Auto-Calibrating EUV/UV Spectrometers) experiment is
prepared to be flown with the ESA SOLAR payload to the International
Space Station as planned for the Shuttle mission E1 in August
2006. Four grazing incidence spectrometers of planar geometry cover
the wavelength range from 16-220 nm with a spectral resolution from
0.5-2.3 nm. These high-efficiency spectrometers will be re-calibrated
by two three-signal ionization chambers to be operated with 44 band
pass filters on routine during the mission. Re-measuring the filter
transmissions with the spectrometers also allows a very accurate
determination of the changing second (optical) order efficiencies of
the spectrometers as well as the stray light contributions to the
spectral recording in different wavelength ranges. In this context
the primary requirements for measurements of high radiometric accuracy
will be discussed in detail. - The absorption gases of the ionization
chambers are neon, xenon and a mixture of 10 % nitric oxide and 90
% xenon. As the laboratory measurements show that by this method
secondary effects can be determined to a high degree resulting in
very accurate irradiance measurements that is ranging from 5 to 3 %
in absolute terms depending on the wavelegth range.
Title: Solar type IV burst spectral fine structures. II. Source model
Authors: Zlotnik, E. Ya.; Zaitsev, V. V.; Aurass, H.; Mann, G.;
Hofmann, A.
Bibcode: 2003A&A...410.1011Z
Altcode:
We discuss a source model for the origin of solar type IV burst fine
structures (FS) using the data of an event in AR 7792 on 25 October
1994. After giving a comprehensive observational treatment of FS (Paper
I), here we repeat the main observed facts to construct a simplified
radio source model. It consists of two interacting loops (named LS1
and EL) with one spatial order of magnitude scale difference (turning
heights 70 and 7 Mm). We consider the implications of this model for
physical mechanisms of broad band pulsations (BBP) and zebra patterns
(ZP). Our analysis leads to the conclusion that meter wave BBP and ZP
originate from a common magnetic source structure - a large asymmetric
coronal loop. It is shown that the BBP result from periodically repeated
injections of fast electrons into the asymmetric magnetic trap. The
excitation of plasma waves is due to the stream instability when these
electrons are propagating along the loop. We demonstrate that a two
percent quasi-periodic modulation of a magnetic field component in EL
is sufficient for it to act as a periodic electron accelerator. The
ZP is due to a plasma wave instability at the levels of double plasma
resonance (DPR) in an inhomogeneous source distributed along the loop
axis of LS1. The DPR frequencies appear at those height levels where the
upper hybrid frequency is equal to a harmonic of the gyrofrequency. Two
Appendices review theoretical details needed to understand the given ZP
interpretation. The gyrofrequency as a function of height was derived
from a force-free extrapolated field line that passes the coronal radio
source. After knowing the loop turning height and the magnetic field
strength we identified for a fixed observing time the harmonic number
of each zebra stripe. The comparison of the calculated DPR levels with
the observed zebra stripe peak frequencies yields a density law for
the ZP source volume. It turns out that this is a barometric law with
a temperature near 106 K. We demonstrate that the drift of
the whole ZP to higher frequencies can be explained as a signature of
magnetic field decrease and/or plasma cooling in the ZP source. The time
delay between BBP and ZP was found to be due to the higher fast particle
threshold of the DPR versus the beam instability. The present analysis
confirms the double plasma resonance model for the ZP fine structure,
and underlines the significance of force-free extrapolated photospheric
fields for coronal magnetic field modelling. Appendices A and B
are only available in electronic form at http://www.edpsciences.org
Title: Current Status of the 1.5m Solar Telescope GREGOR
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
Jürgen; Berkfeld, Thomas; Schmidt, Wolfgang; Soltau, Dirk; Nicklas,
Harald; Wiehr, Eberhardt; Wittman, Axel; Hofmann, Axel; Sobotka,
Michal; Klvana, Miroslav
Bibcode: 2003ANS...324..112V
Altcode: 2003ANS...324..P19V
No abstract at ADS
Title: The Potsdam Echelle Polarimetric and Spectroscopic Instrument
(PEPSI) for the LBT
Authors: Strassmeier, K. G.; Hofmann, A.; Woche, M.
Bibcode: 2003ANS...324...28S
Altcode: 2003ANS...324..D02S
No abstract at ADS
Title: GREGOR: the new 1.5m solar telescope on Tenerife
Authors: Volkmer, Reiner; von der Luehe, Oskar; Kneer, Franz; Staude,
Juergen; Hofmann, Axel; Schmidt, Wolfgang; Sobotka, Michal; Soltau,
Dirk; Wiehr, Eberhardt; Wittmann, Axel; Berkefeld, Thomas
Bibcode: 2003SPIE.4853..360V
Altcode:
The new 1.5 m high resolution telescope will be build up on the reused
solar tower of the German 45 cm Gregory Coudé Telescope at the Teide
Observatory, Izaña, Tenerife. The new telescope is a Gregory type
with open telescope structure, alt-azimuth mount, complete retractable
dome, and a pool of well established and new developed post focus
instruments. An adaptive optics system provides the capability for
diffraction limited observations at visible wavelengths and the
polarimetry device in the secondary focus reduces the perturbation
due to instrumental polarization in an efficient way. We describe the
main optical characteristics and the focal plane instrumentation with
respect to the latest status of the project.
Title: PEPSI spectro-polarimeter for the LBT
Authors: Strassmeier, Klaus G.; Hofmann, Axel; Woche, Manfred F.; Rice,
John B.; Keller, Christoph U.; Piskunov, N. E.; Pallavicini, Roberto
Bibcode: 2003SPIE.4843..180S
Altcode:
PEPSI (Postham Echelle Polarimetric and Spectroscopic Instrument) is
to use the unique feature of the LBT and its powerful double mirror
configuration to provide high and extremely high spectral resolution
full-Stokes four-vector spectra in the wavelength range 450-1100nm. For
the given aperture of 8.4m in single mirror mode and 11.8m in double
mirror mode, and at a spectral resolution of 40,000-300,000 as designed
for the fiber-fed Echelle spectrograph, a polarimetric accuracy between
10-4 and 10-2 can be reached for targets with
visual magnitudes of up to 17th magnitude. A polarimetric accuracy
better than 10-4 can only be reached for either targets
brighter than approximately 10th magnitude together wiht a substantial
trade-off wiht the spectral resolution or with spectrum deconvolution
techniques. At 10-2, however, we will be able to observe
the brightest AGNs down to 17th magnitude.
Title: Polarimetry with GREGOR
Authors: Hofmann, Axel; Rendtel, Juergen
Bibcode: 2003SPIE.4843..112H
Altcode:
GREGOR is the project of a high-resolution solar telescope with an
aperture of 1.5m and an effective focal length of about 55m. It
is designed to support ground-based accurate, high sensitive
spectro-polarimetry at visible and IR wavelengths in the solar
photosphere and chromosphere for studying the dynamics of the solar
atmosphere and the underlying physical processes. The concept of
polarimetric measurements with GREGOR is based on several unique
and highly specialized post-focus polarimeters like POLIS or TIP
and a polarimetric equipment (GPU) situated near the telescope's
secondary focus F2 where the optical properties are still rotationally
symmetric and the telescope can be regarded as polarization free at the
10-4 level. The GPU is an integral part of the telescope,
consisting of a calibration unit and a modulation unit. The calibration
unit allows to calibrate the modulation unit as well as polarimeters
built in any post-focal device. It consists of a linear polarizer and
quarter wave retarders for the visible and the IR spectral range and is
located in front of the modulation unit. The modulation unit is supposed
to permit a very efficient polarimetric analysis in the spectral range
from 400 nm to 700 nm. It consists of two electro-optical modulators
and a prism as linear polarizer.
Title: GREGOR - optical design considerations
Authors: Soltau, D.; Berkefeld, Th.; von der Lühe, O.; Hofmann, A.;
Schmidt, W.; Volkmer, R.; Wiehr, E.
Bibcode: 2003AN....324..292S
Altcode:
After many years of successful work the Gregory Coudé Telescope
(GCT) is going to be replaced by GREGOR. This new 1.5 m telescope is
the result of design considerations which to a wide extent are based
on new technologies. Special aspects of the design are presented,
including measures to compensate for optical aberrations caused by
instrumental and atmospheric effects (Adaptive Optics). First light
is expected for the end of 2004.
Title: Energy release from a large-scale magnetic null point in
the corona?
Authors: Aurass, Henry; Hofmann, Axel; Vršnak, Bojan
Bibcode: 2002ESASP.506..423A
Altcode: 2002svco.conf..423A; 2002ESPM...10..423A
We observed how a flare disturbes a system of interconnecting loops
between neighboring active regions. The disturbed loops brighten in
SOHO/EIT images of the coronal magnetoplasma. They are part of a large
scale structure embracing a weak field range in the photospheric and
coronal magnetic field. Near the weak field site but away from active
regions and from the EIT-detected loopl heating, an initially narrowband
nonthermal meter wave radio source is observed by the AIP spectrometer
and the Nançay radio heliograph. The formation of this source and the
EIT loop brightening can be consequences of current sheet activation
and excessive coronal heating at a disturbed magnetic null point.
Title: GREGOR, a 1.5 m Gregory-type telescope for solar observation
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Soltau, W. Schmidt
D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
Bibcode: 2002NCimC..25..689K
Altcode:
No abstract at ADS
Title: Design study of the PEPSI polarimeter for the LBT
Authors: Hofmann, A.; Strassmeier, K. G.; Woche, M.
Bibcode: 2002AN....323..510H
Altcode:
We present the conceptual design of the two polarimetric channels of the
PEPSI spectropolarimeter for the Large Binocular Telescope (LBT). The
two direct Gregorian f/15 focii of the LBT will take up two identical
but independent full-Stokes IQUV polarimeters that themselves fiberfeed
a high-resolution Echelle spectrograph (see the accompanying paper by
Zerbi et al.). The polarizing units will be based on super-achromatic
Fresnel-rhomb retarders and Foster prisms. A total of four fibers are
foreseen to simultaneously direct two ordinary and two extraordinary
light beams to the Echelle spectrograph. Both polarimetric units are
layed out in a modular design, each one optimized to the polarization
state in which it is used. A number of observing modes can be chosen
that are optimized to the type of polarization that is expected from
the target, e.g. circularly and linearly polarized light simultaneously,
or linearly polarized light in both polarimeters, or integral light from
one and polarized light from the other telescope, a.s.o.. Calibration
would be provided for each polarimeter separately.
Title: Post-focus instrumentation for GREGOR
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.;
Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
Bibcode: 2001AN....322..361K
Altcode:
GREGOR is a high-resolution solar telescope with an aperture of 1.5
m. It will be equipped with an Adaptive Optics system and is designed
for high-precision measurements of magnetic fields and plasma motions
in the solar photosphere and chromosphere with a resolution of 70
km on the Sun. GREGOR will replace the Gregory Coudé Telescope at
the Observatorio del Teide on Tenerife. In concert with the other
solar telescopes at Teide Observatory it will be useful for studying
the dynamics of the solar atmosphere and the underlying physical
processes. GREGOR will also serve as a test bed for next generation
solar telescopes. We discuss briefly the postfocus instrumentation
of GREGOR.
Title: GREGOR, a 1.5 m Gregory-type Telescope for Solar Observation
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.;
Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
Bibcode: 2001AGM....18.P223K
Altcode:
GREGOR is a high-resolution solar telescope with an aperture of 1.5
m. It will be equipped with an Adaptive Optics system and is designed
for high-precision measurements of magnetic fields and plasma motions
in the solar atmosphere and chromosphere with a resolution of 70 km
on the Sun. GREGOR will replace the Gregory Coudé Teleskope at the
Observatorio del Teide on Tenerife. We describe the optical design
and the focal plane instrumentation. In concert with the other solar
telescopes at Teide Observatory it will be useful for studying the
dynamics of the solar atmosphere and the underlying processes. GREGOR
will also serve as a test bed for next generation solar telescopes.
Title: Sonnenforschung in Potsdam - Streiflichter aus der Geschichte
Title: Sonnenforschung in Potsdam - Streiflichter aus der Geschichte
Title: Solar research in Potsdam - sidelights on its history.
Authors: Staude, Jürgen; Hofmann, Axel
Bibcode: 2000AcHA....8..107S
Altcode:
No abstract at ADS
Title: Flares in Sigmoidal Coronal Structures a Case Study
Authors: Aurass, H.; Vršnak, B.; Hofmann, A.; Rudžjak, V.
Bibcode: 1999SoPh..190..267A
Altcode:
We analyze radio observations, magnetograms and extrapolated field
line maps, Hα filtergrams, and X-ray observations of two flare events
(6 February 1992 in AR 7042 and 25 October 1994 in AR 7792) and study
properties, evolution and energy release signatures of sigmoidal loop
systems. During both events, the loop configuration seen in soft X-ray
(SXR) images changes from a preflare sigmoidal shape to a relaxed
post-flare loop system. The underlying magnetic field system consists
of a quadrupolar configuration formed by a sheared arcade core and
a remote field concentration. We demonstrate two possibilities: a
sigmoidal SXR pattern can be due to a single continuous flux tube (the
1992 event). Alternatively, it can be due to a set of independent loops
appearing like a sigmoid (the 1994 event). In both cases, the preflare
and post-flare loops can be well reproduced by a linear force-free
field and potential field, respectively, computed using preflare
magnetograms. We find that thermal and non-thermal flare energy release
indicators of both events become remarkably similar after applying
spatial and temporal scale transformations. Using the spatial scaling
between both events we estimated that the non-thermal energy release
in the second event liberated about 1.7 times more energy per unit
volume. A two-and-a-half times faster evolution indicates that the rate
of the energy release per unit volume is more than four times higher
in this event. A coronal type II burst reveals ignition and propagation
of a coronal shock wave. In contrast, the first event, which was larger
and released about a 10 times more energy during the non-thermal phase,
was associated with a CME, but no type II burst was recorded. During
both events, in addition to the two-ribbon flare process an interaction
was observed between the flaring arcade and an emerging magnetic flux
region of opposite polarity next to the dominant leading sunspot. The
arcade flare seems to stimulate the reconnection process in an `emerging
flux-type' configuration, which significantly contributes to the energy
release. This regime is characterized by the quasiperiodic injection of
electron beams into the surrounding extended field line systems. The
repeated beam injections excite pulsating broadband radio emission in
the decimetric-metric wavelength range. Each radio pulse is due to a
new electron beam injection. The pulsation period (seconds) reflects
the spatial scale of the emerging flux-type field configuration. Since
broadband decimetric-metric radio pulsations are a frequent radio
flare phenomenon, we speculate that opposite-polarity small-scale flux
intrusions located in the vicinity of strong field regions may be an
essential component of the energy release process in dynamic flares.
Title: Determination of the Velocity Vector Field in an Asymmetric
Sunspot Based on Vector Magnetograph Measurements
Authors: Krivtsov, A. M.; Hofmann, A.; Staude, J.; Klvaña, M.;
Bumba, V.
Bibcode: 1999ASPC..184..108K
Altcode:
A new method to determine the distribution of the full velocity
vector in an asymmetric sunspot is presented. Measurements of the
Doppler velocity and of the vector of the magnetic field are used as
initial data for these calculations. The determination is subdivided
into two stages: in a first step we obtain the distribution of the
velocity projection onto the solar surface, and in the second step
the orthogonal component of the velocity field is calculated. The
resulting vector velocity field is in good agreement with the basic
features of the siphon flow model of penumbral flux tubes.
Title: Liquid Crystal Imaging Stokes Polarimeter
Authors: Horn, T.; Hofmann, A.
Bibcode: 1999ASPC..184...33H
Altcode:
The Liquid Crystal based Stokes Polarimeter (LCSP) is a low voltage
complete Stokes polarimeter which is developed for high resolution
spectro-polarimetry. The polarimeter consists of two liquid crystal
variable retarders in front of a polarizing beam splitter. It was
installed and tested in combination with the new image scanner
(Stolpe et al., 1997) at the Gregory Coudé Telescope (GCT) at the
Observatorio del Teide(Izana, Tenerife). We give some information on
the basic design, a method of in situ compensation of cross-talk and
some first observations.
Title: Radio evidence on shock wave formation in the solar corona
Authors: Klassen, A.; Aurass, H.; Klein, K. -L.; Hofmann, A.; Mann, G.
Bibcode: 1999A&A...343..287K
Altcode:
In order to investigate the formation of radio emitting shock
waves above flaring active regions, we combine spectral and imaging
observations of type II radio events with X-ray imaging and full-Sun
observations and, in one case, with the extrapolated magnetic field
configuration in the corona. We confirm and extend earlier findings
that type II bursts are emitted above active region loops seen in
soft X-ray images. Sources at successively lower frequencies are
non-radially displaced from the axis of the active region loops. Two new
radio features identified in high resolution spectrograms establish a
possible link between the type II emission and the preceding activity
in the underlying corona: Groups of fast drift bursts or pulsations
with a restricted bandwidth are observed in coronal loops from the
impulsive flare phase until the onset of the type II emission. These
groups or their high frequency cut-off are found in the spectral range
around the backward extrapolated type II lanes. Envelope features
of the group (starting frequency and/or cut-off frequency) drift
gradually to lower frequencies, at a normalized drift rate similar to
the following type II lanes. The sources are located between the sites
of Hα emission and of the type II emission. We refer to this burst
group and its envelope features as a type II precursor. Immediately
before the type II emission a short (<= 1 min duration) series of
narrow-band bursts occurs at frequencies between the split bands of
the type II lanes. As a whole, the burst sequence has an inverted
U-shaped spectral envelope. We therefore call it an arc. It has
fundamental-harmonic structure as the subsequent type II burst, but
no band split. The source is located near or above the summits of
the coronal loops where the precursor emission occurred before, and
close to the site where the type II emission starts. The arc feature
occurs especially prior to high frequency type II bursts, i.e. type
II shocks formed at comparatively low coronal height. It is concluded
that the type II burst is related to a plasma jet or a blast wave
that originates in closed magnetic structures in the active region,
and is first recognized during the early impulsive phase (between the
onset and main maximum of the hard X-ray emission). This disturbance
becomes or launches a shock wave when it attains the summit of these
structures. The shock is refracted into overlying field regions,
where it becomes visible through the type II emission.
Title: Third Advances in Solar Physics Euroconference: Magnetic
Fields and Oscillations
Authors: Schmieder, B.; Hofmann, A.; Staude, J.
Bibcode: 1999ASPC..184.....S
Altcode:
The third Advances in Solar Physics Euroconference (ASPE) "Magnetic
Fields and Oscillations"concluded a series of three Euroconferences
sponsored by the European Union. The meeting took place in Caputh
near Potsdam, Germany, on September 22-25, 1998, followed by the JOSO
(Joint Organization for Solar Observations) 30th Annual Board Meeting
on September 26, 1998. The ASPE formula is attractive and compares well
with other meetings with "show-and-tell" character. This meeting had
122 participants coming from 26 countries; 36 participants came from
countries formerly behind the Iron Curtain; a "politically incorrect"
estimate says that 48 participants were below 35 years of age,
with an unusually large female-to-male ratio. This characteristic of
youngness is the more striking since solar physics is a perhaps overly
established field exhibiting an overly senior age profile. It was a
good opportunity to train this young generation in Solar Physics. The
conference topic "Magnetic Fields and Oscillations" obviously was
wide enough to cater to many an interest. These proceedings are
organized according to the structure of the meeting. They include
the topics 'High resolution spectropolarimetry and magnetometry',
'Flux-tube dynamics', 'Modelling of the 3-D magnetic field structure',
'Mass motions and magnetic fields in sunspot penumbral structures',
'Sunspot oscillations', 'Oscillations in active regions - diagnostics
and seismology', 'Network and intranetwork structure and dynamics',
and 'Waves in magnetic structures'. These topics covered the first
2.5 days of the conference. The reviews, oral contributions, and
poster presentations were by no means all of the meeting. The ASPE
formula also adds extensive plenary sessions of JOSO Working groups
on topics that involve planning of Europe-wide collaboration. At this
meeting these concerned solar observing techniques, solar data bases,
coordination between SOHO and ground-based observing, and preparations
for August 11, 1999 when more Europeans will be eclipsed than ever
before. The contributions to these sessions have been included into
the present volume as well. The participants of the EU-TMR Research
Network 'Solar Magnetometry Network' came together to discuss in a
special working group session questions of their future collaboration.
Title: Multi-Frequency Observations of the February 6, 1992 Flare
Authors: Ruzdjak, V.; Vrsnak, B.; Aurass, H.; Hofmann, A.; Schroll, A.
Bibcode: 1999HvaOB..23....1R
Altcode:
Observations of the two-ribbon flare of February 6, 1992 which occured
in the active region NOAA 7042 involving a sigmoidal soft X-ray loop
pattern, are described. YOHKOH SXT images, H-alpha filtergrams and
coronal magnetic field extrapolations are used to reveal details of
the preflare coronal magnetic field configuration. The development of
the flare in SXR, H-alpha and dm-m radio wavelength range is followed
in detail to disclose the basic stages of the energy release process.
Title: Energy Release Stages of the Complex Solar Flare of October
25, 1994
Authors: Vrsnak, B.; Aurass, H.; Ruzdjak, V.; Hofmann, A.; Schroll, A.
Bibcode: 1999HvaOB..23...15V
Altcode:
The morphology and evolution of the 1n/C4.7 flare which occured in
the active region NOAA 7792 on October 25, 1994 are studied using the
observations in the SXR, optical and radio range. The preflare coronal
magnetic field structure is inferred comparing the force-free magnetic
field extrapolations of the photospheric longitudinal magnetograms
with structures seen in YOHKOH SXT images and H-alpha filtergrams. The
preflare arcade consisted of a highly sheared system of loops which was
abruptly transformed during the impulsive energy release. The further
evolution was basically following the two-ribbon flare scenario,
including a protrusion of the energy release process into the strong
magnetic field of the major sunspot, as well as an interaction with
an emerging flux region.
Title: Behaviour of the magnetic field in pores, small sunspots and
light bridges (June 3 - 5, 1995).
Authors: Czycykowski, I.; Horn, T.; Hofmann, A.
Bibcode: 1999joso.proc...75C
Altcode:
The active region NOAA 7874 was observed in the Fe I line (6173 Å). The
authors obtained two-dimensional, highly resolved spectra of several
parts of the region in all Stokes parameters. Maps of magnetic field
strength and inclination are presented. The magnetic field in light
bridges shows an inclination angle of about 20°. The penumbral fields
are even stronger inclined.
Title: Determination of the full velocity vector based on vector
magnetograph measurements in an asymmetric sunspot
Authors: Krivtsov, A. M.; Hofmann, A.; Staude, J.; Klvana, M.;
Bumba, V.
Bibcode: 1998A&A...335.1077K
Altcode:
A new method to determine the distribution of the full velocity
vector in an asymmetric sunspot is presented. Measurements of the
Doppler velocity and of the vector of the magnetic field are used as
initial data for these calculations. The determination is subdivided
into two stages: in a first step we obtain the distribution of the
velocity projection onto the solar surface, and in the second step
the orthogonal component of the velocity field is calculated. The
method has been applied to a sunspot observed from two observatories
over 7 days, that is at different positions on the solar disk and in
different phases of its development. The resulting vector velocity
field is in good agreement with the basic features of the siphon flow
model of penumbral flux tubes. The velocities are directed outwards
from the sunspot, and they have small values in the umbra and maximum
values in the central part of the penumbra, while in the outer penumbra
the velocities decrease rapidly. The vertical velocities are mainly
localized at the umbral boundary (upflows) and at the outer parts of
the penumbra (downflows).
Title: The 09 September 1989 gamma -ray flare - multi-site
particle acceleration and shock-excited radio emission during
quasiperpendicular and quasiparallel propagation
Authors: Aurass, H.; Hofmann, A.; Urbarz, H. -W.
Bibcode: 1998A&A...334..289A
Altcode:
This is a case study of the location of particle acceleration sites
during an impulsive flare and its subsequent coronal shock which
occurred on 09 September 1989. Joint radio, hard X-ray/gamma -ray,
and H_alpha observations of the flare impulsive emissions reveal
that electron and ion acceleration results from successive energy
release in different magnetic structures. In this paper we attempt
to identify these magnetic structures, during both the impulsive
flare and the coronal shock propagation. This is done by tracing
essential features identified in high time resolution radio spectral
and hard X-ray/gamma -ray data in radio images, and comparing the
spatial information with sequences of H_alpha images and a static
figure of the coronal magnetic field lines derived by extrapolating
photospheric field measurements. During the impulsive phase signatures
of energy release are found in different coronal height ranges which are
magnetically connected with a small part of an underlying active region
that is invaded by an expanding H_alpha flare ribbon. The magnetic
field configuration comprises two sites at heights between 2 * 10(4)
and 8 * 10(4) km above the primary highly confined flare. These sites
are characterized by coronal magnetic field line systems crossing at
nearly right angle. This provides for potential sites of efficient
energy release and particle acceleration without demanding for extreme
twist. The subsequent shock wave, revealed by type II radio burst
emission, is found to propagate over large distances ( ~ 1 R_sun)
through the corona and to accelerate electrons in regions where it
propagates along and across the ambient magnetic field.
Title: Diverging magnetic arcades and antiparallel currents
Authors: Zuccarello, F.; Vinci, A.; Hofmann, A.
Bibcode: 1998MmSAI..69..715Z
Altcode:
No abstract at ADS
Title: Coronal magnetic-field changes of a strong, very fast
developing compact solar active region and related processes
Authors: Shibasaki, K.; Kosugi, T.; Bogod, V.; Garaimov, V.; Gelfreikh,
G.; Stepanov, A. V.; Kliem, B.; Hildebrandt, J.; Kruger, A.; Hofmann,
A.; Urpo, S.
Bibcode: 1998cee..workE..49S
Altcode:
We have studied the development of the large active region NOAA AR
7321, which suddenly appeared near the central meridian of the Sun,
during the first days of its remarkably fast evolution. To analyse the
structural and evolutional features of the magnetosphere of the active
region spatially resolved spectral-polarization observations of the
large Radioheliograph at Nobeyama (Japan), the RATAN-600 telescope at
Zelenchuk (Russia), and mm-wave data of the Metsahovi Radio Research
Station (Finland) have been used and compared with model calculations
of the radio emission. Our main goals concern the evolution of the
magnetic scale height above the sunspots centre deduced from the
gyromagnetic emission and its relation to the flare activity, as well
as the nature of the energy release responsible for the source of
long-duration coronal mm-wave radiation.
Title: On Shock Wave Formation in the Solar Corona
Authors: Klassen, A.; Aurass, H.; Klein, K. -L.; Hofmann, A.; Mann, G.
Bibcode: 1998cee..workE..45K
Altcode:
In order to investigate the formation of radio emitting shock
waves above flaring active regions, we combine spectral and imaging
observations of type II radio events with X-ray imaging and full-Sun
observations and, in one case, with the extrapolated magnetic field
configuration in the corona. We confirm and extend earlier findings that
type II bursts develop above active region loops seen in soft X-ray
images. Sources at successively lower frequencies are non-radially
displaced from the axis of the active region loops. Two new radio
features identified in high resolution spectrograms establish a
possible link between the type II emission and the preceding activity
in the underlying corona: 1. Fast-drift bursts and pulsations with a
restricted bandwidth are observed in coronal loops from the impulsive
flare phase until the onset of the type II emission. Envelope features
of this burst group (starting frequency and/or cut-off frequency) drift
gradually to lower frequencies, at a normalized drift rate similar to
the following type II lanes. The source sites are located between the
sites of H_alpha emission and of the type II emission. The envelope
features of theburst group therefore appear as an early manifestation
of the disturbance which later gives rise to the type II emission. We
refer to these envelope features as a type II precursor. 2. Immediately
before the type II emission a short (<= 1 min duration) series of
narrow-band bursts occurs at frequencies between the split bands of
the type II lanes. As a whole, the burst sequence has an inverted
U--shaped spectral envelope. We therefore call it an arc. It has
fundamental-harmonic structure as the subsequent type II burst, but
no band splitting. The source is located near or above the summits
of the coronal loops where the precursor emission occurred before,
and close to the site where the type II emission starts. The arc
feature occurs especially prior to high-frequency type II bursts,
i.e. type II shocks formed at comparatively low coronal height. We
discuss qualitatively causes for these features in terms of material
injection in the corona and of the propagation of a magnetohydrodynamic
blast wave through complex magnetic structures. Figure 1. Event on 27
September 1993. Left -- dynamic spectrum with 'Arc' and 'Precursor'
features (arrows). Right -- superposition of the radio sources onto
the Yohkoh -- difference image (12:37--12:00 UT). Positions: black
diamond -- H alpha flare, 'Precursor', dotted box -- 'arc', black box --
type II source trajectory. Insert: Yohkoh-SXT image at 12:00:29 UT.
Title: Coronal Energy Release and Magnetic Fields at Low Solar
Activity
Authors: Krüger, A.; Hildebrandt, J.; Kliem, B.; Hofmann, A.; Nefedev,
V. P.; Agalakov, A. B.; Smolkov, G. Ya.
Bibcode: 1997ESASP.404..469K
Altcode: 1997cswn.conf..469K
No abstract at ADS
Title: Observation and model calculations of sunspot ring structure
at 8.46GHz
Authors: Gopalswarmy, N.; Raulin, J. P.; Kundu, M. R.; Hildebrandt,
J.; Krueger, A.; Hofmann, A.
Bibcode: 1996A&A...316L..25G
Altcode:
We present Very Large Array (VLA) observations of AR 7542 which
demonstrate the existence of definite ring and horse-shoe structures
of a sunspot in intensity (I) and polarization (V) at 8.46GHz (3.5cm
wavelength) and compare them with model calculations of gyroresonance
radiation. The VLA measurements have been made on three different days
in July 1993 when AR 7542 was at three different longitudes which allows
us to study the effect of viewing angle on sunspot-associated microwave
emission. Model calculations of gyroresonance radiation have been
carried out using a modified dipole model corresponding to the observed
photospheric magnetic field strength and average temperature/electron
density distributions consistent with soft X-ray and EUV observations
(for the lower atmosphere) as well as theoretical assumptions (for
the corona). The calculated I and V maps were found to be generally
consistent with the radio observations. We obtain information on the
magnetic scale length in vertical and horizontal directions above
the sunspot and about the distribution of other plasma parameters
(temperature, density) inside the radio source region.
Title: 3D Magnetic Reconnection at an X-Ray Bright Point
Authors: Mandrini, C. H.; Démoulin, P.; Van Driel-Gesztelyi, L.;
Schmieder, B.; Cauzzi, G.; Hofmann, A.
Bibcode: 1996SoPh..168..115M
Altcode:
On May 1, 1993, a flaring X-ray bright point (XBP) was observed for
about 16 hours in the old, disintegrating, bipolar active region (AR)
NOAA 7493. During this period, a minor magnetic bipole (1020
Mx) emerged in the region. We have found observational evidence showing
that the XBP brightenings were due to magnetic reconnection between
the new bipole and pre-existing plage fields. The aim of the present
work is to substantiate with magnetic modelling what has been shown
by the observations. For this purpose we extrapolate the observed
photospheric magnetic fields in the linear force-free approximation
and follow its evolution during the lifetime of the XBP. From the
computed coronal field lines we determine the location of regions of
drastic change in field-line linkage, called `quasi-separatrix layers'
or QSLs. QSLs are open layers that behave physically like separatrices:
the break down of ideal magnetohydrodynamics and the release of free
magnetic energy may occur at these locations when their thickness
is small enough. The extrapolated field lines, with photospheric
footpoints on both sides of QSLs, match the observed chromospheric and
coronal structures (arch filament system, XBP and faint X-ray loops
(FXL)). We study also the evolution of the width of the QSL located
over the new negative polarity pore: the calculated QSL is very thin
(typically less than 100 m) during the lifetime of the XBP, but becomes
much thicker (≥ 104 m) after the XBP has faded. Furthermore
we show that peaks in X-ray brightness propagate along the FXL with a
velocity of ≈ 670 km s-1, starting from the XBP location,
implying that the energy is released where the emerging bipole impacts
against pre-existing coronal loops. We discuss the possible mechanism
of energy transport and conclude that the energy is conducted to the
remote footpoints of the FXL by a thermal front. These results strongly
support the supposition that the XBP brightness and flaring are due
to the interaction of different flux systems, through 3D magnetic
reconnection, at QSLs.
Title: High Resolution Polarimetric Measurements in a Sunspot
Authors: Horn, T.; Hofmann, A.; Balthasar, H.
Bibcode: 1996SoPh..164..321H
Altcode:
A Fabry-Perot interferometer is being used for two-dimensional
spectropolarimetric measurements. We demonstrate the suitability of
the setup for the measurement of the magnetic field and present some
preliminary results from first observations.
Title: High resolution measurement of magnetic field.
Authors: Horn, T.; Hofmann, A.
Bibcode: 1996AGAb...12...94H
Altcode:
No abstract at ADS
Title: X-Ray Bright Point Flares Due to Magnetic Reconnection
Authors: Van Driel-Gesztelyi, L.; Schmieder, B.; Cauzzi, G.; Mein,
N.; Hofmann, A.; Nitta, N.; Kurokawa, H.; Mein, P.; Staiger, J.
Bibcode: 1996SoPh..163..145V
Altcode:
Ground-based optical observations coordinated with Yohkoh/SXT X-ray
observations of an old, disintegrating bipolar active region AR NOAA
7493 (May 1, 1993) provided a multiwavelength data base to study
a flaring `active region' X-ray bright point (XBP) of about 16 hr
lifetime, and the activity related to it in different layers of the
solar atmosphere. The XBP appeared to be related to a new minor bipole
of about 1020 Mx. Superposed on a global evolution of soft
X-ray brightness, the XBP displayed changes of brightness, lasting for
1-10 min. During the brightenings the XBP apparently had a spatial
structure, which was (tiny) loop-like rather than point-like. The
X-ray brightenings were correlated with chromospheric activity: (i)
brightenings of underlying chromospheric faculae, and (ii) appearance
of strong turbulent velocities in the arch filament system. We propose
that the XBP brightenings were due to reconnection of the magnetic
field lines (sketched in 3D) between the new bipole and a pre-existing
plage field induced by the motion of one of the new pores (v = 0.2
km s−1) towards the plage, and that the XBP itself was
a reconnected hot loop between them.
Title: Emerging Flux, Reconnection, and XBP
Authors: van Driel-Gesztelyi, L.; Schmieder, B.; Demoulin, P.;
Mandrini, C.; Cauzzi, G.; Hofmann, A.; Nitta, N.; Kurokawa, H.; Mein,
N.; Mein, P.
Bibcode: 1996mpsa.conf..459V
Altcode: 1996IAUCo.153..459V
No abstract at ADS
Title: Emerging flux seen by Yohkoh.
Authors: van Driel-Gesztelyi, L.; Schmieder, B.; Mandrini, C.;
Démoulin, P.; Cauzzi, G.; Hofmann, A.; Nitta, N.; Kurokawa, H.;
Mein, N.; Mein, P.
Bibcode: 1996joso.proc..124V
Altcode:
No abstract at ADS
Title: Preflare State
Authors: Rust, David M.; Sakurai, Takashi; Gaizauskas, Victor; Hofmann,
Axel; Martin, Sara F.; Priest, Eric R.; Wang, Jing-Xiu
Bibcode: 1994SoPh..153....1R
Altcode:
Discussion on the preflare state held at the Ottawa Flares 22
Workshop focused on the interpretation of solar magnetograms and
of Hα filament activity. Magnetograms from several observatories
provided evidence of significant build up of electric currents in
flaring regions. Images of X-ray emitting structures provided a clear
example of magnetic relaxation in the course of a flare. Emerging
and cancelling magnetic fields appear to be important for triggering
flares and for the formation of filaments, which are associated with
eruptive flares. Filaments may become unstable by the build up of
electric current helicity. Examples of heliform eruptive filaments
were presented at the Workshop. Theoretical models linking filaments
and flares are briefly reviewed.
Title: Relationship between electric currents, photospheric motions,
chromospheric activity, and magnetic field topology
Authors: van Driel-Gesztelyi, L.; Hofmann, A.; Demoulin, P.; Schmieder,
B.; Csepura, G.
Bibcode: 1994SoPh..149..309V
Altcode:
Through coordinated observations made during the Max'91 campaign in June
1989 in Potsdam (magnetograms), Debrecen (white light and Hα), and
Meudon (MSDP), we follow the evolution of the sunspot group in active
region NOAA 5555 for 6 days. The topology of the coronal magnetic field
is investigated by using a method based on the concept of separatrices -
applied previously (Mandriniet al., 1991) to a magnetic region slightly
distorted by field-aligned currents. The present active region differs
by having significant magnetic shear. We find that the Hα flare
kernels and the main photospheric electric current cells are located
close to the intersection of the separatrices with the chromosphere,
in a linear force-free field configuration adapted to the observed
shear. Sunspot motions, strong currents, isolated polarities, or
intersecting separatrices are not in themselves sufficient to produce a
flare. A combination of them all is required. This supports the idea
that flares are due to magnetic reconnection, when flux tubes with
field-aligned currents move towards the separatrix locations.
Title: The Flare of 1989 September 9 09:09 UT: Does Coronal Loop
Collision Initiate Efficient Gamma-Ray Emission?
Authors: Aurass, H.; Hofmann, A.; Rieger, E.
Bibcode: 1994ApJS...90..707A
Altcode: 1994IAUCo.142..707A
Vector magnetogram data and H(alpha) pictures together with data
published by Chupp et al. lead us to conjecture that in the presented
case a contact between the rising two-ribbon flare current sheet and
a coronal loop connecting two nearby plage regions initiates efficient
high-energy gamma-ray emission.
Title: Magnetic field measurements at different levels in the solar
atmosphere.
Authors: Hofmann, A.
Bibcode: 1994imfm.conf..112H
Altcode:
No abstract at ADS
Title: Structural Parameters Derived from Vector Magnetic Field
Measurements
Authors: Hofmann, A.
Bibcode: 1994emsp.conf...85H
Altcode:
No abstract at ADS
Title: Vector-Stokes polarimetry in a sunspot
Authors: Hofmann, A.; Shine, R. A.; Frank, Z. A.; Schmidt, W.;
Balthasar, H.
Bibcode: 1994smf..conf..204H
Altcode:
No abstract at ADS
Title: Twisted Flux Tubes and Sunspot Motions in AR 5555
Authors: van Driel-Gesztelyi, L.; Démoulin, P.; Schmieder, B.;
Hofmann, A.; Csepura, G.
Bibcode: 1994emsp.conf..115V
Altcode:
No abstract at ADS
Title: High resolution vector polarimetry of sunspot magnetic fields.
Authors: Balthasar, H.; Hofmann, A.; Schmidt, W.; Shine, R. A.; Frank,
Z. A.
Bibcode: 1994AGAb...10..112B
Altcode:
No abstract at ADS
Title: Sites of flares and filaments in solar active regions
Authors: Schmieder, B.; van-Driel, L.; Hofmann, A.; Démoulin, P.;
Hénoux, J. C.; Hagyard, M.
Bibcode: 1993AdSpR..13i.119S
Altcode: 1993AdSpR..13..119S
We applied a method developed to derive from magnetic field data the
location of energy release in solar active regions. This method has
been applied to two sets of data (June 13-15 1980 and June 23-25 1989)
using magnetograms (Meudon, MSFC, Potsdam) and Hα data (Debrecen,
MSDP Meudon). The Hα flares are located at intersections of the
separatrices with the chromosphere in places where high density currents
are observed. This supports the idea that the energy released in flares
is due to magnetic reconnection at the location of a separator and
subsequently transported to the chromosphere along field lines. It is
found that strong magnetic shear is a necessary condition for filament
formation. Magnetic shear merely determines the importance of the
flare and is certainly not a necessary condition for flare activity.
Title: Evaporation Causes Flare Related Radio Burst Continuum
Depressions
Authors: Aurass, H.; Hofmann, A.; Magun, A.; Soru-Escaut, I.;
Zlobec, P.
Bibcode: 1993SoPh..145..151A
Altcode:
We study the active region NOAA 6718 and the development of a (2N,
M3.6) flare in radio and Hα. Due to our knowledge of the magnetic
field structure in the active region we are able to associate the
different radio flare burst components with the stages in the Hα
flare evolution. A discussion of the data in terms of chromospheric
flare kernel heating reveals that in the present case the observed
flare-related radio burst continuum switch-off is caused by the
penetration of hot, ablated gas into the coronal radio source.
Title: Book Review: Solar observations: techniques and interpretation
/ Cambridge U Press, 1991
Authors: Sanches, F.; Hofmann, A.
Bibcode: 1993Ap&SS.201..151S
Altcode:
No abstract at ADS
Title: High Resolution Magnetic Field Measurements in the Sunspot
Photosphere (Invited)
Authors: Hofmann, A.; Schmidt, W.; Balthasar, H.; Tarbell, T. T.;
Frank, Z. A.
Bibcode: 1993ASPC...46...11H
Altcode: 1993IAUCo.141...11H; 1993mvfs.conf...11H
No abstract at ADS
Title: Polarimetry and spectroscopy of a simple sunspot. I - On the
magnetic field of a sunspot penumbra
Authors: Schmidt, W.; Hofmann, A.; Balthasar, H.; Tarbell, T. D.;
Frank, Z. A.
Bibcode: 1992A&A...264L..27S
Altcode:
We investigate the magnetic field structure of a medium sized sunspot
using high resolution magnetograms and spectrograms and derive a
relationship between the brightness of penumbral structures and the
inclination of the magnetic field. The field inclination to the spot
normal is larger in the dark structures than in the bright ones. We
show that the field strength does not vary between dark and bright
structures. At the inner penumbral boundary the field strength is 2000
Gauss and about 1000 Gauss at the outer penumbral edge. The line-of
sight component of the material flow decreases rapidly within one
arcsecond at the photospheric boundary of the spot.
Title: A Study on Electric Currents in a Solar Active Region -
a Dynamo Process at a Place of Repeated Flaring
Authors: Hofmann, A.; Ruzdjak, V.; Vrsnak, B.
Bibcode: 1992HvaOB..16...29H
Altcode:
No abstract at ADS
Title: On the magnetic field of a sunspot penumbra.
Authors: Balthasar, H.; Schmidt, W.; Hofmann, A.; Tarbell, T. D.;
Frank, Z. A.
Bibcode: 1992AGAb....7..153B
Altcode:
No abstract at ADS
Title: The Potsdam vector magnetograph: methodical experiences
and results.
Authors: Staude, J.; Hofmann, A.; Bachmann, G.
Bibcode: 1991sopo.work...49S
Altcode:
After a short historical introduction the instrumentation of the double
line vector magnetograph is described as it is working in connection
with the tower telescope of the Solar Observatory "Einsteinturm"
at Potsdam. Subsequently the authors outline the calibration of the
instrumental response to incident polarised light. The theoretical
calibration of deriving the magnetic field components from the
observed Stokes profiles is the next step. Finally the authors give a
few examples of vector magnetograms and outline some of their methods
for deriving further information from the magnetograms.
Title: Electric currents and Lorentz forces derived by vector
magnetographic measurements. I - Electric currents in a flux bundle
Authors: Hofmann, A.
Bibcode: 1991AN....312...49H
Altcode:
The distribution of vertical electric current densities derived from
nearly simultaneous measurements of the photospheric vector magnetic
field made by the magnetographs of the Sayan Observatory (Irkutsk)
and the Potsdam Solar Observatory is investigated. A flux bundle left
the spot investigated nearly horizontally and was isolated enough
to enable the discovery of two current density contours dominated by
azimuthal currents flowing around the bundle. The contours were situated
nearly symmetrically on both sides of the strongly inclined axis of the
bundle. By means of three assumptions concerning the symmetry of the
bundle, a way is given of separating from each other the field-aligned
and the azimuthal components of the currents.
Title: Electric currents and free energy in a flaring twisted field
configuration (NOAA 4263)
Authors: Hofmann, A.; Kalman, B.
Bibcode: 1991A&A...241..203H
Altcode:
In an active region producing repeated flares the required free
energy is accumulated by a strong twist of the field lines due to the
rotation of a sunspot containing one end of the magnetic fluxtube. Using
vector-magnetographic measurements the current flowing in the twisted
field system and the free magnetic energy stored by this current are
derived. This energy is sufficient to produce the observed flares. Two
models are used to describe the process of energy build-up and to
calculate the currents and energies. The model of a thick twisted
fluxtube produces the required current and energy, provided that a
continuous rotation of the sunspot existed for about 5 days; this is
consistent with the observations. The application of the photospheric
dynamo model of Henoux and Somov shows that this process is much more
effective to generate currents of the observed strength, however the
direction of the current is opposite in this model to the observation.
Title: Current Density Effects and the Importance of Charge Transfer
Processes in the Radiolysis of Methane: Astrophysical Implications
Authors: Sack, N. J.; Schuster, R.; Hofmann, A.; Khedim, A.;
Koppmann, R.
Bibcode: 1990ApJ...360..305S
Altcode:
In order to get information about the influence of irradiation
parameters and the addition of inert gases in radiolysis processes
of astrophysical interest, methane gas targets were irradiated with
6.5 MeV protons at a pressure of 1 bar and room temperature. The
yields for higher hydrocarbons like ethane or propane were obtained
by gas chromatography analysis of irradiated gas samples. A series
of experiments was performed adding inert gases like helium, argon,
or nitrogen in various quantities to the methane gas. Charge transfer
reactions were found to play an important role in the reactions
induced by the irradiation. Current density and total absorbed dose
were also proved to have large influence on the yields of produced
hydrocarbons. Estimations of electron, ion, and radical densities
are made, leading to an understanding of the reaction mechanism and
comparisons to other rate coefficients are given. The relevance of
the investigations for the chemistry and the simulation of various
astrophysical systems is given.
Title: Proton irradiation of simple gas mixtures: influence of
irradiation parameters.
Authors: Sack, Norbert J.; Schuster, R.; Hofmann, A.
Bibcode: 1990NASCP3077..411S
Altcode: 1990lrpa.conf..411S
In order to get information about the influence of irradiation
parameters on radiolysis processes of astrophysical interest, methane
gas targets were irradiated with 6.5 MeV protons at a pressure of 1
bar and room temperature.
Title: Vector Magnetic Field and Currents at the Footpoint of a
Loop Prominence
Authors: Hofmann, A.; Ruždjak, V.; Vršnak, B.
Bibcode: 1990LNP...363..233H
Altcode: 1990IAUCo.117..233H; 1990doqp.coll..233H
Using H -filtergrams and vector magnetograms we study the structure of
the magnetic field at the footpoint of a loop prominence rooting deep
in the penumbral photosphere of a sunspot. In the region investigated
the footpoint -field is well marked in the transversal field map. The
field has a predominantly transverse character and is directed parallel
to the axis of the prominence. The flux bundle forming the prominence
left the photosphere by an angle of about 26°, i.e. close to the
horizontal. In the maps of current densities inferred from the vector
magnetic field we find a pair of up- and downflowing currents, being
situated symmetrically to the axis of the prominence. This indicates on
a current ( 3,8 · 1011 A) flowing round the flux bundle and generating
the Lorentz forces causing the concentration of flux at the footpoint
region. The vertical gradients of the longitudinal field hint on an
increase of the field strength with height, i.e. toward the axis of
the prominence.
Title: Currents and energy build-up in a flaring curled field
configuration.
Authors: Hofmann, A.; Kálmán, B.
Bibcode: 1990PDHO....7..112H
Altcode: 1990dysu.conf..112H
One aim of observation of flaring activity complexes is to determine
the field configurations in which flares occur, how these configurations
form and how the energy is stored in them. The authors analysed vector
magnetic field observations of an active region.
Title: Analysis and results of cooperative magnetographic
measurements. III - Vertical gradients of the magnetic field in the
sunspot photosphere
Authors: Hofmann, A.; Rendtel, J.
Bibcode: 1989AN....310...61H
Altcode:
Vertical gradients of sunspot magnetic fields which were derived
for the first time from nearly simultaneous vector magnetograms are
compared. The measurements were obtained in the photospheric lines
Fe I 5250 and Fe I 5253. The gradients derived from the different
measurements show a good correspondence with respect to the numerical
values as well as to the morphological structure of the distributions
in the magnetograph's field of view. In the umbral region of the spot
the vertical gradients amount to about 0.32/Gkm, indicating that the
umbral field can extend with strong field strengths into chromosphere
and corona. A circle-like area with opposite sign to the umbral
gradient was found on the boundary between penumbra and surrounding
photosphere. The physical significance of this area should be the goal
of further observations and interpretations.
Title: Vector Magnetic Field and Currents at the Footpoint of a
Loop Prominence
Authors: Hofmann, A.; Ruzdjak, V.; Vrsnak, B.
Bibcode: 1989HvaOB..13...11H
Altcode:
No abstract at ADS
Title: Cooperative magnetographic and radioastronomical investigations
of the active solar region AR 3804 in July 1982.
Authors: Akhmedov, Sh. B.; Borovik, V. N.; Gel'Frejkh, G. B.; Bogod,
V. M.; Korzhavin, A. N.; Petrov, Z. E.; Hofmann, A.; Bachmann, G.
Bibcode: 1989AISAO..28..111A
Altcode:
An analysis of the radiation peculiar source in the active solar
region AR 3804 is presented using the magnetograms obtained at ZIAP
AW DDR in Potsdam. The source is on the border line of polarities of
the photospheric magnetic field above the filament and has the flux
spectrum increasing with the wavelength, the brightness temperature
at 4 cm being above 6×106K, and the logarithm spectrum
slope in the shortwave region (λ = 2 cm) being about n = 10. The
polarization degree within 2 - 4 cm is close to 30%. An analysis of
magnetograms of the total field vector showed that in the source region
there is observed a density increase of the longitudinal current in
the photosphere, this increase having different signs on different
sides from the zero magnetic line. Near the source there is observed a
magnetic field hill of the reverse polarity relative to the background
field. It is suggested to relate the source with the magnetic loop of
the "twisted" field in the corona in which the local energy emission
and corona heating due to dissipation of magnetic fields of complex
configuration occur (apparently, with the current sheet formation).
Title: Possibilities and Problems of the Interpretation of Solar
Magnetograph Measurements and Applications to Flare-Active Regions
Authors: Staude, J.; Hofmann, A.
Bibcode: 1989ESASP.285..123S
Altcode: 1989rsp..conf..123S
The basic uncertainties encountered in the interpretation of
magnetograph data, and possibilities for deriving more reliable
information are reviewed. The problems are illustrated by describing
the data handling of a vector magnetograph. Examples of observed
flare-active regions demonstrate the state of information which can
be obtained.
Title: Production of amines by proton bombardment of simple gas
mixtures
Authors: Sack, N.; Schuster, R.; Hofmann, A.; Schneider, H. -J.
Bibcode: 1988Icar...76..110S
Altcode:
The production of amines, especially amino acids, has been studied for
different gas mixtures bombarded by protons with incident energies
between 5 and 11 MeV and current densities between 3 and 9 μA/cm
2. We found a strong increase in yield, particular of amino
acids, with increasing current density, whereas no dependence on the
proton incident energy was observed. The production probability of
amines with different functional groups is very sensitive to qualitative
and quantitative variations of the primary gas mixture.
Title: Analysis and results of cooperative magnetographic
measurements. I - Correction, comparison and discussion of
measurements
Authors: Hofmann, A.; Grigor'ev, V. M.; Selivanov, V. L.; Klvana, M.
Bibcode: 1988AN....309..331H
Altcode:
On June 24, 1983 cooperative magnetographic measurements were made
with the vectormagnetographs of the Sayan Observatory (Irkutsk) and
the Potsdam Solar Observatory "Einsteinturm" and with the longitudinal
magnetograph of the Ondřejov Observatory. Additionally, the maximum
field strengths in the sunspot were measured with the photographic
method. A comparison of nearly simultaneous magnetograms shows a good
correspondence in general. Regarding the magnetic field distribution
the magnetograms reflect a substantial nonsymmetric structure in the
spot under study. The magnetic field lines tend to concentrate into
several flux tube clusters.
Title: Analysis and results of cooperative magnetographic
measurements. II - Comparison and interpretation of the current
densities
Authors: Hofmann, A.; Grigor'ev, V. M.; Selivanov, V. L.
Bibcode: 1988AN....309..373H
Altcode:
The authors compare and interpret distributions of vertical electric
current densities derived from nearly simultaneous vector measurements
by the magnetographs of the Sayan Observatory (Irkutsk) and the Potsdam
Solar Observatory. The derived distributions of current densities
of the different observations are in good agreement referring to the
scaling factors and the morphological structure. In a spot under study
the authors found several current filaments indicating that the spot
is clustered by several flux bundles. An azimuthal current between
(5.9±1.4)×1011A and (6.8±1.5)×1011A flowing
around a (relatively isolated) flux bundle can be derived from the
observations.
Title: Flare and Filament Activation in an Unusually Distorted
Field Configuration
Authors: Hofmann, A.; Rendtel, J.; Aurass, H.; Kalman, B.
Bibcode: 1987SoPh..108..151H
Altcode:
Using photospheric and Hα observations and total radio flux data we
study a two-ribbon flare in AR NOAA 4263 which was a part of a flare
event complex on July 31, 1983. We find some facts which illuminate
the special way of flare triggering in the analysed event. Around
a double spot the photospheric vector magnetic field is discussed
with respect to the chromospheric activities. In one of the spots
the feet of long stretched loops are pushed down under steepening
loops rooted in the same spot. This causes energy build-up by twist
and shear in the stretched loops. One foot of the two-ribbon flare
(triggered in the stretched and underpushed loop system) roots in a
part of the spot umbra and penumbra where the field runs in extremely
flat `like a pressed spiral spring'. A strange radio event, starting
before the flares, can be interpreted as a precursor activity of the
flare event complex. The radio data support the view that the analyzed
flare process and the given magnetic field structure, respectively,
are not very effective in energetic particle generation and escape.
Title: Search for dark matter and neutrino sources with the Frejus
detector.
Authors: Kuznik, B.; Arpesella, C.; Bareyre, P.; Barloutaud, R.;
Becker, J.; Becker, K. H.; Benadjal, Y.; Berger, C.; Blum, D.;
Borg, A.; Bourdarios, C.; Chardin, G.; Daum, H. J.; Degrange, B.;
Deuzet, G.; Dudelzak, B.; Ernwein, J.; Eschstruth, P.; Gerbier, G.;
Glicenstein, J. F.; Hofmann, A.; Jullian, S.; Kolton, W.; Lalanne,
D.; Laplanche, F.; Longuemare, C.; Minet, Y.; Mayer, R.; Meyer, H.;
Mosca, L.; Moscoso, L.; Möller, R.; Nguyen-Khac, U.; Paulot, C.;
Raupach, F.; Schleper, P.; Schubnell, M.; Serri, P.; Szklarz, G.;
Tisserant, S.; Tripp, B.; Wei, Y.; Wintgen, P.
Bibcode: 1987nep..conf..215K
Altcode:
A high energy neutrino flux from the sun is expected if the sun contains
significant amounts of dark matter in the form of weak interacting
massive particles, which may be gravitationally bound and annihilate
in the solar core. High energy gamma ray sources could be also sources
for high energy neutrinos due to the production in hadronic cascades. A
search has been done for high energy neutrinos coming from the sun,
Cyg X-3, the galactic center and different other galactic objects.
Title: Electric current density in the sunspot photosphere derived
from vector magnetograms.
Authors: Hofmann, A.; Staude, J.
Bibcode: 1987PAICz..66..105H
Altcode: 1987eram....1..105H
The authors present observational results from which they determine
azimuthal currents flowing across the magnetic field.
Title: Search for muons from the direction of cygnus X-3
Authors: Berger, Ch.; Hofmann, A.; Raupach, F.; Schleper, P.; Schmitz,
G.; Tutas, J.; Voigtländer, B.; Arpesella, C.; Benadjal, Y.; Deuzet,
G.; Dudelzak, B.; Eschtruth, P.; Jullian, S.; Lalanne, D.; Laplanche,
F.; Longuemare, C.; Paulot, C.; Roy, Ph.; Szklarz, G.; Behr, L.; Bland,
R. W.; Degrange, B.; Nguyen-Khac, U.; Serri, P.; Tisserant, S.; Tripp,
R.; Bareyre, P.; Barloutaud, R.; Chardin, G.; di Ciaccio, L.; Edmunds,
D. L.; Ernwein, J.; Gerbier, G.; Jabiol, M. A.; Kolton, W.; Mosca,
L.; Moscoso, L.; Pietrzyk, B.; Becker, K. H.; Daum, H. J.; Demski,
S.; Hinners, R.; Kohrs, W.; Kuznik, B.; Mayer, R.; Meyer, H.; Ortmann,
D.; Peters, J.; Schubnell, M.; Thierjung, J.; Wei, Y.; Wintgen, P.
Bibcode: 1986PhLB..174..118B
Altcode:
Muons and multimuons detected in the Fréjus underground nucleon decay
detector between February 1984 and January 1986 have been analyzed. No
excess events are observed in the direction of Cygnus X-3, which yields
a 90% confidence level upper flux limit of 0.8 × 10-12
cm-2 s-1, for an average rock overburden of 5000
hg cm-2 corresponding to energies >~ 3 TeV. Using the 4.79
h periodicity of Cygnus X-3, no signal is found in any phase interval.
Title: Status of the Frejus experiment and preliminary results on
contained events.
Authors: Bareyre, P.; Barloutaud, R.; Behr, L.; Berger, C.; Bland,
R.; Chardin, G.; Daum, H.; Degrange, B.; Demski, S.; Deuzet, G.;
di Ciaccio, L.; Dudelzak, B.; Edmunds, D.; Ernwein, J.; Eschstruth,
P.; Gerbier, G.; Hinners, R.; Hofmann, A.; Jullian, S.; Kohrs, W.;
Kolton, W.; Kuznick, B.; Lalanne, D.; Laplanche, F.; Longuemarre,
C.; Mayer, R.; Meyer, H.; Mosca, L.; Moscoso, L.; Nguyen-Khac, U.;
Ortmann, D.; Paulot, C.; Peters, J.; Pietrzyk, B.; Raupach, F.; Roy,
P.; Schmitz, G.; Schubnell, M.; Serri, P.; Szklarz, G.; Thierjung,
J.; Tisserant, S.; Tutas, J.; Voigtländer, B.
Bibcode: 1986NCimC...9..159B
Altcode:
The 114-module 912-ton fine-grain tracking-calorimeter facility
installed beneath 1600 m of rock in the Frejus tunnel is described and
illustrated with diagrams and photographs, and preliminary results
are reported. Each module comprises a 6 x 6-m vertical Geiger plane
and eight flash chambers (934,000 5 x 5-mm polypropylene flash tubes
filled with Ne-He and triggered by 40,000 15 x 15-mm-section Al Geiger
tubes filled with Ar-ethanol) separated by 3-mm iron plates; a charged
particle can be located to within 2mm by the detector. From reduction
of data on 30 events recorded since June 1984 and considered to be
within the detector it is inferred that none can be interpreted as
nucleon decay into charged leptons or mesons.
Title: On a strange recurring type I burst pattern.
Authors: Aurass, H.; Kurths, J.; Mann, G.; Hofmann, A.
Bibcode: 1986SoPh..107..123A
Altcode: 1987SoPh..107..123A
Two remarkable intensity-time patterns in the 113 and 64 MHz
single-frequency radio flux records during a type I noise storm and/or
a type IV burst on 31 July, 1983 are studied. A comparison of the
patterns at both frequencies reveals a high degree of resemblance and
inherent common structure although the 64 MHz pattern was seen 40 min
later than the 113 MHz pattern. An interpretation is given assuming
a slowly uprising and thereby expanding clumpy plasma-magnetic field
configuration which is (via accompanying coronal loops) two times
illuminated by energetic electrons coming from the soft X-ray flare
precursor source region of the H-alpha flares F1 and F2 (see Figure 1).
Title: Flare occurence in the complex of activity NOAA/USAF No. 4201,
May 29 - June 12, 1983
Authors: Ruždjak, V.; Vršnak, B.; Hofmann, A.; Křivský, L.;
Markova, E.; Kálmán, B.
Bibcode: 1986CoSka..15..257R
Altcode:
The evolution of a large complex of activity NOAA/USAF No. 4201 in the
period May 29 to June 12, 1983 was investigated. Almost 200 flares
occurring in the complex were reported, most of which clustered at
preferred sites where delta configurations with shear and strong
gradients of the magnetic fields were present or new flux emerged.
Title: Vertical components of magnetic field gradient and current
density in the active region BBR 18474 in July 1982
Authors: Bachmann, G.; Hofmann, A.
Bibcode: 1986CoSka..15..659B
Altcode:
Four vector magnetograms of 15 and 16 July are used to derive
the vertical components of magnetic field gradient and current
density. The two parameters attained maxima of respective 0.30 G/km an
19×10-3 A/m2 within the central delta area. At
the same side of the neutral line current density maxima with opposite
sign are observed close together. In the case of some short compact
filaments agreement of their foot points with a positive and a negative
current density maximum was found, pointing on currents flowing along
the filament.
Title: Preliminary results on atmospheric neutrinos and Cygnus X-3
in the Fréjus detector.
Authors: Bareyre, P.; Barloutaud, R.; Behr, L.; Berger, C.; Bland,
R.; Chardin, G.; Daum, H.; Degrange, B.; Demski, S.; Deuzet, G.;
di Ciaccio, L.; Dudelzak, B.; Edmunds, D.; Ernwein, J.; Eschtruth,
P.; Gerbier, G.; Hinners, R.; Hofmann, A.; Jabiol, M. A.; Jullian,
S.; Kohrs, W.; Kolton, W.; Kuznick, B.; Lalanne, D.; Laplanche,
F.; Longuemare, C.; Mayer, R.; Meyer, H.; Mosca, L.; Moscoso, L.;
Nguyen-Khac, U.; Ortmann, D.; Pietryscz, B.; Paulot, C.; Peters, J.;
Raupach, F.; Roy, P.; Schmitz, G.; Schubnell, M.; Serri, P.; Szklarz,
G.; Thierjung, J.; Tisserant, S.; Tutas, J.; Voigtlander, B.
Bibcode: 1986npdu.conf..146B
Altcode:
The Fréjus nucleon decay detector is a 900 ton fine grain calorimeter
located in the Fréjus road tunnel in the Alps at an average depth of
4800 mwe. The status of the experiment about neutrino interactions and
cosmic ray events is presented, with a special emphasis on Cygnus X-3.
Title: Some aspects of the analysis of vector magnetograms and
application to collaborated measurements
Authors: Hofmann, A.
Bibcode: 1986CoSka..15..469H
Altcode:
Simultaneous vector magnetograms were obtained with the magnetographs of
the Potsdam Solar Observatory "Einsteinturm" and the Sayan Observatory
(Irkutsk). For the study the leading spot is selected in the active
region SD 164 (NOAA 4216) on June 24, 1983. A comparison of the
magnetograms shows a good correpondence in the field strengths as well
as in the field azimuths. The mean vertical gradients in the umbra
were found to be about 0.32 G km-1. The disribution of the
vertical gradients reflects a return flux topology of the magnetic
field around the spot.
Title: Results of vector magnetographic measurements in the active
region SD 228/229 on 15 July 1982.
Authors: Bachmann, G.; Hofmann, A.; Staude, J.
Bibcode: 1983PDHO....5..369B
Altcode: 1984PDHO....5..369B
The authors present measurements of vector magnetic fields in an active
region of 15 July 1982 obtained with the new code-impulse magnetograph
working in the wing of Fe I 5250 line. Linear polarization down to
0.004 corresponding to transverse magnetic fields of about 150 G has
been taken into account. The direction of the transverse magnetic
fields is compared with dark chromospheric structure elements in Hα
filtergrams and with the contours of sunspots obtained by means of
a heliogram. For long-lived stable structures, such as the central
part of the zero-line filament or systems of fibrils and threads,
the correspondence is good, in contrast to short-lived. In regions
with weak magnetic fields no correspondence is found.
Title: Rb-Sr mantle isochrons from oceanic regions
Authors: Brooks, C.; Hart, S. R.; Hofmann, A.; James, D. E.
Bibcode: 1976E&PSL..32...51B
Altcode:
Existing data for 87Sr/ 86Sr and Rb/Sr ratios
of basalts from oceanic islands and mid-ocean spreading ridges
show significant positive correlations on a Rb-Sr isochron diagram
(when data are averaged by island group). Furthermore, tholeiites
and alkali basalts occupy distinct non-overlapping fields on this
plot. The tholeiite correlation is interpreted as a mantle isochron,
and the agreement of this age (1.6 ± 0.2 b.y.) with that reported
for Pb-Pb isochrons from oceanic basalts lends strong support to the
use of such isochrons for tracing mantle evolution. Oceanic basalts
are apparently sampling a mantle in which chemical heterogeneities
have persisted for at least 1.5-2.0 b.y. The data support a kinematic
model for the mantle in which a relatively uniform and non-radiogenic
asthenosphere is penetrated by, and mixed with, blobs or plumes derived
from an isolated (1.5-2 b.y.) and chemically heterogeneous mesosphere.
Title: Diffusion behaviour of low-energy helium ions implanted
in aluminum
Authors: Eberhardt, P.; Hofmann, A.
Bibcode: 1974E&PSL..23..304E
Altcode:
Aluminum foils were bombarded at room temperature with 4He
+ ions in the energy range between 1 and 5 keV. The
irradiation dose varied from 10 12 cm -2 to 10
14cm -2, well below saturation levels. The release
pattern was observed in a stepwise heating experiment. The diffusion
behaviour is strongly dose dependent, higher doses resulting in a shift
of the release pattern to higher temperatures. For a constant dose a
well-defined energy dependence of the gas release exists, which can
be used to deduce the ion energy.
Title: Spektroskopie der Protonen aus der Reaktion Ni( n, p)
Co bei einer Neutronenenergie von 14 MeV mit einem Teleskop aus
Halbleiterdetektoren
Authors: Flohrs, P.; Hofmann, A.
Bibcode: 1966ZPhy..192..264F
Altcode:
Protons from nuclear reactions induced by 14 MeV neutrons are analysed
with solid state detectors. A high background is caused by nuclear
reactions in silicon. The application of a telescope with two dE/dx-
detectors allows to suppress this background extensively. Proton
identification is made by use of a two-dimensional analyser. The
telescope is tested by recoil protons with polyethylene. This
arrangement may also be used as a neutron spectrometer. The proton
spectra from the reaction Ni( n, p)Co in forward direction (0°) and
on an angle of 90° are compared. Nuclear temperature and level-density
parameter are evaluated from statistical theory plots. The contribution
of direct processes is estimated.
Title: «Selenologische Karte des Mondes» (1940)
Authors: Hofmann, A.
Bibcode: 1941C&T....57..420H
Altcode:
No abstract at ADS