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Author name code: robbrecht
ADS astronomy entries on 2022-09-14
author:"Robbrecht, Eva"
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Title: Verification of space weather forecasting at the Regional
Warning Center in Belgium
Authors: Devos, Andy; Verbeeck, Cis; Robbrecht, Eva
2014JSWSC...4A..29D Altcode:
The Solar Influences Data analysis Center (SIDC) in Brussels at the
Royal Observatory of Belgium (ROB) has been providing daily space
weather forecasts for more than a decade. A verification analysis was
applied to evaluate the performance of the SIDC forecasts of fundamental
space weather parameters such as the F10.7 radio flux, solar flare
activity, and local geomagnetic index. Strengths and weaknesses
are determined compared to common numerical models. Descriptive
model statistics, common verification measures, error analysis
and conditional plots related to forecasts and observations are
presented. The verification analysis methods have been designed such
that future improvements and additions can easily be included, for
example with new forecasting models. The SIDC forecast (together with
the persistence model) achieves the best performance for forecasting
F10.7 on day 1, but has potential for improvement for a larger lead
time mainly by applying estimates from the persistence and corrected
recurrence models. The persistence model is superior for the forecast
of flares, though corrected recurrence models are slightly better in
foreseeing M- and X-class flares and the SIDC forecast estimates B-
and C-class flares very well. The SIDC forecast scores better than all
models in forecasting the local K-index. It best reproduces observations
in the range of K = 2-4, but underestimates larger K values. The SIDC
forecast provides a distribution that best matches the observations of
the K-index. The analysis presented here demonstrates the influence of
solar activity on the confidence level of the forecasts, as well as
the hinted influence of the forecaster on duty due to the subjective
nature of forecasting. The output aids to identify the strong and
weak points of the SIDC forecast as well as those of the models
considered. Though the presented analysis needs further extension,
it already illustrates the opportunity to regularly reevaluate space
weather forecasts and to stimulate ideas for improvement and increase
the reliability of space weather forecasting.
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Title: Determining the North-South Displacement of the Heliospheric
Current Sheet from Coronal Streamer Observations
Authors: Robbrecht, E.; Wang, Y. -M.
2012ApJ...755..135R Altcode:
Inferences based on interplanetary field measurements have suggested
a statistical tendency for the heliospheric current sheet (HCS)
to be displaced southward of the heliographic equator during the
past four solar cycles. Here, we use synoptic maps of white-light
streamer structures to determine more directly the longitudinally
averaged latitude of the HCS, after separating out the contribution of
streamers without magnetic polarity reversals ("pseudostreamers"). We
find a strong tendency for the HCS to be shifted southward by a few
degrees during 2007-2011, but no significant shift during the 1996-1997
sunspot minimum. Fluctuations in the magnitude and direction of the
north-south shifts often occur on timescales as short as one or two
Carrington rotations, as a result of changes in the streamer structures
due to active region emergence. The largest shifts occurred during
2010-2011 and were on the order of -6°. Such southward displacements
are consistent with the overwhelming dominance of northern-hemisphere
sunspot activity during the rising phase of the current solar cycle 24,
resulting in a strong axisymmetric quadrupole component whose sign at
the equator matched that of the north polar field; the symmetry-breaking
effect of the quadrupole was further enhanced by the weakness of the
polar fields.
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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.
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).
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Title: On the Nature of the Solar Wind from Coronal Pseudostreamers
Authors: Wang, Y. -M.; Grappin, R.; Robbrecht, E.; Sheeley, N. R., Jr.
2012ApJ...749..182W Altcode:
Coronal pseudostreamers, which separate like-polarity coronal holes, do
not have current sheet extensions, unlike the familiar helmet streamers
that separate opposite-polarity holes. Both types of streamers taper
into narrow plasma sheets that are maintained by continual interchange
reconnection with the adjacent open magnetic field lines. White-light
observations show that pseudostreamers do not emit plasma blobs; this
important difference from helmet streamers is due to the convergence
of like-polarity field lines above the X-point, which prevents the
underlying loops from expanding outward and pinching off. The main
component of the pseudostreamer wind has the form of steady outflow
along the open field lines rooted just inside the boundaries of the
adjacent coronal holes. These flux tubes are characterized by very
rapid expansion below the X-point, followed by reconvergence at greater
heights. Analysis of an idealized pseudostreamer configuration shows
that, as the separation between the underlying holes increases, the
X-point rises and the expansion factor f <SUB>ss</SUB> at the source
surface increases. In situ observations of pseudostreamer crossings
indicate wind speeds v ranging from ~350 to ~550 km s<SUP>-1</SUP>,
with O<SUP>7 +</SUP>/O<SUP>6 +</SUP> ratios that are enhanced compared
with those in high-speed streams but substantially lower than in the
slow solar wind. Hydrodynamic energy-balance models show that the
empirical v-f <SUB>ss</SUB> relation overestimates the wind speeds
from nonmonotonically expanding flux tubes, particularly when the
X-point is located at low heights and f <SUB>ss</SUB> is small. We
conclude that pseudostreamers produce a "hybrid" type of outflow that
is intermediate between classical slow and fast solar wind.
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Title: Asymmetric Sunspot Activity and the Southward Displacement
of the Heliospheric Current Sheet
Authors: Wang, Y. -M.; Robbrecht, E.
2011ApJ...736..136W Altcode:
Observations of the interplanetary magnetic field (IMF) have suggested
a statistical tendency for the heliospheric current sheet (HCS)
to be shifted a few degrees southward of the heliographic equator
during the period 1965-2010, particularly in the years near sunspot
minimum. Using potential-field source-surface extrapolations and
photospheric flux-transport simulations, we demonstrate that this
southward displacement follows from Joy's law and the observed
hemispheric asymmetry in the sunspot numbers, with activity being
stronger in the southern (northern) hemisphere during the declining
(rising) phase of cycles 20-23. The hemispheric asymmetry gives rise
to an axisymmetric quadrupole field, whose equatorial zone has the
sign of the leading-polarity flux in the dominant hemisphere; during
the last four cycles, the polarity of the IMF around the equator thus
tended to match that of the north polar field both before and after
polar field reversal. However, large fluctuations are introduced by
the nonaxisymmetric field components, which depend on the longitudinal
distribution of sunspot activity in either hemisphere. Consistent
with this model, the HCS showed an average northward displacement
during cycle 19, when the "usual" alternation was reversed and
the northern hemisphere became far more active than the southern
hemisphere during the declining phase of the cycle. We propose a new
method for determining the north-south displacement of the HCS from
coronal streamer observations.
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Title: The Evolution of Dark Canopies Around Active Regions
Authors: Wang, Y. -M.; Robbrecht, E.; Muglach, K.
2011ApJ...733...20W Altcode: 2011arXiv1103.4373W
As observed in spectral lines originating from the chromosphere,
transition region, and low corona, active regions are surrounded by an
extensive "circumfacular" area which is darker than the quiet Sun. We
examine the properties of these dark moat- or canopy-like areas using Fe
IX 17.1 nm images and line-of-sight magnetograms from the Solar Dynamics
Observatory. The 17.1 nm canopies consist of fibrils (horizontal fields
containing extreme-ultraviolet-absorbing chromospheric material)
clumped into featherlike structures. The dark fibrils initially
form a quasiradial or vortical pattern as the low-lying field lines
fanning out from the emerging active region connect to surrounding
network and intranetwork elements of opposite polarity. The area
occupied by the 17.1 nm fibrils expands as supergranular convection
causes the active-region flux to spread into the background medium;
the outer boundary of the dark canopy stabilizes where the diffusing
flux encounters a unipolar region of opposite sign. The dark fibrils
tend to accumulate in regions of weak longitudinal field and to become
rooted in mixed-polarity flux. To explain the latter observation,
we note that the low-lying fibrils are more likely to interact with
small loops associated with weak, opposite-polarity flux elements
in close proximity, than with high loops anchored inside strong
unipolar network flux. As a result, the 17.1 nm fibrils gradually
become concentrated around the large-scale polarity inversion lines
(PILs), where most of the mixed-polarity flux is located. Systematic
flux cancellation, assisted by rotational shearing, removes the field
component transverse to the PIL and causes the fibrils to coalesce
into long PIL-aligned filaments.
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Title: Validation of CME Detection Software (CACTus) by Means of
Simulated Data, and Analysis of Projection Effects on CME Velocity
Measurements
Authors: Bonte, K.; Jacobs, C.; Robbrecht, E.; De Groof, A.; Berghmans,
D.; Poedts, S.
2011SoPh..270..253B Altcode: 2011SoPh..tmp...52B; 2011SoPh..tmp...72B
In the context of space weather forecasting, an automated detection
of coronal mass ejections (CMEs) becomes more and more important
for efficiently handling a large data flow which is expected from
recently-launched and future solar missions. In this paper we validate
the detection software package "CACTus" by applying the program to
synthetic data from our 3D time-dependent CME simulations instead of
observational data. The main strength of this study is that we know
in advance what should be detected. We describe the sensitivities
and strengths of automated detection, more specific for the CACTus
program, resulting in a better understanding of CME detection on one
hand and the calibration of the CACTus software on the other hand,
suggesting possible improvements of the package. In addition, the
simulation is an ideal tool to investigate projection effects on CME
velocity measurements.
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Title: The Evolution of Dark Canopies Around Active Regions
Authors: Muglach, Karin; Wang, Y. M.; Robbrecht, E.
2011SPD....42.1718M Altcode: 2011BAAS..43S.1718M
As observed in spectral lines originating from the chromosphere,
transition region, and low corona, active regions are surrounded
by an extensive 'circumfacular' area which is darker than the quiet
Sun. We examine the properties of these dark moat- or canopy-like areas
using Fe IX 17.1 nm images and line-of-sight magnetograms from the
Solar Dynamics Observatory. The 17.1 nm canopies consist of fibrils
(horizontal fields containing EUV-absorbing chromospheric material)
clumped into featherlike structures. The dark fibrils initially
form a quasiradial or vortical pattern as the low-lying field lines
fanning out from the emerging active region connect to surrounding
network and intranetwork elements of the opposite polarity. The area
occupied by the 17.1 nm fibrils expands as supergranular convection
causes the active region flux to spread into the background medium;
the outer boundary of the dark canopy stabilizes where the diffusing
flux encounters a unipolar region of the opposite sign. The dark fibrils
tend to accumulate in regions of weak longitudinal field and to become
rooted in mixed-polarity flux. To explain the latter observation,
we note that the low-lying fibrils are more likely to interact with
small loops associated with weak, opposite-polarity flux elements
in close proximity, than with high loops anchored inside strong
unipolar network flux. As a result, the 17.1 nm fibrils gradually
become concentrated around the large-scale polarity inversion lines
(PILs), where most of the mixed-polarity flux is located. Systematic
flux cancellation, assisted by rotational shearing, removes the field
component transverse to the PIL and causes the fibrils to coalesce
into long PIL-aligned filaments.
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Title: The Temperature-dependent Nature of Coronal Dimmings
Authors: Robbrecht, Eva; Wang, Yi-Ming
2010ApJ...720L..88R Altcode: 2010arXiv1007.5191R
The opening-up of the magnetic field during solar eruptive events
is often accompanied by a dimming of the local coronal emission. From
observations of filament eruptions recorded with the Extreme-Ultraviolet
Imager on STEREO during 2008-2009, it is evident that these dimmings
are much more pronounced in 19.5 nm than in the lower-temperature line
17.1 nm, as viewed either on the disk or above the limb. We conclude
that most of the cooler coronal plasma is not ejected but remains
gravitationally bound when the loops open up. This result is consistent
with Doppler measurements by Imada and coworkers, who found that
the upflow speeds in a transient coronal hole increased dramatically
above a temperature of 1 MK it is also consistent with the quasistatic
behavior of polar plumes, as compared with the hotter interplume regions
that are the main source of the fast solar wind. When the open flux
reconnects and closes down again, the trapped plasma is initially heated
to such high temperatures that it is no longer visible at Fe IX 17.1
nm. Correspondingly, 17.1 nm images show a dark ribbon or "heat wave"
propagating away from the polarity inversion line and coinciding with
the brightened Fe XV 28.4 nm and Fe XII 19.5 nm post-eruptive loops and
their footpoint areas. Such dark ribbons provide a clear example of
dimmings that are not caused by a density depletion. The propagation
of the "heat wave" is driven by the closing-down, not the opening-up,
of the flux and can be observed both off-limb and on-disk.
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Title: Sun to 1 AU propagation and evolution of a slow
streamer-blowout coronal mass ejection
Authors: Lynch, B. J.; Li, Y.; Thernisien, A. F. R.; Robbrecht, E.;
Fisher, G. H.; Luhmann, J. G.; Vourlidas, A.
2010JGRA..115.7106L Altcode: 2010JGRA..11507106L
We present a comprehensive analysis of the evolution of the classic,
slow streamer-blowout CME of 1 June 2008 observed by the STEREO twin
spacecraft to infer relevant properties of the pre-eruption source
region which includes a substantial portion of the coronal helmet
streamer belt. The CME was directed ∼40° East of the Sun-Earth
line and the Heliospheric Imager observations are consistent with
the CME propagating essentially radially to 1 AU. The elongation-time
J-map constructed from the STEREO-A HI images tracks the arrival of two
density peaks that bound the magnetic flux rope ICME seen at STEREO-B on
6 June 2008. From the STEREO-A elongation-time plots we measure the ICME
flux rope radial size R<SUB>c</SUB>(t) and find it well approximated by
the constant expansion value V<SUB>exp</SUB> = 24.5 km/s obtained from
the STEREO-B declining velocity profile within the magnetic cloud. The
flux rope spatial orientation, determined by forward modeling fits to
the STEREO COR2 and HI1 data, approaches the observed 1 AU flux rope
orientation and suggests large-scale rotation during propagation, as
predicted by recent numerical simulations. We compare the ICME flux
content to the PFSS model coronal field for Carrington Rotation 2070
and find sufficient streamer belt flux to account for the observed
ICME poloidal/twist flux if reconnection during CME initiation process
is responsible for the conversion of overlying field into the flux
rope twist component in the standard fashion. However, the PFSS model
field cannot account for the ICME toroidal/axial flux component. We
estimate the field strength of the pre-eruption sheared/axial component
in the low corona and the timescales required to accumulate this
energized pre-eruption configuration via differential rotation and
flux cancelation by supergranular diffusion at the polarity inversion
line. We show that both mechanisms are capable of generating the
desired shear component over time periods of roughly 1-2 months. We
discuss the implications for slow streamer-blowout CMEs arising as
a natural consequence of the corona's re-adjustment to the long term
evolutionary driving of the photospheric fields.
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Title: On the "Extended" Solar Cycle in Coronal Emission
Authors: Robbrecht, E.; Wang, Y. -M.; Sheeley, N. R., Jr.; Rich, N. B.
2010ApJ...716..693R Altcode:
Butterfly diagrams (latitude-time plots) of coronal emission show a
zone of enhanced brightness that appears near the poles just after
solar maximum and migrates toward lower latitudes; a bifurcation seems
to occur at sunspot minimum, with one branch continuing to migrate
equatorward with the sunspots of the new cycle and the other branch
heading back to the poles. The resulting patterns have been likened to
those seen in torsional oscillations and have been taken as evidence
for an extended solar cycle lasting over ~17 yr. In order to clarify
the nature of the overlapping bands of coronal emission, we construct
butterfly diagrams from green-line simulations covering the period
1967-2009 and from 19.5 nm and 30.4 nm observations taken with the
Extreme-Ultraviolet Imaging Telescope during 1996-2009. As anticipated
from earlier studies, we find that the high-latitude enhancements mark
the footpoint areas of closed loops with one end rooted outside the
evolving boundaries of the polar coronal holes. The strong underlying
fields were built up over the declining phase of the cycle through
the poleward transport of active-region flux by the surface meridional
flow. Rather than being a precursor of the new-cycle sunspot activity
zone, the high-latitude emission forms a physically distinct, U-shaped
band that curves upward again as active-region fields emerge at
midlatitudes and reconnect with the receding polar-hole boundaries. We
conclude that the so-called extended cycle in coronal emission is a
manifestation not of early new-cycle activity, but of the poleward
concentration of old-cycle trailing-polarity flux by meridional flow.
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Title: Formation and Evolution of Coronal Holes Following the
Emergence of Active Regions
Authors: Wang, Y. -M.; Robbrecht, E.; Rouillard, A. P.; Sheeley,
N. R., Jr.; Thernisien, A. F. R.
2010ApJ...715...39W Altcode:
The low level of solar activity over the past four years has provided
unusually favorable conditions for tracking the formation and evolution
of individual coronal holes and their wind streams. Employing
extreme-ultraviolet images recorded with the Solar Terrestrial
Relations Observatory during 2007-2009, we analyze three cases
in which small coronal holes first appear at the edges of newly
emerged active regions and then expand via flux transport processes,
eventually becoming attached to the polar holes. The holes form
gradually over timescales comparable to or greater than that for
the active regions to emerge, without any obvious association with
coronal mass ejections. The evolving hole areas coincide approximately
with the footpoints of open field lines derived from potential-field
source-surface extrapolations of the photospheric field. One of these
coronal-hole systems, centered at the equator and maintained by a
succession of old-cycle active regions emerging in the same longitude
range, persists in one form or another for up to two years. The other
two holes, located at midlatitudes and originating from new-cycle
active regions, become strongly sheared and decay away after a few
rotations. The hole boundaries and the small active-region holes, both
of which are sources of slow wind, are observed to undergo continual
short-term (lsim1 day) fluctuations on spatial scales comparable to
that of the supergranulation. From in situ measurements, we identify
a number of plasma sheets associated with pseudostreamers separating
holes of the same polarity.
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Title: Heatwaves on the Sun
Authors: Robbrecht, Eva; Wang, Yi-Ming; Vourlidas, Angelos;
Patsourakos, Spiros
2010cosp...38.1791R Altcode: 2010cosp.meet.1791R
Dimmings have been observed for several years now, but their
interpretation is still problematic. A dimming is an observational
effect of diminished brightness with respect to pre-event images, which
is usually interpreted as a density depletion. But not all dimmings are
what they appear to be. In this paper we report on an unusual "dimming
wave" which is not a density depletion but rather a heat wave. Thanks
to the stereoscopic view from the SECCHI/EUVI imagers we are able not
only to uncover the nature of the wave, but also to understand its
three dimensional evolution and its relationship to a quiet sun CME.
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Title: Sun to 1 AU Propagation of a Slow Streamer-Blowout Coronal
Mass Ejection
Authors: Lynch, B. J.; Li, Y.; Thernisien, A. F.; Robbrecht, E.;
Luhmann, J. G.; Vourlidas, A.
2009AGUFMSH41A1635L Altcode:
We present the time history and evolution of the 3-dimensional size,
shape, and orientation of the slow, classic streamer blowout CME of
2008 Jun 01 by combining STEREO-A remote imaging of its interplanetary
propagation with in situ STEREO-B plasma and field measurements at
1~AU. The STEREO-A HI coverage allows unambigious identification of
the CME white light front-cavity structure and the resulting ICME flux
rope boundaries in running difference images. The elongation-time tracks
predict the arrival of the ICME at STEREO-B on 2008 June 06 remarkably
well. Starting from the simplest in situ flux rope model for the
coherent magnetic cloud field structure, we utilize the unprecedented
coverage of the coronal and heliospheric imaging observations to obtain
important corrections for the ICME flux rope geometry. MHD modeling
results obtained from the NASA Community Coordinated Modeling Center
for the ambient heliopsheric solar wind stream structure and a simple
CME-like density-pulse propagation are used to verify the overall
propagation direction and characterize some of the observed evolutionary
properties. The ICME radial expansion, i.e. the time evolution of
the flux rope radius Rc(t) from the STEREO-A elongation-time plots,
is well approximated by the standard treatment of constant radial
expansion Vexp = 24.5 km/s measured from the in situ bulk velocity
profile. The 3-dimensional spatial orientation of the ICME flux rope
determined by forward modeling in the inner heliosphere shows excellent
agreement with the observed 1~AU flux rope orientation and evidence
for large scale rotation of the ICME flux rope during its propagation,
as predicted by recent numerical simulations. In addition, measurements
of the CME's latitudinal angular width allows us to improve the estimate
the actual flux rope cross-sectional area and measurements of the CME's
longitudinal extent allows us to estimate a more realistic CME “loop
length". These geometric quantities are used to improve the estimates of
the ICME in situ toroidal and poloidal magnetic fluxes, ΦT and ΦP. The
in situ flux values are then compared to the magnetic fluxes inferred
from this event's source region, which includes a substantial portion of
the large scale coronal helmet streamer belt. We conlude by discussing
our results in the context of both CME initiation and the physical
mechanism(s) that energize the pre-eruption configuration. Support
for this work was provided by NASA HGI NNX08AJ04G.
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Title: On the Weakening of the Polar Magnetic Fields during Solar
Cycle 23
Authors: Wang, Y. -M.; Robbrecht, E.; Sheeley, N. R., Jr.
2009ApJ...707.1372W Altcode:
The Sun's polar fields are currently ~40% weaker than they were during
the previous three sunspot minima. This weakening has been accompanied
by a corresponding decrease in the interplanetary magnetic field (IMF)
strength, by a ~20% shrinkage in the polar coronal-hole areas, and by
a reduction in the solar-wind mass flux over the poles. It has also
been reflected in coronal streamer structure and the heliospheric
current sheet, which only showed the expected flattening into the
equatorial plane after sunspot numbers fell to unusually low values
in mid-2008. From latitude-time plots of the photospheric field,
it has long been apparent that the polar fields are formed through
the transport of trailing-polarity flux from the sunspot latitudes
to the poles. To address the question of why the polar fields are
now so weak, we simulate the evolution of the photospheric field and
radial IMF strength from 1965 to the present, employing a surface
transport model that includes the effects of active region emergence,
differential rotation, supergranular convection, and a poleward bulk
flow. We find that the observed evolution can be reproduced if the
amplitude of the surface meridional flow is varied by as little as 15%
(between 14.5 and 17 m s<SUP>-1</SUP>), with the higher average speeds
being required during the long cycles 20 and 23.
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Title: No Trace Left Behind: STEREO Observation of a Coronal Mass
Ejection Without Low Coronal Signatures
Authors: Robbrecht, Eva; Patsourakos, Spiros; Vourlidas, Angelos
2009ApJ...701..283R Altcode: 2009arXiv0905.2583R
The availability of high-quality synoptic observations of the
extreme-ultraviolet (EUV) and visible corona during the SOHO mission
has advanced our understanding of the low corona manifestations of
coronal mass ejections (CMEs). The EUV imager/white light coronagraph
connection has been proven so powerful, it is routinely assumed that if
no EUV signatures are present when a CME is observed by a coronagraph,
then the event must originate behind the visible limb. This assumption
carries strong implications for space weather forecasting but has not
been put to the test. This paper presents the first detailed analysis
of a frontside, large-scale CME that has no obvious counterparts in
the low corona as observed in EUV and Hα wavelengths. The event was
observed by the SECCHI instruments onboard the STEREO mission. The
COR2A coronagraph observed a slow flux-rope-type CME, while an
extremely faint partial halo was observed in COR2B. The event evolved
very slowly and is typical of the streamer-blowout CME class. EUVI A
171 Å images show a concave feature above the east limb, relatively
stable for about two days before the eruption, when it rises into
the coronagraphic fields and develops into the core of the CME. None
of the typical low corona signatures of a CME (flaring, EUV dimming,
filament eruption, waves) were observed in the EUVI B images, which we
attribute to the unusually large height from which the flux rope lifted
off. This interpretation is supported by the CME mass measurements
and estimates of the expected EUV dimming intensity. Only thanks to
the availability of the two viewpoints we were able to identify the
likely source region. The event originated along a neutral line over
the quiet-Sun. No active regions were present anywhere on the visible
(from STEREO B) face of the disk. Leaving no trace behind on the solar
disk, this observation shows unambiguously that a CME eruption does
not need to have clear on-disk signatures. Also it sheds light on the
question of "mystery" geomagnetic storms, storms without clear solar
origin (formerly called problem storms). We discuss the implications
for space weather monitoring. Preliminary inspection of STEREO data
indicates that events like this are not uncommon, particularly during
the ongoing period of deep solar minimum.
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Title: No trace left behind: STEREO Observation of a Coronal Mass
Ejection Lacking Low Coronal Signatures
Authors: Vourlidas, Angelos; Robbrecht, E.; Patsourakos, S.
2009SPD....40.2104V Altcode:
The availability of high quality synoptic observations of the EUV and
visible corona during the SOHO mission has advanced our understanding
of the low corona manifestations of CMEs. The EUV imager/White light
coronagraph connection has been proven so powerful, it is routinely
assumed that if no EUV signatures are present when a CME is observed
by a coronagraph, then the event must originate behind the visible
limb. This assumption carries strong implications for space weather
forecasting but has not been put to the test. This paper presents the
first detailed analysis of a frontside, large-scale CME that has no
obvious counterparts in the low corona. The event was observed by the
SECCHI instruments on the STEREO mission. The COR2A coronagraph observed
the event as a typical flux-rope type CME, while an extremely faint
partial halo was observed in COR2B. The event evolved very slowly and
is typical of the streamer-blowout CME class. EUVI-A 171A images show
a concave feature above the east limb, relatively stable for about
two days before the eruption, when it rises into the coronagraphic
fields and develops into the core of the CME. None of the typical low
corona signatures of a CME (flaring, EUV dimming, filament eruption,
waves) were observed. Thanks to the two STEREO viewpoints we were able
to identify the likely source region. The event originated along a
quiet sun neutral line. No active regions were present anywhere on the
visible face of the disk. Leaving no trace behind, this observation
shows unambiguously that a CME eruption does not need to have clear
on-disk signatures. Also it sheds light on the question of `mystery'
geomagnetic storms; storms without clear solar origin. Preliminary
inspection of STEREO data indicates that events like this are not
uncommon, particularly during the ongoing period of deep solar minimum.
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Title: Automated LASCO CME Catalog for Solar Cycle 23: Are CMEs
Scale Invariant?
Authors: Robbrecht, E.; Berghmans, D.; Van der Linden, R. A. M.
2009ApJ...691.1222R Altcode: 2008arXiv0810.1252R
In this paper, we present the first automatically constructed LASCO
coronal mass ejection (CME) catalog, a result of the application of
the Computer Aided CME Tracking software (CACTus) on the LASCO archive
during the interval 1997 September-2007 January. We have studied the CME
characteristics and have compared them with similar results obtained
by manual detection (CDAW CME catalog). On average, CACTus detects
less than two events per day during solar minimum, up to eight events
during maximum, nearly half of them being narrow (<20°). Assuming
a correction factor, we find that the CACTus CME rate is surprisingly
consistent with CME rates found during the past 30 years. The CACTus
statistics show that small-scale outflow is ubiquitously observed in the
outer corona. The majority of CACTus-only events are narrow transients
related to previous CME activity or to intensity variations in the slow
solar wind, reflecting its turbulent nature. A significant fraction
(about 15%) of CACTus-only events were identified as independent
events, thus not related to other CME activity. The CACTus CME width
distribution is essentially scale invariant in angular span over a
range of scales from 20° to 120° while previous catalogs present a
broad maximum around 30°. The possibility that the size of coronal
mass outflows follow a power-law distribution could indicate that
no typical CME size exists, i.e., that the narrow transients are not
different from the larger well defined CMEs.
---------------------------------------------------------
Title: First STEREO observation of a quiet sun CME
Authors: Robbrecht, E.; Patsourakos, S.; Vourlidas, A.
2008AGUFMSH13B1560R Altcode:
Streamer-blowouts form a particular class of CMEs characterized by a
slow rise and swelling of the streamer that can last for days. While
they are more massive than the average CME, their slow development
complicates their association with features/activity in the low
corona and hampers studies on their initiation mechanism(s). This
paper reports on the first observation from 2 viewpoints of a streamer
blowout CME. The event was observed by the SECCHI/COR2 A instrument
as a typical flux-rope type CME, while a very faint partial halo was
observed in COR2-B. The CME erupted from the east limb in the COR2 A
field of view. EUVI-171 A images show a bright feature above the limb,
traveling from the southern hemisphere towards the equator after which
it slowly rises into the coronagraphic fields of view developing into
the flux-rope structure CME. At the time of eruption the separation
between the two STEREO spacecraft is sufficiently large (54 deg) to
observe the source region face-on in STEREO-B. However, inspection of
EUVI B data didn't reveal any particular source region, other than the
quiet sun. No flaring activity could be related to the eruption. This
observation shows unambiguously that a CME eruption does not necessarily
have clear on-disk signature. Also it sheds light on the long-standing
question of the necessity of having a flare for producing a CME. This
result supplies strong constraints for CME initiation models. This type
of observation could not have been achieved without the multi-viewpoint
observations by STEREO.
---------------------------------------------------------
Title: Preface: SOHO 20 - Transient events on the Sun and in the
heliosphere
Authors: Robbrecht, E.; Hochedez, J. -F.; Fleck, B.; Gurman, J.;
Forsyth, R.
2008AnGeo..26.2953R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: New techniques for the characterisation of dynamical phenomena
in solar coronal images
Authors: Robbrecht, E.
2007PhDT........11R Altcode:
During a total solar eclipse, a narrow strip of the Earth's surface
is shielded completely by the Moon from the disk of the Sun. In this
strip, the corona appears crown-like around the shade of the Moon. It
was uncertain until the middle of the 20th century whether the corona
was a solar phenomenon or if it was related to the Moon or whether
it represented an artifact produced by the Earth's atmosphere. The
answer to this question was provided by Grotrian (1939) and Edlèn
(1942). Based on studies of iron emission lines, they suggested
that the surface of the Sun is surrounded by a hot tenuous gas
having a temperature of million degrees Kelvin and thus in a state
of high ionization. This discovery was a result from spectroscopy,
a field of research which started in 1666 with Sir Isaac Newton's
observations of sunlight, dispersed by a prism. <P />It is now clear
that the hot solar corona is made of a low density plasma, highly
structured by the magnetic field on length scales ranging from the
Sun's diameter to the limit of angular resolution (e.g. Démoulin and
Klein 2000). The need to resolve and study the corona down to such
scales has determined a vigorous scientific and technological impulse
toward the development of solar Ultraviolet (UV) and X-ray telescopes
with high spatial and temporal resolution. With the advent of the
satellite SOHO (Solar and Heliospheric Observatory, see chapter 1),
the picture of a quiet corona was definitely sent to the past. EUV
(Extreme UV) image sequences of the lower solar corona revealed a finely
structured medium constantly agitated by a wide variety of transients
(e.g. Harrison 1998). Active regions consisting of large magnetic
loops with enhanced temperature and density are observed, as well as
"quiet" areas, coronal holes and numerous structures of different
scales such as plumes, jets, spicules, X-ray bright points, blinkers,
all structured by magnetic fields. Launched in 1998, the Transition
Region And Coronal Explorer (TRACE) was an important step on the
way to subarcsecond telescopes. It allows a spatial resolution of 1"
in the EUV and UV bands and, simultaneously, a temporal resolution of
the order of a few seconds. <P />Coronal physics studies are dominated
by two major and interlinked problems: coronal heating and solar wind
acceleration. Above the chromosphere there is a thin transition layer
in which the temperature suddenly increases and density drops. How
can the temperature of the solar corona be three orders of magnitude
higher than the temperature of the photosphere? In order for this
huge temperature gradient to be stationary, non-thermal energy must
be transported from below the photosphere towards the chromosphere
and corona and converted into heat to balance the radiative and
conductive losses. This puzzle of origin, transport and conversion
of energy is referred to as the "coronal heating problem". Due to its
fundamental role in the structuring of the corona, the magnetic field
is supposed to play an important role in the heating. <P />In this
dissertation we describe two aspects of solar coronal dynamics: waves
in coronal loops (Part I) and coronal mass ejections (Part II). We
investigate the influence of (semi-) automated techniques on solar
coronal research. This is a timely discussion since the observation
of solar phenomena is transitioning from manual detection to "Solar
Image Processing". Our results are mainly based on images from the
Extreme UV Imaging Telescope (EIT) and the Large Angle and Spectrometric
Coronagraph (LASCO), two instruments onboard the satellite SOHO (Solar
and Heliospheric Observatory) of which we recently celebrated its 11th
anniversary. The high quality of the images together with the long
timespan created a valuable database for solar physics research. <P
/>Part I reports on the first detection of slow magnetoacoustic waves in
transequatorial coronal loops observed in high cadence image sequences
simultaneously produced by EIT and TRACE (Transition Region And Coronal
Explorer). Ten years of EUV observations made it clear that these
disturbances are a widespread phenomenon in active region loops. The
existence of these waves in the corona had been predicted by the theory
of magnetohydrodynamics (MHD), which we revise briefly. Just like in
helioseismology, coronal seismology uses observations of oscillations
to derive physical parameters which are not directly measurable, such
as the Alfvén speed or the magnetic field strength. The comparison
with helioseismology does not fully hold in the sense that the dense
photosphere does not allow any seeing inside. Instead, for the corona
we do have direct observations, but because of its optical thinness
these observations leave space for many interpretations. <P />At the
end of the forties, it was suggested that the corona could be heated
by the dissipation of acoustic waves (sound waves) driven by the p-mode
oscillations, generated by turbulence in the convection zone. While they
travel upwards, these waves form shocks and heat the plasma by viscous
dissipation. Nowadays, they are believed to be only important for lower
chromospheric heating. By the time the upper chromosphere is reached,
the acoustic waves are heavily damped and what rests is reflected by
the steep temperature and density gradients in the transition zone. As
such, they cannot deposit enough energy in the corona to sufficiently
heat it to the observed temperatures. Dissipation of magnetic energy
by Alfvén waves or directly by the reconnection process in current
sheets are considered to be more likely to heat the corona. <P />Part
II addresses the question of detecting coronal mass ejections (CMEs)
in coronagraphic white light data. The study of CMEs is a rather young
(≲ 30 years) field of research. Coronal mass ejections are sudden
expulsions of mass and magnetic field from the solar corona into the
interplanetary medium. A classical CME carries away some 10^15 g of
coronal mass and can liberate energies of 10^23-10^25 J. They are often
observed n association with low coronal activity, such as flares and
filament eruptions. During the first years of CME observation, it was
believed that a flare was a necessary condition for CME occurrence. The
widely accepted picture today is that flares and CMEs are both
different manifestations of magnetic field restructuring through
reconnection (flare) and the expulsion of mass (CME). Up till now,
the SOHO mission has been the best mission for CME studies because of
the increased resolution, cadence, sensitivity and dynamic range of the
LASCO instruments, but also because of the large array of ground-based
instruments (Howard 2006). The complexity of the CME-picture grew
likewise. The next mission with a coronagraph is the NASA STEREO mission
(Solar Terrestrial Relations Observatory), launched on 26 Oct. 2006. <P
/>In chapter 4 we test the possibility of automatically detecting
CMEs in LASCO data. We describe the algorithm CACTus (Computer Aided
CME Tracking) and test its validity on a short period of 6 days. In
chapter 5 we present our newly constructed CME catalog based on our
automated detection scheme. It is the first automatically generated
catalog which runs over a complete solar cycle (cycle 23). It required
no human interaction, which implies it is totally objective. It includes
all transients obeying the observational definition of CME as a "new,
discrete, bright, white-light feature in the coronagraph field-of-view
moving radially outward" (Hundhausen et al. 1984). As a result, our
catalog contains much more events, mostly narrow, than are included
in the classical CDAW CME catalog (Yashiro et al. 2004) which is
assembled manually. We discuss the CME rate over the solar cycle and
present important new statistics on the CACTus CME parameters (size,
latitude, speed). <P />CME research has gained an increased interest
due to their strong space weather impact. Space weather is defined
by the European Space Agency (ESA) 1 as the "conditions on the Sun
and in the solar wind, magnetosphere, ionosphere and thermosphere
that can influence the performance and reliability of space-borne
and ground-based technological systems and can endanger human life
or health." The significance of space weather lies in its potential
impact on man-made technologies on Earth and in space, for example, on
satellites and spacecraft, electricity power grids, pipelines, radio
and telephone communications and on geophysical exploration. Space
weather also has implications for manned space flight, both in Earth
orbit and further out into space. Solar activity is the main source
of space weather. It is now well established that CMEs are the primary
cause of geomagnetic storms and that their associated shocks accelerate
high energetic particles. These particles can directly and indirectly
influence the operation of spacecraft and affect communication and
navigation. In order to protect systems and people that might be at
risk from space weather effects, we need to understand the causes
of space weather and try to predict its impact on the heliosphere
as soon as possible. A growing field in this respect is Solar mage
Processing (SIP). It allows continuous monitoring and interpretation
of new incoming data. This is not only interesting for space weather
forecasting, but it is also needed to be able to handle efficiently
the large data flow which is expected from recently launched and future
missions. In chapter 6 we revise the current capabilities for automated
detection of CMEs and related phenomena.
---------------------------------------------------------
Title: Current and future space weather services and products from
the SIDC- Brussels
Authors: Lawrence, G.; Kretzschmar, M.; Berghmans, D.; Clete,
F.; Hochedez, J.; van der Linden, R.; Delouille, V.; Gissot, S.;
Marque, C.; Nicula, B.; Patoul, J.; Podladchikova, E.; Robbrecht,
E.; Vanlommel, P.; Dehant, V.
2006AGUFMSA51A..04L Altcode:
The SIDC-Brussels, as WDC for the sunspot index and European RWC of the
ISES, is the European hub for solar data and forecasts. Its services
and products, while long established and widely recognised and used,
are continuously being enhanced and supplemented. We present in
detail the current status and outline the imminent improvements and
additions. The Solar Weather Browser (SWB) is a free, downloadable,
multi-platform visualisation package for real-time browsing of processed
solar images from a variety of space and ground based sources, combined
with context information (events, regions IDs, etc.) via a wide choice
of overlay combinations. The Estimated International Sunspot Number
(EISN) has been produced and distributed daily since January 2006 by the
SIDC. Intended to support operational model predictions of ionospheric
radio propagation, we present some early statistical results. CACTus,
the operational Computer-Aided CME Tracking algorithm, now freely
available to the community via the SSW software framework, is being
tested for its real-time application to the STEREO/SECCHI COR-2 "space
weather beacon" coronagraph telemetry stream. Also NEMO, a software
package for the automated detection and morphological analysis of EIT
waves presently being tested, details the relation between coronal EUV
wave fronts and dimmings and characterizes their evolution; we present
sample results of both developments. The Velociraptor software processes
and interprets movies of the EUV solar corona, an algorithm identifying
outstanding motions such as loop openings that are associated to
space weather events. Sample results using EIT and TRACE data will be
shown. A new flare catalog called B2X is presented, compliled via a
method to detect automatically, and characterise according to time,
localization, size, EUV flares belonging to classes B to X anywhere
on the solar disc and at the limb. In addition we present a summary
of the full range of products available from SIDC which can be chosen
in any combination tailored to the individual, or group's needs. All
products are available via the revamped SIDC website, http://www.sidc.be
---------------------------------------------------------
Title: A Broad Perspective on Automated CME Tracking: Towards Higher
Level Space Weather Forecasting
Authors: Robbrecht, Eva; Berghmans, David
2006GMS...165...33R Altcode:
We discuss our current capabilities to deliver the CME parameters
required for the space weather forecasting process. The ever
growing importance of space weather has lead to new requirements
on the timeliness and objectiveness of CME detection. It has become
indispensable to report the occurrence of Earth-directed CMEs and to
predict their possible impact on the geospace environment. Early 2005,
we are on the eve of a new era in space weather forecasting. We point
out the restricted accuracy on the current forecasts and discuss a
chance for amelioration. This invokes data-driven models (empirical
and numerical), triggered by a real-time CME disturbance, simulating
the propagation and interaction of the ejection with the ambient solar
wind. We discuss the link between the direct observable parameters
(like the CME projected speed and angle around the occulter) and the
required input parameters (like radial speed, direction, …). The only
way to guarantee the real-time value of the simulations is by employing
software which autonomously detect CME parameters in a variety of
data. This paper focusses on the automated CME detection algorithms
that are currently available. Automated CME tracking is yet in its
infancy, therefore this `review' will be an outlook on the potential
of this field rather than looking back on already achieved milestones.
---------------------------------------------------------
Title: A broad Perspective on Automated CME Tracking: towards higher
level space weather forecasting
Authors: Robbrecht, E.; Berghmans, D.
2006GMS...165.....R Altcode:
We discuss our current capabilities to deliver the CME parameters
required for the space weather forecasting process. The ever
growing importance of space weather has lead to new requirements
on the timeliness and objectiveness of CME detection. It has become
indispensable to report the occurrence of Earth-directed CMEs and to
predict their possible impact on the geospace environment. Early 2005,
we are on the eve of a new era in space weather forecasting. We point
out the restricted accuracy on the current forecasts and discuss a
chance for amelioration. This invokes data-driven models (empirical
and numerical), triggered by a real-time CME disturbance, simulating
the propagation and interaction of the ejection with the ambient solar
wind. We discuss the link between the direct observable parameters
(like the CME projected speed and angle around the occulter) and the
required input parameters (like radial speed, direction, ...). The only
way to guarantee the real-time value of the simulations is by employing
software which autonomously detect CME parameters in a variety of
data. This paper focusses on the automated CME detection algorithms
that are currently available. Automated CME tracking is yet in its
infancy, therefore this `review' will be an outlook on the potential
of this field rather than looking back on already achieved milestones.
---------------------------------------------------------
Title: History of the Sunspot Index: 25 years SIDC
Authors: Berghmans, D.; van der Linden, R. A. M.; Vanlommel, P.;
Clette, F.; Robbrecht, E.
2006BGGKP...7..288B Altcode:
The sunspot number is the oldest solar activity index. For a long time,
it was the only index representative of the solar cycle, and many
studies on the cyclical behavior of the Sun were performed using the
sunspot number. The Sunspot Index Data Center (SIDC) was founded in
January 1981 to continue the work of the Swiss Federal Observatory ,
when this institution decided to stop computing and publishing the
sunspot number. The SIDC now also provides daily activity reports and
forecasts of the status of the space environment. This 'space weather'
activity is part of the International Space Environment Services (ISES,
a permanent service of the FAGS) that co-ordinates 10 regional warning
centers (RWC). In this paper we will give an overview of the history
of the sunspot number, as well as a short overview of the 25-year
history of the SIDC.
---------------------------------------------------------
Title: Objective CME detection over the solar cycle: A first attempt
Authors: Robbrecht, E.; Berghmans, D.; van der Linden, R. A. M.
2006AdSpR..38..475R Altcode:
We recently developed a software package to autonomously detect
coronal mass ejections (CMEs) in image sequences from large angle
spectrometric coronagraph (LASCO). The detection is not done in the
original images, instead we detect bright ridges in (time, height) plots
using a modified version of the Hough transform. Experimental results
on real-time data have shown that the developed technique can achieve
excellent results in measuring starting time and principal angle and
good results for the angular width and velocity measurement compared to
the CMEs listed in the manually assembled catalog. The real-time output
of the software can be found online at www.sidc.oma.be/cactus. With
the present paper, we report on the first large scale application of
the software to the LASCO archive. In an ongoing work, we have applied
the software on a first sample of 29 months selected from archive data
in the period July 1997 to December 2002. The results in this paper
show that relevant characteristics of CMEs over the solar cycle are
successfully recovered with the automated procedure. This proves that
a completely automatically produced CME catalog is within reach.
---------------------------------------------------------
Title: A Complete LASCO CME-Catalog based on Automated Detection
Authors: Robbrecht, E.; Berghmans, D.; van der Linden, R. A. M.
2006cosp...36.3564R Altcode: 2006cosp.meet.3564R
We present our new CME catalog a result of the large scale application
of our software CACTus on the LASCO archive We recently improved the
algorithm and upgraded the test-version of the catalog available online
at www sidc be cactus The automatic detection of a CME is done in two
steps and is applied simultaneously on c2 and c3 running difference
images 1 detection of bright features moving radially outward 2
clustering detections into CMEs The recent improvements are undertaken
in the second step of this process Unique for our detection method
is that we use the condition moving radially outward as part of the
detection criterion The detection itself is done using the stroboscopic
method i e in height time slices where height means radial distance
from the sun An outwardly moving feature appears then as a bright ridge
extracted by a modified version of the Hough transform Per month the
output consists of a table containing the CME characteristics and an
overview map in a angle time -coordinate system that clearly shows all
detected CMEs As a result of our method we also have for each CME a
linear speed profile along the angular width of the CME We studied the
characteristics of the CMEs detected with CACTus over the solar cycle
and compared them with results obtained from other catalogs This paper
shows that relevant characteristics of CMEs over the solar cycle are
successfully recovered with the automated procedure Recent tests prove
that in general automated procedures detect far more small features
than human
---------------------------------------------------------
Title: Solar weather monitoring
Authors: Hochedez, J. -F.; Zhukov, A.; Robbrecht, E.; van der Linden,
R.; Berghmans, D.; Vanlommel, P.; Theissen, A.; Clette, F.
2005AnGeo..23.3149H Altcode:
Space Weather nowcasting and forecasting require solar observations
because geoeffective disturbances can arise from three types of solar
phenomena: coronal mass ejections (CMEs), flares and coronal holes. For
each, we discuss their definition and review their precursors in terms
of remote sensing and in-situ observations. The objectives of Space
Weather require some specific instrumental features, which we list
using the experience gained from the daily operations of the Solar
Influences Data analysis Centre (SIDC) at the Royal Observatory of
Belgium. Nowcasting requires real-time monitoring to assess quickly and
reliably the severity of any potentially geoeffective solar event. Both
research and forecasting could incorporate more observations in order
to feed case studies and data assimilation respectively. Numerical
models will result in better predictions of geomagnetic storms
and solar energetic particle (SEP) events. We review the data types
available to monitor solar activity and interplanetary conditions. They
come from space missions and ground observatories and range from
sequences of dopplergrams, magnetograms, white-light, chromospheric,
coronal, coronagraphic and radio images, to irradiance and in-situ
time-series. Their role is summarized together with indications about
current and future solar monitoring instruments.
---------------------------------------------------------
Title: Solar activity: nowcasting and forecasting at the SIDC
Authors: Berghmans, D.; van der Linden, R. A. M.; Vanlommel, P.;
Warnant, R.; Zhukov, A.; Robbrecht, E.; Clette, F.; Podladchikova,
O.; Nicula, B.; Hochedez, J. -F.; Wauters, L.; Willems, S.
2005AnGeo..23.3115B Altcode:
The Solar Influences Data analysis Center (SIDC) is the World Data
Center for the production and the distribution of the International
Sunspot Index, coordinating a network of about 80 stations
worldwide. From this core activity, the SIDC has grown in recent years
to a European center for nowcasting and forecasting of solar activity
on all timescales. This paper reviews the services (data, forecasts,
alerts, software) that the SIDC currently offers to the scientific
community. The SIDC operates instruments both on the ground and in
space. The USET telescope in Brussels produces daily white light and
Hα images. Several members of the SIDC are co-investigators of the
EIT instrument onboard SOHO and are involved in the development of the
next generation of Europe's solar weather monitoring capabilities. While
the SIDC is staffed only during day-time (7 days/week), the monitoring
service is a 24 h activity thanks to the implementation of autonomous
software for data handling and analysis and the sending of automated
alerts. We will give an overview of recently developed techniques for
visualization and automated analysis of solar images and detection of
events significant for space weather (e.g. CMEs or EIT waves). As part
of the involvement of the SIDC in the ESA Pilot Project for Space
Weather Applications we have developed services dedicated to the
users of the Global Positioning System (GPS). As a Regional Warning
Center (RWC) of the International Space Environment Service (ISES),
the SIDC produces daily forecasts of flaring probability, geomagnetic
activity and 10.7 cm radio flux. The accuracy of these forecasts will
be investigated through an in-depth quality analysis.
---------------------------------------------------------
Title: Entering The Era Of Automated Cme Recognition: A Review Of
Existing Tools
Authors: Robbrecht, E.; Berghmans, D.
2005SoPh..228..239R Altcode:
We consider the problem of the observational identification of CMEs. The
ever growing importance of space weather has led to new requirements on
the timeliness and objectiveness of CME detection. It is not sufficient
any more to simply detect CMEs, a complete set of characteristics
(speed, direction, mass, chirality) must be reported as soon as
possible to estimate its geoeffectiveness. Recent developments in
(solar) feature recognition greatly improved the ability to address
these new needs. Progress was achieved in automating the detection of
CMEs in coronagraphic data. This has led to near-real-time messages
alerting the space weather community day and night. In attempting to
generate ever-prompter alerts, we can employ a far broader set of solar
observations than coronagraphic data alone. At present an extensive
set of automatic recognition tools exists for a number of CME-related
phenomena occurring in the lower corona. This paper deals with detection
techniques for disappearing filaments in Hα images, dimmings, EIT waves
and erupting prominences in radio data. We believe that incorporating
all automatically generated alerts into one report per CME can provide
valuable CME information, especially when no coronagraphic images are
available. This paper is thus a quest to reach a maximal success rate
with the help of an integrated system of tools acting on a variety of
data. Future grid-technology systems will greatly facilitate this.
---------------------------------------------------------
Title: Automated recognition of coronal mass ejections (CMEs) in
near-real-time data
Authors: Robbrecht, E.; Berghmans, D.
2004A&A...425.1097R Altcode:
This paper presents a new method and first applications of software that
we have developed to autonomously detect CMEs in image sequences from
LASCO (Large Angle Spectrometric Coronagraph). The crux of the software
is the detection of CMEs as bright ridges in (time, height) maps using
the Hough transform. The next step employs clustering and morphological
closing operations to mark out different CMEs. The output is a list of
events, similar to the classic catalogs, with starting time, principle
angle, angular width and velocity estimation for each CME. In addition
we present a new type of CME overview map that clearly shows all
detected CMEs in a (principal angle, time of occurrence) coordinate
system. In contrast to catalogs assembled by human operators, these CME
detections can be done without any human interference on real-time data
24 h per day (see http://sidc.oma.be/cactus for the real-time output
with data covering the last 4 days). Therefore the detection is not
only more immediate, but, more importantly, also more objective. In
this paper we describe the software and validate its performance by
comparing its output with the SOHO LASCO CME catalog. Experimental
results on real-time data show that the developed technique can achieve
excellent results in measuring starting time and principal angle and
good results for the angular width and velocity measurement compared to
the CMEs listed in the catalog. Its overall success rate is presently
about 94%. The software also reveals CMEs or other features that have
not been listed in the catalog. Such unreported cases might influence
CME statistics and they demonstrate that also the present catalogs do
not have a 100% success rate. This inevitably leads to a discussion
on the definition of a CME. Prospects for improvement and exploitation
are discussed.
---------------------------------------------------------
Title: The expanding space weather services of the SIDC at the Royal
Observatory of Belgium
Authors: van der Linden, R. A. M.; Berghmans, D.; Vanlommel, P.;
Robbrecht, E.; Cugnon, P.; Clette, F.; Wauters, L.; Zhukov, A.
2004cosp...35.2781V Altcode: 2004cosp.meet.2781V
Originally founded in 1981 as the World Data Centre for the Sunspot
Index, the SIDC (Solar Influences Data analysis Centre) shifted to
a higher gear in 2000, when it became a Regional Warning Centre of
the ISES (taking over this activity from Meudon). The obvious link
between space weather and solar activity - a prime research topic of
the solar physics department of the Royal Observatory of Belgium -
and the equally obvious relevance of continuous long-term monitoring
of solar activity have made the SIDC well-placed to embark on this new
future. Thanks to becoming one of the Service Development Activities in
the Space Weather Applications Pilot Project recently set up by ESA, the
SIDC has been able to further improve and expand these activities. In
this paper we discuss some of the new tools, models and data that have
been or will be developed to this purpose. It will also be detailed how
the scientific and operational involvement of the Royal Observatory of
Belgium in many future space missions oriented towards solar physics
and solar monitoring will help the SIDC become an independent European
space weather monitoring and forecasting centre.
---------------------------------------------------------
Title: Objective CME detection over the solar cycle
Authors: Robbrecht, E.; Berghmans, D.
2004cosp...35.2702R Altcode: 2004cosp.meet.2702R
We have developed a software package for 'Computer Aided CME Tracking'
(CACTus), that autonomously detects CMEs in image sequences from
LASCO. The crux of the CACTus software is the detection of CMEs as
bright ridges in [height, time] maps using the Hough transform. The
output is a list of events, similar to the classic catalogs,
with principle angle, angular width and velocity estimation for
each CME. In contrast to catalogs assembled by human operators,
these CME detections by software can be faster and possibly also
more objective, as the detection criterion is written explicitly
in a program. Especially on the timescale of a solar cycle, it is
questionnable whether human, visual CME detection is stable, as the
operator gains experience or personnel is replaced. In this paper we
overview the latest improvements of CACTUS and validate its performance
by comparing the CACTus output with the classical, visually assembled
CME catalogs. Discrepancies between the classical catalogs and the
CACTUS catalogs are discussed. Such discrepancies highlight not
only the performance of CACTUS but also the caveats of the classical
catalogs. Indeed, CACTUS sometimes finds CMEs that are not listed in
the catalogs or interpreted differently (eg halo CME or not). It is
important to know these caveats when using the CME catalogs as input for
statistical CME studies over the solar cycle. The near realtime output
of the software is available on the web(http://sidc.oma.be/cactus)
and is updated daily.
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Title: Spectroscopic Monitoring of 10 Northern SPB Candidates
Authors: Mathias, P.; Le Contel, J. -M.; Aerts, C.; De Cat, P.;
van Winckel, H.; Robbrecht, E.; Briquet, M.; Cuypers, J.
2002ASPC..259..232M Altcode: 2002rnpp.conf..232M; 2002IAUCo.185..232M
No abstract at ADS
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Title: Slow magnetoacoustic waves in coronal loops: EIT and TRACE
Authors: Robbrecht, E.; Verwichte, E.; Berghmans, D.; Hochedez, J. F.;
Poedts, S.; Nakariakov, V. M.
2001A&A...370..591R Altcode:
On May 13, 1998 the EIT (Extreme ultraviolet Imaging Telescope) on board
of SoHO (Solar and Heliospheric Observatory) and TRACE (Transition
Region And Coronal Explorer) instruments produced simultaneous high
cadence image sequences of the same active region (AR 8218). TRACE
achieved a 25 s cadence in the Fe Ix (171 Å) bandpass while EIT
achieved a 15 s cadence (operating in “shutterless mode”, SoHO JOP
80) in the Fe Xii (195 Å) bandpass. These high cadence observations
in two complementary wavelengths have revealed the existence of weak
transient disturbances in an extended coronal loop system. These
propagating disturbances (PDs) seem to be a common phenomenon in
this part of the active region. The disturbances originate from small
scale brightenings at the footpoints of the loops and propagate along
the loops. The projected propagation speeds roughly vary between 65
and 150 km s<SUP>-1</SUP> for both instruments which is close to and
below the expected sound speed in the coronal loops. The measured slow
magnetoacoustic propagation speeds seem to suggest that the transients
are sound (or slow) wave disturbances. This work differs from previous
studies in the sense that it is based on a multi-wavelength observation
of an entire loop bundle at high cadence by two EUV imagers. The
observation of sound waves along the same path shows that they propagate
along the same loop, suggesting that loops contain sharp temperature
gradients and consist of either concentric shells or thin loop threads,
at different temperatures.
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Title: Slow magnetoacoustic waves in coronal loops
Authors: Nakariakov, V. M.; Verwichte, E.; Berghmans, D.; Robbrecht, E.
2000A&A...362.1151N Altcode:
A theoretical model interpreting propagating disturbances of EUV
emission intensity, recently observed in coronal loops, is constructed
in terms of slow magnetoacoustic waves. The model is one-dimensional
and incorporates effects of nonlinearity, dissipation due to finite
viscosity and thermal conduction, and gravitational stratification of
plasma in the loop. It has been found that, for the observationally
detected parameters of the waves, the main factors influencing the
wave evolution are dissipation and stratification. The upwardly
propagating waves of observed periods (5-20 min) are found to decay
significantly in the vicinity of the loop apex, explaining the rarity
of observational detection of downwardly propagating waves. The model
provides a theoretical basis for development of MHD seismology of the
coronal loops.
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Title: Slow magnetoacoustic waves in coronal loops: EIT vs TRACE
Authors: Robbrecht, E.; Verwichte, E.; Berghmans, D.; Hochedez, J. F.;
Poedts, S.
2000AIPC..537..271R Altcode: 2000wdss.conf..271R
On May 13, 1998 the EIT (Extreme-Ultraviolet Imaging Telescope) and
TRACE (Transition Region And Coronal Explorer) instruments produced
simultaneous high cadence image sequences of the same active region
(AR 8218). TRACE achieved a 25 sec cadence in the Fe IX/X (171 Å)
bandpass while EIT achieved a 15 sec cadence (operating in `shutterless
mode,' SOHO JOP 80) in the Fe XII (195 Å) bandpass. These high
cadence observations in two complementary wavelengths have revealed
the existence of weak transient disturbances in an extended coronal
loop system. These propagating disturbances (PDs) seem to be a
common phenomenon in this part of the active region. The disturbances
originate from small scale brightenings at the footpoints of the loops
and propagate along the loops. The apparent propagation speeds roughly
vary between 65 and 150 km s<SUP>-1</SUP> which is close to the expected
sound speed of the coronal loops. The measured propagation speeds seem
to suggest that the transients are sound (or slow) wave disturbances. .
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Title: Multi-Imager Study of Transients and Propagating Disturbances
in Active Region Loops (SOHO JOP80 Campaign)
Authors: Berghmans, D.; Clette, F.; Robbrecht, E.; McKenzie, D.
1999ESASP.448..575B Altcode: 1999ESPM....9..575B; 1999mfsp.conf..575B
No abstract at ADS
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Title: Slow Magnetoacoustic Waves in Coronal Loops?
Authors: Robbrecht, E.; Berghmans, D.; Nakariakov, V.; Poedts, S.
1999ESASP.446..575R Altcode: 1999soho....8..575R
On May 13, 1998 the EIT and TRACE instruments produced simultaneous
high cadence image sequences of the same active region (AR 8218). TRACE
achieved a 25 sec cadence in the 171 deg passband while EIT achieved
a 15 sec cadence (operating in 'shutterless mode', SOHO JOP 80) in the
195 deg passband. These high cadence observations in two complementary
wavelengths have revealed the existence of weak disturbances in an
extended coronal loop system. The disturbances originate from small
scale brightenings at the footpoints of the loops and propagate along
the loops at an apparant speed of the order of 150 km/s which is close
to the expected sound speed. To conclude whether these propagating
disturbances should be interpreted as slow magnetoacoustic waves or as
mass motions ('microflows'), we compare our observational findings with
theoretical models. Our results suggest that the recent discovery of
DeForest and Gurman (1998) of slow MHD waves in polar plumes, are in
fact not typical of polar plumes but occur also in extended coronal
structures elsewhere.
