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1.
In this paper we discuss the two mechanisms by which solar prominences on the limb can manifest themselves when observed in
coronal UV – EUV lines and in the soft X-ray continuum. These mechanisms are the absorption in the resonance continua of hydrogen
and helium on one hand and the reduction of the emissivity in a part of the coronal volume occupied by a prominence on the
other one. We briefly describe earlier observations made with SOHO/SUMER, EIT and Yohkoh/SXT. We then discuss how the instruments on the new Japanese satellite Hinode can be used for more detailed studies of prominences. We also propose some combined observations between the Hinode satellite and the SOHO/SUMER instrument. 相似文献
2.
T. Dudok De Wit 《Solar physics》2006,239(1-2):519-530
A segmentation scheme for identifying large-scale structures (coronal holes, active regions, etc.) in solar extreme ultraviolet images, is presented. Unlike standard approaches, both the image intensity and the relative
contribution of different wavelengths are used. Spectral information is important for compensating luminosity changes. The
approach is illustrated with images taken in the extreme ultraviolet by the EIT telescope onboard SOHO. This supervised segmentation
scheme, which incorporates a Bayesian classifier, is computationally simple, and can easily be used to track in near-real
time structures, such as coronal holes. 相似文献
3.
Markus J. Aschwanden 《Solar physics》2005,228(1-2):339-358
EUV images show the solar corona in a typical temperature range of T >rsim 1 MK, which encompasses the most common coronal structures: loops, filaments, and other magnetic structures in active regions,
the quiet Sun, and coronal holes. Quantitative analysis increasingly demands automated 2D feature recognition and 3D reconstruction,
in order to localize, track, and monitor the evolution of such coronal structures. We discuss numerical tools that “fingerprint”
curvi-linear 1D features (e.g., loops and filaments). We discuss existing finger-printing algorithms, such as the brightness-gradient
method, the oriented-connectivity method, stereoscopic methods, time-differencing, and space–time feature recognition. We
discuss improved 2D feature recognition and 3D reconstruction techniques that make use of additional a priori constraints, using guidance from magnetic field extrapolations, curvature radii constraints, and acceleration and velocity
constraints in time-dependent image sequences. Applications of these algorithms aid the analysis of SOHO/EIT, TRACE, and STEREO/SECCHI
data, such as disentangling, 3D reconstruction, and hydrodynamic modeling of coronal loops, postflare loops, filaments, prominences,
and 3D reconstruction of the coronal magnetic field in general. 相似文献
4.
Extreme-ultraviolet data from EIT/SOHO (1996–2002), soft X-ray data from Yohkoh (1991–2001), and magnetic field data from MDI/SOHO (1996–2002) and Kitt Peak Observatory, NSO/NOAO (1991–2002) are analyzed
together in the form of synoptic maps for the investigation of solar cycle variations of the corona and their relation to
the magnetic field. These results show new interesting relations between the evolution of the topological structure of the
corona, coronal heating and the large-scale magnetic field. The long-lived coronal structures are related to complexes of
solar activity and display quasi-periodic behavior (in the form of impulses of coronal activity) with periods of 1.0–1.5 year,
in the axisymmetric distribution of EUV and X-ray fluxes during the current solar cycle 23. In particular, during the second
maximum of this cycle the solar corona became somewhat hotter than it was in the period of the first maximum. 相似文献
5.
The Recovery of CME-Related Dimmings and the ICME’s Enduring Magnetic Connection to the Sun 总被引：1，自引：1，他引：0
G. D. R. Attrill L. van Driel-Gesztelyi P. Démoulin A. N. Zhukov K. Steed L. K. Harra C. H. Mandrini J. Linker 《Solar physics》2008,252(2):349-372
It is generally accepted that transient coronal holes (TCHs, dimmings) correspond to the magnetic footpoints of CMEs that
remain rooted in the Sun as the CME expands out into the interplanetary space. However, the observation that the average intensity
of the 12 May 1997 dimmings recover to their pre-eruption intensity in SOHO/EIT data within 48 hours, whilst suprathermal
unidirectional electron heat fluxes are observed at 1 AU in the related ICME more than 70 hours after the eruption, leads
us to question why and how the dimmings disappear whilst the magnetic connectivity is maintained. We also examine two other
CME-related dimming events: 13 May 2005 and 6 July 2006. We study the morphology of the dimmings and how they recover. We
find that, far from exhibiting a uniform intensity, dimmings observed in SOHO/EIT data have a deep central core and a more
shallow extended dimming area. The dimmings recover not only by shrinking of their outer boundaries but also by internal brightenings.
We quantitatively demonstrate that the model developed by Fisk and Schwadron (Astrophys. J.
560, 425, 2001) of interchange reconnections between “open” magnetic field and small coronal loops is a strong candidate for the mechanism
facilitating the recovery of the dimmings. This process disperses the concentration of “open” magnetic field (forming the
dimming) out into the surrounding quiet Sun, thus recovering the intensity of the dimmings whilst still maintaining the magnetic
connectivity to the Sun.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
6.
Using SOHO/MDI and SOHO/EIT data we study properties and dynamics of interconnected active regions, and the relations between
the photospheric magnetic fields and coronal structures in active longitudes during the beginning of solar cycle 23. The emergence
of new magnetic flux results in appearance of new interconnecting loops. The existence of stable coronal structures strongly
depends on the photospheric magnetic fluxes and their variations. We present some initial results for a complex of solar activity
observed in April 1997, and discuss the role of reconnection in the formation of the interconnected loops and coronal holes. 相似文献
7.
G. D. R. Attrill L. K. Harra L. van Driel-Gesztelyi M. J. Wills-Davey 《Solar physics》2010,264(1):119-147
We study two CME events on 13 and 14 December 2006 that were associated with large-scale dimmings. We study the eruptions
from pre-event on 11 December through the recovery on 15 December, using a combination of Hinode/EIS, SOHO/EIT, SOHO/MDI, and MLSO Hα data. The GOES X-class flares obscured the core dimmings, but secondary dimmings developed
remote from the active region (AR) in both events. The secondary dimmings are found to be formed by a removal of bright coronal
material from loops in the plage region to the East of the AR. Using Hinode/EIS data, we find that the outflows associated with the coronal-dimming regions are highly structured. The concentrated outflows
are located at the footpoints of coronal loops (which exist before, and are re-established after, the eruptions), and these
are correlated with regions of positive magnetic elements. Comparative study of the Hinode/EIS and SOHO/EIT data shows that the reduction in outflow velocity is consistent with the recovery in intensity of the studied
regions. We find that concentrated downflows develop during the recovery phase of the dimmings and are also correlated with
the same positive magnetic elements that were previously related to outflows. 相似文献
8.
A vertical current sheet is a crucial element in many flare/coronal mass ejection (CME) models. For the first time, Liu et al. reported a vertical current sheet directly imaged during the flare rising phase with the EUV Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). As a follow-up study, here we present the comprehensive analysis and detailed physical interpretation of the observation. The current sheet formed due to the gradual rise of a transequatorial loop system. As the l... 相似文献
9.
Emerging coronal loops were studied with extreme ultraviolet observations performed by SOHO/EIT on 5 and 6 August 1997 for
NOAA 8069. Physical parameters (size and twist) were determined by a new stereoscopic method. The flux tubes were measured
twisted when first observed by EIT. After emerging, they de-twisted as they expanded, which corresponds to a minimization
of the energy. Different scenarios which take into account the conservation of the magnetic helicity are discussed in relation
with structure and temperature variations.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013329626327 相似文献
10.
M. Karovska B. Wood J. Chen J. Cook R. Howard 《Journal of Astrophysics and Astronomy》2000,21(3-4):403-406
We applied advanced image enhancement techniques to explore in detail the characteristics of the small-scale structures and/or
the low contrast structures in several Coronal Mass Ejections (CMEs) observed by SOHO. We highlight here the results from
our studies of the morphology and dynamical evolution of CME structures in the solar corona using two instruments on board
SOHO: LASCO and EIT. 相似文献
11.
Coronal holes (CHs) are regions of open magnetic field lines in the solar corona and the source of the fast solar wind. Understanding the evolution of coronal holes is critical for solar magnetism as well as for accurate space weather forecasts. We study the extreme ultraviolet (EUV) synoptic maps at three wavelengths (195 Å/193 Å, 171 Å and 304 Å) measured by the Solar and Heliospheric Observatory/Extreme Ultraviolet Imaging Telescope (SOHO/EIT) and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) instruments. The two datasets are first homogenized by scaling the SDO/AIA data to the SOHO/EIT level by means of histogram equalization. We then develop a novel automated method to identify CHs from these homogenized maps by determining the intensity threshold of CH regions separately for each synoptic map. This is done by identifying the best location and size of an image segment, which optimally contains portions of coronal holes and the surrounding quiet Sun allowing us to detect the momentary intensity threshold. Our method is thus able to adjust itself to the changing scale size of coronal holes and to temporally varying intensities. To make full use of the information in the three wavelengths we construct a composite CH distribution, which is more robust than distributions based on one wavelength. Using the composite CH dataset we discuss the temporal evolution of CHs during the Solar Cycles 23 and 24. 相似文献
12.
Auchère Frédéric Hassler Donald M. Slater David C. Woods Thomas N. 《Solar physics》2001,202(2):269-280
Two successful sounding rocket flights were launched on 15 May 1997 and 2 November 1998 with an objective of providing inter-calibration with several of the instruments on board SOHO and TRACE. We will discuss here the results of the inter-calibration between the SwRI/LASP rocket imaging instruments and the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO. The Multiple XUV Imager (MXUVI) sounding rocket instrument is a multi-layer mirror telescope equipped with an internal occulter and light trap to provide full-disk images of Feix/x 17.1 nm and off-limb observations of Heii 30.4 nm. The SOHO/EIT instrument is also a full-disk multi-layer imager with four channels, Fe ix/x 17.1 nm, Fexii 19.5 nm, Fexv 28.4 nm and Heii 30.4 nm. By comparison with the EIT observations taken at the same time, we provide new flat-field determinations for EIT which help quantify the sensitivity degradation of the EIT detector, as well as provide a measure of the off-limb stray-light characteristics of the two instruments. We find that the EIT stray-light function is strongly asymmetric, with greater stray light in the 17.1 and 19.5 nm quadrants than the 30.4 and 28.4 nm quadrants. Two possible causes of this asymmetry are the polishing processes of the EIT mirrors and the asymmetric support grid pattern in the foil mesh at the EIT pupil. 相似文献
13.
The structure of the solar corona is dominated by the magnetic field because the magnetic pressure is about four orders of
magnitude higher than the plasma pressure. Due to the high conductivity the emitting coronal plasma (visible, e.g., in SOHO/EIT)
outlines the magnetic field lines. The gradient of the emitting plasma structures is significantly lower parallel to the magnetic
field lines than in the perpendicular direction. Consequently information regarding the coronal magnetic field can be used
for the interpretation of coronal plasma structures. We extrapolate the coronal magnetic field from photospheric magnetic
field measurements into the corona. The extrapolation method depends on assumptions regarding coronal currents, e.g., potential
fields (current-free) or force-free fields (current parallel to magnetic field). As a next step we project the reconstructed
3D magnetic field lines on an EIT-image and compare with the emitting plasma structures. Coronal loops are identified as closed
magnetic field lines with a high emissivity in EIT and a small gradient of the emissivity along the magnetic field. 相似文献
14.
Margarita Karovska Brian Wood John W. Cook James Chen Russ Howard 《Astrophysics and Space Science》2000,273(1-4):7-15
Since January 1996, the Solar and Heliospheric Observatory (SOHO)has been providing unprecedented views of the extended solar coronato heliocentric distances of up to 32 solar radii. During the past threeyears we carried out studies of the morphology and dynamical evolutionof various structures in the solar corona using two instruments on boardSOHO: the Large Angle Spectrometric Coronagraph (LASCO) and the ExtremeUltraviolet Imaging Telescope (EIT). We have applied advanced image resolutionenhancement techniques to explore in detail the characteristics of thesmall-scale structures and/or the low contrast structures in the solarcorona. We describe here the results from these high-angular resolutionstudies, including of the kinematics of several Coronal Mass Ejections andpolar jets. 相似文献
15.
Durgesh Tripathi 《Journal of Astrophysics and Astronomy》2006,27(2-3):193-200
The Large Angle Spectrometric Coronagraph (LASCO) and Extreme-ultraviolet Imaging Telescope (EIT) onboard Solar and Heliospheric
Observatory (SOHO) provide us with unprecedented multi-wavelength observations helping us to understand different dynamic
phenomena on the Sun and in the corona. In this paper we discuss the association between post-eruptive arcades (PEAs) detected
by EIT and white-light coronal mass ejections (CMEs) detected by LASCO/C2 telescope. 相似文献
16.
M. L. Mays O. C. St. Cyr E. Quémerais S. Ferron J.-L. Bertaux S. Yashiro R. Howard 《Solar physics》2007,241(1):113-125
In this study, the possibility that coronal mass ejections (CMEs) may be observed in neutral Lyman-α emission was investigated.
An observing campaign was initiated for SWAN (Solar Wind ANisotropies), a Lyman-α scanning photometer on board the Solar and
Heliospheric Observatory (SOHO) dedicated to monitoring the latitude distribution of the solar wind from its imprints on the
interstellar sky background. This was part of SOHO Joint Observing Program (JOP) 159 and was an exploratory investigation
as it was not known how, or even if, CMEs interact with the solar wind and interstellar neutral hydrogen at this distance
(≈60 and 120 R
S). The study addresses the lack of methods for tracking CMEs beyond the field-of-view of current coronagraphs (30 R
S). In our first method we used LASCO, white-light coronagraphs on SOHO, and EIT, an extreme ultraviolet imaging telescope
also on SOHO, to identify CME candidates which, subject to certain criteria, should have been observable in SWAN. The criteria
included SWAN observation time and location, CME position angle, and extrapolated speed. None of the CME candidates that we
discuss were identified in the SWAN data. For our second method we analyzed all of the SWAN data for 184 runs of the observing
campaign, and this has yielded one candidate CME detection. The candidate CME appears as a dimming of the background Lyman-α
intensity representing ≈10% of the original intensity, moving radially away from the Sun. Multiple candidate CMEs observed
by LASCO and EIT were found which may have caused this dimming. Here we discuss the campaign, data analysis technique and
statistics, and the results. 相似文献
17.
With the use of interplanetary coronal mass ejections (ICMEs) compiled by Richardson and Cane from 1996 to 2007 and the associated
coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric
Observatory (SOHO), we investigate the solar cycle variation of real ICME-associated CME latitudes during solar cycle 23 using
Song et al.’s method. The results show the following:
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Although most of ICME-associated CMEs are distributed at low latitudes, there is a significant fraction of ICME-associated
CMEs occurring at high latitudes. 相似文献
18.
The characteristics of the magnetic field ubiquitously permeating the coronal plasma are still largely unknown. In this paper we analyze some aspects of coronal physics, related to the magnetic field behavior, which forthcoming SOHO UVCS observations can help better understand. To this end, three coronal structures will be examined: streamers, coronal mass ejections (CMEs), and coronal holes.As to streamers and CMEs, we show, via simulations of the Ly- and white-light emission from these objects, calculated on the basis of recent theoretical models (Wang et al., 1995), how new data from SOHO can help in advancing our knowledge of the streamer/CME magnetic configuration. Our discussion highlights also those observational signatures which might offer clues on reconnection processes in streamers' current sheets.Coronal holes (CHs) are discussed in the last section of the paper. Little is known about CH flux tube geometry, which is closely related to the behavior of the solar wind at small heliocentric distances.Indirect evidence for the flux tube spreading factors, within a few solar radii, is here examined. 相似文献
19.
We present an analysis of all the events (around 400) of coronal shocks for which the shock-associated metric type IIs were
observed by many spectrographs during the period April 1997– December 2000. The main objective of this analysis is to give
evidence for the type IIs related to only flare-blast waves, and thus to find out whether there are any type II-associated
coronal shocks without mass ejections. By carefully analyzing the data from multi-wavelength observations (Radio, GOES X-ray,
Hα, SOHO/LASCO and SOHO/EIT-EUV data), we have identified only 30 events for which there were actually no reports of CMEs.
Then from the analysis of the LASCO and EIT running difference images, we found that there are some shocks (nearly 40%, 12/30)
which might be associated with weak and narrow mass ejections. These weak and narrow ejections were not reported earlier.
For the remaining 60% events (18/30), there are no mass ejections seen in SOHO/LASCO. But all of them are associated with
flares and EIT brightenings. Pre-assuming that these type IIs are related to the flares, and from those flare locations of
these 18 cases, 16 events are found to occur within the central region of the solar disk (longitude ≤45^∘). In this case,
the weak CMEs originating from this region are unlikely to be detected by SOHO/LASCO due to low scattering. The remaining
two events occurred beyond this longitudinal limit for which any mass ejections would have been detected if they were present.
For both these events, though there are weak eruption features (EIT dimming and loop displacement) in the EIT images, no mass
ejection was seen in LASCO for one event, and a CME appeared very late for the other event. While these two cases may imply
that the coronal shocks can be produced without any mass ejections, we cannot deny the strong relationship between type IIs
and CMEs. 相似文献
20.
On 13 May 1998, the Extreme-Ultraviolet Imaging Telescope (EIT, on board SOHO) has produced a unique image sequence operating in 'shutterless mode' (SOHO JOP 80). In JOP 80, EIT is the lead instrument, followed by several space-born instruments (SXT, TRACE, MDI, CDS, SUMER), as well as two observatories on the ground (in La Palma and Sac Peak). The target of the campaign was a relatively small but rapidly evolving active region (AR 8218). For the EIT contribution, a 15 s cadence was achieved in the Fexii bandpass at 195 Å by leaving EIT's shutter open for 1 h and operating the CCD in frame-transfer mode. In this paper, we start the analysis of the huge data set, by making an inventory of the transients observed in the EIT image sequence. Besides scatter plots of duration, size and radiative output of the detected EUV brightenings, we discuss in full detail the morphology and evolution of several typical events. These transients range from a B3.5 flare producing a large plasma flow along pre-existing loops, to EUV versions of active region transient brightenings as previously observed by SXT on board Yohkoh. In addition, a new class of weaker footpoint brightenings is discussed that produce wave-like disturbances propagating along quasi-open field lines. This new class of propagating disturbances extends the wide variety of transient phenomena that we discovered in the EIT data, and makes the potential for inter-instrumental studies of the JOP 80 data all the more exciting. We stress the necessity of such forthcoming studies to reach an instrument-independent classification of small-scale solar transients. 相似文献
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