Author name code: vrsnak
ADS astronomy entries on 2022-09-14
author:"Vrsnak, Bojan"
------------------------------------------------------------------------
Title: Determination of CME orientation and consequences for their
propagation
Authors: Martinic, Karmen; Vrsnak, Bojan; Veronig, Astrid; Dumbovic,
Mateja; Temmer, Manuela
Bibcode: 2022cosp...44.2441M
Altcode:
The configuration of the interplanetary magnetic field and features
of the related ambient solar wind in the ecliptic and meridional
plane are different. Therefore, one can expect that the orientation
of the flux rope axis of a coronal mass ejection (CME) influences
the propagation of the CME itself. However, the determination of the
CME's orientation remains a challenging task to perform. This study
aims to provide a reference to different CME orientation determination
methods in the near-Sun environment. Also, it aims to investigate the
non-radial flow in the sheath region of the interplanetary CME (ICME)
in order to provide the first proxy to relate the ICME orientation
with its propagation. We investigated 22 isolated CME-ICME events
in the period 2008-2015. We first determined the CME orientation in
the near-Sun environment using a 3D reconstruction of the CME with
the graduated cylindrical shell (GCS) model applied to coronagraphic
images provided by the STEREO and SOHO missions. The CME orientation
in the near-Sun environment was determined using an ellipse fitting
technique to the CME outer front as determined from the SOHO/LASCO
coronagraph. In the near-Earth environment, we obtained the orientation
of the corresponding ICME using in-situ plasma and field data and also
investigated the non-radial flow in its sheath region. The ability of
GCS and ellipse fitting to determine the CME orientation is found to be
limited to reliably distinguish only between the high or low inclination
of the events. Most of the CME-ICME pairs under investigation were
found to be characterized by a low inclination. The majority of
CME-ICME pairs have a consistent estimation of tilt from remote and
in situ data. The observed non-radial flows in the sheath region show
a greater y-direction to z-direction flow ratio for high-inclination
events indicating that CME orientation could have an impact to the
CME propagation.
Title: Development and evaluation of Drag-Based Ensemble Model (DBEM)
Authors: Čalogović, Jaša; Vrsnak, Bojan; Veronig, Astrid; Dumbovic,
Mateja; Temmer, Manuela
Bibcode: 2022cosp...44.3443C
Altcode:
The Drag-based Model (DBM) is a well-known 2D analytical model for
simulating the heliospheric propagation of Coronal Mass Ejections
(CMEs). Main output is the prediction of the CME arrival time and
speed at Earth or any other given target in the solar system. Due to a
very short computational time of DBM (< 0.01s), the probabilistic
Drag-Based Ensemble Model (DBEM) was developed by making an ensemble
of n different input parameters to account for possible variability
(uncertainties) in the input parameters. Using such an approach
to obtain the distribution and significance of the DBM results,
the DBEM determines the CME hit chance, most probable arrival times
and speeds, quantifies the prediction uncertainties and calculates
the confidence intervals. As an important tool for space weather
forecasters, the fully operational DBM/DBEM web application is
integrated as one of the ESA Space Situational Awareness portal services
(https://swe.ssa.esa.int/current-space-weather). In the last few years,
DBM/DBEM has been constantly improved with various new features such as
Graduated Cylindrical Shell (GCS) option for the CME geometry input,
the CME propagation visualizations as well as a new DBEM version
employing the variable solar wind speeds. The model development,
new features and the corresponding model evaluations will be presented.
Title: Galactic cosmic rays as signatures of interplanetary transients
Authors: Dumbovic, Mateja; Kühl, Patrick; Heber, Bernd; Vrsnak,
Bojan; Temmer, Manuela; Kirin, Anamarija; Hörlöck, Malte; Jensen,
Stefan; Benko, Ilona; Kramaric, Luka
Bibcode: 2022cosp...44.1255D
Altcode:
Coronal mass ejections (CMEs), interplanetary shocks, and corotating
interaction regions (CIRs) drive heliospheric variability, causing
various interplanetary as well as planetary disturbances. One of
their very common in-situ signatures are short-term reductions in
the galactic cosmic ray (GCR) flux (i.e. Forbush decreases), which
are measured by ground-based instruments at Earth and Mars, as well
as various spacecraft throughout the heliosphere (most recently
by Solar Orbiter). In general, interplanetary magnetic structures
interact with GCRs producing depressions in the GCR flux. Therefore,
different types of interplanetary magnetic structures cause different
types of Forbush decreases, allowing us to distinguish between
them. We recently developed and employed two different analytical
models to explain CME-related and CIR-related Forbush decreases,
using an expansion-diffusion and the convection-diffusion approaches,
respectively. We used observation-based generic CME and CIR profiles
as the theoretical background for the models and tested the models on
various case studies. Moreover, the CME-related Forbush decrease model
(ForbMod, Dumbovic et al., 2018; 2020) is brought one step further, as
it also considers the energy dependance of the detector with which the
measurements are made. ForbMod is tested through model-to-observations
comparison to analyse to how many CMEs it is applicable and could
ultimately provide a helpful tool to analyse Forbush decreases. With
new modelling efforts, as well as observational analysis we are one
step closer in utilizing GCR measurements to provide information
on interplanetary transients, especially where other measurements
(e.g. plasma, magnetic field) are lacking.
Title: Determination of coronal mass ejection orientation and
consequences for their propagation
Authors: Martinić, K.; Dumbović, M.; Temmer, M.; Veronig, A.;
Vršnak, B.
Bibcode: 2022A&A...661A.155M
Altcode: 2022arXiv220410112M
Context. The configuration of the interplanetary magnetic field
and features of the related ambient solar wind in the ecliptic
and meridional plane are different. Therefore, one can expect that
the orientation of the flux-rope axis of a coronal mass ejection
(CME) influences the propagation of the CME itself. However, the
determination of the CME orientation, especially from image data,
remains a challenging task to perform. Aim. This study aims to provide
a reference to different CME orientation determination methods in
the near-Sun environment. Also, it aims to investigate the non-radial
flow in the sheath region of the interplanetary CME (ICME) in order
to provide the first proxy to relate the ICME orientation with its
propagation.
Methods: We investigated 22 isolated CME-ICME
events in the period 2008-2015. We determined the CME orientation in
the near-Sun environment using the following: (1) a 3D reconstruction
of the CME with the graduated cylindrical shell (GCS) model applied
to coronagraphic images provided by the STEREO and SOHO missions;
and (2) an ellipse fitting applied to single spacecraft data from
SOHO/LASCO C2 and C3 coronagraphs. In the near-Earth environment,
we obtained the orientation of the corresponding ICME using in situ
plasma and field data and also investigated the non-radial flow in
its sheath region.
Results: The ability of GCS and ellipse
fitting to determine the CME orientation is found to be limited to
reliably distinguish only between the high or low inclination of the
events. Most of the CME-ICME pairs under investigation were found to
be characterized by a low inclination. For the majority of CME-ICME
pairs, we obtain consistent estimations of the tilt from remote and
in situ data. The observed non-radial flows in the sheath region show
a greater y direction to z direction flow ratio for high-inclination
events, indicating that the CME orientation could have an impact on
the CME propagation.
Title: How the area of solar coronal holes affects the properties
of high-speed solar wind streams near Earth: An analytical model
Authors: Hofmeister, Stefan J.; Asvestari, Eleanna; Guo, Jingnan;
Heidrich-Meisner, Verena; Heinemann, Stephan G.; Magdalenic, Jasmina;
Poedts, Stefaan; Samara, Evangelia; Temmer, Manuela; Vennerstrom,
Susanne; Veronig, Astrid; Vršnak, Bojan; Wimmer-Schweingruber, Robert
Bibcode: 2022A&A...659A.190H
Altcode: 2022arXiv220315689H
Since the 1970s it has been empirically known that the area of
solar coronal holes affects the properties of high-speed solar wind
streams (HSSs) at Earth. We derive a simple analytical model for the
propagation of HSSs from the Sun to Earth and thereby show how the
area of coronal holes and the size of their boundary regions affect
the HSS velocity, temperature, and density near Earth. We assume that
velocity, temperature, and density profiles form across the HSS cross
section close to the Sun and that these spatial profiles translate
into corresponding temporal profiles in a given radial direction due
to the solar rotation. These temporal distributions drive the stream
interface to the preceding slow solar wind plasma and disperse with
distance from the Sun. The HSS properties at 1 AU are then given by
all HSS plasma parcels launched from the Sun that did not run into
the stream interface at Earth distance. We show that the velocity
plateau region of HSSs as seen at 1 AU, if apparent, originates from
the center region of the HSS close to the Sun, whereas the velocity
tail at 1 AU originates from the trailing boundary region. Small
HSSs can be described to entirely consist of boundary region plasma,
which intrinsically results in smaller peak velocities. The peak
velocity of HSSs at Earth further depends on the longitudinal width
of the HSS close to the Sun. The shorter the longitudinal width of
an HSS close to the Sun, the more of its "fastest" HSS plasma parcels
from the HSS core and trailing boundary region have impinged upon the
stream interface with the preceding slow solar wind, and the smaller
is the peak velocity of the HSS at Earth. As the longitudinal width
is statistically correlated to the area of coronal holes, this also
explains the well-known empirical relationship between coronal hole
areas and HSS peak velocities. Further, the temperature and density
of HSS plasma parcels at Earth depend on their radial expansion from
the Sun to Earth. The radial expansion is determined by the velocity
gradient across the HSS boundary region close to the Sun and gives
the velocity-temperature and density-temperature relationships at
Earth their specific shape. When considering a large number of HSSs,
the assumed correlation between the HSS velocities and temperatures
close to the Sun degrades only slightly up to 1 AU, but the correlation
between the velocities and densities is strongly disrupted up to 1
AU due to the radial expansion. Finally, we show how the number of
particles of the piled-up slow solar wind in the stream interaction
region depends on the velocities and densities of the HSS and preceding
slow solar wind plasma.
Title: Generic profile of a long-lived corotating interaction region
and associated recurrent Forbush decrease
Authors: Dumbović, M.; Vršnak, B.; Temmer, M.; Heber, B.; Kühl, P.
Bibcode: 2022A&A...658A.187D
Altcode: 2022arXiv220109623D
Context. Corotating interaction regions (CIRs), formed by the
interaction of slow solar wind and fast streams that originate from
coronal holes (CHs), produce recurrent Forbush decreases, which are
short-term depressions in the galactic cosmic ray (GCR) flux.
Aims: Our aim is to prepare a reliable set of CIR measurements to be
used as a textbook for modeling efforts. For that purpose, we observe
and analyse a long-lived CIR, originating from a single CH, recurring
in 27 consecutive Carrington rotations 2057-2083 in the time period
from June 2007-May 2009.
Methods: We studied the in situ
measurements of this long-lived CIR as well as the corresponding
depression in the cosmic ray (CR) count observed by SOHO/EPHIN
throughout different rotations. We performed a statistical analysis,
as well as the superposed epoch analysis, using relative values of the
key parameters: the total magnetic field strength, B, the magnetic
field fluctuations, dBrms, plasma flow speed, v, plasma density, n,
plasma temperature, T, and the SOHO/EPHIN F-detector particle count,
and CR count.
Results: We find that the mirrored CR count-time
profile is correlated with that of the flow speed, ranging from moderate
to strong correlation, depending on the rotation. In addition, we
find that the CR count dip amplitude is correlated to the peak in the
magnetic field and flow speed of the CIR. These results are in agreement
with previous statistical studies. Finally, using the superposed epoch
analysis, we obtain a generic CIR example, which reflects the in situ
properties of a typical CIR well.
Conclusions: Our results are
better explained based on the combined convection-diffusion approach of
the CIR-related GCR modulation. Furthermore, qualitatively, our results
do not differ from those based on different CHs samples. This indicates
that the change of the physical properties of the recurring CIR from one
rotation to another is not qualitatively different from the change of
the physical properties of CIRs originating from different CHs. Finally,
the obtained generic CIR example, analyzed on the basis of superposed
epoch analysis, can be used as a reference for testing future models.
Title: Analytic modeling of recurrent Forbush decreases caused by
corotating interaction regions
Authors: Vršnak, B.; Dumbović, M.; Heber, B.; Kirin, A.
Bibcode: 2022A&A...658A.186V
Altcode: 2022arXiv220109619V
Context. On scales of days, the galactic cosmic ray (GCR) flux is
affected by coronal mass ejections and corotating interaction regions
(CIRs), causing so-called Forbush decreases and recurrent Forbush
decreases (RFDs), respectively.
Aims: We explain the properties
and behavior of RFDs recorded at about 1 au that are caused by CIRs
generated by solar wind high-speed streams (HSSs) that emanate from
coronal holes.
Methods: We employed a convection-diffusion GCR
propagation model based on the Fokker-Planck equation and applied it to
solar wind and interplanetary magnetic field properties at 1 au.
Results: Our analysis shows that the only two effects that are relevant
for a plausible overall explanation of the observations are the enhanced
convection effect caused by the increased velocity of the HSS and the
reduced diffusion effect caused by the enhanced magnetic field and its
fluctuations within the CIR and HSS structure. These two effects that
we considered in the model are sufficient to explain not only the main
signatures of RFDs, but also the sometimes observed "over-recovery"
and secondary dips in RFD profiles. The explanation in terms of the
convection-diffusion GCR propagation hypothesis is tested by applying
our model to the observations of a long-lived CIR that recurred over 27
rotations in 2007-2008.
Conclusions: Our analysis demonstrates
a very good match of the model results and observations.
Title: Influence of the CME Orientation on the ICME Propagation
Authors: Martinic, Karmen; Dumbovic, Mateja; Vrsnak, Bojan
Bibcode: 2021AGUFMSH35B2057M
Altcode:
Beyond certain distance the ICME propagation becomes mostly governed by
the interaction of the ICME and the ambient solar wind. Configuration
of the interplanetary magnetic field and features of the related
ambient solar wind in the ecliptic and meridional plane are
different. Therefore, one can expect that the inclination of the
CME flux rope axis i.e. tilt, influences the propagation of the ICME
itself. In order to study the relation between the tilt parameter and
the ICME propagation we investigated isolated Earth-impacting CME-ICME
evets in the time period from 2006. to 2014. We determined the CME
tilt in the near-Sun environment from the 3D reconstruction of the CME,
obtained by the Graduated Cylindrical Shell model using coronagraphic
images provided by the STEREO and SOHO missions. We determined the
tilt of the ICME in the near-Earth environment using in-situ data. We
constrained our study to CME-ICME events that show no evidence of
rotation while propagating, i.e. have a similar tilt in the near-Sun
and near-Earth environment. For those events we also calculated the drag
parameter to check the implications for space-weather forecasting using
the drag based(ensemble) [DB(E)M] model of heliospheric propagation.
Title: Earth-affecting solar transients: a review of progresses in
solar cycle 24
Authors: Zhang, Jie; Temmer, Manuela; Gopalswamy, Nat; Malandraki,
Olga; Nitta, Nariaki V.; Patsourakos, Spiros; Shen, Fang; Vršnak,
Bojan; Wang, Yuming; Webb, David; Desai, Mihir I.; Dissauer, Karin;
Dresing, Nina; Dumbović, Mateja; Feng, Xueshang; Heinemann, Stephan
G.; Laurenza, Monica; Lugaz, Noé; Zhuang, Bin
Bibcode: 2021PEPS....8...56Z
Altcode: 2020arXiv201206116Z
This review article summarizes the advancement in the studies of
Earth-affecting solar transients in the last decade that encompasses
most of solar cycle 24. It is a part of the effort of the International
Study of Earth-affecting Solar Transients (ISEST) project, sponsored
by the SCOSTEP/VarSITI program (2014-2018). The Sun-Earth is an
integrated physical system in which the space environment of the
Earth sustains continuous influence from mass, magnetic field, and
radiation energy output of the Sun in varying timescales from minutes to
millennium. This article addresses short timescale events, from minutes
to days that directly cause transient disturbances in the Earth's
space environment and generate intense adverse effects on advanced
technological systems of human society. Such transient events largely
fall into the following four types: (1) solar flares, (2) coronal mass
ejections (CMEs) including their interplanetary counterparts ICMEs,
(3) solar energetic particle (SEP) events, and (4) stream interaction
regions (SIRs) including corotating interaction regions (CIRs). In
the last decade, the unprecedented multi-viewpoint observations of
the Sun from space, enabled by STEREO Ahead/Behind spacecraft in
combination with a suite of observatories along the Sun-Earth lines,
have provided much more accurate and global measurements of the size,
speed, propagation direction, and morphology of CMEs in both 3D and over
a large volume in the heliosphere. Many CMEs, fast ones, in particular,
can be clearly characterized as a two-front (shock front plus ejecta
front) and three-part (bright ejecta front, dark cavity, and bright
core) structure. Drag-based kinematic models of CMEs are developed to
interpret CME propagation in the heliosphere and are applied to predict
their arrival times at 1 AU in an efficient manner. Several advanced
MHD models have been developed to simulate realistic CME events from
the initiation on the Sun until their arrival at 1 AU. Much progress
has been made on detailed kinematic and dynamic behaviors of CMEs,
including non-radial motion, rotation and deformation of CMEs, CME-CME
interaction, and stealth CMEs and problematic ICMEs. The knowledge
about SEPs has also been significantly improved. An outlook of how to
address critical issues related to Earth-affecting solar transients
concludes this article.
Title: Coronal Hole Detection and Open Magnetic Flux
Authors: Linker, Jon A.; Heinemann, Stephan G.; Temmer, Manuela;
Owens, Mathew J.; Caplan, Ronald M.; Arge, Charles N.; Asvestari,
Eleanna; Delouille, Veronique; Downs, Cooper; Hofmeister, Stefan J.;
Jebaraj, Immanuel C.; Madjarska, Maria S.; Pinto, Rui F.; Pomoell,
Jens; Samara, Evangelia; Scolini, Camilla; Vršnak, Bojan
Bibcode: 2021ApJ...918...21L
Altcode: 2021arXiv210305837L
Many scientists use coronal hole (CH) detections to infer open
magnetic flux. Detection techniques differ in the areas that they
assign as open, and may obtain different values for the open magnetic
flux. We characterize the uncertainties of these methods, by applying
six different detection methods to deduce the area and open flux of
a near-disk center CH observed on 2010 September 19, and applying
a single method to five different EUV filtergrams for this CH. Open
flux was calculated using five different magnetic maps. The standard
deviation (interpreted as the uncertainty) in the open flux estimate
for this CH ≍ 26%. However, including the variability of different
magnetic data sources, this uncertainty almost doubles to 45%. We
use two of the methods to characterize the area and open flux for
all CHs in this time period. We find that the open flux is greatly
underestimated compared to values inferred from in situ measurements
(by 2.2-4 times). We also test our detection techniques on simulated
emission images from a thermodynamic MHD model of the solar corona. We
find that the methods overestimate the area and open flux in the
simulated CH, but the average error in the flux is only about 7%. The
full-Sun detections on the simulated corona underestimate the model
open flux, but by factors well below what is needed to account for
the missing flux in the observations. Under-detection of open flux in
coronal holes likely contributes to the recognized deficit in solar
open flux, but is unlikely to resolve it.
Title: Probabilistic Drag-Based Ensemble Model (DBEM) Evaluation
for Heliospheric Propagation of CMEs
Authors: Čalogović, Jaša; Dumbović, Mateja; Sudar, Davor; Vršnak,
Bojan; Martinić, Karmen; Temmer, Manuela; Veronig, Astrid M.
Bibcode: 2021SoPh..296..114C
Altcode: 2021arXiv210706684C
The Drag-based Model (DBM) is a 2D analytical model for heliospheric
propagation of Coronal Mass Ejections (CMEs) in ecliptic plane
predicting the CME arrival time and speed at Earth or any other given
target in the solar system. It is based on the equation of motion and
depends on initial CME parameters, background solar wind speed, w and
the drag parameter γ . A very short computational time of DBM (<
0.01 s) allowed us to develop the Drag-Based Ensemble Model (DBEM)
that takes into account the variability of model input parameters
by making an ensemble of n different input parameters to calculate
the distribution and significance of the DBM results. Thus the DBEM
is able to calculate the most likely CME arrival times and speeds,
quantify the prediction uncertainties and determine the confidence
intervals. A new DBEMv3 version is described in detail and evaluated
for the first time determining the DBEMv3 performance and errors by
using various CME-ICME lists and it is compared with previous DBEM
versions, ICME being a short-hand for interplanetary CME. The analysis
to find the optimal drag parameter γ and ambient solar wind speed
w showed that somewhat higher values (γ ≈0.3 ×10−7
km−1, w ≈ 425 km s−1) for both of these DBEM
input parameters should be used for the evaluation than the previously
employed ones. Based on the evaluation performed for 146 CME-ICME pairs,
the DBEMv3 performance with mean error (ME) of −11.3 h, mean absolute
error (MAE) of 17.3 h was obtained. There is a clear bias towards
the negative prediction errors where the fast CMEs are predicted to
arrive too early, probably due to the model physical limitations and
input errors (e.g. CME launch speed). This can be partially reduced by
using larger values for γ resulting in smaller prediction errors (ME
=−3.9 h, MAE = 14.5 h) but at the cost of larger prediction errors
for single fast CMEs as well as larger CME arrival speed prediction
errors. DBEMv3 showed also slight improvement in the performance for
all calculated output parameters compared to the previous DBEM versions.
Title: Drag-based model (DBM) tools for forecast of coronal mass
ejection arrival time and speed
Authors: Dumbović, Mateja; Čalogović, Jaša; Martinić, Karmen;
Vršnak, Bojan; Sudar, Davor; Temmer, Manuela; Veronig, Astrid
Bibcode: 2021FrASS...8...58D
Altcode: 2021arXiv210314292D
Forecasting the arrival time of coronal mass ejections (CMEs) and
their associated shocks is one of the key aspects of space weather
research and predictions. One of the commonly used models is, due
to its simplicity and calculation speed, the analytical drag-based
model (DBM) for heliospheric propagation of CMEs. DBM relies on
the observational fact that slow CMEs accelerate whereas fast CMEs
decelerate, and is based on the concept of MHD drag, which acts to
adjust the CME speed to the ambient solar wind. Although physically
DBM is applicable only to the CME magnetic structure, it is often used
as a proxy for the shock arrival. In recent years, the DBM equation
has been used in many studies to describe the propagation of CMEs
and shocks with different geometries and assumptions. Here we give
an overview of the five DBM versions currently available and their
respective tools, developed at Hvar Observatory and frequently used
by researchers and forecasters. These include: 1) basic 1D DBM, a 1D
model describing the propagation of a single point (i.e. the apex of
the CME) or concentric arc (where all points propagate identically); 2)
advanced 2D self-similar cone DBM, a 2D model which combines basic DBM
and cone geometry describing the propagation of the CME leading edge
which evolves self-similarly; 3) 2D flattening cone DBM, a 2D model
which combines basic DBM and cone geometry describing the propagation
of the CME leading edge which does not evolve self-similarly; 4)
DBEMv1, an ensemble version of the 2D flattening cone DBM which uses
CME ensembles as an input and 5) DBEMv3, an ensemble version of the
2D flattening cone DBM which creates CME ensembles based on the input
uncertainties. All five versions have been tested and published in
recent years and are available online or upon request. We provide an
overview of these five tools, of their similarities and differences,
as well as discuss and demonstrate their application.
Title: Influence of the CME orientation on the ICME propagation
Authors: Martinić, Karmen; Dumbović, Mateja; Vršnak, Bojan
Bibcode: 2021EGUGA..23.2526M
Altcode:
Beyond certain distance the ICME propagation becomes mostly governed by
the interaction of the ICME and the ambient solar wind. Configuration
of the interplanetary magnetic field and features of the related
ambient solar wind in the ecliptic and meridional plane are
different. Therefore, one can expect that the inclination of the
CME flux rope axis i.e. tilt, influences the propagation of the ICME
itself. In order to study the relation between the tilt parameter and
the ICME propagation we investigated isolated Earth-impacting CME-ICME
evets in the time period from 2006. to 2014. We determined the CME tilt
in the "near-Sun" environment from the 3D reconstruction of the CME,
obtained by the Graduated Cylindrical Shell model using coronagraphic
images provided by the STEREO and SOHO missions. We determined the tilt
of the ICME in the "near-Earth" environment using in-situ data. We
constrained our study to CME-ICME events that show no evidence of
rotation while propagating, i.e. have a similar tilt in the "near-Sun"
and "near-Earth" environment. We present preliminary results of our
study and discuss their implications for space-weather forecasting using
the drag-based(ensemble) [DB(E)M] model of heliospheric propagation.
Title: Deriving CME volume and density from remote sensing data
Authors: Temmer, Manuela; Holzknecht, Lukas; Dumbovic, Mateja;
Vrsnak, Bojan; Sachdeva, Nishtha; Heinemann, Stephan G.; Dissauer,
Karin; Scolini, Camilla; Asvestari, Eleanna; Veronig, Astrid M.;
Hofmeister, Stefan
Bibcode: 2021EGUGA..23.2535T
Altcode:
Using combined STEREO-SOHO white-light data, we present a method to
determine the volume and density of a coronal mass ejection (CME) by
applying the graduated cylindrical shell model (GCS) and deprojected
mass derivation. Under the assumption that the CME mass is roughly
equally distributed within a specific volume, we expand the CME
self-similarly and calculate the CME density for distances close to the
Sun (15-30 Rs) and at 1 AU. The procedure is applied on a sample of 29
well-observed CMEs and compared to their interplanetary counterparts
(ICMEs). Specific trends are derived comparing calculated and in-situ
measured proton densities at 1 AU, though large uncertainties are
revealed due to the unknown mass and geometry evolution: i) a moderate
correlation for the magnetic structure having a mass that stays
rather constant and ii) a weak correlation for the sheath density by
assuming the sheath region is an extra mass - as expected for a mass
pile-up process - that is in its amount comparable to the initial
CME deprojected mass. High correlations are derived between in-situ
measured sheath density and the solar wind density and solar wind speed
as measured 24 hours ahead of the arrival of the disturbance. This
gives additional confirmation that the sheath-plasma indeed stems from
piled-up solar wind material. While the CME interplanetary propagation
speed is not related to the sheath density, the size of the CME may
play some role in how much material is piled up.
Title: Validation of Global EUV Wave MHD Simulations and Observational
Techniques
Authors: Downs, Cooper; Warmuth, Alexander; Long, David M.; Bloomfield,
D. Shaun; Kwon, Ryun-Young; Veronig, Astrid M.; Vourlidas, Angelos;
Vršnak, Bojan
Bibcode: 2021ApJ...911..118D
Altcode:
Global EUV waves remain a controversial phenomenon more than 20 yr
after their discovery by SOHO/EIT. Although consensus is growing in the
community that they are most likely large-amplitude waves or shocks,
the wide variety of observations and techniques used to identify
and analyze them have led to disagreements regarding their physical
properties and interpretation. Here, we use a 3D magnetohydrodynamic
(MHD) model of the solar corona to simulate an EUV wave event on 2009
February 13 to enable a detailed validation of the various commonly used
detection and analysis techniques of global EUV waves. The simulated
event exhibits comparable behavior to that of a real EUV wave event,
with similar kinematic behavior and plasma parameter evolution. The
kinematics of the wave are estimated via visual identification and
profile analysis, with both approaches providing comparable results. We
find that projection effects can affect the derived kinematics of the
wave, due to the variation in fast-mode wave speed with height in the
corona. Coronal seismology techniques typically used for estimates
of the coronal magnetic field are also tested and found to estimate
fast-mode speeds comparable to those of the model. Plasma density
and temperature variations of the wave front are also derived using
a regularized inversion approach and found to be consistent with
observed wave events. These results indicate that global waves are
best interpreted as large-amplitude waves and that they can be used
to probe the coronal medium using well-defined analysis techniques.
Title: Analytical and empirical modelling of the origin and
heliospheric propagation of coronal mass ejections, and space
weather applications
Authors: Vršnak, Bojan
Bibcode: 2021JSWSC..11...34V
Altcode:
The focus is on the physical background and comprehension of the
origin and the heliospheric propagation of interplanetary coronal
mass ejections (ICMEs), which can cause most severe geomagnetic
disturbances. The paper considers mainly the analytical modelling,
providing useful insight into the nature of ICMEs, complementary
to that provided by numerical MHD models. It is concentrated on
physical processes related to the origin of CMEs at the Sun, their
heliospheric propagation, up to the effects causing geomagnetic
perturbations. Finally, several analytical and statistical forecasting
tools for space weather applications are described.
Title: Deriving CME Density From Remote Sensing Data and Comparison
to In Situ Measurements
Authors: Temmer, M.; Holzknecht, L.; Dumbović, M.; Vršnak, B.;
Sachdeva, N.; Heinemann, S. G.; Dissauer, K.; Scolini, C.; Asvestari,
E.; Veronig, A. M.; Hofmeister, S. J.
Bibcode: 2021JGRA..12628380T
Altcode: 2020arXiv201106880T
We determine the three dimensional geometry and deprojected mass of 29
well observed coronal mass ejections (CMEs) and their interplanetary
counterparts (ICMEs) using combined Solar Terrestrial Relations
Observatory Solar and Heliospheric Observatory white light data. From
the geometry parameters, we calculate the volume of the CME for the
magnetic ejecta (flux rope type geometry) and sheath structure (shell
like geometry resembling the (I)CME frontal rim). Working under the
assumption that the CME mass is roughly equally distributed within a
specific volume, we expand the CME self similarly and calculate the CME
density for distances close to the Sun (15-30 Rs) and at 1 AU. Specific
trends are derived comparing calculated and in situ measured proton
densities at 1 AU, though large uncertainties are revealed due to the
unknown mass and geometry evolution: (1) a moderate correlation for
the magnetic structure having a mass that stays rather constant (cc
≈ 0.56 - 0.59), and (2) a weak correlation for the sheath density (cc
≈ 0.26) by assuming the sheath region is an extra mass—as expected
for a mass pile up process—that is in its amount comparable to the
initial CME deprojected mass. High correlations are derived between in
situ measured sheath density and the solar wind density (cc ≈ -0.73)
and solar wind speed (cc ≈ 0.56) as measured 24 h ahead of the arrival
of the disturbance. This gives additional confirmation that the sheath
plasma indeed stems from piled up solar wind material. While the CME
interplanetary propagation speed is not related to the sheath density,
the size of the CME may play some role in how much material could be
piled up.
Title: Forecasting the arrival time of coronal mass ejections
Authors: Dumbovic, Mateja; Mays, M. Leila; Riley, Pete; Mierla,
Marilena; Kay, Christina; Vrsnak, Bojan; Veronig, Astrid; Cremades,
Hebe; Čalogović, Jaša; Verbeke, Christine; Temmer, Manuela; Sudar,
Davor; Scolini, Camilla; Hinterreiter, Jürgen; Paouris, Evangelos;
Palmerio, Erika; Balmaceda, Laura
Bibcode: 2021cosp...43E1038D
Altcode:
Forecasting the arrival time of coronal mass ejections (CMEs) and their
associated shocks is one of the key aspects of space weather. In recent
years many models have been developed by various research groups aiming
to forecast CME arrival time. The models differ based on the input,
approach, assumptions and complexity ranging from simple empirical and
analytical to complex numerical and machine learning models. One of the
commonly used models is, due to its simplicity and calculation speed,
the analytical drag-based (ensemble) model [DB(E)M] for heliospheric
propagation of CMEs. DB(E)M relies on the observational fact that
slow CMEs accelerate whereas fast CMEs decelerate, and is based on
the concept of MHD drag, which acts to adjust the CME speed to the
ambient solar wind. However, regardless of the model, forecasting CME
arrival time has proven to be exceedingly challenging. One of the major
setbacks is the uncertainty of the CME observational input, which
is still substantial despite state-of-the-art remote observational
capacities such as high-resolution EUV imagers and stereoscopic
observations. Another major setback is the uncertainty in the CME
propagation itself, due to e.g. unrealistic background solar wind
and/or complex interactions. These limits will be discussed in the
scope of DB(E)M and the CME input analysis performed by the ISSI Bern
team on the "Understanding Our Capabilities In Observing And Modeling
Coronal Mass Ejections".
Title: CME evolution and the corresponding Forbush decrease: modelling
vs multi-spacecraft observation
Authors: Dumbovic, Mateja; Moestl, Christian; Podladchikova, Tatiana;
Guo, Jingnan; Heber, Bernd; Vrsnak, Bojan; Dissauer, Karin; Veronig,
Astrid; Amerstorfer, Tanja; Temmer, Manuela; Carcaboso, Fernando;
Kirin, Anamarija
Bibcode: 2021cosp...43E1747D
Altcode:
One of the very common in-situ signatures of interplanetary coronal
mass ejections (ICMEs), as well as other interplanetary transients are
Forbush decreases (FDs), i.e. short-term reductions in the galactic
cosmic ray (GCR) flux. FD phenomena are caused by the interaction
of GCRs with a magnetic structure, therefore it is expected that
different types of interplanetary substructures cause different types
of GCR time profiles, allowing us to distinguish between shock/sheath,
flux rope and SIR-type of FDs. Moreover, since the interaction of
GCRs and CME magnetic structure (i.e. flux rope) occurs all the
way from Sun to Earth, FDs reflect the evolutionary properties of
CMEs. We apply modelling to different ICME regions in order to obtain
a generic FD profile. We model the shock/sheath-related FD using the
propagating diffusive barrier (PDB) model, the flux-rope-related FD
using the diffusion model for the expanding flux rope (ForbMod),
and the exponential time profile approximates the recovery after
the event. The modeled generic FD profile qualitatively agrees with
our current observation-based understanding of FDs. In addition, we
test ForbMod against a set of multi-spacecraft observations of the
same ICME. We find a reasonable agreement of the ForbMod model with
multi-spacecraft measurements, indicating that modelled FDs reflect
well the flux rope evolution.
Title: Evolution of coronal mass ejections and the corresponding
Forbush decreases: modelling vs. multi-spacecraft observations
Authors: Dumbovic, M.; Vrsnak, B.; Guo, J.; Heber, B.; Dissauer, K.;
Carcaboso-Morales, F.; Temmer, M.; Veronig, A.; Podladchikova, T.;
Moestl, C.; Amerstorfer, T.; Kirin, A.
Bibcode: 2020AGUFMSH046..08D
Altcode:
One of the very common in situ signatures of interplanetary coronal
mass ejections (ICMEs), as well as other interplanetary transients,
are Forbush decreases (FDs), i.e. short-term reductions in the galactic
cosmic ray (GCR) flux. A two-step FD is often regarded as a textbook
example, which presumably owes its specific morphology to the fact that
the measuring instrument passed through the ICME head-on, encountering
first the shock front (if developed), then the sheath and finally the
CME magnetic structure. The interaction of GCRs and the shock/sheath
region, as well as the CME magnetic structure, occurs all the way from
Sun to Earth, therefore, FDs are expected to reflect the evolutionary
properties of CMEs and their sheaths. We apply modelling to different
ICME regions in order to obtain a generic two-step FD profile, which
qualitatively agrees with our current observation-based understanding of
FDs. We next adapt the models for energy dependence to enable comparison
with different GCR measurement instruments (as they measure in different
particle energy ranges). We test these modelling efforts against a
set of multi-spacecraft observations of the same event, using the
Forbush decrease model for the expanding flux rope (ForbMod). We find
a reasonable agreement of the ForbMod model for the GCR depression
in the CME magnetic structure with multi-spacecraft measurements,
indicating that modelled FDs reflect well the CME evolution.
Title: Characteristics of a long-lived CIR and the corresponding
depression in the GCR flux
Authors: Dumbovic, M.; Vrsnak, B.; Temmer, M.; Heber, B.
Bibcode: 2020AGUFMSH0440026D
Altcode:
We observe a long-lived CIR recurring in 27 consecutive Carrington
rotations 2057-2083 in the time period from June 2007 - May 2009. We
characterize the in situ measurements of this long-lived CIR as well as
the corresponding depression in the GCR count observed by SOHO/EPHIN,
and analyze them throughout different rotations. We find that the
behavior of the flow speed peak roughly shows a rising phase and
a declining phase. This is similar to the evolutionary profile of
some observed coronal hole areas, but without a clear peak. The GCR
count evolutionary profile roughly follows that of the flow speed
peak, but moreover we find that the inverted GCR count time-profile
matches very well with that of the flow speed throughout different
rotations. We perform a statistical analysis and find the GCR count
amplitude correlated to the peak in the magnetic field and flow
speed, as expected based on previous statistical studies. In order
to characterize a generic CIR profile for modelling purposes, we
perform the superposed epoch analysis using relative values of the
key parameters. Based on the observed properties we propose a simple
analytical model starting from the basic Fokker-Planck equation.
Title: Solar Flare-CME Coupling throughout Two Acceleration Phases
of a Fast CME
Authors: Gou, Tingyu; Veronig, Astrid M.; Liu, Rui; Zhuang, Bin;
Dumbović, Mateja; Podladchikova, Tatiana; Reid, Hamish A. S.; Temmer,
Manuela; Dissauer, Karin; Vršnak, Bojan; Wang, Yuming
Bibcode: 2020ApJ...897L..36G
Altcode: 2020arXiv200611707G
Solar flares and coronal mass ejections (CMEs) are closely coupled
through magnetic reconnection. CMEs are usually accelerated impulsively
within the low solar corona, synchronized with the impulsive flare
energy release. We investigate the dynamic evolution of a fast CME and
its associated X2.8 flare occurring on 2013 May 13. The CME experiences
two distinct phases of enhanced acceleration, an impulsive one with a
peak value of ∼5 km s-2, followed by an extended phase with
accelerations up to 0.7 km s-2. The two-phase CME dynamics
is associated with a two-episode flare energy release. While the first
episode is consistent with the "standard" eruption of a magnetic flux
rope, the second episode of flare energy release is initiated by the
reconnection of a large-scale loop in the aftermath of the eruption
and produces stronger nonthermal emission up to γ-rays. In addition,
this long-duration flare reveals clear signs of ongoing magnetic
reconnection during the decay phase, evidenced by extended hard X-ray
bursts with energies up to 100-300 keV and intermittent downflows
of reconnected loops for >4 hr. The observations reveal that the
two-step flare reconnection substantially contributes to the two-phase
CME acceleration, and the impulsive CME acceleration precedes the most
intense flare energy release. The implications of this non-standard
flare/CME observation are discussed.
Title: Evolution of Coronal Mass Ejections and the Corresponding
Forbush Decreases: Modeling vs. Multi-Spacecraft Observations
Authors: Dumbović, Mateja; Vršnak, Bojan; Guo, Jingnan; Heber,
Bernd; Dissauer, Karin; Carcaboso, Fernando; Temmer, Manuela; Veronig,
Astrid; Podladchikova, Tatiana; Möstl, Christian; Amerstorfer, Tanja;
Kirin, Anamarija
Bibcode: 2020SoPh..295..104D
Altcode: 2020arXiv200602253D
One of the very common in situ signatures of interplanetary coronal
mass ejections (ICMEs), as well as other interplanetary transients,
are Forbush decreases (FDs), i.e. short-term reductions in the galactic
cosmic ray (GCR) flux. A two-step FD is often regarded as a textbook
example, which presumably owes its specific morphology to the fact that
the measuring instrument passed through the ICME head on, encountering
first the shock front (if developed), then the sheath, and finally the
CME magnetic structure. The interaction of GCRs and the shock/sheath
region, as well as the CME magnetic structure, occurs all the way from
Sun to Earth, therefore, FDs are expected to reflect the evolutionary
properties of CMEs and their sheaths. We apply modeling to different
ICME regions in order to obtain a generic two-step FD profile, which
qualitatively agrees with our current observation-based understanding
of FDs. We next adapt the models for energy dependence to enable
comparison with different GCR measurement instruments (as they measure
in different particle energy ranges). We test these modeling efforts
against a set of multi-spacecraft observations of the same event, using
the Forbush decrease model for the expanding flux rope (ForbMod). We
find a reasonable agreement of the ForbMod model for the GCR depression
in the CME magnetic structure with multi-spacecraft measurements,
indicating that modeled FDs reflect well the CME evolution.
Title: Sun-to-Earth Observations and Characteristics of Isolated
Earth-Impacting Interplanetary Coronal Mass Ejections During 2008
- 2014
Authors: Maričić, D.; Vršnak, B.; Veronig, A. M.; Dumbović, M.;
Šterc, F.; Roša, D.; Karlica, M.; Hržina, D.; Romštajn, I.
Bibcode: 2020SoPh..295...91M
Altcode: 2020arXiv200810265M
A sample of isolated Earth-impacting interplanetary coronal mass
ejections (ICMEs) that occurred in the period January 2008 to August
2014 is analyzed to study in detail the ICME in situ signatures, with
respect to the type of filament eruption related to the corresponding
CME. Observations from different vantage points provided by the
Solar and Heliospheric Observatory (SOHO) and the Solar Terrestrial
Relations Observatory Ahead and Behind (STEREO-A and B) are used to
determine whether each CME under study is Earth directed or not. For
Earth-directed CMEs, a kinematical study was performed using the
STEREO-A and B COR1 and COR2 coronagraphs and the Heliospheric Imagers
(HI1), to estimate the CME arrival time at 1 AU and to link the CMEs
with the corresponding in situ solar wind counterparts. Based on the
extrapolated CME kinematics, we identified interacting CMEs, which
were excluded from further analysis. Applying this approach, a set
of 31 isolated Earth-impacting CMEs was unambiguously identified and
related to the in situ measurements recorded by the Wind spacecraft. We
classified the events into subsets with respect to the CME source
location, as well as with respect to the type of the associated filament
eruption. Hence, the events are divided into three subsamples: active
region (AR) CMEs, disappearing filament (DSF) CMEs, and stealthy
CMEs. The related three groups of ICMEs were further divided into
two subsets: magnetic obstacle (MO) events (out of which four were
stealthy), covering ICMEs that at least partly showed characteristics of
flux ropes, and ejecta (EJ) events, not showing such characteristics. In
this way, 14 MO-ICMEs and 17 EJ-ICMES were identified. The solar
source regions of the non-stealthy MO-ICMEs are found to be located
predominantly (9/10, 90%) within ±30∘ from the solar
central meridian, whereas EJ-ICMEs originate predominantly (16/17, 94%)
from source regions that are outside ±30∘. In the next
step, MO-events were analyzed in more detail, considering the magnetic
field strength and the plasma characteristics in three different
segments, defined as the turbulent sheath (TS), the frontal region
(FR), and the MO itself. The analysis revealed various well-defined
correlations for AR, DSF, and stealthy ICMEs, which we interpreted
considering basic physical concepts. Our results support the hypothesis
that ICMEs show different signatures depending on the in situ spacecraft
trajectory, in terms of apex versus flank hits.
Title: CME evolution and the corresponding Forbush decrease: modelling
vs multi-spacecraft observation
Authors: Dumbovic, Mateja; Vrsnak, Bojan; Guo, Jingnan; Heber, Bernd;
Dissauer, Karin; Carcaboso-Morales, Fernando; Temmer, Manuela; Veronig,
Astrid; Podladchikova, Tatiana; Möstl, Christian; Amerstorfer, Tanja;
Kirin, Anamarija
Bibcode: 2020EGUGA..2210446D
Altcode:
One of the very common in-situ signatures of ICMEs, as well as other
interplanetary transients are Forbush decreases (FDs), i.e. short-term
reductions in the galactic cosmic ray (GCR) flux. A two-step FD is
often regarded as a textbook example, which presumably owns its specific
morphology to the fact that the measuring instrument passed through the
ICME head-on, encountering first the shock front (if developed), then
the sheath and finally the magnetic structure. The interaction of GCRs
and the shock/sheath region as well as CME magnetic structure occurs all
the way from Sun to Earth, therefore, FDs are expected to reflect the
evolutionary properties of CMEs and their sheaths. We apply modelling
to different ICME regions in order to obtain a generic two-step FD
profile, which qualitatively agrees with our current observation-based
understanding of FDs. We next adapt the models for energy dependence
to enable comparison with different GCR measurement instruments
(as they measure in different particle energy ranges). We test these
modelling efforts against a set of multi-spacecraft observations of
the same event.
Title: Relating CME density derived from remote sensing data to CME
sheath solar wind plasma pile up as measured in-situ
Authors: Temmer, Manuela; Holzknecht, Lukas; Dumbovic, Mateja; Vrsnak,
Bojan; Sachdeva, Nishtha; Heinemann, Stephan; Dissauer, Karin; Scolini,
Camilla; Asvestari, Eleanna; Veronig, Astrid; Hofmeister, Stefan
Bibcode: 2020EGUGA..22.3341T
Altcode:
For better estimating the drag force acting on coronal mass ejections
(CMEs) in interplanetary space and ram-pressure at planets, improved
knowledge of the evolution of CME density/mass is highly valuable. We
investigate a sample of 29 well observed CME-ICME events, for which
we determine the de-projected 3D mass (STEREO-A and -B data), and the
CME volume using GCS modeling (STEREO, SoHO). Expanding the volume to
1AU distance, we derive the density and compare the results to in-situ
proton density measurements separately for the ICME sheath and magnetic
structure. A fair agreement between calculated and measured density is
derived for the magnetic structure as well for the sheath if taking
into account mass pile up of solar wind plasma. We give evidence and
observational assessment that during the interplanetary propagation
of a CME 1) the magnetic structure has rather constant mass and 2)
the sheath region at the front of the driver is formed from piled-up
mass that is rather depending on the solar wind density ahead of the
CME, than on the CME speed.
Title: On the Interaction of Galactic Cosmic Rays with Heliospheric
Shocks During Forbush Decreases
Authors: Kirin, Anamarija; Vršnak, Bojan; Dumbović, Mateja; Heber,
Bernd
Bibcode: 2020SoPh..295...28K
Altcode: 2020arXiv200209454K
Forbush decreases (FDs) are depletions in the galactic cosmic ray
(GCR) count rate that last typically for about a week and can be
caused by coronal mass ejections (CMEs) or corotating interacting
regions (CIRs). Fast CMEs that drive shocks cause large FDs that
often show a two-step decrease where the first step is attributed to
the shock/sheath region, while the second step is attributed to the
closed magnetic structure. Since the difference in size of shock and
sheath region is significant, and since there are observed effects
that can be related to shocks and not necessarily to the sheath region
we expect that the physical mechanisms governing the interaction with
GCRs in these two regions are different. We therefore aim to analyze
interaction of GCRs with heliospheric shocks only. We approximate the
shock by a structure where the magnetic field linearly changes with
position within this structure. We assume protons of different energy,
different pitch angle and different incoming direction. We also vary the
shock parameters such as the magnetic field strength and orientation,
as well as the shock thickness. The results demonstrate that protons
with higher energies are less likely to be reflected. Also, thicker
shocks and shocks with stronger field reflect protons more efficiently.
Title: Genesis and impulsive evolution of the fast CME associated
with the X8.2 flare on 2017 September 10
Authors: Veronig, A.; Podladchikova, T.; Dissauer, K.; Temmer, M.;
Seaton, D. B.; Long, D.; Guo, J.; Vrsnak, B.; Harra, L. K.; Kliem, B.
Bibcode: 2019AGUFMSH13A..02V
Altcode:
The X8.2 event of 2017 September 10 provides unique observations to
study the genesis, magnetic morphology, impulsive dynamics and shock
formation in a very fast coronal mass ejection (CME). As will be
discussed in this presentation, fundamental insight in the processes
of magnetic reconnection, CME acceleration and shock formation are
provided through EUV observations of the middle corona. Combining
the large field-of-view and high-cadence imagery from GOES-16/SUVI
and SDO/AIA EUV, respectively, we identify a hot (T ≈ 10-15 MK)
bright rim around a quickly expanding cavity, embedded inside a much
larger CME shell (T ≈ 1-2 MK). The CME shell develops from a dense
set of large AR loops (>0.5Rs) and seamlessly evolves into the
CME front observed in LASCO C2. The strong lateral overexpansion
of the CME shell acts as a piston initiating the fast and globally
propagating EUV shock wave. The hot cavity rim is demonstrated to be
a manifestation of the dominantly poloidal flux and frozen-in plasma
added to the rising flux rope by magnetic reconnection in the current
sheet beneath. The same structure is later observed as the core of the
white-light CME, challenging the traditional interpretation of the CME
three-part morphology (Veronig et al. 2018). The large amount of
added magnetic flux suggested by these observations can explain the
extreme accelerations of the radial and lateral expansion of the CME
shell and cavity, all reaching values up to 5-10 km s-2. The
acceleration peaks occur simultaneously with the first RHESSI 100-300
keV hard X-ray burst of the associated flare, further underlining the
importance of the reconnection process for the impulsive CME evolution
in the low and middle corona.
Title: Heliospheric Evolution of Magnetic Clouds
Authors: Vršnak, B.; Amerstorfer, T.; Dumbović, M.; Leitner, M.;
Veronig, A. M.; Temmer, M.; Möstl, C.; Amerstorfer, U. V.; Farrugia,
C. J.; Galvin, A. B.
Bibcode: 2019ApJ...877...77V
Altcode: 2019arXiv190408266V
The interplanetary evolution of 11 magnetic clouds (MCs) recorded by at
least two radially aligned spacecraft is studied. The in situ magnetic
field measurements are fitted to a cylindrically symmetric Gold-Hoyle
force-free uniform-twist flux-rope configuration. The analysis
reveals that in a statistical sense, the expansion of the studied
MCs is compatible with self-similar behavior. However, individual
events expose a large scatter of expansion rates, ranging from very
weak to very strong expansion. Individually, only four events show an
expansion rate compatible with isotropic self-similar expansion. The
results indicate that the expansion has to be much stronger when
the MCs are still close to the Sun than in the studied 0.47-4.8 au
distance range. The evolution of the magnetic field strength shows a
large deviation from the behavior expected for the case of isotropic
self-similar expansion. In the statistical sense, as well as in most
of the individual events, the inferred magnetic field decreases much
slower than expected. Only three events show behavior compatible
with self-similar expansion. There is also a discrepancy between the
magnetic field decrease and the increase of the MC size, indicating that
magnetic reconnection and geometrical deformations play a significant
role in the MC evolution. About half of the events show a decay of the
electric current as expected for self-similar expansion. Statistically,
the inferred axial magnetic flux is broadly consistent with remaining
constant. However, events characterized by a large magnetic flux show
a clear tendency toward decreasing flux.
Title: Genesis, magnetic morphology and impulsive evolution of
the coronal mass ejection associated with the X8.2 flare on 2017
September 10
Authors: Veronig, Astrid; Podladchikova, Tatiana; Dissauer, Karin;
Temmer, Manuela; Seaton, Daniel; Long, David; Guo, Jingnan; Vrsnak,
Bojan; Harra, Louise; Kliem, Bernhard
Bibcode: 2019EGUGA..21.9243V
Altcode:
The extreme X8.2 event of 2017 September 10 provides unique observations
to study the genesis, magnetic morphology, impulsive dynamics and
shock formation in a very fast coronal mass ejection (CME). Combining
GOES-16/SUVI and SDO/AIA EUV imagery, we identify a hot (T ≈ 10-15
MK) bright rim around a quickly expanding cavity, embedded inside a
much larger CME shell (T ≈ 1-2 MK). The CME shell develops from a
dense set of large AR loops (>0.5Rs) and seamlessly evolves into
the CME front observed in LASCO C2. The strong lateral overexpansion
of the CME shell acts as a piston initiating the fast EUV shock
wave. The hot cavity rim is demonstrated to be a manifestation of
the dominantly poloidal flux and frozen-in plasma added to the rising
flux rope by magnetic reconnection in the current sheet beneath. The
same structure is later observed as the core of the white-light CME,
challenging the traditional interpretation of the CME three-part
morphology. The large amount of added magnetic flux suggested by these
observations explains the extreme accelerations of the radial and
lateral expansion of the CME shell and cavity, all reaching values
up to 5-10 km s-2. The acceleration peaks occur simultaneously with
the first RHESSI 100-300 keV hard X-ray burst of the associated flare,
further underlining the importance of the reconnection process for the
impulsive CME evolution. Finally, the much higher radial propagation
speed of the flux rope in relation to the CME shell causes a distinct
deformation of the white-light CME front and shock.
Title: Observational assessment on CME mass pile up in interplanetary
space
Authors: Temmer, Manuela; Holzknecht, Lukas; Dumbovic, Mateja;
Vrsnak, Bojan
Bibcode: 2019EGUGA..21.9578T
Altcode:
Coronal mass ejections (CMEs) propagating in the heliosphere are
exposed to a drag force due to the ambient solar wind. Mass pile-up
in interplanetary space can have strong effects on the drag force,
and with that on the CME propagation time and energy input to the
magnetosphere. For a sample of well observed events, we determine
the de-projected 3D mass and its evolution up to a distance range
of about 15Rs using combined STEREO-SECCHI COR1 and COR2 data, for
which no pile-up at the CME front is found (see also Bein et al.,
2013). Applying the GCS forward fitting model (Thernisien et al., 2006,
2009) on COR2 data, we obtain the volume of the CMEs. Working under the
assumption that the CME mass is constant beyond 15Rs and that the CME
undergoes self-similar expansion, we estimate the CME density at the
distance of 1AU. The results are compared to in-situ proton density
data measured for the associated ICME's sheath and magnetic structure
for which we derive a trend towards a mass increase at the CME front.
Title: Study of Interplanetary CMEs/Shocks During Solar Cycle 24
Using Drag-Based Model: The Role of Solar Wind
Authors: Suresh, K.; Prasanna Subramanian, S.; Shanmugaraju, A.;
Vršnak, Bojan; Umapathy, S.
Bibcode: 2019SoPh..294...47S
Altcode:
In this paper we analyze a set of 27 fast interplanetary coronal mass
ejections (ICMEs) observed during the period January 2010 - December
2013 in Solar Cycle 24. The arrivals of interplanetary shocks and
CMEs at 1 AU are found from OMNI spacecraft high resolution data
and their travel times are compared with Empirical Shock Arrival
(ESA; Gopalswamy et al. in Adv. Space Res.36, 2289, 2005) and Drag
Based Model (DBM; Vršnak et al. in Solar Phys.285, 295, 2013). The
analysis of the transit time, deceleration, and drag parameter is used
to examine the role of the solar-wind characteristics in the dynamics
of ICMEs. The obtained ICME parameters (deceleration, drag parameter)
are compared with the decelerated events (34 of 91 events in Solar
Cycle 23) from the study of Manoharan et al. (J. Geophy. Res.109,
A06109, 2004). The interplanetary (IP) deceleration shows similar
trend between the cycles. Though the Cycle 24 has weak solar wind,
it does not affect the arrival time behavior. It is concluded that
the solar-wind behavior is considered to be the same in Cycles 23
and 24 for ICMEs. The IP drag parameter is linearly correlated with
CME initial speed. The p-value between CME speed and drag parameter
suggests that they are highly significant. The important result of the
study is that the solar wind showed a similar kind of drag effect for
the propagating CMEs in both cycles.
Title: Multiple EUV wave reflection from a coronal hole
Authors: Podladchikova, Tatiana; Veronig, Astrid M.; Podladchikova,
Olena; Dissauer, Karin; Vršnak, Bojan; Saqri, Jonas; Piantschitsch,
Isabell; Temmer, Manuela
Bibcode: 2019EGUGA..21.9793P
Altcode:
EUV waves are large-scale propagating disturbances in the solar corona
initiated by coronal mass ejections. We investigate the multiple EUV
wave reflections at a coronal hole boundary, as observed by SDO/AIA on 1
April 2017. The EUV wave originates from Active Region (AR) 12645 close
to the disk center and propagates toward the south polar coronal hole
with an average velocity of 430 km/s. The interaction of the EUV wave
with the coronal hole, which represents a region of high Alfven speed,
is observed as a splitting into two wave components: one continues
propagation inside the coronal hole with an increased velocity of 850
km/s (transmitted wave), while the other one moves back toward the AR,
also with an increased velocity of 600 km/s (reflected wave). The
reflected EUV wave is subsequently reflected again from the AR and
propagates toward the coronal hole with an average velocity of 350
km/s, where it is reflected for the second time at the coronal hole
boundary and propagates again toward the AR with a velocity of 300
km/s. These events are observed over an interval of 40 minutes. The
high cadence SDO imagery allows us to study in detail the kinematics
of the direct and multiple times reflected EUV wave. In addition, its
multi-wavelength EUV imagery allows us to derive the plasma properties
of the corona and the EUV wave pulse via Differential Emission Measure
analysis. These results are used to compare the observed characteristics
of the wave interaction with the coronal hole with simulations.
Title: Gradual pre-eruptive phase of solar coronal eruptions
Authors: Vršnak, Bojan
Bibcode: 2019FrASS...6...28V
Altcode:
Physical background of the evolution of a coronal magnetic flux rope
embedded in the magnetic arcade during the gradual-rise pre-eruptive
stage is studied. It is assumed that this stage represents an
externyly-driven evolution of the preeruptive structure through a
series of quasi-equilibrium states, until a point when system losses
equilibrium and erupts. In particular, three driving processes are
considered: twisting motions of the flux-rope footpoints, emergence
of new magnetic flux beneath the flux rope, and the mass leakage down
the flux-rope legs. For that purpose, an analytical flux-rope model is
employed, to inspect how fast the equilibrium height of the structure
rises due to the increase of the poloidal-toaxial field ratio, the
increase of axial electric current, and the decrease of mass. It is
shown that the flux-rope twisting itself is not sufficient to reproduce
the rising speeds observed during the pre-eruptive stage. Yet, it
is essential for the loss-ofequilibrium process. On the other hand,
the considered emerging flux and the mass loss processes reproduce
well the rate at which the pre-eruptive structure rises before the
main acceleration stage of the eruption sets in.
Title: The Physical Processes of CME/ICME Evolution
Authors: Manchester, Ward, IV; Kilpua, Emilia K. J.; Liu, Ying D.;
Lugaz, Noé; Riley, Pete; Török, Tibor; Vršnak, Bojan
Bibcode: 2019sfsw.book..165M
Altcode:
No abstract at ADS
Title: The Origin, Early Evolution and Predictability of Solar
Eruptions
Authors: Green, Lucie M.; Török, Tibor; Vršnak, Bojan; Manchester,
Ward, IV; Veronig, Astrid
Bibcode: 2019sfsw.book..113G
Altcode:
No abstract at ADS
Title: Genesis and Impulsive Evolution of the 2017 September 10
Coronal Mass Ejection
Authors: Veronig, Astrid M.; Podladchikova, Tatiana; Dissauer, Karin;
Temmer, Manuela; Seaton, Daniel B.; Long, David; Guo, Jingnan; Vršnak,
Bojan; Harra, Louise; Kliem, Bernhard
Bibcode: 2018ApJ...868..107V
Altcode: 2018arXiv181009320V
The X8.2 event of 2017 September 10 provides unique observations
to study the genesis, magnetic morphology, and impulsive dynamics
of a very fast coronal mass ejection (CME). Combining GOES-16/SUVI
and SDO/AIA EUV imagery, we identify a hot (T ≈ 10-15 MK) bright
rim around a quickly expanding cavity, embedded inside a much larger
CME shell (T ≈ 1-2 MK). The CME shell develops from a dense set of
large AR loops (≳0.5R s ) and seamlessly evolves into
the CME front observed in LASCO C2. The strong lateral overexpansion
of the CME shell acts as a piston initiating the fast EUV wave. The
hot cavity rim is demonstrated to be a manifestation of the dominantly
poloidal flux and frozen-in plasma added to the rising flux rope by
magnetic reconnection in the current sheet beneath. The same structure
is later observed as the core of the white-light CME, challenging the
traditional interpretation of the CME three-part morphology. The large
amount of added magnetic flux suggested by these observations explains
the extreme accelerations of the radial and lateral expansion of the CME
shell and cavity, all reaching values of 5-10 km s-2. The
acceleration peaks occur simultaneously with the first RHESSI 100-300
keV hard X-ray burst of the associated flare, further underlining
the importance of the reconnection process for the impulsive CME
evolution. Finally, the much higher radial propagation speed of the
flux rope in relation to the CME shell causes a distinct deformation
of the white-light CME front and shock.
Title: Evolution of flux rope, CME and associated EUV wave in the
10-Sep-2018 X8.2 event
Authors: Podladchikova, Tatiana; Veronig, Astrid M.; Dissauer, Karin;
Temmer, Manuela; Seaton, Daniel B.; Long, David; Guo, Jingnan; Vršnak,
Bojan; Harra, Louise; Kliem, Bernhard
Bibcode: 2018csc..confE..38P
Altcode:
We combine the high-cadence and large field-of-view EUV imagery of
the Atmospheric Imaging Assembly (AIA) onboard SDO and the Solar
Ultraviolet Imager (SUVI) onboard GOES-16 to study the origin and
impulsive evolution of the fast CME that originated in the September
10th 2017 X8.2 event as well as the initiation of the associated EUV
wave. In the LASCO field-of-view, the CME reveals speeds >3000
km/s. In the low-to-mid corona, it shows a distinct bubble in the EUV
imagery that reveals a significant lateral overexpansion. In addition,
is also shows a distinct expanding cavity that is interpreted as
manifestation of the flux rope driving the eruption. We present a method
to automatically identify and segment the CME bubble in SUVI images and
to derive its radial and lateral evolution up to about 2 solar radii,
in terms of velocity and acceleration. These measurements are set into
context with the evolution of the embedded flux rope/cavity observed by
AIA. The observations show clear signatures of new poloidal flux added
to the flux rope by magnetic reconnection in the current sheet beneath
the eruptive structure, which is important for the high accelerations
observed in this event. The radial propagation of the CME shell revealed
a peak value of the acceleration of about 5.3 km/s2, whereas the lateral
expansion reached a peak value of 10.1 km/s2, which is the largest value
reported so far. The flux rope/cavity reveals a radial acceleration of
6.7 km/s2 and lateral acceleration of 5.3 km/s2. We note that at this
early evolution phase, the speed of the cavity/flux rope is higher
than that of the CME bubble (front). The EUV wave associated with
this eruption was observed by AIA, SUVI and STEREO-A EUVI, which had
a separation angle with Earth of 128°, and the common field of view
of the spacecraft was 52°. AIA and SUVI images above the solar limb
reveal the initiation of the EUV wave by the accelerating flanks of
the CME bubble, followed by detachment and propagation of the wave
with a speed of 1100 km/s. The EUV wave shows a global propagation
over the full hemisphere visible to Earth view as well as into the
STEREO-A field-of-view. We study the propagation and kinematics of
the direct as well as the various reflected and refracted EUV wave
components on the solar sphere, finding speeds in the range from 370
to 1010 km/s. Finally, we note that this EUV wave is also distinct as
it reveals propagation and transmission through the polar coronal holes.
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: The possible impact of solar activity on extratropical
cyclone activity
Authors: Calogovic, J.; Vrsnak, Bojan; Dumbovic, Mateja; Rieder, Harald
Bibcode: 2018cosp...42E.497C
Altcode:
The mechanism based on the global electric circuit (GEC) flowing
vertically from the ionosphere to the Earth's surface could potentially
provide the link between the solar modulated energetic particles
and Earth's weather and climate. Cosmic ray induced atmospheric
ionization modulates the vertical current density (Jz) and introduces
the changes in GEC that could alter the microphysical properties of
the clouds (Tinsley, 2008). Due to the complexity and scale of the
GEC and its feedbacks, possible implications and importance of this
mechanism are still mostly unknown. One of the possible feedbacks
to GEC alteration could be the proces of storm invigoration and
occurrence of extratropical cyclones. Using 6-hourly sea level pressure
(SLP) fields from the ERA-Interim data, extratropical cyclones are
identified by tracking their low-pressure centers. Daily timescale
epoch-superpositional (composite) analysis is performed to analyze
the occurrence of extratropical cyclones during the biggest Forbush
decrease events in the last three solar cycles. Since autocorrelations
are the common feature of geophysical data, to test the significance
of results we use robust Monte Carlo significance testing. This work
has been supported by the Croatian Science Foundation project 6212
"Solar and Stellar Variability" (SOLSTEL).
Title: Forbush decrease model for expanding CMEs (ForbMod)
Authors: Dumbovic, Mateja; Möstl, Christian; Guo, Jingnan; Heber,
Bernd; Vrsnak, Bojan; Temmer, Manuela
Bibcode: 2018cosp...42E.917D
Altcode:
Forbush decreases (FDs) can be used as one of the "signatures" of an
ICME passage. An analytical diffusion-expansion FD model (ForbMod) was
developed that is based on the widely used approach of an initially
empty, closed magnetic structure (i.e. flux rope) that fills up
slowly with particles by diffusion perpendicular to the magnetic
field of the flux rope. In our approach the FD amplitude is not only
determined by the diffusion process but also by the expansion of the
flux rope. While the first process leads to a smaller amplitude the
second one leads again to a larger effect. Remote CME observations
and 3D reconstruction is used to constrain initial and boundary
conditions. CME evolutionary properties are taken into account by
incorporating the flux rope expansion. Several options of flux rope
expansion are regarded as competing mechanism to diffusion, which can
lead to different FD characteristics, and forward modelling is used
to analyse flux rope expansion and further constrain the model. In
testing the model, a number of spacecraft and planetary observation is
utilised, including those by the Radiation Assessment Detector aboard
the Mars Rover Curiosity. This project has received funding from the
European Union's Horizon 2020 research and innovation programme under
the Marie Sk_odowska-Curie grant agreement No 745782.
Title: Type II solar radio burst band-splitting: Measure of coronal
magnetic field strength
Authors: Mahrous, Ayman; Alielden, Khaled; Vršnak, Bojan; Youssef,
Mohamed
Bibcode: 2018JASTP.172...75M
Altcode:
Studies of the relationship between solar radio bursts and CMEs are
essential for understanding of the nature of type II bursts. In this
study, we examine the type II solar radio burst recorded on 16 March
2016 by the Learmonth radio spectrograph and compare its characteristics
with the kinematics of the associated CMEs observed by STEREO and
SOHO spacecraft. The burst showed a well-defined band-split, which was
used to estimate the magnetic field strength in the solar corona. The
magnetic field decreases from ≈ 4 G at R ≈ 2.6 R⊙ to
0.62 G at R ≈ 3.77 R⊙ depending on the coronal electron
density model employed. We found that two CMEs occurred successively in
a 4-h interval. During this interval, a type II radio burst occurred,
lasting for about 10 min. Tracking of the shock that produced type II
burst and comparison with the CMEs heights as observed by STEREO and
SOHO spacecraft help us to deduce the driver of the shock. According
to the analysis, the type II burst occurrence was associated with the
interaction of the shock driven by the second CME with a streamer
located south of the first CME, since that the type II band-split
significantly increased during the shock-streamer interaction. Our
results show that the analysis of the type II burst band-split
supplemented by the coronagraphic observations of the corona is an
important tool for the understanding of the coronal eruptive processes.
Title: Drag-based ensemble model (DBEM)
Authors: Dumbovic, Mateja; Möstl, Christian; Mays, M. Leila; Vrsnak,
Bojan; Veronig, Astrid; Salogovic, Jara; Piantschitsch, Isabell;
Amerstorfer, Tanja; Temmer, Manuela; Sudar, Davor
Bibcode: 2018cosp...42E.918D
Altcode:
The drag-based model (DBM) for heliospheric propagation of ICMEs is
a widely used simple analytical model which can predict ICME arrival
time and speed at a given heliospheric distance (Vr_nak et al.,
2013, SolPhys). It is based on the assumption that the heliospheric
propagation of ICMEs, is solely under the influence of MHD drag,
where ICME propagation is determined based on CME properties as
well as the properties of the ambient solar wind. The current
version of the DBM is operational as part of ESA's SSA programme
(http://swe.ssa.esa.int/web/guest/graz-dbm-federated). The DBM takes
into account the ICME geometry to track the whole leading edge of an
ICME, it can estimate whether or not an ICME will reach the observer
and calculate the transit time and impact speed. To estimate the
uncertainty for a single event, Drag-Based Ensemble Model (DBEM) was
developed (Dumbovic et al., 2018, ApJ) which utilizes an ensemble of the
observation-based CME input and synthetic values of the ambient solar
wind speed and drag parameter. Using multiple runs with different input
parameters, distributions of predicted arrival times and speeds are
obtained allowing to forecast the confidence in the likelihood of the
ICME arrival. The DBEM was further developed to an on-line application
to provide the real-time CME forecast, which is currently in a test
phase, and will soon be a part of ESA-SSA Heliospheric Weather Expert
Service Group (http://swe.ssa.esa.int/heliospheric-weather). We test
the model and the on-line application using observations and compare
the performance with other CME propagation models.
Title: An Analytical Diffusion-Expansion Model for Forbush Decreases
Caused by Flux Ropes
Authors: Dumbović, Mateja; Heber, Bernd; Vršnak, Bojan; Temmer,
Manuela; Kirin, Anamarija
Bibcode: 2018ApJ...860...71D
Altcode: 2018arXiv180500916D
We present an analytical diffusion-expansion Forbush decrease (FD)
model ForbMod, which is based on the widely used approach of an
initially empty, closed magnetic structure (i.e., flux rope) that
fills up slowly with particles by perpendicular diffusion. The model
is restricted to explaining only the depression caused by the magnetic
structure of the interplanetary coronal mass ejection (ICME). We use
remote CME observations and a 3D reconstruction method (the graduated
cylindrical shell method) to constrain initial boundary conditions
of the FD model and take into account CME evolutionary properties by
incorporating flux rope expansion. Several flux rope expansion modes
are considered, which can lead to different FD characteristics. In
general, the model is qualitatively in agreement with observations,
whereas quantitative agreement depends on the diffusion coefficient and
the expansion properties (interplay of the diffusion and expansion). A
case study was performed to explain the FD observed on 2014 May 30. The
observed FD was fitted quite well by ForbMod for all expansion modes
using only the diffusion coefficient as a free parameter, where the
diffusion parameter was found to correspond to an expected range of
values. Our study shows that, in general, the model is able to explain
the global properties of an FD caused by a flux rope and can thus be
used to help understand the underlying physics in case studies.
Title: Numerical Simulation of Coronal Waves Interacting with Coronal
Holes. III. Dependence on Initial Amplitude of the Incoming Wave
Authors: Piantschitsch, Isabell; Vršnak, Bojan; Hanslmeier, Arnold;
Lemmerer, Birgit; Veronig, Astrid; Hernandez-Perez, Aaron; Čalogović,
Jaša
Bibcode: 2018ApJ...860...24P
Altcode: 2018arXiv181112735P
We performed 2.5D magnetohydrodynamic (MHD) simulations showing the
propagation of fast-mode MHD waves of different initial amplitudes and
their interaction with a coronal hole (CH), using our newly developed
numerical code. We find that this interaction results in, first, the
formation of reflected, traversing, and transmitted waves (collectively,
secondary waves) and, second, in the appearance of stationary features
at the CH boundary. Moreover, we observe a density depletion that
is moving in the opposite direction of the incoming wave. We find a
correlation between the initial amplitude of the incoming wave and
the amplitudes of the secondary waves as well as the peak values of
the stationary features. Additionally, we compare the phase speed of
the secondary waves and the lifetime of the stationary features to
observations. Both effects obtained in the simulation, the evolution
of secondary waves, as well as the formation of stationary fronts at
the CH boundary, strongly support the theory that coronal waves are
fast-mode MHD waves.
Title: Using Forbush decreases to derive the transit time of ICMEs
propagating from 1 AU to Mars
Authors: von Forstner, Johan; Guo, Jingnan; Wimmer-Schweingruber,
Robert F.; Hassler, Donald M.; Temmer, Manuela; Dumbović, Mateja;
Jian, Lan K.; Appel, Jan K.; Čalogović, Jaša; Ehresmann, Bent;
Heber, Bernd; Lohf, Henning; Posner, Arik; Vršnak, Bojan; Zeitlin,
Cary J.
Bibcode: 2018EGUGA..20.9306V
Altcode:
The propagation of 15 interplanetary coronal mass ejections (ICMEs) from
Earth's orbit (1 AU) to Mars (∼1.5 AU) has been studied with their
propagation speed estimated from both measurements and simulations. The
enhancement of magnetic fields related to ICMEs and their shock fronts
cause the so-called Forbush decrease, which can be detected as a
reduction of galactic cosmic ray (GCR) intensity measured on-ground
or on a spacecraft. This effect can be used to detect the passage of
ICMEs at various locations in the heliosphere, for example at Earth
(using neutron monitors), the STEREO A and B spacecraft (HET) as well
the on the surface of Mars using the Radiation Assessment Detector
(RAD) instrument on the Mars Science Laboratory (MSL) rover. A set of
ICME events has been selected during the periods when Earth (or STEREO
A or B) and Mars locations were nearly aligned on the same side of the
Sun in the ecliptic plane (so-called opposition phase). Such lineups
allow us to estimate the ICMEs' transit times between 1 and 1.5 AU by
determining the time delay between the corresponding Forbush decreases
measured at each location. We investigate the evolution of the ICME
propagation speeds before and after passing Earth's orbit and find
that their deceleration due to interaction with the ambient solar
wind may continue beyond 1 AU. We also find a substantial variance of
the speed evolution among different events revealing the dynamic and
diverse nature of eruptive solar events. Furthermore, the results are
compared to simulation data obtained from two CME propagation models,
namely the Drag-Based Model and ENLIL plus cone model.
Title: Forbush decrease model for expanding CMEs (ForbMod)
Authors: Dumbovic, Mateja; Temmer, Manuela; Guo, Jingnan; Heber,
Bernd; Möstl, Christian; Vrsnak, Bojan
Bibcode: 2018EGUGA..2015396D
Altcode:
The Project ForbMod aims to unravel how galactic cosmic rays are
influenced by solar storms in the inner solar system (Sun to Mars)
by developing a new model and utilizing a number of spacecraft and
planetary observation, including those by the Radiation Assessment
Detector aboard the Mars Rover Curiosity. The project focuses on
Forbush decreases (FDs) in the galactic cosmic ray flux, which can
be used as one of the "signatures" of an ICME passage. An analytical
diffusion-expansion FD model was developed that is based on the
widely used approach of an initially empty, closed magnetic structure
(i.e. flux rope) that fills up slowly with particles by perpendicular
diffusion. Remote CME observations and 3D reconstruction is used to
constrain initial and boundary conditions. CME evolutionary properties
are taken into account by incorporating the flux rope expansion. Several
options of flux rope expansion are regarded as competing mechanism
to diffusion, which can lead to different FD characteristics. This
project has received funding from the European Union's Horizon 2020
research and innovation programme under the Marie Skłodowska-Curie
grant agreement No 745782.
Title: Numerical Simulation of Coronal Waves Interacting with Coronal
Holes. II. Dependence on Alfvén Speed Inside the Coronal Hole
Authors: Piantschitsch, Isabell; Vršnak, Bojan; Hanslmeier, Arnold;
Lemmerer, Birgit; Veronig, Astrid; Hernandez-Perez, Aaron; Čalogović,
Jaša
Bibcode: 2018ApJ...857..130P
Altcode: 2018arXiv181112726P
We used our newly developed magnetohydrodynamic (MHD) code to perform
2.5D simulations of a fast-mode MHD wave interacting with coronal holes
(CHs) of varying Alfvén speed that result from assuming different
CH densities. We find that this interaction leads to effects like
reflection, transmission, stationary fronts at the CH boundary,
and the formation of a density depletion that moves in the opposite
direction to the incoming wave. We compare these effects with regard
to the different CH densities and present a comprehensive analysis of
morphology and kinematics of the associated secondary waves. We find
that the density value inside the CH influences the phase speed and
the amplitude values of density and magnetic field for all different
secondary waves. Moreover, we observe a correlation between the
CH density and the peak values of the stationary fronts at the CH
boundary. The findings of reflection and transmission on the one hand
and the formation of stationary fronts caused by the interaction of
MHD waves with CHs on the other hand strongly support the theory that
large-scale disturbances in the corona are fast-mode MHD waves.
Title: The Dependence of the Peak Velocity of High-Speed Solar Wind
Streams as Measured in the Ecliptic by ACE and the STEREO satellites
on the Area and Co-latitude of Their Solar Source Coronal Holes
Authors: Hofmeister, Stefan J.; Veronig, Astrid; Temmer, Manuela;
Vennerstrom, Susanne; Heber, Bernd; Vršnak, Bojan
Bibcode: 2018JGRA..123.1738H
Altcode: 2018arXiv180409579H
We study the properties of 115 coronal holes in the time range
from August 2010 to March 2017, the peak velocities of the
corresponding high-speed streams as measured in the ecliptic at 1
AU, and the corresponding changes of the Kp index as marker of their
geoeffectiveness. We find that the peak velocities of high-speed streams
depend strongly on both the areas and the co-latitudes of their solar
source coronal holes with regard to the heliospheric latitude of
the satellites. Therefore, the co-latitude of their source coronal
hole is an important parameter for the prediction of the high-speed
stream properties near the Earth. We derive the largest solar wind
peak velocities normalized to the coronal hole areas for coronal holes
located near the solar equator and that they linearly decrease with
increasing latitudes of the coronal holes. For coronal holes located
at latitudes ≳60°, they turn statistically to zero, indicating
that the associated high-speed streams have a high chance to miss the
Earth. Similarly, the Kp index per coronal hole area is highest for the
coronal holes located near the solar equator and strongly decreases
with increasing latitudes of the coronal holes. We interpret these
results as an effect of the three-dimensional propagation of high-speed
streams in the heliosphere; that is, high-speed streams arising from
coronal holes near the solar equator propagate in direction toward and
directly hit the Earth, whereas solar wind streams arising from coronal
holes at higher solar latitudes only graze or even miss the Earth.
Title: The Origin, Early Evolution and Predictability of Solar
Eruptions
Authors: Green, Lucie M.; Török, Tibor; Vršnak, Bojan; Manchester,
Ward; Veronig, Astrid
Bibcode: 2018SSRv..214...46G
Altcode: 2018arXiv180104608G
Coronal mass ejections (CMEs) were discovered in the early 1970s
when space-borne coronagraphs revealed that eruptions of plasma
are ejected from the Sun. Today, it is known that the Sun produces
eruptive flares, filament eruptions, coronal mass ejections and failed
eruptions; all thought to be due to a release of energy stored in
the coronal magnetic field during its drastic reconfiguration. This
review discusses the observations and physical mechanisms behind this
eruptive activity, with a view to making an assessment of the current
capability of forecasting these events for space weather risk and impact
mitigation. Whilst a wealth of observations exist, and detailed models
have been developed, there still exists a need to draw these approaches
together. In particular more realistic models are encouraged in order
to asses the full range of complexity of the solar atmosphere and the
criteria for which an eruption is formed. From the observational side,
a more detailed understanding of the role of photospheric flows and
reconnection is needed in order to identify the evolutionary path that
ultimately means a magnetic structure will erupt.
Title: The Drag-based Ensemble Model (DBEM) for Coronal Mass Ejection
Propagation
Authors: Dumbović, Mateja; Čalogović, Jaša; Vršnak, Bojan; Temmer,
Manuela; Mays, M. Leila; Veronig, Astrid; Piantschitsch, Isabell
Bibcode: 2018ApJ...854..180D
Altcode: 2018arXiv180107473D
The drag-based model for heliospheric propagation of coronal mass
ejections (CMEs) is a widely used analytical model that can predict
CME arrival time and speed at a given heliospheric location. It is
based on the assumption that the propagation of CMEs in interplanetary
space is solely under the influence of magnetohydrodynamical drag,
where CME propagation is determined based on CME initial properties
as well as the properties of the ambient solar wind. We present
an upgraded version, the drag-based ensemble model (DBEM), that
covers ensemble modeling to produce a distribution of possible ICME
arrival times and speeds. Multiple runs using uncertainty ranges for
the input values can be performed in almost real-time, within a few
minutes. This allows us to define the most likely ICME arrival times
and speeds, quantify prediction uncertainties, and determine forecast
confidence. The performance of the DBEM is evaluated and compared
to that of ensemble WSA-ENLIL+Cone model (ENLIL) using the same
sample of events. It is found that the mean error is ME = -9.7 hr,
mean absolute error MAE = 14.3 hr, and root mean square error RMSE =
16.7 hr, which is somewhat higher than, but comparable to ENLIL errors
(ME = -6.1 hr, MAE = 12.8 hr and RMSE = 14.4 hr). Overall, DBEM and
ENLIL show a similar performance. Furthermore, we find that in both
models fast CMEs are predicted to arrive earlier than observed, most
likely owing to the physical limitations of models, but possibly also
related to an overestimation of the CME initial speed for fast CMEs.
Title: Properties and relationship between solar eruptive flares and
Coronal Mass Ejections during rising phase of Solar Cycles 23 and 24
Authors: Syed Ibrahim, M.; Shanmugaraju, A.; Moon, Y. -J.; Vrsnak,
B.; Umapathy, S.
Bibcode: 2018AdSpR..61..540S
Altcode:
Statistical relationship between major flares and the associated CMEs
during rising phases of Solar Cycles 23 and 24 are studied. Totally
more than 6000 and 10,000 CMEs were observed by SOHO/LASCO (Solar
and Heliospheric Observatory/Large Angle Spectrometric Coronagraph)
during 23rd [May 1996-June 2002] and 24th [December 2008-December 2014]
solar cycles, respectively. In particular, we studied the relationship
between properties of flares and CMEs using the limb events (longitude
70-85°) to avoid projection effects of CMEs and partial occultation of
flares that occurred near 90°. After selecting a sample of limb flares,
we used certain spatial and temporal constraints to find the flare-CME
pairs. Using these constraints, we compiled 129 events in Solar Cycle 23
and 92 events in Solar Cycle 24. We compared the flare-CME relationship
in the two solar cycles and no significant differences are found between
the two cycles. We only found out that the CME mean width was slightly
larger and the CME mean acceleration was slightly higher in cycle 24,
and that there was somewhat a better relation between flare flux and
CME deceleration in cycle 24 than in cycle 23.
Title: Using Forbush Decreases to Derive the Transit Time of ICMEs
Propagating from 1 AU to Mars
Authors: Freiherr von Forstner, Johan L.; Guo, Jingnan;
Wimmer-Schweingruber, Robert F.; Hassler, Donald M.; Temmer, Manuela;
Dumbović, Mateja; Jian, Lan K.; Appel, Jan K.; Čalogović, Jaša.;
Ehresmann, Bent; Heber, Bernd; Lohf, Henning; Posner, Arik; Steigies,
Christian T.; Vršnak, Bojan; Zeitlin, Cary J.
Bibcode: 2018JGRA..123...39F
Altcode: 2017arXiv171207301V
The propagation of 15 interplanetary coronal mass ejections (ICMEs) from
Earth's orbit (1 AU) to Mars (∼1.5 AU) has been studied with their
propagation speed estimated from both measurements and simulations. The
enhancement of magnetic fields related to ICMEs and their shock fronts
causes the so-called Forbush decrease, which can be detected as a
reduction of galactic cosmic rays measured on ground. We have used
galactic cosmic ray (GCR) data from in situ measurements at Earth, from
both STEREO A and STEREO B as well as GCR measurements by the Radiation
Assessment Detector (RAD) instrument on board Mars Science Laboratory
on the surface of Mars. A set of ICME events has been selected during
the periods when Earth (or STEREO A or STEREO B) and Mars locations
were nearly aligned on the same side of the Sun in the ecliptic plane
(so-called opposition phase). Such lineups allow us to estimate the
ICMEs' transit times between 1 and 1.5 AU by estimating the delay time
of the corresponding Forbush decreases measured at each location. We
investigate the evolution of their propagation speeds before and after
passing Earth's orbit and find that the deceleration of ICMEs due to
their interaction with the ambient solar wind may continue beyond 1
AU. We also find a substantial variance of the speed evolution among
different events revealing the dynamic and diverse nature of eruptive
solar events. Furthermore, the results are compared to simulation data
obtained from two CME propagation models, namely the Drag-Based Model
and ENLIL plus cone model.
Title: CME volume calculation from 3D GCS reconstruction
Authors: Holzknecht, L.; Temmer, M.; Dumbović, M.; Wellenzohn, S.;
Krikova, K.; Heinemann, S. G.; Rodari, M.; Vršnak, B.; Veronig, A. M.
Bibcode: 2018CEAB...42....3H
Altcode: 2019arXiv190411418H
The mass evolution of a coronal mass ejection (CME) is an important
parameter characterizing the drag force acting on a CME as it propagates
through interplanetary space. Spacecraft measure in-situ plasma
densities of CMEs during crossing events, but for investigating the
mass evolution, we also need to know the CME geometry, more specific,
its volume. Having derived the CME volume and mass from remote sensing
data using 3D reconstructed CME geometry, we can calculate the CME
density and compare it with in-situ proton density measurements near
Earth. From that we may draw important conclusions on a possible
mass increase as the CME interacts with the ambient solar wind in the
heliosphere. In this paper we will describe in detail the method for
deriving the CME volume using the graduated cylindrical shell (GCS)
model tep[][see \ref{fig:GCSModel}]{thernisien06,thernisien09}. We show
that, assuming self-similar expansion, one can derive the volume of the
CME from two GCS parameters and that it furthermore can be expressed
as a function of distance.
Title: Multi-spacecraft observations of ICMEs propagating beyond
Earth orbit during MSL/RAD flight and surface phases
Authors: von Forstner, J.; Guo, J.; Wimmer-Schweingruber, R. F.;
Hassler, D.; Temmer, M.; Vrsnak, B.; Čalogović, J.; Dumbovic, M.;
Lohf, H.; Appel, J. K.; Heber, B.; Steigies, C. T.; Zeitlin, C.;
Ehresmann, B.; Jian, L. K.; Boehm, E.; Boettcher, S. I.; Burmeister,
S.; Martin-Garcia, C.; Brinza, D. E.; Posner, A.; Reitz, G.; Matthiae,
D.; Rafkin, S. C.; weigle, G., II; Cucinotta, F.
Bibcode: 2017AGUFMSH53A2543V
Altcode:
The propagation of interplanetary coronal mass ejections (ICMEs)
between Earth's orbit (1 AU) and Mars ( 1.5 AU) has been studied
with their propagation speed estimated from both measurements and
simulations. The enhancement of the magnetic fields related to ICMEs
and their shock fronts cause so-called Forbush decreases, which can
be detected as a reduction of galactic cosmic rays measured on-ground
or on a spacecraft. We have used galactic cosmic ray (GCR) data from
in-situ measurements at Earth, from both STEREO A and B as well
as the GCR measurement by the Radiation Assessment Detector (RAD)
instrument onboard Mars Science Laboratory (MSL) on the surface of
Mars as well as during its flight to Mars in 2011-2012. A set of ICME
events has been selected during the periods when Earth (or STEREO A
or B) and MSL locations were nearly aligned on the same side of the
Sun in the ecliptic plane (so-called opposition phase). Such lineups
allow us to estimate the ICMEs' transit times between 1 AU and the
MSL location by estimating the delay time of the corresponding Forbush
decreases measured at each location. We investigate the evolution of
their propagation speeds after passing Earth's orbit and find that the
deceleration of ICMEs due to their interaction with the ambient solar
wind continues beyond 1 AU. The results are compared to simulation data
obtained from two CME propagation models, namely the Drag-Based Model
(DBM) and the WSA-ENLIL plus cone model.
Title: A Numerical Simulation of Coronal Waves Interacting with
Coronal Holes. I. Basic Features
Authors: Piantschitsch, Isabell; Vršnak, Bojan; Hanslmeier, Arnold;
Lemmerer, Birgit; Veronig, Astrid; Hernandez-Perez, Aaron; Čalogović,
Jaša; Žic, Tomislav
Bibcode: 2017ApJ...850...88P
Altcode: 2018arXiv181112073P
We have developed a new numerical code that is able to perform 2.5D
simulations of a magnetohydrodynamic (MHD) wave propagation in the
corona, and its interaction with a low-density region, such as a
coronal hole (CH). We show that the impact of the wave on the CH
leads to different effects, such as reflection and transmission of the
incoming wave, stationary features at the CH boundary, or formation
of a density depletion. We present a comprehensive analysis of the
morphology and kinematics of primary and secondary waves, I.e.,
we describe in detail the temporal evolution of density, magnetic
field, plasma flow velocity, phase speed, and position of the wave
amplitude. Effects like reflection, refraction, and transmission of
the wave strongly support the theory that large-scale disturbances
in the corona are fast MHD waves and distinguish that theory from the
competing pseudo-wave theory. The formation of stationary bright fronts
was one of the main reasons for the development of pseudo-waves. Here,
we show that stationary bright fronts can be produced by interactions
of an MHD wave with a CH. We find secondary waves that are traversing
through the CH and we show that one part of these traversing waves
leaves the CH again, while another part is being reflected at the CH
boundary inside the CH. We observe a density depletion that is moving
in the opposite direction of the primary wave propagation. We show
that the primary wave pushes the CH boundary to the right, caused by
the wave front exerting dynamic pressure on the CH.
Title: The Physical Processes of CME/ICME Evolution
Authors: Manchester, Ward; Kilpua, Emilia K. J.; Liu, Ying D.; Lugaz,
Noé; Riley, Pete; Török, Tibor; Vršnak, Bojan
Bibcode: 2017SSRv..212.1159M
Altcode: 2017SSRv..tmp...90M
As observed in Thomson-scattered white light, coronal mass ejections
(CMEs) are manifest as large-scale expulsions of plasma magnetically
driven from the corona in the most energetic eruptions from the
Sun. It remains a tantalizing mystery as to how these erupting magnetic
fields evolve to form the complex structures we observe in the solar
wind at Earth. Here, we strive to provide a fresh perspective on the
post-eruption and interplanetary evolution of CMEs, focusing on the
physical processes that define the many complex interactions of the
ejected plasma with its surroundings as it departs the corona and
propagates through the heliosphere. We summarize the ways CMEs and
their interplanetary CMEs (ICMEs) are rotated, reconfigured, deformed,
deflected, decelerated and disguised during their journey through the
solar wind. This study then leads to consideration of how structures
originating in coronal eruptions can be connected to their far removed
interplanetary counterparts. Given that ICMEs are the drivers of most
geomagnetic storms (and the sole driver of extreme storms), this work
provides a guide to the processes that must be considered in making
space weather forecasts from remote observations of the corona.
Title: Geomagnetic Effects of Corotating Interaction Regions
Authors: Vršnak, Bojan; Dumbović, Mateja; Čalogović, Jaša;
Verbanac, Giuliana; Poljančić Beljan, Ivana
Bibcode: 2017SoPh..292..140V
Altcode:
We present an analysis of the geoeffectiveness of corotating interaction
regions (CIRs), employing the data recorded from 25 January to 5 May
2005 and throughout 2008. These two intervals in the declining phase
of Solar Cycle 23 are characterised by a particularly low number of
interplanetary coronal mass ejections (ICMEs). We study in detail how
four geomagnetic-activity parameters (the Dst, Ap, and AE indices,
as well as the Dst time derivative, dDst /d t ) are related to three
CIR-related solar wind parameters (flow speed, V , magnetic field, B ,
and the convective electric field based on the southward Geocentric
solar magnetospheric (GSM) magnetic field component, VBs)
on a three-hour time resolution. In addition, we quantify statistical
relationships between the mentioned geomagnetic indices. It is found
that Dst is correlated best to V , with a correlation coefficient of cc
≈0.6 , whereas there is no correlation between dDst /d t and V . The
Ap and AE indices attain peaks about half a day before the maximum of V
, with correlation coefficients ranging from cc ≈0.6 to cc ≈0.7 ,
depending on the sample used. The best correlations of Ap and AE are
found with VBs with a delay of 3 h, being characterised
by cc ≳0.6 . The Dst derivative dDst /d t is also correlated with
VBs, but the correlation is significantly weaker cc
≈0.4 - 0.5, with a delay of 0 - 3 h, depending on the employed
sample. Such low values of correlation coefficients indicate that
there are other significant effects that influence the relationship
between the considered parameters. The correlation of all studied
geomagnetic parameters with B are characterised by considerably lower
correlation coefficients, ranging from cc =0.3 in the case of dDst /d
t up to cc =0.56 in the case of Ap. It is also shown that peak values
of geomagnetic indices depend on the duration of the CIR-related
structures. The Dst is closely correlated with Ap and AE (cc =0.7 ),
Dst being delayed for about 3 h. On the other hand, dDst /d t peaks
simultaneously with Ap and AE, with correlation coefficients of 0.48
and 0.56, respectively. The highest correlation (cc =0.81 ) is found
for the relationship between Ap and AE.
Title: Validation of the CME Geomagnetic Forecast Alerts Under the
COMESEP Alert System
Authors: Dumbović, Mateja; Srivastava, Nandita; Rao, Yamini K.;
Vršnak, Bojan; Devos, Andy; Rodriguez, Luciano
Bibcode: 2017SoPh..292...96D
Altcode:
Under the European Union 7th Framework Programme (EU FP7) project
Coronal Mass Ejections and Solar Energetic Particles (COMESEP,
http://comesep.aeronomy.be), an automated space weather alert system has
been developed to forecast solar energetic particles (SEP) and coronal
mass ejection (CME) risk levels at Earth. The COMESEP alert system
uses the automated detection tool called Computer Aided CME Tracking
(CACTus) to detect potentially threatening CMEs, a drag-based model
(DBM) to predict their arrival, and a CME geoeffectiveness tool (CGFT)
to predict their geomagnetic impact. Whenever CACTus detects a halo or
partial halo CME and issues an alert, the DBM calculates its arrival
time at Earth and the CGFT calculates its geomagnetic risk level. The
geomagnetic risk level is calculated based on an estimation of the
CME arrival probability and its likely geoeffectiveness, as well as an
estimate of the geomagnetic storm duration. We present the evaluation
of the CME risk level forecast with the COMESEP alert system based on a
study of geoeffective CMEs observed during 2014. The validation of the
forecast tool is made by comparing the forecasts with observations. In
addition, we test the success rate of the automatic forecasts (without
human intervention) against the forecasts with human intervention using
advanced versions of the DBM and CGFT (independent tools available at
the Hvar Observatory website, http://oh.geof.unizg.hr). The results
indicate that the success rate of the forecast in its current form is
unacceptably low for a realistic operation system. Human intervention
improves the forecast, but the false-alarm rate remains unacceptably
high. We discuss these results and their implications for possible
improvement of the COMESEP alert system.
Title: Investigation on M-class Flare-Associated Coronal Mass
Ejections with and Without DH Type II Radio Bursts
Authors: Selvarani, G.; Shanmugaraju, A.; Vrsnak, Bojan; Lawrance,
M. Bendict
Bibcode: 2017SoPh..292...74S
Altcode:
We perform a statistical analysis on 157 M-class soft X-ray flares
observed during 1997 - 2014 with and without deca-hectometric (DH)
type II radio bursts aiming at the reasons for the non-occurrence
of DH type II bursts in certain events. All the selected events are
associated with halo Coronal Mass Ejections (CMEs) detected by the
Solar and Heliospheric Observatory (SOHO) / Large Angle Spectrometric
and COronograph (LASCO). Out of 157 events, 96 (61%; "Group I")
events are associated with a DH type II burst observed by the Radio
and Plasma Wave (WAVES) experiment onboard the Wind spacecraft and 61
(39%; "Group II") events occur without a DH type II burst. The mean
CME speed of Group I is 1022 km/s and that of Group II is 647 km/s. It
is also found that the properties of the selected M-class flares such
as flare intensity, rise time, duration and decay time are greater
for the DH associated flares than the non-DH flares. Group I has a
slightly larger number (56%) of western events than eastern events
(44%), whereas Group II has a larger number of eastern events (62%)
than western events (38%). We also compare this analysis with the
previous study by Lawrance, Shanmugaraju, and Vršnak (Solar Phys.290,
3365L, 2015) concerning X-class flares and confirm that high-intensity
flares (X-class and M-class) have the same trend in the CME and flare
properties. Additionally we consider aspects like acceleration and the
possibility of CME-streamer interaction. The average deceleration of
CMEs with DH type II bursts is weaker (a =−4.39 m/s2) than
that of CMEs without a type II burst (a =−12.21 m/s2). We
analyze the CME-streamer interactions for Group I events using the
model proposed by Mancuso and Raymond (Astron. Astrophys.413, 363,
2004) and find that the interaction regions are the most probable
source regions for DH type II radio bursts.
Title: Validation of the CME Geomagnetic forecast alerts under
COMESEP alert system
Authors: Dumbovic, Mateja; Srivastava, Nandita; Khodia, Yamini;
Vršnak, Bojan; Devos, Andy; Rodriguez, Luciano
Bibcode: 2017EGUGA..1914917D
Altcode:
An automated space weather alert system has been developed under the
EU FP7 project COMESEP (COronal Mass Ejections and Solar Energetic
Particles: http://comesep.aeronomy.be) to forecast solar energetic
particles (SEP) and coronal mass ejection (CME) risk levels at
Earth. COMESEP alert system uses automated detection tool CACTus
to detect potentially threatening CMEs, drag-based model (DBM)
to predict their arrival and CME geo-effectiveness tool (CGFT) to
predict their geomagnetic impact. Whenever CACTus detects a halo or
partial halo CME and issues an alert, DBM calculates its arrival time
at Earth and CGFT calculates its geomagnetic risk level. Geomagnetic
risk level is calculated based on an estimation of the CME arrival
probability and its likely geo-effectiveness, as well as an estimate
of the geomagnetic-storm duration. We present the evaluation of the
CME risk level forecast with COMESEP alert system based on a study
of geo-effective CMEs observed during 2014. The validation of the
forecast tool is done by comparing the forecasts with observations. In
addition, we test the success rate of the automatic forecasts (without
human intervention) against the forecasts with human intervention using
advanced versions of DBM and CGFT (self standing tools available at Hvar
Observatory website: http://oh.geof.unizg.hr). The results implicate
that the success rate of the forecast is higher with human intervention
and using more advanced tools. This work has received funding from the
European Commission FP7 Project COMESEP (263252). We acknowledge the
support of Croatian Science Foundation under the project 6212 „Solar
and Stellar Variability".
Title: Characteristics of Low-latitude Coronal Holes near the Maximum
of Solar Cycle 24
Authors: Hofmeister, Stefan J.; Veronig, Astrid; Reiss, Martin A.;
Temmer, Manuela; Vennerstrom, Susanne; Vršnak, Bojan; Heber, Bernd
Bibcode: 2017ApJ...835..268H
Altcode: 2017arXiv170202050H
We investigate the statistics of 288 low-latitude coronal holes
extracted from SDO/AIA-193 filtergrams over the time range of
2011 January 01-2013 December 31. We analyze the distribution of
characteristic coronal hole properties, such as the areas, mean AIA-193
intensities, and mean magnetic field densities, the local distribution
of the SDO/AIA-193 intensity and the magnetic field within the coronal
holes, and the distribution of magnetic flux tubes in coronal holes. We
find that the mean magnetic field density of all coronal holes under
study is 3.0 ± 1.6 G, and the percentaged unbalanced magnetic flux
is 49 ± 16%. The mean magnetic field density, the mean unsigned
magnetic field density, and the percentaged unbalanced magnetic flux of
coronal holes depend strongly pairwise on each other, with correlation
coefficients cc > 0.92. Furthermore, we find that the unbalanced
magnetic flux of the coronal holes is predominantly concentrated in
magnetic flux tubes: 38% (81%) of the unbalanced magnetic flux of
coronal holes arises from only 1% (10%) of the coronal hole area,
clustered in magnetic flux tubes with field strengths >50 G (10
G). The average magnetic field density and the unbalanced magnetic
flux derived from the magnetic flux tubes correlate with the mean
magnetic field density and the unbalanced magnetic flux of the overall
coronal hole (cc > 0.93). These findings give evidence that the
overall magnetic characteristics of coronal holes are governed by the
characteristics of the magnetic flux tubes.
Title: Understanding the Physical Nature of Coronal "EIT Waves"
Authors: Long, D. M.; Bloomfield, D. S.; Chen, P. F.; Downs, C.;
Gallagher, P. T.; Kwon, R. -Y.; Vanninathan, K.; Veronig, A. M.;
Vourlidas, A.; Vršnak, B.; Warmuth, A.; Žic, T.
Bibcode: 2017SoPh..292....7L
Altcode: 2016arXiv161105505L
For almost 20 years the physical nature of globally propagating waves in
the solar corona (commonly called "EIT waves") has been controversial
and subject to debate. Additional theories have been proposed over the
years to explain observations that did not agree with the originally
proposed fast-mode wave interpretation. However, the incompatibility
of observations made using the Extreme-ultraviolet Imaging Telescope
(EIT) onboard the Solar and Heliospheric Observatory with the fast-mode
wave interpretation was challenged by differing viewpoints from the twin
Solar Terrestrial Relations Observatory spacecraft and data with higher
spatial and temporal resolution from the Solar Dynamics Observatory. In
this article, we reexamine the theories proposed to explain EIT waves
to identify measurable properties and behaviours that can be compared
to current and future observations. Most of us conclude that the
so-called EIT waves are best described as fast-mode large-amplitude
waves or shocks that are initially driven by the impulsive expansion
of an erupting coronal mass ejection in the low corona.
Title: Solar eruptions: The CME-flare relationship
Authors: Vršnak, B.
Bibcode: 2016AN....337.1002V
Altcode:
Coronal mass ejections (CMEs), caused by large-scale eruptions of the
coronal magnetic field, often are accompanied by a more localized
energy release in the form of flares, as a result of dissipative
magnetic-field reconfiguration. Morphology and evolution of such
flares, also denoted as dynamical flares are often explained as a
consequence of reconnection of the arcade magnetic field, taking place
below the erupting magnetic flux rope. A close relationship of the
CME acceleration and the flare energy release is evidenced by various
statistical correlations between parameters describing CMEs and flares,
as well as by the synchronization of the CME acceleration phase with
the impulsive phase of the associated flare. Such behavior implies
that there must be a feedback relation between the dynamics of the CME
and the flare-associated reconnection process. From the theoretical
standpoint, magnetic reconnection affects the CME dynamics in several
ways. First, it reduces the tension of the overlying arcade magnetic
field and increases the magnetic pressure below the flux rope, and
in this way enhances the CME acceleration. Furthermore, it supplies
the poloidal magnetic flux to the flux rope, which helps sustaining
the electric current in the rope and prolonging the action of the
driving Lorentz force to large distances. The role of these processes,
directly relating solar flares and CMEs, is illustrated by employing a
simple model, where the erupting structure is represented by a curved
flux rope anchored at both sides in the dense/inert photosphere,
being subject to the kink and torus instability. It is shown that
in most strongly accelerated ejections, where values on the order of
10 km s-2 are attained, the poloidal flux supplied to the
erupting rope has to be several times larger than was the initial flux.
Title: Understanding the Physical Nature of Coronal "EIT Waves"
Authors: Long, D. M.; Bloomfield, D. S.; Chen, P. -F.; Downs,
C.; Gallagher, P. T.; Kwon, R. -Y.; Vanninathan, K.; Veronig, A.;
Vourlidas, A.; Vrsnak, B.; Warmuth, A.; Zic, T.
Bibcode: 2016usc..confE..24L
Altcode:
For almost 20 years the physical nature of globally-propagating waves
in the solar corona (commonly called "EIT waves") has been controversial
and subject to debate. Additional theories have been proposed throughout
the years to explain observations that did not fit with the originally
proposed fast-mode wave interpretation. However, the incompatibility
of observations made using the Extreme-ultraviolet Imaging Telescope
(EIT) on the Solar and Heliospheric Observatory with the fast-mode
wave interpretation have been challenged by differing viewpoints
from the Solar Terrestrial Relations Observatory spacecraft and higher
spatial/temporal resolution data from the Solar Dynamics Observatory. In
this paper, we reexamine the theories proposed to explain "EIT waves"
to identify measurable properties and behaviours that can be compared
to current and future observations. Most of us conclude that "EIT
waves" are best described as fast-mode large-amplitude waves/shocks,
which are initially driven by the impulsive expansion of an erupting
coronal mass ejection in the low corona.
Title: On the propagation of a geoeffective coronal mass ejection
during 15-17 March 2015
Authors: Wang, Yuming; Zhang, Quanhao; Liu, Jiajia; Shen, Chenglong;
Shen, Fang; Yang, Zicai; Zic, T.; Vrsnak, B.; Webb, D. F.; Liu, Rui;
Wang, S.; Zhang, Jie; Hu, Qiang; Zhuang, Bin
Bibcode: 2016JGRA..121.7423W
Altcode: 2016arXiv160707750W
The largest geomagnetic storm so far, called 2015 St. Patrick's Day
event, in the solar cycle 24 was produced by a fast coronal mass
ejection (CME) originating on 15 March 2015. It was an initially
west-oriented CME and expected to only cause a weak geomagnetic
disturbance. Why did this CME finally cause such a large geomagnetic
storm? We try to find some clues by investigating its propagation from
the Sun to 1 AU. First, we reconstruct the CME's kinematic properties
in the corona from the SOHO and Solar Dynamics Observatory imaging
data with the aid of the graduated cylindrical shell model. It is
suggested that the CME propagated to the west ∼33°±10° away from
the Sun-Earth line with a speed of about 817 km s-1 before
leaving the field of view of the SOHO/Large Angle and Spectrometric
Coronagraph (LASCO) C3 camera. A magnetic cloud (MC) corresponding
to this CME was measured in situ by the Wind spacecraft 2 days after
the CME left LASCO's field of view. By applying two MC reconstruction
methods, we infer the configuration of the MC as well as some kinematic
information, which implies that the CME possibly experienced an
eastward deflection on its way to 1 AU. However, due to the lack of
observations from the STEREO spacecraft, the CME's kinematic evolution
in interplanetary space is not clear. In order to fill this gap, we
utilize numerical MHD simulation, drag-based CME propagation model
(DBM) and the model for CME deflection in interplanetary space (DIPS)
to recover the propagation process, especially the trajectory, of
the CME from 30RS to 1 AU under the constraints of the
derived CME's kinematics near the Sun and at 1 AU. It is suggested
that the trajectory of the CME was deflected toward the Earth by
about 12°, consistent with the implication from the MC reconstruction
at 1 AU. This eastward deflection probably contributed to the CME's
unexpected geoeffectiveness by pushing the center of the initially
west-oriented CME closer to the Earth.
Title: VizieR Online Data Catalog: Predicting coronal mass ejections
transit times (Sudar+, 2016)
Authors: Sudar, D.; Vrsnak, B.; Dumbovic, M.
Bibcode: 2016yCat..74561542S
Altcode:
We compiled a list of 153 CMEs for which their ICME counterparts were
detected and their TT to Earth was measured. We used only the events
for which the CME source position could be determined. All CME-ICME
pairings were taken from the catalogue provided by Richardson &
Cane (2010, Sol. Phys., 264, 189). (1 data file).
Title: Detailed Analysis of Solar Data Related to Historical Extreme
Geomagnetic Storms: 1868 - 2010
Authors: Lefèvre, Laure; Vennerstrøm, Susanne; Dumbović, Mateja;
Vršnak, Bojan; Sudar, Davor; Arlt, Rainer; Clette, Frédéric;
Crosby, Norma
Bibcode: 2016SoPh..291.1483L
Altcode: 2016SoPh..tmp...64L
An analysis of historical Sun-Earth connection events in the context
of the most extreme space weather events of the last ∼150 years
is presented. To identify the key factors leading to these extreme
events, a sample of the most important geomagnetic storms was selected
based mainly on the well-known aa index and on geomagnetic parameters
described in the accompanying paper (Vennerstrøm et al., Solar Phys. in
this issue, 2016, hereafter Paper I). This part of the analysis focuses
on associating and characterizing the active regions (sunspot groups)
that are most likely linked to these major geomagnetic storms.
Title: Extreme Geomagnetic Storms - 1868 - 2010
Authors: Vennerstrom, S.; Lefevre, L.; Dumbović, M.; Crosby, N.;
Malandraki, O.; Patsou, I.; Clette, F.; Veronig, A.; Vršnak, B.;
Leer, K.; Moretto, T.
Bibcode: 2016SoPh..291.1447V
Altcode: 2016SoPh..tmp...73V
We present the first large statistical study of extreme geomagnetic
storms based on historical data from the time period 1868 - 2010. This
article is the first of two companion papers. Here we describe how the
storms were selected and focus on their near-Earth characteristics. The
second article presents our investigation of the corresponding solar
events and their characteristics. The storms were selected based
on their intensity in the aa index, which constitutes the longest
existing continuous series of geomagnetic activity. They are analyzed
statistically in the context of more well-known geomagnetic indices,
such as the Kp and Dcx/Dst index. This reveals that neither Kp nor
Dcx/Dst provide a comprehensive geomagnetic measure of the extreme
storms. We rank the storms by including long series of single magnetic
observatory data. The top storms on the rank list are the New York
Railroad storm occurring in May 1921 and the Quebec storm from March
1989. We identify key characteristics of the storms by combining
several different available data sources, lists of storm sudden
commencements (SSCs) signifying occurrence of interplanetary shocks,
solar wind in-situ measurements, neutron monitor data, and associated
identifications of Forbush decreases as well as satellite measurements
of energetic proton fluxes in the near-Earth space environment. From
this we find, among other results, that the extreme storms are very
strongly correlated with the occurrence of interplanetary shocks (91 -
100 %), Forbush decreases (100 %), and energetic solar proton events
(70 %). A quantitative comparison of these associations relative to less
intense storms is also presented. Most notably, we find that most often
the extreme storms are characterized by a complexity that is associated
with multiple, often interacting, solar wind disturbances and that they
frequently occur when the geomagnetic activity is already elevated. We
also investigate the semiannual variation in storm occurrence and
confirm previous findings that geomagnetic storms tend to occur less
frequently near solstices and that this tendency increases with storm
intensity. However, we find that the semiannual variation depends on
both the solar wind source and the storm level. Storms associated
with weak SSC do not show any semiannual variation, in contrast to
weak storms without SSC.
Title: Forbush Decrease Prediction Based on Remote Solar Observations
Authors: Dumbovic, Mateja; Vrsnak, Bojan; Calogovic, Jasa
Bibcode: 2016EGUGA..18.6369D
Altcode:
We study the relation between remote observations of coronal mass
ejections (CMEs), their associated solar flares and short-term
depressions in the galactic cosmic-ray flux (so called Forbush
decreases). Statistical relations between Forbush decrease magnitude
and several CME/flare parameters are examined. In general we find
that Forbush decrease magnitude is larger for faster CMEs with larger
apparent width, which is associated with stronger flares that originate
close to the center of the solar disk and are (possibly) involved in
a CME-CME interaction. The statistical relations are quantified and
employed to forecast expected Forbush decrease magnitude range based
on the selected remote solar observations of the CME and associated
solar flare. Several verification measures are used to evaluate
the forecast method. We find that the forecast is most reliable
in predicting whether or not a CME will produce a Forbush decrease
with a magnitude >3 %. The main advantage of the method is that
it provides an early prediction, 1-4 days in advance. Based on the
presented research, an online forecast tool was developed (Forbush
Decrease Forecast Tool, FDFT) available at Hvar Observatory web page:
http://oh.geof.unizg.hr/FDFT/fdft.php. We acknowledge the support of
Croatian Science Foundation under the project 6212 "Solar and Stellar
Variability" and of European social fond under the project "PoKRet".
Title: Forecasting the Arrival of Coronal Mass Ejections: The
Drag-Based Model
Authors: Vršnak, B.; Temmer, M.; Zic, T.; Dumbović, M.; Čalogović,
J.
Bibcode: 2016ASPC..504..209V
Altcode:
Arrival-time predictions based on the numerical "WSA-ENLIL+Cone model"
and the analytical "Drag-based model" (DBM) are analyzed, employing
a sample of 50 well observed CMEs. The best match between the two
models is obtained if the background solar-wind speed of w = 400 km
s-1 is applied in DBM. It is also demonstrated that both
models show similar prediction accuracy.
Title: Predicting coronal mass ejections transit times to Earth with
neural network
Authors: Sudar, D.; Vršnak, B.; Dumbović, M.
Bibcode: 2016MNRAS.456.1542S
Altcode: 2015arXiv151107620S
Predicting transit times (TT) of coronal mass ejections (CMEs) from
their initial parameters is a very important subject, not only from the
scientific perspective, but also because CMEs represent a hazard for
human technology. We used a neural network (NN) to analyse TT for 153
events with only two input parameters: initial velocity of the CME, v,
and central meridian distance, CMD, of its associated flare. We found
that transit time dependence on v is showing a typical drag-like pattern
in the solar wind. The results show that the speed at which acceleration
by drag changes to deceleration is v ≈ 500 km s-1. TT are
also found to be shorter for CMEs associated with flares on the western
hemisphere than those originating on the eastern side of the Sun. We
attribute this difference to the eastward deflection of CMEs on their
path to 1 AU. The average error of the NN prediction in comparison to
observations is ≈12 h which is comparable to other studies on the
same subject.
Title: Forbush Decrease Prediction Based on Remote Solar Observations
Authors: Dumbović, M.; Vršnak, B.; Čalogović, J.
Bibcode: 2016SoPh..291..285D
Altcode: 2015SoPh..tmp..169D; 2015arXiv151003282D
We employ remote observations of coronal mass ejections (CMEs)
and the associated solar flares to forecast the CME-related Forbush
decreases, i.e. short-term depressions in the galactic cosmic-ray
flux. The relation between the Forbush effect at Earth and remote
observations of CMEs and associated solar flares is studied via a
statistical analysis. Relations between Forbush decrease magnitude and
several CME/flare parameters were found: the initial CME speed, apparent
width, source position, associated solar-flare class, and the effect of
successive-CME occurrence. Based on the statistical analysis, remote
solar observations are employed to forecast a Forbush-decrease. For
this purpose, an empirical probabilistic model is constructed that
uses selected remote solar observations of the CME and associated solar
flare as input and gives the expected Forbush-decrease magnitude range
as output. The forecast method is evaluated using several verification
measures, indicating that as the forecast tends to be more specific, it
is less reliable, which is its main drawback. However, the advantages
of the method are that it provides an early prediction and that the
input does not necessarily depend on using a spacecraft.
Title: Formation of Coronal Large-Amplitude Waves and the
Chromospheric Response
Authors: Vršnak, B.; Žic, T.; Lulić, S.; Temmer, M.; Veronig, A. M.
Bibcode: 2016SoPh..291...89V
Altcode: 2015SoPh..tmp..175V
An in-depth analysis of numerical simulations is performed to obtain
a deeper insight into the nature of various phenomena occurring in the
solar atmosphere as a consequence of the eruption of unstable coronal
structures. Although the simulations take into account only the most
basic characteristics of a flux-rope eruption, the simulation analysis
reveals important information on various eruption-related effects. It
quantifies the relation between the eruption dynamics and the evolution
of the large-amplitude coronal magnetohydrodynamic wave and the
associated chromospheric downward-propagating perturbation. We show that
the downward propagation of the chromospheric Moreton-wave disturbance
can be approximated by a constant-amplitude switch-on shock that moves
through a medium of rapidly decreasing Alfvén velocity. The presented
analysis reveals the nature of secondary effects that are observed
as coronal upflows, secondary shocks, various forms of wave-trains,
delayed large-amplitude slow disturbances, transient coronal depletions,
etc. We also show that the eruption can cause an observable Moreton
wave and a secondary coronal front only if it is powerful enough and
is preferably characterized by significant lateral expansion. In weaker
eruptions, only the coronal and transition-region signatures of primary
waves are expected to be observed. In powerful events, the primary
wave moves at an Alfvén Mach number significantly larger than 1 and
steepens into a shock that is due to the nonlinear evolution of the
wavefront. After the eruption-driven phase, the perturbation evolves
as a freely propagating simple wave, characterized by a significant
deceleration, amplitude decrease, and wave-profile broadening. In weak
events the coronal wave does not develop into a shock and propagates
at a speed close to the ambient magnetosonic speed.
Title: Flare-CME Models: An Observational Perspective (Invited Review)
Authors: Schmieder, B.; Aulanier, G.; Vršnak, B.
Bibcode: 2015SoPh..290.3457S
Altcode: 2015SoPh..tmp...64S
Eruptions, flares, and coronal mass ejection (CMEs) are due to physical
phenomena mainly driven by an initially force-free current-carrying
magnetic field. We review some key observations relevant to the
current theoretical trigger mechanisms of the eruption and to the
energy release via reconnection. Sigmoids observed in X-rays and UV,
as well as the pattern (double J-shaped) of electric currents in
the photosphere show clear evidence of the existence of currents
parallel to the magnetic field and can be the signature of a flux
rope that is detectable in CMEs. The magnetic helicity of filaments
and active regions is an interesting indirectly measurable parameter
because it can quantify the twist of the flux rope. On the other hand,
the magnetic helicity of the solar structures allows us to associate
solar eruptions and magnetic clouds in the heliosphere. The magnetic
topology analysis based on the 3D magnetic field extrapolated from
vector magnetograms is a good tool for identifying the reconnection
locations (null points and/or the 3D large volumes - hyperbolic flux
tube, HFT). Flares are associated more with quasi-separatrix layers
(QSLs) and HFTs than with a single null point, which is a relatively
rare case. We review various mechanisms that have been proposed to
trigger CMEs and their observable signatures: by "breaking" the field
lines overlying the flux rope or by reconnection below the flux rope
to reduce the magnetic tension, or by letting the flux rope to expand
until it reaches a minimum threshold height (loss of equilibrium or
torus instability). Additional mechanisms are commonly operating in
the solar atmosphere. Examples of observations are presented throughout
the article and are discussed in this framework.
Title: Investigation of X-class Flare-Associated Coronal Mass
Ejections with and without DH Type II Radio Bursts
Authors: Lawrance, M. Bendict; Shanmugaraju, A.; Vršnak, Bojan
Bibcode: 2015SoPh..290.3365L
Altcode: 2015SoPh..tmp..163L
A statistical analysis of 135 out of 141 X-class flares observed during
1997 - 2012 with and without deca-hectometric (DH) type II radio bursts
has been performed. It was found that 79 events (X-class flares and
coronal mass ejections - Group I) were associated with DH type II radio
bursts and 62 X-class flare events were not. Out of these 62 events
without DH type IIs, 56 events (Group II) have location information,
and they were selected for this study. Of these 56 events, only 32
were associated with CMEs. Most of the DH-associated X-class events
(∼79 % ) were halo CMEs, in contrast to 14 % in Group II. The
average CME speed of the X-class flares associated with DH type IIs
is 1555 km s−1, which is nearly twice that of the X-class
flare-associated CMEs without DH event (744 km s−1). The
X-class flares associated with DH radio bursts have a mean flare
intensity (3.63 ×10−4Wm−2) that is 38 %
greater than that of X-class flares without DH radio bursts (2.23
×10−4Wm−2). In addition to the greater
intensity, it is also found that the the duration and rise time of
flares associated with DH radio emission (DH flares) is more than
twice than that of the flares without DH radio emission. When the
events were further divided into two categories with respect to their
source locations in eastern and western regions, 65 % of the events
in the radio-loud category (with DH radio bursts) are from the western
hemisphere and the remaining 35 % are from the eastern hemisphere. On
the other hand, in the radio-quiet category (without DH radio bursts),
nearly 60 % of the events are from the eastern hemisphere in contrast to
those of the radio-loud category. It is found that 81 % of the events
from eastern regions have flare durations > 30 min in the DH-flare
category, in contrast to a nearly equal number from the western side
for flare durations longer/shorter than 30 min. Similarly, the eastern
events in the DH-flare category have a longer average rise-time of
34 min, while the western events have an average flare rise-time of
26 min. On the other hand, the CME speed and flare strength are found
to be nearly equal among east and west side events, except that both
these parameters are greater for events with DH type IIs.
Title: Division II: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
Bibcode: 2015IAUTB..28..106V
Altcode:
The Business Meeting of Commission 10 was held as part of the Business
Meeting of Division II (Sun and Heliosphere), chaired by Valentin
Martínez-Pillet, the President of the Division. The President of
Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took
the chair for the business meeting of C10. She summarised the activities
of C10 over the triennium and the election of the incoming OC.
Title: Heliospheric Propagation of Coronal Mass Ejections: Drag-based
Model Fitting
Authors: Žic, T.; Vršnak, B.; Temmer, M.
Bibcode: 2015ApJS..218...32Z
Altcode: 2015arXiv150608582Z
The so-called drag-based model (DBM) simulates analytically the
propagation of coronal mass ejections (CMEs) in interplanetary space
and allows the prediction of their arrival times and impact speeds at
any point in the heliosphere (“target”). The DBM is based on the
assumption that beyond a distance of about 20 solar radii from the
Sun, the dominant force acting on CMEs is the “aerodynamic” drag
force. In the standard form of DBM, the user provisionally chooses
values for the model input parameters, by which the kinematics of the
CME over the entire Sun-“target” distance range is defined. The
choice of model input parameters is usually based on several previously
undertaken statistical studies. In other words, the model is used
by ad hoc implementation of statistics-based values of the input
parameters, which are not necessarily appropriate for the CME under
study. Furthermore, such a procedure lacks quantitative information
on how well the simulation reproduces the coronagraphically observed
kinematics of the CME, and thus does not provide an estimate of the
reliability of the arrival prediction. In this paper we advance the DBM
by adopting it in a form that employs the CME observations over a given
distance range to evaluate the most suitable model input parameters
for a given CME by means of least-squares fitting. Furthermore, the
new version of the model automatically responds to any significant
change of the conditions in the ambient medium (solar wind speed,
density, CME-CME interactions, etc.) by changing the model input
parameters according to changes in the CME kinematics. The advanced
DBM is shaped in a form that can be readily employed in an operational
system for real-time space-weather forecasting by promptly adjusting
to a successively expanding observational data set, thus providing a
successively improving prediction of the CME arrival.
Title: Strong coronal channelling and interplanetary evolution of
a solar storm up to Earth and Mars
Authors: Möstl, Christian; Rollett, Tanja; Frahm, Rudy A.; Liu,
Ying D.; Long, David M.; Colaninno, Robin C.; Reiss, Martin A.;
Temmer, Manuela; Farrugia, Charles J.; Posner, Arik; Dumbović,
Mateja; Janvier, Miho; Démoulin, Pascal; Boakes, Peter; Devos, Andy;
Kraaikamp, Emil; Mays, Mona L.; Vršnak, Bojan
Bibcode: 2015NatCo...6.7135M
Altcode: 2015arXiv150602842M; 2015NatCo...6E7135M
The severe geomagnetic effects of solar storms or coronal mass
ejections (CMEs) are to a large degree determined by their propagation
direction with respect to Earth. There is a lack of understanding of
the processes that determine their non-radial propagation. Here we
present a synthesis of data from seven different space missions of a
fast CME, which originated in an active region near the disk centre
and, hence, a significant geomagnetic impact was forecasted. However,
the CME is demonstrated to be channelled during eruption into a
direction +37+/-10° (longitude) away from its source region, leading
only to minimal geomagnetic effects. In situ observations near Earth
and Mars confirm the channelled CME motion, and are consistent with
an ellipse shape of the CME-driven shock provided by the new Ellipse
Evolution model, presented here. The results enhance our understanding
of CME propagation and shape, which can help to improve space weather
forecasts.
Title: Real-Time Solar Wind Prediction Based on SDO/AIA Coronal
Hole Data
Authors: Rotter, T.; Veronig, A. M.; Temmer, M.; Vršnak, B.
Bibcode: 2015SoPh..290.1355R
Altcode: 2015arXiv150106697R; 2015SoPh..tmp...37R
We present an empirical model based on the visible area covered by
coronal holes close to the central meridian with the aim to predict
the solar wind speed at 1 AU with a lead time of up to four days in
advance with a time resolution of one hour. Linear prediction functions
are used to relate coronal hole areas to solar wind speed. The function
parameters are automatically adapted by using the information from the
previous three Carrington Rotations. Thus the algorithm automatically
reacts to the changes of the solar wind speed during different phases
of the solar cycle. The adaptive algorithm was applied to and tested
on SDO/AIA-193 Å observations and ACE measurements during the years
2011 - 2013, covering 41 Carrington Rotations. The solar wind needs
on average 4.02±0.5 days to reach Earth. The algorithm produces good
predictions for the 156 solar wind high-speed streams peak amplitudes
with correlation coefficients of cc≈0.60. For 80 % of the peaks,
the predicted arrival matches the ACE in situ measurements within a
time window of 0.5 days. The same algorithm, using linear predictions,
was also applied to predict the magnetic field strength in wind streams
originating from coronal hole areas, but it did not give reliable
predictions (cc≈0.15).
Title: Strong coronal deflection of a CME and its interplanetary
evolution to Earth and Mars
Authors: Möstl, Christian; Rollett, Tanja; Frahm, Rudy A.; Liu, Ying
D.; Long, David M.; Colaninno, Robin C.; Reiss, Martin A.; Temmer,
Manuela; Farrugia, Charles J.; Posner, Arik; Dumbovic, Mateja; Janvier,
Miho; Demoulin, Pascal; Boakes, Peter; Devos, Andy; Kraaikamp, Emil;
Mays, Mona L.; Vrsnak, Bojan
Bibcode: 2015EGUGA..17.1366M
Altcode:
We discuss multipoint imaging and in situ observations of the coronal
mass ejection (CME) on January 7 2014 which resulted in a major false
alarm. While the source region was almost at disk center facing Earth,
the eruption was strongly deflected in the corona, and in conjunction
with its particular orientation this CME missed Earth almost entirely,
leading to no significant geomagnetic effects. We demonstrate this
by a synthesis of data from 7 different heliospheric and planetary
space missions (STEREO-A/B, SOHO, SDO, Wind, Mars Express, Mars
Science Laboratory). The CMEs ecliptic part was deflected by 37
± 10° in heliospheric longitude, a value larger than previously
thought. Multipoint in situ observations at Earth and Mars confirm
the deflection, and are consistent with an elliptical interplanetary
shock shape of aspect ratio 1.4 ± 0.4. We also discuss our new method,
the Ellipse Evolution (ElEvo) model, which allows us to optimize the
global shape of the CME shock with multipoint in situ observations of
the interplanetary CME arrival. ElEvo, which is an extension to the
Drag-Based-Model by Vrsnak et al., may also be used for real time space
weather forecasting. The presented results enhance our understanding
of CME deflection and shape, which are fundamental ingredients for
improving space weather forecasts.
Title: Geoeffectiveness of Coronal Mass Ejections in the SOHO Era
Authors: Dumbović, M.; Devos, A.; Vršnak, B.; Sudar, D.; Rodriguez,
L.; Ruždjak, D.; Leer, K.; Vennerstrøm, S.; Veronig, A.
Bibcode: 2015SoPh..290..579D
Altcode: 2014arXiv1410.3303D
The main objective of the study is to determine the probability
distributions of the geomagnetic Dst index as a function of the
coronal mass ejection (CME) and solar flare parameters for the purpose
of establishing a probabilistic forecast tool for the intensity of
geomagnetic storms. We examined several CME and flare parameters as well
as the effect of successive CME occurrence in changing the probability
for a certain range of Dst index values. The results confirm some
previously known relationships between remotely observed properties of
solar eruptive events and geomagnetic storms: the importance of the
initial CME speed, apparent width, source position, and the class of
the associated solar flare. We quantify these relationships in a form
that can be used for future space-weather forecasting. The results of
the statistical study are employed to construct an empirical statistical
model for predicting the probability of the geomagnetic storm intensity
based on remote solar observations of CMEs and flares.
Title: Initiation and Evolution of Global Coronal Waves
Authors: Vršnak, B.; Muhr, N.; Žic, T.; Lulić, S.; Kienreich,
I. W.; Temmer, M.; Veronig, A. M.
Bibcode: 2015CEAB...39...65V
Altcode:
Some essential outcomes of a detailed analysis of the formation and
evolution of the coronal EUV wave of 15 February 2011 are presented,
focused on the relationship between the source region expansion, wave
kinematics, and the evolution of the wave amplitude. The observations
are explained in terms of the results of the numerical MHD simulations,
providing new insights into the physical background of coronal waves,
especially considering the nature of the relationship of the wave
amplitude and propagation velocity in different phases of the wave
evolution.
Title: Forbush decreases associated to Stealth Coronal Mass Ejections
Authors: Heber, B.; Wallmann, C.; Galsdorf, D.; Herbst K.; Kühl,
P.; Dumbovic, M.; Vršnak, B.; Veronig, A.; Temmer, M.; Möstl, C.;
Dalla, S.
Bibcode: 2015CEAB...39...75H
Altcode:
Interplanetary coronal mass ejections (ICMEs) are structures in the
solar wind that are the counterparts of coronal mass ejections (CMEs)
at the Sun. It is commonly believed that enhanced magnetic fields
in interplanetary shocks and solar ejecta as well as the increased
turbulence in the solar wind sheath region are the cause of Forbush
decreases (FDs) representing decreases of galactic cosmic ray (GCR)
intensities. Recently, stealth CMEs i.e.~CMEs with no apparent solar
surface association have become a subject in recent studies of solar
activity. Whether all of such stealth CMEs can drive a FD is difficult
to investigate on the basis of neutron monitor NM measurements because
these measurements not only reflect the GCR intensity variation in
interplanetary space but also the variation of the geomagnetic field as
well as the conditions in the Earth atmosphere. Single detector counter
from spacecraft instrumentation, here SOHO and Chandra EPHIN, exceed
counting statistic of NMs allowing to determine intensity variation of
less than 1 permil in interplanetary space on the basis of 30 minute
count rate averages. Here we present the ongoing analysis of eleven
stealth CMEs.
Title: Statistical Analysis of Large-Scale EUV Waves Observed by
STEREO/EUVI
Authors: Muhr, N.; Veronig, A. M.; Kienreich, I. W.; Vršnak, B.;
Temmer, M.; Bein, B. M.
Bibcode: 2014SoPh..289.4563M
Altcode: 2014arXiv1408.2513M; 2014SoPh..tmp..126M
We statistically analyzed the kinematical evolution and wave pulse
characteristics of 60 strong large-scale EUV wave events that
occurred during January 2007 to February 2011 with the STEREO
twin spacecraft. For the start velocity, the arithmetic mean
is 312±115 km s−1 (within a range of 100 - 630 km
s−1). For the mean (linear) velocity, the arithmetic
mean is 254±76 km s−1 (within a range of 130 - 470
km s−1). 52 % of all waves under study show a distinct
deceleration during their propagation (a≤−50 m s−2),
the other 48 % are consistent with a constant speed within the
uncertainties (−50≤a≤50 m s−2). The start velocity
and the acceleration are strongly anticorrelated with c≈−0.8,
i.e. initially faster events undergo stronger deceleration than
slower events. The (smooth) transition between constant propagation
for slow events and deceleration in faster events occurs at an EUV wave
start-velocity of v≈230 km s−1, which corresponds well to
the fast-mode speed in the quiet corona. These findings provide strong
evidence that the EUV waves under study are indeed large-amplitude
fast-mode MHD waves. This interpretation is also supported by the
correlations obtained between the peak velocity and the peak amplitude,
impulsiveness, and build-up time of the disturbance. We obtained
the following association rates of EUV wave events with other solar
phenomena: 95 % are associated with a coronal mass ejection (CME),
74 % to a solar flare, 15 % to interplanetary type II bursts, and 22
% to coronal type II bursts. These findings are consistent with the
interpretation that the associated CMEs are the driving agents of the
EUV waves.
Title: Interaction Between Two CMEs During 14 - 15 February 2011
and Their Unusual Radio Signature
Authors: Shanmugaraju, A.; Prasanna Subramanian, S.; Vrsnak, Bojan;
Ibrahim, M. Syed
Bibcode: 2014SoPh..289.4621S
Altcode: 2014SoPh..tmp..125S
We report a detailed analysis of an interaction between two coronal
mass ejections (CMEs) that were observed on 14 - 15 February 2011
and the corresponding radio enhancement, which was similar to the
"CME cannibalism" reported by Gopalswamy et al. (Astrophys. J.548,
L91, 2001). A primary CME, with a mean field-of-view velocity of 669 km
s−1 in the Solar and Heliospheric Observatory (SOHO)/Large
Angle Spectrometric Coronagraph (LASCO), was more than as twice as
fast as the slow CME preceding it (326 km s−1), which
indicates that the two CMEs interacted. A radio-enhancement signature
(in the frequency range 1 MHz - 400 kHz) due to the CME interaction
was analyzed and interpreted using the CME data from LASCO and from
the Solar Terrestrial Relations Observatory (STEREO) HI-1, radio data
from Wind/Radio and Plasma Wave Experiment (WAVES), and employing known
electron-density models and kinematic modeling. The following results
are obtained: i) The CME interaction occurred around 05:00 - 10:00 UT
in a height range 20 - 25 R⊙. An unusual radio signature
is observed during the time of interaction in the Wind/WAVES dynamic
radio spectrum. ii) The enhancement duration shows that the interaction
segment might be wider than 5 R⊙. iii) The shock height
estimated using density models for the radio enhancement region is
10 - 30 R⊙. iv) Using kinematic modeling and assuming a
completely inelastic collision, the decrease of kinetic energy based on
speeds from LASCO data is determined to be 0.77×1023 J, and
3.67×1023 J if speeds from STEREO data are considered. vi)
The acceleration, momentum, and force are found to be a=−168 m
s−2, I=6.1×1018 kg m s−1, and
F=1.7×1015 N, respectively, using STEREO data.
Title: Heliospheric Propagation of Coronal Mass Ejections: Comparison
of Numerical WSA-ENLIL+Cone Model and Analytical Drag-based Model
Authors: Vršnak, B.; Temmer, M.; Žic, T.; Taktakishvili, A.;
Dumbović, M.; Möstl, C.; Veronig, A. M.; Mays, M. L.; Odstrčil, D.
Bibcode: 2014ApJS..213...21V
Altcode:
Real-time forecasting of the arrival of coronal mass ejections (CMEs) at
Earth, based on remote solar observations, is one of the central issues
of space-weather research. In this paper, we compare arrival-time
predictions calculated applying the numerical "WSA-ENLIL+Cone
model" and the analytical "drag-based model" (DBM). Both models use
coronagraphic observations of CMEs as input data, thus providing an
early space-weather forecast two to four days before the arrival of
the disturbance at the Earth, depending on the CME speed. It is shown
that both methods give very similar results if the drag parameter Γ =
0.1 is used in DBM in combination with a background solar-wind speed
of w = 400 km s-1. For this combination, the mean value
of the difference between arrival times calculated by ENLIL and DBM
is \overline{Δ }=0.09+/- 9.0 hr with an average of the absolute-value
differences of \overline{\vert Δ \vert }=7.1 hr. Comparing the observed
arrivals (O) with the calculated ones (C) for ENLIL gives O - C = -0.3
± 16.9 hr and, analogously, O - C = +1.1 ± 19.1 hr for DBM. Applying
Γ = 0.2 with w = 450 km s-1 in DBM, one finds O - C =
-1.7 ± 18.3 hr, with an average of the absolute-value differences
of 14.8 hr, which is similar to that for ENLIL, 14.1 hr. Finally,
we demonstrate that the prediction accuracy significantly degrades
with increasing solar activity.
Title: Combined Multipoint Remote and in situ Observations of the
Asymmetric Evolution of a Fast Solar Coronal Mass Ejection
Authors: Rollett, T.; Möstl, C.; Temmer, M.; Frahm, R. A.; Davies,
J. A.; Veronig, A. M.; Vršnak, B.; Amerstorfer, U. V.; Farrugia,
C. J.; Žic, T.; Zhang, T. L.
Bibcode: 2014ApJ...790L...6R
Altcode: 2014arXiv1407.4687R
We present an analysis of the fast coronal mass ejection (CME) of 2012
March 7, which was imaged by both STEREO spacecraft and observed in
situ by MESSENGER, Venus Express, Wind, and Mars Express. Based on
detected arrivals at four different positions in interplanetary space,
it was possible to strongly constrain the kinematics and the shape of
the ejection. Using the white-light heliospheric imagery from STEREO-A
and B, we derived two different kinematical profiles for the CME
by applying the novel constrained self-similar expansion method. In
addition, we used a drag-based model to investigate the influence of
the ambient solar wind on the CME's propagation. We found that two
preceding CMEs heading in different directions disturbed the overall
shape of the CME and influenced its propagation behavior. While the
Venus-directed segment underwent a gradual deceleration (from ~2700 km
s-1 at 15 R ⊙ to ~1500 km s-1 at 154
R ⊙), the Earth-directed part showed an abrupt retardation
below 35 R ⊙ (from ~1700 to ~900 km s-1). After
that, it was propagating with a quasi-constant speed in the wake of
a preceding event. Our results highlight the importance of studies
concerning the unequal evolution of CMEs. Forecasting can only be
improved if conditions in the solar wind are properly taken into
account and if attention is also paid to large events preceding the
one being studied.
Title: Shock ahead ICMEs analyzed from in situ data
Authors: Maricic, D.; Rosa, D.; Karlica, M.; Dumbovic, M.; Vrsnak,
B.; Hrzina, D.; Romstajn, I.
Bibcode: 2014simi.conf....7M
Altcode:
We analyzed solar wind disturbances recorded by WIND satellite
which can be connected with corresponding ICMEs recorded in Solar
Terrestrial Relations Observatory (STEREO) data, in the period from
2008 to 2013. Detailed analysis of the in situ data reveals complex and
different internal structures of the disturbances, where signatures
of the initially independent ICMEs could be recognized. Separating
the disturbances in different types, we focus on the disturbances
in which we can analyze shock ahead the ICMEs. Correlations of the
magnetic field strength, solar-wind speed, proton density and thermal
velocity are presented. Finally, we employ the ground-based cosmic
ray observations and Dst index, to investigate the connection between
CME manifestations with the Forbush decrease (FD) and its influence
on the Earth's magnetic field. The results presented provide a better
understanding of the ICME morphology and reveal effects that should
be taken into account in forecasting of the arrival of such compound
structures.
Title: Connecting Speeds, Directions and Arrival Times of 22 Coronal
Mass Ejections from the Sun to 1 AU
Authors: Möstl, C.; Amla, K.; Hall, J. R.; Liewer, P. C.; De Jong,
E. M.; Colaninno, R. C.; Veronig, A. M.; Rollett, T.; Temmer, M.;
Peinhart, V.; Davies, J. A.; Lugaz, N.; Liu, Y. D.; Farrugia, C. J.;
Luhmann, J. G.; Vršnak, B.; Harrison, R. A.; Galvin, A. B.
Bibcode: 2014ApJ...787..119M
Altcode: 2014arXiv1404.3579M
Forecasting the in situ properties of coronal mass ejections (CMEs)
from remote images is expected to strongly enhance predictions of
space weather and is of general interest for studying the interaction
of CMEs with planetary environments. We study the feasibility of using
a single heliospheric imager (HI) instrument, imaging the solar wind
density from the Sun to 1 AU, for connecting remote images to in situ
observations of CMEs. We compare the predictions of speed and arrival
time for 22 CMEs (in 2008-2012) to the corresponding interplanetary
coronal mass ejection (ICME) parameters at in situ observatories
(STEREO PLASTIC/IMPACT, Wind SWE/MFI). The list consists of front-
and backsided, slow and fast CMEs (up to 2700 km s-1). We
track the CMEs to 34.9 ± 7.1 deg elongation from the Sun with J maps
constructed using the SATPLOT tool, resulting in prediction lead times
of -26.4 ± 15.3 hr. The geometrical models we use assume different
CME front shapes (fixed-Φ, harmonic mean, self-similar expansion) and
constant CME speed and direction. We find no significant superiority
in the predictive capability of any of the three methods. The absolute
difference between predicted and observed ICME arrival times is 8.1 ±
6.3 hr (rms value of 10.9 hr). Speeds are consistent to within 284 ±
288 km s-1. Empirical corrections to the predictions enhance
their performance for the arrival times to 6.1 ± 5.0 hr (rms value
of 7.9 hr), and for the speeds to 53 ± 50 km s-1. These
results are important for Solar Orbiter and a space weather mission
positioned away from the Sun-Earth line.
Title: Morphology of an ICME-event derived by Multi-point in Situ
and Heliospheric Imaging Data
Authors: Rollett, Tanja; Möstl, Christian; Temmer, Manuela; Veronig,
Astrid M.; Frahm, Rudy A.; Davies, Jackie A.; Vrsnak, Bojan; Farrugia,
Charles J.; Amerstorfer, Ute V.
Bibcode: 2014EGUGA..1610892R
Altcode:
We show the analysis of an outstanding fast interplanetary coronal
mass ejection (ICME) of 07 March 2012, which has been observed
stereoscopically from both STEREO spacecraft. Assuming self-similar
expansion and constant direction of motion we derive the kinematical
profiles for the eastern and the western part of the roughly
Earth-directed ICME. As additional constraints we use the huge
advantage of in situ measurements at various locations during the
ICME's propagation, namely from Venus Express, Messenger, Wind and
Mars Express. We found that the eastern part of the ICME had a much
higher propagation speed than its western part. Using the drag-based
model, a model for the propagation of ICMEs in the inner heliosphere,
we analyzed the influence of the drag on both sides of the ICME due
to the surrounding solar wind conditions. These different solar wind
conditions could have been the reason for the differing velocities
and therefore for a distortion of the ICME front. These studies are
fundamental in order to deepen the understanding of ICME evolution
and to enhance existing forecasting methods.
Title: Connecting speeds, directions and arrival times of 22 coronal
mass ejections from the Sun to 1 AU
Authors: Möstl, Christian; Amla, Keshav; Hall, Jeff R.; Liewer,
Paulett C.; DeJong, Eric M.; Colaninno, Robin C.; Veronig, Astrid M.;
Rollett, Tanja; Temmer, Manuela; Peinhart, Vanessa; Davies, Jackie
A.; Lugaz, Noé; Liu, Ying; Farrugia, Charles J.; Luhmann, Janet G.;
Vrsnak, Bojan; Harrison, Richard A.; Galvin, Antoinette B.
Bibcode: 2014EGUGA..16.1755M
Altcode:
Forecasting in situ properties of coronal mass ejections (CMEs) from
remote images is expected to strongly enhance predictions of space
weather, and is of general interest for studying the interaction of
the solar wind with planetary environments. We study the feasibility of
using a heliospheric imager (HI) instrument, which is able to image the
solar wind density along the full Sun to 1 AU distance, for connecting
remote images to in situ observations of CMEs. Such an instrument
is currently in operation on each of the two STEREO spacecraft. We
compare the predictions for speed and arrival time for 22 different
CME events (between 2008-2012), each observed remotely by one STEREO
spacecraft, to the interplanetary coronal mass ejection (ICME) speed and
arrival time observed at in situ observatories (STEREO PLASTIC/IMPACT,
Wind SWE/MFI). We use croissant modeling for STEREO/COR2, and with a
single-spacecraft STEREO/HI instrument, we track each CME to 34.9 ± 7.1
degree elongation from the Sun with J-maps constructed with the SATPLOT
tool. We then fit geometrical models to each track, assuming different
CME front shapes (Fixed-Φ, Harmonic Mean, Self-Similar Expansion),
and constant CME speed and direction. We find no significant preference
in the predictive capability for any of the three geometrical modeling
methods used on the full event list, consisting of front- and backsided,
slow and fast CMEs (up to 2700 km s-1). The absolute difference between
predicted and observed ICME arrival times is 8.1 ± 6.4 hours (rms
value of 10.9h), and speeds are consistent within 284 ± 291 km s-1,
including the geometric effects of CME apex or flank encounters. We
derive new empirical corrections to the imaging results, enhancing
the performance of the arrival time predictions to 6.1 ± 5.0 hours
(rms value of 7.9h), and the speed predictions to 53 ± 50 km s-1,
for this particular set of events. The prediction lead time is around
1 day (-26.4 ± 15.3h). CME directions given by the HI methods differ
considerably, and biases are found on the order of 30-50 degree in
heliospheric longitude, consistent with theoretical expectations. These
results are of interest concerning future missions such as Solar Orbiter
or a dedicated space weather mission positioned remotely from the Earth.
Title: Asymmetry in the CME-CME Interaction Process for the Events
from 2011 February 14-15
Authors: Temmer, M.; Veronig, A. M.; Peinhart, V.; Vršnak, B.
Bibcode: 2014ApJ...785...85T
Altcode: 2014arXiv1402.6891T
We present a detailed study of the interaction process of two coronal
mass ejections (CMEs) successively launched on 2011 February 14 (CME1)
and 2011 February 15 (CME2). Reconstructing the three-dimensional
shape and evolution of the flux ropes, we verify that the two CMEs
interact. The frontal structure of both CMEs, measured along different
position angles (PAs) over the entire latitudinal extent, reveals
differences in the kinematics for the interacting flanks and the
apexes. The interaction process is strongly PA-dependent in terms of
timing as well as kinematical evolution. The central interaction occurs
along PA-100°, which shows the strongest changes in kinematics. During
interaction, CME1 accelerates from ~400 km s-1 to ~700 km
s-1 and CME2 decelerates from ~1300 km s-1 to ~600
km s-1. Our results indicate that a simplified scenario such
as inelastic collision may not be sufficient to describe the CME-CME
interaction. The magnetic field structures of the intertwining flux
ropes and the momentum transfer due to shocks each play an important
role in the interaction process.
Title: Investigation of the Coronal Magnetic Field Using a Type II
Solar Radio Burst
Authors: Vasanth, V.; Umapathy, S.; Vršnak, Bojan; Žic, Tomislav;
Prakash, O.
Bibcode: 2014SoPh..289..251V
Altcode: 2013arXiv1305.1760V
The type II solar radio burst recorded on 13 June 2010 by the Hiraiso
Solar Observatory Radio Spectrograph was employed to estimate
the magnetic-field strength in the solar corona. The burst was
characterized by a well-pronounced band splitting, which we used to
estimate the density jump at the shock and Alfvén Mach number using the
Rankine-Hugoniot relation. We convert the plasma frequency of the type
II burst into height [R] in solar radii using an appropriate density
model, and then we estimated the shock speed [Vs], coronal
Alfvén velocity [VA], and the magnetic-field strength at
different heights. The relative bandwidth of the band splitting was
found to be in the range 0.2 - 0.25, corresponding to a density jump
of X=1.44 - 1.56, and an Alfvén Mach number of MA=1.35 -
1.45. The inferred mean shock speed was on the order of V≈667 km
s−1. From the dependencies V(R) and MA(R)
we found that the Alfvén speed slightly decreases at R≈1.3 - 1.5
R⊙. The magnetic-field strength decreases from a value
between 2.7 and 1.7 G at R≈1.3 - 1.5 R⊙, depending on
the coronal-density model employed. Our results are in good agreement
with the empirical scaling by Dulk and McLean (Solar Phys. 57, 279,
1978) and Gopalswamy et al. (Astrophys. J.744, 72, 2012). Our results
show that the type II band-splitting method is an important tool for
inferring the coronal magnetic field, especially when independent
measurements are made from white-light observations.
Title: Transit Time of Coronal Mass Ejections under Different Ambient
Solar Wind Conditions
Authors: Shanmugaraju, A.; Vršnak, Bojan
Bibcode: 2014SoPh..289..339S
Altcode:
The speed [v(R)] of coronal mass ejections (CMEs) at various distances
from the Sun is modeled (as proposed by Vršnak and Gopalswamy in
J. Geophys. Res. 107, 2002, doi:10.1029/2001/JA000120) by using the
equation of motion adrag=γ(v−w) and its quadratic form
adrag=γ(v−w)|v−w|, where v and w are the speeds of
the CME and solar wind, respectively. We assume that the parameter γ
can be expressed as γ=αRβ, where R is the heliocentric
distance, and α and β are constants. We extend the analysis of
Vršnak and Gopalswamy to obtain a more detailed insight into the
dependence of the CME Sun-Earth transit time on the CME speed and the
ambient solar-wind speed, for different combinations of α and β. In
such a parameter-space analysis, the results obtained confirm that
the CME transit time depends strongly on the state of the ambient
solar wind. Specifically, we found that: i) for a particular set
of values of α and β, a difference in the solar-wind speed causes
larger transit-time differences at low CME speeds [v0],
than at high v0; ii) the difference between transit times of
slow and fast CMEs is larger at low solar-wind speed [w0]
than at high w0; iii) transit times of fast CMEs are
only slightly influenced by the solar-wind speed. The last item is
especially important for space-weather forecasting, since it reduces
the number of key parameters that determine the arrival time of fast
CMEs, which tend to be more geo-effective than the slow ones. Finally,
we compared the drag-based model results with the observational data
for two CME samples, consisting of non-interacting and interacting
CMEs (Manoharan et al. in J. Geophys. Res.109, 2004). The comparison
reveals that the model results are in better agreement with the
observations for non-interacting events than for the interacting
events. It was also found that for slow CMEs (v0<500
km s−1), there is a deviation between the observations
and the model if slow-wind speeds (≈ 300 - 400 km s−1)
are taken for the model input. On the other hand, the model values and
the observed data agree for both the slow and the fast CMEs if higher
solar-wind speeds are assumed. It is also found that the quadratic
form of the drag equation reproduces the observed transit times of
fast CMEs better than the linear drag model.
Title: Kinematics of Interacting ICMEs and Related Forbush Decrease:
Case Study
Authors: Maričić, D.; Vršnak, B.; Dumbović, M.; Žic, T.; Roša,
D.; Hržina, D.; Lulić, S.; Romštajn, I.; Bušić, I.; Salamon, K.;
Temmer, M.; Rollett, T.; Veronig, A.; Bostanjyan, N.; Chilingarian,
A.; Mailyan, B.; Arakelyan, K.; Hovhannisyan, A.; Mujić, N.
Bibcode: 2014SoPh..289..351M
Altcode:
We study heliospheric propagation and some space weather aspects of
three Earth-directed interplanetary coronal mass ejections (ICMEs),
successively launched from the active region AR 11158 in the period
13 - 15 February 2011. From the analysis of the ICME kinematics,
morphological evolution, and in situ observations, we infer that the
three ICMEs interacted on their way to Earth, arriving together at 1
AU as a single interplanetary disturbance. Detailed analysis of the
in situ data reveals complex internal structure of the disturbance,
where signatures of the three initially independent ICMEs could be
recognized. The analysis also reveals compression and heating of the
middle ICME, as well as ongoing magnetic reconnection between the
leading and the middle ICME. We present evidence showing that the
propagation of these two, initially slower ICMEs, was boosted by the
fastest, third ICME. Finally, we employ the ground-based cosmic ray
observations, to show that this complex disturbance produced a single
cosmic ray event, i.e., a simple Forbush decrease (FD). The results
presented provide a better understanding of the ICME interactions and
reveal effects that should be taken into account in forecasting of
the arrival of such compound structures.
Title: Initiation of Coronal Mass Ejections by Sunspot Rotation
Authors: Valori, G.; Török, T.; Temmer, M.; Veronig, A. M.; van
Driel-Gesztelyi, L.; Vršnak, B.
Bibcode: 2014IAUS..300..201V
Altcode:
We report observations of a filament eruption, two-ribbon flare, and
coronal mass ejection (CME) that occurred in Active Region NOAA 10898
on 6 July 2006. The filament was located South of a strong sunspot that
dominated the region. In the evolution leading up to the eruption, and
for some time after it, a counter-clockwise rotation of the sunspot of
about 30 degrees was observed. We suggest that the rotation triggered
the eruption by progressively expanding the magnetic field above the
filament. To test this scenario, we study the effect of twisting
the initially potential field overlying a pre-existing flux rope,
using three-dimensional zero-β MHD simulations. We consider a magnetic
configuration whose photospheric flux distribution and coronal structure
is guided by the observations and a potential field extrapolation. We
find that the twisting leads to the expansion of the overlying field. As
a consequence of the progressively reduced magnetic tension, the flux
rope quasi-statically adapts to the changed environmental field, rising
slowly. Once the tension is sufficiently reduced, a distinct second
phase of evolution occurs where the flux rope enters an unstable regime
characterized by a strong acceleration. Our simulation thus suggests
a new mechanism for the triggering of eruptions in the vicinity of
rotating sunspots.
Title: The Wave-Driver System of the Off-Disk Coronal Wave of 17
January 2010
Authors: Temmer, M.; Vrsnak, B.; Veronig, A. M.
Bibcode: 2013SoPh..287..441T
Altcode: 2012arXiv1207.2857T; 2012SoPh..tmp..194T
We study the 17 January 2010 flare-CME-wave event by using
STEREO/SECCHI-EUVI and -COR1 data. The observational study is combined
with an analytic model that simulates the evolution of the coronal wave
phenomenon associated with the event. From EUV observations, the wave
signature appears to be dome shaped having a component propagating
on the solar surface (\overline{v}≈280~km s^{-1}) as well as one
off-disk (\overline{v}≈ 600~km s^{-1}) away from the Sun. The off-disk
dome of the wave consists of two enhancements in intensity, which
conjointly develop and can be followed up to white-light coronagraph
images. Applying an analytic model, we derive that these intensity
variations belong to a wave-driver system with a weakly shocked wave,
initially driven by expanding loops, which are indicative of the early
evolution phase of the accompanying CME. We obtain the shock standoff
distance between wave and driver from observations as well as from
model results. The shock standoff distance close to the Sun (< 0.3
R⊙ above the solar surface) is found to rapidly increase
with values of ≈ 0.03 - 0.09 R⊙, which gives evidence
of an initial lateral (over)expansion of the CME. The kinematical
evolution of the on-disk wave could be modeled using input parameters
that require a more impulsive driver (duration t=90 s, acceleration
a=1.7 km s−2) compared to the off-disk component (duration
t=340 s, acceleration a=1.5 km s−2).
Title: Comparison of geoeffectiveness of coronal mass ejections and
corotating interaction regions
Authors: Verbanac, G.; Živković, S.; Vršnak, B.; Bandić, M.;
Hojsak, T.
Bibcode: 2013A&A...558A..85V
Altcode:
Context. A detailed comparison of the geomagnetic responses to
interplanetary coronal mass ejection (ICMEs) and corotating interaction
regions (CIRs) during solar cycle 23 was performed using geomagnetic
indices Dst, Ap, and AE.
Aims: We aim to find out if there
are relative differences in the response of various magnetospheric
current systems to the impact of ICMEs and CIRs. In addition, we are
exploring the possibility of forecasting geomagnetic activity using the
coronagraphic observations of the ICME take-off.
Methods: The
peak values of the plasma characteristics of ICMEs and CIRs (velocity V,
magnetic field B, and BV related to the electric field), and geomagnetic
indices were investigated by applying the linear and power-law cross
correlation analysis. The influence of the time-resolution on the
results was performed for two time resolutions obtained by one-hour
(three-hour for Ap) and six-hour data averaging.
Results:
For ICMEs the power-law fits are found to be important only for the
relationships between BV and geomagnetic indices. For Ap and Dst, there
is no difference between the one-hour (three-hour for Ap) and six-hour
option. For AE, the one-hour data distribution shows more clearly the
non-linear dependence on BV. Our data set shows that below BV ~ 5 mV
m-1 ICMEs have practically no geomagnetic effect at low and
mid latitudes, but at high latitudes at least some geomagnetic activity
will be triggered. For all HSS/CIRs dependencies, a power law is found
to better describe the data than the linear fit. The data distributions
show that BV has to reach ~4 mV m-1 in order to drive at
least some geomagnetic activity at all latitudes. We observed that
there are fast CMEs that have almost no geomagnetic effect at low and
mid latitudes. On the other hand, at high latitudes, fast CMEs always
trigger some geomagnetic activity. This might be have implications
for space weather forecasting.
Conclusions: Magnetospheric
response to both solar drivers (ICMEs and CIRs) is different at various
latitudes, thus results in different development of various current
systems within the Earth's magnetosphere and ionosphere. Furthermore,
we show that ICMEs and CIRs cause different geomagnetic activity. In
the case of ICMEs equatorial current system responses in a linear
manner, while the response of the polar-current system is likely to
be non-linear. For HSS/CIRs, apparently all current systems respond
in a non-linear way, especially the polar current system.
Title: Initiation of Coronal Mass Ejections by Sunspot Rotation
Authors: Török, T.; Temmer, M.; Valori, G.; Veronig, A. M.; van
Driel-Gesztelyi, L.; Vršnak, B.
Bibcode: 2013SoPh..286..453T
Altcode: 2014arXiv1401.2922T
We study a filament eruption, two-ribbon flare, and coronal mass
ejection (CME) that occurred in NOAA Active Region 10898 on 6 July
2006. The filament was located South of a strong sunspot that dominated
the region. In the evolution leading up to the eruption, and for some
time after it, a counter-clockwise rotation of the sunspot of about
30 degrees was observed. We suggest that the rotation triggered the
eruption by progressively expanding the magnetic field above the
filament. To test this scenario, we study the effect of twisting
the initially potential field overlying a pre-existing flux-rope,
using three-dimensional zero-β MHD simulations. We first consider
a relatively simple and symmetric system, and then study a more
complex and asymmetric magnetic configuration, whose photospheric-flux
distribution and coronal structure are guided by the observations and a
potential field extrapolation. In both cases, we find that the twisting
leads to the expansion of the overlying field. As a consequence of the
progressively reduced magnetic tension, the flux-rope quasi-statically
adapts to the changed environmental field, rising slowly. Once the
tension is sufficiently reduced, a distinct second phase of evolution
occurs where the flux-rope enters an unstable regime characterised by
a strong acceleration. Our simulations thus suggest a new mechanism
for the triggering of eruptions in the vicinity of rotating sunspots.
Title: Formation of Coronal Shock Waves
Authors: Lulić, S.; Vršnak, B.; Žic, T.; Kienreich, I. W.; Muhr,
N.; Temmer, M.; Veronig, A. M.
Bibcode: 2013SoPh..286..509L
Altcode: 2013arXiv1303.2786L
Magnetosonic wave formation driven by an expanding cylindrical
piston is numerically simulated to obtain better physical insight
into the initiation and evolution of large-scale coronal waves caused
by coronal eruptions. Several very basic initial configurations are
employed to analyze intrinsic characteristics of MHD wave formation
that do not depend on specific properties of the environment. It turns
out that these simple initial configurations result in piston/wave
morphologies and kinematics that reproduce common characteristics of
coronal waves. In the initial stage, the wave and the expanding source
region cannot be clearly resolved; i.e. a certain time is needed before
the wave detaches from the piston. Thereafter, it continues to travel
as what is called a "simple wave." During the acceleration stage of the
source region inflation, the wave is driven by the piston expansion, so
its amplitude and phase-speed increase, whereas the wavefront profile
steepens. At a given point, a discontinuity forms in the wavefront
profile; i.e. the leading edge of the wave becomes shocked. The
time/distance required for the shock formation is shorter for a more
impulsive source-region expansion. After the piston stops, the wave
amplitude and phase speed start to decrease. During the expansion,
most of the source region becomes strongly rarefied, which reproduces
the coronal dimming left behind the eruption. However, the density
increases at the source-region boundary, and stays enhanced even after
the expansion stops, which might explain stationary brightenings
that are sometimes observed at the edges of the erupted coronal
structure. Also, in the rear of the wave a weak density depletion
develops, trailing the wave, which is sometimes observed as weak
transient coronal dimming. Finally, we find a well-defined relationship
between the impulsiveness of the source-region expansion and the wave
amplitude and phase speed. The results for the cylindrical piston are
also compared with the outcome for a planar wave that is formed by a
one-dimensional piston, to find out how different geometries affect
the evolution of the wave.
Title: Solar TErrestrial Relations Observatory-A (STEREO-A) and
PRoject for On-Board Autonomy 2 (PROBA2) Quadrature Observations of
Reflections of Three EUV Waves from a Coronal Hole
Authors: Kienreich, I. W.; Muhr, N.; Veronig, A. M.; Berghmans, D.;
De Groof, A.; Temmer, M.; Vršnak, B.; Seaton, D. B.
Bibcode: 2013SoPh..286..201K
Altcode: 2012SoPh..tmp..138K
We investigate the interaction of three consecutive large-scale coronal
waves with a polar coronal hole, simultaneously observed on-disk by the
Solar TErrestrial Relations Observatory (STEREO)-A spacecraft and on
the limb by the PRoject for On-Board Autonomy 2 (PROBA2) spacecraft on
27 January 2011. All three extreme ultraviolet (EUV) waves originate
from the same active region, NOAA 11149, positioned at N30E15 in the
STEREO-A field of view and on the limb in PROBA2. For the three primary
EUV waves, we derive starting velocities in the range of ≈ 310 km
s−1 for the weakest up to ≈ 500 km s−1
for the strongest event. Each large-scale wave is reflected at the
border of the extended coronal hole at the southern polar region. The
average velocities of the reflected waves are found to be smaller than
the mean velocities of their associated direct waves. However, the
kinematical study also reveals that in each case the ending velocity
of the primary wave matches the initial velocity of the reflected
wave. In all three events, the primary and reflected waves obey the
Huygens-Fresnel principle, as the incident angle with ≈ 10° to
the normal is of the same magnitude as the angle of reflection. The
correlation between the speed and the strength of the primary EUV waves,
the homologous appearance of both the primary and the reflected waves,
and in particular the EUV wave reflections themselves suggest that the
observed EUV transients are indeed nonlinear large-amplitude MHD waves.
Title: Propagation of Interplanetary Coronal Mass Ejections: The
Drag-Based Model
Authors: Vršnak, B.; Žic, T.; Vrbanec, D.; Temmer, M.; Rollett, T.;
Möstl, C.; Veronig, A.; Čalogović, J.; Dumbović, M.; Lulić, S.;
Moon, Y. -J.; Shanmugaraju, A.
Bibcode: 2013SoPh..285..295V
Altcode: 2012SoPh..tmp..124V
We present the "Drag-Based Model" (DBM) of heliospheric propagation
of interplanetary coronal mass ejections (ICMEs). The DBM is based on
the hypothesis that the driving Lorentz force, which launches a CME,
ceases in the upper corona and that beyond a certain distance the
dynamics becomes governed solely by the interaction of the ICME and
the ambient solar wind. In particular, we consider the option where
the drag acceleration has a quadratic dependence on the ICME relative
speed, which is expected in a collisionless environment, where the
drag is caused primarily by emission of magnetohydrodynamic (MHD)
waves. In this paper we present the simplest version of DBM, where
the equation of motion can be solved analytically, providing explicit
solutions for the Sun-Earth ICME transit time and impact speed. This
offers easy handling and straightforward application to real-time
space-weather forecasting. Beside presenting the model itself, we
perform an analysis of DBM performances, applying a statistical and
case-study approach, which provides insight into the advantages and
drawbacks of DBM. Finally, we present a public, DBM-based, online
forecast tool.
Title: Evolution of CMEs in the inner heliosphere - observations
versus models
Authors: Temmer, Manuela; Vrsnak, Bojan; Möstl, Christian; Veronig,
Astrid; Rollett, Tanja; Bein, Bianca
Bibcode: 2013EGUGA..15.1328T
Altcode:
With the SECCHI instrument suite aboard STEREO, coronal mass ejections
(CMEs) can be observed from multiple vantage points during their entire
propagation all the way from the Sun to 1 AU. The propagation behavior
of CMEs in the interplanetary space is mainly influenced by the ambient
solar wind flow. CMEs that are faster than the ambient solar wind get
decelerated, whereas slower ones are accelerated until the CME speed
is finally adjusted to the solar wind speed. On a statistical basis,
empirical models taking into account the drag force acting on CMEs,
are able to describe the observed kinematical behaviors. For several
well observed events, we will present a comparative study showing
the kinematical evolution of CMEs derived from remote sensing and
in situ data, as well as from empirical models using 2D and 3D input
parameters. From this we aim to obtain the distance regime at which
the solar wind drag force is dominating the CME propagation.
Title: Radial evolution of magnetic cloud properties
Authors: Rollett, Tanja; Veronig, Astrid M.; Leitner, Martin; Vrsnak,
Bojan; Möstl, Christian; Farrugia, Charles J.; Temmer, Manuela
Bibcode: 2013EGUGA..15.2710R
Altcode:
Magnetic clouds (MCs) are characterized as intervals of enhanced,
smoothly rotating interplanetary magnetic field, low plasma beta and
temperature in spacecraft in situ data and can be part of ICMEs. In
this study we analyze the radial evolution of MCs using a sample of
events detected by radial aligned spacecrafts at different heliocentric
distances. The data-sets are fitted with a force-free, constant-alpha
flux rope model. Using the outcome of this fitting model we calculate
the estimated cross section diameter (assuming a cylindrical flux tube),
the poloidal and the axial magnetic field, the current, the magnetic
flux and the inductance. All these parameter are further studied as a
function of heliocentric distance. Strong variations of the current or
the magnetic flux could be a hint for magnetic reconnection between
the MC and the solar wind. This work has received funding from the
European Commission FP7 Project COMESEP (263252).
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: The Successive CME on 13th; 14th and 15th February 2011 and
Forbush decrease on 18 February 2011
Authors: Maričić, D.; Bostasyan, N.; Dumbović, M.; Chilingarian,
A.; Mailyan, B.; Rostomyan, H.; Arakelyan, K.; Vršnak, B.; Roša,
D.; Hržina, D.; Romštajn, I.; Veronig, A.
Bibcode: 2013JPhCS.409a2158M
Altcode:
Aims. We analyze the kinematics of three interplanetary coronal mass
ejections (ICMEs) that occurred on 13th, 14th and 15th February
2011 in the active region AR 11155 and have shown that they
appeared at the Earth orbit on February, 18th and caused Forbush
decrease (FD). Methods. The solar coordinates of flares are (S19W03),
(S20W14) and (S21W18). The kinematic curves were obtained using STEREO
(A&B) data. Additionally, we explore the possibility of the CME-CME
interaction for these three events. We compare obtained estimates of
ICME arrival with the in-situ measurements from WIND satellite at
L1 point and with ground-based cosmic ray data obtained from SEVAN
network. Results. The acceleration of each CME is highly correlated
with the associated SXR flares energy release. CMEs that erupted at 13
and 14 Feb 2011 are not associated with prominence eruption; maximum
velocity was vmax550 ± 50 km/s and vmax400
± 50 km/s, respectively. However, 15 Feb 2011 CME is connected with
much more violent eruption associated with a prominence, with maximum
velocity of vmax 1400 ± 50 km/s. The last overtakes 13th
and 14th Feb CMEs at distances of 32 and 160 Rsolar, respectively.
Title: Solar eruptive filament studies at USO for the COMESEP project
Authors: Srivastava, N.; Crosby, N.; Veronig, A.; Robrrecht, E.;
Vršnak, B.; Vennerstrom, S.; Malandraki, O.; Dalla, S.; Rodriguez,
L.; Hesse, M.; Odstrcil, D.
Bibcode: 2013ASInC..10...67S
Altcode:
The Coronal Mass Ejections and Solar Energetic Particles (COMESEP)
project is developing tools for forecasting geomagnetic storms and solar
energetic particle (SEP) radiation storms. By analysis of historical
solar and interplanetary data, complemented with the extensive data
coverage of solar cycle 23, the key ingredients that lead to geomagnetic
storms, SEP events and the factors that are responsible for false
alarms are being identified. Based on the insights gained, and making
use of algorithms for the automated detection of CMEs, forecasting
tools for geomagnetic and SEP radiation storms will be developed and
optimised. Validation and implementation of the developed tools into
an operational Space Weather Alert system will be performed. COMESEP
is a unique cross collaboration effort and bridges the gap between the
SEP, CME and terrestrial effects scientific communities. The role of
the Udaipur Solar Observatory (USO) in addressing some of the goals
of this project are highlighted in this paper. In particular, USO is
engaged in studying the CMEs associated with eruptive filaments. We
describe the studies undertaken to understand space weather effects
related to these CMEs.
Title: Relation Between Coronal Hole Areas on the Sun and the Solar
Wind Parameters at 1 AU
Authors: Rotter, T.; Veronig, A. M.; Temmer, M.; Vršnak, B.
Bibcode: 2012SoPh..281..793R
Altcode: 2012SoPh..tmp..202R
We analyze the relationship between the coronal hole (CH)
characteristics on the Sun (area, position, and intensity levels)
and the corresponding solar wind parameters (solar wind speed v,
proton temperature T, proton density n, and magnetic field strength B)
measured in situ at 1 AU with a 6-h time resolution. We developed a
histogram-based intensity thresholding method to obtain fractional CH
areas from SOHO/EIT 195 Å images. The algorithm was applied to 6-h
cadence EIT 195 Å images for the year 2005, which were characterized
by a low solar activity. In calculating well-defined peaks of the solar
wind parameters corresponding to the peaks in CH area, we found that the
solar wind speed v shows a high correlation with correlation coefficient
cc=0.78, medium correlation for T and B with cc=0.41 and cc=0.41. No
significant correlation was found with the proton density n. Applying
an intensity-weighted CH area did not improve the relations, since
the size and the mean intensity of the CH areas are not independent
parameters but strongly correlated (cc=− 0.72). Comparison of the
fractional CH areas derived from GOES/SXI and SOHO/EIT and the related
solar wind predictions shows no systematic differences (cc=0.79).
Title: Coronal Mass Ejection of 26 February 2000: Complete analysis
of the three-part CME structure
Authors: Maričić, D.; Vršnak, B.; Roša, D.; Hržina, D.
Bibcode: 2012SunGe...7...85M
Altcode:
We analyze the kinematics and morphology of the limb coronal mass
ejection (CME) of 26 February 2000, utilizing observations from Mauna
Loa Solar Observatory (MLSO), the Solar and Heliospheric Observatory
(SOHO) and the Geostationary Operational Environmental Satellite
(GOES). Also, we analyze the relation between dynamics of the CME and
the energy release in the associated flare. An intricate structure
(prominence, prominence-like absorbing feature, cavity and bright
overlying arcade) is clearly recognizable in the low corona during
the pre-eruption phase of slow rise. This provided measurements of
kinematics of verious features from the very beginning of the eruption
up to the post-acceleration phase which was followed up to 32 solar
radii. Such events are observed only occasionaly, and are of great
importance for the comprehension of the nature of forces driving
CMEs. The acceleration maximum was attained at the radial distance
of 2.4 solar radii from the solar center and ceased beyond 12 solar
radii. The time profiles of the acceleration of various features
of CME are showing "self-similar" expansion and implying a common
driver. The acceleration phase was synchronized to a certain degree with
the impulsive phase of the associated two-ribbon flare. Observations
provide clear evidence that CME eruption caused a global restructuring
of the magnetic field in the outer and inner corona. Furthermore,
kinematics and morphological properties of this CME show possibility
that in some events the prominence can evolve into a structure which
looks like three-part structure CME, i.e. where the frontal rim is
just a part of helically twisted prominence.
Title: Impulsive Acceleration of Coronal Mass Ejections. II. Relation
to Soft X-Ray Flares and Filament Eruptions
Authors: Bein, B. M.; Berkebile-Stoiser, S.; Veronig, A. M.; Temmer,
M.; Vršnak, B.
Bibcode: 2012ApJ...755...44B
Altcode: 2012arXiv1206.2144B
Using high time cadence images from the STEREO EUVI, COR1, and COR2
instruments, we derived detailed kinematics of the main acceleration
stage for a sample of 95 coronal mass ejections (CMEs) in comparison
with associated flares and filament eruptions. We found that CMEs
associated with flares reveal on average significantly higher peak
accelerations and lower acceleration phase durations, initiation
heights, and heights, at which they reach their peak velocities and peak
accelerations. This means that CMEs that are associated with flares are
characterized by higher and more impulsive accelerations and originate
from lower in the corona where the magnetic field is stronger. For CMEs
that are associated with filament eruptions we found only for the CME
peak acceleration significantly lower values than for events that were
not associated with filament eruptions. The flare rise time was found
to be positively correlated with the CME acceleration duration and
negatively correlated with the CME peak acceleration. For the majority
of the events the CME acceleration starts before the flare onset (for
75% of the events) and the CME acceleration ends after the soft X-ray
(SXR) peak time (for 77% of the events). In ~60% of the events, the
time difference between the peak time of the flare SXR flux derivative
and the peak time of the CME acceleration is smaller than ±5 minutes,
which hints at a feedback relationship between the CME acceleration and
the energy release in the associated flare due to magnetic reconnection.
Title: Forecasting Geomagnetic Storms and Solar Energetic Particle
Events: the COMESEP Project
Authors: Crosby, N.; Veronig, A.; Robbrecht, E.; Vrsnak, B.;
Vennerstrøm, S.; Malandraki, O.; Dalla, S.; Srivastava, N.; Hesse,
M.; Odstrcil, D.
Bibcode: 2012EGUGA..1412544C
Altcode:
COMESEP (COronal Mass Ejections and Solar Energetic Particles),
funded by the European Union Framework 7 programme, is a three-year
collaborative project that has been running for one year. Tools for
forecasting geomagnetic storms and solar energetic particle (SEP)
radiation storms are being developed under the project. By analysis
of historical data, complemented by the extensive data coverage of
solar cycle 23, the key ingredients that lead to magnetic storms and
SEP events and the factors that are responsible for false alarms are
being identified. To enhance our understanding of the 3D kinematics
and interplanetary propagation of coronal mass ejections (CMEs), the
structure, propagation and evolution of CMEs are being investigated. In
parallel, the sources and propagation of SEPs are being examined and
modeled. Based on the insights gained, and making use of algorithms for
the automated detection of CMEs, forecasting tools for geomagnetic and
SEP radiation storms are being developed and optimised. Validation and
implementation of the produced tools into an operational Space Weather
Alert system will be performed. Geomagnetic and SEP radiation storm
alerts will be based on the COMESEP definition of risk. COMESEP is a
unique cross-collaboration effort and bridges the gap between the SEP
and CME scientific communities. For more information about the project,
see the COMESEP website http://www.comesep.eu/ . This work has received
funding from the European Commission FP7 Project COMESEP (263252).
Title: STEREO-A and PROBA2 Quadrature Observations of Reflections
of three EUV Waves from a Coronal Hole
Authors: Kienreich, Ines Waltraud; Muhr, Nicole; Veronig, Astrid;
Berghmans, David; de Groof, Anik; Temmer, Manuela; Vršnak, Bojan;
Seaton, Dan
Bibcode: 2012arXiv1204.6472K
Altcode: 2012arXiv1204.6472W
We investigate the interaction of three consecutive large-scale coronal
waves with a polar coronal hole, simultaneously observed on-disk by the
Solar TErrestrial Relations Observatory (STEREO)-A spacecraft and on
the limb by the PRoject for On-Board Autonomy 2 (PROBA2) spacecraft on
January 27, 2011. All three extreme-ultraviolet(EUV) waves originate
from the same active region NOAA 11149 positioned at N30E15 in the
STEREO-A field-of-view and on the limb in PROBA2. We derive for the
three primary EUV waves start velocities in the range of ~310 km/s for
the weakest up to ~500 km/s for the strongest event. Each large-scale
wave is reflected at the border of the extended coronal hole at the
southern polar region. The average velocities of the reflected waves
are found to be smaller than the mean velocities of their associated
direct waves. However, the kinematical study also reveals that in each
case the end velocity of the primary wave matches the initial velocity
of the reflected wave. In all three events the primary and reflected
waves obey the Huygens-Fresnel principle, as the incident angle with
~10° to the normal is of the same size as the angle of reflection. The
correlation between the speed and the strength of the primary EUV waves,
the homologous appearance of both the primary and the reflected waves,
and in particular the EUV wave reflections themselves implicate that the
observed EUV transients are indeed nonlinear large-amplitude MHD waves.
Title: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
Bibcode: 2012IAUTA..28...69V
Altcode:
Commission 10 of the International Astronomical Union has more than
650 members who study a wide range of activity phenomena produced by
our nearest star, the Sun. Solar activity is intrinsically related
to solar magnetic fields and encompasses events from the smallest
energy releases (nano- or even picoflares) to the largest eruptions
in the Solar System, coronal mass ejections (CMEs), which propagate
into the Heliosphere reaching the Earth and beyond. Solar activity is
manifested in the appearance of sunspot groups or active regions, which
are the principal sources of activity phenomena from the emergence of
their magnetic flux through their dispersion and decay. The period
2008-2009 saw an unanticipated extended solar cycle minimum and
unprecedentedly weak polar-cap and heliospheric field. Associated with
that was the 2009 historical maximum in galactic cosmic rays flux since
measurements begun in the middle of the 20th Century. Since then Cycle
24 has re-started solar activity producing some spectacular eruptions
observed with a fleet of spacecraft and ground-based facilities. In
the last triennium major advances in our knowledge and understanding
of solar activity were due to continuing success of space missions as
SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched
by the breathtaking images of the solar atmosphere produced by the
Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the
framework of NASA's Living with a Star program. In August 2012, at the
time of the IAU General Assembly in Beijing when the mandate of this
Commission ends, we will be in the unique position to have for the
first time a full 3-D view of the Sun and solar activity phenomena
provided by the twin STEREO missions about 120 degrees behind and
ahead of Earth and other spacecraft around the Earth and ground-based
observatories. These new observational insights are continuously
posing new questions, inspiring and advancing theoretical analysis
and modelling, improving our understanding of the physics underlying
magnetic activity phenomena. Commission 10 reports on a vigorously
evolving field of research produced by a large community. The number
of refereed publications containing `Sun', `heliosphere', or a synonym
in their abstracts continued the steady growth seen over the preceding
decades, reaching about 2000 in the years 2008-2010, with a total of
close to 4000 unique authors. This report, however, has its limitations
and it is inherently incomplete, as it was prepared jointly by the
members of the Organising Committee of Commission 10 (see the names
of the primary contributors to the sections indicated in parentheses)
reflecting their fields of expertise and interest. Nevertheless, we
believe that it is a representative sample of significant new results
obtained during the last triennium in the field of solar activity.
Title: CME-CME interaction during the 2010 August 1 events
Authors: Temmer, M.; Vrsnak, B.; Rollett, T.; Bein, B.; deKoning,
C. A.; Liu, Y.; Bosman, E.; Davies, J. A.; Möstl, C.; Zic, T.;
Veronig, A. M.; Bothmer, V.; Harrison, R.; Nitta, N.; Bisi, M.; Flor,
O.; Eastwood, J.; Odstrcil, D.; Forsyth, R.
Bibcode: 2012EGUGA..14.1677T
Altcode:
We study a CME-CME interaction that occurred during the 2010 August 1
events using STEREO/SECCHI data (COR and HI). The CMEs were Earth
directed where clear signatures of magnetic flux ropes could be
measured from in situ Wind data. To give evidence of the actual
interaction we derive the direction of motion for both CMEs applying
several independent methods. From this we obtain that both CMEs head
into similar directions enabling us to actually observe the merging
in the HI1 field-of-view (and rule out the possibility that this is
just a line of sight effect). The full de-projected kinematics of the
faster CME from Sun to Earth is derived when combining data points from
remote observations with in situ parameters of the ICME measured at
1 AU. We study the evolution of the kinematical profile of the faster
CME by applying a drag based model.
Title: Characteristics of Kinematics of a Coronal Mass Ejection
during the 2010 August 1 CME-CME Interaction Event
Authors: Temmer, Manuela; Vršnak, Bojan; Rollett, Tanja; Bein, Bianca;
de Koning, Curt A.; Liu, Ying; Bosman, Eckhard; Davies, Jackie A.;
Möstl, Christian; Žic, Tomislav; Veronig, Astrid M.; Bothmer, Volker;
Harrison, Richard; Nitta, Nariaki; Bisi, Mario; Flor, Olga; Eastwood,
Jonathan; Odstrcil, Dusan; Forsyth, Robert
Bibcode: 2012ApJ...749...57T
Altcode: 2012arXiv1202.0629T
We study the interaction of two successive coronal mass ejections (CMEs)
during the 2010 August 1 events using STEREO/SECCHI COR and heliospheric
imager (HI) data. We obtain the direction of motion for both CMEs by
applying several independent reconstruction methods and find that the
CMEs head in similar directions. This provides evidence that a full
interaction takes place between the two CMEs that can be observed in the
HI1 field of view. The full de-projected kinematics of the faster CME
from Sun to Earth is derived by combining remote observations with in
situ measurements of the CME at 1 AU. The speed profile of the faster
CME (CME2; ~1200 km s-1) shows a strong deceleration over
the distance range at which it reaches the slower, preceding CME (CME1;
~700 km s-1). By applying a drag-based model we are able
to reproduce the kinematical profile of CME2, suggesting that CME1
represents a magnetohydrodynamic obstacle for CME2 and that, after
the interaction, the merged entity propagates as a single structure
in an ambient flow of speed and density typical for quiet solar wind
conditions. Observational facts show that magnetic forces may contribute
to the enhanced deceleration of CME2. We speculate that the increase
in magnetic tension and pressure, when CME2 bends and compresses the
magnetic field lines of CME1, increases the efficiency of drag.
Title: Flare-generated Type II Burst without Associated Coronal
Mass Ejection
Authors: Magdalenić, J.; Marqué, C.; Zhukov, A. N.; Vršnak, B.;
Veronig, A.
Bibcode: 2012ApJ...746..152M
Altcode:
We present a study of the solar coronal shock wave on 2005 November
14 associated with the GOES M3.9 flare that occurred close to the
east limb (S06° E60°). The shock signature, a type II radio burst,
had an unusually high starting frequency of about 800 MHz, indicating
that the shock was formed at a rather low height. The position of the
radio source, the direction of the shock wave propagation, and the
coronal electron density were estimated using Nançay Radioheliograph
observations and the dynamic spectrum of the Green Bank Solar Radio
Burst Spectrometer. The soft X-ray, Hα, and Reuven Ramaty High Energy
Solar Spectroscopic Imager observations show that the flare was compact,
very impulsive, and of a rather high density and temperature, indicating
a strong and impulsive increase of pressure in a small flare loop. The
close association of the shock wave initiation with the impulsive
energy release suggests that the impulsive increase of the pressure
in the flare was the source of the shock wave. This is supported by
the fact that, contrary to the majority of events studied previously,
no coronal mass ejection was detected in association with the shock
wave, although the corresponding flare occurred close to the limb.
Title: Cosmic ray modulation by different types of solar wind
disturbances
Authors: Dumbović, M.; Vršnak, B.; Čalogović, J.; Župan, R.
Bibcode: 2012A&A...538A..28D
Altcode:
Context. Solar wind disturbances such as interplanetary coronal
mass ejections (ICMEs) and corotating interaction regions (CIRs)
cause short-term cosmic ray depressions, generally denoted as Forbush
decreases.
Aims: We conduct a systematic statistical study of
various aspects of Forbush decreases. The analysis provides empirical
background for physical interpretations of short-term cosmic ray
modulations.
Methods: Firstly, we analyzed the effects of
different types of solar wind disturbances, and secondly, we focused
on the phenomenon of over-recovery (the return of the cosmic ray
count to a value higher than the pre-decrease level). The analysis
is based on ground-based neutron monitor data and the solar wind
data recorded by the Advanced Composition Explorer. The correlations
between various cosmic ray depressions and solar wind parameters as
well as their statistical significance are analyzed in detail. In
addition, we performed a normalized superposed epoch analysis for
depressions and magnetic field enhancements.
Results: The
analysis revealed differences in the relationship between different
solar wind disturbances and cosmic ray depression parameters. The
amplitude of the depression for ICMEs was found to correlate well
with the amplitudes of magnetic field strength and fluctuations,
whereas for CIRs we found only the correlation between the amplitude
of the depression and the solar wind disturbance dimension proxy
vtB. Similar behavior was found for shock and no-shock
events, respectively. The CIR/ICME composites show a specific
behavior that is a mixture of both ICMEs and CIRs. For all analyzed
categories we found that the duration of the depression correlates
with the duration of the solar wind disturbance. The analysis of the
over-recovery showed that there is no straightforward relationship to
either "branching-effect" or geomagnetic effects, therefore we propose
a scenario where the "branching-effect" is caused by several factors
and is only indirectly related to the over-recovery.
Title: Solar influences on the short-term cosmic ray modulation
Authors: Dumbović, M.; Vršnak, B.; Čalogović, J.
Bibcode: 2012CEAB...36...65D
Altcode:
This study aims to provide a quantitative basis for physical
interpretations of Forbush decreases (FDs) caused by disturbances in the
interplanetary magnetic field. A superposed epoch analysis is applied
to the magnetic field strength and fluctuations data obtained from the
Advanced Composition Explorer, and to the cosmic ray data obtained
from ground level neutron monitors. We found that the morphology,
as well as the amplitude and duration of FDs, are dependent on the
type of the disturbance that caused the FD.
Title: Characteristics of DH type II bursts, CMEs and flares with
respect to the acceleration of CMEs
Authors: Prakash, O.; Umapathy, S.; Shanmugaraju, A.; Pappa Kalaivani,
P.; Vršnak, Bojan
Bibcode: 2012Ap&SS.337...47P
Altcode: 2011Ap&SS.tmp..608P
A detailed investigation on DH-type-II radio bursts recorded in
Deca-Hectometer (hereinafter DH-type-II) wavelength range and their
associated CMEs observed during the year 1997-2008 is presented. The
sample of 212 DH-type-II associated with CMEs are classified into three
populations: (i) Group I (43 events): DH-type-II associated CMEs are
accelerating in the LASCO field view ( a>15 m s-2);
(ii) Group II (99 events): approximately constant velocity CMEs
(-15< a<15 m s-2) and (iii) Group III (70 events):
represents decelerating CMEs ( a<-15 m s-2). Our study
consists of three steps: (i) statistical properties of DH-type-II
bursts of Group I, II and III events; (ii) analysis of time lags
between onsets of flares and CMEs associated with DH-type-II bursts
and (iii) statistical properties of flares and CMEs of Group I, II
and III events. We found statistically significant differences between
the properties of DH-type-II bursts of Group I, II and III events. The
significance ( P a ) is found using the one-way ANOVA-test
to examine the differences between means of groups. For example, there
is significant difference in the duration ( P a =5%),
ending frequency ( P a =4%) and bandwidth ( P a
=4%). The accelerating and decelerating CMEs have more kinetic
energy than the constant speed CMEs. There is a significant difference
between the nose height of CMEs at the end time of DH-type-IIs ( P
a ≪1%). From the time delay analysis, we found: (i) there is
no significant difference in the delay (flare start—DH-type-II start
and flare peak—DH-type-II start); (ii) small differences in the time
delay between the CME onset and DH-type-II start, delay between the
flare start and CME onset times. However, there are high significant
differences in: flare duration ( P a =1%), flare rise time
( P a =0.5%), flare decay time ( P a =5%) and
CMEs speed ( P a ≪1%) of Group I, II and III events. The
general LASCO CMEs have lower width and speeds when compared to the DH
CMEs. It seems there is a strong relation between the kinetic energy
of CMEs and DH-type-II properties.
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: Plasma Diagnostics of an EIT Wave Observed by Hinode/EIS
and SDO/AIA
Authors: Veronig, A. M.; Gömöry, P.; Kienreich, I. W.; Muhr, N.;
Vršnak, B.; Temmer, M.; Warren, H. P.
Bibcode: 2011ApJ...743L..10V
Altcode: 2011arXiv1111.3505V
We present plasma diagnostics of an Extreme-Ultraviolet
Imaging Telescope (EIT) wave observed with high cadence in
Hinode/Extreme-Ultraviolet Imaging Spectrometer (EIS) sit-and-stare
spectroscopy and Solar Dynamics Observatory/Atmospheric Imaging
Assembly imagery obtained during the HOP-180 observing campaign on 2011
February 16. At the propagating EIT wave front, we observe downward
plasma flows in the EIS Fe XII, Fe XIII, and Fe XVI spectral lines
(log T ≈ 6.1-6.4) with line-of-sight (LOS) velocities up to 20
km s-1. These redshifts are followed by blueshifts with
upward velocities up to -5 km s-1 indicating relaxation
of the plasma behind the wave front. During the wave evolution, the
downward velocity pulse steepens from a few km s-1 up to 20
km s-1 and subsequently decays, correlated with the relative
changes of the line intensities. The expected increase of the plasma
densities at the EIT wave front estimated from the observed intensity
increase lies within the noise level of our density diagnostics from
EIS Fe XIII 202/203 Å line ratios. No significant LOS plasma motions
are observed in the He II line, suggesting that the wave pulse was not
strong enough to perturb the underlying chromosphere. This is consistent
with the finding that no Hα Moreton wave was associated with the
event. The EIT wave propagating along the EIS slit reveals a strong
deceleration of a ≈ -540 m s-2 and a start velocity of v
0 ≈ 590 km s-1. These findings are consistent
with the passage of a coronal fast-mode MHD wave, pushing the plasma
downward and compressing it at the coronal base.
Title: Influence of the Ambient Solar Wind Flow on the Propagation
Behavior of Interplanetary Coronal Mass Ejections
Authors: Temmer, Manuela; Rollett, Tanja; Möstl, Christian; Veronig,
Astrid M.; Vršnak, Bojan; Odstrčil, Dusan
Bibcode: 2011ApJ...743..101T
Altcode: 2011arXiv1110.0827T
We study three coronal mass ejection (CME)/interplanetary coronal mass
ejection (ICME) events (2008 June 1-6, 2009 February 13-18, and 2010
April 3-5) tracked from Sun to 1 AU in remote-sensing observations
of Solar Terrestrial Relations Observatory Heliospheric Imagers and
in situ plasma and magnetic field measurements. We focus on the ICME
propagation in interplanetary (IP) space that is governed by two
forces: the propelling Lorentz force and the drag force. We address
the question: which heliospheric distance range does the drag become
dominant and the CME adjust to the solar wind flow. To this end,
we analyze speed differences between ICMEs and the ambient solar
wind flow as a function of distance. The evolution of the ambient
solar wind flow is derived from ENLIL three-dimensional MHD model
runs using different solar wind models, namely, Wang-Sheeley-Arge and
MHD-Around-A-Sphere. Comparing the measured CME kinematics with the
solar wind models, we find that the CME speed becomes adjusted to the
solar wind speed at very different heliospheric distances in the three
events under study: from below 30 R ⊙, to beyond 1 AU,
depending on the CME and ambient solar wind characteristics. ENLIL can
be used to derive important information about the overall structure of
the background solar wind, providing more reliable results during times
of low solar activity than during times of high solar activity. The
results from this study enable us to obtain greater insight into the
forces acting on CMEs over the IP space distance range, which is an
important prerequisite for predicting their 1 AU transit times.
Title: Propagation behavior of interplanetary CMEs: driving versus
drag force
Authors: Temmer, M.; Rollett, T.; Moestl, C.; Veronig, A. M.;
Vrsnak, B.
Bibcode: 2011AGUFMSH23C1968T
Altcode:
The evolution of coronal mass ejections (CMEs) is governed by the
Lorentz and the drag force. Initially, the CME is launched and driven
by the Lorentz force, whereas the drag force owing to the ambient solar
wind controls the CME kinematics as it propagates into interplanetary
(IP) space. The subject of the current study is to infer a heliospheric
distance at which the drag force starts to prevail over the driving
force. With the SECCHI instrument suite aboard STEREO, CMEs can be
observed during their entire propagation all the way from Sun to 1
AU. In combination with in-situ measurements at 1 AU we are able to
derive the direction and speed of a CME. This information is used as
input to derive the kinematical behavior of well observed CME events
in the IP distance regime, which is subsequently compared to the output
from ENLIL (NASA/CCMC) MHD model runs for the ambient solar wind flow.
Title: Characteristics of Type-II Radio Bursts Associated with Flares
and CMEs
Authors: Vasanth, V.; Umapathy, S.; Vršnak, Bojan; Anna Lakshmi, M.
Bibcode: 2011SoPh..273..143V
Altcode: 2011SoPh..tmp..360V
We present a statistical study of the characteristics of type-II radio
bursts observed in the metric (m) and deca-hectometer (DH) wavelength
range during 1997-2008. The collected events are divided into two
groups: Group I contains the events of m-type-II bursts with starting
frequency ≥ 100 MHz, and group II contains the events with starting
frequency of m-type-II radio bursts < 100 MHz. We have analyzed
both samples considering three different aspects: i) statistical
properties of type-II bursts, ii) statistical properties of flares
and CMEs associated with type-II bursts, and iii) time delays between
type-II bursts, flares, and CMEs. We find significant differences
in the properties of m-type-II bursts in duration, bandwidth, drift
rate, shock speed and delay between m- and DH-type-II bursts. From
the timing analysis we found that the majority of m-type-II bursts in
both groups occur during the flare impulsive phase. On the other hand,
the DH-type-II bursts in both groups occur during the decaying phase
of the associated flares. Almost all m-DH-type-II bursts are found to
be associated with CMEs. Our results indicate that there are two kinds
of shock in which group I (high frequency) m-type-II bursts seem to
be ignited by flares whereas group II (low frequency) m-type-II bursts
are CME-driven.
Title: Analysis of Characteristic Parameters of Large-scale Coronal
Waves Observed by the Solar-Terrestrial Relations Observatory/Extreme
Ultraviolet Imager
Authors: Muhr, N.; Veronig, A. M.; Kienreich, I. W.; Temmer, M.;
Vršnak, B.
Bibcode: 2011ApJ...739...89M
Altcode:
The kinematical evolution of four extreme ultraviolet waves,
well observed by the Extreme Ultraviolet Imager on board the
Solar-Terrestrial Relations Observatory (STEREO), is studied by visually
tracking wave fronts as well as by a semi-automatized perturbation
profile method, which leads to results matching each other within
the error limits. The derived mean velocities of the events under
study lie in the range of 220-350 km s-1. The fastest of
the events (2007 May 19) reveals a significant deceleration of ≈ -
190 m s-2, while the others are consistent with a constant
velocity during wave propagation. The evolution of maximum-intensity
values reveals initial intensification of 20%-70% and decays to original
levels within 40-60 minutes, while the widths at half-maximum and
full-maximum of the perturbation profiles broaden by a factor of two
to four. The integral below the perturbation profile remains basically
constant in two cases, while it shows a decrease by a factor of three
to four in the other two cases. From the peak perturbation amplitudes,
we estimate the corresponding magnetosonic Mach numbers M ms,
which range from 1.08-1.21. The perturbation profiles reveal three
distinct features behind the propagating wave fronts: coronal dimmings,
stationary brightenings, and rarefaction regions. All features appear
after the wave passage and only slowly fade away. Our findings indicate
that the events under study are weak-shock fast-mode magnetohydrodynamic
waves initiated by the CME lateral expansion.
Title: Solar wind high-speed streams and related geomagnetic activity
in the declining phase of solar cycle 23
Authors: Verbanac, G.; Vršnak, B.; Živković, S.; Hojsak, T.;
Veronig, A. M.; Temmer, M.
Bibcode: 2011A&A...533A..49V
Altcode:
Context. Coronal holes (CHs) are the source of high-speed streams
(HSSs) in the solar wind, whose interaction with the slow solar wind
creates corotating interaction regions (CIRs) in the heliosphere.
Aims: We investigate the magnetospheric activity caused by CIR/HSS
structures, focusing on the declining phase of the solar cycle 23
(years 2005 and 2006), when the occurrence rate of coronal mass
ejections (CMEs) was low. We aim to (i) perform a systematic analysis
of the relationship between the CH characteristics, basic parameters of
HSS/CIRs, and the geomagnetic indices Dst, Ap and AE; (ii) study how the
magnetospheric/ionospheric current systems behave when influenced by
HSS/CIR; (iii) investigate if and how the evolution of the background
solar wind from 2005 to 2006 affected the correlations between CH,
CIR, and geomagnetic parameters.
Methods: The cross-correlation
analysis was applied to the fractional CH area (CH) measured in the
central meridian distance interval ± 10°, the solar wind velocity (V),
the interplanetary magnetic field (B), and the geomagnetic indices Dst,
Ap, and AE.
Results: The performed analysis shows that Ap and
AE are better correlated with CH and solar wind parameters than Dst,
and quantitatively demonstrates that the combination of solar wind
parameters BV2 and BV plays the central role in the process
of energy transfer from the solar wind to the magnetosphere.
Conclusions: We provide reliable relationships between CH properties,
HSS/CIR parameters, and geomagnetic indices, which can be used in
forecasting the geomagnetic activity in periods of low CME activity.
Title: Impulsive Acceleration of Coronal Mass Ejections. I. Statistics
and Coronal Mass Ejection Source Region Characteristics
Authors: Bein, B. M.; Berkebile-Stoiser, S.; Veronig, A. M.; Temmer,
M.; Muhr, N.; Kienreich, I.; Utz, D.; Vršnak, B.
Bibcode: 2011ApJ...738..191B
Altcode: 2011arXiv1108.0561B
We use high time cadence images acquired by the STEREO EUVI and
COR instruments to study the evolution of coronal mass ejections
(CMEs) from their initiation through impulsive acceleration to the
propagation phase. For a set of 95 CMEs we derived detailed height,
velocity, and acceleration profiles and statistically analyzed
characteristic CME parameters: peak acceleration, peak velocity,
acceleration duration, initiation height, height at peak velocity,
height at peak acceleration, and size of the CME source region. The CME
peak accelerations we derived range from 20 to 6800 m s-2
and are inversely correlated with the acceleration duration and the
height at peak acceleration. Seventy-four percent of the events reach
their peak acceleration at heights below 0.5 R sun. CMEs that
originate from compact sources low in the corona are more impulsive and
reach higher peak accelerations at smaller heights. These findings can
be explained by the Lorentz force, which drives the CME accelerations
and decreases with height and CME size.
Title: Evolution of Solar and Geomagnetic Activity Indices, and
Their Relationship: 1960 - 2001
Authors: Verbanac, G.; Mandea, M.; Vršnak, B.; Sentic, S.
Bibcode: 2011SoPh..271..183V
Altcode: 2011SoPh..tmp..133V; 2011SoPh..tmp..190V; 2011SoPh..tmp..259V
We employ annually averaged solar and geomagnetic activity indices for
the period 1960 - 2001 to analyze the relationship between different
measures of solar activity as well as the relationship between solar
activity and various aspects of geomagnetic activity. In particular,
to quantify the solar activity we use the sunspot number Rs,
group sunspot number Rg, cumulative sunspot area Cum, solar
radio flux F10.7, and interplanetary magnetic field strength IMF. For
the geomagnetic activity we employ global indices Ap, Dst and Dcx, as
well as the regional geomagnetic index RES, specifically estimated for
the European region. In the paper we present the relative evolution of
these indices and quantify the correlations between them. Variations
have been found in: i) time lag between the solar and geomagnetic
indices; ii) relative amplitude of the geomagnetic and solar activity
peaks; iii) dual-peak distribution in some of solar and geomagnetic
indices. The behavior of geomagnetic indices is correlated the best
with IMF variations. Interestingly, among geomagnetic indices, RES
shows the highest degree of correlation with solar indices.
Title: Analysis of characteristic parameters of large-scale coronal
waves observed by STEREO/EUVI
Authors: Muhr, N.; Veronig, A. M.; Kienreich, I. W.; Temmer, M.;
Vrsnak, B.
Bibcode: 2011arXiv1107.0921M
Altcode:
The kinematical evolution of four EUV waves, well observed by the
Extreme UltraViolet Imager (EUVI) onboard the Solar-Terrestrial
Relations Observatory (STEREO), is studied by visually tracking the
wave fronts as well as by a semiautomatized perturbation profile
method leading to results matching each other within the error
limits. The derived mean velocities of the events under study lie
in the range of 220-350 km/s. The fastest of the events (May 19,
2007) reveals a significant deceleration of \approx -190 m s-2 while
the others are consistent with a constant velocity during the wave
propagation. The evolution of the maximum intensity values reveals
initial intensification by 20 up to 70%, and decays to original
levels within 40-60 min, while the width at half maximum and full
maximum of the perturbation profiles are broadening by a factor of
2 - 4. The integral below the perturbation profile remains basically
constant in two cases, while it shows a decrease by a factor of 3 -
4 in the other two cases. From the peak perturbation amplitudes we
estimate the corresponding magnetosonic Mach numbers Mms which are
in the range of 1.08-1.21. The perturbation profiles reveal three
distinct features behind the propagating wave fronts: coronal dimmings,
stationary brightenings and rarefaction regions. All of them appear
after the wave passage and are only slowly fading away. Our findings
indicate that the events under study are weak shock fast-mode MHD
waves initiated by the CME lateral expansion.
Title: Cosmic ray modulation by solar wind disturbances
Authors: Dumbović, M.; Vršnak, B.; Čalogović, J.; Karlica, M.
Bibcode: 2011A&A...531A..91D
Altcode:
Aims: We perform a systematic statistical study of the
relationship between characteristics of solar wind disturbances, caused
by interplanetary coronal mass ejections and corotating interaction
regions, and properties of Forbush decreases (FDs). Since the mechanism
of FDs is still being researched, this analysis should provide a firm
empirical basis for physical interpretations of the FD phenomenon.
Methods: The analysis is based on the ground-based neutron monitor
data and the solar wind data recorded by the Advanced Composition
Explorer, where the disturbances were identified as increases in proton
speed, magnetic field, and magnetic field fluctuations. We focus on
the relative timing of FDs, as well as on the correlations between
various FD and solar wind parameters, paying special attention to
the statistical significance of the results.
Results: It was
found that the onset, the minimum, and the end of FDs are delayed
after the onset, the maximum, and the end of the magnetic field
enhancement. The t-test shows that at the 95% significance level the
average lags have to be longer than 3, 7, and 26 h, respectively. FD
magnitude (| FD|) is correlated with the magnetic field strength
(B), magnetic field fluctuations (δB), and speed (v), as well as
with combined parameters, BtB, Bv, vtB, and
BvtB, where tB is the duration of the magnetic
field disturbance. In the |FD|(B) dependence, a "branching" effect
was observed, i.e., two different trends exist. The analysis of the FD
duration and recovery period reveals a correlation with the duration of
the magnetic field enhancement. The strongest correlations are obtained
for the dependence on combined solar wind parameters of the product
of the FD duration and magnitude, implying that combined parameters
are in fact true variables themselves, rather than just a product of
variables.
Conclusions: From the time lags we estimate that
"the penetration depth" in the disturbance, at which FD onset becomes
recognizable, is on the order of 100 Larmor radii and is comparable
to a typical shock-sheath dimension. The results for the FD time
profile indicate "shadow effect" of the solar wind disturbance before
and after it passes the observer. The importance of reduced parallel
diffusion during the passage of the disturbance is discussed, along
with the influence of terrestrial effects on the observed "branching
effect". Appendices A-C are available in electronic form at http://www.aanda.org
Title: The Drag Based Model of ICME Propagation
Authors: Dumbović, M.; Vršnak, B.; Žic, T.; Vrbanec, D.; Veronig,
A.; Temmer, M.; Rollett, T.; Moestl, C.; Moon, Y. -J.
Bibcode: 2011simi.confR...2D
Altcode:
One of central issues of space weather is the propagation of
interplanetary coronal mass ejections (ICMEs). At the heliospheric
distances beyond R=20 solar radii the "aerodynamic" drag is presumably
the dominant force governing ICME propagation; therefore, a drag based
model (DBM) was established, which can be used to forecast the ICME
arrival at the Earth. First, the model was tested on a sample
of CMEs by combining remote observations of the CME take-off gained
by the LASCO onboard SOHO, and in situ measurements from ACE and Wind
satellites. The results of the DBM were compared to observational data
and a fairly good agreement of the two was found. The model was then
tested against STEREO observations. The ICME kinematics was inferred
from STEREO observations by applying the Harmonic Mean method and
compared to the DBM results. In this way we were able to reproduce
the propagation of both slow and fast ICMEs, as well as to identify
ICME-ICME interactions and a transition from fast-to-slow solar wind
regimes. Finally, a statistical study was performed, where parameters
were varied within a model in order to obtain optimal values, for
which the average difference in the observed and calculated TT is zero
(O-C=0) and the O-C scatter gets minimum. The source of the scatter
in O–C values was investigated. The research leading to the
results presented in this paper has received funding from European
Community's Seventh Framework Programme (FP7/2007-2013) under grant
agreement No. 218816.
Title: Cosmic Ray Modulation by Solar Wind Disturbances
Authors: Dumbović, M.; Vršnak, B.; Čalogović, J.; Karlica, M.
Bibcode: 2011simi.confQ...2D
Altcode:
Solar wind disturbances (SWDs), namely interplanetary coronal mass
ejections (ICMEs) and corotating interaction regions (CIRs), cause
short-term depressions in galactic cosmic ray (GCR) flux. The mechanism
of this modulation is still a matter of research from observational
point and theoretical modeling. Since GCR flux reflects solar activity,
solving this problem represents an important aspect of space weather. We analyze the influence of different SWD parameters on the amplitude
and the duration of the depressions, using ground-based neutron monitor
data and in situ solar wind data from the ACE satellite. We test
correlations between GCR depression amplitudes and solar wind speed,
IMF and IMF fluctuations. Time profiles are also examined. The analysis
is performed for SWDs in general, ICMEs, CIRs, mixed ICME/CIR events,
events associated with an interplanetary shock, and events without
shock. A statistical analysis is also performed regarding the
delay of the depression after the onset of the IMF increase. We find
that in the majority of cases the decrease follows the onset in IMF
increase with an average delay on the order of the typical shock-sheath
thickness. High correlation between the depression magnitude and the
increase in IMF fluctuations and strength was found, favoring reduced
diffusion as a modulation mechanism. Furthermore, the proxies of time
integrals are found to behave as physical quantities. The differences
were observed between the data sorted by type (ICME, CIR, and mixed)
and shock association. Obtained results can be used to test theoretical
models.
Title: Coronal Mass Ejection of 12 June 2010: CME Kinematical
Parameters
Authors: Maričić, D.; Vršnak, B.; Roša, D.; Hržina, D.;
Romštajn, I.
Bibcode: 2011simi.confQ...5M
Altcode:
We investigate the initiation and development of the limb coronal mass
ejection (CME) which launched 12 June 2010, utilizing observations from
Mauna Loa Solar Observatory (MLSO), Solar and Heliospheric Observatory
(SOHO), Solar Terrestrial Relations Observatory (STEREO) and Solar
Dynamic Observatory (SDO). The goal of this study is to investigate
the new, relatively fast method for determining true geometric and
kinematical CME parameters from simultaneous observation of CMEs by
various satellites and ground based MLSO observatory. These parameters
are direction of CME motion, velocity and acceleration of the different
parts of CME (the frontal rim, the cavity, and the prominence) and
CME angular size. Furthermore, we investigate the driving mechanisms
of CME and infer the magnetic field properties at the onset of the
instability. To determine the driving mechanism, we quantitatively and
qualitatively compared the observationally obtained kinematic evolution
with that predicted by various CME models, mainly based on toroidal
geometry (cf., Chen, 1989; Vršnak, 1990; Forbes and Isenberg, 1991;
Amari et al., 2000; Wu et al., 2000; T r k and Kliem, 2005).
Title: Comparison between Linear and Quadratic Drag Models for
ICME Propagation
Authors: Moon, Yong-Jae; Vrsnak, B.; Gopalswamy, N.; Yashiro, S.
Bibcode: 2011SPD....42.2318M
Altcode: 2011BAAS..43S.2318M
In this paper, we have examined a recent issue what kinds of drag
form (linear or quadratic drag) is proper for interplanetary coronal
mass ejections (ICMEs). For this work, we have examined well-observed
LASCO CMEs associated with DH Type II bursts satisfying the following
conditions: (1) the CMEs speeds are larger than 600 km/s, (2) their
longitudes are larger than 60 degrees, (3) the numbers of their LASCO
data points are larger than 6, and (4) their accelerations are smaller
than -1 m/s2. We find that their accelerations (Log a) in
the LASCO field of view has a very good quadratic relationship with
the CME relative speeds Log (Vcme-400) with the correlation coefficient
of R=0.83, supporting the quadratic drag force. Another test has been
made by applying two drag models to two well-observed STEREO/SECCHI
events. As a result, we found that (1) while two speed profiles are
well fitted by the quadratic drag model, one speed profile can not be
fitted by the linear model; (2) while the physical parameters for the
quadratic model are well consistent with observations, the kinematic
viscosity for the linear model should be four orders larger than its
observed value. From this study, we conclude that the quadratic drag
model for ICME propagation should be proper than the linear drag model.
Title: Correlation between CME and Flare Parameters (with and without
Type II Bursts)
Authors: Shanmugaraju, A.; Moon, Y. -J.; Vršnak, Bojan
Bibcode: 2011SoPh..270..273S
Altcode: 2011SoPh..tmp...58S; 2011SoPh..tmp...81S
CMEs and flares are the two energetic phenomena on the Sun responsible
for generating shocks. Our main aim is to study the relation between the
physical properties of CMEs and flares associated with and without type
II radio bursts. We considered a set of 290 SOHO/LASCO CMEs associated
with GOES X-ray flares observed during the period from January 1997
to December 2000. The relationship between the flares and CMEs is
examined for the two sets i) with metric-type IIs and ii) without
metric-type IIs. Physical properties such as rise time, duration,
and strength of the flares and width, speed, and acceleration of CMEs
are considered. We examined the energy relationship and temporal
relationship between the CMEs and flares. First, all the events in
each group were considered, and then the limb events in each group
were considered separately. While there is a relationship between the
temporal characteristics of flares and CME properties in the case of
with-type IIs, it is absent in the case of all without-type IIs. Among
all the relations studied, the correlation between flare duration
and CME properties is found to be highly significant compared to the
other relations. Also, the relationship between flare strength and CME
speed found in the with-type II events is absent in the case of all
without-type II events. However, when the limb without-type II events
(with reduced time window between flare and CME) are studied separately,
we found the energy relationship and the temporal relationship.
Title: Equatorial coronal holes, solar wind high-speed streams,
and their geoeffectiveness
Authors: Verbanac, G.; Vršnak, B.; Veronig, A.; Temmer, M.
Bibcode: 2011A&A...526A..20V
Altcode:
Context. Solar wind high-speed streams (HSSs), originating in equatorial
coronal holes (CHs), are the main driver of the geomagnetic activity in
the late-declining phase of the solar cycle.
Aims: We analyze
correlations between CH characteristics, HSSs parameters, and the
geomagnetic activity indices, to establish empirical relationships
that would provide forecasting of the solar wind characteristics,
as well as the effect of HSSs on the geomagnetic activity in periods
when the effect of coronal mass ejections is low.
Methods:
We apply the cross-correlation analysis to the fractional CH area
(CH) measured between central meridian distances ±10°, solar wind
parameters (flow velocity V, proton density n, temperature T, and
the magnetic field B), and the geomagnetic indices Dst and Ap.
Results: The cross-correlation analysis reveals a high degree
of correlation between all studied parameters. In particular, we
show that the Ap index is considerably more sensitive to HSS and CH
characteristics than Dst. The Ap and Dst indices are most tightly
correlated with the solar wind parameter BV2.
Conclusions: From the point of view of space weather, the most
important result is that the established empirical relationships
provide a few-days-in-advance forecasting of the HSS characteristics
and the related geomagnetic activity at the six-hour resolution. Appendices, Figs. 9-14, and table 4 are only available in electronic
form at http://www.aanda.org
Title: Case Study of Four Homologous Large-scale Coronal Waves
Observed on 2010 April 28 and 29
Authors: Kienreich, I. W.; Veronig, A. M.; Muhr, N.; Temmer, M.;
Vršnak, B.; Nitta, N.
Bibcode: 2011ApJ...727L..43K
Altcode: 2011arXiv1101.5232K
On 2010 April 28 and 29, the Solar TErrestrial Relations Observatory
B/Extreme Ultraviolet Imager observed four homologous large-scale
coronal waves, the so-called EIT-waves, within 8 hr. All waves emerged
from the same source active region, were accompanied by weak flares and
faint coronal mass ejections, and propagated into the same direction
at constant velocities in the range of ~220-340 km s-1. The
last of these four coronal wave events was the strongest and fastest,
with a velocity of 337 ± 31 km s-1 and a peak perturbation
amplitude of ~1.24, corresponding to a magnetosonic Mach number of M
ms ~ 1.09. The magnetosonic Mach numbers and velocities of
the four waves are distinctly correlated, suggestive of the nonlinear
fast-mode magnetosonic wave nature of the events. We also found a
correlation between the magnetic energy buildup times and the velocity
and magnetosonic Mach number.
Title: Inventorying the Solar System with LSST
Authors: Jones, R. Lynne; Brown, M. E.; Abel, P. A.; Chesley, S. R.;
Durech, J.; Fernandez, Y. R.; Harris, A. W.; Holman, M. J.; Ivezic,
Z.; Jedicke, R.; Kaasalainen, M.; Kaib, N. A.; Knevezic, Z.; Milani,
A.; Park, A.; Ragozzine, D.; Ridgway, S. T.; Trilling, D. E.; Vrsnak,
B.; LSST Solar System Science Collaboration
Bibcode: 2011AAS...21725210J
Altcode: 2011BAAS...4325210J
Near the ecliptic, LSST is expected to detect approximately 4000
moving objects per 9.6 square degree field of view. Each pointing
(with mag limits r 24.5) will be revisited within 30-45 minutes,
several times per month. Automated software will provide the means
to link these individual detections into orbits. The result will be
publicly available catalogs of hundreds of thousands of NEOs and Jupiter
Trojans, millions of asteroids, tens of thousands of TNOs, as well as
thousands of other objects such as comets and irregular satellites of
the major planets. These catalogs will contain final orbits as well
as the individual (multi-color) observations, calibrated to high
precisision in astrometry ( 50 mas) and photometry ( 0.01 mag). With these large datasets, LSST will provide new insights into
links between populations of moving objects, such as the relationship
between Main Belt asteroids and NEOs. Models of solar system evolution,
such as the Nice model, can be tested against an order of magnitude
larger statistical sample, providing much stronger constraints than
are currently possible. With high accuracy multi-color photometry,
lightcurves and colors will be determined for a significant fraction
of the objects detected. Using sparse lightcurve inversion, spin state
and shape models will be derived for tens of thousands of main belt
asteroids. Derivation of proper elements for Main Belt asteroids
will greatly enlarge existing asteroid families, particularly at
smaller sizes, and precise color information will facilitate further
division. More unpredictable discoveries, such as the potential for
observing a real-time collision, could lead to new insights into
physical properties, the size distribution at very small diameters,
the orbital evolution of asteroids, or the discovery of possible space
mission targets.
Title: Application of data assimilation to solar wind forecasting
models
Authors: Innocenti, M.; Lapenta, G.; Vrsnak, B.; Temmer, M.; Veronig,
A.; Bettarini, L.; Lee, E.; Markidis, S.; Skender, M.; Crespon, F.;
Skandrani, C.; Soteria Space-Weather Forecast; Data Assimilation Team
Bibcode: 2010AGUFMSM54A..08I
Altcode:
Data Assimilation through Kalman filtering [1,2] is a powerful
statistical tool which allows to combine modeling and observations
to increase the degree of knowledge of a given system. We apply this
technique to the forecast of solar wind parameters (proton speed, proton
temperature, absolute value of the magnetic field and proton density) at
1 AU, using the model described in [3] and ACE data as observations. The
model, which relies on GOES 12 observations of the percentage of the
meridional slice of the sun covered by coronal holes, grants 1-day and
6-hours in advance forecasts of the aforementioned quantities in quiet
times (CMEs are not taken into account) during the declining phase
of the solar cycle and is tailored for specific time intervals. We
show that the application of data assimilation generally improves
the quality of the forecasts during quiet times and, more notably,
extends the periods of applicability of the model, which can now provide
reliable forecasts also in presence of CMEs and for periods other than
the ones it was designed for. Acknowledgement: The research leading
to these results has received funding from the European Commission’s
Seventh Framework Programme (FP7/2007-2013) under the grant agreement
N. 218816 (SOTERIA project: http://www.soteria-space.eu). References:
[1] R. Kalman, J. Basic Eng. 82, 35 (1960); [2] G. Welch and G. Bishop,
Technical Report TR 95-041, University of North Carolina, Department
of Computer Science (2001); [3] B. Vrsnak, M. Temmer, and A. Veronig,
Solar Phys. 240, 315 (2007).
Title: On the Origin of the Solar Moreton Wave of 2006 December 6
Authors: Balasubramaniam, K. S.; Cliver, E. W.; Pevtsov, A.; Temmer,
M.; Henry, T. W.; Hudson, H. S.; Imada, S.; Ling, A. G.; Moore, R. L.;
Muhr, N.; Neidig, D. F.; Petrie, G. J. D.; Veronig, A. M.; Vršnak,
B.; White, S. M.
Bibcode: 2010ApJ...723..587B
Altcode:
We analyzed ground- and space-based observations of the eruptive flare
(3B/X6.5) and associated Moreton wave (~850 km s-1 ~270°
azimuthal span) of 2006 December 6 to determine the wave driver—either
flare pressure pulse (blast) or coronal mass ejection (CME). Kinematic
analysis favors a CME driver of the wave, despite key gaps in coronal
data. The CME scenario has a less constrained/smoother velocity versus
time profile than is the case for the flare hypothesis and requires an
acceleration rate more in accord with observations. The CME picture is
based, in part, on the assumption that a strong and impulsive magnetic
field change observed by a GONG magnetograph during the rapid rise phase
of the flare corresponds to the main acceleration phase of the CME. The
Moreton wave evolution tracks the inferred eruption of an extended
coronal arcade, overlying a region of weak magnetic field to the west
of the principal flare in NOAA active region 10930. Observations of
Hα foot point brightenings, disturbance contours in off-band Hα
images, and He I 10830 Å flare ribbons trace the eruption from 18:42
to 18:44 UT as it progressed southwest along the arcade. Hinode EIS
observations show strong blueshifts at foot points of this arcade
during the post-eruption phase, indicating mass outflow. At 18:45
UT, the Moreton wave exhibited two separate arcs (one off each flank
of the tip of the arcade) that merged and coalesced by 18:47 UT to
form a single smooth wave front, having its maximum amplitude in
the southwest direction. We suggest that the erupting arcade (i.e.,
CME) expanded laterally to drive a coronal shock responsible for the
Moreton wave. We attribute a darkening in Hα from a region underlying
the arcade to absorption by faint unresolved post-eruption loops.
Title: Modeling UV and X-ray Emission in a Post-coronal Mass Ejection
Current Sheet
Authors: Ko, Yuan-Kuen; Raymond, John C.; Vršnak, Bojan; Vujić, Eugen
Bibcode: 2010ApJ...722..625K
Altcode: 2010arXiv1008.1732K
A post-coronal mass ejection (CME) current sheet (CS) is a
common feature developed behind an erupting flux rope in CME
models. Observationally, white light observations have recorded
many occurrences of a thin ray appearing behind a CME eruption
that closely resembles a post-CME CS in its spatial correspondence
and morphology. UV and X-ray observations further strengthen this
interpretation by the observations of high-temperature emission at
locations consistent with model predictions. The next question then
becomes whether the properties inside a post-CME CS predicted by a
model agree with observed properties. In this work, we assume that
the post-CME CS is a consequence of Petschek-like reconnection and
that the observed ray-like structure is bounded by a pair of slow mode
shocks developed from the reconnection site. We perform time-dependent
ionization calculations and model the UV line emission. We find that
such a model is consistent with SOHO/UVCS observations of the post-CME
CS. The change of Fe XVIII emission in one event implies an inflow
speed of ~10 km s-1 and a corresponding reconnection rate
of MA ~ 0.01. We calculate the expected X-ray emission for
comparison with X-ray observations by Hinode/XRT, as well as the ionic
charge states as would be measured in situ at 1 AU. We find that the
predicted count rate for Hinode/XRT agrees with what was observed in
a post-CME CS on 2008 April 9, and the predicted ionic charge states
are consistent with high ionization states commonly measured in the
interplanetary CMEs. The model results depend strongly on the physical
parameters in the ambient corona, namely the coronal magnetic field,
the electron density, and temperature during the CME event. It is
crucial to obtain these ambient coronal parameters and as many facets
of the CS properties as possible by observational means so that the
post-CME CS models can be scrutinized more effectively.
Title: Type-II Bursts in Meter and Deca - Hectometer Wavelengths
and Their Relation to Flares and CMEs: II
Authors: Prakash, O.; Umapathy, S.; Shanmugaraju, A.; Pappa kalaivani,
P.; Vršnak, Bojan
Bibcode: 2010SoPh..266..135P
Altcode: 2010SoPh..tmp..148P
A study of the relationship between 38 type-II bursts recorded in meter
and deca-hectometer (hereinafter m and DH) wavelength range and the
associated flares and CMEs observed during the years 2000 - 2005 was
carried out by Prakash et al. (2009). These events were divided into two
classes: i) Class I, representing events where DH-type-II bursts are not
a continuation of m-type-II bursts and ii) Class II, where DH-type-II
bursts are a continuation of m-type-II bursts. In the present work,
we extend the analysis of this sample of 38 events in three different
steps: i) statistical properties of m- and DH-type-II bursts; ii)
analysis of time lags between onsets of flares and CMEs associated with
type-II bursts; and iii) statistical properties and relation between
flares and CMEs of Class I and Class II events. We found a significant
difference between the properties of m- and DH-type-II bursts of Class
I and Class II events. For example, there are significant differences
in starting and ending frequencies, bandwidth and speed. From the
time delay analysis, we found the following. i) In 64% of Class I
events, flares start after the onset of CMEs and the remaining 36%
of flares start before the onset of CMEs. On the other hand, in the
case of Class II events, the values are 83% and 17%, respectively. ii)
The difference between the mean values of delay between flare start
and DH start has high statistical significance (probability P of null
hypothesis <1%). The time delays between the start of m-type-II burst
and the CME onset are considerably larger for Class I events (P=7%)
than Class II events. iii) There are notable differences in: (a) delay
between the flare and CME onset times (P<1%); (b) flare rise time of
Class I and Class II events (P<5%). iv) While the flare rise time is
well correlated with the lag between the flare start and the CME onset
in Class I events, there is no such correlation for Class II events.
Title: Observations of Chromospheric Flare Re-brightenings
Authors: Miklenic, C. H.; Veronig, A. M.; Vršnak, B.; Bárta, M.
Bibcode: 2010ApJ...719.1750M
Altcode:
We investigate an active region that produced three C-class flares
and one M-class flare within 2.5 hr. The morphology and location of
the C-flares indicate that these events constitute a set of homologous
flares. Radio observations indicate the occurrence of a downward-moving
plasmoid during the impulsive phase of the M flare. We use TRACE
1700 Å filtergrams and SOHO Michelson Doppler Imager magnetograms
to examine the character of the UV brightenings; i.e., we search
for re-brightenings of former flare areas both across the series of
events and within one and the same event. We find that essentially the
same footpoints re-brighten in each C flare. Based on the progression
of both the derived magnetic flux change rate and the observed Radio
Solar Telescope Network microwave emission, we speculate about a further
re-brightening during the decay phase of the M flare as a further member
of the series of homologous flares. We conclude that the "postflare"
field is driven to repeated eruption by continuous, shear-increasing,
horizontal, photospheric flows, as one end of the involved magnetic
arcade is anchored in the penumbra of a large sunspot. The observed
motion pattern of the UV kernels indicates that the arcade evolves
during the series of events from a both highly sheared and heavily
entangled state to a still sheared but more organized state.
Title: Multiwavelength Imaging and Spectroscopy of Chromospheric
Evaporation in an M-class Solar Flare
Authors: Veronig, A. M.; Rybák, J.; Gömöry, P.; Berkebile-Stoiser,
S.; Temmer, M.; Otruba, W.; Vršnak, B.; Pötzi, W.; Baumgartner, D.
Bibcode: 2010ApJ...719..655V
Altcode: 2010arXiv1007.0930V
We study spectroscopic observations of chromospheric evaporation mass
flows in comparison with the energy input by electron beams derived
from hard X-ray (HXR) data for the white-light M2.5 flare of 2006 July
6. The event was captured in high-cadence spectroscopic observing mode
by SOHO/CDS combined with high-cadence imaging at various wavelengths
in the visible, extreme ultraviolet, and X-ray domain during the joint
observing campaign JOP171. During the flare peak, we observe downflows
in the He I and O V lines formed in the chromosphere and transition
region, respectively, and simultaneous upflows in the hot coronal
Si XII line. The energy deposition rate by electron beams derived
from RHESSI HXR observations is suggestive of explosive chromospheric
evaporation, consistent with the observed plasma motions. However, for
a later distinct X-ray burst, where the site of the strongest energy
deposition is exactly located on the Coronal Diagnostics Spectrometer
(CDS) slit, the situation is intriguing. The O V transition region
line spectra show the evolution of double components, indicative of
the superposition of a stationary plasma volume and upflowing plasma
elements with high velocities (up to 280 km s-1) in single
CDS pixels on the flare ribbon. However, the energy input by electrons
during this period is too small to drive explosive chromospheric
evaporation. These unexpected findings indicate that the flaring
transition region is much more dynamic, complex, and fine structured
than is captured in single-loop hydrodynamic simulations.
Title: Coronal Shocks Associated with Impulsive and Decaying Phases
of Solar Flares
Authors: Suresh, K.; Umapathy, S.; Shanmugaraju, A.; Vršnak, B.
Bibcode: 2010SoPh..264..353S
Altcode: 2010SoPh..tmp..108S
We have analyzed a set of 147 metric Type II radio bursts observed by
Culgoora radio spectrograph from November 1997 to December 2006. These
events were divided into two sets: The first subset contains Type
II events that started during the impulsive phase of the associated
solar flares and the second subset contains those starting during
the decaying phase of flares. Our main aim is to differentiate the
metric Type IIs, flares and coronal mass ejections (CMEs) of these
two subsets. It is found that while Type II burst characteristics
of both subsets are very similar, there are significant differences
between flare and CME properties for these two subsets. Considering
all analyzed relationships between the characteristics of Type IIs,
flares and CMEs in these two Type II subsets, we conclude that most of
the coronal shocks causing metric Type II bursts are driven by CMEs,
but that a fraction of events are probably ignited by solar flares.
Title: Origin of Coronal Shock Waves Associated with Slow Coronal
Mass Ejections
Authors: Magdalenić, J.; Marqué, C.; Zhukov, A. N.; Vršnak, B.;
Žic, T.
Bibcode: 2010ApJ...718..266M
Altcode:
We present a multiwavelength study of five coronal mass ejection/flare
events (CME/flare) and associated coronal shock waves manifested as type
II radio bursts. The study is focused on the events in which the flare
energy release, and not the associated CME, is the most probable source
of the shock wave. Therefore, we selected events associated with rather
slow CMEs (reported mean velocity below 500 km s-1). To
ensure minimal projection effects, only events related to flares
situated close to the solar limb were included in the study. We used
radio dynamic spectra, positions of radio sources observed by the
Nançay Radioheliograph, GOES soft X-ray flux measurements, Large Angle
Spectroscopic Coronagraph, and Extreme-ultraviolet Imaging Telescope
observations. The kinematics of the shock wave signatures, type II
radio bursts, were analyzed and compared with the flare evolution
and the CME kinematics. We found that the velocities of the shock
waves were significantly higher, up to one order of magnitude, than
the contemporaneous CME velocities. On the other hand, shock waves
were closely temporally associated with the flare energy release that
was very impulsive in all events. This suggests that the impulsive
increase of the pressure in the flare was the source of the shock
wave. In four events the shock wave was most probably flare-generated,
and in one event results were inconclusive due to a very close temporal
synchronization of the CME, flare, and shock.
Title: Investigations of the sensitivity of a coronal mass ejection
model (ENLIL) to solar input parameters
Authors: Falkenberg, T. V.; Vršnak, B.; Taktakishvili, A.; Odstrcil,
D.; MacNeice, P.; Hesse, M.
Bibcode: 2010SpWea...8.6004F
Altcode:
Understanding space weather is not only important for satellite
operations and human exploration of the solar system but also to
phenomena here on Earth that may potentially disturb and disrupt
electrical signals. Some of the most violent space weather effects
are caused by coronal mass ejections (CMEs), but in order to predict
the caused effects, we need to be able to model their propagation
from their origin in the solar corona to the point of interest, e.g.,
Earth. Many such models exist, but to understand the models in detail
we must understand the primary input parameters. Here we investigate
the parameter space of the ENLILv2.5b model using the CME event of 25
July 2004. ENLIL is a time-dependent 3-D MHD model that can simulate
the propagation of cone-shaped interplanetary coronal mass ejections
(ICMEs) through the solar system. Excepting the cone parameters
(radius, position, and initial velocity), all remaining parameters are
varied, resulting in more than 20 runs investigated here. The output
parameters considered are velocity, density, magnetic field strength,
and temperature. We find that the largest effects on the model output
are the input parameters of upper limit for ambient solar wind velocity,
CME density, and elongation factor, regardless of whether one's main
interest is arrival time, signal shape, or signal amplitude of the
ICME. We find that though ENLILv2.5b currently does not include the
magnetic cloud of the ICME, it replicates the signal at L1 well in the
studied event. The arrival time difference between satellite data and
the ENLILv2.5b baseline run of this study is less than 30 min.
Title: First Observations of a Dome-shaped Large-scale Coronal
Extreme-ultraviolet Wave
Authors: Veronig, A. M.; Muhr, N.; Kienreich, I. W.; Temmer, M.;
Vršnak, B.
Bibcode: 2010ApJ...716L..57V
Altcode: 2010arXiv1005.2060V
We present first observations of a dome-shaped large-scale
extreme-ultraviolet coronal wave, recorded by the Extreme Ultraviolet
Imager instrument on board STEREO-B on 2010 January 17. The main
arguments that the observed structure is the wave dome (and not the
coronal mass ejection, CME) are (1) the spherical form and sharpness of
the dome's outer edge and the erupting CME loops observed inside the
dome; (2) the low-coronal wave signatures above the limb perfectly
connecting to the on-disk signatures of the wave; (3) the lateral
extent of the expanding dome which is much larger than that of the
coronal dimming; and (4) the associated high-frequency type II burst
indicating shock formation low in the corona. The velocity of the upward
expansion of the wave dome (v ~ 650 km s-1) is larger than
that of the lateral expansion of the wave (v ~ 280 km s-1),
indicating that the upward dome expansion is driven all the time,
and thus depends on the CME speed, whereas in the lateral direction it
is freely propagating after the CME lateral expansion stops. We also
examine the evolution of the perturbation characteristics: first the
perturbation profile steepens and the amplitude increases. Thereafter,
the amplitude decreases with r -2.5 ± 0.3, the width
broadens, and the integral below the perturbation remains constant. Our
findings are consistent with the spherical expansion and decay of a
weakly shocked fast-mode MHD wave.
Title: Four decades of geomagnetic and solar activity: 1960-2001
Authors: Verbanac, Giuli; Vršnak, Bojan; Temmer, Manuela; Mandea,
Mioara; Korte, Monika
Bibcode: 2010JASTP..72..607V
Altcode:
We analyze the relationship between some space weather indices (Dst,
Ap, F10.7) and geomagnetic effects on the regional (European) scale,
over the period 1960-2001. The remaining external field signal (RES)
detected in the Northward magnetic component of the European observatory
annual means are used as an indicator of the regional geomagnetic
activity. Relationship RES-F10.7 suggests correction factors for getting
the geomagnetic annual means of the Northern component less affected by
the external sources. We have found some time lags among investigated
parameters. These delays may suggest that the Ap responds to the solar
activity in a differently than Dst and RES, Ap being more sensitive
to the high-speed streams (HSS) and the Alfvenic waves present in HSS,
while Dst and RES being more influenced by the coronal mass ejections
activity (CME).
Title: Combined STEREO/RHESSI Study of Coronal Mass Ejection
Acceleration and Particle Acceleration in Solar Flares
Authors: Temmer, M.; Veronig, A. M.; Kontar, E. P.; Krucker, S.;
Vršnak, B.
Bibcode: 2010ApJ...712.1410T
Altcode: 2010arXiv1002.3080T
Using the potential of two unprecedented missions, Solar Terrestrial
Relations Observatory (STEREO) and Reuven Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI), we study three well-observed fast coronal
mass ejections (CMEs) that occurred close to the limb together with
their associated high-energy flare emissions in terms of RHESSI hard
X-ray (HXR) spectra and flux evolution. From STEREO/EUVI and STEREO/COR1
data, the full CME kinematics of the impulsive acceleration phase up to
~4 R sun is measured with a high time cadence of <=2.5
minutes. For deriving CME velocity and acceleration, we apply and
test a new algorithm based on regularization methods. The CME maximum
acceleration is achieved at heights h <= 0.4 R sun,
and the peak velocity at h <= 2.1 R sun (in one case,
as small as 0.5 R sun). We find that the CME acceleration
profile and the flare energy release as evidenced in the RHESSI HXR flux
evolve in a synchronized manner. These results support the "standard"
flare/CME model which is characterized by a feedback relationship
between the large-scale CME acceleration process and the energy release
in the associated flare.
Title: The role of aerodynamic drag in propagation of interplanetary
coronal mass ejections
Authors: Vršnak, B.; Žic, T.; Falkenberg, T. V.; Möstl, C.;
Vennerstrom, S.; Vrbanec, D.
Bibcode: 2010A&A...512A..43V
Altcode:
Context. The propagation of interplanetary coronal mass ejections
(ICMEs) and the forecast of their arrival on Earth is one of the
central issues of space weather studies.
Aims: We investigate
to which degree various ICME parameters (mass, size, take-off speed)
and the ambient solar-wind parameters (density and velocity) affect
the ICME Sun-Earth transit time.
Methods: We study solutions of
a drag-based equation of motion by systematically varying the input
parameters. The analysis is focused on ICME transit times and 1 AU
velocities.
Results: The model results reveal that wide ICMEs
of low masses adjust to the solar-wind speed already close to the
sun, so the transit time is determined primarily by the solar-wind
speed. The shortest transit times and accordingly the highest 1 AU
velocities are related to narrow and massive ICMEs (i.e. high-density
eruptions) propagating in high-speed solar wind streams. We apply the
model to the Sun-Earth event associated with the CME of 25 July 2004
and compare the results with the outcome of the numerical MHD modeling.
Title: Quasi-Periodic Oscillations in Lasco Coronal Mass Ejection
Speeds
Authors: Shanmugaraju, A.; Moon, Y. -J.; Cho, K. -S.; Bong, S. C.;
Gopalswamy, N.; Akiyama, S.; Yashiro, S.; Umapathy, S.; Vrsnak, B.
Bibcode: 2010ApJ...708..450S
Altcode:
Quasi-periodic oscillations in the speed profile of coronal mass
ejections (CMEs) in the radial distance range 2-30 solar radii are
studied. We considered the height-time data of the 307 CMEs recorded
by the Large Angle and Spectrometric Coronagraph (LASCO) during 2005
January-March. In order to study the speed-distance profile of the CMEs,
we have used only 116 events for which there are at least 10 height-time
measurements made in the LASCO field of view. The instantaneous CME
speed is estimated using a pair of height-time data points, providing
the speed-distance profile. We found quasi-periodic patterns in at
least 15 speed-distance profiles, where the speed amplitudes are larger
than the speed errors. For these events we have determined the speed
amplitude and period of oscillations. The periods of quasi-periodic
oscillations are found in the range 48-240 minutes, tending to
increase with height. The oscillations have similar properties as
those reported by Krall et al., who interpreted them in terms of the
flux-rope model. The nature of forces responsible for the motion of
CMEs and their oscillations are discussed.
Title: Inventorying the Solar System with LSST
Authors: Jones, R. Lynne; Chesley, S. R.; Abell, P. A.; Brown, M. E.;
Durech, J.; Fernandez, Y. R.; Harris, A. W.; Holman, M. J.; Ivezic,
Z.; Jedicke, R.; Kaasaleinen, M.; Kaib, N. A.; Knevezic, Z.; Milani,
A.; Parker, A.; Ridgway, S. T.; Trilling, D. E.; Vrsnak, B.; LSST
Solar System Science Collaboration
Bibcode: 2010AAS...21540107J
Altcode: 2010BAAS...42..218J
LSST's extremely wide sky coverage (>30,000 square degrees),
coupled with a faint limiting magnitude (r 24.7 per image), and a
rapid observational cadence -- each field is observed twice per night,
4-5 times each month -- result in a survey telescope with powerful
potential for detecting small moving objects. Near the ecliptic,
LSST is expected to detect approximately 4000 moving objects per 9.6
square degree field of view; automated software will provide the
means to link these individual detections into orbits. The result
will be catalogs of hundreds of thousands of NEOs and Jupiter
Trojans, millions of asteroids, tens of thousands of TNOs, and
thousands of other objects such as comets and irregular satellites
of the major planets. These catalogs will be publicly available,
both final orbits and the underlying multi-color observations, with
highly accurate measurements in astrometry ( 50 mas) and photometry (
0.01-0.02 mag). With these large datasets, LSST will provide new
insights into links between populations of moving objects, such as the
relationship between Main Belt asteroids and NEOs. Models of solar
system evolution, such as the Nice model, can be tested against an
order of magnitude larger statistical sample, providing much stronger
constraints than are currently possible. Detection of populations of
objects beyond Neptune at a wide range of ecliptic latitudes as well as
a well-characterized measurement of cometary populations will permit
measurements of the nature of the inner and outer Oort cloud. Using
high accuracy multicolor photometry, lightcurves and colors will be
determined for a significant fraction of the objects detected. Through
sparse lightcurve inversion, spin state and shape models will be
derived for tens of thousands of main belt asteroids. Derivation of
proper elements for Main Belt asteroids will greatly enlarge existing
asteroid families, particularly at smaller sizes, and precise color
information will facilitate further divisions.
Title: Analysis of a Global Moreton Wave Observed on 2003 October 28
Authors: Muhr, N.; Vršnak, B.; Temmer, M.; Veronig, A. M.;
Magdalenić, J.
Bibcode: 2010ApJ...708.1639M
Altcode: 2009arXiv0911.4405M
We study the well-pronounced Moreton wave that occurred in association
with the X17.2 flare/CME event of 2003 October 28. This Moreton wave is
striking for its global propagation and two separate wave centers, which
implies that two waves were launched simultaneously. The mean velocity
of the Moreton wave, tracked within different sectors of propagation
direction, lies in the range of v ≈ 900-1100 km s-1 with
two sectors showing wave deceleration. The perturbation profile analysis
of the wave indicates amplitude growth followed by amplitude weakening
and broadening of the perturbation profile, which is consistent with
a disturbance first driven and then evolving into a freely propagating
wave. The Extreme-Ultraviolet Imaging Telescope wave front is found to
lie on the same kinematical curve as the Moreton wave fronts indicating
that both are different signatures of the same physical process. Bipolar
coronal dimmings are observed on the same opposite east-west edges of
the active region as the Moreton wave ignition centers. The radio type
II source, which is cospatially located with the first wave front,
indicates that the wave was launched from an extended source region
(gsim60 Mm). These findings suggest that the Moreton wave is initiated
by the coronal mass ejection expanding flanks.
Title: Flare-generated coronal shock on 14 November 2005
Authors: Magdalenic, Jasmina; Marque, Christophe; Zhukov, Andrei;
Veronig, Astrid; Vrsnak, Bojan
Bibcode: 2010cosp...38.1798M
Altcode: 2010cosp.meet.1798M
Origin of coronal shock waves is still not completely understood. Since
the flare impulsive phase and the acceleration phase of a CME are
usually well synchronized, it is difficult to give a conclusive
answer on the shock wave origin in flare/CME events. We present
multiwave-length study of a shock wave associated with the flare event
recorded on 14 November 2005. The evolution of the shock wave signature
-type II radio burst -is analysed using dynamic spectra recorded by the
Green Bank Solar Radio Bursts Spectrometer and Nançay Radioheliograph
imaging. The observations of the plasma dynamics in the low and high
corona were provided by EIT and LASCO instruments onboard SOHO. The
strong type II emission starts at unusually high frequency of 700
MHz. The obtained values for the shock velocity, Alfven velocity and
Alfven Mach number are in the range of typical shock parameters. The
shock wave was closely associated with the impulsive phase of the
compact M3.9 flare in the NOAA AR 10822 (located at S06E60). The
short impulsive phase of the flare (4 minutes), suggests that a
strong pressure pulse was ignited by the flare. Additionally, RHESSI
observations show compact event of a rather high density and high
temperature which gives indication of a strong, impulsive increase
of pressure in the small flare loop. SOHO/LASCO observations do not
show any CME associated with this event. Since the active region
is rather close to the limb, the possibility that the corresponding
CME is not observed due to the unfavorable geometry is unlikely. We
therefore conclude that the shock wave recorded on 14 November 2005
was a blast wave launched by the impulsive energy release in the course
of the flare.
Title: Study of the kinematics and driver of the global Moreton wave
observed on 2003 October 28
Authors: Muhr, Mmag. Nicole; Vrsnak, Bojan; Temmer, Manuela; Veronig,
Astrid; Magdalenic, Jasmina
Bibcode: 2010cosp...38.1844M
Altcode: 2010cosp.meet.1844M
We analyze the evolution and kinematics of the fast, globally
propagating Moreton wave of 2003 October 28 associated with the extreme
X17.2 solar flare/CME event. This Moreton wave is distinct due to its
strengths and azimuthal span of span 360. We study the wave kinematics
in different propagation directions, and compare it with the following
associated phenomena: EIT wave, coronal dimmings, fast halo CME, flare,
and type II burst. The sectoral analysis yield mean velocity values
in the range 900-1000 km/s; two sectors show wave deceleration. The
perturbation profile evolution indicates an amplitude growth followed
by amplitude weakening and broadening, which is consistent with a
disturbance first driven and then evolving into a freely propagating
wave. We find two `'radiant points" for the Moreton wave fronts on
opposite east-west edges of the source region, roughly co-spatial with
the bipolar coronal dimming. The co-spatiality of the associated radio
type II burst source and the first Moreton wave fronts indicate that
the wave was launched from an extended region. These findings indicate
that the wave is initiated by the CME expanding flanks.
Title: LSST Science Book, Version 2.0
Authors: LSST Science Collaboration; Abell, Paul A.; Allison, Julius;
Anderson, Scott F.; Andrew, John R.; Angel, J. Roger P.; Armus, Lee;
Arnett, David; Asztalos, S. J.; Axelrod, Tim S.; Bailey, Stephen;
Ballantyne, D. R.; Bankert, Justin R.; Barkhouse, Wayne A.; Barr,
Jeffrey D.; Barrientos, L. Felipe; Barth, Aaron J.; Bartlett, James
G.; Becker, Andrew C.; Becla, Jacek; Beers, Timothy C.; Bernstein,
Joseph P.; Biswas, Rahul; Blanton, Michael R.; Bloom, Joshua S.;
Bochanski, John J.; Boeshaar, Pat; Borne, Kirk D.; Bradac, Marusa;
Brandt, W. N.; Bridge, Carrie R.; Brown, Michael E.; Brunner, Robert
J.; Bullock, James S.; Burgasser, Adam J.; Burge, James H.; Burke,
David L.; Cargile, Phillip A.; Chandrasekharan, Srinivasan; Chartas,
George; Chesley, Steven R.; Chu, You-Hua; Cinabro, David; Claire,
Mark W.; Claver, Charles F.; Clowe, Douglas; Connolly, A. J.; Cook,
Kem H.; Cooke, Jeff; Cooray, Asantha; Covey, Kevin R.; Culliton,
Christopher S.; de Jong, Roelof; de Vries, Willem H.; Debattista,
Victor P.; Delgado, Francisco; Dell'Antonio, Ian P.; Dhital, Saurav;
Di Stefano, Rosanne; Dickinson, Mark; Dilday, Benjamin; Djorgovski,
S. G.; Dobler, Gregory; Donalek, Ciro; Dubois-Felsmann, Gregory;
Durech, Josef; Eliasdottir, Ardis; Eracleous, Michael; Eyer, Laurent;
Falco, Emilio E.; Fan, Xiaohui; Fassnacht, Christopher D.; Ferguson,
Harry C.; Fernandez, Yanga R.; Fields, Brian D.; Finkbeiner, Douglas;
Figueroa, Eduardo E.; Fox, Derek B.; Francke, Harold; Frank, James
S.; Frieman, Josh; Fromenteau, Sebastien; Furqan, Muhammad; Galaz,
Gaspar; Gal-Yam, A.; Garnavich, Peter; Gawiser, Eric; Geary, John;
Gee, Perry; Gibson, Robert R.; Gilmore, Kirk; Grace, Emily A.; Green,
Richard F.; Gressler, William J.; Grillmair, Carl J.; Habib, Salman;
Haggerty, J. S.; Hamuy, Mario; Harris, Alan W.; Hawley, Suzanne L.;
Heavens, Alan F.; Hebb, Leslie; Henry, Todd J.; Hileman, Edward;
Hilton, Eric J.; Hoadley, Keri; Holberg, J. B.; Holman, Matt J.;
Howell, Steve B.; Infante, Leopoldo; Ivezic, Zeljko; Jacoby, Suzanne
H.; Jain, Bhuvnesh; R; Jedicke; Jee, M. James; Garrett Jernigan,
J.; Jha, Saurabh W.; Johnston, Kathryn V.; Jones, R. Lynne; Juric,
Mario; Kaasalainen, Mikko; Styliani; Kafka; Kahn, Steven M.; Kaib,
Nathan A.; Kalirai, Jason; Kantor, Jeff; Kasliwal, Mansi M.; Keeton,
Charles R.; Kessler, Richard; Knezevic, Zoran; Kowalski, Adam;
Krabbendam, Victor L.; Krughoff, K. Simon; Kulkarni, Shrinivas;
Kuhlman, Stephen; Lacy, Mark; Lepine, Sebastien; Liang, Ming;
Lien, Amy; Lira, Paulina; Long, Knox S.; Lorenz, Suzanne; Lotz,
Jennifer M.; Lupton, R. H.; Lutz, Julie; Macri, Lucas M.; Mahabal,
Ashish A.; Mandelbaum, Rachel; Marshall, Phil; May, Morgan; McGehee,
Peregrine M.; Meadows, Brian T.; Meert, Alan; Milani, Andrea; Miller,
Christopher J.; Miller, Michelle; Mills, David; Minniti, Dante; Monet,
David; Mukadam, Anjum S.; Nakar, Ehud; Neill, Douglas R.; Newman,
Jeffrey A.; Nikolaev, Sergei; Nordby, Martin; O'Connor, Paul; Oguri,
Masamune; Oliver, John; Olivier, Scot S.; Olsen, Julia K.; Olsen,
Knut; Olszewski, Edward W.; Oluseyi, Hakeem; Padilla, Nelson D.;
Parker, Alex; Pepper, Joshua; Peterson, John R.; Petry, Catherine;
Pinto, Philip A.; Pizagno, James L.; Popescu, Bogdan; Prsa, Andrej;
Radcka, Veljko; Raddick, M. Jordan; Rasmussen, Andrew; Rau, Arne; Rho,
Jeonghee; Rhoads, James E.; Richards, Gordon T.; Ridgway, Stephen
T.; Robertson, Brant E.; Roskar, Rok; Saha, Abhijit; Sarajedini,
Ata; Scannapieco, Evan; Schalk, Terry; Schindler, Rafe; Schmidt,
Samuel; Schmidt, Sarah; Schneider, Donald P.; Schumacher, German;
Scranton, Ryan; Sebag, Jacques; Seppala, Lynn G.; Shemmer, Ohad;
Simon, Joshua D.; Sivertz, M.; Smith, Howard A.; Allyn Smith, J.;
Smith, Nathan; Spitz, Anna H.; Stanford, Adam; Stassun, Keivan G.;
Strader, Jay; Strauss, Michael A.; Stubbs, Christopher W.; Sweeney,
Donald W.; Szalay, Alex; Szkody, Paula; Takada, Masahiro; Thorman,
Paul; Trilling, David E.; Trimble, Virginia; Tyson, Anthony; Van
Berg, Richard; Vanden Berk, Daniel; VanderPlas, Jake; Verde, Licia;
Vrsnak, Bojan; Walkowicz, Lucianne M.; Wandelt, Benjamin D.; Wang,
Sheng; Wang, Yun; Warner, Michael; Wechsler, Risa H.; West, Andrew
A.; Wiecha, Oliver; Williams, Benjamin F.; Willman, Beth; Wittman,
David; Wolff, Sidney C.; Wood-Vasey, W. Michael; Wozniak, Przemek;
Young, Patrick; Zentner, Andrew; Zhan, Hu
Bibcode: 2009arXiv0912.0201L
Altcode:
A survey that can cover the sky in optical bands over wide fields to
faint magnitudes with a fast cadence will enable many of the exciting
science opportunities of the next decade. The Large Synoptic Survey
Telescope (LSST) will have an effective aperture of 6.7 meters and an
imaging camera with field of view of 9.6 deg^2, and will be devoted
to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each
pointing will be imaged 2000 times with fifteen second exposures in six
broad bands from 0.35 to 1.1 microns, to a total point-source depth
of r~27.5. The LSST Science Book describes the basic parameters of
the LSST hardware, software, and observing plans. The book discusses
educational and outreach opportunities, then goes on to describe
a broad range of science that LSST will revolutionize: mapping the
inner and outer Solar System, stellar populations in the Milky Way and
nearby galaxies, the structure of the Milky Way disk and halo and other
objects in the Local Volume, transient and variable objects both at low
and high redshift, and the properties of normal and active galaxies at
low and high redshift. It then turns to far-field cosmological topics,
exploring properties of supernovae to z~1, strong and weak lensing,
the large-scale distribution of galaxies and baryon oscillations, and
how these different probes may be combined to constrain cosmological
models and the physics of dark energy.
Title: Relative Kinematics of the Leading Edge and the Prominence
in Coronal Mass Ejections
Authors: Maričić, Darije; Vršnak, Bojan; Roša, Dragan
Bibcode: 2009SoPh..260..177M
Altcode:
We present a statistical analysis of the relationship between the
kinematics of the leading edge and the eruptive prominence in coronal
mass ejections (CMEs). We study the acceleration phase of 18 CMEs in
which kinematics was measured from the pre-eruption stage up to the
post-acceleration phase. In all CMEs, the three part structure (the
leading edge, the cavity, and the prominence) was clearly recognizable
from early stages of the eruption. The data show a distinct correlation
between the duration of the leading edge (LE) acceleration and eruptive
prominence (EP) acceleration. In the majority of events (78%) the
acceleration phase onset of the LE is very closely synchronized (within
± 20 min) with the acceleration of EP. However, in two events the LE
acceleration started significantly earlier than the EP acceleration
(> 50 min), and in two events the EP acceleration started earlier
than the LE acceleration (> 40 min). The average peak acceleration of
LEs (281 m s−2) is about two times larger than the average
peak acceleration of EPs (136 m s−2). For the first time,
our results quantitatively demonstrate the level of synchronization of
the acceleration phase of LE and EP in a rather large sample of events,
i.e., we quantify how often the eruption develops in a "self-similar"
manner.
Title: Analytic Modeling of the Moreton Wave Kinematics
Authors: Temmer, M.; Vršnak, B.; Žic, T.; Veronig, A. M.
Bibcode: 2009ApJ...702.1343T
Altcode: 2009arXiv0908.3746T
The issue whether Moreton waves are flare-ignited or coronal mass
ejection (CME)-driven, or a combination of both, is still a matter of
debate. We develop an analytical model describing the evolution of a
large-amplitude coronal wave emitted by the expansion of a circular
source surface in order to mimic the evolution of a Moreton wave. The
model results are confronted with observations of a strong Moreton
wave observed in association with the X3.8/3B flare/CME event from
2005 January 17. Using different input parameters for the expansion
of the source region, either derived from the real CME observations
(assuming that the upward moving CME drives the wave), or synthetically
generated scenarios (expanding flare region, lateral expansion of the
CME flanks), we calculate the kinematics of the associated Moreton
wave signature. Those model input parameters are determined which
fit the observed Moreton wave kinematics best. Using the measured
kinematics of the upward moving CME as the model input, we are not able
to reproduce the observed Moreton wave kinematics. The observations
of the Moreton wave can be reproduced only by applying a strong and
impulsive acceleration for the source region expansion acting in a
piston mechanism scenario. Based on these results we propose that the
expansion of the flaring region or the lateral expansion of the CME
flanks is more likely the driver of the Moreton wave than the upward
moving CME front.
Title: Type II bursts in Meter and Decameter - Hectometer Wavelength
Ranges and Their Relation to Flares and CMEs
Authors: Prakash, O.; Umapathy, S.; Shanmugaraju, A.; Vršnak, Bojan
Bibcode: 2009SoPh..258..105P
Altcode:
Statistical analysis of the relationship between type II radio
bursts appearing in the metric (m) and decameter-to-hectometer
(DH) wavelength ranges is presented. The associated X-ray flares
and coronal mass ejections (CMEs) are also reported. The sample is
divided into two classes using the frequency-drift plots: Class I,
representing those events where DH-type-II bursts are not continuation
of m-type-II bursts and Class II, where the DH-type-II bursts are
extensions of m-type-II bursts. Our study consists of three steps:
i) comparison of characteristics of the Class I and II events; ii)
correlation of m-type-II and DH-type-II burst characteristics with
X-ray flare properties and iii) correlation of m-type-II and DH-type-II
burst characteristics with CME properties. We have found no clear
correlation between properties of m-type-II bursts and DH-type-II
bursts. For example, there is no correlation between drift rates
of m-type-II bursts and DH-type-II bursts. Similarly there is no
correlation between their starting frequencies. In Class I events we
found correlations between X-ray flare characteristics and properties of
m-type-II bursts and there is no correlation between flare parameters
and DH-type-II bursts. On the other hand, the correlation between CME
parameters and m-type-II bursts is very weak, but it is good for CME
parameters and DH-type-II bursts. These results indicate that Class
I m-type-II bursts are related to the energy releases in flares,
whereas DH-type-II bursts tend to be related to CMEs. On the contrary,
for Class II events in the case of m-type-II and DH-type-II bursts we
have found no clear correlation between both flare and CMEs.
Title: Radial Evolution of Well-Observed Slow CMEs in the Distance
Range 2 - 30 R ⊙
Authors: Shanmugaraju, A.; Moon, Y. -J.; Vrsnak, Bojan; Vrbanec, Dijana
Bibcode: 2009SoPh..257..351S
Altcode:
We performed a detailed analysis of 27 slow coronal mass ejections
(CMEs) whose heights were measured in at least 30 coronagraphic images
and were characterized by a high quality index (≥4). Our primary aim
was to study the radial evolution of these CMEs and their properties
in the range 2 - 30 solar radii. The instantaneous speeds of CMEs
were calculated by using successive height - time data pairs. The
obtained speed - distance profiles [v(R)] are fitted by a power law v
= a(R−b)c. The power-law indices are found to be in the
ranges a=30 - 386, b=1.95 - 3.92, and c=0.03 - 0.79. The power-law
exponent c is found to be larger for slower and narrower CMEs. With
the exception of two events that had approximately constant velocity,
all events were accelerating. The majority of accelerating events
shows a v(R) profile very similar to the solar-wind profile deduced
by Sheeley et al. (Astrophys. J.484, 472, 1997). This indicates that
the dynamics of most slow CMEs are dominated by the solar wind drag.
Title: Analyses of magnetic field structures for active region 10720
using a data-driven 3D MHD model
Authors: Wu, S. T.; Wang, A. H.; Gary, G. Allen; Kucera, Ales; Rybak,
Jan; Liu, Yang; Vrśnak, Bojan; Yurchyshyn, Vasyl
Bibcode: 2009AdSpR..44...46W
Altcode:
In order to understand solar eruptive events (flares and CMEs) we
need to investigate the changes at the solar surface. Thus, we use
a data-driven, three-dimensional magnetohydrodynamic (MHD) model to
analyze a flare and coronal mass ejection productive active region,
AR 10720 on January 15, 2005. The measured magnetic field from Big
Bear Solar Observatory (BBSO) digital vector magnetograph (DGVM) was
used to model the non-potential coronal magnetic field changes and the
evolution of electric current before and after the event occurred. The
numerical results include the change of magnetic flux ( Φ), the net
electric current ( IN), the length of magnetic shear of the
main neutral line ( Lss), the flux normalized measure of
the field twist (α={μIN}/{Φ}) with μ being the magnetic
permeability. The current helicity ( Hc) injected into the
corona and the photospheric surface velocity are also computed. The
characteristic parameters of the buildup process before the event and
the decay process after the event are investigated and the amount of
magnetic energy converted to drive the event is estimated.
Title: Morphology and density structure of post-CME current sheets
Authors: Vršnak, B.; Poletto, G.; Vujić, E.; Vourlidas, A.; Ko,
Y. -K.; Raymond, J. C.; Ciaravella, A.; Žic, T.; Webb, D. F.;
Bemporad, A.; Landini, F.; Schettino, G.; Jacobs, C.; Suess, S. T.
Bibcode: 2009A&A...499..905V
Altcode: 2009arXiv0902.3705V
Context: Eruption of a coronal mass ejection (CME) drags and “opens”
the coronal magnetic field, presumably leading to the formation
of a large-scale current sheet and field relaxation by magnetic
reconnection.
Aims: We analyze the physical characteristics
of ray-like coronal features formed in the aftermath of CMEs, to
confirm whether interpreting this phenomenon in terms of a reconnecting
current sheet is consistent with observations.
Methods: The study
focuses on measurements of the ray width, density excess, and coronal
velocity field as a function of the radial distance.
Results: The
morphology of the rays implies that they are produced by Petschek-like
reconnection in the large-scale current sheet formed in the wake of
CME. The hypothesis is supported by the flow pattern, often showing
outflows along the ray, and sometimes also inflows into the ray. The
inferred inflow velocities range from 3 to 30 km s-1, and
are consistent with the narrow opening-angle of rays, which add up to a
few degrees. The density of rays is an order of magnitude higher than
in the ambient corona. The density-excess measurements are compared
with the results of the analytical model in which the Petschek-like
reconnection geometry is applied to the vertical current sheet, taking
into account the decrease in the external coronal density and magnetic
field with height.
Conclusions: The model results are consistent
with the observations, revealing that the main cause of the density
excess in rays is a transport of the dense plasma from lower to higher
heights by the reconnection outflow.
Title: Temporal comparison of nonthermal flare emission and
magnetic-flux change rates
Authors: Miklenic, C. H.; Veronig, A. M.; Vršnak, B.
Bibcode: 2009A&A...499..893M
Altcode: 2009arXiv0910.1701M
Context: To understand the mechanisms that trigger solar flares,
we require models describing and quantifying observable responses
to the original energy release process, since the coronal energy
release site itself cannot be resolved with current technical
equipment. Testing the usefulness of a particular model requires the
comparison of its predictions with flare observations.
Aims:
To test the standard flare model (CSHKP-model), we measured the
magnetic-flux change rate in five flare events of different GOES
classes using chromospheric/photospheric observations and compared
its progression with observed nonthermal flare emission. We calculated
the cumulated positive and negative magnetic flux participating in the
reconnection process, as well as the total reconnection flux. Finally,
we investigated the relations between the total reconnection flux,
the GOES class of the events, and the linear velocity of the
flare-associated CMEs.
Methods: Using high-cadence Hα and
TRACE 1600 Å image time-series data and MDI/SOHO magnetograms, we
measured the required observables (newly brightened flare area and
magnetic-field strength inside this area). RHESSI and INTEGRAL hard
X-ray time profiles in nonthermal energy bands were used as observable
proxies for the flare-energy release rate.
Results: We detected
strong temporal correlations between the derived magnetic-flux change
rate and the observed nonthermal emission of all events. The cumulated
positive and negative fluxes, with flux ratios of between 0.64 and 1.35,
were almost equivalent to each other. Total reconnection fluxes ranged
between 1.8×1021 Mx for the weakest event (GOES class B9.5)
and 15.5×1021 Mx for the most energetic one (GOES class
X17.2). The amount of magnetic flux participating in the reconnection
process was higher in more energetic events than in weaker ones. Flares
with more reconnection flux were associated with faster CMEs.
Title: The role of aerodynamic drag in dynamics of coronal mass
ejections
Authors: Vršnak, Bojan; Vrbanec, Dijana; Čalogović, Jaša; Žic,
Tomislav
Bibcode: 2009IAUS..257..271V
Altcode:
Dynamics of coronal mass ejections (CMEs) is strongly affected by the
interaction of the erupting structure with the ambient magnetoplasma:
eruptions that are faster than solar wind transfer the momentum and
energy to the wind and generally decelerate, whereas slower ones
gain the momentum and accelerate. Such a behavior can be expressed in
terms of “aerodynamic” drag. We employ a large sample of CMEs to
analyze the relationship between kinematics of CMEs and drag-related
parameters, such as ambient solar wind speed and the CME mass. Employing
coronagraphic observations it is demonstrated that massive CMEs are
less affected by the aerodynamic drag than light ones. On the other
hand, in situ measurements are used to inspect the role of the solar
wind speed and it is shown that the Sun-Earth transit time is more
closely related to the wind speed than to take-off speed of CMEs. These
findings are interpreted by analyzing solutions of a simple equation
of motion based on the standard form for the drag acceleration. The
results show that most of the acceleration/deceleration of CMEs on
their way through the interplanetary space takes place close to the
Sun, where the ambient plasma density is still high. Implications for
the space weather forecasting of CME arrival-times are discussed.
Title: Cosmic ray modulation by corotating interaction regions
Authors: Čalogović, Jaša; Vršnak, Bojan; Temmer, Manuela; Veronig,
Astrid M.
Bibcode: 2009IAUS..257..425C
Altcode:
We analyzed the relationship between the ground-based modulation of
cosmic rays (CR) and corotating interaction regions (CIRs). Daily
averaged data from 8 different neutron monitor (NM) stations were
used, covering rigidities from Rc = 0 - 12.91 GeV. The in
situ solar wind data were taken from the Advanced Composition Explorer
(ACE) database, whereas the coronal hole (CH) areas were derived from
the Solar X-Ray Imager onboard GOES-12. For the analysis we have
chosen a period in the declining phase of solar cycle 23, covering
the period 25 January-5 May 2005. During the CIR periods CR decreased
typically from 0.5% to 2%. A cross-correlation analysis showed a
distinct anti-correlation between the magnetic field and CR, with the
correlation coefficient (r) ranging from -0.31 to -0.38 (mean: -0.36)
and with the CR time delay of 2 to 3 days. Similar anti-correlations
were found for the solar wind density and velocity characterized by
the CR time lag of 4 and 1 day, respectively. The relationship was also
established between the CR modulation and the area of the CIR-related CH
with the CR time lag of 5 days after the central-meridian passage of CH.
Title: Type II Radio Bursts with High and Low Starting Frequencies
Authors: Shanmugaraju, Annamalai; Moon, Y. -J.; Vrsnak, Bojan
Bibcode: 2009SoPh..254..297S
Altcode:
We report on the detailed analysis of i) differences between the
properties of type IIs with various starting frequencies (high: ≥100
MHz; low: ≤50 MHz; mid: 50 MHz ≤f≤ 100 MHz) and ii) the properties
of CMEs and flares associated with them. For this study, we considered
a sample of type II radio bursts observed by Culgoora radio spectrograph
from January 1998 to December 2000. The X-ray flares and CMEs associated
with these events are identified using GOES and SOHO/LASCO data. The
secondary aim is to study the frequency dependence on other properties
of type IIs, flares, and CMEs. We found that the type IIs with high
starting frequencies have larger drift rate, relative drift rate,
and shock speed than the type IIs with low starting frequencies. The
flares associated with high frequency type IIs are of impulsive in
nature with shorter rise time, duration and delay between the flare
start and type II start times than the low frequency type IIs. There
is a distinct power - law relationship between the flare parameters
and the starting frequencies of type II bursts, whereas the trend in
the CME parameters shows low correlation. While the mean speed of CMEs
is larger for the mid-frequency group, it is nearly the same for the
high and low frequency groups. On the other hand, the percentage of
CME association (90%) is larger for low frequency type IIs than for
the high frequency type IIs (75%).
Title: Magnetic Flux Change Rates and Nonthermal Flare Emission
Authors: Miklenic, C.; Veronig, A.; Vršnak, B.
Bibcode: 2009CEAB...33..197M
Altcode:
We tested the standard flare model by measuring the magnetic flux
change rate in five flares of different GOES classes and compared it
to the observed nonthermal flare hard X-ray emission. In addition we
calculated the cumulated positive and negative magnetic reconnection
flux, as well as the total reconnection flux. We also investigated the
relations between the total reconnection flux, the GOES importance of
the events, and the linear velocity of the flare-associated CMEs. The
required observables (newly brightened flare area and magnetic field
strength inside this area) were measured using high-cadence Hα and
TRACE 1600 Å image time series along with MDI/SOHO magnetograms. RHESSI
and INTEGRAL hard X-ray time profiles in nonthermal energy bands served
as observable proxies for the flare energy release rate. We found
good temporal correlations between the derived magnetic flux change
rate and the observed nonthermal emission in all events. Cumulated
positive and negative fluxes were roughly balanced. The amount of
magnetic reconnection flux was larger in more energetic events than in
weaker ones. Flares with more reconnection flux were associated with
faster CMEs. The findings indicate that the standard flare model is
applicable to the analysed events.
Title: Commission 10: Solar Activity
Authors: Klimchuk, James A.; van Driel-Gesztelyi, Lidia; Schrijver,
Carolus J.; Melrose, Donald B.; Fletcher, Lyndsay; Gopalswamy,
Natchimuthuk; Harrison, Richard A.; Mandrini, Cristina H.; Peter,
Hardi; Tsuneta, Saku; Vršnak, Bojan; Wang, Jing-Xiu
Bibcode: 2009IAUTA..27...79K
Altcode: 2008arXiv0809.1444K
Commission 10 deals with solar activity in all of its forms,
ranging from the smallest nanoflares to the largest coronal mass
ejections. This report reviews scientific progress over the roughly
two-year period ending in the middle of 2008. This has been an exciting
time in solar physics, highlighted by the launches of the Hinode and
STEREO missions late in 2006. The report is reasonably comprehensive,
though it is far from exhaustive. Limited space prevents the inclusion
of many significant results. The report is divided into the following
sections: Photosphere and chromosphere; Transition region; Corona and
coronal heating; Coronal jets; flares; Coronal mass ejection initiation;
Global coronal waves and shocks; Coronal dimming; The link between low
coronal CME signatures and magnetic clouds; Coronal mass ejections in
the heliosphere; and Coronal mass ejections and space weather. Primary
authorship is indicated at the beginning of each section.
Title: Cylindrical and Spherical Pistons as Drivers of MHD Shocks
Authors: Žic, Tomislav; Vršnak, Bojan; Temmer, Manuela; Jacobs, Carla
Bibcode: 2008SoPh..253..237Z
Altcode: 2008SoPh..tmp..153Z
We consider an expanding three-dimensional (3-D) piston as a driver
of an MHD shock wave. It is assumed that the source-region surface
accelerates over a certain time interval to achieve a particular maximum
velocity. Such an expansion creates a large-amplitude wave in the
ambient plasma. Owing to the nonlinear evolution of the wavefront, its
profile steepens and after a certain time and distance a discontinuity
forms, marking the onset of the shock formation. We investigate
how the formation time and distance depend on the acceleration phase
duration, the maximum expansion velocity (defining also acceleration),
the Alfvén velocity (defining also Mach number), and the initial size
of the piston. The model differs from the 1-D case, since in the 3-D
evolution, a decrease of the wave amplitude with distance must be taken
into account. We present basic results, focusing on the timing of the
shock formation in the low- and high-plasma-beta environment. We find
that the shock-formation time and the shock-formation distance are
(1) approximately proportional to the acceleration phase duration;
(2) shorter for a higher expansion velocity; (3) larger in a higher
Alfvén speed environment; (4) only weakly dependent on the initial
source size; (5) shorter for a stronger acceleration; and (6) shorter
for a larger Alfvén Mach number of the source surface expansion. To
create a shock causing a high-frequency type II burst and the Moreton
wave, the source region expansion should, according to our results,
achieve a velocity on the order of 1000 km s−1 within a
few minutes, in a low Alfvén velocity environment.
Title: A Flare-Generated Shock during a Coronal Mass Ejection on 24
December 1996
Authors: Magdalenić, J.; Vršnak, B.; Pohjolainen, S.; Temmer, M.;
Aurass, H.; Lehtinen, N. J.
Bibcode: 2008SoPh..253..305M
Altcode: 2008SoPh..tmp..120M
We present a multiwavelength study of the large-scale coronal
disturbances associated with the CME - flare event recorded on 24
December 1996. The kinematics of the shock wave signature, the type
II radio burst, is analyzed and compared with the flare evolution
and the CME kinematics. We employ radio dynamic spectra, position
of the Nançay Radioheliograph sources, and LASCO-C1 observations,
providing detailed study of this limb event. The obtained velocity of
the shock wave is significantly higher than the contemporaneous CME
velocity (1000 and 235 km s−1, respectively). Moreover,
since the main acceleration phase of the CME took place 10 - 20 min
after the shock wave was launched, we conclude that the shock wave
on 24 December 1996 was probably not driven by the CME. However,
the shock wave was closely associated with the flare impulsive phase,
indicating that it was ignited by the energy release in the flare.
Title: Investigation of geomagnetic and solar activity over time
span 1960-2001
Authors: Verbanac, G.; Vrsnak, B.; Korte, M.; Mandea, M.; Temmer, M.
Bibcode: 2008AGUFMSH13A1502V
Altcode:
The solar-terrestrial relationship has been widely studied by means
of space weather indices and various solar wind parameters. In this
study we perform such an investigation over four decades (1960--2001)
on the global scale, and for the first time on the regional scale as
well. The variations regarded as the remaining external field signal
(thereafter RES) and present in the European observatory annual means
of the Northward, X, magnetic component is used as an indicator of
the regional geomagnetic activity and are investigated regarding
different processes occurring on the Sun. In order to understand how
various geomagnetic activity indices respond to the interplanetary
disturbances during different periods of the solar cycle, the annual
means of the solar activity index, F10.7, and geomagnetic Ap and Dst
indices, are studied. The indices are inherently complex since they
include contributions from different solar phenomena. By studying
the cross-correlations we aim to distinguish between different
generation mechanisms. The relationships between the solar parameter
F10.7 and RES, Dst, Ap, characterized by high cross-correlation
coefficients, suggest the possibility to evaluate the behaviour of
these geomagnetic parameters on short timescale. We found finite
time lags among the investigated parameters. Our study suggests a
one year delay of both RES and Dst after F10.7. The Ap is clearly
delayed for about two years with respect to F10.7 and about one year
with respect to Dst and RES. This indicate that the Ap responds to
the solar activity in a different manner than Dst and RES, which are
dominated by the coronal mass ejections activity. On the other hand,
it seems that Ap is more sensitive to the high--speed streams (HSS)
and Alfvenic waves present in HSS. The existence of time lags gives
us the possibility to forecast different parameters. Importantly, the
proposed forecasting procedure offers the possibility to reconstruct
the F10.7 from the inferred geomagnetic activity for the epochs prior
to solar activity monitoring. This study contributes to understanding
some physical processes on the Sun that cause the perturbations in the
near-Earth interplanetary space and consequently in the Earth's magnetic
field. Moreover such investigations may provide a better insight into
the time evolution of the open and closed solar magnetic field, and into
long term changes in the solar activity with related physical processes.
Title: Origin of Coronal Shock Waves. Invited Review
Authors: Vršnak, Bojan; Cliver, Edward W.
Bibcode: 2008SoPh..253..215V
Altcode: 2008SoPh..tmp..142V
The basic idea of the paper is to present transparently and confront
two different views on the origin of large-scale coronal shock waves,
one favoring coronal mass ejections (CMEs), and the other one preferring
flares. For this purpose, we first review the empirical aspects of the
relationship between CMEs, flares, and shocks (as manifested by radio
type II bursts and Moreton waves). Then, various physical mechanisms
capable of launching MHD shocks are presented. In particular, we
describe the shock wave formation caused by a three-dimensional piston,
driven either by the CME expansion or by a flare-associated pressure
pulse. Bearing in mind this theoretical framework, the observational
characteristics of CMEs and flares are revisited to specify advantages
and drawbacks of the two shock formation scenarios. Finally,
we emphasize the need to document clear examples of flare-ignited
large-scale waves to give insight on the relative importance of flare
and CME generation mechanisms for type II bursts/Moreton waves.
Title: Dynamics of coronal mass ejections. The mass-scaling of the
aerodynamic drag
Authors: Vršnak, B.; Vrbanec, D.; Čalogović, J.
Bibcode: 2008A&A...490..811V
Altcode:
Context: Coronal and interplanetary propagation of coronal mass
ejections (CMEs) is strongly affected by aerodynamic drag.
Aims:
The dependence of the drag acceleration on the mass of the CMEs is
investigated to establish a quantitative empirical relationship, which
might be important in semi-empirical space-weather forecasting.
Methods: We employ a large sample of CMEs observed in the radial
distance range of 2-30 solar radii by the Large Angle and Spectrometric
Coronagraph on board the SoHO mission to statistically analyze the
acceleration-velocity relationship in subsamples of various classes of
CME masses.
Results: It is demonstrated that the slope and the
v-axis intercept of the anti-correlation of the CME acceleration a and
velocity v depend on the mean mass of CMEs included in the sample. The
slope k of the correlation is less steep for subsamples of higher
masses, revealing that massive CMEs are less affected by the aerodynamic
drag. Furthermore, it is found that the v-axis intercept is shifted
to higher velocities for subsamples of higher masses. This indicates
that, on average, the driving force is greater in more massive CMEs.
Conclusions: The empirically established dependence of the a(v) slope
on the CME mass is very close to the dependence kpropto m-1/3
which follows from the physical characteristics of the aerodynamic drag.
Title: Processes and mechanisms governing the initiation and
propagation of CMEs
Authors: Vršnak, B.
Bibcode: 2008AnGeo..26.3089V
Altcode:
The most important observational characteristics of coronal mass
ejections (CMEs) are summarized, emphasizing those aspects which are
relevant for testing physical concepts employed to explain the CME
take-off and propagation. In particular, the kinematics, scalings,
and the CME-flare relationship are stressed. Special attention is paid
to 3-dimensional (3-D) topology of the magnetic field structures,
particularly to aspects related to the concept of semi-toroidal
flux-rope anchored at both ends in the dense photosphere and embedded
in the coronal magnetic arcade. Observations are compared with physical
principles and concepts employed in explaining the CME phenomenon, and
implications are discussed. A simple flux-rope model is used to explain
various stages of the eruption. The model is able to reproduce all basic
observational requirements: stable equilibrium and possible oscillations
around equilibrium, metastable state and possible destabilization
by an external disturbance, pre-eruptive gradual-rise until loss of
equilibrium, possibility of fallback events and failed eruptions,
relationship between impulsiveness of the CME acceleration and the
source-region size, etc. However, it is shown that the purely ideal
MHD process cannot account for highest observed accelerations which
can attain values up to 10 km s-2. Such accelerations can be
achieved if the process of reconnection beneath the erupting flux-rope
is included into the model. Essentially, the role of reconnection is in
changing the magnetic flux associated with the flux-rope current and
supplying "fresh" poloidal magnetic flux to the rope. These effects
help sustain the electric current flowing along the flux-rope, and
consequently, reinforce and prolong the CME acceleration. The model
straightforwardly explains the observed synchronization of the flare
impulsive phase and the CME main-acceleration stage, as well as the
correlations between various CME and flare parameters.
Title: Two-spacecraft reconstruction of a magnetic cloud and
comparison to its solar source
Authors: Möstl, C.; Miklenic, C.; Farrugia, C. J.; Temmer, M.;
Veronig, A.; Galvin, A. B.; Vršnak, B.; Biernat, H. K.
Bibcode: 2008AnGeo..26.3139M
Altcode:
This paper compares properties of the source region with those inferred
from satellite observations near Earth of the magnetic cloud which
reached 1 AU on 20 November 2003. We use observations from space
missions SOHO and TRACE together with ground-based data to study
the magnetic structure of the active region NOAA 10501 containing
a highly curved filament, and determine the reconnection rates and
fluxes in an M4 flare on 18 November 2003 which is associated with
a fast halo CME. This event has been linked before to the magnetic
cloud on 20 November 2003. We model the near-Earth observations with
the Grad-Shafranov reconstruction technique using a novel approach in
which we optimize the results with two-spacecraft measurements of the
solar wind plasma and magnetic field made by ACE and WIND. The two
probes were separated by hundreds of Earth radii. They pass through
the axis of the cloud which is inclined -50 degree to the ecliptic. The
magnetic cloud orientation at 1 AU is consistent with an encounter with
the heliospheric current sheet. We estimate that 50% of its poloidal
flux has been lost through reconnection in interplanetary space. By
comparing the flare ribbon flux with the original cloud fluxes we infer
a flux rope formation during the eruption, though uncertainties are
still significant. The multi-spacecraft Grad-Shafranov method opens
new vistas in probing of the spatial structure of magnetic clouds in
STEREO-WIND/ACE coordinated studies.
Title: Large-scale Coronal Waves Observed with EUVI/STEREO
Authors: Veronig, A.; Temmer, M.; Vrsnak, B.
Bibcode: 2008ESPM...12.2.97V
Altcode:
We report first observations and analysis of flare/CME associated
large-scale coronal waves (so-called "EIT waves") observed with
high time cadence by the EUVI instruments onboard the recent STEREO
mission. The EIT instrument onboard SOHO for the first time directly
imaged global disturbances in the solar corona, but the observations
are severely hampered by the low cadence of EIT (12-15 min). Thus,
the nature and origin of these large-scale disturbances are still
not sufficiently constraint by observations, and it is an intense
matter of debate whether EIT waves: a) are the coronal counterparts of
Moreton waves observed in the chromosphere; b) are caused by the flare
explosive energy release or by the erupting CME; c) are waves at all or
rather propagating disturbances related to magnetic field line opening
and restructuring associated with the CME lift-off. The high cadence
full-disk coronal imaging by the EUVI instruments on the twin STEREO
spacecraft provide us with the unprecedented opportunity to study the
dynamics and origin of flare/CME associated coronal waves. We present
first studies of global coronal waves observed with EUVI finding wave
deceleration, indicative of an MHD blast wave (Veronig et al. 2008,
ApJ Lett., in press).
Title: Relation between CME SchmiederAcceleration Profile and Flare
Energy Release derived from Combined STEREO and RHESSI Observations
Authors: Temmer, M.; Veronig, A. M.; Vrsnak, B.
Bibcode: 2008ESPM...12.2.96T
Altcode:
In the standard flare/CME picture magnetic reconnection occurs in
a current sheet formed behind the CME, which may provide a feedback
relationship between both phenomena. To study the relationship of the
large-scale CME acceleration and the energy release in the associated
flare we analyze three well observed events. The observations cover
the early (low corona) evolution of the CMEs with the EUVI instruments
aboard the twin STEREO spacecraft and the RHESSI hard X-ray emission
of the associated flare. Since the flare hard X-rays are due to fast
electrons, they provide the most direct indicator of the evolution of
the flare energy release in the flare. The results are compared to case
studies for halo-CMEs where a close synchronization between the CME
acceleration and the flare energy release was found (Temmer et al.,
ApJ, 2008, 673, L95).
Title: High-Cadence Observations of a Global Coronal Wave by
STEREO EUVI
Authors: Veronig, Astrid M.; Temmer, Manuela; Vršnak, Bojan
Bibcode: 2008ApJ...681L.113V
Altcode: 2008arXiv0806.0710V
We report a large-scale coronal wave (so-called EIT wave) observed
with high cadence by EUVI on board STEREO in association with the GOES
B9.5 flare and double CME event on 2007 May 19. The EUVI instruments
provide us with the unprecedented opportunity to study the dynamics of
flare/CME associated coronal waves. The coronal wave under study reveals
deceleration, indicative of a freely propagating MHD wave. Complementary
analysis of the associated flare and erupting filament/CME hint at wave
initiation by the CME expanding flanks, which drive the wave only over
a limited distance. The associated flare is very weak and occurs too
late to account for the wave initiation.
Title: Analysis of a Moreton Wave Associated with the X17.2/4B
Flare/CME of 28-10-2003
Authors: Muhr, M.; Temmer, M.; Veronig, A.; Vršnak, B.; Hanslmeier, A.
Bibcode: 2008CEAB...32...79M
Altcode:
The fast Moreton wave of 28-Oct-2003 associated with the extreme X17.2
solar flare/CME event is studied. It can be followed in four sectors,
spanning almost over 360° on the visible solar disc. The mean wave
velocity lies in the range of v∼900-1000 km s^{-1}. We find two
wave ignition centres on opposite edges of the source region, which
may indicate that the wave is driven by the CME expanding flanks.
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: Acceleration in Fast Halo CMEs and Synchronized Flare HXR
Bursts
Authors: Temmer, M.; Veronig, A. M.; Vršnak, B.; Rybák, J.; Gömöry,
P.; Stoiser, S.; Maričić, D.
Bibcode: 2008ApJ...673L..95T
Altcode:
We study two well-observed, fast halo CMEs, covering the full CME
kinematics including the initiation and impulsive acceleration phase,
and their associated flares. We find a close synchronization between the
CME acceleration profile and the flare energy release as indicated by
the RHESSI hard X-ray flux onsets, as well as peaks occur simultaneously
within 5 minutes. These findings indicate a close physical connection
between both phenomena and are interpreted in terms of a feedback
relationship between the CME dynamics and the reconnection process in
the current sheet beneath the CME.
Title: Large-scale coronal waves observed with STEREO/EUVI
Authors: Veronig, Astrid; Temmer, Manuela; Vrsnak, Bojan
Bibcode: 2008cosp...37.3328V
Altcode: 2008cosp.meet.3328V
The EUVI instruments onboard the twin STEREO spacecraft provide
high-cadence full-disk imaging of the solar atmosphere with four
different filters at EUV wavelengths. These observations are highly
suitable to study the kinematics and dynamics of flare/CME associated
coronal waves, so-called "EIT waves". We present a detailed analysis
of one coronal wave captured by the EUVI instruments, with particular
emphasis on the wave dynamics and its connection to the associated
flare (RHESSI hard X-rays) and CME (STEREO COR1) in terms of blast
wave versus driven wave scenario.
Title: Dynamics of plasmoids formed by the current sheet tearing
Authors: Bárta, M.; Vršnak, B.; Karlický, M.
Bibcode: 2008A&A...477..649B
Altcode:
Context: Moving blob-like features observed in the soft X-ray and
EUV range above flare-loops are often interpreted as signatures of
plasmoids formed by the current sheet tearing in the flare-associated
reconnection process.
Aims: We investigate the evolution of
the flare-associated current sheet numerically in order to analyse
the kinematics and dynamics of plasmoids. The goal is to explain the
broad diversity of kinematical properties of the plasmoid signatures
recorded by various observational techniques.
Methods: We
performed a 2-dimensional resistive-MHD numerical simulation of
the reconnection starting from the Harris-type current sheet. After
identifying the plasmoids, we followed their motion to determine basic
kinematical parameters (velocity and acceleration), and we analysed
the associated magnetic field topology.
Results: The simulation
reveals a broad variety of the kinematical/dynamical properties of
plasmoids - after formation, a plasmoid can move upward, downward, or
can even change its direction of propagation. The highest velocities,
in the range of the ambient Alfvén speed, are found in the case of
upward propagating plasmoids. The acceleration is determined by the
net magnetic field tension of the reconnected field lines. Downwardly
propagating plasmoids achieve only a fraction of the ambient Alfvén
speed. They strongly decelerate during the coalescence with low-lying
flare-loops, when distinct energy-release peaks occur and loop system
oscillations are excited.
Conclusions: The presented results
explain, qualitatively and quantitatively, the broad spectrum of
kinematical properties of various observational features attributed
to the current-sheet plasmoids.
Title: The role of reconnection in the initiation, acceleration,
and interplanetary propagation of CMEs
Authors: Vrsnak, Bojan
Bibcode: 2008cosp...37.3368V
Altcode: 2008cosp.meet.3368V
The central issue of the presentation is the role of reconnection in the
acceleration of coronal mass ejections (CMEs). Following basic physical
concepts it is demonstrated that impulsive acceleration of fast CMEs is
not possible without reconnection. In the absence of reconnection the
magnetic flux associated with the erupting structure remains conserved,
causing the inductive decay of the electric current and cessation
of the Lorentz force. Under such circumstances, the acceleration is
constrained to values in the order of 100 m s-2 and acts over distances
too short to provide velocities ∼ 1000 km s-1 . However, if the
flare-associated reconnection in the wake of the erupting flux-rope
supplies the rope with additional poloidal flux, the Lorentz-force
acceleration can attain values up to 10 km s-2 and prolong to large
heights. The results of a simple analytical model of a semitoroidal
line-tied flux-rope are confronted with observations: various aspects
of the CME-flare relationship are addressed, particularly emphasizing
proxy measurements of the reconnected flux. Beside this central
topic, the role of reconnection in the evolution and destabilization
of the pre-eruptive structure is discussed, focusing on scenarios
that lead to loss of equilibrium. Finally, the effect of reconnection
ahead/aside of the CME during its propagation through the upper corona
and interplanetary space is addressed.
Title: Synchronization between the CME acceleration and the energy
release in the associated flare
Authors: Temmer, Manuela; Veronig, Astrid; Vrsnak, Bojan
Bibcode: 2008cosp...37.3167T
Altcode: 2008cosp.meet.3167T
In the standard flare/CME picture magnetic reconnection occurs in a
current sheet formed behind the CME, which is indicative of a feedback
relationship between both phenomena. We analyze two X-class flare/CME
events which were well covered by RHESSI hard X-ray observations,
and the early evolution of the CMEs could be observed in TRACE and
GOES/SXI images. Since the flare hard X-rays are due to fast electrons,
they provide the most direct indicator of the evolution of the energy
release in the flare. This data set enables us to study in detail
the relationship of the large-scale CME acceleration and the energy
release in the associated flare.
Title: Projection effects in coronal mass ejections studied with
STEREO and SoHO
Authors: Temmer, Manuela; Preiss, Stefanie; Veronig, Astrid; Vrsnak,
Bojan
Bibcode: 2008cosp...37.3168T
Altcode: 2008cosp.meet.3168T
The STEREO mission consists of two identical satellites, positioned
ahead (A) and behind (B) the Earth, which observe the Sun from viewing
angles different from that of LASCO aboard SoHO (positioned at L1). The
kinematics (speed) and width of a coronal mass ejection (CME) is derived
by measuring distinct CME features observed in projection against the
plane of sky. As STEREO-A, STEREO-B, and LASCO/SoHO, observe a CME
from three different viewing angles, the resulting CME kinematics and
widths differ. By combining the observations from the three satellites
we study for several well observed CMEs the importance of projection
effects for the CME kinematics and expansion.
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: The magnetic flux and self-inductivity of a thick toroidal
current
Authors: Žic, Tomislav; Vršnak, Bojan; Skender, Marina
Bibcode: 2007JPlPh..73..741Z
Altcode: 2007JPlPh..73..741A
We investigate numerically the magnetic flux and
self-inductivity of a toroidal current I of arbitrary aspect
ratio (R0/r0 = 1/η, where R0
and r0 are the major and the minor torus radii,
respectively). The total flux Ψ is represented by the sum of the
flux outside the torus envelope (Ψo) and the internal
flux within the torus body (Ψi). Analogously, the total
inductivity is expressed as L = Lo + Li. The
outside self-inductivity is determined directly from the magnetic
flux Ψo, utilizing Ψo = Lo
I. On the other hand, the internal inductivity is evaluated as the
magnetic energy contained in the poloidal field. The calculations are
performed for three different radial profiles of the current density,
j(r). It is found that Ψo(η) and Lo
(η) depend only very weakly on the form of j(r). On the other hand,
Ψi and Li do not depend on η, but depend on
the form of j(r). In the range 0.02 η 0.5, the numerical values of
Lo can be very well fitted by the function of the form
Lofit1(η) = -A log(η) - B. Such a relation
is analogous to that for a slender torus, although the coefficients
are different. For η 0.01 the slender-torus approximation
(Lo*) matches the numerical results better than our
function Lofit1, whereas for thicker tori,
Lofit1 becomes more appropriate. It is shown
that, beyond η 0.1, the departure of the slender-torus analytical
expression from the numerical values becomes greater than 10%, and the
difference becomes larger than 100% at η 0.55. In the range η 0.5,
the numerical values of Lo can be very well expressed
by the function Lofit2(η)=c1
(1 - η)c2. Furthermore, since the internal
flux and inductivity become larger than that outside the envelope,
Ψi and Li become larger than Ψo and
Lo. The total inductivity Ltotfit
= Lofit + Li, calculated by
appropriately employing our functions Lofit1
and Lofit2, never deviates by more than 1%
from the numerically determined values of Ltot.
Title: Transit times of interplanetary coronal mass ejections and
the solar wind speed
Authors: Vršnak, B.; Žic, T.
Bibcode: 2007A&A...472..937V
Altcode:
Aims: The Sun-Earth transit time of interplanetary coronal
mass ejections (ICMEs) is one of central issues of space weather
forecasting. Our aim is to find out to what degree the ICME transit
time depends on the solar wind speed. Methods. Two samples of coronal
mass ejections (CMEs) and the associated ICMEs are used to analyze
the relationship between transit times, TT, and the solar wind speed,
w, measured at 1 AU ahead and behind the ICME. Results. We found a
distinct correlation TT(w), clearly showing that the transit time is
dependent not only on the ICME take-off speed vCME, but also
on the solar wind speed. After dividing the samples into the solar wind
speed bins w≤400, 400 < w ≤ 500, w > 500 km s^-1, we compared
the corresponding TT(vCME) correlations to find that the
transit times in the case of w ≤ 400 km s^-1 subset are longer, on
average, for about 20-30 hours than in the case of the w > 500 km
s^1 subset. Conclusions. Since the ICME transit time is significantly
influenced by the solar wind speed, this effect should be included in
statistical and kinematical methods of the space weather forecast.
Title: Large amplitude oscillatory motion along a solar filament
Authors: Vršnak, B.; Veronig, A. M.; Thalmann, J. K.; Žic, T.
Bibcode: 2007A&A...471..295V
Altcode: 2007arXiv0707.1752V
Context: Large amplitude oscillations of solar filaments is a phenomenon
that has been known for more than half a century. Recently, a new mode
of oscillations, characterized by periodical plasma motions along
the filament axis, was discovered.
Aims: We analyze such an
event, recorded on 23 January 2002 in Big Bear Solar Observatory Hα
filtergrams, to infer the triggering mechanism and the nature of the
restoring force.
Methods: Motion along the filament axis of a
distinct buldge-like feature was traced, to quantify the kinematics of
the oscillatory motion. The data were fitted by a damped sine function
to estimate the basic parameters of the oscillations. To identify the
triggering mechanism, morphological changes in the vicinity of the
filament were analyzed.
Results: The observed oscillations of the
plasma along the filament were characterized by an initial displacement
of 24 Mm, an initial velocity amplitude of 51 km s-1,
a period of 50 min, and a damping time of 115 min. We interpret
the trigger in terms of poloidal magnetic flux injection by magnetic
reconnection at one of the filament legs. The restoring force is caused
by the magnetic pressure gradient along the filament axis. The period of
oscillations, derived from the linearized equation of motion (harmonic
oscillator) can be expressed as P=π√{2}L/v_Aϕ≈4.4L/v_Aϕ, where
v_Aϕ =Bϕ0/√μ_0ρ represents the Alfvén speed based
on the equilibrium poloidal field Bϕ0.
Conclusions:
Combination of our measurements with some previous observations of
the same kind of oscillations shows good agreement with the proposed
interpretation. Movie to Fig. 1 is only available in electronic
form at http://www.aanda.org
Title: Projection effects in coronal mass ejections
Authors: Vršnak, B.; Sudar, D.; Ruždjak, D.; Žic, T.
Bibcode: 2007A&A...469..339V
Altcode:
Context: Basic observational parameters of a coronal mass ejection
(CME) are its speed and angular width. Measurements of the CME speed
and angular width are severely influenced by projection effects.
Aims: The goal of this paper is to investigate a statistical
relationship between the plane-of-sky speeds of CMEs and the direction
of their propagation, hopefully providing an estimate of the true
speeds of CMEs.
Methods: We analyze the correlation between
the plane-of-sky velocity and the position of the CME source region,
employing several non-halo CME samples. The samples are formed applying
various restrictions to avoid crosstalk of relevant parameters. For
example, we select only CMEs observed to radial distances larger than
10 solar radii; we omit CMEs showing a considerable acceleration in the
considered distance range and treat CMEs of different angular widths
separately. Finally, we combine these restriction criteria, up to the
limits beyond which the statistical significance of the results becomes
ambiguous.
Results: A distinct anti-correlation is found between
the angular width of CMEs and their source-region position, clearly
showing an increasing trend towards the disc center. Similarly, all of
the considered subsamples show a correlation between the CME projected
speed and the distance of the source region from the disc center. On
average, velocities of non-halo limb-CMEs are 1.5-2 times higher than
in the case of non-halo CMEs launched from regions located close to
the disc center.
Conclusions: Unfortunately, the established
empirical relationships provide only a rough estimate of the velocity
correction as a function of the source-region location. To a certain
degree, the results can be explained in terms of CME cone models, but
only after taking crosstalk of various parameters and observational
artifacts into account.
Title: Periodic Appearance of Coronal Holes and the Related Variation
of Solar Wind Parameters
Authors: Temmer, Manuela; Vršnak, Bojan; Veronig, Astrid M.
Bibcode: 2007SoPh..241..371T
Altcode:
We compared the variability of coronal hole (CH) areas (determined
from daily GOES/SXI images) with solar wind (daily ACE data) and
geomagnetic parameters for the time span 25 January 2005 until 11
September 2005 (late declining phase of solar cycle 23). Applying
wavelet spectral analysis, a clear 9-day period is found in the
CH time series. The GOES/SXI image sequence suggests that this
periodic variation is caused by a mutual triangular distribution of
CHs ∼120° apart in longitude. From solar wind parameters a 9-day
periodicity was obtained as well, simultaneously with the 9-day period
in the CH area time series. These findings provide strong evidence
that the 9-day period in solar wind parameters, showing up as higher
harmonic of the solar rotation frequency, is caused by the "periodic"
longitudinal distribution of CHs on the Sun recurring for several
solar rotations. The shape of the wavelet spectrum from the Dst index
matches only weakly with that from the CH areas and is more similar to
the wavelet spectrum of the solar wind magnetic field magnitude. The
distinct 9-day period does not show up in sunspot group areas which
gives further evidence that the solar wind modulation is strongly
related to CH areas but not to active region complexes. The wavelet
power spectra for the whole ACE data range (∼1998 - 2006) suggest that
the 9-day period is not a singular phenomenon occurring only during
a specific time range close to solar minimum but is occasionally also
present during the maximum and decay phase of solar cycle 23. The main
periods correspond to the solar rotation (27d) as well as
to the second (13.5d) and third (9d) harmonic.
Title: Acceleration Phase of Coronal Mass Ejections: I. Temporal
and Spatial Scales
Authors: Vršnak, Bojan; Maričić, Darije; Stanger, Andrew L.;
Veronig, Astrid M.; Temmer, Manuela; Roša, Dragan
Bibcode: 2007SoPh..241...85V
Altcode:
We study kinematics of 22 coronal mass ejections (CMEs) whose
motion was traced from the gradual pre-acceleration phase up to
the post-acceleration stage. The peak accelerations in the studied
sample range from 40, up to 7000 m s−2, and are inversely
proportional to the acceleration phase duration and the height range
involved. Accelerations and velocities are, on average, larger in CMEs
launched from a compact source region. The acceleration phase duration
is proportional to the source region dimensions; i.e., compact CMEs
are accelerated more impulsively. Such behavior is interpreted as
a consequence of stronger Lorentz force and shorter Alfvén time
scales involved in compact CMEs (with stronger magnetic field and
larger Alfvén speed being involved at lower heights). CMEs with
larger accelerations and velocities are on average wider, whereas the
widths are not related to the source region dimensions. Such behavior is
explained in terms of the field pile-up ahead of the erupting structure,
which is more effective in the case of a strongly accelerated structure.
Title: Acceleration Phase of Coronal Mass Ejections:
II. Synchronization of the Energy Release in the Associated Flare
Authors: Maričić, Darije; Vršnak, Bojan; Stanger, Andrew L.;
Veronig, Astrid M.; Temmer, Manuela; Roša, Dragan
Bibcode: 2007SoPh..241...99M
Altcode:
We analyze the relationship between the acceleration of coronal mass
ejections (CMEs) and the energy release in associated flares, employing
a sample of 22 events in which the CME kinematics were measured from
the pre-eruption stage up to the post-acceleration phase. The data
show a distinct correlation between the duration of the acceleration
phase and the duration of the associated soft X-ray (SXR) burst rise,
whereas the CME peak acceleration and velocity are related to the
SXR peak flux. In the majority of events the acceleration started
earlier than the SXR burst, and it is usually prolonged after the
SXR burst maximum. In about one half of the events the acceleration
phase is very closely synchronized with the fastest growth of the SXR
burst. An additional one quarter of the events may be still considered
as relatively well-synchronized, whereas in the remaining quarter of the
events there is a considerable mismatch. The results are interpreted
in terms of the feedback relationship between the CME dynamics and
the reconnection process in the wake of the CME.
Title: Coronal Holes and Solar Wind High-Speed Streams:
II. Forecasting the Geomagnetic Effects
Authors: Vršnak, Bojan; Temmer, Manuela; Veronig, Astrid M.
Bibcode: 2007SoPh..240..331V
Altcode:
We present a simple method of forecasting the geomagnetic storms caused
by high-speed streams (HSSs) in the solar wind. The method is based
on the empirical correlation between the coronal hole area/position
and the value of the Dst index, which is established in a period of
low interplanetary coronal mass ejection (ICME) activity. On average,
the highest geomagnetic activity, i.e., the minimum in Dst, occurs
four days after a low-latitude coronal hole (CH) crosses the central
meridian. The amplitude of the Dst dip is correlated with the CH area
and depends on the magnetic polarity of the CH due to the Russell -
McPherron effect. The Dst variation may be predicted by employing the
expression Dst(t)=(−65±25×cos λ)[A(t*)]0.5,
where A(t*) is the fractional CH area measured in the
central-meridian slice [−10°,10°] of the solar disc, λ is the
ecliptic longitude of the Earth, ± stands for positive/negative CH
polarity, and t−t*=4 days. In periods of low ICME activity,
the proposed expression provides forecasting of the amplitude of the
HSS-associated Dst dip to an accuracy of ≈30%. However, the time of
occurrence of the Dst minimum cannot be predicted to better than ±2
days, and consequently, the overall mean relative difference between
the observed and calculated daily values of Dst ranges around 50%.
Title: Coronal Holes and Solar Wind High-Speed Streams: I. Forecasting
the Solar Wind Parameters
Authors: Vršnak, Bojan; Temmer, Manuela; Veronig, Astrid M.
Bibcode: 2007SoPh..240..315V
Altcode:
We analyze the relationship between the coronal hole (CH) area/position
and physical characteristics of the associated corotating high-speed
stream (HSS) in the solar wind at 1 AU. For the analysis we utilize the
data in the period DOY 25 - 125 of 2005, characterized by a very low
coronal mass ejection (CME) activity. Distinct correlations between
the daily averaged CH parameters and the solar wind characteristics
are found, which allows us to forecast the solar wind velocity v,
proton temperature T, proton density n, and magnetic field strength B,
several days in advance in periods of low CME activity. The forecast
is based on monitoring fractional areas A, covered by CHs in the
meridional slices embracing the central meridian distance ranges
[−40°,−20°], [−10°,10°], and [20°,40°]. On average, the
peaks in the daily values of n, B, T, and v appear delayed by 1, 2,
3, and 4 days, respectively, after the area A attains its maximum
in the central-meridian slice. The peak values of the solar wind
parameters are correlated to the peak values of A, which provides
also forecasting of the peak values of n, B, T, and v. The most
accurate prediction can be obtained for the solar wind velocity, for
which the average relative difference between the calculated and the
observed peak values amounts to \overline{\vertδ\vert}≈10 %. The
forecast reliability is somewhat lower in the case of T, B, and n (
\overline{\vertδ\vert}≈20 , 30, and 40%, respectively). The space
weather implications are discussed, including the perspectives for
advancing the real-time calculation of the Sun - Earth transit times
of coronal mass ejections and interplanetary shocks, by including more
realistic real-time estimates of the solar wind characteristics.
Title: Observational Characteristics of Magnetic Reconnection in a
Two-Ribbon Flare
Authors: Miklenic, C. H.; Veronig, A. M.; Vršnak, B.; Hanslmeier, A.
Bibcode: 2007CEAB...31...39M
Altcode:
A well-observed GOES M3.9 two-ribbon flare was analysed in order
to derive the local reconnection rate (coronal electric field) and
the global reconnection rate (magnetic flux change rate), as well
as the energy release rate (Poynting flux) in a two-ribbon flare
from chromospheric/photospheric observations, using TRACE 1600 Å,
Kanzelhöhe Hα, SOHO/MDI, and RHESSI hard X-ray (HXR) data. We found
good temporal correlations between the derived time profiles and
observed HXR flux. Furthermore, it was confirmed that equal shares of
positive and negative magnetic flux participated in the reconnection
process. The findings indicate that the 2D reconnection model is
applicable to the analysed flare.
Title: Analysis of the Flare Wave Associated with the 3B/X3.8 Flare
of January 17, 2005
Authors: Thalmann, J. K.; Veronig, A. M.; Temmer, M.; Vršnak, B.;
Hanslmeier, A.
Bibcode: 2007CEAB...31..187T
Altcode:
The flare wave associated with the 3B/X3.8 flare and coronal mass
ejection (CME) of January 17, 2005 are studied using imaging data
in the Hα and EUV spectral channels. Due to the high-cadence Hα
observations from Kanzelhöhe Solar Observatory (KSO), a distinct
Moreton wave can be identified in ∼40 Hα frames over a period
of ∼7 minutes. The associated coronal EIT wave is identifiable in
only one EUV frame and appears close to the simultaneously observed
Moreton wave front, indicating that they are closely associated
phenomena. Beside the morphology of the wave across the solar disc
(covering an angular extend of ∼130°), the evolution in different
directions is studied to analyse the influence of a coronal hole (CH)
on the wave propagation. The Moreton wave shows a decelerating character
which can be interpreted in terms of a freely propagating fast-mode MHD
shock. The parts of the wave front moving towards the CH show a lower
initial and mean speed, and a greater amount of deceleration than the
segments moving into the undisturbed direction. This is interpreted
as the tendency of high Alfvén velocity regions to influence the
propagation of wave packets.
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: Helical Eruptive Prominence Associated with a Pair of
Overlapping CMEs on 21 April 2001
Authors: Ali, Syed Salman; Uddin, Wahab; Chandra, Ramesh; Mary, D. L.;
Vršnak, Bojan
Bibcode: 2007SoPh..240...89A
Altcode:
The eruption of limb prominence on 21 April 2001 associated with
two coronal mass ejections (CMEs) is investigated. Hα images
reveal two large-scale eruptions (a prominence body and a southern
foot-point arch), both showing helical internal structure. These two
eruptions are found to be spatially and temporally associated with
the corresponding CMEs. The kinematics and the study of geometrical
parameters of the prominence show that the eruption was quite impulsive
(with peak acceleration ≈470 m s−2) and has taken place
for relatively low pitch angle of helical threads, not exceeding tan
θ≈1.2. The stability criteria of the prominence are revisited in
the light of the model of Vršnak (1990, Solar Phys.129, 295) and the
analysis shows that the eruption violates the instability criteria of
that model. Finally, the energy stored in the prominence circuit and
the energies (kinetic, potential, and magnetic) of the associated CMEs
are estimated and it is found that there was enough energy stored in
the prominence to drive the two CMEs.
Title: Spatial Restriction to HXR Footpoint Locations by Reconnection
Site Geometries
Authors: Temmer, M.; Vršnak, B.; Veronig, A.; Miklenic, M.
Bibcode: 2007CEAB...31...49T
Altcode: 2007astro.ph..1203T
It is assumed that HXR sources map to the primary energy release site in
flares where particle acceleration occurs. Strong HXR sources are mostly
observed at confined regions along the reconnecting magnetic arcade. We
make a general approach on how the geometry of the reconnecting current
sheet (CS) may influence the strength and localization of observed HXR
sources. For this we use results from an analysis on the 3B/X3.8 flare
on January 17, 2005 (Temmer et al., 2007), as well as measurements from
the associated CME. Due to the close match of the CME acceleration
profile and the flare HXR flux, we suppose that the CME might play a
certain role in modifying the geometry of the CS (``symmetric'' versus
``asymmetric'' vertically stretched CS). This could be the driver for
``guiding'' the accelerated particles to confined areas along the
flaring arcade and might explain the spatially limited occurrence of
strong HXR sources in comparison to elongated ribbons as seen in Hα
and UV.
Title: Reconnection and energy release rates in a two-ribbon flare
Authors: Miklenic, C. H.; Veronig, A. M.; Vršnak, B.; Hanslmeier, A.
Bibcode: 2007A&A...461..697M
Altcode:
Aims:The aim of this study was to derive the local reconnection
rate (coronal electric field) and the global reconnection rate
(magnetic flux change rate), as well as the energy release rate
(Poynting flux), in a two-ribbon flare from chromospheric/photospheric
observations. Furthermore, we tested whether equal shares of positive
and negative magnetic flux are involved in the flare process.
Methods: A well-observed GOES M3.9 two-ribbon flare was analyzed. The
required observables (ribbon expansion velocity, newly brightened area,
and magnetic field strength at the ribbon front) were extracted from
the TRACE 1600 Å and Kanzelhöhe Hα image time series, and a SOHO
MDI magnetogram. Furthermore, the ratio of the converted positive
vs. negative magnetic flux was determined. Both RHESSI hard X-ray
20-60 keV full-disk time profiles and subregion time profiles derived
from a time series of RHESSI images in the same energy range were used
as independent, observable proxies for the energy release rate. The
RHESSI images were also used to localize the sites where the bulk
of the energy was deposited by fast electrons.
Results: We
found good temporal correlations between the derived time profiles
(local and global reconnection rate, Poynting flux) and observed
HXR flux. The local reconnection-rate peak values ranged from 2.7 {V
cm-1} to 11.8 {V cm-1}, whereas the positive
and the negative magnetic flux covered by the flare emission were
equal within 5-10%.
Conclusions: .The results indicate that
the local reconnection rate, the global reconnection rate, and the
energy release rate in a simple two-ribbon flare can be derived from
chromospheric/photospheric observations. Furthermore, it was confirmed
that equal shares of positive and negative magnetic flux participated
in the reconnection process.
Title: Recent Research: Large-scale Disturbances, their Origin
and Consequences
Authors: Mann, Gottfried; Vršnak, Bojan
Bibcode: 2007LNP...725..203M
Altcode:
This article gives a flavour of recent research dedicated to the
large-scale coronal disturbances and the related interplanetary
phenomena. The discussions include the take-off and propagation of
coronal mass ejections (CMEs); the CME-flare relationship; the origin
and propagation of shocks; the role of flares, CMEs, and shocks in
particle acceleration; radio signatures of CMEs and shocks; coronal
and IP plasma diagnostics offered by the radio emission excited by
these phenomena.
Title: Energy Release Rates along Hα Flare Ribbons and the Location
of Hard X-Ray Sources
Authors: Temmer, M.; Veronig, A. M.; Vršnak, B.; Miklenic, C.
Bibcode: 2007ApJ...654..665T
Altcode:
Local reconnection and energy release rates for an X3.8 flare that
occurred on 2005 January 17 are derived. In particular, we distinguish
between Hα flare ribbon segments that were accompanied by RHESSI
hard X-ray (HXR) footpoints and those without HXRs. We find that the
reconnection and energy release rates are not uniform along the flare
ribbons but much larger at the locations where the HXR footpoints are
observed. The difference is about 2 orders of magnitude in the case of
the energy release rates and 1 order of magnitude for the reconnection
rates (with peak values up to 8 kV m-1). These differences
are enough to explain the different flare morphologies typically
observed in HXRs (compact footpoints) and Hα/UV (extended ribbons)
by the limited dynamic range of present HXR instruments. Our results
are consistent with a scenario in which the electrons are accelerated
primarily along a certain subsystem of magnetic loops as outlined by
the HXR footpoints, and only a minor fraction (for the 2005 January
17 flare estimated to be about 1/15) go into the large flare arcade
outlined by the Hα ribbons and EUV postflare loops.
Title: MHD Waves and Shocks Generated during Magnetic Field
Reconnection
Authors: Bárta, M.; Karlický, M.; Vršnak, B.; Goossens, M.
Bibcode: 2007CEAB...31..165B
Altcode:
We use a 2D MHD model of magnetic field reconnection to investigate
if and how bursts of reconnection activity, changes of the magnetic
field and shock wave generation are related. We found that major
bursts of power dissipated into Joule heat occur during topological
transitions of the magnetic field structure. These bursts are followed
by shocks and waves. Along the plasma outflow jet not only MHD waves,
but also ion-sound shocks are formed. After the phase of more or
less quiet reconnection (Petschek-type) the tearing mode produces
plasmoids. The interactions of these plasmoids are associated with
further bursts of the reconnection activity and a complex structure
of shock waves. Finally, all these processes are discussed as possible
sources of various radio bursts.
Title: Millisecond solar radio bursts in the metric wavelength range
Authors: Magdalenić, J.; Hillaris, A.; Zlobec, P.; Vršnak, B.
Bibcode: 2006AIPC..848..224M
Altcode: 2010arXiv1009.4477M
A study and classification of super-short structures (SSSs) recorded
during metric type IV bursts is presented. The most important property
of SSSs is their duration, at half power ranging from 4-50 ms, what is
up to 10 times shorter than spikes at corresponding frequencies. The
solar origin of the SSSs is confirmed by one-to-one correspondence
between spectral recordings of Artemis-IV1 and high time
resolution single frequency measurements of the TSRS2. We have divided the SSSs in the following categories: 1. Broad-Band SSSs: They were partitioned in two subcategories, the
SSS-Pulses and Drifting SSSs; 2. Narrow-band: They appear either
as Spike-Like SSSs or as Patch-Like SSSs; 3. Complex SSS: They
consist of the absorption-emission segments and were morphologically
subdivided into Rain-drop Bursts (narrow-band emission head and a
broad-band absorption tail) and Blinkers.
Title: Interaction of a Moreton/EIT Wave and a Coronal Hole
Authors: Veronig, Astrid M.; Temmer, Manuela; Vršnak, Bojan; Thalmann,
Julia K.
Bibcode: 2006ApJ...647.1466V
Altcode: 2006astro.ph..4613V
We report high-cadence Hα observations of a distinct Moreton wave
observed at Kanzelhöhe Solar Observatory associated with the 3B/X3.8
flare and coronal mass ejection (CME) event of 2005 January 17. The
Moreton wave can be identified in about 40 Hα frames over a period of
7 minutes. The EIT wave is observed in only one frame, but the derived
propagation distance is close to that of the simultaneously measured
Moreton wave fronts, indicating that they are closely associated
phenomena. The large angular extent of the Moreton wave allows us to
study the wave kinematics in different propagation directions with
respect to the location of a polar coronal hole (CH). In particular, we
find that the wave segment whose propagation direction is perpendicular
to the CH boundary (``frontal encounter'') is stopped by the CH, which
is in accordance with observations reported from EIT waves. However,
we also find that at a tongue-shaped edge of the coronal hole, where
the front orientation is perpendicular to the CH boundary (the wave
``slides along'' the boundary), the wave signatures can be found up
to 100 Mm inside the CH. These findings are briefly discussed in the
frame of recent modeling results.
Title: Reconnection and Energy Release Rates in aTwo-Ribbon Flare
Authors: Miklenic, Christiane H.; Veronig, A. M.; Vrsnak, B.
Bibcode: 2006SPD....37.0801M
Altcode: 2006BAAS...38..230M
We tried to verify whether the local reconnection rate (coronal
electric field) and the global reconnection rate (magnetic flux
change rate) as well as the energy release rate (Poynting flux) in
a two-ribbon flare can be derived from chromospheric/photospheric
observations. Furthermore, we tested whether equal shares of positive
and negative magnetic flux are involved in the flare process.A
well observed GOES M3.9 two-ribbon flare was analyzed. The required
observables (ribbon expansion velocity, magnetic field strength at the
ribbon front, and newly brightened area) were extracted from TRACE
1600 Å and Kanzelhöhe H-alpha image time series, and a SOHO MDI
magnetogram, respectively. Furthermore, the ratio of the converted
positive vs. negative magnetic flux was determined. RHESSI Hard X-ray
20 - 60 keV full-disk time profiles as well as subregion imaging light
curves derived from a time series of RHESSI images in the same energy
range, were used as independent, observable proxies of the reconnection
and the energy release rate, respectively. The RHESSI images were also
used to localize the sites where the bulk of the energy was deposited
by fast electrons.We found good temporal correlations between derived
time profiles (local and global reconnection rate, Poynting flux) and
observed HXR flux. The local reconnection rate peak values ranged from
1.4 V/cm to 4.6 V/cm, and the ratio of converted positive vs. negative
magnetic flux deviated from the theoretically expected value by
only 5 - 10%.The results indicate that the local reconnection rate,
the global reconnection rate as well as the energy release rate in a
simple two-ribbon flare can be derived from chromospheric/photospheric
observations. Furthermore, it was confirmed that equal shares of
positive and negative magnetic flux participated in the reconnection
process.This work is supported by the Austrian ‘Fonds zur Förderung
der wissenschaftlichen Forschung’ under project P15344.
Title: Classification and Properties of Supershort Solar Radio Bursts
Authors: Magdalenić, J.; Vršnak, B.; Zlobec, P.; Hillaris, A.;
Messerotti, M.
Bibcode: 2006ApJ...642L..77M
Altcode:
Characteristics of supershort structures (SSSs) occurring in the
metric solar type IV radio bursts are described. The most important
property of SSSs is their duration, which, at half-power, ranges from
4 to 60 ms and is thus much shorter than generally expected for the
bursts in the metric range. The comparison of the distributions of SSS
durations with those of the spikes confirms that these are completely
different classes of bursts. Our analysis is focused on the frequency
range 200-450 MHz, providing us with the one-to-one identification
of individual SSSs in single-frequency records of the INAF-Trieste
Astronomical Observatory (Italy) and in the high-resolution spectral
data of Artemis IV (Greece). The analysis reveals a number of different
bursts that are classified as simple broadband, simple narrowband, and
complex SSSs. The diversity of SSSs has a resemblance to the variety
of the well-known metric radio bursts characterized by a 1 s timescale.
Title: Shrinking and Cooling of Flare Loops in a Two-Ribbon Flare
Authors: Vršnak, Bojan; Temmer, Manuela; Veronig, Astrid; Karlický,
Marian; Lin, Jun
Bibcode: 2006SoPh..234..273V
Altcode:
We analyze the evolution of the flare/postflare-loop system in the
two-ribbon flare of November 3, 2003, utilizing multi-wavelength
observations that cover the temperature range from several tens of
MK down to 104 K. A non-uniform growth of the loop system
enables us to identify analogous patterns in the height-time, h(t),
curves measured at different temperatures. The "knees," "plateaus,"
and "bends" in a higher-temperature curve appear after a certain time
delay at lower heights in a lower-temperature curve. We interpret such
a shifted replication as a track of a given set of loops (reconnected
field lines) while shrinking and cooling after being released from the
reconnection site. Measurements of the height/time shifts between h(t)
curves of different temperatures provide a simultaneous estimate of
the shrinkage speed and cooling rate in a given temperature domain,
for a period of almost ten hours after the flare impulsive phase. From
the analysis we find the following: (a) Loop shrinkage is faster at
higher temperatures - in the first hour of the loop-system growth,
the shrinkage velocity at 5 MK is 20 - 30 km s−1, whereas
at 1 MK it amounts to 5 km s−1; (b) Shrinking becomes
slower as the flare decays - ten hours after the impulsive phase,
the shrinkage velocity at 5 MK becomes 5 km s−1; (c) The
cooling rate decreases as the flare decays - in the 5 MK range it is
1 MK min−1 in the first hour of the loop-system growth,
whereas ten hours later it decreases to 0.2 MK min−1;
(d) During the initial phase of the loop-system growth, the cooling
rate is larger at higher temperatures, whereas in the late phases
the cooling rate apparently does not depend on the temperature; (e)
A more detailed analysis of shrinking/cooling around one hour after
the impulsive phase reveals a deceleration of the loop shrinkage,
amounting to ā ≈ 10 m s−2 in the T < 5 MK range;
(f) In the same interval, conductive cooling dominates down to T ≈
3 MK, whereas radiation becomes dominant below T ≈ 2 MK; (g) A few
hours after the impulsive phase, radiation becomes dominant across the
whole T < 5 MK range. These findings are compared with results of
previous studies and discussed in the framework of relevant models.
Title: Multi-wavelength study of coronal waves associated with the
CME-flare event of 3 November 2003
Authors: Vršnak, B.; Warmuth, A.; Temmer, M.; Veronig, A.;
Magdalenić, J.; Hillaris, A.; Karlický, M.
Bibcode: 2006A&A...448..739V
Altcode:
The large flare/CME event that occurred close to the west solar limb on
3 November 2003 launched a large-amplitude large-scale coronal wave that
was observed in Hα and Fe xii 195 Å spectral lines, as well as in the
soft X-ray and radio wavelength ranges. The wave also excited a complex
decimeter-to-hectometer type II radio burst, revealing the formation of
coronal shock(s). The back-extrapolation of the motion of coronal wave
signatures and the type II burst sources distinctly marks the impulsive
phase of the flare (the hard X-ray peak, drifting microwave burst,
and the highest type III burst activity), favoring a flare-ignited
wave scenario. On the other hand, comparison of the kinematics of
the CME expansion with the propagation of the optical wave signatures
and type II burst sources shows a severe discrepancy in the CME-driven
scenario. However, the CME is quite likely associated with the formation
of an upper-coronal shock revealed by the decameter-hectometer type II
burst. Finally, some six minutes after the launch of the first coronal
wave, another coronal disturbance was launched, exciting an independent
(weak) decimeter-meter range type II burst. The back-extrapolation
of this radio emission marks the revival of the hard X-ray burst,
and since there was no CME counterpart, it was clearly ignited by the
new energy release in the flare.
Title: Coronal Mass Ejection of 28 June 2000: Coupling of the CME
Evolution and the Flare Energy Release
Authors: Maričić, D.; Roša, D.; Vršnak, B.
Bibcode: 2006SunGe...1a..35M
Altcode:
We study the initiation and development of the limb Coronal Mass
Ejection (CME) of 28 Jun 2000, utilizing observations from Mauna
Loa Solar Observatory (MLSO), the Solar and Heliospheric Observatory
(SOHO), the Geostationary Operational Environmental Satellite (GOES)
and Yohkoh. Also, we analyze the relation between dynamics of the
Coronal Mass Ejection (CME) and the energy release in the associated
flare. The basic structure of the CME (prominence imbedded in bright
coronal arcade) is clearly recognizable already in the low corona during
the pre-eruption phase of slow rise. This provided measurements of
kinematics of various features from the very beginning of the eruption
up to the post-acceleration phase which was followed up to 32 solar
radii. Such events are observed only occasionally, and are of great
importance for the comprehension of the nature of forces driving
CMEs. The acceleration maximum was attained at the radial distance
of 1.45 solar radii from the Sun's center and ceased beyond 4 solar
radii. The acceleration phase was synchronized with the impulsive
phase of the associated two-ribbon flare. Observations provide clear
evidence that the CME eruption caused a global restructuring of the
magnetic field in the outer and inner corona.
Title: X-ray sources and magnetic reconnection in the X3.9 flare of
2003 November 3
Authors: Veronig, A. M.; Karlický, M.; Vršnak, B.; Temmer, M.;
Magdalenić, J.; Dennis, B. R.; Otruba, W.; Pötzi, W.
Bibcode: 2006A&A...446..675V
Altcode:
Context: .Recent RHESSI observations indicate an apparent altitude
decrease of flare X-ray loop-top (LT) sources before changing to the
commonly observed upward growth of the flare loop system.
Aims:
.We performed a detailed study of the LT altitude decrease for one well
observed flare in order to find further hints on the physics of this
phenomenon and how it is related to the magnetic reconnection process in
solar flares.
Methods: .RHESSI X-ray source motions in the 2003
November 3, X3.9 flare are studied together with complementary data
from SXI, EIT, and Kanzelhöhe Hα. We particularly concentrate on the
apparent altitude decrease of the RHESSI X-ray LT source early in the
flare and combine kinematical and X-ray spectral analysis. Furthermore,
we present simulations from a magnetic collapsing trap model embedded
in a standard 2-D magnetic reconnection model of solar flares.
Results: .We find that at higher photon energies the LT source is
located at higher altitudes and shows higher downward velocities
than at lower energies. The mean downward velocities range from
14 km s-1 in the RHESSI 10-15 keV energy band to 45 km
s-1 in the 25-30 keV band. For this flare, the LT altitude
decrease was also observed by the SXI instrument with a mean speed of
12 km s-1. RHESSI spectra indicate that during the time
of LT altitude decrease the emission of the LT source is thermal
bremsstrahlung from a "superhot" plasma with temperatures increasing
from 35 MK to 45 MK and densities of the order of 1010
cm-3. The temperature does not significantly increase
after this early (pre-impulsive superhot LT) phase, whereas the
LT densities increase to a peak value of (3-4) × 1011
cm-3.
Conclusions: .Modeling of a collapsing magnetic
trap embedded in a standard 2D magnetic reconnection model can reproduce
the key observational findings in case that the observed emission is
thermal bremsstrahlung from the hot LT plasma. This agrees with the
evaluated RHESSI spectra for this flare.
Title: Model of Motion of the X-Ray Loop-Top Source at the Beginning
of Cusp-Type Flares
Authors: Karlický, M.; Veronig, A.; Vršnak, B.
Bibcode: 2006CEAB...30...85K
Altcode:
A model with a 1-D collapsing magnetic trap is proposed for
an explanation of the motion of the X-ray loop-top source at the
beginning of cusp-type flares. Considering plasma heating due to the
betatron mechanism an analytic formula for the temporal and spatial
evolution of heated plasma temperature in the trap is derived. Using
the formula and the relation for thermal bremsstrahlung flux, the
time evolution of the X-ray intensity profile in the trap is computed
numerically. The model explains not only the downward motion of the
X-ray loop-top source observed at the beginning of cusp-type flares,
but also the upward motion which follows.
Title: New Families of Super Short Radio Bursts (Abstract)
Authors: Magdalenić, J.; Vrsnak, B.; Zlobec, P.; Hillaris, A.
Bibcode: 2006pre6.conf..399M
Altcode: 2006pre4.conf..399M
Several new families of super-short Solar radio features occurring
in type IV bursts are presented. We discovered them in the high time
resolution data (1 ms) recorded by the multichannel radiopolarimeter
of the Trieste Astronomical Observatory. The reliability of these
super-short structures (SSSs) is confirmed by one-to-one identification
of individual SSS events in the corresponding Artemis-IV (University
of Athens) spectral recordings. The most important characterists of
SSSs is that they are faster then spikes, up to now considered the
shortest Solar radio bursts [Guedel and Benz, A&A 231, 1, 202-212,
1990]. SSSs are 1-6 times shorter than the spikes, and it seems
that their duration is not frequency dependent. Another distinction
in respect to spikes is a stochastic change of the polarization
degree (0-100%), sometimes also including the change of the sense of
polarization within a given group of SSSs. Analysis of the spectral
and single-frequency morphology reveals at least 3 distinct categories:
"rain drops", broad-band SSSs, and spike-like SSSs. It seems that some
subclasses also exist.
Title: Theoretical and Observational Features of Magnetic Reconnection
Authors: Biernat, H. K.; Semenov, V. S.; Penz, T.; Miklenic, C.;
Veronig, A.; Hanslmeier, A.; Vršnak, B.; Heyn, M. F.; Ivanov, I. B.;
Ivanova, V. V.; Kiehas, S. A.; Langmayr, D.
Bibcode: 2006CEAB...30...75B
Altcode:
We present general solutions of the time--dependent Petschek--type
model of magnetic reconnection for a compressible plasma. The
disruption of a tangential discontinuity because of a localized
decrease of the resistivity leads to the generation of several
MHD wave modes. By solving the Riemann problem, the behavior of
these modes can be visualized. Additionally, disturbances of the
ambient plasma environment by the propagating shock structures
can be modeled. As an observational feature, the determination
of the reconnection rate in a two--ribbon flare is presented.
Title: Forces governing coronal mass ejections
Authors: Vršnak, B.
Bibcode: 2006AdSpR..38..431V
Altcode:
Kinematics of CMEs is analyzed to get an insight into the properties
of forces partaking in the eruption. It is demonstrated that the
Lorentz force plays a dominant role within a distance of a few
solar radii. In the distance range 1 30 solar radii, the inferred
values of the Lorentz-force acceleration aL on average
decrease with the heliocentric distance roughly as aL ∝
R-2. Generally, the values are found to be in the range g ≲
aL < 30g, where g(R) is the acceleration of gravity. In
fast events, the aerodynamic drag is comparable to the Lorentz force,
and on average also decreases with the distance. However, the decrease
is more gradual, so the drag becomes essential in the upper corona
and interplanetary space. Inspecting the CME flare relationship it is
demonstrated that the reconnection beneath the CME plays an active role
in the CME acceleration. Statistically, the CME speeds are correlated
with the importance of the associated flare. The distribution of
non-flare CME speeds is very similar to that of CMEs associated with
flares of the soft X-ray importance C, and includes a considerable
fraction of fast events. This implies that the concept of two distinct
classes of CMEs (flare and non-flare) should be reformulated.
Title: X-Ray Sources and Magnetic Reconnection in AN X-Class Flare
Authors: Veronig, A. M.; Vršnak, B.; Karlický, M.; Temmer, M.;
Magdalenić, J.; Dennis, B. R.; Otruba, W.; Pötzi, W.
Bibcode: 2005ESASP.600E..32V
Altcode: 2005ESPM...11...32V; 2005dysu.confE..32V
No abstract at ADS
Title: Properties of Very Short Duration Solar Radio Bursts
Authors: Magdalenić, J.; Vršnak, B.; Zlobec, P.; Aurass, H.
Bibcode: 2005ESASP.600E.131M
Altcode: 2005dysu.confE.131M; 2005ESPM...11..131M
No abstract at ADS
Title: Wave Phenomena Associated with the X3.8 Flare/cme of
17-JAN-2005
Authors: Temmer, M.; Veronig, A.; Vršnak, B.; Thalmann, J.;
Hanslmeier, A.
Bibcode: 2005ESASP.600E.144T
Altcode: 2005ESPM...11..144T; 2005dysu.confE.144T
No abstract at ADS
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 CME-flare relationship: Are there really two types of CMEs?
Authors: Vršnak, B.; Sudar, D.; Ruždjak, D.
Bibcode: 2005A&A...435.1149V
Altcode:
We present a statistical analysis of 545 flare-associated CMEs and 104
non-flare CMEs observed in the heliocentric distance range 2-30 solar
radii. We found that both data sets show quite similar characteristics,
contradicting the concept of two distinct (flare/non-flare) types of
CMEs. In both samples there is a significant fraction of CMEs showing
a considerable acceleration or deceleration and both samples include a
comparable ratio of fast and slow CMEs. We present kinematical curves of
several fast non-flare CMEs moving at a constant speed or decelerating,
i.e., behaving as expected for flare-associated CMEs. Analogously,
we identified several slow flare-CMEs showing the acceleration peak
beyond a height of 3 solar radii. On the other hand, it is true that
CMEs associated with major flares are on average faster and broader than
non-flare CMEs and small-flare CMEs. There is a well-defined correlation
between the CME speed and the importance of the associated flare. In
this respect, the non-flare CMEs show characteristics similar to CMEs
associated with flares of soft X-ray class B and C, which is indicative
of a "continuum" of events rather than supporting the existence of
two distinct CME classes. Furthermore, we inferred that CMEs whose
source region cannot be identified with either flares or eruptive
prominences are on average slowest. The results indicate that the
magnetic reconnection taking place in the current sheet beneath the
CME significantly influences the CME dynamics.
Title: Broadband Metric-Range Radio Emission Associated with a
Moreton/EIT Wave
Authors: Vršnak, B.; Magdalenić, J.; Temmer, M.; Veronig, A.;
Warmuth, A.; Mann, G.; Aurass, H.; Otruba, W.
Bibcode: 2005ApJ...625L..67V
Altcode:
We present the evolution and kinematics of a broadband radio source
that propagated collaterally with an Hα/EIT wave, linking it with the
type II burst that was excited higher up in the corona. The NRH wave
emission extended from the frequency f~327 to f<151 MHz and was
considerably weaker than the flare-related type IV burst. The emission
centroid propagated at a height of 0-200 Mm above the solar limb and
was intensified when the disturbance passed over enhanced coronal
structures. We put forward the ad hoc hypothesis that the NRH wave
signature is optically thin gyrosynchrotron emission excited by the
passage of the coronal MHD fast-mode shock. The identification of radio
emission associated with the coronal wave front is important since it
offers us new diagnostic information that could provide us with better
insight into the physical conditions in the disturbance itself.
Title: Terminology of Large-Scale Waves in the Solar Atmosphere
Authors: Vršnak, Bojan
Bibcode: 2005EOSTr..86..112V
Altcode:
This is the fourth in a series of essays on terms used in
solar-terrestrial physics that are thought to be in need of
clarification. Terms are identified and essays are commissioned by
a committee chartered by Division II (Sun and Heliosphere) of the
International Astronomical Union. Terminology Committee members include
Ed Cliver (chair), Jean-Louis Bougeret, Hilary Cane, Takeo Kosugi, Sara
Martin, Rainer Schwenn, and Lidia van Driel-Gestelyi. Authors are asked
to review the origins of terms and their current usage/misusage. The
goals are to inform the community and to open a discussion. The
following article by Bojan Vršnak focuses on terms used to describe
large-scale waves in the solar atmosphere, an area of research that
has been given great impetus by the images of waves from the Extreme
ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric
Observatory (SOHO). The committee welcomes suggestions for other terms
to address in this forum.
Title: 2 1/2 -D compressible reconnection model
Authors: Skender, M.; Vršnak, B.
Bibcode: 2005ASSL..320..167S
Altcode: 2005smp..conf..167S
No abstract at ADS
Title: Solar Flares — Observations and Theory
Authors: Vršnak, Bojan
Bibcode: 2005ASSL..320...27V
Altcode: 2005smp..conf...27V
No abstract at ADS
Title: Exact Solution of Jump Relations at Discontinuities in a
Two-And-Half-Dimensional Compressible Reconnection Model
Authors: Skender, Marina; Vršnak, Bojan; Martinis, Mladen
Bibcode: 2005IAUS..226..274S
Altcode:
Two-and-half-dimensional reconnection is examined for a compressible
plasma: Exact solution of jump relations in the system of
discontinuities is used to investigate how the outflowing jet and the
conditions in the intermediate region depend on the characteristics of
the inflow. The most significant implications concerning large-scale
eruptive phenomena of solar atmosphere are presented.
Title: Properties of Type IV Radio Bursts with Periodical Fine
Structures
Authors: Magdalenić, J.; Vršnak, B.; Zlobec, P.; Messerotti, M.;
Temmer, M.
Bibcode: 2005ASSL..320..259M
Altcode: 2005smp..conf..259M
No abstract at ADS
Title: The Acceleration-Velocity Relationship in 5000 LASCO-CME's
Authors: Ruždjak, D.; Vršnak, B.; Sudar, D.
Bibcode: 2005ASSL..320..195R
Altcode: 2005smp..conf..195R
No abstract at ADS
Title: 2 1/2-Dimensional Reconnection Model and Energy Release in
Solar Flares
Authors: Vršnak, Bojan; Skender, Marina
Bibcode: 2005SoPh..226...97V
Altcode:
We employ a 2 1/2-dimensional reconnection model to analyse different
aspects of the energy release in two-ribbon flares. In particular,
we investigate in which way the systematic change of inflow region
variables, associated with the vertical elongation of current sheet,
affects the flare evolution. It is assumed that as the transversal
magnetic field decreases, the ambient plasma-to-magnetic pressure ratio
increases, and the reconnection rate diminishes. As the transversal
field decreases due to the arcade stretching, the energy release
enhances and the temperature rises. Furthermore, the magnetosonic
Mach number of the reconnection outflow increases, providing the
formation of fast mode standing shocks above the flare loops and
below the erupting flux rope. Eventually, in the limit of a very
small transversal field the reconnection becomes turbulent due to
a highly non-linear response of the system to small fluctuations of
the transversal field. The turbulence results in the energy release
fragmentation which increases the release efficiency, and is likely
to be responsible for the impulsive phase of the flare. On the other
hand, as the current sheet stretches to larger heights, the ambient
plasma-to-magnetic pressure ratio increases which causes a gradual
decrease of the reconnection rate, energy release rate, and temperature
in the late phase of flare. The described magnetohydrodynamical changes
affect also the electron distribution function in space and time. At
large reconnection rates (impulsive phase of the flare) the ratio of the
inflow-to-outflow magnetic field strength is much smaller than at lower
reconnection rates (late phase of the flare), i.e., the corresponding
loss-cone angle becomes narrower. Consequently, in the impulsive phase
a larger fraction of energized electrons can escape from the current
sheet downwards to the chromosphere and upwards into the corona -
the dominant flare features are the foot-point hard X-ray sources and
type III radio bursts. On the other hand, at low reconnection rates,
more particles stay trapped in the outflow region, and the thermal
conduction flux becomes strongly reduced. As a result, a superhot
loop-top, and above-the-loop plasma appears, as sometimes observed,
to be a dominant feature of the gradual phase.
Title: Loop-Top Altitude Decrease in an X-Class Flare
Authors: Veronig, A.; Vršnak, B.; Karlický, M.; Temmer, M.;
Magdalenić, J.; Dennis, B. R.; Otruba, W.; Pötzi, W.
Bibcode: 2005HvaOB..29..127V
Altcode:
We study RHESSI X-ray source motions in the X3.9 flare of 2003 November
3. Particular attention is drawn to the apparent altitude decrease
of a distinct loop-top (LT) source at the early flare phase before
then changing to the commonly observed upward expansion of the flare
loop system. We obtain that the downward motion is more pronounced
at higher X-ray energies (peak values up to 50 km s^{-1}) consistent
with recent findings by Sui et al. (2004). RHESSI spectra indicate
that the emission process in the LT source is thermal bremsstrahlung
from a super hot plasma (∼40 MK) with high densities increasing from
∼10^{10} cm^{-3} early in the flare to several times 10^{11} cm^{-3}
at the end of RHESSI observations.
Title: Coronal Mass Ejection of 15 May 2001: II. Coupling of the
Cme Acceleration and the Flare Energy Release
Authors: Vršnak, B.; Maričić, D.; Stanger, A. L.; Veronig, A.
Bibcode: 2004SoPh..225..355V
Altcode:
We analyze the relationship between the dynamics of the coronal mass
ejection (CME) of 15 May 2001 and the energy release in the associated
flare. The flare took place behind the east limb and was disclosed by a
growing system of hot soft X-ray (SXR) loops that appeared from behind
the limb around the onset of the rapid acceleration of the CME. The
highly correlated behavior of the SXR light-curve derivative and the
time profile of the CME acceleration reveals an intrinsic relationship
between the CME dynamics and the flare energy release. Furthermore,
we found that the CME acceleration peak occurs simultaneously with the
fastest growth (100 km s-1) of X-ray loops, indicating that
the reconnection plays an essential role in the eruption. Inspecting
the CME/flare morphology we recognized in the Yohkoh-SXT images an
oval feature that formed within the rising structure at the onset
of the rapid acceleration phase, simultaneously with the appearance
of the X-ray loops. The eruptive prominence was imbedded within
the lower half of the oval, suggestive of a flux-rope/prominence
magnetic configuration. We interpret the observed morphological
evolution in terms of a reconnection process in the current sheet that
presumably formed below the erupting flux-rope at the onset of the CME
acceleration. Measurements of the tip-height of the cusped X-ray loop
system and the height of the lower edge of the oval, enable us to trace
the stretching of the current sheet. The initial distance between the
oval and the loops amounted to 35 - 40 Mm. In about 1 h the inferred
length of the current sheet increased to 150 - 200 Mm, which corresponds
to a mean elongation speed of 35 - 45 km s-1. The results
are discussed in the framework of CME models that include the magnetic
reconnection below the erupting flux-rope.
Title: Coronal Mass Ejection of 15 May 2001: I. Evolution of
Morphological Features of the Eruption
Authors: Maričić, D.; Vršnak, B.; Stanger, A. L.; Veronig, A.
Bibcode: 2004SoPh..225..337M
Altcode:
We study the initiation and development of the limb coronal mass
ejection (CME) of 15 May 2001, utilizing observations from Mauna Loa
Solar Observatory (MLSO), the Solar and Heliospheric Observatory (SOHO),
and Yohkoh. The pre-eruption images in various spectral channels show
a quiescent prominence imbedded in the coronal void, being overlaid
by the coronal arch. After the onset of rapid acceleration, this
three-element structure preserved its integrity and appeared in the
MLSO MK-IV coronagraph field of view as the three-part CME structure
(the frontal rim, the cavity, and the prominence) and continued its
motion through the field of view of the SOHO/LASCO coronagraphs up
to 30 solar radii. Such observational coverage allows us to measure
the relative kinematics of the three-part structure from the very
beginning up to the late phases of the eruption. The leading edge and
the prominence accelerated simultaneously: the rapid acceleration of
the frontal rim and the prominence started at approximately the same
time, the prominence perhaps being slightly delayed (4 - 6 min). The
leading edge achieved the maximum acceleration amax≈ 600
± 150 m s−2 at a heliocentric distance 2.4 -2.5 solar
radii, whereas the prominence reached amax≈ 380± 50 m
s−2, almost simultaneously with the leading edge. Such a
distinct synchronization of different parts of the CME provides clear
evidence that the entire magnetic arcade, including the prominence,
erupts as an entity, showing a kind of self-similar expansion. The
CME attained a maximum velocity of vmax≈ 1200 km
s−1 at approximately the same time as the peak of the
associated soft X-ray flare. Beyond about 10 solar radii, the leading
edge of the CME started to decelerate at a≈−20 m s−2,
most likely due to the aerodynamic drag. The deceleration of the
prominence was delayed for 10 -30 min, which is attributed to its
larger inertia.
Title: Kinematics of coronal mass ejections between 2 and 30 solar
radii. What can be learned about forces governing the eruption?
Authors: Vršnak, B.; Ruždjak, D.; Sudar, D.; Gopalswamy, N.
Bibcode: 2004A&A...423..717V
Altcode:
Kinematics of more than 5000 coronal mass ejections (CMEs)
measured in the distance range 2-30 solar radii is investigated. A
distinct anticorrelation between the acceleration, a, and the
velocity, v, is found. In the linear form, it can be represented
as a=-k1(v-v0), where v0=400 km
s-1, i.e., most of CMEs faster than 400 km s-1
decelerate, whereas slower ones generally accelerate. After grouping
CMEs into the width and mean-distance bins, it was found that the
slope k1 depends on these two parameters: k1
is smaller for CMEs of larger width and mean-distance. Furthermore,
the obtained CME subsets show distinct quadratic-form correlations,
of the form a= -k2 (v-v0)| v-v0|. The
value of k2 decreases with increasing distance and width,
whereas v0 increases with the distance and is systematically
larger than the slow solar wind speed by 100-200 km s-1. The
acceleration-velocity relationship is interpreted as a consequence of
the aerodynamic drag. The excess of v0 over the solar wind
speed is explained assuming that in a certain fraction of events the
propelling force is still acting in the considered distance range. In
most events the inferred propelling force acceleration at 10 solar radii
ranges between aL=0 and 10 m s-2, being on average
smaller at larger distances. However, there are also events that show
aL>50 m s-2, as well as events indicating
aL<0. Implications for the interplanetary motion of
CMEs are discussed, emphasizing the prediction of the 1 a.u. arrival
time. Appendices A and B are only available in electronic form
at http://www.edpsciences.org
Title: A multiwavelength study of solar flare waves. II. Perturbation
characteristics and physical interpretation
Authors: Warmuth, A.; Vršnak, B.; Magdalenić, J.; Hanslmeier, A.;
Otruba, W.
Bibcode: 2004A&A...418.1117W
Altcode:
The study of solar flare waves - globally propagating wave-like
disturbances usually observed in Hα as Moreton waves - has recently
come back into focus prompted by the observation of coronal waves in
the EUV with the SOHO/EIT instrument (``EIT waves"), and in several
additional wavelength channels. We study 12 flare wave events in
order to determine their physical nature, using Hα, EUV, helium I,
SXR and radioheliographic data. In the companion Paper I, we have
presented the observational data and have discussed the morphology,
spatial characteristics and the kinematics of the different flare
wave signatures. The wavefronts observed in the various spectral bands
were found to follow kinematical curves that are closely associated,
implying that they are signatures of the same physical disturbance. In
the present paper, we continue the study with a close examination of
the evolution of the common perturbation that causes the different wave
signatures, and with a detailed analysis of the metric type II radio
bursts that were associated with all flare wave events. The basic
characteristics of the waves are deceleration, perturbation profile
broadening, and perturbation amplitude decrease. This behavior can
be interpreted in terms of a freely propagating fast-mode MHD shock
formed from a large-amplitude simple wave. It is shown that this
scenario can account for all observed properties of the flare waves
in the various spectral bands, as well as for the associated metric
type II radio bursts.
Title: A multiwavelength study of solar flare waves. I. Observations
and basic properties
Authors: Warmuth, A.; Vršnak, B.; Magdalenić, J.; Hanslmeier, A.;
Otruba, W.
Bibcode: 2004A&A...418.1101W
Altcode:
Propagating wave-like disturbances associated with solar flares -
commonly observed in the chromosphere as Moreton waves - have been
known for several decades. Recently, the phenomenon has come back into
focus prompted by the observation of coronal waves with the SOHO/EIT
instrument (``EIT waves"). It has been suggested that they represent
the anticipated coronal counterpart to Moreton waves, but due to some
pronounced differences, this interpretation is still being debated. We
study 12 flare wave events in order to determine their physical nature,
using Hα, EUV, He I 10 830 Å, SXR and radioheliographic data. The
flare wave signatures in the various spectral bands are found to
lie on closely associated kinematical curves, implying that they are
signatures of the same physical disturbance. In all events, and at
all wavelengths, the flare waves are decelerating, which explains
the apparent ``velocity discrepancy" between Moreton and EIT waves
which has been reported by various authors. In this paper, the focus
of the study is on the morphology, the spatial characteristics and the
kinematics of the waves. The characteristics of the common perturbation
which causes the wave signatures, as well as the associated type II
radio bursts, will be studied in companion Paper II, and a consistent
physical interpretation of flare waves will be given.
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: Exact solution of jump relations at discontinuities in a
2.5-D reconnection model
Authors: Skender, M.; Vrsnak, B.; Martinis, M.
Bibcode: 2004cosp...35..192S
Altcode: 2004cosp.meet..192S
Exact solution of the complete set of jump relations at the rotational
discontinuity and the slow mode shock in a two-and-half dimensional
(2 1/2 - D) symmetric reconnection model is presented. The solution is
used for analyzing the outflow jet characteristics in dependence on the
speed and the incidence angle of the inflowing plasma, for a given shear
of the inflow magnetic field. It is found that the outflow magnetosonic
Mach number depends significantly on the incidence angle, the effect
being more prominent at larger reconnection rates. Significant dynamical
changes in the flow/field geometry are found in the transition to the
2-D regime: In the region between the rotational discontinuity and the
slow mode shock the direction of flow and the magnetic field become
extremely sensitive to the degree of the magnetic field shear in the
inflow. Implications for evolutionary systems like solar flares are
discussed. M. Skender, B. Vrsnak, M.Martinis, Phys. Rev. E 68 (2003)
046405
Title: Forces governing coronal mass ejections
Authors: Vrsnak, B.
Bibcode: 2004cosp...35..697V
Altcode: 2004cosp.meet..697V
The general morphology and kinematics of coronal mass ejections (CMEs)
are briefly described to outline physical aspects that are relevant for
the understanding of CME propagation. The CME dynamics can be explained
combining characteristics of the Lorentz force, the aerodynamic drag,
and the gravity with the geometrical properties of CMEs. Whereas the
Lorentz force plays a dominant role during the main acceleration
stage that takes place within the distance of a few solar radii,
the drag becomes essential in the upper corona and interplanetary
space. Utilizing a large sample of more than 5000 CMEs measured
in the distance range of 2--30 solar radii, the properties of the
drag force are inspected. Analyzing the quadratic-form correlation,
a =-k(v-v_0)| v-v_0|, between the acceleration, a, and the velocity,
v, it is found that k is smaller for wider CMEs and decreases with
heliocentric distance. Furthermore, v_0 increases with distance and
is systematically larger than the slow solar wind speed by 100--200 km
s-1. Such an excess of v_0 is explained assuming that in a
certain fraction of events the propelling force is still acting in the
considered distance range. In most events the inferred propelling-force
acceleration at R=10 ranges between a_L=0 and 10 m s-2,
being on average smaller at larger distances. However, there are also
events showing a_L>50 m s-2, as well as the events
indicating a_L<0. The prolonged acceleration of fast events is
attributed to the reconnection below the erupting flux rope, which
might also cause a two-ribbon flare if the associated energy release
is powerful enough. Implications for the interplanetary motion of CMEs
are discussed, emphasizing the prediction of the Sun-Earth transit time.
Title: Are there really two types of cmes?
Authors: Vrsnak, B.
Bibcode: 2004cosp...35..700V
Altcode: 2004cosp.meet..700V
A statistical analysis of coronal mass ejections (CMEs) is performed
to compare the kinematical properties of flare-associated (F)
and non-flare (nF) CMEs. The study is based on measurements of
545 F-CMEs and 211 nF-CMEs in the distance range R=2--30 solar
radii. No statistically significant difference is found: e.g.,
the mean accelerations and mean velocities amount to a_F=-1.8± 17 m
s-2, anF=-0.3± 19 m s-2 and v_F=581±
314 km s-1, vnF=576± 323 km s-1,
respectively. Such an outcome indicates that the division of CMEs into
two distinct categories is highly doubtful. The argument is strengthened
by showing examples of fast F-CMEs significantly accelerating beyond
R=2, and fast nF-CMEs moving at a constant speed or decelerating in the
considered distance range. These examples disprove the scheme according
to which the flare-associated CMEs are fast and move at a constant speed
(or decelerate) in the coronagraph field of view, whereas the non-flare
CMEs expose a gradual acceleration. Yet, it is shown that there is a
well-defined relationship between the kinematical properties of CMEs
and importance of associated flares: CMEs related to more powerful
flares tend to be faster. However, the distribution of events along
the speed-importance correlation is homogeneous, i.e., there is no
grouping of CMEs into distinguishable classes. On the other hand, it
is also found that wider CMEs are faster on average, and tend to show
a prolonged acceleration phase. When only CMEs of a given width-class
are isolated, the non-flare CMEs show kinematical parameters similar
to CMEs associated with the flares of lowest importances. The results
are interpreted in terms of the erupting flux rope model.
Title: Magnetic reconnection in solar flares
Authors: Vršnak, B.; Skender, M.
Bibcode: 2004HvaOB..28..103V
Altcode:
Various aspects of solar flares are considered in the framework of
analytic solutions of the complete set of MHD equations describing the
2 1/2 -- D Petschek's reconnection model. In particular, we compare the
theoretical results with characteristics of two-ribbon flares since they
provide spatially resolved measurements of various plasma parameters. We
emphasize the evolutionary aspect of the process, focusing on the
effects of the change of the current sheet length-to-width ratio,
longitudinal-to-transversal field ratio, and the ambient value of
the plasma-to-magnetic pressure ratio. These parameters determine the
conditions for the onset of fast reconnection, the transition to the
turbulent reconnection regime, the spatial distribution of the density
and temperature, and the efficiency of the particle acceleration.
Title: Band-splitting of coronal and interplanetary type
II bursts. III. Physical conditions in the upper corona and
interplanetary space
Authors: Vršnak, B.; Magdalenić, J.; Zlobec, P.
Bibcode: 2004A&A...413..753V
Altcode:
We analyse properties of 58 type II radio bursts recorded in the
meter-to-kilometer wavelength range, focusing on episodes of band-split
emission. The basic two parameters utilized are the frequency drift
Df=df/dt and the relative band-split BDW=Δ f/f of type
II burst emission lanes. On average, in the meter-to-kilometer
wavelength range Df increases with the emission frequency
as Df∝ f1.83, revealing that source velocities
are smaller at larger heliocentric distances. The relative band-split
shows a weak but statistically significant dependence on the emission
frequency, BDW∝ f-0.06, indicating an increase of BDW with
the heliocentric distance. Combining the shock velocity estimated from
the frequency drift, with the Mach number inferred from the band-split,
the Alfvén speed and the magnetic field in the ambient plasma can
be estimated as a function of the heliocentric distance r. However,
the outcome directly depends on the coronal/interplanetary density
model used, which is poorly known in the upper corona and the near-Sun
interplanetary space. So, we invert the problem: utilizing the results
of the previous paper where it was shown that beyond the heliocentric
distance of two solar radii (r/r⊙=R>2) the average
magnetic field decreases approximately as B∝ R-2, we
infer the density n(R) in the upper corona and near-Sun interplanetary
space. The obtained empirical dependence n(R) is presented in the
analytical form as a four-degree polynomial of 1/R, and is compared
with some theoretical n(R) models, considering also a deviation from
the B∝ 1/R2 scaling used. The model matches the five-fold
Saito density model (representing the active region corona) with the
n∝ R-2 regime in the interplanetary space. Furthermore, it
is shown that on average the magnetosonic speed attains a local minimum
of vms≈ 400 km s-1 around R=3 and a broad local
maximum of vms≈ 500 km s-1 in the range R=4-6,
beyond which it gradually decreases to several tens km s-1
at 1 a.u. The local minimum becomes even deeper if the super-radial
expansion of the magnetic field is taken into account. The implications
regarding the formation and evolution of shocks in the corona and upper
corona are discussed in the framework of CME-piston and flare-blast
scenarios. The inferred general decrease of type II burst source
velocities and broadening of band-splits with distance is interpreted
in terms of the deceleration of mass ejections driving the shocks in
the decreasing vms environment. Appendices A and B
are only available in electronic form at http://www.edpsciences.org
Title: Interaction of an Erupting Filament with the Ambient
Magnetoplasma and Escape of Electron Beams
Authors: Vršnak, Bojan; Warmuth, Alexander; Maričić, Darije;
Otruba, Wolfgang; Ruždjak, Vladimir
Bibcode: 2003SoPh..217..187V
Altcode:
A huge filament eruption of 12 September 2000 associated with a
two-ribbon spotless flare is described. During the acceleration phase
the shape of the filament changed, and signatures of topological
restructuring of large-scale coronal magnetic fields were inferred
by tracking changes of nearby coronal holes. At the same time
electron beams associated with the flare impulsive phase escaped
into interplanetary space. Based on the time-spatial relationships a
hypothesis is put forward, according to which the reconnection between
the arcade magnetic field and the ambient field provides a temporary
link between the open field lines and the flare energy release site,
enabling the escape of electron beams into interplanetary space.
Title: Dynamics of coronal mass ejections in the near-Sun
interplanetary space
Authors: Vrsnak, B.; Ruzdajak, D.; Sudar, D.; Gopalswamy, N.
Bibcode: 2003ESASP.535..517V
Altcode: 2003iscs.symp..517V
Kinematics of more than 5000 coronal mass ejections (CMEs) measured
between 2 and 30 solar radii is investigated. A distinct relationship
between the late-phase acceleration of CMEs and their velocities is
found. It can be represented in the form a[m s-2]
= -0.02(v-400)[km s-1]. The relationship is
interpreted in terms of coupling of the CME motion and the solar wind,
i.e., by the action of the aerodynamic drag. The results indicate that
in the considered radial distance range the Lorentz force acceleration
becomes weak, in the majority of the events spanning between 0 and 10
m s-2. Implications for the interplanetary motion of CMEs
are discussed, emphasising the prediction of the 1 a.u. arrival time.
Title: Initiation and development of two coronal mass ejections
Authors: Maričić, D.; Vršnak, B.; Stanger, A. L.; Roša, D.;
Hržina, D.
Bibcode: 2003ESASP.535..441M
Altcode: 2003iscs.symp..441M
We analyze the initiation and development of coronal mass ejections
(CMEs) launched on 2001 May 15 and 2001 May 25. Both CMEs show clearly
recognizable three-part structure already at low heights during
the initial gradual rise in the pre-eruptive phase. This provides
measurements of relative kinematics of the three-part structure from
the very beginning of the eruption up to the post-acceleration phase. In
both CMEs the frontal rim starts accelerating some 10-20 min before the
prominence. However, the acceleration maximum of the prominence and the
frontal rim was synchronized. CMEs attained the maximum acceleration
of ≍350 m s-2 and ≍250 m s-2, at the leading
edge heliocentric distance R=2.4 and R=2.1 solar radii, respectively. In
both cases the acceleration time profile of the frontal rim was broader
than that of the prominence, although the peak values were similar. The
CMEs attained velocities in excess of 1000 km s-1, showing
a weak deceleration at large heights. The leading edge of CME starts
decelerating before the prominence in both cases, indicating that the
deceleration is caused by the aerodynamic drag force.
Title: Radio signatures of fast oscillatory phenomena in the solar
corona
Authors: Magdalenic, Jasmina; Zlobec, P.; Vršnak, B.; Messerotti,
M.; Aurass, H.; Temmer, M.
Bibcode: 2003ESASP.535..619M
Altcode: 2003iscs.symp..619M
During type IV solar radio bursts different types of periodic fine
structures (PFSs) are frequently observed, which can be interpreted
as radio signatures of fast oscillatory phenomena in the coronal
plasma. We analyze a large set of type IV bursts containing PFSs,
recorded with high time resolution at single frequencies in the
metric and decimetric bands. Focusing on the association with flares
and flare-like phenomena we found: PFSs can be found in about 50% of
type IV bursts characterized by fine structures; 10% of PFS-containing
events are weak/short type IV-like radio bursts that occur in absence
of any flare-like activity. In the weakest events the whole radio
burst was in fact just one short PFS-episode recorded at only one
observing frequency; In flare associated events we found two distinct
classes of PFSs - impulsive phase and decay phase related PFSs; yet,
no statistically significant difference in the characteristic periods
and amplitudes is found between the two classes; PFS-rich radio events
are characterized by large SXR and radio peak fluxes - neither one
of the weak type IV bursts was PFS-rich. The opposite is not true:
mainly powerful bursts are PFS-poor.
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: Analysis of periodical fine structures in type IV solar
radio bursts
Authors: Magdaleníc, J.; Zlobec, P.; Vršnak, B.; Messerotti, M.;
Auras, H.; Veronig, A.
Bibcode: 2003HvaOB..27..131M
Altcode:
Superimposed on the continuum of type IV solar radio bursts
fine structures are often observed. We analysed a large data set
of periodical fine structures recorded with high time resolution
at single frequencies (metric to decimetric bands) by the Trieste
Solar Radio System. Diverse classes of fine structures with similar
periodical behaviour but different time profiles and polarization
characteristics were identified. It is demonstrated that on the average
pulsation periods decrease with increasing observing frequency, and
that pulsations recorded above ≈ 600 MHz could be of different nature
than pulsations below ≈ 400 MHz.
Title: Flare waves revisited
Authors: Warmuth, A.; Vršnak, B.; Hanslmeier, A.
Bibcode: 2003HvaOB..27..139W
Altcode:
Recently, the flare wave phenomenon has come back into focus prompted
by the observation of coronal waves with the SOHO/EIT instrument
(EIT waves). It has been suggested that they represent the coronal
counterpart to the chromospheric Moreton waves, but this interpretation
has remained a matter of debate. In order to make inferences on the
physical nature of the phenomenon, 12 flare wave events are studied
using Hα, EUV, Helium I, SXR and radio data. The wavefronts in
the various spectral ranges are following very similar kinematical
curves, which suggests that they are caused by the same physical
disturbance. The characteristics of this disturbance -- deceleration,
perturbation profile broadening, and perturbation amplitude decrease
-- can be interpreted in terms of a freely propagating fast-mode MHD
shock which decays to an ordinary fast-mode wave. This scenario also
accounts for the associated metric type II radio bursts.
Title: Magnetic 3-D Configurations of Energy Release in Solar Flares
Authors: Vrsnak, B.
Bibcode: 2003LNP...612...28V
Altcode: 2003ecpa.conf...28V
Basic concepts and principles usually used to interpret the solar
flare phenomenon are summarized, and traditional classification schemes
based on 2-D magnetic field representations are briefly reviewed. The
extension to 3-D opens new aspects, some of which are sketched in this
paper: In particular two-loop interactions, two-ribbon flare geometry,
and the plasmoid formation in a current sheet are considered. It
is shown that alternative onset processes exist beside the standard
two-ribbon flare scenario in which the pre-flare arcade evolves slowly
through a series of equilibrium states until the eruption. For example,
the arcade can become unstable after an abrupt reformation of its core
through a sequence of loop interactions. The restructuration results in
an impulsive compact flare and the formation of an unstable `sigmoid'
whose eruption provides the two-ribbon phase aftermath. Possible
modalities of main phase are emphasized. Especially the secondary
plasmoid formation is considered and its fate discussed stressing
the 3-D aspect of the process and the effect of line-tying. Finally,
complex events composed of several distinct, but causally related
energy release processes are described.
Title: Solar type II radio bursts: emission from shock segments with
a collapsing trap geometry?
Authors: Magdalenić, J.; Vršnak, B.; Aurass, H.
Bibcode: 2002ESASP.506..335M
Altcode: 2002svco.conf..335M; 2002ESPM...10..335M
The nature of the band-split type II burst radio emission is
discussed. Examples are shown in which the extrapolations of band-split
lanes of type II bursts recorded in the hekto-kilometric wavelength
range fit to the local frequency jump caused by the passage of the
associated interplanetary MHD shock wave at 1 AU. Such cases favour
the interpretation of band-split in terms of the emission from the
upstream and downstream shock region. On the other hand type II bursts
sometimes show relative band-splits smaller than 10%, indicating that
even low amplitude shocks can excite the type II burst emission. This
implies that an additional mechanism is accelerating electrons at
such shocks. It is proposed that the band-split type II emission is
excited at quasi-perpendicular shock segments forming a collapsing trap
geometry. In such a configuration a particular magnetic field line
intersects the shock front twice forming a system of two approaching
magnetic mirrors in which electrons can be accelerated.
Title: Energy release from a large-scale magnetic null point in
the corona?
Authors: Aurass, Henry; Hofmann, Axel; Vršnak, Bojan
Bibcode: 2002ESASP.506..423A
Altcode: 2002svco.conf..423A; 2002ESPM...10..423A
We observed how a flare disturbes a system of interconnecting loops
between neighboring active regions. The disturbed loops brighten in
SOHO/EIT images of the coronal magnetoplasma. They are part of a large
scale structure embracing a weak field range in the photospheric and
coronal magnetic field. Near the weak field site but away from active
regions and from the EIT-detected loopl heating, an initially narrowband
nonthermal meter wave radio source is observed by the AIP spectrometer
and the Nançay radio heliograph. The formation of this source and the
EIT loop brightening can be consequences of current sheet activation
and excessive coronal heating at a disturbed magnetic null point.
Title: Physical conditions in the reconnection outflow
Authors: Skender, M.; Vršnak, B.; Martinis, M.
Bibcode: 2002ESASP.506..757S
Altcode: 2002svco.conf..757S; 2002ESPM...10..757S
The amplitude of the standing fast mode shock formed in the reconnection
outflow is studied. The dependence on the Mach number of the inflow,
the direction of the inflow, the transversal magnetic field, and the
plasma to magnetic pressure ratio is investigated in 2-D and 2 1/2-D
model separately. The results are presented stressing the observable
quantities such as the band-split of the radio emission excited in the
upstream and downstream regions of the fast mode shock. For the range
of parameter values appropriate for the occurrence of solar flares it
is found that the relative band-split should be in the range 10-50%.
Title: Coronal and interplanetary magnetic fields inferred from
band-splitting of type II bursts
Authors: Vršnak, B.; Magdalenić, J.; Aurass, H.; Mann, G.
Bibcode: 2002ESASP.506..409V
Altcode: 2002svco.conf..409V; 2002ESPM...10..409V
Adopting that the band-splitting of type II bursts is a consequence
of the plasma emission from the upstream and downstream shock region,
the band-split reveals the shock Mach number. On the other hand,
the shock speed can be inferred from the frequency drift. Combining
these two parameters the Alfvén velocity and the magnetic field in
the ambient plasma can be estimated. The results of such an analysis
applied to 44 type II bursts recorded from decimetric to kilometric
wavelengths are presented. The inferred magnetic field decreases as
R-2 beyond the radial distance of 1.5 solar radii (R >
1.5) if the two-fold Saito density model is applied at R < 10 and
Leblanc model beyond R ≍ 30. In that case the Alfvén velocity shows
a shallow local minimum of vA ≍ 400 km s-1
at 2 < R < 3 and a broad local maximum of vA ≍ 500
km s-1 at 4 < R < 6.
Title: Band-splitting of coronal and interplanetary type II
bursts. II. Coronal magnetic field and Alfvén velocity
Authors: Vršnak, B.; Magdalenić, J.; Aurass, H.; Mann, G.
Bibcode: 2002A&A...396..673V
Altcode:
Type II radio bursts recorded in the metric wavelength range are
excited by MHD shocks traveling through the solar corona. They often
expose the fundamental and harmonic emission band, both frequently
being split in two parallel lanes that show a similar frequency drift
and intensity behaviour. Our previous paper showed that band-splitting
of such characteristics is a consequence of the plasma emission from
the upstream and downstream shock regions. Consequently, the split
can be used to evaluate the density jump at the shock front and to
estimate the shock Mach number, which in combination with the shock
speed inferred from the frequency drift provides an estimate of the
Alfvén velocity and the magnetic field in the ambient plasma. In this
paper such a procedure is applied to 18 metric type II bursts with the
fundamental band starting frequencies up to 270 MHz. The obtained values
show a minimum of the Alfvén velocity at the heliocentric distance R~ 2
amounting to vA~ 400-500 km s-1. It then increases
achieving a local maximum of vA~ 450-700 km s-1
at R~ 2.5. The implications regarding the process of formation and decay
of MHD shocks in the corona are discussed. The coronal magnetic field in
the range 1.3<R<3 decreases as R-3 to R-4,
or H-1.5 to H -2 if expressed as a function
of the height. The results are compared with other estimates of the
coronal magnetic field in the range 1<R<10. Combined data show
that below H<0.3 the magnetic field is dominated by active region
fields, whereas above H=1 it becomes radial, behaving roughly as B=2x
R-2 with a plausible value of B~ 5 nT at 1 a.u.
Title: The Neupert effect and the electron-beam-driven evaporation
model
Authors: Veronig, A.; Vršnak, B.; Dennis, B. R.; Temmer, M.;
Hanslmeier, A.; Magdalenić, J.
Bibcode: 2002ESASP.506..367V
Altcode: 2002svco.conf..367V; 2002ESPM...10..367V
Based on a sample of ~1100 solar flares observed simultaneously in
hard and soft X-rays, we performed a statistical analysis of the
Neupert effect. For a subset of ~500 events, supplementary Hα flare
data were considered. The timing behavior of >50% of the events
is consistent with the Neupert effect. A high correlation between
the soft X-ray peak flux and the hard X-ray fluence is obtained,
being indicative of electron-beam-driven evaporation. However, about
one fourth of the events (predominantly weak flares) reveal strong
deviations from the predicted timing, with a prolonged increase of
the thermal emission beyond the end of the hard X-rays. These findings
suggest that electron-beam-driven evaporation plays an important role
in solar flares. Yet, in a significant fraction of events there is also
evidence for an additional energy transport mechanism from the energy
release site other than electron beams, presumably thermal conduction.
Title: Morphology and polarization of metric and decimetric solar
radio pulsations: a statistical approach
Authors: Magdalenić, J.; Zlobec, P.; Messerotti, M.; Vršnak, B.
Bibcode: 2002ESASP.506..331M
Altcode: 2002ESPM...10..331M; 2002svco.conf..331M
We analyzed a large data set of fine structures observed during type
IV solar radio events recorded with high time resolution at single
frequencies in the metric and decimetric bands by the Trieste Solar
Radio System. Varieties of fine structures with periodical behavior
but different temporal morphologies and polarization characteristics
were identified. For selected events general statistics was derived
in order to describe the evolution in time and at different observing
frequencies, i.e. to infer the evolution of the related coronal plasma
disturbances in time and height. To better characterize the observed
radio pulsations in the framework of the flaring process, we derived
the positions of the associated active regions, and studied the timing
of such radio phenomena with respect to the associated SXR flares.
Title: The Neupert effect in solar flares and implications for
coronal heating
Authors: Veronig, A.; Vrsnak, B.; Dennis, B. R.; Temmer, M.;
Hanslmeier, A.; Magdalenić, J.
Bibcode: 2002ESASP.505..599V
Altcode: 2002solm.conf..599V; 2002IAUCo.188..599V; 2002astro.ph..8089V
Based on simultaneous observations of solar flares in hard and soft
X-rays we studied several aspects of the Neupert effect. About half of
1114 analyzed events show a timing behavior consistent with the Neupert
effect. For these events, a high correlation between the soft X-ray
peak flux and the hard X-ray fluence is obtained, being indicative of
electron-beam-driven evaporation. However, for about one fourth of the
events there is strong evidence for an additional heating agent other
than electron beams. We discuss the relevance of these findings with
respect to Parker's idea of coronal heating by nanoflares.
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: Investigation of the Neupert effect in solar
flares. I. Statistical properties and the evaporation model
Authors: Veronig, A.; Vršnak, B.; Dennis, B. R.; Temmer, M.;
Hanslmeier, A.; Magdalenić, J.
Bibcode: 2002A&A...392..699V
Altcode: 2002astro.ph..7217V
Based on a sample of 1114 flares observed simultaneously in hard X-rays
(HXR) by the BATSE instrument and in soft X-rays (SXR) by GOES, we
studied several aspects of the Neupert effect and its interpretation
in the frame of the electron-beam-driven evaporation model. In
particular, we investigated the time differences (Delta t) between
the maximum of the SXR emission and the end of the HXR emission,
which are expected to occur at almost the same time. Furthermore,
we performed a detailed analysis of the SXR peak flux - HXR fluence
relationship for the complete set of events, as well as separately for
subsets of events which are likely compatible/incompatible with the
timing expectations of the Neupert effect. The distribution of the time
differences reveals a pronounced peak at Delta t = 0. About half of the
events show a timing behavior which can be considered to be consistent
with the expectations from the Neupert effect. For these events, a high
correlation between the SXR peak flux and the HXR fluence is obtained,
indicative of electron-beam-driven evaporation. However, there is also
a significant fraction of flares (about one fourth), which show strong
deviations from Delta t = 0, with a prolonged increase of the SXR
emission distinctly beyond the end of the HXR emission. These results
suggest that electron-beam-driven evaporation plays an important role
in solar flares. Yet, in a significant fraction of events, there is
also clear evidence for the presence of an additional energy transport
mechanism other than nonthermal electron beams, where the relative
contribution is found to vary with the flare importance.
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: Relative timing of solar flares observed at different
wavelengths
Authors: Veronig, A.; Vršnak, B.; Temmer, M.; Hanslmeier, A.
Bibcode: 2002SoPh..208..297V
Altcode: 2002astro.ph..8088V
The timing of 503 solar flares observed simultaneously in hard X-rays,
soft X-rays and Hα is analyzed. We investigated the start and the peak
time differences in different wavelengths, as well as the differences
between the end of the hard X-ray emission and the maximum of the
soft X-ray and Hα emission. In more than 90% of the analyzed events,
a thermal pre-heating seen in soft X-rays is present prior to the
impulsive flare phase. On average, the soft X-ray emission starts 3 min
before the hard X-ray and the Hα emission. No correlation between the
duration of the pre-heating phase and the importance of the subsequent
flare is found. Furthermore, the duration of the pre-heating phase
does not differ for impulsive and gradual flares. For at least half of
the events, the end of the non-thermal emission coincides well with
the maximum of the thermal emission, consistent with the beam-driven
evaporation model. On the other hand, for ∼ 25% of the events there
is strong evidence for prolonged evaporation beyond the end of the
hard X-rays. For these events, the presence of an additional energy
transport mechanism, most probably thermal conduction, seems to play
an important role.
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: Shock-excited radio burst from reconnection outflow jet?
Authors: Aurass, H.; Vršnak, B.; Mann, G.
Bibcode: 2002A&A...384..273A
Altcode:
Models of dynamic (two-ribbon-, arcade) flares involve the formation
of a system of standing slow and possibly also fast mode shock waves
associated with the fast reconnection process below the erupting
filament. These shocks are anticipated theoretically, but are not
unambiguously confirmed by observations. In this paper we identify for
the first time the radio signature of a fast mode outflow termination
shock in a dynamic radio burst spectrogram. The standing fast mode
shock is revealed by a zero-drift type II burst recorded between 300
and 400 MHz. It started almost 1 hour after the impulsive phase of
the 7 April 1997 flare and lasted for more than 30 min. The burst
shows a characteristic herringbone fine structure and a band split
of (10+/-6)% of the emission frequency. No fundamental-harmonic
pattern was observed, and we argue that the feature is fundamental
mode emission. Simultaneous imaging observations (Hα , Yohkoh SXT,
SOHO EIT) show a relaxed postflare loop arcade with a bright soft X-ray
cusp commonly interpreted as a typical reconnection pattern. Conditions
for termination shock formation and excitation of radio emision are
investigated. Favourable circumstances for the radio detection of a
termination shock in the reconnection outflow are a comparatively large
height of the diffusion region, a low plasma to magnetic pressure
ratio beta upstream of the slow shocks, and a small angle between
the reconnecting field lines. Finally, we discuss why similar radio
signatures are not observed more frequently, and why it appeared
so late in the event. We stress the implications and point to some
inconsistencies which might be a consequence of commonly practiced ad
hoc application of idealized model results to realistic conditions.
Title: Moreton waves and their relation with EIT waves
Authors: Warmuth, Alexander; Vršnak, Bojan; Aurass, Henry; Hanslmeier,
Arnold
Bibcode: 2002ESASP.477..195W
Altcode: 2002scsw.conf..195W
Moreton waves, observed in Hα, and the recently discovered coronal
transients known as "EIT waves" have remained fairly poorly understood
phenomena. In particular, the issues of their mutual association and of
the nature of their driver are not resolved. We discuss seven Moreton
waves observed in Hα and derive their basic characteristics. Four of
these events were observed simultaneously in Hα and EUV. A deceleration
of the disturbances is found in all cases. In the 2 May 1998 event,
the cospatiality of Moreton and EIT wave fronts is established and
a detailed analysis of the evolution of the Hα wave, its kinematics
and perturbation profile is carried out. The results - deceleration,
broadening, and decrease of intensity of the profiles - favor the
fast-mode shock ("blast wave") scenario over the CME-associated magnetic
field evolution hypothesis.
Title: Influence of the aerodynamic drag on the motion of
interplanetary ejecta
Authors: Vršnak, Bojan; Gopalswamy, Nat
Bibcode: 2002JGRA..107.1019V
Altcode:
A simple semi-empirical model for the motion of interplanetary ejecta is
proposed to advance the prediction of their arrival times at Earth. It
is considered that the driving force and the gravity are much smaller
than the aerodynamic drag force. The interaction with the ambient solar
wind is modeled using a simple expression for the acceleration $[\dot
\upsilon \]$ = -γ(υ-w), where w = w(R) is the distance-dependent
solar wind speed. It is assumed that the coefficient γ decreases
with the heliocentric distance as γ = αR-β, where α
and β are constants. The equation of motion is integrated numerically
to relate the Earth transit time and the associated in situ velocity
with the velocity of coronal mass ejection. The results reproduce well
the observations in the whole velocity range of interest. The model
values are compared with some other models in which the interplanetary
acceleration is not velocity dependent, as well as with the model where
the drag acceleration is quadratic in velocity $\[\dot \upsilon \]$
= -γ2(υ - w)|υ - w|.
Title: Radio Shocks from Reconnection Outflow Jet? - New Observations
Authors: Aurass, H.; Karlicky, M.; Thompson, B. J.; Vršnak, B.
Bibcode: 2002mwoc.conf..401A
Altcode:
The common analysis of dynamic radio spectrograms with Yohkoh X-ray
images yields information about possible associations between
nonthermal electron acceleration and changes in hot and dense
plasma-magnetic field structures of the corona. Examples are correlated
X-ray-jet--electron beam injections (type III/U bursts), motions of
X-ray blobs and correlated shock-driven (type II) radio bursts, and
sigmoid evolution associated with characteristic type IV burst spectral
fine structures. Here, we demonstrate the first identification of the
radio signature of a reconnection outflow termination shock during a
dynamic flare. Reconnection of magnetic fields is one flare energy
release mechanism. During dynamic flares there is formed a system
of standing slow and - sometimes - also fast mode shock waves in the
space around the diffusion region. This standing fast mode shock is
revealed by a zero-drift type II burst between 300 and 400 MHz. It
exists more than 30 min starting 1 hour after the impulsive flare on
07 April 1997 in AR 8027. It shows herringbone fine structure and 10%
band splitted lanes. We argue for having detected fundamental mode
emission. No fundamental-harmonic pattern was observed. Simultaneous
imaging observations (Hα, Yohkoh SXT, SOHO EIT) show a postflare loop
arcade with a bright soft X-ray cusp. Preferable conditions for the
radio detection of the termination shock are a low plasma to magnetic
pressure ratio eta upstream of the slow shocks, a low diffusion region
rise velocity, and a low reconnection rate. The occurrence of the
termination shock is most probable in late stage of flares.
Title: Processes Governing Coronal and Interplanetary Ejecta
Authors: Vrsnak, B.
Bibcode: 2001AGUFMSH12A0741V
Altcode:
Basic features of coronal mass ejections (CMEs) are surveyed, relating
the observations in chromospheric and transition region spectral lines
with white light coronographic observations. The pre-eruptive prominence
evolution, including the development of internal (helical) structure
is emphasized. Various processes causing destabilization of the
arcade/filament system are noted. Properties of the acceleration phase,
including the relationship between the dynamics of prominence, cavity,
and leading edge of CMEs are stressed revealing various acceleration
scenarios. The decrease of the helical fine structure pitch angle
and the mass leakage estimated in some prominences are presented as
a function of time and compared with acceleration curves. The events
showing a deceleration in late phases of cronographic observations
are used to estimate the influence of the drag force on the flux rope
motion and to evaluate the propelling Lorentz force. The CME velocities,
earth transit times, and in situ measured velocities are analyzed to
infer the intreplanetary dynamics of ejecta.\The observational features
described are explained considering the Lorentz force acting in the
line-tied semi-toroidal flux rope, the gravity, and the aerodynamic
drag. At low heights (initial stage) dominant forces are the Lorentz
force and gravity. After the ejection attains velocity in the order of
100 km s-1 the influence of the aerodynamic drag becomes
important. Since the cold and dense prominence material is leaking
down the rope's legs, and the height increases, the gravity soon
becomes negligible. The Lorentz force also starts decreasing beyond
radial distances of several solar radii. Its action might be enhanced
and prolonged by effects of the reconnection below the rope (long
duration flare association) and by the redistribution of twist into
the expanded uppermost parts of the rope. At large distances the drag
becomes the dominant force. Fast ejecta start decelerating, whereas
those in which the Lorentz force was too weak and the ejection did not
surpass the solar wind speed get an additional acceleration: In the
interplanetary space ejecta asymptotically approach the wind velocity.
Title: Dynamics of solar coronal eruptions
Authors: Vršnak, Bojan
Bibcode: 2001JGR...10625249V
Altcode:
The kinematics of 87 solar eruptive events (flare sprays, eruptive
prominences, and coronal transients) observed above the solar limb
are studied. The data reveal a clear statistical trend for the
highest measured value of the acceleration to be lower in the events
taking place at a larger radial distance. The majority of events
(84%) show a phase of exponential-like growth of the velocity. The
growth rate decreases with the height at which this regime sets in. A
phase of constant acceleration was found only in 11% of cases. In the
postacceleration phase a constant velocity regime was found in 57% of
events. A considerable number of eruptions (32%) exposed a deceleration,
most often showing an exponential-like decay of the velocity. The
related theoretical models are confronted with the observations,
and the implications are discussed.
Title: Solar flares and coronal shock waves
Authors: Vršnak, Bojan
Bibcode: 2001JGR...10625291V
Altcode:
The ignition of coronal shock waves by solar flares is
investigated confronting the results of a simple MHD model with the
observations. Characteristics of type II bursts recorded in the metric
and decametric wavelength range are compared with the properties of
the microwave and soft X-ray bursts of the associated flares. The time
delay of the type II burst beginning after the onset of a flare is
found to be shorter for shorter rise time of a flare burst and for
a more powerful flare. Starting frequencies of type II bursts are
higher for more impulsive flares. Instantaneous relative bandwidths
of the type II bursts are larger for more powerful flares, indicating
a higher Mach number of the shock. Consistent with this, a positive
correlation between the flare importance and the inferred shock velocity
is found. The results are fully consistent with the implications of
the model describing the shock formation by a flare-ignited blast wave.
Title: Evolution of Two EIT/Hα Moreton Waves
Authors: Warmuth, A.; Vršnak, B.; Aurass, H.; Hanslmeier, A.
Bibcode: 2001ApJ...560L.105W
Altcode:
Since the discovery of EIT waves, questions have remained about
the driver of these disturbances and their association with the
chromospheric Moreton waves. In order to resolve some of these issues,
two flare-associated transient events (1997 November 3 and 1998 May 2)
observed simultaneously in Hα and EIT are analyzed. The cospatiality
of Moreton and EIT wave fronts is established, and a deceleration of
the disturbances is found in both events. In the case of 1998 May 2, a
detailed analysis of the evolution of the Moreton wave, its kinematics,
and perturbation profile is carried out. The results-deceleration,
broadening, and decrease of intensity of the profiles-favor the
fast-mode shock (``blast wave'') scenario over the coronal mass
ejection-associated magnetic field evolution hypothesis.
Title: Band-splitting of coronal and interplanetary type II
bursts. I. Basic properties
Authors: Vršnak, B.; Aurass, H.; Magdalenić, J.; Gopalswamy, N.
Bibcode: 2001A&A...377..321V
Altcode:
Patterns analogous to the band-splitting of metric type II bursts are
found in a number of type II bursts observed in the dekameter-kilometer
wavelength range. A similarity of morphological and frequency-time
characteristics of two emission components are indicative of a
common source. Relative frequency splits span in the range Delta
f/f=0.05-0.6. At radial distances between 2 and 4 Rsun only
small splits around 0.1 can be found. In the interplanetary space the
relative split on average increases with the radial distance, whereas
the inferred shock velocity decreases. In three events extrapolations
of the split components point to the base and the peak of the jump
in the local plasma frequency caused by the associated shock passage
at 1 AU. This is suggestive of the plasma radiation from the regions
upstream and downstream of the shock. Adopting this interpretation, one
finds that the drop of Delta f/f at 2-4 Rsun is congruent
with the Alfvén velocity maximum expected there. The split increase
and the velocity decrease at larger distances can be explained as a
consequence of declining Alfvén speed in the interplanetary space.
Title: Comparative Analysis of Type ii Bursts and of Thermal and
non-Thermal Flare Signatures
Authors: Vršnak, B.; Magdalenić, J.; Aurass, H.
Bibcode: 2001SoPh..202..319V
Altcode:
The relationship between metric type II radio bursts and solar flares is
studied. Well-defined correlations between the properties of type II
bursts and the characteristics of associated microwave and soft X-ray
bursts are established in two entirely independent data sets. It is
shown that the correlations are strongly affected by the wide range
of coronal Alfvén velocities involved, comprising values from only
150 up to 800 km s−1, with a typical value of 400 km
s−1. After careful data analysis it was inferred that
type II bursts are more closely related to the soft X-ray bursts than
they are to microwave bursts. The correlations indicate that type II
burst shocks are preferably generated by flares with a relatively
strong thermal component, and that the shocks are probably ignited
by the plasma expansion associated with the 'evaporation' process in
the transition region. Although the results imply that the majority
of metric type II bursts are caused by flares, a simple geometrical
consideration shows that a fraction of non-flare type II bursts cannot
be explained by behind-limb events and that roughly 10% of metric type
II bursts should be attributed to non-flare coronal mass ejections.
Title: Deceleration of Coronal Mass Ejections
Authors: Vršnak, Bojan
Bibcode: 2001SoPh..202..173V
Altcode:
Decelerated motion of 12 coronal eruptions is studied. It is found that
the measured decelerations and deceleration rates depend on the events'
plane-of-sky velocities and heights. The dependence of deceleration on
the velocity is described better by a quadratic function then by linear
fit. Results are interpreted in terms of a viscous drag. An empirical
relation expressing the decrease of the drag effectiveness with the
projected height is established. The interplay between the Lorentz
force, viscous drag, and gravity is discussed. Several examples are
considered to illustrate the relative contributions of these forces
under various circumstances.
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: Onset of Metric and Kilometric Type II Bursts
Authors: Vršnak, B.
Bibcode: 2001ASSL..259..255V
Altcode: 2001dysu.conf..255V
A model governing the evolution of a large amplitude magnetosonic
wave is applied to coronal and interplanetary conditions, with the
aim to investigate the starting frequencies and the onset times of
metric and kilometric type II bursts. The results are compared with
the statistical properties of a sample of metric type II bursts and
the associated flares.
Title: Formation of Coronal Shock Waves
Authors: Vršnak, B.
Bibcode: 2001ASSL..259..251V
Altcode: 2001dysu.conf..251V
The evolution of the leading edge of a large amplitude perturbation
is studied to investigate the formation of a perpendicular MHD shock
wave. The results are applicable to metric and kilometric solar
type~II bursts.
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: 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: Statistical Study of Solar Flares Observed in Soft X-Ray,
Hard X-Ray and Hα Emission
Authors: Veronig, A.; Vršnak, B.; Temmer, M.; Magdalenić, J.;
Hanslmeier, A.
Bibcode: 2001HvaOB..25...39V
Altcode:
Correlations among statistical properties of solar flares observed
in soft X-rays, hard X-rays and Hα are studied. We investigate
corresponding HXR flares measured by BATSE, SXR flares observed by GOES
and Hα flares reported in the SGD for the period 1997--2000. Distinct
correlations are found among the SXR peak flux and Hα area, as well
as between the SXR peak flux and HXR fluence. This can be comprehended
in the frame of the chromospheric evaporation model of flares.
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: Formation of coronal MHD shock waves - II. The Pressure
Pulse Mechanism
Authors: Vršnak, B.; Lulić, S.
Bibcode: 2000SoPh..196..181V
Altcode:
The ignition of coronal shock waves by flares is investigated. It
is assumed that an explosive expansion of the source region caused
by impulsive heating generates a fast-mode MHD blast wave which
subsequently transforms into a shock wave. The solutions of 1-D
MHD equations for the flaring region and for the external region
are matched at their boundary. The obtained results show under what
conditions flares can ignite shock waves that excite the metric type
II bursts. The heat input rate per unit mass has to be sufficiently
high and the preflare value of the plasma parameter β in the flaring
region has to be larger than β0crit. The critical
values depend on the flare dimensions and impulsiveness. Larger and more
impulsive flares are more effective in generating type II bursts. Shock
waves of a higher Mach number require a higher preflare value of β
and a more powerful heating per unit mass. The results demonstrate why
only a small fraction of flares is associated with type II bursts and
why the association rate increases with the flare importance.
Title: Formation Of Coronal Mhd Shock Waves - I. The Basic Mechanism
Authors: Vršnak, B.; Lulić, S.
Bibcode: 2000SoPh..196..157V
Altcode:
The formation and evolution of a large amplitude MHD perturbation
propagating perpendicular to the magnetic field in a perfectly
conducting low β plasma is studied. The perturbation is generated
by an abrupt expansion of the source region. Explicit expressions
for the time and the distance needed for the transformation of the
perturbation's leading edge into a shock wave are derived. The results
are applied to coronal conditions and the dynamic spectra of the radio
emission excited by the shock are synthesized, reproducing metric and
kilometric type II bursts. The features corresponding to the metric
type II burst precursor and the moving type IV burst in the case of
kilometric type II bursts are identified. A specific radio signature
that is sometimes observed at the onset of a metric type II burst is
found to appear immediately before the shock wave formation due to
the associated growth of the magnetic field gradient. Time delays and
starting frequencies of bursts' onsets are calculated and presented
as a function of the impulsiveness of the source-region expansion,
using different values of the ambient Alfvén velocity and various time
profiles of the expansion velocity. The results are confronted with
the observations of metric and kilometric type II solar radio bursts.
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: Correlations Between Properties of Type II Bursts and
Associated Flares
Authors: Magdalenic, J.; Vrsnak, B.
Bibcode: 2000HvaOB..24....1M
Altcode:
Relations between characteristics of metric type II bursts and the
associated microwave bursts are investigated to test the model of the
shock wave formation via a flare ignited blast. It is found that the
time delay of the type II burst start after the onset of the microwave
burst is shorter for a more powerful microwave burst. A considerably
lower correlation coefficient is found for the microwave burst
impulsiveness. The starting frequency of a type II burst increases
with the microwave burst impulsiveness. The instantaneous relative
bandwidths and the propagation velocities are positively correlated,
and are larger when associated with stronger microwave bursts. All of
the obtained correlations are fully consistent with the flare ignited
blast wave scenario for the shock wave formation.
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: On the Origin and Propagation of Coronal and Interplanetary
MHD Shock Waves
Authors: Vrsnak, B.; Lulic, S.
Bibcode: 2000HvaOB..24...17V
Altcode:
Various scenaria for the formation of coronal and interplanetary MHD
shock waves are surveyed. The main characteristics of shocks generated
by different mechanisms are summarized and observational signatures are
illustrated by several examples. An explanation is proposed showing
how a flare ignited shock can sustain in propagating through the
interplanetary space. Signatures that might reveal the shock source
and the shock formation mechanisms are discussed.
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: 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: Flares in Sigmoidal Coronal Structures a Case Study
Authors: Aurass, H.; Vršnak, B.; Hofmann, A.; Rudžjak, V.
Bibcode: 1999SoPh..190..267A
Altcode:
We analyze radio observations, magnetograms and extrapolated field
line maps, Hα filtergrams, and X-ray observations of two flare events
(6 February 1992 in AR 7042 and 25 October 1994 in AR 7792) and study
properties, evolution and energy release signatures of sigmoidal loop
systems. During both events, the loop configuration seen in soft X-ray
(SXR) images changes from a preflare sigmoidal shape to a relaxed
post-flare loop system. The underlying magnetic field system consists
of a quadrupolar configuration formed by a sheared arcade core and
a remote field concentration. We demonstrate two possibilities: a
sigmoidal SXR pattern can be due to a single continuous flux tube (the
1992 event). Alternatively, it can be due to a set of independent loops
appearing like a sigmoid (the 1994 event). In both cases, the preflare
and post-flare loops can be well reproduced by a linear force-free
field and potential field, respectively, computed using preflare
magnetograms. We find that thermal and non-thermal flare energy release
indicators of both events become remarkably similar after applying
spatial and temporal scale transformations. Using the spatial scaling
between both events we estimated that the non-thermal energy release
in the second event liberated about 1.7 times more energy per unit
volume. A two-and-a-half times faster evolution indicates that the rate
of the energy release per unit volume is more than four times higher
in this event. A coronal type II burst reveals ignition and propagation
of a coronal shock wave. In contrast, the first event, which was larger
and released about a 10 times more energy during the non-thermal phase,
was associated with a CME, but no type II burst was recorded. During
both events, in addition to the two-ribbon flare process an interaction
was observed between the flaring arcade and an emerging magnetic flux
region of opposite polarity next to the dominant leading sunspot. The
arcade flare seems to stimulate the reconnection process in an `emerging
flux-type' configuration, which significantly contributes to the energy
release. This regime is characterized by the quasiperiodic injection of
electron beams into the surrounding extended field line systems. The
repeated beam injections excite pulsating broadband radio emission in
the decimetric-metric wavelength range. Each radio pulse is due to a
new electron beam injection. The pulsation period (seconds) reflects
the spatial scale of the emerging flux-type field configuration. Since
broadband decimetric-metric radio pulsations are a frequent radio
flare phenomenon, we speculate that opposite-polarity small-scale flux
intrusions located in the vicinity of strong field regions may be an
essential component of the energy release process in dynamic flares.
Title: 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: Multi-Frequency Observations of the February 6, 1992 Flare
Authors: Ruzdjak, V.; Vrsnak, B.; Aurass, H.; Hofmann, A.; Schroll, A.
Bibcode: 1999HvaOB..23....1R
Altcode:
Observations of the two-ribbon flare of February 6, 1992 which occured
in the active region NOAA 7042 involving a sigmoidal soft X-ray loop
pattern, are described. YOHKOH SXT images, H-alpha filtergrams and
coronal magnetic field extrapolations are used to reveal details of
the preflare coronal magnetic field configuration. The development of
the flare in SXR, H-alpha and dm-m radio wavelength range is followed
in detail to disclose the basic stages of the energy release process.
Title: Energy Release Stages of the Complex Solar Flare of October
25, 1994
Authors: Vrsnak, B.; Aurass, H.; Ruzdjak, V.; Hofmann, A.; Schroll, A.
Bibcode: 1999HvaOB..23...15V
Altcode:
The morphology and evolution of the 1n/C4.7 flare which occured in
the active region NOAA 7792 on October 25, 1994 are studied using the
observations in the SXR, optical and radio range. The preflare coronal
magnetic field structure is inferred comparing the force-free magnetic
field extrapolations of the photospheric longitudinal magnetograms
with structures seen in YOHKOH SXT images and H-alpha filtergrams. The
preflare arcade consisted of a highly sheared system of loops which was
abruptly transformed during the impulsive energy release. The further
evolution was basically following the two-ribbon flare scenario,
including a protrusion of the energy release process into the strong
magnetic field of the major sunspot, as well as an interaction with
an emerging flux region.
Title: 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: Prominence Eruptions (Review)
Authors: Vrsnak, B.
Bibcode: 1998ASPC..150..302V
Altcode: 1998npsp.conf..302V; 1998IAUCo.167..302V
No abstract at ADS
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: Sunspot activity and tree growth in Northern Croatia.
Authors: Vršnak, B.; Božić, H.; Roša, J.; Roša, D.; Korica, S.
Bibcode: 1997joso.proc...91V
Altcode:
No abstract at ADS
Title: Correlation of Impulsive Microwave Bursts with Type M Bursts
and Solar Flares
Authors: Ruzdjak, V.; Vrsnak, B.; Pohjolainen, S.; Urpo, S.; Urbarz,
H. W.
Bibcode: 1996ASPC...93..381R
Altcode: 1996ress.conf..381R
No abstract at ADS
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: Soft X-ray radiation associated with flares developing in
strong magnetic fields.
Authors: Vršnak, B.; Ruždjak, V.; Altas, L.; Özgüç, A.; Zlobec, P.
Bibcode: 1996joso.proc..165V
Altcode:
No abstract at ADS
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: Ignition of MHD Shocks Associated with Solar Flares
Authors: Vrsnak, B.; Ruzdjak, V.; Zlobec, P.; Aurass, H.
Bibcode: 1995SoPh..158..331V
Altcode:
We have selected single frequency recordings of 28 `high-frequency'
type II bursts characterized by a starting frequency greater than
237 MHz to estimate as accurately as possible the `launch-time' of
the flare-associated MHD shocks. We established the time associations
between metric type II burst onsets and the time characteristics of
the microwave and X-ray fluxes of the associated flares. The associated
flares were impulsive events with rise times most often about 1 min in
the hard X-ray range and 1-2 min in the microwave wavelength range. The
majority of the type II bursts from our sample started about 1 min
after the maximum of the microwave burst. Launch times of MHD shocks
producing type II bursts were obtained using the 10 × Saito coronal
model and shock velocities estimated from burst characteristics at
different frequencies. Back-extrapolations of type II recordings
indicate that MHD shocks are launched in the time interval prior to
the maximum of the first peak in the associated microwave burst, most
probably at the beginning of the rapid increase of the microwave burst.
Title: Projected Heliographic Coordinates of Objects Located in the
Solar Atmosphere
Authors: Rosa, D.; Vrsnak, B.; Bozic, H.
Bibcode: 1995HvaOB..19...23R
Altcode:
The relation between the real and apparent (projected) heliographic
coordinates of an object located in the solar atmosphere is presented
as a function of the object's height relative to the solar surface.The
derived relations are expressed in a explicit form providing simple
algorhytms which can be applied to various problems in solar research,
e.g. the solar rotation, the geometry of the coronal features, motions
in coronal structures, etc.
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: Soft X-Ray Radiation Associated with Flares Developing in
Strong Magnetic Fields
Authors: Vrsnak, B.; Ruzdjak, V.; Altas, L.; Ozguc, A.; Zlobec, P.
Bibcode: 1995HvaOB..19...15V
Altcode:
A sample of 30 Imp =1 Z-flares (H alpha flare emission protruding over
sunspot umbrae) observed at Kandilli Observatory was analyzed. It was
found that the soft X-ray emission reached its maximum approximately
3-4 minutes after the chromospheric flare emission has protruded over
the umbra. It can be concluded that the most powerful energy release
takes place when the flare process starts to occure in strong magnetic
field regions. This interpretation is also supported by the fact that
the peaks of microwave emission at about 3 GHz (revealing the presence
of accelerated electrons) were grouped approximately about the time
of the protrusion. A positive correlation was found between the peak
fluxes of the soft X-ray bursts and the magnetic field strengths of
the protruded spots.
Title: Interaction of Two Intertwined Helicoidal Current Filaments
Authors: Lulic, S.; Vrsnak, B.; Rosa, D.
Bibcode: 1994HvaOB..18...29L
Altcode:
No abstract at ADS
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: Prominence Oscillations and Stability
Authors: Vrsnak, B.; Ruzdjak, V.
Bibcode: 1994scs..conf..329V
Altcode: 1994IAUCo.144..329V
A number of high-amplitude prominence oscillations of different modes,
triggered by various perturbations, are presented and classified
as membrane-like, string-like, sausage-like and elastic-spring-like
oscillations.
Title: Interaction of Two Intertwined Helicoidal Magnetic Tubes
Authors: Lulić, S.; Vršnak, B.; Roša, D.
Bibcode: 1994HvaOB..18...21L
Altcode:
No abstract at ADS
Title: The Relation Between Solar Activity Tree Growth in Northern
Croatia
Authors: Rosa, J.; Rosa, D.; Bozic, H.; Vrsnak, B.
Bibcode: 1994HvaOB..18...21R
Altcode:
No abstract at ADS
Title: Structure and Stability of Helicoidal Magnetic Structures in
the Solar Corona
Authors: Vrsnak, B.; Ruzdjak, V.
Bibcode: 1994HvaOB..18....1V
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: The Relation Between Solar Activity and Tree Growth in
Northern Croatia
Authors: Roša, J.; Roša, D.; BožiĆ, H.; Vršnak, B.
Bibcode: 1994HvaOB..18...29R
Altcode:
No abstract at ADS
Title: Structure and stability of helicoidal magnetic configurations
in the solar corona
Authors: Vršnak, B.; Ruždjak, V.
Bibcode: 1994smf..conf..393V
Altcode:
No abstract at ADS
Title: Kinematics and evolution of twist in the eruptive prominence
of August 18, 1980
Authors: Vrsnak, B.; Ruzdjak, V.; Rompolt, B.; Rosa, D.; Zlobec, P.
Bibcode: 1993SoPh..146..147V
Altcode:
The prominence which erupted at the SE limb on August 18, 1980 is one
of the best observed disparition brusque events: high-resolution
monochromatic ground-based observations in the Hα line were
supplemented by the SMM and Solwind satellite coronographic
observations; the radio wavelength range was well covered by
single-frequency and spectral observations, and the prominence magnetic
fields were measured two days before the eruption.
Title: Changes of Helical Structures During the Eruption of Two
Prominences
Authors: Rosa, D.; Vrsnak, B.; Ruzdjak, V.; Ozguc, A.; Rusin, V.
Bibcode: 1993HvaOB..17...15R
Altcode:
No abstract at ADS
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: Classification of Prominence Oscillations
Authors: Vrsnak, B.
Bibcode: 1993HvaOB..17...23V
Altcode:
No abstract at ADS
Title: Magnetic structure of solar prominences.
Authors: Vrsnak, B.
Bibcode: 1992AnGeo..10..344V
Altcode: 1992AnG....10..344V
The cold and dense prominence plasma is suspended in the core of
an arcade of magnetic field lines above the solar photosphere. The
observations reveal a rather uniform horizontal component of the
magnetic field, predominantly directed close to the prominence long
axis. On the other hand, monochromatic observations of the prominence
fine structure provide a visualization of the magnetic configuration in
the core of an arcade revealing intricate morphological structures and
chaotic plasma flows. Prominences are usually modelled in cylindrical
or slab geometry, assuming that the fine structure is only superposed
on a larger scale magnetic field structure. MHD models often treat a
prominence as a 2-D static feature but it seems that 3-D dynamical
models, including real geometry, are necessary to comprehend the
prominence phenomenon completely. The models predict a number of
possible MHD instabilities which can cause a prominence eruption. When
the critical conditions in an arcade are met the large scale magnetic
structure erupts, together with the embedded prominence, and this
eruption is known as a coronal mass ejection.
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: A Study on Electric Currents in a Solar Active Region -
a Dynamo Process at a Place of Repeated Flaring
Authors: Hofmann, A.; Ruzdjak, V.; Vrsnak, B.
Bibcode: 1992HvaOB..16...29H
Altcode:
No abstract at ADS
Title: 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: Stability of Prominences Exposing Helical like Patterns
Authors: Vrsnak, B.; Ruzdjak, V.; Rompolt, B.
Bibcode: 1991SoPh..136..151V
Altcode:
The internal structure of prominences appearing as twisted tubes was
studied. The sample embraced 15 stable and 13 eruptive prominences,
exposing patterns which possibly reflect a helical configuration. The
equivalent pitch angles (ϑ) of twisted fine structure features were
measured. In some cases the evolution of the internal structure was
followed and 49 independent measurements of the parameter ϑ were
performed in total. The results are presented in the plane relating
the parameter ϑ and the normalized prominence height. The eruptive
prominences occupy the region characterized by ϑ > 50° and h
> 0.8d, where h and d are the prominence height and the footpoint
half-separation, respectively. All prominences characterized by h <
0.6d or by ϑ < 35° were stable. Such a result is in good agreement
with an order of magnitude treatment of the forces acting in a curved
magnetic tube, anchored at both ends in the photosphere.
Title: Calcium Plage Intensity and Solar Irradiance Variations
Authors: Vršnak, B.; Plačko, D.; Ruždjak, V.
Bibcode: 1991SoPh..133..205V
Altcode:
We have established a statistical relation between the facular
contribution to the solar irradiance and the intensity of the associated
calcium plage. For the solar irradiance data the ACRIM measurements
were used. The quiet-Sun level of the irradiance was determined as a
function of the time for the period studied. A sample of plages in the
period of the solar activity minimum was selected, during the periods
when no spots were present on the solar disc. We have expressed the
dependence studied through the parameter Cp in the `proxy'
PFI concept. The parameter Cp could be related to the plage
intensity (I) as Cp = 0.006I + 0.003. The mean value of
the parameter Cp ranged between 0.015 and 0.017 depending
on the choice of samples.
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: Microwave and Soft X-ray Radiation During Flares Evolving in
Strong Magnetic Fields
Authors: Vrsnak, B.; Ruzdjak, V.; Zlobec, P.; Jurac, S.
Bibcode: 1991HvaOB..15...11V
Altcode:
No abstract at ADS
Title: Launch-times of MHD Shocks Observed as Type II Bursts
Authors: Vrsnak, B.; Zlobec, P.; Ruzdjak, V.
Bibcode: 1991HvaOB..15....1V
Altcode:
No abstract at ADS
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: The Role of the Magnetic Field in Intensity and Geometry in
the Type-Iii Burst Generation
Authors: Zlobec, P.; Ruždjak, V.; Vršnak, B.; Karlický, M.;
Messerotti, M.
Bibcode: 1990SoPh..130...31Z
Altcode:
We study the association of type III bursts related to Hα flares
in different magnetic environments in the period 1970-1981. Special
attention is paid to flares which partly cover a major spot umbra
(Z-flares). In particular we consider the location of the spots in the
active regions and the magnetic field intensities of spots covered by
a ribbon. The association rate with type III bursts decreases to 17%
when the flare is located inside the bipolar pattern of a large active
region, compared with an association rate of 54% when the flare is
situated outside it. The association rate increases with the magnetic
field intensity of the spot covered by Hα emission; this is most
clearly revealed for the flares occurring outside the bipolar pattern
of active regions. Ninety-three percent of the flare-associated type
III burst were accompanied by 10 cm radio bursts. For the most general
case in which a flare is developing anywhere in an active region,
the association with type III bursts generation increases with the
increasing magnetic field intensity of the main spot of the group.
Title: Eruptive instability of cylindrical prominences
Authors: Vrsnak, B.
Bibcode: 1990SoPh..129..295V
Altcode: 1990SoPh..129..295B
The stability of prominences and the dynamics of an eruption are
studied. The prominence is represented by an uniformly twisted, curved,
magnetic tube, anchored at both ends in the photosphere. Several stages
of the eruption are analyzed, from the pre-eruptive phase and the onset
of the instability, up to the late phases of the process. Before the
eruption, the prominence evolves through a series of equilibrium states,
slowly ascending either due to an increase of the electric current or
to mass loss. The eruption starts when the ratio of the current to
the total mass attains a critical value after which no neighbouring
equilibrium exists. The linearized equation of motion was used to
obtain the instability threshold, which is presented in a form enabling
comparison with the observations. The height at which the prominence
erupts depends on the twist, and is typically comparable with the
footpoint half-separation. Low-lying prominences are stable even for
large twists. The importance of the external field reconnection below
the filament, and the mass loss through the legs in the early phases
of the eruption is stressed. The oscillations of stable prominences
with periods on the Alfvén time-scale are discussed. The results are
compared with the observations.
Title: Eruptive Instability of Magnetic Arcades
Authors: Vrsnak, B.
Bibcode: 1990Ap&SS.170..141V
Altcode:
Eruptive prominences trace disruptions of magnetic arcades in which
they are embedded. The stability of an arcade containing an electric
current filament at its axis is discussed. The model provides criteria
for the onset of the eruptive instability in terms of prominence twist
and overall geometry, i.e., the parameters which could be measured
directly. The evolution of the eruption is analyzed, and the dependence
of the acceleration and the pitch of field-lines on the height is
established. The model is compared with the observations of one eruptive
prominence where the development of helical structure was followed.
Title: Solar Irradiance Variations and Nonthermal Processes in the
Solar Atmosphere
Authors: Vrsnak, B.; Ruzdjak, V.; Zlobec, P.
Bibcode: 1990BAICz..41..243V
Altcode:
The association of solar irradiance decreases with flare and
noise storm activity is analyzed. The studied irradiance dips were
accompanied by enhanced flare activity, but increased flare activity
did not necessarily imply the occurrence of a dip. On the other hand,
longlasting broadband noise storm activity was always accompanied by
an irradiance dip. An upper limit to the amplitude of the irradiance
perturbation caused by a sunspot group of a given area and flare
activity is estimated. Also, a lower limit to the irradiance
perturbation amplitude with respect to noise storm activity is
established. The estimated energies associated with the considered
processes were found to be much smaller than the energies related to
the irradiance decreases. Nevertheless, the analysis indicates the
importance of nonthermal processes in large agglomerations of magnetic
fields related to irradiance decreases.
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: Dynamics and Internal Structure of an Eruptive Prominence
Authors: Vrsnak, B.
Bibcode: 1990SoPh..127..129V
Altcode:
The kinematics and the development of the internal structure in the
eruptive prominence of August 16, 1988 are described. The prominence
exposed helical structure, and the pitch of the fine structure filaments
was measured. The evolution of the pitch was measured in the legs of
the prominence and at its summit from the pre-eruptive phase up to the
late phases of the eruption. The pitch angle was decreasing in the legs
as well as at the summit. However, the observations indicate that the
integral twist remained constant. The prominence was twisted more at
the summit where it was wider than in the legs. The `effective' twist
at the prominence summit was approximately 20 π and in the legs it
amounted to about 8 π. Such a ratio did not change during the eruption,
i.e., no redistribution of the twist was observed within the accuracy
of measurements. The nature of the instability causing the eruption
is discussed and the energetics of the process is considered.
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: Vector Magnetic Field and Currents at the Footpoint of a
Loop Prominence
Authors: Hofmann, A.; Ruždjak, V.; Vršnak, B.
Bibcode: 1990LNP...363..233H
Altcode: 1990IAUCo.117..233H; 1990doqp.coll..233H
Using H -filtergrams and vector magnetograms we study the structure of
the magnetic field at the footpoint of a loop prominence rooting deep
in the penumbral photosphere of a sunspot. In the region investigated
the footpoint -field is well marked in the transversal field map. The
field has a predominantly transverse character and is directed parallel
to the axis of the prominence. The flux bundle forming the prominence
left the photosphere by an angle of about 26°, i.e. close to the
horizontal. In the maps of current densities inferred from the vector
magnetic field we find a pair of up- and downflowing currents, being
situated symmetrically to the axis of the prominence. This indicates on
a current ( 3,8 · 1011 A) flowing round the flux bundle and generating
the Lorentz forces causing the concentration of flux at the footpoint
region. The vertical gradients of the longitudinal field hint on an
increase of the field strength with height, i.e. toward the axis of
the prominence.
Title: Time Profiles of Solar Irradiance Dips
Authors: Vrsnak, B.; Ruzdjak, V.; Ruzic, Z.
Bibcode: 1990SoPh..125...13V
Altcode:
ACRIM data have been analyzed to study the time profiles of simple
irradiance dips caused by single active regions. Comparison of the
average characteristics of the dips appearing in the minimum and maximum
of the solar cycle shows that there are no significant differences. In
both periods we disclosed the facular irradiance excess in the profile
wings having typical duration of two to three days and an amplitude
of about 20% of the dip amplitude. The profiles were asymmetric, with
a stronger and longer excess in the trailing wing. We determined an
`average' profile which was attributed to an idealized active region,
and we calculated the luminosity perturbation caused by it. Excess
radiation in the wings of the profile compensates about 1/3 of
the deficit in the dip. In the most simple case from our sample we
compared the profile based on ACRIM measurements and the proxy profile
estimated using sunspot and plage areas published in Solar Geophysical
Data catalogues. The comparison indicates that the facular excess was
compensating instantaneously about 2/3 of the luminosity deficit caused
by sunspots.
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: Soft X-Ray Emission from a Hot Turbulent Current Sheet and
the Precursor Phase of Solar Flares
Authors: Vrsnak, B.
Bibcode: 1989SoPh..120...79V
Altcode:
The properties and development of a high-temperature current sheet
characterized by increasing merging velocity are studied and related to
the early phases of solar flares. It is shown that the system can be
described by the Petschek-type geometry for a wide range of merging
velocities. In the diffusion region and the standing MHD shocks a
certain low-frequency plasma microturbulence is generated from the very
beginning of the reconnection process. We present qualitative solutions
for the case of ion-acoustic turbulence in marginally stable state,
which provide a comparison with observations. The increasing merging
velocity leads to the appearance of the soft X-ray precursor. The
precursor temperature maximum should appear during the current sheet
formation, before the Petschek regime is established. In the Petschek
regime the temperature of the hot plasma decreases due to the decrease
of the magnetic field strength at the diffusion region boundary, while
the soft X-ray radiation still increases, reaching precursor maximum
for merging velocities about 1% of the external Alfvén velocity. The
precursor phase ends when the value of the merging velocity surpasses
the upper limit for the Petschek regime and the system enters into the
pile-up regime, causing a new increase of plasma temperature and soft
X-ray radiation.
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: Vector Magnetic Field and Currents at the Footpoint of a
Loop Prominence
Authors: Hofmann, A.; Ruzdjak, V.; Vrsnak, B.
Bibcode: 1989HvaOB..13...11H
Altcode:
No abstract at ADS
Title: Solar irradiance variations and their relation with solar
flares.
Authors: Pap, J.; Vrsnak, B.
Bibcode: 1989sasf.confP.243P
Altcode: 1988sasf.conf..243P; 1989IAUCo.104P.243P
A clear association is demonstrated between the dips in the total solar
irradiance and flare occurrence. It is found that both the irradiance
dips and flares are related to emerging new activity.
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: Solar Irradiance Perturbations Caused by Active Regions
Authors: Vrsnak, B.; Ruzdjak, V.; Ruzic, Z.
Bibcode: 1988HvaOB..12....1V
Altcode:
No abstract at ADS
Title: Solar irradiance variations and magnetic field emergence
Authors: Vrsnak, B.; Ruzdjak, V.
Bibcode: 1988AdSpR...8g..35V
Altcode: 1988AdSpR...8...35V
We have studied the association between different aspects of the
magnetic field behaviour in large active regions and the solar
irradiance variations. It was found that irradiance decreases were
accompanied by enhanced flare and coronal mass ejection activity. A
still better association seems to exist with broadband, long-lasting
noise storm activity. This indicates that the enhanced generation
of weak MHD shock waves causing noise storm in large interconnecting
coronal loops is related to the appearance of irradiance dips.
Title: H-alpha Flares Recorded at Hvar Observatory During the "Solar
International Month-September 1988"
Authors: Ruzdjak, V.; Vrsnak, B.; Zloch, F.
Bibcode: 1988HvaOB..12...11R
Altcode:
No abstract at ADS
Title: Reconnection driven by an erupting filament in the May 14,
1981 flare
Authors: Vršnak, B.; Ruždjak, V.; Messerotti, M.; Mouradian, Z.;
Urbarz, H.; Zlobec, P.
Bibcode: 1987SoPh..114..289V
Altcode:
We present observations of the flare of May 14, 1981, which can
be classified as a three-ribbon flare. After a detailed analysis
in metric, decimetric, microwave, optical, and X-ray ranges we
propose that the event was caused by a reconnection process driven
by erupting filament. The energy was liberated in the current sheet
above the filament in the region between the erupting flux and the
overlying field. It is shown that plasma microinstabilities develop
as the plasma enters the current sheet. The observations indicate
that during the precursor phase a certain low-frequency turbulence,
such as ion-accoustic turbulence had to be present.
Title: Evidence for interacting loop process in a phase of the May
16, 1981 flare
Authors: Vrsnak, B.; Ruzdjak, V.; Messerotti, M.; Zlobec, P.
Bibcode: 1987SoPh..111...23V
Altcode:
The behaviour of the flare in the period of enhancement and maximum of
hard X-ray, microwave and decimetric type IV continuum is analysed. The
elongation of the Hα ribbons and microwave source disclose that
the energy release site was shifting through a system of loops with
a velocity less than 200 km s-1, and that the energy
was carried down the field lines with a velocity of about 1000 km
s-1, implying the thermal conduction front mechanism of
energy transport. Several processes of energy release are considered and
it is concluded that an explanation in terms of succeeding interactions
of neighbouring loops, involving fast reconnection of their poloidal
components is in best agreement with the observations.
Title: Spotless flares and the associated radio continuum emission.
Authors: Ruždjak, V.; Messerotti, M.; Nonino, M.; Schroll, A.;
Vršnak, B.; Zlobec, P.
Bibcode: 1987SoPh..111..103R
Altcode: 1987SoPh..111..103F
We studied 24 spotless flares of Ha importance ≥ 1 which occurred
during the 21st cycle of solar activity. The spotless flares could
be grouped in three categories according to their location and time
history of the associated active region. Our association of the
flares with radio events was based on relative timing and on the
flare importances. Weak microwave gradual rise and fall events were
frequently recorded during the occurrence of the spotless flares. A few
flares from our sample could be associated with impulsive and complex
microwave bursts. Only in one case an association of a spotless flare
with a significant metric type II/IV event seems to be justified.
Title: The Effects of Diferent Basic Process in Solar Flares
Authors: Vrsnak, B.
Bibcode: 1987HvaOB..11...91V
Altcode:
Current flare classifications are presented briefly, to illustrate
observational evidence of the differences among flares. Possible
mechanisms of energy release processes in different magnetic structures
are discussed and the interplay between ideal and resistive MHD
processes and plasma microinstabilities is emphasized. The interacting
loop flare model is taken as an example to show that there are
observational evidences for at least two different mechanisms of energy
release processes (coalescence instability and driven reconnection)
in this kind of flares.
Title: PC Based Image Processing of Solar Activity Features
Authors: Dzubur, A.; Caldarevic, M.; Ruzdjak, V.; Vrsnak, B.
Bibcode: 1987HvaOB..11..143D
Altcode:
A short description of digital image, i.e. the image which is after
digitalization process stored in the PC computer memory and to which
digital processing can be applied is given. The procedure is applied
to images of various solar activity features.
Title: Behaviour of the Polarization at Dm-M Wavelengths during the
Evolution of Five Two-Ribbon Flares
Authors: Ruzdjak, V.; Vrsnak, B.; Zlobec, P.; Schroll, A.
Bibcode: 1986SoPh..104..169R
Altcode:
The general behaviour of the circular polarization at dm-m wavelengths
during the evolution of five two-ribbon flares is investigated. The
changes of polarization, if present, occurred 10 to 20 min after the
impulsive phases. Increases of the radio and X-ray fluxes occurred at
the moments when the Hα ribbons started to extend over spot umbrae.
Title: Changes of Polarization in the dm-m Range During the Flare
of May 16, 1981
Authors: Ruzdjak, V.; Vrsnak, B.; Zlobec, P.
Bibcode: 1986HvaOB..10...11R
Altcode:
The polarization behaviour in the dm-m radio range during the flare of
May 16, 1981 is studied and related to other observed phenomena. The
evolution of the flare is discussed.
Title: Flare occurence in the complex of activity NOAA/USAF No. 4201,
May 29 - June 12, 1983
Authors: Ruždjak, V.; Vršnak, B.; Hofmann, A.; Křivský, L.;
Markova, E.; Kálmán, B.
Bibcode: 1986CoSka..15..257R
Altcode:
The evolution of a large complex of activity NOAA/USAF No. 4201 in the
period May 29 to June 12, 1983 was investigated. Almost 200 flares
occurring in the complex were reported, most of which clustered at
preferred sites where delta configurations with shear and strong
gradients of the magnetic fields were present or new flux emerged.
Title: Microwave limb sources and their relation to solar flares
Authors: Urpo, S.; Terasranta, H.; Pohjolainen, S.; Ruzdjak, V.;
Vrsnak, B.
Bibcode: 1986AdSpR...6f.267U
Altcode: 1986AdSpR...6..267U
Solar microwave maps (at 22 and 37 GHz) performed at Metsähovi
(Finland) in the period 1979-1982 have revealed about a dozen
radiation sources, microwave limb sources (MW LS), located up to 50
000 km above the optical limb of the Sun (in normal conditions the
sources for microwave emission are locate 2 000 - 4 000 km above the
photosphere). It could be established that MW LS showed up after
the occurrence of two ribbon flares situated close to the limb of
the visible hemisphere or that they have been connected to flares
occuring behind the limb, which was documented by the appearance of
Hα post flare loop systems. The life time of the MW LS was
typically about 3 hours and they have been correlated to long decay
events (LDE) in X-rays. In one case the active region where the MW LS
appeared was also tracked at Metsähovi and a typical gradual rise and
fall event was recorded. It is proposed that the MW LS are of thermal
origin and that they originate from condensed flaring loops appearing
at successive higher altitudes.
Title: The Two Ribbon Flare of May 14, 1981
Authors: Vrsnak, B.; Ruzdjak, V.; Zlobec, P.
Bibcode: 1986HvaOB..10...17V
Altcode:
The flare behaviour at radio waves, X-rays and in H-alpha is
described. The flare geometry is discussed and compared with the model
of Heyvaerts et al. (1977).
Title: Microwave emission from the limb of the Sun: the events of
September 8, 1980 and May 10, 1981
Authors: Urpo, S.; Terasranta, H.; Ruzdjak, V.; Vrsnak, B.; Rompolt,
B.; Kren, G.
Bibcode: 1985HvaOB...9...25U
Altcode:
Solar microwave maps at 8 mm performed at the Metsähovi Radio
Research Station in the period 1978 - 1982 revealed some radiation
sources located up to 50000 km outside the optical Sun. The authors
describe two characteristic events which were associated with large
flares occurring close to the solar limb.
Title: Helical prominences III: the prominence of July 29, 1980
Authors: Vrsnak, B.
Bibcode: 1985HvaOB...9...61V
Altcode:
A prominence, observed at Hvar Observatory on July 29, 1980, is
described and interpreted. The lower limit of the magnetic field
strength is estimated to 10 G, while lower limits of the total internal
current and current density are estimated as 1011A and
8×10-5A/m2, respectively.
Title: The oscillating loop prominence of July 17, 1981
Authors: Vrsnak, B.
Bibcode: 1984SoPh...94..289V
Altcode:
Oscillatory motions of a loop prominence observed on July 17, 1981 are
analysed. The oscillations were mainly horizontal, with a period of 8
min. Restoring force was a result of magnetic tension, and assuming a
simplified magnetic field configuration the expression for frequencies
of oscillations is derived and compared with the observations. Taking
the observed period, the strength of the magnetic field permeating
the prominence is estimated as 45 G. Finally, the stability of the
prominence is discussed.
Title: The Flares of May 14 and 16, 1981, August 19, 1981, October
14, 1983 and the Associated Radio Events
Authors: Ruzdjak, V.; Vrsnak, B.; Kotrc, P.; Merciar, C.; Schober,
H. J.; Schroll, A.; Urpo, S.; Zlobec, P.
Bibcode: 1984HvaOB...8...25R
Altcode:
Observational data for the flares of May 14 and 16, 1981,
August 19, 1981 and October 14, 1983 in the optical and
radio ranges are presented. It seems that the complexity of the
photospheric-chromospheric aspect and its time evolution is reflected
in the radio range.
Title: The Hellical Prominence of May 26, 1982
Authors: Vrsnak, B.
Bibcode: 1984HvaOB...8...13V
Altcode:
The observation of a prominence with helical structure is described
and interpreted. Dynamics, stability and the internal structure of
the prominence are discussed.
Title: The Determination of the Reduction Factors for Sunspot
Observations at the Astronomical Observatory Zagreb
Authors: Kren, G.; Vrsnak, B.; Ruzdjak, V.
Bibcode: 1984HvaOB...8....1K
Altcode:
Sunspot observations performed at the Astronomical Observatory Zagreb
in the period May 1979 - June 1983 were investigated. The reduction
factors of the observed Wolf numbers to the international Wolf numbers
have been obtained. The dependence of the reduction factors on the
seeing has been analysed.
Title: Correlation of noise storm and H-alpha activity for the CONS
period Sept. 1 - 4, 1980.
Authors: Vršnak, B.; Ruždjak, V.
Bibcode: 1983srs..work..315V
Altcode:
The Ca plage index was found to be highly correlated with the 260
MHz radio flux. The noise storm enhancement followed the increase in
the number of major flares with the delay of one day. The noise storm
behaviour was correlated with the three hour flare index, but not with
the one hour flare index.
Title: Oscillatory motions of prominences
Authors: Vrsnak, B.
Bibcode: 1982HvaOB...6..129V
Altcode:
Oscillatory motions of a loop prominence observed on July 17, 1981
are discussed. Stability of the loop, origin of the restoring force
and magnetic field strength are discussed.
Title: Correlation of noise storm and Hα activity for the CONS
period September 1-4, 1980
Authors: Vršnak, B.; Ruždjak, V.
Bibcode: 1982srs..work..315V
Altcode:
No abstract at ADS
Title: The helical prominence of March 17, 1977
Authors: Vrsnak, B.; Ruzdjak, V.
Bibcode: 1982HvaOB...6..123V
Altcode:
The dynamics of a prominence with helical structure is studied. The
slow rising motion accompanied by detwisting indicates that mass loss
probably caused the prominence rising and decrease of the azimuthal
component of the magnetic field. The radial dependence of current
density within the prominence cylinder is discussed.
Title: Post SMY observations at Hvar Observatory, May-June 1981
Authors: Ruzdjak, V.; Vrsnak, B.; Novak, N.
Bibcode: 1981HvaOB...5...41R
Altcode:
No abstract at ADS
Title: Some internal mass motions in prominences indicated by special
spectral features.
Authors: Ruzdjak, V.; Vrsnak, B.
Bibcode: 1981PAOS....1..191R
Altcode:
No abstract at ADS
Title: Observations carried out at Hvar Observatory during SMY,
August 1979 - February 1981
Authors: Ruzdjak, V.; Vrsnak, B.; Novak, N.
Bibcode: 1981HvaOB...5...31R
Altcode:
No abstract at ADS
Title: Internal mass motions in three eruptive prominences
Authors: Vrsnak, B.
Bibcode: 1980HvaOB...4...17V
Altcode:
No abstract at ADS