Author name code: brajsa
ADS astronomy entries on 2022-09-14
author:"Brajsa, Roman"
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Title: The quiet Sun at mm Wavelengths as Seen by ALMA
Authors: Alissandrakis, Costas; Bastian, Timothy; Brajša, Roman
Bibcode: 2022arXiv220902569A
Altcode:
Solar observations at sub-mm, mm and cm wavelengths offer a
straightforward diagnostic of physical conditions in the solar
atmosphere because they yield measurement of brightness temperature
which, for optically thick features, equals intrinsic temperature -
much unlike solar diagnostics in other spectral ranges. The Atacama
Large Millimeter and sub-millimeter Array (ALMA) has therefore opened a
new, hitherto underexplored, spectral window for studying the enigmatic
solar chromosphere. In this review we discuss initial ALMA studies of
the quiet chromosphere that used both single-dish and compact-array
interferometric observing modes. We present results on the temperature
structure of the chromosphere, comparison with classic empirical models
of the chromosphere, and observations of the chromospheric network and
spicules. Furthermore, we discuss what may be expected in the future,
since the ALMA capabilities continuously expand and improve towards
higher angular resolution, wavelength coverage, and polarization
measurement for magnetometry.
Title: Variation in solar differential rotation and activity in the
period 1964-2016 determined by the Kanzelhöhe data set
Authors: Poljančić Beljan, I.; Jurdana-Šepić, R.; Jurkić, T.;
Brajša, R.; Skokić, I.; Sudar, D.; Ruždjak, D.; Hržina, D.;
Pötzi, W.; Hanslmeier, A.; Veronig, A. M.
Bibcode: 2022A&A...663A..24P
Altcode: 2022arXiv220407396P
Aims: Theoretical calculations predict an increased equatorial
rotation and more pronounced differential rotation (DR) during
the minimum of solar magnetic activity. However, the results of
observational studies vary, some showing less and some more pronounced
DR during the minimum of solar magnetic activity. Our study aims to gain
more insight into these discrepancies.
Methods: We determined
the DR parameters A and B (corresponding to the equatorial rotation
velocity and the gradient of the solar DR, respectively) by tracing
sunspot groups in sunspot drawings of the Kanzelhöhe Observatory
for Solar and Environmental Research (KSO; 1964-2008, for solar
cycles 20-23) and KSO white-light images (2009-2016, for solar cycle
24). We used different statistical methods and approaches to analyse
variations in DR parameters related to the cycle and to the phase of
the solar cycle, together with long-term related variations.
Results: The comparison of the DR parameters for individual cycles
obtained from the KSO and from other sources yield statistically
insignificant differences for the years after 1980, meaning that the
KSO sunspot group data set is well suited for long-term cycle to cycle
studies. The DR parameters A and B show statistically significant
periodic variability. The periodicity corresponds to the solar cycle
and is correlated with the solar activity. The changes in A related
to solar cycle phase are in accordance with previously reported
theoretical and experimental results (higher A during solar minimum,
lower A during the maximum of activity), while changes in B differ
from the theoretical predictions as we observe more negative values
of B, that is, a more pronounced differential rotation during activity
maximum. The main result of this paper for the long-term variations in
A is the detection of a phase shift between the activity flip (in the
1970s) and the equatorial rotation velocity flip (in the early 1990s),
during which both A and activity show a secular decreasing trend. This
indicates that the two quantities are correlated in between 1970 and
1990. Therefore, the theoretical model fails in the phase-shift time
period that occurs after the modern Gleissberg maximum, while in the
time period thereafter (after the 1990s), theoretical and experimental
results are consistent. The long-term variations in B in general yield
an anticorrelation of B and activity, as a rise of B is observed during
the entire time period (1964-2016) we analysed, during which activity
decreased, with the exception of the end of solar cycle 22 and the
beginning of solar cycle 23.
Conclusions: We study for the first
time the variation in solar DR and activity based on 53 years of KSO
data. Our results agree well with the results related to the solar cycle
phase from corona observations. The disagreement of the observational
results for B and theoretical studies may be due to the fact that we
analysed the period immediately after the modern Gleissberg maximum,
where for the phase-shift period, A versus activity also entails a
result that differs from theoretical predictions. Therefore, studies
of rotation versus activity with data sets encompassing the Gleissberg
extremes should include separate analyses of the parts of the data set
in between different flips (e.g., before the activity flip, between
the activity and the rotation flip, and after the rotation flip).
Title: A Genetic Algorithm to Model Solar Radio Active Regions From
3D Magnetic Field Extrapolations
Authors: de Oliveira e Silva, Alexandre José; Selhorst, Caius Lucius;
Costa, Joaquim E. R.; Simões, Paulo J. A.; Giménez de Castro,
Carlos Guillermo; Wedemeyer, Sven; White, Stephen M.; Brajša, Roman;
Valio, Adriana
Bibcode: 2022FrASS...9.1118D
Altcode: 2022arXiv220503385S; 2022arXiv220503385J
In recent decades our understanding of solar active regions (ARs)
has improved substantially due to observations made with better
angular resolution and wider spectral coverage. While prior AR
observations have shown that these structures were always brighter
than the quiet Sun at centimeter wavelengths, recent observations
at millimeter and submillimeter wavelengths have shown ARs with well
defined dark umbrae. Given this new information, it is now necessary to
update our understanding and models of the solar atmosphere in active
regions. In this work, we present a data-constrained model of the AR
solar atmosphere, in which we use brightness temperature measurements
of NOAA 12470 at three radio frequencies: 17, 100 and 230 GHz. The
observations at 17 GHz were made by the Nobeyama Radioheliograph (NoRH),
while the observations at 100 and 230 GHz were obtained by the Atacama
Large Millimeter/submillimeter Array (ALMA). Based on our model, which
assumes that the radio emission originates from thermal free-free and
gyroresonance processes, we calculate radio brightness temperature
maps that can be compared with the observations. The magnetic field at
distinct atmospheric heights was determined in our modelling process by
force-free field extrapolation using photospheric magnetograms taken by
the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO). In order to determine the best plasma temperature
and density height profiles necessary to match the observations,
the model uses a genetic algorithm that modifies a standard quiet Sun
atmospheric model. Our results show that the height of the transition
region (TR) of the modelled atmosphere varies with the type of region
being modelled: for umbrae the TR is located at 1080 ± 20 km above
the solar surface; for penumbrae, the TR is located at 1800 ± 50 km;
and for bright regions outside sunspots, the TR is located at 2000 ±
100 km. With these results, we find good agreement with the observed
AR brightness temperature maps. Our modelled AR can be used to estimate
the emission at frequencies without observational coverage.
Title: The Solar Activity Monitor Network - SAMNet
Authors: Erdélyi, Robertus; Korsós, Marianna B.; Huang, Xin; Yang,
Yong; Pizzey, Danielle; Wrathmall, Steven A.; Hughes, Ifan G.;
Dyer, Martin J.; Dhillon, Vikram S.; Belucz, Bernadett; Brajša,
Roman; Chatterjee, Piyali; Cheng, Xuewu; Deng, Yuanyong; Domínguez,
Santiago Vargas; Joya, Raúl; Gömöry, Peter; Gyenge, Norbert G.;
Hanslmeier, Arnold; Kucera, Ales; Kuridze, David; Li, Faquan; Liu,
Zhong; Xu, Long; Mathioudakis, Mihalis; Matthews, Sarah; McAteer,
James R. T.; Pevtsov, Alexei A.; Pötzi, Werner; Romano, Paolo; Shen,
Jinhua; Temesváry, János; Tlatov, Andrey G.; Triana, Charles; Utz,
Dominik; Veronig, Astrid M.; Wang, Yuming; Yan, Yihua; Zaqarashvili,
Teimuraz; Zuccarello, Francesca
Bibcode: 2022JSWSC..12....2E
Altcode:
The Solar Activity Magnetic Monitor (SAMM) Network (SAMNet) is a
future UK-led international network of ground-based solar telescope
stations. SAMNet, at its full capacity, will continuously monitor
the Sun's intensity, magnetic, and Doppler velocity fields at
multiple heights in the solar atmosphere (from photosphere to upper
chromosphere). Each SAMM sentinel will be equipped with a cluster of
identical telescopes each with a different magneto-optical filter (MOFs)
to take observations in K I, Na D, and Ca I spectral bands. A subset
of SAMM stations will have white-light coronagraphs and emission line
coronal spectropolarimeters. The objectives of SAMNet are to provide
observational data for space weather research and forecast. The goal
is to achieve an operationally sufficient lead time of e.g., flare
warning of 2-8 h and provide many sought-after continuous synoptic
maps (e.g., LoS magnetic and velocity fields, intensity) of the lower
solar atmosphere with a spatial resolution limited only by seeing or
diffraction limit, and with a cadence of 10 min. The individual SAMM
sentinels will be connected to their master HQ hub where data received
from all the slave stations will be automatically processed and flare
warning issued up to 26 h in advance.
Title: A prediction for the 25th solar cycle maximum amplitude
Authors: Brajša, Roman; Verbanac, Giuli; Bandić, Mario; Hanslmeier,
Arnold; Skokić, Ivica; Sudar, Davor
Bibcode: 2022AN....34313960B
Altcode: 2022arXiv220311293B
The minimum-maximum method, belonging to the precursor class of the
solar activity forecasting methods, is based on a linear relationship
between relative sunspot number in the minimum and maximum epochs of
solar cycles. In the present analysis, we apply a modified version of
this method using data not only from the minimum year, but also from
a couple of years before and after the minimum. The revised 13-month
smoothed monthly total sunspot number data set from Sunspot Index and
Long-term Solar Observations/SIDC is used. Using data for solar cycle
nos. 1-24 the largest correlation coefficient (CC) is obtained when
correlating activity level 3 years before solar cycle minimum with the
subsequent maximum (CC=0.82), independent of inclusion or exclusion
of the solar cycle no. 19. For the next solar maximum of the cycle
no. 25, we predict: Rmax=121±33. Our results indicate that the next
solar maximum (of the cycle no. 25) will be of the similar amplitude
as the previous one, or even something lower. This is in accordance
with the general middle-term lowering of the solar activity after the
secular maximum in the 20th century and consistent with the Gleissberg
period of the solar activity. The reliability of the "3 years before
the minimum" predictor is experimentally justified by the largest CC
and verified with the Student t-test. It is satisfactorily explained
with the two empirical well-known findings: the extended solar cycle
and the Waldmeier effect. Finally, we successfully reproduced the
maxima of the last four solar cycles, nos. 21-25, using the 3 years
before the minimum method.
Title: On the Determination of the Solar Rotation Elements i, {\Omega}
and Period using Sunspot Observations by Ruđer Bošković in 1777
Authors: Husak, Mirko; Brajša, Roman; Špoljarić, Dragan
Bibcode: 2022arXiv220302289H
Altcode:
In September 1777, Ruđer Bošković observed sunspots for
six days. Based on these measurements, he used his own methods to
calculate the elements of the solar rotation, the longitude of the
node, the inclination of the solar equator and the period. He published
a description of the methods, the method of observation and detailed
instructions for calculations in the second chapter of the fifth part of
the Opera in 1785. In this paper, Bošković original calculations and
repeated calculations by his procedure are published. By analysing the
input quantities, procedures, and results, the input quantities of the
error, and the calculation results are discussed. The reproduction of
Bošković calculations is successfully reproduced and we obtained very
similar results. The conclusion proposes a relationship of Bošković
research with modern astronomy.
Title: Solar rotation elements $i, \Omega$ and period determined
using sunspot observations by Ruđer Bošković in 1777
Authors: Husak, M.; Brajša, R.; ŠPoljarić, D.
Bibcode: 2022arXiv220311745H
Altcode:
Ruđer Bošković developed methods for determination of solar
rotation elements: the solar equator inclination i, the longitude
of the node {\Omega} and the period of solar rotation. In his last
work Opera pertinentia ad opticam et astronomiam, published in 1785,
in the chapter Opuscule II he described his methods, the formulae
with figure descriptions and an example for calculation of the solar
rotation elements with detailed numerical explanation using his own
observations performed in September 1777. The original numerical
procedure was performed using logarithmic formulae. In present work
we give a description of the original results of R. Bošković and
compare them with our recalculated values.
Title: Determination of the Solar Rotation Elements and Period from
Ruđer Bo\vsković's Sunspot Observations in 1777
Authors: Roša, D.; Hržina, D.; Husak, M.; Brajša, R.; Špoljarić,
D.; Skokić, I.; Maričić, D.; Šterc, F.; Romštajn, I.
Bibcode: 2021simi.conf...86R
Altcode:
This paper focuses on the observations of sunspots made by Ruđer
Bo\vsković in 1777. We derived the expressions needed to calculate
the elements of the Sun's rotation and period from observations. We
used modern ephemeris data in the processing of the observation
results. Obtained results are very similar to Bo\vsković's original
calculations. In addition to historical significance, they also provide
scientifically valuable data on the Sun's differential rotation,
which plays a significant role in generating and maintaining solar
magnetic activity.
Title: ALMA small-scale features in the quiet Sun and active regions
Authors: Brajša, R.; Skokić, I.; Sudar, D.; Benz, A. O.; Krucker,
S.; Ludwig, H. -G.; Saar, S. H.; Selhorst, C. L.
Bibcode: 2021A&A...651A...6B
Altcode: 2021arXiv210503644B
Aims: The main aim of the present analysis is to decipher (i)
the small-scale bright features in solar images of the quiet Sun and
active regions obtained with the Atacama Large Millimeter/submillimeter
Array (ALMA) and (ii) the ALMA correspondence of various known
chromospheric structures visible in the Hα images of the Sun.
Methods: Small-scale ALMA bright features in the quiet Sun region
were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz)
and in an active region using interferometric ALMA measurements (3
mm, 100 GHz). With the single-dish observations, a full-disk solar
image is produced, while interferometric measurements enable the
high-resolution reconstruction of part of the solar disk, including
the active region. The selected quiet Sun and active regions are
compared with the Hα (core and wing sum), EUV, and soft X-ray images
and with the magnetograms.
Results: In the quiet Sun region,
enhanced emission seen in the ALMA is almost always associated with a
strong line-of-sight magnetic field. Four coronal bright points were
identified, while other small-scale ALMA bright features are most likely
associated with magnetic network elements and plages. In the active
region, in 14 small-scale ALMA bright features randomly selected and
compared with other images, we found five good candidates for coronal
bright points, two for plages, and five for fibrils. Two unclear cases
remain: a fibril or a jet, and a coronal bright point or a plage. A
comparison of the Hα core image and the 3 mm ALMA image of the analyzed
active region showed that the sunspot appears dark in both images
(with a local ALMA radiation enhancement in sunspot umbra), the four
plage areas are bright in both images and dark small Hα filaments are
clearly recognized as dark structures of the same shape also in ALMA.
Title: Magnetic structure above solar active regions
Authors: Brajša, R.; Skokić, I.; Sudar, D.
Bibcode: 2020CEAB...44....1B
Altcode:
ALMA images of the whole solar disk (single-dish measurements, 248
GHz, $\lambda = 1.21$ mm) and of an active region (interferometric
measurements, 100 GHz, $\lambda$ = 3.0 mm) are compared with LOS
photospheric magnetograms and potential field source surface model of
extrapolated magnetic field in the solar chromosphere and corona. While
a general magnetic structure is consistent with ALMA observations,
a detailed analysis and comparison with ALMA small-scale features is
not possible with the model used and requires a more detailed magnetic
field extrapolation model, which is left for a future work.
Title: Correlation between the solar magnetic field strength and
the millimeter brightness temperature
Authors: Skokić, I.; Sudar, D.; Brajša, R.
Bibcode: 2020CEAB...44....2S
Altcode: 2022arXiv220311747S
Images of the Sun at millimeter wavelengths obtained by ALMA show
a significant correspondence with the magnetograms. In this paper,
we investigate this correspondence by comparing ALMA full-disk solar
image taken at 1.2 mm with a SDO/HMI magnetogram and analyze their
correlation. It is found that chromospheric network and active regions
show a positive correlation where brightness temperature is increasing
with the line-of-sight magnetic field strength, while sunspots have a
negative correlation. Quiet Sun regions do not show any dependence
of the brightness temperature with the magnetic field. Thermal
bremsstrahlung is given as the best explanation for the observed
correlations.
Title: Centre-to-Limb Brightness Variations from the Atacama Large
Millimeter-Submillimeter Array (ALMA) Full-Disk Solar Images
Authors: Sudar, Davor; Brajša, Roman; Skokić, Ivica; Benz, Arnold O.
Bibcode: 2019SoPh..294..163S
Altcode:
Science Verification (SV) data of solar observations with the Atacama
Large Millimeter-submillimeter Array (ALMA) telescope have been
released to the scientific community. Understanding the centre-to-limb
brightness function is necessary to compare features in full-disk
images. Our goals are to find the empirical centre-to-limb brightness
functions in two available spectral bands and create flattened
images with centre-to-limb brightness variations removed. We used a
second-order polynomial fit of the cosine of incidence angle to data
points as a function of radial distance to the centre of the solar
disk. The method also includes iterative removal of outliers based
on the inter-quartile range. Fitting functions for all available
images proved to describe the data adequately with comparatively
small errors in the fitting coefficients. In both bands we found
brightening towards the limb, which is a consequence of the increase
in electron temperature with radial distance in this region of the
solar atmosphere. This study found that the brightness temperature
[Tb] of an active region has about 180 K difference between
estimates with and without the limb brightening at radial distance
≈0.75 R⊙ in Band 6. We also made flattened images with
limb brightening removed. The limb-brightening effect in ALMA images
is significant enough (of the order of 10% for Band 3 and about 15%
in Band 6) that it cannot be neglected in further analyses. Since the
effect of the side lobes was not included in this study, these values
probably represent the lower limit of the limb brightening. The shape
of the limb-brightening function can also be used to constrain electron
densities and temperatures in various layers of the solar atmosphere.
Title: Centre to limb brightness variations from ALMA full disk
solar images
Authors: Sudar, Davor; Brajša, Roman; Skokić, Ivica; Benz, Arnold O.
Bibcode: 2019arXiv190908952S
Altcode:
Science Verification (SV) data of solar observations with Atacama Large
Millimeter-submillimeter Array (ALMA) telescope were released to the
scientific community. Understanding the centre to limb brightness
function is necessary to compare features in full disk images. Our
goals are to find the empirical centre to limb brightness functions
in two available spectral bands and create flattened images with
centre to limb brightness variations removed. We used second-order
polynomial fit of the cosine of incidence angle to data points as
a function of radial distance to the centre of the solar disk. The
method also includes iterative removal of outliers based on the
interquartile range. Fitting functions for all available images proved
to adequately describe the data with comparatively small errors in the
fitting coefficients. In both bands we found brightening towards the
limb which is a consequence of increase in electron temperatures with
radial distance in this region of the solar atmosphere. This study
found that the $T_{b}$ of an active region has about 180 K difference
between with and without the limb brightening at radial distance
$\approx$0.75$R_{\odot}$ in Band 6. We also made flattened images with
limb brightening removed. The limb brightening effect in ALMA images
is significant enough (of the order of 10% for Band 3 and about 15%
in Band 6) that it can not be neglected in further analyses. Since the
effect of the side lobes was not included in this study, these values
probably represent the lower limit of the limb brightening. The shape
of the limb brightening function can also be used to constrain electron
densities and temperatures in various layers of the solar atmosphere.
Title: Turbulent Diffusion Derived from the Motions of SDO/AIA
Coronal Bright Points
Authors: Skokić, I.; Brajša, R.; Sudar, D.; Ruždjak, D.; Saar, S. H.
Bibcode: 2019ApJ...877..142S
Altcode:
Diffusion of magnetic elements on the Sun has an important role
in current solar dynamo models as a part of the mechanism for
redistribution of the magnetic field and as an important part for
maintaining the solar activity cycle. The main goal is to determine the
character of solar magnetic diffusivity and a value of the diffusion
coefficient by analyzing the motions of coronal bright points (CBPs)
within the frame of the random walk model. We tracked positions of
CBPs in Solar Dynamics Observatory/Atmospheric Imaging Assembly images
for a period of 5 months and examined their displacement spectrum. We
calculated spectral index and diffusion coefficient from the spectrum
and investigated their variation with temporal and spatial scale. For
the first time, variations of the spectral index with heliographic
latitude and time were analyzed. Our results indicate subdiffusion with
the spectral index γ = 0.70 ± 0.01 and the corresponding diffusion
coefficient with a value decreasing from 400 to 100 km2
s-1 for temporal scales of 103-105
s and spatial scales of (1.5-7) × 103 km. Seemingly
random variations around the mean value of spectral index were found,
with peak-to-peak amplitudes <0.30 with time and <0.10 with
latitude. The main conclusion is that CBP motions are consistent with
a subdiffusion process.
Title: ALMA Solar Ephemeris Generator
Authors: Skokić, I.; Brajša, R.
Bibcode: 2019arXiv190408263S
Altcode:
An online software tool for the easy preparation of ephemerides of the
solar surface features is presented. It was developed as a helper tool
for the preparation of observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), but it can be used at other
observatories as well. The tool features an easy to use point-and-click
graphical user interface with the possibility to enter or adjust input
parameters, while the result is a table of predicted positions in the
celestial equatorial coordinate system, suitable for import into the
ALMA Observing Tool software. The tool has been successfully used for
the preparation and execution of solar observations with ALMA.
Title: Polar Brightening in ALMA full-disk solar images
Authors: Sudar, D.; Skokić, I.; Brajša, R.
Bibcode: 2019CEAB...43....1S
Altcode:
We have analysed polar brightening effect in two images recorded by
the ALMA telescope in 2015 in Bands 3 ($\lambda=2.8$ mm, $\nu=107$
GHz) and 6 ($\lambda=1.2$ mm, $\nu=248$ GHz). After removing the limb
brightening effect we found that the amplitude of variations is $\approx
180$ K in both Bands.
Title: Solar Polar Brightening and Radius at 100 and 230 GHz Observed
by ALMA
Authors: Selhorst, Caius L.; Simões, Paulo J. A.; Brajša, Roman;
Valio, Adriana; Giménez de Castro, C. G.; Costa, Joaquim E. R.;
Menezes, Fabian; Rozelot, Jean Pierre; Hales, Antonio S.; Iwai,
Kazumasa; White, Stephen
Bibcode: 2019ApJ...871...45S
Altcode: 2018arXiv181112158S
Polar brightening of the Sun at radio frequencies has been studied
for almost 50 years and yet a disagreement persists between solar
atmospheric models and observations. Some observations reported
brightening values much smaller than the expected values obtained from
the models, with discrepancies being particularly large at millimeter
wavelengths. New clues to calibrate the atmospheric models can be
obtained with the advent of the Atacama Large Millimeter/submillimeter
Array (ALMA) radio interferometer. In this work, we analyzed the lower
limit of the polar brightening observed at 100 and 230 GHz by ALMA,
during its Science Verification period, 2015 December 16-20. We find
that the average polar intensity is higher than the disk intensity
at 100 and 230 GHz, with larger brightness intensities at the south
pole in eight of the nine maps analyzed. The observational results
were compared with calculations of the millimetric limb brightening
emission for two semi-empirical atmospheric models, FAL-C and SSC. Both
models presented larger limb intensities than the average observed
values. The intensities obtained with the SSC model were closer to the
observations, with polar brightenings of 10.5% and 17.8% at 100 and
230 GHz, respectively. This discrepancy may be due to the presence of
chromospheric features (like spicules) at regions close to the limb.
Title: Photospheric and chromospheric observations with solar
telescope at Hvar Observatory
Authors: Calogovic, J.; Brajsa, Roman; Vrsnak, Bojan; Dumbovic,
Mateja; Skokic, Ivica
Bibcode: 2018cosp...42E.496C
Altcode:
The double solar telescope at the Hvar Observatory consists of two
Carl Zeiss refractors, one with 217 mm objective diameter used for
photospheric observations and the second one with 130 mm objective used
for chromospheric observations. Hvar solar telescope aims to produce
the high-resolution and high-cadence imaging of active regions on the
Sun using a field of view of about 11 arcmin for the photosphere and
7 arcmin for the chromosphere. The modern Pulnix TM-4200GE 12-bit 4
megapixel CCD cameras recording seven frames per second together with
the software that automatically selects the sharpest frames allow
to study the rapid changes on the Sun in great detail. High-cadence
ground-based observations are an important tool to identify and study
solar flares, filaments and other solar phenomena that are associated
with coronal mass ejections and their propagation to the Earth. Aiming
to improve the space weather forecasts using ground-based observations,
we compiled the catalogue of Hvar solar telescope observations in the
solar cycle 24. In addition, expansion of this catalogue in future
will be used for comparison with ALMA-SSALMON observations. This work
has been supported by the Croatian Science Foundation project 6212
"Solar and Stellar Variability" (SOLSTEL).
Title: Observations of the solar chromosphere with ALMA and comparison
with theoretical models
Authors: Brajsa, Roman; Sudar, Davor; Skokic, Ivica; Benz, Arnold O.;
Kuhar, Matej; Kobelski, Adam; Wedemeyer, Sven; White, Stephen M.;
Ludwig, Hans-G.; Temmer, Manuela; Saar, Steven H.; Selhorst, Caius L.
Bibcode: 2018csss.confE..37B
Altcode: 2018arXiv181207293B
In this work we use solar observations with the ALMA radio telescope
at the wavelength of 1.21 mm. The aim of the analysisis to improve
understanding of the solar chromosphere, a dynamic layer in the
solar atmosphere between the photosphere andcorona. The study has
an observational and a modeling part. In the observational part
full-disc solar images are analyzed.Based on a modied FAL atmospheric
model, radiation models for various observed solar structures are
developed. Finally, theobservational and modeling results are compared
and discussed.
Title: First analysis of solar structures in 1.21 mm full-disc ALMA
image of the Sun
Authors: Brajša, R.; Sudar, D.; Benz, A. O.; Skokić, I.; Bárta,
M.; De Pontieu, B.; Kim, S.; Kobelski, A.; Kuhar, M.; Shimojo, M.;
Wedemeyer, S.; White, S.; Yagoubov, P.; Yan, Y.
Bibcode: 2018A&A...613A..17B
Altcode: 2017arXiv171106130B
Context. Various solar features can be seen in emission or absorption
on maps of the Sun in the millimetre and submillimetre wavelength
range. The recently installed Atacama Large Millimetre/submillimetre
Array (ALMA) is capable of observing the Sun in that wavelength range
with an unprecedented spatial, temporal and spectral resolution. To
interpret solar observations with ALMA, the first important step is to
compare solar ALMA maps with simultaneous images of the Sun recorded in
other spectral ranges.
Aims: The first aim of the present work
is to identify different structures in the solar atmosphere seen in the
optical, infrared, and EUV parts of the spectrum (quiet Sun, active
regions, prominences on the disc, magnetic inversion lines, coronal
holes and coronal bright points) in a full-disc solar ALMA image. The
second aim is to measure the intensities (brightness temperatures) of
those structures and to compare them with the corresponding quiet Sun
level.
Methods: A full-disc solar image at 1.21 mm obtained on
December 18, 2015, during a CSV-EOC campaign with ALMA is calibrated and
compared with full-disc solar images from the same day in Hα line, in
He I 1083 nm line core, and with various SDO images (AIA at 170 nm, 30.4
nm, 21.1 nm, 19.3 nm, and 17.1 nm and HMI magnetogram). The brightness
temperatures of various structures are determined by averaging over
corresponding regions of interest in the calibrated ALMA image.
Results: Positions of the quiet Sun, active regions, prominences on
the disc, magnetic inversion lines, coronal holes and coronal bright
points are identified in the ALMA image. At the wavelength of 1.21
mm, active regions appear as bright areas (but sunspots are dark),
while prominences on the disc and coronal holes are not discernible
from the quiet Sun background, despite having slightly less intensity
than surrounding quiet Sun regions. Magnetic inversion lines appear as
large, elongated dark structures and coronal bright points correspond
to ALMA bright points.
Conclusions: These observational results
are in general agreement with sparse earlier measurements at similar
wavelengths. The identification of coronal bright points represents
the most important new result. By comparing ALMA and other maps,
it was found that the ALMA image was oriented properly and that the
procedure of overlaying the ALMA image with other images is accurate
at the 5 arcsec level. The potential of ALMA for physics of the solar
chromosphere is emphasised.
Title: Meridional Motions and Reynolds Stress Determined by Using
Kanzelhöhe Drawings and White Light Solar Images from 1964 to 2016
Authors: Ruždjak, Domagoj; Sudar, Davor; Brajša, Roman; Skokić,
Ivica; Poljančić Beljan, Ivana; Jurdana-Šepić, Rajka; Hanslmeier,
Arnold; Veronig, Astrid; Pötzi, Werner
Bibcode: 2018SoPh..293...59R
Altcode: 2018arXiv180401344R
Sunspot position data obtained from Kanzelhöhe Observatory for
Solar and Environmental Research (KSO) sunspot drawings and white
light images in the period 1964 to 2016 were used to calculate the
rotational and meridional velocities of the solar plasma. Velocities
were calculated from daily shifts of sunspot groups and an iterative
process of calculation of the differential rotation profiles was used
to discard outliers. We found a differential rotation profile and
meridional motions in agreement with previous studies using sunspots as
tracers and conclude that the quality of the KSO data is appropriate
for analysis of solar velocity patterns. By analyzing the correlation
and covariance of meridional velocities and rotation rate residuals
we found that the angular momentum is transported towards the solar
equator. The magnitude and latitudinal dependence of the horizontal
component of the Reynolds stress tensor calculated is sufficient to
maintain the observed solar differential rotation profile. Therefore,
our results confirm that the Reynolds stress is the dominant mechanism
responsible for transport of angular momentum towards the solar equator.
Title: Exploring the Sun with ALMA
Authors: Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; Pontieu,
B. D.; Gary, D. E.; Fleishman, G. D.; Hales, A. S.; Iwai, K.; Hudson,
H.; Kim, S.; Kobelski, A.; Loukitcheva, M.; Shimojo, M.; Skokić,
I.; Wedemeyer, S.; White, S. M.; Yan, Y.
Bibcode: 2018Msngr.171...25B
Altcode:
The Atacama Large Millimeter/submillimeter Array (ALMA) Observatory
opens a new window onto the Universe. The ability to perform continuum
imaging and spectroscopy of astrophysical phenomena at millimetre and
submillimetre wavelengths with unprecedented sensitivity opens up new
avenues for the study of cosmology and the evolution of galaxies, the
formation of stars and planets, and astrochemistry. ALMA also allows
fundamentally new observations to be made of objects much closer
to home, including the Sun. The Sun has long served as a touchstone
for our understanding of astrophysical processes, from the nature of
stellar interiors, to magnetic dynamos, non-radiative heating, stellar
mass loss, and energetic phenomena such as solar flares. ALMA offers
new insights into all of these processes.
Title: A comparison of solar ALMA observations and model based
predictions of the brightness temperature
Authors: Brajša, R.; Kuhar, M.; Benz, A. O.; Skokić, I.; Sudar,
D.; Wedemeyer, S.; Báarta, M.; De Pontieu, B.; Kim, S.; Kobelski,
A.; Shimojo, M.; White, S.; Yagoubov, P.; Yan, Y.; Ludwig, H. G.;
Temmer, M.; Saar, S. H.; Selhorst, C. L.; Beuc, R.
Bibcode: 2018CEAB...42....1B
Altcode:
The new facility Atacama Large Millimeter/submillimeter Array (ALMA) is
capable of observing the Sun in the wavelength range from 0.3 mm to 10
mm with an unprecedented spatial, temporal and spectral resolution. The
first aim of the present work is to identify different structures
in the solar atmosphere (quiet Sun, active regions, filaments on the
disc, and coronal holes) in a full disc solar ALMA image at 1.21 mm
obtained on December 18, 2015 during a CSV-EOC campaign. It is compared
with full disc solar images from the same day in the Hα line (Cerro
Tololo Observatory, NISP), and at three EUV wavelengths (30.4 nm,
21.1 nm, 17.1 nm; a composite SDO image). Positions of the quiet Sun
areas, active regions, filaments on the disc, and coronal holes are
identified in the ALMA image. To interpret solar observations with ALMA
it is important to compare the measured and calculated intensities
of various solar structures. So, the second aim of this work is to
calculate the intensity (brightness temperature) for those structures
(quiet Sun, active regions, filaments on the disc, and coronal holes)
for a broad wavelength range (from 0.3 mm to 10 mm), closely related
to that of the ALMA, and to compare the results with available
ALMA observations. Thermal bremsstrahlung is the dominant radiation
mechanism for explanation of the observed phenomena. A procedure for
calculating the brightness temperature for a given wavelength and
model atmosphere, which integrates the radiative transfer equation
for thermal bremsstrahlung, is used. At the wavelength of 1.21 mm
active regions appear as bright areas, while filaments on the disc and
coronal holes are not discernible from the quiet Sun background. The
models generally agree with the observed results: Active regions are
bright primarily due to higher densities, filaments can appear bright,
dark or not at all and coronal holes cannot be easily identified.
Title: A Relationship Between the Solar Rotation and Activity Analysed
by Tracing Sunspot Groups
Authors: Ruždjak, Domagoj; Brajša, Roman; Sudar, Davor; Skokić,
Ivica; Poljančić Beljan, Ivana
Bibcode: 2017SoPh..292..179R
Altcode: 2017arXiv171103723R
The sunspot position published in the data bases of the Greenwich
Photoheliographic Results (GPR), the US Air Force Solar Optical
Observing Network and National Oceanic and Atmospheric Administration
(USAF/NOAA), and of the Debrecen Photoheliographic Data (DPD) in the
period 1874 to 2016 were used to calculate yearly values of the solar
differential-rotation parameters A and B . These differential-rotation
parameters were compared with the solar-activity level. We found that
the Sun rotates more differentially at the minimum than at the maximum
of activity during the epoch 1977 - 2016. An inverse correlation between
equatorial rotation and solar activity was found using the recently
revised sunspot number. The secular decrease of the equatorial rotation
rate that accompanies the increase in activity stopped in the last part
of the twentieth century. It was noted that when a significant peak
in equatorial rotation velocity is observed during activity minimum,
the next maximum is weaker than the previous one.
Title: Solar differential rotation in the period 1964-2016 determined
by the Kanzelhöhe data set
Authors: Poljančić Beljan, I.; Jurdana-Šepić, R.; Brajša, R.;
Sudar, D.; Ruždjak, D.; Hržina, D.; Pötzi, W.; Hanslmeier, A.;
Veronig, A.; Skokić, I.; Wöhl, H.
Bibcode: 2017A&A...606A..72P
Altcode:
Context. Kanzelhöhe Observatory for Solar and Environmental Research
(KSO) provides daily multispectral synoptic observations of the Sun
using several telescopes. In this work we made use of sunspot drawings
and full disk white light CCD images.
Aims: The main aim of
this work is to determine the solar differential rotation by tracing
sunspot groups during the period 1964-2016, using the KSO sunspot
drawings and white light images. We also compare the differential
rotation parameters derived in this paper from the KSO with those
collected fromf other data sets and present an investigation of the
north - south rotational asymmetry.
Methods: Two procedures
for the determination of the heliographic positions were applied: an
interactive procedure on the KSO sunspot drawings (1964-2008, solar
cycles Nos. 20-23) and an automatic procedure on the KSO white light
images (2009-2016, solar cycle No. 24). For the determination of the
synodic angular rotation velocities two different methods have been
used: a daily shift (DS) method and a robust linear least-squares fit
(rLSQ) method. Afterwards, the rotation velocities had to be converted
from synodic to sidereal, which were then used in the least-squares
fitting for the solar differential rotation law. A comparison of
the interactive and automatic procedures was performed for the
year 2014.
Results: The interactive procedure of position
determination is fairly accurate but time consuming. In the case
of the much faster automatic procedure for position determination,
we found the rLSQ method for calculating rotational velocities to
be more reliable than the DS method. For the test data from 2014,
the rLSQ method gives a relative standard error for the differential
rotation parameter B that is three times smaller than the corresponding
relative standard error derived for the DS method. The best fit solar
differential rotation profile for the whole time period is ω(b) =
(14.47 ± 0.01)-(2.66 ± 0.10)sin2b (deg/day) for the DS
method and ω(b) = (14.50 ± 0.01)-(2.87 ± 0.12)sin2b
(deg/day) for the rLSQ method. A barely noticeable north - south
asymmetry is observed for the whole time period 1964-2016 in the
present paper. Rotation profiles, using different data sets, presented
by other authors for the same time periods and the same tracer types,
are in good agreement with our results.
Conclusions: The KSO
data set used in this paper is in good agreement with the Debrecen
Photoheliographic Data and Greenwich Photoheliographic Results and is
suitable for the investigation of the long-term variabilities in the
solar rotation profile. Also, the quality of the KSO sunspot drawings
has gradually increased during the last 50 yr.
Title: Identification of features in solar ALMA images and comparison
with solar atmospheric models
Authors: Skokić, I.; Brajša, R.; Sudar, D.; Kuhar, M.; Benz, A. O.
Bibcode: 2017simi.conf..121S
Altcode:
Various solar features were identified in the full-disc single-dish ALMA
images taken at wavelength of 1.2 and 2.8 mm and compared with Hα, Ca
II K and EUV images and magnetograms. Assuming thermal bremsstrahlung
as the dominant radiation mechanism, several models were constructed
to calculate the brightness temperature of various solar features
(quiet and active solar regions, filaments and coronal holes) in a
wavelength range from 0.3 to 10 mm. It is found that model predictions
are in a qualitative agreement with ALMA images where active regions
appear brighter, while filaments and coronal holes are not discernible
from the quiet Sun level. It is also found that in ALMA images regions
around magnetic neutral lines appear darker than the quiet Sun. Models
predict a critical frequency below which filaments should appear in
absorption and above it in emission.
Title: Solar Commissioning Observations of the Sun with ALMA
Authors: White, Stephen M.; Shimojo, Masumi; Bastian, Timothy S.;
Iwai, Kazumasa; Hales, Antonio; Brajsa, Roman; Skokic, Ivica; Kim,
Sujin; Hudson, Hugh S.; Loukitcheva, Maria; Wedemeyer, Sven
Bibcode: 2017SPD....4820402W
Altcode:
PI-led science observations have commenced with the Atacama
Large Millimeter-submillimeter Array (ALMA) following an extensive
commissioning effort. This talk will summarize that effort and discuss
some of the scientific results derived from the commissioning data. As
the solar cycle declines, ALMA observations will mainly address
chromospheric science topics. Examples of data obtained during
commissioning, both from the interferometer and from single-dish
observations, will be presented. The temperatures of the layers that
ALMA is most sensitive to have been determined for the two frequency
bands currently used for solar observations. Curious behavior in a
sunspot umbra and an observations of a small chromospheric ejection
will be discussed.
Title: Solar research with ALMA: Czech node of European ARC as your
user-support infrastructure
Authors: Bárta, M.; Skokić, I.; Brajša, R.; Czech ARC Node Team
Bibcode: 2017simi.conf..127B
Altcode:
ALMA (Atacama Large Millimeter/sub-millimeter Array) is by far the
largest project of current ground-based observational facilities
in astronomy and astrophysics. It is built and operated in the
world-wide cooperation (ESO, NRAO, NAOJ) at altitude of 5000m in the
desert of Atacama, Chile. Because of its unprecedented capabilities,
ALMA is considered as a cutting-edge research device in astrophysics
with potential for many breakthrough discoveries in the next decade
and beyond. In spite it is not exclusively solar-research dedicated
instrument, science observations of the Sun are now possible and has
recently started in the observing Cycle 4 (2016-2017). In order to
facilitate user access to this top-class, but at the same moment very
complicated device to researchers lacking technical expertise, a network
of three ALMA Regional Centers (ARCs) has been formed in Europe, North
America, and East Asia as a user-support infrastructure and interface
between the observatory and users community. After short introduction
to ALMA the roles of ARCs and hint how to utilize their services will
be presented, with emphasis to the specific (and in Europe unique)
mission of the Czech ARC node in solar research with ALMA. Finally,
peculiarities of solar observations that demanded the development
of the specific Solar ALMA Observing Modes will be discussed and the
results of Commissioning and Science Verification observing campaigns
(solar ALMA maps) will be shown.
Title: A comparison of solar ALMA maps with solar images obtained
at other wavelengths
Authors: Brajsa, Roman; Sudar, Davor; Skokic, Ivica; Benz, Arnold;
Kuhar, Matej; White, Stephen M.
Bibcode: 2017SPD....4820403B
Altcode:
We use recently released Commissioning and Science Verification
data of the Sun from the observing campaigns perfomed with the ALMA
radio telescope in December 2014 and in December 2015. The dataset
consists of calibrated maps of the Sun recorded in ALMA observing
band 3 (corresponding to a wavelength of 3 mm) and band 6 (1.2 mm)
which show both bright and dark areas and a background of highly
structured pattern. Solar ALMA maps are compared with images in EUV
(SDO/AIA), H-alpha (NISP, Cerro Tololo) and He 1083 nm (NSO/SOLIS),
as well as with magnetograms (SDO/HMI) recorded at times closest
to the ALMA observations. With a special software the images
are overlapped and a correspondence of identified structures is
checked in both ways. The visibility of active regions, sunspots,
inversion lines of global magnetic field, prominences on the disc,
coronal holes and coronal bright points is investigated in ALMA
images at mm wavelengths. Single-dish ALMA images of the Sun reveal
large-scale structures in the solar atmosphere, while high resolution
interferometric images are used to analyse the fine-scale chromospheric
structure. The intensities (the brightness temperatures) of identified
structures were determined and compared with selected model-based
predictions.
Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric
Imaging
Authors: Shimojo, M.; Bastian, T. S.; Hales, A. S.; White, S. M.;
Iwai, K.; Hills, R. E.; Hirota, A.; Phillips, N. M.; Sawada, T.;
Yagoubov, P.; Siringo, G.; Asayama, S.; Sugimoto, M.; Brajša, R.;
Skokić, I.; Bárta, M.; Kim, S.; de Gregorio-Monsalvo, I.; Corder,
S. A.; Hudson, H. S.; Wedemeyer, S.; Gary, D. E.; De Pontieu, B.;
Loukitcheva, M.; Fleishman, G. D.; Chen, B.; Kobelski, A.; Yan, Y.
Bibcode: 2017SoPh..292...87S
Altcode: 2017arXiv170403236S
Observations of the Sun at millimeter and submillimeter wavelengths
offer a unique probe into the structure, dynamics, and heating of the
chromosphere; the structure of sunspots; the formation and eruption
of prominences and filaments; and energetic phenomena such as jets
and flares. High-resolution observations of the Sun at millimeter and
submillimeter wavelengths are challenging due to the intense, extended,
low-contrast, and dynamic nature of emission from the quiet Sun,
and the extremely intense and variable nature of emissions associated
with energetic phenomena. The Atacama Large Millimeter/submillimeter
Array (ALMA) was designed with solar observations in mind. The
requirements for solar observations are significantly different from
observations of sidereal sources and special measures are necessary
to successfully carry out this type of observations. We describe the
commissioning efforts that enable the use of two frequency bands,
the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum
interferometric-imaging observations of the Sun with ALMA. Examples of
high-resolution synthesized images obtained using the newly commissioned
modes during the solar-commissioning campaign held in December 2015
are presented. Although only 30 of the eventual 66 ALMA antennas
were used for the campaign, the solar images synthesized from the
ALMA commissioning data reveal new features of the solar atmosphere
that demonstrate the potential power of ALMA solar observations. The
ongoing expansion of ALMA and solar-commissioning efforts will continue
to enable new and unique solar observing capabilities.
Title: Solar differential rotation in the period 1964 - 2016
determined by the Kanzelhöhe data set
Authors: Poljančić Beljan, I.; Jurdana-Šepić, R.; Brajša, R.;
Sudar, D.; Ruždjak, D.; Hržina, D.; Pötzi, W.; Hanslmeier, A.;
Veronig, A.; Skokić, I.; Wöhl, H.
Bibcode: 2017arXiv170707886P
Altcode:
The main aim of this work is to determine the solar differential
rotation by tracing sunspot groups during the period 1964-2016, using
the Kanzelhöhe Observatory for Solar and Environmental Research
(KSO) sunspot drawings and white light images. Two procedures for
the determination of the heliographic positions were applied: an
interactive procedure on the KSO sunspot drawings (1964 - 2008, solar
cycles nos. 20 - 23) and an automatic procedure on the KSO white light
images (2009 - 2016, solar cycle no. 24). For the determination of the
synodic angular rotation velocities two different methods have been
used: a daily shift (DS) method and a robust linear least-squares fit
(rLSQ) method. Afterwards, the rotation velocities had to be converted
from synodic to sidereal, which were then used in the least-squares
fitting for the solar differential rotation law. For the test
data from 2014, we found the rLSQ method for calculating rotational
velocities to be more reliable than the DS method. The best fit solar
differential rotation profile for the whole time period is $\omega(b)$
= (14.47 $\pm$ 0.01) - (2.66 $\pm$ 0.10) $\sin^2b$ (deg/day) for the
DS method and $\omega(b)$ = (14.50 $\pm$ 0.01) - (2.87 $\pm$ 0.12)
$\sin^2b$ (deg/day) for the rLSQ method. A barely noticeable north -
south asymmetry is observed for the whole time period 1964 - 2016
in the present paper. Rotation profiles, using different data sets
(e.g. Debrecen Photoheliographic Data, Greenwich Photoheliographic
Results), presented by other authors for the same time periods and the
same tracer types, are in good agreement with our results. Therefore,
the KSO data set is suitable for the investigation of the long-term
variabilities in the solar rotation profile.
Title: Meridional Motion and Reynolds Stress from Debrecen
Photoheliographic Data
Authors: Sudar, Davor; Brajša, Roman; Skokić, Ivica; Poljančić
Beljan, Ivana; Wöhl, Hubertus
Bibcode: 2017SoPh..292...86S
Altcode: 2017arXiv170500912S
The Debrecen Photoheliographic Data catalogue is a continuation of
the Greenwich Photoheliographic Results providing daily positions of
sunspots and sunspot groups. We analyse the data for sunspot groups
focussing on meridional motions and transfer of angular momentum towards
the solar equator. Velocities are calculated with a daily shift method
including an automatic iterative process of removing the outliers. Apart
from the standard differential rotation profile, we find meridional
motion directed towards the zone of solar activity. The difference in
measured meridional flow in comparison to Doppler measurements and some
other tracer measurements is interpreted as a consequence of different
flow patterns inside and outside of active regions. We also find a
statistically significant dependence of meridional motion on rotation
velocity residuals confirming the transfer of angular momentum towards
the equator. Analysis of horizontal Reynolds stress reveals that the
transfer of angular momentum is stronger with increasing latitude
up to about 40∘, where there is a possible maximum in
absolute value.
Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
Authors: White, S. M.; Iwai, K.; Phillips, N. M.; Hills, R. E.; Hirota,
A.; Yagoubov, P.; Siringo, G.; Shimojo, M.; Bastian, T. S.; Hales,
A. S.; Sawada, T.; Asayama, S.; Sugimoto, M.; Marson, R. G.; Kawasaki,
W.; Muller, E.; Nakazato, T.; Sugimoto, K.; Brajša, R.; Skokić, I.;
Bárta, M.; Kim, S.; Remijan, A. J.; de Gregorio, I.; Corder, S. A.;
Hudson, H. S.; Loukitcheva, M.; Chen, B.; De Pontieu, B.; Fleishmann,
G. D.; Gary, D. E.; Kobelski, A.; Wedemeyer, S.; Yan, Y.
Bibcode: 2017SoPh..292...88W
Altcode: 2017arXiv170504766W
The Atacama Large Millimeter/submillimeter Array (ALMA) radio
telescope has commenced science observations of the Sun starting
in late 2016. Since the Sun is much larger than the field of view
of individual ALMA dishes, the ALMA interferometer is unable to
measure the background level of solar emission when observing the
solar disk. The absolute temperature scale is a critical measurement
for much of ALMA solar science, including the understanding of energy
transfer through the solar atmosphere, the properties of prominences,
and the study of shock heating in the chromosphere. In order to provide
an absolute temperature scale, ALMA solar observing will take advantage
of the remarkable fast-scanning capabilities of the ALMA 12 m dishes
to make single-dish maps of the full Sun. This article reports on the
results of an extensive commissioning effort to optimize the mapping
procedure, and it describes the nature of the resulting data. Amplitude
calibration is discussed in detail: a path that uses the two loads in
the ALMA calibration system as well as sky measurements is described
and applied to commissioning data. Inspection of a large number of
single-dish datasets shows significant variation in the resulting
temperatures, and based on the temperature distributions, we derive
quiet-Sun values at disk center of 7300 K at λ =3 mm and 5900 K at
λ =1.3 mm. These values have statistical uncertainties of about 100
K, but systematic uncertainties in the temperature scale that may be
significantly larger. Example images are presented from two periods
with very different levels of solar activity. At a resolution of about
25″, the 1.3 mm wavelength images show temperatures on
the disk that vary over about a 2000 K range. Active regions and plages
are among the hotter features, while a large sunspot umbra shows up as
a depression, and filament channels are relatively cool. Prominences
above the solar limb are a common feature of the single-dish images.
Title: Hvar Observatory - First 45 Years
Authors: Brajša, R.
Bibcode: 2017CEAB...41....1B
Altcode:
Hvar Observatory is the only professional astrophysical observatory
in the Republic of Croatia. It is one of departments of the Faculty
of Geodesy, University of Zagreb and the main activities of Hvar
Observatory are scientific research in astronomy and astrophysics and
education of young scientists in these research areas. Four telescopes
are installed at Hvar Observatory, for observations of the Sun, the
bodies of the solar system, stars and galaxies. Besides the astronomical
and astrophysical research, geodetic and geophysical measurements
are also systematically performed at Hvar Observatory. In this paper
we briefly describe the foundation of Hvar Observatory in the year
1972 and activities of the Observatory in the last 45 years. Also,
the telescopes of Hvar Observatory, scientific research and public
outreach are presented.
Title: Extreme Space Weather in Extra-Solar Systems - a Flare
Alert Program
Authors: Hanslmeier, A.; Leitzinger, M.; Greimel, R.; Odert, P.;
Ratzka, T.; Brajša, R.; Guenther, E. W.; Korhonen, H.; Pribulla, T.;
Šlechta, M.; Vida, K.; Güdel, M.; Heinzel, P.; Lammer, H.; Ribas, I.
Bibcode: 2017CEAB...41...67H
Altcode:
Highly energetic stellar activity phenomena, such as outbreaks of
radiations (flares) and mass expulsions into the helio-/astro-sphere
(coronal mass ejections/CMEs), may have a severe impact on
the atmospheric evolution of planets. Stellar flare have been
studied for decades now and their typical parameters are determined
statistically. However, observations of stellar CMEs are rare, yielding
only a handful of detections so far. The detection of CMEs on stars
demands more observational effort than the detection flares, because
the circumstellar environment cannot be spatially resolved like in the
Solar System. The most convincing detections up to now were found as
Doppler-shifted Balmer line extra-emission/absorption features emerging
close to flare events, indicating the rise end ejection of prominence
material embedded in the CME core. Dedicated programs aiming for a
statistical determination of occurrence rates and the parameters of
stellar CMEs are still lacking. Therefore, we propose an innovative
and efficient observational approach to search for and characterize
CMEs on Sun-like and late-type pre-main sequence and main sequence
stars to determine stellar CME parameters and their occurrence rates
as a function of stellar spectral type and age, as well as establish
a stellar flare/CME association rate, for the first time ever.
Title: Solar Observations with the Atacama Large
Millimeter/submillimeter Array (ALMA)
Authors: Kobelski, A.; Bastian, T. S.; Bárta, M.; Brajša, R.; Chen,
B.; De Pontieu, B.; Fleishman, G.; Gary, D.; Hales, A.; Hills, R.;
Hudson, H.; Hurford, G.; Loukitcheva, M.; Iwai, K.; Krucker, S.;
Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S.; Yan, Y.; ALMA
Solar Development Team
Bibcode: 2016ASPC..504..327K
Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a
joint North American, European, and East Asian project that opens
the mm-sub mm wavelength part of the electromagnetic spectrum for
general astrophysical exploration, providing high-resolution imaging
in frequency bands currently ranging from 84 GHz to 950 GHz (300
microns to 3 mm). It is located in the Atacama desert in northern
Chile at an elevation of 5000 m. Despite being a general purpose
instrument, provisions have been made to enable solar observations
with ALMA. Radiation emitted at ALMA wavelengths originates mostly
from the chromosphere, which plays an important role in the transport
of matter and energy, and the in heating the outer layers of the solar
atmosphere. Despite decades of research, the solar chromosphere remains
a significant challenge: both to observe, owing to the complicated
formation mechanisms of currently available diagnostics; and to
understand, as a result of the complex nature of the structure and
dynamics of the chromosphere. ALMA has the potential to change the
scene substantially as it serves as a nearly linear thermometer at
high spatial and temporal resolution, enabling us to study the complex
interaction of magnetic fields and shock waves and yet-to-be-discovered
dynamical processes. Moreover, ALMA will play an important role in
the study of energetic emissions associated with solar flares at
sub-THz frequencies.
Title: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
Bibcode: 2016SSRv..200....1W
Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.
Title: Meridional motions and Reynolds stress from SDO/AIA coronal
bright points data
Authors: Sudar, D.; Saar, S. H.; Skokić, I.; Poljančić Beljan,
I.; Brajša, R.
Bibcode: 2016A&A...587A..29S
Altcode: 2016arXiv160102406S
Context. It is possible to detect and track coronal bright points (CBPs)
in Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA)
images. A combination of high resolution and high cadence provides
a wealth of data that can be used to determine velocity flows on the
solar surface with very high accuracy.
Aims: We derived a very
accurate solar rotation profile and investigated meridional flows,
torsional oscillations, and horizontal Reynolds stress based on ≈6
months of SDO/AIA data.
Methods: We used a segmentation algorithm
to detect CBPs in SDO/AIA images. We also used invariance of the solar
rotation profile with central meridian distance (CMD) to determine
the height of CBPs in the 19.3 nm channel.
Results: The best
fit solar rotation profile is given by ω(b) = (14.4060 ± 0.0051 +
(-1.662 ± 0.050)sin2b + (-2.742 ± 0.081)sin4b)°
day-1. The height of CBPs in the SDO/AIA 19.3 nm channel was
found to be ≈6500 km. Meridional motion is predominantly poleward for
all latitudes, while solar velocity residuals show signs of torsional
oscillations. Horizontal Reynolds stress was found to be smaller than in
similar works, but still showed transfer of angular momentum towards the
solar equator.
Conclusions: Most of the results are consistent
with Doppler measurements rather than tracer measurements. The fairly
small calculated value of horizontal Reynolds stress might be due to
the particular phase of the solar cycle. Accuracy of the calculated
rotation profile indicates that it is possible to measure changes in
the profile as the solar cycle evolves. Analysis of further SDO/AIA CBP
data will also provide a better understanding of the temporal behaviour
of the rotation velocity residuals, meridional motions, and Reynolds
stress. Table 1 is only available at the CDS via anonymous ftp to
http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/587/A29
Title: An Application of the Random Walk Model to Proper Motions of
Coronal Bright Points from SDO Data
Authors: Skokić, I.; Sudar, D.; Saar, S. H.; Brajša, R.;
Poljančić-Beljan, I.
Bibcode: 2016CEAB...40...23S
Altcode:
Atmospheric Imaging Assembly (AIA) images from the Solar Dynamics
Observatory (SDO) were used to follow the motions of coronal bright
points (CBPs) in the period 1 January - 19 May 2011 with a cadence of
10 minutes. This resulted in a data set of 80966 CBPs with measured
lifetimes and mean velocities which were used in a random walk model
to calculate the diffusion coefficient, D. The results show that D has
a value of ≈260 km^2 s^{-1} for CBPs with lifetime below 6 hours,
decreasing to ≈170 km^2 s^{-1} for lifetimes above 12 hours, with
a mean value of ≈230 km^2 s^{-1}.
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
Bibcode: 2016arXiv160100587W
Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.
Title: VizieR Online Data Catalog: Time and positions of coronal
bright points (Sudar+, 2016)
Authors: Sudar, D.; Saar, S. H.; Skokic, I.; Poljancic Beljan, I.;
Brajsa, R.
Bibcode: 2016yCat..35870029S
Altcode:
Positional information about CBPs detected by the segmentation
algorithm are presented. For each CBP time in Julian days (JD) and x
and y coordinates in pixels are given. (1 data file).
Title: Solar Simulations for the Atacama Large Millimeter Observatory
Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.;
Shimojo, M.
Bibcode: 2015ASPC..499..341W
Altcode: 2015arXiv150206379W
The Atacama Large Millimeter/submillimeter Array (ALMA) will
be a valuable tool for observing the chromosphere of our Sun at
(sub-)millimeter wavelengths at high spatial, temporal and spectral
resolution and as such has great potential to address long-standing
scientific questions in solar physics. In order to make the best use
of this scientific opportunity, the Solar Simulations for the Atacama
Large Millimeter Observatory Network has been initiated. A key goal
of this international collaboration is to support the preparation and
interpretation of future observations of the Sun with ALMA.
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
Bibcode: 2015AdSpR..56.2679W
Altcode: 2015arXiv150205601W
The Solar Simulations for the Atacama Large Millimeter Observatory
Network (SSALMON) was initiated in 2014 in connection with two ALMA
development studies. The Atacama Large Millimeter/submillimeter Array
(ALMA) is a powerful new tool, which can also observe the Sun at
high spatial, temporal, and spectral resolution. The international
SSALMONetwork aims at co-ordinating the further development of solar
observing modes for ALMA and at promoting scientific opportunities
for solar physics with particular focus on numerical simulations,
which can provide important constraints for the observing modes and
can aid the interpretation of future observations. The radiation
detected by ALMA originates mostly in the solar chromosphere - a
complex and dynamic layer between the photosphere and corona, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Potential targets
include active regions, prominences, quiet Sun regions, flares. Here,
we give a brief overview over the network and potential science cases
for future solar observations with ALMA.
Title: Solar ALMA Observations - A New View of Our Host Star
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Shimojo,
M.; Hales, A.; Yagoubov, P.; Hudson, H.
Bibcode: 2015ASPC..499..345W
Altcode: 2015arXiv150206397W
ALMA provides the necessary spatial, temporal and spectral resolution to
explore central questions in contemporary solar physics with potentially
far-reaching implications for stellar atmospheres and plasma physics. It
can uniquely constraint the thermal and magnetic field structure in
the solar chromosphere with measurements that are highly complementary
to simultaneous observations with other ground-based and space-borne
instruments. Here, we highlight selected science cases.
Title: The Atacama Large Millimeter/Submillimeter Array: a New Asset
for Solar and Heliospheric Physics
Authors: Bastian, Timothy S.; Barta, Miroslav; Brajsa, Roman; Chen,
Bin; De Pontieu, Bart; Fleishman, Gregory; Gary, Dale; Hales, Antonio;
Hills, Richard; Hudson, Hugh; Iwai, Kazamasu; Shimojo, Masumi; White,
Stephen; Wedemeyer, Sven; Yan, Yihua
Bibcode: 2015IAUGA..2257295B
Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint
North American, European, and East Asian interferometric array that
opens the mm-submm wavelength part of the electromagnetic spectrum
for general astrophysical exploration, providing high-resolution
imaging in frequency bands ranging from 86 to 950 GHz. Despite being
a general purpose instrument, provisions have been made to enable
solar observations with ALMA. Radiation emitted at ALMA wavelengths
originates mostly from the chromosphere, which plays an important
role in the transport of energy and matter and the heating of the
outer layers of the solar atmosphere. In this paper we describe
recent efforts to ensure that ALMA can be usefully exploited by
the scientific community to address outstanding questions in solar
physics. We summarize activities under North American and European
ALMA development studies, including instrument testing, calibration
and imaging strategies, a science simulations. With the support of
solar observations, ALMA joins next-generation groundbased instruments
that can be used alone or in combination with other ground-based and
space-based instruments to address outstanding questions in solar
and heliospheric physics. Opportunities for the wider community to
contribute to these efforts will be highlighted.
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, Sven; Bastian, Timothy S.; Brajsa, Roman; Barta,
Miroslav
Bibcode: 2015IAUGA..2257466W
Altcode:
The Atacama Large Millimeter/submillimeter Array (ALMA) provides
a new powerful tool for observing the solar chromosphere at high
spatial, temporal, and spectral resolution, which will allow for
addressing fundamental scientific questions. Based on first solar test
observations, observing strategies for regular solar campaigns are
currently under development. State-of-the-art numerical simulations of
the solar atmosphere and modeling of instrumental effects can help in
this respect, constraining and optimizing future observing modes for
ALMA. On September 1st, 2014, the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) has been initiated
with the aim to co-ordinate related activities and to promote the
scientific potential of ALMA observations of the Sun. The network
is connected to two currently ongoing ALMA development studies. As
of March 18th, 57 scientists from 15 countries have joined the
international SSALMONetwork. Among the affiliations are NRAO, ESO,
NAOJ, the Czech ALMA ARC node at Ondrejov, ESA and many more. Since
March 2015, we are building up expert teams, which work on specific
tasks in preparation of future regular ALMA observations (expected to
start in late 2016) and their interpretation. Registration and more
information at http://www.ssalmon.uio.no.
Title: Solar ALMA observations - A revolutionizing new view at our
host star
Authors: Wedemeyer, Sven; Brajsa, Roman; Bastian, Timothy S.; Barta,
Miroslav; Hales, Antonio; Yagoubov, Pavel; Hudson, Hugh; Loukitcheva,
Maria; Fleishman, Gregory
Bibcode: 2015IAUGA..2256732W
Altcode:
Observations of the Sun with the Atacama Large Millimeter/submillimeter
Array (ALMA) have a large potential for revolutionizing our
understanding of our host star with far reaching implications
for stars in general. The radiation emitted at ALMA wavelengths
originates mostly from the chromosphere - a complex and dynamic layer
between the photosphere and the corona, which plays an important
role in the transport of energy and matter and the heating of the
outer layers of the solar atmosphere.Despite decades of intensive
research, the chromosphere is still elusive and challenging to
observe owing to the complicated formation mechanisms of currently
available diagnostics. ALMA will change the scene substantially as
it serves as a nearly linear thermometer at high spatial, temporal,
and spectral resolution, enabling us to study the complex interaction
of magnetic fields and shock waves and yet-to-be-discovered dynamical
processes. Furthermore, radio recombination and molecular lines
may have great diagnostic potential but need to be investigated
first. These unprecedented capabilities promise important new findings
for a large range of topics in solar physics including the structure,
dynamics and energy balance of quiet Sun regions, active regions and
sunspots, flares and prominences. As a part of ongoing development
studies, an international network has been initiated, which aims at
defining and preparing key solar science with ALMA through simulation
studies: SSALMON -- Solar Simulations for the Atacama Large Millimeter
Observatory Network (http://ssalmon.uio.no). Here, we give an overview
of potential science cases.
Title: Steps towards a high precision solar rotation profile:
Results from SDO/AIA coronal bright point data
Authors: Sudar, D.; Skokić, I.; Brajša, R.; Saar, S. H.
Bibcode: 2015A&A...575A..63S
Altcode: 2015arXiv150101285S
Context. Coronal bright points (CBP) are ubiquitous small brightenings
in the solar corona associated with small magnetic bipoles.
Aims:
We derive the solar differential rotation profile by tracing the motions
of CBPs detected by the Atmospheric Imaging Assembly (AIA) instrument
aboard the Solar Dynamics Observatory (SDO). We also investigate
problems related to the detection of CBPs resulting from instrument
and detection algorithm limitations.
Methods: To determine
the positions and identification of CBPs we used a segmentation
algorithm. A linear fit of their central meridian distance and latitude
vs time was used to derive velocities.
Results: We obtained
906 velocity measurements in a time interval of only 2 days. The
differential rotation profile can be expressed as ωrot =
(14.47° ± 0.10° + (0.6° ± 1.0°) sin2(b) = (-4.7° ±
1.7°) sin4(b)) d-1. Our result is in agreement
with other work and it comes with reasonable errors in spite of the
very short time interval used. This was made possible by the higher
sensitivity and resolution of the AIA instrument compared to similar
equipment as well as high cadence. The segmentation algorithm also
played a crucial role by detecting so many CBPs, which reduced the
errors to a reasonable level.
Conclusions: Data and methods
presented in this paper show a great potential for obtaining very
accurate velocity profiles, both for rotation and meridional motion and,
consequently, Reynolds stresses. The amount of CBP data that could be
obtained from this instrument should also provide a great opportunity
to study changes of velocity patterns with a temporal resolution of
only a few months. Other possibilities are studies of evolution of
CBPs and proper motions of magnetic elements on the Sun.
Title: A comparison between the observed and predicted amplitude of
the 24th solar cycle
Authors: Brajša, R.; Verbanac, G.; Sudar, D.; Skokić, I.; Žic,
T.; Hanslmeier, A.; Wöhl, H.; Roth, M.; Mursula, K.; Zhang, L.
Bibcode: 2015CEAB...39..135B
Altcode:
In present work we compared the measured and predicted amplitudes of
the 24th solar cycle. The modified minimum--maximum method,
belonging to the precursor class of methods, was applied to the smoothed
monthly sunspot number values (the ``old'' data set, used before the
change introduced on July 1st, 2015). The maximum of the
24th solar cycle occurred in April 2014 with an amplitude of
R=82 and this observed value is very close to our mean predicted value
R=83. The maximum was significantly weaker than in several previous
cycles. Additionally, a curious solar activity minimum of 2008,
between the solar cycles no.~23 and no.~24 was analysed, as well as
the shape of the maximum profile. The maximum of the 24th
solar cycle had a double-peak, the second one being higher than the
first one. The obtained results represent a strong indication that
the minimum--maximum method is a reliable tool for the solar cycle
prediction, using data available already 3 years before the preceding
minimum of solar activity.
Title: Solar velocity field determined tracking coronal bright points
Authors: Brajša, R.; Sudar, D.; Skokić, I.; Saar, S. H.; Žic, T.
Bibcode: 2015CEAB...39...21B
Altcode:
Preliminary data from Atmospheric Imaging Assembly (AIA) instrumenton
board Solar Dynamics Observatory (SDO) satellite were used to determine
solar differential rotation and related phenomena. A segmentation
algorithm, which uses multiple AIA channels in search for intensity
enhancements in EUV and X-ray parts of the spectrum compared to the
background intensity, was applied to obtain positional information of
coronal bright points (CBPs). More than 60000 position measurements
of more than 10000 identified CBPs from the period 1 - 2 January 2011
were analyzed. Rotational and meridional velocities were determined
by tracking identified CBPs and various filters were used to exclude
erroneous results. Also, proper motions of CBPs were calculated from
rotation velocity residuals and meridional velocities. Proper motions
of CBPs were investigated using a random walk model and the diffusion
constant was calculated. These results were compared with the previous
ones obtained by other instruments and methods (especially with the
SOHO-EIT and Hinode data) and a striking agreement of the obtained
diffusion constant with results from other studies was found.
Title: Future mmVLBI Research with ALMA: A European vision
Authors: Tilanus, R. P. J.; Krichbaum, T. P.; Zensus, J. A.; Baudry,
A.; Bremer, M.; Falcke, H.; Giovannini, G.; Laing, R.; van Langevelde,
H. J.; Vlemmings, W.; Abraham, Z.; Afonso, J.; Agudo, I.; Alberdi,
A.; Alcolea, J.; Altamirano, D.; Asadi, S.; Assaf, K.; Augusto, P.;
Baczko, A-K.; Boeck, M.; Boller, T.; Bondi, M.; Boone, F.; Bourda,
G.; Brajsa, R.; Brand, J.; Britzen, S.; Bujarrabal, V.; Cales, S.;
Casadio, C.; Casasola, V.; Castangia, P.; Cernicharo, J.; Charlot,
P.; Chemin, L.; Clenet, Y.; Colomer, F.; Combes, F.; Cordes, J.;
Coriat, M.; Cross, N.; D'Ammando, F.; Dallacasa, D.; Desmurs, J-F.;
Eatough, R.; Eckart, A.; Eisenacher, D.; Etoka, S.; Felix, M.; Fender,
R.; Ferreira, M.; Freeland, E.; Frey, S.; Fromm, C.; Fuhrmann, L.;
Gabanyi, K.; Galvan-Madrid, R.; Giroletti, M.; Goddi, C.; Gomez, J.;
Gourgoulhon, E.; Gray, M.; di Gregorio, I.; Greimel, R.; Grosso, N.;
Guirado, J.; Hada, K.; Hanslmeier, A.; Henkel, C.; Herpin, F.; Hess,
P.; Hodgson, J.; Horns, D.; Humphreys, E.; Hutawarakorn Kramer, B.;
Ilyushin, V.; Impellizzeri, V.; Ivanov, V.; Julião, M.; Kadler, M.;
Kerins, E.; Klaassen, P.; van 't Klooster, K.; Kording, E.; Kozlov,
M.; Kramer, M.; Kreikenbohm, A.; Kurtanidze, O.; Lazio, J.; Leite,
A.; Leitzinger, M.; Lepine, J.; Levshakov, S.; Lico, R.; Lindqvist,
M.; Liuzzo, E.; Lobanov, A.; Lucas, P.; Mannheim, K.; Marcaide, J.;
Markoff, S.; Martí-Vidal, I.; Martins, C.; Masetti, N.; Massardi,
M.; Menten, K.; Messias, H.; Migliari, S.; Mignano, A.; Miller-Jones,
J.; Minniti, D.; Molaro, P.; Molina, S.; Monteiro, A.; Moscadelli,
L.; Mueller, C.; Müller, A.; Muller, S.; Niederhofer, F.; Odert,
P.; Olofsson, H.; Orienti, M.; Paladino, R.; Panessa, F.; Paragi,
Z.; Paumard, T.; Pedrosa, P.; Pérez-Torres, M.; Perrin, G.; Perucho,
M.; Porquet, D.; Prandoni, I.; Ransom, S.; Reimers, D.; Rejkuba, M.;
Rezzolla, L.; Richards, A.; Ros, E.; Roy, A.; Rushton, A.; Savolainen,
T.; Schulz, R.; Silva, M.; Sivakoff, G.; Soria-Ruiz, R.; Soria, R.;
Spaans, M.; Spencer, R.; Stappers, B.; Surcis, G.; Tarchi, A.; Temmer,
M.; Thompson, M.; Torrelles, J.; Truestedt, J.; Tudose, V.; Venturi,
T.; Verbiest, J.; Vieira, J.; Vielzeuf, P.; Vincent, F.; Wex, N.;
Wiik, K.; Wiklind, T.; Wilms, J.; Zackrisson, E.; Zechlin, H.
Bibcode: 2014arXiv1406.4650T
Altcode:
Very long baseline interferometry at millimetre/submillimetre
wavelengths (mmVLBI) offers the highest achievable spatial
resolution at any wavelength in astronomy. The anticipated inclusion
of ALMA as a phased array into a global VLBI network will bring
unprecedented sensitivity and a transformational leap in capabilities
for mmVLBI. Building on years of pioneering efforts in the US and
Europe the ongoing ALMA Phasing Project (APP), a US-led international
collaboration with MPIfR-led European contributions, is expected to
deliver a beamformer and VLBI capability to ALMA by the end of 2014
(APP: Fish et al. 2013, arXiv:1309.3519). This report focuses on
the future use of mmVLBI by the international users community from
a European viewpoint. Firstly, it highlights the intense science
interest in Europe in future mmVLBI observations as compiled from
the responses to a general call to the European community for future
research projects. A wide range of research is presented that includes,
amongst others: - Imaging the event horizon of the black hole at the
centre of the Galaxy - Testing the theory of General Relativity an/or
searching for alternative theories - Studying the origin of AGN jets
and jet formation - Cosmological evolution of galaxies and BHs, AGN
feedback - Masers in the Milky Way (in stars and star-forming regions) -
Extragalactic emission lines and astro-chemistry - Redshifted absorption
lines in distant galaxies and study of the ISM and circumnuclear gas -
Pulsars, neutron stars, X-ray binaries - Testing cosmology - Testing
fundamental physical constants
Title: Validity of the Relations Between the Synodic and Sidereal
Rotation Velocities of the Sun
Authors: Skokić, I.; Brajša, R.; Roša, D.; Hržina, D.; Wöhl, H.
Bibcode: 2014SoPh..289.1471S
Altcode: 2013arXiv1310.0778S
Existing methods for conversion between synodic and sidereal
rotation velocities of the Sun are tested for validity using
state-of-the-art ephemeris data. We found that some of them agree well
with ephemeris calculations while others show a discrepancy of almost
0.01∘ day−1. This discrepancy is attributed
to a missing factor and a new corrected relation is given.
Title: Tracing sunspot groups to determine angular momentum transfer
on the Sun
Authors: Sudar, D.; Skokić, I.; Ruždjak, D.; Brajša, R.; Wöhl, H.
Bibcode: 2014MNRAS.439.2377S
Altcode: 2014MNRAS.tmp..355S; 2014arXiv1401.5641S
In this paper, our goal is to investigate Reynolds stress and to
check whether it is plausible that this is responsible for angular
momentum transfer towards the solar equator. We have also analysed
meridional velocity, rotation velocity residuals and correlation
between the velocities. We have used the position measurements of
sunspot groups from the Greenwich Photographic Result and the Solar
Observing Optical Network/United States Air Force/National Oceanic
and Atmospheric Administration data bases, covering the period
1878-2011. In order to calculate the velocities, we used the daily
motion of sunspot groups. The sample was also limited to ±58° in the
central meridian distance in order to avoid solar limb effects. We have
mainly investigated velocity patterns depending on the solar cycle phase
and latitude. We have found that the meridional motion of sunspot groups
is towards the centre of activity from all available latitudes and in
all phases of the solar cycle. The range of meridional velocities is
±10 m s-1. Horizontal Reynolds stress is negative at all
available latitudes and indicates that there is a minimum value (q ≈
-3000 m2 s-2) located at b ≈ ±30°. In our
convention, this means that angular momentum is transported towards
the solar equator, in agreement with the observed rotational profile
of the Sun.
Title: An analysis of the solar differential rotation from the
Kanzelhöhe sunspot drawings
Authors: Poljančić Beljan, I.; Jurdana-Šepić, R.; Čargonja, M.;
Brajša, R.; Hržina, D.; Pötzi, W.; Hanslmeier, A.
Bibcode: 2014CEAB...38...87P
Altcode:
We present here the results of the behaviour of the solar differential
rotation during solar cycles no. 20 and no. 22, derived from Kanzelhöhe
sunspot drawings (Kanzelhöhe Observatory for Solar and Environmental
Research, University of Graz, Austria). The positions of sunspot groups
were determined using a special software Sungrabber. Sunspot groups were
identified with the help of the Greenwich Photoheliographic Results
(GPR) and Debrecen Photoheliographic Data (DPD) databases, covering
solar cycles no. 20 and no. 22, respectively. In order to calculate
the sidereal angular rotation rate ω and subsequently solar rotation
parameters A and B we used two procedures: a) daily motion of sunspot
groups and b) linear least-square fit from the function CMD(t) for each
tracer, where CMD denotes the Central Meridian Distance. The sample
was limited to ±58° in CMD in order to avoid solar limb effects. We
mainly investigated velocity patterns depending on the solar cycle
phase and latitude.
Title: Preliminary results on the solar rotation determined tracing
SDO/AIA coronal bright points
Authors: Brajša, R.; Sudar, D.; Skokič, I.; Saar, S. H.
Bibcode: 2014CEAB...38..105B
Altcode:
In this paper we present preliminary results on the solar differential
rotation measured tracing coronal bright points in SDO/AIA images. An
automatic recognition method was applied to the series of images taken
in the test period, 1-2 January 2011. Coronal bright points are suitable
tracers for the determination of the solar differential rotation,
because they are localized objects which are very well distributed
over solar disc. Results presented in this paper show that the SDO/AIA
data are very useful for that aim, due to the high spatial and temporal
resolution of the images.
Title: High-latitude opportunities for flow measurements with the
EUI instrument onboard Solar Orbiter
Authors: Gissot, Samuel; Brajsa, Roman
Bibcode: 2014cosp...40E1000G
Altcode:
The Solar Orbiter mission will offer a unique out-of-ecliptic
vantage point which represents an excellent opportunity to address
the cyclic nature of solar magnetic activity and key questions
for our understanding of solar dynamo such as transport of flux at
high latitudes and the properties of the polar magnetic field. Our
current knowledge of solar activity at high latitudes is limited
by the ecliptic position of past and current instruments so that
characterizing the solar flows remains difficult because of this
ecliptic viewing angle. This is a major limitation to our knowledge
of meridional circulation and differential rotation at all latitudes
and radii. During the out-of-the-ecliptic phase of the mission,
Solar Orbiter will reach heliographic latitudes of 25(°) during the
nominal mission and as high as 34(°) during the extended mission and
thus contribute to new constraints and understanding for solar dynamo
models. Following past work on tracer tracking of coronal bright points
and the inter-instrument measurements of flows, we will present the
lessons learned to address these issues using the Solar Orbiter EUI and
PHI remote sensing instruments. In particular, we will present a method
for the detection and tracking of high-latitude tracers from which one
can derive requirements on the specific high solar inclination science
windows of Solar Orbiter. We will illustrate how this will contribute
to refine our current knowledge of differential rotation and meridional
flows and discuss how the successive Solar Orbiter passes at perihelion
could extend these solar activity indices over cycle time scales.
Title: The chaotic solar cycle. II. Analysis of cosmogenic
10Be data
Authors: Hanslmeier, A.; Brajša, R.; Čalogović, J.; Vršnak,
B.; Ruždjak, D.; Steinhilber, F.; MacLeod, C. L.; Ivezić, Ž.;
Skokić, I.
Bibcode: 2013A&A...550A...6H
Altcode: 2014arXiv1402.2776H
Context. The variations of solar activity over long time intervals using
a solar activity reconstruction based on the cosmogenic radionuclide
10Be measured in polar ice cores are studied.
Aims:
The periodicity of the solar activity cycle is studied. The solar
activity cycle is governed by a complex dynamo mechanism. Methods
of nonlinear dynamics enable us to learn more about the regular
and chaotic behavior of solar activity. In this work we compare
our earlier findings based on 14C data with the results
obtained using 10Be data.
Methods: By applying
methods of nonlinear dynamics, the solar activity cycle is studied
using solar activity proxies that have been reaching into the past
for over 9300 years. The complexity of the system is expressed by
several parameters of nonlinear dynamics, such as embedding dimension
or false nearest neighbors, and the method of delay coordinates is
applied to the time series. We also fit a damped random walk model,
which accurately describes the variability of quasars, to the solar
10Be data and investigate the corresponding power spectral
distribution. The periods in the data series were searched by the
Fourier and wavelet analyses.
Results: The solar activity on the
long-term scale is found to be on the edge of chaotic behavior. This
can explain the observed intermittent period of longer lasting solar
activity minima. Filtering the data by eliminating variations below
a certain period (the periods of 380 yr and 57 yr were used) yields a
far more regular behavior of solar activity. A comparison between the
results for the 10Be data with the 14C data shows
many similarities. Both cosmogenic isotopes are strongly correlated
mutually and with solar activity. Finally, we find that a series of
damped random walk models provides a good fit to the 10Be
data with a fixed characteristic time scale of 1000 years, which is
roughly consistent with the quasi-periods found by the Fourier and
wavelet analyses.
Conclusions: The time series of solar activity
proxies used here clearly shows that solar activity behaves differently
from random data. The unfiltered data exhibit a complex dynamics that
becomes more regular when filtering the data. The results indicate
that solar activity proxies are also influenced by other than solar
variations and reflect solar activity only on longer time scales.
Title: Observing the Sun with ALMA
Authors: Benz, A. O.; Brajsa, R.; Shimojo, M.; Karlicky, M.; Testi, L.
Bibcode: 2012IAUSS...6E.205B
Altcode:
The Atacama Large Millimeter Array (ALMA) is in the commissioning
phase for solar observations. A filter reduces the solar radiation
to a level suitable for solar observations. First observations
with one antenna have mapped the Sun by scanning. The results look
promising. The image of the quiet chromosphere shows large spatial
variations in emissivity. Interferometry is much more demanding and
not yet achieved. The current state and problems will be summarized. It
is clear that solar ALMA observations will take more developing time,
but will eventually be possible. The goal is subarcsecond resolution
of the quiet and active submillimeter continuum radiation originating
in the chromosphere and possibly in the flaring corona. A limiting
factor will be the temporal variability of the solar emission.
Title: Solar Hα and white light telescope at Hvar Observatory
Authors: Čalogović, J.; Dumbović, M.; Novak, N.; Vršnak, B.;
Brajša, R.; Pötzi, W.; Hirtenfellner-Polanec, W.; Veronig, A.;
Hanslmeier$, A.; Klvaňa, M.; Ambrož, P.
Bibcode: 2012CEAB...36...83C
Altcode:
Recently, the double solar telescope at Hvar Observatory was equipped
with the fourth generation of acquisition hardware and software. It
provides a valuable instrument to study rapid changes of chromospheric
and photospheric features in great detail. The telescope consists of
two Carl Zeiss refractors (photosphere d=217mm, chromosphere d=130mm)
mounted as one unit on a German parallax mounting. Using a field
of view of about 7 and 11 arcmin, it aims to produce high-resolution
high-cadence imaging of active regions on the Sun. New Pulnix TM-4200GE
12-bit CCD cameras allow to obtain time series with a cadence up to
30 images per minute.
Title: A relationship between the solar rotation and activity in the
period 1998-2006 analysed by tracing small bright coronal structures
in SOHO-EIT images
Authors: Jurdana-Šepić, R.; Brajša, R.; Wöhl, H.; Hanslmeier,
A.; Poljančić, I.; Svalgaard, L.; Gissot, S. F.
Bibcode: 2011A&A...534A..17J
Altcode:
Aims: The study aims to find a relationship between the rotation
of the small bright coronal structures (SBCS) described by the solar
rotation parameters and indices of solar activity on monthly and yearly
temporal scales.
Methods: We analyse precise measurements of
the solar differential rotation determined by tracing SBCS in SOHO-EIT
images and compare the derived solar rotation parameters with the
status of solar activity in the period 1998 - 2006. Full-disc solar
images obtained with the Extreme ultraviolet Imaging Telescope (EIT)
on board the Solar and Heliospheric Observatory (SOHO) were used to
analyse solar differential rotation determined by tracing SBCS. An
automatic method to identify and track the SBCS in EIT full-disc images
with a six hour cadence is applied. We performed a statistical analysis
of the monthly and yearly values of solar sidereal rotation velocity
parameters A and B (corresponding to the equatorial rotation velocity
and the gradient of the solar differential rotation, respectively)
as a function of various solar activity indices.
Results:
The dependence of the solar rotation on the phase of the solar cycle
was found. It is clearly visible for the solar rotation parameter A,
whilst the results are not conclusive for parameter B. The relationship
between the solar rotation and activity, expressed by the monthly
relative sunspot number, the smoothed monthly relative sunspot number,
the yearly relative sunspot number, and the interdiurnal variability
(IDV) index was investigated. The statistically significant correlation
was found for the solar rotation parameter A, whilst a very low and
insignificant correlation was obtained for the rotation parameter
B.
Conclusions: During the maximum of the solar cycle 23 and
just after it, the equatorial solar rotation velocity was lower than
in other phases of the cycle, when there was less activity. This is
consistent with other observational findings, obtained by different
tracers and methods.
Title: Differences in heliographic positions and rotation velocities
of sunspot groups from various observatories
Authors: Poljančić, I.; Brajša, R.; Hržina, D.; Wöhl, H.;
Hanslmeier, A.; Pötzi, W.; Baranyi, T.; Özgüç, A.; Singh, J.;
Ruždjak, V.
Bibcode: 2011CEAB...35...59P
Altcode:
Measured positions of sunspot groups that differ in format, precision
and observing procedure are collected from various data sets:
GPR (Greenwich Photoheliographic Results), SOON/USAF/NOAA (Solar
Optical Observing Network/United States Air Force/National Oceanic and
Atmospheric Administration), as well as from the Kodaikanal and Debrecen
observatories. Kanzelh&{uml;o}he and Kandilli Observatory currently
provide the digitized sunspot drawings, from which the positions
of selected sunspot groups are determined with a special software
Sungrabber. The rotation velocities are calculated from the position
data. The aim of this work is to compare and to check the precision
of the mentioned data sets using the Kanzelh&{uml;o}he Observatory
data set as the reference basis of sunspot position measurements. The
selected groups (about 40% consist of single sunspots Z&{uml;u}rich
types H and J) are from the years 1972 and 1993 belonging to similar
declining phases of two solar activity cycles. The occurrence of some
systematic differences of the sunspot group positions and rotation
velocities suggests the need for a more detailed analysis of the data
accumulation procedures.
Title: Heating of the quiet solar corona from measurements of the
FET/TESIS instrument on-board the KORONAS-FOTON satellite
Authors: Rybák, J.; Gömöry, P.; Benz, A.; Bogachev, P.; Brajša, R.
Bibcode: 2010nspm.conf..107R
Altcode:
The paper presents the first results of the observations of time
evolution of the quiet solar corona brightenings obtained due to
very rapid photography of the corona with full-disk EUV telescopes
of the FET/TESIS instrument onboard the KORONA FOTON satellite. The
measurements were performed simultaneously in the emission of the Fe
IX / X 17.1 and Fe VIII 13.1 spectral lines with 10 second temporal
cadence and spatial scale of 1.7 arc seconds within one hour. This
test observation, carried out on 15 July 2009, was analyzed in order
to determine whether this type of observation can be used to identify
individual microevents in the solar corona heating that are above the
tresholds of spatial and temporal resolutions of the observations
of non-active regions in the solar atmosphere. For this purpose,
a simple method was used involving cross-correlation of the plasma
emission time evolution at different temperatures, each time from
observations of identical elements. The results obtained are confronted
with the expected observable manifestations of the corona heating
via nanoflares. TESIS is a set of instruments for the Sun photography
developed in the Lebedev Physics Institute of the Russian Academy of
Sciences that was launched into orbit in January 2009.
Title: A Comparison of Sunspot Position Measurments from Different
Data Sets
Authors: Poljancic, I.; Brajsa, R.; Ruzdjak, D.; Hrzina, D.;
Jurdana-Sepic, R.; Wohl, H.; Otruba, W.
Bibcode: 2010SunGe...5...52P
Altcode:
There are several data sets containing information about the positions
of sunspots and sunspot groups that differ in format and precision,
e.g. GPR (Greenwich Photoheliographic Results) and SOON/USAF/NOAA
(Solar Optical Observing Network/United States Air Force/National
Oceanic and Atmosferic Administration). The aim of this paper is to
check the precision of the SOON/USAF/NOAA data set and compare it with
the GPR data set. For this purpose, we have used a third data set,
the Kanzelhöhe Observatory data set, as a basis of sunspot position
measurments comparison. The positions of selected sunspot groups in the
digitized Solar Observatory Kanzelhöhe drawings were determined with
a special software Sungrabber. The selected groups consisted mostly of
single sunspots (Zürich types H and J) from the years 1972 and 1993
belonging to the similar phases of the solar activity cycles. The
determined Kanzelhöhe sunspot group coordinates were compared with
those from GPR for the year 1972 and SOON/USAF/NOAA for the year
1993. The rotation velocities calculated for sunspot groups observed
at Kanzelhöhe were compared with the ones obtained from the two data
sets mentioned above in the same observing periods. With the assumption
of constant precision of Kanzelhöhe drawings, it was established that
SOON/USAF/NOAA data are somewhat less precise than the GPR ones.
Title: A precise measurement of the solar differential rotation by
tracing small bright coronal structures in SOHO-EIT images. Results
and comparisons for the period 1998-2006
Authors: Wöhl, H.; Brajša, R.; Hanslmeier, A.; Gissot, S. F.
Bibcode: 2010A&A...520A..29W
Altcode:
Aims: We precisely determine the solar rotation velocity during
most of the 23rd solar cycle, in the years 1998-2006. We measure the
solar differential rotation by tracing small bright coronal structures
(SBCS) in SOHO-EIT images.
Methods: The 28.4 nm EIT channel was
used and positions of more than 55 000 structures were measured applying
an interactive and improved automatic method of data reduction.
Results: We achieve the closest representation of the observational
data when all three solar differential rotation parameters are used
and obtain the formula ω (b) = 14.499 (±0.006) - 2.54 (±0.06)
sin2 b - 0.77 (±0.09) sin4b. This result
represents the sidereal rotation velocity in deg day-1
and is produced by the automatic method applied in 1998-2006. A
north-south rotational asymmetry and a rigid component of the solar
rotation at high latitudes were found.
Conclusions: A more
differential rotation profile of SBCS than of sunspots and sunspot
groups was found. The rotation velocity of SBCS is very similar
to those obtained by small photospheric magnetic features. The
north-south rotational asymmetry of SBCS was interpreted with a model
of the relationship between solar rotation and activity. The rigid
component of the solar rotation at high latitudes, identifiable only
from the results of the automatic method, was related to larger
structures mostly identified by that method, in contrast to the
interactive method, which detected smaller structures. Tables
3-11 and Figs. 4, 5 are only available in electronic form at http://www.aanda.org
Title: The chaotic solar cycle. I. Analysis of cosmogenic
14C-data
Authors: Hanslmeier, A.; Brajša, R.
Bibcode: 2010A&A...509A...5H
Altcode:
Context. The study of solar activity over long time intervals using
proxies.
Aims: The periodicity of the solar activity cycle
is studied. The solar activity cycle is governed by a complex dynamo
mechanism. Methods of nonlinear dynamics enable us to learn more about
the regular and chaotic behavior of solar activity.
Methods:
By applying methods of nonlinear dynamics, the solar activity cycle is
studied by using solar activity proxies that have been reaching into the
past for over 10 000 years. The complexity of the system is expressed by
several parameters of nonlinear dynamics, such as embedding dimension or
false nearest neighbors, and the method of delay coordinates is applied
to the time series.
Results: The solar activity cycle is found
to be on the edge of chaotic behavior. This can explain the observed
intermittent period of longer lasting solar activity minima. Filtering
the data by eliminating variations below a certain period (the periods
380 yr and 57 yr were used) yields a far more regular behavior of
solar activity.
Conclusions: The solar time series of solar
activity proxies used here clearly shows that solar activity behaves
differently from random data. The unfiltered data exhibit a complex
dynamics requiring an embedding dimension >15. The variations become
more regular when filtering the data. The results also indicate that
solar activity proxies are influenced by other than solar variations
and reflect solar activity only on longer time scales.
Title: Solar activity proxies - geomagnetic variations
Authors: Hanslmeier, A.; Brajša, R.
Bibcode: 2010CEAB...34..109H
Altcode:
Solar activity proxies are needed for studies of the long term
behaviour of the solar activity cycles. Cosmogenic isotopes like
^{14}C are influenced also by geomagnetic field variations. We study
the influence of such variations on the global behaviour of solar
activity measured by methods of non linear dynamics.
Title: Comparison of the sidereal angular velocity of subphotospheric
layers and small bright coronal structures during the declining
phase of solar cycle 23
Authors: Zaatri, A.; Wöhl, H.; Roth, M.; Corbard, T.; Brajša, R.
Bibcode: 2009A&A...504..589Z
Altcode: 2009arXiv0907.4099Z
Context: We compare solar differential rotation of subphotospheric
layers derived from local helioseismology analysis of GONG++
dopplergrams and the one derived from tracing small bright coronal
structures (SBCS) using EIT/SOHO images for the period August
2001-December 2006, which correspond to the declining phase of solar
cycle 23.
Aims: The study aims to find a relationship between
the rotation of the SBCS and the subphotospheric angular velocity. The
north-south asymmetries of both rotation velocity measurements are also
investigated.
Methods: Subphotospheric differential rotation was
derived using ring-diagram analysis of GONG++ full-disk dopplergrams of
1 min cadence. The coronal rotation was derived by using an automatic
method to identify and track the small bright coronal structures in
EIT full-disk images of 6 h cadence.
Results: We find that the
SBCS rotate faster than the considered upper subphotospheric layer
(3 Mm) by about 0.5 deg/day at the equator. This result joins the
results of several other magnetic features (sunspots, plages, faculae,
etc.) with a higher rotation than the solar plasma. The rotation rate
latitudinal gradients of the SBCS and the subphotospheric layers
are very similar. The SBCS motion shows an acceleration of about
0.005°day-1/month during the declining phase of solar cycle
23, whereas the angular velocity of subsurface layers does not display
any evident variation with time, except for the well known torsional
oscillation pattern. Finally, both subphotospheric and coronal rotations
of the southern hemisphere are predominantly larger than those of the
northern hemisphere. At latitudes where the north-south asymmetry of
the angular velocity increases (decreases) with activity for the SBCS,
it decreases (increases) for subphotospheric layers.
Title: On solar cycle predictions and reconstructions
Authors: Brajša, R.; Wöhl, H.; Hanslmeier, A.; Verbanac, G.;
Ruždjak, D.; Cliver, E.; Svalgaard, L.; Roth, M.
Bibcode: 2009A&A...496..855B
Altcode:
Context: Generally, there are two procedures for solar cycle
predictions: the empirical methods - statistical methods based on
extrapolations and precursor methods - and methods based on dynamo
models.
Aims: The goal of the present analysis is to forecast
the strength and epochs of the next solar cycle, to investigate proxies
for grand solar minima and to reconstruct the relative sunspot number
in the Maunder minimum.
Methods: We calculate the asymmetry of
the ascending and descending solar cycle phases (Method 1) and use this
parameter as a proxy for solar activity on longer time scales. Further,
we correlate the relative sunspot numbers in the epochs of solar
activity minima and maxima (Method 2) and estimate the parameters of
an autoregressive moving average model (ARMA, Method 3). Finally,
the power spectrum of data obtained with the Method 1 is analysed
and the Methods 1 and 3 are combined.
Results: Signatures of
the Maunder, Dalton and Gleissberg minima were found with Method 1. A
period of about 70 years, somewhat shorter than the Gleissberg period
was identified in the asymmetry data. The maximal smoothed monthly
sunspot number during the Maunder minimum was reconstructed and found
to be in the range 0-35 (Method 1). The estimated Wolf number (also
called the relative sunspot number) of the next solar maximum is in
the range 88-102 (Method 2). Method 3 predicts the next solar maximum
between 2011 and 2012 and the next solar minimum for 2017. Also, it
forecasts the relative sunspot number in the next maximum to be 90
± 27. A combination of the Methods 1 and 3 gives for the next solar
maximum relative sunspot numbers between 78 and 99.
Conclusions:
The asymmetry parameter provided by Method 1 is a good proxy for solar
activity in the past, also in the periods for which no relative sunspot
numbers are available. Our prediction for the next solar cycle No. 24
is that it will be weaker than the last cycle, No. 23. This prediction
is based on various independent methods.
Title: Solar Differential Rotation Determined by Tracing Low and
High Brightness Temperature Regions at 8 mm
Authors: Romštajn, I.; Brajša, R.; Wöhl, H.; Benz, A. O.; Temmer,
M.; Roša, D.; Ruždjak, V.
Bibcode: 2009CEAB...33...79R
Altcode:
At the wavelength of 8 mm absorption features (Low brightness
Temperature Regions, LTRs) and emission features (High brightness
Temperature Regions, HTRs) can be traced for determination of solar
rotation. From earlier studies it is known that about two thirds of LTRs
are associated with Hα filaments. The goal of the present analysis is
to determine the heights of these solar structures and their rotational
velocities. We used the method for the simultaneous determination of
the solar synodic rotation velocity and the height of tracers. The
rotation velocities were determined by the linear least-square fit of
their central meridian distances as a function of time. The mean value
of the low brightness temperature regions' heights is about 45 600
km. The results of solar rotation determined by tracing LTRs and HTRs
are mutually compared and also compared with the results using other
tracers and methods. The method for the simultaneous determination
of the solar synodic rotation velocity and the height of the tracers
could be applied properly only on LTRs, since a wide distribution
over latitudes and central meridian distances of a large data set is
necessary, which was not available for HTRs. Observational findings that
HTRs rotate systematically faster than LTRs and the possibility that
they can be observed at and outside the solar limb are consistent with
relatively high altitudes of HTRs. It was concluded that the radiation
mechanism of HTRs is thermal bremsstrahlung, probably associated with
flaring active regions.
Title: Measurements of the He I 1083 nm Line on the Sun During
Different Solar Activity Phases: in the Years 1993 and 1995
Authors: Jurdana-Šepić, R.; Brajša, R.; Šaina, B.; Wöhl, H.
Bibcode: 2009CEAB...33..337J
Altcode:
Latitudinal variations and North--South asymmetry of the He I 1083 nm
line across the solar disc are investigated, based on 69 solar scans
obtained during two observational campaigns in 1993 and 1995. In both
time intervals the He I 1083 nm line was observed in absorption. The
ratio of average intensity in the line and the average intensity of
the nearby quasi-continuum was calculated and cautiously used
as a measure of the line absorption and represented as a function of
position on the solar disc. Results indicate a distinct dependence of
the He I 1083 nm average relative line intensity on the solar latitude,
its North--South asymmetry and well pronounced differences for the two
campaigns belonging to different solar cycle phases. The latitudinal
dependence is analysed with and without discriminating quiet Sun and
active regions. The observed behaviour (the latitudinal dependence
and a North--South asymmetry) is to a large extent a consequence
of the presence/absence of active regions. The differences found
for the two observing campaigns are not only due to decreased solar
activity in 1995 as compared to 1993, but also due to different and
much more pronounced telluric water-vapour contamination during the
second observing interval. Finally, in the Appendix we describe all
He scans used here in more detail and compare the numbers of solar
features identified in other spectral regions along the scans for the
two observational campaigns.
Title: A Prediction for the 24th Solar Cycle
Authors: Brajša, R.; Wöhl, H.; Hanslmeier, A.; Verbanac, G.;
Ruždjak, D.; Cliver, E.; Svalgaard, L.; Roth, M.
Bibcode: 2009CEAB...33...95B
Altcode:
The aim of the present analysis is to forecast the strength of the
next solar maximum of the 24th cycle. We correlate the
relative sunspot numbers in the epochs of solar activity minima and
maxima. Using this method, the estimated relative sunspot number (also
called the Wolf number) of the next solar maximum is in the range 67-81,
i.e., about 40 % below the peak sunspot number of 121 for cycle No. 23.
Title: Heights of solar tracers observed at 8 mm and an interpretation
of their radiation
Authors: Brajša, R.; Romštajn, I.; Wöhl, H.; Benz, A. O.; Temmer,
M.; Roša, D.
Bibcode: 2009A&A...493..613B
Altcode:
Context: At the wavelength of 8 mm, emissive features (high
brightness-temperatrue regions, HTRs) and absorptive features
(low brightness-temperature regions, LTRs) can be traced for the
determining the solar rotation. From earlier studies it is known
that about two thirds of LTRs are associated with Hα filaments.
Aims: Thermal bremsstrahlung and gyromagnetic (cyclotron) radiation
mechanism can be important for explaining the observed phenomena,
so we determine the heights of solar structures and interpret their
radiation mechanism(s).
Methods: We use the method of simultaneous
determination of the solar synodic rotation velocity and the height
of tracers. The rotation velocities were determined by the linear
least-square fit of their central meridian distance as a function of
time. We used a procedure for calculating the brightness temperature for
a given wavelength and model atmosphere, which integrates the radiative
transfer equation for the thermal bremsstrahlung.
Results:
The mean value of the low brightness-temperature regions' heights
is about 45 600 km. This height was used as input for constructing
prominence and coronal condensation models, which, when assuming thermal
bremsstrahlung as the radiation mechanism, yield a decrease in the
brightness temperature of 2-14%, in agreement with observations. If the
same radiation mechanism is considered, the models of the solar corona
above active regions give an increase in the brightness temperature of
5-19%, also in agreement with observations. In this case an indirect
indication (from the rotational analysis) that the HTRs are located
higher in the solar atmosphere than the LTRs was taken into account.
Conclusions: The method for simultaneously determining the solar
synodic rotation velocity and the height of tracers could have only
been properly applied on LTRs, since a homogeneous distribution over
latitudes and central meridian distances of a large enough data set is
necessary. Thermal bremsstrahlung can explain both the LTR (prominences
and coronal condensations) and HTR (ordinary active regions) phenomena
observed at 8 mm. At this wavelength, thermal gyromagnetic emission
is almost surely excluded as a possible radiation mechanism.
Title: On the Solar Rotation and Activity in the Years 1998 - 2003
Authors: Brajsa, R.; Woehl, H.; Hanslmeier, A.; Gissot, S. F.
Bibcode: 2008ESPM...122.114B
Altcode:
Full-disc full-resolution solar images obtained by the Extreme
Ultraviolet Imaging Telescope on board the Solar and Heliospheric
Observatory are used to analyse solar differential rotation by tracing
coronal bright points. We applied an interactive and an improved
automatic method of data reduction to process data obtained during
the period 1998 - 2003 with the Fe XV filter (28.4 nm). Using the
interactive method, the data obtained during 8 months of observation
were reduced, while the automatic method was applied to analyse the
whole 57-month period of observations, since the interactive method is
much more time consuming than the automatic one. In the present work we
especially focus on a possible relationship between the solar rotation
(57 monthly values of the solar rotation parameters determined with
the automatic method) and activity (relative sunspot numbers for the
same months) during the maximum phase of the solar cycle 23.
Title: Proper Motions of Coronal Bright Points
Authors: Brajša, R.; Wöhl, H.; Vršnak, B.; Ruždjak, V.; Clette,
F.; Hochedez, J. -F.; Verbanac, G.; Skokić, I.; Hanslmeier, A.
Bibcode: 2008CEAB...32..165B
Altcode:
Full-field full-resolution solar images obtained by the Extreme
Ultraviolet Imaging Telescope on board the Solar and Heliospheric
Observatory are used to analyse proper motions, velocity distributions,
lifetimes, and diffusion coefficient of coronal bright points. The
results obtained by the interactive method for three tracer subtypes
(point-like structures, small loops, and small active regions)
of coronal bright points for the period 4 June 1998 to 22 May 1999
are presented and compared. Distributions of meridional velocities,
residual azimuthal velocities and velocities of proper motions are
presented for the three tracer subtypes. Lifetimes up to 54 hours
are found for 98% of all observed coronal bright points. Small active
regions last on the average longer than point-like structures and small
loops. The correlation between the absolute velocity of proper motion
and lifetime is investigated and the mean free path (in the range from
3000 km to 15000 km) and the diffusion coefficient (approximately 200
km2/s) of coronal bright points are estimated. Finally,
characteristics of the random walk process associated to the motions
of coronal bright points are discussed in the Appendix.
Title: Coronal Bright Points as Tracers for Solar Rotation in
October-November 1999
Authors: Brajša, R.; Mulec, M.; Hanslmeier, A.; Wöhl, H.; Ruždjak,
V.; Hochedez, J. -F.
Bibcode: 2008CEAB...32..117B
Altcode:
Whole-disc full-resolution solar images taken in the extreme ultraviolet
part of the spectrum (Fe XV line at 28.4 nm) with the EIT instrument
on board the SOHO spacecraft were used to visually identify coronal
bright points appropriate for solar rotation determination. From the
time differences in successive tracer positions amounting to six hours
the solar rotation velocity was determined tracing coronal bright
points in images obtained in October and November 1999. The resulting
parameters and profiles of the solar rotation are presented.
Title: On the solar rotation and activity
Authors: Brajša, R.; Wöhl, H.; Ruždjak, D.; Vršnak, B.; Verbanac,
G.; Svalgaard, L.; Hochedez, J. -F.
Bibcode: 2007AN....328.1013B
Altcode:
The interaction between differential rotation and magnetic fields in
the solar convection zone was recently modelled by Brun (2004). One
consequence of that model is that the Maxwell stresses can oppose the
Reynolds stresses, and thus contribute to the transport of the angular
momentum towards the solar poles, leading to a reduced differential
rotation. So, when magnetic fields are weaker, a more pronounced
differential rotation can be expected, yielding a higher rotation
velocity at low latitudes taken on the average. This hypothesis
is consistent with the behaviour of the solar rotation during the
Maunder minimum. In this work we search for similar signatures of
the relationship between the solar activity and rotation determined
tracing sunspot groups and coronal bright points. We use the extended
Greenwich data set (1878-1981) and a series of full-disc solar images
taken at 28.4 nm with the EIT instrument on the SOHO spacecraft
(1998-2000). We investigate the dependence of the solar rotation on
the solar activity (described by the relative sunspot number) and
the interplanetary magnetic field (calculated from the interdiurnal
variability index). Possible rotational signatures of two weak solar
activity cycles at the beginning of the 20th century (Gleissberg
minimum) are discussed.
Title: An Interpretation of the Coronal Holes' Visibility in the
Millimeter Wavelength Range
Authors: Brajša, R.; Benz, A. O.; Temmer, M.; Jurdana-Šepić, R.;
Šaina, B.; Wöhl, H.
Bibcode: 2007SoPh..245..167B
Altcode:
Various observations indicate that coronal holes generally appear as low
brightness temperature regions (LTRs) in the centimeter and millimeter
wavelength ranges. However, within their borders local enhancements of
radiation, that is, high brightness temperature regions (HTRs), often
occur. The theory behind the described behavior is not fully understood
and therefore we analyze full-disk solar images obtained at a wavelength
of 8 mm at Metsähovi Radio Observatory and compare them with data
simultaneously taken in other wavelength ranges. The observational
finding that the average brightness temperature of coronal holes is
not much different from the quiet-Sun level (with localized deviations
toward higher and lower intensities on the order of a few percent)
is compared with theoretical models of the thermal bremsstrahlung
radiation originating in the solar chromosphere, transition region,
and corona. Special attention is devoted to the interpretation of the
localized enhancements of radiation observed inside coronal holes at
millimeter wavelengths. The main conclusion is that the most important
contribution to the brightness temperature comes from an increased
density in the transition region and low corona (i.e., at the heights
where the temperature is below 106 K). This can explain
both the LTRs and HTRs associated with coronal holes.
Title: Are There Radio-quiet Solar Flares?
Authors: Benz, Arnold O.; Brajša, Roman; Magdalenić, Jasmina
Bibcode: 2007SoPh..240..263B
Altcode: 2007astro.ph..1570B
Some 15% of solar flares having a soft X-ray flux above GOES class C5
are reported to lack coherent radio emission in the 100 - 4000 MHz
range (type I - V and decimetric emissions). A detailed study of 29
such events reveals that 22 (76%) of them occurred at a radial distance
of more than 800″ from the disk center, indicating that radio waves
from the limb may be completely absorbed in some flares. The remaining
seven events have statistically significant trends to be weak in GOES
class and to have a softer non-thermal X-ray spectrum. All of the
non-limb flares that were radio-quiet above 100 MHz were accompanied by
metric type III emission below 100 MHz. Out of 201 hard X-ray flares,
there was no flare except near the limb (R>800″) without coherent
radio emission in the entire meter and decimeter range. We suggest
that flares above GOES class C5 generally emit coherent radio waves
when observed radially above the source.
Title: Solar Rotation Velocity Determined by Coronal Bright Points -
New Data and Analysis
Authors: Mulec, M.; Brajša, R.; Wöhl, H.; Hanslmeier, A.; Vršnak,
B.; Ruždjak, V.; Hochedez, J. -F.; Engler, J.
Bibcode: 2007CEAB...31....1M
Altcode:
Full-disc solar images obtained with the Extreme Ultraviolet Imaging
Telescope on board the Solar and Heliospheric Observatory were used
to analyse solar differential rotation determined by tracing coronal
bright points. Rotation velocity residuals, meridional motions and
their relationship are investigated for a new data set from October
1, 1999 to March 31, 2000. Further we take care for the evolution
of the single structures, dividing them into Point-Like-Structures,
Small Loops and Small Active Regions and analysing their variation in
intensity and size.
Title: Sungrabber - Software for Measurements on Solar Synoptic Images
Authors: Hržina, D.; Roša, D.; Hanslmeier, A.; Ruždjak, V.;
Brajša, R.
Bibcode: 2007CEAB...31..273H
Altcode:
Measurement of positions of the tracers on synoptic solar images and
conversion to heliographic coordinates is a time-consuming procedure
with different sources of errors. To make measurements faster and
easier, the application "Sungrabber" was developed. The data of the
measured heliographic coordinates are stored in text files which are
linked to the related solar images, which allows also a fast and simple
comparison of the measurements from different sources. Extension of
the software is possible and therefore Sungrabber can be used for
different purposes (e.g. determining the solar rotation rate, proper
motions of the tracers on the Sun, etc.).
Title: On the Visibility of Coronal Holes in Microwaves
Authors: Brajša, R.; Benz, A. O.; Temmer, M.; Jurdana-Šepić, R.;
Šaina, B.; Wöhl, H.; Ruždjak, V.
Bibcode: 2007CEAB...31..219B
Altcode:
Previous observations indicate that coronal holes generally appear as
low brightness temperature regions in microwaves. However, within their
borders local enhancements of radiation often occur. This is confirmed
by comparing a full-disc solar image obtained at 37 GHz on 27 May 1993
with full-disc solar images obtained at various wavelengths. Microwave
brightness temperatures of three coronal holes are determined and
interpreted.
Title: Are there Radio-quiet Solar Flares?
Authors: Benz, Arnold O.; Brajsa, Roman; Magdalenic, Jasmina
Bibcode: 2007astro.ph..1566B
Altcode:
This submission has been withdrawn by arXiv administrators because it
is a duplicate of astro-ph/0701570.
Title: Temporal Variations of the Solar Rotation Determined by
Sunspot Groups
Authors: Brajša, R.; Ruždjak, D.; Wöhl, H.
Bibcode: 2006SoPh..237..365B
Altcode: 2006SoPh..tmp...37B
The extended Greenwich data set consisting of positions of sunspot
groups is used for the investigation of cycle-related variations of the
solar rotation in the years 1874-1981. Applying the residual method,
which yields a single number for each year describing the average
deviation from the mean value of the solar rotation, the dependence
of the rotation velocity residual on the phase of the solar cycle is
investigated. A secular deceleration of the solar rotation was found:
the slope being statistically significant at the 3σ level. Periods
of 33, 22, 11, 5.2, and 3.5 years can be identified in the power
spectra. The rotation velocity residuals were averaged for all years
with the same solar cycle phase relative to the nearest preceding
sunspot minimum. The variation pattern reveals a higher than average
rotation velocity in the minimum of activity and, to a lesser extent,
also around the maximum of activity. The analysis was repeated with
several changes in the reduction method, such as elimination of the
secular trend, application of statistical weights, different cutoffs of
the central meridian distance, division of the latitude into subregions
and treating data from the years of activity minima separately. The
results obtained are compared with those from the literature, and an
interpretation of the observed phenomena is proposed.
Title: Periodical patterns in major flare occurrence and their
relation to magnetically complex active regions
Authors: Temmer, M.; Veronig, A.; Rybák, J.; Brajša, R.; Hanslmeier,
A.
Bibcode: 2006AdSpR..38..886T
Altcode:
A periodical occurrence rate of major solar flares (observed in hard
X-rays) of ∼24 days (synodic) was first reported by Bai (1987)
[Bai, T. Distribution of flares on the sun superactive regions and
active zones of 1980 1985. ApJ 314, 795 807, 1987] for the years 1980
1985. Here, we report a significant relation between the appearance of
the 24-day period in major Hα flares and magnetically complex sunspot
groups (i.e., including a γ and/or δ configuration). From synoptic
maps of magnetograms (NSO/KP) patterns in the magnetic flux evolution
are traced which might be the cause of the 24-day period observed in
flare activity.
Title: Spatial Distribution and North-South Asymmetry of Coronal
Bright Points from Mid-1998 to Mid-1999
Authors: Brajša, R.; Wöhl, H.; Vršnak, B.; Rušdjak, V.; Clette,
F.; Hochedez, J. -F.; Verbanac, G.; Temmer, M.
Bibcode: 2005SoPh..231...29B
Altcode:
Full-disc full-resolution (FDFR) solar images obtained with the Extreme
Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric
Observatory (SOHO) were used to analyse the centre-to-limb function and
latitudinal distribution of coronal bright points. The results obtained
with the interactive and the automatic method, as well as for three
subtypes of coronal bright points for the time period 4 June 1998 to 22
May 1999 are presented and compared. An indication of a two-component
latitudinal distribution of coronal bright points was found. The
central latitude of coronal bright points traced with the interactive
method lies between 10∘ and 20∘. This is
closer to the equator than the average latitude of sunspots in the
same period. Possible implications for the interpretation of the
solar differential rotation are discussed. In the appendix, possible
differences between the two solar hemispheres are analysed. More coronal
bright points were present in the southern solar hemisphere than in
the northern one. This asymmetry is statistically significant for the
interactive method and not for the automatic method. The visibility
function is symmetrical around the central meridian.
Title: The Influence of the Evolution of Sunspot Groups on the
Determination of the Solar Velocity Field
Authors: Ruždjak, D.; Brajša, R.; Sudar, D.; Wöhl, H.
Bibcode: 2005SoPh..229...35R
Altcode:
Meridional motions and differential rotation of stable recurrent
sunspot groups from the Greenwich data set are investigated. Simple
and complex, as well as younger and older sunspot groups are treated
separately. There is no difference in behavior of the meridional
motions for the simple and complex sunspot groups, while complex groups
rotate faster than the simple ones. If we attribute the differences of
rotational velocities to the errors in position determination, it can be
concluded that the rotational velocities determined by using sunspot
groups as tracers are slightly overestimated. Both the meridional
motions and differential rotation show the same dependence on the age,
when simple and complex recurrent sunspot groups are considered.
Title: The Two Complexes of Activity Observed in the Northern
Hemisphere during 1982 and the 24-Day Periodicity of Flare Occurrence
Authors: Ruždjak, V.; Ruždjak, D.; Brajša, R.; Temmer, M.;
Hanslmeier, A.
Bibcode: 2005HvaOB..29..117R
Altcode:
Daily numbers of solar Hα flares of importance classes ≥ 1 for
the northern solar hemisphere in 1982 are studied applying wavelet
power spectra (WPS). Special attention is paid to the occurrence of a
24-day period in the WPS. The wavelet power spectra method is combined
with synoptic maps of the magnetic fields. Separately, flare indices
of two activity complexes mainly contributing to flare occrrence in
this period are examined. It is found that the detected 24-day signal
in the WPS is mainly a consequence of the presence of the two flare
activity complexes separated by about 45° in longitude during several
succesive Carrington rotations.
Title: On the 24-day period observed in solar flare occurrence
Authors: Temmer, M.; Veronig, A.; Rybák, J.; Brajša, R.; Hanslmeier,
A.
Bibcode: 2004SoPh..221..325T
Altcode:
Time series of daily numbers of solar Hα flares from 1955 to
1997 are studied by means of wavelet power spectra with regard to
predominant periods in the range of ∼ 24 days (synodic). A 24-day
period was first reported by Bai (1987) for the occurrence rate of
hard X-ray flares during 1980-1985. Considering the northern and
southern hemisphere separately, we find that the 24-day period is not
an isolated phenomenon but occurs in each of the four solar cycles
investigated (No. 19-22). The 24-day period can be established also
in the occurrence rate of subflares but occurs more prominently in
major flares (importance classes ≥ 1). A comparative analysis of
magnetically classified active regions subdivided into magnetically
complex (i.e., including a γ and/or δ configuration) and non-complex
(α, β) reveals a significant relation between the appearance of
the 24-day period in Hα flares and magnetically complex sunspot
groups, whereas it cannot be established for non-complex groups. It is
suggested that the 24-day period in solar flare occurrence is related
to a periodic emergence of new magnetic flux rather than to the surface
rotation of sunspots.
Title: Deceleration of the rotational velocities of sunspot groups
during their evolution
Authors: Ruždjak, D.; Ruždjak, V.; Brajša, R.; Wöhl, H.
Bibcode: 2004SoPh..221..225R
Altcode:
Using the Greenwich Photoheliographic Results for the years
1874-1976 the daily rotational velocities for 955 recurrent and 13169
non-recurrent sunspot groups from the first day of their appearance and
during their evolution have been determined. The rotational velocities
were divided in six latitude strips with a width of five degrees and
grouped according to the age of the groups. It was established that
the rotational velocities of recurrent and non-recurrent sunspot
groups decrease with time in all studied latitude strips. At their
birth the recurrent spot groups rotate faster by about 0.15°
day−1 than the non-recurrent ones and settle, within
the errors of measurements, to an about 0.5° day−1
slower velocity value during the second disc passage. A comparison
of our results with helioseismology measurements indicates that in
the frame of the anchoring hypothesis, the recurrent sunspot groups
at their birth could be coupled to the fast rotating layer at about
r=0.93 R⊙.
Title: Height correction in the measurement of solar differential
rotation determined by coronal bright points
Authors: Brajša, R.; Wöhl, H.; Vršnak, B.; Ruždjak, V.; Clette,
F.; Hochedez, J. -F.; Roša, D.
Bibcode: 2004A&A...414..707B
Altcode:
Full-disc solar images obtained with the Extreme Ultraviolet Imaging
Telescope (EIT) on board the Solar and Heliospheric Observatory
(SOHO) are used to analyse solar differential rotation by tracing
coronal bright points for the period June 4, 1998 to May 22, 1999. A
method for the simultaneous determination of the true solar synodic
rotation velocity and the height of the tracers is applied to data
sets analysed with interactive and automatic methods. The calculated
height of coronal bright points is on average 8000-12000 km above
the photosphere. Corrected rotation velocities are transformed
into sidereal ones and compared with results from the literature,
obtained with various methods and tracers. The differential rotation
profile determined by coronal bright points with the interactive method
corresponds roughly to the profile obtained by correlating photospheric
magnetic fields and the profile obtained from the automatic method
corresponds roughly to the rotation of sunspot groups. This result is
interpreted in terms of the differences obtained in the latitudinal
distribution of coronal bright points using the two methods.
Title: Importance of magnetically complex active regions on solar
flare occurrence
Authors: Temmer, M.; Veronig, A.; Rybák, J.; Brajša, R.; Hanslmeier,
A.
Bibcode: 2004HvaOB..28...95T
Altcode:
Daily numbers of solar Hα flares from 1955 to 1997 and daily numbers
of magnetically classified active regions for the time span 1964--1997
are studied applying wavelet power spectra. The occurrence of dominant
periods in the range of ∼24 days (synodic) is investigated considering
the northern and southern hemisphere separately. From the flare events
it is revealed that the 24-day period occurs in each of the four solar
cycles investigated (no. 19--22). The 24-day period can be established
also in the occurrence rate of subflares but occurs more prominently
in major flares (importance classes ≥1). Magnetically complex active
regions, i.e. including a γ and/or δ configuration, show the 24-day
period closely related to those found for major Hα flares, whereas
it cannot be established for non-complex α, β groups.
Title: Variation of the solar rotation during the activity cycle
applying the residual method to Greenwich data
Authors: Brajša, R.; Wöhl, H.; Ruždjak, D.; Schawinski-Guiton, K.
Bibcode: 2004HvaOB..28...55B
Altcode:
The Greenwich data set consisting of positions of sunspot groups was
used for the investigation of possible cycle-related variations of the
solar rotation in the years from 1874 to 1976. The measurements were
extended with the USAF/SOON and NOAA data for the years 1977--1981. The
residual method providing yearly deviations from the mean rotation
velocity (averaged over all years) for each 5-deg latitude band was
applied. These deviations were averaged over latitudes and yearly
residuals were calculated. A dependence of the rotation velocity
residual on the phase of the solar cycle was found and compared with
results from the literature.
Title: Proper motions of sunspots - new data and further results
Authors: Wöhl, H.; Brajša, R.; Kučera, A.; Ruždjak, V.; Rybák, J.
Bibcode: 2004HvaOB..28...47W
Altcode:
The attempts to investigate patterns of motions of main spot components
and small pores in complex sunspot groups were continued. The data
discussed stem mainly from observations at Hvar/HR and Tatranská
Lomnica/SK. Two historic collections of photographic plates are
included: The plate archives of the former Fraunhofer Institute and
that of photographic plates taken at Skalnate Pleso/SK.
Title: Periodical patterns in major flare occurrence and their
relation to magnetically complex active regions
Authors: Temmer, M.; Veronig, A.; Rybak, J.; Brajsa, R.; Hanslmeier, A.
Bibcode: 2004cosp...35.1395T
Altcode: 2004cosp.meet.1395T
A periodical occurrence rate of solar major flares (observed in
hard X-rays) of about 24 days (synodic) was first reported by Bai
(1987) for the years 1980--1985. Its origin is still far from being
understood. Applying wavelet analyses for daily numbers of Hα flare
events covering almost four entire solar cycles (no. 19--22) reveals a
24-day period in each of the cycles studied. This can be established
primarily in the occurrence rate of major flares but is also seen
in subflares. Since large flares occur preferentially in association
with active regions of complex magnetic configuration a comparative
study of magnetically classified active regions, subdivided into
magnetically complex (i.e. including a γ and/or δ configuration)
and non-complex (α, β) was performed. A significant relation
between the appearance of the 24-day period in major Hα flares and
magnetically complex sunspot groups is found, whereas it cannot be
established for non-complex groups. From solar rotation studies based
on tracing sunspots practically no siderial rotation velocities as high
as 16 deg/day (which corresponds to a synodic period of ∼24 days)
are reported. Thus the cause of the 24-day period is very likely not
related to solar surface rotation. Alternatively it might be due to
periodical patterns in magnetic flux emergence which is an important
driver of flare eruptions. We test this hypothesis by investigating
synoptic maps of magnetograms (National Solar Observatory/Kitt Peak)
for selected time ranges in which the 24-day period is revealed for
both flares and magnetically complex active regions.
Title: Extracting the apparent motion from two successive EIT images
Authors: Gissot, Samuel F.; Hochedez, J. -F.; Dibos, F.; Brajša,
R.; Jacques, L.; Berghmans, D.; Zhukov, A.; Clette, F.; Wöhl, H.;
Antoine, J. -P.
Bibcode: 2003ESASP.535..853G
Altcode: 2003iscs.symp..853G
The EIT observations cover more than seven years of the 23rd solar
cycle. The main synoptic dataset, usually refered to as the "CME
Watch", is a nearly uninterrupted sequence of images taken in the Fe
XII bandpass at a cadence of four images per hour. In this work we
study motion tracking methods in order to estimate displacements from
frame to frame. We have implemented a novel optical flow algorithm,
and tested it on a couple of successive images. We have linked the
apparent motion occurring between two frames to the expected rotation
rate. On this short time scale (20 minutes), we are able to retrieve
the global parameters of the solar differential rotation. A strategy
for the extraction of region with reliable motion will be discussed.
Title: Properties of the solar velocity field indicated by motions
of coronal bright points
Authors: Vršnak, B.; Brajša, R.; Wöhl, H.; Ruždjak, V.; Clette,
F.; Hochedez, J. -F.
Bibcode: 2003A&A...404.1117V
Altcode:
Full-disc solar images obtained with the Extreme Ultraviolet Imaging
Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO)
are used to analyse properties of the solar velocity field by tracing
coronal bright points from June 4, 1998 to May 22, 1999. Rotation
velocity residuals, meridional motions and their relationship are
investigated. Zones of slow and fast rotation found in motions of
coronal bright points are consistent with the pattern of torsional
oscillations, indicating that the statistical velocity pattern
of bright point motions reflects the large-scale plasma flows. A
complex pattern of meridional motion is deduced: The equatorward
flows are found to dominate at low (B<10deg) and high
(B>40deg) latitudes, whereas at mid-latitudes (B~
10deg-40deg) a poleward flow is inferred. The
complete data set shows no significant correlation between rotation
residuals and meridional motions. However, when a subsample of
coronal bright points including only the ``point-like structures''
(predominantly young bright points) is considered, a statistically
significant correlation is found. On average, faster tracers show
equatorward motion and the slower ones show poleward motion. Such
a segregation is reflected in a statistically significant
covariance of the rotation residuals and meridional velocities
in the order of -1000 m2 s-2, revealing an
equatorward transport of angular momentum. The negative value of the
covariance is provided by the high velocity tail in the velocity
distribution of point-like structures, representing less than 15%
of the population. The latitude dependence of the covariance can be
expressed as Q=-62 B + 200 m2 s-2 covering the
range B=0deg-60deg.
Title: Solar rotation velocity determined by coronal bright points
Authors: Brajša, R.; Wöhl, H.; Vršnak, B.; Ruždjak, V.; Clette,
F.; Hochedez, J. F.; Roša, D.; Hržina, D.
Bibcode: 2003HvaOB..27...13B
Altcode:
Full-disc solar images in the extreme ultraviolet part of the spectrum
from the SOHO spacecraft (instrument EIT, data in the spectral line of
Fe XV at a wavelength of 28.4 nm) are used to identify and trace coronal
bright points with the interactive and automatic method. The Solar
rotation was determined for the period June 4, 1998 to May 22, 1999
and a two-step velocity filter was applied. Histograms of latitudinal
and central meridian distance distributions of coronal bright points,
for both solar hemispheres treated together (north and south, east and
west), are presented and compared for different reduction procedures.
Title: Proper motions of sunspot groups
Authors: Wöhl, H.; Brajša, R.; Kučera, A.; Ruždjak, V.; Rybák, J.
Bibcode: 2003HvaOB..27....1W
Altcode:
Attempts to investigate typical patterns of motions of main spot
components and small pores in complex sunspot groups are described. The
data used up to now stem from observations at Hvar/HR, Tatranská
Lomnica/SK, Izaña/E, Debrecen/H and from the MDI instrument on
SoHO. Preliminary results are presented.
Title: Flare waves observed in Helium I 10 830 Å. A link between
Hα Moreton and EIT waves
Authors: Vršnak, B.; Warmuth, A.; Brajša, R.; Hanslmeier, A.
Bibcode: 2002A&A...394..299V
Altcode:
Three traveling disturbances recorded in the absorption line of
Helium I at 10 830 Å, (He I), analogous to Hα Moreton waves,
are analyzed. The morphology and kinematics of the wavefronts are
described in detail. The He I wave appears as an expanding arc of
increased absorption roughly corresponding to the Hα disturbance,
although not as sharply defined. He I perturbations consist of a
relatively uniform diffuse component and a patchy one that appears as
enhanced absorption in He I mottles. It leads the Hα front by some
20 Mm and can be followed to considerably larger distances than in
Hα observations. Behind the front stationary areas of reduced He I
absorption develop, resembling EUV coronal dimming. The observed He I
as well as the Hα disturbances show a deceleration of the order of
100-1000 m s-2. Moreover, in the event where Hα , He I,
and EUV wavefronts are observed, all of them follow closely related
kinematical curves, indicating that they are a consequence of a common
disturbance. The analysis of spatial perturbation profiles indicates
that He I disturbances consist of a forerunner and a main dip, the
latter being cospatial with the Hα disturbance. The properties and
behavior of the wavefronts can be comprehended as a consequence of a
fast-mode MHD coronal shock whose front is weakly inclined to the solar
surface. The Hα disturbance and the main He I dip are a consequence
of the pressure jump in the corona behind the shock front. The He I
forerunner might be caused by thermal conduction from the oblique shock
segments ahead of the shock-chromosphere intersection, or by electron
beams accelerated in the quasi-perpendicular section of the shock.
Title: Solar differential rotation determined by tracing coronal
bright points in SOHO-EIT images. II. Results for 1998/99 obtained
with interactive and automatic methods
Authors: Brajša, R.; Wöhl, H.; Vršnak, B.; Ruždjak, V.; Clette,
F.; Hochedez, J. -F.
Bibcode: 2002A&A...392..329B
Altcode:
Full-disc solar images obtained with the Extreme Ultraviolet Imaging
Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO)
were used to analyse solar differential rotation by tracing coronal
bright points. The results obtained with the interactive and the
automatic method for the time period June 4, 1998 to May 22, 1999 are
presented and compared. A possible north-south rotational asymmetry
and differences in the rotation velocity curves for various subtypes
of tracers are investigated.
Title: Differential Rotation of Stable Recurrent Sunspot Groups
Authors: Brajša, R.; Wöhl, H.; Vršnak, B.; Ruždjak, D.; Sudar,
D.; Roša, D.; Hržina, D.
Bibcode: 2002SoPh..206..229B
Altcode:
Stable recurrent sunspot groups from the Greenwich data set which
were identified in at least two subsequent solar rotations were
traced. The solar rotation was determined by the period method from
the time difference of the two central meridian passages of each of
the 327 identified groups. Sidereal rotation periods were calculated
from the synodic ones by a seasonal-dependent procedure taking into
account the details of the Earth's motion around the Sun. Growing
recurrent sunspot groups rotate on the average faster than decaying
recurrent sunspot groups, while sunspot groups of all types taken
together rotate faster than both growing and decaying recurrent sunspot
groups. A north-south rotational asymmetry and a cycle-dependence of
rotational velocity of recurrent sunspot groups were analyzed. Positive
rotation velocity deviations are larger, but less numerous than the
negative ones. Signatures of torsional oscillations were not found
analyzing the rotation velocity residual of recurrent sunspot groups
as a function of the distance from the average latitude of activity.
Title: Solar differential rotation determined by tracing coronal
bright points in SOHO-EIT images. I. Interactive and automatic
methods of data reduction
Authors: Brajša, R.; Wöhl, H.; Vršnak, B.; Ruždjak, V.; Clette,
F.; Hochedez, J. -F.
Bibcode: 2001A&A...374..309B
Altcode:
Full-disc solar images obtained with the Extreme Ultraviolet Imaging
Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO)
were used to analyse solar differential rotation determined by tracing
coronal bright points. Two different procedures were developed and
compared: an interactive and an automatic method. The interactive method
is based on the visual tracing of coronal bright points in consecutive
images using computer programs written in the Interactive Data Language
(IDL). The automatic method relies on the IDL procedure ``Regions Of
Interest (ROI) segmentation'' which is used to detect and follow bright
points in triplets of consecutive images. The test-results obtained
applying both methods by different persons who performed tracing are
presented and compared. The advantages and disadvantages of the two
methods are discussed.
Title: On the Rigid Component in the Solar Rotation
Authors: Brajša, R.; Ruždjak, V.; Vršnak, B.; Wöhl, H.;
Pohjolainen, S.; Urpo, S.
Bibcode: 2001ASSL..259..263B
Altcode: 2001dysu.conf..263B
A rigid component in the rotation velocity determined by tracing low
brightness temperature regions in the microwave regime was found and
interpreted in terms of their association rate (39%) with rigidly
rotating ``pivot-points".
Title: Meridional Motions of Stable Recurrent Sunspot Groups
Authors: Wöhl, H.; Brajša, R.
Bibcode: 2001SoPh..198...57W
Altcode:
Stable recurrent sunspot groups from the Greenwich data set which
were identified in at least two subsequent solar rotations were traced
and meridional motions were determined from the two central meridian
passages. In total, 327 meridional velocities were calculated and
the results for the northern and the southern solar hemisphere were
compared. A dependence of the solar meridional velocity vectors on the
development status, latitude and position respectively to the activity
belt of sunspots is investigated. The results indicate that sunspot
groups are moving on the average away from the center of activity. This
was found for sunspot groups growing and decreasing in area.
Title: Determination of the Solar Rotation Tracing EUV Bright Points
with the Automatic Method
Authors: Wöhl, H.; Brajša, R.; Vršnak, B.; Ruždjak, V.; Clette,
F.; Hochedez, J. -F.
Bibcode: 2001HvaOB..25...27W
Altcode:
Full-disc solar images in the extreme ultraviolet part of the spectrum
from the SOHO spacecraft (instrument EIT, data in the spectral
line of Fe XV at the wavelength of 28.4 nm) are used for the solar
rotation determination tracing coronal bright points. From the time
differences in tracer positions, approximately six hours, the solar
rotation velocity is determined automatically for image sequences in
several time intervals from June 4, 1998 to May 22, 1999. The resulting
rotational profiles are mutually compared.
Title: Determination of the Solar Rotation Tracing EUV Bright Points
with the Interactive Method
Authors: Brajša, R.; Wöhl, H.; Schuck, T. J.; Schawinski-Guiton,
K.; Wegner, A.; Vršnak, B.; Ruždjak, V.; Clette, F.; Hochedez, J. -F.
Bibcode: 2001HvaOB..25...13B
Altcode:
Full-disc solar images in the extreme ultraviolet part of the spectrum
from the SOHO spacecraft (instrument EIT, data in the spectral line of
Fe XV at a wavelength of 28.4 nm) are used to visually identify coronal
bright points appropriate for the solar rotation determination. From the
time differences in successive tracer positions, about six hours, the
solar rotation velocity is determined tracing coronal bright points in
several time intervals from June 4, 1998 to May 22, 1999. The resulting
rotational profiles obtained by five observers are mutually compared.
Title: An Analysis of the Solar Rotation Velocity by Tracing Coronal
Features
Authors: Brajsa, R.; Vrsnak, B.; Ruzdjak, V.; Rosa, D.; Hrzina, D.;
Wöhl, H.; Clette, F.; Hochedez, J. -F.
Bibcode: 2001IAUS..203..377B
Altcode:
Full-disc solar images in the extreme ultraviolet part of the spectrum
from the SOHO spacecraft (EIT) are used to identify various coronal
structures appropriate for the solar rotation determination (e.g. bright
points and coronal holes). From the time differences in tracer positions
(more than 1 image per day) solar rotation velocities are measured,
primarily by well-defined tracers, such as coronal bright points, whose
large number and broad coverage of latitudes may provide an unique
opportunity for a solar rotation analysis. The analysis started using
the SOHO data from 1997-1999 and preliminary experiences obtained
measuring solar rotation from the full-disc images in soft X-rays
from the YOHKOH (SXT) satellite were taken into account. This work is
connected to the SOHO EIT Proposal Brajsas.
Title: Statistical Weights and Selective Height Corrections in the
Determination of the Solar Rotation Velocity
Authors: Brajša, R.; Ruždjak, V.; Vršnak, B.; Wöhl, H.;
Pohjolainen, S.; Upro, S.
Bibcode: 2000SoPh..196..279B
Altcode:
Observations of the Sun performed at 37 GHz with the 14-m radio
telescope of the Metsähovi Radio Observatory were analyzed. Rotation
velocities were determined, tracing Low Temperature Regions (LTRs) in
the years 1979-1980, 1981-1982, 1987-1988, and 1989-1991. Statistical
weights were ascribed to the determined rotation velocities of LTRs,
according to the number of tracing days. Measured changes of the
rotation velocity during the solar activity cycle, as well as a
north-south rotation asymmetry, are discussed. The results obtained
with and without the statistical weights procedure are compared, and it
was found that the statistical significance of the solar differential
rotation parameters' changes is higher when the statistical weights
procedure is applied. A selective application of the height correction
on LTR's positions has not removed the cycle-related changes nor the
north-south asymmetry of the solar rotation measured tracing LTRs. So,
projection effects cannot explain these changes. The differential
rotation of LTRs is more rigid than the differential rotation obtained
tracing magnetic features and measuring Doppler shifts, which can be
explained by the association rate of the LTRs' positions with rigidly
rotating `pivot points'. The observed cycle-related changes and the
north-south asymmetry of the rotation velocity of LTRs are consistent
with the cycle-related changes and the north-south asymmetry of the
association rate between LTRs and pivot points.
Title: Characteristics of Flares with Hα Emission Protruding over
Major Sunspot Umbrae
Authors: Vršnak, B.; Ruždjak, V.; Brajša, R.; Zlobec, P.; Altaş,
L.; Özgüç, A.; Aurass, H.; Schroll, A.
Bibcode: 2000SoPh..194..285V
Altcode:
A sample of 47 importance ≥ 1 flares whose Hα emission occurred
or protruded over umbrae of major sunspots (so called Z-flares) was
studied to investigate characteristics of the associated dm - m radio,
microwave and soft X-ray emission as the energy release site permeats
into regions of strong magnetic fields. A close time association was
found between the microwave burst peak and the `contact' of the Hα
emission with the sunspot umbra. The Hα emission attained maximum
close to or a few minutes after the contact. The soft X-ray bursts
were delayed more, attaining maximum 0-10 min after the contact. The
onset of bursts in the dm - m wavelength range was associated with the
period of growth or the peak of the microwave burst. Two categories
of type III and IV bursts could be recognized: the ones starting some
ten minutes before the microwave peak, and those that begin close to
the microwave burst peak. Type III bursts occur preferably when the
microwave burst peaks simultaneously with or after the contact. The
results are explained presuming that the contact reveals a permeation
of the energy release process into a region of strong magnetic fields,
where the process intensifies, and where the accelerated particles
have access to magnetic field lines extending to large coronal
heights. Different manifestations of the energy release process in
various magnetic field topologies are considered to account for the
various time sequences observed.
Title: Position Measurements on Synoptic Solar Images
Authors: Rosa, D.; Hrzina, D.; Vrsnak, B.; Brajsa, R.
Bibcode: 2000HvaOB..24..145R
Altcode:
Procedures for finding the solar sidereal rotation rate, which are
taking into account the height of tracers are described. The related
algorithms for data reduction, including detailed corrections for the
Earth's motion, are presented.
Title: Measurements of Solar Rotation Using EUV Bright Points -
Preliminary Results
Authors: Brajsa, R.; Woehl, H.; Kasabasic, M.; Rodmann, J.; Vrsnak,
B.; Ruzdjak, V.; Rosa, D.; Hrzina, D.; Clette, F.; Hochedez, J. -F.
Bibcode: 2000HvaOB..24..153B
Altcode:
Full-disc solar images in the extreme ultraviolet part of the spectrum
from the SOHO spacecraft (instrument EIT, data in the spectral line
of Fe XV at the wavelength of 28.4 nm) are used to identify visually
various small-scale coronal structures appropriate for the determination
of the solar rotation. From the time differences in tracer positions,
approximately six hours, the solar rotation velocity is determined
tracing coronal bright points in the period June 4-14, 1998 by four
observers. The resulting rotational profiles are mutually compared
and the reduction methods are discussed.
Title: On the Meridional Motions of Recurrent Stable Sunspot Groups
Authors: Brajsa, R.; Woehl, H.
Bibcode: 2000HvaOB..24..125B
Altcode:
Meridional motions of recurrent stable sunspot groups are analysed as
a function of the distance from the average latitude of activity in
each solar hemisphere. Results were obtained for two data subsets,
consisting of recurrent sunspot groups decreasing and growing in
area respectively, and are compared with the ones published in the
literature. An interpretation of the results in the frame of the model
of magnetic flux-transport on the Sun is discussed.
Title: Correlated Radio Bursts Observed at Metric and Millimetric
Wavelengths
Authors: Zlobec, P.; Urpo, S.; Vrsnak, B.; Brajsa, R.; Ruzdjak, V.
Bibcode: 2000HvaOB..24...41Z
Altcode:
Characteristics of the bursts that occur almost contemporaneously at
metric and millimetric wavelengths are presented. It is found that
such events are rather rare. The correlated impulsive bursts observed
at 237 MHz and at 37 GHz start in average almost simultaneously (time
difference 0.3 - 2.8 s). The first peaks at 37 GHz are delayed few
seconds (3.1 - 3.0 s) in respect to the 237 MHz peaks, whereas for the
bursts maxima the delay is about 1s in average (1.0 - 2.7 s). A weak
correlation between peak fluxes at 37 GHz and 237 MHz is found. Spectral
characteristics of these events indicate electron beams of extremely
high energies. The association of the gradual bursts observed at 37
GHz and the phenomena at 237 MHz is not so clear, however in some
cases a relationship was established.
Title: Height of Tracers and the Correction of the Measured Solar
Synodic Rotation Rate: Demonstration of the Method
Authors: Vršnak, B.; Roša, D.; Božić, H.; Brajša, R.; Ruždjak,
V.; Schroll, A.; Wöhl, H.
Bibcode: 1999SoPh..185..207V
Altcode:
Two large stable solar filaments were used as test tracers to determine
the apparent synodic rotation rate as a function of the central meridian
distance for several filaments' segments at different heights. An
analytic fitting procedure was applied to determine simultaneously
the real synodic rotation rate and the height of the traced filament
segments. The determined heights were compared with the values obtained
from the widths of filament contours on the solar disk and with the
values obtained by direct measurements at the solar limb. Furthermore,
the obtained rotation rates and heights of the filaments' segments close
to the filaments' pivot points were compared with the values obtained
using two successive central meridian passages. Finally, sources and
scales of errors were investigated and possible implications on the
previous studies of the solar differential rotation were considered.
Title: An Estimate of Microwave low-Brightness-Temperature Regions'
Heights Obtained Measuring Their Rotation Velocity
Authors: Brajša, R.; Ruždjak, V.; Vršnak, B.; Wöhl, H.;
Pohjolainen, S.; Urpo, S.
Bibcode: 1999SoPh..184..281B
Altcode:
Daily full-disk solar maps obtained at 37 GHz in the years 1979,
1980, 1981, 1982, 1987, 1988, 1989, 1990, and 1991 are analysed
and compared with full-disk solar maps in Hα. A search for a
difference in the measured angular rotation velocity for two classes
of microwave low-brightness-temperature regions (LTRs), associated and
not associated with Hα filaments, is performed. Procedures with and
without statistical weights, assigned to angular rotation velocities
according to the tracing time, are applied and the statistical
significance of the results is discussed. A higher angular rotation
velocity is measured for LTRs associated with Hα filaments than for
the not-associated ones. This angular velocity difference is interpreted
as a consequence of a height difference between these two types of LTR
tracers. Changes of the solar differential rotation velocity during
the activity cycle measured using LTRs as tracers are explained by
the measured cycle-dependence of the association rate between LTRs
and Hα filaments. Similarly, the north-south asymmetry in the solar
rotation velocity measured tracing LTRs is explained by the measured
north-south asymmetry in the association rate between LTRs and Hα
filaments. The rotation velocity of LTRs and Hα filaments is on the
average more rigid in comparison with sunspots.
Title: An analysis of the solar rotation velocity determined tracing
microwave features and an estimate of their heights.
Authors: Brajša, R.; Ruždjak, V.; Vršnak, B.; Wöhl, H.;
Pohjolainen, S.; Urpo, S.
Bibcode: 1999joso.proc..156B
Altcode:
Measurements of the Sun performed at 37 GHz (8 mm) with the 14 m
radio telescope of the Metsähovi Radio Observatory were analysed
and rotation velocities tracing microwave Low brightness Temperature
Regions in the years 1979 - 1980, 1981 - 1982, 1987 - 1988, and 1989 -
1991 were determined. The statistical weights method was applied and
possible changes of the measured rotation velocity values are discussed.
Title: A Method to Determine the Solar Synodic Rotation Rate and
the Height of Tracers
Authors: Roša, D.; Vršnak, B.; Božić, H.; Brajša, R.; Ruždjak,
V.; Schroll, A.; Wöhl, H.
Bibcode: 1998SoPh..179..237R
Altcode:
The dependence of the measured apparent synodic solar rotation rate
on the height of the chosen tracer is studied. A significant error
occurs if the rotation rate is determined by tracing the apparent
position of an object above the photospheric level projected on the
solar disc. The centre-to-limb variation of this error can be used to
determine simultaneously the height of the object and the true synodic
rotation rate. The apparent (projected) heliographic coordinates
are presented as a function of the height of the traced object and
the coordinates of its `footpoint'. The relations obtained provide
an explicit expression for the apparent rotation rate as a function
of the observed heliographic coordinates of the tracer, enabling an
analytic least-squares fit expression to determine simultaneously the
real synodic rotation rate and the height of the tracer.
Title: On the Possible Changes of the Solar Differential Rotation
during the Activity Cycle Determined Using Microwave Low-Brightness
Regions and Hα Filaments as Tracers
Authors: Brajša, R.; Ruždjak, V.; Vršnak, B.; Pohjolainen, S.;
Urpo, S.; Schroll, A.; Wöhl, H.
Bibcode: 1997SoPh..171....1B
Altcode:
The solar rotation rate obtained using the microwave
Low-brightness-Temperature Regions (LTRs) as tracers in the heliographic
range ± 55° from the years 1979-1980, 1981-1982, 1987-1988, and
1989-1991 varied from 3% to 4% in medium latitudes, and below 1%
at the equator. Using Hα filaments as tracers at higher latitudes
from the years 1979, 1980, 1982, 1984, and 1987, the solar rotation
rate variation was between 2% and 8%. This represents an upper limit
on the rotation rate variation during the solar activity cycle. Such
changes could be caused by short-lived, large-scale velocity patterns
on the solar surface. The Sun revealed a higher rotation rate on the
average during the maxima of the solar activity cycles 21 and 22,
i.e., in the periods 1979-1980 and 1989-1991, respectively, which
differs from the rotation rates (lower on the average) in some years,
1981-1982 and 1987-1988, between the activity maximum and minimum
(LTR data). Simultaneous comparison of rotation rates from LTRs and
Hα filament tracings was possible in very limited time intervals
and latitude bands only, and no systematic relationship was found,
although the rotation rates determined by LTRs were mostly smaller than
the rotation rates determined by Hα filaments. The errors obtained by
applying different fitting procedures of the LTR data were analyzed, as
well as the influence of the height correction. Finally, the north-south
asymmetry in the rotation rate investigated by LTRs indicates that
the southern solar hemisphere rotated slower in the periods under
consideration, the difference being about 1%. The reliability of all
obtained results is discussed and a comparison with other related
studies was performed.
Title: Soft X-ray, Microwave and He I Measurements of Coronal Holes
Authors: Brajsa, R.; Ruzdjak, V.; Vrsnak, B.; Pohjolainen, S.; Urpo,
S.; Sakurai, T.; Wohl, H.
Bibcode: 1997IAUJD..19E..15B
Altcode:
The Poster presents and describes microwave signatures of three
coronal holes that were recorded on May 27, 1993. Differences in the
brightness temperatures between an equatorial and two polar coronal
holes were found. The measurements in the He I 10830 AA absorption
line for that day were also analyzed and compared with the microwave
and soft X-ray data.
Title: On the Determination of the Height of Microwave Low Temperature
Regions from Solar Rotation Measurements
Authors: Brajsa, R.; Ruzdjak, V.; Vrsnak, B.; Pohjolainen, S.; Urpo,
S.; Woehl, H.
Bibcode: 1997HvaOB..21...67B
Altcode:
A larger angular rotation velocity was measured for microwave LTRs
associated with H-alpha filaments than for the not associated ones. This
implies that LTRs not associated with H-alpha filaments are located
at lower heights above the solar photosphere than LTRs associated with
H-alpha filaments. Data from three intervals were analysed (1979-1980,
1981-1982 and 1987-1988) with different percentages of association
between LTRs and H-alpha filaments.
Title: A New Method for Numerical Data Reduction of Solar Microwave
Measurements
Authors: Brajsa, R.; Ruzdjak, V.; Vrsnak, B.; Woehl, H.; Pohjolainen,
S.; Urpo, S.
Bibcode: 1996HvaOB..20...15B
Altcode:
Numerical data reduction of full-disk solar measurement taken in
the microwave part of the spectrum (37 GHz) at the Metsahovi Radio
Research Station is described. The basic parameter distinguishing
between various features on the microwave solar maps is the brightness
temperature. Regions on the Sun with a lower brightness temperature
than the quiet Sun level are called Low Temperature Regions (LTRs),
and in the present paper mainly LTRs are considered. On the other
hand, High Temperature Regions (HTRs) have a brightness temperature
higher than the quiet Sun level. The data reduction includes: to
obtain a circular solar picture, to construct twelve radial vectors
from the preliminary solar disk center in order to determinate the
coordinates of the solar limb, to remove "erroneous" limb points,
to determine a circle trough the limb points by the least-squares
method, to obtain corrected coordinates of the solar disk center and
the radius and to repeat the procedures using the obtained results
as input parameters. The quiet Sun level was determined as the mean
value of all data points on the disk for every map separately. Several
numerical criteria were tested, and the minima of relative intensities
for all data points were determined. Taking these minima as centers,
circles of different radii were drawn with criterion that the mean value
inside the circle is less than the quiet Sun level. The latitudinal
distribution of LTRs, as well as the solar rotation rate as determined
by the LTRs, were investigated using automatic numerical procedures. It
was established how the parameters which describe the solar rotation
rate and corresponding errors depend upon various numerical criteria
including: definition of LTR's size, allowing different deviations
of LTR's positions and rotation rate, confining the set of obtained
rotation rates according to each error of the rotation rate and
confining the set of obtained rotation rates according to allowance
of a specific rotation rate at a specific latitude.
Title: Helium 10830 Å measurements of the Sun
Authors: Brajša, R.; Pohjolainen, S.; Ruždjak, V.; Sakurai, T.;
Urpo, S.; Vršnak, B.; Wöhl, H.
Bibcode: 1996SoPh..163...79B
Altcode:
Measurements of the Sun in the near-infrared He I 10830 Å absorption
line were performed using the echelle spectrograph with a dispersion
of 6.71 mÅ per pixel at the Vacuum Tower Telescope (German Solar
Telescopes, Teide Observatory, Izaña, Tenerife, Spain) on May 26,
1993. These measurements were compared with full-disc soft X-ray images
of the Sun (Japanese solar satellite Yohkoh), full-disc solar images
in Hα (Big Bear Solar Observatory), full-disc solar images in the
He I 10830 Å line (National Solar Observatory, Kitt Peak) and with
full-disc microwave solar maps at 37 GHz (Metsähovi Radio Research
Station). In the He 10830 Å line the Sun displays a limb darkening
similar to that in the visible part of the spectrum. Active regions
and Hα filaments show a strong absorption in the He 10830 Å line,
whereas the absorption is weak in coronal holes.
Title: The Relation between the Synodic and Sidereal Rotation Period
of the Sun
Authors: Roša, D.; Brajša, R.; Vršnak, B.; Wöhl, H.
Bibcode: 1995SoPh..159..393R
Altcode:
The relation between the synodic and sidereal rotation period of
the Sun for an arbitrary date of observation is derived taking into
account details of the Earth's motion. The transformation procedure
between the synodic (apparent) and sidereal rotation period presented
here can be performed without using the annual ephemerides.
Title: An Investigation of Cycle-Related Changes of the Solar Rotation
by Tracing Microwave Low Brightness Temperature Regions
Authors: Brajsa, R.; Ruzdjak, V.; Vrsnak, B.; Pohjolainen, S.; Urpo,
S.; Woehl, H.
Bibcode: 1995HvaOB..19....1B
Altcode:
Indications of possible changes of the solar rotation rate during
several phases of the solar activity cycle (the years analyzed were
1979, 1980, 1981, 1982, 1987, 1988, 1989, 1990 and 1991) were found. The
solar rotation rates were determined by tracing microwave Low brightness
Temperature Regions in the latitude range 55 deg. These changes of the
rotation rate, although of low statistical significance, indicate that
the Sun has nearly equal rotation rates during successive cycle maxima,
which are different from the measured rotation rates in the periods
between the maxima.
Title: Solar Centre-to-Limb Functions in Optical and Radio Wavelength
Ranges
Authors: Brajsa, R.; Ruzdjak, V.; Vrsnak, B.; Pohjolainen, S.; Urpo,
S.; Woehl, H.
Bibcode: 1994HvaOB..18....9B
Altcode:
No abstract at ADS
Title: A comparison of large-scale patterns outlined by low brightness
temperature microwave regions and magnetic fields on the Sun
Authors: Brajša, R.; Pohjolainen, S.; Ruždjak, V.; Teräsranta,
H.; Urpo, S.; Vršnak, B.; Wöhl, H.
Bibcode: 1994smf..conf...62B
Altcode:
No abstract at ADS
Title: A Comparison of Hα and Microwave Full-Disc
Solar Maps
Authors: Brajša, R.; Ruždjak, V.; Wöhl, H.
Bibcode: 1994emsp.conf..125B
Altcode:
No abstract at ADS
Title: On the Possible Physical Conditions which Result in Appearance
of Low Brightness Temperature Microwave Regions on the Sun
Authors: Brajsa, R.
Bibcode: 1993SoPh..144..199B
Altcode:
The physical conditions causing the appearance of the low brightness
temperature regions at 37 and 22 GHz are discussed. The source radiation
stems from free-free processes (bremsstrahlung), and passes through the
transparent chromosphere and corona. The absorption occurs either in
the prominences or in coronal condensations with physical parameters
between those of prominences and corona. A deficit in emission of the
chromosphere-corona transition region in the vicinity of the inversion
lines of the longitudinal component of the photospheric magnetic field
could also result in the appearance of low temperature region.
Title: Erratum: "Variations of solar global rotation during the
polarity reversal" [Hvar Obs. Bull., Vol. 16, No. 1, p. 13 - 22
(1992)].
Authors: Brajša, R.; Vršnak, B.; Ruždjak, V.; Božić, H.;
Pohjolainen, S.; Teräsranta, H.; Urpo, S.
Bibcode: 1993HvaOB..17...65B
Altcode:
No abstract at ADS
Title: Variations of Solar Global Rotation During the Polarity
Reversal
Authors: Brajsa, R.; Vrsnak, B.; Ruzdjak, V.; Bozic, H.; Pohjolainen,
S.; Urpo, S.; Terasranta, H.
Bibcode: 1992HvaOB..16...13B
Altcode:
No abstract at ADS
Title: Cycle Dependent Rotation of Solar Large Scale Patterns as
Determined from Millimeter-Range Observations
Authors: Brajsa, R.; Ruzdjak, V.; Vrsnak, B.; Jurac, S.; Pohjolainen,
S.; Terasranta, H.; Urpo, S.
Bibcode: 1992ASPC...27..274B
Altcode: 1992socy.work..274B
No abstract at ADS
Title: Giant Cells on the Sun Revealed by Low Temperature Microwave
Regions?
Authors: Brajsa, R.; Vrsnak, B.; Ruzdjak, V.; Jurac, S.; Pohjolainen,
S.; Urpo, S.; Terasranta, H.
Bibcode: 1992HvaOB..16....1B
Altcode:
No abstract at ADS
Title: Large-scale patterns on the Sun observed in the millimetric
wavelength range
Authors: Vrsnak, B.; Pohjolainen, S.; Urpo, S.; Terasranta, H.;
Brajsa, R.; Ruzdjak, V.; Mouradian, Z.; Jurac, S.
Bibcode: 1992SoPh..137...67V
Altcode:
The nature and behaviour of large-scale patterns on the solar surface,
indicated by the areas of brightness-temperature depressions in
the millimetric wavelength range, is studied. A large sample of 346
individual, low-temperature regions (LTRs) was employed to provide
reliable statistical evidence. An association of 99% was found between
the locations of LTRs and the large-scale magnetic field inversion
lines, and 60% of the LTRs were associated with the inversion
line filaments. A tentative physical association with filaments is
reconsidered, and one particularly well-observed case is presented. The
heights of the perturbers causing brightness-temperature depressions are
discussed. The long-term evolution of the latitudinal distribution of
LTRs is presented in a butterfly diagram. Two belts of low-temperature
regions outline the active region belts, shifting with them towards the
equator during the solar activity cycle. The low-temperature region
belts of the forthcoming cycle appear already at the maximum of the
actual cycle at latitudes of about 55 °. The superpositions of the
temperature minima distributions in the synoptic maps show patterns
appearing as `giant cells' and compatible with indications inferred
from magnetographic data. The reliability of the inferred cells is
considered, and a statistical analysis reveals a negligible probability
for an accidental distribution appearing in the form of giant cells.
Title: Solar Differential Rotation Determined by Polar Crown Filaments
Authors: Brajša, R.; Vršnak, B.; Ruždjak, V.; Schroll, A.;
Pohjolainen, S.; Urpo, S.; Teräsranta, H.
Bibcode: 1991SoPh..133..195B
Altcode:
The rotation rates obtained by tracing 124 polar crown filaments are
presented in comparison with previous results. Higher filament rotation
rate in polar regions was detected and discussed in terms of the various
phenomena such as: the projection effect due to the height of measured
tracers, the connection of polar filaments with the magnetic field
patterns which show an increase of the rotation rate at high latitudes,
rigid rotation of polar filaments which form pivot points, and eventual
change of the differential rotation law during the cycle. However, when
the height correction for an average height of 1% of the solar radius is
applied, the filament rotation rate in polar regions decreases. Then
the rotation law becomes: Ω(φ) = 14.45 − 0.11 sin2
φ − 3.69 sin4 φ (° day−1, sidereal).
Title: Intensity Variations and Short Time Evolution of Solar
Microwave Low Temperature Regions
Authors: Pohjolainen, S.; Urpo, S.; Terasranta, H.; Vrsnak, B.;
Brajsa, R.; Ruzdjak, V.; Jurac, S.
Bibcode: 1991HvaOB..15...21P
Altcode:
No abstract at ADS
Title: Rotation of large scale patterns on the solar surface as
determined from filament and millimeter data
Authors: Pohjolainen, S.; Vršnak, B.; Teräsranta, H.; Urpol, S.;
Brajša, R.; Ruždjak, V.; Jurač, S.; Schroll, A.
Bibcode: 1991LNP...380..279P
Altcode: 1991IAUCo.130..279P; 1991sacs.coll..279P
The rotation of large scale solar magnetic field patterns was studied
using quiescent filaments and low temperature regions observed at 37
GHz as tracers.
Title: Large scale patterns on the solar surface indicated by
microwave observations
Authors: Vršnak, B.; Pohjolainen, S.; Teräsranta, H.; Urpol, S.;
Brajša, R.; Ruždjak, V.; Schroll, A.; Jurač, S.
Bibcode: 1991LNP...380..282V
Altcode: 1991IAUCo.130..282V; 1991sacs.coll..282V
A large set of observations of the Sun at 37 GHz is analysed. An
association of 99% is found between the regions of brightness
temperature depression and the magnetic field inversion
lines. Observations indicate a possible existence of giant cells with
duration of 1-2 years and a longitudinal extension up to 90°.
Title: Oscillatory Motions in an Active Prominence
Authors: Vrsnak, B.; Ruzdjak, V.; Brajsa, R.; Zloch, F.
Bibcode: 1990SoPh..127..119V
Altcode:
Different types of oscillatory motions were detected in the late
phases of eruption of a prominence. We found oscillations of
the prominence axis and diameter with periods of 4.3 and 9.1 min,
corresponding to the eigenmodes m = 4 and m = 8 with a damping factor
4.6 × 10−3 s−1. A period about 4.5 min was
found for oscillations of the pitch angle of the helically twisted
filaments. The m = 2 and m = 3 eigenmodes could be also identified
and they led to the final relaxation of the prominence axis. The
observations are compared with a model in which we consider forces
acting in a curved, cylindrical magnetic tube anchored at both ends
in the photosphere and carrying an electric current. The stability of
the prominence is discussed.
Title: Polar Crown Filaments and Solar Differential Rotation at
High Latitudes
Authors: Brajša, R.; Vršnak, B.; Rundjak, V.; Schroll, A.
Bibcode: 1990LNP...363..293B
Altcode: 1990doqp.coll..293B; 1990IAUCo.117..293B
No abstract at ADS
Title: Motion of High Latitude Solar Microwave Sources and Comparison
with Polar Prominences
Authors: Urpo, S.; Pohjolainen, S.; Teräsranta, H.; Vrsnak, B.;
Ruzdjak, V.; Brajsa, R.; Schroll, A.
Bibcode: 1990LNP...363..292U
Altcode: 1990doqp.coll..292U; 1990IAUCo.117..292U
Solar microwave sources at high solar latitudes have been observed
with a 14 m radio telescope at the Metsahovi Radio Research Station in
Finland. Several periods for observations were organized in 1986-1989
in order to detect sources close to the north and south pole of
the Sun. Measurements at 22 and 37 GHz (wavelengths 14 and 8 mm
respectively) have revealed the existence of high temperature and low
temperature regions (relative to the quiet Sun level) at latitudes 50-80
degrees. The motions of these regions have been studied and compared
with optical measurements of polar prominences. The temperature
enhancement at 37 GHz is typically 100-400 K above the quiet Sun
level (7800 K) at that frequency. Although in most cases temperature
depression in a low temperature area amounts 50-300 K, at 37 GHz,
the temperature drop in the low temperature area which was observed in
July 1982 was as low as 900 K. The results of the radio measurements
of the Sun at 22 and 37 GHz on high solar latitudes imply that high
temperature areas correspond to polar faculae while low temperature
areas correspond to polar prominences. The principal cause of the
observed lower temperature area is the absorbtion by the filament.
Title: On the Appearances, Intensities and Motions of Solar Microwave
Low Temperature Areas
Authors: Pohjolainen, S.; Brajša, R.; Urpo, S.; Teräsranta, H.;
Vršnak, B.; Ruždjak, V.; Jurač, S.
Bibcode: 1990PDHO....7...56P
Altcode: 1990ESPM....6...56P; 1990dysu.conf...56P
Solar microwave sources at 37 GHz have been observed at Metsähovi since
1978. The solar maps have revealed the existence of low temperature
regions, i.e. areas where the brightness temperature is typically 100 -
400K lower than the quiet Sun level, throughout the solar cycle. The
authors have investigated the appearances, intensities and motions
of these temperature depressions and compared them with the activity
features in the optical part of the spectrum.
Title: Oscillatory Relaxation of an Eruptive Prominence
Authors: Vršnak, B.; Ruždjak, V.; Brajša, R.; Zloch, F.
Bibcode: 1990LNP...363..256V
Altcode: 1990IAUCo.117..256V; 1990doqp.coll..256V
Different types of oscillatory motions were detected in the late phases
of eruption of a prominence. We found oscillations of the prominence
axis and diameter with periods of 4.3 and 9.1 minutes corresponding
to the eigenmodes m=4 and m=8 with a damping factor 4.6 10-3 s{-1}. A
period of about 4.5 minutes was found for oscillations of the pitch
angle of the helically twisted filaments. The m=2 and m=3 eigenmodes
could be also identified and they led to the final relaxation of the
prominence axis. The observations are interpreted in analogy with
damped oscillations of an elastic string. The lowest eigenmode was
not excited due to >hile the m=2 and m=3 eigenmodes were highly
damped. The frequency of free oscillations due to restoring forces
and the decay constant were inferred using the dispersion relation for
oscillation of the elastic string and the observed frequentes in the
m=4 and m=8 modes to =3.1x10-3-1, corresponding to a period of T=34
min. and S =4.6x10-3s-1.
Title: A Comparison of Hα and Soft X-Ray Characteristics of Spotless
and SPOT Group Flares
Authors: Ruždjak, V.; Vršnak, B.; Schroll, A.; Brajša, R.
Bibcode: 1989SoPh..123..309R
Altcode:
A comparative analysis of spotless and spot group flares recorded
at Hvar and Kanzelhöhe Observatories during the 21st cycle of solar
activity is presented. The rate of occurrence of two-ribbon flares was
found to be significantly higher for the spotless flares. In comparison
with spot group flares of corresponding Hα importance, the soft X-ray
peak values have been systematically lower for the spotless flares. The
highest peak values and the energy released in soft X-rays was found
for flares with a Hα ribbon protruding over a major spot umbra. It
was found that the effective plasma temperatures in spotless flares
have been considerably lower than the temperatures in spot group flares.
Title: Motion of High Latitude Solar Microwave Sources and Comparison
with Solar Prominences
Authors: Urpo, S.; Pohjolainen, S.; Terasranta, H.; Vrsnak, B.;
Ruzdjak, V.; Brajsa, R.; Schroll, A.
Bibcode: 1989HvaOB..13..437U
Altcode:
No abstract at ADS
Title: Oscillatory Relaxation of an Eruptive Prominence
Authors: Vrsnak, B.; Ruzdjak, V.; Brajsa, R.; Zloch, F.
Bibcode: 1989HvaOB..13..137V
Altcode:
No abstract at ADS
Title: Polar Crown Filaments and Solar Differential Rotation at
High Latitudes
Authors: Brajsa, R.; Vrsnak, B.; Ruzdjak, V.; Schroll, A.
Bibcode: 1989HvaOB..13..449B
Altcode:
No abstract at ADS
Title: Structure and Stability of Prominences with Helical Structure
Authors: Vrsnak, B.; Ruzdjak, V.; Brajsa, R.; Dzubur, A.
Bibcode: 1988SoPh..116...45V
Altcode:
Observations of internal structure and development of four helical
prominences are presented. We assume that the helically twisted fine
structure threads are outlining magnetic field lines and we found
that it is possible to describe the magnetic fields by the uniform
twist configuration, with the twists ranging between 2π and 7π. The
estimated lower limits for the magnetic fields were about 20 G which
give lower limits for the currents flowing along the prominences in
the range between 2 × 1010 A and 2 × 1011 A and
current densities at the axis of the prominences about 10-4
A m-2. The upper limit of electron drift velocity could
be estimated as 1 m s-1, which is far below the critical
velocities for the onset of plasma microinstabilities.
Title: Analysis of errors in visual brightness estimates of variable
stars.
Authors: Brajša, R.; Baćani, K.
Bibcode: 1983Vasio..31...17B
Altcode:
No abstract at ADS