共查询到20条相似文献，搜索用时 31 毫秒
1.
The space mission Solar TErrestrial RElations Observatory (STEREO) will provide images from two viewpoints. An important aim
of the STEREO mission is to get a 3D view of the solar corona. We develop a program for the stereoscopic reconstruction of
3D coronal loops from images taken with the two STEREO spacecraft. A pure geometric triangulation of coronal features leads
to ambiguities because the dilute plasma emissions complicates the association of features in image 1 with features in image
2. As a consequence of these problems, the stereoscopic reconstruction is not unique and multiple solutions occur. We demonstrate
how these ambiguities can be resolved with the help of different coronal magnetic field models (potential, linear, and non-linear
force-free fields). The idea is that, due to the high conductivity in the coronal plasma, the emitting plasma outlines the
magnetic field lines. Consequently, the 3D coronal magnetic field provides a proxy for the stereoscopy, which allows to eliminate
inconsistent configurations. The combination of stereoscopy and magnetic modelling is more powerful than one of these tools
alone. We test our method with the help of a model active region and plan to apply it to the solar case as soon as STEREO
data become available. 相似文献
2.
Automatic Detection and Classification of Coronal Holes and Filaments Based on EUV and Magnetogram Observations of the Solar Disk 总被引：1，自引：0，他引：1
A new method for the automated detection of coronal holes and filaments on the solar disk is presented. The starting point is coronal images taken by the Extreme Ultraviolet Telescope on the Solar and Heliospheric Observatory (SOHO/EIT) in the Fe ix/x 171 Å, Fe xii 195 Å, and He ii 304 Å extreme ultraviolet (EUV) lines and the corresponding full-disk magnetograms from the Michelson Doppler Imager (SOHO/MDI) from different phases of the solar cycle. The images are processed to enhance their contrast and to enable the automatic detection of the two candidate features, which are visually indistinguishable in these images. Comparisons are made with existing databases, such as the He i 10830 Å NSO/Kitt Peak coronal-hole maps and the Solar Feature Catalog (SFC) from the European Grid of Solar Observations (EGSO), to discriminate between the two features. By mapping the features onto the corresponding magnetograms, distinct magnetic signatures are then derived. Coronal holes are found to have a skewed distribution of magnetic-field intensities, with values often reaching 100?–?200 gauss, and a relative magnetic-flux imbalance. Filaments, in contrast, have a symmetric distribution of field intensity values around zero, have smaller magnetic-field intensity than coronal holes, and lie along a magnetic-field reversal line. The identification of candidate features from the processed images and the determination of their distinct magnetic signatures are then combined to achieve the automated detection of coronal holes and filaments from EUV images of the solar disk. Application of this technique to all three wavelengths does not yield identical results. Furthermore, the best agreement among all three wavelengths and NSO/Kitt Peak coronal-hole maps occurs during the declining phase of solar activity. The He ii data mostly fail to yield the location of filaments at solar minimum and provide only a subset at the declining phase or peak of the solar cycle. However, the Fe ix/x 171 Å and Fe xii 195 Å data yield a larger number of filaments than the Hα data of the SFC. 相似文献
3.
O. G. Badalyan 《Astronomy Letters》2012,38(1):51-61
The 1943–2001 data on the brightness of the coronal green λ530.3 nm line are used to investigate the surface distribution of the north-south (N-S) asymmetry index A. Synoptic maps of the asymmetry index in 784 successive Carrington rotations have been constructed. The results are presented
in the form of a movie that visualizes the time variation in the spatial distribution of the asymmetry. Examination of a series
of synoptic maps shows that the time variation in the general distribution of the A index over the solar surface has a number of peculiar features. In particular, the latitude-longitude regions with the dominance
of the green line brightness in one of the hemispheres are replaced by similar (in shape) regions with its dominance in the
other hemisphere after 14–18 rotations-in other words, the map, as it were, turns into its negative. This may be a manifestation
of the quasi-biennial oscillations in the N-S asymmetry. The synodic rotation period of the asymmetry “structures” has been
determined. It has turned out to be equal to the period of the fast coronal rotation mode found previously from the large-scale
brightness distribution of the coronal green line, i.e., this is 27 days on the equator and slightly more than 28 days at
high latitudes. The N-S asymmetry and its characteristics should be taken into account when considering the dynamo mechanism. 相似文献
4.
A three-dimensional coronal magnetic field is reconstructed for the NOAA active region 11158 on 14 February 2011. A GPU-accelerated direct boundary integral equation (DBIE) method is implemented which is approximately 1000 times faster than the original DBIE used on solar non-linear force-free field modeling. Using the SDO/HMI vector magnetogram as the bottom boundary condition, the reconstructed magnetic field lines are compared with the projected EUV loop structures as observed in the front-view (SDO/AIA) and the side-view (STEREO-A/B) images for the first time; they show very good agreement three-dimensionally. A quantitative comparison with some stereoscopically reconstructed coronal loops shows that the average misalignment angles in our model are at the same order as the state-of-the-art results obtained from reconstructed coronal loops. It is found that the observed coronal loop structures can be grouped into a number of closed and open field structures with some central bright coronal loop features around the polarity inversion line. The reconstructed highly sheared magnetic field lines agree very well with the low-lying sigmoidal filament along the polarity inversion line. This central low-lying magnetic field loop system must have played a key role in powering the flare. It should be noted that while a strand-like coronal feature along the polarity inversion line may be related to the filament, one cannot simply interpret all the coronal bright features along the polarity inversion line as manifestation of the filament without any stereoscopic information. 相似文献
5.
B. I. Ryabov V. P. Maksimov S. V. Lesovoi K. Shibasaki A. Nindos A. Pevtsov 《Solar physics》2005,226(2):223-237
Microwave maps of solar active region NOAA 8365 are used to derive the coronal magnetograms of this region. The technique is based on the fact that the circular polarization of a radio source is modified when microwaves pass through the coronal magnetic field transverse to the line of sight. The observations were taken with the Siberian Solar Radio Telescope (SSRT) on October 21 – 23 and with the Nobeyama Radio Heliograph (NoRH) on October 22 – 24, 1998. The known theory of wave mode coupling in quasi-transverse (QT) region is employed to evaluate the coronal magnetograms in the range of 10 – 30 G at the wavelength 5.2 cm and 50 – 110 G at 1.76 cm, taking the product of electron density and the scale of coronal field divergence to be constant of 1018 cm–2. The height of the QT-region is estimated from the force-free field extrapolations as 6.2 × 109 cm for the 20 G and 2.3 × 109 cm for 85 G levels. We find that on large spatial scale, the coronal magnetograms derived from the radio observations show similarity with the magnetic fields extrapolated from the photosphere. 相似文献
6.
Using Fe ix/x 17.1 nm observations from the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), we have identified many coronal plumes inside low-latitude coronal holes as they transited the solar limb during the late
declining phase of cycle 23. These diffuse, linear features appear to be completely analogous to the familiar polar plumes.
By tracking them as they rotate from the limb onto the disk (or vice versa), we confirm that EUV plumes seen against the disk appear as faint, diffuse blobs of emission surrounding a brighter core.
When the EIT images are compared with near-simultaneous magnetograms from the SOHO Michelson Doppler Imager (MDI), the low-latitude, on-disk plumes are found to overlie regions of mixed polarity, where small
bipoles are in contact with unipolar flux concentrations inside the coronal hole. The birth and decay of the plumes are shown
to be closely related to the emergence of ephemeral regions, their dispersal in the supergranular flow field, and the cancellation
of the minority-polarity flux against the dominant-polarity network elements. In addition to the faint polar and nonpolar
plumes associated with ephemeral regions, we note the existence of two topologically similar coronal structures: the giant
plume-like features that occur above active regions inside coronal holes, and the even larger scale “pseudostreamers” that
separate coronal holes of the same polarity. In all three cases, the basic structure consists of open field lines of a given
polarity overlying a photospheric region of the opposite polarity; ongoing interchange reconnection at the X-point separating
the open field domains from the underlying double-arcade system appears to result in the steady evaporation of material from
the closed into the open region. 相似文献
7.
The common observation that where photospheric magnetic fields are strong, the overlying corona is bright is examined quantitatively. White light coronal brightness is employed because it is independent of coronal temperature and is directly related to coronal electron density. Brightness data for the inner corona on 7 March, 1970, taken from isodensitometer traces having a resolution of 10 arc sec, are utilized. The data were obtained from photographs which exhibit distinct chromospheric features 5 arc sec or smaller. These data are quantitatively compared, using cross correlation and scatter plot techniques, with the corresponding photospheric magnetic field data provided by Kitt Peak National Observatory. Cross correlation coefficients are computed between latitudes ±55° at specified heights. In general, a statistically significant positive correlation is found. The correlation decreases with height in the corona. However, a range of values in several parameters remains to be investigated so the physical significance of the presently observed correlation is not yet entirely clear. We expect that refinement of our input parameters will produce a more sensitive correlation and lead to an expression for the relationship of electron density to photospheric magnetic field strength.NAS/NRC Research Associate. 相似文献
8.
N. Narukage T. Sakao R. Kano H. Hara M. Shimojo T. Bando F. Urayama E. DeLuca L. Golub M. Weber P. Grigis J. Cirtain S. Tsuneta 《Solar physics》2011,269(1):169-236
The X-Ray Telescope (XRT) onboard the Hinode satellite is an X-ray imager that observes the solar corona with unprecedentedly high angular resolution (consistent with its 1?? pixel size). XRT has nine X-ray analysis filters with different temperature responses. One of the most significant scientific features of this telescope is its capability of diagnosing coronal temperatures from less than 1 MK to more than 10 MK, which has never been accomplished before. To make full use of this capability, accurate calibration of the coronal temperature response of XRT is indispensable and is presented in this article. The effect of on-orbit contamination is also taken into account in the calibration. On the basis of our calibration results, we review the coronal-temperature-diagnostic capability of XRT. 相似文献
9.
B. A. Lindblad 《Astrophysics and Space Science》1990,170(1-2):55-61
A correlative study is made between inferred solar sources of high-speed solar wind streams and extended white-light coronal features. The solar wind data used in the study consists of 110 co-rotating high-speed plasma streams observed from spacecraft at 1 AU in the period February 1971-December 1974; the coronal data consists of 144 equatorward extensions of polar coronal holes and 15 equatorial coronal holes, derived fromK-coronometer maps of the white-light corona during the same period. Of 110 observed solar wind streams 88 could directly be associated with an equatorward extension of a polar-cap coronal hole and 14 could be associated with a low-latitude equatorial coronal hole. In 8 cases no visible coronal feature was identified. Of 144 identified polar-cap extensions 102 were associated with a high-speed stream observed at 1 AU; 19 coronal features were related in time to data gaps in the solar wind measurements, while 38 features did not give rise to solar wind streams observed at Earth orbit. The probability of an association depended on the heliographic co-latitude of a polar hole extension, being 50% for a polar lobe extending down to 45° co-latitude and 100% for a polar coronal hole extending to 80° co-latitude or more.Paper presented at the 11th European Regional Astronomical Meeting of the IAU on New Windows to the Univese, held 3–8 July, 1989, Tenerife, Canary Islands, Spain. 相似文献
10.
Various observations indicate that coronal holes generally appear as low brightness temperature regions (LTRs) in the centimeter and millimeter wavelength ranges. However, within their borders local enhancements of radiation, that is, high brightness temperature regions (HTRs), often occur. The theory behind the described behavior is not fully understood and therefore we analyze full-disk solar images obtained at a wavelength of 8 mm at Metsähovi Radio Observatory and compare them with data simultaneously taken in other wavelength ranges. The observational finding that the average brightness temperature of coronal holes is not much different from the quiet-Sun level (with localized deviations toward higher and lower intensities on the order of a few percent) is compared with theoretical models of the thermal bremsstrahlung radiation originating in the solar chromosphere, transition region, and corona. Special attention is devoted to the interpretation of the localized enhancements of radiation observed inside coronal holes at millimeter wavelengths. The main conclusion is that the most important contribution to the brightness temperature comes from an increased density in the transition region and low corona (i.e., at the heights where the temperature is below 106 K). This can explain both the LTRs and HTRs associated with coronal holes. 相似文献
11.
Ester Antonucci 《Solar physics》1974,34(2):471-476
A cross-correlation analysis of coronal green line intensity (5303 Å) and interplanetary magnetic field polarity for the period 1947–1970 shows that the coronal features are organized in a constant pattern with respect to the 4-sector structure through the solar cycle. A sudden inversion of the coronal pattern with respect to the sector structure takes place at the solar minima. The high emission regions of the green corona are located near the solar magnetic sector boundaries having polarities (?, +), (+, ?), (?, +) during cycles 18, 19, 20 respectively in the northern hemisphere, and (+, ?), (?, +), (+, ?) in the southern hemisphere. 相似文献
12.
L. K. Harra T. Magara H. Hara S. Tsuneta T. J. Okamoto A. J. Wallace 《Solar physics》2010,263(1-2):105-119
Active region magnetic flux that emerges to the photosphere from below will show complexity in the structure, with many small-scale fragmented features appearing in between the main bipole and then disappearing. Some fragments seen will be absorbed into the main polarities and others seem to cancel with opposite magnetic field. In this paper we investigate the response of the corona to the behaviour of these small fragments and whether energy through reconnection will be transported into the corona. In order to investigate this we analyse data from the Hinode space mission during flux emergence on 1?–?2 December 2006. At the initial stages of flux emergence several small-scale enhancements (of only a few pixels size) are seen in the coronal line widths and diffuse coronal emission exists. The magnetic flux emerges as a fragmented structure, and coronal loops appear above these structures or close to them. These loops are large-scale structures – most small-scale features predominantly stay within the chromosphere or at the edges of the flux emergence. The most distinctive feature in the Doppler velocity is a strong ring of coronal outflows around the edge of the emerging flux region on the eastern side which is either due to reconnection or compression of the structure. This feature lasts for many hours and is seen in many wavelengths. We discuss the implications of this feature in terms of the onset of persistent outflows from an active region that could contribute to the slow solar wind. 相似文献
13.
Neupert W.M. Newmark J. Delaboudinière J.-P. Thompson B.J. Catura R.C. Moses J.D. Gurman J.B. Portier-Fozzani F. Maucherat A.J. Defise J.M. Jamar C. Rochus P. Dere K.P. Howard R.A. Michels D.J. Freeland S. Lemen J.R. Stern R.A. 《Solar physics》1998,183(2):305-321
Solar EUV images recorded by the EUV Imaging Telescope (EIT) on SOHO have been used to evaluate temperature and density as a function of position in two largescale features in the corona observed in the temperature range of 1.0–2.0 MK. Such observations permit estimates of longitudinal temperature gradients (if present) in the corona and, consequently, estimates of thermal conduction and radiative losses as a function of position in the features. We examine two relatively cool features as recorded in EIT's Feix/x (171 Å) and Fexii (195 Å) bands in a decaying active region. The first is a long-lived loop-like feature with one leg, ending in the active region, much more prominent than one or more distant footpoints assumed to be rooted in regions of weakly enhanced field. The other is a near-radial feature, observed at the West limb, which may be either the base of a very high loop or the base of a helmet streamer. We evaluate energy requirements to support a steady-state energy balance in these features and find in both instances that downward thermal conductive losses (at heights above the transition region) are inadequate to support local radiative losses, which are the predominant loss mechanism. The requirement that a coronal energy deposition rate proportional to the square of the ambient electron density (or pressure) is present in these cool coronal features provides an additional constraint on coronal heating mechanisms. 相似文献
14.
15.
Bidirectional coronal type III bursts are modeled by combining a model of coronal electron heating and beam generation via time-of-flight effects with semiquantitative estimates of quasilinear relaxation. Electromagnetic emissivities are estimated by extending the recently developed theory of interplanetary type III bursts to coronal emissions, including its features of stochastic Langmuir-wave growth and three-wave interactions. The results are investigated for heating on open and closed coronal field lines and are compared with observations of normal, reverse-slope, bidirectional, and inverted-J and -U coronal type III radio bursts. Harmonic emission is predicted to dominate at plasma frequencies above roughly 100 MHz where the efficiency of fundamental emission falls off steeply, while its free-free reabsorption rises. The model also explains the observed trends in the likelihood of occurrence of normal, reverse-slope, and bidirectional coronal type III bursts. 相似文献
16.
New X-ray observations of the north polar region taken from the X-ray Telescope (XRT) of the Hinode spacecraft are used to analyze several time sequences showing small loop brightenings with a long ray above. We focus on
the formation of the jet and discuss scenarios to explain the main features of the events: the relationship with the expected
surface magnetism, the rapid and sudden radial motion, and possibly the heating, based on the assumption that the jet occurs
above a null point of the coronal magnetic field. We conclude that 2-D reconnection models should be complemented in order
to explain the observational details of these events and suggest that alternative scenarios may exist. 相似文献
17.
We report on initial observations of coronal holestructure and evolution by the Coronal Diagnostic Spectrometer (CDS) instrument
on board the Solar and Heliospheric Observatory (SOHO). The data show that there is coronal activity on time scales of tens
of minutes, manifested as brightenings at chromospheric network cell junctions in extreme ultraviolet (EUV) wavelengths. There
are also significant differences in structure seen in different wavelengths in coronal holes. Finally, we show coronal hole
EUV spectra and compare them to quiet-Sun spectra, also taken by CDS. 相似文献
18.
The photospheric vector magnetic fields, H and soft X-ray images of AR 7321 were simultaneously observed with the Solar Flare Telescope at Mitaka and the Soft X-ray Telescope of Yohkoh on October 26, 1992, when there was no important activity in this region. Taking the observed photospheric vector magnetic fields as the boundary condition, 3D magnetic fields above the photosphere were computed with a new numerical technique. Then quasi-separatrix layers (QSLs), i.e., regions where 3D magnetic reconnection takes place, were determined in the computed 3D magnetic fields. Since Yohkoh data and Mitaka data were obtained in well-arranged time sequences during the day, the evolution of 3D fields, H features and soft X-ray features in this region can be studied in detail. Through a comparison among the 3D magnetic fields, H features and soft X-ray features, the following results have been obtained: (a) H plages are associated with the portions of QSLs in the chromosphere; (b) diffuse coronal features (DCFs) and bright coronal features (BCFs) are morphologically confined by the coronal linkage of the field lines related to the QSLs; (c) BCFs are associated with a part of the magnetic field lines related to the QSLs. These results suggest that as the likely places where energy release may occur by 3D magnetic reconnection, QSLs play an important role in the chromospheric and coronal heating in this active region. 相似文献
19.
Formation of a Flare-Productive Active Region: Observation and Numerical Simulation of NOAA AR 11158
We present a comparison of the Solar Dynamics Observatory (SDO) analysis of NOAA Active Region (AR) 11158 and numerical simulations of flux-tube emergence, aiming to investigate the formation process of this flare-productive AR. First, we use SDO/Helioseismic and Magnetic Imager (HMI) magnetograms to investigate the photospheric evolution and Atmospheric Imaging Assembly (AIA) data to analyze the relevant coronal structures. Key features of this quadrupolar region are a long sheared polarity inversion line (PIL) in the central δ-sunspots and a coronal arcade above the PIL. We find that these features are responsible for the production of intense flares, including an X2.2-class event. Based on the observations, we then propose two possible models for the creation of AR 11158 and conduct flux-emergence simulations of the two cases to reproduce this AR. Case 1 is the emergence of a single flux tube, which is split into two in the convection zone and emerges at two locations, while Case 2 is the emergence of two isolated but neighboring tubes. We find that, in Case 1, a sheared PIL and a coronal arcade are created in the middle of the region, which agrees with the AR 11158 observation. However, Case 2 never builds a clear PIL, which deviates from the observation. Therefore, we conclude that the flare-productive AR 11158 is, between the two cases, more likely to be created from a single split emerging flux than from two independent flux bundles. 相似文献
20.
Randolph H. Levine 《Solar physics》1982,79(2):203-230
Models of open magnetic structures on the Sun are presented for periods near solar minimum (CR 1626–1634) and near solar maximum (CR 1668–1678). Together with previous models of open magnetic structures during the declining phase (CR 1601–1611) these calculations provide clues to the relations between open structures, coronal holes, and active regions at different times of the solar cycle. Near solar minimum the close relation between active regions and open structures does not exist. It is suggested that near solar minimum the systematic emergence of new flux with the proper polarity imbalance to maintain open magnetic structures may occur primarily at very small spatial scales. Near solar maximum the role of active regions in maintaining open structures and coronal holes is strong, with large active regions emerging in the proper location and orientation to maintain open structures longer than typical active region lifetimes. Although the use of He I 10830 Å spectroheliograms as a coronal hole indicator is shown to be subject to significant ambiguity, the agreement between calculated open structures and coronal holes determined from He I 10830 Å spectroheliograms is very good. The rotation properties of calculated open structures near solar maximum strongly suggest two classes of features: one that rotates differentially similar to sunspots and active regions and a separate class that rotates more rigidly, as was the case for single large coronal holes during Skylab. 相似文献